Compact disc manufacturing
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Compact disc manufacturing is the process by which commercial compact discs (CDs) are replicated in mass quantities using a master version created from a source recording. This may be either in audio form (CD-Audio) or data form (CD-ROM). This process is used in the mastering of CDs, and does not include CD-Rs or DVDs, although these are made using similar methods.
A compact disc (CD) can be used to store audio, video, and data in various formats which are defined in the Rainbow Books. A CD is usually manufactured in a class 100 or better clean room, and can usually be manufactured with strict manufacturing tolerances for only a few US cents.
CD mastering differs from burning, as the pits and lands of a mastered CD are moulded into the CD, rather than being 'burn marks' (phase changes) by a CD burner.
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[edit] Premastering
All CDs are pressed from a digital source, with the most common sources being low error-rate CD-Rs or files from an attached computer hard drive containing the finished product. Some CD pressing systems can use digital master tapes, either Digital Audio Tapes, Exabytes or Umatics. However such sources can only be used for production of audio CDs. If the source is not a CD, the table of contents for the CD to be pressed must also be prepared and stored on the tape or hard drive. In all cases except CD-R, the tape must be uploaded to a media mastering system in order for the TOC (Table Of Contents) to be created.
[edit] Mastering
[edit] Glass mastering
Glass mastering is performed in a class 100 or better clean room or a self-enclosed clean environment within the mastering system. If introduced during critical stages of manufacturing, contaminants such as dust, pollen, hair and smoke can render the master unusable. Once completed, a CD will resist effects caused by these contaminants.
During glass mastering, glass is used as a substrate to hold the CD master, hence the name, while it is created and processed. Glass substrates, noticeably larger than a CD, are round plates of glass approximately 240 mm in diameter and 6 mm thick. They often also have a small, steel hub on one side to facilitate handling. The substrates are created especially for CD mastering and one side is polished until it is exceptionally smooth. Even microscopic scratches in the glass will affect CD quality. The extra area on the substrate allows for easier handling of the glass master and prevention of damage to the pit structure when the "father" stamper is removed from the glass substrate.
Once the glass substrate is cleaned using detergents and ultrasonic baths, the glass is placed in a spin coater. The spin coater spins the glass and rinses it first with a solvent and then applies either photoresist or dye-polymer depending on the mastering process. The rotation spreads the photoresist or dye-polymer across the face of the glass in an even coating. The substrate is removed and baked to dry the coating and the glass substrate is ready for mastering.
Mastering is performed by a Laser Beam Recorder (LBR) machine. These use one of two recording methodologies for CD mastering, photo resist and non-photoresist mastering. Photoresist also comes in two variations positive photoresist and negative photoresist.
While nearly all mastering to glass is done at multiple speeds for sake of plant efficiency (8X or higher is standard), single speed glass mastering [1] (also referred to as 1X glass cutting or 1x glass mastering) is offered by a few CD replication plants, such as U.S.-based Oasis Disc Manufacturing [2]. A large number of audiophiles believe this results in truer reproduction although the competing claims have gone back and forth for many years.
[edit] Photoresist mastering
Photoresist mastering uses a light-sensitive material (photoresist) to create the pits and lands in the CD master.
The laser beam recorder uses a deep blue or ultraviolet laser to write the master. When exposed to the laser light, the photoresist undergoes a chemical reaction which hardens it. The exposed area is then soaked in a developer solution which removes the exposed positive photoresist or the unexposed negative photoresist.
Once the mastering is complete, the glass master is removed from the LBR and it is chemically developed.
Once developing is complete, the glass master must be metalized to provide a surface for the stamper to be formed on.
[edit] Non-photoresist (NPR) or Dye-Polymer mastering
Once the glass is ready for mastering, it is placed in a Laser Beam Recorder (LBR). LBRs are capable of mastering at greater than x1 speed, but due to the weight of the glass substrate and the requirements of a CD master they are typically mastered at no greater than 8X playback speed. The LBR uses a laser to write the information, with a wavelength and final lens NA (numerical aperture) chosen to produce the required pit size on the recording. For example, DVD pits are smaller than CD pits, so a shorter wavelength or higher NA (or both) is needed for DVD mastering. When a laser is used to record on the dye-polymer used in NPR mastering, the dye-polymer absorbs a large quantity of the laser energy focused in a precise spot, that vapourises and forms a pit in the surface of the dye-polymer. This pit can be scanned by a red laser beam that follows the cutting beam, and the quality of the recording can be directly and immediatetly assessed, the audio can also be played straight from the glass master as it is being recorded. The pit geometry and quality of the playback can all be adjusted while the CD is being mastered, as the blue writing laser and the red read laser are typically connected via a feedback system to optimise the recording. This allows the dye-polymer LBR to produce very consistent pits even if there are variations in the dye-polymer layer. Another advantage of this method is that pit depth variation can be programmed during recording to compensate for downstream problems such as poor molding. This cannot be done with photoresist mastering because the pit depth is set by the PR coating thickness, whereas dye-polymer pits are cut into a coating that is thicker than the desired pits.
This type of mastering is called Direct Read After Write or DRAW and is the main advantage of some non-photoresist recording systems. Any problems with the quality of the glass substrate, scratches or an uneven coating of the dye-polymer are immediately detected and if required the mastering can be halted, thereby saving time and increasing throughput.
[edit] Post-mastering
After mastering, the glass master is baked to harden the developed surface material and it is ready for metalisation. Metalisation is a critical step prior to the electrogalvanic manufacture (electroplating).
The developed glass master is placed in a vapour deposition metaliser which uses a combination of mechanical vacuum pumps and cryopumps to lower the pressure inside a chamber to a hard vacuum. A piece of nickel wire is then heated in a tungsten boat to white hot temperature and the nickel vapour is deposited onto the rotating glass masters. The glass masters are coated with the nickel vapour up to a typical thickness of around 400nm before they are removed.
The glass masters are removed and inspected for stains, pinholes or incomplete coverage of the nickel coating.
[edit] Electroforming
Electroforming occurs in "Matrix", the adopted name for the electroforming process area and is still a class 100 cleanroom. The information contained on the metalised glass master is extremely fragile and must be transferred to a more resilient form for use in the injection moulding equipment.
The metalised master is clamped in a conductive plating frame with the information side facing outwards and lowered into a plating tank. The tank contains a nickel salt solution (Nickel Sulfamate) at a specific concentration. The solution is carefully buffered to maintain the pH and detergents are added to maintain a specific surface tension. If the surface tension is too high, the solution cannot flow around the features on the surface sufficiently to deposit nickel evenly. The bath is heated to approximately 40 °C.
The glass master is rotated in the plating tank while a pump circulates the nickel solution over the surface of the master. As the electroforming progresses, nickel is drawn out of the solution galvanically and must be replenished. This is achieved using high purity nickel pellets (99.99% pure) suspended in the solution in non-conductive polypropylene bags called anode bags. The plating solution flows through the bag and over the glass master. The anode bags stop sediment formed during the nickel decomposition from being plated on to the part. The nickel is packed firmly into the bag and forms part of the electric circuit.
A DC current is applied to the glass master travelling through the nickel contained in the anode bags, through the solution, and into the nickel surface of the glass master. The electrons flow in the opposite direction to the current, from the cathode to the anode via the solution. The electrons are stripped from the nickel in the anode bag, travelling through the external circuit before combining with the Nickel ions in the solution at the cathode end and forming metallic nickel on the surface of the glass master.
The current must start off quite low and be increased slowly and evenly to prevent the metalised surface from overheating and burning, like an electrical fuse. As the thickness of the nickel on the glass master increases the current can be increased. After approximately 1 hour the electroforming is complete. Typical stampers are 0.300 mm thick. The part is removed from the tank and the metal part peeled off the glass substrate. The metal part, now called a "father", has the information side as a series of bumps rather than pits. The father is washed with deionised water and other chemicals such as sodium hydroxide or acetone to remove any trace of resist or other contaminant. The glass can be sent for reclaiming, to be cleaned and checked before it is used again. If a defect is detected it will be discarded or sent for recycling.
Once cleaned of any loose nickel and resist, the father is electrolysed, washed and clamped back into a frame and returned to the plating tank. This time the metal part that is grown is the mirror image of the father and is called a "mother". From the mother all the stampers used to manufacture the CDs are made. Mothers can be regrown from Fathers if they become damaged, however if handled correctly, 10 - 20 stampers can be grown from a mother before the quality of the stamper is called into question. Mothers are regrown from the father if it still exists, otherwise a new glass master is made.
If the recording is going to be part of a long production run, the father may be archived, however it is generally cut down with a hyper-accurate hydraulic punch and used as a stamper for moulding runs. Stampers and fathers are the same "polarity", the information surface is made up of a series of bumps. Mothers are the reverse and are made up of pits.
A father, mother, and a collection of stampers (sometimes called "sons") are known collectively as a "family". Fathers and mothers are the same size as a glass substrate, typically 300 μm in thickness. Stampers do not require the extra space around the outside of the program area and they are punched to remove the excess nickel from the outside and inside the information area in order to fit the mould of the injection moulding machine (IMM). The physical dimensions of the mould vary from machine to machine but some typical dimensions are common throughout the industry.
[edit] Replication
CD moulding machines are specifically designed high temperature polycarbonate injection moulders. They have an average throughput of 550-900 discs per hour, per moulding line. Clear polycarbonate pellets are first dried at around 130 degrees Celsius for nominally three hours (dependent on which optical grade resin is in use) and are fed via vacuum transport into the one end of the injection moulder's barrel (the feed throat) and are transported to the injection chamber via a large screw inside the barrel. The barrel, wrapped with heater bands ranging in temperature from circa 210 to 320 degrees Celsius melts the polycarbonate. When the mould is closed the screw moves forward to inject molten plastic into the mould cavity. When the mould is full, cool water running through mould halves, outside the cavity, cools the plastic so it solidifies somewhat. The entire process from the mould closing, injection and opening again takes approximately 3 to 5 seconds.
The moulded "disc" (referred to as a 'green' disc, i.e. unprocessed) is removed from the mould by vacuum high-speed robots with vacuum suction caps and moved onto the infeed conveyor or cooling station of the finishing line before metallisation. At this point the discs are clear and contains all the digital information however it cannot be played because there is no reflective layer.
The discs then pass, one at a time into the metaliser, a small chamber operating at approximately 10E-3 Torr vacuum. This process is called 'sputtering'. The metaliser contains a metal "target" made of an alloy of mostly aluminium and some small amounts of other metals. There is a system of a load-lock (like an airlock) so that the process chamber can maintain high vacuum as the discs are exchanged. When the disc is rotated into the processing position by the swivel arm in the vacuum chamber, a small dose of argon gas is injected into the process chamber and a 700 Volt DC electrical current at up to 20 kW is applied to the target. This results in a plasma igniting and the aluminium target evaporates onto the disc (anode - cathode reaction). The metal coats the information side of the disc (upper surface) and covers the pits. This metal layer is the reflective surface that can be seen on the reverse of a CD. This thin layer of metal is unstable and will oxidise if it is not protected by a lacquer.
After metalisation the discs pass onto a spin-coater, where UV curable lacquer is dispensed on to the metal layer and spun rapidly to coat the entire disc in a very thin layer (circa 70 nm). After the lacquer is applied it passes under a high intensity UV lamp which cures the lacquer. The lacquer also provides a surface for the screen printing or offset printing ink to adhere to.
[edit] Testing
For quality control, both the stamper and the moulded discs are tested before a production run. Samples of the disc (test pressings) are taken during long production runs and tested to ensure consistency of quality. The pressed discs are analyzed on a signal analysis machine. The metal stamper can also be tested on a signal analysis machine which has been adapted for this purpose. The machine will "play" the disc or stamper and will measure various physical and electrical parameters. Errors can be introduced by the moulding process, however both CD and stamper sources of errors can be located and compensated for. If the errors are too severe then the stamper is rejected and must be made again. An experienced operator can interpret the report from the analysis system and optimise the moulding process to make a disc that meets the required Rainbow Book specification (e.g. Red Book for Audio, other colors for other formats).
If no defects are found the CD continues into printing to have a label screen or offset printed on the top of the disc and then onward to be packaged, and passed to distribution.
