A charge-coupled device (CCD) is a device (described as an
"analog shift register") made up of semiconductors that enables the
transmission of analog signals (electric charges) through successive
stages (capacitors), controlled by a clock signal. "CCD" refers to the
way the image signal is read out from the chip. Under the control of an
external circuit, each capacitor can transfer its electric charge to one
or other of its neighbors. CCDs can be used as a form of memory or for
delaying samples of analog signals.
oday, charge-coupled devices are most widely used in arrays of
photoelectric light sensors, to serialize parallel analog signals. CCDs
are used in digital photography, astronomy (particularly in photometry
and "lucky imaging"), sensors, electron microscopy, medical fluoroscopy,
and optical and UV spectroscopy. (Not all image sensors use CCD
technology; for example, CMOS chips are also commercially available.)
History
In 1961, Eugene F. Lally of the Jet Propulsion Laboratory published a
paper titled "Mosaic Guidance for Interplanetary Travel," illustrating a
mosaic array of optical detectors that formed a photographic image
using digital processing. This paper gave birth to the concept of
digital photography. Lally noted such an optical array required
development so digital cameras could be produced.
The required array consisting of CCD technology was invented in 1969 by
Willard Boyle and George E. Smith at AT&T Bell Labs. The lab was
working on the picture phone and the development of semiconductor bubble
memory. Merging these two initiatives, Boyle and Smith conceived of the
design of what they termed "Charge 'Bubble' Devices." The essence of
the design was the ability to transfer charge along the surface of a
semiconductor.
As the CCD started its life as a memory device, one could only "inject"
charge into the device at an input register. However, it was immediately
clear that the CCD could receive charge via the photoelectric effect
and electronic images could be created. By 1969, Bell researchers were
able to capture images with simple linear devices; thus the CCD was
born.
Several companies, including Fairchild Semiconductor, RCA and Texas
Instruments, picked up on the invention and began development programs.
Fairchild was the first with commercial devices and by 1974 had a linear
500 element device and a 2-D 100 x 100 pixel device. Under the
leadership of Kazuo Iwama, Sony also started a big development effort on
CCDs involving a lot of money. Eventually, Sony managed to mass produce
CCDs for their camcorders. Before this happened, Iwama died in August
1982. Subsequently, a CCD chip was placed on his tombstone to
acknowledge his contribution.
In January 2006, Boyle and Smith were awarded the National Academy of
Engineering Charles Stark Draper Prize for their work on the CCD.
Basics of operation
In a CCD for capturing images, there is a photoactive region (an
epitaxial layer of silicon), and a transmission region made out of a
shift register (the CCD, properly speaking).
An image is projected by a lens on the capacitor array (the photoactive
region), causing each capacitor to accumulate an electric charge
proportional to the light intensity at that location. A one-dimensional
array, used in line-scan cameras, captures a single slice of the image,
while a two-dimensional array, used in video and still cameras, captures
a two-dimensional picture corresponding to the scene projected onto the
focal plane of the sensor. Once the array has been exposed to the
image, a control circuit causes each capacitor to transfer its contents
to its neighbor. The last capacitor in the array dumps its charge into a
charge amplifier, which converts the charge into a voltage. By
repeating this process, the controlling circuit converts the entire
semiconductor contents of the array to a sequence of voltages, which it
samples, digitizes and stores in some form of memory.
Detailed physics of operation
The photoactive region of the CCD is, generally, an epitaxial layer of
silicon. It has a doping of p+ (Boron) and is grown upon the substrate
material, often p++. In buried channel devices, the type of design
utilized in most modern CCDs, certain areas of the surface of the
silicon are ion implanted with phosphorus, giving them an n-doped
designation. This region defines the channel in which the photogenerated
charge packets will travel. The gate oxide, that is, the capacitor
dielectric, is grown on top of the epitaxial layer and substrate. Later
on in the process polysilicon gates are deposited by chemical vapor
deposition, patterned with photolithography, and etched in such a way
that the separately phased gates lie perpendicular to the channels. The
channels are further defined by utilization of the LOCOS process to
produce the channel stop region. Channel stops are thermally grown
oxides that serve to isolate the charge packets in one column from those
in another. These channel stops are produced before the polysilicon
gates are, as the LOCOS process utilizes a high temperature step that
would destroy the gate material. The channel's stops are parallel to,
and exclusive of, the channel, or "charge carrying," regions. Channel
stops often have a p+ doped region underlying them, providing a further
barrier to the electrons in the charge packets (this discussion of the
physics of CCD devices assumes an electron transfer device, though hole
transfer is possible).
One should note that the clocking of the gates, alternately high and
low, will forward and reverse bias the diode that is provided by the
buried channel (n-doped) and the epitaxial layer (p-doped). This will
cause the CCD to deplete, near the p-n junction and will collect and
move the charge packets beneath the gates—and within the channels—of the
device.
It should be noted that CCD manufacturing and operation can be optimized
for different uses. The above process describes a frame transfer CCD.
While CCDs may be manufactured on a heavily doped p++ wafer it is also
possible to manufacture a device inside p-wells that have been placed on
an n-wafer. This second method, reportedly, reduces smear, dark
current, and infrared and red response. This method of manufacture is
used in the construction of interline transfer devices.
Architecture
The CCD image sensors can be implemented in several different
architectures. The most common are full-frame, frame-transfer and
interline. The distinguishing characteristic of each of these
architectures is their approach to the problem of shuttering.
In a full-frame device, all of the image area is active and there is no
electronic shutter. A mechanical shutter must be added to this type of
sensor or the image will smear as the device is clocked or read out.
With a frame transfer CCD, half of the silicon area is covered by an
opaque mask (typically aluminum). The image can be quickly transferred
from the image area to the opaque area or storage region with acceptable
smear of a few percent. That image can then be read out slowly from the
storage region while a new image is integrating or exposing in the
active area. Frame-transfer devices typically do not require a
mechanical shutter and were a common architecture for early solid-state
broadcast cameras. The downside to the frame-transfer architecture is
that it requires twice the silicon real estate of an equivalent
full-frame device; hence, it costs roughly twice as much.
The interline architecture extends this concept one step further and
masks every other column of the image sensor for storage. In this
device, only one pixel shift has to occur to transfer from image area to
storage area; thus, shutter times can be less than a microsecond and
smear is essentially eliminated. The advantage is not free, however, as
the imaging area is now covered by opaque strips dropping the fill
factor to approximately 50 percent and the effective quantum efficiency
by an equivalent amount. Modern designs have addressed this deleterious
characteristic by adding microlenses on the surface of the device to
direct light away from the opaque regions and on the active area.
Microlenses can bring the fill factor back up to 90 percent or more
depending on pixel size and the overall system's optical design.
The choice of architecture comes down to one of utility. If the
application cannot tolerate an expensive, failure prone, power hungry
mechanical shutter, then an interline device is the right choice.
Consumer snap-shot cameras have used interline devices. On the other
hand, for those applications that require the best possible light
collection and issues of money, power and time are less important, the
full-frame device will be the right choice. Astronomers tend to prefer
full-frame devices. The frame-transfer falls in between and was a common
choice before the fill-factor issue of interline devices was addressed.
Today, the choice of frame-transfer is usually made when an interline
architecture is not available, such as in a back-illuminated device.
CCDs containing grids of pixels are used in digital cameras, optical
scanners and video cameras as light-sensing devices. They commonly
respond to 70 percent of the incident light (meaning a quantum
efficiency of about 70 percent) making them far more efficient than
photographic film, which captures only about 2 percent of the incident
light.
Most common types of CCDs are sensitive to near-infrared light, which
allows infrared photography, night-vision devices, and zero lux (or near
zero lux) video-recording/photography. For normal silicon based
detectors the sensitivity is limited to 1.1μm. One other consequence of
their sensitivity to infrared is that infrared from remote controls will
often appear on CCD-based digital cameras or camcorders if they don't
have infrared blockers.
Cooling reduces the array's dark current, improving the sensitivity of
the CCD to low light intensities, even for ultraviolet and visible
wavelengths. Professional observatories will often cool their detectors
with liquid nitrogen, to reduce the dark current, and hence the thermal
noise, to negligible levels.
CCDs in astronomy
CCDs offer high quantum efficiencies, linearity of output (one count for
one photon of light), and ease of use compared to photographic plates.
For these and a variety of other reasons, CCDs were rapidly adopted by
astronomers for nearly all UV-to-infrared applications.
Thermal noise, dark current, and cosmic rays may alter the pixels in the
CCD array. To counter such effects, astronomers take an average of
several exposures with the CCD shutter closed and opened. The average of
images taken with the shutter closed is necessary to lower the random
noise. Once developed, the “dark frame” average image is then subtracted
from the open-shutter image to remove the dark current and other
systematic defects in the CCD (dead pixels, hot pixels, and so forth).
The Hubble Space Telescope, in particular, has a highly developed series
of steps (“data reduction pipeline”) used to convert the raw CCD data
to useful images. See for a more in-depth description of the steps in
processing astronomical CCD data.
CCD cameras used in astrophotography often require sturdy mounts to cope
with vibrations and breezes, along with the tremendous weight of most
imaging platforms. To take long exposures of galaxies and nebulae, many
astronomers use a technique known as auto-guiding. Most autoguiders use a
second CCD chip to monitor deviations during imaging. This chip can
rapidly detect errors in tracking and command the mount's motors to
correct for them.
An interesting unusual astronomical application of CCDs, called
"drift-scanning," is to use a CCD to make a fixed telescope behave like a
tracking telescope and follow the motion of the sky. The charges in the
CCD are transferred and read in a direction parallel to the motion of
the sky, and at the same speed. In this way, the telescope can image a
larger region of the sky than its normal field of view. The Sloan
Digital Sky Survey is the most famous example of this, using the
technique to produce the largest uniform survey of the sky yet.
Color cameras
Digital color cameras generally use a Bayer mask over the CCD. Each
square of four pixels has one filtered red, one blue, and two green (the
human eye is more sensitive to green than either red or blue). The
result of this is that luminance information is collected at every
pixel, but the color resolution is lower than the luminance resolution.
Better color separation can be reached by three-CCD devices (3CCD) and a
dichroic beam splitter prism, that splits the image into red, green,
and blue components. Each of the three CCDs is arranged to respond to a
particular color. Some semi-professional digital video camcorders (and
most professionals) use this technique. Another advantage of 3CCD over a
Bayer mask device is higher quantum efficiency (and therefore higher
light sensitivity for a given aperture size). This is because in a 3CCD
device most of the light entering the aperture is captured by a sensor,
while a Bayer mask absorbs a high proportion (about 2/3) of the light
falling on each CCD pixel.
Since a very-high-resolution CCD chip is very expensive as of 2005, a
3CCD high-resolution still camera would be beyond the price range even
of many professional photographers. There are some high-end still
cameras that use a rotating color filter to achieve both color-fidelity
and high-resolution. These multi-shot cameras are rare and can only
photograph objects that are not moving.
Sensor sizes
Sensors (CCD/CMOS) are often referred to with an imperial fraction
designation such as 1/1.8" or 2/3," this measurement actually originates
back in the 1950s and the time of Vidicon tubes. Compact digital
cameras and Digicams typically have much smaller sensors than a Digital
SLR and are thus less sensitive to light and inherently more prone to
noise.