ICMCTF2007 Session F3/E1: Nanotribology Instrumentation and Diagnostics

Tuesday, April 24, 2007 1:30 PM in Room Sunset

Tuesday Afternoon

Time Period TuA Sessions | Abstract Timeline | Topic F Sessions | Time Periods | Topics | ICMCTF2007 Schedule

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1:30 PM F3/E1-1 Tribological and Electrical Behavior of Coatings for Microstystems
M.T. Dugger (Sandia National Laboratories)

Microelectromechanical system (MEMS) surfaces that have impacting and sliding contacts frequently necessitate modification by coatings to improve the tribological or electrical performance. The principal challenges associated with surface modification for microsystems are 1) limited ability to insert surface modification steps during manufacturing, requiring post-fabrication surface treatment; 2) small gaps between structures (<1 micron) that limit coating thickness and tolerance for variation; and 3) contacting interfaces that require modification may be deeply buried in complex structures. Although contacting surfaces of polycrystalline silicon structures have arithmetic roughness of less than 50 nm, the contact forces applied (typically below 1 mN) prescribe that real contact occurs at only a few asperities. Therefore, relating composition and structure of the contacting surfaces to tribological behavior is quite challenging due to the limited size and number of real contact spots. MEMS ohmic switches present similar challenges in terms of coating application and minimization of adhesion and friction, with the added requirement that surface modification must also enable low-loss electrical performance.

A variety of surface treatment approaches have been investigated for microsystems, including chemisorbed monolayers, atomic layer deposited films, and physical vapor deposited materials. An understanding of the mechanical and chemical degradation mechanisms of tribological coatings is of primary importance for the reliable operation of microsystems. Micromachined diagnostic structures such as tribometers and electrical contact switches have been employed to investigate changes in adhesive and friction forces due to coating degradation. Relationships between tribological behavior, processing, and degradation of surface treatments for dynamic MEMS interfaces will be discussed.

2:10 PM F3/E1-3 Nano-Channel Topography for Reduction of Adhesive and Friction Forces
R.A. Singh, D.C. Pham, E.-S. Yoon (Korea Institute of Science and Technology, Korea); H.E. Jeong, K.Y. Suh (Seoul National University, Korea)
Topographical modification of thin polymeric surfaces has proved to be an effective way in enhancing tribological properties at small-scales. Engineering of nano/micro-topography on flat polymeric surfaces using soft-lithographic technique is a novel route to reduce surface forces like adhesion and friction at nano/micro-scales. In this work, we demonstrate the potential of nano-channel topography to improve nano-scale tribological properties. A simple soft-lithographic technique called capillary force lithography was used to fabricate nano-channels with varying thickness and pitch. This lithographic method involves application of the molding process in which a uniform polymer-coated surface is molded by a patterned mold by means of capillary force above the glass transition temperature of the polymer. The nano-channel topography was fabricated on polymethyl methacrylate (PMMA) thin film spin-coated on silicon wafers. Poly (dimethylsiloxane) (PDMS) stamps (master molds) were used to fabricate these patterns. Characterization of static water contact of the patterned surfaces showed that the surfaces were hydrophobic in nature, whereas the non-patterned thin film was hydrophilic. The patterned surfaces were investigated for their adhesion and friction properties at nano-scale using Atomic Force Microscopy (AFM). Glass (Borosilicate) balls mounted on cantilever (Contact Mode type NPS) were used as tips. The tribological behavior of the patterned surfaces was compared with that of non-patterned thin film. It was observed that patterned samples exhibited superior tribological properties when compared to the non-patterned samples. Increased hydrophobicity and reduction in contact area are some of the parameters that contribute towards the improved tribological performance of these patterned surfaces. Influence of the variation in pitch and thickness of the channels on the tribological behavior of these patterned surfaces are also studied.
2:30 PM F3/E1-4 Lubrication of RF MEMS Switches using Nanoparticle Fluids
S.T. Patton (University of Dayton Research Institute); S. Diamanti, R. Vaia, A.A. Voevodin (Air Force Research Laboratory)
RF MEMS switches hold great promise in a myriad of commercial, aerospace, and military applications including cellular phones and phased array antennas. However, there is limited understanding of the factors that determine the performance and reliability of these devices. In particular, there has been little development of lubricants to improve switch performance and reliability. Self-assembled monolayers of diphenyl disulfide were found to decompose in metallic MEMS switch contacts, which led to growth in contact resistance. In this study, ionic liquid nanoparticle fluids are used to lubricate metallic MEMS switch contacts. For these studies, novel fluids comprised of 30 vol.% Au and Pt nanoparticles with a covalently attached organic ionic corona of mercaptoethanesulfonate and quartnearyammonium were used. The role of nanoparticle addition was studied from the two perspectives: i) increasing electrical conductivity between lubricated gold contact interfaces without organic component degradation and contact melting; ii) preventing adhesive failure of the contact by introduction of nanosized asperities. Experiments were conducted in dry nitrogen at MEMS-scale forces using a micro/nanoadhesion apparatus as a switch simulator. For high current (1 mA) switching, the material prevented shorting and extended lifetime by five orders of magnitude over that of self-assembled monolayers. The eventual failure mechanism was increased contact resistance due to growth of a liquid degradation product film on contact surfaces. At low current, no degradation of contact resistance was observed through 106 cycles. Detailed physical and chemical analysis of fresh and worn contact surfaces was conducted and results are summarized along with ionic lubricant degradation mechanisms.
2:50 PM F3/E1-5 Ionic Liquids as Lubricants for Macro and Micro Scale Devises
B.S. Phillips (Air Force Research Laboratory); J. Nainaparampil (UES); K.C. Eapen (UDRI); A.A. Voevodin, J.H. Sanders (Air Force Research Laboratory)
Ionic liquids have shown great promise for use as a lubricant. Lubricant properties have been studied on the macro scale as well as the micro scale. In this study, comparisons of micro scale testing, specifically MEMS devices, will be made to macro-scale testing and surface analysis. Lubricating properties of ionic liquids have been analyzed by atomic force microscope by incorporating a liquid cell into scan setting. Macro scale testing includes the use of a pin-on-disk tribometer. Study of molecules with different geometrical structures and physical properties showed that six member ring based pyridinium ethyl sulfate exhibit promising characteristics. This study focuses on pyridinium based ionic liquids for further evaluation of their lubrication nature. 1- ethyl-3-methyl-pyridinium ethyl sulfate and 1-Ethyl-3-hydroxymethylpyridinium ethyl sulfate which are low melting (~ -60°C) and stable under humidity, are characterized under varied loads in interfaces ranging from nano to micro scale in cross section. Multiple substrates were evaluated in this study. Failure life evaluation of MEMS devices lubricated with the selected ionic liquids will also be presented for comparison purpose. XPS, FTIR and XANES spectrograms will be used to analyze the chemical changes due to rubbing interfaces (tribochemistry).
3:10 PM F3/E1-6 Tip-Based Simulations of Nanotribology of Self-Assembled Monolayers
M. Chandross (Sandia National Laboratories); C.D. Lorenz (Iowa State University); M.J. Stevens, G.S. Grest (Sandia National Laboratories)

While nanotribological simulations are generally performed for opposing parallel surfaces, the AFM experiments to which they are often compared measurethe interactions between a curved probe tip and a sample. These simulations cannot replicate effects seen in experiments, including load-dependent contact areas and transfer of material from the substrate to the tip. We present the results of true dynamical simulations of alkylsilane self-assembled monolayers (SAMs) with realistic tip/substrate geometries. The adhesion and friction of tips matching experimental dimensions in contact with SAM-coated amorphous silica substrates were studied with massively parallel molecular dynamics simulations. We discuss the contact mechanics of the tip/substrate geometry and compare to previous simulations and AFM measurments. The effects of SAM morphology will also be discussed.

Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.

3:50 PM F3/E1-8 Tribological and Mechanical Properties of HfB2/Hf-B-N Multilayers on the Nanoscale
A. Chatterjee (University of Illinois Urbana Champaign); S. Jayaraman (Intel Corp.); J.E Gerbi, N. Kumar, J.R. Abelson (University of Illinois Urbana Champaign); J.P. Chevalier (Centre d'Etudes Chimie-Metallurgie, France); P. Bellon (University of Illinois Urbana Champaign)
Multilayer nitride/boride coatings offer an excellent combination of high toughness, high hardness and high wear resistance, thus making them attractive materials for hard and wear resistant coatings. In this work, HfB2/Hf-B-N multilayer films are grown by CVD with intermittent supply of N atoms under conditions that afford highly conformal and smooth films (precursor pressure ~ 0.1 mTorr, substrate temperature ~ 275 C). The thickness of the nitride layers was varied and some of the as-deposited films were annealed in-situ. X-ray diffraction reveals strong crystallinity for annealed films while as-deposited films are X-ray amorphous with traces of crystallinity. The present work seeks to explore the tribological behavior of multilayers in the nanoscale regime in terms of nanohardness, nanoscratch and nanowear as well as chemical characterization of those films after the nanowear process. Nanoindentation tests reveal high hardness, ranging from 12 GPa for the as-deposited films to 23 GPa for the annealed films; HfB2/Hf-B-N multilayer films of thickness ~ 1 µm deposited on Si substrates were subjected to reciprocating nano-wear testing against a conospherical diamond tip of radius 700 nm. The wear scars were recorded by AFM just after wear and later characterized by SEM (both in SE as well as BSE mode). Very high wear resistance of the films were recorded especially for annealed films, with wear rates comparable to that of standard TiN coatings. The films were also subjected to single and multipass nanoscratch measurements for friction measurements. Very low friction coefficients of the order of 0.05 were observed for all the films. The adhesive strength of the films, leading to delamination was recorded and XPS, EDS as well as AES characterization was used to shed light on the chemical information at the site of the delaminated films. Finally, we analyze the effect of the thickness of the nitride layers on the wear resistance, friction coefficient and hardness.
4:10 PM F3/E1-9 Characterization of Antistiction Layer for Nanoimprint Lithography (NIL) by Vapor SAM
J.G. Park, K.C. Kim, N.G. Cha, J.Y. Kim (Hanyang University, Korea)
Nanoimprint lithography (NIL) is the method to transfer patterns from a stamp onto a substrate. Nanoimprint lithography has a potential to be such a method allowing low-cost, high-throughput production of nanopatterns over large areas employing a single lithographic step. The most catastrophic problem of NIL is a stiction occurred during process. This stiction problem can prevent by using the antistiction layer. Various fabrication methods are existed such as liquid, vapor and plasma type. Especially, vapor SAM (self-assembled monolayer) method can apply to coat on deep and narrow patterns which can not be coated by liquid SAM method. And it shows better antisticking properties on silicon and oxide materials than plasma deposition method. In this study, we deposited and characterized the antistiction layer by vapor SAM with FOTS (perfluorooctyltrichlorosilane). Vapor SAM were deposited as a function of temperature to find out optimized conditions. Static contact angles and dynamic contact angles were measured on Si and SiO2 substrates. Surface energies of these samples were calculated using static contact angle data. The thickness of antistiction layer and optical properties was measured by spectroscopic ellipsometry. AFM/LFM (atomic/lateral force microscope) was conducted to compare morphology and friction force between samples. We got a thermal area about 110°C that changed contact angle, surface energy and friction force from experimental results even if the thickness changes did not occur. We could be able to confirm the process condition which had become optimum VSAM formation.
Time Period TuA Sessions | Abstract Timeline | Topic F Sessions | Time Periods | Topics | ICMCTF2007 Schedule