First Test Measurements of a 64k pixel readout chip ... · First Test Measurements of a 64k pixel...

First Test Measurements of a 64k pixel First Test Measurements of a 64k pixel readout chip working in single photon readout chip working in single photon

counting modecounting mode

X. Llopart

CERN, 1211 Geneva 23, Switzerland

On behalf of the Medipix2 Collaboration*

* See: http://medipix.web.cern.ch/MEDIPIX/

September 2002 Xavier Llopart 2

OutlineOutline

u Introductionu Motivation for the chip designu The Medipix2 pixel cellu The Medipix2 chip architectureu Electrical measurementsu Conclusionsu Future work


Medipix1 image of a sardineMedipix1 image of a sardine

X-ray tubeMo target 30 µm Mo filter25 kV5 mAs50 cm from sourceRaw data


MotivationsMotivations

u Medipix I proved potential for photon counting pixel detectors.

u Deep sub-micron CMOS (0.25µm) is available and well characterized.

u 170 µm to 55 µm pixel side and 64x64 to 256x256 pixels per chip. Pixel dimensions competitive with film-screen systems.

u More functionality can be added to a smaller pixel.


Characteristics of Medipix2 ChipCharacteristics of Medipix2 Chip

u Square pixel size of 55 µm u Sensitive to positive or negative input charge (different

detector materials can be used)u Pixel by pixel detector leakage current compensation u Window in energy as precise as possibleu 13-bit counter per pixel u Count rates of 1 MHz/pixel (0.33 GHz/mm2)u 256 x 256 pixels u 3-side buttableu serial or parallel I/O


Medipix2 Pixel CellMedipix2 Pixel Cell

Preamp

DiscL

Double Disc logic

Vth Low

Vth High

13-bits

Shift Register

Input

Ctest

Testbit

Test Input

Maskbit

3 bits Threshold Adjust

3 bits Threshold Adjust

Shutter

Mux

Mux

ClockOut

Previous Pixel

Next Pixel

Conf

8 bits configuration

Polarity

Analog Digital

DiscH


Medipix2 Pixel Cell LayoutMedipix2 Pixel Cell Layout

55 µm

55 µm

55 µm

55 µm


Medipix2 Chip Architecture (I)Medipix2 Chip Architecture (I)14111 µm

IO

Logic

LVDS

Input

LVDS

Output32-bit CMOS Output

256-bit Fast Shift Register

3328

-bit

Pix

el C

olum

n-0

13 8-bit DACs

14111 µm

3328

-bit

Pix

el C

olum

n-1

3328

-bit

Pix

el C

olum

n-1

1612

0 µ

m

IO

Logic

LVDS

Input

LVDS

Output32-bit CMOS Output

256-bit Fast Shift Register

3328

-bit

Pix

el C

olum

n-0

13 8-bit DACs

14111 µm

3328

-bit

Pix

el C

olum

n-1

3328

-bit

Pix

el C

olum

n-1

1612

0 µ

m


Medipix2 Chip Architecture (II)Medipix2 Chip Architecture (II)14111 µm

14111 µm

1612

0 µ

m


Measurement SetupMeasurement Setup


Preamplifier and discriminator measurementPreamplifier and discriminator measurement

u Measurements have been done using the Test output Pads and applying a voltage test pulse to the on-pixel injection capacitance.

Preamp Out (e-)Qin=9.28 Ke-

Y=20mVX=500ns

Preamp Out (h+)Qin=9.28 Ke-

Y=20mVX=500ns

Disc OutTHL=7Ke-

Y=2VX=500ns

Disc OutTHL=7Ke-

Y=2VX=500ns


Measurements and CalibrationMeasurements and Calibrationu All the reported measurements are done using the electronic

calibration (Injection capacitor + external voltage pulse). u The 8fF injection capacitor nominal value has a tolerance of 10%.u The dedicated Muros2 readout system had been used

0

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5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 10000

e-

Cou

nts THL

Elec Noise L Elec Noise H

THH


Threshold LinearityThreshold Linearity

0

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0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Threshold IN (Ke-)

Thre

shol

d M

easu

red

(Ke-

)

THLLinear (THL)

Using internal THL 8-bit DAC

Using external 12-bit DAC

Maximum injection charge is 57.5 Ke-


Threshold Equalization (I)Threshold Equalization (I)

0

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14000

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18000

20000

0 2000 4000 6000 8000 10000 12000 14000e-

Co

un

ts

THL 3bit ONTHL 3bit OFFTHH 3bit OFFTHH 3bit ONTHL AdjustedTHH Adjusted

Unadjusted Thresholds ~ 500 e- rms

Adjusted Thresholds~ 110 e- rms


Threshold Equalization (II)Threshold Equalization (II)

1 row of Unadjusted pixels 1 row of Adjusted pixels

0

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0 1 2 3 4 5 6 7 8 9 10 11 12

Ke-

Co

un

ts

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Ke-

Co

un

ts


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900000

1000000

0 2000 4000 6000 8000 10000 12000 14000

e-

Sum

of c

ount

s 30

x30

UnadjustedAdjusted

Threshold Equalization (III)Threshold Equalization (III)•THL=120•THL=100•Matrix unmasked•30x30 pixels active


0

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200000

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1000000

0 2000 4000 6000 8000 10000 12000 14000

e-

Sum

of c

ount

s 30

x30

UnadjustedAdjusted

Threshold Equalization (IV)Threshold Equalization (IV)•THL=120•THL=110•Matrix unmasked•30x30 pixels active


Threshold Equalization (V)Threshold Equalization (V)•THL=140 (2.8 Ke-)•Injection of 1000 pulses of 3.6 Ke-

•Matrix unmasked•30x30 pixels active


Summary of the Electrical MeasurementsSummary of the Electrical Measurements

σnTHL~ 110 e- σnTHH~ 110 e-Adjusted Threshold dispersion

~8 µW/channel for a 2.2 V supplyAnalog power dissipation

σnTHL~ 500 e- σnTHH~ 500 e-Threshold dispersion

σnL~ 105 e- σnH~ 105 e-Electronic Noise

<1µs for Qin <50 ke-Return to baseline

<200 nsPeaking time

<3% to 80 ke-<3% to 100 ke-Non linearity

13.25 mV/ke-12.5 mV/ke-Gain

Holes Collection

Electron Collection


Periphery MeasurementsPeriphery Measurements

u The 13 DACs perform as simulationsu Fast shift register works at > 100 Mhz*u Peripheral logic works to > 100 Mhz*u Serial/parallel I/O worku LVDS drivers and receivers work to > 100 MHz*

* IC-Tester maximum clock frequency


Radiation Tolerance MeasurementsRadiation Tolerance Measurements

u 10 keV X-ray source

u Chip under bias conditions

u Applied dose rates:u 3.9 krad/min up to 150 krad

u 8.04 krad/min from 150 krad to 500 krad

u Analog power supply current increase from 200mA to 260 mA

u Digital power supply current increase sharply @ 200 krad reaching 1100 mA @ 500 krad

u After 1 week of annealing at 100°C the power supplies current recovered to pre-irradiation values

u Chip showed normal behavior untill 200 krad and still functioning after annealing at 500 krad


ConclusionsConclusions

u A prototype chip consisting of 256x256 pixels has been produced with a square pixel size of 55 µm. Each pixel has around 500 transistors.

u Using the dedicated Medipix2 readout system (Muros2 and Medisoft4) complete electronic measurements and threshold calibration have been done.

u Adjusted threshold variation ~110 e- rms for both levels of discrimination.

u Electronic Noise ~105 e- rms.

u Difficulties to lower the threshold under 2.5 Ke- with the present setup.

u The chip is radiation tolerant until at least 200 Krad.


OnOn--going workgoing worku A new chipboard card is ready to be tested with

improved decoupling and power distribution.

u Probe tested wafers have been sent for bump bonding to silicon detectors.

u This should allow an absolute calibration with radioactive sources.

u Other materials will be tried later ( CdTe, GaAs, etc…)


Future ProspectsFuture Prospects

u With pixel shrinking charge sharing starts to dominate the detector behavior.

u Hexagonal pixels (on the detector side) become attractive.

u New front-end electronics architectures are needed.

u Some ideas are presented in a recently accepted paper for publication in the NSS/MIC IEEE journal.

u A whole spectrum of new possibilities opens with e.g. time-resolved measurements, very high dynamic range applications, colour X-ray imaging…

First Test Measurements of a 64k pixel readout chip ... · First Test Measurements of a 64k pixel...

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