Effect of post-release sidewall morphology on the fracture and fatigue properties of polycrystalline silicon structural films

Surface properties can markedly affect the mechanical behavior of structural thin films used in microelectromechanical systems (MEMS) applications. This study highlights the striking difference in the sidewall surface morphology of n⁺-type polysilicon films from two popular MEMS processes and its effect on fracture and fatigue properties. The sidewall surface roughness was measured using atomic force microscopy, whereas silicon oxide thickness and grain size were measured using (energy-filtered) transmission electron microscopy. These measurements show that the oxide layers are not always thin native oxides, as often assumed; moreover, the roughness of the silicon/silicon oxide interface is significantly influenced by the oxidation mechanism. Thick silicon oxides (20 ± 5 nm) found in PolyMUMPs™ films are caused by galvanic corrosion from the presence of gold on the chip, whereas in SUMMiT V™ films a much thinner (3.5 ± 1.0 nm) native oxide was observed. The thicker oxide layers, in combination with differences in sidewall roughness (14 ± 5 nm for PolyMUMPs™ and 10 ± 2 nm for SUMMiT V™), can have a significant effect on the reliability of polysilicon structures subjecting to bending loads; this is shown by measurements of the fracture strength (3.8 ± 0.3 GPa for PolyMUMPs™ and 4.8 ± 0.2 GPa for SUMMiT V™) and differences in the stress-lifetime cyclic fatigue behavior.