Novel high precision microlithographic techniques applicable to distributed feedback laser grating manufacture
Distributed feedback, DFB, lasers are a major source for long haul optical fibre based telecommunication systems. They rely on ultrahigh precision gratings to reduce mode competition within the laser cavity, thereby making single frequency operation possible. Techniques applied to DFB grating manufacture tend to be either holographic, or a direct serial write process using slow and expensive electron beam lithographic machines. An entirely new approach is proposed which compliments the accuracy of the electron beam systems with the high throughput and relatively low cost of a conventional contact mask regime. The new process relies on a group of materials which are able to retain an embossed relief upon curing with ultraviolet radiation. The main manufacturing stages of this new technique are presented including details of the embossing tool fabrication using reactive ion etching, and characterisation of material properties. It is proposed that the technique, whilst intended for DFB manufacture, could find applications in many other areas, for instance as a route to low cost replication of photomasks. In addition a new grating assessment technique, based on normal reflectance measurements, has been developed. The technique has been used to investigate hitherto unmeasured effects in electron beam lithography on a variety of substrate materials before and after semiconductor overgrowth. Details of a theoretical exercise which agrees closely with experimental results are included. The technique is non-destructive and should be of interest to all areas of microlithography and holographic imaging where it is important to have control over a wide range of process stages or where the feature size rules out the use of conventional optical microscopes.