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  • DEPFET Pixel: A Pixel Device with Integrated Amplification

    Johannes Ulrici Bonn 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, Dortmund University,

    funded by the DFG and by the NRW department of Science

  • p+

    n

    p n+

    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+

    p n+

    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 depleted n--substrate

    internal gate

    rear contact

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

    V

    potential minimum for electrons

    p-channel p+

    -

    -

    - - +

    +

    + +

    -- -- ~1 µm

    ~300 µm

  • DEPFET - Clear Mechanism

    • Internal gate filled by signal charges and thermally generated electrons

    • „Reset“ needed

    • One possibility: pulsed clear!

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

    p+ n+

    rear contact

    drain bulksource

    p

    sy m

    m et

    ry a

    xi s

    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 @ 300K 2 3 4 5 6 7

    0

    50

    100

    150

    200

    250

    300

    350

    FWHM = 138 eV

    @ 300 K

    k β

    k α

    photo-

    escape

    peak

    55 Fe k

    α = 5,89 keV

    55 Fe kβ = 6,49 keV

    co un

    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

    64x64 pixel

    IDRAIN

    DEPFET-matrix

    VGATE, OFF

    off

    off

    on

    off

    VGATE, ON

    64 -b

    it- sh

    ift r

    eg is

    te r

    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

    FiFo 64k x 18bit

    Hybrid ADC -Card

    PCI - Card (in PCI Slot)

    Power Supply Card

    DDC - Digital DEPFET Card

    CARLOS (64

    channel AMP & MUX)

    SWITCHER (control chip)

    SWITCHER (control chip)

    64 x 64 pulsed clear DEPJFET-

    matrix

    8

    3

    1MB static RAM

    2

    XILINX

    X IL

    IN X

    32

    8

    OP Amp

    40 M

    H z

    XILINX XC4010

    151 2

    1 image (64x64 Pixel) per 1ms

  • The DEPFET Pixel Bioscope

  • X-Ray images: 64x64 Matrix, 50x50µm2 Pixel Shadow 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

    S ig

    na l [

    E le

    kt ro

    ne n]

    X Achse [µm]

    DEPFET - Charge collection efficiency

    Laser Scan across matrix with bricked 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 K α

    1 0 9Cd K β

    Mo Rb

    55Fe

    A D

    C -W

    er t

    Energie [keV]

    DEPFET - linearity and noise

    All 4096 Pixels:

    -400 -200 0 200 400 0

    50

    100

    150

    200

    250

    300

    350

    400

    σ Gauss

    = (119 ± 8) e

    H äu

    fig ke

    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-Source 75µm thick tungsten sample, 55Fe-6keV-source

    500 1000 1500 2000 2500 3000 0

    50

    100

    150

    200 6.67 LP/mm (75 µ m )

    5 LP/mm (100 µ m )

    10 LP/mm (50 µm )

    20 LP/mm (25 µ m )

    c o

    u n

    ti n

    g r

    a te

    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 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 DEPFET Matrix: – 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

  • DEPF

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