article on ccd

8/14/2019 Article on CCD

http://slidepdf.com/reader/full/article-on-ccd 1/6

Charge Coupled Devices (CCDs): The Workhorses of Modern Electronic ImagingDevices

By

Shivaprasad M Khened, Curator, Nehru Science Centre, Mumbai

Introduction

From the incredible pictures of the outer space taken by the Hubble SpaceTelescope, to the digital images generated by the modern professional photographers,all use an invincible technological spectacle, the Charge Coupled Device, CCD for short.Rapid developments that have seen the light of the day in the ever developingmicroelectronic technology have presented humankind with newfound opportunities for ahitherto unknown view of the heavens, and that too with an unprecedented sensitivity.The CCDs are an integral part of every modern electronic digital imaging device and arewoven into the very fabric of these devices, which encompass every perceivable modernday imaging application. CCDs have been used in wide applications ranging from thestudio camera work and motion picture industry to the law enforcement and X-ray filmscanning. CCDs have also been used for exploration of the universe and earth andeverything in between. Cockpit displays and aerial photography are some other applications that use a wide spectrum of digital electronic imaging. To sum it all CCDsare inextricably linked to the modern day electronic imaging devices that are used inproviding extraordinary electronic imaging solutions for virtually every modern imagingapplication.

Need for CCDs

The significance of the CCDs can be measured by understanding the capabilities

of our eyes as a source of light detectors. The eye in fact is a very good light detector,perfectly tailored to its everyday uses, but it has its limitations especially for astronomicalstudies. Its efficiency can be as high as a few percent, which is respectable even whencompared with some of the light detecting devices. However the eye responds only to alimited range of colours. Radiations in the neighbouring ultraviolet and infrared regions of the electromagnetic spectrum are invisible to the naked eye. Eye is also a poor judge of absolute brightness. It cannot store light for more than a few tenths of a second. The eyetherefore has proved to be an ineffective imaging tool for satisfying the unending questof humankind to understand the vast expanse of nature and to try and unravel itsmysteries. It was this necessity that has led to the invention of the modernmicroelectronic wonder marvel, the Charge Coupled Device. The CCD is a light-sensitive integrated circuit used in a wide variety of applications, primarily in imaging.

Advances in solid-state physics led to the invention of CCD technology, which nowserves science from the bottom of the ocean to outer space.

CCD and Digital Cameras

Digital cameras are now the most sought after devices that have revolutionizedthe photographic industry. Just as any other modern electronic gadgets, the digitalcameras are also becoming cheaper and cheaper and have followed a path of other electronic gadgets of remaining anti inflationary while at the same time offering more and



more features. The popularity of the digital cameras has reached a new zenith. Theability to take hundreds or even thousands of pictures without having to worry about filmand developing costs and the ability to send these pictures by email to friends, familyand business contacts all over the world, are just some of the many reasons why digitalcameras are becoming more and more popular. While many digital cameras look liketraditional point-and-shoot film cameras, what’s inside is quite different and the main

difference is the presence of the ubiquitous CCD.

Digital cameras produce high quality digital images that are captured usingspecial light sensitive Charge Coupled Device integrated circuits. Digital cameras cancontain many thousands or even millions of these microscopically small light sensitiveelements. One or more of these elements are considered to represent a single point of light, or pixel. Light falling on a CCD causes a minute electrical charge, proportional tothe brightness of the light, to be held within each element. Absence of any chargecorresponds to black, and a presence of full charge to white. Depending on the chargecontent the nature of the colour can be interpreted by analyzing the charge producedfrom these devices. Each element in the CCD holds an analog signal (since there areinfinite variations between no charge and full charge). To convert each charge to its

digital equivalent, an analog-to-digital converter is used which divides the range of charges into a discrete number of stages or steps to which the charge may beapproximated. If the converter has allowed 8-bits to store the charge of each pixeldigitally, 256 discrete levels of brightness can be described (2 raised to the power 8).Some systems devote more bits to each pixel, allowing for more brightness levels andtherefore smoother gradation between levels. Higher the bits the system uses the better are the gradations between the levels in the brightness and there fore a better resolutionwill be the result.

The beginning of CCD

The seeds for the utility of the concept of usage of the CCD lay in another device

the Bucket Bridge Device. In 1969 F.Sangster and K.Teer of the Philips Research Labsinvented the Bucket-Brigade Device or BBD. The function of this device was to transfer acharge packet from one transistor to another. One year later, Willard Boyle and George.Smith of the Bell Laboratories extended this concept by inventing a transport mechanismfrom one capacitor to another one. This new device got the name Charge CoupledDevice or simply CCD for short. George Smith worked at Bell Labs from 1959 to 1986.For much of this time, he led research aimed at creating novel lasers and other semiconductor devices. Willard Boyle was at Bell Labs from 1953 to 1979. He worked onthe research in optical and satellite communications, digital and quantum electronics,computing, and radio astronomy; he also helped NASA choose a site for the Apollolanding on the moon. Willard Boyle and George. Smith have been honored for their accomplishment of the invention of CCDs with the C&C Prize, one of the world's top

honours in computing and communications. The prize, awarded by the nonprofitFoundation for C&C Promotion in Japan, was established in 1985 by the NECCorporation to honour individuals who have made major contributions to thedevelopment of the global electronics industry. In his award acceptance speech Smithsaid "It is extremely satisfying to have a thirty-year old accomplishment recognized asbeing significant". His co inventor Boyle said “I am always surprised when I hear of anew application for the CCD. It has become an ubiquitous device in such diverse areasas cosmology and internal medicine.”



Most common applications of the CCD in modern days are its utility in thecameras. Interestingly though, the CCDs were not intended to be used in the electronicimagery when they were invented. These devices were conceptualized and developed toreplace the conventional costly memory devices of the time and to combat the ever-

growing problems of the memory devices for the early versions of the computers. Themain inspiration for Smith and Boyle was the challenge of creating a new kind of semiconductor memory for computers, which could store more memory in smaller spaceand cost less. Memory its cost and limitation were the main concern for the computer industry in its infancy. There was another challenge facing the electronic industry that of developing a technology for the video telephone service, which required solid-statecameras. In the space of an hour on October 17, 1969, Smith and Boyle while working atthe Bell labs sketched out the CCD's basic structure, defined its principles of operation,

and outlined its applications concentrating however on the memory part. The devicethey invented stores information, represented by discrete packets of electric charge, incolumns of closely spaced semiconductor capacitors. With multiple columns side byside, a CCD chip can record images. Reading out the information - for processing,

display, or more permanent storage - is accomplished by shifting stored charges downthe columns, one position at a time. The CCD's sensitivity to light, coupled to thismethod of storing and reading out information, makes it a versatile and robust opticaldetector. By 1970, the Bell Labs researchers had built the CCD into the world's firstsolid-state video camera. In 1975, they demonstrated the first CCD camera with animage quality sharp enough for broadcast television.

Omnipresence of CCDs

Talking about the significance of the invention of CCDs, at Bell Labs, ArunNetravali, executive vice president of research at Bell Labs, who shared a 1997 C&CPrize for his contributions in digital image and video compression said, "The story of theCCD highlights two beautiful aspects of research, one is the possibility that you may, inthe process of solving today's problem, create something unexpected that will have ahuge impact in the future. The other is the extent to which frontier science and frontier technologies depend on each other. Advances in solid-state physics led to the inventionof CCD technology, which now serves science from the bottom of the ocean to outer space."

Though CCD were initially designed to serve as a memory device in computers,they soon became a good candidate for immense other applications, prominent amongthem all, was their utility in photography. Since the CCD chip was sensitive to light andthe content of the charge depended directly on the nature of the light falling on thesedevices, inference were soon drawn that they could be used as good image sensors.The first to recognize the potential of the CCD for producing high quality scientificimages were the astronomers who foresaw their quest for understanding the vastuniverse being fulfilled from the use of these devices in their telescopes. The CCD it wassoon realized had a significantly higher sensitivity than photographic film and the thenprevalent vidicon tube devices. With sensitivity as high as 100 times greater than thephotographic films, the CCD soon displaced other sensors within a few years of itsinvention. In the history of physics the invention of new or better equipment usuallycauses a cascading affect which lead to other discoveries. The same was true for the



invention of the CCD as well. Soon, CCD found their use for discovering previouslyinvisible objects and thus increasing our knowledge and understanding of the vast andunending expanse of the universe. Based on new data, theoretical models could beverified or were newly developed.

Today, CCD technology is all encompassing and has found its use not only in

astronomy and broadcasting, but also in, video applications that range from securitymonitoring to high-definition television (HDTV), and from endoscopies to desktopvideoconferencing. Facsimile machines, copying machines, image scanners, digital stillcameras, and bar code readers have employed CCDs to turn patterns of light into usefulinformation. State-of-the-art CCDs can make sense out of light trickling in at rates as lowas one photon per minute. Their efficiency as detectors and their sensitivity across awide band of wavelengths - together with the light weight, low power consumption, highstability, and long life they share with other microchips - have made CCDs omnipresentin all modern day imaging devices. Some of the latest applications of CCDs include theVic Nalwa's 360-degree camera, called the Full Circle camera by Lucent. Other recentapplications of CCDs include their use by Tony Tyson and colleagues to map thedistribution of dark matter in the universe.

CCDs in astronomical observations

Since 1983, when telescopes were first outfitted with solid-state cameras, CCDshave enabled astronomers to study objects thousands of times fainter than what themost sensitive photographic plates could capture, and to image in seconds what wouldhave taken hours before. Today all optical observatories, including the Hubble SpaceTelescope, rely on digital information systems built around "mosaics" of ultra sensitiveCCD chips. Researchers in other fields have put CCDs to work in applications as diverseas observing chemical reactions in the lab and studying the feeble light emitted by hotwater gushing out of vents in the ocean floor. CCD cameras also are used in satelliteobservation of the earth for environmental monitoring, surveying, and surveillance.

CCD Chips

The CCD chip is an array of Metal-Oxide-Semiconductor capacitors (MOScapacitors) each capacitor represents a pixel. By applying an external voltage to the topplates of the MOS structure, charges (electrons (e-) or holes (h+)) can be stored in theresulting potential well. These charges can be shifted from one pixel to another pixel bydigital pulses applied to the top plates (gates). In this way the charges can be transferredrow by row to a serial output register, which could be analysed for producing the desiredoutput. The resulting picture produced from the serial output registers is the display of the electron charge distribution. CCD can be used as a light sensor for cameras, sinceelectrons can be optically generated or more precisely excited from the valence in theconduction band, the Cameras where the light penetrates through the gate structure toreach the region where electrons are collected are called front-illuminated. Moresophisticated in the production, but with a higher sensitivity, are cameras where the CCDchip is exposed from the opposite side. These cameras are called back-illuminated. Toinsure charge transport from the back to the front side where the electrons are collected,the silicon bulk is thinned.



How do charge couple devices (CCDs) work?

A CCD is nothing but a semiconductor like device with one of its face sensitive tothe incident light. The light sensitive face is rectangular in shape and subdivided into agrid of discrete rectangular areas called the picture elements or pixels, each about 10-30micron across. The CCD is placed in the focal plane of an imaging device so that thelight-sensitive surface is illuminated and an image of the object being viewed is formedon it. The light falling on the pixel, in the form of a photon, generates a small electricalcharge, which is stored for later read-out and interpretation. The quantity of chargegenerated by the CCD depends directly illumination level with brighter illuminationproducing greater charge and vice versa. The CCD pixel grids are usually square andthe number of pixels on each side varies from a minimum of 64x64 elements to a high of 2048x2048 elements. The pixel grids are formed to comply with the binary systemformat, which forms the basis for any digital electronics. During the early period of theintroduction of the CCD in the 1970s, 64x64 elements were used in the grid. The 1980sadopted an improved 256x256 or 512x512 element chips in the grid matrix. The modernday imaging devices commonly use 1024x1024 or 2048x2048 element chips. A CCD inisolation is just a semiconductor chip. In order to turn it into a usable imaging device itneeds to be connected to an electronics circuitry to power it, control it, read it andinterpret it. By using a few clocking circuits, an amplifier and a fast analogue-to-digitalconverter (ADC), usually of 16-bit accuracy, it is possible to estimate the amount of lightthat has fallen onto each pixel by examining the amount of charge it has stored. Thus,the charge stored in each of the pixels is converted by the electronic circuitry intonumbers. These numbers are expressed as Analogue data units (ADUs). ADUs are notyet calibrated into physical units. The Analogue data Converter (ADC) factor is theconstant of proportionality to convert ADUs into the amount of charge (expressed as a

number of electrons) stored in each pixel. This factor is needed during the data reductionand is usually included in the documentation for the instrument. The chip will usually beplaced in an insulating flask and cooled (often with liquid nitrogen) to reduce the noiselevel. The electronics controlling the CCD chip are interfaced to a computer, which inturn controls them. Thus, the images observed by the CCD are transferred directly tocomputer memory, with no intermediate analogue stage, and thus they can be directlyplotted on an image display device or stored directly in any of the computer memorydevices.

Anatomy of CCDThe average CCD consists of three sections namely, photo diodes, shift gates

and solid-state capacitors. Arrays of photo diodes are positioned at the output of theprism. As varying amounts of light strike the diodes, those that are illuminated become"forward biased", and a current flows that is proportional to the intensity of the light. Theshift gate acts as a switch. This permits the current from each diode to be stored in asolid-state capacitor in the CCD. The CCD analog shift register deals with the chargescoming from the capacitors. Each of these registers has an address decoder that allowseach portion of the image to be individually addressed. An address encoder cyclesthrough the field of photosensitive registers, and reads out the analog voltages for eachpixel. The speed of operation of this decoder is synchronized to the scan rate of



television. The CCD analog shift register actually performs two functions. First, all of theshift gates can provide their light intensity signals at once. This is somewhat like acamera shutter opening for a brief instant to let light pass through, and closing andstaying closed while the film is advanced. And the variable "shutter" control on a CCDcamera lets you perform high-speed video sampling - great for sporting events featuringblur-free slow motion playbacks. Second, the serial reading of all of the CCD chip's

individual storage elements provides image scanning, somewhat automatically - noelectron beam required. The actual transfer of the voltages out to the real world is thekey to why CCDs are so ingenious. The CCD unit can transfer the voltage from cell tocell without any loss. This is called charge coupling, which is how the CCD gets itsname: Charge Coupled Device. When the transfer gate of a CCD image sensor isactivated, the CCD's clocking circuitry moves the contents of each picture cell to theadjacent cell. Clocking the shift registers in this manner transfers the light input value of each cell to the output, one value at a time. The CCD chips provide their own scanningcircuitry, in a way. The last cell in the chain sends its voltage, in turn, to the output circuitof the chip. As an added bonus, cycling through all of the cells this way will not only sendout all of the stored voltages, but also discharges all of the cells, too. Everything goesback to normal and the cells are ready to take in a new analog voltage value.

Making of a CCD

The technique used in the making of the CCDs is the same as the one used inmaking other silicon integrated chips: a piece of silicon is covered with a layer of 'photoresist'. Light is shone onto this photoresist through a mask, which maps out tracksand various details to be incorporated onto the chip. The light does not affect areas of photoresist that lie behind the mask, whereas, areas exposed, undergo a chemicalchange. Washing this assembly with a developing chemical, removes the exposedareas, and leaves the unexposed areas intact. It's a bit like producing a photograph withthe mask acting as the negative. A few further washings with various other chemicalsetch away the exposed silicon and leave the unexposed silicon intact. The intact silicon

forms the structure of the chip. Most chips have several layers built up in this way.

Conclusion

From humble beginnings as a compact and inexpensive means to delay videosignals, the charge-coupled device (CCD) has now encompassed every aspect of modern digital imagery. It has traveled a long way in realizing the dream of developing asolid-state imager which is rugged, possesses high sensitivity and image quality,demand for little power, and has resistance to wearing out unlike the imaging tubes of the past. CCD imagers have made tremendous strides in a short time. They are nowomnipresent in modern technological devices and equipments with wide and diverseapplications. In fact since the beginning of the recorded history of photography some150 years before, the year 2003 was a landmark year, the number of digital camerassold during 2003 were more than the conventional film based cameras. The usage of CCD in wide ranging application has continued unabated. Its applications will continue togrow for years to come.

##############################################################

article on ccd

Documents