In this article, written for a non-specialized audience, she explains what a CCD is, how it works, and why it is so important.

What allows us to see to the edges of the Universe, yet also lets us explore and build objects smaller than a hundredth of the width of a human hair? Here is another hint: it has revolutionized astronomy in a little over two decades, and is utilized in many imaging devices. Give up? The answer is the charge-coupled device, or the CCD.

This small, electrical device is familiar to astronomers, physicists and engineers but now even some biologists and chemists are beginning use CCDs in their research. You have likely encountered it before, as CCDs are used in facsimile machines, photocopiers, bar-code readers, closed-circuit television cameras, video cameras, regular photographic cameras, or other sorts of sensitive light detectors. In fact, CCDs have a wide range of applications - everything from reconnaissance and aerial mapping to medicine, microtechnology and astronomy.^6,7

What is a CCD?

A CCD is an electrical device that is used to create images of objects, store information (analogous to the way a computer stores information), or transfer electrical charge (as part of larger device). It receives as input light from an object or an electrical charge. The CCD takes this optical or electronic input and converts it into an electronic signal - the output. The electronic signal is then processed by some other equipment and/or software to either produce an image or to give the user valuable information.^1,2

A CCD chip is a metal oxide semiconductor (MOS) device. This means that its base, which is constructed of a material which is a good conductor under certain conditions, is topped with a layer of a metal oxide. In the case of the CCD, usually silicon is used as the base material and silicon dioxide is used as the coating. The final, top layer is also made of silicon - polysilicon.^1,7

The importance of applying voltages to the depletion region (called biasing the p-n junction) is that it precisely allows us to control applied current through any p-n material. When the p-n junction is reversed-biased, an only infinitesimal amount of applied current can flow, which for all practical purposes is zero. This corresponds to the "off" state. When the p-n junction is forward-biased, current easy flows through the junction (because the smaller electric field does not impede the flow of charges as much). In fact, by plotting a graph of applied voltage versus current flow - an I-V Curve - we can see that the dependence of current flow on applied voltage across the junction is exponential. Forward-bias corresponds to the "on" state. Thus, biasing of the junction through the application of voltages can be used to precisely control the motion of electrical charge.

How does the CCD work?

In a CCD, the electrical field at different parts of the surface is controlled by an array or matrix of electrodes; these electrodes are called the gates. (CCD arrays can be either one-dimensional or two-dimensional, but here we will consider the one-dimensional array in detail, and then apply that information to understand the two-dimensional array.) This array of electrodes biases each small part of the surface differently, which allows any flow of charge on the CCD to be controlled. ^1,2

The surface of the CCD is further broken down into smaller regions called pixels, or picture elements.⁸ This name is appropriate because they represent a single "grain" of the imaged object (just like you can see that your TV images appear to be made up of tiny "grains"). The array of electrodes apply a positive potential, (+V_g, a positive electric field) to two-thirds of each pixel, thus forward-biasing that portion of the pixel. Let's represent the first third of the pixel by (1, the second third by (2, and the last third by (3. So, (1 and (3 are at a positive potential of +V_g, and (2 is at a lower potential, V_s.^1,2,3

When light or photons of high enough energy strike the surface, electrons are usually liberated from the surface.² (The quantum efficiency or the ratio of electrons liberated per incident photon is about 0.70-0.80.⁴) For every electron liberated, a hole is created simply by the act of the electron leaving. Thus, incident photons create electron-hole pairs.^4,5 The hole, being effectively positive, is repelled by the applied positive potential (1 and (3, and eventually escapes into the base of the chip.⁴ The electron, however, is captured in the nearest potential well (2. The more light incident on a pixel, the more electrons captured in the potential wells. Thus, differences in the intensity of incoming light are "recorded" by the number of electrons collected in each potential well.^1,4,5

So now the challenge is to extract information from these "electron-collecting bins" (which may also be thought of as tiny capacitors). To do this, the charge packets (the collection of electrons in each well) must be transferred to another device for data processing. This is accomplished by sequentially changing the applied voltage at the three parts of each pixel. First, the level of the potential barrier (V₃) closest to the data processing device is lowered to the same potential as (2. This causes the electrons to divide between the two wells. The primary mechanism for this electron diffusion is induced self-drift from Coulomb repulsions, which acts to separate the charge. Then, the potential of (2 is raised over a finite time interval (corresponding to the diffusion rate of the electrons from (2 to (3) so that (2 now becomes a potential barrier. The remaining charge is transferred from (2 to (3 by these changes in potential. (1 is maintained at constant potential during this entire process to keep the charge packets separate from one another. Now, each charge packet in the row has moved over one-third of a pixel closer to the data processing device. This cycle is repeated over and over in fractions of a second to transfer all the charge off the chip to a detector which usually uses a load resistance to measure the amount of charge collected in each "bin". This is how the three-phase CCD works. ^1,3,4,7 The term charge-coupling in charged-coupled device comes from the coupling of electrical potentials.⁴

A two-dimensional CCD is composed of channels, or rows along which charge is transferred. Each channel is essentially a one-dimensional CCD array. Charge is prevented from moving sideways by channel stops, which are the narrow barriers between the closely spaced channels of the CCD.⁴

Why is the CCD chip so great?

The answer to the above question lies in its resolution.⁸ CCDs provide extraordinary detail for objects either very far away or very small in size - resolution which was hitherto impossible to attain.⁴ This resolution is a result of the large number of pixels in the CCD array - the more pixels, the finer the detail that can be achieved.^2,5 Typically, modern CCDs comprise anywhere from about one thousand to about half a million pixels.⁸ As each pixel is a few microns square, the active area of a two-dimensional CCD array is usually a few millimeters by a few millimeters.^1,3,4,6,8

How do CCDs record color or distinguish among photons of different energies?

In imagers or CCD cameras, CCDs are only part of the whole device. A lens is required to properly focus the incident radiation from the object onto the array. For astronomical applications, the lens is an optical telescopic lens. In addition, since the pixels themselves are monochrome, there must be a way to select for the wavelengths of light incident on the array. Colored filters are thus used to record colors in the case of visible light. In most digital cameras, a color filter array registers the intensity of a single color at each pixel. By interpolation, algorithms use the color intensities at nearby pixels to estimate the intensity of a color at each pixel. This is how a full-color image is generated.⁵ A single picture made by a CCD imager that is only 500 pixels by 500 pixels holds the same amount of raw information as a 100,000 word book!⁴ In addition, the number of electrons collected is proportional to the energy of the incident photons. So mathematically, the energies of the photons liberated can be calculated.

What if an electron is "randomly" liberated? ( i.e. Aren't There Sources of Noise?)

Noise manifests itself during two main processes: the collection of electrons and the transfer of charge packets. During the collection of electrons, noise stems from thermal processes, light pollution, and the generation of electron-hole pairs in the depletion regions.^1,4,7,8 During charge-transfer, noise stems from transfer inefficiency.⁷

During the collection of electrons, thermal noise or dark current is one of the biggest sources of error. Over time, thermal processes fill the depletion region with electrons, which masks stored information.^1,7 To minimize the effects of thermal noise, CCDs are cooled to very low temperatures, which are typically around 150 K.⁴ Other sources of error present during input include random noise from electron-hole pair generation in the depletion region and light from sources other than the desired one (light pollution).^7,8

During charge transfer, efficiency is a major concern. Whenever charge is transferred, a small amount is left behind.⁷ This "residue" blurs the image.⁵ Charge transfer efficiency values greater than 99.9% are common, which translates into only 10% of the original charge being lost after 100 transfers.⁷

Electron traps - which are most common at the surface - collect charge when exposed to a large charge packet and also pose a problem for efficient charge transfer. Over time, they slowly release the charge that they have trapped during later cycles of smaller charge packets. This smears the image. This problem, however, can be minimized by "fat zero". In fat zero, anywhere between 10-25% of the total capacity of the well is filled with electrons before data is even collected. This reduces trapping to tolerable levels. Alternatively, the buried channel CCD structure - a different construction from the surface channel CCD (the one so far described) - can reduces the occurrence of electron traps, since in this design, charge is not exposed to surface-state traps.⁷

What are some applications of the CCD?

CCDs are used in a variety of different imaging devices. Linear imaging devices (LIDs) employing CCDs are typically used in facsimile machines, photocopiers, mail sorters, and bar code readers, in addition to being used for aerial mapping and reconnaissance. Area imaging devices (AIDs) employing CCDs are used for closed-circuit television cameras, video cameras, and vision systems for robotics and as film for astronomical observations.⁷ CCDs are also used in drug discovery in combination with DNA arrays in a process called Multi-Array Plate Screening (MAPS).⁶

Applications: Astronomy

In astronomy, CCD cameras have revolutionized the way that astronomers take and record images. They have extended the range of faint objects astronomers can study, and thus how far across the Universe astronomers are able to "see". CCD cameras have supplanted photographic plates, which were previously used to record astronomical images. One of the other huge advantages to using CCD cameras in astronomy is the ability to convert the gathered analogue information into digital information, which can be analyzed using computer software.⁴

Detection of very faint objects is achieved by minimizing noise from thermal photons by cooling the CCD to temperatures around 160 K. When the CCD operates at such low temperatures, it is said to be functioning in "slow scan" mode, which permits long exposures of up to several hours.^7,8 For galaxies and other astronomical objects as faint as 25 (visual magnitude), this is the only way to detect the light emitted by these objects.⁴ For comparison, the human eye can only see objects brighter than about 5 or 6 (visual magnitude).

Applications: Multi-Array Plate Screening (MAPS)

In MAPS, potential drugs are applied to their targets in a microtiter plates. Their degree of binding to the target - and thus their potential ability as effective drugs - is assessed by the amount of light emitted from the well through either laser-induced fluorescence, radioactive scintillation, or chemiluminescence. As for astronomical applications, the CCD must be cooled to cryogenic temperatures of about -100°C to minimize noise from thermal photons. CCD cameras are able to detect both RNA and DNA in amounts as low as 30,000 molecules, which is about 5.0 x 10-20 moles. The benefit of using CCD cameras in MAPS is that no amplification of the nucleotides is required, which reduces cost and error and saves time.⁶

What Are the Non-Imaging Applications of the CCD?

Non-imaging applications of CCDs include signal processing, memory applications, video integration, transversal filtering, and MTI filtering for radar. Again, non-imaging applications fall under the categories of either signal processing or delay line applications.¹

What Does the Future Hold for CCDs?

The future of imaging devices, however, is not likely to be the CCD. The CMOS or Complementary Metal Oxide Semiconductor image sensor appears to be the future of imaging, because it is fabricated using essentially the same CMOS process as the large majority of modern integrated circuits, which include microprocessors and dynamic random-access memories (RAMs). CCDs, on the other hand, are fabricated using a variant of practically obsolete N-MOS fabrication technology, which is basically only used now in the fabrication of CCDs. What has kept CMOS image devices from replacing CCDs is the trade-off in image quality; there is more noise in CMOS devices, and unwanted signals from various sources degrade the input signal. For many applications requiring that every photon counts, the CMOS is simply not sensitive enough. For the meantime, CCDs will be used in imaging devices where resolution is very important, and CMOS imaging devices will be used when their cheaper cost outweighs the benefit of higher resolution.⁵