21 May 2015 Optical characterization of MEMS-based multiple air-dielectric blue-spectrum distributed Bragg reflectors
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The optical performance of a distributed Bragg reflector (DBR) is typically the determining factor in many optical MEMS devices and is mainly limited by the number of the periods (number of layers) and the refractive index contrast (RIC) of the materials used. The number of suitable available materials is limited and implementing a large number of periods increases the process complexity. Using air as a low-index material improves the RIC by almost 50% as compared with most conventional layer combinations and hence provides a higher optical performance at a given number of layers. This paper presents the design, fabrication, and optical characterization of multiple air-SiO2 Bragg reflectors with two airgap layers designed for the visible spectrum. Alternate polysilicon deposition and silicon-dioxide growth on the wafers followed by the selective etching of polysilicon layers in a TMAH-based solution results in a layer stack according to the optical design. However, unlike the conventional MEMS processes, fabrication of a blue-band airdielectric DBR demands several sacrificial layers in the range of 100 nm. Therefore, a successful release of the membrane after wet-etching is critical to the successful performance of the device. In this study, several DBRs with two periods have been fabricated using a CO2 supercritical drying process. The wide-area reflection measurements showed a peak reflectance of 65% and an FWHM of about 100 nm for a DBR centered at 500 nm. DBRs centered on 400 nm gave a much wider spectral response. This paper presents preliminary optical characterization results and discusses the challenges for a reflector design in the blue-visible range.
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M. Ghaderi, M. Ghaderi, N. P. Ayerden, N. P. Ayerden, G. de Graaf, G. de Graaf, R. F. Wolffenbuttel, R. F. Wolffenbuttel, "Optical characterization of MEMS-based multiple air-dielectric blue-spectrum distributed Bragg reflectors", Proc. SPIE 9517, Smart Sensors, Actuators, and MEMS VII; and Cyber Physical Systems, 95171M (21 May 2015); doi: 10.1117/12.2178609; https://doi.org/10.1117/12.2178609

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