Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53UC Regents Spring 2007 © UCBCS 250: Silicon Imagers2007-3-22John Lazzaro www.cs.berkeley.edu/~lazzaroCS 250 - VLSI SystemsCMOS Imagerswww.cs.berkeley.edu/~johnw/courses/cs250/UC Regents Spring 2007 © UCBCS 250: Silicon ImagersSilicon imagers are everywhere.2006: 1.9 billion units, 7 billion USDSource: www.icinsights.com60% based on a CMOS process2008: 80% of imagers will be CMOS90% of imager revenue will be CMOS2010: 3.6 billion units, 12.8 billion USDToday’s Lecture: Markets & TechnologyUC Regents Spring 2007 © UCBCS 250: Silicon ImagersSource: www.elecdesign.comMotorola Q Smart Phone0.45inchesMoto predicted 3M shipped Q4 2006.UC Regents Spring 2007 © UCBCS 250: Silicon ImagersCamera module cost: $74.4% of total cost ($158)UC Regents Spring 2007 © UCBCS 250: Silicon ImagersFrontBackSource: Portelligent Inc.UC Regents Spring 2007 © UCBCS 250: Silicon ImagersCamera module slides into PC board cutoutUC Regents Spring 2007 © UCBCS 250: Silicon ImagersTypical camera module for the Micron MT9M1110.27 inches deep.Fixed-focus.No “optical” zoom.0.37 inch x 0.37 inch squareSource: www.asia-optical.com.twUC Regents Spring 2007 © UCBCS 250: Silicon ImagersUC Regents Spring 2007 © UCBCS 250: Silicon ImagersMicron MT9M111* - 1.3 MPixel CMOS Imager* Photo a closerelative (MT9M011)0.25 inchesPixel size: 3.6 μm x 3.6 μm 1280 x 1024 pixels. Each pixel is R, G, or B. So, 2/3 ofRGB imagedata is interpolated.UC Regents Spring 2007 © UCBCS 250: Silicon ImagersSilicon PhotosensitivityThe physics of sensing photons ...UC Regents Spring 2007 © UCBCS 250: Silicon ImagersIntrinsic silicon: a weak conductorHoles (positive carriers)ElectronsValence bandConduction bandelectron energyBand gap energy.Silicon Eg = 1.1 eVValence bandConduction bandelectron energy hν > EgSufficiently energetic photons createelectron-holepairs.Si is an “indirect” material - falling e- creates heat, not light.UC Regents Spring 2007 © UCBCS 250: Silicon ImagersBand-gap bounds the λ sensitivityHoles (positive carriers)ElectronsValence bandConduction bandelectron energyBand gap energy.Silicon Eg = 1.1 eV700 nm600 nm 500 nm 400 nmλ = 1000 nmSince Eg = hc/λ, photons with λ < 1000 nm can excite a valence band electron to the conduction band.From infrared through the visible band.InfraredCS 152 L5: Timing UC Regents Fall 2006 © UCBThis is how “photo cells” work ...SemiconductorA mainstay of 100-in-1 experimenter kits hνSchematic symbolA photosensitive resistor. The semiconductor CdS is often used, as its band gap falls in blue-green, and thus blocks infrared w/o a filter.Surface open to light.UC Regents Spring 2007 © UCBCS 250: Silicon ImagersFor ICs, photodiodes a better match n+p-Depletion region hνoxideoxideMetal shieldMetal shieldReadout circuits go hereOptically-generated electrons near depletion region swept to the right. hνn+ regionp- regiondepletion regionelectron energyOptically-generated holes near depletion region swept to the left. hνUC Regents Spring 2007 © UCBCS 250: Silicon ImagersPhotodiode I-V curves ...+-I V“Dark current” When no photons are present.Quadrant for photosensing.Quadrant for solar cells.I = Io (eV/Vo - 1) - IphIo : 1-20 fAVo : 25-60 mVUC Regents Spring 2007 © UCBCS 250: Silicon ImagersSpectral response (0.35μm n-well CMOS)Source: “A 640 512 CMOS Image Sensor with Ultrawide Dynamic Range Floating-Point Pixel-Level ADC”,David X. D. Yang, Abbas El Gamal, Boyd Fowler, and Hui Tian, JSSC, Dec 1999.Data shown is for a standard 0.35μ CMOS logic process.Quantum efficiency can be improved by modifying the process.42% of photons that fall on the photodiode are converted to electrons (“quantum efficiency”)UC Regents Spring 2007 © UCBCS 250: Silicon ImagersProcess improvements for Q.E. 75%Claim: These slopes represent technology limits.Designed for machine vision. Extended IR response is a feature.UC Regents Spring 2007 © UCBCS 250: Silicon ImagersColorSi photodiodes “see” gray scale ...UC Regents Spring 2007 © UCBCS 250: Silicon ImagersColor filters deposited on pixel arraySource: Eric Fossum, IEEE Micro, and Micron Data Sheets“RGB Bayer”Why?UC Regents Spring 2007 © UCBCS 250: Silicon ImagersSource: Hofer et al, J. Neuroscience,25(42):9669-9679Human cone array, imaged through the eye.UC Regents Spring 2007 © UCBCS 250: Silicon ImagersPixel ScalingUC Regents Spring 2007 © UCBCS 250: Silicon ImagersCamera optics limits pixel scaling5μm5μmpixel5μm pixels match the optical resolving power of practical camera optical systems (1997, Fossum). 2007 figure may be smaller. Shrinking pixels beyond limit does not add resolution.Larger die sizes are the path to higher resolution.Process scaling helps imager arrays in a more subtle way ...UC Regents Spring 2007 © UCBCS 250: Silicon ImagersRecall: Photodiode design ...n+p- hνoxideoxideReadout circuits go here hν hνPhotons that reflect off metal shielding are lost.Fill factor = Photodiode areaPixel areaAs process shrinks, readout circuits shrink and diode grows. So, fill factor increases and fewer photons lost.UC Regents Spring 2007 © UCBCS 250: Silicon ImagersSource: www.bioschool.co.uk, BBC“Compound eyes” of an insect - “microlenses”UC Regents Spring 2007 © UCBCS 250: Silicon ImagersSource: http://micro.magnet.fsu.edu/UC Regents Spring 2007 © UCBCS 250: Silicon ImagersSource: Fraunhofer ISITUC Regents Spring 2007 © UCBCS 250: Silicon ImagersReadout CircuitsUC Regents Spring 2007 © UCBCS 250: Silicon ImagersThree-Transistor Active Pixel CellResetRow SelColumn SenseCdStep 1: Fill Cd, andsense column current. Edge circuitry samples current I(Qf) for later use.VddCdVddQfVddVddI(Qf)Parasitic photodiode capacitance.UC Regents Spring 2007 © UCBCS 250: Silicon ImagersOpening the electronic shutter ...Step 2: “Electronic shutter” is open, photodiode empties Cd.CdQf - ∫Qd(t) ResetRow SelColumn SenseGndCdGndQd(t) Too much ∫Qd(t), and we empty bucket before shutter closes. Not enough ∫Qd(t), and we capture temporal noise.Limits dynamic range and signal-to-noise.UC Regents