Medical X-Ray Imaging with Energy Windowing Hans Krger Bonn - - PowerPoint PPT Presentation
Pixel 2000 International Workshop on Semiconductor Pixel Detectors for Particles and X-Rays Genova, June 5-8, 2000 Medical X-Ray Imaging with Energy Windowing Hans Krger Bonn University Overview ! Overview of the MPEC chip family
Pixel 2000 International Workshop on Semiconductor Pixel Detectors for Particles and X-Rays Genova, June 5-8, 2000
Hans Krüger Bonn University
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 2
! Overview of the MPEC chip family
– Assemblies with Si- and GaAs sensors
! The MPEC 2.1 chip
– Energy windowing – Performance – Sensors
! Conclusion and outlook
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 3
!
Multi Picture Element Counters
!
First chips (MPEC 1.0, 1.1) derived from ATLAS Pixelchip Prototype
(Bier & Pastis, Phys. Inst. Uni Bonn, CPPM Marseille )
GaAs sensors
GaAs sensors
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 4
! Solder bump bonding
– Used for ATLAS pixel modules – Fine pitch (50 µ) possible – Solderwettable under bump metallization requires processing of whole wafers
! MPEC 1.1 assembled with
– Si sensors – GaAs sensors
Solder bumps before and after reflow (IZM, Berlin) Solder bumping process (IZM, Berlin)
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 5
Radiogram of a small slit to determine the Line spread function LSF Single chip assembly with MPEC 1.1 and bump bonded Si sensor
1200 1400 1600 1800 2000 2200 2400 1000 2000 3000 4000 5000 6000 7000
Model: LineSpreadFunction P1 1752.25 P2 4750 P3 25 P4 550 P5 43.22 P6 300
Counts Position [µm]
LSF fitted to image data !
NEP ≈ ≈ ≈ ≈ 9.5 lp/mm (noise equivalent pass band, MTF = 0.3) in direction of 50 µ pixel edge
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 6
Test setup at European Synchroton Radiation Facility (ESFR)
(in collaboration with the Paul Scherrer Institute, PSI) Scan with collimated beam
50 100 150 200 250 500 1000 1500 2000 2500 3000 3500 4000
18 keV Beam, 2500e Threshold Counts Beam position [µm]
!
Inhomogeneity < 2 %
!
Efficiency
3000 4000 5000 6000 7000 8000 50 100 150 200
events 501-1000 sigma: 107 counts with flatfield correction
# Pixel Counts
Countrate distribution
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 7
! Si Sensor ! GaAs Sensor
5000 10000 15000 20000 25000 25 50 75 100 125 150 # Events Counts
Data Gaußfit GaAs_4_Am_250V 4000e Threshold 300 Events Gauß: µ = 12116 ± 47 σ = 1069
500 1000 1500 2000 2500 25 50 75 100 125 150 # Events Counts
Data Gaußfit Si_5_Am_120V 4000e Threshold 300 Events Gauß: µ = 1067 ± 3 σ = 53
!
Count rate improves by a factor 10 for GaAs (60 keV γ γ γ γ)
!
GaAs sensors show inhomogeneous efficiency
× 10
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 8
!
0.8 µ CMOS active area 6.4 mm x 6.4 mm
!
32 x 32 pixel, size 200 µ x 200 µ
!
Preamp, two discriminators and two counters in each pixel
!
Serial readout for four columns each
DAC / Control / Buffer Active Area
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 9
!
Absorption for different tissues is energy dependent
!
Contrast enhancement with energy window
10 20 30 40 50 60 70 80 0,0 0,5 1,0 1,5 2,0 2,5 3,0 Upper Threshold Lower Threshold tube spectrum after 1 cm tissue after 1 cm bone Flux [a. u.] Energy [keV]
Transmission spectrum with energy window Contrast between two different tissues
10 20 30 40 50 60 70 80 1,0 1,2 1,4 1,6 1,8 2,0 Lower Threshold = 20 keV 200 µ GaAs
Upper Threshold [keV]
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 10
18 bit counter 18 bit counter INJECT EXTCOUNT MODE1 MODE2 ENABLE1 ENABLE2 PREV COUNTER NEXT COUNTER Bump pad VCCD2 VTH2 VTH1
Window logic
VCCD1
WRITE VCORR SEL
ENABLE INJECT SEL
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 11
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 12
Analog out for different feedback currents (a), and different signal charges (b), (Rise time limited by output-buffer)
! Charge sensitive preamplifier
– Design adopted from ATLAS prototype chip Bier & Pastis – Current feedback – No additional shaper – Return to baseline ≈ ≈ ≈ ≈ 500 ns
Bump pad Analog out If Folded cascode
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 13
5 10 15 20 25 30 20 40 60 80
errors Delay / s
Counter tests with different refresh delays !
18 bit LFSR counter
!
FF with 6 transistors
!
!
Dynamic logic, refresh needed (low frequency, < 1Hz)
1
D Q C
2 3 18 17
D Q C
18 bit linear feedback shift register counter Single phase Flip Flop
ctrl
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 14
3200 3300 3400 3500 3600 3700 3800 3900 4000 4100 4200 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
upper threshold lower threshold lower counter upper counter sum Counts [%] Charge [electrons]
!
Window discriminator
!
Count signals above upper threshold only in upper counter
!
Increase dynamic range
!
Decrease crosstalk between counters and preamplifier
Threshold scan shows the sum of both counters is 100 % (no loss)
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 15
512 1024 1536 2048 50 100 150 200 250
upper discriminator lower discriminator
Noise [electrons] Pixel number 50 100 150 200 250 50 100 150 200 250 300
upper discriminator lower discriminator
# Pixel Electrons
!
Average equivalent noise charge is 42 electrons (measured with upper discriminator)
!
Upper discriminator shows slightly higher noise (74 electrons)
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 16
512 1024 1536 2048 1000 2000 3000 4000 5000
upper discriminator upper discriminator, adjusted lower discriminator lower discriminator, adjusted
Threshold [electrons] Pixel number
1000 2000 3000 4000 5000 25 50 75 100 125 150 175 200 225mean: 2511 e rms: 327 e mean: 3832 e rms: 233 e # Pixel Threshold [electrons]
1000 2000 3000 4000 5000 25 50 75 100 125 150 175 200
mean: 4003 e rms: 11 e mean: 2008 e rms: 13e # Pixel Threshold [electrons]
Dispersion without threshold adjust Dispersion with threshold adjust !
Window discriminator demands low threshold dispersion
!
Thresholds dispersion < 13 electrons with adjust
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 17
VCORR, IN WRITE VCORR, stored (from external source) (to discriminator)
Drift compensation with buffer and double switch
120 240 360 480 600 720 840 960 1080 2740 2750 2760 2770 2780 2790 VCORR switched off
Drift: -13,7 µV/s @ IL=30 IL=02 IL=75 IL=50 IL=30 IL=20 IL=10 IL=04 VCORROUT [mV] time [s]
!
Global threshold and individual correction voltage for each discriminator
!
Correction voltage stored on a capacitor
!
Ultra low leakage of drift compensation (0.03 fA)
!
Threshold drift only 0.2 electrons per second
!
Refresh during normal exposure times not necessary
Drift of the stored compensation voltages
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 18
CdTe sensor bonded with gold studs to a dummy chip MPEC 2.1 with gold studs
Al Pads Al Pads + add. Metallization Chip Substrate Chip Substrate Chip Substrate Sensor Sensor Force, Temp. Chip Substrate Sensor Assembly Au Ball Studs
Mechanical gold stud bumping (IZM, Berlin)
! Gold stud bump bonding – Mechanical process – Assembly of single chips ! Sensors for MPEC 2.1 – Si (successful assembled, chip failure) – GaAs (assembly failure, sensor met.) – CdTe (sensor available)
Metallization of the GaAs sensor
Pixel 2000, Genua 8.6.00 Hans Krüger, Bonn University 19
!
First MPEC counting pixel chips derived from ATLAS electronics
!
Chip assembly with Si and GaAs sensors
!
Features individual threshold adjust and energy windowing
!
Outlook – Assembly MPEC 2.1 with Si, GaAs and CdTe sensors – Next MPEC generation in deep-submicron technology
Papers and additional information: http://xray.physik.uni-bonn.de/