It
was a historic moment for digital cameras when their shutters opened
for the fraction of a second to capture the committee, award the coveted
Nobel Prize to the ones who invented the electronic eyes. Had it not
been the duo Willard Boyle and George Smith, those cameras might still
have been shuttering light on a photosensitive film and then drying them
on rails in the dark room. By inventing the Charge Coupled Device
and predicting its applications way back in 1969, Boyle and Smith
opened up the domain of solid state devices for imaging and memory
applications.
Though
Boyle and Smith invented the method to convert light into an electrical
signal, the entire research would not have been possible had it not
been Albert Einstein’s groundbreaking explanation of the Photoelectric
effect. Einstein explained how materials could absorb incident radiation
and the eventual knocking out of the electrons from the surface. Boyle
and Smith studied how this incidence of light caused disturbances in
electrons and how it could be utilized. They succeeded in doing this by
grouping a number of capacitors into an array, something which makes up
the pixels of a digital image.
Initially named as Charge Bubble Devices,
the device’s operation as a shift register and a linear imaging device
was recorded. It was based on a principle similar to Bucket-Brigade
Devices, where charge is transferred from one capacitor to another along
the semiconductor surface. Bell labs succeeded in building the first
solid state video camera in 1970. By 1971, Michael F. Tompsett and other
researchers at Bell Labs succeeded in capturing images with linear CCD arrays. After that, many semiconductor majors like Texas Instruments, Fairchild Semiconductors, Sony etc. started investing in CCD technologies. One of the first commercial CCD devices
was built by Fairchild in 1974 which was a 100x 100 pixel device having
about 500 CCDs array. The first CCD based reconnaissance satellite
KH-11 KENNAN was launched in 1976. By 1983, CCDs had started to replace
photographic plates in astronomical telescopes too. Companies like Kodak
had been manufacturing CCD based professional cameras since 1985, but
by 1995, cost effective high resolution CCD cameras started flooding the
markets.
A CCD
can be thought of as a subset of Charge Transfer Devices. These are
based upon Metal Oxide Semiconductor (MOS) capacitors. Two types, viz
surface channel and buried channel MOS capacitors have been used in CCD,
but primarily buried channel capacitors have been used for
manufacturing since these do not have problems caused by surface
irregularities at oxide-semiconductor interface. A thin n-type buried
channel is formed on a p-type substrate through ion implantation. The
Silicon Dioxide insulator layer is grown on top of the n-region, and to
complete the capacitor, gates of metal or heavily doped polycrystalline
silicon are placed on top of the insulating SiO2 using CVD
process. To isolate the charge packets of one column from other,
thermally grown oxide ‘channel stops’ are placed parallel to the
channels.
Buried Channel MOS
But
if a CCD cell were to pass along charges, it would just be any other
MOS capacitor. The additional property that it is sensitive to light
makes a CCD unique. The response to photons is through the epitaxial
layers of doped silicon grown on the substrate. When photons are
incident on the semiconductor surface, they dislodge electrons which
create charge that is proportional to the light falling on the surface. A
single CCD cell performs four functions:
1. Receive charge from the cell above it in the array.
2. Hold that charge for sometime without much loss.
3. Pass that charge to the cell below it in array.
4. Respond to outside stimulus like light and generate its own charge.