Vapour HF Release of Airgap-based UV-visible Optical Filters

Abstract

The design and CMOS-compatible fabrication of airgap-based optical filters in a surface micromachining process with sacrificial release using thevapour phase is presented. An airgap-dielectric layer combination offers a higher refractive index contrast, as compared to the conventional all-dielectriclayer based filters, which results in a higher peak reflectance and a wider bandwidth in a distributed Bragg reflector (DBR). Several DBRs and Fabry-Perot filters with multiple (low-n) airgap layers separating a stack of (high-n) layers of a dielectric material were designed for operation in the UV-visible spectrum. The fabrication was based on an Al2O3 and SiO2 system. The lateral design includes a set of anchor points and access holes. Finally, a vapour phase HF etching was used to remove the SiO2 layers and release the Al2O3 membranes. This method prevents the stiction of the membranes and provides a higher control and uniformity over the filter area.