Thermomechanical behavior and microstructural evolution of SiN x /Al bimaterial microcantilevers

Bimaterial microcantilevers are used in numerous applications in microelectromechanical systems (MEMS) for thermal, mechanical, optical, tribological and biological functionalities. Unfortunately, the residual stress-induced curvature and combined effects of creep and stress relaxation in the thin film significantly compromises the performance of these structures. To fully understand the themomechanical deformation and microstructural evolution of such microcantilevers, SiNx/Al bilayer cantilever beams were studied in this work. These microcantilevers were heated and subsequently cooled for five cycles between room temperature and 250 ◦C, with the peak temperature in each successive cycle increased in increments of 25 ◦C using a custom-built micro-heating stage. The in situ curvature change was monitored using an interferometric microscope. The general behavior of the bimaterial microcantilever beams can be characterized by linear thermoelastic regimes with (dκ/dT)ave = 0.079 mm−1 ◦C−1 and inelastic regimes. After thermal cycling with a maximum temperature of 225 ◦C, upon returning to room temperature, the bimaterial microcantilever beams were flattened and the curvature decreased by 99%. The thermoelastic deformation during thermal cycling was well described by the Kirchhoff plate theory. Deformation of bimaterial microcantilevers during long-term isothermal holding was studied at temperatures of 100 ◦C, 125 ◦C and 150 ◦C with a holding period of 70 h. The curvature of bimaterial microcantilever beams decreased more for higher holding temperatures. Finite element analysis (FEA) with power-law creep in Al was used to simulate the creep and stress relaxation and thus the curvature change of the bimaterial microcantilever beams. The microstructure evolutions due to isothermal holding in SiNx/Al microcantilevers were studied using an atomic force microscope (AFM). The grain growth in both the vertical and lateral directions was present due to isothermal holding. As the isothermal holding temperature increased, the surface roughness of the film increased with more prominent grain structures. (Some figures in this article are in colour only in the electronic version)