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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. Q. V ± s v. Events. Count. ADC. (x,y,t).

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mid ir photon counting array using hgcdte apds and the medipix2 roic

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

slide2

Q

V ± sv

Events

Count

ADC

(x,y,t)

Threshold

Events

± sEvents

Photon counting

Charge integrating

motivation for photon counting
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)

signal in presence of noise

1000 photons

100 photons

10 photons

8 x 8Noiseless35% QE

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 noise
imaging ir photon counting detector concept
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)
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”
hdvip ir apds from drs
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)
gain vs bias voltage
Gain vs. bias voltage

l = 4.3 mm, 77K, 53 of 54 in array

excess noise factor
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
medipix2 roic
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

medipix readout of semiconductor arrays
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

medipix2 readout architecture

3584 bit Pixel Column 0

3584 bit Pixel Column 1

3584 bit Pixel Column 255

256 bit fast shift register

32 bit CMOS output

LVDS out

Medipix2 readout architecture

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

hdvip medipix2 hybrid
HDVIP - Medipix2 Hybrid

Characteristics well matched:

HDVIP Medipix2

64 m pixel (8x8) 55 mm pixel

Gain up to 1000 Minimum threshold 900e-

Backside output Frontside input

Low dark current 10nA/pxl compensation

However

77K operation Room temp. design

IR sensitive Very active chip

test setup
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
feasibility test at drs
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
future work
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
summary
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