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 Krüger Bonn University

Overview ! Overview of the MPEC chip family – Assemblies with Si- and GaAs sensors ! The MPEC 2.1 chip – Energy windowing – Performance – Sensors ! Conclusion and outlook Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 2

Overview of the MPEC Chip Family MPEC 1.0 MPEC 1.1 MPEC 2.0 MPEC 1.0 MPEC 1.1 MPEC 2.0 • 12 x 63 pixel • Minor bugfix to MPEC 1.0 • 32 x 32 pixel • 12 x 63 pixel • Minor bugfix to MPEC 1.0 • 32 x 32 pixel • 50 µ x 433 µ • Bump bonded to Si and • 200 µ x 200 µ • 50 µ x 433 µ • Bump bonded to Si and • 200 µ x 200 µ • 15 bit shift counter GaAs sensors • 18 bit shift counter • 15 bit shift counter GaAs sensors • 18 bit shift counter MPEC 1D MPEC 2.1 MPEC 1D MPEC 2.1 • Energy windowing • Energy windowing • Energy windowing • Energy windowing • 32 x 32 pixel • 2x 15 bit shift counter • 32 x 32 pixel • 2x 15 bit shift counter • 200 µ x 200 µ • 200 µ x 200 µ • 2x 18 bit shift counter • 2x 18 bit shift counter ! Multi Picture Element Counters First chips (MPEC 1.0, 1.1) derived from ATLAS Pixelchip Prototype ! (Bier & Pastis, Phys. Inst. Uni Bonn, CPPM Marseille ) Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 3

Sensor Assembly (1) ! 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 Solder bumping process (IZM, Berlin) – Si sensors – GaAs sensors Solder bumps before and after reflow (IZM, Berlin) Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 4

Imaging Resolution Radiogram of a small slit to determine the Line spread function LSF 7000 Model: LineSpreadFunction P1 1752.25 P2 4750 6000 P3 25 P4 550 5000 P5 43.22 P6 300 Counts 4000 3000 2000 1000 Single chip assembly with MPEC 1.1 0 1200 1400 1600 1800 2000 2200 2400 and bump bonded Si sensor 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 Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 5

Synchrotron Beam Measurements ! Inhomogeneity < 2 % 200 events 501-1000 sigma: 107 counts with flatfield correction 150 # Pixel 100 50 0 3000 4000 5000 6000 7000 8000 Counts Countrate distribution ! Efficiency 4000 18 keV Beam, 2500e Threshold 3500 Test setup at European Synchroton 3000 2500 Radiation Facility (ESFR) Counts 2000 (in collaboration with the Paul Scherrer Institute, PSI) 1500 1000 500 0 0 50 100 150 200 250 Beam position [µm] Scan with collimated beam Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 6

Performance of Si and GaAs Sensor ! Si Sensor ! GaAs Sensor 150 150 Data Data 125 125 Gaußfit Gaußfit Si_5_Am_120V GaAs_4_Am_250V 100 100 4000e Threshold 4000e Threshold # Events # Events 300 Events 300 Events 75 75 Gauß: Gauß: µ = 12116 ± 47 µ = 1067 ± 3 σ = 1069 50 50 σ = 53 25 25 × 10 0 0 0 5000 10000 15000 20000 25000 0 500 1000 1500 2000 2500 Counts Counts Count rate improves by a factor 10 for GaAs (60 keV γ γ γ γ ) ! GaAs sensors show inhomogeneous efficiency ! Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 7

MPEC 2.1 Architecture ! 0.8 µ CMOS active area 6.4 mm x 6.4 mm 32 x 32 pixel, ! size 200 µ x 200 µ Active Area Preamp, two discriminators ! and two counters in each pixel Serial readout for four ! columns each DAC / Control / Buffer Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 8

Energy Windowing 3,0 tube spectrum Lower Upper 2,0 Lower Threshold = 20 keV after 1 cm tissue Threshold Threshold 200 µ GaAs 2,5 after 1 cm bone 1,8 2,0 Norm. Contrast 1,6 Flux [a. u.] 1,5 1,4 1,0 1,2 0,5 1,0 0,0 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 Energy [keV] Upper Threshold [keV] Transmission spectrum with energy window Contrast between two different tissues ! Absorption for different tissues is energy dependent Contrast enhancement with energy window ! Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 9

MPEC 2.1 Pixel Schematic PREV MODE1 EXTCOUNT COUNTER VTH1 Window 18 bit logic counter VCCD1 ENABLE1 Bump pad MODE2 VTH2 18 bit counter VCCD2 SEL ENABLE2 NEXT ENABLE INJECT COUNTER INJECT SEL WRITE VCORR Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 10

Pixel Layout Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 11

Preamplifier ! Charge sensitive preamplifier – Design adopted from ATLAS prototype chip Bier & Pastis I f – Current feedback Bump pad Analog out – No additional shaper – Return to baseline ≈ ≈ ≈ 500 ns ≈ Folded cascode Analog out for different feedback currents (a), and different signal charges (b), (Rise time limited by output-buffer) Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 12

Counter 18 bit LFSR counter ! FF with 6 transistors ! ctrl Max. clock frequency > 10Mhz ! D Q 1 2 3 17 18 C Dynamic logic, refresh needed ! (low frequency, < 1Hz) 18 bit linear feedback shift register counter 80 60 D Q errors 40 C 20 0 Single phase Flip Flop 0 5 10 15 20 25 30 Delay / s Counter tests with different refresh delays Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 13

Window Logic Window discriminator ! 150 140 lower counter upper counter 130 sum 120 Count signals above upper ! upper threshold 110 lower threshold 100 threshold only in upper counter 90 Counts [%] 80 70 60 50 ! Increase dynamic range 40 30 20 10 Decrease crosstalk between ! 0 3200 3300 3400 3500 3600 3700 3800 3900 4000 4100 4200 counters and preamplifier Charge [electrons] Threshold scan shows the sum of both counters is 100 % (no loss) Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 14

Noise Performance 250 300 upper discriminator upper discriminator lower discriminator 250 lower discriminator 200 Noise [electrons] 200 150 # Pixel 150 100 100 50 50 0 0 0 50 100 150 200 250 0 512 1024 1536 2048 Electrons Pixel number ! Average equivalent noise charge is 42 electrons (measured with upper discriminator) Upper discriminator shows slightly higher noise (74 electrons) ! Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 15

Threshold Dispersion 5000 225 200 Threshold [electrons] mean: 3832 e 175 rms: 233 e 4000 150 125 # Pixel mean: 2511 e 3000 100 rms: 327 e 75 50 2000 25 0 upper discriminator 0 1000 2000 3000 4000 5000 1000 Threshold [electrons] upper discriminator, adjusted lower discriminator Dispersion without threshold adjust lower discriminator, adjusted 0 0 512 1024 1536 2048 200 Pixel number 175 150 ! Window discriminator demands low 125 mean: 4003 e threshold dispersion # Pixel 100 rms: 11 e mean: 2008 e 75 rms: 13e 50 25 Thresholds dispersion < 13 electrons ! 0 0 1000 2000 3000 4000 5000 with adjust Threshold [electrons] Dispersion with threshold adjust Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 16

Threshold Adjust ! Global threshold and individual correction voltage for each discriminator V CORR , IN V CORR , stored (from external source) (to discriminator) Correction voltage stored on a ! WRITE capacitor Drift compensation with buffer and double switch Ultra low leakage of drift ! 2790 compensation (0.03 fA) V CORR switched off 2780 I L =02 V CORROUT [mV] I L =04 2770 Threshold drift only 0.2 ! I L =10 I L =20 2760 electrons per second I L =30 I L =50 2750 Drift: -13,7 µV/s @ I L =30 I L =75 ! Refresh during normal exposure 2740 0 120 240 360 480 600 720 840 960 1080 time [s] times not necessary Drift of the stored compensation voltages Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 17

Sensor Assembly (2) Al Pads + add. ! Gold stud bump bonding Al Pads Metallization Chip Substrate Sensor – Mechanical process Au Ball Studs – Assembly of single chips Chip Substrate Force, Temp. ! Sensors for MPEC 2.1 Assembly Sensor Sensor Si (successful assembled, chip failure) – Chip Substrate Chip Substrate – GaAs (assembly failure, sensor met.) Mechanical gold stud bumping (IZM, Berlin) – CdTe (sensor available) MPEC 2.1 with gold studs Metallization of the GaAs CdTe sensor bonded with sensor gold studs to a dummy chip Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 18

Conclusion ! 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/ Hans Krüger, Bonn University Pixel 2000, Genua 8.6.00 19