CD & DVD Glass Mastering
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.
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.
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.