Theoretical and experimental analysis of the fabrication tolerance on deeply etched silicon/air Bragg micromirrors

Modelling and analysis of the technological tolerance and artifacts of deeply-etched Bragg micromirrors and their impact on the performance is presented. Si/Air Bragg micromirrors with 150 μm etching depth and Si/Air layer thickness of about 3.8 μm / 3.6 μm are fabricated on Si wafer using the Deep Reactive Ion Etching technology (DRIE); designed to operate at a central wavelength of 1600 nm. For such high aspect ratio DRIE, the side-effects of the technology such as over-etching and surface roughness can be significant. Optical measurements for the multi-band spectral response are used to characterize the performance of the fabricated micromirrors and estimate the technological parameters. It was found that the central wavelength shifts monotonically with the over-etching at a rate of 0.45 nm / nm. The dependence of the 3-dB bandwidth and mirror reflectivity is non-monotonic with the over-etching. The surface roughness leads to shift in the central wavelength in addition to degradation in the bandwidth and mirror reflectivity. The presented results are important to understand the technological impact on optical filters and sensors based on the Bragg micromirrors.