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PhD Thesis defense Michal Szelezniak ULP, Strasbourg 25 February 2008

Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider. PhD Thesis defense Michal Szelezniak ULP, Strasbourg 25 February 2008. Outline .

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PhD Thesis defense Michal Szelezniak ULP, Strasbourg 25 February 2008

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  1. Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider PhD Thesis defense Michal Szelezniak ULP, Strasbourg 25 February 2008

  2. Outline Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider • The new vertex detector for the STAR experiment • Development of Monolithic Active Pixel Sensors (MAPS) at IPHC • MAPS prototype for PIXEL detector • 3-sensor telescope system with prototype readout for PIXEL detector • Future development plans • Summary and Conclusions

  3. STAR experiment STAR was constructed to study Quark-Gluon Plasma created in heavy-ion collisions at Relativistic Heavy Ion Collider (RHIC) • Lorentz contracted ions before the collision • Hard interactions between partons of incoming nuclei • New, high-density state of matter (QGP?) • Hadronization and freezout Location of the new vertex detector End view of tracks registered by the STAR TPC in a heavy-ion collision

  4. QGP in heavy-ion collisions • Penetrating probes (created early in a collision) are sensitive to the evolution of the medium • Particles with very high transverse momentum • Heavy particles containing charm or bottom quarks • To study next: • Charm flow to test thermalization of light quarks at RHIC • Charm energy loss to test pQCD in a hot and dense medium at RHIC (from HFT proposal) The D0 signal, after topological cuts, is shown by the solid black circles. The original spectrum, before software cuts, is shown by the line of open circles.

  5. HFT: new vertex detector for STAR Heavy Flavor Tracker PIXEL at 2.5 and 8 cm • Goal: increasing pointing resolution from the outside in • TPC pointing resolution at the SSD is ~ 1 mm • SSD pointing at the IST is ~ 300 µm • IST pointing at the PIXEL is ~ 250 µm • PIXEL pointing at the VTX is ~ 30 µm IST at 14 cm Secondary vertex ~100 µm SSD at 23 cm D0 (cū) Primary vertex To measure heavy flavor production it is necessary to measure charm and bottom hadrons through direct topological reconstruction New Vertex Detector is needed! PIXEL: spatial resolution < 10 μm radiation length ~ 0.3 % VXD3 0.4%, ALICE pixel detector ~1%

  6. PIXEL Detector • Quick extraction and sensor replacement • Monolithic Active Pixel Sensors • Thinned to 50 μm thickness • 30 μm x 30 μm pixels • 640 x 640 pixel array • Integration time <200 μs at L=8×1027 • Power disspation <100 mW/cm2 PIXEL characteristics: • Two layers at 2.5 & 8 cm radius • 10+30 ladders • 10 sensors/ladder • Nearly 164 M pixels • 0.28 % radiation length/layer • Air cooled Ladder with 10 MAPS sensors

  7. Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider • The new vertex detector for the STAR experiment • Development of Monolithic Active Pixel Sensors (MAPS) at IPHC • Simulations and tests of in-pixel voltage amplifiers, • Tests of advanced pixel structures with in-pixel memories • Tests and study of AC coupling for in-pixel amplifiers • Tests and study of MAPS operated in current mode (PhotoFET) • MAPS prototype for PIXEL detector • 3-sensor telescope system with prototype readout for PIXEL detector • Future development plans • Summary and Conclusions

  8. Monolithic Active Pixel Sensors MAPS pixel cross-section (not to scale) Properties: • Standard commercial CMOS technology • Sensor and signal processing integrated in the same silicon wafer • Signal created in low-doped epitaxial layer (typically ~10-15 μm) • Charge collection mainly through thermal diffusion (~100 ns), reflective boundaries at p-well and substrate • Charge sensing in n-well/p-epi junction • 100% fill-factor • High granularity • Low power dissipation • Substantial radiation tolerance • Thinning available as standard post-processing • Only NMOS transistors inside pixels • Thin active volume → MIP signal limited to <1000 electrons • Thermal diffusion → cluster size of ~10 pixels (20-30 μm pitch) • sensitivity to charge of a few tens of electrons  ←noise at the level of 10 e-

  9. MAPS vs. other technologies Hybrid Pixel Sensors: detector bump bonded to readout chip CCD: integrated detector and readout, external processing MAPS: integrated detector/readout/processing 8” wafer with MAPS prototypes MAPS Hybrid Pixel Sensors CCD + - + + - + + ++ - + ++ - • High granularity (several μm pitch) • Small material budget • Fast readout • Radiation tolerance

  10. Simple pixel architectures Classical diode with reset read Reset noise, offset Continuous reverse bias (self-biased) No reset noise, no offset read

  11. Pixel sensor architectures • Typical sensor readout • Raster scan • Charge integration time = array readout time • Multiplexed sub-arrays to decrease integration time • Column parallel readout architecture • All columns readout in parallel and then multiplexed to one output • Charge integration time = column readout time Analog readout – simpler architecture but ultimately slower readout Digital readout – offers increased speed but requires on-chip discriminators or ADCs On-chip signal processing requires high S/N – signal amplification is needed

  12. Example of a simple in-pixel amplifier (0.35 μm CMOS process) • Amplifier in cascode configuration (only NMOS transistors) • Switches for switched-power operation • Typical gain: 4-6 • Cascode transistor to reduce the Miller effect that is present in a common-source configuration: Cin = Cgs + Cgd(1+G) • Typical power consumption (3.3 V) P=20 μW Lower input capacitance  higher charge-to-voltage conversion factor • Typical biasing voltage: ~0.7 V

  13. Optimization of pixel design Typical connection AC-coupling • Improves CCE (5%) • Degrades ENC (25%) Compact layout implementation of AC coupling • DC coupling gives better ENC performance

  14. Investigated in-pixel amplifiers Design gain = 8 Measured gain < 4.5 ENC = 20 e- Design gain = 9 Measured gain < 5 ENC = 18 e- Basic Design gain = 5 Measured gain = 4 ENC = 12 e- Pixel with 2 internal memories E.g. memory discharge time: MOSFET capacitor 7μm x 7μm (200 fF) 5s/div and 200 mV/div Promising structure for on-chip CDS processing

  15. MAPS operated in current mode • PhotoFET cell – collected charge modulates current in the PMOS transistor • Early prototypes: single cell ENC ~ 5e- Signal distribution from one pixel • Tested in pixel array configuration • Two in-pixel current memory cells Noisy prototype (ENC 50-60 e-) due to large noise bandwidth Coupling of digital signals to memory nodes during sensor operation prevented the use of the integrated CDS

  16. CDS in current mode • Two CDS performing circuits validated (in discrete implementation) • Capacitance arithmetic (integrator + amplifier) • Subtraction on an operational amplifier (two integrators + amplifier) More compact architecture Lower power consumption Simpler subtraction – faster operation More amplifiers – higher power consumption PhotoFET – interesting concept and promising results BUT Not ready to provide a reliable solution for a vertex detector

  17. Increased tolerance to ionizing radiation Shot Noise Contribution @ 30°C and @4 ms integration time ENCshot = 39 electrons ENCshot = 12 electrons standard diode layout thin-oxide diode layout

  18. Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider • The new vertex detector for the STAR experiment • Development of Monolithic Active Pixel Sensors (MAPS) at IPHC • MAPS prototype for PIXEL detector • Tests and study of performance as a function of ionizing radiation dose • Tests and study of sensor’s susceptibility to latch up • 3-sensor telescope system with prototype readout for PIXEL detector • Future development plans • Summary and Conclusions

  19. ADC CDS CDS Disc. Data sparsification readout to DAQ Pixel Sensors On-chip data processing and complementary RDO Data sparsification reduction of the amount of data transferred, typically through zero-suppression Correlated Double Sampling (CDS) = subtraction of two consecutive signal samples reduces low frequency noise extracts signal accumulated during integration time Complementary detector readout digital signals analog signals digital analog MimoSTAR sensors 4 ms integration time Phase-1 sensors 640 μs integration time Ultimate sensors < 200 μs integration time First prototypes in hand and tested 2010 Install 3-module demonstrator (based on Phase1) 2011 Install final detector Few years back it was planned to built a demonstrator detector based on sensors with 4 ms integration time.

  20. MAPS Prototype for STAR MimoSTAR2: • Analog readout • Radiation tolerant diode design • JTAG* controlled configuration *Joint Test Action Group (JTAG) is the IEEE 1149.1 standard entitled Standard Test Access Port and Boundary-Scan Architecture

  21. MimoSTAR2 performance – ionizing radiation 60Co 55Fe signal collected in central pixels Degradation of noise performance • Significant improvement in resistance to ionizing radiation • Satisfies initial PIXEL detector requirements Peak corresponds to the full charge collection (1640 e-)

  22. MimoSTAR2 performance – latch up Setup at the Tandem Van der Graff accelerator facility at BNL Parasitic thyristor • No latch ups observed up to energies equivalent to 6000 MIPs

  23. STD 0.8 ms STD 4.0 ms RAD 0.8 ms RAD 4.0 ms STD 0.8 ms STD 4.0 ms RAD 0.8 ms RAD 4.0 ms MimoSTAR2 performance – beam tests Standard setup for tests with minimum ionizing particles (5 GeV e- @ DESY) • detection efficiency > 99.8 % when S/N >12 Analysis by Auguste Besson, IPHC

  24. Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider • The new vertex detector for the STAR experiment • Development of Monolithic Active Pixel Sensors (MAPS) at IPHC • MAPS prototype for PIXEL detector • 3-sensor telescope system with prototype readout for PIXEL detector • Construction and tests of the telescope head • FPAG and software programming for JTAG communication • Study of efficiency of the proposed hit finding algorithm • Laboratory calibrations, ALS test and sensors alignment, tests in the STAR environment • Future development plans • Summary and Conclusions

  25. Motivation for the 3-sensor telescope • The telescope is a small prototype and contains all elements easily scalable to meet the requirements of the PIXEL • Test functionality of a prototype MIMOSTAR2 detector in the environment at STAR 2006-2007: • Charged particle environment near the interaction region in STAR. • The noise environment in the area in which we expect to put the final PIXEL. • Performance of the MIMOSTAR2 sensors. • Performance of our hit finding algorithm. • Performance of our hardware / firmware as a system. • Functionality of our tested interfaces to the other STAR subsystems.

  26. Implementation of the 3-sensor telescope MOTHER BOARD DAUGHTER CARD MimoStar2 chips on kapton cables Control PC (Win) STRATIX Acquisition Server (Linux) RORC SIU

  27. Zero suppression through on-the-fly hit finding Functionally equivalent to a raster scan Checks 9 pixel window at each clock cycle Only pixel addresses are saved Hits are recognized when: • signal in the central pixel exceeds high threshold • and any one of the neighboring 8 pixels exceeds low threshold. Efficiency and accidental rates are comparable to the traditional ADC sum method.

  28. Cluster Finder Efficiency Sum method Two Threshold FPGA method Cut on the central pixel goes from 14 to 8 ADC counts (left to right) every 1 ADC = 7.1 e- Detection efficiency >99% and accidental hit rate <10-4 achievable for a range of settings Expected close to 3 orders of magnitude data rate reduction for a 4 ms PIXEL detector

  29. MimoSTAR2 Telescope test at the ALS 1.2 GeV electrons at the ALS Booster Test Facility Due to not decoupled DAC pads on the sensor, our noise level was double the value achieved under normal conditions. Decoupled  11-15 e- Not decoupled  30-35 e- @ 30º C MPV = 49 (Standard) and 43 (Radtol) ADC counts at ~230 electrons Sensors aligned based on straight tracks reconstructed in all 3 planes Scan of threshold levels to calibrate the system for the next stage of tests in the STAR environment • High cut 25 ADC • Low cut 14 ADC

  30. signals originating at the collision point Background tracks parallel to the beam (magnified) theoretical projection of the beam diamond Increased width from multiple Coulomb scattering in the beam pipe MimoSTAR2 Telescope test at STAR (Run 200 GeV Au-Au) Magnet Pole Tip • No environmentally induced noise observed • Operation in magnetic field of 0.5 T • Average RHIC luminosity 8×1026 cm-2s-1 • On average 25 clusters per cm2 per frame (1.7 ms) • Operation of the complete system was validated The interraction point is ~2 m away Telescope head 145 cm from interaction point 5 cm below beam pipe. Beam Pipe Electronics Box View of TPC end cap Analysis by Xiangming Sun, LBL

  31. Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider • The new vertex detector for the STAR experiment • Development of Monolithic Active Pixel Sensors (MAPS) at IPHC • MAPS prototype for PIXEL detector • 3-sensor telescope system with prototype readout for PIXEL detector • Future development plans • Summary and Conclusions

  32. What will a pixel for the PIXEL look like? MAPS developed for STAR started with a very simple pixel architecture Currently, the most promising architecture developed by IPHC and CEA-Saclay • The simplest pixel • Sequential pixel readout • In-pixel amplifier • In-pixel CDS • Column parallel readout • On-chip discriminators Meets PIXEL requirements There is always room for improvements … and we still have a little bit of time Mimosa 16

  33. Data sparsification readout to DAQ Pixel Sensors Final detector system Currently in the testing phase Under development + Pixel analog signals digital signals Disc. CDS Phase-1 sensors – 640 μs integration time Ultimate sensors – <200 μs integration time 2010 Install 3-module demonstrator (based on Phase1) 2011 Install final detector

  34. Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider • The new vertex detector for the STAR experiment • Development of Monolithic Active Pixel Sensors (MAPS) at IPHC • MAPS prototype for PIXEL detector • 3-sensor telescope system with prototype readout for PIXEL detector • Future development plans • Summary and Conclusions

  35. Summary and Conclusions • MAPS development is keeping pace with requirements for STAR • Development of pixels for on chip CDS processing (in-pixel amplifiers, on chip CDS, alternative current mode) • MimoSTAR2 prototype was a necessary precursor to the final STAR PIXEL sensor • Validation of the technology based on the first prototypes • Development and testing of the PIXEL detector readout system • The existing sensor architecture with column parallel readout should satisfy PIXEL detector requirements • IPHC-LBL development plan leads us to achieving the design goals in the next few years (2010 – detector demonstrator, 2011 final installation) • PIXEL detector is going to be the first vertex detector built with MAPS technology – significant impact on the HEP field

  36. Thank you for your attention

  37. Backup Slides

  38. z x Introduction to the STAR experiment • Penetrating probes (created early in a collision) are sensitive to the evolution of the medium • Particles with very high transverse momentum • Heavy particles containing charm or bottom quarks • Some of the observed physics: • To study next: • Production of heavy quarks • Elliptic flow of heavy quarks Suppression of the side-away jets Flow source source

  39. QGP in heavy-ion collisions • Penetrating probes (created early in a collision) are sensitive to the evolution of the medium • Particles with very high transverse momentum • Heavy particles containing charm or bottom quarks • To study next: • Charm flow to test thermalization of light quarks at RHIC • Charm energy loss to test pQCD in a hot and dense medium at RHIC • Selected result: spectra of heavy quarks The corresponding heavy flavor decayed electron spectra are shown as black curves. Single electron/positron spectra from semileptonic decays are not sufficient. S. Batsouli et al. Phys. Lett. B557, 26 (2003)

  40. D0 reconstruction (from HFT proposal) The D0 signal, after topological cuts, is shown by the solid black circles. The original spectrum, before software cuts, is shown by the line of open circles.

  41. STAR pointing resolution Pointing resolution of the TPC alone Pointing resolution at the vertex by the TPC+SSD+IST+PIXEL detectors

  42. PIXEL development plan • Original plan (2006) • New plan (2007) 08 2007 Wafers of full-reticule MimoSTAR4 08 2008 Install 4ms detector (based on MimoSTAR4) 06 2011 Install final detector • analog readout • 4 ms integration time • binary readout • 640 μs integration time 03 2008 Submit Phase1 for fabrication 08 2010 Install 3-module demonstrator (based on Phase1) 06 2011 Install final detector • binary readout • 640 μsintegration time • binary readout • 640 μs integration time • binary readout • On-chip zero suppression • 200 μs integration time

  43. MimoSTAR2 Telescope test at the ALS Electronic noise background Merged cluster data – typically 2-3 hits per cluster. Increased noise in sensors results in reduced performance.

  44. PIXEL Data Rates for a 4ms detector 168 MB/s 63 GB/s 42 GB/s • Rate @ R1 (2.5 cm) = 52.9 / cm2 • Rate @ R2 (8 cm) = 7.3 / cm2 (at L = 1027 cm-2s-1) • Average event size = 168 kB * • Data Rate = 168 MB/s at 1 kHz * • On average 2.5 pixels per cluster *Bit rate without any overhead

  45. PIXEL ladder

  46. Telescope results • RDO system with on-the-fly data sparsification implemented and functional for Mimostar2 sensors. • Prototype system fully functional and characterized. • Fully functioning interfaces between the prototype system and STAR detector infrastructure. • Completed measurements of detector environment at STAR.

  47. Fast, column-parallel architecture Developed in IPHC - DAPNIAcollaboration A1 Voff1 A2, Voff2 Vin1,2 VC VS_READ VREAD,CALIB CDS at column level (reduces Fixed Pattern Noise below temporal noise)

  48. Next generation of prototypes • Radiation tolerant diode design • Column parallel readout with on-chip discriminators • Binary readout • JTAG controlled configuration • On-chip zero suppression(currently at prototyping stage)

  49. Summary and Conclusions • An architecture of the MAPS sensor that should comply with the final PIXEL detector requirements exists and provides very promising initial results • The on-going development of pixel architectures and in particular in-pixel amplifiers has a potential of further improving the established performance • Readout architecture for the PIXEL detector has been prototyped and validated • Reading out sensors with binary output will require adjustments w.r.t. the existing solution (fast LVDS readout) • Detector dead-time is primarily limited by the number of externally allocated readout buffers • The next mile-stone for MAPS and PIXEL development will integrate the new full-size (640×640 pixels) sensor prototype (Phase-1 under development), prototype mechanical support and new readout system for fast binary sensor readout

  50. Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider • The new vertex detector for the STAR experiment • Introduction to the STAR experiment • HFT: new vertex detector for STAR • PIXEL detector • Development of Monolithic Active Pixel Sensors (MAPS) at IPHC • MAPS prototype for PIXEL detector • 3-sensor telescope system with prototype readout for PIXEL detector • Future development plans • Summary and Conclusions

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