Electroforming
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 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. The bath is heated to approximately 50 °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 electroplated onto the surface in a method identical to normal electroplating. The main difference with normal plating 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 master increases, the current can be increased. The full plating 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. 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 plating 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 for plating 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 to be stored is plated again and the sandwich 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.
Read more about this topic: Compact Disc Manufacturing