Visible Light CMOS Image Sensors

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Visible LightCMOS Image SensorsDr. Eric R. FossumSenior Fellow, Micron ImagingMicron Technology, Inc.1

Outline4 Introduction to CMOS image sensor pixels4 Examples of sensors4 Customer trends4 CMOS image sensor technology trend4 Thoughts on next 5 yearsSeptember 032

3Transistor (3T) CMOS APS4 Use source-followerM1RST“amplifier” to drivecolumn busIpdVpixM2SFVpdCpdColBusM3RSVssVoutILSeptember 033

Voltage on 3T Pixel vs. TimePhotodiodeVoltageintegration startspixel reset level sampled3VNet2Vsignal1Vsignal sampled0V33 ms66 msPixel sampled twice to remove variations in threshold voltageof pixel source-follower and reset levelSeptember 034

Simplified Signal ChainPhotodiodeVoltageintegration startsPIXELVDDpixel reset level sampled3VNet2VsignalRST1Vsignal sampled0V33 msRSCOL BUSSHS66 msGLOBALCSSIGVLPHOR BUSSHRRSTVLNPER COLUMNSeptember 035VLP

Imaging System on a ChipDigital Logic for User Interface Sensor Setup Timing Generator Digital SignalProcessing–Color Processing–White Balance–Image Enhancement Data OutputFormattingCMOS Active Pixel Color Imaging ArrayAnalog SignalProcessing Data Sampling Noise Reduction GainSeptember 03Analog-To-Digital Conversion6

Relative responseSpectral Response of Eye and Silicon10.90.80.70.60.50.40.30.20.10400Human EyeSilicon500600700800900Wavelength (nm)UVSeptember 03NIR710001100

Fill Factor4 A pixel is divided into a sensing portion and a readoutportion4 Fill factor is the ratio of sensing area to total area andis typically about 20-30%Total pixel areaSensitive areaSeptember 038

Color Filter Arrays and MicrolensesMicrolens layerColor filter layerMetal opaque layerPhotodiodeSilicon substrateSeptember 039

Resolution and Sensitivity4 More pixels high resolution4 Smaller pixels less sensitivitySceneImagePhotons/pixelPixel pitchSeptember 0310

Optical Formats7.8 mm105.6 mm3.6 mmPixel Size (um)CIFVGA1.3M2.1M“1/4 in”3.3M4.1M2.8 mm11/71/61/51/41/3Lens FormatSeptember 03111/22/3

Pictures from PC Camera SensorsSeptember 03CIF ResolutionVGA Resolution(352 x 288)(640 x 480)12

1.3Mpixel High Speed Sensor41280 ADCs per chip45T pixel4500 pictures persecond at fullresolution4Shutter from 1/30th secto 1/100,000th sec4Shutter efficiency 99.99%4Rotating fan image ‡1/33,000 sec 30 usecFreeze frame shutterSeptember 0313

Pill CameraSensor and US 0.10Pill Camera and US 0.254 Pixel Format: 256 X 2564 Pixel Size: 10 µm X 10 µm4 Frame Rate: 2 fps4 ADC: On-Chip, 8 bits4 Power Supply: 2.8 VKrymski 1998September 034 Power: 3 mW14View inside smallintestine

4 Mpixel sensor 240fps ERS44444442352 x 17287 mm x 7 mm pixel pitch16x10b digital output240 fps ERS960 Mbytes/sec at 66 MHz4000 bits/lx-sec3.3 volt operationKrymski 2001September 0315

Customer Drivers4 Incredibly fast price point erosion (2Mpixel CCDnow sells for 8 in Japan)4 Voltage scaling required to be compatible withcompanion chips (3.3V - 2.8V - 1.8V)4 Power dissipation 200 mW/megapixel forportable apps4 CCD performance sets benchmark for DSCapplications (SNR, dark current, etc.)4 CMOS performance fine for PC and wirelessapplicationsSeptember 0316

A Very Small Digital Camera in 19954 2nd generation APScamera4 JPL 256x256 element APS4 On-chip timing/control4 On-chip FPN suppression4 Separate 10-bit ADC4 Separate FPGA4 Serial digital camera I/O17

Cell Phone Cameras Are SmallSeptember 0318

Technology Challenges4 Smaller pixels4 Maintain or improve SNR with smaller pixel4 Maintain or increase dynamic range with loweroperating voltages4 Reduce dark current4 Reduce number of defective pixels4 Reduce power dissipation in analog and digital circuitswhile increasing functionalitySeptember 0319

Diffraction Limit201816LENSSize (microns)14Cheap Lens Resolution(30 lp/mm)F/1112Airy Disk DiameterD 2.44 F#108F/2.86420400High Performance Lens Resolution(120 lp/mm)BBGG500RR600700800Wavelength (nm)September 032090010001100

Alignment to Mainstream Technology4 CMOS APS starts diverging from mainstream CMOS toSystem Miniaturization Costimprove pixel performanceMaCCDsinstreamCMOSTechnologCMOS APSyWindow ofOpportunity1970September 0319801990212000

Buried PhotodiodesConventionalPhotodiodeNSeptember 03BuriedPhotodiodePP22NP

4T Buried PhotodiodeTGBPDTGFDBPDFD4Reset Pixel4Integrate4Readout4 Reset FDTGBPDTGFDBPD4 Read FDFD4 Transfer Q4 Read FDDVfdSeptember 0323

Comparison of PixelsThree-transistor (3T)Four-transistor (4T)photodiode-type pixelpinned-photodiode-type pixel fewer transistors easier to implement better under good light- poorer under low lightRST- more transistors- challenge to implement- poorer under good light better under low lightRST“Amp”“Amp”TGPhotodiodeSeptember 03SELSELBuriedPhotodiodeCOLBUS24COLBUS

Image Sensor Revenue Projections 4,335 4,500.00 4,000.00 3,477 3,500.00 3,000.00 2,468 2,500.00 2,000.00 1,500.00 1,645 1,379 1,000.00 500.00 0.00200020012002Sources: CD analysis based on OEM and industry research reportsSeptember 032520032004

CMOS Image Sensor Market Growth 1,500 1,443ToysMachine VisionSecurity/Biometrics in Millions 1,000Optical MiceVideo CamerasOtherPC CamerasMobile PhonesDigital Still Cameras 500 020012002Sources: CD analysis based on OEM and industry research reportsSeptember 032620032004

CMOS Vs. CCD Market r 0320032004200527200620072008

4CMOS APS starts diverging from mainstream CMOS to improve pixel performance 1970 1980 1990 2000 CCDs CMOS APS Mainstream CMOS Technology Window of Opportunity System Miniaturization Cost. September 03 22 Buried Photodiodes N P P N P Conventional Photodiode Buried Photodiode.

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