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CD & DVD Glass Mastering
Glass mastering is the most crucial and
complicated part of CD and DVD replication, requiring technology
and skill |
Both CD and DVD glass mastering require a laser beam recorder whereby
a laser is used to expose minute areas of a photo-resist layer on top of
a rotating glass master disc. The lengths and spacing of these areas is
defined by the data to be stored on the disc. These areas are then
developed to form pits and coated in a thin layer of nickel, on which a
nickel Father is grown using an electroforming process. A Mother is then
grown from the Father and the stampers grown from the Mother. The
premastered data for the CD to be mastered is used as the input data
during the glass mastering process. This process has several steps
listed below.
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
 The
CD or DVD data is normally read from the appropriate source media (see
above) and copied to a server for checking prior to laser beam
recording. CD and DVD require different hardware and software to format
the data for recording.
- CD source media include CD-R, Exabyte and U-matic tape (CD
Audio only).
- DVD source media will include DLT (digital linear tape) and
DVD-R. For DVD-9 discs and where copy protection and/or region coding
are required only DLT will suffice. DVD-9 and DVD-10 discs require two
DLTs to create two stampers.
The glass master with photo-resist coating is placed on the turntable
of the laser beam recorder (LBR), where a gas laser is used to expose
the photo-resist, where the pits are to be moulded in the final discs.
The laser can be blue, violet or (for DVD mastering)
ultra violet. The laser beam is modulated to expose the photo-resist 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.
Laser beam recording for DVD requires a smaller spot size, higher
numerical aperture and tighter tolerances than for CDs. The use of an
ultra violet laser is crucial to ensure good pits in the final disc,
instead of the blue or violet lasers normally used for CD mastering.
Also the turntable and associated mechanics must ensure constant linear
velocity and track pitch as the pits are recorded.
DVD-9 (dual layer) discs require the upper layer (layer 1) to be
mastered with the laser beam recorder 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.
Data Formatter/Encoder
The LBR is controlled by a formatter or encoder 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.
Copy Protection
Copy protection is added during laser beam recording.
- CD audio copy protection is implemented at the glass
mastering stage. Since the audio cannot be encrypted, copy protection
works by preventing the reading and transfer of CD audio data via the
CD-ROM drive of a PC. To allow the audio to be played on a PC,
compressed audio files are added to a second session. This is also
carried out during the glass mastering stage, which requires dedicated
hardware and/or software.
- CD-ROM copy protection uses encryption and scrambling
technology and needs a premastering process to be carried out prior to
laser beam recording. This creates a digital 'signature', which is
stored on the disc in a way that makes it difficult to copy. Different
systems require different hardware and/or software to implement the
encoding process.
- For DVD-Video both CSS (Content Scrambling System) digital
copy protection and Macrovision analogue copy protection are carried
out at the mastering stage by the data formatting system. For CSS the
data is combined with the encrypted keys and the audio and video data
scrambled using these keys, which are hidden on the DVD disc. The CSS
keys are provided on diskette. Each set of CSS keys or MKB is used
only once and then discarded. The premastering process must ensure
that the application is prepared for copy protection and the necessary
region coding information is provided. For DVD-9 opposite track discs,
where a contiguous video spans both layers, the same set of CSS keys
must be used for both layers. Apart from this requirement the keys are
generally only used once, for security reasons.
- For DVD-Audio discs, CPPM (Content Protection for
Pre-recorded Media) is used and involves a more complicated process
than for CSS. A media key block (MKB) is used for each disc side
mastered and is stored on the disc within the dummy files (DVDAUDIO.MKB
and DVDAUDIO.BUP) that were create during premastering. The formatter
must obtain the media key from the MKB and its unique device key
before encrypting the audio data.
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.
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.
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