Suppression of inelastic deformation of nanocoated thin film microstructures

We study the suppression of time-dependent inelastic deformation due to creep, stress relaxation, and microstructural evolution in multilayer thin film microstructures by the use of alumina nanocoatings realized by atomic layer deposition (ALD). Gold (0.5 μm thick)/polysilicon (1.5 or 3.5 μm thick) beam and plate microstructures were fabricated using surface micromachining. The microstructures were then coated on each side with a 40-nm-thick amorphous Al₂O₃ layer by ALD. The beam and plate microstructures were initially thermal cycled between room temperature and 190 °C to partially stabilize the gold microstructure. After the initial thermal cycles, the microstructures were cooled from 190 to 120 °C and held at 120 °C for about 700 h (4 weeks). We measured, using an interferometric microscope with a custom-built temperature chamber, full-field deformed shapes (and from these determined the average curvatures in the x and y directions) of the microstructures during the initial thermal cycles, during the cooling process from 190 to 120 °C, and during the isothermal hold. Measurements were made on both coated and uncoated microstructures to assess the influence of the coating. We find that while the 40-nm-thick coating has a small effect on the thermoelastic response of the microstructure, it significantly reduces the extent of inelastic deformation during the isothermal hold. We modeled the curvature evolution with time assuming the inelastic deformation mechanism can be described by power-law creep in the gold, Є˙=Aσⁿ, and that the polysilicon and alumina deform elastically. The simple model describes the observed behavior reasonably well for the uncoated microstructures (when the power-law parameters are fit using the measured curvature), however, for the coated microstructure- s, the model predicts a decrease in the inelastic deformation, but nowhere near the magnitude observed. This suggests not only an alternation of the stress state in the gold film by the nanoscale coating, but also a change in the fundamental inelastic deformation mechanisms. © 2004 American Institute of Physics.