Tailoring the optical properties of holey Si thin films

Si thin films with holes are composite materials with interesting optical properties that can be fabricated and modified by state-of-the-art Si process technologies. Adjusting the volumetric air fraction of these films allows control over their effective refractive indices. This work demonstrates a novel scalable method that combines charged sphere colloidal lithography (CSCL) and dry etching to pattern spatially disordered nanoholes in Si thin films. The method can also be adapted to dielectric materials other than silicon. We show controlled tuning of the effective refractive index by lateral dry-etching of the holes. Utilizing this process, a progressive widening of the average hole diameter was obtained, expanding from initial diameters of 60 nm and 100 nm to dimensions reaching 118 nm and 168 nm, respectively. Consequently, the refractive index of the holey films decreased to approximately 2.1 determined via ellipsometry and Bruggeman's model in the near-infrared (NIR) - mid-infrared (MIR) range, in contrast to the unstructured Si refractive index of 3.42. The systematic optical modification was also observed in the reflection spectra of the fabricated films using Fourier-transform infrared spectroscopy (FTIR). Dielectric holey thin films can be attractive for potential applications in MIR photonic devices such as filters and waveguides.