Micromechanical investigation of MEMS-based short-wave infrared tunable Fabry-Perot filters

Byron Walmsley

    Research output: ThesisDoctoral Thesis

    91 Downloads (Pure)

    Abstract

    [Truncated abstract] This study investigates the mechanical and physical properties of low-temperature (100-300 ?C) plasma enhanced chemical vapour deposited (PECVD) silicon nitride (SiNxHy) thin films for the fabrication of short-wave infrared tunable Fabry-Perot filters with high fill factor, high cavity finesse and low actuation voltages. It has been the intensions of this work to fabricate a tunable filter that can be monolithically integrated with temperature-sensitive substrates, namely mercury cadmium telluride (Hg(1-x)CdxTe) photoconductors and photodiodes. A range of methods have been utilised to determine the Young's modulus (E), residual stress ([sigma]0), density ([rho]) and Poisson's ratio ([nu]) of PECVD SiNxHy thin films. In order to understand how E, [sigma]0, [rho] and [nu] are affected by process conditions, a range of SiNxHy thin films deposited with varying chuck temperatures, RF powers and chamber pressures were measured. The resonance method was used to determine E and [nu] of SiNxHy thin films deposited under varying process conditions. The resonance method involves exciting the bending and torsional vibration modes of a microcantilever beam fabricated from a film. The E and G values can be extracted directly from the bending and torsional vibration modes and the [nu] value can be determined from the calculated E and G values. The density of the films was determined using the quartz crystal microbalance method. In order to determine the validity of the resonance method, finite element modelling was used to determine its dependence on microcantilever beam dimensions. ... Increasing the temperature also increases the tensile residual stress of the films. This study also reveals that increasing the RF power and decreasing the chamber pressure increases E and [rho], as well as increasing the compressive residual stress of the films. The theoretical design and analysis, as well as the fabrication of a new surface micromachined short-wave infrared tunable Fabry-Perot filter for adaptive infrared photon detectors is also presented in this study. The proposed structure, termed the suspension filter, uses broad spectral range, high reflectivity distributed Bragg reflector (DBR) mirrors, resulting in very high finesse filters. The device utilises multiple sacrificial layers to define the resonant cavity spacer and the separation of the top mirror from the supporting flexures. The flexures were fabricated from low-temperature (PECVD) SiNxHy thin films. Separation of the top mirror from the supporting flexures allows for improved fill-factors (up to 79%), as well as increased tuning range. Theoretical optical and electromechanical results shows large wavelength tuning ranges (1.5-2.5 [mu]m) at low actuation voltages (
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2007

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