1. Mid-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

2. Photon counting Charge integrating Events Q V ± sv Events ± sEvents
Threshold
Events
Count
(x,y,t)
Charge integrating Q
ADC
V ± sv
Events
± sEvents

3. Motivation 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)

4. Signal in presence of noise
8 x 8 Noiseless 35% QE
10 photons -
100 photons
1000 photons
8 x e- rms 90% QE
6 x e- rms 90% QE
4 x e- rms 90% QE

5. Imaging 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
55m pixels, 256x256 format
Readout binary data at 100MHz fast (~1 kHz framerate)

6. 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”

7. High Density Vertically Integrated Photodiode (HDVIP)
DRS Infrared Technologies

8. HDVIP 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 < 4.3m)
Excess noise ~ 1 !
Arrays have been fabricated (128x128)

9. Gain vs. bias voltage
l = 4.3 mm, 77K, 53 of 54 in array

10. Excess 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

11. Medipix2 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

12. Medipix readout of semiconductor arrays
Developed at CERN for Medipix collaboration (xray)
radiography
tomography
mammography
neutron detection
gamma imaging
MCP readout
gaseous detectors
electron microscope

13. Medipix2 readout architecture
3584 bit Pixel Column 0
3584 bit Pixel Column 255
3584 bit Pixel Column 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

14. HDVIP - Medipix2 Hybrid Characteristics well matched: HDVIP Medipix2
64 m pixel (8x8) mm pixel
Gain up to Minimum threshold 900e-
Backside output Frontside input
Low dark current nA/pxl compensation
However
77K operation Room temp. design
IR sensitive Very active chip

15. Test 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

16. Ceramic Header & Thermal Testing

17. Test thermal profile

18. Individual DACs vs. Temp.

19. Threshold Variation (noise)

20. Threshold Variation (noise)

21. Threshold Variation (noise)

22. Feasibility 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

23. Medipix2 and APD array
APD array
Medipix2
Wirebonds

24. Test Hybrid in dewar

25. IR photodiode to illuminate APD

26. Future 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

27. If successful, this effort could lead to a sensor with:
Summary
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