Suvorova, A.; Lawn, B. R.; Liu, Y.; Hu, X. Z.; Dell, J. M.; and Faraone, L., "Effect of deposition conditions on mechanical properties of low-temperature PECVD silicon nitride films" (2006 AbstractThe effect of deposition conditions on characteristic mechanical properties -elastic modulus and hardness -of low-temperature PECVD silicon nitrides is investigated using nanoindentation. It is found that increase in substrate temperature, increase in plasma power and decrease in chamber gas pressure all result in increases in elastic modulus and hardness. Strong correlations between the mechanical properties and film density are demonstrated. The silicon nitride density in turn is shown to be related to the chemical composition of the films, particularly the silicon/nitrogen ratio.

This paper reports an investigation on techniques for determining elastic modulus and intrinsic stress gradient in plasma-enhanced chemical vapor deposition (PECVD) silicon nitride thin films. The elastic property of the silicon nitride thin films was determined using the nanoindentation method on silicon nitride/silicon bilayer systems. A simple empirical formula was developed to deconvolute the film elastic modulus. The intrinsic stress gradient in the films was determined by using micrometric cantilever beams, cross-membrane structures and mechanical simulation. The deflections of the silicon nitride thin film cantilever beams and cross-membranes caused by in-thickness stress gradients were measured using optical interference microscopy. Finite-element beam models were built to compute the deflection induced by the stress gradient. Matching the deflection computed under a given gradient with that measured experimentally on fabricated samples allows the stress gradient of the PECVD silicon nitride thin films introduced from the fabrication process to be evaluated.

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This study investigates the mechanical and physical properties of low-temperature plasma-enhanced chemical-vapor-deposited silicon nitride thin films, with particular respect to the effect of deposition temperature. The mechanical properties of the films were evaluated by both nanoindentation and microcantilever beam-bending techniques. The cantilever beam specimens were fabricated from silicon nitride thin films deposited on (100) silicon wafer by bulk micromachining. The density of the films was determined from quartz crystal microbalance measurements, as well as from the resonant modes of the cantilever beams, which were mechanically excited using an atomic force microscope. It was found that both the Young’s modulus and density of the films were significantly reduced with decreasing deposition temperature. The decrease in Young’s modulus is attributed to the decreasing material density. The decrease in density with decreasing deposition temperature is believed to be due to the slower diffusion rates of the deposited species, which retarded the densification of the film during the deposition process.

The construction of self-supporting and suspended structures is one of the fundamental challenges of microelectromechanical systems (MEMS). Many technologies have been developed for the fabrication of such structures, which can be categorized into bulk or surface micromachining. Generally surface micromachining techniques rely on a high-temperature deposition process such as low pressure chemical vapour deposition to produce high-quality films. Plasma enhanced chemical vapour deposition (PECVD) can be used to deposit films at temperatures less than 300 • C. PECVD of silicon nitride has not been used extensively in MEMS structures due to the material limitations created via the deposition technique, primarily controllability of the intrinsic stress and etch selectivity of the deposited film. We show here that PECVD silicon nitride can be used successfully in MEMS structures, and that the intrinsic stress is controllable through variations in the PECVD deposition parameters. A MEMS-based Fabry-Pérot cavity has been fabricated using PECVD silicon nitride as the membrane layer with ZnS as the sacrificial material. Devices with an initial 1 µm cavity length typically provide a displacement of 240 nm across a 380 µm membrane span for an applied bias of only 1.6 V.