Anodization of electroformed nickel for the manufacture of compact disc stampers

Abstract

Compact discs are manufactured using electroformed nickel stampers. The process of making stampers involves the anodization of the metal works in alkaline solutions. During the anodization process, a layer of nickel oxide is formed on the surface of the electroformed metal works that subsequently is removed by the next process. This process may be repeated several times for each metal work in the course of CD manufacturing. In the present work, the effect of the growth and removal of NiO film on the quality of the stampers has been investigated. This work shows that although the nickel oxide layer formed under normal conditions of the anodizing bath is extremely thin, the quality of the stampers can be affected, by repeating the standard anodizing process for a number of times or by prolonged and vigorous anodization. A model has been presented for NiO formation and safe conditions for anodization have been proposed. Compact disc (CD-CDROM-DVD) manufacture involves a complex series of processes including engraving the digital data onto a glass coated with photoresist using a laser beam, in the form of little pits and lands. The dimensions of these pits and lands are critical and should be kept within a very narrow range. Both pits and lands vary in length but their depths and widths remain constant. In fact, in a typical compact disk there are pits and lands with 9 different lengths, conventionally referred to as 3T to 11T. The depth of the pits are adjusted to be a quarter of the playback laser’s wavelength, which works out to be about 110 to 113 nm. The data image face of the glass master is then sputtered with a thin layer of nickel/vanadium allowing the subsequent electroforming process. The next process in CD manufacture is the transfer the image from the so-called “glass master” onto a metal work, which has a hardness that is suitable for usage as a stamper in molding machines. The stamper, now having the negative image of the data is placed in an injection molding machine, pressing a molten transparent polycarbonate against it, replicating the original positive image. The polycarbonate disc is then covered with lacquer and together with artwork it forms the familiar CD/CDROM/ DVD shape. The electroforming process is, however, far from a straightforward process, involving several intermediate stages, including the anodization process. There are two reasons for anodizing the Ni metal works: