A charge-coupled device (CCD) is an
integrated circuit containing an array of linked, or coupled,
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
s. Under the control of an external circuit, each capacitor can transfer its
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
to a neighboring capacitor. CCD sensors are a major technology used in
digital imaging
Digital imaging or digital image acquisition is the creation of a digital representation of the visual characteristics of an object, such as a physical scene or the interior structure of an object. The term is often assumed to imply or include t ...
.
In a CCD
image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of c ...
,
pixel
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device.
In most digital display devices, pixels are the ...
s are represented by
p-doped metal–oxide–semiconductor
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(MOS)
capacitors. These
MOS capacitor
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
s, the basic building blocks of a CCD,
are biased above the threshold for inversion when image acquisition begins, allowing the conversion of incoming
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
s into electron charges at the semiconductor-oxide interface; the CCD is then used to read out these charges.
Although CCDs are not the only technology to allow for light detection, CCD image sensors are widely used in professional, medical, and scientific applications where high-quality image data are required.
In applications with less exacting quality demands, such as consumer and professional
digital camera
A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile devices ...
s,
active pixel sensors, also known as
CMOS sensor
An active-pixel sensor (APS) is an image sensor where each pixel sensor unit cell has a photodetector (typically a pinned photodiode) and one or more active transistors. In a metal–oxide–semiconductor (MOS) active-pixel sensor, MOS field-effec ...
s (complementary MOS sensors), are generally used.
However, the large quality advantage CCDs enjoyed early on has narrowed over time and since the late 2010s CMOS sensors are the dominant technology, having largely if not completely replaced CCD image sensors.
History
![Nobel Prize 2009-Press Conference KVA-19](https://upload.wikimedia.org/wikipedia/commons/6/63/Nobel_Prize_2009-Press_Conference_KVA-19.jpg)
The basis for the CCD is the
metal–oxide–semiconductor
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(MOS) structure,
with
MOS capacitor
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
s being the basic building blocks of a CCD,
and a
depleted MOS structure used as the
photodetector
Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or ...
in early CCD devices.
In the late 1960s,
Willard Boyle and
George E. Smith
George Elwood Smith (born May 10, 1930) is an American scientist, applied physicist, and co-inventor of the charge-coupled device (CCD). He was awarded a one-quarter share in the 2009 Nobel Prize in Physics for "the invention of an imaging semico ...
at Bell Labs were researching MOS technology while working on
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
bubble memory
Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as ''bubbles'' or ''domains'', each storing one bit of data. The material is arranged to form a series o ...
. They realized that an electric charge was the analogy of the magnetic bubble and that it could be stored on a tiny MOS capacitor. As it was fairly straightforward to
fabricate a series of MOS capacitors in a row, they connected a suitable voltage to them so that the charge could be stepped along from one to the next.
This led to the invention of the charge-coupled device by Boyle and Smith in 1969. They conceived of the design of what they termed, in their notebook, "Charge 'Bubble' Devices".
[See and ]
The initial paper describing the concept in April 1970 listed possible uses as
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered ...
, a delay line, and an imaging device. The device could also be used as a
shift register
A shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one loc ...
. The essence of the design was the ability to transfer charge along the surface of a semiconductor from one storage capacitor to the next. The concept was similar in principle to the
bucket-brigade device A bucket brigade or bucket-brigade device (BBD) is a discrete-time analogue delay line, developed in 1969 by F. Sangster and K. Teer of the Philips Research Labs in the Netherlands. It consists of a series of capacitance sections C0 to Cn. The stor ...
(BBD), which was developed at
Philips Research Labs during the late 1960s.
The first experimental device demonstrating the principle was a row of closely spaced metal squares on an
oxidized
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
surface electrically accessed by wire bonds. It was demonstrated by
Gil Amelio
Gilbert Frank Amelio (born March 1, 1943) is an American technology executive. Amelio worked at Bell Labs, Fairchild Semiconductor, and the semiconductor division of Rockwell International, and was also the CEO of National Semiconductor and Ap ...
,
Michael Francis Tompsett Michael Tompsett (born 1939) is a British-born physicist, engineer, and inventor, and the founder director of the US software company TheraManager. He is a former researcher at the English Electric Valve Company, who later moved to Bell Labs in th ...
and George Smith in April 1970. This was the first experimental application of the CCD in
image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of c ...
technology, and used a depleted MOS structure as the photodetector.
The first
patent
A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A ...
() on the application of CCDs to imaging was assigned to Tompsett, who filed the application in 1971.
The first working CCD made with
integrated circuit technology was a simple 8-bit shift register, reported by Tompsett, Amelio and Smith in August 1970. This device had input and output circuits and was used to demonstrate its use as a shift register and as a crude eight
pixel
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device.
In most digital display devices, pixels are the ...
linear imaging device. Development of the device progressed at a rapid rate. By 1971, Bell researchers led by Michael Tompsett were able to capture images with simple linear devices.
Several companies, including
Fairchild Semiconductor,
RCA
The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Comp ...
and
Texas Instruments
Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globa ...
, 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 2D 100 × 100 pixel device.
Steven Sasson, an electrical engineer working for
Kodak
The Eastman Kodak Company (referred to simply as Kodak ) is an American public company that produces various products related to its historic basis in analogue photography. The company is headquartered in Rochester, New York, and is incorpor ...
, invented the first
digital still camera
A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile devices ...
using a Fairchild CCD in 1975.
The interline transfer (ILT) CCD device was proposed by L. Walsh and R. Dyck at Fairchild in 1973 to reduce smear and eliminate a mechanical
shutter. To further reduce smear from bright light sources, the frame-interline-transfer (FIT) CCD architecture was developed by K. Horii, T. Kuroda and T. Kunii at
Matsushita (now Panasonic) in 1981.
The first
KH-11 KENNEN
The KH-11 KENNEN (later renamed CRYSTAL,p.199-200 then Evolved Enhanced CRYSTAL System, and codenamed 1010 and Key Hole) is a type of reconnaissance satellite first launched by the American National Reconnaissance Office (NRO) in December 1 ...
reconnaissance satellite equipped with charge-coupled device array ( pixels) technology for imaging was launched in December 1976. Under the leadership of
Kazuo Iwama,
Sony
, commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professiona ...
started a large development effort on CCDs involving a significant investment. Eventually, Sony managed to mass-produce CCDs for their
camcorder
A camcorder is a self-contained portable electronic device with video and recording as its primary function. It is typically equipped with an articulating screen mounted on the left side, a belt to facilitate holding on the right side, hot-swa ...
s. Before this happened, Iwama died in August 1982. Subsequently a CCD chip was placed on his tombstone to acknowledge his contribution. The first mass-produced consumer CCD
video camera
A video camera is an optical instrument that captures videos (as opposed to a movie camera, which records images on film). Video cameras were initially developed for the television industry but have since become widely used for a variety of oth ...
, the CCD-G5, was released by Sony in 1983, based on a prototype developed by
Yoshiaki Hagiwara in 1981.
Early CCD sensors suffered from
shutter lag In photography, shutter lag is the delay between triggering the shutter and when the photograph is actually recorded. This is a common problem in the photography of fast-moving objects or animals and people in motion. The term narrowly refers only ...
. This was largely resolved with the invention of the
pinned photodiode
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
(PPD).
It was invented by
Nobukazu Teranishi
is a Japanese engineer who researches image sensors, and is known for inventing the pinned photodiode, an important component of modern digital cameras. He was one of four recipients of the 2017 Queen Elizabeth Prize for Engineering. His industr ...
, Hiromitsu Shiraki and Yasuo Ishihara at
NEC
is a Japanese multinational information technology and electronics corporation, headquartered in Minato, Tokyo. The company was known as the Nippon Electric Company, Limited, before rebranding in 1983 as NEC. It provides IT and network soluti ...
in 1980.
They recognized that lag can be eliminated if the signal carriers could be transferred from the
photodiode to the CCD. This led to their invention of the pinned photodiode, a photodetector structure with low lag, low
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
, high
quantum efficiency and low
dark current.
It was first publicly reported by Teranishi and Ishihara with A. Kohono, E. Oda and K. Arai in 1982, with the addition of an anti-blooming structure.
The new photodetector structure invented at NEC was given the name "pinned photodiode" (PPD) by B.C. Burkey at Kodak in 1984. In 1987, the PPD began to be incorporated into most CCD devices, becoming a fixture in
consumer electronic video cameras
A video camera is an optical instrument that captures videos (as opposed to a movie camera, which records images on film). Video cameras were initially developed for the television industry but have since become widely used for a variety of other ...
and then
digital still camera
A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile devices ...
s. Since then, the PPD has been used in nearly all CCD sensors and then
CMOS sensor
An active-pixel sensor (APS) is an image sensor where each pixel sensor unit cell has a photodetector (typically a pinned photodiode) and one or more active transistors. In a metal–oxide–semiconductor (MOS) active-pixel sensor, MOS field-effec ...
s.
In January 2006, Boyle and Smith were awarded the
National Academy of Engineering
The National Academy of Engineering (NAE) is an American nonprofit, non-governmental organization. The National Academy of Engineering is part of the National Academies of Sciences, Engineering, and Medicine, along with the National Academy of ...
Charles Stark Draper Prize
The U.S. National Academy of Engineering annually awards the Draper Prize, which is given for the advancement of engineering and the education of the public about engineering. It is one of three prizes that constitute the "Nobel Prizes of Enginee ...
, and in 2009 they were awarded the
Nobel Prize for Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
for their invention of the CCD concept. Michael Tompsett was awarded the 2010
National Medal of Technology and Innovation, for pioneering work and electronic technologies including the design and development of the first CCD imagers. He was also awarded the 2012
IEEE Edison Medal
The IEEE Edison Medal is presented by the Institute of Electrical and Electronics Engineers (IEEE) "for a career of meritorious achievement in electrical science, electrical engineering, or the electrical arts." It is the oldest medal in this fi ...
for "pioneering contributions to imaging devices including CCD Imagers, cameras and thermal imagers".
Basics of operation
![CCD charge transfer animation](https://upload.wikimedia.org/wikipedia/commons/6/66/CCD_charge_transfer_animation.gif)
In a CCD for capturing images, there is a photoactive region (an
epitaxial
Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...
layer of silicon), and a transmission region made out of a
shift register
A shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one loc ...
(the CCD, properly speaking).
An image is projected through a
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...
onto the capacitor array (the photoactive region), causing each capacitor to accumulate an electric charge proportional to the
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
intensity at that location. A one-dimensional array, used in line-scan cameras, captures a single slice of the image, whereas 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 A charge amplifier is an electronic current integrator that produces a voltage output proportional to the integrated value of the input current, or the total charge injected.
The amplifier offsets the input current using a feedback reference cap ...
, which converts the charge into a
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
. By repeating this process, the controlling circuit converts the entire contents of the array in the semiconductor to a sequence of voltages. In a digital device, these voltages are then sampled, digitized, and usually stored in memory; in an analog device (such as an analog video camera), they are processed into a continuous analog signal (e.g. by feeding the output of the charge amplifier into a low-pass filter), which is then processed and fed out to other circuits for transmission, recording, or other processing.
Detailed physics of operation
Charge generation
Before the MOS capacitors are exposed to light, they are
biased into the depletion region; in n-channel CCDs, the silicon under the bias gate is slightly ''p''-doped or intrinsic. The gate is then biased at a positive potential, above the threshold for strong inversion, which will eventually result in the creation of an ''n'' channel below the gate as in a
MOSFET. However, it takes time to reach this thermal equilibrium: up to hours in high-end scientific cameras cooled at low temperature. Initially after biasing, the holes are pushed far into the substrate, and no mobile electrons are at or near the surface; the CCD thus operates in a non-equilibrium state called deep depletion.
[ Chapter 13.6.]
Then, when
electron–hole pair
In the solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Carrier generation and recombination processes are ...
s are generated in the depletion region, they are separated by the electric field, the electrons move toward the surface, and the holes move toward the substrate. Four pair-generation processes can be identified:
* photo-generation (up to 95% of
quantum efficiency),
* generation in the depletion region,
* generation at the surface, and
* generation in the neutral bulk.
The last three processes are known as dark-current generation, and add noise to the image; they can limit the total usable integration time. The accumulation of electrons at or near the surface can proceed either until image integration is over and charge begins to be transferred, or thermal equilibrium is reached. In this case, the well is said to be full. The maximum capacity of each well is known as the well depth, typically about 10
5 electrons per pixel.
Design and manufacturing
The photoactive region of a CCD is, generally, an
epitaxial
Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...
layer of
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
. It is lightly ''p'' doped (usually with
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
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, giving them an n-doped designation. This region defines the channel in which the photogenerated charge packets will travel.
Simon Sze
Simon Min Sze, or Shi Min (; born 1936), is a Chinese-American electrical engineer. He is best known for inventing the floating-gate MOSFET with Korean electrical engineer Dawon Kahng in 1967.
Biography
Sze was born in Nanjing, Jiangsu, and grew ...
details the advantages of a buried-channel device:
This thin layer (= 0.2–0.3 micron) is fully depleted and the accumulated photogenerated charge is kept away from the surface. This structure has the advantages of higher transfer efficiency and lower dark current, from reduced surface recombination. The penalty is smaller charge capacity, by a factor of 2–3 compared to the surface-channel CCD.
The gate oxide, i.e. the
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
, is grown on top of the epitaxial layer and substrate.
Later in the process,
polysilicon
Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.
Polysilicon is produce ...
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
LOCOS, short for LOCal Oxidation of Silicon, is a microfabrication process where silicon dioxide is formed in selected areas on a silicon wafer having the Si-SiO2 interface at a lower point than the rest of the silicon surface. As of 2008 it was ...
process to produce the
channel stop In semiconductor device fabrication, channel-stopper or channel-stop is an area in semiconductor devices produced by implantation or diffusion of ions, by growing or patterning the silicon oxide Silicon oxide may refer to either of the following: ...
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 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
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
transfer device, though hole transfer is possible).
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.
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
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
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
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
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 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-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 when an interline architecture is not available, such as in a back-illuminated device.
CCDs containing grids of
pixel
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device.
In most digital display devices, pixels are the ...
s are used in
digital camera
A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile devices ...
s,
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
''Top:'' tree photographed in the near infrared range. ''Bottom:'' same tree in the Visible spectrum, visible part of the spectrum.
In infrared photography, the photographic film, film or image sensor used is sensitive to infrared light ...
,
night-vision
Night vision is the ability to see in low-light conditions, either naturally with scotopic vision or through a night-vision device. Night vision requires both sufficient spectral range and sufficient intensity range. Humans have poor night v ...
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 control
In electronics, a remote control (also known as a remote or clicker) is an electronic device used to operate another device from a distance, usually wirelessly. In consumer electronics, a remote control can be used to operate devices such a ...
s 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
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
, to negligible levels.
Frame transfer CCD
![IECCD55-20](https://upload.wikimedia.org/wikipedia/commons/a/a7/IECCD55-20.jpg)
The frame transfer CCD imager was the first imaging structure proposed for CCD Imaging by Michael Tompsett at Bell Laboratories. A frame transfer CCD is a specialized CCD, often used in
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
and some
professional video camera
A professional video camera (often called a television camera even though its use has spread beyond television) is a high-end device for creating electronic moving images (as opposed to a movie camera, that earlier recorded the images on film). ...
s, designed for high exposure efficiency and correctness.
The normal functioning of a CCD, astronomical or otherwise, can be divided into two phases: exposure and readout. During the first phase, the CCD passively collects incoming
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
s, storing
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
s in its cells. After the exposure time is passed, the cells are read out one line at a time. During the readout phase, cells are shifted down the entire area of the CCD. While they are shifted, they continue to collect light. Thus, if the shifting is not fast enough, errors can result from light that falls on a cell holding charge during the transfer. These errors are referred to as "vertical smear" and cause a strong light source to create a vertical line above and below its exact location. In addition, the CCD cannot be used to collect light while it is being read out. Unfortunately, a faster shifting requires a faster readout, and a faster readout can introduce errors in the cell charge measurement, leading to a higher noise level.
A frame transfer CCD solves both problems: it has a shielded, not light sensitive, area containing as many cells as the area exposed to light. Typically, this area is covered by a reflective material such as aluminium. When the exposure time is up, the cells are transferred very rapidly to the hidden area. Here, safe from any incoming light, cells can be read out at any speed one deems necessary to correctly measure the cells' charge. At the same time, the exposed part of the CCD is collecting light again, so no delay occurs between successive exposures.
The disadvantage of such a CCD is the higher cost: the cell area is basically doubled, and more complex control electronics are needed.
Intensified charge-coupled device
An intensified charge-coupled device (ICCD) is a CCD that is optically connected to an image intensifier that is mounted in front of the CCD.
An image intensifier includes three functional elements: a
photocathode
A photocathode is a surface engineered to convert light ( photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron ...
, a
micro-channel plate (MCP) and a
phosphor screen. These three elements are mounted one close behind the other in the mentioned sequence. The photons which are coming from the light source fall onto the photocathode, thereby generating photoelectrons. The photoelectrons are accelerated towards the MCP by an electrical control voltage, applied between photocathode and MCP. The electrons are multiplied inside of the MCP and thereafter accelerated towards the phosphor screen. The phosphor screen finally converts the multiplied electrons back to photons which are guided to the CCD by a fiber optic or a lens.
An image intensifier inherently includes a
shutter functionality: If the control voltage between the photocathode and the MCP is reversed, the emitted photoelectrons are not accelerated towards the MCP but return to the photocathode. Thus, no electrons are multiplied and emitted by the MCP, no electrons are going to the phosphor screen and no light is emitted from the image intensifier. In this case no light falls onto the CCD, which means that the shutter is closed. The process of reversing the control voltage at the photocathode is called ''gating'' and therefore ICCDs are also called gateable CCD cameras.
Besides the extremely high sensitivity of ICCD cameras, which enable single photon detection, the gateability is one of the major advantages of the ICCD over the
EMCCD
A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
cameras. The highest performing ICCD cameras enable shutter times as short as 200
picoseconds.
ICCD cameras are in general somewhat higher in price than EMCCD cameras because they need the expensive image intensifier. On the other hand, EMCCD cameras need a cooling system to cool the EMCCD chip down to temperatures around . This cooling system adds additional costs to the EMCCD camera and often yields heavy condensation problems in the application.
ICCDs are used in
night vision devices
A night-vision device (NVD), also known as a night optical/observation device (NOD), night-vision goggle (NVG), is an optoelectronic device that allows visualization of images in low levels of light, improving the user's night vision. The de ...
and in various scientific applications.
Electron-multiplying CCD
An electron-multiplying CCD (EMCCD, also known as an L3Vision CCD, a product commercialized by e2v Ltd., GB, L3CCD or Impactron CCD, a now-discontinued product offered in the past by Texas Instruments) is a charge-coupled device in which a gain register is placed between the shift register and the output amplifier. The gain register is split up into a large number of stages. In each stage, the electrons are multiplied by
impact ionization
Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in semiconductors, an electron (or Electron hole, hole) with enough kinetic energy can kno ...
in a similar way to an
avalanche diode
In electronics, an avalanche diode is a diode (made from silicon or other semiconductor) that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current co ...
. The gain probability at every stage of the register is small (''P'' < 2%), but as the number of elements is large (N > 500), the overall gain can be very high (
), with single input electrons giving many thousands of output electrons. Reading a signal from a CCD gives a noise background, typically a few electrons. In an EMCCD, this noise is superimposed on many thousands of electrons rather than a single electron; the devices' primary advantage is thus their negligible readout noise. The use of
avalanche breakdown
Avalanche breakdown (or avalanche effect) is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good ...
for amplification of photo charges had already been described in the in 1973 by George E. Smith/Bell Telephone Laboratories.
EMCCDs show a similar sensitivity to
intensified CCDs (ICCDs). However, as with ICCDs, the gain that is applied in the gain register is stochastic and the ''exact'' gain that has been applied to a pixel's charge is impossible to know. At high gains (> 30), this uncertainty has the same effect on the
signal-to-noise ratio (SNR) as halving the
quantum efficiency (QE) with respect to operation with a gain of unity. This effect is referred to as the Excess Noise Factor (ENF). However, at very low light levels (where the quantum efficiency is most important), it can be assumed that a pixel either contains an electron—or not. This removes the noise associated with the stochastic multiplication at the risk of counting multiple electrons in the same pixel as a single electron. To avoid multiple counts in one pixel due to coincident photons in this mode of operation, high frame rates are essential. The dispersion in the gain is shown in the graph on the right. For multiplication registers with many elements and large gains it is well modelled by the equation:
where ''P'' is the probability of getting ''n'' output electrons given ''m'' input electrons and a total mean multiplication register gain of ''g''. For very large numbers of input electrons, this complex distribution function converges towards a Gaussian.
Because of the lower costs and better resolution, EMCCDs are capable of replacing ICCDs in many applications. ICCDs still have the advantage that they can be gated very fast and thus are useful in applications like
range-gated imaging. EMCCD cameras indispensably need a cooling system—using either
thermoelectric cooling
Thermoelectric cooling uses the Peltier effect to create a heat flux at the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side o ...
or liquid nitrogen—to cool the chip down to temperatures in the range of . This cooling system unfortunately adds additional costs to the EMCCD imaging system and may yield condensation problems in the application. However, high-end EMCCD cameras are equipped with a permanent hermetic vacuum system confining the chip to avoid condensation issues.
The low-light capabilities of EMCCDs find use in astronomy and biomedical research, among other fields. In particular, their low noise at high readout speeds makes them very useful for a variety of astronomical applications involving low light sources and transient events such as
lucky imaging
Lucky imaging (also called lucky exposures) is one form of speckle imaging used for astrophotography. Speckle imaging techniques use a high-speed camera with exposure times short enough (100 ms or less) so that the changes in the Earth's a ...
of faint stars, high speed
photon counting
Photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). A single-photon detector emits a pulse of signal for each detected photon, in contrast to a normal photodetector, which generates an analo ...
photometry,
Fabry-Pérot spectroscopy and high-resolution spectroscopy. More recently, these types of CCDs have broken into the field of biomedical research in low-light applications including
small animal imaging
Preclinical imaging is the visualization of living animals for research purposes, such as drug development. Imaging modalities have long been crucial to the researcher in observing changes, either at the organ, tissue, cell, or molecular level, i ...
,
single-molecule imaging,
Raman spectroscopy,
super resolution microscopy as well as a wide variety of modern
fluorescence microscopy
A fluorescence microscope is an optical microscope that uses fluorescence instead of, or in addition to, scattering, reflection, and attenuation or absorption, to study the properties of organic or inorganic substances. "Fluorescence microscop ...
techniques thanks to greater SNR in low-light conditions in comparison with traditional CCDs and ICCDs.
In terms of noise, commercial EMCCD cameras typically have clock-induced charge (CIC) and dark current (dependent on the extent of cooling) that together lead to an effective readout noise ranging from 0.01 to 1 electrons per pixel read. However, recent improvements in EMCCD technology have led to a new generation of cameras capable of producing significantly less CIC, higher charge transfer efficiency and an EM gain 5 times higher than what was previously available. These advances in low-light detection lead to an effective total background noise of 0.001 electrons per pixel read, a noise floor unmatched by any other low-light imaging device.
Use in astronomy
Due to the high quantum efficiencies of charge-coupled device (CCD) (the ideal
quantum efficiency is 100%, one generated electron per incident photon), linearity of their outputs, 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 ray
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s 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 pixel
A defective pixel is a pixel on a liquid crystal display (LCD) that is not functioning properly. The ISO standard ISO 13406-2 distinguishes between three different types of defective pixels, while hardware companies tend to have further disting ...
s, hot pixels, etc.) in the CCD. Newer Skipper CCDs counter noise by collecting data with the same collected charge multiple times and has applications in precision light
Dark Matter
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...
searches and
neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
measurements.
The
Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
, in particular, has a highly developed series of steps (“data reduction pipeline”) to convert the raw CCD data to useful images.
[
]
(Hainaut is an astronomer at th
European Southern Observatory
CCD cameras used in
astrophotography
Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1840, but it was no ...
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 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 a survey of over a quarter of the sky.
In addition to imagers, CCDs are also used in an array of analytical instrumentation including
spectrometer
A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
s and
interferometers.
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
Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...
,
green
Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a combi ...
and
blue
Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when ...
components. Each of the three CCDs is arranged to respond to a particular color. Many
professional video camcorders, and some semi-professional camcorders, use this technique, although developments in competing CMOS technology have made CMOS sensors, both with beam-splitters and bayer filters, increasingly popular in high-end video and digital cinema cameras. Another advantage of 3CCD over a Bayer mask device is higher
quantum efficiency (higher light sensitivity), because most of the light from the lens enters one of the silicon sensors, while a Bayer mask absorbs a high proportion (more than 2/3) of the light falling on each pixel location.
For still scenes, for instance in microscopy, the resolution of a Bayer mask device can be enhanced by
microscanning Microscanning is a method for increasing resolution of digital cameras. With the color co-site sampling identically colored pixels in several frames of the specimen, obtained by moving the sensor with a piezo mechanism in a regular raster, are c ...
technology. During the process of
color co-site sampling
Colour co-site sampling is a system of photographic colour sensing, wherein 4, 16 or 36 images are collected from the sensor and merged to form a single image. Each subsequent image physically moves the sensor by exactly one pixel, in order to col ...
, several frames of the scene are produced. Between acquisitions, the sensor is moved in pixel dimensions, so that each point in the visual field is acquired consecutively by elements 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 each color and the resolution of the three channels become equivalent (the resolutions of red and blue channels are quadrupled while the green channel is doubled).
Sensor sizes
Sensors (CCD / CMOS) come in various sizes, or image sensor formats. These sizes are often referred to with an inch fraction designation such as 1/1.8″ or 2/3″ called the
optical format Optical format is a hypothetical measurement approximately 50% larger than the true diagonal size of a solid-state photo sensor. The use of the optical format means that a lens used with a particular size sensor will have approximately the same ang ...
. This measurement originates back in the 1950s and the time of
Vidicon tubes.
Blooming
When a CCD exposure is long enough, eventually the electrons that collect in the "bins" in the brightest part of the image will overflow the bin, resulting in blooming. The structure of the CCD allows the electrons to flow more easily in one direction than another, resulting in vertical streaking.
Some anti-blooming features that can be built into a CCD reduce its sensitivity to light by using some of the pixel area for a drain structure.
[
]
James M. Early developed a vertical anti-blooming drain that would not detract from the light collection area, and so did not reduce light sensitivity.
See also
References
External links
Journal Article On Basics of CCDsMore statistical properties
{{DEFAULTSORT:Charge-Coupled Device
American inventions
Integrated circuits
Image processing
Image sensors
Image scanners
Astronomical imaging
MOSFETs