1. Field of the Invention
This invention relates to a method of and an apparatus for manufacturing a disk-like medium such as a compact disc (CD) or a video disc (VD), and particularly to a method of and an apparatus for manufacturing a pattern such as round tracks (grooves) formed on a disc medium by unevenness or information pits on the tracks, by the lithography technique.
2. Related Background Art
As a method of manufacturing a CD or a VD of this kind or the like, it has heretofore been practised to form pattern information which will provide grooves or pits on a master disc (mother) by direct cutting by the use of a laser spot, reproduce it as a replica onto an electrocasting stamper, and further make a CD or a VD which will be a product by stamping by the use of the electrocasting stamper.
In the presentday CD or VD, the line width of grooves or pits is about 1 .mu.m, and even by the stamping method, pattern transfer can be accomplished sufficiently accurately, and throughput and cost reduction are also good.
Accordingly, in a CD or a VD, round tracks (grooves) on the disc are concentrically or spirally formed at a pitch of the order of 2 .mu.m in the diametrical direction thereof.
However, if an attempt is made to make the information density per unit area on the disc higher than the existing one, there will arise the necessity of making the width of and the interval between the grooves or pits as minute as 0.5 .mu.m-0.3 .mu.m. Along therewith, the wavelength .lambda. of an information reading beam will be shortened. By shortening the wavelength .lambda. of the beam, it becomes possible to make the diameter of the spot applied onto the disc minute. Of course, the depth of the grooves or pits becomes smaller as the line width or the interval becomes more minute, because the optical distance (depth x refractive index) becomes 1/4 of the wavelength .lambda. of the beam.
If the grooves or pits are thus made minute, there will arise the problem that accurate transfer by the existing stamper system becomes difficult. That is, the material forming the disc has more or less elasticity and therefore, there will arise the problem that transfer cannot be successfully accomplished even if an attempt is made to apply pressure to form the pattern of grooves or pits, because the grooves or pits are made too minute.
So, a method of and an apparatus for manufacturing a disc medium by the lithography technique has been proposed by Japanese Laid-Open Patent Application No. 62-217443. This known technique is to project the pattern of a disc medium formed on a photomask or a reticle onto a partial area of a disc substrate having photoresist applied thereto in advance and rotate this substrate continuously or intermittently to thereby expose the substrate and complete exposure by one full rotation of the substrate.
However, the above-mentioned publication has merely proposed a basic technique of manufacturing a disc medium by the use of the lithography technique, and various improvements therein have been necessary to complete it as an actually practical apparatus.