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1085-T01-04

Nano-scale Tribological Behavior of Polycrystalline Silicon Structural Films in Ambient Air Daan Hein Alsem1, Ruben van der Hulst2, Eric A. Stach3, Michael T. Dugger4, Jeff Th. M. de Hosson2, and Robert O. Ritchie1,5 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 2 Department of Applied Physics, University of Groningen, Groningen, 9747 AG, Netherlands 3 School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN, 47907 4 Materials Science and Engineering Center, Sandia National Laboratories, Albuquerque, NM, 87185 5 Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, CA, 94720 ABSTRACT 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 ~105 cycles. Additionally, the average nanoscale wear coefficient sharply increased in the first ~105 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). INTRODUCTION Microelectromechanical systems (MEMS) can be fabricated at low cost using batch processing; they are found in a wide variety of consumer products and defence/space applications like sensors, projection displays, inkjet printers and optical switches [1,2]. Due to their large surface-to-volume ratio, it not always possible to simply extrapolate known macroscale failure modes to MEMS on the micro- and nano-meter scale. Although a growing amount of research has provided insight into how to overcome problems in their fabrication and design [2], MEMS can fail prematurely by such failure modes as adhesion, wear and fatigue [3]. Adhesion and wear are more common in devices with contacting surfaces (see recent review [4]), but fatigue is of particular interest for silicon MEMS because large-scale specimens of silicon are not susceptible to this failure mode [5]. In this paper, we measure the dynamic coefficient of friction and nano-scale wear volumes as function of wear cycles in order to gain more insight in the evolution of friction and wear of polycrystalline silicon (polysilicon) sidewalls during wear. Furthermore, we use the static friction coefficient to investigate reoxidation of worn polysilicon sidewall surfaces. Results are discussed in t