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Glass mastering
is the most crucial and complicated part of CD and DVD
replication, requiring technology and skill
Glass mastering
is the first stage needed to create a stamper from the
source audio or CD-ROM data and comprises the following
stages:
Glass Master
Preparation ready for laser beam recording
Laser Beam Recording from the source CD or tape
Development & Metallisation ready for electroforming
Electroforming making the stampers from the glass master
Stamper Finishing polishing and punching the stamper
Mastering of CDs and DVDs is a complex process carried
out in a class 1,000 clean room. Operators wear special
clothing including face masks and footwear to minimise
particles that could affect the quality of stampers
and therefore of pressed discs.
There are
differences when mastering DVDs compared with CDs, mainly
due to the smaller geometries and tighter specifications.
Glass
Master Preparation
Glass Master Preparation of the 240 cm diameter 6mm
thick glass master starts by stripping the old photo
resist from its surface (since the glass blanks can
be recycled). This is followed by cleaning and final
washing using de-ionised water. The blank glass master
is then dried carefully ready for the next stage.
The surface of the clean glass master is coated with
a primer and then a photo resist layer 140 to 150 microns
thick by spin coating. The thickness should be matched
to the moulding cycle time. Shorter cycle times imply
a thicker resist layer to ensure good pit geometry.
The uniformity of the layer is measured with an infra
red laser.
The photo
resist coated glass master is then baked at about 80º
C for 30 minutes. This hardens the photo resist layer
ready for exposing by laser light.
Laser
Beam Recording
A Laser Beam Recorder (LBR) is used to expose the photoresist
layer on the glass master where the final pits are required.
This is carried out in a class 100 controlled environment
using a high power gas laser from the premastered source
audio or CD-ROM data.
The laser can be blue, violet or (for DVD mastering)
ultra violet. The laser beam is modulated to expose
the photoresist where pits should be while the glass
master spins at exactly the correct linear velocity
and is moved gradually and smoothly to maintain the
correct track pitch and linear velocity.
The LBR is
controlled by a formatter which formats the source data
ready for laser beam recording. Usually the data is
transferred to a server from the source media and checked
for any errors that would not allow laser beam recording
to complete. The data is then transferred to the LBR
and associated controller via a high speed network.
Several LBRs can be connected to the network and mastering
jobs can be scheduled in advance. The result is higher
speed, more reliable mastering.
The formatter
takes the data as sectors or blocks and adds the error
protection and modulation before sending signals to
the LBR to modulate the laser beam.
For CD the
formatter adds the CIRC error correction, combines the
main and subcode channel data, formats the resultant
data into frames and then adds EFM modulation before
outputting the raw data to the LBR. (See CD data coding).
For DVD the formatter formats the data according to
the RSPC coding with error correction, then adds EFM+
modulation before outputting the raw data to the LBR.
(See DVD specifications).
Speed of laser beam recording depends on the machine
and glass used, but 4x is the normal maximum speed used
for CD and 2x for DVD. The absolute limit of speed is
dictated by the robustness of the glass. For 240 mm
glass plates, the practical limit is around 4x for CD
mastering. Smaller glass dimensions and higher speeds
are also being introduced.
There are
other differences between CD and DVD mastering (see
below).
Copy protection
for CD Audio, CD-ROM and DVD, if required, is added
during the laser beam recording stage by encrypting
or other wise modifying the data and/or adding signatures
or other data to the disc. The process is normally real-time
and carried out on the fly.
Development
and Metallisation
The exposed photoresist surface is developed to remove
the photoresist exposed by the laser, creating pits
in the surface. These pits should extend right through
the photoresist to the glass underneath to achieve good
pit geometries as specified in the Red Book. The glass
itself is unaffected by this process and acts merely
as a carrier for the photoresist.
The active surface (ie containing pits) of the developed
glass master is then metallised either with nickel or
nickel alloy created by sputtering.
Electroforming
Nickel fathers, mothers and stampers are created from
the metallised glass master by electroforming in a class
1000 clean room environment.
The father is electroformed from the metallised glass
master (see diagram) and then the surface containing
the 'bumps' is oxidised ready for the next stage. (This
allows the mother to be separated from the father).
The mother
is then electroformed from the father and is an essential
intermediate stage from which the stamper(s) are then
electroformed in a similar way.
After the
mother has been created, the father can then be used
as a stamper. Only mothers are needed for creating subsequent
stampers. Additional stampers are created for long runs.
The photoresist
on the glass master is then removed and the glass cleaned
ready to be used again.
Stamper
Finishing
Each stamper is checked visually, the back polished,
it is punched to the required outside diameter, a hole
accurately punched in the centre and finally it is checked
on a stamper player before being fitted to the press.
Stamper finishing is an important stage as it will affect
the quality of the final disc. The centre hole must
be accurately cut to avoid eccentricity which could
affect the playability of CD-ROMs or DVD-ROMs using
modern high speed drives. Also the stamper thickness
must be uniform to avoid unbalance problems in the finished
discs.
DVD
vs CD Glass Mastering
The differences between DVD and CD means that much of
the mastering process for DVD needs new equipment including
improved glass master preparation, laser beam recording
and developing.
The photo-resist
layer should, ideally be about 120 nm in thickness (instead
of 140 nm for CD) but successful mastering using the
same thickness as for CDs is possible. Any defects or
variations in thickness of this layer must be kept very
small.
Laser beam recording requires a smaller spot size, higher
numerical aperture and tighter tolerances than for CDs.
Many LBRs designed for DVD mastering use a UV laser
(instead of the blue or violet laser used for CDs).
To handle CD and DVD mastering, it is necessary to change
the numerical aperture from 0.6 for CD to 0.9 for DVD
mastering.
DVD data is formatted differently from CDs and requires
new formatting hardware/software to handle the RSPC
error correction, 8 to 16 modulation and the higher
channel data rate.
Stamper finishing requires more care than for CDs, since
tilt (variations in flatness of the final disc) is critical
for DVD.
DVD-9 (dual layer) discs require the upper layer (layer
1) to be mastered with the turntable rotating in the
reverse direction. Also, the direction of writing will
be either from the inside to outside (parallel track)
or outside to inside (opposite track), depending on
the application requirements.
CSS (Content Scrambling System) copy protection is carried
out at the mastering stage. The data on DLT is combined
with the encrypted keys and the audio and video data
scrambled using these keys, which are hidden on the
DVD disc.
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