CHAPTER 13

690 Chapter 13 Optical Storage
has a physical track pitch of about 125 microns. When viewed from the reading side (the bottom),
the disc rotates counterclockwise. If you examined the spiral track under a microscope, you would see
that along the track are raised bumps, called pits, and flat areas between the pits, called lands. It seems
strange to call a raised bump a pit, but that is because when the discs are pressed, the stamper works
from the top side. So, from that perspective, the pits are actually depressions made in the plastic.
The laser used to read the disc would pass right through the clear plastic, so the stamped surface is
coated with a reflective layer of metal (usually aluminum) to make it reflective. Then, the aluminum
is coated with a thin protective layer of acrylic lacquer, and finally a label or printing is added.
Note
CD-ROM media should be handled with the same care as a photographic negative. The CD-ROM is an optical device
and degrades as its optical surface becomes dirty or scratched. Also it is important to note that, although discs are read
from the bottom, the layer containing the track is actually much closer to the top of the disc. Writing on the top surface of
a disc with a ballpoint pen, for example, easily damages the recording underneath. You need to be careful even when
using a marker to write on the disc. The inks and solvents used in some markers can damage the print and lacquer overcoat
on the top of the disc, and subsequently the information layer right below. Use only markers designed for writing on
CDs. The important thing is to treat both sides of the disc carefully, especially the top (label) side.
Mass-Producing CD-ROMs
Commercial mass-produced CDs are stamped or pressed and not burned by a laser as many people
believe (see Figure 13.1). Although a laser is used to etch data onto a glass master disc that has been
coated with a photosensitive material, using a laser to directly burn copies would be impractical for
the reproduction of hundreds or thousands of copies.
The steps in manufacturing CDs are as follows (use Figure 13.1 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. 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 can 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 3 seconds
in a modern stamping machine.