Introduction To CMOS Image Sensor Design

1 Introduction ToCMOS Image Sensor Design2 CCD vs. cmos Charge-Coupled Device Image Sensor Higher Quality Lower Dark Current Lower Noise Higher Dynamic Range (because of higher VDD) cmos Image Sensor Easier to Integrate with Periphery Lower Power Dissipation Higher Speed (Higher Frame Rate)3 Outline Introduction Basic Architecture Photo-Sensing Operation Design Characteristics Logarithmic Sensor High Dynamic-Range Sensor Digital Pixel Sensor (DPS)4 Architecture Of cmos Image SensorimagesensorarrayCDS(correlated double sampling)

2 MultiplexorAnalog-to-DigitalConverterdec oderandcontroller8-bit digital outputclockenablepixel5 Resolution Resolution CIF (352x288 ~ 10 ), VGA (640x480 ~ 30 ) SVGA (800x600 ~ 48 ), XGA (1024x768 ~ 79 ) Typical pixel size m process ( m) m process (5 mx5 m) m process (3 mx3 m)63-Transistor Sensor Cell Operation of a cell (1) Reset (2) Sensor cellresetRScolumnlinesourcefollowerVDDVD DRS: row selectphotodiodeXIph7 The Operation Of A CellXResetTime1 msweak illuminationstrongerilluminationVoltagea tnode XABCDE integration time8 Photo-Electrical CharacteristicIllumination (Lux)analogoutputafter CDS(volt) region5000250001000020000150000DR.

3 Dynamic RangeLinearregion9 Scanning 1 Row 2 Row 3 Last timeReset Signal For Each Row10 Sensor CharacteristicsQuality ofCMOS Image SensorResolutionPixel TypePixel SizeFill FactorADC (Quantum Efficiency)Conversion GainConfigurationResponsivityTransferFun ctionNoiseSensitivitySaturation LevelDynamic RangeDark SignalTemporal noiseFixed-Pattern Noise11 Pixel LayoutPhotodiodePhotodiodeResetResetVddV ddOutputOutputnn++P-substrate is groundedFOXFOXFOXFOXAA Reset-Ring isolates diode from FOX Helps reduce the dark currentCross-Sectional ViewTop ViewXxRSRS12 Fill Factor The ratio between the light sensitive pixel area and the total pixel pixel area.

4 5 m x 5 mFill factor 40%PhotoSensingArea13 Signal Path: Light To Digital CoderesetcolumnlineVDDVDDXn+n+P-substrat eLight Source (Lux)ElectronsAnalog Voltage (V)Digital Code (8-bit)Quantum Efficiency(e-/ photon)Conversion GainAnalog-To-Digital ConversionRS14 Sensor CharacteristicsQuality ofCMOS Image SensorResolutionPixel TypePixel SizeFill FactorADC (Quantum Efficiency)Conversion GainConfigurationResponsivityTransferFun ctionNoiseSensitivitySaturation LevelDynamic RangeDark SignalTemporal noiseFixed-Pattern Noise15 Sensitivity VResetOutputTintLight intensity (lx)Voltage Drop (V/s)Slope = sensitivity( V / lx-s) Sensitivity is the ratio of voltage response to the photo energy Current Dark current is the photo-detector leakage current under no illumination.

5 Dark signalResetResetOutputOutputTTintintnn++ LOCOSLOCOSpp--subsub17 Noise of cmos Imager Temporal noise : Thermal noise ~ reset, readout. Shot noise, 1/f noise ~ integration. Substrate noise ~ readout. Spatial noise : Fixed pattern noise ~ process Noise FPN is the spatial variations at pixel outputs Due to the device parameter variations (non-uniformity) Pixel FPNP ixel FPNC olumn FPNC olumn FPN19 Cell Array and CDSresetRSCDSCDSCDSCDSC olumn-parallel Correlated Double Sampling (CDS) Circuitry20 CDS (Correlated Double Sampling)Goal.

6 To eliminate the Fixed-Pattern Noise(FPN) due to pixel mismatchtimevoltageintegration phaseResetVxV = Vx VresetV = 0 Vreset VxpixeloutputCDSoutputSHSELCLC2C1resetRS CLSH vreset21 CCD vs. cmos Charge-Coupled Device Image Sensor Higher Quality Lower Dark Current Lower Noise Higher Dynamic Range (because of higher VDD) cmos Image Sensor Easier to Integrate with Periphery Lower Power Dissipation Higher Speed (Higher Frame Rate)22 Optimization of Transfer Curve (Step 1) Logarithmic Output Response (Step 2) Dynamic Range EnhancementIllumination (Lux)Output(volt)enhanceresponseunderlow -lightIllumination (Lux)Output(volt)23 Logarithmic Sensor (1/2)24 Logarithmic Sensor (2/2)

7 Linear ADCmulti-resolution ADC25 Optimization Of Transfer Curve (Step 1) Logarithmic Output Response (Step 2) Dynamic Range EnhancementIllumination (Lux)Output(volt)enhanceresponseunderhig h-lightIllumination (Lux)Output(volt)higherdynamicrange26 High Dynamic-Range ImageOriginalHigh-Dynamic-Range27 Outline Introduction Logarithmic Sensor Logarithmic Sensor Cell Multi-Resolution ADC High Dynamic-Range Sensor Digital Pixel Sensor (DPS)28 Logarithmic Sensor CellcolumnlinephotodiodeXVDDVDDIphCircle d transistor is in sub-threshold region: Vx= VDD ln (Iph/I0)Main Problems.

8 (1) mismatches harder to resolve(2) small output swing29 Low-Cost Nearly Logarithmic Sensor (1) Pixel-Level Logarithm much larger pixel(2) Chip-Level Logarithm via multi-resolution ADCI llumination (Lux)CDS outputcodeIllumination (Lux)x=outputcodeanalog inputto ADC25530 Multi-Resolution ADCL inear ADCM ulti-Resolution ADC31 Spectrum of ADCS peedAccuracyintegratingSuccessiveApproxi mationFlash ADC,Pipelined ADC8~10 bits3 M-sample/s32 The Problem of ADCD igital Code0168412 Input: Given an analog voltage, say : What is the digital output code?

9 Analog Voltage0 V1 V33 Successive Approximation (SA)Narrow down the possible code rangesstep-by-step0 V1 V0 V1 V0 V1 V0 V1 VOutput bitD3=0D2=1D1=0D0=1 Final output code for V is D3 D2 D1 D0 = 0101 Architecturecom-parator10-bitDACS/HclkVi ncontrollogic10-bitSARclkLinear ADC10-bitcode8-bit10-bit1024 x 8mappingtableTable lookup8-bitcodeAnalogPivot Voltage35 Interleaved Architecturecom-parator10-bitDACS/HclkVi ncontrollogic8-bitSARclk256 x 10mappingtable8-bitcodeAdvantages.

10 (1) conversion timeis reduced from 11 to 9 cycles(2) table sizeis reduced to ~ 1/48-bit10-bit36 Modified Successive ApproximationDigital CodeAnalog Pivot (110)21st pivot3rd pivot2nd pivot81 LayoutCDSBufPeripheryCon-troller176 x 144 SensorArrayADCBISTBIST: Built-In Self-Test38 Outline Introduction Logarithmic Sensor High Dynamic-Range Sensor (1) Well Capacity Adjusting (2) Conditional Reset (3) Two-Frame Composition Digital Pixel Sensor (DPS)39A High Dynamic-Range Sensor CellVoutRef: Steven Deckeret.