Monolithic pixel sensors

1. Monolithic pixel sensors Ivan Perić

2. Introduction – classification of CMOS sensors • Pixel type • Simple pixels (no particle detection in pixel, analog output, rolling shutter readout) • Intelligent pixels (particle detection in pixel, digital output, zero-suppressed readout) • Technology • Standard (CMOS, Opto- or HV-CMOS) or special technology (Hi-resistive CMOS, backside depleted, SOI) • Bias voltage • Low (charge collection: diffusion based, sensor: epi layer) • High (charge collection: drift based, sensor: depleted layer)

3. Introduction – classification of CMOS sensors • Simple pixels • CMOS pixel sensors • Standard MAPS • Intelligent pixels • INMPAS • TWELL MAPS • Depleted MAPS with intelligent pixels • HVCMOS (Hi-Res CMOS included) • Espros MAPS • T3 MAPS • SOI

4. Commercial CMOS monolithic pixel sensors

5. CMOS monolithic sensors • The original application of CMOS sensors – consumer electronics • Imaging sensors for digital cameras and mobile phones • Improvements are necessary for HEP Phone with 41 M pixel sensor, 1.4um pixel size

6. Commercial imaging sensors • Imaging sensors for digital cameras and mobile phones 29 mm Canon 120 M pixels CMOS, 2 µm pixel size

7. MOS technology • MOS Technology – Integrated circuit technology based on Metal Oxide Semiconductor field effect transistors „Metal“ Electrode Insulator n type region “diffusion” Silicon p type Samsung 32nm process

8. PN junction • The simplest building element – PN junction • N-diffusion – potential minimum for electrons • P-substrate –potential barrier for electrons Silicon n type Silicon p type Free electrons

9. PN junction • Reversely biased – large depleted layer • Detector mode + Silicon n type Silicon p type Depleted

10. PN junction as sensor of radiation • PN junction as sensor • 1. step - ionization Photons orparticles Ionisation Free e- Atoms

11. PN junction as sensor of radiation • PN junction as sensor • 2. step – charge collection • Two possibilities for charge collection – drift (through E-force) and by diffusion (density gradient) Collection of electrons Atoms

12. PN junction as sensor of radiation • PN junction as sensor • 3. step – charge to voltage conversion • Collection of the charge signal leads to the potential change Potential change Atoms

13. CMOS pixel • Pixel sensor in MOS technology Sensor-junction MOS FET N-type region Diffusion (shallow) Or well (deep) Gate MOS FET Sensor-junction

14. CMOS pixel • N in P diode acts as sensor element – signal collection electrode Sensor-junction MOS FET N-type region Diffusion (shallow) Or well (deep) Gate MOS FET Sensor-junction

15. CMOS pixel • Charge generated by ionization is collected by the N-diffusion • This leads to the potential change of the N-diffusion • The potential change is transferred to transistor gate – it modulates the transistor current Sensor-junction MOS FET N-type region Diffusion (shallow) Or well (deep) Gate MOS FET Sensor-junction

16. Rolling shutter readout • Readout principle: Many pixels (usually one row) share one readout line • Additional MOSFET used as switch • The readout lines are connected to the electronics at the chip periphery that does signal processing Switch Switch A Periphery of the chip Pixel i Pixel i+1 A A A A A

17. CMOS monolithic pixel sensors for particle tracking

18. CMOS sensors for particle tracking • Can CMOS structure be used for detection of high energy particles in particle tracking? • Yes, but fill factor is an issue – ratio of the sensitive versus insensitive area Detected Not detected Absorbed by electronics Charge collection by drift

19. Fill-factor • Partial signal collection in the regions without E-field

20. Fill-factor • Partial signal collection in the regions without E-field Recombination

21. Overview • Partial signal collection in the regions without E-field Recombination

22. Overview • Partial signal collection in the regions without E-field Charge collection by diffusion

23. Fill-factor • Partial signal collection in the regions without E-field Charge collection by diffusion

24. Fill-factor • In the case visible light imaging, the insensitive regions do not impose a serious problem • Light can be focused by lenses • Exposure time can be increased • In the case of particle tracking, any insensitive region should be avoided

25. CMOS pixel sensor with 100% fill factor • MOS sensor with 100% fill-factor • Based on epi-layer • Monolithic active pixel sensor - “MAPS” MOS FET NMOS N-diffusion or N-well Heavily p-doped P-well Lightly p-doped epi-layer

26. CMOS pixel sensor with 100% fill factor • Ionization in the epi-layer • Charge collection by diffusion Particle N-diffusion or N-well

27. MAPS

28. CMOS pixel sensor with 100% fill factor - MAPS NMOS transistor in p-well N-well (collecting region) Pixel i P-type epi-layer P-type substrate Energy (e-) Charge collection (diffusion) MAPS

29. MAPS • Many institutes are developing MAPS, for instance: IPHC Strasbourg (PICSEL group) • Family of MIMOSA chips • Applications:, STAR-detector (RHIC Brookhaven), Eudetbeam-telescope and ALICE inner tracker upgrade http://www.iphc.cnrs.fr/Monolithic-Active-Pixel-Sensors.html

30. MAPS • MIMOSAs are based on rolling shutter RO but use more complex pixel electronics • Continuous reset and double correlated sampling MIMOSA 26 forEudettelescope Ultimate chipfor STAR http://www.iphc.cnrs.fr/Monolithic-Active-Pixel-Sensors.html

31. Charge collection & technology studies – simple demonstrators Real size prototype - yield studies Reticule 2x 2 cm 2006 Pixel Array Mimosa22 Discriminators Zero Suppression Suze 2007 Sara 2006 2008 2007 Bias Readout Final circuits – sub-blocs integration Data compression - digitization MAPS 1999 Production

32. Advanced CMOS pixel sensors with intelligent pixels

33. Frame readout • - Simple pixels • Signal and leakage current is collected • No time information is attached to hits • The whole frames are readout •  Small pixels •  Low power consumption •  Slow readout

34. 3 6 9 9 9 9 Sparse readout 3 6 • - Intelligent pixels • FPN is tuned inside pixels • Leakage current is compensated • Hit detection on pixel level • Time information is attached to hits •  Larger pixels •  Larger power consumption •  Fast (trigger based) readout

35. Intelligent pixel CR-RC Comparator Latch Bus driver CSA RAM Readout bus 4-bit tune DAC

36. Sparse readout Sparse readout Sensor Comp. out Rolling shutter readout Sensor Reset RO enable Comp. out

37. CMOS electronics • Two transistor types n-channel NMOS and p-channel PMOS are needed for the realization of complex circuits „Metal“ Electrode „Metal“ Electrode Insulator Insulator Silicon n type Silicon p type NMOS PMOS Silicon p type Silicon n type Holes PMOS Holes NMOS Free e-

38. CMOS electronics • Example: A good voltage amplifier can only be realized with CMOS „Metal“ Electrode „Metal“ Electrode Insulator Insulator Silicon n type Silicon p type NMOS PMOS Silicon p type Silicon n type PMOS Holes NMOS Free e-

39. MAPS structure with CMOS pixel electronics • If PMOS transistors are introduced, signal loss can happen N-well (collecting region) Pixel i NMOS transistor in p-well PMOS transistor in n-well P-type epi-layer P-type substrate Energy (e-) Signal loss Signal collection MAPS with a PMOS transistor in pixel

40. Advanced structures: INMAPS

41. INMPAS • Deep P-layer is introduced to shield the PMOS transistors from epi layer • No charge loss occurs • This is not a CMOS standard process • Only one producer so far: Tower Jazz Pixel PMOS in a shallow p-well NMOS shielded by a deep p-well N-well (collecting region) P-doped epi layer INMAPS

42. Overview • INMAPS Tower Jazz process is gaining popularity in particle physics community • /ALICE inner tracker) • It was originally developed by the foundry and the Detector Systems Centre, Rutherford Appleton Laboratory 2 Megapixels, large area sensor Designed for high-dynamic range X-ray imaging 40 µm pixel pitch 1350 x 1350 active pixels in focal plane Analogue readout Region-of-Reset setting 140 dB dynamic range 20 frames per second FORTIS chip http://dsc.stfc.ac.uk/Capabilities/CMOS+Sensors+Design/Follow+us/19816.aspx

43. Overview • Detector Systems Centre, Rutherford Appleton Laboratory – some examples Wafer scale 120 x 145 mm chip for medical imaging http://dsc.stfc.ac.uk/Capabilities/CMOS+Sensors+Design/Follow+us/19816.aspx

44. TWELL - MAPS

45. TWELL MAPS • Collection electrode is a deep n-well • To avoid crosstalk, secondary n-well is used for digital electronics • Rely on diffusion, implemented in low voltage CMOS processes • Collaboration: INFN Pisa, Pavia, Trieste, Padova, Torino, Bologna P-well Pixel Deep n-well 2. n-well NMOS PMOS Signal collection Signal loss Epi-layer Diffusion Triple-well MAPS Energy (e-)

46. TWELL MAPS • APSEL Chips for B-factories The APSEL4D MAPS chip bonded to the chip carrier. Schematic drawing of the full Layer0 made of 8 pixel modules mounted around the beam pipe with a pinwheel arrangement. “Thin pixel development for the SuperB silicon vertex tracker”, NIMA vol. 650, 2011

47. Fast CMOS detectors based on drift charge collection:detectors in HVCMOS-processes and the CMOS processes with a high resistive wafer

48. Drift based detector: HVMAPS • HVMAPS rely on the charge collection by drift • Fast charge collection – high radiation tolerance • The key is the use of a high voltage n-well in a relatively highly doped substrate • Pixel electronics is embedded in the n-well • Two concepts: • High Ohmic Monolithic Pixels - LePIX – relies on a special CMOS process with high resistive substrate (CERN, Geneve) • HVCMOS (or smart diode arrays - SDAs) – use a commercial HVCMOS process

49. HVCMOS detectors (smart diode arrays)

50. SDA • Smart diode array Pixel “Smart” Diode n-Well Drift Energy(e-) Depleted P-Substrate “Smart diode” Detector