March 11, 2010

Complementary Metal-Oxide

Semiconductor Sensors

Prepared for

Ann Holms

University of California Santa Barbara

Prepared by

Alvin Quach

University of California Santa Barbara

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Table of Contents

Abstract ............................................................................................................................................2

Introduction ......................................................................................................................................3

CMOS Sensor Functionality ............................................................................................................3

Description of Parts and Functions ......................................................................................4

Color Filter ...............................................................................................................4

Pixel Array ...............................................................................................................5

Digital Control .........................................................................................................6

Analog-to-Digital .....................................................................................................6

Theory of Operation .............................................................................................................6 CCD Functionality ...........................................................................................................................6

Description of Parts and Functions ......................................................................................7

Color Filter ...............................................................................................................7

Pixel Array ...............................................................................................................7

Theory of Operation .............................................................................................................8

CMOS vs. CCD ...............................................................................................................................8

Technical Comparison .........................................................................................................9

Pixel Sensor Array Comparison...............................................................................9

Other Differences .....................................................................................................9

Analysis from a Technical Viewpoint ...................................................................10

Practical Comparison .........................................................................................................10

Speculations of CMOS Sensor Technology ..................................................................................11

Conclusion .....................................................................................................................................12

References ......................................................................................................................................13

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Abstract

The complementary metal-oxide-semiconductor sensor, or CMOS sensor, powers the digital

camera in a camera phone and webcam. This report discusses the components and functionality

of CMOS sensors and its rival technology, the charged-coupled device (CCD). In addition, the

report discusses the typical applications of CMOS sensors and CCDs.

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Introduction

The complementary metal-oxide-semiconductor (CMOS) sensor, also known as the active pixel

sensor (APS), is a type of image sensor used in many consumer devices. Unlike its technological

competitor, the charged-coupled device (CCD), CMOS sensors have most of their required

circuitry and components integrated onto the sensor, resulting in a smaller and a less power

consuming system overall. This makes CMOS sensors better suited for smaller consumer

electronics, such as cell phone cameras and webcams. It is also used in some digital cameras and

scanners.

CMOS sensors have greatly impacted the social lifestyle of the 21st century. Camera phones,

which use CMOS sensors, allow users to take photos and record videos wherever they go and

easily share them with friends and family. By integrating the camera with the phone, most people

can now carry one less device. CMOS sensors have also found their way into the webcams of

notebook computers. This affected the way that many business meetings are run. Participants can

now meet without physically being in the room. This not only saves time, but can also be more

convenient for the meeting participants.

The purpose of this report is to explain the basic functionality of CMOS sensors. The basic

functionality of CCDs will also be explained to point out the differences between the two

technologies. The manufacturing process of CMOS sensors will not be explained. Technologies

exclusive to specific CMOS sensor models will not be covered either; only the general

technologies found in all CMOS sensors will be discussed.

The paper begins with an overview of what CMOS sensors are and what they are used for,

including a basic timeline. The next section explains the functionality of CMOS sensors,

including the main parts of CMOS sensors and how the parts work together to make the sensor

and the system functional. Some explanation of the functionality of CCDs will also be provided.

The next section discusses the differences between CMOS sensors and CCDs in terms of

functionality, requirements, and real world performance and applications. The report concludes

with some drawbacks of introducing the technology into society.

CMOS Sensor Functionality

A CMOS sensor is an image sensor that contains most of its functioning parts on a single circuit.

It adds image sensing capabilities to devices, such as cell phone cameras and scanners, and

allows the user to convert a real life scene into a digital image.

A typical CMOS is an integrated circuit with an array of pixel sensors. Each pixel sensor

contains its own light sensor and active amplifier. An analog-to-digital converter and other

components critical to the operation of the pixel sensors are located on the CMOS sensor.

Light comes through the lens and is processed by the color filter before reaching the pixel sensor

array. When the filtered light reaches the pixel array, each pixel sensor converts the light into an

amplified voltage signal that can be further processed by the rest of the CMOS sensor.

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The CMOS sensor contains four main parts: the color filters, the pixel array, the digital controller,

and the analog to digital convertor (Figure 1).

Figure 1. Parts of a CMOS sensor [Litwiller, 2005]

Description of Parts and Functions

The following section discusses the main parts of the CMOS sensor.

Color Filter An array or a mosaic of tiny color filters is placed over the pixel sensor array to capture color

information. Each filter in the array corresponds to a pixel sensor in the pixel array and only

allows certain colors of light to pass through to the pixel sensor. This is achieved by filtering out

other the wavelengths of unwanted color. Color filters are required because the pixel sensors can

only detect light intensity, and not wavelength, which dictates the color of a light [Zumdahl,

2008].

The individual filters in a color filter array can be designed to permit the transmittance of any

single color, but the most effective colors are red, green, blue, cyan, magenta, and yellow. A

combination of different colored filters and their corresponding pixel sensors must be used to

properly determine the color for one pixel of an image. The combination is a pattern of color

filters called a sub-mosaic, which is most commonly made up of a 2-by-2 grid of color filters.

The most common type of color filter array is the Bayer filter. The sub-mosaic pattern of a Bayer

filter is 2x2 with one red, one blue and two green filters. Two green filters are used for each red

and blue filter to mimic the human’s eye greater sensitivity of green light (Figure 2) [McHugh,

2010].

Co

lor

Filt

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Analog-to-Digital

Converter OU

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CIR

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AR

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DIGITAL SIGNAL ANALOG SIGNAL

Pixel Array

Digital Control

INPUT

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CMOS Sensor

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Figure 2. A Bayer filter over the pixel sensor array [Wikimedia, 2006]

Pixel Array The pixel array consists of millions of active pixel sensors responsible for capturing the intensity

of the pre-filtered light passing through. Each individual pixel sensor then converts the detected

intensity level into a voltage signal before passing it down to the another part of the chip, such as

the analog-to-digital convertor (Figure 3).

Figure 3. The pixel sensor array of a CMOS sensor [Litwiller, 2005]

The functionality of a pixel sensor is based on the principle of the photoelectric effect. The

photoelectric effect is a phenomenon in which electrons are emitted from matter, particularly

metals, when energy from an electromagnetic radiation of very short wavelength is absorbed. For

image sensors, the source of the electromagnetic radiation is light, which takes the form of a

wave of particles called photons. The intensity of a light is proportional to the amount of photons

Pixel Sensor –photons to electrons

conversion

Voltage Signal Output

Electron charge to voltage conversion (inside pixel sensor)

Bayer Filter Pixel Sensor Array

2-by-2 RGGB Sub-mosaic

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associated with it. Similarly, the amount of electrons emitted by the metal pixel sensor is relative

to the amount of photons striking the pixel sensor [Fowler, 1997].

The resulting charge of the electrons emitted is converted into a voltage signal by the pixel

sensor. The voltage signals from the pixel sensors in the pixel array are then combined and

outputted as a single signal.

Digital Control The digital controller is the set of circuitry integrated on the CMOS sensor that controls the pixel

array. It consists of multiple components, including the clock/timing generator and oscillator to

ensure that every pixel in the array is in sync with each other. It is responsible for telling the

pixel array to when to start capturing light. Any other circuitry that is required for the array of

pixels to function orderly as a whole is included in the digital controller.

Analog to Digital Converter (ADC)

The ADC takes the analog voltage signals from the pixel sensor array and converts them into a

digital signal. The final digital signal is then outputted to an image processor or another device

independent of the CMOS sensor which converts or further processes the digital signal into

something viewable by the end user.

Theory of Operation When the pixel array receives the signal from the digital controller, the pixel sensors captures the

intensity levels of the wavelength-filtered light and outputs the result as an analog voltage signal.

The analog signal is converted into digital by the ADC so that the final signal leaving the CMOS

sensor can be used and further processes by other digital components on the printed circuit board

of the device.

CCD Functionality

A CCD is an image sensor that is consists of an array of photoelectric devices, or pixel sensors. It

adds image sensing capabilities to devices, such as cameras, and allows the user to convert a real

life scene into a digital image.

A typical CCD is an array of pixel sensors with extra circuitry to output the captured data as an

analog voltage signal.

Light comes through the lens and is processed by the color filter before reaching the pixel sensor

array. When the filtered light reaches the pixel array, it captures the light intensity data which is

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converted into an analog voltage signal that can be further processed by other chips on an

external circuit board.

The CCD contains two main parts: the color filter and the pixel array (Figure 4).

Figure 4. Parts of a Charged-Coupled Device [Litwiller, 2005]

Description of Parts and their Functions The following section discusses the main parts of the CMOS sensor.

Color Filter

The color filter of a charged-coupled device functions in the same way as the color filter in a

CMOS sensor. For more information, refer to “Color Filter” under the section “CMOS Sensor

Functionality” on page 4 of this report.

Pixel Array

The pixel array consists of millions of passive pixel sensors responsible for capturing the

intensity of the light passing through. The light intensity data is then combined before being

converted into an analog voltage signal, which is outputted to an external circuit board to be

further processed (Figure 5).

Co

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Fil

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UTP

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ANALOG SIGNAL

Pixel Array

INPUT FROM CIRCUIT BOARD

LIG

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FRO

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Figure 5. The pixel sensor array of a CCD [Litwiller, 2005]

The functionality of a pixel sensor is based on the principle of the photoelectric effect. The

photoelectric effect is a phenomenon in which electrons are emitted from matter, particularly

metals, when energy from an electromagnetic radiation of very short wavelength is absorbed. For

image sensors, the source of the electromagnetic radiation is light, which takes the form of a

wave of particles called photons. The intensity of a light is proportion to the amount of photons

associated with it. Similarly, the amount of electrons emitted by the metal pixel sensor is relative

to the amount of photons striking the pixel sensor [Fowler, 1997].

The resulting charge of the electrons emitted is converted into a voltage signal by the pixel

sensor. The voltage signals from the pixel sensors in the pixel array are then combined and

outputted as a single signal.

Theory of Operation When the pixel array receives the signal from an external component on the printed circuit board,

the pixel sensors captures the intensity levels of the wavelength-filtered light and outputs the

result as an analog voltage signal. The analog signal is outputted to the printed circuit board to be

converted to a digital signal and further processed.

CMOS vs. CCD

CMOS sensors and charged-coupled devices are both widely used for capturing images digitally.

Each technology has its unique strengths and weaknesses, giving them advantages in different

applications.

Pixel Sensor –photons to electrons

conversion

Voltage Signal Output

Electron charge to voltage conversion

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Technical Comparison

The technical differences between CMOS sensor and CCDs include the on-chip functions, the

type of output signal, and the pixel sensor array (Figure 6).

Figure 6. The pixel array of a CMOS sensor and a CCD [Litwiller, 2005]

Pixel Sensor Array Comparison The pixel sensor arrays in both image sensors function by converting the light photons into an

electric charge and processing it into useful electronic signals. The main differences are where

the photon to electric charge conversion takes place on the pixel array.

Both types of pixel sensor arrays convert the photons striking the pixel sensor into electrons (an

electrical charge) using the photoelectric effect. In a CMOS sensor, each pixel sensor also

converts the electrons into a voltage signal. In a CCD, the electrical charge of each pixel sensor

is transferred to an output node to be converted into a voltage signal [Litwiller, 2001].

Other Differences In addition to the pixel sensor array, a CMOS sensor also includes other circuitries such as a

digital controller and an analog-to-digital convertor on the chip itself. Other functions include a

built-in amplifier and noise-corrector, which may or may not be included in a CMOS sensor. The

analog-to-digital convertor allows a CMOS sensor to output digital signals.

A CCD, on the other hand, outputs an analog voltage signal and utilizes other chips located on an

external circuit board to convert the signal to digital and apply noise-correction, etc.

Pixel Sensor –photons to electrons

conversion

Voltage Signal Output

Electron charge to voltage conversion

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Table I summarizes the technical differences between CMOS sensors and CCDs.

Table I. Feature Comparison [DALSA, 2010]

Feature CMOS CCD

Signal out of pixel Voltage Electrical Charge

Signal out of chip Bits (digital) Voltage (analog)

Signal out of system Bits (digital) Bit (digital)

System noise Moderate Low

Chip complexity High Low

System complexity Low High

System components Sensor, lens, few

additional support

chips

Sensor, lens,

multiple support

chips

Analysis from a Technical Viewpoint The added functions to the CMOS sensor reduce the amount of off-chip circuitry required for

operation, but they clutter the chip and reduce the area available for capturing light. In addition,

with each pixel sensor performing its own electron to voltage conversion, the uniformity across

the whole array will be lower.

All of the CCD’s area can be used for capturing light, because it has no extra circuitry that take

up space on the chip. The uniformity of the output is also relatively high compared to CMOS

sensors. However, due to the lack of extra functions, CCDs require extra off-chip circuitry to

operate properly [DALSA, 2010].

Practical Comparison The technical differences between CMOS sensors and CCDs affect the way each technology is

used in the real world. The following sections will discuss such effects, and how the technical

aspects are involved.

Image Quality Typical CMOS sensors have a lower uniformity than CCDs due to the design of its pixel sensor

array. The result is that the images captured by a CMOS sensor will generally have more noise

than a similar image captured by a CCD.

CCDs have generally been the preferred choice in the photographic, scientific, and industrial

applications that demand the highest image quality in terms of noise and quantum efficiency,

although both CCDs and CMOS sensors can offer excellent imaging performance when designed

properly [DALSA, 2010].

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Costs The manufacturing costs of both technologies are similar at the chip level. CMOS sensors were

speculated to have much lower manufacturing costs than CCDs because CMOS sensors were

manufactured using the same technologies as existing mainstream logic chips or memory chips

(which was called the CMOS process, hence CMOS sensors).

However, the manufacturing process had to be revised because CMOS sensors need to be

manufactured at higher qualities then traditional chips in order to achieve good imaging

capabilities, reducing the cost advantages that CMOS sensors had over CCDs. In addition, the

manufacturing process for CCDs is more mature than the revised CMOS manufacturing process,

making the CCDs generally less expensive to manufacture.

CMOS sensors can still be produced in higher volumes due to the use of a larger silicon wafer

(200mm for CMOS, versus 150mm for CCDs) [DALSA, 2010]. Overall, this balanced out the

total manufacturing costs for both technologies, and their manufacturing costs turned to be very

comparable to each other.

Development costs, which are separate from manufacturing costs, are higher for CMOS sensors

because they are more complicated to design. CCDs are relatively simple and easier to design

than CCDs.

Applications

The main advantages that CMOS sensors have over CCDs are the allowance for a smaller system

and lower power draw. Therefore, CMOS sensors are dominant in camera phones. It is also the

favored technology for other small, low-power devices.

The advantages of a CDD are its higher imaging quality, so it is found in most high-performance

professional and industrial cameras.

However, exceptions are being introduced as both CMOS sensor and CCD technologies are

improving; CCDs are used in some low-power cellphone cameras while CMOS sensors are used

in some high-performance industrial cameras. For other applications, there is no clear line to

segregate the two technologies.

Speculations of CMOS Sensor Technology

One of the goals for CMOS sensor producers is to reduce the disadvantages that the CMOS

sensor has against CCDs. This includes image quality shortcomings such as low uniformity and

noisy images, and development issues, such as the comparatively high cost to engineer a sensor.

However, just like any other technologies, CCDs will also continue to improve and make up for

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its shortcomings against CMOS sensors. As a resulted, it is speculated that CMOS sensors will

not replace CCDs, but will exist in parallel as an alternative technology.

Applications for CMOS sensor will also expand in the future. Right now, cell phones are the

biggest application for CMOS sensors in terms of unit volume, but CMOS sensors are beginning

to find their way into new applications. For example, the usage of CMOS sensors is increasing in

the automotive industry, specifically for safety systems. Security industries are also gaining

interest in sensors with on-board intelligence [Litwiller, 2005].

Conclusion

The CMOS sensor allows images to be captured digitally and functions based on the principle of

the photoelectric effect. CCDs are another type of digital image capture technology that is also

based on the photoelectric effect. Other than that, both technologies operate differently and have

different characteristics.

The main difference is CMOS sensors have most of their required circuitry and components

integrated onto the sensor, resulting in a smaller, less energy consuming system than CCD based

systems. This makes CMOS sensors better suited for smaller consumer electronics, such as cell

phone cameras and webcams. It is also used in some digital cameras and scanners. CCDs yield

better image results overall, and have lower development costs (although manufacturing costs

are comparable between the two).

In the future, both technologies will continue to improve, and their differences in characteristics

will ensure that nether technologies will replace one another.

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References

DALSA. (2010). CCD vs. CMOS. Retrieved February 12, 2010 from http://www.dalsa.com/

sensors/Products/ccd_vs_cmos.aspx

Fowler, M. (1997). The Photoelectric Effect. Retrieved March 4, 2010 from

http://galileo.phys.virginia.edu/classes/252/photoelectric_effect.html

Kender, D. (November 2006). The ClearVID CMOS, Sony’s Chip of Choice. Retrieved March 4,

2010 from http://www.camcorderinfo.com/content/The-ClearVID-CMOS-Sonys-Chip-

of-Choice.htm

Litwiller, D. (January 2001). CCD vs. CMOS: Facts and Fiction. Retrieved February 26, 2010

from http://www.dalsa.com/public/corp/Photonics_Spectra_CCDvsCMOS_ Litwiller.pdf

Litwiller, D. (August 2005). CCD vs. CMOS: Maturing Technologies, Maturing Markets.

Retrieved January 26, 2010 from http://www.dalsa.com/public/corp/CCD_vs_CMOS_

Litwiller_2005.pdf

McHugh, S. (February 2010). Digital Camera Sensors. Retrieved March 4, 2010 from

http://www.cambridgeincolour.com/tutorials/camera-sensors.htm

Zumdahl, S. S. (2008) Chemical Principles (6th ed.).Boston: Houghton Mifflin Custom

Publishing.