The movie links below show a simple demonstration of the operation of a CCD. You can think of CCDs as an array of
buckets (pixels) out to catch rainfall (photons of light). The red buckets
are just like pixels of a CCD imaging area. The purple buckets are the
readout or serial register. At the end of the row of
purple buckets is a metering station (the output amplifier) where the rain water is measured. The
values of each bucket are read out to a computer. The computer then
assigns gray-scale values to each number (in the movie, 1 is white, 3 is black,
with variations of gray in between) and reconstructs the image.
Backside access of light (photons) into silicon is the main reason
charge-coupled devices are thinned or back-illuminated. Front-illuminated
CCDs absorb and reflect much of the incident light due to pixel structures and
electrical circuits near the frontside surface. If the CCD is flipped over
to expose its backside to light, the photons are better able to enter the
silicon. The device is
then made very thin so that the electrons generated by the incident light are
not impeded by traps within the silicon. These electrons can then easily
travel to the pixels near the front side of the CCD where they are detected to
form an image. The application of an antireflection coating to the back
surface reduces the number of photons lost to reflection before entering the
silicon.
The Effect of Temperature on Thin Silicon
The movie link below shows the effect of temperature on a wafer of thin silicon. The wafer is first at room temperature (25 C), then heated to about 150 C (boiling water), and then cooled. Initially it is warped, but as it heats it expands, stretching the crystalline silicon tight. When stretched it appears like a smooth mirror. As it cools back down to room temperature it once again becomes warped. The pattern seen in the wafer surface is the reflection of a grid of straight lines from standard graph paper.
