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Amorphous Bilayer Thin Films for Optical Storage

IP.com Disclosure Number: IPCOM000050215D
Original Publication Date: 1982-Sep-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 65K

Publishing Venue

IBM

Related People

Ahn, KY: AUTHOR [+3]

Abstract

These optical storage media are ternary alloy bilayers of amorphous thin films deposited on substrates. Such media differ from ternary alloy, single layer amorphous films, such as GeTeAs, as interaction between the two layers of the bilayer causes a reflectivity change for laser writing. The bilayer structure has improved life-time reliability.

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Amorphous Bilayer Thin Films for Optical Storage

These optical storage media are ternary alloy bilayers of amorphous thin films deposited on substrates. Such media differ from ternary alloy, single layer amorphous films, such as GeTeAs, as interaction between the two layers of the bilayer causes a reflectivity change for laser writing. The bilayer structure has improved life-time reliability.

Thin film media for optical storage must have lifetimes of about 10 years at ambient environment, with good thermal stability and corrosion resistance. Safe disposal of waste disks and used disks eliminates use of toxic materials, such as Te and Se, and their alloys, in disk fabrication. Since the semiconductor GaAs laser is a cheap source of writing energy, an ideal optical storage film requires no more than say 16 mW of power for writing with a typical pulse width of 50 to 100 ns. No thin film media meet all these requirements. For example, bilayers of films of silicide-forming materials, such as Pt, Pd, and Rh on Si, for optical storage show excellent lifetimes and corrosion resistance, and they are nontoxic. These materials require a writing power in excess of 20 mW to produce a micron- sized spot.

Fig.1 shows a bilayer structure consisting of a binary alloy and amorphous Si. A suitable bilayer combination is a layer of a binary amorphous alloy 12, such as Gd (.85) Co(.15) or Rh(.80) B(.20),on a layer of amorphous Si or Ge 11 carried on a substrate 10, as shown in Fig.1. Alternatively, Fig.2 shows a bilayer structure consisting of a layer of a ternary amorphous alloy 16, such as Au(.60) Pd(.20) Si(.20) deposited on a layer of amorphous Si 15 which is carried on a substrate 14. Such structures are suitable for use in laser writing through a transparent substrate. A ternary Au alloy is employed because a binary amorphous Au(.70) Si(.30) alloy is very unstable near room temperature because its latent heat is comparable to its heat of crystallization; therefore, Pd is added to stabilize the structure. With a neighboring amorphous Si layer, the alloy can react with Si to form silicide phases, and produce a large change in reflectivity. Without the amorphous Si layer, it crystallizes the ternary amorphous alloy, yet the reflectivity change between the amorphous and crystallized regions is too small.

A bilayer was made by fabricating one class of bilayer structures, namely boron-doped Rh deposited on an amorphous Si. Boron is attractive technologically as it has a vapor pressure close to that of Rh, and is evaporated from an electron gun using an alloy slug with the desired composition. Using a composition with 10 weight percent boron, alloying reduces reflectivity. At the GaAs laser wavelength, reflectivity drops from 70 percent for the pure Rh to 26 percent for the alloy. When an amorphous film of Si is deposited over the alloy to form the desired bilayer structur, the reflectance is reduced significantly. The minmum reflectance shifts toward...