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

Photon counting

Threshold (x,y,t) Count Events

Charge integrating

Q V  s v ADC Events  s Events

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)

SPIE 2007 San Diego

Signal in presence of noise 1000 photons 100 photons 8 x 8 Noiseless 35% QE 8 x 8 2.5 e rms

-

90% QE 6 x 6 2.5 e rms

-

90% QE 4 x 4 2.5 e rms

-

90% QE 10 photons

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)

SPIE 2007 San Diego

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”

SPIE 2007 San Diego

High Density Vertically Integrated Photodiode (HDVIP) SPIE 2007 San Diego

DRS Infrared Technologies

HDVIP IR APDs from DRS

 HgCd 1-x Te x 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)

SPIE 2007 San Diego

Gain vs. bias voltage SPIE 2007 San Diego



= 4.3



m, 77K, 53 of 54 in array

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

SPIE 2007 San Diego

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!

Previous Pixel Amplifier noise 110 e rms Mask bit Shutter Mux .

Lower Thre sh.

Clock out Polarity Disc.

Input Preamp Disc.

Disc.

log ic Mux .

13 bit coun ter – Shift Register Uppe r Thresh .

Mask bit

~ 500 transistors/pixel

Analog Next Pixel Digital

SPIE 2007 San Diego

SPIE 2007 San Diego 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

SPIE 2007 San Diego Medipix2 readout architecture

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

HDVIP - Medipix2 Hybrid

Characteristics well matched: 64  HDVIP m pixel (8x8) Gain up to 1000 Backside output Low dark current However Medipix2 55 mm pixel Minimum threshold 900e Frontside input 10nA/pxl compensation 77K operation IR sensitive

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Room temp. design Very active chip

Test Setup

 Simple test - drop Medipix2 chip into LN 2 –

Mounted on ceramic header used for 350C tests

–

Attached to brass heat sink and copper cold finger

–

Accurate diode thermometer glued to header

SPIE 2007 San Diego

SPIE 2007 San Diego Ceramic Header & Thermal Testing

SPIE 2007 San Diego

300 250 200 150 100 50 0 15:00

Test thermal profile LN2 Thermal test of MXR2 E07 on ceramic header

16:00 Time 17:00 18:00

Individual DACs vs. Temp. IKRUM

2 1.8

1.6

1.4

1.2

1 0.8

0.6

0.4

0.2

0 1 299K 170K 123K 101K 77K 14 27 40 53 66 79 92 105 118 131 144 157 170 183 196 209 222 235 248

DAC Value PREAMP

1.4

1.2

1 0.8

0.6

0.4

299K 170K 123K 101K 77K 0.2

0 1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253

DAC Value DISC

1.4

1.2

1 0.8

0.6

0.4

299K 170K 123K 101K 77K 0.2

0 1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253

DAC Value THS

2 1.8

1.6

1.4

1.2

1 0.8

0.6

0.4

0.2

0 1 299K 170K 123K 101K 77K 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253

DAC Value SPIE 2007 San Diego

SPIE 2007 San Diego Threshold Variation (noise) 299K

20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 100 150 200 250 300 350 400

SPIE 2007 San Diego Threshold Variation (noise) 98K

10000 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 100 150 200 250 300 350 400

SPIE 2007 San Diego Threshold Variation (noise) 77k-Reoptimized

5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 100 150 200 250 300 350 400

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

SPIE 2007 San Diego

SPIE 2007 San Diego Medipix2 and APD array Medipix2 Wirebonds APD array

SPIE 2007 San Diego Test Hybrid in dewar

SPIE 2007 San Diego IR photodiode to illuminate APD

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

SPIE 2007 San Diego

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

SPIE 2007 San Diego