CMOS Image Sensor

companyscourgeAI and Robotics

Oct 19, 2013 (3 years and 9 months ago)

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Daniel Aguirre

Utah State University

Mechatronics

MAE/ECE5320

CMOS Image Sensor

CMOS Image Sensor


CMOS
-

Complimentary Metal Oxide Semiconductor


Primary uses:


Digital Cameras


Digital Camcorders


Cellular Phones


Security Cameras


Surveillance application


Also used in a large number of industrial, medical, media, and
consumer applications.

APS (Active Pixel Sensor)


A CMOS image sensor is an
APS that uses a CMOS
process.

An APS is an integrated
circuit that contains an
array of pixels each
containing a photo detector
and system to amplify the
signal from the photo
detector.



CMOS image sensor

(ref 1)

What is a pixel?

In simple terms a pixel is a dot.



A digital picture is made up of many, often
millions of the these dots.


The resolution of the these pictures
depend on; the size of pixels, the
distance between pixels, and the
number of pixels that comprise the
image.


Large vs. Small Pixels


The image on the right has small pixels where the image on the
left has larger pixels or it can also be explained that the image on
the right has a higher number of pixels per area than the image on
the left.
2

What is a CMOS Process?

“Complementary”


Use of complementary and symmetric
pairs or
p
-
type (increased number of free
charge carriers) and
n
-
type (containing
extra conduction electrons) metal oxide
semiconductor field effect transistors for
logic functions.
3

“Metal
-
Oxide
-
Semiconductor”


References to the physical structure of
certain field effect transistors, having a
metal gate electrode placed on top of an
oxide insulator, which is on top of a
semiconductor material.
3


How does it work?

An array of light sensitive diodes collect light, kind of like a solar cell, and
emit a signal. That signal is amplified and send to a decoder that resolves
the signal into a dot. And a computer program places the dot in it’s
respective place from the array into the image.


Array set
-
up for CMOS sensor
4

Image Taking Structure


Array of pixels, each containing a photodiode to convert

photons into electrons and transistors to generate voltage.



Each pixel is controlled by a reset switch, which clears the

device of all charge prior to the next exposure.



At the time of capture, the pixel is charged by the light.

This charge is then converted to a voltage at the pixel

level.



The voltage signal then passes through the automatic

gain controller (AGC) and analog
-
to
-
digital converter

(
ADC) before leaving the CMOS chip.

ref. 5

Color

Color is determined much like a printer.


Three colors are used either
Red,Blue
, and Green scheme or
Magenta, Cyan, Yellow scheme. Determined by either additive
coloration or subtractive coloration respectively.


As the pixels get smaller the three color scheme blends the
three colors into a color that matches the color that was seen by
the one taking the picture. As seen in slide 5


Pixels and Color

Each pixel is covered further with an array of light filters allowing only certain
colors of light through. The light that is passed through hits a sensor that sends
the signal to tell the processor that this color is at this dot.


Ref. 6

2 Methods of
L
ight Gathering

1.
Front
-
illuminated structure

2.
Back
-
illuminated structure

7

Front
-
illuminated

structure

In front
-
illuminated structure
sensors the actual light collecting
system is semi
-
buried deep in the
structure of the silicone wafer.
Even though light can reach the
diodes and light is gathered fairly
abundantly, the light is
obstructed through the thickness
of the wafer.

The reason this is done is because
it easier to place the diodes at the
bottom of the wafer and it gives
the light receiving surface
protection.


B
ack
-
illuminated

structure

Back illuminated structure places the
photodiodes and light receiving surface
above the wiring and on top the wafer.
This method, though slightly more
expensive and extensive to assemble,
allows more true light to get to the
light receiving surface and into to
photodiode. Creating a more true to
real color coming through the lenses
because there is less obstruction.

CCD (Charge Coupling Device)

The major competitor in the digital image sensors is the CCD
sensor


Though both sensors were theorized and developed at the same
time, CCD technology received the most momentum from the
start:


Due to ease of construction and available materials


At the beginning CMOS sensors were expensive and
yeilded

very poor
imaging


Since CMOS sensors have been further developed:


Yielding higher resolution images because of decreased wafer thickness
and pixel size available to put on wafer


Generally consume less power


Single voltage power supply (reducing the number of amplifiers required.

Ref 8

Consumer Products or Industry




Y
ou
can find CCDs in low
-
cost low
-
power cellphone cameras and
CMOS sensors in high
-
performance professional and industrial
cameras


CCD sensors can be produced quickly and inexpensively but
quality and pixel size, like one would think, are sacrificed with
cost


CMOS sensors, although initially thought to be less expensive
because the chip has similar make
-
up as the mainstream memory
wafer, have become very extensive in fabrication processes and
material sciences.


However, both are found abundant in both arenas and are subject
to performance and cost relativity.

Characteristics of a typical CMOS
sensor

CMOS Sensors


Typical Characteristics

• Zero or First Order System depending on how photodiode

capacitance is used.

• Power Consumption


200
mW

• Dynamic Range


70+ dB (Human DR is ~90dB)

• Spectral Range


200nm to 1100nm with 700nm peak.

• Response time can be as little a 1ms.

• Fixed Pattern Noise
-

~16% p
-
p

• Fill Space


30
-
40% of each pixel can be used to capture light.

• Automatic Gain Control


driven by feedback from other optical

components, and image processing algorithms.

• Generally low SNR due to various noise sources

• Low Power Consumption

• Additional Digital Logic Capability

Ref 9


Electrical noise always exists and will
affect whatever sensor you are using


Noise Sources


Temporal Noise


random noise often caused by the random arrival
of photons
onto
the sensor. Similar noise can be induced by the
random arrival of electrons to
an
electrical element of the sensor. This
includes
arrival of pixel voltages to column
amplifiers
, AGCs and
ADCs.


Amplifier Noise


Thermal noise is generated by uncorrelated
sampling
operations
.


Analog
-
Digital Conversion


Inherent noise in quantization of analog
signal.


Fixed Pattern Noise


Spatial noise due to mismatches in pixels or
CFAs. This can
also
be caused by variations in various amplifiers or
converters

Ref 9

CMOS Sensor Price Examples

Cypress 6.6 MP APS: $377

Used for machine vision, biometry and

document scanning.

Cypress

Aptina

5MP APS: $33

Used for digital video cameras and still

cameras

Omnivision

5MP APS: $15

Used for mobile phones, PC multimedia,

toys, and digital still cameras

Ref 9

References

1.
Wikipedia
:
http://en.wikipedia.org/wiki/CMOS_sensor

2.
http://mashable.com/2012/01/20/pixel
-
art/

3.
Wikipedia
:
http://en.wikipedia.org/wiki/CMOS

4.
http://www.siliconimaging.com/ARTICLES/CMOS%20PRIM
ER.htm

5.
http://www.moglik.com/i/show/4255/camera
-
sensor
-
cmos
-
diagrama.htm

6.
http://www.sensorcleaning.com/whatisasensor.php

7.
http://
www.i
-
micronews.com/lectureArticle.asp?id=1607

8.
http://
micro.magnet.fsu.edu/primer/digitalimaging/cmosima
gesensors.html

9.
http://mech207.engr.scu.edu/SensorPresentations/Patel%20
-
%20CMOSImageSensor%20Combined.pdf