Image Sensor Technologies Chris Soltesz SSE Deluxe Sony Electronics, Inc. BPSD

Image Sensor Technologies An introductory guide to CCD and CMOS imagers

Image Sensors An image sensor is an electronic device that converts a image (light) to an electric signal. Theyre used in digital cameras and other imaging devices. Imagers are typically an array of charge-coupled devices (CCD) or CMOS sensors such as active-pixel sensors (APS).

Color Principles

Electromagnetic Spectrum

Spectral Characteristics

Color Primaries

Dichroic Prism White Light Blue Imager Green Imager Red Imager

Additive Color

Image Sensor Technologies

What is the difference between CCD & CMOS imagers? a) Generate and Collect Charge b) Measure Charge and turn into voltage or current c) Output the signal d) Transfer Mechanism The difference is in the strategies and mechanisms developed to carry out those functions.

Light-to-charge Conversion Charge Accumulation Photo Sensor (Light-sensitive Region) of a Pixel Charge Transfer Vertical and Horizontal CCD Amplifier behind Horizontal CCD Charge-to-voltage Conversion /Amplification Signal Wire (Micro Wire) Voltage Transfer Charge-to-voltage Conversion /Amplification CCD Image Sensor CMOS Image Sensor Amplifier within Pixel Mechanism Differences or Capacitance Equation C = Capacitance Q = Charge V = Voltage

Light Charge (Electrons) Photo Sensor (b) (Light-sensitive Region) Vertical CCD (c) Horizontal CCD ( d) Amplifier (x) Pixel (a) Output CCD Image Sensor

CMOS Image Sensor

Basic Mechanism of CCD Image Sensors

Light Charge (Electrons) Photo Sensor (b) (Light-sensitive Region) Vertical CCD (c) Horizontal CCD ( d) Amplifier (x) Pixel (a) Output CCD Image Sensor

Light Photo Sensor Gate Charge (Electrons) Gate Opens Vertical CCD Charge (Electrons) Charge Transfer- Photo Sensor to Vertical CCD

The transfer of charge in a CCD is similar to a bucket-brigade moving water CCD Charge Charge Transfer Charge

Gate Output Gate Gate Horizontal CCD Floating Diffusion (FD) Amplifier of CCD Image Sensor Charge Micro Wire Voltage Generated on Surface of FD Amplifier Output

Basic Mechanism of CMOS Image Sensors

CMOS Image Sensor Photodiode Active-Pixel Architecture (APS) Actual Photodiode Active-Pixel Architecture

CMOS Image Sensor

High 0 V Voltage Surface Voltage Photo Sensor Light Surface Voltage to Amplifier High 0 V Voltage Surface Voltage Photo Sensor Surface Voltage to Amplifier When Charge is NOT Accumulated in Photo Sensor When Charge is Accumulated in Photo Sensor Fig. AFig. B Charge Voltage Detection

0 V Amplified Voltage High V1 Gate Current Surface Voltage from Photo Sensor When Charge is NOT Accumulated in Photo Sensor 0 V Amplified Voltage High V2 V1 Surface Voltage from Photo Sensor Gate Lifts Signal Voltage When Charge is Accumulated in Photo Sensor Fig. CFig. D Voltage Detection

APS Block Diagram

CCD & APS Performance Improvements

CCD Image Sensor with 2-channel Horizontal CCDs Light Charge (Electrons) Photo Sensor (Light-sensitive Area) Vertical CCD Horizontal CCD 2 Amplifier 1 Pixel Channel 1 Channel 2 Horizontal CCD 1 Amplifier 2 Output

CMOS Image Sensor with 3-channel Outputs Column Signal Wire (Micro Wire) Photo Sensor (Light-sensitive Region) Light Charge Signal Amplifier Pixel Pixel-select Switch ON Column-select Switch Channel 1 Channel 3 Channel 2 ON Row Signal Wire (Micro Wire) ON Column Circuit Output

Technologies Used to Improve Performance of Image Sensors

Signal Charge HAL Signal Charge Free Electron Conventional Photo Sensor Buried-type Photo Sensor Free Electron

P+ N NNN Photo Sensor Read- out Gate Floating Diffusion (FD) FD Reset Gate FD Reset Drain Amplifier Pixel-select Switch N N P-type Si (Substrate) P+ (HAL) Gate Drain Signal Wire HAD-type CMOS Image Sensor Source SiO 2 Poly-Si

Micro Condensing Lens

CCD Sensor Structure On-chip Micro-lens Hyper HAD CCD Power HAD CCD Micro Condensing Lens

Power HAD EX Imager Performance Improvement With New CCD construction Improvement of Smear with thinner insulation membrane Power HAD CCD camera -125dB (Typical) Power HAD EX CCD camera -140dB (Typical) Thinner Insulation Film

Pros and Cons of Image Sensors

Pros & Cons of Imagers Feature Comparison FeatureCCDCMOS Signal out of pixelElectron packetVoltage Signal out of chipVoltage (analog)Bits (digital) Signal out of cameraBits (digital) Fill factorHighModerate Amplifier mismatchN/AModerate System NoiseLowModerate System ComplexityHighLow Sensor ComplexityLowHigh Camera componentsSensor + multiple support chips + lensSensor + lens possible, but additional support chips common Relative R&D costLowerHigher Relative system costDepends on Application

Pros & Cons of Imagers Performance Comparison PerformanceCCDCMOS ResponsivityModerateSlightly better Dynamic RangeHighModerate UniformityHighLow to Moderate Uniform ShutteringFast, commonPoor UniformityHighLow to Moderate SpeedModerate to HighHigher WindowingLimitedExtensive AntibloomingHigh to noneHigh Biasing and ClockingMultiple, higher voltageSingle, low-voltage

Pros & Cons of Imagers Winding Path of CMOS Development's Initial Prediction for CMOSTwistOutcome Equivalence to CCD in imaging performance Required much greater process adaptation and deeper submicron lithography than initially thought High performance available in CMOS, but with higher development cost than CCD On-chip circuit integrationLonger development cycles, increased cost, tradeoffs with noise, flexibility during operation Greater integration in CMOS, but companion chips still required for both CMOS and CCD Reduced power consumptionSteady improvement in CCDsAdvantage for CMOS, but margin diminished Reduced imaging subsystem sizeOptics, companion chips and packaging are often the dominant factors in imaging subsystem size CCDs and CMOS comparable Economies of scale from using mainstream logic and memory foundries Extensive process development and optimization required CMOS imagers use legacy production lines with highly adapted processes akin to CCD fabrication

Image Distortion with CMOS Camera CMOS Camera