Surface-micromachined Bragg Reflectors Based on Multiple Airgap/SiO2 Layers for CMOS-compatible Fabry-perot Filters in the UV-visible Spectral Range
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Abstract
In CMOS-compatible optical filter designs, SiO2 is often used as the low-index material, limiting the optical contrast (nHi/nLo) to about 2. Using the air as low-index material improves the optical contrast by about 50%, thus increasing the reflectivity and bandwidth at a given design complexity. The design and fabrication of a 4-layered air-dielectric distributed Bragg reflector (DBR) centered at 400 nm using surface micromachining techniques, is presented here. Fabrication is based on the deposition of poly-Si thin films and subsequent thermal oxidation. Selectively removing the Si layers, produces an air/SiO2 filter stack according to the optical design. The width of the air-gaps needed in this application is typically smaller as compared to conventional MEMS devices. The number of layers is also higher, while no electrical contact is required. The concept, the fabrication of 4-layer DBRs and preliminary measurement results are discussed in this paper.