- All CDs are pressed from a digital data source, with the most common sources being low error-rate CD-R’s or files from an attached computer hard drive containing the finished data (e. g., music or computer data). Some CD pressing systems can use digital master tapes, either in Digital Audio Tape, Exabyte or Umatic formats. However such sources are suitable only for production of audio CDs due to error detection and correction issues. 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 sources, the tape must be uploaded to a media mastering system to create the TOC (Table of Contents) for the CD.
- Glass mastering is performed in a class 100 or better clean room or a self-enclosed clean environment within the mastering system. Contaminants introduced during critical stages of manufacturing (e.g., dust, pollen, hair, or smoke) can cause sufficient errors to make a master unusable. Once successfully completed, a CD master will be less susceptible to the effects of these contaminants.
- During glass mastering, glass is used as a substrate to hold the CD master image while it is created and processed; hence the name. 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 specially for CD mastering and one side is polished until it is extremely smooth. Even microscopic scratches in the glass will affect the quality of CDs pressed from the master image. The extra area on the substrate allows for easier handling of the glass master and reduces risk of damage to the pit and land 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 rinses the glass black with a solvent and then applies either photoresist or dye-polymer depending on the mastering process. Rotation spreads the photoresist or dye-polymer coating evenly across the surface of the glass. 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 techniques; 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 common), single speed glass mastering (also referred to as 1X glass cutting or 1x glass mastering) is offered by a few CD replication plants as a higher quality process. A large number of audiophiles believe this results in truer reproduction although this has remained a matter of controversy for many years.
- Photoresist mastering uses a light-sensitive material (a photoresist) to create the pits and lands on the CD master blank.
- 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 chemically ‘developed’.
- Once developing is finished, the glass master is metalized to provide a surface for the stamper to be formed onto.
Non-photoresist (NPR) or Dye-Polymer mastering
- Once the glass is ready for mastering, it is placed in a Laser Beam Recorder (LBR). Most 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 master blank. 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 laser energy focused in a precise spot; this vaporizes 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 immediately assessed; for instance, audio signals being recorded can also be played straight from the glass master in real time. 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 optimize 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 characteristics of the local production process (e.g., marginal molding performance). 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 thicker than the intended pits.
- This type of mastering is called Direct Read After Write or DRAW and is the main advantage of some non-photoresist recording systems. Problems with the quality of the glass blank master, such as scratches, or an uneven dye-polymer coating, can be immediately detected. If required the mastering can be halted, saving time and increasing throughput.
- After mastering, the glass master is baked to harden the developed surface material to prepare it for metallization. Metallization is a critical step prior to electrogalvanic manufacture (electroplating).
- The developed glass master is placed in a vapor deposition metalize which uses a combination of mechanical vacuum pumps and cryopumps to lower the total vapor 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 vapor deposited onto the rotating glass master. The glass master is coated with the nickel vapor up to a typical thickness of around 400nm.
- The finished glass masters are inspected for stains, pinholes or incomplete coverage of the nickel coating and passed to the next step in the mastering process.
- Electroforming occurs in “Matrix”, the name used for the electroforming process area in many plants; it is also a class 100 cleanroom. The data (music, computer data, etc) on the metalized glass master is extremely easy to damage and must be transferred to a tougher form for use in the injection molding equipment which actually produces the end-procut optical disks.
- The metalized master is clamped in a conductive plating frame with the data side facing outwards and lowered into a plating tank. The tank contains a nickel salt solution (usually nickel sulfamate) at a particular concentration which may be adjusted slightly in different plants depending on the characteristics of the prior steps. The solution is carefully buffered to maintain its pH, and detergents are added to maintain a specific surface tension. If the surface tension is too high, the solution cannot flow around the very small features (i.e., the pits and lands) on the surface of the glass master sufficiently well to deposit metal properly. The bath is heated to approximately 40 °C.
- The glass master is rotated in the plating tank while a pump circulates the plating solution over the surface of the master. As the electroforming progresses, nickel is galvanically drawn out of the solution and must be replenished to maintain a constant concentration in the plating bath. 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 reaching the solution and perhaps being plated onto to a master’s surface. The nickel is packed firmly into the bag and forms part of the electric circuit.
- A DC current applied to the glass master is the source of the galvanic potential which forces nickel from the anode pellets in the bags into solution as ions and ultimately onto the master’s surface as an electrically neutral metallic layer. The electrons flow in the opposite direction to the current, from the cathode to the anode via the solution. Electrons are stripped from the nickel in the anode bag, travel through the external circuit before combining with nickel ions in the solution at the cathode end thus 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 metalized surface from overheating damage. As the thickness of the nickel on the glass master increases, the current can be increased. The electroplating step is finished after approximately 1 hour. Typical stampers are 0.300 mm thick. The part is removed from the tank and the metal layer peeled off the glass substrate. The metal part, now called a “father”, has the desired data as a series of bumps rather than pits; it is a negative master. The father is washed with deionised water and other chemicals such as sodium hydroxide or acetone to remove all trace of resist or other contaminants. The glass master can be sent for reclamation, cleaning and checking before reuse. If defects are detected, it will be discarded or recycled.
- Once cleaned of any loose nickel and resist, the father is electrolyzed, 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”; this is a ‘positive’ master. All the stampers used to manufacture the CDs are made from a mother. Mothers can sometimes be regrown from fathers if they become damaged, however if handled correctly, 10 – 20 stampers can be grown from a single mother before the quality of the stamper is reduced unacceptably. Mothers are regrown from the father if it still exists; otherwise a new glass master is made.
- If the CD is 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 molding runs. Stampers and fathers are the same (negative) “polarity”; the information surface is made up of a series of bumps. Mothers are the reverse and have pits on their surfaces.
- 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 outside and inside the information area in order to fit the mould of the injection molding machine (IMM). The physical dimensions of the mould vary somewhat from machine to machine.
- CD molding machines are specifically designed high temperature polycarbonate injection molders. They have an average throughput of 550-900 discs per hour, per molding line. Clear polycarbonate pellets are first dried at around 130 degrees Celsius for three hours (nominal; this depends on which optical grade resin is in use) and are fed via vacuum transport into one end of the injection molder’s barrel (i.e., the feed throat) and are moved to the injection chamber via a large screw inside the barrel. The barrel, wrapped with heater bands ranging in temperature from ca 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 somewhat solidifies. The entire process from the mould closing, injection and opening again takes approximately 3 to 5 seconds.
- The molded “disc” (referred to as a ‘green’ disc, lacking final processing) is removed from the mould by vacuum handling; high-speed robot arms with vacuum suction caps. They are moved onto the finishing line infeed conveyor, or cooling station, in preparation for metallization. At this point the discs are clear and contain all the digital information desired; however they cannot be played because there is no reflective layer.
- The discs pass, one at a time, into the metalize, a small chamber at approximately 10E-3 Torr vacuum. The process is called ‘sputtering’. The metalize contains a metal “target” — almost always an alloy of (mostly) aluminum and small amounts of other metals. There is a load-lock system (similar to an airlock) so the process chamber can be kept at high vacuum as the discs are exchanged. When the disc is rotated into the processing position by a 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 produces a plasma from the target, and the plasma vapor is deposited onto the disc; it is an anode – cathode transfer. The metal coats the data side of the disc (upper surface), covering the pit and lands. This metal layer is the reflective surface which can be seen on the reverse (non label side) of a CD. This thin layer of metal is subject to corrosion from various contaminants and so is protected by a thin layer of lacquer.
- After metallization, the discs pass on to a spin-coater, where UV curable lacquer is dispensed onto the newly metalized layer. By rapid spinning, the lacquer coats the entire disc with a very thin layer (circa 70 nm). After the lacquer is applied, the disks pass under a high intensity UV lamp which cures the lacquer rapidly. The lacquer also provides a surface for a label, generally screen printed or offset printed. The printing ink(s) must be chemically compatible with the lacquer used. Markers used by consumers to write on blank surfaces are not always, which can lead to breaks in the protective lacquer layer, to corrosion of the reflective layer, and failure of the CD.
- For quality control, both the stamper and the molded discs are tested before a production run. Samples of the disc (test pressings) are taken during long production runs and tested for quality consistency. Pressed discs are analyzed on a signal analysis machine. The metal stamper can also be tested on a signal analysis machine which has been specially adapted (larger diameter, more fragile, …). The machine will “play” the disc or stamper and measure various physical and electrical parameters. Errors can be introduced at every step of production, but the molding process is the least subject to adjustment. Error sources of errors are more readily identified and compensated for during mastering. If the errors are too severe then the stamper is rejected and a replacement installed. An experienced machine operator can interpret the report from the analysis system and optimize the molding process to make a disc that meets the required Rainbow Book specification (e.g. Red Book for Audio from the Rainbow Books series).
- If no defects are found, the CD continues to printing so a label can be screen or offset printed on the top surface of the disc. Thereafter, disks are counted, packaged, and shipped