MAPS for a Tera-Pixel ECAL at the International

MAPS for a “Tera-Pixel” ECAL at the International Linear Collider J. P. Crooks Y. Mikami, O. Miller, V. Rajovic, N. K. Watson, J. A. Wilson University of Birmingham J. A. Ballin, P. D. Dauncey, A. -M. Magnan, M. Noy Imperial College London J. P. Crooks, B. Levin, M. Lynch, M. Stanitzki, K. D. Stefanov, R. Turchetta, M. Tyndel, E. G. Villani STFC-Rutherford Appleton Laboratory

Introduction Si. W ECAL for ILC • 30 layers silicon & tungsten • Prove Monolithic Active Pixel Sensor (MAPS) as a viable solution for the silicon! Machine operation • 189 ns min bunch spacing • 199 ms between bunch trains for readout Sensor Specification • Sensitive to MIP signal • Binary readout from 50 micron pixels • Store timestamp & location of “hits” • Noise rate 10 -6 • Design to hold data for 8 k bunch crossings before readout 2625 bunches

INMAPS Process • • Standard 0. 18 micron CMOS 6 metal layers Analog & Digital VDD @ 1. 8 v 12 micron epitaxial layer • Additional module: Deep P-Well – – – Developed by foundry for this project Added beneath all active circuits in the pixel Should reflect charge, preventing unwanted loss in charge collection efficiency • Device simulations show conservation of charge • Test chip processing variants – Sample parts were manufactured with/without deep p-well for comparison

Device Simulations TCAD model of 3 x 3 pixels Charge injected in 21 reference points Response at each diode in 3 x 3 pixels recorded • Charge collected • Collection time (to 90%) Profile mirrored to create full 150 x 150 um terrain • 60 50 Profile B; through cell 45 40 % total signal • • 35 30 25 GDS+DPW 20 GDS-DPW 15 10 Profile F; through cell 5 50 0 % total signal 0 40 30 GDS-DPW 10 F 0 0 10 20 30 Position in cell (microns) 40 20 30 Position in cell (microns) Pixel profiles GDS+DPW 20 10 50 B 40 50 Simulation points

Pixel Architectures pre. Shape • • • Gain 94 u. V/e Noise 23 e. Power 8. 9 u. W • 150 ns “hit” pulse wired to row logic Shaped pulses return to baseline • pre. Sample • • • Gain 440 u. V/e Noise 22 e. Power 9. 7 u. W • 150 ns “hit” pulse wired to row logic Per-pixel selfreset logic •

Pixel Layouts pre. Shape Pixel • • • 4 diodes 160 transistors 27 unit capacitors • Configuration SRAM – – • Mask Comparator trim (4 bits) 2 variants: subtle changes to capacitors pre. Sample Pixel • • • 4 diodes 189 transistors 34 unit capacitors 1 resistor (4 Mohm) Configuration SRAM – – • Mask Comparator trim (4 bits) 2 variants: subtle changes to capacitors Deep p-well Diodes Circuit N-Wells

Test Chip Architecture • • • 8. 2 million transistors 28224 pixels; 50 microns; 4 variants Sensitive area 79. 4 mm 2 – • Four columns of logic + SRAM – – – • of which 11. 1% “dead” (logic) Logic columns serve 42 pixels Record hit locations & timestamps Local SRAM Data readout – – Slow (<5 Mhz) Current sense amplifiers Column multiplex 30 bit parallel data output

Sensor Testing: Overview Test pixels • • pre. Sample pixel variant Analog output nodes Fe 55 stimulus IR laser stimulus Single pixel in array • • • Per pixel masks Fe 55 stimulus Laser Stimulus Full pixel array • • • pre. Shape (quad 0/1) Pedestals & trim adjustment Gain uniformity Crosstalk Beam test quad 0 quad 1

Test pixels: Laser Stimulus • 1064 nm pulsed laser • 2 x 2 um square area of illumination at focal point • Simulates point-charge deposit in pixel • Illuminate back of sensor • Silicon is ~transparent at this λ • Adjust focus to hit the EPI layer • Account for refractive index! • Scan XY position to 1 um accuracy • Test pixels & laser run asynchronously • Oscilloscope triggered by laser sync pulse shows analog response from test pixel • Measure (histogram) • Amplitude • Time delay = (System Delay) + (charge collection) 100 400 Bulk silicon wafer EPI Pixel Circuits 12

Test pixels: Laser Stimulus • First look (Nov ‘ 07) • without / with DPW • 4 x 4 um spot, 5 um steps • Poor focussing! Recent scans • Optimised Focus • 2 x 2 um spot, 2 um steps 120 Signal Magnitude (m. V) • 250 200 150 NO DPW 100 WITH DPW 50 Amplitude Position Scan Through Diodes: Sensor WITH DPW 0 100 Signal Magnitude (m. V) Preliminary. Comparison: With/Without DPW 0 80 20 40 60 80 100 120 Y posistion (microns) [X position fixed ~pixel diodes] 140 60 • 40 20 0 522 532 542 552 562 572 X Position (microns) [Y position fixed ~pixel diodes 582 592 Automated laser profile of full test pixel area begins… • With/without DPW • Different depths epi 160

Test pixels: Laser Stimulus Timing measurement TCAD Simulation 17 um (30 m. V threshold) t(Q=90%) values Focus 1200 250 400 350 200 Delay (ns) 300 78 ns 250 90 ns 150 200 150 50 100 50 0 522 532 542 552 562 572 582 592 60 70 80 90 100

Evaluating single pixel performance • Binary readout from pixels in the array • Can mask individual pixels • • Record #hits for a given threshold setting 1 threshold unit ~0. 4 m. V Low thresholds noise hits Max #hits defined by memory limit (=19 per row) Comparator is edge-triggered o Very small or negative thresholds don’t trigger comparator Signal should generate hits at higher thresholds than the noise No hits expected for very high thresholds Number of hits • Evaluated with a threshold scan… Single active pixel with/without laser firing

Single Pixel in Array: Laser/Alignment • Use laser for alignment • Back of sensor has no features for orientation • Mounting is not necessarily square to <1 um • Laser position scans in X & Y • Threshold scan technique • Estimate signal magnitude from drop-off • By eye • By function fit?

Single Pixel in Array: Laser Stimulus • Amplitude results • With/without deep pwell • Compare • Simulations “GDS” • Measurements “Real” 50 Profile B; through cell 45 % total signal 40 35 30 GDS+DPW 25 GDS-DPW 20 Real+DPW 15 60 Profile F; through cell 10 5 50 % total signal real-DPW 0 0 40 GDS+DPW 30 Real+DPW real-DPW 10 F 0 0 10 20 30 Position in cell (microns) 40 20 30 Position in cell (microns) Pixel profiles GDS-DPW 20 10 50 B 40 50 Simulation points

Single Pixel in Array: • 55 Fe Source gives 5. 9 ke. V photon • Deposits all energy in ~1 mm 3 volume in silicon; 1640 e− • Sometimes will deposit maximum energy in a single diode and no charge will diffuse absolute calibration! • Binary readout from pixel array • Need to differentiate distribution to get signal peak in threshold units (TU) • Differential approximation

Array of Pre. Shape Pixels: Pedestals • • Threshold scan of individual pixels Note differing threshold scans of noise! • Plot the distribution of pedestals • Falling edge Calculate necessary trim adjustment Per-pixel trim file • uni-directional adjustment Re-scan pixels individually with trims Re-plot the distribution of pedestals • •

Array of Pre. Shape Pixels: Gains • • • Use laser to inject fixed-intensity signal into many pixels Relative position should be equivalent for each pixel scanned Adjust/trim for known pixel pedestals • Gain uniform to 12% • Quad 1 ~40% more gain than Quad 0 • Quad 1 ~20% better S/N than Quad 0

Array of Pre. Shape Pixels: Beam Test • Took advantage of beam-test opportunity • • • very soon after receiving sensors before long shut-down at DESY Proof of 4 -sensor system Did see particles in multiple layers Sensor pedestals were not trimmed at this time • Little usable data

Immediate Future • • • Characterisation of v 1. 0 is still ongoing • Automated laser tests • Cosmics stack Version 1. 1 due back late September • One pixel variant selected (pre. Shape quad 1) • Upgrade trim adjustment from 4 bits to 6 bits • Compatible format: size, pins, pcb, daq etc. • Minor bugs fixed • Additional test pixels & devices Version 1. 1 Full Characterisation • Automated laser tests • test pixels • array • Source tests • Cosmic tests • Beam test early 2009 • With trims this time!

Long Term future • Version 2 is part of a proposal submitted last week! • Larger sensor 25 x 25 mm • Tiled to create a 125 x 125 mm layer of pixels • Minimum dead space between sensors • Wire bonded through PCB holes • Stacked in 16 layers to ultimately prove the Digital ECAL concept

Conclusions • • • First Sensor • Successful operation of highly complex pixels • See α & β radioactive sources • See laser injection of charge • See beam particles (albeit with low efficiency at the time) • Proved viability of the Deep P-Well for applying MAPS to particle physics • Selected a preferred pixel design to take forward • Some minor bugs • Low level data corruption • Some coupling between power domains generating false hits Revised Sensor • Uniform array of improved pixels • Full characterisation ready to go! Exciting future • Prove “Digital ECAL” concept using CMOS sensors