A
charge-coupled device (
CCD) is
a device for the movement of electrical charge, usually from within
the device to an area where the charge can be manipulated, for
example conversion into a digital value. This is achieved by
"shifting" the signals between stages within the device one at a
time. Technically, CCDs are implemented as
shift registers that move charge between
capacitive
bins in the device, with the shift allowing for
the transfer of charge between bins.
Often the device is integrated with a sensor, such as a
photoelectric device to produce the charge that is being read, thus
making the CCD a major technology where the conversion of images
into a digital signal is required. Although CCDs are not the only
technology to allow for light detection, CCDs are widely used in
professional, medical, and scientific applications where
high-quality image data is required.
History
The
charge-coupled device was invented in 1969 at AT&T Bell Labs
by Willard Boyle and
George E. Smith. The lab was working on
semiconductor bubble
memory when Boyle and Smith conceived of the design of what
they termed, in their notebook, 'Charge "Bubble" Devices'.The
essence of the design was the ability to transfer charge along the
surface of a semiconductor.
The first working CCD was an 8-bit shift register.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 soon clear
that the CCD could also accumulate charge via the
photoelectric effect and electronic
images could be created. By 1971, Bell researchers
Michael F. Tompsett et al. were able to capture
images with simple linear devices;thus the CCD imager was
born.
Several
companies, including Fairchild
Semiconductor, RCA and Texas
Instruments
, picked up
on the invention and began development programs. Fairchild's
effort, led by ex-Bell researcher
Gil
Amelio, 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 significant investment. 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, and in 2009 they were awarded the
Nobel Prize for Physics, for their
work on the CCD.
Basics of operation
The charge packets (electrons, blue)
are collected in potential wells (yellow) created by applying
positive voltage at the gate electrodes (G).
Applying positive voltage to the gate electrode in the correct
sequence transfers the charge packets.
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 through a
lens
onto 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 (operating as a
shift register). 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
contents of the array in the semiconductor to a sequence of
voltages, which it samples, digitizes, and stores in 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 a
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, i.e. 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 channels 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 to 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.
Another version of CCD is called a peristaltic CCD. In a
peristaltic charge-coupled device, the charge packet transfer
operation is analogous to the peristaltic contraction and dilation
of the
digestive system. The
peristaltic CCD has an additional implant that keeps the charge
away from the silicon/
silicon
dioxide interface and generates a large lateral electric field
from one gate to the next. This provides an additional driving
force to aid in transfer of the charge packets.
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 smears 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
aluminium). 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-intensive mechanical shutter, 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 is 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, frame-transfer is
usually chosen 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 1.1 μm. One other consequence of
their sensitivity to infrared is that infrared from
remote controls often appears on CCD-based
digital cameras or camcorders if they do not 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 often cool their detectors with liquid
nitrogen to reduce the dark current, and therefore the thermal
noise, to negligible levels.
Usage in astronomy
Due to the high quantum efficiencies of CCDs, linearity of their
outputs (one count for one photon of light), ease of use compared
to photographic plates, and a variety of other reasons, CCDs were
very rapidly adopted by astronomers for nearly all UV-to-infrared
applications. Thermal noise and
cosmic
rays may alter the pixels in the CCD array. To counter such
effects, astronomers take 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 (
dead pixels, hot pixels, etc.) in the CCD. The
Hubble Space Telescope, in
particular, has a highly developed series of steps (“data reduction
pipeline”) to convert the raw CCD data to useful images. See the
references for a more in-depth description of the steps in
astronomical CCD image-data correction and processing.
(Hainaut is an astronomer at the European Southern Observatory)
CCD cameras used in
astrophotography often require sturdy
mounts to cope with vibrations from wind and other sources, 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 motors to correct for
them.
An interesting unusual astronomical application of CCDs, called
drift-scanning, uses 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
accomplished.
In addition to astronomy, CCDs are also used in laboratory
analytical instrumentation such as
monochromators,
spectrometers, and
N-slit interferometers.
Color cameras
CCD-Colorsensor
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 professional camcorders) 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.
For still scenes, for instance in microscopy, the resolution of a
Bayer mask device can be enhanced by
micro-scanning technology. During the
process of
color co-site sampling
several frames of the scene are produced. Between acquisitions, the
sensor is moved in pixel dimensions, so the one and the same point
in the scene is acquired consecutively by parts of the mask that
are sensitive to the red, green and blue components of its color.
Eventually every pixel in the image has been scanned at least once
in all colors and the resolution is therefore identical in all
three color channels.
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. Some examples of the CCDs found in modern cameras can be
found in
this table in a Digital Photography Review
article
|
Type
|
Aspect Ratio
|
Width
mm
|
Height
mm
|
Diagonal
mm
|
Area
mm2
|
Relative Area
|
| 1/6" |
4:3 |
2.300 |
1.730 |
2.878 |
3.979 |
1.000 |
| 1/4" |
4:3 |
3.200 |
2.400 |
4.000 |
7.680 |
1.930 |
| 1/3.6" |
4:3 |
4.000 |
3.000 |
5.000 |
12.000 |
3.016 |
| 1/3.2" |
4:3 |
4.536 |
3.416 |
5.678 |
15.495 |
3.894 |
| 1/3" |
4:3 |
4.800 |
3.600 |
6.000 |
17.280 |
4.343 |
| 1/2.7" |
4:3 |
5.270 |
3.960 |
6.592 |
20.869 |
5.245 |
| 1/2" |
4:3 |
6.400 |
4.800 |
8.000 |
30.720 |
7.721 |
| 1/1.8" |
4:3 |
7.176 |
5.319 |
8.932 |
38.169 |
9.593 |
| 2/3" |
4:3 |
8.800 |
6.600 |
11.000 |
58.080 |
14.597 |
| 1" |
4:3 |
12.800 |
9.600 |
16.000 |
122.880 |
30.882 |
| 4/3" |
4:3 |
18.000 |
13.500 |
22.500 |
243.000 |
61.070 |
| Other image sizes as a
comparison |
| APS-C |
3:2 |
25.100 |
16.700 |
30.148 |
419.170 |
105.346 |
| 35mm |
3:2 |
36.000 |
24.000 |
43.267 |
864.000 |
217.140 |
| 645 |
4:3 |
56.000 |
41.500 |
69.701 |
2324.000 |
584.066 |
See also
References
External links
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