Mid-IR photon counting array using HgCdTe APDs and the Medipix2 ROIC
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Transcript of Mid-IR photon counting array using HgCdTe APDs and the Medipix2 ROIC
Mid-IR photon counting array using HgCdTe APDs and the Medipix2 ROICMid-IR photon counting array using
HgCdTe APDs and the Medipix2 ROIC
John Vallerga and Jason McPhate
Space Sciences Laboratory
University of California, Berkeley
Larry Dawson and Maryn Stapelbroek
DRS Sensors & Targeting Systems, Cypress CA
Photon counting
QADC
V v
EventsEvents
Charge integrating
Threshold
EventsCount(x,y,t)
SPIE 2007 San Diego
Motivation for photon countingMotivation for photon counting
Reduction of readout noise in infrared imaging
Advantageous in applications where imaging is not background dominated:
High frame rate (adaptive optics, interferometry)
Short integration times (Lidar etc.)
Low background (spectrophotometry, space based)
8 x 8Noiseless35% QE
10 photons
- - -
100 photons
1000 photons
8 x 82.5 e- rms90% QE
6 x 62.5 e- rms90% QE
4 x 42.5 e- rms90% QE
Signal in presence of noiseSignal in presence of noise
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Imaging IR photon counting detector conceptImaging IR photon counting detector concept
Use an IR sensitive absorber with gain– HgCdTe APDs– Large arrays
Count events at the pixel level– “Medipix2” CMOS ASIC– 55m pixels, 256x256 format
Readout binary data at 100MHz fast (~1 kHz framerate)
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Avalanche Photodiodes (APDs)Avalanche Photodiodes (APDs)
Geiger mode– Biased above breakdown– High, saturated gain - easy to count– Long recovery time per event– Afterpulsing and higher background
Linear mode– Biased near breakdown– Lower gain -harder to count– Distribution of pulse sizes - “excess noise”
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High Density Vertically Integrated Photodiode (HDVIP)High Density Vertically Integrated Photodiode (HDVIP)
DRS Infrared Technologies
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HDVIP IR APDs from DRSHDVIP IR APDs from DRS
HgCd1-xTex with adjustable c
Electron induced avalanche
Ion-milled via allows backside readout
Linear gains as high as 1000 (c < 4.3m)
Excess noise ~ 1 !
Arrays have been fabricated (128x128)
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Gain vs. bias voltageGain vs. bias voltage
= 4.3 m, 77K, 53 of 54 in array
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Excess noise factorExcess noise factor
k=0, only electrons involved in amplification
Excess noise factor of 1.0 implies a deterministic amplification process
Low noise factor allows a higher threshold in pulse sensing electronics
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Medipix2 ROICMedipix2 ROIC
Each pixel has amp, discriminator, gate & counter. 256 x 256 with 55 µm pixels (buttable to 512 x 512). Counts integrated at pixel. No charge transfer! Amplifier noise 110 e- rms
~ 500 transistors/pixel
Input
Preamp
Disc.
Disc. logic Mux. 13 bit
counter –ShiftRegister
Clock out
Shutter
Lower Thresh.
Disc.
Mux.
Previous Pixel
Mask bit
Analog Digital
Upper Thresh.
Next Pixel
Mask bit
Polarity
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Medipix readout of semiconductor arraysMedipix readout of semiconductor arrays
Developed at CERN for Medipix collaboration (xray)
radiography
tomography
mammography
neutron detection
gamma imaging
MCP readout
gaseous detectors
electron microscope
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Medipix2 readout architectureMedipix2 readout architecture35
84 b
it P
ixel
Col
umn
0
3584
bit
Pix
el C
olum
n 25
5
3584
bit
Pix
el C
olum
n 1
256 bit fast shift register
32 bit CMOS output LVDS out
• Pixel values are digital (14 bit)
• Bits are shifted into fast shift register
• Choice of serial or 32 bit parallel output
• Maximum designed bandwidth is 100MHz
• Corresponds to 284µs frame readout
SPIE 2007 San Diego
HDVIP - Medipix2 HybridHDVIP - Medipix2 Hybrid
Characteristics well matched:
HDVIP Medipix264 m pixel (8x8) 55 mm pixelGain up to 1000 Minimum threshold 900e-
Backside output Frontside inputLow dark current 10nA/pxl compensation
However
77K operation Room temp. design IR sensitive Very active chip
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Test SetupTest Setup
Simple test - drop Medipix2 chip into LN2
– Mounted on ceramic header used for 350C tests
– Attached to brass heat sink and copper cold finger
– Accurate diode thermometer glued to header
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Ceramic Header & Thermal TestingCeramic Header & Thermal Testing
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Test thermal profile Test thermal profile
LN2 Thermal test of MXR2 E07 on ceramic header
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15:00 16:00 17:00 18:00
Time
Temp [K]
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Individual DACs vs. Temp. Individual DACs vs. Temp. IKRUM
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1 14 27 40 53 66 79 92 105 118 131 144 157 170 183 196 209 222 235 248
DAC Value
Volts 299K
170K
123K
101K
77K
PREAMP
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0.6
0.8
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1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253
DAC Value
Volts 299K
170K
123K
101K
77K
DISC
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0.8
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1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253
DAC Value
Volts 299K
170K
123K
101K
77K
THS
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1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253
DAC Value
Volts 299K
170K
123K
101K
77K
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Threshold Variation (noise)Threshold Variation (noise)
299K
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Threshold Variation (noise)Threshold Variation (noise)
98K
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10000
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Threshold Variation (noise)Threshold Variation (noise)
77k-Reoptimized
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Feasibility Test at DRSFeasibility Test at DRS
Used existing 8x8 APD array mounted on fan-out header
Wirebonded 8 APD outputs to 8 Medipix2 input pads
Hybrid assembly mounted on larger header
Large header mounted in test dewar
– Expect higher amplifier noise due to increased capacitance
– Use IR photodiode as photon light source to input light pulses
– Use photon-transfer curve to characterize gain and noise
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Medipix2 and APD arrayMedipix2 and APD array
APD array
Medipix2 Wirebonds
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Test Hybrid in dewarTest Hybrid in dewar
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IR photodiode to illuminate APDIR photodiode to illuminate APD
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Future workFuture work
Start/continue feasibility tests– Quantify noise, gain and threshold sensitivity
Extrapolate results to realistic APD mounting Investigate APD fabrication techniques onto
Medipix wafer Model/simulate APD pixel to match Medipix
Seek funding to pursue full chip fabrication
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SummarySummary
If successful, this effort could lead to a sensor with:– HgCdTe QE (c < 4.3 m)
– Large arrays (512 x n*256)– Zero readout noise– kHz frame rates or higher– Electronic shutter
Which should prove very useful for many niche applications with low background in the IR