Wear mechanisms and friction parameters for sliding wear of micron-scale polysilicon sidewalls

As tribological properties are critical factors in the reliability of silicon-based microelectromechanical systems, it is important to understand what governs wear and friction. Average dynamic friction, wear volumes and morphology have been studied for polysilicon devices fabricated using the Sandia SUMMiT V™ process and actuated in room-temperature air at μN loads. A total of seven devices was tested for total life. Three of the total-life experiments showed a global peak in the friction coefficient at three times the initial value with failure after 10⁵ cycles. Four other total-life experiments ran similarly up to 10⁵ cycles; however, following this global peak in the friction coefficient these devices continued to operate and achieved a lower steady-state friction regime with no failure for millions of cycles. Coincident with the friction coefficient increase, the nano-scale wear coefficient and surface roughness increased sharply in the first 10⁵ cycles and then decayed over several million cycles. These results are considered in terms of a mechanistic understanding of wear and friction: after an initial short adhesive wear regime with early failures attributed to local spikes in friction caused by differences in the local nano-scale surface morphology, three-body abrasive wear becomes the governing mechanism, allowing the devices to achieve a steady-state friction regime. Changing the relative humidity, sliding speed and load in the steady-state regime, in which devices spend the majority of their operating life, is found to influence the friction coefficient, but re-oxidation of worn polysilicon surfaces was only found to have an effect on the friction coefficient after periods of inactivity.