Trapezoidal smooth grooves for video disc

DETAILED DESCRIPTION An initial disc suitable for recording video information may be formed by a number of methods similar to those employed in the audio record reproduction art.
In prior video record reproduction methods as well as that described herein, the following preliminary steps may be followed in producing an initial disc.
A base disc of 1/2 inch thick aluminum 14 inches in diameter is machined flat and a protective coating is applied to the aluminum surface to prevent chemical attack of the aluminum base prior to depositing a layer of bright copper or lacquer thereon.
A uniform layer approximately 0.
005 inch thick of such material is deposited on the disc surface.
This layer is thereafter machined flat to within 0.
0002 inches utilizing a diamond cutting tool.
The surface is then further machined to improve the smoothness and a spiral groove is cut therein utilizing an appropriately shaped diamond cutting stylus.
The depth of the cut typically is about 0.
0001 inches and the spiral groove pitch is generally from about 2000 to 8000 groove convolutions per inch.
In prior art systems for producing an information storing disc, a V-shaped groove is cut in the manner described above into a copper or lacquered surface as shown in FIG.
1.
This groove has a typical included angle of 90.
degree.
and is formed so as to cause adjacent convolutions of the spiral groove to abut one another.
Successive negative and positive replications of the grooved, lacquer or copper surface are thereafter made by nickel deposition and separation steps known in the audio recording art.
The positive replica is coated with a layer of viscous material such as photoresist, electron beam sensitive material or other suitable material by spinning the disc at a velocity between 100 and 2000 rpm (typically 450 rpm for a material having a viscosity of 4.
5 cps).
Disc spinning velocity is selected in proportion to material viscosity.
Rotation is then continued (e.
g.
, for 1 minute) so as to remove excess material.
The rotational speed is then decreased to about 5 rpm until the coating has dried