Image Sensor Technologies

Chris SolteszSSE DeluxeSony 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.

They’re 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

Color Primaries

BLUE

GREEN

RED

R

G B

Y

C

MWHITE

1

111 1

1111

111

000

0000

0000

0

Dichroic Prism

White Light

Blue Imager

Green Imager

Red Imager

Additive Color

Image Sensor Technologies

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 MechanismThe difference is in the strategies and mechanisms developed tocarry out those functions.

Light-to-charge Conversion

ChargeAccumulation

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

V

QC VCQ 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

Photo Sensor (b) (Light-sensitive Region)

Light

Charge

Signal

Amplifier

(y)Pixel

(a)

Column Signal Wire (f) (Micro Wire)

Pixel-select Switch (e)

ONON ON

Output ON

Column

Circuit (h)Row Signal Wire (i) (Micro Wire)

Column-select Switch

(g)

Pixel Row (j)

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 CCDCCDCCD

Charge

Charge Transfer

ChargeCharge

Charge

Gate GateOutput 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

Photo Sensor (b) (Light-sensitive Region)

Light

Charge

Signal

Amplifier

(y)Pixel

(a)

Column Signal Wire (f) (Micro Wire)

Pixel-select Switch (e)

ONON ON

Output

ON

Column

Circuit (h)Row Signal Wire (i) (Micro Wire)

Column-select Switch

(g)

Pixel Row (j)

CMOS Image Sensor

High

0 V

Vo

ltag

e

Surface Voltage

Photo Sensor

Light

Surface Voltage to Amplifier

High

0 V

Vo

ltag

e

Surface Voltage

Photo Sensor

Surface Voltage to Amplifier

When Charge is NOT Accumulated in Photo Sensor

When Charge is Accumulated in Photo Sensor

Fig. A Fig. 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. C Fig. 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

AmplifierPixel

Pixel-select Switch

ONON ON

Column-select Switch

Channel 1

Channel 3

Channel 2ON

Row Signal Wire (Micro Wire)ON

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

P+N N N

P+

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 Gate

Gate

Gate

Drain Drain

Signal Wire

HAD-type CMOS Image Sensor

Source

SiO2

Poly-Si

Micro Condensing Lens

CCD SensorStructure

On-chipMicro-lens

Hyper HAD CCD Power HAD CCD

Micro Condensing Lens

Power HAD EX ImagerPerformance 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)

Fig.-1 Power HAD CCD Sensor Construction

Poly Si

V-register

On-Chip-Micro lens

Poly Si

Sensor V-register

Fig.-2 New CCD Sensor Construction

Internal Lens

On-Chip-Micro lens

Sensor

Photo-Shieldingfilm

Photo-Shieldingfilm

Thinner Insulation Film

Pros and Cons of Image Sensors

Pros & Cons of Imagers

Feature Comparison

Feature CCD CMOS

Signal out of pixel Electron packet Voltage

Signal out of chip Voltage (analog) Bits (digital)

Signal out of camera Bits (digital) Bits (digital)

Fill factor High Moderate

Amplifier mismatch N/A Moderate

System Noise Low Moderate

System Complexity High Low

Sensor Complexity Low High

Camera components Sensor + multiple support chips + lens Sensor + lens possible, but additional support chips common

Relative R&D cost Lower Higher

Relative system cost Depends on Application Depends on Application

Pros & Cons of Imagers

Performance Comparison

Performance CCD CMOS

Responsivity Moderate Slightly better

Dynamic Range High Moderate

Uniformity High Low to Moderate

Uniform Shuttering Fast, common Poor

Uniformity High Low to Moderate

Speed Moderate to High Higher

Windowing Limited Extensive

Antiblooming High to none High

Biasing and Clocking Multiple, higher voltage Single, low-voltage

Pros & Cons of Imagers

Winding Path of CMOS Development's

Initial Prediction for CMOS Twist Outcome

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 integration Longer 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 consumption Steady improvement in CCDs Advantage for CMOS, but margin diminished

Reduced imaging subsystem size Optics, 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