1

Visible LightCMOS Image Sensors

Dr. Eric R. FossumSenior Fellow, Micron Imaging

Micron Technology, Inc.

September 03 2

Outline

4 Introduction to CMOS image sensor pixels4 Examples of sensors4 Customer trends4 CMOS image sensor technology trend4 Thoughts on next 5 years

September 03 3

3Transistor (3T) CMOS APS

M2SF

M1RST

M3RS

Vpd

Vout

ColBus

Vpix

IL

CpdIpd

Vss

M2SF

M1RST

M3RS

Vpd

Vout

ColBus

Vpix

IL

CpdIpd

Vss

4 Use source-follower“amplifier” to drivecolumn bus

September 03 4

Voltage on 3T Pixel vs. Time

Pixel sampled twice to remove variations in threshold voltageof pixel source-follower and reset level

integration starts

signal sampled

pixel reset level sampled

PhotodiodeVoltage

0V

1V

2V

3V

33 ms 66 ms

Netsignal

September 03 5

Simplified Signal Chain

SHS

SHR

SIG

RST

COL BUS

RS

RST

VDD

CS

HOR BUS

VLN

VLP

VLP

PIXEL

PER COLUMN

GLOBAL

integration starts

signal sampled

pixel reset level sampled

Photodiode Voltage

0V

1V

2V

3V

33 ms 66 ms

Netsignal

integration starts

signal sampled

pixel reset level sampled

Photodiode Voltage

0V

1V

2V

3V

33 ms 66 ms

Netsignal

September 03 6

Imaging System on a ChipCMOS Active Pixel Color Imaging ArrayDigital Logic for

• User Interface• Sensor Setup• Timing Generator• Digital Signal Processing

–Color Processing–White Balance–Image Enhancement

• Data Output Formatting

Analog Signal Processing• Data Sampling• Noise Reduction• Gain Analog-To-Digital Conversion

September 03 7

00.10.20.30.40.50.60.70.80.9

1

400 500 600 700 800 900 1000 1100Wavelength (nm)

Rela

tive

resp

onse

Human Eye

Silicon

NIR

Spectral Response of Eye and Silicon

UV

September 03 8

Fill Factor4 A pixel is divided into a sensing portion and a readout

portion4 Fill factor is the ratio of sensing area to total area and

is typically about 20-30%

Total pixel areaSensitive area

September 03 9

Color Filter Arrays and Microlenses

Microlens layer

Color filter layerMetal opaque layerPhotodiodeSilicon substrate

September 03 10

Scene Image

Resolution and Sensitivity

Photons/pixel

Pixel pitch

4 More pixels = high resolution4 Smaller pixels = less sensitivity

September 03 11

1

10

1/7 1/6 1/5 1/4 1/3 1/2 2/3

Lens Format

Pix

el S

ize

(um

) CIF

VGA

1.3M

2.1M

3.3M

4.1M

Optical Formats7.8 mm5.6 mm3.6 mm

2.8 mm

“1/4 in”

September 03 12

Pictures from PC Camera Sensors

CIF Resolution(352 x 288)

VGA Resolution(640 x 480)

September 03 13

1.3Mpixel High Speed Sensor

1/33,000 sec = 30 usecFreeze frame shutter

41280 ADCs per chip45T pixel4500 pictures per

second at fullresolution

4Shutter from 1/30th secto 1/100,000th sec

4Shutter efficiency >99.99%

4Rotating fan image ‡

September 03 14

Pill Camera

4 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 V4 Power: 3 mW

Sensor and US$0.10 Pill Camera and US$0.25 View inside smallintestine

Krymski 1998

September 03 15

4 Mpixel sensor 240fps ERS

4 2352 x 17284 7 mm x 7 mm pixel pitch4 16x10b digital output4 240 fps ERS4 960 Mbytes/sec at 66 MHz4 4000 bits/lx-sec4 3.3 volt operation

Krymski 2001

September 03 16

Customer Drivers4 Incredibly fast price point erosion (2Mpixel CCD

now sells for $8 in Japan)4 Voltage scaling required to be compatible with

companion chips (3.3V -> 2.8V ->…1.8V)4 Power dissipation <200 mW/megapixel for

portable apps4 CCD performance sets benchmark for DSC

applications (SNR, dark current, etc.)4 CMOS performance fine for PC and wireless

applications

17

A Very Small Digital Camera in 1995

4 2nd generation APScamera

4 JPL 256x256 element APS4 On-chip timing/control4 On-chip FPN suppression4 Separate 10-bit ADC4 Separate FPGA4 Serial digital camera I/O

September 03 18

Cell Phone Cameras Are Small

September 03 19

Technology Challenges

4 Smaller pixels4 Maintain or improve SNR with smaller pixel4 Maintain or increase dynamic range with lower

operating voltages4 Reduce dark current4 Reduce number of defective pixels4 Reduce power dissipation in analog and digital circuits

while increasing functionality

September 03 20

Diffraction Limit

0

2

4

6

8

10

12

14

16

18

20

400 500 600 700 800 900 1000 1100

Wavelength (nm)

Size

(micr

ons)

Airy Disk DiameterD = 2.44 F#

Cheap Lens Resolution(30 lp/mm)

High Performance Lens Resolution(120 lp/mm)

B G R

F/2.8

F/11

B G R

LENS

September 03 21

Alignment to Mainstream Technology

4 CMOS APS starts diverging from mainstream CMOS toimprove pixel performance

1970 1980 1990 2000

CCDs

CMOS APS

Mainstream CMOS Technology

Window ofOpportunity

Syst

em M

inia

turiz

atio

n Co

st

September 03 22

Buried Photodiodes

N P NP P

ConventionalPhotodiode

BuriedPhotodiode

September 03 23

4T Buried Photodiode

BPD FD BPD FD

BPD FD

TG TG

TG

BPD FD

TG

DVfd

4Reset Pixel4Integrate4Readout4 Reset FD4 Read FD4 Transfer Q4 Read FD

September 03 24

Comparison of Pixels

RST

SELCOLBUS

Photodiode

“Amp” RST

SELCOLBUS

TG

BuriedPhotodiode

“Amp”

Three-transistor (3T) photodiode-type pixel

Four-transistor (4T) pinned-photodiode-type pixel

+ fewer transistors+ easier to implement+ better under good light- poorer under low light

- more transistors- challenge to implement- poorer under good light+ better under low light

September 03 25

$1,379$1,645

$2,468

$3,477

$4,335

$0.00

$500.00

$1,000.00

$1,500.00

$2,000.00

$2,500.00

$3,000.00

$3,500.00

$4,000.00

$4,500.00

2000 2001 2002 2003 2004

Image Sensor Revenue Projections

Sources: CD analysis based on OEM and industry research reports

September 03 26

$0

$500

$1,000

$1,500

2001 2002 2003 2004

ToysMachine VisionSecurity/BiometricsOptical MiceVideo CamerasOtherPC CamerasMobile PhonesDigital Still Cameras

CMOS Image Sensor Market Growth

Sources: CD analysis based on OEM and industry research reports

$ in

Mill

ions

$1,443

September 03 27

CMOS Vs. CCD Market Growth

0

10

20

30

40

50

60

70

2002 2003 2004 2005 2006 2007 2008

CMOS CCD

Perc

enta

ge