M. Campbell, R. Ballabriga , E. Heijne, X. Llopart, L. Tlustos , W. Wong CERN

1. A Circuit Topology Suitable for the Readout of Ultra Thin Pixel Detectors at the SLHC and elsewhere M. Campbell, R. Ballabriga, E. Heijne, X. Llopart, L. Tlustos, W. Wong CERN Geneva, Switzerland TWEPP 2007, Prague

2. Outline • Why hybrid pixels? • Performance and limitations • The example of Medipix2 • Improving spectral resolution for X-ray imaging • The charge summing and allocation architecture (Medipix3) • Vertex tracking in thin detectors • Design study of new architecture for SLHC • Summary and conclusions

3. Why hybrid pixels? • Any CMOS commercial process can be used • The detector can be optimised for application • thin EPI or 3D Si for SLHC • Diamond • GaAs for mammography etc.. • Gas…. • Sensor is usually fully depleted – prompt charge collection • Optimal signal to noise at high rates – essential for clean pattern recognition….

4. 10-1 10-2 10-4 fn/fb 10-5 10-6 10-7 0 1 2 3 4 5 Qth/sn Noise hit rate for a discriminator 100 fn= noise hit rate fb = system bandwidth Qth= threshold sn = noise In a large bandwidth system (such as an HEP experiment) noise and threshold variation must be kept very far from the threshold to produce clean event information.

5. Signal, Threshold, Noise Signal Threshold a. u. Noise Charge A good separation of signal, threshold and noise is achieved with hybrid pixels. However, this argument does not take charge sharing into account…

6. Medipix2 Cell Schematic Charge sensitive preamplifier with individual leakage current compensation 2 discriminators with globally adjustable threshold 3-bit local fine tuning of the threshold per discriminator 1 test and1 mask bit External shutter activates the counter 13-bit pseudo-random counter 1 Overflow bit

7. Medipix2 Cell Layout

8. Medipix2 Chip Architecture 256 x 256 pixels 10ms readout time (serial) 300ms readout time (parallel)

9. High resolution X-ray imaging using a micro-focus X-ray source(1)

10. Edges are enhanced by phase contrast effect High resolution X-ray imaging using a micro-focus X-ray source(2) Needle holding the sample

11. Increasing threshold Calibration at ESRF – monochromatic pencil beam at pixel centre 8kev plus harmonics….

12. Increasing threshold Effect of Charge Sharing on Energy Resolution – monochromatic 1mm2 beam

13. Increasing threshold Effect of Charge Sharing on Energy Resolution – monochromatic 1mm2 beam Th2 Th1

14. Performance and limitations of Medipix2 as an X-ray sensor • Single photon counting provides excellent noise free images • Ideal in photon starved situations • However, charge sharing in the sensor is an issue: • Flat field correction is sensitive to incoming spectrum • Energy resolution is limited by charge sharing tail

15. Medipix3 – charge summing concept The winner takes all • The incoming quantum is assigned as a single hit • Charge processed is summed in every 4 pixel cluster on an event-by-event basis 55m

16. Medipix3 – pixel block diagram

17. DIGITAL CIRCUITRY • Control logic (124) • 2x15bit counters / shift registers (480) • Configuration latches (152) • Arbitration circuits (100) • Total digital 856 4 5 6 7 55m • ANALOG CIRCUITRY • Preamplifier (24) • Shaper (134) • Discriminators and Threshold Adjustment Circuits (72) • Total analog 230 2 1 3 Michael Campbell 55m

18. The Medipix3 prototype chip • 0.13mm technology • 8 metal layers • 8x8 pixel matrix 2 mm 1 mm

19. Pre-amp and shaper measurements 200ns 10mV Response to a 3.71 Ke- input charge Nominal Conditions ICSA=2mA IRESET=2.5nA ISHAPER=500nA

20. Medipix3 – charge summing measurements Input charge: 2.78Ke- (30 DAC pulses) 4000 pulses (0,2) (1,2) (2,2) 8 8 (0,1) (1,1) (2,1) 7 7 (0,0) (1,0) (2,0)

21. Medipix3 – charge summing measurements Input charge: 2.78Ke- (30 DAC pulses) 4000 pulses (0,2) (1,2) (2,2) 30 (0,1) (1,1) (2,1) (0,0) (1,0) (2,0)

22. Medipix3 – charge summing measurements Input charge: 2.78Ke- (30 DAC pulses) 4000 pulses (0,2) (1,2) (2,2) 30 (0,1) (1,1) (2,1) (0,0) (1,0) (2,0)

23. Medipix3 – electrical measurements summary

24. How about tracking at SLHC? • Thin sensors desirable – less mass better spatial resolution • Low thresholds • Clean readout still essential • Good separation threshold-noise • Time walk can be an issue: K. Einsweiller, ATLAS Pixel Detector, LBL Instrumentation Colloquim , 13 April 2005 See: http://instrumentationcolloquium.lbl.gov/The_ATLAS_pixel_detector.pdf

25. Charge deposition with MIPs – unsegmented Si Sensor 50mm thick 10 000 electrons 20MeV 0 deg angle of incidence

26. Charge detection – Conventional Readout All hits Sensor 50mm thick Pixel 20mm x 150mm

27. Charge detection – Conventional Readout Single hits only Sensor 50mm thick Pixel 20mm x 150mm

28. Charge detection – Charge summing Readout All hits Sensor 50mm thick Pixel 20mm x 150mm

29. Comparison Charge deposition – Charge summing Readout All hits Sensor 50mm thick Pixel 20mm x 150mm

30. Is such an approach feasible at SLHC? • Design example • Assumptions: • 25 ns shaping time • Power budget 1 W/cm2 • ½ of power budget for analog • Pixel dimensions 55mm x 55mm (equivalent in area to 20mm x 150mm) • Therefore 10mA available for analog • Noise 100 e-rms per channel (200 e-rms after charge summing)

31. A B C Previous Block diagram of pixel E Pixel SR1/2 D E F G H I From adjacent MUX pixels (A, B, D) From adjacent pixels (F, H, I) x6 x3 C x2 F TH1 15 bits Input Shift DISC Cluster x1 Pad Register common SR1 g x2 control m g logic m B_TH1<0:4> V x3 + FBK C TH2 TEST Arbitration circuitry DISC x1 TestBit PolarityBit Next Pixel Test GainMode SR1/2 Input B_TH2<0:4> SummingMode x2 ModeContRW x6 To adjacent To adjacent DisablePixelCom AdjustTHH pixels (A, B, D) pixels (F, H, I) SpectroscopicMode ANALOG DIGITAL

32. Noise calculation RFB The transistors are modeled with the EKV equations (valid weak-strong inversion) The gm stage is implemented as a differential pair. I2lknetwork and i2pfb are calculated considering the architecture in [1] CFB VOUT ts IPR CP ID CDET

33. ENC vs preamp Current and W of IP transistor Cin = 80fF

34. ENC vs preamp Current and W of IP transistor x Possible design value for a target ENC of 200 e- rms after summing IPR=2mA, ISH=10mA-IPR=8mA

35. The Shaper Circuit calculation Transfer Function Equation Shaper calculations VBIAS_SHAPER MPS1 CD CD VD vD VIN MIN+ CGS gm gm(VIN-VD) IBIAS_SH vIN vI VI CI MNL1 gmI CI gmI Integrator Transistor Size Calculation Differential version 1m/7.7m N: Number of neighbours (4 in rectangular pixels) T: Number of thresholds Calculated based on op point 0.8mA

36. The Shaper Circuit calculation Transfer Function Equation Shaper calculations (contd) VBIAS_SHAPER MPS1 CD CD vD VD VIN MIN+ CGS gm gm(VIN-VD) IBIAS_SH vIN vI VI CI MNL1 gmI CI gmI Differentiator Transistor Size Calculation CD = 600fF (300fF in differential version) W/L = 11m/0.12m

37. Preamplifier and Shaper Time Waveforms

38. Summary • Hybrid pixels enable the separate optimisation of sensor and readout and permits the use of the most advanced CMOS process available • Charge sharing caused by diffusion distorts the spectrum seen by segmented sensors • This applies to single photon counting systems such as Medipix2 as well as to the MIP spectrum at an HEP experiment • A new architecture (Medipix3) is proposed whereby a clean spectrum is reproduced prior to a YES/NO decision being taken - improved energy resolution • This same approach should enable the use of highly segmented and very thin (rad hard?) sensors to for vertex tracking • For HEP the YES/NO decision could be used to ‘freeze’ the analog hit info in the 4 pixels concerned • A preliminary design study suggests that the architecture is suitable for SLHC within a reasonable power budget

39. Acknowledgements Fellow members of the Medipix2 and Medipix3 Consortia See: www.cern.ch/medipix Stanislav Pospisil and co-workers at CTU, Prague