Reducing stiction in microelectromechanical systems by rough nanometer-scale films grown by atomic layer deposition

Atomic layer deposition (ALD) can be used to grow pinhole-free nanometer-thin conformal inorganic films at low temperatures, making it of interest for many applications in MEMS (microelectromechanical systems). Stiction during device operation remains one of the mechanisms leading to permanent failure of operating silicon-based MEMS. This work investigated whether stiction in MEMS could be decreased by applying rough thin inorganic ALD films. Test structures based on the cantilever-beam-array (CBA) method were fabricated and coated with 2-50 nm thick ALD layers varying in chemical nature and roughness: Al₂O₃, TiO₂, MoN and Ta(Al)N. Smooth ALD films (AFM rms roughness below or equal to ca. 0.5 nm) were not observed to decrease stiction markedly. Crystalline films with roughness in the nanometer range (AFM rms ca. 1-3 nm) decreased stiction, resulting in a decrease of adhesion energy by up to four orders of magnitude as compared to Si and other smooth films. On the basis of this work, rough crystalline nanoscale ALD films are candidates for anti-stiction layers in MEMS.