PanSTARRS Gigapixel Camera System

1. PanSTARRS Gigapixel Camera System John Tonry (IFA), Gerard Luppino (IFA), Peter Onaka (IFA), Barry Burke (MITLL) • Scope of Gigapixel Camera effort. • WIYN telescope collaboration. • OTA development status • OTA test plan • OTA package, focal plane and cryostat design. • OTA controller electronics design and development. Outline

2. Scope of Gigapixel Camera Effort Part of Detector/Camera System Scope?

3. WIYN Telescope Collaboration • Non Binding Collaboration • WIYN Developing One-Degree Imager (ODI) • Camera System Nearly Identical to one of the PanSTARRS Gigapixel Cameras.

4. Outline • Overview of OTA • Principal design and process challenges • Pixel layout • Logic design, fabrication, and recent results • Metallization and interconnects • Other OTA design issues and options • Die size and pads • Enhanced substrate E fields • Compatibility of design with CMOS option • Mini-OTA (MOTA) options • Process options and lot-1 splits • Design status and summary

5. Orthogonal Transfer Array (OTA) OTCCD cells • 88 array of small OTCCDs , each ~500  500 pixels* • OTCCD cells independently clocked via on-chip control logic • Cells read out one row at a time at 1-MHz read rate; readout time ~ 2 s • Subset of cells (typically five) selectable for tracking guidestars at ~30-Hz rates *(12 um: 480 x 496; gaps 9 x 28, 10 um: 574 x 594; gaps 11 x 35)

6. DetectorDetails – Overview Each CCD cell of a 4K  4K OTA • Independent ~500  500-pixel CCDs • Individual or collective addressing • 1 arcmin field of view • Dead cells excised, yield >50% • Bad columns confined to cells • Cells with bright stars for guiding • 8 output channels per OTA • Fast readout (8 amps, 2 sec) • Disadvantage – 0.1 mm gaps, but gaps and dead cells are dithered out anyway 5 cm

7. Outline • Overview of OTA • Principal design and process challenges • Pixel layout • Logic design, fabrication, and recent results • Metallization and interconnects • Other OTA design issues and options • Die size and pads • Enhanced substrate E fields • Compatibility of design with CMOS option • Mini-OTA (MOTA) options • Process options and lot-1 splits • Design status and summary

8. Principal Challenges • OTCCD • OTCCD yield was low until recently (lot 4) • Pixel layout appears to be critical to high yield • Control logic • Except for some preliminary test structures nMOS has not been part of Lincoln CCD process • Design and simulation learning curve • Metallization • Extensive use of two levels of metal • New planarized metal process recently developed on other programs; test-structure data are encouraging

9. OTCCD Process Issues • Polysilicon stackup must be minimized • High stresses may be a factor • Metallization process requires low gate profile (see subsequent chart) • OTCCD lot 4 had high yield after pixel re-design • Design rules • Max of two poly layers over channel stops • Max of three poly layers over channel regions

10. Poly Pileup Areas of three- and four-poly stacks Three-poly stacks only Revised pixel layout Original pixel layout

11. OTCCD Types • Type 1 OTCCD • Most experience with this style: 2K  4K, 1024  1320, 512  512 (all 15-µm pixels) • 2K  4K now has good yield (CCID-28, lot 4) • Type 2 • Higher symmetry but no obvious fabrication advantage • Only 512  512 (15-µm pixels) made thus far

12. Poly stackup Minimum poly linewidth Well capacity OTA Pixel Designs • Experience to date is with 15-µm pixels • Desired pixel sizes are ≤ 12 µm • 12-µm pixel: low risk • 10 µm: moderate risk • 8 µm: high risk • Tight trade space: Poly1, poly2 layout Channel stops Poly3, poly4 layout Example of 10-µm pixel design

13. Pixel Design Choices • Current plan: four versions of OTA, each with different pixel layout • Two 12-µm-pixel designs as lowest risk • OTA-a: type 1 • OTA-b: type 2 • Two 10-µm pixel designs, both scaled from 12-µm layouts, as somewhat higher risk but closer to desired pixel size • OTA-c: type 1 (scaled OTA-a pixel) • OTA-d: type 2 (scaled OTA-b pixel) Pixel a Pixel b

14. Logic Design Considerations • Requirements • Cells must be independently addressable • Parallel clocks for each cell can be set to one of three states • Active (image readout, update pixel shifts, etc.) • Standby (image acquisition between updates) • Floating (shorted phases) • Cell outputs read out one row at a time • Maximum of one cell/column read at any time • Desired • Low FET count, compact layout • Low power • Compatible with parallel shift rate > 100 kHz

15. Control Logic Overview • NMOS logic chosen • Easy to add to n-channel CCD process(+) • Power inefficient (-) • Early start on development • Oct 2002: SPICE parameter extraction from test MOSFETs on CCID-28, lot 4 • February 2003: preliminary OTA logic designs added to SST mask set • June 2003: logic designs on SST wafers in test!

16. Addressing and Control Logic:Current Design • Data are latched at each cell until addressed again • Flexible operation • Cell can be clocked with video on or off • Standby mode during science image acquisition • Defective parallel gates can float

17. Final Control Logic Design • 39 FETs, 0.6–0.8 mW power dissipation • Operation with VDD=5.0 V; accepts inputs with 5-V CMOS-compatible levels • Further variants under study for inclusion as test structures

18. Logic Designs on SST Wafers • Designs • Simple building-block circuits • Preliminary OTA addressing and control logic (since modified) • Advanced aggressive designs • Important validation of design and simulation methodologies Original OTA logic design (61 FETs) Closeup of circuit

19. Q Q First Test Circuit Results • Latch circuit works for VDD= 5.0 V • Two versions (different design rules) successful • Threshold voltages somewhat higher than expected • Fix is simple implant-dose adjustment D Q CLK D CLK

20. Additional NMOS Test Results • First version of OTA control logic works! • Design from January; uses 61 FETs • Functions with VDD=5 V

21. Metallization • OTA uses large amount of wiring • Two levels of metal (light and dark blue) • Design rules kept deliberately relaxed • Line widths and spacings  5 µm

22. Metallization Process: Planarized Dielectrics with W Plugs • Same approach as used in current CMOS processes • Process outline: • Thick oxide deposited and planarized (CMP) • Contact holes etched, filled with W (damascene process) • Metal straps • Repeat for second metal • Advantages • W plugs can be narrower and deeper than conventional contacts for AlSi • Planar surface better for fine lithography • Shorts yield from test wafers is high • Poly stackups must be minimized to maintain poly/metal isolation Cross section from CCID-34 test wafer

23. Metal Strapping Experiments • Used existing large imager (~20 cm2) mask set and added metal straps over chanstops • Critical test (gate shorts induced by straps) showed high yield • Significant differences from standard LL CCD process • Dry-etched metal (vs. wet) • Ti/TiN/AlSi (vs. AlSi) • W plugs (new) Metal straps

24. Metallization Options • Placing digital-related lines over pixel arrays increases fill factor by 3.3% • Is this a worthwhile gain? • Process implications • Greater chance of metal/poly shorts • More room for metal lines: fewer metal/metal shorts • Metal lines can be wider: faster clocking, less vulnerable to line breaks • Metal-over-pixels option requires three mask changes

25. Outline • Overview of OTA • Principal design and process challenges • Pixel layout • Logic design, fabrication, and recent results • Metallization and interconnects • Other OTA design issues and options • Die size and pads • Enhanced substrate E fields • Compatibility of design with CMOS option • Mini-OTA (MOTA) options • Process options and lot-1 splits • Design status and summary

26. Die Size • Exclusion zone > 5 mm chosen  die size = 49.5  49.5 mm • Compatible with STA/Dalsa requirements • Saw kerf ≈ 70 µm  sawn die size = 49.43  49.43 mm

27. Pad Layout • Mirror symmetric pad layout: FI and BI devices use same package and I/O Center Right side Left side

28. Deep Depletion with Substrate Bias • Chanstop and bottom substrate electrically connected: bottom substrate cannot be biased • Chanstop and bottom substrate electrically isolated: bottom substrate can be biased

29. Advantages of Deep Depletion Vsub Vsub Front-illuminated – Improved high-energy x-ray response Back illuminated – Increased vertical E fields  tighter PSF

30. Measured Depletion Depths • Deep-depletion CCD (CCID-42) • 512512, 15-µm-pixel frame-transfer device • Same pinouts as all previous 5122 devices • Data taken from capacitance-voltage test structure that represents CCD layout • Depletion depths > 150 µm easily achieved

31. X-ray Data from MIT/CSR • Si absorption length at 22 keV=1370 µm • QE  depletion depth • Improved detection of high-quality events • ~2 count increase consistent with ~2 increase in depletion depth

32. Deep Depletion • Current plans • Thin four CCID-42 chips to varying thicknesses (60 – 150 µm) • Measure QE and PSF vs. thickness and substrate bias (data relevant to Pan-STARRS) • Devices ready for test in 2 – 3 months • Backside treatment: II/LA or chemisorption charging? • OTA design can include deep-depletion option

33. Why OTA/CMOS Hybrid? • Advantages • Risk reduction if monolithic OTA fails • Simplifies OTCCD processing (no NMOS, large network of metal lines) • Pathfinder for more on-sensor processing in CMOS • Possible improvement in fill factor • Disadvantages • Additional costs: CMOS design, fab & test, bump bonding • Adjustments to thinning process

34. Hybrid OTA OTA chip CMOS control chip Bump bonding Hybrid OTA/CMOS sensor

35. OTA Design for CMOS Hybrid • Optional metal mask brings all OTA cell functions to pads for bumping • Can be done as process split (one mask change, two mask steps deleted) • Fill factor uncertain; better than baseline but maybe not as good as metal-over-pixels • Space allows double pads/function for redundancy OTA cell

36. CMOS/OTA Attachment 3. Attach handle wafer (with epoxy) 4. Flush-thin CCD wafer 1. Attach CMOS die to OTA wafer 2. Underfill with epoxy

37. OTA/CMOS Hybrid Sensor After CCD thinning, pad-via etch, back-surface treatment After bonding

38. Mini OTAs • MOTA is 22-cell version of OTA • Opportunity to try advanced features • Two-phase serial register • Dual output gates • Higher performance logic designs • Useful as monitor devices (QE, PSF, etc.) • ~12 die possible • Fits in conventional PGA Tentative wafer layout

39. 12m 12m Type 1 Type 2 10m 10m Type 1 Type 2 Process Splits for Lot 1

40. Test Plan for OTAs • Device selection phase (Mar –Jun 2004) • Approximately 16 different devices • Pixel size, CMOS logic, deeper depletion, shutter, etc. • MOTAs with more exotic choices • Serious, thorough testing necessary during a short time which will lead to final, production OTA design choices. • Production testing (Oct 2004 – Oct 2006) • Wafer probe frontside for S/O (powered up?); excise cells? • Wafer probe backside, cold (packages are valuable) • OTA test, tune, and select • 300 OTAs need testing (20,000 CCDs!), so we need well designed, stable test setups, software, and analysis tools.

41. Design Selection Phase • Clocking / pixel performance • CTI • Pocket density • Cosmetics • Dark current • Image persistence • QE non-uniformity, fringing • QE • Full well • Clocking time constants • Charge diffusion • Goodies • Electronic shutter • 2-phase serial

42. Design Selection Phase • Amplifier performance • Read noise • 1/f knee – noise versus speed • Linearity • Amplifier / logic glow • Cross – talk: inter-cell, inter-OTA • Goodies: • pJFET?

43. Design Selection Phase • Package/cells • CR and radioactivity rate • Addressing • Cell excision: • Laser/ultrasonic excision? • Tri-state logic? • OTA metrology • Flatness • Placement of dice on package • Epoxy squeeze-out, epoxy voids • Bond wire masking / covering of bright scatterers

44. Hardware • Controller • New controllers must be thoroughly tested and understood! • Features • Alter clocking / bias voltages • Read out full chip or arbitrary binned subarray • 1 Mpix readout at 4e- noise • Slow readout for better noise • Dewars and focal planes • 1 OTA, quick swap • 2 x 4 OTA, 2 controller: test 2x2, QUOTA/OCTOTA for sky • MOTA package / testing • Test setup • X-ray source • Monochromator / photodiode (inside dewar?) • Optical images focussed on detectors (sub pixel?)

45. Testing Setup • Replace normal dewar window with “Snoutus Maximus” • Simultaneous x-ray stimulation from Fe55 or optical light • Complete test sequence in one 5 hour cool-down cycle

46. X-ray Testing – Qualitative

47. X-ray Testing - Quantitative Parallel CTI Pixel Value Pixel Value Row Row Serial CTI Column Column Perfect CTE Bad CTE

48. X-ray Testing - Quantitative • Slop2 • Determine and subtract bias level • Generate and subtract “local sky” • Reconstruct split pixels (if requested) • Identify K-alpha pixel values as a function of (x,y) • Fit a plane • Gain derived from plane intercept at (0,0) • Noise derived from overclocks • CTE parallel from y slope, CTE serial from x slope • Other features sometimes visible in pixel value plots

49. X-ray Testing - Quantitative • Plot pixel values as a function of column (x) and row (y) • K-alpha (1620 e-) and K-beta (1780 e-) are visible • Junk on right comes from a bright defect and associated bleed column.

50. X-ray Testing - Quantitative • Here’s the associated histogram of pixel values. K K