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) plasmaenhanced 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.
We have developed a microspectrometer based on monolithic integration of a Fabry-Pérot optical filter directly with a Hg x Cd 1-x Te-based infrared detector. The tunable Fabry-Pérot is created by a parallel plate MEMS fabricated from two dielectric mirror stacks separated by an initial air gap of 1.4 µm. We have measured linewidths as low as 55 nm, switching times of 40 µs and a tuning range of 380 nm. However this tuning corresponds to only 42% of the desired tuning range, from 1.6-2.5 µm (900 nm). The tuning range is limited by a process called "snap down" which occurs when the MEMS is drive by a voltage source. It can be shown that for a parallel plate snap down occurs at 1/3 the initial gap; complete tuning across the SWIR band requires a physical deflection of at least 60% of the gap. We have developed a modified actuator design which allows 60% tuning of the moveable mirror. Further, the method minimizes actuation-induced stress gradients which can lead to substantial bowing of the mirror and subsequently broad optical linewidths. We will compare the results of our current microspectrometer with our new extended tuning designs. These designs are based on Coventorware and analytical mechanical models combined with optical models for the FabryPérot.
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