We present an experimental demonstration of a novel, integrated readout approach for measuring the suspended height of micro-electro-mechanical systems (MEMS) structures. The approach is based on creating a resonant optical cavity between the suspended MEMS structure and the substrate that the MEMS structure is anchored to. The resulting interferometric effect causes modulation of an optical laser signal which is strongly dependent on the position of the MEMS device.
This work focuses on the development of a cryogenic optical profilometry system for the measurement of material properties of thin films across a wide temperature range. A cryostat was machined and integrated with a Zygo NewView 600K optical profilometer and vacuum system. Curvature data were taken for a SiNx thin film on a GaAs substrate from 300 K down to 80 K. From the curvature data, the coefficient of thermal expansion was calculated. The cryogenic optical profilometry system was benchmarked with a three beam curvature technique, and demonstrated excellent agreement across the full temperature range from 300 K to 80 K
Recent progress in short wavelength infrared MEMS based Fabry-Pérot microspectrometers at The University of
Western Australia is presented. The original monolithic approach has been replaced with a hybrid one due to HgCdTe
restricting the thermal budget and affecting the quality of structural silicon nitride films. The spectral resolution has been
improved by introducing five layer Bragg mirrors and by limiting the electrostatically actuated top mirror bowing and
tilting using stress balancing between layers. In effect the FWHM has been reduced to 30nm at ~2.0μm in comparison to
the ideal theoretical mid-range value of 9nm. Analysis of mirror profiles shows that this difference is a result of
remaining mirror imperfections.
Thin-film MEMS are essential to realization of intelligent integrated microsystems. Of critical importance in such microsystems is the determination and control of mechanical properties in the thin films used for construction of the MEMS, which can be the decisive factor in the realization and subsequent performance, reliability, and long-term stability of the system. In future microsystems the need to fabricate MEMS on temperature sensitive, non-standard substrates will be of particular importance. In this work, mechanical properties of low-temperature (50-300°C) plasma-enhanced chemical vapour deposited silicon nitride thin films have been investigated using depth sensing indentation. Young’s modulus, E, and hardness, H, values obtained for the examined film/substrate bilayers were found to vary asymptotically from the thin film properties for shallow indents to the substrate properties for deep indents. A simple empirical formulation is shown to relate E and H obtained for the film/substrate bilayers to corresponding material properties of the constituent materials via a power-law relation. The temperature of the deposition process was found to strongly influence the thin film mechanical properties. Values of E ~ 150-160GPa and H ~ 14-15GPa were observed for depositions above 225°C. Decreasing the deposition temperature initially caused a moderate and linear decrease in E and H parameters, which was followed by an abrupt decrease in E and H once the deposition temperature was lowered below 100°C, such that E ~ 50GPa and H ~ 3.5GPa at a deposition temperature of 50°C.
Two experimental techniques have been investigated to examine residual stress in low temperature plasma enhanced chemical vapour deposited (PECVD) SiNx thin films: one that measures the stress induced substrate curvature, and the other that takes advantage of the stress induced deformation of freestanding diagnostic microstructures. A general linear dependence of residual stress on deposition temperature is observed, with the magnitude of stress changing linearly from circa 300MPa tensile stress to circa 600MPa compressive stress as the deposition temperature is decreased from 300°C to 100°C. However, the results deviate from the linear dependence by a different degree for both measurement techniques at successively lower deposition temperatures. The stress values obtained via the substrate curvature method deviate from the linear dependence for deposition temperatures below 200°C, whereas the values obtained via the diagnostic microstructures method deviate from the linear dependence for deposition temperatures below 100°C. Stress uniformity over the deposition area is also investigated.