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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