DEPFET Pixel: A Pixel Device withIntegrated Amplification

Johannes UlriciBonn University

FAUST Semiconductor Lab

Pixel 2000 Conference Genova, 4.6.-8.6. 2000

Outline

• The DEPFET-Principle

• Measurements on DEPFET Single Pixel Devices

• First Measurements with a DEPFET Pixel Array (64x64)

(Applications: Autoradiography, High Energy Physics,

X-Ray Astronomy)

• Summary

In cooperation with the MPI semiconductor Lab Munich, Bochum University, DortmundUniversity,

funded by the DFG and by the NRW department of Science

p+

n

pn+

S DG

n+

internal gate

Integration of JFET

DEPFET

Advantage:

• Small input capacitance

• no stray capacitance

=> Large Signal to Noise

Ratio

DEPFET - principle idea

p+

n

n+

pn+

S DG

Sensor Diode

Preamplifier(p-JFET)

Kemmer, Lutz (1987):

• integrate preamplifier into

Sensor Si- Substrate

DEPFET - charge collection

• sidewards depletion

• electrons collected in internal gate

• channel current of JFET modulated by signal charges

Ionising particle

p+

p+ n+

n

n+

totally depletedn--substrate

internal gate

rear contact

source top gate drain bulk potential via axistop-gate / rear contact

V

potential minimumfor electrons

p-channelp+

-

-

- -+

+

++

-- --~1 µm

~300 µm

DEPFET - Clear Mechanism

• Internal gate filled bysignal charges andthermally generatedelectrons

• „Reset“ needed

• One possibility: pulsed clear!

• Other clear mechanisms:cont. clear, gate clear, ... p+

p+ n+

rear contact

drain bulksource

p

sym

met

ry a

xis

n+

n

internal gate

top gate clear

n -

n+p+

0V

+15V

0V

pulsed clear: dead time less than 0,1% of measuring time

- -- ---

DEPFET - Measurements on Single Pixel Devices

Spectrum of 55Fe-Source:

• At room temperature (300 K)

• Shapingtime: 10µs

• Noise Peak σ = (6,1 +/- 0.1) e-

• Kα-Gaussfit σ = 16,1 e- or

FWHM = 138 eV

• low energy tail due to split

events (pixel size 50x50 µm)

Noise: ENC = 6.1e @ 300K2 3 4 5 6 7

0

50

100

150

200

250

300

350

FWHM = 138 eV

@ 300 K

kβ

kα

photo-

escape

peak

55Fe k

α = 5,89 keV

55Fe kβ = 6,49 keV

coun

ts

Energy [keV]

Layout of DEPFET-Matrix

• Source, Gate and Clear connected row-wise

• Drain column-wise (zig-zag)

• bond pads on end of columns/rows

DEPFET - Matrix Data Acquisition

64x64pixel

IDRAIN

DEPFET-matrix

VGATE, OFF

off

off

on

off

VGATE, ON

64-b

it-sh

ift r

egis

ter

gate

drain VCLEAR, OFF

off

off

clear

off

VCLEAR, ON

64-bit-shift register

clear

output

• switch on one row through gate contacts, get pedestal current

• after doing this for all rows, switch row on again, get signal current

clear this row

ext.gate

source

drain

clear

signal charge in int. Gate ~

signal-current -pedestal-current

Developed ASICs:

CARLOS:• low-noise 64-channel amplifier• track & hold• 10 MHz Serializer (64 to 1)

SWITCHER:• 64 channels, 65MHz• AMS 25V HV-technology

DEPFET - Bioscope

64 x 64 DEPFET matrix

CARLOS

Gate-SWITCHER Clear-SWITCHER

further components:• analog PBC with 12-bit ADC• digital PBC with XILINX for data acquisition

1 image (64x64 Pixel) per 1ms

DEPFET Pixel Bioscope System

ADC(12bit)40MHz

FiFo64k x 18bit

Hybrid ADC -Card

PCI - Card(in PCI Slot)

Power SupplyCard

DDC - DigitalDEPFET Card

CARLOS(64

channelAMP &MUX)

SWITCHER(control chip)

SWITCHER(control chip)

64 x 64pulsed clearDEPJFET-

matrix

8

3

1MBstatic RAM

2

XILINX

XIL

INX

32

8

OPAmp

40M

Hz

XILINXXC4010

1512

1 image (64x64 Pixel) per 1ms

The DEPFET Pixel Bioscope

X-Ray images: 64x64 Matrix, 50x50µm2 PixelShadow image of toothed wheel of watch, 55Fe-γ-Source (6keV)

1,2 mm

Pedestal Image

digital information

linear interpolation

0 50 100 150 200 250 300-1000

0

1000

2000

3000

4000

5000

6000

7000

L

M N

O P

R T Total

Sig

nal [

Ele

ktro

nen]

X Achse [µm]

DEPFET - Charge collection efficiency

Laser Scan across matrix withbricked pixel layout (50x50µm2)

homogeneous charge collection efficiency! (no charge loss into clear contacts)

0 10 20 30 40 50 60

0

100

200

300

400

500

600 2 4 1Am

Ba

Tb

Cu

1 0 9Cd Kα

1 0 9Cd Kβ

MoRb

55Fe

AD

C-W

ert

Energie [keV]

DEPFET - linearity and noise

All 4096 Pixels:

-400 -200 0 200 4000

50

100

150

200

250

300

350

400

σGauss

= (119 ± 8) e

Häu

figke

it

Elektronen [e]

B Gauss Fit

very good linearity [6 - 60keV]low noise of single image: σnoise = 84 ±6 e

Tritium-Detection: 64x64 DEPFET-Matrix

Tritium-detection possible (8σ threshold) !!!

3H-radiograph

3H-Microscale (30 Bq per block)(18keV β-endpoint-energy)

24h measurement

Spatial Resolution with 55Fe-Source75µm thick tungsten sample, 55Fe-6keV-source

500 1000 1500 2000 2500 30000

50

100

150

2006.67 LP/mm(75 µ m )

5 LP/mm(100 µ m )

10 LP/mm(50 µm )

20 LP/mm(25 µ m )

co

un

tin

g r

ate

p r o j e c t i o n a x i s [ µ m ]

digital (50x50 µm2)

FAUST

100 um 50 um 25 um75 um

2 mm

3 mm

linear interpolation(1Pixel =12.5 x 12.5µm2)

28 LP/mm resolution!

Spatial Resolution

σgauss= 9,0 ± 0,6 µm

DEPFET - Performance Summary:

• very good SNR (> 80 @ 6keV)

• 200 nm thin, homogeneous entrance window

• non-destructive readout -> multiple readout possible

• very fast readout, partial readout of matrix possible

• small deadtime ( < 0,1 % of Data Acquisition time)

-> high efficiency

• small pixelsize (50 µm)

DEPFET - Applications:Biomedicine (Autoradiography):

• Real time detection of 3H at room temperature without vacuum

• Good energy resolution -> different radioactive markers

X-Ray Astronomy (sucessor of XMM: XEUS)

• low energy γ’s

• high rates, nearly no dead time, no ghost hits

Particle Physics (TESLA)

• thin detectors (30µm)

• small pixel size (30x30 µm2)

Biomedical Application: Autoradiography

Good spatial resolution, no time or energy information

radioactively marked sample

14C: β-decay, 50keV (mean)3H: β-decay, 6keV (mean)

AgBr-Emulsion (d<10µm)

Digital Autoradiography:• time resolved -> real time observation of dynamic processes -> no development of film necessary (up to months)

-> not sensitive to exposure time• energy resolution -> different radioactive markers• DEPFET -> detection of 3H (room temperature, no vacuum!)

X-ray astronomy: XEUS (2010-2015)XMM-successor: „X-ray Evolving Universe Spectroscopy“

pn-CCDs too slow for „Wide Field Imager“

DEPFET-Advantages:• high rates• no „ghost hits“• rad hard (no transfer of signal charges necessary)• fast partial readout of matrix areas• multiple readout possible• nearly no dead time

Summary

• DEPFET- single pixel:– ENC = 6,1 e @ room temperature

• DEPFET Pixel Bioscope with 64 x 64 DEPFETMatrix:– Readout time for 1 image: ~ 1ms

– spatial resolution with simple linear interpolation: ~ 9µm– noise of single image > 10 e

– Tritium detected!

– homogeneous charge collection efficiency!

• future: time resolved

measurements, various applications

DEPFET: InformationRecent papers:

W. Neeser, M. Böcker, P. Buchholz, P. Fischer, P. Holl, J. Kemmer, P. Klein, H. Koch, M. Löcker, G. Lutz, H. Matthäy, L. Strüder, M. Trimpl, J. Ulrici, N. Wermes; "DEPFET - a pixel device with integrated amplification", submitted to NIM A

W. Neeser, M. Böcker, P. Buchholz, P. Fischer, P. Holl, J. Kemmer, P. Klein, H. Koch, M. Löcker, G. Lutz, H. Matthäy, L. Strüder, M. Trimpl, J. Ulrici, N. Wermes; "The DEPFET Pixel Bioscope", submitted to IEEE Trans. on Nucl. Sci.

P. Klein, T. Aurisch, P. Fischer, W. Neeser, L. Strüder, M. Trimpl, J. Ulrici, J. Vocht, N. Wermes; "A DEPFET Pixel Bioscope for the Use in Autoradiography", submitted to NIM A

Homepage:

depfet.physik.uni-bonn.de