Cmos Primer

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An Introduction to CMOS Image Sensor Technology

Preface

THE PURPOSE of this primer is to introduce the technology used in CMOS image sensors

as well as promoting a better understanding of the consumer product potential unleashed

as the Internet, digital imaging and integrated information appliances converge. It isassumed that the reader is familiar with some basic physics and semiconductor

technology, but most of this overview covers entry level imaging technology information.

THIS DOCUMENT starts with the basics of light and builds on this information to bring anunderstanding of CMOS imagers. Also included are some terms and phrases you may run

into in the digital imaging industry.

Table of Contents

LightPhotons

Color

The Human Eye

Color Mixing and Reproduction

Binary Representation of PicturesStandard Spatial Resolutions

Digital Representation of Pixels

Image SensorsCMOS vs. CCD

CMOS Imager Characteristics

Light

Visible light is the band of electromagnetic radiation that can be sensed by the human

eye. Electromagnetic radiation is the type of energy, traveling in a wave that is produced

by an oscillating charge or energy source. Electromagnetic ("EM") waves also include

radio waves, x-rays and gamma rays.

An electromagnetic wave can be defined by its wavelength (measure of lengthpeak-to-peak) or its frequency (number of cycles per second). The multiplication of these

two characteristics is a constant - the speed of light - so the two are inversely

proportional to one another. That is, the shorter the wavelength, the faster the frequency,the longer the wavelength, the slower the frequency.

OS PRIMER http://www.siliconimaging.com/ARTICLES/CMOS PRIMER.ht

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Visible light is usually referred to in wavelength (instead of frequency) and includes

wavelengths of 400 nanometers (10

-9

meters, abbreviated "nm") to 750 nm. Ultravioletand infrared refer to the wavelengths just beyond the ends of the visible electromagnetic

spectrum:

Wavelength Band Type of RadiationFrequency (cycles persecond)

~10 -14 meters Gamma Rays ~10 22

~10 -9 meters(nanometer)

X Rays ~10 17

~10 -7 meters Ultraviolet ~10 15

~10 -6 meters Visible Light ~10 14

~10 -5 meters Infrared ~10 13

~10 -2 meter (centimeter) Short Wave Radio ~10 10

~1 meter TV and FM Radio ~10 8

~10 2 meters AM Radio ~10 6 (megahertz)

~10 6 meters Long Wave Radios ~10 2

Measurements approximate. The product of the wavelength times the frequency equals

3x108 meters per second, the speed of light.

Everyday usage of this chart is heard in the terms "short wave radio" and "900MHzcordless phone".

Photons

While light has properties of waves, the energy carried by light is not distributed in a

wave, but carried in discrete bundles (or "quantized"), giving light some properties likeparticles. These light "particles" are called photons, and are referred to when explaining

how light transfers energy and are used to explain how CMOS Imagers transfer light

energy to electrical energy.


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Color

The Visible Light band in the EM spectrum can be broken down into a series of colors,each color corresponding to a different wavelength. The typical spectrum that is

displayed is seven colors - red, orange, yellow, green, blue, indigo, and violet. In reality

the band represents a continuum of colors, each corresponding to a differentwavelength, but seven colors are historically displayed. The bands outside this region -

ultraviolet and infrared - are said to be beyond the range of the human eye, although inexperiments both ultraviolet and infrared light can be seen unaided in certain conditions.

Visible Band

400nm 500nm 600nm

The Human Eye

The human eye can discriminate between hundreds of wavelengths as well as the

intensity of the light source being received. The ability to distinguish these characteristics

is through two main types of sensory cells in the retina:

rods - Rods convert photons into an electrical impulse that is processed by the brain.Rod cells are stimulated by the intensity of light and are responsible for perceiving the

size, shape, and brightness of visual images. They do not perceive color and fine detail;tasks performed by the other major type of light-sensitive cell, the cone. The rods are

what are in use when you are in the dark, meaning a red stop sign looks gray when you

look at one without the aid of your car's headlights.

cones - Cones are less sensitive to low illumination levels, but give us our color vision.

There are three types of cones, each of which contains a distinctive type of pigment. One

cone absorbs red light, another green, and the third type blue. A given color stimulatesall three types of receptors with varying effectiveness; the pattern of these responses

determines the color perceived. This color breakdown is explained more in color

reproduction, below.

The mnemonic to remember which cells does what is that Cones receive Color and both

start with a "C"! Rod cells are not only much more sensitive to light than cones but arealso much more numerous. The human eye contains about 130 million rods to about 7

million cones. This means that the human eye is much more sensitive to the intensity of

the light than its color. This characteristic is taken advantage of in color processing,

which will be covered later.

Color Mixing and Reproduction

This basic problem of color reproduction comes down to the question of how to create all

the colors that are possible in the color spectrum. Over the years, it has been discovered


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that all the colors in the spectrum can be recreated from only a sub sample of only three

other colors by mixing them in varying degrees.

The fact that the whole range of colors may be synthesized from only three primary

colors is essentially a description of the process by which the eye processes colors. Thisis a fortunate property of vision since it allows three colors to represent any of the

10,000 or more colors (and brightness) that may be distinguished by human vision. If

this was not the case and vision was dependent on the energy and wavelength

relationship of light described above, it is doubtful that color reproduction could beincorporated in any mass-communication system.

The three main ways to reproduce color are as follows:

Primary Colors - Red, Green, Blue or "RGB" - Most people remember this from their childhood art

classes. This is an "additive" method of adding the three primary colors in different amounts to

recreate other colors, usually used in systems that project light. A mixture of these three primaries

- red, green, and blue - may match any other color if their relative intensities are varied. White is

made by adding ALL the colors (remember that "white light" represents the entire visible EM

spectrum). The RGB scheme is used by televisions, computer monitors and other devices that

project with light.

1.

Complementary Colors - Cyan, Magenta, Yellow or "CMY" - This method is "subtractive" and is

primarily used in printing since ink pigment "subtracts" the light falling on it. For example, a yellow

pigment "absorbs" blue and violet light and "reflects" yellow (along with green and blue which

together make more yellow). Since RGB is the best method for adding colors, then "negative red",

or a pigment which absorbs the most red, "negative green", or a pigment which absorbs the most

green, and "negative blue", which absorbs the most blue, are the best colors for subtracting. These

colors, respectively, are Cyan, Magenta and Yellow. This method is used in inkjet printers and other

methods that print (rather than project, which uses RGB). In practice, most inkjet printers not only

use C, M, and Y ink, but also black ink since black, the combination of all the colors, would use up

those inks very quickly. Since "B" already means "Blue", the last letter of the word "black" is used,meaning this method is referred to as "CMYk".

Subtractive filters are used in consumer cameras since they absorb less light. Professional cameras

use additive filters since additive produces more accurate color.

2.

YCRCB - Luminance, Chrominance (Red), Chrominance (Blue) - The third way to characterize l ight

makes use of the RGB concept above, but breaks down the color components in different fashion.Any color can be broken down into two qualities:

3.

Luminance - Its brightness or intensity. Remember that the human eye that is more

sensitive to brightness than to color. The luminance value, stated with the letter "Y", is

1.


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the brightness breakdown of the color.

Chrominance - the color "remaining" once luminance is removed. This can be broken

down again into two independent qualities:

2.

Hue - This is the color of the light, in this case red or blue.

Saturation - the relative purity, or the amount of white light mixed with a hue. High

saturation colors contain little or no white light.

The translation from RGB to YCRCB is done with a "look-up table" which takes any RGB value and

matches it to its corresponding YCRCB components.

Binary Representation of Pictures

The fact that color can be broken down into individual components is extremely

important to digital imaging - the process of breaking down a picture into the "1"s and

"0" of digital communications. The process of breaking down a picture into individual

components can be done in two basic steps:

Breaking down the picture into a pixel grid - For a picture to be described as a series of1s and 0s, it first must be broken down into a grid, or array. This process simply places a

grid over a picture and assigns a single color for each square in the grid. This single color

grid square is called a "pixel" (short for picture element).

The number of pixels used for the picture breakdown is called the "spatial resolution"

and is usually referred to by its horizontal and vertical number of pixels, such as"640x480", meaning 640 pixels horizontally and 480 pixels vertically.

For a given picture, the number of pixels will determine the quality of the digital picture.That is, the smaller number of pixels, the larger they must be and the lower the picture

quality. The higher number of pixels, the smaller each pixel is and the better the picturequality:

Low Spatial

Resolution

High Spatial

Resolution

Large Pixel Size

Fewer PixelsLow Picture Quality

Small Pixel Size

More PixelsHigh Picture Quality


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Standard Spatial Resolutions

There are a number of standard resolutions, or arrays, in the sensor industry. Most of

these formats come from the display monitor industry, which drives the number of pixels

you see on computer monitors. Since sensors typically display on monitors, they

commonly match monitor resolutions. Term one will hear in regards to spatial format

include:

CIF - Common Intermediate Format - 352 x 288 pixels for a total of 101,376pixels (commonly referred rounded at 100,000 pixels). This format was

developed for PC video conferencing. The number of pixels is fairly small, butwas needed in order to get full motion video at 30 frames per second.

QCIF - Quarter CIF - One quarter of a CIF format, so 176x144 for a total of

about 25,000.

VGA - Video Graphics Array - 640x480 pixels for a total of 307,200 pixels. TheVGA format was developed for computer monitors by IBM and become the

standard for monitors for many years. Although monitor resolutions today are

higher, VGA is still lowest "common" display which all PCs will support.

SVGA - Super VGA - 800x600 pixels for a total of 480,000 pixels. The next

highest monitor resolution developed for PCs.

XGA - "Xtended" Graphics Array - 1024x768 for a total of 786,432 pixels.

Another monitor standard.

If a sensor is not one of these standards, its resolution is simply displayed as vertical byhorizontal (200x300, for example). Typically, if a sensor has more than 1 million total

pixels (anything more than 1000x1000 pixels), it is termed a "megapixel" sensor, which

has come to mean any sensor with more than one million pixels.

Digital Representation of Pixels

Now that the picture is represented as an array of pixels, each pixel needs to be

described digitally. To do this, each pixel is assigned two main components: its location in

the picture and its color. Its location is usually just represented by its "x and y"coordinate in the grid. Its color is represented by its color resolution, which is the

method of describing a color digitally.

Using the RGB method of color representation, a color can be divided into an arbitrary

number of levels of that color. For example, red can be broken down from total red to no

red (or white):

No Red Total Red

Each step in the arbitrary breakdown is called a "gray level" (even though the color isnot gray). The same breakdown can be done for green and blue.


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By experiment, the naked eye can distinguish about 250 shades of each color. Using

binary math, the closest binary number is 256, which is 28, gray levels can be used for

each color. This means for each color component of a picture, there are 8-bits used foreach R, G, B element, for a total of 24 bits of color representation. The complete R, G,

breakdown of 224 colors represents about 16.7 million colors that can be represented

digitally. The number of colors represented by a pixel is called its "tonal resolution" orits "color dynamic range". If fewer bits are used, the number of colors represented is

smaller, so its dynamic range is smaller.

Image Sensors

Image sensors are devices that take an image and directly convert it to a digital image.Referred to in marketing literature as "silicon firm" or "silicon eyes", these devices are

made of silicon since silicon has the properties of both being sensitive to light in the

visible spectrum and being able to have circuitry integrated on-board. Silicon imagesensors come in two broad classes:

Charge-Coupled Devices (CCD) - Currently the most commonly used image sensor,

CCDs capture light onto an array of light-sensitive diodes, each diode representing onepixel. For color imagers, each pixel is coated with a film of red, green, or blue (or

complementary color scheme) so that each particular pixel captures that one particularcolor.

The pixel, made up of a light sensitive diode, converts the light photon into a charge, and

the value of that charge is moved to a single location in a manner similar to a row ofpeople passing buckets of water. At the end, the charge is amplified. Since this "bucket

brigade" is accomplished by applying different voltages to the pixels in a succession, the

process is called charge-coupling. Because the value in the pixel is moved by applyingdifferent voltages, CCD sensors must be supported by several external voltage

generators. In addition, CCDs require a specialized manufacturing process that cannot be

used by any other device.

Graphical representation

of CCD

Image source: Digital Photography

Review

CMOS Imagers - Like CCDs, these imagers are made from silicon, but as the name

implies, the process they are made in is called CMOS, which stands for Complementary


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Metal Oxide Semiconductor. This process is today the most common method of making

processors and memories, meaning CMOS Imagers take advantage of the process and

cost advancements created by these other high-volume devices.

Like CCDs, CMOS imagers include an array of photo-sensitive diodes, one diode withineach pixel. Unlike CCDs, however, each pixel in a CMOS imager has its own individual

amplifier integrated inside. Since each pixel has its own amplifier, the pixel is referred to

as an "active pixel". (note: There are also "passive pixel sensors" (pps) that do not

contain this amplifier). In addition, each pixel in a CMOS imager can be read directly onan x-y coordinate system, rather than through the "bucket-brigade" process of a CCD.This means that while a CCD pixel always transfers a charge, a CMOS pixel always

detects a photon directly, converts it to a voltage and transfers the information directlyto the output. This fundamental difference in how information is read out of the imager,

coupled with the manufacturing process, gives CMOS Imagers several advantages over

CCDs.

CMOS Sensor Array

CMOS vs. CCD

Due to both design and manufacturing considerations, there are a number of advantages

that CMOS Imagers have over CCD:

Integration - Because CMOS Imagers are created in the same process as

processors, memories and other major components, CMOS Imagers can integrated

with these same components onto a single piece of silicon. In contrast, CCDs aremade in a specialized process and require multiple clocks and inputs. This feature

limits CCDs to discrete systems, which in the long run will put CMOS Imagers at a

cost advantage, as well as limit what kinds of portable devices CCDs can beintegrated into.

1.


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Reduced Power Consumption - because of all the external clocks needed to "bucketbrigade" each pixel, CCDs are inherently power hungry. Every clock is essentiallycharging and discharging large capacitors in the CCD array. In contrast CMOS

imagers require only a single voltage input and clock, meaning they consume muchless power than CCDs, a feature that is critical for portable, battery operated

devices.

Pixel Addressibility - CCDs use of the bucket brigade to transfer pixel values means

that individual pixels in a CCD cannot be read individually. CMOS imagers on the

other hand have the pixels in an x-y grid allowing pixels to be read individually. This

means that CMOS imagers will be able to do functions such as "windowing", whereonly a small sample of the imager is read, image stabilization to remove jitters from

camcorders, motion tracking and other advanced imaging techniques internally that

CCDs cannot do.

2.

Manufacturing Cost - Since CMOS imagers are manufactured in the same process as

memories, processors and other high-volume devices, CMOS imagers can take

advantage of process improvements and cost reductions these devices drivethroughout the industry.

3.

CMOS Imager Characteristics


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There are a number of phrases and terms for describing the functional capability, physical

features or competitive characteristics of an imager:

Active Pixel Sensor (also APS) - As explained above, an active CMOS Imager pixel has

its own amplifier for boosting the pixel's signal. Active Pixels are the dominant type ofCMOS Imagers in the commercial market today. The other type of CMOS Imager, a

passive pixel sensor (PPS), consists of only the photo detector without a local amplifier.

While very sensitive to low light conditions, these types of sensors are not suitable for

commercial applications due to their high amount of noise and poor picture quality whencompared to active pixels.

Fill Factor - The amount of a CMOS Pixel that is actually capturing light. In an activepixel, both the photo detector and the amplifier take up "real estate" in the pixel. The

amplifier is not sensitive to light, so this part of the pixel area is lost when taking a

picture.

The fill factor is simply the percentage of the area of the pixel that is sensitive to light. In

the picture above, this is about 40%. As semiconductor process technologies get smallerand smaller, the amount of area taken up by the amplifier is taking up less space, so low

fill factors are becoming less of an issue with active pixels. Note that in passive pixels -

where there is no amplifier at all - fill factors typically reach over 80%. The reason they

do not reach 100% is due to routing and pixel selection circuitry that are also needed in a

CMOS imager.

Microlenses - In some pixel designs, the fill factor becomes too small to be effective.

For example, if a fill factor in an imager were 25%, this would mean that 75% of the lightfalling on a pixel would be lost, reducing the pixel's capability. To get around thissituation, some CMOS imagers have small lenses manufactured directly above the pixel

to focus the light towards the active portion that would otherwise fall on the non-lightsensitive portion of the pixel. Microlenses typically can increase the effective fill factor by

two to three times.


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Color Filter Array (also CFA or just "color filter") - CMOS Pixels are sensitive to light

photons but are not, by themselves, sensitive to color. Unaided, the pixels will capture

any kind of light, creating a black and white image. In order to distinguish betweencolors, filters are put on top of a pixel to allow only certain colors to pass, turning the

"rods" of the array into "cones". Since all colors can be broken down into an RGB orCMYk pattern, individual primary or complementary color schemes are deposited on top

of the pixel array. After being read from the sensor, software takes the different values ofthe pattern and recombines the colors to match the original picture. There are a variety

of different filters, the most popular being the Bayer Filter Pattern (also known as

RGBG). Note the large amount of green in the pattern, due to the fact that the eye ismost sensitive to color in the green part of the spectrum.

Bayer Color FilterPattern

Noise - The same as static in a phone line or "snow" in a television picture, noise is any

unwanted electrical signal that interferes with the image being read and transferred bythe imager. There are two main types of noise associated with CMOS Sensors:

Read Noise (also called temporal noise) - This type of noise occurs randomly and isgenerated by the basic noise characteristics of electronic components. This type of

noise looks like the "snow" on a bad TV reception.

1.

Fixed Pattern Noise (also FPN) - This noise is a result of each pixel in an imager

having its own amplifier. Even though the design of each amplifier is the same, whenmanufactured, these amplifiers may have slightly different offset and gain

characteristics. This means for any picture given, if certain pixels are boosting thesignal for every picture taken, they will create the same pattern again and again,

hence the name.

2.

Blooming - The situation where too many photons are being produced to be received by

a pixel. The pixel overflows and causes the photons to go to adjacent pixels. Blooming is

similar to overexposure in film photography, except that in digital imaging, the result is a


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number of vertical and/or horizontal streaks appearing from the light source in the

picture.

This photo illustrates two undesirable characteristics:

blooming, the slight vertical line running from the topto the bottom of the picture and lens flare, the star

shape light which is a function of the lens and not the

imager.

Optical Format - is a number in inches that is calculated by taking the diagonal

measurement of a sensor array in millimeters and dividing by 16. For example, a CMOS

Imager that has a diagonal measurement of 4mm has an optical format of 4/16, or ".

What Optical Format calculates is the type of lens system that must be used with theimager. In the lens industry, there are standard sets of ", ", ", etc. lens systems. By

using Optical Format, a user of imagers can use standard, mass-produced (andinexpensive) lens systems rather than having to design and custom build a special lens

system. The terms and measurement comes from the days of electron tubes and

pre-dates solid-state electronics. Generally speaking, larger optics are more expensive,so a " lens system is less than a 1/3" lens system.

Aspect Ratio - The ratio between the height and width of a sensor or display. It is found

by dividing the vertical number of pixels (height) by the horizontal number of pixels

(width) leaving it in fractional format.

For example, a pixel with resolution of 640x480 would have an aspect ration of 480/640=

.

The most common aspect ratios are and 9/16. The aspect ratio is the ratio for

computer monitors and TVs. The newer 9/16 aspect ratio is used for High Definition

Television (HDTV)

Quantum Efficiency (or QE) - Imagers create digital images by converting photonenergy to electrical energy. The efficiency in which each photon is converted to an

electron is the imager's quantum efficiency. The number is calculated by simply dividing

electrons by photons, or E/P. If no electrons are created, the efficiency is obviously zero,

while if each photon creates one electron the efficiency is 100%. Typically, a sensor has

different efficiency at different light frequencies, so a graph of the quantum efficiencyover the different wavelengths is typically shown:

Dark Current - A situation in CMOS imagers where the pixels fill with thermally created

electrons without any illumination. This problem is a function of the manufacturing

process and layout and increases with increasing temperature.


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