Hardware Reference
In-Depth Information
The steps in manufacturing CDs are as follows. (Use Figure 11.1 as a visual.)
1. Photoresist coating —A circular 240mm diameter piece of polished glass 6mm thick
is spin-coated with a photoresist layer about 150 microns thick and then hardened by
baking at 80°C (176°F) for 30 minutes.
2. Laser recording —A Laser Beam Recorder (LBR) fires pulses of blue/violet laser
light to expose and soften portions of the photoresist layer on the glass master.
3. Master development —A sodium hydroxide solution is spun over the exposed glass
master, which then dissolves the areas exposed to the laser, thus etching pits in the
photoresist.
4. Electroforming —The developed master is then coated with a layer of nickel alloy
through a process called electroforming . This creates a metal master called a father .
5. Master separation —Themetalmasterfatheristhenseparatedfromtheglassmaster.
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 ac-
tual stampers. This enables many more discs to be stamped without ever having to go
through the glass mastering process again.
6. Disc-stamping operation —Ametalstamperisusedinaninjectionmoldingmachine
topressthedataimage(pitsandlands)intoapproximately18gramsofmolten(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
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