imec 2007 1
Radiometric Performance Enhancement of APS 3 rd Microelectronic Presentation Days, Estec, March 7-8, 2007
Outline Introduction Backside illuminated APS detector Approach CMOS APS (readout) design: Sensor and pixel architecture Readout modes Technological developments Thin wafer processing Special features on hybrid arrays Backside surface treatment AR coating Conclusions imec 2007 3
Introduction European Space Agency funded project Hybrid APS (ITT AO/1-3970/02/NL/EC) Partners: FillFactory/Cypress: design IMEC: technology development Galileo Avionica: radiometric characterization Aim: snapshot shutter CMOS APS demonstrator for high-end spaceborne imaging Hyperspectral imaging 2-D sensor with spectrometer slit: Spatial information on x-axis Spectral information on y-axis Spatial information by scanning image cube imec 2007 4
Backside illuminated detector: approach Monolithic approach backside thinned CMOS readout array mounted on MCM substrate using Au stud bumps MCM substrate Hybrid approach ROIC backside thinned diode array flipchip integrated using In bumps CMOS readout array imec 2007 5
Backside illuminated detector: CMOS APS design Sensor architecture synchronous pipelined shutter 22.5 µm pixel pitch stitched design: 512 x 512 pixels stitch blocks up to 2048 x 2048 pixels pseudo-differential output per 256 columns 20 Mpixels/s per output SPI interface for upload: addressing, gain & offset, NDR, non-linear amplifier, etc. imec 2007 6
Backside illuminated detector: CMOS APS design monolithic C ph determines charge handling capacity (FWC) on-chip CDS with full well synchronous and conversion charge pipelined gain shutter (x 1000 electrons) (FWC) C 1,2,3 determine read noise and dynamic range hybrid in-pixel storage capacitance (ff) 350 350 150-200 250-1000 read noise (electrons) 15-20 25-100 dynamic range (FWC/dark noise) (db) 80 80 maximum SNR 388-447 500-1000 imec 2007 7
Backside illuminated detector: CMOS APS design single pixel photodiode pad for hybridization sample capacitors imec 2007 8
Backside illuminated detector: normal readout mode synchronous shutter: all pixels integrate in parallel pipelined: readout while integrate correlated double sampling T int < T read C 1 and C 3 : reset level C 2 : signal level T int > T read imec 2007 9
Backside illuminated detector: Optimized readout modes Non-destructive readout (NDR) Line by line variable integration time Ref.: J. Bogaerts et al.: 2005 IEEE Workshop on CCD and Advanced Image Sensors, Nagano, Japan, June 2005 imec 2007 10
Technology development & challenges Realized through stitching stepper lithography 512 2, 1024 2 and 2048 2 pixel arrays Readout: 0.35 µm technology @ commercial foundry Hybrid diode array: 0.13 µm technology @ Imec imec 2007 11
Technology development & challenges: thin wafer processing (post-) processing of thin wafers (35 um) Backside thinning with excellent thickness (uniformity) control (< 1 um) use of temporary carrier for thin wafer handling Ref.: K. De Munck et al., IMAPS Device Packaging 2006, IEDM 2006 Device wafer 1 st Glue layer 1 st Carrier Glue layer deposition Wafer bonding Wafer thinning Debonding & frontside processing 2 nd Carrier 2 nd Glue layer Wafer bonding Backside processing imec 2007 12
Technology development & challenges: special features Hybrid diode array: special features Graded epi for built-in electrical field: enhanced charge collection at low voltage operation Doped poly-si filled trenches for cross-talk reduction frontside backside built-in electric field doped poly-si filled trenches 22.5 µm built-in electric field imec 2007 13
Technology development & challenges: backside passivation Backside surface treatment after thinning Damage removal by dry/wet etch Shallow (50nm) backside implantation Dopant activation by laser annealing 100 90 80 implant + anneal: simulated measured before treatment implant + anneal: measured Quantum efficiency [%] 70 60 50 40 30 20 10 nm 20 nm after treatment 10 um thick epi before treatment 25 um thick epi 10 50 nm doped layer thickness 100 nm 0 200 300 400 500 600 700 800 900 1000 1100 1200 Wavelength [nm] imec 2007 14
Technology development & challenges: AR coating Optimized broadband ARC Reflectivity <3% @ 400 to 850nm Spectral response: 60% >80% Reflectivity (%) 10 1 ARC: 50nm ZnS + 100nm MgF2 Measured Simulated Spectral response spec achieved @ 450-850nm 400 600 800 Wavelength (nm) 1000 imec 2007 15
Technology development & challenges Working detectors realized: COB package Hybrid: both 512 2 and 1024 2 pixels Monolithic: 1024 2 pixels imager 40 µm MCM > 99.9 % pixel yield Readout: 2048 2 pixels imec 2007 16
Conclusions Thinned backside illuminated imagers realized Hybridized and monolithic CMOS APS: synchronous pipelined shutter with true CDS New 3D process technology for thin wafer handling and processing Use of temporary carriers and glues 200 mm thin wafer processing on carrier with standard equipment Performance enhancing concepts implemented Graded EPI lower cross-talk and improved QE for same voltage Poly-Si filled pixel separating trenches low cross-talk Working demonstrators Detailed characterization is currently ongoing imec 2007 17
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