Michael T. Dugger

Nanotribology and MEMS

The tribological phenomena of adhesion, friction, and wear arise when solid objects make contact. As the size of devices shrinks to micro- and nanoscales, the surface-to-volume ratio increases and the effects of body forces (gravity and inertia) become insignificant compared with those of surface forces (van der Waals, capillary, electrostatic, and chemical bonding). In microelectromechanical systems (MEMS), tribological and static interfacial forces are comparable with forces driving device motion. In this situation, macroscale lubrication and wear mitigation methods, such as the use of bulk fluids and micrometer thick coatings, are ineffective; new nano-engineering approaches must be employed for MEMS devices with moving structures. We review fundamental tribological problems related to micro- and nanoscale mechanical contacts and developments in MEMS lubrications.

Structural and tribological studies of MoS2 solid lubricant films having tailored metal-multilayer nanostructures

Abstract Molybdenum disulfide (MoS 2 ) solid lubricant films were prepared by r.f. magnetron sputtering on 440C steel, 52100 steel, and silicon substrates. This study concentrated on films that were multilayer coatings of MoS 2 with either nickel or Au-(20%)Pd metal interlayers. Multilayer thicknesses ranged from 0.2 nm to 1.0 nm while the multilayer periodic spacing ranged from 3 to 10 nm. Scanning electron microscopy and X-ray diffraction revealed that the multilayer films had dense microstructures that, in some cases, exhibited significant orientation of their basal planes parallel to the substrate. Film endurance was assessed in sliding contact using thrust washer tests and in rolling contact using thrust bearing tests. Some film microstructures exhibited excellent endurance. Brale indentation indicated that the metal layers can improve film fracture toughness. Friction in air and ultrahigh vacuum (UHV) was investigated using a UHV-compatible test apparatus. Friction coefficients between 0.05 and 0.08 were measured in UHV.

Materials issues in microelectromechanical devices : science, engineering, manufacturability and reliability.

MicroElectroMechanical Systems (MEMS) technology offers considerable potential throughout the manufacturing sector, because of certain intrinsic advantages in terms of low cost, reliability, and small size. Relatively simple MEMS are used in applications ranging from automobile air bag sensors to electronic games. Considerably more complex devices have been designed for defense applications, for which government funding is available; however, the fledgling industry suffers from insufficient knowledge of materials physics at micrometer size and from the fact that currently commercialized MEMS devices are designed for specialized and rather disparate purposes, do not have a broad user base, and therefore have not generated industry standards or the design and process software that would be built upon those industry standards. In addition to industry standards, further advances in MEMS technology require a more complete understanding of the physics underlying performance and reliability. The first half of this paper reviews general issues related to fabrication and commercialization; the second half addresses the technical materials issues that relate to MEMS performance and reliability.

The effect of humidity on the reliability of a surface micromachined microengine

Humidity is shown to be a strong factor in the wear of rubbing surfaces in polysilicon micromachines. We demonstrate that very low humidity can lead to very high wear without a significant change in reliability. We show that the volume of wear debris generated is a function of the humidity in an air environment. As the humidity decreases, the wear debris generated increases. For the higher humidity levels, the formation of surface hydroxides may act as a lubricant. The dominant failure mechanism has been identified as wear. The wear debris has been identified as amorphous oxidized silicon. Large slivers (approximately 1 /spl mu/m in length) of debris observed at the low humidity level were also amorphous oxidized silicon. Using transmission electron microscopy (TEM), we observed that the wear debris forms spherical and rod-like shapes. We compared two surface treatment processes: a fluorinated silane chain (FTS) process and supercritical CO/sub 2/ dried (SCCO/sub 2/) process. The microengines using the SCCO/sub 2/ process were found to be less reliable than those released with the FTS process under two humidity levels.

Friction and wear in surface micromachined tribological test devices

We report on the design, construction, and initial testing of surface micromachined devices for the measurement of friction and wear. The devices measure friction coefficients on both horizontal deposited polysilicon surfaces and vertical etched polysilicon surfaces. The contact geometry of the rubbing surfaces is well-defined, and a method is presented for the determination of the normal and frictional forces. Initial observations on test devices which have been dried with supercritical CO2 and devices coated with octadecyltrichlorosilane suggest that the coatings increase the lifetime of the devices and the repeatability of the results.

The effect of frequency on the lifetime of a surface micromachined microengine driving a load

Experiments have been performed on surface micromachined microengines driving load gears to determine the effect of the rotation frequency on median cycles to failure. We did observe a frequency dependence and have developed a model based on fundamental wear mechanisms and forces exhibited in resonant mechanical systems. Stressing loaded microengines caused observable wear in the rotating joints and, in a few instances, led to fracture of the pin joint in the drive gear.

Electrical Contact Resistance Degradation of a Hot-Switched Simulated Metal MEMS Contact

Electrical contact resistance testing was performed by hot-switching a simulated gold-platinum metal microelectromechanical systems contact. The experimental objective was to determine the sensitivity of the contact resistance degradation to current level and environment. The contact resistance increased sharply after 100hot-switched cycles in air. Hot-switching at a reduced current and in nitrogen atmosphere curtailed contact resistance degradation by several orders of magnitude. The mechanism responsible for the resistance degradation was found to be arc-induced decomposition of adsorbed surface contaminants

Wear Mechanisms in a Reliability Methodology

The main thrust in any reliability work is identifying failure modes and mechanisms. This is especially true for the new technology of MicroElectroMechanical Systems (MEMS). The methods are sometimes just as important as the results achieved. This paper will review some of the methods developed specifically for MEMS. Our methodology uses statistical characterization and testing of complex MEMS devices to help us identify dominant failure modes. We strive to determine the root cause of each failure mode and to gain a fundamental understanding of that mechanism. Test structures designed to be sensitive to a particular failure mechanism are typically used to gain understanding. The development of predictive models follows from this basic understanding. This paper will focus on the failure mechanism of wear and how our methodology was exercised to provide a predictive model. The MEMS device stressed in these studies was a Sandia-developed microengine with orthogonal electrostatic linear actuators connected to a gear on a hub. The dominant failure mechanism was wear in the sliding/contacting regions. A sliding beam-on-post test structure was also used to measure friction coefficients and wear morphology for different surface coatings and environments. Results show that a predictive model of failure-time as a function of drive frequency based on wear fits the functional form of the reliability data quite well, and demonstrates the benefit of a fundamental understanding of wear. The results also show that while debris of similar chemistry and morphology was created in the two types of devices, the dependence of debris generation on the operating environment was entirely different. The differences are discussed in terms of wear maps for ceramics, and the mechanical and thermal contact conditions in each device.

Atomic layer deposition of tungsten disulphide solid lubricant thin films

The synthesis and characterization of crystalline tungsten disulphide (WS 2 ) solid lubricant thin films grown by atomic layer deposition (ALD) using WF 6 and H 2 S gas precursors was studied. A new catalytic route was established to promote nucleation and growth of WS 2 films on silicon surfaces with native oxide. Scanning electron microscopy with energy dispersive spectroscopy and Raman spectroscopy were used to determine the film morphology, composition, and crystallinity. The films exhibited solid lubricating behavior with a steady-state friction coefficient of 0.04 in a dry nitrogen environment.

Tribological degradation of fluorocarbon coated silicon microdevice surfaces in normal and sliding contact

Reported here is a study of the tribological degradation of the contact interface of a fluorocarbon monolayer-coated polycrystalline silicon microdevice. A surface micromachined silicon tribometer is employed to track changes in the adhesion and friction properties during repetitive normal and sliding contacts. Evidence for tribological degradation commences immediately for parallel sliding contact motion, and is slightly delayed in the case of repetitive impact loading normal to the surface. The observed changes in interfacial behavior indicate dramatic changes in the chemical (i.e., surface energy) and physical (i.e., roughness, real contact area, etc.) nature of the contacting surfaces. Results from microscale sliding and impact experiments are interpreted in the light of the primary physical and chemical degradation mechanisms of monolayer-coated silicon microdevices.