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A

SeminarOn

CMOS IMAGE SENSOR

Guided By: - Submitted By: -

D. Boolchandani YogeshChouhan

Sr. Lecturer VIII Sem. ID:

012376

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Contents

General Information

CCD Image Sensor: Concept

CMOS Image Sensor: Concept

Comparison CCD-CMOS

Improvements for the CMOS sensor

Technology Modifications References

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General Information

Optics: Optical arrangement to focus the real world

picture on the sensor.

Sensor: Sensor is a device that senses the imageand converts it to a desired out put.

Real world Optics Sensor

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Types of sensors:

CCD = Charge Coupled Device

CMOS = Complementary Metal-Oxide Semiconductor

Similar operation: the incident light is transferredto a electrostatic charge (like a photodiode). Thesecharges are an indication for the intensity of the

light on that pixel.

The difference between both imaging technologiesis the way of transporting these charges.

Continued

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CCD sensor: Concepts

A CCD is composed of a series connection of capacitors. All of

the capacitors have one plate in common: the silicon substrate.The second plate of each capacitor is biased by means of digital

pulses.

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Continued CCD sensor concept

A two-dimensional CCD imaging array-

Serial connection of the CCD cells. Six (vertical) columns,each of 6pixels, compose the plane.

Operation: All light-sensitive top part of the device arebiased in the integrating mode. Parts of the CCD gates

are connected to a high DC level, creating individualpotential wells in the silicon substrate.

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Continued CCD sensor concept

Example: Frame transfer CCD

Charge

Image Area

Storage Area

Horizontal Register

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CCD transport of charge

CCD imaging technology

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Continued CCD transport of charge

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CCD: pros and cons

Superior quality.

Efficient use of the available surface

High sensitivity for light

Specific manufacturing process higher cost

Numerous external signals lot of power

No extra logic available on-chip

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Characteristics of thephotodiode

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CMOS-sensor: concepts

Capture of the light - similar

C i d CMOS

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ContinuedCMOS-sensor: concepts

Difference- Current flowing

MOSFETs - as an amplifying element

It is possible to point every single pixel separately.

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Sensing Process

S

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Continued Sensing Process

Reset step

Integration step

Read step

Conversion step

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Opto-electrical characteristic

Two common deficiencies:

Often inadequate when taking indoor pictures

Dynamic range

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CMOS: pros and cons

CMOS consumes very little power

CMOS sensors can be manufactured likeordinary silicon chips lower cost

Easy integration with external logic

Images of lower quality

Smaller fill factor less sensitive to light

I t f th CMOS

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Improvements for the CMOS

sensor

Increasing the fill factor:

Micro-lenses

High fill factor technology

Capture color images

I i th fill f t

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Increasing the fill factor:1.Micro Lenses

Increase the fill factorup to 90%

For perpendicularincoming light

Not yield a significantimprovement for light

striking under a very lowangle

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2.High fill factor technology

Redirect the freeelectrons, caused by

incoming photons,towards thephotodiode-structure

Done by applying aslightly differentdoped silicon layer.

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Capture color images

There are different possibilities to enable CMOS

sensors to capture colored images:

Sequential illumination using 3 filters Dichromatic lenses

RGB-filters

Vertically stacked photodiodes

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Sequential illumination

Use of standard sensor

Easy to implement

Three exposure is needed

Slow only recommendedwith static objects

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Dichromatic lenses

Solve the delay problem

Optimal resolution

Demands high accuracy

No standard construction

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RGB-filter

Mostly used

Bayer Filter is used

Standard sensors are used

Only one color per pixelloss of resolution

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Vertically stacked photodiodes

Different wave length absorbedin CMOS on different depths

All info in one pixel.

Bad color separation

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Technology Modifications

Modifications to standard CMOS technologies toimprove their imaging performance:

To increase photo responsivitynon silicided deep junction

diodes and integrated micro lens arrays are used

To increase quantum efficiencydoping profiles of these

diodes are optimized To increase light transmissiondifferent Si materials are

being tried out

To reduce leakage defectsHydrogen annealing is done

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Conclusion

CCDs are still superior to CMOS image sensors, as far assignal-to-noise and dynamic range are concerned

Efforts are being done to further optimization of the noiseperformance of CMOS imagers

New pixel designs

fine tuning of the semiconductor processes

CMOS possesses many advantages over CCDs:

low power of CMOS

low driving voltages of CMOS

on chip functionality

selective read-out mechanism

cost advantages

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References:

Comparison of modern CCD and CMOS image sensortechnologies and systems for low resolution imaging, Carlson,B.S.; Sensors, 2002. Proceedings of IEEE , Volume: 1 , 12-14June 2002 Pages:171 - 176 vol.1

A CMOS image sensor with a simple FPN-reduction technologyand a hole accumulated diode,Yonemoto, K.; Sumi, H.; Suzuki,R.; Ueno, T.;Solid-State Circuits Conference, 2000. Digest ofTechnical Papers. ISSCC. 2000 IEEE International , 7-9 Feb.

2000 Pages:102 103 http://www.opencontent.org/openpub/

www.dalsa.com

www.howstuffworks.com

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