Characterization of Microscale Wear in a Polysilicon-Based MEMS Device Using AFM and PEEM–NEXAFS Spectromicroscopy
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- Grierson, D.S., Konicek, A.R., Wabiszewski, G.E. et al. Tribol Lett (2009) 36: 233. doi:10.1007/s11249-009-9478-7
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Mechanisms of microscale wear in silicon-based microelectromechanical systems (MEMS) are elucidated by studying a polysilicon nanotractor, a device specifically designed to conduct friction and wear tests under controlled conditions. Photoelectron emission microscopy (PEEM) was combined with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and atomic force microscopy (AFM) to quantitatively probe chemical changes and structural modification, respectively, in the wear track of the nanotractor. The ability of PEEM–NEXAFS to spatially map chemical variations in the near-surface region of samples at high lateral spatial resolution is unparalleled and therefore ideally suited for this study. The results show that it is possible to detect microscopic chemical changes using PEEM–NEXAFS, specifically, oxidation at the sliding interface of a MEMS device. We observe that wear induces oxidation of the polysilicon at the immediate contact interface, and the spectra are consistent with those from amorphous SiO2. The oxidation is correlated with gouging and debris build-up in the wear track, as measured by AFM and scanning electron microscopy (SEM).
KeywordsMicroscale wearMicroelectromechanical systems (MEMS)NanotractorPhotoelectron emission microscopy (PEEM)Atomic force microscopy (AFM)
Micro-/nanoelectromechanical systems (MEMS/NEMS) promise to transform a number of industries by combining micro-/nanoelectronics with versatile micro-/nanofabrication technology. A multitude of MEMS technologies have been successfully fabricated and implemented, such as inertial and non-inertial sensors, actuators, relays, resonators, oscillators, filters, and switches . However, because of their large surface-to-volume ratio, micro- and nanoscale devices designed to operate with sliding, contacting components are especially susceptible to premature failure due to adhesion, friction, and wear, thus hindering their commercialization [2, 3]. In order to overcome these tribological issues, we must understand the micro- and nanoscopic mechanisms by which surfaces become modified via interfacial interaction.
The nanotractor can be used to measure both static and sliding friction coefficients, as well as interfacial adhesion , and was specifically designed to study the effect of applied load on friction and wear at the microscale. By virtue of the micromachining processes used to fabricate this microtribometer, the contacting surfaces can be roughened, oxidized, or coated with self-assembled monolayers (SAMs) to alter the strength of interfacial interactions. Thus, the structure and chemistry of the contacting surfaces can be tailored to conduct fundamental studies and to simulate the tribological conditions found in more complicated systems intended for commercial production.
Here, we present measurements of the morphology and chemistry of unworn and worn surfaces of a polysilicon MEMS device coated with a hydrophobic perfluorinated monolayer. Coating the polysilicon surfaces of the nanotractor with this film is known to improve wear performance , though premature device failure still occurs. Understanding the failure mechanism(s) of this microfabricated tribometer may guide future design of microscale tribosystems.
Nanotractor devices, all contained on a single chip, were microfabricated at Sandia National Laboratory using the SUMMiT™ V process. A perfluorinated monolayer lubricant coating, (tridecafluoro-1,1,2,2-tetrahydrooctyl)tris(dimethylamino)-silane (FOTAS), was applied to the entire chip by a vapor deposition technique . This technique allows FOTAS to conformally coat both contacting surfaces that comprise the sliding interface. Sliding tests were conducted on two different nanotractors in ambient air, at a relative humidity of 20–30%, with the specific intent to induce wear. Each test nominally consisted of moving a nanotractor ~40 μm (i.e., to the left or to the right in Fig. 1a) in ~50 nm steps, thereby deflecting the guide springs and applying a lateral force to the friction clamps. A fixed voltage of 20 V was then applied between the friction clamps and the clamp electrodes, which corresponds to a normal force of ~47.2 μN on the load clamps. The device was then allowed to relax back to its initial position by lowering the normal force. Typical friction coefficients measured during these tests can vary from 0.3 to 4.0 depending on the sample preparation and the degree of wear . This process was repeated in both directions for at least 500 complete cycles (i.e., an interval consisting of motion in both directions) for each device. After testing both nanotractors, the silicon chip was packaged and sealed to prevent contamination until the spectroscopic measurements were performed.
Photoelectron emission microscopy with near-edge x-ray absorption fine structure (PEEM–NEXAFS) spectroscopy measurements were performed ex-situ with the PEEM-II instrument at the Advanced Light Source at Lawrence Berkeley National Laboratory on Beamline 220.127.116.11. The clamps were removed from the silicon chip with adhesive tape prior to being inserted into the vacuum chamber. Electron emission images and x-ray absorption spectra were acquired from worn and unworn regions of the nanotractors at the C 1s, F 1s, and O 1s absorption edges. Unfortunately, this beamline could not accommodate Si spectroscopy. Because of the geometry of the nanotractor device and the 60° angle of incidence of the x-rays (from normal), special care had to be taken to properly align the sample to illuminate the P0 layer, which is ~4.5 µm deeper than the surrounding clamp electrodes. In addition, because of the susceptibility of FOTAS to radiation damage, we employed photon shutters to minimize the exposure of the surfaces to x-rays. All NEXAFS spectra were normalized by division to set the spectral intensity to 1.0 in the pre-edge region of each spectrum, which eliminated topographically-induced differences in the electron emission intensity. This allows spectra taken at different regions on the sample to be compared and ensures that relative peak heights correlate with atomic bonding concentrations. The sampling depth of the spectroscopy is less than 10 nm.
Atomic force microscopy (AFM) imaging was performed on the P0 layer with a Digital Instruments Multimode AFM with a Nanoscope IV controller. Areas of interest examined with the PEEM were imaged with AFM to correlate the topography and wear formation with the surface chemistry.
3 Results and Discussion
The PEEM–NEXAFS results are shown in Fig. 4. Figure 4a depicts a PEEM image acquired at 530 eV of a portion of the P0 layer viewed normal to the surface. The bright vertical line along the middle of the image is the wear scar. The other vertical lines arise from microfabricated changes in height of the substrate. The dashed rectangular region drawn in the PEEM image in Fig. 4a corresponds to the region imaged by AFM in Fig. 3a. Figure 4b is a plot of the oxygen (O 1s) spectra obtained from the highlighted ROIs drawn in Fig. 4a.
The most striking observation is that the O 1s spectra extracted from the debris and the wear track (spectra A and B, respectively) have similar shapes and intensities, and are substantially greater in intensity than the ROI unmodified by wear (spectrum C). This feature is indicative of the O 1s → σ* transition and clearly demonstrates increased oxidation of the near-surface region of the worn interface, with the oxygen primarily bonded in a single-bonded state. Furthermore, none of the O 1s spectra exhibit a pre-edge feature at ~532 eV. This absent pre-edge feature corresponds to the O 1s→ π* transition, and therefore, the lack of this feature demonstrates that no significant π-bonded oxygen, which would be expected for a double-bonded state, is present. These spectra have identical characteristics with those of amorphous SiO2 found in the literature [17–19] and show no indication of crystalline Si–O bonding or oxides that deviate from the stoichiometry of SiO2 . NEXAFS spectra were also obtained at the carbon and fluorine edges (data not shown). In all cases, the C 1s and F 1s signals were weak everywhere (i.e., in both worn and unworn regions), indicating that their concentrations were at or below the detection threshold of the PEEM. There was no detectable difference in the C 1s and F 1s spectra between modified and unmodified regions for this wear track, although we have observed slight decreases in fluorine intensity in worn regions of other nanotractors, suggestive of removal of the FOTAS molecules.
The observed increase in the amount of Si–O bonding is in good general agreement with results found in the literature [21–23]. For example, Alsem et al.  used transmission electron microscopy (TEM) and energy dispersive x-ray (EDX) spectroscopy analyses to analyze debris particles formed in a polysilicon MEMS side-wall friction test device. Their TEM-EDX results confirmed that the debris particles and the wear track had higher oxygen contents than the surrounding unworn polysilicon, but they were unable to determine the exact oxidation state at the surface due to limitations of the EDX technique. However, they were able to estimate Si:O stoichiometry ratios of 34:66 and 80:20 in a debris particle and on a worn surface layer, respectively. From our spectroscopic results, we can conclude that oxidation of the debris and wear track due to wear in ambient laboratory conditions occurs solely in the form of SiO2 bonding. The surface sensitivity and the sensitivity to chemical bonding states that PEEM–NEXAFS possesses allows us to advance our understanding of the nature of wear-induced oxidation of polysilicon by clearly showing that only amorphous SiO2 is formed on the immediate contact interface. Future study investigating the Si 1s and 2p edges will help inform the interpretation of the spectra further.
A more complete story of polysilicon wear emerges by combining PEEM spectroscopy and AFM topography. Based on previous studies of nanotractor friction and wear, we hypothesize that wear of the sliding interface begins as the FOTAS coating breaks down  within the contact zone and advances via moderate gouging, Si material removal (including the removal of individual grains as observed in the sidewall friction tests ), and debris formation within and around the contacting regions. Debris is mechanically polished and becomes oxidized to amorphous SiO2, as does the wear scar. As elucidated by Flater et al., oxidized Si surfaces are known to exhibit higher friction , and therefore the wear scar and trapped oxidized debris may cause higher adhesion and friction, leading to further increase in wear. Device failure ultimately occurs at the time when the restoring force of the guide spring cannot exceed the lateral resistance produced by the SiO2 debris formations and worn, oxidized surfaces.
Experiments using the nanotractor provide insight into the wear mechanisms for polycrystalline Si surfaces. PEEM–NEXAFS reveals that worn regions and debris formations have an increased amount of oxygen bonding, demonstrating that substantial oxidation in the form of amorphous SiO2 takes place on mechanically modified polysilicon regions. AFM topography measurements confirm that the wear scar exhibits moderate gouging. More notably, debris, formed from gouging and from the removal of either entire grains or portions of grains from the substrate, can be trapped in the interface. The formation, smoothing, and oxidation of those particles, as well as the oxidation of the wear-scar gouges, characterize the wear process of the nanotractor.
By combining PEEM–NEXAFS with AFM, we are able to describe the wear processes that may lead to device failure from a combined chemical and mechanical perspective. This approach leads to an understanding of reliability and performance issues of a polysilicon MEMS device. Many of the concerns with MEMS/NEMS involving tribological contact can be addressed by applying these techniques to the materials and geometries currently employed in micro- and nanoscale wear studies.
This study was partly funded by Air Force grant FA9550-08-1-0024, and partly by Sandia—a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The authors thank Dr. Scholl and Dr. Doran for their help with PEEM II at the Advanced Light Source (ALS). The ALS and use of the Center for Nanoscale Materials facility are supported by the DOE under Contract DE-AC02-05CH11231 and Contract DE-AC02-06CH11357, respectively.