Usage of dry processes for the formation of diffractive structures on Ti and Ti/Si films

The paper describes methods for manufacturing of diffractive optical elements by means of only "dry" processes starting from direct laser writing on titanium-containing films. According to first approach, direct laser writing onto thin Ti film forms surface oxide mask. Reactive ion etching removes non-oxidized Ti film and develops "latent" oxidized image. Subsequent thermal annealing of the oxidized Ti structure in air makes the mask more stable for following reactive ion etching of fused silica substrate to ensure proper phase depth of the binary diffractive structure. This makes it possible to avoid liquid etching, which reduces the yield and accuracy. The phase structure of the diffractive elements manufactured using the described method consists of the grooves etched in the fused silica substrate between ridges covered by TiO₂ between them. We found out also that covering the Ti film by very thin Si layer helps to increase laser energy absorption at direct writing and creates quite resistant masking layer TiSi₂ for the reactive ion etching. Preliminary estimates show that dual layer Si/Ti films can be used to create amplitude reflective DOEs. Possible application area for the developed methods is manufacturing of the diffractive optical elements used for precision generation of reference wavefronts in interferometric measurements of spherical and aspherical surfaces.