Shannon J. Timpe

An experimental study of sidewall adhesion in microelectromechanical systems

A surface micromachine was designed specifically for studying sidewall adhesion in microelectromechanical systems (MEMS). The dependence of surface adhesion on contact load and ambient conditions was investigated under quasistatic normal loading conditions. Insight was obtained into the relative contributions of van der Waals and capillary forces to the measured adhesion force. Several shortcomings in previous adhesion studies of MEMS were overcome, and measurement of the true adhesion force was achieved under different testing conditions. The present experimental procedure enables the isolation of the van der Waals component of the adhesion force and the determination of the contributions of both contacting and noncontacting asperities to the total adhesion force at the inception of surface separation. The major benefits of the developed experimental methodology and surface micromachine are discussed in the context of adhesion results obtained for different values of apparent contact pressure, ambient pressure, and relative humidity.

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.

The Effect of Adhesion on the Static Friction Properties of Sidewall Contact Interfaces of Microelectromechanical Devices

Static friction between sidewall contact surfaces of polycrystalline silicon micromachines was investigated under different contact pressures, vacuum conditions, relative humidity levels, and temperatures. The static coefficient of friction exhibited a nonlinear dependence on the external contact pressure. A difference between in-contact and pull-out adhesion forces was observed due to the elastic recovery of the deformed asperities at the contact interface. The true static coefficient of friction was determined by considering the effects of the dominant adhesion forces (i.e., van der Waals and capillary forces) on the normal force applied at the sidewall contact interface. The roles of van der Waals and capillary forces in the sidewall friction behavior were analyzed in light of results for the interfacial shear strength and the adhesion force. The major benefits of the present friction micromachine and the developed experimental scheme are discussed in the context of static coefficient of friction and adhesion force results obtained under different environmental and loading conditions

Microdevice for measuring friction and adhesion properties of sidewall contact interfaces of microelectromechanical systems

A microdevice was specifically designed to characterize the static and dynamic friction and adhesion characteristics of sidewall contact interfaces of microelectromechanical systems (MEMS). The microdevice was fabricated by surface micromachining and tested under conditions that accurately mimic those of typical MEMS contacts. The developed experimental scheme enables the direct measurement of the critical normal force at the instant of surface separation and the friction force at the onset of sliding. Additional capabilities include evaluation of the dynamic friction behavior, measurement of the electrical characteristics across the contact interface, and characterization of the tribological response under impact contact loading. The microdevice can operate over a wide range of normal forces and different environmental conditions. Because the design is independent of process environment, the microdevice can be used to study the effects of different surface treatments and variations in fabrication process steps on the tribological properties of MEMS contact interfaces. Characteristic results of static and dynamic friction behaviors, electrical contact resistance, and response to dynamic impact loading illustrate the experimental capabilities and versatility of the designed microdevice.

Wear of Polysilicon Surface Micromachines Operated in High Vacuum

The evolution of wear at sidewall surfaces of polysilicon microelectromechanical systems was investigated in high vacuum under controlled normal load and sliding speed conditions. The static adhesion force was used as an indicator of the changes in wear characteristics occurring during oscillatory sliding contact. Measurements of the static adhesion force as a function of sliding cycles and scanning electron microscopy observations of micromachines from the same batch process subjected to nominally identical testing conditions revealed two distinctly different tribological patterns, namely, low-adhesion/high-wear behavior and high-adhesion/low-wear behavior. The static adhesion force and wear behavior were found to be in direct correlation with the micromachine operational lifetime. Transmission electron microscopy, selected area diffraction, and energy dispersive X-ray spectroscopy yielded insight into the origin, microstructure, and composition of wear debris and agglomerates adhered onto the sliding surfaces. Results demonstrate a strong dependence of micromachine operational life on the removal of the native oxide film and the organic monolayer coating as well as the formation of agglomerates consisting of organic coating material and wear debris.

Microscale friction phenomena in oscillatory sliding contacts

Microscale friction phenomena encountered in oscillatory sliding contacts were examined with a special reciprocating surface micromachine. Variations in static and dynamic friction forces were tracked in situ throughout testing under controlled loading and environmental conditions. Stick-slip surface interactions emerged at high numbers of sliding cycles. An unexpected binary friction behavior occurred as sliding transitioned between two-body and three-body conditions due to the formation of fine wear particles. The dominant friction mechanisms arising at the asperity scale are interpreted in the context of temporal evolutions of the static and dynamic friction forces and the decrease of the static and dynamic operational safety factors with accumulating sliding cycles. An important finding is that oscillating microdevices tend to fail in static friction mode rather than in dynamic friction mode. The results of this study illustrate the important role of microscale stick-slip phenomena in high-speed oscil...

Evolution of interfacial adhesion force in dynamic micromachines due to repetitive impact loading

A contact-mode surface micromachine was used to study the effect of repetitive impact loading on the evolution of the adhesion force at sidewall contact interfaces. Low and stable adhesion force was encountered during the initial stage of impact testing (run-in phase). A surface degradation phase occurred subsequently in which the adhesion force increased logarithmically with impact cycles. The experimental trend was used to derive a method for predicting micromachine failure due to excessive interfacial adhesion. High-magnification scanning electron microscopy did not reveal any modification of the surface topography even after 5.5×107 impact cycles, despite the significant enhancement of the adhesion force, attributed to the increase of the real contact area and the higher surface energy produced as a result of the removal of thin surface layers. The dominant surface degradation mechanisms are interpreted in the context of adhesion force measurements and microscopy images of the impacted surfaces.

Effects of electrical and thermal phenomena on the evolution of adhesion at contact interfaces of electrostatically activated surface microstructures

A contact-mode microstructure fabricated by surface micromachining was used to study the development of adhesion at sidewall contact surfaces during electrical actuation. Temporary and permanent changes in the adhesion force were monitored for different voltages applied across the contact interface. Relatively low current flow across the interface yielded a significant increase in the adhesion force. A portion of the increase was attributed to thermal heating of the contacting asperities. Current flow through asperity contacts lead to the accumulation of trapped charges in the insulating oxide layer, resulting in electrostatic attraction that was maintained after surface separation and with grounded surfaces. High current flow across asperity contacts due to dielectric breakdown of the native oxide layer at a critical voltage resulted in interfacial bonding that caused permanent adhesion of the sidewall surfaces.

Tribological Studies of Microelectromechanical Systems

Understanding and controlling friction in micromachine interfaces is critical to the reliability and operational efficiency of microelectromechanical systems (MEMS). The relatively high adhesion forces and friction forces encountered in these devices often present major obstacles to the design of reliable MEMS devices. Using surface micromachining, arrays of microstructures are being designed and tested to examine the adhesion characteristics, static friction behavior, and dynamic friction response. Emphasis is also being given to the control and actuation of the test structures and the modeling of the dynamic response and contact mechanics at the interface. Specifically, the purpose of the research is to fabricate and test MEMS devices in order to obtain insight into the effect of surface topography, material properties, surface chemical state, environmental conditions, and contact load on the static and dynamic characteristics of the contact interface.

Electrical Effect on the Adhesion Force at MEMS Contact Interfaces

A special microdevice was designed and tested to characterize electrical effects on interfacial adhesion in microelectromechanical systems (MEMS). The adhesion force was measured as a function of the voltage applied across the contact interface. An increase in the adhesion force was attributed to electrostatic attraction resulting from trapped charges. A thermal effect at contacting asperities enhanced the adhesion force significantly. At a critical voltage, microwelding at asperity contacts caused the surfaces to adhere strongly to each other. The adhered device could not be released within its operation limits, indicating the development of a very high adhesion force.Copyright