Asymmetrical radiation exposure of spin coated photoresist to obtain uniform thickness coating used to replicate spiral grooves in plastic substrate

In a method of replicating a spiral groove pattern comprising the steps of:

• spin coating a layer of positive photoresist on a substrate;
• recording the pattern in the photoresist layer with actinic radiation;
• developing the photoresist layer by removing areas of said layer which have been exposed to actinic radiation so that the groove pattern is reproduced in the photoresist layer;
• transferring the pattern to a metal layer;
• and p1 forming the pattern in the surface of a plastic substrate;
• the improvement which comprises preparing a photoresist layer having uniform groove depth from a photoresist layer wherein the thickness of said layer varies as a function of the distance from the center point of spin coating by the additional steps of:
• asymmetrically irradiating the photoresist surface layer with actinic radiation so that thicker regions receive a greater exposure than thinner regions;
• and
• removing the irradiated photoresist surface layer so that the thickness of the remaining photoresist layer is substantially uniform.