Fabrication of sub-100nm thick Nanoporous silica thin films

Low refractive index thin films have generated substantial interest for use in optical devices particularly in Light Emitting Diodes (LEDs). Applications include photonic cavities, mirrors, etc. Such films are required to have precise control over both thickness and refractive index. Here, we investigate fabrication of Omni Directional Reflectors (ODRs) using nanoporous silica films. Sub-100 nm thick nanoporous silica films of low refractive index (high porosity) are fabricated by spin coating on III-V compound semiconductor substrates. Thicker nanoporous silica films (~1 μm) on silicon substrates have been fabricated and characterized in past (Jain et al, 2001). We have modified the process to fabricate excellent quality sub-100nm thick nanoporous silica films, which were used in successful fabrication of more efficient ODRs. Interfacial forces between the substrate and nanoporous silica solution are believed to affect the adhesion of the film on the substrate and to play an important role in developing the final film properties like the thickness and the refractive index. In particular, the role of interfacial slip during films spinning to explain abovementioned results is investigated in this paper. Both theoretical and experimental studies support the assertion that interfacial slip occurs due to an alteration in intermolecular forces that exist between the substrate and the film.