# Presentation: 932137

## Visible LightCMOS Image Sensors

**Dr. Eric R. Fossum**
**Senior Fellow, Micron Imaging**
**Micron Technology, Inc.**

## Outline

**Introduction to CMOS image sensor pixels**
**Examples of sensors**
**Customer trends**
**CMOS image sensor technology trend**
**Thoughts on next 5 years**

## 3Transistor (3T) CMOS APS

**Use source-follower “amplifier” to drive column bus**

## Voltage on 3T Pixel vs. Time

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

## Simplified Signal Chain

**SHS**

**SHR**

**SIG**

**RST**

**COL BUS**

**RS**

**RST**

**VDD**

**CS**

**HOR BUS**

**VLN**

**VLP**

**VLP**

**PIXEL**

**PER COLUMN**

**GLOBAL**

## Imaging System on a Chip

- CMOS Active Pixel Color Imaging Array

- Digital 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

## Spectral Response of Eye and Silicon

**NIR**

**Spectral Response of Eye and Silicon**

**UV**

## Fill Factor

**A pixel is divided into a sensing portion and a readout portion**
**Fill factor is the ratio of sensing area to total area and is typically about 20-30%**

- Total pixel area

- Sensitive area

## Color Filter Arrays and Microlenses

- Microlens layer

- Color filter layer

- Metal opaque layer

- Photodiode

- Silicon substrate

## Resolution and Sensitivity

- Scene

- Image

**Resolution and Sensitivity**

- Photons /pixel

- Pixel pitch

-  More pixels = high resolution
-  Smaller pixels = less sensitivity

## Optical Formats

**Optical Formats**

- 7.8 mm

- 5.6 mm

- 3.6 mm

- 2.8 mm

- “1/4 in”

## Pictures from PC Camera Sensors

- CIF Resolution
- (352 x 288)

- VGA Resolution
- (640 x 480)

## 1.3Mpixel High Speed Sensor

- 1/33,000 sec = 30 usec
- Freeze frame shutter

**1280 ADCs per chip**
**5T pixel **
**500 pictures per second at full resolution**
**Shutter from 1/30****th**** sec to 1/100,000****th**** sec**
**Shutter efficiency > 99.99%**
**Rotating fan image ******

## Pill Camera

**Pixel Format: 256 X 256**
**Pixel Size: 10 μm X 10 μm**
**Frame Rate: 2 fps**
**ADC: On-Chip, 8 bits**
**Power Supply: 2.8 V**
**Power:  3 mW**

- Sensor and US$0.10

- Pill Camera and US$0.25

- View inside small intestine

*Krymski 1998*

## 4 Mpixel sensor 240fps ERS

-  2352 x 1728
-  7 m x 7 m pixel pitch
-  16x10b digital output
-  240 fps ERS
-  960 Mbytes/sec at 66 MHz
-  4000 bits/lx-sec
-  3.3 volt operation

*Krymski 2001*

## Customer Drivers

**Incredibly fast price point erosion (2Mpixel CCD now sells for $8 in Japan)**
**Voltage scaling required to be compatible with companion chips (3.3V -> 2.8V ->...1.8V)**
**Power dissipation <200 mW/megapixel for portable apps**
**CCD performance sets benchmark for DSC applications (SNR, dark current, etc.)**
**CMOS performance fine for PC and wireless applications**

## A Very Small Digital Camera in 1995

**2nd generation APS camera**
**JPL 256x256 element APS**
**On-chip timing/control**
**On-chip FPN suppression**
**Separate 10-bit ADC**
**Separate FPGA**
**Serial digital camera I/O**

- Micron Confidential

- March 1999

## Cell Phone Cameras Are Small

## Technology Challenges

**Smaller pixels**
**Maintain or improve SNR with smaller pixel**
**Maintain or increase dynamic range with lower operating voltages**
**Reduce dark current**
**Reduce number of defective pixels**
**Reduce power dissipation in analog and digital circuits while increasing functionality**

## Diffraction Limit

- B

- G

- R

**LENS**

## Alignment to Mainstream Technology

**CMOS APS starts diverging from mainstream CMOS to improve pixel performance**

**1970**

**1980**

**1990**

**2000**

**CCDs**

**CMOS APS**

**Mainstream CMOS Technology**

**System Miniaturization Cost**

## Buried Photodiodes

- N

- P

- N

- P

- P

_**Conventional**_
_**Photodiode**_

_**Buried**_
_**Photodiode**_

## 4T Buried Photodiode

- BPD

- FD

- BPD

- FD

- BPD

- FD

- TG

- TG

- TG

- BPD

- FD

- TG

- Vfd

- Reset Pixel
- Integrate
- Readout
-    Reset FD
-    _Read FD_
-    Transfer Q
-    _Read FD_

## Comparison of Pixels

- RST

- SEL

- COL
- BUS

- TG

- Buried
- Photodiode

- “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

## Image Sensor Revenue Projections

**Image Sensor Revenue Projections**

- Sources:  CD analysis based on OEM and industry research reports

## CMOS Image Sensor Market Growth

**CMOS Image Sensor Market Growth**

- Sources:  CD analysis based on OEM and industry research reports

**$ in Millions**

- $1,443

## CMOS Vs. CCD Market Growth

**CMOS**

**CCD**

**Percentage**