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R&D on Monolithic Active Pixel Sensors for the TESLA Vertex Detector

R&D on Monolithic Active Pixel Sensors for the TESLA Vertex Detector. - overview: past, present & future work -. Devis Contarato University of Hamburg. Outline. Introduction: TESLA requirements MAPS: a technology for the TESLA VXD Basic concepts on Monolithic Active Pixel Sensors

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R&D on Monolithic Active Pixel Sensors for the TESLA Vertex Detector

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  1. R&D on Monolithic Active Pixel Sensors for the TESLA Vertex Detector - overview: past, present & future work - Devis Contarato University of Hamburg

  2. Outline • Introduction: TESLA requirements • MAPS: a technology for the TESLA VXD • Basic concepts on Monolithic Active Pixel Sensors • Overview of the first MAPS prototypes • Status of the project • Involvement of the DESY/UNIHH group • Current developments and future prospects • Conclusions D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  3. High impact parameter resolution • need for spatial resolution<5 m, multiple scattering<0.1% X0 • need for thin layers, with excellent mechanical stability • High granularity due to high jets multiplicity • typical pixel pitch of around 20x20 m2 • High occupancy due to e+e- pairs background • need for fast read-out and on-line data sparsification • Relatively harsh radiation environment: • neutron5 109 n(1 MeV)/cm2/5 years • Dionisation= 100 kRad/5 years Requirements of TESLA VXD Need to combine high granularity, little multiple scattering, high read-out speed and radiation hardness D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  4. MAPS: a technology for the TESLA VXD? • Technological options for the TESLA VXD • CCDs: good granularity and low material budget, but the read-out speed needs to be improved and the radiation tolerance to the neutron background is not yet ascertained • Hybrid pixels: fast and (presumably) more radiation-hardened, but they are thick (high material budget) and their granularity is poor • MAPSseem to combine the positive features of both: • - high granularity and possibility ofthinning down (~50 m) • improved read-out speed thanks to the integration of many functionalities on the same sensor substrate • radiation tolerance, as the charge is not clocked through the sensor bulk • low power consumption, as the circuitry in each pixel is active only during the read-out • use of a standard, cheap and easily available CMOS technology D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  5. Principle of operation of MAPS • double-well CMOS process with epitaxial layer • the charge generated by the impinging particle is reflected by the potential barriers due to doping differences and collected by thermal diffusion by the n-well/p-epi diode • 100% fill factor, as the active volume is underneath the readout electronics • possibility of integrating the circuitry electronics on the same sensor substrate MIMOSA I read-out architecture block schematic diagram D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  6. First prototypes: MIMOSA I – II MIMOSA I: technology demonstration • standard 0.6mm CMOS of AMS (tox=12.7nm) • 14m thick EPI layer (1014cm-3) • 4 arrays 64x64 pixels • pixel pitch 20x20 m2 • diode (nwell/p-epi) size 3x3 m2 - 3.1fF • 1 and 4 diodes/pixel • serial analogue readout - max. clock freq.: 5MHz • power supply 5V die size 3.6x4.2mm2 MIMOSA II: noise and radiation tolerance studies (use of different pixel layouts) • standard 0.35mm CMOS of MIETEC (tox=7.4nm) • 4.2 mthick EPI layer (1015cm-3) • 6 arrays 64x64 pixels • pixel pitch 20x20 m2 • diode (nwell/p-epi) size 1.7x1.7 m2 - 1.65fF • 1 and 2 diodes/pixel • radiation toleranttransistor design • serial analogue readout - max. clock freq.: 25MHz • power supply 3.3V die size 4.9x3.5 mm2 D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  7. Work done on first prototypes • Device simulationand optimisation of the sensor design • Test and measurement of the tracking performances: visible and IR light, soft X-rays photons (55Fe source), high-energy particle beams. • Goal: estimation of • properties of charge collection • detection efficiency • noise performance and S/N ratio • spatial resolution • Radiation hardness tests: protons, neutrons and X-ray photons • Tests with a strong magnetic field (preliminary) D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  8. Performances of MIMOSA I-II Device simulation: charge collection properties as a function of the sensor design parameters - Collected charge: charge spread onto adjacent pixels, the cluster size increase for thicker epilayer and decrease for increased number of diodes/pixels; increased substrate contribution with decreasing the epilayer thickness - Charge collection time: 90% of charge <150 nsec; 4-diode charge collection ~3 times faster; charge collection faster for thinner epitaxial layer Tracking performances: test beam measurements @ CERN SPS, - 120 GeV/c ~99.5%, R~1.5-2.5 m, ENC~10 e, S/N~30 Measured values of collected charge and of charge collection times (IR laser tests) are in good agreement with simulations D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  9. Neutron radiation tolerance Chips (MIMOSA I and II) have been irradiated with neutron sources at JINR (Dubna) and CEA-Saclay reactors and tested with 55Fe X-ray source. Collected charge as a function of fluence Noise as a function of fluence Minor changes in the leakage current, but charge losses at higher fluences(up to ~60% for MIMOSA II at 1013 n/cm2). Open question of the charge losses: where do they come from? Charge losses are anyway observed only for fluences>1011 n/cm2, that is 2 orders of magnitude more than it is expected for TESLA D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  10. Ionising radiation tolerance Irradiation with 30 MeV protons (Karlsruhe) with fluences up to 5 1011 p/cm2(~50 krad, ~1012 n(1 MeV)/cm2): charge losses (up to ~40%) and increase of leakage current (~5 times) Laeakage current [ADC units] Collection efficiency [%] Irradiation dose [p/cm2] Temperature [°C] Irradiation with 10 keV X-ray photons (100-400 krad): charge losses are slight (~6%) after 100 krad, but become considerable (~40%) after 400 krad. Leakage current increases by an order of magnitude after 100 krad (interface state modification) The degradation is observed at doses significantly higher than expected at TESLA, but further studies are needed to understand the mechanisms D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  11. MIMOSA III: trial of a deep-submicronic process First prototypes : MIMOSA III – IV • standard 0.25 mCMOS of IBM (tox=5.84 nm) • 2.5 m thick EPI layer (~1015 cm-3) • 2 arrays 128x128 pixels - pixel pitch 8x8 m2 • diode (nwell/p-epi) size 1x1 m2 - 2.1 fF • 1 diode/pixel • serial analogue readout - max. clock freq.: 40 MHz • power supply 2.5 V 128x128 APS array optimum noise maximum 40 MHz readout test APS arrays die size 4.0x2.0 mm2 MIMOSA IV: study of substrate contribution and of new charge sensing elements • standard 0.35 m CMOS of AMS (tox=7.5 nm) • p-substrate process(~1014cm-3) – no EPI layer • 4 arrays 64x64 pixels - pixel pitch 20x20 m2 • diode (nwell/p-epi) size 2x2 m2 - 1.8 fF • 1 and 3 diodes/pixel • radiation tolerant transistor design • new structures of charge sensing elements: charge spill-gate, photoFET, self-biasing diodes • serial analogue readout - max. clock freq.: 40 MHz • power supply 3.3 V die size 3.7x3.8 mm2 D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  12. Performances of MIMOSA IV • Test beam results: • Detection efficiency ~99.7% • S/N~30 but charge spread is wider (due to wider diffusion) • Spatial resolution R~4 m (20 m pitch) • Important observations: • Self-reverse polarisation of charge collecting diode implemented • Dependence of charge collection on pixel layout: only the prototypes with the self-bias structure work, the matrices with standard pixel structure doesn’t! • Collected charge is a function of the temperature: decreasing T of 20 °C results in 50% more collected charge! Technology without epitaxial layer seems worth investigating and optimizing! D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  13. MIMOSA V: large scale detector • standard 0.6mm CMOS of AMS (tox=12.7nm) with 14m thick EPI layer (1014cm-3)– same process as MIMOSA I • 4 independent matrices of 510x512 pixels read-out in parallel; pixel pitch 17x17m2 • serial analogue readout (but external hardware processing) - max. clock freq.: 40 MHz • first real size prototype: 3.5 cm2, 1M pixels • coarse stitching (~100 mm); back-thinning down to 120 mm • Preliminary test-beam results • Detection efficiency ~99.3%, spatial resolution R~1.7 mm, gain non-uniformity gain2-3 % • Noise mean ENC~20 e, single pixel S/N~23 Performances close to MIMOSA I, but double noise (bigger chip, faster readout…) Problem of low fabrication yield (~30%)- problems of wide area chips… D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  14. MIMOSA VI: integrated data processing • standard 0.35mm CMOS of MIETEC (tox=7.4nm) • 4.2 mthick EPI layer (1015cm-3) • 1 array30x128 pixels – 29 transistors/pixel • pixel pitch 28x28m2 • diode (nwell/p-epi) size 4.0x3.7 m2 – 3.5fF • 24 columns read-out in parallel - max. clock freq.: 30 MHz • Amplification and Correlated Double Sampling on-pixel • Discriminators integrated on chip periphery • Power dissipation ~500 mW per column AC coupling capacitor Charge storage capacitors Chip layout Single pixel layout Single pixel layout First design attempt for on-pixel signal amplification and double-sampling pixel operation; chips are available and will be tested soon D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  15. Summary of first prototypes • The first 4 generations of small scale MAPS prototypes have shown good and promising performances in charged particle tracking: ~99%, sp~1.5-2.5 m, S/N~30. The preliminary results obtained with the first large scale prototype (MIMOSA V, reticle size~3.5 cm2, 120 m thin) indicate that these performances are reproducible with real size detectors • Tolerance to neutrons exceeds TESLA requirements by more than 2 orders of magnitude • The prototype (MIMOSA IV) built in a technology without EPI layer (but with low resistivity substrate) has also shown good perfomances • The first chips with integrated signal processing functions (CDS+Amplification+Discriminators) are already available and will be soon tested. • R&D collaboration started from Strasbourg and joined by several centers in Europe (F-D-UK-CH-NL) D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  16. Current developments & future trends • Current plans • MAPS with integrated functionalities (amplification, CDS, column-level discrimination) • MAPS with on-pixel data processing (charge-to-current conversion, analogue information storage allowing on-line data sparsification) • Study of new charge-sensing elements: photoFET (current-mode CSE) • Basic radiation studies (test structures) to understand radiation damage mechanism, with the aid of device simulation • Exploration of fabrication processes (optimum EPI thickness ~8 m) • Goals • Optimisation of the intrinsic detector performances • Optimise granularity, read-out speed (single-pixel read-out frequency ~50 MHz achievable), material and power budget • Search for an efficient solution for the read-out architecture (CP) and on-chip data processing • Fabrication of ladders, improvement of fabrication yield and thinning processes: the goal is to achieve <50 m thinning on real-scale chip • Improvement of radiation tolerance (starting from careful sensor design) D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  17. DESY/UNIHH contribution • Sensor development (engineering) • - Mechanical support and sensor cooling: CAD design of VXD layers layout, exploration of various alternatives for VXD cooling; optimization in terms of material budget • Radiation hardness studies • Simulations • Basic studies on dedicated test structures (measurement of macroscopic and microscopic parameters) • Chip tests and test-beam measurements • General detector design and optimization • - Simulation • - Physics studies D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  18. Device simulation (ISE-TCAD) Issue for device simulation: feasibility of MAPS in deep-submicron technology • reduced epilayer thickness: smaller sensitive volume • presence of isolation structures (Shallow Trench Isolation) • radiation hardness: build-up of radiation-induced interface states Goal of the simulations: study of the influence of these parameters on the sensor charge collection properties Standard configuration of the collecting diode in a sub-m process with STI Example of simulated structure and geometrical parameters used in the simulations D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  19. Device simulation (II) Epi 2 um Epi 8 um Epi 5 um (Interface traps concentrations: 1011, 1012 1/eVcm2~500 krad) • Simulation results • Thinner epilayer: poor signal, but limited charge spreading, shorter collection times (faster collection, <50 nsec), and reduced effect of interface damage • Thicker epilayer: better signal, collection times still ok, but bigger charge losses after introduction of interface states • In the presence of interface damage, there is a sensible dependence of the collected charge on the trench geometry (mainly on depth) Problems: lack of information on technological parameters, reliability of TCAD built-in physical models D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  20. Test stand • Test stand: hardware and software back from Strasbourg. The test-stand is being built. We will start working on a real-size sensor (MIMOSA V chip). • Planned tests: • 55Fe source • IR laser light injection • test-beam • temperature dependence • influence of B-field • simulation of running conditions with cycling power First MIMOSA V read-out at DESY! D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  21. Open questions • Radiation-tolerance: mechanisms not yet fully understood  need for basic studies: • test structures: basic measurements on diodes, capacitors, transistors. Problem of obtaining (separated) test structures from the foundry • aid of improved device simulations based on measured microscopic parameters. Two trends of ideas of the mechanism (parasitic collection, interface states). Problems: lack of technological details, and reliability of simulation software • high energy electrons: might represent an issue? It seems so for CCDs, MAPS may be less sensitive. • Mechanics: backthinning (chemical etching), material budget, mechanical support, handling (problems in common with CCDs) • Cooling: the power dissipation for the full detector ~1 kW. Is it possible to switch off between bunch trains?  switch on/off tests with MIMOSA V D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

  22. Conclusions • The technology has shown good and promising performances on small-scale prototypes: the effort needed for the TESLA VXD is to reproduce this performances on large-scale detectors, in parallel with optimisation of the read-out and on-chip data processing • Many open questions deserve further investigation and offer room for collaboration, in which the DESY/UNIHH group can be effective in several fields, in particular radiation studies, chip tests, physics simulation and engineering issues • Status of the collaboration: not many centers (F-D-CH-UK) are actively working on MAPS for TESLA. Strasbourg is also interested in imaging applications (SUCIMA collaboration): they will continue to work on TESLA, but at some point our paths may depart. Do we have the technological skill to develop the technology on our own? • Technological choice for the TESLA VXD not before 2004: will MAPS be ready? D. Contarato R&D on MAPS for the TESLA Vertex Detector 9 January 2003

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