Performance limits of a 55 m pixel CdTe detector G.Pellegrini, M. Lozano, R. Martinez, M. Ullan Centro Nacional de Microelectronica, Barcelona, 08193,

Performance limits of a 55Performance limits of a 55mm pixel CdTe detector pixel CdTe detector

G.Pellegrini, M. Lozano , R. Martinez, M. Ullan Centro Nacional de Microelectronica, Barcelona, 08193, Spain

M. Chmeissani , M. Maiorino, G. Blanchot, J. Garcia C. Puigdengoles

Institut de Física d'Altes Energies, UAB Campus, 08193 Bellaterra, Spain

2004 IEEE NSSGiulio Pellegrini

Objectives of the researchObjectives of the research

CdTe is one of the most widely studied material in the field of X-ray detection for energies above 10keV, due to its high absorption efficiency.

CdTe detectors for imaging applications are usually fabricated in thick substrates and with pixel size smaller than 100μm.

However, reducing the pixel size might lead to an increase in the charge shared among neighboring pixels due to the low mobility of the major carriers.

The operation of a pixel detector is strongly influenced by the ratio of the pixel size to the thickness of the detector.

We present charge sharing results from simulation and experimental data obtained with a specific purpose circuitry using Medipix-II with CdTe detectors.


X-ray absorptionX-ray absorption

Mammography

0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,100,0

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sorb

tio

(%)

Thickness (cm)

5keV 10keV 15keV 20keV 60keV

CdTe0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10

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Ab

sorp

tio

n (

%)

x(cm)

5kev 10kev 15kev 20kev 60keV

Fu

ll D

ep

leti

on

Bia

s (

V)

Silicon

Absorption )exp1(0 xIxI

General radiography

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25

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0 20 40 60 80 100 120 140Energy (keV)

Abs

ortio

n (%

)

Si, 0.8 mmCdTe, 0.3 mmCdTe, 1 mmKodak film 0.1 mm


Charge sharing problemCharge sharing problem

Charge sharing depends strongly on the position of the interaction of the incident X-ray.

Since detectors are illuminated from the back contact, low energy x-rays will interact “far” from the pixel side.

Charges generated in point 1 will be shared between different pixels.

Charges generated in point 2 will be collected only in the central pixel.

Mean free path* = 80m for a 20 KeV X-ray in CdTe. 80% of the X-rays interact in the first 120m

*Average distance traveled before the interaction take place

Pla tinum c o nta c t Insula tio n la ye r

X-ra y

C d Te

Ele

ctri

c fi

eld

X1

X2

Pixe ls

Ba c k e le c tro d e


CdTe detectorCdTe detector

55m

45m 15m

Solder bumps CdTe pixel electrodes

from ACRORAD


MEDIPIX-II chipMEDIPIX-II chip

Chip Specifications 55 µm pixel pitch 65536 (256×256) pixels

(14.1×14.1 mm active area)

14 DAC settings Very good energy

resolution (0.75keV) 1 Mbit data Serial & Parallel read-out

bus (input bus is Serial only)

Maximum estimated communication clock: 80 MHz

Pixel Specifications 55×55 µm area Dynamic range: -1 ÷ 8000 3-bits (8 levels) threshold

adjustment

Schematic view of a detector pixel bump bonded to Medipix II chip

Single photon counting mode


X-ray X-ray ImagingImaging

Image taken with Medipix2 chip with CdTe detector with low temperature bump bonding.

Microfocus X-ray tube: 40 keV, 10 µA, 1 sec.

Pixel size 55 µm

Detector bias –100 V (electron collection)


Experimental setupExperimental setup

3×3 pixels matrix at the bottom of MPX2 chip (pixels 120,0 to 122,2)

Spectrum 241Am source Bias voltage :100V Chip analog outputs, got via DEar-MaMa

read-out system, were digitised and read through GPIB bus

Collected 500 events per bias value Events analyzed:

values 3 above noise peak in the central pixel

NOTE: the chip threshold level is NOT involved in this operation

M. Chmeissani et al., [1] ”, Proc. of the 21st IEEE Instr. and Meas. Tech. Conf., Vol. 1, pp. 787-791, Como, 18-20 May 2004, Italy, ISBN 0-7803-8249-8


Charge sharing Charge sharing measurementsmeasurements

The maximum charge collected in the central pixel is 60% at 400V.

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2

3

120121

122

00.05

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llect

ed C

har

ge

(%)

Relative Charge Sharing at 50V

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00.10.20.3

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llect

ed C

har

ge

(%)

Relative Charge Sharing at 400V

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Bias(V)

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arg

e co

llect

ed (

%)

Simulation Experimetal data


Software packageSoftware package

Electrical simulators: 3D simulator : ISE TCAD

software Monte Carlo simulator:

Geant4

Parameters:

Carrier carrier scattering Recombination High field saturation

No k-shell effect No traps added to the

model.


Simulation resultsSimulation results

Charge collection efficiency is calculated for pixel 5 generating the charge in different points and at various depths.

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Ch

arg

e c

olle

cte

d (

%)

Depth interaction (m)

point 1 point 2 point 3 point 4 point 5 point 6

Charge collected on pixel 5at bias -100V

Distance from pixel electrodes

4 5 m

m

m

m

m

5 5 m

45m 7

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45

6

2-4

2-21-4

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2 -31 -3


Integrated chargeIntegrated charge

Minuit fit

20

20

2

1

2

1exp

yx

yyxxAz

m 28

Charge cloud profile

Current density profile

Bias =100 V x-ray generated in point 1 and at a depth of 100um from the back contact


Charge sharing comparisonCharge sharing comparison

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Simulation results at V=-100V

Ch

arge co

llected (%

)

Pix

els

pixels

Simulation results combining DESSIS and Geant4 Monte Carlo simulation.

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3

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Pix

els

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arge co

llected (%

)

Pixels

Experimetal results at V=-100V

Experimental result using an Americium source. The detector was biased at –100V.

Average relative charge sharing for the 3x3 matrix simulated and measured experimentally.


ConclusionsConclusions

The data collected with the pixilated CdTe coupled to Medipix-II chip and the simulated results by DESSIS are in good agreement , thus one can use such a model to optimize the design of the pixilated CdTe detector for photon counting readout ASICS.

A 1mm thick CdTe with small pixel pitch will have poor performance when coupled to Photon Counting (PC) ASIC. Either it does not trigger, if the threshold is relatively too high, or many pixels count the same photon if the threshold is very low.


CdTe spectroscopyCdTe spectroscopy

Documents

Performance limits of a 55 m pixel CdTe detector G.Pellegrini, M. Lozano, R. Martinez, M. Ullan Centro Nacional de Microelectronica, Barcelona, 08193,