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)MultiplexorAnalog-to-DigitalCon verterdecoderandcontroller8-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)

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: 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:5 m x 5 mFill factor 40%PhotoSensingArea13 Signal Path.

3 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. Dark signalResetResetOutputOutputTTintintnn++ LOCOSLOCOSpp--subsub17 Noise of CMOS Imager Temporal noise : Thermal noise ~ reset, readout. Shot noise, 1/f noise ~ integration.

4 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: 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)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)

5 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:(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?

6 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:(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.

7 Al., A 256x256 CMOS Imaging Array WithWide Dynamic Range Pixels and Column-Parallel Digital Output, IEEE Journal of Solid-State Circuits, Vol. 33, No. 12, Dec. (t)v1 High Dynamic Range is achieved by dynamic reset40 Cross-Section Viewn+n+n+n+n+photo-diodechargespill gatelateraloverflow gatechargesenseDiffusion VDDrowselectcolumnoutput p-substrate(1) Rest is now called lateral overflow gate(2) Another charge spill gateis added41 Well Capacity Adjusting (WCA)n+n+n+n+n+ VDDC ross-Section0V5 Vpotential 1Vb(t)Spill gateOverflowgatedrainThe well capacity at is controlled by b(t)42 Transfer Function of WCA Overflow barriertimeLight illuminationCharge atsense diffusion xtimeb(t) Control Signalb(t)43 Outline Introduction Logarithmic Sensor High Dynamic-Range Sensor (1) Well Capacity Adjusting (2) Conditional Reset (3) Two-Frame Composition Digital pixel Sensor (DPS)44 Conditional ResetRef: Sung-Hyun Yang, Kyoung-Rok Cho, High Dynamic Range CMOS Image Sensor with Conditional Reset.

8 IEEE 2002. High Dynamic Range was achieved by (1) Multiple Sampling (2) Conditional ResetxA pixel is reset againwheneverits value is lower than a referencevoltage at certain sample Sensor Cell with Conditional ResetLightVDDRowSelectC-Reset(Vsample> Vref)ColumnLineVDDP ixelResetThe pixel is conditionally reset whenC-reset andRow-Selectare both on46 Extend Dynamic Range By N TimesT2T4T8 TIntegrationtimeI2I4I8 IPixelVoltage1st2nd4th3T5T6T7 TNormal integration period is 8 TInformation to be recorded: The first conditional : pixel ResponseIntegrationtimePixelVoltageInteg rationtimePixelVoltageVrefVrefT2T4T8T3T5 T6T7TT2T4T8T3T5T6T7 TFinal output = (V1) * 8/2 = 4V1V1 Final output = (V2) * 8/2 = 4V2V248 Outline Introduction Logarithmic Sensor High Dynamic-Range Sensor (1) Well Capacity Adjusting (2) Conditional Reset (3) Two-Frame Composition Digital pixel Sensor (DPS)49 Effect of Integration TimeLight intensityPixel outputCurve 1: (longer integration time)Curve 2: (shorter integration time)DR of curve 2DR of curve 1DR.

9 Dynamic range50 Logarithmic ResponsesLight intensityPixel outputLow-FrameHigh-Frame51 Two-Frame Composition An Image is composed of two frames Low Frame (focuses on low-light part) High Frame (focuses high-light part)Low-frame value xHigh-frame value y+=Final value zSensitive to dark objectsAnd also responsive to bright objectsSensitive to dark objectsResponsive to bright object52 Composition RuleLow-frame value xHigh-frame value y+=Final value zComposition Ruley81x87z+=Low-framehi0255 Code Range Distribution22353 Composite ImageRef: O. Schrey, J. Huppertz, G. Filimonovie, A. Bu mann, W. Brockherde, Hosticka A 1k x 1k High Dynamic Range CMOS Image Sensor with on-chip Programmable Region-of-Interest readout. IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 37, NO. 7, JULY Introduction Logarithmic Sensor High Dynamic-Range Sensor Digital pixel Sensor (DPS)55 Digital pixel Sensor (DPS) Granularity of ADC Chip Level Column Level pixel Level, ( , one ADC per pixel ) Digital pixel Sensor Digital pixel Output Snap-shot Image acquisition (High Speed) Scalable Lower Power Immune to the read-out noise (or column FPN)ADC8-bitmemoryDigital PixelDigitalOutput56 pixel Schematic for DPS* thick-oxidetransistorsstorageStuart Kleinfelder, SukHwan Lim, Xinqiao Liu, and Abbas El Gamal, A 10000 Frames/sCMOS Digital pixel Sensor , IEEE Journal of Solid-State, Vol.

10 36, pp. 2049-2059, Dec. ADCC ounter value is loaded into memorywhen ramp meets pixel valuePixel value58 Operation SequencesResetIntegrateADCReadSingle Sampling:ResetIntegrateADCReadDigital Correlated Double Sampling:ADCReadMultiple Sampling (To Enhance Dynamic Range) of Digital CDSF ixed-Pattern NoiseWithout Digital CDSWith Digital CDSThe integration time is set to 1 ms60 Characteristics Of A DPS ChipnMOS Photo-GatePhoto-Detector37No. of millionNumber of Transistors64-bit (167 MHz)Readout Architecture> GB/sMax Output Data Rate> 10,000 frames/sMax. Frame Rate15% Sensor Fill x umPixel Size> 1 G-pixels/sMax. pixel Rate352 x 288 pixels (CIF)Array Size5x5 mmDie 5-metal CMOST echnology61 Conclusions CIS (CMOS Image Sensor ) over CCD Ease of integration with periphery Lower power dissipation CIS Optimization Just like human eyes Logarithmic Sensitivity High Dynamic Range (50dB 70dB 90dB) Low-Noise and High Speed via DPS