A glass master,
also referred to as a 'stamper' is used to punch
all of the data pits into a CD or DVD during the
process of replication.
The reason why
it is called a glass master
is because the information is copied onto a special
chemical coating on a circular block of glass. The
block of glass is actually much larger than a CD
(they are typically 240mm in diameter and 6mm deep)
to facilitate handling and to avoid the sensitive
data area from being touched or damaged.
The glass master
is polished until it is ultra smooth as even microscopic
scratches can affect the quality of the CDs being produced.
Glass mastering is performed in a Class 100 clean
room (10 times cleaner than an operating theatre).
A
Class 100 cleanroom is designed to never allow more
than 100 particles (0.5 microns or larger) per cubic
foot of air (typical office
building air contains from 500,000 to 1,000,000 particles
per cubic foot of air). This is because dust, pollen
and smoke particles can all affect the quality of
a CD glass master whilst it is being prepared, so
the mastering facilities are kept as clean as possible.
There are three principle steps involved in creating
a glass master:
Step 1 - Photoresist mastering and Laser Beam Recording
(LBR)
The first step in producing the glass master
is to clean the glass plate with detergents and to
then apply a photoresistive light-sensitive material
of about 140 to 150 microns that is then burnt using
a Laser Beam Recorder (LBR) which is a deep blue
or ultraviolet laser. When exposed
to the laser light, the photoresist undergoes a chemical
reaction which hardens it.
After
mastering, the glass master is baked at about 80°C
for 30 minutes to harden the developed surface material
and prepare it for metalisation. Metalisation is
a critical step prior to electroplating with nickel.
Step 2 - Metalisation of the glass
master
After the photoresist mastering, the developed glass
master is placed in a vapour deposition metaliser
which lowers the pressure inside a chamber
to an extreme vacuum. A piece of nickel wire
is then heated to white hot temperature
and the nickel vapour is deposited onto the rotating
glass master. The glass master is coated with
the nickel vapour up to a typical thickness of around
400nm before being removed.
Step 3 - Electroforming
The information contained on the metalised
glass master is extremely fragile and it must
be transferred to a more resilient form for
use in the injection moulding equipment.
The metalised master is therefore rotated
in a plating tank containing a nickel salt
solution (Nickel Sulfamate). The electroforming
process takes approximately 1 hour to create
a 0.3mm thick uniform nickel layer.
This master is then called the 'father' and
a negative of the father, the 'mother' needs
to be created to be able to punch the pits
and grooves into the membranes on the final
CDs or DVDs that the customer will receive.
|
 |
| A polycarbonate layer
of an actual DVD disc prior to application
of the reflective aluminium surface |
|
The mother glass master is created from the father
using electroforming and the mothers are then used
to punch holes in the membrane layer on replicated
CDs or DVDs that will then allow light through to
reflect off the silver aluminium layer above the
membrane layer in the centre of a CD or DVD.
View other CD
and DVD production FAQs.
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