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

Michal Szelezniak - PhD thesis defense - 25 February Outline 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 Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider

Michal Szelezniak - PhD thesis defense - 25 February STAR experiment End view of tracks registered by the STAR TPC in a heavy-ion collision STAR was constructed to study Quark-Gluon Plasma created in heavy-ion collisions at Relativistic Heavy Ion Collider (RHIC) a) Lorentz contracted ions before the collision b) Hard interactions between partons of incoming nuclei c) New, high-density state of matter (QGP?) d) Hadronization and freezout Location of the new vertex detector

Michal Szelezniak - PhD thesis defense - 25 February 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 QGP in heavy-ion collisions (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.

Michal Szelezniak - PhD thesis defense - 25 February HFT: new vertex detector for STAR – 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 To measure heavy flavor production it is necessary to measure charm and bottom hadrons through direct topological reconstruction New Vertex Detector is needed! D 0 (cū) Heavy Flavor Tracker ~100 µm Secondary vertex PIXEL at 2.5 and 8 cm Primary vertex IST at 14 cm SSD at 23 cm PIXEL:spatial resolution < 10 μm radiation length ~ 0.3 % VXD3 0.4%, ALICE pixel detector ~1%

Michal Szelezniak - PhD thesis defense - 25 February PIXEL Detector PIXEL characteristics: Two layers at 2.5 & 8 cm radius – ladders – 10 sensors/ladder Nearly 164 M pixels 0.28 % radiation length/layer Air cooled 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=8×10 27 – Power disspation <100 mW/cm 2 Ladder with 10 MAPS sensors

Michal Szelezniak - PhD thesis defense - 25 February 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 Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider

Michal Szelezniak - PhD thesis defense - 25 February Monolithic Active Pixel Sensors 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 MAPS pixel cross-section (not to scale) 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 -

Michal Szelezniak - PhD thesis defense - 25 February MAPS vs. other technologies High granularity (several μm pitch) Small material budget Fast readout Radiation tolerance 8” wafer with MAPS prototypes MAPS Hybrid Pixel Sensors CCD Hybrid Pixel Sensors: detector bump bonded to readout chip CCD: integrated detector and readout, external processing MAPS: integrated detector/readout/processing

Michal Szelezniak - PhD thesis defense - 25 February Simple pixel architectures Continuous reverse bias (self-biased) Classical diode with reset Reset noise, offset No reset noise, no offset read

Michal Szelezniak - PhD thesis defense - 25 February 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 On-chip signal processing requires high S/N – signal amplification is needed Analog readout – simpler architecture but ultimately slower readout Digital readout – offers increased speed but requires on-chip discriminators or ADCs

Michal Szelezniak - PhD thesis defense - 25 February Example of a simple in-pixel amplifier Amplifier in cascode configuration (only NMOS transistors) Typical gain: 4-6 Switches for switched-power operation Cascode transistor to reduce the Miller effect that is present in a common- source configuration: C in = C gs + C gd (1+G) Lower input capacitance  higher charge-to-voltage conversion factor Typical biasing voltage: ~0.7 V Typical power consumption (3.3 V) P=20 μW (0.35 μm CMOS process)

Michal Szelezniak - PhD thesis defense - 25 February Optimization of pixel design Typical connectionAC-coupling Compact layout implementation of AC coupling Improves CCE (5%) Degrades ENC (25%)  DC coupling gives better ENC performance

Michal Szelezniak - PhD thesis defense - 25 February Investigated in-pixel amplifiers E.g. memory discharge time: MOSFET capacitor 7μm x 7μm (200 fF) 5s/div and 200 mV/div Pixel with 2 internal memories 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 - Promising structure for on-chip CDS processing

Michal Szelezniak - PhD thesis defense - 25 February Noisy prototype (ENC e - ) due to large noise bandwidth Coupling of digital signals to memory nodes during sensor operation prevented the use of the integrated CDS MAPS operated in current mode PhotoFET cell – collected charge modulates current in the PMOS transistor Early prototypes: single cell ENC ~ 5e - Tested in pixel array configuration Two in-pixel current memory cells Signal distribution from one pixel

Michal Szelezniak - PhD thesis defense - 25 February CDS in current mode Two CDS performing circuits validated (in discrete implementation) – Capacitance arithmetic (integrator + amplifier) – Subtraction on an operational amplifier (two integrators + amplifier) PhotoFET – interesting concept and promising results BUT Not ready to provide a reliable solution for a vertex detector Simpler subtraction – faster operation More amplifiers – higher power consumption More compact architecture Lower power consumption

Michal Szelezniak - PhD thesis defense - 25 February Increased tolerance to ionizing radiation standard diode layout thin-oxide diode layout Shot Noise 30°C ms integration time ENC shot = 39 electrons ENC shot = 12 electrons

Michal Szelezniak - PhD thesis defense - 25 February 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 Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider

Michal Szelezniak - PhD thesis defense - 25 February On-chip data processing and complementary RDO 2011 Install final detector 2010 Install 3-module demonstrator (based on Phase1) First prototypes in hand and tested Correlated Double Sampling (CDS) = subtraction of two consecutive signal samples  reduces low frequency noise  extracts signal accumulated during integration time Data sparsification reduction of the amount of data transferred, typically through zero-suppression Few years back it was planned to built a demonstrator detector based on sensors with 4 ms integration time. Pixel Sensors CDS ADC Data sparsification readout to DAQ analog signals Phase-1 sensors 640 μs integration time Complementary detector readout MimoSTAR sensors 4 ms integration time Ultimate sensors < 200 μs integration time analog digital digital signals Disc. CDS

Michal Szelezniak - PhD thesis defense - 25 February MAPS Prototype for STAR MimoSTAR2: * Joint Test Action Group (JTAG) is the IEEE standard entitled Standard Test Access Port and Boundary-Scan ArchitectureIEEE Analog readout Radiation tolerant diode design JTAG* controlled configuration

Michal Szelezniak - PhD thesis defense - 25 February MimoSTAR2 performance – ionizing radiation 60 Co Significant improvement in resistance to ionizing radiation Satisfies initial PIXEL detector requirements 55 Fe signal collected in central pixelsDegradation of noise performance Peak corresponds to the full charge collection (1640 e-)

Michal Szelezniak - PhD thesis defense - 25 February MimoSTAR2 performance – latch up Setup at the Tandem Van der Graff accelerator facility at BNL No latch ups observed up to energies equivalent to 6000 MIPs Parasitic thyristor

Michal Szelezniak - PhD thesis defense - 25 February MimoSTAR2 performance – beam tests Standard setup for tests with minimum ionizing particles (5 GeV DESY) detection efficiency > 99.8 % when S/N >12 Analysis by Auguste Besson, IPHC 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

Michal Szelezniak - PhD thesis defense - 25 February 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 Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider

Michal Szelezniak - PhD thesis defense - 25 February 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 : – 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.

Michal Szelezniak - PhD thesis defense - 25 February Implementation of the 3-sensor telescope STRATIX DAUGHTER CARD RORC SIU MimoStar2 chips on kapton cables MOTHER BOARD Acquisition Server (Linux) Control PC (Win)

Michal Szelezniak - PhD thesis defense - 25 February Zero suppression through on-the-fly hit finding Hits are recognized when: 1. signal in the central pixel exceeds high threshold 2. and any one of the neighboring 8 pixels exceeds low threshold. Efficiency and accidental rates are comparable to the traditional ADC sum method. Functionally equivalent to a raster scan Checks 9 pixel window at each clock cycle Only pixel addresses are saved

Michal Szelezniak - PhD thesis defense - 25 February Cluster Finder Efficiency Sum methodTwo 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

Michal Szelezniak - PhD thesis defense - 25 February 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  e - Not decoupled  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

Michal Szelezniak - PhD thesis defense - 25 February The interraction point is ~2 m away MimoSTAR2 Telescope test at STAR Telescope head 145 cm from interaction point 5 cm below beam pipe. Magnet Pole Tip Electronics BoxBeam Pipe 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 No environmentally induced noise observed Operation in magnetic field of 0.5 T Average RHIC luminosity 8×10 26 cm -2 s -1 On average 25 clusters per cm 2 per frame (1.7 ms) Operation of the complete system was validated Analysis by Xiangming Sun, LBL View of TPC end cap (Run 200 GeV Au-Au)

Michal Szelezniak - PhD thesis defense - 25 February 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 Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider

Michal Szelezniak - PhD thesis defense - 25 February What will a pixel for the PIXEL look like? The simplest pixel Sequential pixel readout In-pixel amplifier In-pixel CDS Column parallel readout On-chip discriminators MAPS developed for STAR started with a very simple pixel architecture Currently, the most promising architecture developed by IPHC and CEA-Saclay There is always room for improvements … and we still have a little bit of time Meets PIXEL requirements Mimosa 16

Michal Szelezniak - PhD thesis defense - 25 February Final detector system 2011 Install final detector 2010 Install 3-module demonstrator (based on Phase1) Under development+ Currently in the testing phase Pixel Sensors CDS Disc. Data sparsification readout to DAQ analog signals Phase-1 sensors – 640 μs integration time Ultimate sensors – <200 μs integration time digital signals Pixel

Michal Szelezniak - PhD thesis defense - 25 February 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 Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider

Michal Szelezniak - PhD thesis defense - 25 February 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

Michal Szelezniak - PhD thesis defense - 25 February Thank you for your attention

Michal Szelezniak - PhD thesis defense - 25 February Backup Slides

Michal Szelezniak - PhD thesis defense - 25 February 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 x z Flow Suppression of the side-away jets  source

Michal Szelezniak - PhD thesis defense - 25 February 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 QGP in heavy-ion collisions 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)

Michal Szelezniak - PhD thesis defense - 25 February D 0 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.

Michal Szelezniak - PhD thesis defense - 25 February STAR pointing resolution Pointing resolution of the TPC alone Pointing resolution at the vertex by the TPC+SSD+IST+PIXEL detectors

Michal Szelezniak - PhD thesis defense - 25 February PIXEL development plan Original plan (2006) New plan (2007) Install final detector  binary readout  640 μs integration time Wafers of full-reticule MimoSTAR Install 4ms detector (based on MimoSTAR4)  analog readout  4 ms integration time Install final detector  binary readout  On-chip zero suppression  200 μ s integration time Submit Phase1 for fabrication Install 3-module demonstrator (based on Phase1)  binary readout  640 μs integration time  binary readout  640 μs integration time

Michal Szelezniak - PhD thesis defense - 25 February MimoSTAR2 Telescope test at the ALS Merged cluster data – typically 2-3 hits per cluster. Increased noise in sensors results in reduced performance. Electronic noise background

Michal Szelezniak - PhD thesis defense - 25 February PIXEL Data Rates for a 4ms detector R1 (2.5 cm) = 52.9 / cm 2 R2 (8 cm) = 7.3 / cm 2 (at L = cm -2 s -1 ) Average event size = 168 kB * Data Rate = 168 MB/s at 1 kHz * On average 2.5 pixels per cluster 63 GB/s42 GB/s 168 MB/s *Bit rate without any overhead

Michal Szelezniak - PhD thesis defense - 25 February PIXEL ladder

Michal Szelezniak - PhD thesis defense - 25 February 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.

Michal Szelezniak - PhD thesis defense - 25 February Fast, column-parallel architecture CDS at column level (reduces Fixed Pattern Noise below temporal noise) V READ,CALIB VCVC V in1,2 V S_READ A 1 V off1 A 2, V off2 Developed in IPHC - DAPNIA collaboration

Michal Szelezniak - PhD thesis defense - 25 February 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)

Michal Szelezniak - PhD thesis defense - 25 February 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

Michal Szelezniak - PhD thesis defense - 25 February 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 Development of pixel detectors with integrated signal processing for the Vertex Detector in the STAR experiment at the RHIC collider