Snapshot: Janesick and Blouke provide a back-to-

basics article on charge-coupled devices and outline

their current and potential applications.

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PAST, PRESENT, &FUTURE By James R. Janesick and Morley Blouke

Philips Research Laboratories introduced to the world the bucket-brigade device (BBD)—a remark­able invention—in 1966. When Bell Laboratories

reshaped the B B D in 1970 the charge-coupled device (CCD) was born. Early C C D s were influenced by three key factors. First, the C C D had to compete with photo­graphic film as an imaging detector. Fi lm was a tough contender because its performance, cost, and ease of use were well established in the scientific community. Second, the commercial imaging market wanted an electronic solid-state imaging detec­tor to replace the vacuum tube. Developers saw an opportunity in the C C D ' s size, weight, low-power consumption, ultra-low-noise, linearity, dynamic range, photometric accura­cy, broad spectral response, geometric stability, reliabil­ity, and durability. They also attempted to match tube characteristics in for­mat, frame rate, cosmetic quality, and cost. The third factor that influenced C C D development was the need for a new detector in space imaging applications. Two early Jet Propulsion Lab (JPL) C C D programs, the Galileo Orbiter Solid-State Imager and the in i t ia l Hubble Space Telescope (HST) Wide Field/Plane­tary C a m e r a ( W F / P C ) , required several detectors.

In 1972, JPL initiated an advanced program to develop the scientific C C D . Three U.S. C C D manufactur­ers were considered: R C A , F a i r c h i l d , and Texas Instruments (TI). Fairchild was a pioneer in developing small buried channel devices. The first Fairchild sensor exhibited a read noise floor (for a definition of this and other key terms see page 19) of 30 e- rms and a charge transfer efficiency (CTE) better than 0.99995 per pixel transfer. These early devices were a tremendous benefit in getting the scientific C C D off the ground. For example, in 1974 a 100 X 100 C C D was used in conjunction with an eight-inch Celestron telescope to generate the first astro­nomical image of the moon (i.e., "first light") for the

C C D . Unfortunately, the architectural philosophy that Fairchild followed (i.e., interline transfer) was not opti­mized for scientific performance primarily in achieving high quantum efficiency.

R C A took a different approach initially: a full frame, backside i l lumina ted C C D that, in theory, w o u l d achieve the highest quantum efficiency (QE) possible. R C A was developing the largest C C D at the time, a 512 (Vertical) X 320 (Horizontal) frame transfer device intended for commercial T V applications. Unfor tu ­

nately, these early devices were based o n surface channel technology. Tests performed on these units at JPL showed that C T E per fo rmance was p o o r (<0.99) and h i g h read noise (100 e- rms). Both R C A a n d F a i r c h i l d focused on commercia l pixel formats with limit­ed spat ial r e s o l u t i o n , unsatisfactory for most scientific requirements.

It became clear from these early studies that a special R & D project was necessary to combine the best attributes of all C C D technologies known at the time. JPL then contracted TI to work on a scientific C C D sensor based o n backside i l l u m i n a t i o n , full-frame, buried channel technology, wi th pixel counts equivalent to or greater than the V id icon tube (i.e., 1024 X 1024).

In 1976, it was announced that a buried channel, back­side illuminated, 800 X 800 device was being fabricated for the Galileo mission to Jupiter. Collaboration between JPL and TI progressed for over a decade and significantly advanced the scientific C C D we use today.

General operation C C D s were initially conceived as an electronic analogue of the magnetic bubble device. To function as memory, there must be a physical quantity that represents a bit of information, a means of recognizing the presence or absence of the bit (reading), and a means of creating

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Optics & Photonics News/April 1995 17

and destroying the information (writing and erasing). In the CCD, a bit of information is represented by a packet of electrons. These charges are stored in the depletion region of a metal insulator semiconductor (MIS) capacitor, an important CCD structure to be described below. Charges are moved about in the CCD circuit by placing the MIS capacitors so as to allow the charge to spill from one capacitor to the next: thus the name charge-coupled device. A charge detection ampli­fier detects the presence of the charge packet providing a useful voltage to the outside world. Charge packets can be created by injecting charge from a diode or optically to generate images. Like the magnetic bubble device, the CCD is a serial device where charge packets are read one at a time.

The operation of a CCD is quite simple in principle. An analogy to describe CCD operation is shown in Figure 1. Deter­mining the brightness dis­tribution in a CCD image can be likened to measur­ing the rainfall at differ­ence points in a field with an array of buckets. Once the rain has stopped, the buckets in each row are moved down vertically across the field on convey­or belts. As the buckets in each column reach the end of the conveyor, they are emptied into another bucket system on a hori­zontal belt that carries it to a metering station where its contents are measured.

Technically speaking, the CCD must perform four tasks in generating an image. These functions are referred to as: (1) charge generation, (2) charge collection, (3) charge transfer, and (4) charge detection. The first operation relies on a physical process known as the photoelectric effect. This process occurs when free electrons are liberated when photons or particles interact with certain materials (typically silicon for CCDs).

In the second step, charge collection, the photoelec­trons are collected in the nearest discrete collecting site. These sites are referred to as picture elements or pixels, for short. Pixels are defined by electrodes, called gates, formed on the surface of the CCD. In our rainfall anal­ogy, the buckets represent the pixels.

The third operation, charge transfer, is accomplished by manipulating the voltage on the gates in a systematic way so that the signal electrons move down vertically from one pixel to the next in a conveyor-belt-like fash­ion. At the end of the columns is a horizontal register of pixels. This register collects a line at a time and then transports the charge packets in a serial fashion to an output amplifier.

The final operating step that the CCD must perform is when the charge packet from the horizontal register is converted to a useful output voltage by the on-chip

amplifier. This voltage is amplified, processed, and digitally encoded off chip. The digital data is then stored in a computer and an image reconstructed on a television monitor.

Current technology CCD technology, now cel­ebrating its 25th birthday, has matured to a remark­able degree. Through much of the 1970s, CCD fabrication was beset by countless difficulties, including contamination, variable and incompletely understood processing steps, rudimentary design tools, and deficient model­ing of device performance. As a consequence, a great deal of trial and error was involved. Technological progress took place despite these instabilities and unknowns, but the price was high. As an example, Galileo and HST entailed a total of approximately 150 production lots to develop and produce suitable CCDs. In contrast, today it

is not uncommon for a new custom CCD to be success­fully produced on the first attempt. This maturation has enabled the production of CCDs at prices that are more widely affordable and has spawned an explosion in CCD applications.

Indicators of this maturity are numerous: 1. CTE (0.9999995) and production yield have progressed

to the point where CCDs having 108 pixels are being planned—10,000 times more pixels than the first CCDs tested. Device yield has improved where it is now feasi­ble to build wafer-scale arrays. For example, a 7000 X 9000, 12-μm pixel CCD that occupies an entire 6-in. silicon wafer is currently in the planning stage.

Figure 1 (Above). An analogy often used to explain how a CCD (shown below) operates. A 3 x 3 bucket (pixel) array is shown with vertical and horizontal conveyor belts (CCD registers) that transfer water (electrons) to a measuring device (amplifier).

18 Optics & Photonics News/April 1995

2. Read noise below one electron rms has been achieved. Well capacity has been improved almost a factor of 10 compared to CCDs fabricated 15 years ago. Dynamic range for some CCDs today is enor­mous, greater than 106.

3. For back-illuminated, thinned devices, the quantum efficiency is remarkably high (> 50%) over an unprecedented range of wavelengths, approximately 1-10,000 Å.

Applications CCD development continues to make great strides as more and more applications open up to the sensor. It is only recently that CCDs are beginning to be accepted for general scientific imaging applications. These devices have been used for many years in consumer electronics, notably in the nearly ubiquitous camcorder, first as a black and white sensor and finally as a full color, single-chip imager. Today it is possible to pur­chase a complete miniature NTSC- or PAL-compatible CCTV camera for a few hundreds of dollars. CCDs have virtually replaced vidicons in all but perhaps the most demanding of broadcast TV applications. A little less well known, but an important medical application, is as the sensor for laproscopic, endoscopic, and arthro­scopic applications. These devices tend to have similar format and smaller physical size than the normal TV sensors.

In spite of its success, the widespread application of CCDs to solve high-speed image problems has been delayed. Now, over 50% of new CCD applications are oriented to high-speed imaging. This delay part may be attributed, in large part, to the lack of availability of high-speed computers with lots of memory and rela­tively easy-to-use image processing software packages. Because of its rectilinear format, the CCD is the natural input device for robotic, scientific, medical, and indus­trial applications where quantitative output is desired. After integrating a scene, the image is naturally spatially quantized and the output is normally digitized. For example, a 1024 X 1024 pixel array produces 2 MB of data for each frame at 16 bits/pixel. CCD groups are currently designing 1024 X 1024 sensors that readout at 1000 frames/sec (over a billion pixels/sec). At this frame rate, an enormous amount of data can accumu­late rapidly, thus the need for rapid data analysis. In addition, there are the slope and offset corrections for each pixel that are required to correct the data.

There are both scientific and medical applications for CCDs involving the detection of x-ray photons. For imaging x-ray applications, the feature of the photode­tection process that is important is the fact that, on average, one electron is generated per 3.65 eV of inci­dent energy. As a consequence, if one forms an image of an x-ray scene, then the device produces not only the location of the source, but also the energy of the inci­dent photon. This results in a two-dimensional imaging spectrometer. Measurements have shown that CCDs

have energy resolution of 0.2 keV in the 1-10 kev range. Such devices will be used in the focal plane of NASA's Advanced X-ray Facility (AXAF) satellite and the European Space Agency's XMM mission.

One of the more interesting biological applications is the use of back-illuminated CCDs when building x-ray microscopes. In the region between 238-530 Ev, biological materials are relatively transparent due to the fact that water absorbs weakly in this region. Fresnel zone plates that focus x-rays and tunable sources such as a synchrotron give researchers another tool to ana­

lyze the topography and structure of cells that is not possible with other higher energy, more destructive methods. With this technique, it is possible to image relatively thick samples in aqueous solutions and study cellular content and distribution of proteins, nucleic acids, and carbohydrates.

For medical uses, the devices are either lens coupled or fiber optically coupled to a phosphor. The phosphor absorbs and detects the x-ray quanta. This has two advantages: First, the phosphor, rather than the CCD, absorbs the x-ray energy, and second, with proper selection of the phosphor, the wavelength of the emit­ted photons matches the region of the CCD's highest sensitivity. Spot mammography systems now exist that are used to perform stereotactic images of microcalcifi­cations and suspicious lesions in breast tissue. In the process, two stereo images, spatially separated by 30°, are taken a few seconds apart. The location of the lesion is determined from these data by triangulation. If required, a biopsy can be taken immediately. The whole

Glossary

Buried channel CCD: A C C D technology where signal charge is transferred in the

bulk silicon away from the surface to prevent signal trapping. Charge transfer effi­

ciency (CTE) performance dramatically improved when this technology was imple­

mented.

CCD—Charge-coupled device: A micro-electronic device that is used in memo­

ry, signal processing, and imaging applications.

CTE—Charge transfer efficiency: The ability to transfer charge from pixel to

pixel in a C C D array (e.g, CTE=0.9999 means that 99.99% of the charge is trans­

ferred in one pixel transfer).

Frame transfer, Interline transfer: C C D technologies where charge is trans­

ferred rapidly into onboard C C D storage registers that are not light sensitive. The

storage registers are read out while the image region collects another image. These

technologies are popular in cameras where fast electronic shuttering is required.

Surface channel: The first C C D technology fabricated where charge is transferred

at the surface between the gate electrode and silicon. Replaced by buried channel

technology because of charge trapping problems.

Pixel: Short for "picture element"—small discrete elements that together constitute

an image.

Read noise: The uncertainty in charge measurement generated in the C C D ampli­

fier (expressed in units of rms electrons). It sets the limit for the smallest charge

packet that can be measured.

Optics & Photonics News/April 1995 19

process takes place in the physician's office rather than in a hospital. These systems are being used routinely today. Full breast imaging systems are now under devel­opment and prototype systems have been demonstrat­

ed. Intraoral devices, using 1k X 3k pixels arrays and a PC, now produce quality x-ray images of the mouth, replac­ing x-ray dental film. In both these applications, the patient is subjected to approximately 1/10th the x-ray dose that would be used in the alterna­tive procedure.

CCDs provide the multi­channel advantage offered by array detectors. The fact that it is a two-dimensional array leads to the possibility of tak­ing multiple spectra simulta­neously. This feature is used in several applications using

optical fibers. In one method, an array of single fibers are distributed throughout a process cell, sampling var­ious regions of interest. The exit pupils of the fibers are then arranged to fill the input slit of a spectrometer. In this manner, tens to hundreds of spectra can be record­ed simultaneously, allowing one to monitor the progress of chemical reactions in various portions of the sample. If a second set of fibers is arranged such that each fiber conveys power at a different wavelength (for example, at the output of a monochrometer), it becomes possible to measure emission/excitation matrix of a material.

One is not limited to measuring the properties with­in a single sample with this method. Astronomers use the technique to measure spectra of multiple objects simultaneously by placing the input end of the fiber at the location in the focal plane where the star of galaxy of interest is imaged. Systems now exist that use thinned devices for highest quantum efficiency; these can record the spectra of up to 600 objects simultaneously.

Another area of spectroscopy where the large for­mat, two-dimensional nature of CCDs has found use is in high-resolution or echelle spectroscopy. In these applications, the spectrum is analyzed by a grating to produce high-resolution spectra with overlapping orders, and the orders (which contain the spectral information in successive spectral regions) are separat­ed in the orthogonal direction by a cross-dispersing ele­ment. The resulting two-dimensional spectrum is imaged onto the CCD. In this way, it is possible to obtain spectra covering the UV to the near-IR range with 0.01 nm resolution.

The ability to make very large format devices with small pixels has lead to the development of a number of systems designed for electron still photography. Thirty-five millimeter format cameras with an electronic focal

plane and a medium for storing images has been avail­able for over 10 years. With the availability of larger for­mat devices, systems for professional cameras are now appearing. Some of these systems are replacement packages for the film pack on existing cameras while others represent completely new cameras. The resolu­tion of the images varies from 1.3 Mpixel to arrays of 5000 X 7000 elements. One system using a square for­mat 2000 X 2000 sensor is designed specifically for stu­dio quality still photography. An extension of this activ­ity is the movie industry's interest in digital photogra­phy. In this instance high-resolution images of 4k X 4k or more pixels are desired, with sensors capable of 30-60 frames per second data rates. Such devices are now in the offing.

Time-delay and integration (TDI) is a method of reading the CCD which capitalizes on the fact that the charge packets must move down columns to the output register before being read out. This method makes it possible to image moving targets, such as film of mater­ial or documents, by clocking the device in synchrony with the motion of the scene across the image plane. In this way, not only is high-speed linear imaging possible, but a better signal can be achieved since the effective integration time on the target is increased by the num­ber of rows. A number of such cameras and systems exist including RS170 output for industrial applica­tions, high-resolution systems with the resolutions of >1000 dpi for document scanning, and color scanners for graphic arts and digital archiving of artistic works.

Scientific CCDs have amazed the scientific commu­nity and are truly a remarkable invention. It is certain that the device will continue to impress those who use it by generating even more applications and scientific data.

James R. Janesick is group leader, CCD Advanced Development Group at the Jet Propulsion Laboratory, Pasadena, Calif. Morley Blouke is director of technology at Scientific Imaging Technologies Inc., Beaverton, Ore.

Scientific CCDs have amazed the scientific community and are truly a remarkable invention.

20 Optics & Photonics News/April 1995