J. Krim

Study of contacts in an electrostatically actuated microswitch

Surface micromachined, electrostatically actuated microswitches have been developed at Northeastern University. Microswitches have an initial contact resistance of 0.5-1 /spl Omega/, and current handling capability of about 20 mA. Typically, contact resistance degrades progressively when the switches are cycled beyond approximately 10/sup 6/ cycles. In this work, the microswitch contact resistance is studied on the basis of a simple, clean metal contact resistance model. Comparison of measured contact resistance (measured as a function of contact force) with the characteristics predicted by the model shows the measured resistance to be higher than the prediction, approximately by an order of magnitude, suggesting that insulating films at the contact interface need to be taken into account. Microswitches with a large number of parallel contacts have also been developed, and measurement data is presented showing that these devices have a current handling capability greater than 150 mA.

Experimental Observations of Self-Affine Scaling and Kinetic Roughening at Sub-Micron Lengthscales

Experimental observations of self-affine scaling and kinetic roughening at sub-micron length scales are reviewed for thin solid films and ion-beam eroded surfaces.

Foundations of Nanomechanics: From Solid-State Theory to Device Applications

1. Introduction: Linear Atomic Chains.- 2. Two- and Three-Dimensional Lattices.- 3. Properties of the Phonon Gas.- 4. Stress and Strain.- 5. Elasticity Relations.- 6. Static Deformations of Solids.- 7. Dynamical Behavior of Solids.- 8. Dissipation and Noise in Mechanical Systems.- 9. Experimental Nanostructures.- 10. Nanostructure Fabrication I.- 11. Nanostructure Fabrication II.- A. Mathematical Tools.- A.1 Scalars, Vectors, Tensors.- A.1.1 Vectors.- A.1.2 Tensors.- A.2 Eigenvectors and Eigenvalues.- A.3 The Dirac Delta Function.- B. Compatibility Relations for Stress and Strain.- C. Notation.

Fundamentals of Friction

By most recent estimates, improved attention to friction and wear would save developed countries up to 1.6% of their gross national product—over

Surface science and the atomic-scale origins of friction: what once was old is new again

100 billion annually in the United States alone. It is thus not surprising that tribomaterials, materials designed for use in moving contact (sliding, rolling, abrasive, etc.) have for decades attracted the interests of materials scientists and mechanical and chemical engineers. However the field of tribology is hardly a recent one. Such tribological advances as Leonardo da Vincis design of intricate gears and bearings (some of which were not built until the Industrial Revolution provided sufficiently strong materials) and the landmark 18th century development of a timepiece allowing accurate longitudinal positioning of ships at sea (accomplished via a self-lubricating wooden gear) could easily be termed “modern,” given the overall longevity of the field. As important as tribomaterials are to technology, their discovery has usually been serendipitous. Materials scientists have frequently been able to provide explanations for why tribomaterials perform as well as they do. They have also been able to substantially improve the performance of tribomaterials through the development of new alloys, composites, and/or improved surface-engineering methods. They have however been far less successful at a priori design of tribomaterials with improved performance, largely because friction and wear processes have not been understood at the fundamental level. The late 1980s marked the advent of renewed interest in fundamental areas of tribology, sparked by a number of new experimental and theoretical techniques that made it possible to study the force of friction in geometries that are well-defined at the nanometer scale.

Scanning tunneling microscope measurements of the amplitude of vibration of a quartz crystal oscillator

Long neglected by physicists, the study of friction’s atomic-level origins, or nanotribology, indicates that sliding friction stems from various unexpected sources, including sound energy, and static friction may arise from physisorbed molecules. Progress in this field will be discussed, with an emphasis on how the field of surface science has influenced our understanding of friction. 2001 Elsevier Science B.V. All rights reserved.

Friction and energy dissipation mechanisms in adsorbed molecules and molecularly thin films

We report highly accurate measurements of the vibrational amplitude of a transverse shear mode quartz resonator, obtained by directly imaging the surface oscillatory motion with a scanning tunneling microscope. Amplitude measurements, performed over a range of resonator drive levels and quality factors, agree with theoretical predictions to within a factor of two.

Surface roughness, asperity contact and gold RF MEMS switch behavior

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Comparison of Au and Au–Ni Alloys as Contact Materials for MEMS Switches

Modeling predictions and experimental measurements were obtained to characterize the electro-mechanical response of radio frequency (RF) microelectromechanical (MEM) switches due to variations in surface roughness and finite asperity deformations. Three-dimensional surface roughness profiles were generated, based on a Weierstrass–Mandelbrot fractal representation, to match the measured roughness characteristics of contact bumps of manufactured RF MEMS switches. Contact asperity deformations due to applied contact pressures were then obtained by a creep constitutive formulation. The contact pressure is derived from the interrelated effects of roughness characteristics, material hardening and softening, temperature increases due to Joule heating and contact forces. This modeling framework was used to understand how contact resistance evolves due to changes in the real contact area, the number of asperities in contact, and the temperature and resistivity profiles at the contact points. The numerical predictions were qualitatively consistent with the experimental measurements and observations of how contact resistance evolves as a function of deformation time history. This study provides a framework that is based on integrated modeling and experimental measurements, which can be used in the design of reliable RF MEMS devices with extended life cycles.

Slippage of simple liquid films adsorbed on silver and gold substrates

This paper reports on a comparison of gold and gold-nickel alloys as contact materials for microelectromechanical systems (MEMS) switches. Pure gold is commonly used as the contact material in low-force metal-contact MEMS switches. The top two failure mechanisms of these switches are wear and stiction, which may be related to the material softness and the relatively high surface adhesion, respectively. Alloying gold with another metal introduces new processing options to strengthen the material against wear and reduce surface adhesion. In this paper, the properties of Au-Ni alloys were investigated as the lower contact electrode was controlled by adjusting the nickel content and thermal processing conditions. A unique and efficient switching degradation test was conducted on the alloy samples, using pure gold upper microcontacts. Solid-solution Au-Ni samples showed reduced wear rate but increased contact resistance, while two-phase Au-Ni (20 at.% Ni) showed a substantial improvement of switching reliability with only a small increase of contact resistance. Discussion of the effects of phase separation, surface topography, hardness, and electrical resistivity on contact resistance and switch degradation is also included.