Author: Dhirendra Kumar Tripathi

Publisher:

Published: 2016

Total Pages:

ISBN-13:

Optical microelectromechanical systems (Optical MEMS) offer a myriad of advantages, such as system miniaturization, robustness, cost effectiveness and high portability. Microspectrometers are optical MEMS based low cost, portable and robust spectroscopic systems which can be employed for soil characterization, food quality inspection, chemical mapping, and spectral imaging. The Microelectronics Research Group at The University of Western Australia has been involved in the design and development of various microspectrometers technologies for well over a decade. The heart of a microspectrometer is a tunable Fabry-Perot filter, which usually consists of two distributed Bragg reflectors(DBRs), one of which is fixed and the other is actuated to modulate the wavelength transmitted through the filter.The objective of this work is to fabricate and characterize silicon based DBRs and filters as building blocks for microspectrometers operating invisible to mid-infrared wavelengths (450-5000 nm). In order to achieve this objective, first, thin films of inductively coupled chemical vapour deposited (ICPCVD) silicon were optimized for their residual stress and optical properties. Second, the optimal structural design and fabrication process for DBRs and filters was identified and, third, the performance of fabricated structures was assessed through optical and mechanical characterizations. It was found that higher deposition temperature leads to better quality of ICPCVD silicon films. Furthermore, at higher deposition temperatures, the decrease in the inductively coupled power results in films with low tensile stress, higher refractive index and low extinction coefficients. The experimental work shows that formation of stable monohydride (Si-H) bonds between the hydrogen and silicon leads to improved optical and mechanical quality of ICPCVD silicon thin films. Thus, it is concluded that the hydrogen concentration and the specific hydrogen-silicon bonding nature together play a vital role in improving optical and mechanical quality of the silicon thin films. This thesis presents a simple process to fabricate quarter wavelength silicon-air-silicon based surface micromachined DBRs. A precise control of stress in the silicon thin films by in-situ and pre-release annealing was highly effective in fabricating DBRs ranging in area from 200 ?m 200 ?m to 5000 ?m 5000 ?m size with only a 5-15 nm variation in at-ness across the surface of the suspended membrane. DBRs from visible to MWIR wavelengths show near theoretical reflectance properties. In the visible and near infrared (NIR) wavelengths, silicon-air-silicon based DBRs demonstrate higher than 90% reflectance, despite the relatively high absorbance of silicon in these wavelength ranges. A new notch based actuation structure is presented. It is demonstrated that this structure allows a suspended membrane to actuate beyond50% of the initial tunable air-gap. This planer actuation structure is robust against the effects of out of plane stress on the suspended mirrors, and can be applied across a wide array of mirror materials and sizes. A tunable multi-spectral MWIR filter was fabricated and optically characterized. The filter shows near ideal performance with close to 70% peak transmittance and 380 nm full width at half maximum (FWHM), and a 800 nm tuning range. This thesis also present the fabrication process and optical characterization of an air-gap mirror based short-wave infrared (SWIR) filter. The optical transmittance measurement indicates a first order peak with a peak transmission of 70% and 90 nm FWHM at a wavelength of 2240 nm. The second order peak of the measured spectrum shows 53% peak transmission and 35 nm FWHM at 1550 nm,which compares very well with the predicted 44% peak transmission and 30 nm FWHM. In the NIR wavelength range a silicon-silicon oxide-silicon DBR based filter was fabricated. The peak transmission of the filter at 1140 nm center wavelength is around 54% with 40 nm FWHM, which is in close agreement with simulation. The fabricated filter shows an excellent out of band extinction. In order to assess optical uniformity, transmission measurements of the optical MEMS devices were undertaken as a two dimensional optical transmission map of fabricated devices.