Read-Only Memory (ROM) optical discs are designed to store large amounts of digital information that is accessible over many years of lifetime of the product. Such discs should be readable by a drive device provided it meets certain specification requirements that do not change appreciably over the lifetime of the product.
The reflectivity of optical ROM discs is normally enhanced by the addition of a thin, highly reflective coating made of metal or metal alloys at the data surface. The choice of material is primarily dependent upon its reflectivity at the wavelength of the read back laser, its stability in the operating environment over the lifetime of the product, and the cost of the material, though other factors are also involved, e.g. roughness and uniformity of the coating.
Silver alloys have been used in optical discs for several years. Although silver alloys perform well in accelerated life tests, silver is expensive and does not have the highest reflectivity per unit thickness compared to some other metals. The reflectivity of silver alloy is even less optimal at the 405 nm wavelength, the wavelength that “Blue-Ray” discs are read at. Other metals such as aluminum have higher reflectivity at 405 nm per unit thickness and lower cost, but they suffer from corrosion and have poor durability. Corrosion can be a particularly difficult factor for miniature discs that are designed for mobile devices. Such discs may be subjected to higher temperatures as they are carried outside and are exposed to higher heat and humidity levels.
One factor that affects the quality and performance of the read back signal of an optical disc is the thickness of the reflective layer. The thickness of the reflective layer needs to be sufficient to meet the minimum reflectivity requirement, and this is about 15 nm for a conventional silver alloy reflective layer. The thickness of the reflective layer should not be so thick that it causes a distortion in the read back signal. In a conventional silver alloy reflective layer, if the thickness is greater than approximately 25 nm the jitter of the data signal increases and the error rates increase to unacceptable values. This limitation in the maximum practical thickness of the reflective layer limits the maximum reflectivity of the optical disc. Optical discs work better with high signal-to-noise ratios, since this will minimize data errors. So it is desirable to have discs with higher reflectivity.
Herein, an optical disc is provided that includes a substrate layer, a cover layer, and an aluminum alloy layer that is between the substrate layer and cover layer, and is adjacent to the substrate layer. The aluminum alloy layer includes a majority amount of aluminum and an additional metal selected from the group consisting of: chromium, titanium, tantalum, and any combination thereof.
Herein, a method is provided for making an optical disc including the steps of: forming a substrate layer; sputtering an aluminum alloy target onto the substrate layer to form an aluminum alloy layer; and forming a cover layer; wherein the aluminum alloy layer comprises a majority amount of aluminum and an additional metal selected from the group consisting of: chromium, titanium, tantalum, and any combination thereof.
A method for making an optical disc is also provided that includes the steps of: forming a substrate layer; sputtering a first target including a majority amount of an additional metal onto the substrate layer, wherein the additional metal is chromium, titanium, tantalum, and any combination thereof, thereby forming an additional metal layer; sputtering a second target including a metal onto the additional metal layer, wherein the metal is aluminum, thereby forming an aluminum layer; and forming a cover layer. The aluminum alloy layer comprises a majority amount of aluminum and an additional metal selected from the group consisting of: chromium, titanium, tantalum, and any combination thereof.