a cmos 8x8 spad array for time-of- flight measurement and ... · a cmos 8x8 spad array for...
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WASC, June 3-4, 2013, Seville 1
I. Vornicu, R. Carmona-Galán, Á. Rodríguez-Vázquez
A CMOS 8x8 SPAD Array for Time-of-Flight Measurement and Light-Spot
Statistics
Institute of Microelectronics of Seville (IMSE-CNM)CSIC-University of Seville
Workshop on Architecture of Smart Camera (WASC), Seville, Spain
June 3-4, 2013
WASC, June 3-4, 2013, Seville 2
Outline
• SPADs in Positron Emission Tomography
• 8 x 8 SPAD array for ToF and spot characterization
• Architecture
• Functionality
• Simulation results
• Conclusions
WASC, June 3-4, 2013, Seville 3
Creates images by measuring electro-magnetic radiation emitted by the radiotracer molecule
Positron emission tomography (PET)
Single photon emission tomography (SPECT)
[Cherry 2003][Wernick 2004]
The key element to the successful nuclear medicine imaging is proper selection of the radioactive tracer [Saha 2005].
Nuclear medicine
PET: principles of operation
PET combines diagnosis with treatment
[Iniewski 2009]
An atomic electron Positron annihilation = interacts with
a positron
WASC, June 3-4, 2013, Seville 4
The detection of the two events establishes a line-of-response (LOR)
PET: principles of operation
The required time (Tbin) and spatial resolution are below few nanoseconds and
4x4mm² respectively
LOR
rayγ
Detection improvement byMinimizing the uncertainty along LOR by ToF estimation
Increasing spatial resolution by detecting the position of the maximum spot
Increase the diameter of detectors rings
Improve spatial resolution (LSS)
[Iniewski 2009]
Minimize parallax error
WASC, June 3-4, 2013, Seville 5
Depth of interaction (DOI)γ-ray
Visible photons
High-energy photon
DOI
Scintillatorcrystal
• Inaccurate determination of DOI results in incorrectly positioned LOR
• Incorrectly positioned LOR leads to parallax errors and, hence:
• Imprecise reconstruction of the image
• DOI in continuous scintillatorcrystals can be inferred from width of light-distribution
[Kao et al. 2000]
[Hoffman et al. 1989]
[Lerche et al. 2005]
WASC, June 3-4, 2013, Seville 6
Aggregated row and column detections
Centre of mass of the light spot:
Variance (second moment):
Relation between DOI and variance:
Focal-plane calculation of the DOI
∑=
=N
iiip
11μ
( )∑=
−=N
iipi
1
21
2 μσ
( )200 σσ −∝− zz[Pozas et al. 2011]
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D1=L+d; T1=D1/c;
∆T=2d/c – is the time difference in detection between detector 1 and 2.
D2=L-d; T2=D2/c;
[Lewellen 1998]
[Conti 2008] Less data is needed to infer the actual position of the radio tracer
ToF minimize uncertainty along LOR
Tbin=200ps, ∆x=3cm
WASC, June 3-4, 2013, Seville 8
PET detectors: PMT vs. SPAD
Photomultiplier tubes:mature technology, widely employedaffected by magnetic fields
limited spatial resolutionmultiple crystal arrays are expensiveundetermined DOI
Single-photon avalanche diodes:experimentalcan operate in magnetic fields
better spatial resolutioncheaperfocal-plane image processing
rayγ
visible spectrum
WASC, June 3-4, 2013, Seville 9
DV
DI
1
2
3
Single-photon avalanche diode
1) A photodiode biased beyond breakdown stays at zero current until avalanche is initiated
2) Avalanche can be triggered by one single photon absorption event, DC or AP
3) Quenching circuit (active or passive) limits the avalanche current by keeping |V0-VA|<VBD
Avalanche must be quenched to avoid device destruction
The simplest scheme is passive quenching
Active quenching and recharge can be employed to decrease dead time
DV+
−DI
2
3 11
Passive quenching
Active quenching
1) Dark counts
2) After-pulsingUnwanted events
WASC, June 3-4, 2013, Seville 10
State of the art
Ref.
#
Techno
logy
DCR Time resolution
Pixel size Array size Incorporated functionality
Tunable
dead time (DT)
VbiasSPAD
[Tisa et al. 2008]
CMOS 0.35um
Up to 30Khz
617ps 50 x 100um - Tofmeasurements
40ns –2us
24V
[Fara. et al. 2008]
CMOS 0.18um
40Khz - 90 x 100um - Tofmeasurements
30ns 10.2V
[Braga et al. 2011]
CMOS 0.35um
1-2Khz expected
325ps 145 x 215um
14x10 mini SiPM with 32 SPADs
Data compression for
PET
- -
This work
CMOS 0.18um
25kHz 150ps expected
32 x 32um 8 x 8
Tofmeasurements
Statistics of the spot for
PET
3ns –40ns
10.8V
WASC, June 3-4, 2013, Seville 11
Features double functionality:ToF measurements Focal plane statistics
Array architecture
Row
cou
nter
Col decoderR
ow d
ecod
er
Col counters
Data serialiserD
ata
seria
liser
8x8 SPADs +AQR
Row
cou
nter
Col decoderR
ow d
ecod
er
Col counters
Data serialiserD
ata
seria
liser
8x8 SPADs +AQR
CMOS SPAD
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Array architecture
Vspad+K
A
AQR
GNDspad+
Vspad+K
A
AQR
GNDspad+
Vspad+K
A
AQR
GNDspad+
Vspad+K
A
AQR
GNDspad+
ENRST
ENRST
ENRST
VDD VDD
OVFB12
B1
OVFB12
B1
A. Time of flight configuration
Each SPAD is multiplexed to a single output
B. Light spot statistics configuration
MAveraging M measurements, TR improves by
Time gated measurement
Row/column counters are collecting data to build up statistics of the actual position
of the impinging light beam
Very low light illumination
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Active quenching and recharge
AQR – this work
AQR principle [Tisa 2008]Variable-load quenching circuit [Tisa 2008]
AQR principle [Faramarzpour 2008]
WASC, June 3-4, 2013, Seville 14
Post layout simulations
0
0.5
1
1.5
2 /Vout pixel /Vout array /Vout PAD
Vol
tage
(V)
348 350 352 354 356 3580
0.5
1
1.5
/Anode current spike
Cur
rent
(mA
)
time(ns)
pixelarrayPADT1=374ps
T2=2.8ns
Worst case signal path delay
The output buffer introduces an systematic error
The delay (T1) introduced by the AQRcircuit is much smaller than T2
a) Shape of the output voltage along the signal pathb) Current spike that flows through the SPAD
SPAD are diodes working beyond the breakdown (BD) region
Fast quenching circuits are mandatory to avoid device destruction
WASC, June 3-4, 2013, Seville 15
Post layout simulations
0
0.5
1
1.5
2/Vreset /VA
Vol
tage
(V)
248 249 250 251 252 253 254 255 256 2570
0.5
1
1.5
/I(A)-anode current spike
Cur
rent
(mA
)
time(ns)
Vreset
VA
Active quenching
Active reset
Due to the current that flowsthrough M3 and M4
VA is the anode voltage
Vreset is a very short pulse (~300ps)it restores the state of the SPADthat is ready to trigger another avalanche
WASC, June 3-4, 2013, Seville 16
Post layout simulations
0
0.5
1
/Vcap
0
1
2/Vout
Vol
tage
(V)
250 255 260 265 270 275 280 285 2900
0.5
1
1.5/I(A)-anode current spike
Cur
rent
(mA
)
time(ns)
DT=5.6ns DT=41ns
DT=5.6nsDT=41ns
Unwanted pulses can be triggered by:- dark count events (DC)- after pulsing events (AP)
AP rate can be decreased by increasingDT
DT of the SPAD can be tuned between 4 and 41ns
DCR and AP are also depending on theexcess voltage (Ve)
Lower sensitivity
Ve
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8x8 SPAD array (ToF and Spot Stats)1.5mm
1.5m
m
CMOS technology UMC 0.18um
No. of SPADs 8 x 8 test structure
Die area 1.5mm x 1.5mm
Sensor area 256um x 256um
SPAD cell pitch 32um x 32um
SPAD diameter 12um (14um external TWELL)
Counters depth 13b
Time resolution <150ps
WASC, June 3-4, 2013, Seville 18
8x8 SPAD array (ToF and Spot Stats)32um
32u
m
CMOS technology UMC 0.18um
No. of SPADs 8 x 8 test structure
Die area 1.5mm x 1.5mm
Sensor area 256um x 256um
SPAD cell pitch 32um x 32um
SPAD diameter 12um (14um external TWELL)
Counters depth 13b
Time resolution <150ps
WASC, June 3-4, 2013, Seville 19
Preliminary results
The output of one Individual cell
Laser modulating signal
Events triggered by the laser
Events triggered by DC, AP and minimum illumination power
The light beam reaches themaximum power level
The light beam reaches theminimum power level
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Conclusions
• We have designed an array of SPADs in order to improve accuracy in PET imaging
• Rows and columns of the array behave as independent digital Silicon Photo-Multipliers.
• The outcome of the SiPM processing are the histograms of the rows and columns of the array.
• This information is useful to compute the DOI and therefore to achieve a clearer picture of the sample.