Nanotribological Behavior of Ultra-thin Al2O3 Films Prepared by Atomic Layer Deposition

Si-based nano/micro-electromechanical system (NEMS/MEMS) devices with contacting and rubbing structures cannot run reliably due to their poor tribological performance. A thin alumina (Al₂O₃) film is a promising candidate for the protective coating in the applications of NEMS/MEMS devices. In this study, nanotribological behavior of ultra-thin Al₂O₃ films prepared by atomic layer deposition on a Si (100) substrate was investigated in comparison with that of Si (100). X-ray photoelectron spectroscopy was used to determine the composition of Al₂O₃ films. Atomic force microscopy with different tips was employed to measure the scratch resistance, adhesion and friction forces of various samples. The results show that Al₂O₃ films have larger scratch resistance than that of Si (100). In addition, the adhesion and friction forces of Al₂O₃ films are smaller than that of Si (100). Thus, the Al₂O₃ films are capable of a wide application in Si-based NEMS/MEMS devices. The improved tribological performance of Al₂O₃ films is attributed to their hydrophobic properties.     

Nano/Microtribological Properties of Ultrathin Functionalized Imidazolium Wear-Resistant Ionic Liquid Films on Single Crystal Silicon

Ionic liquids (ILs) are considered as a new kind of lubricant for micro/nanoelectromechanical system (M/NEMS) due to their excellent thermal and electrical conductivity. However, so far, only few reports have investigated the tribological behavior of molecular thin films of various ILs. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a critical step for their application in MEMS/NEMS devices. To this end, four kinds of ionic liquid carrying methyl, hydroxyl, nitrile, and carboxyl group were synthesized and these molecular thin films were prepared on single crystal silicon wafer by dip-coating method. Film thickness was determined by ellipsometric method. The chemical composition and morphology were characterized by the means of multi-technique X-ray photoelectron spectrometric analysis, and atomic force microscopic (AFM) analysis, respectively. The nano- and microtribological properties of the ionic liquid films were investigated. The morphologies of wear tracks of IL films were examined using a 3D non-contact interferometric microscope. The influence of temperature on friction and adhesion behavior at nanoscale, and the effect of sliding frequency and load on friction coefficient, load bearing capacity, and anti-wear durability at microscale were studied. Corresponding tribological mechanisms of IL films were investigated by AFM and ball-on-plane microtribotester. Friction reduction, adhesion resistance, and durability of IL films were dependent on their cation chemical structures, wettability, and ambient environment.     

Nanotribological properties of silicon nano-pillars coated by a Z-DOL lubricating film

This paper reports a novel approach for improving the nanotribological properties of silicon (Si) surfaces by topographically and chemically modifying the surfaces. In the first step, Si (100) wafers were topographically modified into nano-pillars by using the photolithography and reactive ion etching (RIE) techniques. Various patterns, including nano-pillars of varying diameters and pitches (distance between pillars), were fabricated. Then, the patterns were coated with a Z-DOL (perfluoropolyether (PFPE)) lubricating film using a dipcoating technique, and this process was followed by thermal treatment. These modified surfaces were tested for their nanotribological properties, namely adhesion and friction forces, using an atomic force microscope (AFM). The results showed that the topographical modification and Z-DOL coating each independently reduced the adhesion and friction forces on the Si surfaces. However, the combination of the two surface treatments was most effective in reducing these forces. This is attributed to the combined effects of the reduction in the real area of contact due to patterning and the low surface energy of the Z-DOL lubricant. Further, it was found that adhesion and friction forces of the surfaces with combined modification varied significantly depending on the diameter of the pillars and the pitch. It is proposed that such a combination of surface modifications promises to be an effective method to improve the nanotribological performance of miniaturized devices, such as MEMS, in which Si is a typical material.     

Wear durability study on self-lubricating SU-8 composites with perfluoropolyther,multiply-alkylated cyclopentane and base oil as the fillers

Though SU-8 has become a useful material for micro-fabrication of MEMS/NEMS components using the micro-fabrication route, its poor tribological properties limit its wider applications. From our previous study [1], it was observed that adding PFPE lubricant to SU-8 possibly promoted chemical reaction between the molecules and helped in the boundary lubrication enhancing the wear durability of SU-8 by more than four orders of magnitude. For further investigation, another two different lubricants, a base oil and a multiply-alkylated cyclopentane (MAC) oil, were also added to SU-8. Both lubricants are hydrocarbons, chemically inert and have no polar reactive terminal groups unlike PFPE which has –OH polar terminal groups. SU-8+PFPE composite exhibited higher wear life than all SU-8 composites at all wt% of the lubricant content. Proper dispersion and possible chemical bonding of PFPE molecules with SU-8 are responsible for the superior tribological properties of SU-8+PFPE composite when compared with other SU-8 composites.     

Reducing Friction Force of Si Material by Means of Atomic Layer-Deposited ZnO Films

With excellent lubricating property, zinc oxide (ZnO) films are promising candidates to act as protective coatings in Si-based microelectromechanical system devices for the purpose of decreasing friction forces of silicon (Si) material. In this paper, the nanotribological behavior of ZnO films prepared by atomic layer deposition on a Si (100) substrate is investigated by an atomic force microscope. The ZnO films have various thicknesses ranging from 10.0 to 182.1 nm. With the increase of film thickness, the root-mean-square roughness of the films increases, while the ratio of hardness to Young’s modulus (H/E) decreases. Due to their large surface roughness, the thick ZnO films are low in adhesion force. The friction force of the ZnO films is smaller than that of the Si (100) substrate and is greatly influenced by their adhesion force and mechanical property. In a low-load condition, the friction force is dominated by the adhesion force, and thus, the friction force of the ZnO films decreases as film thickness increases. While in a high-load condition, the friction force is dominated by plowing. Films with higher H/E possess smaller friction force, and thus, the friction force increases with the decreasing film thickness.     

Adhesion Properties of Nanometer-Thick Perfluoropolyether Films Confined Between Solid Surfaces: A Coarse-Grained Molecular Dynamics Study

Lubrication with thin liquid films is essential to ensure the tribological reliability of technologically advanced devices, such as micro-electro-mechanical systems and hard disk drives. However, the adhesion and friction properties of thin films and the underlying mechanism remain elusive due to our limited understanding of film structures and motions at the molecular scale. Here, we investigate the adhesion behavior of nanometer-thick perfluoropolyether (PFPE) films confined between two solid surfaces as a function of film thickness using coarse-grained molecular dynamics simulations. Consistent with typical experimental results, our simulations show that the adhesive force exerted by the PFPE films reaches a maximum and then decreases with increasing solid–solid spacing. The maximum adhesive force increases sharply for PFPE films thinner than 4 nm. When exhibiting the maximum adhesive force, PFPE films are slightly stretched within a solid–solid spacing a little larger than the initial film thickness and thereby show lower density than the original equilibrium density. Conventional theories of adhesion, which assume equilibrium density for liquid films, are not applicable in such case. Therefore, we construct a theoretical model that takes decreasing liquid density into account to discuss the underlying mechanism of the adhesive force exerted by nanometer-thick PFPE films on solid surfaces. We infer from the theoretical analyses that the maximum adhesive force originates mainly from solid–liquid interaction for thin films and liquid–liquid interaction for thick films.     

Effects of film thickness and contact load on nanotribological properties of sputtered amorphous carbon thin films

The nanotribological properties of amorphous carbon (a-C) films of thickness in the range of 5-85 nm sputtered on Si(1 0 0) substrates were investigated with a surface force microscope (SFM), using a Berkovich diamond tip of nominal radius of curvature approximately equal to 200 nm and contact (normal) loads between 10 and 1200 μN. The dependence of the friction and wear behaviors of the a-C films on normal load and film thickness was studied in terms of nanomechanical properties, images of scratched surfaces, and numerical results obtained from a previous analytical friction model. The increase of the contact load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it either remained constant or decreased slightly. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and plowing mechanisms contributed to the friction behavior in the intermediate- and high-load ranges. Thinner (thicker) a-C films yielded higher (lower) friction coefficients for normal loads less than 50 μN (low-load range) and lower (higher) friction coefficients for normal loads greater than 150 μN (high-load range). Elastic and plastic deformation, microcracking, and delamination of the a-C films occurred, depending on the contact load and film thickness ranges. The reduced load-carrying capacity, relatively low effective hardness (strength) obtained with thinner films, and dominant friction and wear mechanisms at each load range illustrate the film thickness and contact load dependence of the nanotribological properties of the sputtered a-C films.     

A Tribological Study of Multiply-Alkylated Cyclopentanes and Perfluoropolyether Lubricants for Application to Si-MEMS Devices

Lubrication of Micro-Electro-Mechanical Systems (MEMS) is a major constraint in MEMS applications, restricting the designs and practical usages of such devices. Possible lubricants and methods have been investigated in this paper, comparing perfluoropolyether (PFPE) lubricant with multiply-alkylated cyclopentanes (MACs). The effectiveness of both the lubricants in reducing friction and enhancing the wear life was investigated in a new method of MEMS lubrication known as Localised-Lubrication or “Loc-Lub.” Friction and wear tests were conducted in a flat-on-flat test geometry under a normal load of 50 g and a sliding velocity of 5 mm s^?1 in reciprocation, with Si as the substrate. Further tests were conducted at higher loads, to compare wear durability between lubricants and methods. It was found that MACs have a propensity to remain cohesive during the tests due to higher surface tension and provide better friction and wear properties when tested under reciprocating sliding conditions, as a complete film is present between the two surfaces. The results show that MAC lubricant is more effective in extending the wear life and reducing friction under the tested conditions compared to PFPE.     

Nanotribological behavior of tethered reinforced polymer nanolayer coatings

We fabricated molecularly thick thermoplastic elastomeric films with organized microdomain structure and intriguing nanotribological properties. Molecular films from poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) were obtained by a melt/solution grafting to a functionalized silicon surface modified with epoxy-terminated self-assembling monolayers. We varied the thickness of grafted block–polymer films from 1.35 nm (disordered polymer layer) to 9 nm (well defined nanophase structure) and tested their friction, adhesion, shear and wearing properties on a microscale with scanning probe microscopy. Tethered SEBS monolayers, composed of a rubber matrix reinforced by a two-dimensional net of glassy polystyrene (PS) microdomains, possess a friction coefficient as low as 0.02 and shear strength in the range 0.15–1.5 GPa. Chemically tethered SEBS monomolecular films are much more stable under shear stresses than conventional molecular coatings.     

Contributions of Mobile and Bonded Molecules to Dynamic Friction of Nanometer-Thick Perfluoropolyether Films Coated on Magnetic Disk Surfaces

To protect the interface against intermittent head–disk contact in hard disk drives, nanometer-thick perfluoropolyether (PFPE) films consisting of both “bonded” and “mobile” molecules are applied on the disk surfaces. Because of their different adsorption states and mobility, the bonded and mobile molecules are supposed to contribute differently to friction properties, which directly impact the stability of ultra-low flying head–disk interfaces. By measuring the friction force at light loads and low to high speeds as a function of bonded and mobile film thicknesses, we studied the contributions of bonded and mobile molecules to the dynamic friction of nanometer-thick PFPE films. We found that the friction coefficient of lubricant films without or with less bonded molecules increased as a power function of sliding speed, whereas that of lubricant films with more bonded molecules increased logarithmically with sliding speed. We suggest that these results can be explained by the following mechanisms: the dynamic friction of lubricant films without and with less bonded molecules is dominated by shear thinning behavior of mobile molecules, while that of lubricant films with more bonded molecules is governed by bonded molecules which lead to boundary lubrication.     

Nanotribological characterization of digital micromirror devices using an atomic force microscope

Texas Instruments’ digital micromirror device (DMD) comprises an array of fast digital micromirrors, monolithically integrated onto and controlled by an underlying silicon memory chip. The DMD is one of the few success stories in the emerging field of MEMS. In this study, an atomic force microscope (AFM) has been used to characterize the nanotribological properties of the elements of the DMD. An AFM methodology was developed to identify and remove micromirrors of interest. The surface roughness, adhesion, friction, and stiffness properties of the DMD elements were studied. The influence of relative humidity and temperature on the behavior of the DMD element surfaces was also investigated. Potential mechanisms for wear and stiction are discussed in light of the findings.     

Analysis of PFPE lubricating film in NEMS application via molecular dynamics simulation

Interfacial lubrication plays an important role in the functional performance of nanoelectrome-chanical (NEMS) systems. Here, we used molecular dynamics simulation to analyze the lubricating effect of a perfluoropolyether (PFPE) film to reveal the mechanism behind our experimental observations and understand the performance of the film. There was good agreement in the trends of the coefficients of friction between our simulation results and experimental characterizations. By studying the atomic motion, interfacial mechanics and polymer chain deformation, we found that PFPE films provide good lubrication because their linear flowability promotes surface reconstruction. Our simulations suggest that a high performance lubricant film needs to have low resistance to shear deformation, possess high linear flowability, promote surface reconstruction and adhere effectively to the substrates.     

微/纳机电系统

微机电系统（MEMS）和纳机电系统（NEMS）是微米／纳米技术的重要组成部分。MEMS已在产业化道路上发展，NEMS还处于基础研究阶段，分析了微／纳机电系统的发展特点，简要地介绍了典型的MEMS和NEMS器件及系统后，讨论了MEMS和NEMS发展中的几个问题以及它们的发展前景。     

Nanotribological properties of novel lubricants for magnetic tapes

Two classes of novel lubricants, perfluoropolyethers (PFPE) and ionic liquids (ILs), were deposited on metal film magnetic tapes. The adhesive force and coefficient of friction of lubricated and unlubricated tapes were investigated at the nanoscale with an atomic force microscope (AFM) as a function of various humidity and temperature conditions. Microscale tests with a ball-on-flat tribometer were also performed in order to study the length-scale effects on friction. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated by monitoring the friction force, surface potential and contact resistance with the AFM. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) experiments were conducted to determine the chemical species that affect intermolecular bonding and as an aid in interpreting how the lubricant film tribological properties vary with the environmental conditions. Z-TETRAOL, one of the PFPEs, was found to exhibit the lowest adhesion and friction among the lubricant films studied. The ionic liquid 1,1′-(pentane-1,5-diyl)bis(3-hydroxyethyl-1H-imidazolium-1-yl) di[bis(trifluoromethanesulfonyl)imide)] exhibited comparable nanotribological properties with the PFPEs. This is attributed to the presence of hydroxyl groups at its chain ends, which can hydrogen bond with the surface similar to PFPEs.     

Investigation of nanotribological properties of self-assembled monolayers with alkyl and biphenyl spacer chains (invited)

Understanding the relationships between molecular structure and nanotribological performance of self-assembled monolayers (SAMs) are quite important for molecular tailoring for efficient lubrication. For this purpose, SAMs, having alkyl and biphenyl spacer chains with different surface terminal groups (-CH3, -COOH, and -OH), and head groups (-SH and -OH), were prepared. The influence of spacer chains, surface terminal groups, and head groups on adhesion, friction and wear properties were investigated by contact mode atomic force microscopy (AFM). The relative stiffness of SAMs was determined by force modulation mode AFM and indentation experiments using load-displacement curves. The friction properties of SAMs are explained using a molecular spring model in which local stiffness governs the friction properties. Micropatterned SAMs specimen were fabricated and studied to verify the molecular spring model. The influence of relative humidity, temperature and velocity on adhesion and friction was studied. The failure mechanisms of SAMs and substrates were investigated by wear and continuous microscratch AFM technique. Based on these studies, the adhesion, friction and wear mechanisms of SAMs at molecular scale are discussed.     

Tribological Challenges in Micromechanical Systems

Despite much progress in surface micromachining technology, adhesion, friction and wear remain key issues, severely limiting the realization and reliability of many microelectromechanical systems (MEMS) devices. In this article, we focus on the use of molecularly thin organic films as release and anti-stiction coatings for MEMS. The various classes of organic films explored for MEMS are reviewed here, followed by a discussion of the current limitations and areas for improvements for this coating technology.     

Nanostructure-Textured Surfaces with Low Friction and High Deformation Resistance

Nanotextured surfaces can effectively reduce friction and adhesion, especially in applications with micro- and nanoscale contact interactions. However, for these surfaces, a common weakness is a lack of structural integrity of the individual nanotextures when subjected to contact loading, resulting in permanent deformation at even the moderate contact forces encountered in microscale systems. Nanostructure-textured surfaces (NSTSs), composed of arrays of novel Al/a-Si core–shell nanostructures (CSNs), have been developed with a desirable combination of low friction and high deformation resistance. When subjected to nanoscratch testing, these surfaces are shown to have extremely low coefficients of friction (as low as ～0.015), as well as no detectable nanostructure deformation at contact forces up to 8,000 μN (estimated contact pressure greater than 1 GPa). In addition, the NSTSs have low adhesion (pull-off) forces on the order of less than 1 μN. The unique properties of these NSTSs provide avenues for designing low-friction, deformation-resistant surfaces that could benefit a variety of fields, including micro/nanoelectromechanical systems (MEMS/NEMS), microelectronics, magnetic recording, or any other application where the mechanical integrity of nanostructures is important.     

Tribological behaviors of composite molecular deposition films

Polyallylamine hydrochloride (PAH)/graphite oxides (GO) ultrathin film and the multilayer film of PAH incorporated with TiO₂ enwrapped by polyacrylic acid (PAA), namely PAH/PAA(TiO₂) composite film, were prepared by molecular deposition (MD) method in laboratory. Both of them were heated to change the film forming dynamic force from electrostatic force to covalent bond so as to increase the bonding strength of the films. The structure and nanotribological properties of the films were analyzed by atomic force microscope (AFM) and ultraviolet-visible (UV) spectroscopy. It was found that these films had a much smaller friction force than their substrates and the friction force was dependent on the morphology and/or hardness of the films.     

Assessing Nanotribological Performance and Surface Energies of Inconel-ZrN,Cr-ZrN,Nb-ZrN,and ZrN Thin Films

The nanotribological performance for three groups of metal-ZrN, including Inconel-ZrN, Cr-ZrN, Nb-ZrN, and polycrystalline ZrN thin films has been investigated and results were correlated with surface energy evaluations. Metal-ZrN and ZrN thin films were deposited using direct current (DC) unbalanced magnetron sputtering and their elemental composition was investigated using X-ray photoelectron spectroscopy (XPS). Both nanomechanical and nanotribological properties were evaluated using a triboscope interfaced with an atomic force microscope (AFM) and the surface energies were calculated from the contact angle measurements. The present research reports for the first time on the nanowear behavior, surface roughness, and friction coefficients correlated with surface energies of metal-ZrN and ZrN thin films.

All metal-ZrN thin films showed improved nanotribological performance compared to the polycrystalline ZrN. Results indicate that several of the Inconel-ZrN thin film compositions have both superior nanotribological behavior and good wettability and thus have high potential use for wear resistant applications.