Hardware Reference
In-Depth Information
process called electroforming. This creates a metal master called a
father
.
5.
Master separation
—The metal master father is then separated from the glass master. The
father is a metal master that can be used to stamp discs, and for short runs, it may in fact be used
that way. However, because the glass master is damaged when the father is separated, and
because a stamper can produce only a limited number of discs before it wears out, the father
often is electroformed to create several reverse image mothers. These mothers are then
subsequently electroformed to create the actual stampers. This enables many more discs to be
stamped without ever having to go through the glass mastering process again.
6.
Disc-stamping operation
—A metal stamper is used in an injection molding machine to press
the data image (pits and lands) into approximately 18 grams of molten (350°C or 662°F)
polycarbonate plastic with a force of about 20,000psi. Normally, one disc can be pressed every
2-3 seconds in a modern stamping machine.
7.
Metalization
—The clear stamped disc base is then sputter-coated with a thin (0.05-0.1
micron) layer of aluminum to make the surface reflective.
8.
Protective coating
—The metalized disc is then spin-coated with a thin (6-7 micron) layer of
acrylic lacquer, which is then cured with UV (ultraviolet) light. This protects the aluminum
from oxidation.
9.
Finished product
—Finally, a label is affixed or printing is screen-printed on the disc and
cured with UV light.
Although the manufacturing process shown here was for CDs, the process is almost identical for other
types of optical media.
Pits and Lands
Reading the information back from a disc is a matter of bouncing a low-powered laser beam off the
reflective layer in the disc. The laser shines a focused beam on the underside of the disc, and a
photosensitive receptor detects when the light is reflected back. When the light hits a land (flat spot)
on the track, the light is reflected back; however, when the light hits a pit (raised bump), no light is
reflected back.
As the disc rotates over the laser and receptor, the laser shines continuously while the receptor sees
what is essentially a pattern of flashing light as the laser passes over pits and lands. Each time the
laser passes over the edge of a pit, the light seen by the receptor changes in state from being reflected
to not reflected, or vice versa. Each change in state of reflection caused by crossing the edge of a pit
is translated into a 1 bit digitally. Microprocessors in the drive translate the light/dark and dark/light
(pit edge) transitions into 1 bits, translate areas with no transitions into 0 bits, and then translate the
bit patterns into actual data or sound.
The individual pits on a CD are 0.125 microns deep and 0.6 microns wide. Both the pits and lands
vary in length from about 0.9 microns at their shortest to about 3.3 microns at their longest. The track
is a spiral with 1.6 microns between adjacent turns (see
Figure 11.2
)
.
