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

SPIE 2007 San Diego

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)

SPIE 2007 San Diego

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”

SPIE 2007 San Diego

High Density Vertically Integrated Photodiode (HDVIP)High Density Vertically Integrated Photodiode (HDVIP)

DRS Infrared Technologies

SPIE 2007 San Diego

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)

SPIE 2007 San Diego

Gain vs. bias voltageGain vs. bias voltage

= 4.3 m, 77K, 53 of 54 in array

SPIE 2007 San Diego

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

SPIE 2007 San Diego

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

SPIE 2007 San Diego

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

SPIE 2007 San Diego

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

SPIE 2007 San Diego

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

SPIE 2007 San Diego

Ceramic Header & Thermal TestingCeramic Header & Thermal Testing

SPIE 2007 San Diego

Test thermal profile Test thermal profile

LN2 Thermal test of MXR2 E07 on ceramic header

0

50

100

150

200

250

300

15:00 16:00 17:00 18:00

Time

Temp [K]

SPIE 2007 San Diego

Individual DACs vs. Temp. Individual DACs vs. Temp. IKRUM

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

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

SPIE 2007 San Diego

Threshold Variation (noise)Threshold Variation (noise)

299K

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

100 150 200 250 300 350 400

SPIE 2007 San Diego

Threshold Variation (noise)Threshold Variation (noise)

98K

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

100 150 200 250 300 350 400

SPIE 2007 San Diego

Threshold Variation (noise)Threshold Variation (noise)

77k-Reoptimized

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

100 150 200 250 300 350 400

SPIE 2007 San Diego

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

SPIE 2007 San Diego

Medipix2 and APD arrayMedipix2 and APD array

APD array

Medipix2 Wirebonds

SPIE 2007 San Diego

Test Hybrid in dewarTest Hybrid in dewar

SPIE 2007 San Diego

IR photodiode to illuminate APDIR photodiode to illuminate APD

SPIE 2007 San Diego

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

SPIE 2007 San Diego

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