The Glass mastering process is the stage where one transfers the
information from the premastered source to a glass plate which has a chemically
treated layer on it. Why glass? Because it is the most economical medium on
which you can put a chemical layer without the surface reacting with the
chemical, and it can be washed and reused in the process again. And the best
part is, it can be re-polished to extend its life too. Hence it is the best
choice currently available.
Note: Currently some newer systems do offer pre-grooved Polycarbonate
substrates/Nickel substrates as an alternative, but a large percentage of major
replication houses still use the glass plate method. There are two major types
of Glass mastering methods currently in wide use, viz. Dye Polymer coating and
the Photo resist type . The major difference between the two systems is in the
way the chemical treatment of the glass master is done. Firstly the glass plate
is cleaned thoroughly, to remove any dirt, chemicals, impurities etc. Then in
the Photoresist method, an adhesive layer is first applied upon which, a layer
of photo resist is applied. The adhesive layer is to help the photo resist
layer to adhere better. This glass substrate is then baked in an oven before
being used in an LBR. (More on this later).
After the recording is done on the LBR, the glass master is
chemically developed and metallised to create a master. This technique is an
older technique, and is prevalent from the times of manufacturing Laser Discs.
An outline of the process is given below
The Dye polymer method is a recent invention, and is much more convenient. Here,
after the glass master is cleaned, it is directly coated with a dye polymer
chemical. Then it is put in a LBR for the recording and then the glass master
is dried, and then metalised.
Most major reputed replication facilities have one of the systems mentioned
above. The glass mastering process along with the electroforming process is
necessarily done in a Class 10 clean room. Even the utilities used here, the
compressed air, water etc have to be of a very pure quality to ensure a master
which is of a reasonably good quality. This is important as the final CDs are
as good as their master and a good master will avoid problems later on. The LBR
(Laser Beam Recorder) is the equipment in which the main work of mastering is
done. Here at one side, the chemically treated glass plate is put and at the
same time, in another part of the LBR, the pre-mastered Exabyte tape or CDR is
put, and the recording started.
This LBR then reads from the source and with a laser beam, burns holes ("pits")
corresponding to the information read from the source, on the chemical layer on
the glass master. Some systems have a feature where they read what has been
written, to compare with the source so as to have a closed loop quality control
over the mastering process. This step has the added advantage the defects if
any, can be located immediately, instead of correcting this at the end of the
process. At the end of the program on the source master, the glass plate is
taken out and it is checked visually for any surface defects, and then the
glass master is put in a metallising machine, where a layer of nickel is
applied on it. This is then baked in an oven before being sent to the galvanics
or the electroforming department.
Announcements
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