an innovative electro-optical nose for artificial...
TRANSCRIPT
PhD Dissertation
International Doctorate School in Information andCommunication Technologies
DIT - University of Trento
An Innovative Electro-Optical Nose for
Artificial Olfaction Applications
Arianna Tibuzzi
Advisor:
Prof. Giovanni Soncini
Universita degli Studi di Trento
February 2005
Abstract
Electronic noses, array of chemical cross-responsive gas sensors able to de-
tect and classify complex compounds mixtures (”odors”), are being increas-
ingly employed in food and agriculture industry, industrial process control,
environmental monitoring and as non invasive diagnostic instruments in
medical applications. Nevertheless in order to achieve a mass diffusion in
the market, a cheaper and more portable system needs to be developed. In
this dissertation work I studied, fabricated and tested a prototype of an
electro-optical nose based on a matrix of silicon integrated photodiodes and
phototransistors, employed as optical sensor transducers, coated by metal-
loporphyrins as sensing layers able to change the peak amplitude and peak
wavelength in their adsorption spectrum on exposure to volatile organic
compounds (VOCs). Since the most significant variation occurs around
440nm, new silicon photodetectors with enhanced responsivity in the blue
spectral range have been designed. Due to the employment of integrated de-
vices and on its room temperature operation, such a system offers important
advantages with respect to the existing electronic noses: low cost, weight
and size; further integration of sensors and signal processing electronics on
the same chip; low power consumption.
Keywords
[electronic nose,silicon photodiode,BJT phototransistor,metalloporphyrin,gas
sensor,ethanol]
Contents
1 Introduction 1
1.1 The Context . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 The Human Olfactory System . . . . . . . . . . . . 2
1.1.2 Natural and Artificial Olfaction . . . . . . . . . . . 2
1.1.3 Electronic Nose Applications . . . . . . . . . . . . . 5
1.2 The Problem and the Proposed Solution . . . . . . . . . . 7
1.2.1 Working in the Soret Band . . . . . . . . . . . . . . 8
1.3 Innovative Aspects . . . . . . . . . . . . . . . . . . . . . . 8
1.4 Structure of the Thesis . . . . . . . . . . . . . . . . . . . . 9
2 State of the Art 11
2.1 Electronic Nose Sensors Technology . . . . . . . . . . . . . 11
2.2 MO Metal Oxide . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 KAMINA: Chemical Gas Detector Sensor (SPECS
Inc.) . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 CP Conductive and non-Conductive Polymers . . . . . . . 13
2.4 SAW and BAW Surface and Bulk Acoustic Wave . . . . . 14
2.5 FET Field Effect Transistor . . . . . . . . . . . . . . . . . 14
2.6 QMB Quartz Micro Balance . . . . . . . . . . . . . . . . . 15
2.6.1 LIBRA NOSE (TechnoBioChip) . . . . . . . . . . . 15
2.7 FO Fiber Optic . . . . . . . . . . . . . . . . . . . . . . . . 17
2.8 Optical Technologies . . . . . . . . . . . . . . . . . . . . . 18
i
2.8.1 SMELLSEEING: A Colorimetric Electronic Nose (Chem-
Sensing Inc.) . . . . . . . . . . . . . . . . . . . . . 18
2.8.2 Optical NoseTM and BeadArrayTM (Illumina Inc.) 19
3 An Electro-Optical Nose E-ON 23
3.1 Metalloporphyrins . . . . . . . . . . . . . . . . . . . . . . . 23
3.1.1 Optical Properties . . . . . . . . . . . . . . . . . . 25
4 Silicon Integrated Photodetector Transducers 33
4.1 Metalloporphyrins Deposition Methods . . . . . . . . . . . 33
4.2 Conventional Photodiodes . . . . . . . . . . . . . . . . . . 35
4.3 Finger Photodiodes . . . . . . . . . . . . . . . . . . . . . . 39
4.3.1 Modelling . . . . . . . . . . . . . . . . . . . . . . . 40
4.3.2 Design and Fabrication . . . . . . . . . . . . . . . . 47
4.3.3 Electrical Characterization . . . . . . . . . . . . . . 53
4.3.4 Optical Characterization . . . . . . . . . . . . . . . 56
4.4 Finger BJT Phototransistors . . . . . . . . . . . . . . . . . 61
4.4.1 Electrical Characterization . . . . . . . . . . . . . . 62
4.4.2 Modelling . . . . . . . . . . . . . . . . . . . . . . . 66
4.4.3 Optical Characterization . . . . . . . . . . . . . . . 69
5 The E-O Nose System 81
5.1 Package: the Nose Nostril . . . . . . . . . . . . . . . . . . 82
5.2 The Nose Box . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2.1 Read-Out Circuit . . . . . . . . . . . . . . . . . . . 85
6 Sensors Experimental Testing 89
6.1 First campaign of Measurements . . . . . . . . . . . . . . . 92
6.2 Second campaign of Measurements . . . . . . . . . . . . . 96
6.2.1 Finger photodiodes . . . . . . . . . . . . . . . . . . 96
ii
6.3 Third campaign of Measurements . . . . . . . . . . . . . . 102
6.3.1 Finger phototransistors . . . . . . . . . . . . . . . . 102
6.4 Parasitic Porphyrin Resistance . . . . . . . . . . . . . . . 109
7 Conclusion 119
Bibliography 125
A Photodetectors Layout 129
B Nostril Packaging & Bonding 131
B.1 Sensors Board . . . . . . . . . . . . . . . . . . . . . . . . . 131
B.2 Sensors package . . . . . . . . . . . . . . . . . . . . . . . . 134
iii
List of Tables
1.1 Mimicking the Human Olfactory System . . . . . . . . . . 4
1.2 EN application fields . . . . . . . . . . . . . . . . . . . . . 6
4.1 Wavelength indexes for the spectral responsivity peaks. . . . 51
4.2 Measured Dark Current and Breakdown Voltage for all the
fabricated photodiodes. . . . . . . . . . . . . . . . . . . . . 55
4.3 Maximum measured responsivity values for all photodiodes
in the blue spectral range at 5V reverse bias. . . . . . . . . 57
4.4 Measured responsivity values for N+PD40 and N+PDstand
for increasing reverse bias (ionization multiplication). . . . 61
4.5 Maximum β current gain and correspondent base current for
all phototransistors. . . . . . . . . . . . . . . . . . . . . . . 63
6.1 Conversion from percentages of Ethanol vapor in the flow to
ppm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
v
List of Figures
1.1 Schematic image of the human olfactory system. . . . . . . 3
1.2 The multidimensional sensor space of the EN output. . . . 5
1.3 Plot of the first 2 principal components providing a good
clustering and separation of different olive oils. . . . . . . . 6
2.1 Image of a quartz crystal with gold electrodes. . . . . . . . 16
2.2 Equivalent circuit of a QMB. . . . . . . . . . . . . . . . . 16
2.3 Image of Tor Vergata Libra Nose, commercially available
from Technobiochip. . . . . . . . . . . . . . . . . . . . . . . 17
2.4 Zoom of the sensors nostril, showing the 8 QMB inserted in
8 oscillating circuits. . . . . . . . . . . . . . . . . . . . . . 17
2.5 Photograph of the latest Tor Vergata nose upgrade: En-Qube. 17
2.6 Schematics of the Colpitts sinusoidal oscillator employed to
extract the quartz frequency variation. . . . . . . . . . . . . 17
2.7 Comparison of Zn(TPP) spectral shifts upon exposure to
ethanol and pyridine (py). a) In methylene chloride so-
lution; b) on the reverse phase support. In both a) and
b), the bands correspond, from left to right, to Zn(TPP),
Zn(TPP)(C2H5OH) and Zn(TPP)(py), respectively. . . . . 19
vii
2.8 Color change profiles of a metalloporphyrin sensor array. A)
Color change profiles of the metalloporphyrin sensor array
as a function of exposure time to nbutylamine vapor; B)
Color change profiles for a series of vapors: the degree of
ligand softness increases from left to right, top to bottom. . 20
2.9 Left: optical fiber bundle; Middle: microscopic etched well
at the end of each individual fiber; Right: one bead is drawn
in each well. . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1 Block diagram of the electro-optical nose system with the two
types of sensors employed: (a) photodiodes, and (b) BJT
phototransistors. . . . . . . . . . . . . . . . . . . . . . . . 24
3.2 Basic molecular structure of a porphyrin. The central metal
can host many of the metal of the periodic table, while at the
R, R’ position lateral groups can be linked. . . . . . . . . . 25
3.3 Characteristic absorption spectrum of metalloporphyrins. . 26
3.4 Picture of the experimental set-up used for spectroscopic mea-
surements of metalloporphyrins absorption spectrum varia-
tions with VOCs. . . . . . . . . . . . . . . . . . . . . . . . 27
3.5 Transmission spectra of Zn-T(heptyloxy)PP (a) with no TEA
and (b) with no EtOH and both after 2’,4’,12’ from injection.
The spectrum variations are reversible. . . . . . . . . . . . 28
3.6 Transmission spectra of Mn-T(hexadodecyloxy)PP with no
EtOH and after 2’,4’,12’ from injection. The spectrum vari-
ation is reversible. . . . . . . . . . . . . . . . . . . . . . . . 29
3.7 Molecular structure of a T(heptyloxy)PP, a porphyrin with
added alkyl chains. . . . . . . . . . . . . . . . . . . . . . . 30
viii
3.8 Sensor response time versus the length of the alkyl chain of
the sensitive molecule. The response time is defined as the
time necessary to reach a steady sensor signal after analyte
introduction into the sensor chamber. . . . . . . . . . . . . 31
3.9 UV-VIS spectra of Co-TPP (-), Co-TPP-7 (.-.), CoTPP-12
(...), CoTPP-18 (- -). . . . . . . . . . . . . . . . . . . . . 31
4.1 One of the conventional photodiodes employed in the first
matrix after deposition of Mn-TPP-Cl by air brush. . . . . 34
4.2 One of the conventional photodiodes employed in the first
matrix after deposition of Zn-T(heptyloxy)PP by evaporation. 35
4.3 Image of part of the silicon photodiodes matrix. . . . . . . 36
4.4 Responsivity spectra of several types of fabricated photodi-
odes. The responsivity of the one employed in the E-ON
project is marked with a star. . . . . . . . . . . . . . . . . 36
4.5 Absorbed optical power by silicon for 440nm light vs distance
from Si incident surface according to the Lambert-Beer Law:
P0 is the incident optical power, α is the absorption coeffi-
cient, R1 and R2 are the reflectivities without and with the
SiO2 ARC (Anti Reflective Coating) on top. . . . . . . . . 38
4.6 First packaged matrix of silicon photodiodes . . . . . . . . 38
4.7 Drawing of the package metal line and pads and the sensor
matrix. The capital letters show the locations of the spray-
coated photodiodes. . . . . . . . . . . . . . . . . . . . . . . 38
4.8 Cross-section of the two types of finger-junction photodiodes
designed: a)P+PDxx, p+-finger anode in the n-substrate,
and b) N+PDxx, n+-finger cathode in a p-well implanted in
the n-substrate. . . . . . . . . . . . . . . . . . . . . . . . . 39
ix
4.9 2D plot, produced by SILVACO, of the distribution of elec-
trons concentration in a finger photodiode with 10µm inter-
digit distance. . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.10 2D plot, produced by SILVACO, of the distribution of holes
concentration in a finger photodiode with 10µm interdigit
distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.11 Electric field, electrons and holes concentrations plots result-
ing from 2 X-cuts at (a) 10nm and (b) 100nm (distance from
Si surface) of the 2D structure in Fig. 4.10. . . . . . . . . 43
4.12 Electric field, electrons and holes concentrations plots result-
ing from 2 X-cuts at (a) 200nm and (b) 300nm (distance
from Si surface) of the 2D structure in Fig. 4.10. . . . . . 44
4.13 Electric field, electrons and holes concentrations plots result-
ing from an Y-cut between the second and third finger of the
2D structure in Fig. 4.10 . . . . . . . . . . . . . . . . . . . 45
4.14 2D plots of electron density in a finger (left side) and stan-
dard photodiode (right side) with no bias applied (results
obtained with ISE-TCAD after running DESSIS and TEC-
PLOT for viewing): the blue color represents a high holes
concentration (p-well), green represents the junction area
and holes depletion region. . . . . . . . . . . . . . . . . . . 46
4.15 Simulated responsivity spectra of standard and finger pho-
todiodes with different interdigit distance (results obtained
with ISE-TCAD after running DESSIS and OPTIK for op-
tical generation). Only the latters show high peaks in the
blue region, around 400nm, with a maximum value for the
maximum interdigit distance, d=40µm. . . . . . . . . . . . 47
x
4.16 Simulation results of the dark current dependence on (a) S0,
surface recombination velocity, for finger photodiodes and
(b) τg, carriers lifetime for conventional photodiodes. . . . 48
4.17 Image of the layout of PD20, finger photodiode with d=20µm
interfinger distance. . . . . . . . . . . . . . . . . . . . . . . 50
4.18 Micrographs of two finger photodiodes after Zn-T(heptyloxy)PP
evaporation. . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.19 Micrograph of the standard photodiode after Zn-T(heptyloxy)PP
evaporation. . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.20 Schematic cross section of the finger photodiode (cut in the
middle of the area in Fig. 4.17) at the end of the fabrication
process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.21 Measured IV curves for the two types of photodiodes, (a)
N+PDxx, and (b) P+PDxx and comparison with the stan-
dard fully implanted photodiode. . . . . . . . . . . . . . . . 54
4.22 Comparison between the IV curves of the two types of pho-
todiodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.23 Breakdown voltage versus impurity concentration for one-
sided abrupt doping profile with cylindrical and spherical
junction geometries, where rj is the radius of curvature. . . 56
4.24 Finger photodiodes spectral responsivity at 5V reverse bias
for increasing interdigit distance and standard photodiode:
a) for n+-finger cathode in a p-well photodiode, N+PDxx;
b) for p+-finger anode in the n-substrate photodiode, P+PDxx. 58
4.25 PD40 spectral responsivity for increasing reverse bias volt-
age: a) for n+-finger cathode in a p-well photodiode, N+PD40
(Vbreakdown=22.5V); b) for p+-finger anode in the n-substrate
photodiode, P+PD40 (Vbreakdown=53V). . . . . . . . . . . . 59
xi
4.26 Spectral responsivity for increasing reverse bias voltage for
the standard photodiode N+PDstand, n+ fully implanted in
the p-well. . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.27 Cross-section of the npn BJT with the finger-shaped Emit-
ter/Base junction. . . . . . . . . . . . . . . . . . . . . . . 62
4.28 Experimental Gummel plots (Ib and Ic vs Vb) for 3 different
BJTs: BJT0 standard, BJT10 and BJT40. . . . . . . . . . 63
4.29 BJT current gain Beta vs collector current Ic for increasing
interfinger distance and the standard BJT. . . . . . . . . . 64
4.30 Dependence of the fingers perimeter on the fingers number. 65
4.31 Dependence of the base current on the fingers perimeter. . 66
4.32 Dependence of the current gain on the fingers perimeter. . 66
4.33 2D plots, generated by ISE-TCAD Tools with the software
Tecplot, representing the holes current density in the vicinity
of the E/B junction for the BJT0 (top left), BJT10 (top
right) and BJT40 (bottom left). . . . . . . . . . . . . . . . 67
4.34 Holes density along X resulting from an Y-cut at 1µm dis-
tance from Si surface (just below the finger implants) of the
three structures in Fig 4.33. . . . . . . . . . . . . . . . . . 68
4.35 SRH recombination distribution in the three structures of
Fig 4.33: the prevailing yellow color in BJT40 around the
E/B junction stands for a smaller recombination and con-
sequently a smaller Ir component of total Ib. . . . . . . . . 69
4.36 Finger phototransistors spectral responsivity at Vcc=5V for
increasing interdigit distance and standard BJT, with small
light beam (only E/B junction contribution). . . . . . . . . 70
xii
4.37 Comparison between spectral responsivities of finger photo-
diodes and phototransistors with respectively Vrev=5V and
Vcc=5V for the standard devices and the finger photodetec-
tors with d=10µm and d=40µm. . . . . . . . . . . . . . . . 71
4.38 Comparison between spectral responsivities of finger photo-
diodes and phototransistors with respectively Vrev=5V and
Vcc=5V for: (a) PD10 and PT10; (b) PD20 and PT20; (c)
PD30 and PT30; (d) PD40 and PT40. . . . . . . . . . . . 73
4.39 Phototransistors spectral responsivity at Vcc=5V and 15V
for standard BJT and (a) PT10 with small light beam (only
E/B junction contribution); (b) PT10 and PT40 with large
light beam (also B/C lateral junction contribution). . . . . 75
4.40 Spatial responsivity of BJT10 at different wavelengths when
scanned horizontally by a 200µm beam with a 23µm step. . 78
4.41 Zoom of the curves at 350 and 400nm in Fig. 4.40. . . . . 78
5.1 Picture of the assembled sensors matrix after die and wire
bonding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2 Image of the package used as nose nostril to lodge the sen-
sor matrix: (a) first prototype; (b) second prototype, for a
differential measurements configuration. . . . . . . . . . . . 82
5.3 Two images of the first chamber where the sensor matrix was
placed, provided with holes for electrical connections access
and gas flow. . . . . . . . . . . . . . . . . . . . . . . . . . 83
xiii
5.4 Picture of the top of the open nose box. The circuit board
is on top left with the three operational amplifiers and their
respective RC feedback networks; the batteries container (for
op amps power supply) is on bottom left, close to the LED
intensity selector, which in turn is near the 3 outputs BNCs
(vertical); the castle with the sensors and LED board in on
the right (only the backside of the LED board is visible) with
the 2 plastic tubes for gas flow coming in and out; on top of
the picture, on the left the 3 connectors for voltage biasing of
the 3 sensors rows are visible, together with the 3 selectors
(vertical) of the sensor in each row. . . . . . . . . . . . . . 84
5.5 Schematic image of the castle: left, explosion of the three
boards: the one mechanically fixed to the metal box, from
which all electrical connections to the sensors matrix and
the LED source start; middle, removable part of the castle,
made up by the sensors board and the LED board (Fig. 5.6);
right, complete close castle. . . . . . . . . . . . . . . . . . . 85
5.6 Picture of the top part of the castle: the LED board is at-
tached to the sensors board through a series of connectors
and screws, easily removable. The two pieces of hose for gas
flow are visible on the lateral sides of the sensors package,
going in and out. . . . . . . . . . . . . . . . . . . . . . . . 86
5.7 Schematic of the circuit configuration for output signal ex-
traction of (a) photodiodes and (b) phototransistors sensors. 87
5.8 Electrical scheme of a single signal extraction channel. The
4 BJTs are placed in the matrix package (Fig. 5.2(b)) on the
sensors board (Fig. 5.1), while the I-V converter is mounted
on a separated circuit board (Fig. 5.4). . . . . . . . . . . . 87
xiv
6.1 Picture of the experimental set up: first version of the E-
ON metal box, coupled to a manual monochromator, which
in turn is coupled to a white halogen lamp. The gas bench
is completely visible, with the 4-channels flow meter, the
ethanol bubbler and the hoses. . . . . . . . . . . . . . . . . 90
6.2 Output voltage versus time of 7 measurement cycles at de-
creasing EtOH concentrations with a standard photodiode-
based sensor, spray-coated with Co-T(hexadodecyloxy)PP. . 93
6.3 Output voltage versus time of 7 measurement cycles at de-
creasing EtOH concentrations with a standard photodiode-
based sensor, spray-coated with Zn-T(butyloxy)PP. . . . . . 93
6.4 Four voltage output variations for the same sensor at 33%
EtOH concentration for four different emission wavelengths
around the metalloporphyrin transmission peak. . . . . . . 94
6.5 Output voltage versus time of 2 measurement cycles at 20%
and 5% EtOH concentrations, by employing a photodiode de-
tector spray-coated by Zn-T(heptyloxy)PP and a light source
with 426nm emission wavelength. . . . . . . . . . . . . . . 95
6.6 Output voltage versus time of 4 measurement cycles at 2.5%,
5%, 10% and 20% EtOH concentrations by employing a pho-
todiode detector evaporated by Zn-T(heptyloxy)PP and a light
source with 440nm emission wavelength. . . . . . . . . . . 95
6.7 Output voltage versus time of 4 measurement cycles at 10%,
5%, 2%, 1% and 0.5% EtOH concentrations by employing
the finger-photodiode sensor evaporated by Zn-T(heptyloxy)PP.
The light source set-up employed is the white lamp+monochromator. 96
6.8 N+PD10 sensor response to increasing ethanol concentra-
tion for 3 different reverse bias conditions. . . . . . . . . . 97
xv
6.9 4 measurement cycles for N+PD10 sensor at 50%, 40%,
30% and 20% ethanol concentration (a) before and (b) after
applying the differential drift cancellation. . . . . . . . . . 99
6.10 4 measurement cycles for N+PD10 sensor at 10%, 5%, 2%
and 1% ethanol concentration, after applying the differential
drift cancellation. . . . . . . . . . . . . . . . . . . . . . . . 100
6.11 Zoom of the voltage output increase on exposure to 20%
EtOH concentration (Fig. 6.9). CoTPP rise is faster than
ZnTPP rise. . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.12 6 measurement cycles for BJT10 sensor at 20%, 10%, 5%,
2%, 1% and 0.5% ethanol concentration (a) before and (b)
after applying the differential drift cancellation. . . . . . . 103
6.13 6 measurement cycles for BJT10 sensor at 20%, 10%, 5%,
2%, 1% and 0.5% ethanol concentration, after 8 months
from the first experiments (Fig. 6.12(b)). . . . . . . . . . . 104
6.14 Repeatability test: 3 measurement cycles are repeated for the
same EtOH concentration, at 10%, 5%, 2%, 1% and 0.5%. 105
6.15 Response curve for photodiode and phototransistor sensor:
the latter exhibits higher response and higher sensitivity at
low concentrations (the line is steeper). . . . . . . . . . . . 106
6.16 4 measurement cycles for all types of phototransistor sen-
sors at 10%, 5%, 2% and 1% ethanol concentration, con-
ducted with the latest nose box set-up. In these plotted results
BJT10 (a) and BJT20 (b) were coated with Zn-T(heptyloxy)PP;
BJT30 (c) and BJT40 (d) were coated with Co-T(hexadodecyloxy)PP.107
6.17 Response curve for sensor N+PD10 and all phototransis-
tor sensors, BJT10, BJT20, BJT30 AND BJT40, with Zn-
T(heptyloxy)PP coating. . . . . . . . . . . . . . . . . . . . 108
xvi
6.18 Response curve for the phototransistor sensors BJT30 AND
BJT40, with Co-T(hexadodecyloxy)PP coating. . . . . . . . 109
6.19 (a) Layout of one of the finger photodetector; (b) schematic
cross view of the critical electrical path that gives place to
the parasitic parallel resistor. . . . . . . . . . . . . . . . . . 111
6.20 Measurement cycles at a wide range of ethanol concentra-
tions, from 50% till 0.5%, for different photodiode reverse
bias voltage: (a) 5V, (b) 10V. . . . . . . . . . . . . . . . . 113
6.21 Different zooms of the plot in Fig. 6.20(a): (a) the ”spike” is
in fact a change in the response variation (from decreasing
to increasing output voltage); (b) low EtOH concentrations
cycles: from 20% recovery phase, the optical sensing mech-
anism becomes dominant and the parasitic resistor disappears.114
6.22 Measurement cycles at the same EtOH concentrations em-
ployed in Fig. 6.20(a) and Fig. 6.20(b) with the LED off in
order to test only the conductivity increase of metallopor-
phyrin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
6.23 Parasitic response to 40% EtOH concentration of the sensor
BJT40 coated with (a) Co-T(hexadodecyloxy)PP and (b) Zn-
T(heptyloxy)PP, at 20C. . . . . . . . . . . . . . . . . . . . 116
6.24 Parasitic response to 40% EtOH concentration of the sensor
BJT40 coated with (a) Co-T(hexadodecyloxy)PP and (b) Zn-
T(heptyloxy)PP, at 10C. . . . . . . . . . . . . . . . . . . . 117
A.1 Images of the layout of four photodiodes. PD20 layout is
shown in Fig. 4.17. . . . . . . . . . . . . . . . . . . . . . . 130
B.1 Schematic top view of the photodiodes sensors matrix board
with the wall isolating the bottom row for differential mea-
surements. . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
xvii
B.2 Schematic top (left) and backside (right) view of the photo-
transistors sensors matrix board. . . . . . . . . . . . . . . . 133
B.3 Schematic design of the matrix package: (a) first prototype;
(b) second prototype, for a differential measurements config-
uration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
B.4 Measures of the matrix package: (a) first prototype; (b) sec-
ond prototype, for a differential measurements configuration. 134
xviii
Chapter 1
Introduction
1.1 The Context
Human sense can be divided in two main categories: physical senses (tac-
tile, sight, hear) and chemical senses (smell and taste). The latter operate
at unconscious level and the perceptions are not fully expressed. For ages,
the human nose has been an important tool in assessing the quality of
many products. In 1982 the possibility to mimic the human olfaction, by
the development of an ”objective” means for using ”subjective” informa-
tion confined in the smell, became real: Persaud and Dodd introduced the
concept of the Electronic Nose [1]. Since then, several efforts have been car-
ried on to find out new technologies and suitable data analysis instruments
to face this big interdisciplinary scientific challenge. The development and
build-up of an electronic nose bring together several skills belonging to
different scientific fields: electronics, physics, chemistry, biology, computer
science, mathematics and even medicine and telecommunications in some
cases. This derives from being a complex system that integrates microelec-
tronic devices with chemical sensing layers and signal processing electronics
with pattern recognition algorithms and data analysis techniques, with a
big variety of applications.
The contribution given by this dissertation work belongs mainly to the
1
CHAPTER 1. INTRODUCTION
electronic research area, more specifically dealing with the design, fabrica-
tion and testing of a new class of chemical gas sensors to be assembled in
a matrix to build up the integrated nostril of an innovative electro-optical
nose.
1.1.1 The Human Olfactory System
In order to develop an electronic nose, it is useful to examine the phys-
iology behind olfaction since biological olfactory systems contain many
of the desired properties for electronic noses. The mammalian olfactory
system uses a variety of chemical sensors, known as olfactory receptors,
combined with an automated pattern recognition system incorporated in
the olfactory bulb and higher portions of the brain. Fig. 1.1 illustrates
the major components and function of the mammalian olfactory system.
When we inhale, odors reach the nasal chamber. The odorant molecules
interact with the olfactory receptors, distributed in the epithelium; there
are approximately ten million receptor cells in the human nose and each
receptor is sensitive to a great number of compounds. They have to pro-
vide a bio-chemical transduction and amplification, they are not specific
and they are redundant. After the chemical stimulation, they produce an
electrical stimulus (the response time is in the order of seconds), that is
transmitted to the neurons of the olfactory bulb. The neurons in the bulb
form a network able to perform a first step processing of the information.
A second network of neurons located in the olfactory cortex is responsible
for the final processing which makes us experience consciously the odor
perception [2] [3] [4] [5].
1.1.2 Natural and Artificial Olfaction
The fundamental olfaction components are:
2
1.1. THE CONTEXT
Figure 1.1: Schematic image of the human olfactory system.
• Sampling and Delivering system
• Measurement chamber
• Sensors
• Signal processing
• Pattern recognition
The comparative Table 1.1 summarizes the main features of the natural
and artificial olfactory systems [6].
The architecture of an Electronic Nose (EN) mimics the biological ol-
factory components. It consists of an array of chemical cross-responsive
sensing elements, packaged to make up the measurement chamber, an
electronic interface providing signal conditioning and pre-processing and
a pattern recognition system able to create a database of signatures of the
different odorants. Each odorant or volatile compound presented to the
sensor array produces a characteristic pattern. By exposing many distinct
odorants to the sensor array, the database is built up and then used to
3
CHAPTER 1. INTRODUCTION
Table 1.1: Mimicking the Human Olfactory System
NATURAL OLFACTION ARTIFICIAL OLFACTION
Receptors
• Non selective
• Ultra High Redundancy (108)
• Biochemical Transduction
• Signal: pattern of spikes
Sample Delivery
• Actuation of sniffing
• Two sources of odor (outside and
mouth)
Signal Processing
• Data synthesis
Data Analysis
• Ultra Wide Database
• Drift compensation
• High integration with other senses
Sensors
• Non selective
• Low Redundancy (10)
• Chemical Transduction
• Signal: steady signal
Sample Delivery
• Continuous sniffing
• One source of odor (outside)
Signal Processing
• One sensor-one signal
Data Analysis
• Limited Database
• Poor Drift compensation
• Integrability with other instruments
train the pattern recognition system. The goal of this training process
is to configure the recognition system to produce unique classifications or
clusterings of each odorant so that an automated identification can be im-
plemented.
4
1.1. THE CONTEXT
Data Analysis
A multisensor system output belongs to a multidimensional space (Fig. 1.2)
and a great care must be taken with Electronic Nose data [5]. In the sensor
space a single measure is a n-dimensional vector and only the employment
of suitable data analysis techniques (Principal or Independent Components
Analysis PCA-ICA, Artificial Neural Networks ANN) allow to visualize this
space as a 2D plot, where odors are mapped and represented in clusters
according to their similarities and differences (e.g. in Fig. 1.3).
Figure 1.2: The multidimensional sensor space of the EN output.
1.1.3 Electronic Nose Applications
The EN can be employed in every environment where the atmosphere com-
position or a characteristic odor gives relevant information. Table 1.2 re-
ports the main application areas, among which the medical ones represent
an important field in which recently electronic noses have been employed
as non invasive fast instruments for disease diagnostics [7].
5
CHAPTER 1. INTRODUCTION
Figure 1.3: Plot of the first 2 principal components providing a good clustering and sepa-
ration of different olive oils.
Table 1.2: EN application fields
INDUSTRIAL FOOD &
AGRICULTURE
MEDICAL
• Leather industry
• Olfactory Impact
• Packaging
• Automotive industry
• Space aircraft
• Perfumes
• Tobacco
• Coffee
• Wines
• Drinks
• Vegetables quality
after harvest
• Food quality
• Fish products
• Flavor enhancement
• Diabetes
• Hepatic diseases
• Bacterial infections
Skin
Breath channels
Urine
• Lung cancer
• Schizophrenia
6
1.2. THE PROBLEM AND THE PROPOSED SOLUTION
1.2 The Problem and the Proposed Solution
The several existing electronic noses are mostly based on the technologies
described in Chapter 2 and some of them are bulky, some are power con-
suming, some are operated at high temperature, some are too sensitive
to environmental parameters and become unstable in long time periods.
The motivation and goals of the EN prototype presented here match the
following needs:
• low cost, weight and size
• low power consumption
• possibility of integration of transducers and signal processing electron-
ics on the same chip
• room temperature operation
The main issue is the realization of a cost effective portable ”sniffing” de-
vice.
Moreover, lately, some experiments of odors transmission between two re-
mote places have been conducted at the University of Rome Tor Vergata
and in a future scenario the availability of a portable nose would allow to
register and transmit odors in any kind of situation and environment [8].
The solution proposed in this dissertation consists of building up a very
small nose nostril made up of a matrix of integrated transducers photode-
tectors coated with a thin layer of several different metalloporphyrins, in
order to convert the change in their absorption spectrum, related to VOCs
concentration in air, into a photogenerated current and a final output volt-
age.
7
CHAPTER 1. INTRODUCTION
1.2.1 Working in the Soret Band
On exposure to VOCs (Volatile Organic Compounds) metalloporphyrins
modify their absorption spectrum by shifting their peak towards higher/lower
wavelengths and by changing the peak amplitude value. The most signif-
icant peak variation occurs in the most intensive absorption band, the so
called Soret Band, around 440nm, therefore the sensors to be employed
have to be operated in the blue spectral region to maximize the response
to gases.
The main problem related to working with silicon photodetectors in this
spectral region is the short penetration of blue photons into the silicon sub-
strate, and consequently low responsivity and low conversion efficiency. In
order to optimize the matching between metalloporphyrin optical response
and transducer photocurrent response, novel silicon integrated photodiodes
and phototransistors have been investigated and successfully developed.
Moreover, during the thesis work both the synthesis and the deposition of
metalloporphyrins have been optimized in order to achieve a more porous
layer with a highly resolved absorption peak and a uniform deposited sens-
ing layer with a controlled and reproducible thickness. These improvements
have significantly contributed to the optimization of the sensor response.
1.3 Innovative Aspects
In the EN technology the attention and interest towards optical systems
and optical working mechanisms has grown only recently, especially after
the work done by Suslick on the ”Smell-Seeing” system (Chapter2) [9].
The approach reported in this work is based on the same sensing layer,
metalloporphyrin, and on same optical properties adopted and considered
by Suslick, taking advantage of the significant peak changes induced in
metalloporphyrins absorption spectrum by VOCs ligand binding. Differ-
8
1.4. STRUCTURE OF THE THESIS
ently, while Suslick developed an easy colorimetric technique thus minimiz-
ing the need for extensive signal transduction hardware by using a simple
commercial scanner, the EN proposed here employs metalloporphyrins as
optical filters able to modulate incident blue light according to the VOC
concentration in air, mostly due to the peak amplitude variations rather
than peak wavelength shifts (color change). The transduction is built-in
within the sensor by evaporating metalloporphyrins on the active area of
silicon photodetectors. Such a system is extremely miniaturized, low cost
and simple also with respect to similar technologies in which metallopor-
phyrins are employed with optical fibers [10].
In this dissertation, novel silicon photodetectors with enhanced responsiv-
ity in the blue spectral region have been designed and modelled starting
from a previous work by Ghazi [11]. Differently from Ghazi’s photodiodes,
two types of finger photodiodes have been implemented here, p+-anode in
n-substrate-cathode and n+-cathode in a p-well anode (implanted in the
n-substrate).
For first time finger shaped junction npn BJT have been developed with
current gains much higher than the ones exhibited by standard BJTs.
1.4 Structure of the Thesis
This dissertation has a Chapters structure in which every Chapter is di-
vided into Sections and Subsections, where necessary. After this first chap-
ter, which intends to introduce briefly the context, the motivations and the
novel sensors matrix adopted, Chapter 2 on the State of the Art gives a
much closer overview of the existing sensor technology for EN development
and highlights a few significant examples of commercial or innovative ENs.
Chapter 3 is completely devoted to the Electro-Optical nose, from the
schematics of the complete system to its working principle and chemical
9
CHAPTER 1. INTRODUCTION
description of metalloporphyrins and their optical properties.
The silicon photodetectors are extensively described in the following Chap-
ter 4, divided into 3 main Sections through which standard photodiodes,
finger-shaped photodiodes and finger-shaped phototransistors are presented
and compared, illustrating all the experimental results of their electrical
and optical characterization.
Chapter 5 rises above the sensor/device level up to a higher system level
by presenting the complete Electro-Optical nose system, from the nostril
packaging to the output signal extraction circuitry.
Chapter 6 collects all the experimental results recorded during successive
measurements campaigns carried on at different alternate times, according
to testing phases following upgrades in design and fabrication process, but
here reported all together for sake of clarity and easiness of comparison.
It is divided into 3 main sections according to the type of photodetector
employed in the sensor matrix: standard photodiode, finger photodiode,
finger phototransistor. The experimental set-up is also described in detail.
At the end of this chapter an extensive description and study of a parasitic
effect observed in the experimental results is included, being relevant for
research in metalloporphyrins and their interaction with gases.
In the last Chapter 7, conclusions are drawn and future development and
improvements are suggested.
After the Bibliography, Appendixes deal with more detailed information
about the sensor layout design and matrix packaging and bonding, infor-
mation extracted from laboratory and internal reports related to the thesis
project.
10
Chapter 2
State of the Art
2.1 Electronic Nose Sensors Technology
Sensors to be employed as receptors in an EN must meet key design pa-
rameters, such as sensitivity, speed of operation, cost, size, manufactura-
bility, the ability to operate in diverse environments, and the ability to be
automatically and quickly cleaned. The sensors must be able to adsorb
large numbers of molecules of a particular species to produce a measur-
able change. After the odorant is identified, the process must be reversed
through a cleaning process. The choice of chemical sensors to meet these
requirements is large and includes metal-oxide semiconductors (MOS),
conductive polymers (CP), conducting oligomers (CO), surface and bulk
acoustic wave (SAW, BAW) devices, quartz crystal microbalance (QMB),
chemical field effect transistors (ChemFET), fiber optic (FO) sensors, and
discotic liquid crystal (DLC) sensors. In addition, GCs and spectrome-
ters can also be used alone or in combination with the above mentioned
chemical sensors [12] [13].
11
CHAPTER 2. STATE OF THE ART
2.2 MO Metal Oxide
A metal-oxide semiconductor (MOS) sensor is a resistive device made from
a metal-oxide film (e.g., tin oxide, SnO2 [14]) deposited onto two differ-
ent types of substrates, taking into account the AppliedSensors technology
[15]: alumina substrates (thick-film sensors) and Si-micromachined sub-
strates (micro sensors). Both substrates are provided with electrodes that
enable measurement of the resistance of the sensing layer, and heaters pro-
viding for the heating of the sensing layer which needs to be operated at
high temperature, 200-400oC. The odorant molecules undergo a reduction
reaction on the film surface producing a conductivity change in the sensor.
To remove the odorant molecules, an oxidation reaction must take place.
Heaters within the sensors aid in the oxidation process.
The advantages of metal oxides include low cost, longevity, low response
to humidity, and electronic simplicity. The disadvantages include the ne-
cessity to operate at high temperatures, restrictive selectivity, high power
requirements, and modest sensitivity (5-500ppm).
2.2.1 KAMINA: Chemical Gas Detector Sensor (SPECS Inc.)
An interesting example of EN based on metal-oxide semiconductor tech-
nology is the KAMINA nose, developed at the Karlsruhe Research Center
in Germany [16]. It employs a SnO2 : Pt sensitive thin film segmented
by Pt electrodes to create a microarray of 38 sensors, heated up by 4 Pt
heaters on the backside of the substrate. By supplying the heaters with
different voltages a temperature gradient is generated along the sensor seg-
ments and the temperature distribution along the chip is controlled via 2
Pt thermoresistors on the front side of the chip.
The implementation of a temperature gradient improves gas discrimination
and sensors sensitivity and selectivity, by matching each sensitive layer with
12
2.3. CP CONDUCTIVE AND NON-CONDUCTIVE POLYMERS
its best performing temperature. Other solutions reported in literature for
generating a temperature difference along a microarray are fabricating a
PolySi variable microheater on a SiO2 released membrane [17] and plac-
ing the microheater on top of a SiO2 layer with a gradually decreasing
thickness (thinner means hotter) [16].
2.3 CP Conductive and non-Conductive Polymers
A conductive polymer (CP) sensor is a semiconducting polymer film coated
to adsorb specific species of molecules. When the odorant molecules in-
teract with the coating, the conductivity of the sensor changes. On the
contrary, in the commercially available ”Cyranose 320” Electronic Nose
(from CyranoTM Sciences [18]), each individual detector of the sensor ar-
ray is a composite material consisting of a non-conducting polymer homo-
geneously blended throughout conductive carbon graphite. The detector
materials are deposited as thin films on an alumina substrate, each across
two electrical leads thus creating conducting chemoresistors. The output
from the device is an array of resistance values. When a composite is
exposed to a vapor-phase analyte, the polymer matrix ”swells up” while
absorbing the analyte. The increase in volume causes an increase in resis-
tance because the conductive carbon-black pathways through the material
are broken. When the analyte is removed, the polymer ”sponge” off-gasses
and ”dries out”.
The advantages of conductive polymers are wide selectivity, high sensitiv-
ity (0.1-100ppm), stability and operation at ambient temperatures. The
biggest disadvantage is a strong sensitivity to humidity.
13
CHAPTER 2. STATE OF THE ART
2.4 SAW and BAW Surface and Bulk Acoustic Wave
Surface and bulk acoustic wave (SAW, BAW) sensors are piezoelectric
quartz crystals coated with selective coatings which adsorb species of mole-
cules [19]. The adsorbed molecules increase the mass of the sensor changing
its resonance frequency. By measuring this shift (also in term of phase de-
lay), a concentration of odorant can be derived.
The advantages of SAWs and BAWs include high selectivity, high sensitiv-
ity, stability over wide temperature ranges, low response to humidity and
good reproducibility. The disadvantage is the complexity in the interface
electronics.
2.5 FET Field Effect Transistor
A chemical field effect transistor (ChemFET) is based on a field effect
transistor with a catalytic metal as gate contact [8] [15]. The gate volt-
age controls the current through the MOSFET. The interaction of gases
with the catalytic gate, which adsorbs odorant molecules, induces dipoles
or charges generation, which adds up to the gate bias thus changing the
current through the transistor.
For the MOSFET sensor, gate and drain are connected and the sensor op-
erates as a two-terminal device. The voltage (around 2V) at a constant
current (100A) is monitored. The gas response is recorded as a voltage
change in the sensor signal. The operation temperatures are 150-200oC
and 200-600oC for devices respectively based on silicon and silicon carbide
as semiconductor.
The advantages include high sensitivity (ppm), high selectivity and ease
of integration with other electronics. The disadvantages include lack of
suppliers and the necessary penetration of the odorant molecules into the
14
2.6. QMB QUARTZ MICRO BALANCE
transistor gate.
2.6 QMB Quartz Micro Balance
Quartz microbalance (QMB) sensor technology is based on measuring the
frequency of quartz crystals coated by a sensing layer. The frequency is
influenced by bulk absorption of analyte molecules into the sensing ma-
trix because it is a function of the graviting mass. The sensitivity and
selectivity of the QMB sensors can be varied through the selection of dif-
ferent coatings, having different functional groups in the side chains. In a
Thickness Shear Mode Resonator, made up by an AT-cut quartz crystal
(Fig. 2.1), a linear relationship between variation of mass and variation of
resonance frequency exists (Sauerbrey Law), for small amount of mass:
∆f = − f 20
2νSρcA∆m = − f 2
0
2νSρcA×mmol × nabsorbed(pi) (2.1)
The fundamental frequency is in the range 5-30MHz and the typical sen-
sitivity is of the order of 10 ng/Hz.
The equivalent circuit of a QMB sensor consists of a serial connection of
an inductance, a capacitor and a resistor (Fig. 2.2). The additional shunt
capacitor CO refers to stray capacitance due to soldering and housing ef-
fects.
2.6.1 LIBRA NOSE (TechnoBioChip)
The design of the innovative electro-optical nose that is presented in this
thesis project taps on the long experience of the Sensors Group of the
University of Rome Tor Vergata, partner in this research. They have been
15
CHAPTER 2. STATE OF THE ART
working on the electronic nose since 1991 and they have been studying met-
alloporphyrins as sensing films since 1994; they have developed a very suc-
cessful commercialized Libra Nose [20], based on QMB technology. Fig. 2.3
shows the whole cylindrical nose, made up of two main parts: the odorant
pumping system and valves at the bottom, the nostril at the top (Fig. 2.4).
The nostril, which represents the measurement chamber, consists of eight
QMBs inserted in eight Colpitts oscillator circuits (Fig. 2.6), in order to
read-out the frequency shift of each of them when exposed to the odor-
ant. The output signals are then sent to a software via a serial connection,
able to display them simultaneously and to register them. Successively the
data array will be processed by PCA algorithms by using Matlab pattern
recognition tools and matrix calculus.
Each quartz has a resonance frequency of 20MHz and it is spray-coated by
a distinct metalloporphyrin, a not selective sensing layer able to quickly
adsorb odorant molecules and to release them at room temperature.
Fig. 2.5 shows the latest Libra version, called EN-Qube, where the gas
pumping system has been optimized in order to reduce the instrument
weight and size.
Figure 2.1: Image of a quartz crystal with
gold electrodes.
Figure 2.2: Equivalent circuit of a QMB.
16
2.7. FO FIBER OPTIC
Figure 2.3: Image of Tor Vergata Libra Nose, commercially available from Technobiochip.
Figure 2.4: Zoom of the sensors nostril,
showing the 8 QMB inserted in 8 oscillating
circuits.
Figure 2.5: Photograph of the latest Tor Ver-
gata nose upgrade: En-Qube.
Figure 2.6: Schematics of the Colpitts
sinusoidal oscillator employed to extract
the quartz frequency variation.
2.7 FO Fiber Optic
A fiber optic (FO) sensor is a conventional optical fiber coated with a
fluorescent coating which interacts with the odorant molecules [21]. An
17
CHAPTER 2. STATE OF THE ART
optical pulse is applied to the sensor and is adsorbed by the coating. The
interaction of the odorant molecules and fluorescent dyes produces a fre-
quency shift in the returned fluorescent signal. The returned signal is then
analyzed to determine the properties of the odorant molecules.
2.8 Optical Technologies
This approach was introduced by Walt in 1996 by employing fluorescent
polymers sensing layer with optical fibers [21] and started in Europe with
the work by D’Amico and Di Natale on metalloporphyrins [22], on which
this thesis project is based. The latest significant examples, reported in
the following sections, can be found in U.S.A. research institutes and com-
panies.
2.8.1 SMELLSEEING: A Colorimetric Electronic Nose (Chem-
Sensing Inc.)
This simple approach is based on sensor array detection and utilizes the
colorimetric response from a library of immobilized vapor-sensing metal-
containing dyes, metalloporphyrins [9]. They provide a way of reporting
the presence and concentration of odors by changes in color (Fig. 2.7).
Once a two-dimensional display (6x6) is arranged, a digital image before
and after exposure to the analyte is registered and a final difference map is
produced as ”molecular fingerprint” (Fig. 2.8). Suslick’s SmellSeeing has
been addressed as ”intriguing” by Lundstrom in [23] because the possibil-
ity of identifying different smells by eye could be used to monitor levels of
insecticides in the environment or to sniff out bacteria causing infections.
The replacement of pattern recognition routines and computer-made de-
cisions with the eyes of an experienced operator will have advantages in
many other situations.
18
2.8. OPTICAL TECHNOLOGIES
Figure 2.7: Comparison of Zn(TPP) spectral shifts upon exposure to ethanol and pyridine
(py). a) In methylene chloride solution; b) on the reverse phase support. In both a)
and b), the bands correspond, from left to right, to Zn(TPP), Zn(TPP)(C2H5OH) and
Zn(TPP)(py), respectively.
2.8.2 Optical NoseTM and BeadArrayTM (Illumina Inc.)
In this approach, bead sensors are fabricated by either adsorbing or co-
valently immobilizing fluorescent dyes in a polymer microsphere matrix
[24]. Responses are generated by measuring intensity changes, spectral
shift and time-dependent variations associated with the fluorescent sensors.
The bead array is assembled on an optical fiber bundle of the diameter of
1.5mm (Fig. 2.9, left), consisting of about 50000 individual fibers (Fig. 2.9,
middle), successively dipped into a chemical solution and then into a pool
of coated beads (Fig. 2.9, right).
19
CHAPTER 2. STATE OF THE ART
Figure 2.8: Color change profiles of a metalloporphyrin sensor array. A) Color change
profiles of the metalloporphyrin sensor array as a function of exposure time to nbuty-
lamine vapor; B) Color change profiles for a series of vapors: the degree of ligand softness
increases from left to right, top to bottom.
20
2.8. OPTICAL TECHNOLOGIES
Figure 2.9: Left: optical fiber bundle;
Middle: microscopic etched well at the
end of each individual fiber; Right: one
bead is drawn in each well.
21
Chapter 3
An Electro-Optical Nose E-ON
The system architecture of the electro-optical nose developed in this dis-
sertation is schematically presented in Fig.3.1: the blue light source is
a LED with emission peak around 440nm, in the Soret band, where the
absorption spectrum of metalloporphyrins exhibits a maximum peak; met-
alloporphyrins are deposited directly on the active region of the silicon
integrated photodiodes, which collect the photons coming from the LED
and generating electron-hole pairs. These charges produce a current flow
when generated in the depletion region of the p-n junction, where an elec-
tric field is present. The output current is then converted into a voltage
signal and processed to have a good S/N (Signal/Noise) ratio. A multi-
plexer can be employed to monitor and register the voltage output of each
photodiode in the array with a certain switching rate and, after an Analog
to Digital conversion, data are sent to a Pattern Recognition software for
the final mapping and response.
3.1 Metalloporphyrins
Tetra Phenyl Porphyrins (TPP) are among the most important class of
chemical families [25] [22]. They have been selected by Nature for impor-
tant biological functions such as oxygen transport in blood and photosyn-
23
CHAPTER 3. AN ELECTRO-OPTICAL NOSE E-ON
(a) Photodiodes
(b) Phototransistors
Figure 3.1: Block diagram of the electro-optical nose system with the two types of sensors
employed: (a) photodiodes, and (b) BJT phototransistors.
thesis in plants. A great number of features makes porphyrins eligible as
good sensing material able to detect the volatile organic compounds present
in the environment. Porphyrins in fact are rather stable compounds and
their properties can be finely tuned by simple modifications of their basic
molecular structure: this is planar, formed by four pyrrolic rings and me-
thine bridges making up an aromatic system of 18 π -electrons. This basic
structure can be modified complexing a metal atom at the center of the
cycle and/or adding peripheral groups (Fig.3.2). Eight metals are usually
considered for the electronic nose sensing films: Zn, Co, Cu, Mn, Ru, Rh,
Fe, Sn.
The coordinated metal, the peripheral substituents, the conformations
of the macrocyclic skeleton influence the coordination and the related sens-
ing properties of these compounds. All together these characteristics in-
24
3.1. METALLOPORPHYRINS
Figure 3.2: Basic molecular structure of
a porphyrin. The central metal can host
many of the metal of the periodic table,
while at the R, R’ position lateral groups
can be linked.
crease the versatility of these molecules and different transducers have been
proposed for porphyrin-based chemical sensors, all showing outstanding
properties of these materials in terms of stability, chemical sensitivity and
reproducibility. The adsorption properties of solid state porphyrins are
characterized by large sensitivities and wide selectivities: both of these
features are particularly appealing for electronic nose applications.
The metalloporphyrins employed in this dissertation work are the same
used in the LIBRA nose at the Univeristy of Rome Tor Vergata (Subsec-
tion 2.6.1, Chapter 2) and they have been synthesized at the Department
of Chemical Sciences and Technologies of the same university.
3.1.1 Optical Properties
The optical features shown by porphyrins and related compounds make
these molecules particularly appealing for optical sensing purposes. The
absorption and luminescence properties derive from the aromatic charac-
ter of these materials and are related to electronic transitions within their
π-aromatic system. The characteristic absorption spectrum of a metallo-
porphyrin is reported in Fig. 3.3, where the main peak in the Soret band
(blue region) and the secondary peaks in the Q-bands (green-red region)
25
CHAPTER 3. AN ELECTRO-OPTICAL NOSE E-ON
Figure 3.3: Characteristic absorption
spectrum of metalloporphyrins.
are presented.
Furthermore, due the presence of ππ interactions between macrocycles,
solid state aggregation of porphyrins can result in broadening, splitting and
shifts of the bands to respect solution spectra. Interactions with VOCs can
induce a decrease of these interactions, leading to additional modifications
of the spectra.
Variations of the absorption spectrum of the metalloporphyrins employed
in this thesis have been experimentally investigated at the Electro-Optical
Laboratory at the Department of Information and Communication Tech-
nology of the University of Trento, Povo, by conducting spectroscopic mea-
surements with the spectrometer Avaspec-2048. The light source employed
was a tungsten halogen lamp (HL-2000-FHSA, Avantes, 17mW maximum
power), coupled via optical fiber to a glass cuvette containing the solid
state metalloporhyrin deposited on a small glass by evaporation (same
thickness as the material evaporated afterwards on the sensors). The cu-
vette was kept isolated from ambient light by a special black housing. The
output light was coupled to the spectrometer via another optical fiber. The
described set-up is shown in the picture of Fig. 3.4. Fig. 3.5 and Fig.3.6 re-
port three of the measured transmission spectra of Zn-T(heptyloxy)PP and
Mn-T(hexadodecyloxy)PP, before and after the injection, by a common
26
3.1. METALLOPORPHYRINS
Figure 3.4: Picture of the experimental set-up used for spectroscopic measurements of
metalloporphyrins absorption spectrum variations with VOCs.
syringe, of saturated vapors of triethylamine (TEA) and ethanol (EtOH):
three measures at three different times after exposure (2’, 4’, 12’) have
been performed with no significant distinction in the resulted spectrum.
The absorption process by the porphyrins is totally reversible, at room
temperature and 1atm pressure. In Fig. 3.5 a big variation of the peak
amplitude is registered, while in Fig. 3.6 also a shift of the peak wave-
length towards lower values is shown.
The metalloporphyrins used in the previous measurements and eventu-
ally deposited on the sensors have been modified in their structure with a
synthesis procedure that inserts alkyl chains of variable length at the pe-
ripheral sites; depending on the number n of the CH2 groups in the chains,
metalloporphyrins are named T(butyloxy)PP if n=4, T(heptyloxy)PP if
n=7 (Fig. 3.7), T(hexadodecyloxy)PP if n=17.
27
CHAPTER 3. AN ELECTRO-OPTICAL NOSE E-ON
(a) Zn-T(heptyloxy)PP and TEA
(b) Zn-T(heptyloxy)PP and EtOH
Figure 3.5: Transmission spectra of Zn-T(heptyloxy)PP (a) with no TEA and (b) with no
EtOH and both after 2’,4’,12’ from injection. The spectrum variations are reversible.
28
3.1. METALLOPORPHYRINS
Figure 3.6: Transmission spectra of Mn-T(hexadodecyloxy)PP with no EtOH and after
2’,4’,12’ from injection. The spectrum variation is reversible.
29
CHAPTER 3. AN ELECTRO-OPTICAL NOSE E-ON
Alkyl chains increase the mutual distance between porphyrin rings, thus
reducing the mutual interactions and giving rise to a less compact film with
improved morphological properties able to provide a faster absorption of
the VOC molecules [26]. Fig. 3.8 reports the sensor response time versus
the length of the alkyl chain for similar metalloporphyrins studied by Di
Natale and Paolesse in [26]: response time is inversely proportional to the
length of the corresponding alkyl chain (e.g. for n=4 tr=850s, for n=7
tr=700s).
Moreover these modified sensing layers exhibit a more resolved absorption
spectrum, with an increase of sharpness of the Soret band with the length
of the chain, like shown in Fig. 3.9. This property affect the sensor perfor-
mance by allowing for higher resolution and sensitivity in gas detection.
Figure 3.7: Molecular structure of a
T(heptyloxy)PP, a porphyrin with added
alkyl chains.
N HNHN
NNHNH
O(CH2)7CH3O(CH2)7CH3
H3C(H2C)7OH3C(H2C)7O O(CH2)7CH3O(CH2)7CH3
O(CH2)7CH3O(CH2)7CH3
30
3.1. METALLOPORPHYRINS
Figure 3.8: Sensor response time versus the length of the alkyl chain of the sensitive
molecule. The response time is defined as the time necessary to reach a steady sensor
signal after analyte introduction into the sensor chamber.
Figure 3.9: UV-VIS spectra of Co-TPP (-), Co-TPP-7 (.-.), CoTPP-12 (...), CoTPP-18
(- -).
31
Chapter 4
Silicon Integrated Photodetector
Transducers
In the thesis work different types of silicon integrated photodetectors have
been employed as signal transducers and they have been all fabricated in
the Microfabrication Laboratory of ITC-Irst, Trento. They are responsible
for converting the chemical interaction between metalloporphyrin and VOC
molecules into an electric signal, easy to extract, store and analyze.
The main part of the thesis work has dealt with the optimization of the
photodetectors, that make up the heart of the E-ON, with the goal of taking
the biggest advantage of the spectral absorption change of porphyrins.
Since such a variation is not particularly high, the need for ad hoc designed
photodetectors has been relevant, together with the study and development
of a good deposition method for metalloporphyrins coating.
4.1 Metalloporphyrins Deposition Methods
While employing the first type of standard photodiodes, metalloporphyrins
have been deposited by spray-coating through a thin metal shadow mask
with an air-brush, mixing them in a liquid chloroform solution. Since chlo-
roform, the solvent, is very volatile, it evaporates immediately on touching
33
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
the chip target and only the porphyrin solid thin film remains on the sen-
sor. This method does not provide uniformity of the film thickness over
the photodiode area and reproducibility. Moreover common film thickness
is high, around 1µm. Fig. 4.1 shows a picture of one of the sensors after
metalloporphyrin deposition.
Figure 4.1: One of the conventional photodiodes employed in the first matrix after depo-
sition of Mn-TPP-Cl by air brush.
One of the most significant improvements of the sensors response has
been achieved by switching to deposition by evaporation: metalloporphyrin
powder is put in a small quartz cylinder, surrounded by a wire in which a
current flow provides Joule effect heating. Everything is placed in a reactor
with high vacuum and the porphyrin powder evaporates to hit the photo-
diodes dies attached to a metal target right in front. Fig. 4.2 shows the
picture of a sensor evaporated with Zn-T(heptyloxy)PP (Zn-TPP-7), one
of the two sensing layers employed in the successive experimental testing
(the second metalloporphyrin is Co-T(dodecyloxy)PP, Co-TPP-12). The
coating appears much more uniform with a controllable and thin thickness
(150nm).
34
4.2. CONVENTIONAL PHOTODIODES
Figure 4.2: One of the conventional photodiodes employed in the first matrix after depo-
sition of Zn-T(heptyloxy)PP by evaporation.
Chapter 6 reports relevant experimental differences in sensors perfor-
mance between photodiodes coated with the first and the second method
(Fig. ??).
4.2 Conventional Photodiodes
The first sensor matrix of the E-ON has been assembled by employing
”off-the-shelf” silicon conventional p-n photodiodes previously fabricated
in ITC-Irst. The picture in Fig. 4.3 shows part of the matrix. The photo-
diode active area is 710µm×710µm, while the single die, included lateral
aluminum contacts, measures 1010µm×750µm. Data sheet reports a re-
verse dark current of 100pA and reverse breakdown voltage of 52V.
Their responsivity spectrum is reported in Fig. 4.4: the maximum peak
of 0.65A/W occurs at 880nm and the spectral bandwidth is 600-1000nm
at 50% peak value. This conventional responsivity curve provides only
0.14A/W in the blue spectral region, around 430nm, in the Soret band,
which is the desired sensor working region. The absorbed optical power in
35
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.3: Image of part of the silicon photodiodes matrix.
Figure 4.4: Responsivity spectra of several types of fabricated photodiodes. The responsivity
of the one employed in the E-ON project is marked with a star.
silicon is ruled by the Lambert-Beer Law:
P = (1−R)P0e−αx (4.1)
where
P=absorbed optical power at distance x in the silicon photodiode from the
36
4.2. CONVENTIONAL PHOTODIODES
incident surface
P0=incident optical power
α=absorption coefficient with α440nm=2.7×104cm−1
R=reflectivity calculated by the following formula:
R =((n− 1)2 + e∗2)((n + 1)2 + e∗2)
(4.2)
where
n=4.823 real part of Si refractive index at 440nm
e∗=imaginary part of Si refractive index at 440nm
Fig. 4.5 reports the calculated curve of the absorbed optical power for
440nm incident light. Short wavelength photons, like the blue ones, have
a short penetration into the silicon substrate and to achieve an efficient
collection a depletion region (p-n junction) and electric field should be
create as close as possible to the silicon surface. For instance, 45% of the
incident optical power is absorbed within the first 200nm below the surface.
For a conventional p-n photodiode made in standard CMOS process almost
all of the incident photons are absorbed at a typical depth of 0.2-0.4µm
and the majority of the photogenerated carriers recombine in the p+-anode,
with a consequent low responsivity for blue light.
This is one of the main reasons of the bad experimental performance of
these first sensors, forced to work in a region with very low photon-electron
conversion efficiency.
Fig. 4.6 shows a picture of the first 3×7 matrix, already packaged: different
metalloporhyrins have been sprayed alternately on the 21 dies, to make the
selective deposition easier. In this way only 8 sensors in the matrix were
completely coated and electrically connected (like sketched in the drawing
in Fig. 4.7).
37
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.5: Absorbed optical power by silicon for 440nm light vs distance from Si incident
surface according to the Lambert-Beer Law: P0 is the incident optical power, α is the
absorption coefficient, R1 and R2 are the reflectivities without and with the SiO2 ARC
(Anti Reflective Coating) on top.
Figure 4.6: First packaged matrix of silicon
photodiodes
Figure 4.7: Drawing of the package metal line
and pads and the sensor matrix. The capital
letters show the locations of the spray-coated
photodiodes.
38
4.3. FINGER PHOTODIODES
Figure 4.8: Cross-section of the two types of finger-junction photodiodes designed:
a)P+PDxx, p+-finger anode in the n-substrate, and b) N+PDxx, n+-finger cathode in
a p-well implanted in the n-substrate.
4.3 Finger Photodiodes
In order to improve the sensor performance and overcome the undesired
low spectral responsivity explained in the previous section, new photodi-
odes have been studied and designed with space-charge regions just below
the incident surface. A finger cathode has been adopted as a solution to
move the p-n junction as close as possible to the surface: space-charge
regions between neighboring n+-fingers merge together creating a series of
photosensitive areas at the sides of the n+ implants (Fig. 4.8). A similar
design strategy was reported by Ghazi et al. [11], who inspired the develop-
ment of this new photodetector. Main differences between the two works
are mentioned in Subsection 4.3.2.
39
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
4.3.1 Modelling
Modelling was carried on with two softwares, SILVACO (ATHENA) and
ISE-TCAD, and different photodiodes geometries have been analyzed by
varying the distance between two fingers (10, 20, 30 and 40µm) while keep-
ing the finger width (5µm) and the device area constant.
Fig. 4.9 and 4.10 represent the 2D plots produced by SILVACO of the
electrons and holes concentrations, respectively, in the n+-fingers/p-well
photodiode with an interdigit distance of 10µm. Only 4 finger-implants
have been reproduced and one p-well contact on the right. The first plot
shows a series of electrons depletion regions between the fingers in the p-
well, in correspondence of a concentration of 1016 holes/cm3 (yellow region)
in Fig. 4.10, with an increase and decrease of electrons and holes densities,
respectively, when approaching the Si surface. Right below the surface
the colors legends show 108 electrons/cm3 and 104 holes/cm3, thus proving
an inversion of majority carriers and the presence of a p-n junction and a
consequent space charge region within the first 200nm below the surface
between the fingers implants. This simulation result confirmed the merg-
ing occurring between depletion regions around adjacent fingers.
In order to find some coordinates and identify the surface depletion region,
Fig. 4.11 and 4.12 report electric field and electrons and holes concentra-
tions along 4 different horizontal sections of the 2D structure in Fig. 4.10
at 10, 100, 200 and 300nm below the surface, and Fig. 4.13 shows a vertical
section between the second and third finger.
In the horizontal X-cuts plots, the curves follow the fingers shape and in the
regions between the fingers at 10nm the electrons concentration is higher
than holes, while at 100nm holes become to increase and to overcome elec-
trons till reaching the p-well concentration value of 1016 /cm3 at 300nm.
Fig. 4.13 confirms the creation of the majority carriers (holes) depletion
40
4.3. FINGER PHOTODIODES
Figure 4.9: 2D plot, produced by SILVACO, of the distribution of electrons concentration
in a finger photodiode with 10µm interdigit distance.
region very close to the Si surface by showing the intersection between the
carriers concentrations curves at about 125nm: holes increase and reach
the p-well concentration at about 400nm and the electric field linearly de-
creases to zero through the surface space charge region of about 350nm
width.
41
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.10: 2D plot, produced by SILVACO, of the distribution of holes concentration in
a finger photodiode with 10µm interdigit distance.
In the simulations performed with ISE-TCAD, particular care was taken
in the comparison between the conventional photodiode and the n+-fingers/p-
well photodiode with an interdigit distance of 40µm. Simulation results
were again successful in proving the effectiveness of the adopted solution,
confirming the creation of the surface depletion region and the consequent
responsivity increase in the blue spectral range. Fig. 4.14 shows the main
42
4.3. FINGER PHOTODIODES
(a) x=10nm
(b) x=100nm
Figure 4.11: Electric field, electrons and holes concentrations plots resulting from 2 X-cuts
at (a) 10nm and (b) 100nm (distance from Si surface) of the 2D structure in Fig. 4.10.
43
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
(a) x=200nm
(b) x=300nm
Figure 4.12: Electric field, electrons and holes concentrations plots resulting from 2 X-cuts
at (a) 200nm and (b) 300nm (distance from Si surface) of the 2D structure in Fig. 4.10.
44
4.3. FINGER PHOTODIODES
Figure 4.13: Electric field, electrons and holes concentrations plots resulting from an Y-cut
between the second and third finger of the 2D structure in Fig. 4.10
difference between a finger and a conventional photodiode, represented in
cross-section respectively with a zoom of half of the finger and half of the
n+-implant contact. Electron density is related to a color legend: red rep-
resents a high electron concentration, that is 1020/cm3 inside the finger
implant; blue represents a lack of electrons, corresponding to the p-well
zone; green represents the junction depletion layer. On the finger right
side, holes concentration decreases in the p-well thus creating a surface
depletion region < 0.1µm, with a minimum concentration of 104/cm3 at
y=0 (Si surface), while in the standard device the junction area starts only
under 0.5µm.
Fig. 4.15 reports the responsivity spectra for all the implemented de-
vices, with four different interdigit distances of 10, 20, 30, 40µm and 5µm
finger width, calculated after running cycles of electrical and optical sim-
ulations at different incident light wavelengths. The finger photodiodes
exhibit three main peaks at 420nm, 480nm and 560nm, with amplitude in-
45
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.14: 2D plots of electron density in a finger (left side) and standard photodiode
(right side) with no bias applied (results obtained with ISE-TCAD after running DESSIS
and TECPLOT for viewing): the blue color represents a high holes concentration (p-well),
green represents the junction area and holes depletion region.
creasing with interdigit distance, while the standard device presents lower
responsivity values with a maximum around 560nm. The best result is
obtained for an interdigit distance of 40µm because in this device the pho-
todetector active area is mostly covered by the surface depletion region
rather than by numerous n+-finger implants (maximum value of d/w ra-
tio). In the finger photodiodes the carriers surface recombination at the
SiO2/Si interface is fundamental while in the standard device the reverse
current is dominated by generation in the bulk. These two phenomena
are ruled by two parameters, respectively, S0, the surface recombination
velocity, and τg, the carriers lifetime, which are present in the expression
46
4.3. FINGER PHOTODIODES
Figure 4.15: Simulated responsivity spectra of standard and finger photodiodes with differ-
ent interdigit distance (results obtained with ISE-TCAD after running DESSIS and OP-
TIK for optical generation). Only the latters show high peaks in the blue region, around
400nm, with a maximum value for the maximum interdigit distance, d=40µm.
of the reverse current with a surface and bulk contribution. Simulations
run with different values of S0 (4, 6, 10, 30, 60cm/s) and τg (10−4, 10−3,
5×10−3, 10−2, 2×10−2s) for both the devices confirmed the strong depen-
dence of the finger photodiodes from S0 and the weak dependence from
τg, and vice versa for the standard detector. The simulation results are
plotted in Fig. 4.16.
4.3.2 Design and Fabrication
The design of the new photodiodes layout followed the simulations and ac-
cording to the modelling results, 4 different geometries of finger-photodiodes
have been designed together with a conventional fully implanted photodi-
ode for reference.
47
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
(a) finger photodiode: dependence on S0
(b) standard photodiode: dependence on τg
Figure 4.16: Simulation results of the dark current dependence on (a) S0, surface re-
combination velocity, for finger photodiodes and (b) τg, carriers lifetime for conventional
photodiodes.
The description of the devices is listed below with the main differences
between this work and Ghazi’s previous work:
48
4.3. FINGER PHOTODIODES
• two types of photodiode have been implemented here: p+-finger an-
ode in the n-silicon substrate cathode (named P+PD10/20/30/40),
and n+-finger cathode in a p-well anode implanted in the n-substrate
(named N+PD10/20/30/40) (Fig. 4.8), while Ghazi studied and fab-
ricated only the former type;
• in both works, in order to find the optimized interdigitated structure,
finger photodiodes with different numbers of fingers for a constant area
of the photodiodes have been processed to study different combina-
tions of d/w ratio (d=interfinger distance, w=finger implant width).
Here the finger width is kept constant (in both cases it has been cho-
sen minimal according to the design rules, 5µm here and 1.2µm by
Ghazi) and the interfinger distance varies, while Ghazi changed both
the geometrical parameters, keeping the ratio between 4 and 5; the
d/w ratios considered here are the following:
40/5=8
30/5=6
20/5=4
10/5=2
with the highest being the most performing in the blue/UV spectral
range (Fig. 4.24(a)), as expected by Ghazi’s considerations [11].
• the creation of the surface depletion region is here significantly affected
by the positive charge trapped in the SiO2 passivation film on top of
the photodiode, with a concentration of about 5×1011cm−2;
• Ghazi’s goal was optimizing the finger photodiode for the UV/blue
and red spectral ranges at the same time, by varying the doping con-
centration of the epitaxial layer, and he was concerned with both
responsivity and response time, while here the target spectral range
49
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.17: Image of the layout of PD20, finger photodiode with d=20µm interfinger
distance.
is the blue one (and only one doping concentration has been used for
the epitaxial layer) and response speed is not an issue (due to the slow
chemical interactions in the sensor). In this thesis work more care has
been taken in the study of the responsivity dependence on interfin-
ger distance d and on the photodiode type (finger-shaped anode or
finger-shaped cathode?).
Fig. 4.17 shows the image of the layout of one of the finger photodiode,
with 20µm interfinger distance. The analogue layouts of the other devices,
PDstand, PD10, PD30, PD40, are collected in Appendix A. For a detailed
description of the layout, dies names and identification on wafer, see the
internal technical report [27]. The fabrication process is based on the
conventional planar silicon process technology: it is a 6 masks run and
low resolution optical photolitography has been used. The active area of
the photodiode measures 714µm× 744µm (p-well area). The Si substrate
is 600µm thick and has a high resistivity, with a donors concentration
of 1011/cm3; the implanted p-well is 2µm thick and boron doped with a
50
4.3. FINGER PHOTODIODES
maximum concentration of 4×1016/cm3 at 0.9µm; the n+-fingers have been
implanted in the p-well with 1020/cm3 atoms of arsenic. Aluminum metal
lines contacting the fingers have been designed in order to allow a fast
collection of the carriers through the whole active region and two pads
give access to anode and finger-cathode (Fig. 4.17). The final LTO (Low
Temperature Oxide) layer deposited on top of the photodiodes is 1µm
thick and is not optimized as ARC (Anti Reflective Coating) because its
thickness has been set by the characteristics of other devices on the same
wafer and employed for passivation. Its exact thickness (1068nm) has been
calculated from the spectral responsivity peaks experimentally found from
the optical measurements described in Subsection 4.3.4 and by employing
the following known formula:
tox =1
2nox(
1
λr− 1
λr+1)−1 (4.3)
and
mr =tox4nox
λr
where
tox=top oxide thickness
nox=oxide refraction index=1.46
λr and λr+1=wavelengths of two adjacent peaks in the responsivity spec-
trum
mr=wavelength index, whose values have been calculated in Table 4.1.
Table 4.1: Wavelength indexes for the spectral responsivity peaks.
λr λpeak [nm] Calculated mr
λ11 560 11.1
λ13 480 13.0
λ15 420 14.9
λ17 370 16.9
51
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
(a) PD10, d=10µm (b) PD40, d=40µm
Figure 4.18: Micrographs of two finger photodiodes after Zn-T(heptyloxy)PP evaporation.
Figure 4.19: Micrograph of the
standard photodiode after Zn-
T(heptyloxy)PP evaporation.
At the end of the fabrication in the Clean Room of ITC-Irst, two sets of
dies have been completely coated with 170nm of two different metallopor-
phyrins, Zn-T(heptyloxy)PP and Co-T(dodecyloxy)PP, with two succes-
sive evaporations in the Clean Room of IMM-CNR, Tor Vergata Research
Area, in Rome (Fig. 4.18 and 4.19).
Fig. 4.20 shows a schematic cross section of the finger photodiode at the
end of the fabrication process, after metalloporphyrin evaporation.
52
4.3. FINGER PHOTODIODES
Figure 4.20: Schematic cross section of the finger photodiode (cut in the middle of the
area in Fig. 4.17) at the end of the fabrication process.
4.3.3 Electrical Characterization
Electrical testing has been conducted over all the photodiodes types by
measuring IV curve with a variable reverse bias of the finger-shaped p-n
junction. Fig. 4.21(a) and 4.21(b) show the IV curves for N+PDxx and
P+PDxx devices respectively together with the standard photodiode curve.
Average measured dark current and breakdown voltage for N+PDxx and
P+PDxx devices are respectively around 2pA 22.5V, and 13pA 53V. More
detailed experimental values are reported in Table 4.2 (all measurements
have been conducted with the positive output probe on the anode/p-well
contact).
Both simulations and experimental results show dark currents values
higher for the finger photodiodes than the standard devices: this is due to
the surface current that generates within 1µm around the finger implant
and adds up to the bulk current, proportionally to the fingers perimeter.
Fig. 4.22 shows a direct comparison between the two types of photodiodes
N+PDxx and P+PDxx.
53
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
(a) N+PDxx, n+fingers implanted in a p-well on n-Si substrate
(b) P+PDxx, p+fingers implanted in the n-Si substrate
Figure 4.21: Measured IV curves for the two types of photodiodes, (a) N+PDxx, and (b)
P+PDxx and comparison with the standard fully implanted photodiode.
According to Eq. 4.4,
VB =εSE2
max
2q(NB)−1 (4.4)
the breakdown voltage is inversely proportional to impurity concentra-
tion and from the plot in Fig. 4.23 from Sze [28], it is possible to ver-
54
4.3. FINGER PHOTODIODES
Table 4.2: Measured Dark Current and Breakdown Voltage for all the fabricated photodi-
odes.Photodiode Name Reverse Dark
Current [pA]
Reverse Break-
down Voltage
[V]
P+PDstand -7 63
P+PD10 -15 55
P+PD20 -13 54
P+PD30 -16 53
P+PD40 -13 52.5
N+PDstand +2.6 at 0V, 1 at -1V 23.5
N+PD10 -1.5 22.5
N+PD20 -3 22.5
N+PD30 -2.5 22.5
N+PD40 +0.13 at 0V, -3.5 at
1V
22.5
Figure 4.22: Comparison between the IV curves of the two types of photodiodes.
55
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.23: Breakdown voltage versus impurity concentration for one-sided abrupt dop-
ing profile with cylindrical and spherical junction geometries, where rj is the radius of
curvature.
ify the measured VB for the N+PDxx, about 22V (Boron concentration=
4×1016atoms/cm3), smaller than 53V of the undoped substrate photodiode
P+PDxx. The same plot reports the strong dependence of the breakdown
voltage on the junction radius, which dramatically decreases Vb, especially
for spherical junctions at low impurity concentrations (for P+PDxx with
such a low n-substrate concentration, 1011/cm3, 53V couldn’t be acceptable
without taking into account the radius of curvature).
4.3.4 Optical Characterization
Optical testing of the photodiodes has been conducted at ITC-Irst with a
series of spectral responsivity measurements at different reverse bias volt-
ages. Fig. 4.24 report the two experimental curves of the spectral responsiv-
ity for the two types of photodiodes at 5V reverse bias. Both plots exhibit
peaks in the blue region, located at 370, 420, 480, 560nm, with responsivity
value around the blue peak higher for the finger-photodiode with respect
to the standard reference. For a clear comparison, maximum responsivity
56
4.3. FINGER PHOTODIODES
Table 4.3: Maximum measured responsivity values for all photodiodes in the blue spectral
range at 5V reverse bias.
Photodiode
Name
Max Respon-
sivity [A/W] @
5Vrev, 420nm
Photodiode
Name
Max Respon-
sivity [A/W] @
5Vrev, 420nm
N+PDstand 0.14 P+PDstand 0.15
N+PD10 0.20 P+PD10 0.21
N+PD20 0.21 P+PD20 0.23
N+PD30 0.21 P+PD30 0.23
N+PD40 0.22 P+PD40 0.24
values at 5V reverse bias are reported in Table 4.3 for both N+PDxx and
P+PDxx devices. At 420nm and 5V reverse bias the measured responsivity
values for N+PD40(d=40µm) and P+PD40 and the conventional photodi-
ode are respectively 0.22, 0.24 and 0.14A/W, which means that the finger
junction provides an increment of 60%. Responsivity values at low reverse
bias voltages do not vary significantly between N+PDxx and P+PDxx de-
vices neither with increasing interdigit distance, even if a slightly higher
value is observed for 40µm (Fig. 4.24). At high reverse bias voltages, only
in the N+PDxx devices (Fig. 4.25), an impact ionization phenomenon oc-
curs due to the heavy doping of the n+ fingers and the p-well (Fig. 4.25(a))
and it causes a photocurrent multiplication, responsible for the exceptional
increase in responsivity, which becomes even higher than the value corre-
sponding to 100% quantum efficiency (Table 4.4). Since this effect is based
on the doping levels, it has been observed also for the standard photodiode
N+PDstand (Fig. 4.26 and Table 4.4).
57
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
(a)
(b)
Figure 4.24: Finger photodiodes spectral responsivity at 5V reverse bias for increasing in-
terdigit distance and standard photodiode: a) for n+-finger cathode in a p-well photodiode,
N+PDxx; b) for p+-finger anode in the n-substrate photodiode, P+PDxx.
58
4.3. FINGER PHOTODIODES
(a)
(b)
Figure 4.25: PD40 spectral responsivity for increasing reverse bias voltage: a) for n+-
finger cathode in a p-well photodiode, N+PD40 (Vbreakdown=22.5V); b) for p+-finger anode
in the n-substrate photodiode, P+PD40 (Vbreakdown=53V).
59
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.26: Spectral responsivity for increasing reverse bias voltage for the standard pho-
todiode N+PDstand, n+ fully implanted in the p-well.
The plot in Fig. 4.24(a) consistently matches the simulation results of
Fig. 4.15, with slightly lower experimental responsivity values (e.g. for
d=40µm at 420nm: 0.22A/W measured vs 0.27A/W simulated).
The significant difference in the N+PDxx and P+PDxx devices spectra is
due to the presence of the p-well in the former, with a thickness of 2µm.
Only a part of the spectrum wavelengths can be absorbed by the p-well
that eventually will cut off photons beyond red. Therefore these detectors
are expected to have the responsivity peaks shifted towards the green/blue
region, while the P+PDxx, directly implanted in the n-substrate, which
constitutes a much thicker cathode, are expected to present the classical
responsivity curve, increasing with wavelength, but with higher responsiv-
ity values in the blue region (due to the finger-shaped p+implant).
Mentioning again Ghazi’s work, as experimental results concern, he found
responsivity peaks in the blue range at 400nm of 0.23A/W and in the red
60
4.4. FINGER BJT PHOTOTRANSISTORS
Table 4.4: Measured responsivity values for N+PD40 and N+PDstand for increasing re-
verse bias (ionization multiplication).
Reverse Voltage [A/W] N+PD40
@420nm
(Q.E.=0.34A/W)
[A/W]
N+PDstand
@420nm
[A/W]
N+PDstand
@490nm
(Q.E.=0.40A/W)
5V 0.22 0.14 0.20
10V 0.23 0.15 0.23
15V 0.27 0.17 0.32
18V 0.34 0.20 0.42
20V 0.44 0.24 0.54
spectrum at 638nm of 0.49A/W, with ARC, while here the correspondent
peaks are located at 420nm with a maximum value of 0.24A/W (P+PD40)
and at 700nm with 0.45A/W, without ARC. Ghazi compared this high re-
sponsivity value with a reference photodiode performance, with the same
antireflecting coating, registering an increment by a factor of 2.8, but the
reference photodiode was not fabricated with the same technology process,
while all the comparisons shown in this thesis take into account refer-
ence conventional photodiodes fabricated in the same run and wafer of
the finger-shaped photodiodes, in order to minimize the parameters that
may affect a reliable comparison. At 420nm the responsivity value for the
reference device P+PDstand is 0.15A/W with an increment factor of 1.7,
without ARC optimization.
4.4 Finger BJT Phototransistors
After performing several measurements campaigns with the finger pho-
todiodes and Ethanol vapor at different concentrations (for details and
experimental results, see Chapter 6) in order to test the sensors response
and sensitivity, I decided to use the same N+PDxx photodetectors as npn-
61
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.27: Cross-section of the npn BJT with the finger-shaped Emitter/Base junction.
BJTs Bipolar Junction Phototransistors. The n-Si substrate is contacted
from the backside and it serves as the n−-collector, while the p-well and
the n+-fingers constitute base and emitter respectively, like presented in
Fig. 4.27.
4.4.1 Electrical Characterization
Output Characteristics and Gummel plots have been experimentally mea-
sured for the new BJTs at ITC-Irst. The most significant results are the
following: a larger current gain in finger-type devices with respect to the
standard one has been observed and, among the finger BJTs, an increasing
beta for larger interfinger distance has been recorded. Fig. 4.28 presents
a comparative Gummel plot of the base Ib and the collector Ic currents
vs the base bias voltage Vb for the standard BJT and two finger-BJTs,
named BJT10 and BJT40 (interdigit distance 10µm and 40µm respec-
tively). These three devices have been taken into particular account in
the testing and simulation phases because a comparison among them can
include also the trend observed for BJT20 and BJT30. While the collector
current remains constant for all devices, a significant decrease in the base
current is registered for a larger interfinger distance and in general the high-
62
4.4. FINGER BJT PHOTOTRANSISTORS
Figure 4.28: Experimental Gummel plots (Ib and Ic vs Vb) for 3 different BJTs: BJT0
standard, BJT10 and BJT40.
est base current is exhibited by the standard transistor. This trend highly
affects the beta internal current gain of the BJTs, which consequently
increases for increasing interfinger distance and for the finger-emitter tran-
sistor with respect to the standard one (Fig. 4.29). Table 4.5 reports the
beta maximum values and the correspondent base currents for all five types
of devices.
BJT base current is therefore affected by the interdigit distance or by the
Table 4.5: Maximum β current gain and correspondent base current for all phototransis-
tors.DUT β Ibase [A]
BJT0 72 3.85× 10−7
BJT10 109 3.00× 10−7
BJT20 132 2.50× 10−7
BJT30 148 2.16× 10−7
BJT40 158 2.00× 10−7
63
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.29: BJT current gain Beta vs collector current Ic for increasing interfinger dis-
tance and the standard BJT.
number of emitter n+-fingers in the p-well area. This dependence has been
further studied by calculating the total fingers perimeter for each device
and plotting the base current Ib and the current gain β as a function of
the perimeter itself, that in turn grows with the fingers number (Fig. 4.30)
according to a second order polynomial function
Y = A + B1X + B2X2
with the following fitting coefficients:
Fitting Coeffients for P/]-fingers plot
Parameter Value Error
A −14094.28139 4593.50752
B1 168.44193 30.28672
B2 −0.06679 0.04292
Fig. 4.31 and 4.32 show the functions Ib and β versus the fingers perime-
ter: the former has been fitted by a second order polynomial fit
Y = A + B1X + B2X2
64
4.4. FINGER BJT PHOTOTRANSISTORS
Figure 4.30: Dependence of the fingers perimeter on the fingers number.
and the latter by a Holliday function (with no weighting)
Y =a
1 + bX + cX2
with the following coefficients for the two expressions:
Fitting Coeffients for Ib/P plot
Parameter Value Error
A 1.59351× 10−7 1.85084× 10−9
B1 3.25792× 10−12 1.3131× 10−13
B2 −1.2597× 10−17 1.87431× 10−18
Fitting Coeffients for β/P plot
Parameter Value Error
a 183.04446 2.69711
b 0.00001 1.311× 10−6
c 2.146× 10−12 1.7305× 10−11
The β dependence and proper fitting has been found by taking into
account the following relationships:
Ib = f(A + B1X + B2X2),
β =Ic
Ib=
const
f(A + B1X + B2X2)=
a
1 + bX + cX2
65
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.31: Dependence of the base current
on the fingers perimeter.
Figure 4.32: Dependence of the current gain
on the fingers perimeter.
where X=number of fingers, and Ic is considered constant as observed
from the experimental results (Fig. 4.28).
4.4.2 Modelling
In order to further study the behavior observed from experiments, electri-
cal simulations of the three types of transistors BJT10, BJT40 and BJT0
standard have been performed by ISE-TCAD, which allowed to generate
2D plots of the structures, representing the hole current density, dominant
component of the base current. Fig. 4.33 collects the three plots corre-
spondent to the standard, BJT10 and BJT40 devices, where five fingers
and two fingers have been implemented for BJT10 and BJT40, in order
to keep the cross section of 82µm width and 600µm height constant, thus
allowing a correct straight comparison. According to the color legend and
gradient, all the three structures show a holes current density decreasing
with the distance from the p-well base contact on the right, but this gradi-
66
4.4. FINGER BJT PHOTOTRANSISTORS
X
Y
0 25 50 75
-1
0
1
2
3
4
5
6
holes current densityd10beta4_mdr.grd - d10beta4_000010_des.dat
X
Y
0 25 50 75
-1
0
1
2
3
4
5
6
holes current densityd40beta3_mdr.grd - d40beta3_000010_des.dat
X
Y
0 25 50 75
-1
0
1
2
3
4
5
6
Abs(hCurrentDensity0.316740.2006480.1271060.08051930.05100730.03231210.0204690.01296670.008214150.005203490.003296310.002088140.001322790.0008379630.0005308330.0003362710.0002130210.0001349448.54845E-055.41527E-053.43046E-052.17313E-051.37663E-058.72067E-065.52437E-063.49957E-062.21691E-061.40437E-068.89636E-07
holes current densitystandbeta4_mdr.grd - standbeta4_000010_des.dat
Figure 4.33: 2D plots, generated by ISE-TCAD Tools with the software Tecplot, repre-
senting the holes current density in the vicinity of the E/B junction for the BJT0 (top
left), BJT10 (top right) and BJT40 (bottom left).
ent is affected by the finger-implant configuration and is higher for larger
interdigit distance. This is confirmed quantitatively by the three plots in
Fig. 4.34, resulting from a cross cut at Y=1µm of the structures presented
in the previous Fig. 4.33: for sake of comparison, taking as reference level
the holes current density value 10−2, the three curves fall below this value
at X=78µm for BJT0, at X=75µm for BJT10 and at X=69µm for BJT40,
meaning that the current decreasing gradient is faster for a higher interdigit
distance.
The different base currents can be also affected by the SRH recombina-
tion, that is smaller for BJT40, like shown in Fig. 4.35, which leads to a
smaller contribution of the Ibr recombination current in the neutral base
67
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.34: Holes density along X resulting from an Y-cut at 1µm distance from Si
surface (just below the finger implants) of the three structures in Fig 4.33.
region, component of the total base current, like recalled in the expression
below [29]:
Ib = Ipe + Ire − Igc + Ibr (4.5)
where
Ipe =hole component of the emitter current
Ire =recombination current in the depletion region of the emitter/base
junction
Igc =generation current in the depletion region of the collector/base junc-
tion
68
4.4. FINGER BJT PHOTOTRANSISTORS
X
Y
20 40 60 80
-1
0
1
2
SRH Recombinationd40beta3_mdr.grd - d40beta3_000010_des.dat
X
Y
20 40 60 80
-1
0
1
2
SRH Recombinationd10beta4_mdr.grd - d10beta4_000010_des.dat
X
Y
20 40 60 80
-1
0
1
2
srhRecombinatio7.06367E+184.08033E+182.357E+181.36152E+187.86482E+174.54311E+172.62433E+171.51594E+178.75684E+165.05839E+162.92198E+161.68788E+169.75003E+155.6321E+153.25338E+151.87932E+151.08559E+156.27089E+143.62237E+142.09246E+141.20869E+146.98177E+134.03261E+132.32872E+131.34395E+13
SRH Recombinationstandbeta4_mdr.grd - standbeta4_000010_des.dat
Figure 4.35: SRH recombination distribution in the three structures of Fig 4.33: the pre-
vailing yellow color in BJT40 around the E/B junction stands for a smaller recombination
and consequently a smaller Ir component of total Ib.
Ibr =recombination current in the neutral base region
4.4.3 Optical Characterization
A spectral responsivity measurements campaign has been conducted at
ITC-Irst also for the phototransistors in order to study the role of the E/B
and B/C junctions in the blue photons collection and to make a compari-
son with the photodiodes. BJTs have been kept with floating base and two
collector voltages have been applied, 5V and 15V. Three measurements se-
ries have been done: in the first series a small light beam (300µm diameter)
has been used and it lighted only the central area of the phototransistor,
69
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
therefore only the E/B junction took part to the photons collection; in the
second series a larger light beam (1250µm diameter) has been employed
in order to light all the phototransistor die, thus including also the B/C
lateral junction surrounding the p-well perimeter; finally, ”spatial” respon-
sivity measurements have been performed at 8 different light wavelengths
(350, 400, 450, 500, 550, 600, 650, 700nm) by scanning the device horizon-
tally (23µm step) with a small beam of 200µm diameter.
Even if the E/B finger-shaped junction is almost forward biased in the
BJTs case (opposite of the reverse bias applied to photodiodes), it affects
the blue photons collection as shown by the spectral responsivity curves in
Fig.4.36, measured at 5V collector bias voltage. They still differ one from
another, even if the trend is not the same observed for the photodiodes:
PT10 exhibits the highest responsivity at 420nm, while PT40 the lowest.
From the direct comparison of three of the photodiodes with the corre-
Figure 4.36: Finger phototransistors spectral responsivity at Vcc=5V for increasing inter-
digit distance and standard BJT, with small light beam (only E/B junction contribution).
70
4.4. FINGER BJT PHOTOTRANSISTORS
Figure 4.37: Comparison between spectral responsivities of finger photodiodes and photo-
transistors with respectively Vrev=5V and Vcc=5V for the standard devices and the finger
photodetectors with d=10µm and d=40µm.
spondent phototransistors in Fig. 4.37, standard, d10 and d40 devices, the
responsivity peaks occur almost at the same wavelengths, with a possi-
ble 5-10nm shift (in both directions), and the main difference is the curve
trend that for BJTs keeps increasing for increasing wavelengths. This is
due the bulk contribution to the collection of higher wavelengths photons,
totally absent for the photodiodes, whose n-substrate is kept floating (cut-
off wavelength set by the 2µm p-well thickness). The phototransistors have
a more resolved (less broad) and higher peaks in the blue spectral region,
like clearly shown in the four comparative plots of Fig. 4.38, meaning that
the former can better sense small changes, especially wavelength shifts in
71
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
metalloporphyrins absorption spectrum (Fig. 3.5 and Fig. 3.6, Chapter 3).
For this reason phototransistors are expected to be more performing than
photodiodes in the experimental response to ethanol. The registered incre-
ment in responsivity is not constant for all types of phototransistors and
it is calculated below for each BJT at 420nm:
∆Resp(PT0-N+PDstand)=(0.20-0.14)=0.06A/W
∆Resp(PT10-N+PD10)=(0.24-0.20)=0.04A/W
∆Resp(PT20-N+PD20)=(0.25-0.21)=0.04A/W
∆Resp(PT30-N+PD30)=(0.24-0.22)=0.02A/W
∆Resp(PT40-N+PD40)=(0.23-0.22)=0.01A/W
72
4.4. FINGER BJT PHOTOTRANSISTORS
(a) (b)
(c) (d)
Figure 4.38: Comparison between spectral responsivities of finger photodiodes and photo-
transistors with respectively Vrev=5V and Vcc=5V for: (a) PD10 and PT10; (b) PD20
and PT20; (c) PD30 and PT30; (d) PD40 and PT40.
73
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
The improvement in responsivity introduced by the phototransistor de-
teriorates with increasing interdigit distance, in fact PT40 responsivity is
almost equal to its correspondent photodiode, N+PD40, while the greatest
variation has been observed for the standard BJT. This can be explained
by recalling the two main mechanisms of formation of the surface depletion
region between neighboring finger implants: the positive charge trapped
in the passivation oxide layer and the reverse bias of the n+-fingers/p-well
junction, applied to the photodiodes during optical testing. In addition,
interdigit distance is also important for the merging of neighboring deple-
tion regions around adjacent fingers to occur. In the phototransistors case
the second mechanism ceased to be, in fact the E/B junction can be con-
sidered slightly forward biased, and the first mechanism only can’t provide
the same amount of surface depletion layer for the efficient collection of the
blue photons and the interdigit distance start to play a fundamental role:
if the fingers are too far (from calculations, d=20µm seems like a thresh-
old distance), responsivity decreases for a lack of active surface collection
region, the merging doesn’t occur. This effect is in competition with the
improvement brought by the phototransistor device and tend to almost
compensate it for PT30 and PT40.
Fig. 4.39 shows the comparison between the spectral responsivity curves at
5V and 15V collector voltage for measurements done with small (Fig. 4.39(a))
and large (Fig. 4.39(b)) light beam.
74
4.4. FINGER BJT PHOTOTRANSISTORS
(a)
(b)
Figure 4.39: Phototransistors spectral responsivity at Vcc=5V and 15V for standard BJT
and (a) PT10 with small light beam (only E/B junction contribution); (b) PT10 and PT40
with large light beam (also B/C lateral junction contribution).
75
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
A straight comparison between the responsivity measurements of the
first and second series, with small and large beam, is not reliable and can’t
be done due to the lack of homogeneity of the two beams employed and
some set-up differences. Nevertheless, three relevant results and qualitative
differences regarding the blue spectral region in the two cases of Fig. 4.39
can be highlighted:
1. an increase of the responsivity occurs only in the second case; the in-
crements from 5V to 15V are calculated below for each BJT at 420nm:
∆Resp(PT015V cc-PT05V cc)=(0.22-0.20)=0.02A/W
∆Resp(PT1015V cc-PT105V cc-)=(0.24-0.19253)=0.05A/W
∆Resp(PT2015V cc-PT205V cc-)=(0.23-0.19364)=0.04A/W
∆Resp(PT3015V cc-PT305V cc-)=(0.24-0.18914)=0.05A/W
∆Resp(PT4015V cc-PT405V cc-)=(0.22367-0.17659)=0.05A/W
Below follow the increments calculated taking into account the shift
of the peak from 420 to 425nm at 15V and making the difference be-
tween the two peak values:
∆Resp(PT015V cc,425nm-PT05V cc,420nm)=(0.24-0.20)=0.04A/W
∆Resp(PT1015V cc,425nm-PT105V cc,420nm-)=(0.25-0.19253)=0.06A/W
∆Resp(PT2015V cc,425nm-PT205V cc,420nm-)=(0.26-0.19364)=0.06A/W
∆Resp(PT3015V cc,425nm-PT305V cc,420nm-)=(0.25-0.18914)=0.06A/W
∆Resp(PT4015V cc,425nm-PT405V cc,420nm-)=(0.23-0.17659)=0.05A/W
2. only in the second case a relevant change in the standard BJT re-
sponsivity spectrum occurs: it becomes similar to the other spectra,
exhibiting peaks at the same wavelengths and with the similar ampli-
76
4.4. FINGER BJT PHOTOTRANSISTORS
tude, for both collector voltages;
3. with respect to the spectra measured for the same devices with the
small beam in the first case, in the second case the responsivity spectra
exhibit closer peaks, 360, 390 and 420nm in the blue region, with
respect to 365 and 425nm of the first plot.
All the results can be explained by considering that by employing the large
light beam also the B/C depletion region around the p-well perimeter is
lighted: the first and the second result prove that it provides a lateral sur-
face space-charge region able to collect blue photons, which widens for in-
creasing collector voltage (increasing junction reverse bias) thus providing
higher responsivity values at Vcc=15V. As for the amount of these incre-
ments, no differences are expected among BJTs with different interfinger
distance, since the p-well perimeter is constant for all types and conse-
quently the extension of this lateral active region is constant for all: from
the calculation reported above, the same increment has been found in both
cases (0.05 and 0.06A/W).
This lateral B/C active region, being the p-well perimeter the same also
for the standard BJT, is completely responsible for the appearing of the
peaks in the blue region of its spectrum (second result), in spite of not
having finger-shaped E/B junction.
The third result is related to a fabrication detail: the thickness of the oxide
layer on top of the p-well area (1068nm) is smaller than the thickness of the
oxide layer on the rest of the die and therefore also on the lateral collector
Si-substrate, due to a photolitography step. According to Eq. 4.3,
tox ∝ λrλr+1
λr+1−λr∝ 1
∆λ
if the oxide thickness becomes larger, ∆λ becomes smaller and adjacent
responsivity peaks become closer to each other.
The third and last series of optical measurements done with a 200µm beam
77
CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS
Figure 4.40: Spatial responsivity of BJT10 at
different wavelengths when scanned horizon-
tally by a 200µm beam with a 23µm step.
Figure 4.41: Zoom of the curves at 350 and
400nm in Fig. 4.40.
scanned horizontally along the BJT10 die gave further information on the
B/C lateral depletion region contribution and on the most responsive ar-
eas of the photodetector. Fig. 4.40 reports all the results for the 8 beam
wavelengths used: 350, 400, 450, 500, 555, 600, 650, 700nm. The pho-
tocurrent on the Y-axis has not been normalized to the current gain and
it is the straight output value read by the HP4145 analyzer. The zoom for
the smaller curves at 350 and 400nm in Fig. 4.41 shows the clear contri-
bution of the lateral B/C junction to the blue photons collection: the two
highest peaks (right and left) occur at the p-well border with the collector.
Such lateral peaks become less relevant for higher wavelengths, except for
600 and 700nm, where in the former case they even mark the maximum
current values. This might be explained by the different oxide thickness
outside the p-well zone that gives place at different maximum transmission
peaks. All the curves present a series of small peaks and valley due to
the geometry of the metal lines (emitter contacts) shown in the layout of
Fig. 4.17: the zone in the curve with minimum signal correspond to the
78
4.4. FINGER BJT PHOTOTRANSISTORS
Al lines. On the other hand all the curves present a central maximum due
to the absence of finger implants in the central area of the device layout
(Fig. 4.17): in the BJT10 case, this area is 250µm wide and the peak in
the middle of the curve in Fig. 4.40 is about the same. That central part of
the photodetector is the most responsive, where the surface space charge
region is more extended.
79
Chapter 5
The E-O Nose System
At the end of the fabrication process, the photodetectors have been evap-
orated with different metalloporphyrins (with the method described in
Chapter 4, Section 4.1, Fig. 4.2), cut into single die and bonded to a black
plastic board with three lines of gold metal contacts and pads to attach
4 sensors per row, 12 per package (Fig. 5.1; see Appendix B for bonding
schematics and pin-out details, Fig. B.1 for photodiodes and Fig. B.2 for
phototransistors). Every photodiode has two contacts, p-well/anode and
n+-fingers cathode, while phototransistors need only one single contact
per die, the n+-fingers emitter, because the collector is contacted from the
backside of the chip and is common for all the sensors in the same row,
while the p-well base is left floating.
Figure 5.1: Picture of the assembled sen-
sors matrix after die and wire bonding.
81
CHAPTER 5. THE E-O NOSE SYSTEM
(a) (b)
Figure 5.2: Image of the package used as nose nostril to lodge the sensor matrix: (a) first
prototype; (b) second prototype, for a differential measurements configuration.
5.1 Package: the Nose Nostril
In order to assemble the nose nostril, an ”ad hoc” package has been built
to provide all the necessary features: transparent glass top cover to let
the blue light pass and two lateral holes for gas flowing (to attach the
small hoses). It measures 15mm×15mm×8.5mm. Fig. 5.2 shows the two
versions of the package: the second one (Fig.5.2(b)) differs from the first
(Fig. 5.2(a) and Fig. 4.6 in Chapter 4) for the position of the lateral hole,
which has been moved up, and for the presence of a wall to isolate the
sensors bottom line from gas flowing and take advantage of a differential
measurement configuration, in which one of a couple of sensors with the
same metalloporphyrin is not exposed to VOCs. This configuration is
extremely useful for chemical sensors to cancel baseline drift, sensing layer
aging and any environmental dependence (temperature, relative humidity,
etc...). Details on the geometry and all dimensions of the two packages can
be found in Appendix B.
82
5.2. THE NOSE BOX
(a) Top view with the LED in front of the packagedsensors matrix
(b) Side view with electrical wires coming out
Figure 5.3: Two images of the first chamber where the sensor matrix was placed, provided
with holes for electrical connections access and gas flow.
5.2 The Nose Box
For the first experimental measurements campaign, the assembled matrix
was lodged in a cylindrical PVC black chamber (Fig.5.3), containing just
the sensors package and the blue LED source, placed in front of it, while the
read-out circuit board was kept separated and connected to the chamber
via electrical wires.
This first set-up did not provide a box for the full system and conse-
quently a good SNR for the output voltage neither a good isolation from
environmental light. The first metal box, perfectly isolating the optical
sensors from environmental light and electronic noise (when connected to
ground) is shown in Fig. 6.1 in Chapter 6 [30]: it contained also the read-
out circuit board, it had two lateral holes for the hoses carrying the gas in
and out and it had a large hole at one side to couple to a monochromator
output, which replaced the LED source in some measurements campaigns.
The final optimized box, making up the complete E-ON, LED included, is
shown in Fig. 5.4 and is fully described in detail in the internal Technical
report [31]. Every board in this final flexible system has been optimized to
83
CHAPTER 5. THE E-O NOSE SYSTEM
Figure 5.4: Picture of the top of the open nose box. The circuit board is on top left with
the three operational amplifiers and their respective RC feedback networks; the batteries
container (for op amps power supply) is on bottom left, close to the LED intensity selector,
which in turn is near the 3 outputs BNCs (vertical); the castle with the sensors and LED
board in on the right (only the backside of the LED board is visible) with the 2 plastic
tubes for gas flow coming in and out; on top of the picture, on the left the 3 connectors
for voltage biasing of the 3 sensors rows are visible, together with the 3 selectors (vertical)
of the sensor in each row.
be easy to mount and removable, especially the sensors and the LED source
boards, in order to quickly change the sensors matrix and proceed with the
testing of another set of sensors (coated with different metalloporphyrins)
or change the LED type (another wavelength emission, another brand,...)
(Fig. 5.6). These two boards are mounted on a third one, mechanically
fixed to the box, making up a so-called ”castle” unit (Fig. 5.5). The
LED mounted is a Pink Super Bright LED, with emission wavelength of
440nm; Candle Power: 1.1 (1100 mcd); Viewing Angle: 15 degrees (from
84
5.2. THE NOSE BOX
Figure 5.5: Schematic image of the castle: left, explosion of the three boards: the one
mechanically fixed to the metal box, from which all electrical connections to the sensors
matrix and the LED source start; middle, removable part of the castle, made up by the
sensors board and the LED board (Fig. 5.6); right, complete close castle.
LedsDirect.com).
5.2.1 Read-Out Circuit
The circuit for the sensor signal extraction is made up by three equal I-V
converters, one per sensors row in the matrix, allowing the real time mon-
itoring of one sensor per row. Each circuit reads out the photogenerated
current (photodiode reverse current or BJT emitter current) and converts
it into an output voltage by employing a low input bias current low noise
operational amplifier (AD 549JH). The feedback network is flexible and
variable according to the sensors to test: if photodiodes are employed,
the feedback network provides a transimpedence gain of 200MΩ and a low
pass filter for white noise reduction with 1.4Hz cut-off frequency; if photo-
transistors are employed, the output signal needs a smaller amplification,
thanks to the BJT internal gain, and the feedback network provides a
gain of 470KΩ and 1Hz cut-off frequency. Both the feedback networks are
mounted on the board and selected with a switch for each row. Fig. 5.7
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CHAPTER 5. THE E-O NOSE SYSTEM
Figure 5.6: Picture of the top part of the castle: the LED board is attached to the sensors
board through a series of connectors and screws, easily removable. The two pieces of hose
for gas flow are visible on the lateral sides of the sensors package, going in and out.
shows the PSPICE schematics of both the circuit configurations.
Fig. 5.8 presents the electrical scheme of a single signal extraction chan-
nel for the phototransistors case: the four BJTs have the collector in com-
mon, connected to the power supply (5V), the base floating and the emitter
is connected to the inverting input of the op amp when the switch is closed
(a manual selector per row performs the switching task). The op amp is
dually powered by two 9V batteries placed in a special container in the
nose metal box.
86
5.2. THE NOSE BOX
(a) Photodiodes sensors (b) Phototransistors sensors
Figure 5.7: Schematic of the circuit configuration for output signal extraction of (a) pho-
todiodes and (b) phototransistors sensors.
Figure 5.8: Electrical scheme of a single signal extraction channel. The 4 BJTs are placed
in the matrix package (Fig. 5.2(b)) on the sensors board (Fig. 5.1), while the I-V converter
is mounted on a separated circuit board (Fig. 5.4).
87
Chapter 6
Sensors Experimental Testing
Numerous experimental measurements to assess the sensors performance
have been conducted in the Electro-Optical Laboratory at the Dept. of
Information and Communications Tech. of the University of Trento. The
VOC used in the tests is Ethanol, because it is an easy to find, not danger-
ous and very volatile compound. This means that it does not bind easily
to the porphyrin molecules or to any other sensing layer, therefore it can
be considered as a challenging VOC in testing a prototype. It is poured in
a liquid phase in a bubbler and brought from its saturation concentration
in air to different decreasing concentrations by means of a 4-channels mass
flow controller, able to mix sature ethanol vapor to a neutral carrier, dry
air. Dry air is stored in a tank outside the lab building for security reasons
and a pipeline system carries it to the gas bench inside. All the experi-
mental set-up is shown in the picture of Fig. 6.1, where the first type of
metal box is presented and the light source set-up is made up by a white
lamp coupled with a manual monochromator (see Section 6.1).
The three sensors output voltages are monitored on the PC by the
NI 4350 high-precision voltage meter acquisition card and the software
Labview 6.1, which is the basis also for the flow meter remote control
application. Measurements are done in continuous mode (cc).
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CHAPTER 6. SENSORS EXPERIMENTAL TESTING
Figure 6.1: Picture of the experimental set up: first version of the E-ON metal box, coupled
to a manual monochromator, which in turn is coupled to a white halogen lamp. The gas
bench is completely visible, with the 4-channels flow meter, the ethanol bubbler and the
hoses.
In order to better understand the ethanol concentrations referred to in
the following measurements results, the Antoine Equation is here recalled,
which allows to find the saturation pressure in air of any compound at a
certain temperature [32]:
lg(p/p0) = A−B/(T + C) (6.1)
where
p0=ambient pressure=1bar('1atm)
p=vapor or saturation pressure [bar]
T=temperature=293K
Antoine Coefficients for Ethanol calculated by NIST:
A=5.37229; B=1670.409; C=-40.191
90
Table 6.1: Conversion from percentages of Ethanol vapor in the flow to ppm.
percentage % ppm
100 58196
50 29098
40 23280
30 17460
20 11640
10 5820
5 2910
2 1164
1 582
0.5 291
Therefore
p=0.058196bar
p×106=58196ppm(@100% EtOH flow)
The conversion to ppm, parts per million, is important because ppm is the
standard unit of measure used for VOCs concentrations in sensors science.
In the experimental measurements performed in this thesis different con-
centrations have been used and Table 6.1 reports the value in ppm of the
corresponding ethanol percentages in the total flow (100sccm, square cubic
cm) that have been used in the experiments. In the rest of the text and in
the measurements plots, percentages will be used just for shortness, even
if ppm should be always taken into account.
The measurements results plotted in this chapter have been chosen among
the most representative and significant out of numerous experimental re-
sults collected in three years sensors testing. They are presented chronolog-
ically, starting with the first type of standard photodiodes used as sensors
till the latest E-ON system, based on phototransistors.
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CHAPTER 6. SENSORS EXPERIMENTAL TESTING
6.1 First campaign of Measurements
The first sensors employed have been the standard photodiodes already
available at ITC-Irst (Section 4.2, Chapter 4), spray-coated with simple
metalloporphyrins, with the molecular structure of Fig. 3.2, Chapter 3.
Performances of the sensors were very poor and the minimum detectable
response corresponded to an ethanol concentration of 67%, also due to the
employment of the first PVC chamber described in Section 5.2, Chapter 5
(Fig. 5.3).
The first improvement came from the chemistry of the sensing layer, whose
molecular structure was modified like described in Section 3.1.1, Chap-
ter 3. Fig. 6.2 and 6.3 show the first results obtained by using sensors
spray-coated with Co-T(hexadodecyloxy)PP and Zn-T(butyloxy)PP, and
a blue LED with an emission wavelength of 470nm, operated in a contin-
uous mode. Both the plots present seven measure cycles performed with
ethanol at decreasing concentrations: 100%, 80%, 60%, 50%, 40%, 20%,
10%. Even if they didn’t show a significant response in term of output
voltage variation for 10% ethanol, an extremely relevant improvement was
achieved.
The second optimization came from the blue light source, when another
set-up replaced the LED: a halogen white lamp HL-2000-FHSA (Avantes,
360-1700nm range, 17mW bulb output) coupled to a manual monochro-
mator, aligned in front of the sensors nostril (Fig. 6.1). Even if bulkier and
noisier than the simple small LED, such a set-up proved the importance of
adjusting the light source on the emission wavelength correspondent to the
maximum variation of metalloporphyrins transmission spectrum. Even a
few nm can make a difference, like shown by the results plotted in Fig. 6.4
for four wavelengths. The highest response signal is obtained at 426nm.
Fig. 6.5 shows two measurements cycles at 20% and 5% EtOH concentra-
92
6.1. FIRST CAMPAIGN OF MEASUREMENTS
Figure 6.2: Output voltage versus time of 7 measurement cycles at decreasing
EtOH concentrations with a standard photodiode-based sensor, spray-coated with Co-
T(hexadodecyloxy)PP.
Figure 6.3: Output voltage versus time of 7 measurement cycles at decreasing EtOH con-
centrations with a standard photodiode-based sensor, spray-coated with Zn-T(butyloxy)PP.
tions conducted with the same sensor previously used, spray-coated with
Zn-T(heptyloxy)PP, but here with a light source emission of 426nm. A
good response is obtained down to 5% EtOH.
The third important improvement came from metalloporphyrins deposi-
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CHAPTER 6. SENSORS EXPERIMENTAL TESTING
Figure 6.4: Four voltage output variations for the same sensor at 33% EtOH concentration
for four different emission wavelengths around the metalloporphyrin transmission peak.
tion method (Section 4.1, Chapter 4): the better results achieved with the
evaporated sensing layer (Fig. 6.6) proved that the porphyrins spectral be-
havior and in general their optical properties strictly depend on the state
of aggregation of the deposited film, on its uniformity and thickness.
94
6.1. FIRST CAMPAIGN OF MEASUREMENTS
Figure 6.5: Output voltage versus time of 2 measurement cycles at 20% and 5% EtOH
concentrations, by employing a photodiode detector spray-coated by Zn-T(heptyloxy)PP
and a light source with 426nm emission wavelength.
Figure 6.6: Output voltage versus time of 4 measurement cycles at 2.5%, 5%, 10%
and 20% EtOH concentrations by employing a photodiode detector evaporated by Zn-
T(heptyloxy)PP and a light source with 440nm emission wavelength.
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CHAPTER 6. SENSORS EXPERIMENTAL TESTING
6.2 Second campaign of Measurements
The introduction of the optimized fingers-photodetectors with enhanced
responsivity in the Soret band allowed for further important improvement:
the minimum detectable ethanol concentration achieved is 0.5% correspon-
dent to less than 300ppm. In addition to this, the adoption of the nose
metal box described in Section 5.2, Chapter 5 (Fig. 6.1 and 5.4) proved to
be fundamental in improving both the sensor response and the S/R ratio.
6.2.1 Finger photodiodes
Fig. 6.7 shows 4 measurements cycles at decreasing ethanol concentrations,
down to 0.5%, performed with the finger-photodiode N+PD10. A zero re-
verse bias has been applied in this case. Only the N+PDxx have been
experimentally tested with ethanol (not P+PDxx). Measurements at dif-
Figure 6.7: Output voltage versus time of 4 measurement cycles at 10%, 5%, 2%, 1%
and 0.5% EtOH concentrations by employing the finger-photodiode sensor evaporated by
Zn-T(heptyloxy)PP. The light source set-up employed is the white lamp+monochromator.
96
6.2. SECOND CAMPAIGN OF MEASUREMENTS
Figure 6.8: N+PD10 sensor response to increasing ethanol concentration for 3 different
reverse bias conditions.
ferent reverse bias voltages have been also conducted to find out the best
sensor working point. Fig. 6.8 reports the calculated N+PD10 sensor re-
sponse for three different reverse bias conditions, 0, 1 and 5V, and the
highest voltage output variations are observed for the unbiased sensor.
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CHAPTER 6. SENSORS EXPERIMENTAL TESTING
The previous measurements have been conducted by employing the
lamp+monochromator light source set-up, while the results plotted in
Fig. 6.9 and 6.10 have been measured by using the latest nose system
and the pink LED at 440nm. The nostril mounted on the sensors board
contained a row (bottom) of finger-photodiodes sensors coated with Co-
T(hexadodecyloxy)PP (”CoTPP” for shortness in the figures) and a row
(top) with Zn-T(heptyloxy)PP sensors (”ZnTPP” for shortness in the fig-
ures). The package used for this matrix is not the one with the isolation
wall for differential configuration measurements, but the useful differential
cancellation is allowed by the middle row containing uncoated photodiodes.
Fig. 6.9(a) reports two series of measurement cycles for both the sensors
affected by baseline drift, while Fig. 6.9(b) shows the same cycles after
applying the differential drift cancellation with the uncoated devices.
98
6.2. SECOND CAMPAIGN OF MEASUREMENTS
(a) Before differential cancellation
(b) After differential cancellation
Figure 6.9: 4 measurement cycles for N+PD10 sensor at 50%, 40%, 30% and 20% ethanol
concentration (a) before and (b) after applying the differential drift cancellation.
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CHAPTER 6. SENSORS EXPERIMENTAL TESTING
Figure 6.10: 4 measurement cycles for N+PD10 sensor at 10%, 5%, 2% and 1% ethanol
concentration, after applying the differential drift cancellation.
The low concentrations measurement cycles are shown in Fig. 6.10, al-
ready adjusted by the drift cancellation.
From these experimental results it is clear that the most responsive sens-
ing layer is Co-T(hexadodecyloxy)PP, as expected from its chemical struc-
ture, because the alkyl chains synthetized at its peripheral sites are longer
(n=17) than the chains in Zn-T(heptyloxy)PP (n=7), thus providing an im-
proved morphological layer and more resolved spectrum (Subsection 3.1.1,
Fig. 3.9, Chapter 3).
In the same chapter and subsection of the thesis a comparison among re-
sponse time for different alkyl chains lengths has been shown. Even if a
direct absolute comparison can’t be done between the response times shown
in Fig. 3.8 and the performance of the two metalloporphyrins used in these
experiments, due to the different thickness, deposition method (spray-
coating vs evaporation) and transduction mechanism (QMB vs photodetec-
tors), Co-T(hexadodecyloxy)PP proved to be faster than Zn-T(heptyloxy)PP,
100
6.2. SECOND CAMPAIGN OF MEASUREMENTS
Figure 6.11: Zoom of the voltage output increase on exposure to 20% EtOH concentration
(Fig. 6.9). CoTPP rise is faster than ZnTPP rise.
like expected from the plot of Fig. 3.8. The zoom of Fig. 6.11 shows that
the derivative of the CoTPP slope is higher (steeper) than the derivative
of ZnTPP. Response time for both sensors at 20% EtOH concentration has
been calculated in order to make a comparison with the response times in
Fig. 3.8 for spray-coated films deposited on Quartz Microbalance sensors
[26]. For uniformity, also here response time is considered as the time in-
terval necessary to achieve 90% of the complete signal transition.
tr(ZnTPP)=80s with tot transition=3.44mV, ∆Vout,90%=53.7mV
tr(CoTPP)=120s (n=17) with tot transition=9.56mV, ∆Vout,90%=47.9mV
These numbers, especially the latter, having the same coordinated metal,
can be compared with the response time in literature [26] and they result
much smaller: even taking the longest alkyl chain metalloporphyrin tested
by Di Natale, CoTPP-18 (n=18), the minimum response time is 400s. This
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CHAPTER 6. SENSORS EXPERIMENTAL TESTING
is a relevant result because it proves the success of optical transduction in
being faster (about 4 times decrease in response time) than QMB transduc-
tion, like expected from optical sensors, though taking into account that
the sensing layer here used has been evaporated, so the morphology and
thickness are different.
6.3 Third campaign of Measurements
The last measurements campaign was performed with the BJT-based sen-
sors and a final comparison between photodiodes and phototransistors per-
formance is presented at the end of this section.
6.3.1 Finger phototransistors
The first measurements with phototransistors sensors have been conducted
by employing the lamp+monochromator light source set-up. Fig. 6.12
shows 6 measurement cycles at decreasing ethanol concentration down to
0.5% (291ppm) by employing the sensor BJT10. Again the improvement
gained with the differential configuration is highlighted.
102
6.3. THIRD CAMPAIGN OF MEASUREMENTS
(a) Before differential cancellation
(b) After differential cancellation
Figure 6.12: 6 measurement cycles for BJT10 sensor at 20%, 10%, 5%, 2%, 1% and 0.5%
ethanol concentration (a) before and (b) after applying the differential drift cancellation.
103
CHAPTER 6. SENSORS EXPERIMENTAL TESTING
Figure 6.13: 6 measurement cycles for BJT10 sensor at 20%, 10%, 5%, 2%, 1% and 0.5%
ethanol concentration, after 8 months from the first experiments (Fig. 6.12(b)).
The same measurement with the same sensor was repeated after 8
months and the results are plotted in Fig. 6.13. The sensor exhibits a de-
teriorated response and a smaller sensitivity at the lowest concentrations:
0.5% can’t be properly distinguished from 1%, but this could be also due
to the different total flow employed, 100sccm in this case and 200sccm in
the previous experiment. Since the mass flow controller accuracy is 0.5%,
a 100sccm flow is not very reliable at very low ethanol concentrations.
After this reproducibility experiment, Fig. 6.14 shows a repeatability
measurement conducted by repeating the flowing of the same ethanol con-
centration for three times successively with the same sensor, for 5 ethanol
concentrations (10%, 5%, 2%, 1% and 0.5%). Even if the measure is still
affected by baseline drift, the output signal variations for the same concen-
tration are constant for the three cycles.
The response curves of the two sensors N+PD10 and BJT10 have been
104
6.3. THIRD CAMPAIGN OF MEASUREMENTS
Figure 6.14: Repeatability test: 3 measurement cycles are repeated for the same EtOH
concentration, at 10%, 5%, 2%, 1% and 0.5%.
calculated and plotted together in Fig. 6.15: the phototransistor exhibits
higher response values and higher sensitivity at low concentrations.
Fig. 6.16 collects four measurements plots for all types of finger- pho-
totransistors, tested with the latest set-up and nose system (metal box
with incorporated pink LED). Sensors with different coatings are shown
because BJT10 coated with CoTPP resulted damaged and BJT20 coated
with CoTPP didn’t work properly and exhibited a very noisy response. For
these two sensors only the devices coated with Zn-T(heptyloxy)PP have
been taken into account. For BJT30 and BJT40 only measurements with
Co-T(hexadodecyloxy)PP coating are shown, but both the coatings have
105
CHAPTER 6. SENSORS EXPERIMENTAL TESTING
Figure 6.15: Response curve for photodiode and phototransistor sensor: the latter exhibits
higher response and higher sensitivity at low concentrations (the line is steeper).
been tested and results with the response curves are reported in Fig. 6.17
and Fig. 6.18.
106
6.3. THIRD CAMPAIGN OF MEASUREMENTS
(a) (b)
(c) (d)
Figure 6.16: 4 measurement cycles for all types of phototransistor sensors at 10%, 5%, 2%
and 1% ethanol concentration, conducted with the latest nose box set-up. In these plotted
results BJT10 (a) and BJT20 (b) were coated with Zn-T(heptyloxy)PP; BJT30 (c) and
BJT40 (d) were coated with Co-T(hexadodecyloxy)PP.
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CHAPTER 6. SENSORS EXPERIMENTAL TESTING
Figure 6.17: Response curve for sensor N+PD10 and all phototransistor sensors, BJT10,
BJT20, BJT30 AND BJT40, with Zn-T(heptyloxy)PP coating.
The plotted response curves for all sensors coated with Zn-T(heptyloxy)PP,
included the photodiode N+PD10, refer to the latest measurements cam-
paign in which all the presented sensors were bounded in the same matrix
and mounted in the complete nose box. The comparison between N+PD10
and BJT10 is very important and reliable in this case because exactly the
same device has been tested in the two cases but with different bias condi-
tion and output signal probe. Again, like in Fig. 6.15, the most performing
sensor is the phototransistor based sensor with higher response and higher
sensitivity at low ethanol concentrations, even if a low S/N ratio is ob-
served with respect to the low noise photodiodes measurements.
Among the BJT sensors, again BJT10 exhibits the best response and sen-
sitivity, while the worst is BJT40. This result matches with the results
of the spectral responsivity measurements, in which the highest and the
lowest responsivity values are registered for BJT10 and BJT40 respectively
108
6.4. PARASITIC PORPHYRIN RESISTANCE
Figure 6.18: Response curve for the phototransistor sensors BJT30 AND BJT40, with
Co-T(hexadodecyloxy)PP coating.
(with both the small and large light beam set-up, Fig. 4.36 and Fig. 4.39,
Chapter 4).
6.4 Parasitic Porphyrin Resistance
This section is dedicated to the illustration and explanation of a par-
asitic phenomenon that has been observed in the results of all experi-
mental measurements with ethanol concentration higher than 20%. The
semiconductor-like behavior of porphyrins has been investigated in litera-
ture [33] and D’Amico and Di Natale [22] demonstrated porphyrins conduc-
tivity dependence on the presence of volatile compounds, such as amines
and organic acid and reducing gases, by performing tests at room tempera-
ture with films deposited by solvent casting onto interdigitated electrodes.
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CHAPTER 6. SENSORS EXPERIMENTAL TESTING
Also at high ethanol concentrations in air, the metalloporphyrin film cease
to be an insulator to become conductive. Since the film is deposited here
on the whole sensor die, contact pads included, like schematically shown in
Fig. 6.19, macroscopically a resistance in parallel to the p-n junction pho-
todiode and to the B/E junction in the phototransistor is observed. This
could be considered a second sensing mechanism in the same transducer
but in this work it is called parasitic because undesired and unexpected.
In fact the response generated by the conductivity variation is opposite in
sign to the optical response.
For ethanol concentrations of 50%, 40%, 30%, the observed parallel resis-
tance, even if still high, drives a current comparable to the output current
of the photodevices and makes up the most significant sensor response, be-
ing responsible for a variation in the response curve of the optical sensor.
In order to better explain this parasitic effect, Fig. 6.20(a) reports the
measurement cycles where it was observed for first time with the finger-
photodiode sensor. It occurs only when the photodiode is biased and re-
sults in Fig. 6.20(a) and Fig. 6.20(b) have been measured at 5V and 10V
reverse voltage respectively. Only in this biased case the current through
the parasitic resistor generates, adding up to the photocurrent and causing
the output signal to increase.
Two zooms of the plot in Fig. 6.20(a) are presented in Fig. 6.21 to show
the variation in the output voltage curve versus time. At the beginning the
signal response is only due to the optical mechanism: the output voltage
value decreases on increasing of the absorption peak amplitude of metallo-
porphyrin, when it starts to absorb ethanol molecules.
110
6.4. PARASITIC PORPHYRIN RESISTANCE
A A’A A’
(a) Layout
(b) AA’ cross section
Figure 6.19: (a) Layout of one of the finger photodetector; (b) schematic cross view of the
critical electrical path that gives place to the parasitic parallel resistor.
111
CHAPTER 6. SENSORS EXPERIMENTAL TESTING
The second response mechanism, metalloporphyrin conductivity increase,
starts to manifest after a few seconds (Fig. 6.21(a)), with a sudden change
in the voltage curve: the first zoom proves that the strange signals that
look like noisy spikes in fact are not, but are the consequence of the sud-
den response variation. The voltage output increases slowly as long as the
conductivity of the sensing layer increases on further ethanol absorption.
Therefore this second sensor response is opposite to the optical response
and at first it compensates it but quickly becomes the dominant effect, thus
producing a rise in the signal, proportional to the reverse bias applied. Only
at 20% ethanol concentration the parasitic response starts to be overcome
by the optical response and when air flows to clean the nostril, the out-
put voltage doesn’t decrease like in the previous cycles, but it increases,
like expected from the recovery phase of the optical sensing mechanism
(Fig. 6.21(b)). The successive 6 cycles at 15%, 10%, 5%, 2%, 1% and 0.5%
are totally dominated by the optical response, being the absorbed ethanol
molecules not enough to allow a conductivity rise in metalloporphyrin, and
the output signal decreases on exposure to ethanol.
Fig. 6.22 reports dark measurements at 5V and 10V reverse bias: here
the only sensing mechanism is the conductivity increase because no optical
interaction can be observed. No spikes, no sudden changes in the voltage
curve are registered because no other competitive response is present in the
darkness set-up. For low ethanol concentrations, no significant response are
registered because of a lack of a good amount of ethanol molecules in air.
Only 50%, 40% and 30% responses are clearly visible while the 20% vari-
ation is very small, even if higher for 10V, proportionally to the reverse
bias.
112
6.4. PARASITIC PORPHYRIN RESISTANCE
(a) Vrev=5V
(b) Vrev=10V
Figure 6.20: Measurement cycles at a wide range of ethanol concentrations, from 50% till
0.5%, for different photodiode reverse bias voltage: (a) 5V, (b) 10V.
113
CHAPTER 6. SENSORS EXPERIMENTAL TESTING
(a) Spike zoom
(b) 20% EtOH: the optical sensing mechanism appears
Figure 6.21: Different zooms of the plot in Fig. 6.20(a): (a) the ”spike” is in fact a
change in the response variation (from decreasing to increasing output voltage); (b) low
EtOH concentrations cycles: from 20% recovery phase, the optical sensing mechanism
becomes dominant and the parasitic resistor disappears.
114
6.4. PARASITIC PORPHYRIN RESISTANCE
Figure 6.22: Measurement cycles at the same EtOH concentrations employed in
Fig. 6.20(a) and Fig. 6.20(b) with the LED off in order to test only the conductivity
increase of metalloporphyrin.
More investigations have been conducted lately with the phototransis-
tor sensors and the pink LED set-up, focusing on BJT40 only, with both
coatings and with measurements performed at two different temperatures,
20C and 10C. The latter have also proved that the observed parasitic re-
sistor in parallel is not due to the ethanol itself, which could condense on
the device when flowing.
Fig. 6.23 and Fig. 6.24 show the variation of the output voltage on exposure
to 40% ethanol concentration: the parasitic effect is dominant.
115
CHAPTER 6. SENSORS EXPERIMENTAL TESTING
(a) CoTPP
(b) ZnTPP
Figure 6.23: Parasitic response to 40% EtOH concentration of the sensor BJT40 coated
with (a) Co-T(hexadodecyloxy)PP and (b) Zn-T(heptyloxy)PP, at 20C.
116
6.4. PARASITIC PORPHYRIN RESISTANCE
(a) CoTPP
(b) ZnTPP
Figure 6.24: Parasitic response to 40% EtOH concentration of the sensor BJT40 coated
with (a) Co-T(hexadodecyloxy)PP and (b) Zn-T(heptyloxy)PP, at 10C.
117
CHAPTER 6. SENSORS EXPERIMENTAL TESTING
Voltage increase rate, proportional to the conductivity increase rate, has
been calculated for both the sensors at both operating temperatures:
@ 20C: CoTPP: 7.7µV/s, ZnTPP: 7.2µV/s
@ 10C: CoTPP: 3.9µV/s, ZnTPP: 5.3µV/s
These rates show that metalloporphyrin conductivity depends on temper-
ature and on the alkyl chain length. It decreases if temperature decreases,
exhibiting a NTC (Negative Temperature Coefficient) character, accord-
ing to the porphyrin semiconductor behavior, described in literature [33].
While at room temperature the rates of the two sensing layers are com-
parable, at 10C, Co-T(hexadodecyloxy)PP conductivity increases slowly
with respect to Zn-T(heptyloxy)PP: this could be explained by consider-
ing the longer alkyl chains at the peripheral sites of the aromatic ring.
Longer chains means farther molecules (more porous film) and more dif-
ficult interactions among them. Further investigations on this interesting
dependence will be conducted in the future in order to better understand
how the alkyl chain affects the porphyrin conductivity and how it can im-
prove its insulator character when needed.
The described phenomenon is not considered an issue for the E-ON system
because it appears only at high VOC concentrations, in a range far from
the usual operating range of the sensors. Moreover it is possible in the fu-
ture to selectively evaporate metalloporphyrins only on the photodetector
active area and shadowing the aluminum contacts by employing a metal
mask.
118
Chapter 7
Conclusion
The research work reported and summarized in this dissertation thesis con-
tributed to the field of Electronic Nose for artificial olfaction applications,
which nowadays are gaining more and more importance due to the employ-
ment of electronic noses in medical diagnosis, environmental and industrial
process control and standardization, quality of food assessment.
Europe is leader in electronic nose research and this work has been devel-
oped in collaboration with the University of Rome Tor Vergata, where the
LIBRA nose was born almost 10 years ago and a long time work on gas
sensors matrix and metalloporphyrins sensing films has been done. The
intent of this thesis was to develop the sensors system of a new Electro-
Optical Nose, smaller, lighter, faster, less power consuming and cheaper
than the existing electronic noses.
Since metalloporphyrins had proved to work particularly well as sensing
films for VOCs (Volatile Organic Compounds) in gas sensors array, they
have been chosen as responsible of the first chemical interaction with gas
molecules, and silicon integrated photodetectors, photodiodes at the begin-
ning and phototransistors later, have been employed as signal transducers.
For first time in literature metalloporphyrins have been directly deposited
on the active area of the photodevices and served as blue light modulation
119
CHAPTER 7. CONCLUSION
filter, due to the change in their spectral absorption peak in the Soret band
(440nm) on exposure to VOCs.
Modified metalloporphyrins with alkyl chains in the peripheral sites of their
molecular structure have been evaporated on the sensors and they proved
to be responsible for a high improvement of the sensors response, provid-
ing a more porous layer with a more resolved spectral absorption peak.
Sensors response became also faster and together with the evaporation de-
position, which provided more uniform and controllable films, an ethanol
concentration in air of 1455ppm (parts per million) could be detected in
the first experiments (ethanol saturation vapor in air=58196ppm).
The most innovative solution proposed and studied in this thesis is the
adoption of silicon photodetectors with enhanced responsivity in the blue
spectral range. A fingers-shaped n+-p junction has been designed in order
to move the depletion region very close to the light incident surface to effi-
ciently collect the blue photons, with small penetration in the Si substrate
(100-200nm). Depletion regions around neighboring fingers merge together
by providing a continuous surface space charge region. Two types of pho-
todetectors have been fabricated and electrically and optically tested: 5µm
wide n+-finger implants in a p-well in the undoped n−-Si substrate and 5µm
p+-finger implants in the n−-Si substrate. Both the structures proved suc-
cessful in improving spectral responsivity around 440nm with a series of
peaks in the blue region, with slightly increasing amplitude for increas-
ing interfinger distance (10, 20, 30, 40µm). The maximum responsivity
value for the finger-photodiodes with 5V reverse bias applied is 0.24A/W
at 420nm, with an increment factor of 1.7 over the responsivity value of
the fabricated reference device (0.15A/W), without taking advantage of
any ARC (Anti Reflective Coating) optimization.
npn BJT phototransistors have been also developed with the same pho-
todevice structure by using the n+-fingers as the emitter, the p-well as the
120
base and the n−-substrate as the collector (contacted from the backside).
Such a simple upgrade of the original device brought some important ad-
vantages, like a higher spectral responsivity in the blue range and the beta
current gain which internally amplifies the photocurrent generated in the
floating base.
Electrical testing of the BJTs demonstrated higher current gains for in-
creasing interfinger distance, with a maximum of 158 for BJT40, more
than double with respect to 70 of the standard BJT.
The finger-implants significantly affect the spectral responsivity of both
photodiodes and phototransistors and the base current of the latter.
For BJTs an important contribution of the p-well perimeter has been
demonstrated in the collection of blue photons: the depletion region of the
lateral B/C junction contributes to the surface collection and it widens with
higher applied collector voltage, thus increasing the spectral responsivity.
This effect is registered only for the phototransistors and in the future a
new design for the p-well geometry will be implemented: a fingers-shaped
perimeter. In fact the fingers-E/B junction, if not reversely biased like in
the photodiodes case, doesn’t contribute much to the blue responsivity in-
crease, while implanting a p-well with fingers all around its perimeter will
provide an extended surface depletion region all around the B/C lateral
junction.
As regards sensing performance, the employment of the finger-photodiodes
and phototransistors improves the response till a minimum detectable sig-
nal of 291ppm. BJT-based sensors proved to be more performing than
photodiodes sensors, even if with a low S/N ratio, with higher response
and higher sensitivity to ethanol.
Repeatability and reproducibility have been experimentally tested during
8 months with a small deterioration of the sensor response in time, proba-
bly due to the aging of the sensing layer.
121
CHAPTER 7. CONCLUSION
Investigations of the role of metalloporphyrin conductivity variation with
ethanol have been also conducted as soon as a second sensing mecha-
nism appeared in the measurements results for high ethanol concentrations
(29000-12000ppm). It was demonstrated that metalloporphyrin conduc-
tivity is dependent on temperature and on the length of the alkyl chains
inserted at its peripheral groups, opening a new research on porphyrin con-
ductivity modulation and control through alkyl chains synthesis.
The final system developed in this dissertation is not yet a real electro-
optical nose, it is a prototype which proved the success of this optical
transduction approach and served to optimize the matching between the
sensing layer maximum response to gases and the photodetctor transduc-
tion.
Future work must concentrate on the experimental testing with a broader
range of VOCs and metalloporphyrins deposited, with more care on envi-
ronmental parameters dependence (e.g. relative humidity response).
All the data analysis part is still missing and it is going to be taken in
charge by the group of prof. Di Natale at the University of Tor Vergata.
The system box is not completed yet because the circuit board should con-
tain a multiplexer to allow the real-time extraction of all the sensors signals
in the matrix, and not only one per row. Moreover, in order to avoid the
mass flow controller, it will be necessary to mount valves and pumps in
the nose box itself. The same system adopted for the LIBRA nose can be
used for this.
This thesis work proved to be successful in providing a miniaturized nos-
tril completely optimized for VOCs sensing with metalloporphyrins and
in addition, interesting phototransistors with high spectral responsivity in
the blue range, which can be employed in different application fields, such
as new optical storage systems (DVD-ROMs and DVRs), which require
shorter wavelength laser diodes and read-out sensors with good sensitivity
122
in the UV/blue spectrum.
123
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Appendix A
Photodetectors Layout
The layout of the fingers photodetectors is made up by 5 mask layers:
1. p-well implant litho
2. p+ohmic contacts to p-well litho
3. n+-fingers implant litho
4. metal litho
5. contact holes litho
The layout images of all types of photodetectors designed are reported in
Fig. A.1 and Fig. 4.17, Chapter 4. For further details on the layout and
dies labeling on wafer, see the internal Technical Report [27].
129
APPENDIX A. PHOTODETECTORS LAYOUT
(a) PDstand (b) PD10
(c) PD30 (d) PD40
Figure A.1: Images of the layout of four photodiodes. PD20 layout is shown in Fig. 4.17.
130
Appendix B
Nostril Packaging & Bonding
For complete description of the E-ON box, pin out of all the boards and
schematics, see the internal Technical Reports [30] and [31], from which
the pictures below have been taken.
B.1 Sensors Board
Fig. B.1 represents the schematics of the die and wire bonding of the pho-
todiodes N+PDxx, 4 for each row. The sensors in the same row have the
p-well anode (red square contact) in common (only one sensor per row at a
time is monitored), short-circuited by a bridging wire, and the n-substrate
left floating. The p-well is connected to ground or the voltage generator
(negative reverse bias voltage), while the n+-fingers cathode (black square
contact) of each sensor is separately connected to the inverting input of
the op amp.
Fig. B.2 represents the schematics of the die and wire bonding of the
phototransistors: PTxx stands for BJTxx, the top row contains the 4 types
of finger sensors coated with Co-T(hexadodecyloxy)PP, the bottom row 4
finger sensors coated with Zn-T(heptyloxy)PP, the middle row contains 4
uncoated sensors for reference. The red square contact on each die rep-
resents the p-well/base contact, while the black square represents the n+-
131
APPENDIX B. NOSTRIL PACKAGING & BONDING
Figure B.1: Schematic top view of the photodiodes sensors matrix board with the wall
isolating the bottom row for differential measurements.
fingers/emitter contact. All the BJT bases are left floating, while the col-
lector is common for all sensors in the same row and is contacted from the
substrate backside. On the right of the figure, the sensors board backside
shows the bonding pads for soldering the wires to the circuit board. The
emitter is connected to the inverting input of the op amp and the collector
to the voltage generator (5V).
132
B.1. SENSORS BOARD
Figure B.2: Schematic top (left) and backside (right) view of the phototransistors sensors
matrix board.
(a) (b)
Figure B.3: Schematic design of the matrix package: (a) first prototype; (b) second pro-
totype, for a differential measurements configuration.
133
APPENDIX B. NOSTRIL PACKAGING & BONDING
B.2 Sensors package
Fig. B.3 and Fig. B.4 show the technical designs of the two nostril packages
employed during the thesis work.
(a) (b)
Figure B.4: Measures of the matrix package: (a) first prototype; (b) second prototype, for
a differential measurements configuration.
134