Tribological behavior of micron-scale polycrystalline silicon structural films in ambient air

Abstract

As tribological properties are critical factors in the reliability of microelectromechanical systems, it is important to understand the physical processes and parameters governing wear and friction in silicon structural films. Dynamic friction, wear volumes and wear morphology have been studied for polysilicon devices from the Sandia SUMMiT V^TM process actuated in ambient air at μN loads. A total of seven devices were tested. Roughly half of the devices showed a peak in the friction coefficient at three times the initial value with failure after 105 cycles. The other half of the devices behaved similarly initially; however, following the friction coefficient peak they displayed a lower steady-state friction regime with no failure for millions of cycles. Additionally, the nanoscale wear coefficient and roughness increased in the first ~10⁵ cycles and then slowly decayed over several million cycles. Transmission electron microscopy studies revealed amorphous oxygen-rich debris. These measurements show that after a short adhesive wear regime, abrasive wear is the governing mechanism with failures attributed to differences in the local nanoscale surface morphology. Changing the relative humidity, sliding speed and load was found to influence the friction coefficient, but re-oxidation of worn polysilicon surfaces was only found to have an effect after periods of inactivity.