W. Robert Ashurst

Self-assembled monolayers as anti-stiction coatings for MEMS: characteristics and recent developments

Despite significant advances in surface micromachining technology, stiction remains a key problem, severely limiting the realization and reliability of many micro-electro-mechanical systems (MEMS) devices. In this article, we focus on self-assembled monolayers as release and anti-stiction coatings for MEMS. Their formation mechanism, the microstructure coating process, and the characteristics of the coated microstructures are described, followed by a discussion of the current limitations, areas for improvements and recent progress for this coating technology.

Alkene based monolayer films as anti-stiction coatings for polysilicon MEMS

Abstract This paper describes a new class of anti-stiction coatings for polysilicon MEMS. This class of molecular film is based on the free radical reaction of a primary alkene (e.g. 1-octadecene C16H33CHCH2) with hydrogen terminated silicon [1] , [2] . The new coating has several key advantages over the previously reported octadecyltrichlorosilane (OTS) and perfluorodecyltrichlorosilane (FDTS) based self-assembled monolayers (SAM) [3] , [4] : (1) the coating does not produce HCl at any stage in the monolayer formation whereas chlorosilane based chemistry does. (2) The coating does not require the formation of an intervening charge-trapping oxide layer. (3) The film formation procedure for alkene based monolayers is simpler than for chlorosilane based SAMs for two main reasons. First, the surface re-oxidation step is entirely eliminated. Second, the coating solution does not need to be conditioned before use, since water is not a reagent in this process. (4) The coating process is much more robust since it is essentially insensitive to relative humidity. (5) The coated structures have many fewer particulates in comparison to those coated with OTS. (6) The coating process can be made selective to coat only exposed silicon by generating radicals using a radical initiator. The coating has been evaluated in several ways, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle analysis, work of adhesion by cantilever beam array technique and coefficient of static friction using a sidewall testing device. The octadecene film is compared to the OTS SAM with respect to anti-stiction properties. XPS data confirm the absence of oxygen in both freshly prepared samples and in samples aged for more than 4 months in laboratory ambient. Water and hexadecane contact angles, and work of adhesion data are similar to those of OTS. AFM shows that the samples, which receive 1-octadecene films, accumulate far fewer particles during processing than those which receive the OTS SAM treatment. Based upon the data presented here, we find that the anti-stiction properties of films produced with the alkene chemistry are indeed comparable to those produced with the chlorosilane SAMs, but without many of the limitations imposed by the chlorosilane chemistry.

A low-temperature CVD process for silicon carbide MEMS

A low-temperature, single precursor CVD process for the realization of SiC-based MEMS and SiC-coated MEMS is described using 1,3-disilabutane. With this deposition method, the fabrication of an all-SiC cantilever beam array is demonstrated using standard microfabrication processes. Also, SiC coating of released Si micromechanical structures is realized using this process. The SiC-coated microstructures are shown to have superior chemical stability when compared to their Si analogs, as well as exhibit highly favorable mechanical properties.

An investigation of sidewall adhesion in MEMS

Adhesion or stiction is a key problem in surface micromachining technology, affecting the reliability of most MEMS. To date, the quantitative analysis of the phenomenon has been limited to in-plane adhesion. Since many micromechanisms involve contacts between sidewalls, we have designed a microinstrument to measure sidewall adhesion. Here, we describe the design, modeling, results and problems encountered with this first generation of sidewall adhesion devices.

Rate-state friction in microelectromechanical systems interfaces: Experiment and theory

A microscale, multi-asperity frictional test platform has been designed that allows for wide variation of normal load, spring constant, and puller step frequency. Two different monolayer coatings have been applied to the surfaces—tridecafluorotris(dimethylamino)silane (FOTAS, CF3(CF2)5(CH2)2 Si(N(CH3)2)3) and octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3). Static friction aging was observed for both coatings. Simulating the platform using a modified rate-state model with discrete actuator steps results in good agreement with experiments over a wide control parameter subspace using system parameters extracted from experiments. Experimental and modeling results indicate that (1) contacts strengthen with rest time, exponentially approaching a maximum value and rejuvenating after inertial events, and (2) velocity strengthening is needed to explain the shorter than expected length of slips after the friction block transitions from a stick state. We suggest that aging occurs because tail groups in the monolayer coatings reconfigure readily upon initial contact with an opposing countersurface. The reconfiguration is limited by the constraint that head groups are covalently bound to the substrate.

Investigation of a vapor-deposited thin silica film: morphological and spectral characterization.

Surface modification reactions by organosilicon compounds have demonstrated great success in a wide variety of applications. However, they are of limited usefulness in that they only proceed appreciably on surfaces that have an abundance of reactive hydroxyl groups, thus preventing their application to some materials of technological relevance, such as plastics and polymers. A process capable of depositing a surface rich in reactive hydroxyl groups onto a wide variety of substrates could potentially enable the extension of organosilane surface modification reactions to new materials, but conventional processes for depositing oxide layers require temperatures that are too high for most polymers and plastics. It has been shown that silica layers can be deposited from the vapor-phase hydrolysis of tetrachlorosilane at room temperature, but little if any work has been done to characterize the resulting films. In this work, ellipsometry, atomic force microscopy, and Fourier transform infrared spectroscopy are employed to study the characteristics of films formed from this process. Interestingly, very different film morphologies can be obtained by changing key processing parameters. Furthermore, isotopic exchange experiments and dehydration studies show that the surfaces of the silica films obtained by this method are composed entirely of hydrogen-bonded silanol groups and do not exhibit any freely vibrating surface silanol groups, a result that is in contrast with conventionally prepared silica materials. Still, this layer has been shown to behave very similarly to conventional silica materials with respect to surface reactions. Finally, infrared spectral data and contact angle data demonstrate that this method can be employed to deposit silica layers onto poly(methyl methacrylate) and polystyrene surfaces.

Characterization of gas-expanded liquid-deposited gold nanoparticle films on substrates of varying surface energy.

Dodecanethiol-stabilized gold nanoparticles (AuNPs) were deposited via a gas-expanded liquid (GXL) technique utilizing CO(2)-expanded hexane onto substrates of different surface energy. The different surface energies were achieved by coating silicon (100) substrates with various organic self-assembled monolayers (SAMs). Following the deposition of AuNP films, the films were characterized to determine the effect of substrate surface energy on nanoparticle film deposition and growth. Interestingly, the critical surface tension of a given substrate does not directly describe nanoparticle film morphology. However, the results in this study indicate a shift between layer-by-layer and island film growth based on the critical surface tension of the capping ligand. Additionally, the fraction of surface area covered by the AuNP film decreases as the oleophobic nature of the surfaces increases. On the basis of this information, the potential exists to engineer nanoparticle films with desired morphologies and characteristics.

In situ oxygen plasma cleaning of microswitch surfaces-comparison of Ti and graphite electrodes

Ohmic micro- and nanoswitches are of interest for a wide variety of applications including radio frequency communications and as low power complements to transistors. In these switches, it is of paramount importance to maintain surface cleanliness in order to prevent frequent failure by tribopolymer growth. To prepare surfaces, an oxygen plasma clean is expected to be beneficial compared to a high temperature vacuum bakeout because of shorter cleaning time (<5 min compared to ~24 h) and active removal of organic contaminants. We demonstrate that sputtering of the electrode material during oxygen plasma cleaning is a critical consideration for effective cleaning of switch surfaces. With Ti electrodes, a TiO x layer forms that increases electrical contact resistance. When plasma-cleaned using graphite electrodes, the resistance of Pt-coated microswitches exhibit a long lifetime with consistently low resistance (<0.5 Ω variation over 300 million cycles) if the test chamber is refilled with ultra-high purity nitrogen and if the devices are not exposed to laboratory air. Their current–voltage characteristic is also linear at the millivolt level. This is important for nanoswitches which will be operated in that range.

FUNCTIONALIZED CELLULOSE FIBERS FOR DEWATERING AND ENERGY EFFICIENCY IMPROVEMENTS

Paper drying accounts for nearly 80% of the energy used in the papermaking process. This is due to the high energy requirements for the process of drying by vaporization. Because the cost-effectiveness of the various physical means of dewatering far exceeds that of thermal drying, significant energy savings can be expected if the physical dewatering effectiveness is improved. To that end, a novel method of enhancing the physical dewatering process that involves the addition of hydrophobic fibers to the pulp furnish is described and evaluated. Freeness and water retention measurements indicate that the addition of hydrophobic fibers at even a few weight percent may have a significant impact on the freeness and water retention properties of the furnish and therefore a significant improvement in the effectiveness of the physical dewatering of webs made using the hydrophobically tailored furnish material.

Silica-Encapsulated Nanoparticle Films as Surface Modifications for MEMS

In an effort to improve the reliability of microelectromechanical systems (MEMS), silica thin films deposited by chemical vapor deposition were used to encapsulate gold nanoparticle coatings. These composite coatings were shown to provide extremely durable films that significantly reduce the adhesion energy of silicon-based microcantilever beams. The results discussed suggest that encapsulating nanoparticle films with a durable silica thin film may lead to improved MEMS reliability.