Dynamic friction, wear volumes and wear morphology have been studied for sliding wear in polysilicon in ambient air at µN normal loads using on-chip micron-scale test specimens. With increasing number of wear cycles, the friction coefficients show two distinct types of behavior: (i) an increase by a factor of two and a half to a steady-state regime after peaking at three times the initial value of about 0.10 ± 0.04, with no failure after millions of cycles; (ii) an increase by a factor larger than three followed by failure after ~10 5 cycles. Additionally, the average nanoscale wear coefficient sharply increased in the first ~10 5 cycles up to about 10 -4 and then decayed by an order of magnitude over the course of several million cycles. For both modes of behavior, abrasive wear is the governing mechanism, the difference being attributed to variations in the local surface morphology (and wear debris) between the sliding surfaces. The oxidation of worn polysilicon surfaces only affects the friction coefficient after periods of inactivity (>30 min).
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 10 5 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 ~10 5 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.
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