mechanical sensor instrumentation 1

Stewart SmithBiosensors and InstrumentationU-Tokyo Special Lectures

Mechanical Sensor Instrumentation 1

Lecture 6

1


Micro-Mechanical Sensors

• Piezoresistive Sensors‣ Strain gauges

‣ Applications

• Piezoelectric Sensors‣ Quartz crystal microbalances

‣ Surface Acoustic Wave Sensors

• Capacitive sensors (next lecture)

2


Piezoresistance

• A change in resistance of a material as it is subjected to mechanical force

• Discovered by Lord Kelvin (1856)

• Also referred to as a strain gauge effect

• Thin film deposited or diffused resistors are particularly sensitive

• Greater effect in semiconductors than metals

3


Strain Gauge

• Typically a metal foil on a flexible insulating support

• Bending in long direction causes largest change in resistance

• Tension makes conductors longer and thinner, R increases

• Compression makes conductors shorter and wider, R decreases

4


Strain Gauge

• Characteristics defined by the Gauge Factor, GF:

• ∆R: Change in resistance with applied strain of ε

• RG: Resistance of gauge with no applied strain.

5

GF =�R

�RG


Strain Gauge Instrumentation

• Wheatstone bridges

• Two possible arrangements:

‣ R1 - active gauge, R2 - dummy gauge

‣ R1 and R4 - Strain gauges in compressionR2 and R3 - Strain gauges in tension

• Both temperature compensated, second gives “push-pull” action, greater Vo

6

A

B

C

DVV

RR

R R

1

os

24

3

I1I2


Murphy’s Law

• Normally quoted as “If anything can go wrong it will”

• Named for Edward A. Murphy Jr.

• Story is murky but involves early strain gauges and rocket sleds

• Book by Nick T. Spark contains the full story

7


Silicon Strain Gauges

• Piezoresistive effect in semiconductors is more complicated than with metals

• Resistivity of material has dependence on applied stress due to effects on mobility

• Gauge factor is higher than with metals

• Strain effects are exploited in BJTs and advanced CMOS microelectronics

8


Silicon Strain Gauges

• Diffused resistor

• Function for voltage drop under strain:

• R0 - stress free resistance, I - applied current

• πL,πT - Piezoresistive coefficients in longitudinal and transverse directions

• σij - tensile stress components in x, y, z

9

V = R0I[1 + ⇡L

�xx

+ ⇡T

(�yy

+ �zz

)]


Applications of Piezoresistors

• Diffused or polysilicon resistors are easily integrated with micro-mechanical sensors

• Examples include accelerometers and gyros for motion and orientation sensing

• Pressure sensors with piezoresistive transducers have been developed here

10


Pressure Sensor

• Micromachined silicon sensor

• Sealed cavity with flexible membrane

• Detect membrane movement

11

Bonded

BondedSilicon Sealed Cavity

Flexible Membrane

Silicon


Pressure Sensor

• Polysilicon strain gauges on membrane

• Wheatstone bridge arrangement

• Two resistors will change with pressure

12


Pressure Sensor

• Designed at the ISLI in Livingston

• Fabricated at the SMC

• Chamber etched with wet process (TMAH)

13


Pressure Sensor

• Standard Dual-In-Line packaging with gold wire bonding to pads

• Hole cut in package to allow pressure connection to chip

14

Pressure Sensing Chip

Package Cover

Ceramic DIL Package

Push Fit Vacuum Connector


Pressure Sensor• Wheatstone bridge

with added potentiometer

• Allows zeroing of the sensor at set pressure

15

Vout

V+

−

+

+−


MEMS Accelerometer

• Developed at Heriot-Watt and fabricated at the SMC

• 3-Axis linear motion sensing operation

• Biocompatible packaging for implantation on heart to monitor post-bypass

16

medical staff in post-surgery to provide “real-time”monitoring of the heart and give early warning of regionalcardiac ischemia.The sensors that have used in our studies so far are

biocompatibally packaged so that the sensor is safe tobe implanted in the human body. This packaging of theaccelerometer was carried out using a silicone mouldingtechnique (Imenes et al., 2007). In order to operate on theheart, the thorax of the patient is opened prior to the bypasssurgery. Following the bypass procedure, the sensor is suturedto the surface of the heart before closing the thorax.The studies carried out to date have utilized commercially

available three-axis accelerometers, such as the KionixKXM52-1050 (Kionix Inc., Ithaca, New York, USA). Onesuch sensor can be seen in situ in Figure 1. The studies haveproved that this type of sensor is capable of measuring theheart function in great detail and provide early recognition ofischemia (Elle et al., 2005). These sensors have been removedfrom the heart before the chest was closed after the surgery.For post-surgery monitoring, the sensor would be left on

the heart for a few days after the surgery, i.e. after the chest isclosed. The sensor is then pulled out through a small hole inthe patient’s chest using its cable before they leave thehospital. The present sensor is too large for this procedure,and there is a need for a dedicated smaller acceleration sensorfor this application.This paper describes the design and fabrication of such a

sensor. The dedicated sensor measures approximately 3mmin width, 5mm in length, and 1.5mm in height.

Accelerometer structure

The various fabricated sensor designs all have the sameconfiguration consisting of a basic seismic mass and cantileverbeam structure. Four monolithic designs were submitted,each of which consisted of four full wafer thickness masses,each supported by either a single or two thin stress-sensitivebeams.To realise the seismic masses, silicon-on-insulator (SOI)

wafers were processed using a deep dry etch process stepThis process allows the silicon handle layer to be etched forthe definition of the full wafer thickness masses (380mmthick). It also permits the definition of near vertical sidewalls.

The buried oxide (BOX) layer represents the etch-stop fromthe backside due to etch selectivity. Finally, the silicon devicelayer can be used for the cantilever beams and these werechosen to be 4mm thick. The schematic of one of the sensorsis shown in Figure 2 (note that this does not include thebonding and bond pad areas).The use of a deep dry etch versus an anisotropic wet etch

has particular benefits for this sensor design. The etch rate isno longer dependent on crystal planes, which would otherwiseresult in angled sidewalls. Larger masses can therefore befitted into a given footprint, which increases the sensitivity ofthe sensor. In addition, the position of the centre of gravity ofthe masses in the vertical direction lies deeper inside thewafer, which further increases the sensitivity to in-planeaccelerations. There is also no requirement for compensationstructures that are required in anisotropic etching to protectfaster etching convex corners.

Figure 1 Heart sensor stitched to a heart during animal studies

Figure 2 Deformation of sensor masses when: (a) in-plane (x-axis); and(b) out-of-plane (z-axis) accelerations are applied

1 3

4 2

x

zy

13

24

x

zy

(a)

(b)

Fabrication of a MEMS accelerometer

Craig Lowrie, Marc P.Y. Desmulliez, Lars Hoff, Ole Jakob Elle and Erik Fosse

Sensor Review

Volume 29 · Number 4 · 2009 · 319–325

320


MEMS Accelerometer

• Four moving masses suspended by cantilevers

• Diffused piezoresistors sense bending of cantilevers

• 2 resistors in each cantilever

• 16 in total, 4 Wheatstone bridges

17

6 Etching is then performed from the front-side of the waferfirst. A reactive ion etch is used to define the outline of themasses and the beams by etching through the silicondevice layer. The BOX layer is used as an etch stop.

7 The final lithography step is then done on the backside ofthe wafer. The etch process uses an inductively coupledplasma (ICP) etch and is used to define the masses andthe supports for the base of the beams. The 380mm thicksilicon handle layer is etched first using the BOX layer asan etch stop. Finally, the BOX layer is then etched itselfand this releases the structures, leaving the massessupported only by the beams.

Packaging

The decision was made to perform the packaging at the chiplevel. To do this, it was necessary to remove each chip fromwafer-level following completion of fabrication. Conventional

dicing was not considered as a viable approach due to the highrisk of damage from vibration and debris to the fully releasedsensitive structures. To overcome this issue, tabs wereincorporated around the sensors, which held the chips inplace once processing was complete. An example of these tabsis shown on the left-hand side of Figure 7. A small amount ofdownward pressure “snaps” the chip free from the wafer.To protect the sensitive structures of the sensor for handling

and further biocompatible packaging, glass caps have beenbonded to the front and back of the die. This was performedusing a polymer based, low-temperature bonding process.Benzocyclobutane (BCB) is deposited and patterned onto theglass wafers (Wang et al., 2008). No cavities have been etchedinto the glass caps so the thickness of the deposited BCBforms the gap for the movement of the masses underacceleration. The three components have been aligned undera microscope and pressed together by a weight. A heatingcycle is then ramped up and down with the complete

Figure 6 Microscope images of the front-side of (a) design I, (b) design II, (c) design III and (d) design IV

(a) (b)

(c) (d)



Sensor Review

Volume 29 · Number 4 · 2009 · 319–325

323

temperature-loading cycle taking approximately 30min.A packaged sensor is shown in Figure 8.To achieve a larger gap between the glass caps and the top

and bottom of the masses, the possibility of using sandblastedglass spacer layers was investigated. The gap can bedetermined by the thickness of the glass used and puts lesspressure on processing thicker BCB. BCB was also used tobond these layers in a glass-glass-silicon-glass-glass structureand was proven to be a viable approach.With the commercial sensor solution used during the

animal studies, almost one-third of the volume was occupiedby the cable termination. An approach has been investigatedinvolving using a ribbon cable instead of a round multi-wiredcable. Two techniques that have been considered for bondingthe ribbon cable to the sensor are wedge bonding and non-conductive adhesive bonding (Imenes et al., 2008). Two setsof bondpads have been used in the design layouts, smallerpads for using with the methods described above and largerbondpads to be used with traditional wire bonding for thepurpose of characterization. An example of a wire-bondedsensor can be seen in Figure 9. These would not be requiredin a further process run and would further reduce the size ofthe sensors. In addition, all the designs were made to be thesame size in the wafer layout in case the tabs were not

successful and a different approach had to be used. The tabswere a success so further miniaturisation would be possible ina further run.

Conclusions

Four accelerometer designs have been designed andfabricated to be used as a sensor solution for measuring theheart wall motion of patients who have just undergonecoronary artery bypass surgery. The feasibility of using athree-axis accelerometer in this application has been provenusing a commercially available sensor. Approaches for thepackaging have been demonstrated that allow for furtherminiaturisation and biocompatibility.The designs are theoretically capable of having matching in-

plane and out-of-plane sensitivities by taking advantage of thebenefits offered by using an inductively ICP etch togetherwith SOI wafers.The packaged sensors have been glued into a ceramic chip

carrier and wire bonded so that they are ready forcharacterization. It is hoped that the sensors will bedynamically characterized using a scanning laser-Dopplervibrometre. Piezoelectric actuators to provide excitation,makes it possible to integrate the use of a vacuum chamberwith a vibrometre to measure the sensor in differentenvironments. Another approach being investigated is to useacoustic excitation using an air-coupled ultrasonic transducer.

References

Elle, O.J., Halvorsen, S., Gulbrandsen, M.G. and Fosse, E.(2005), “Early recognition of regional cardiac ischemiausing a 3-axis accelerometer sensor”, PhysiologicalMeasurement, Vol. 26 No. 4, pp. 429-40.

Imenes, K., Aasmundtveit, K. and Husa, E.M. (2007),“Assembly and packaging of a three-axis microaccelerometer used for detection of heart infarction”,Springer Biomedical Microdevices, Vol. 9 No. 6, pp. 951-7.

Imenes, K., Aasmundtveit, K. and Moreno, P. (2008),“Micro ribbon cable bonding for an implantable device”,Proceedings of the 2nd Electronics Systemintegration Technology

Figure 8 Packaged sensor placed on top of a one-pence piece

Figure 7 Microscope image of the back-side of design II Figure 9 Packaged sensor glued and wire bonded in ceramic chipcarrier



Sensor Review

Volume 29 · Number 4 · 2009 · 319–325

324


Single Crystal Silicon Strain Gauges

• Equal and opposite piezoresistive coefficients for longitudinal or transverse bending

• When under longitudinal tension:

• Then under longitudinal compression:

18

RL = R+�R,RT = R��R

RL = R��R,RT = R+�R


In-Plane Acceleration

• Movement in X or Y

• One mass moves up, opposite mass down

• One RL and one RT on each proof mass

19













1 3

4 2

x

zy

13

24

x

zy

(a)

(b)



Sensor Review

Volume 29 · Number 4 · 2009 · 319–325

320


Out of Plane Acceleration

• Movement in Z

• All 4 masses move up or down together

• Looking at 3&4

20













1 3

4 2

x

zy

13

24

x

zy

(a)

(b)



Sensor Review

Volume 29 · Number 4 · 2009 · 319–325

320

RL3 = R+�R

RT3 = R��R

RL4 = R+�R

RT4 = R��R


Out of Plane Acceleration

• What are the outputs of these?

21

VoutVS

RL2

RT1

RL1

RT2

VoutVS

RT4

RT3

RL3

RL4


Piezoelectric Sensors

• Principle of Operation (Pierre and Jaques Curie, 1880)

‣ Ability of materials to generate a electrical potential (or electric field) when mechanically stressed

‣ Conversely applying a potential to a piezoelectric material can cause mechanical deformation

‣ Ultrasound systems typically use piezoelectric actuators and sensors

22


Piezoelectric Materials

• Typically crystalline, they will contain dipoles with some form of polarisation

• Applying mechanical force changes the polarisation, moving charge around

• Materials include: quartz, bone, lead zirconate titanate (PZT), lithium niobate and aluminium nitride.

23


Piezoelectric Materials24

+- +

+-

-

Si

O2

SiSi

O2

O2

Fx Fx

• Schematic of atoms in a quartz crystal

• Applying a force will rearrange the charge


Quartz Crystal Microbalance

• Thin slice of quartz with thin film electrodes

• Applying AC signal creates standing wave in crystal

• High Q (frequency/bandwidth) resonance

• Detection of electrical signal generated by resonance

25


Quartz Crystal Microbalance

• QCM resonant frequency is sensitive to mass

• Often used as thickness monitors in deposition

• Resonant frequency and bandwidth will change with mass and viscoelasticity

26


QCM and QCM-D

• Pure f changes due to deposition of hard materials do not apply in biosensing

• Biological materials and operation in liquid requires QCM-D (dissipative QCM)

• Q-Sense hold patents on QCM-D technique. Analysis of shifts in f and D (1/Q)

• Detection of binding events such as immuno-sensing or DNA attachment

27


QCM Equivalent Circuit

• CP - Electrical capacitance of the QCM and connections

• L, CS, R - characteristic of crystal resonance and load

• At resonance L and CS cancel to leave R representing losses

• Low R equivalent to high Q

28

L

C

R

CP S


QCM Instrumentation

• Drive QCM into resonance

• Switch off power and measure decay

• Dissipation (D = 1/Q)

• Useful for biosensor measurements

29

Relay

Oscilloscope

Oscillator

Freq. counter

Computer

QCM


Surface Acoustic Waves

• QCMs are “Bulk Acoustic Wave” devices

• Rayleigh waves are acoustic vibrations in the surface of a material - SAW

30


SAW Devices

• Interdigitated electrode on piezoelectric substrate generates SAW vibrations

31

Piezoelectric Substrate


Interdigitated Transducers (IDT)

AC

Gen

erat

or

Load


AC

Gen

erat

orSAW Devices

• Frequency f = v/d where v is wave velocity in the material and d is the pitch of the IDT

32




Load


SAW Sensors

• Temperature, strain, pressure, force or added film thickness can be sensed as ∆f

33




AC

Gen

erat

or

Load


SAW Biosensors34

SensorFrequency

DifferenceFrequency

ReferenceFrequency

Amplifier

Amplifier

SAW biosensor with specific

receptor layer

Reference SAW device without receptor layer


Wireless SAW

• SAW devices are used in passive RFID tags

• Excited by an RF pulse they have with reflectors spaced like a barcode for ID

• A wireless SAW sensor would instead have a single reflector with a sensing zone

• Changes in delay or spread of pulse contains information from the sensor

35


Surface Acoustic Wave (SAW) Microfluidics

36

• RF signal applied to interdigitated transducer generates a surface acoustic wave in piezoelectric material (ZnO)

• Acoustic energy couples into a droplet on the hydrophobic surface causing it to vibrate and then move

mechanical sensor instrumentation 1

Documents