Example of a glass master used in the compact disc replication process
Electroforming occurs in “Matrix”, the name used for the electroforming process area in many plants; it is also a class 100 (ISO 5) or better clean room. The data (music, computer data, etc.) on the metalised glass master is extremely easy to damage and must be transferred to a tougher form for use in the injection moulding equipment which actually produces the end-product optical disks.
The metalised master is clamped in a conductive electrodeposition frame with the data side facing outwards and lowered into an electroforming tank. The specially prepared and controlled tank water 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 organic contaminants must be kept below one part in five million for good results. The bath is heated to approximately 50 °C.
The glass master is rotated in the electroforming tank while a pump circulates the electroforming solution over the surface of the master. As the electroforming progresses, nickel is not electroplated onto the surface of the glass master, since that would preclude separation. Plating is rather eschewed through passivation and, initially, because the glass is not electroconductive. Instead, the metal coating on the glass disc, actually reverse-plates onto the nickel (not the mandrel) which is being electrodeposited by the attraction of the electrons on the cathode, which presents itself as the metal-coated glass mistress, or, premaster mandrel. Electroplating, on the other hand, would have entailed electrodepostion directly to the mandrel along with the intention of it staying adhered. That, and the more rigorous requirements of temperature control and purity of bathwater, are the main differences between the two disciplines of electrodeposition (invented by Luigi Brugnatelli and often credited to another Luigi – Sr. Galvani). The metal stamper first struck from the metal-coated glass is the metal master (and we shouldn’t make a master from another master as that would not follow the nomenclature of the sequence of siring that is germane to electroforming) This is clearly a method opposite to normal electroplating. Another difference to electroplating is that the internal stress of the nickel must be controlled carefully, or the nickel stamper will not be flat. The solution cleanliness is important but is achieved by continuous filtration and usual anode bagging systems. Another large difference is that the stamper thickness must be controlled to ±2% of the final thickness so that it will fit on the injection moulding machines with very high tolerances of gassing rings and centre clamps. This thickness control requires electronic current control and baffles in the solution to control distribution
The current must start off quite low as the metallised layer is too thin to take large currents, and is increased steadily. As the thickness of the nickel on the glass “mistress” increases, the current can be increased. The full electroforming current density is very high with the full thickness of usually 0.3 mm taking approximately one hour. The part is removed from the tank and the metal layer carefully separated from the glass substrate. If plating occurs, the process must be begun anew, from the glass mastering phase. The metal part, now called a “father”, has the desired data as a series of bumps rather than pits. The injection moulding process works better by flowing around high points rather than into pits on the metal surface. The father is washed with deionised water and other chemicals such as ammonical hydrogen peroxide, 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 repolished recycled.
Once cleaned of any loose nickel and resist, the father surface is washed and the passivated, either electrically or chemically, which allows the next plated layer to separate from the father. This layer is an atomic layer of absorbed oxygen that does not alter the physical surface. The father is 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”; as this is now pits, it cannot be used for moulding.
The mother-father sandwich is carefully separated and the mother is then washed, passivated and returned to the electroforming baths to have a mirror image produced on it called a son. Most moulded CDs are produced from sons.
Mothers can be regrown from fathers if they become damaged, or a very long run. If handled correctly, there is no limit to the number of stampers that can be grown from a single mother before the quality of the stamper is reduced unacceptably. Fathers can be used as a stamper, directly, if a very fast turnaround is required, or if the yield is 100%, in which case the father would be wastefully stored. At the end of a run, the mother is certainly to be stored.
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 moulding machine (IMM). The physical dimensions of the mould vary depending of the injection tooling being used.
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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 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 moulder’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 moulded “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 metallisation. 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 metaliser, a small chamber at approximately 10−3 Torr (130 mPa) vacuum. The process is called ‘sputtering’. The metaliser contains a metal “target” — almost always an alloy of (mostly) aluminium 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 electric 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 metalisation, the discs pass on to a spin-coater, where UV curable lacquer is dispensed onto the newly metallized layer. By rapid spinning, the lacquer coats the entire disc with a very thin layer (approx. 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 can lead to breaks in the protective lacquer layer, which may lead to corrosion of the reflective layer, and failure of the CD.
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 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 moulding process is the least subject to adjustment. 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 optimise the moulding 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, discs are counted, packaged, and shipped.
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