Project Report

Our research deals with the tribology (the study of friction, wear and lubrication properties) of silicon surfaces. The tribology of silicon is important because this material is used for the production of microelectromechanical systems (MEMS), tiny machines produced using the lithography and surface machining techniques developed by the semiconductor industry. MEMS devices are already prevalent in everyday life: they are the acceleration sensors that activate airbags in cars and detect movement in video game controllers and cell phones, and they are the actuators to control the flow of ink in InkJet printers. It is possible to make more complicated machines with complex moving parts (such as interlocking gears), which would be useful for myriad applications such as a lab-on-a-chip, but the tribological properties of silicon unfortunately do not allow for complicated motion – the friction between two silicon surfaces is so high that the device will not move. Our research aims to use alcohol vapor as a lubricant for silicon to reduce this prohibitively high friction, thereby enabling complex movements in MEMS devices. The group in Chengdu is studying a specific phenomenon, wherein sliding a sharp silicon probe against a flat silicon surface causes the formation of a bump, Figure 1 (a), rather than the expected result of surface wear, Figure 1 (b). This frictionally induced "hillock" is due to mechanical restructuring of the silicon crystal lattice. This effect occurs in environments that reduce the probe-surface chemical interactions, i.e. vacuum and inert gases, whereas the surface will wear under the probe in air or humid environments, where the probe-surface interactions are quite strong. Since our alcohol lubrication method functions by shutting down the chemical interactions between silicon surfaces, we wanted to investigate whether the alcohol vapor would also shut down the wear mechanism and allow hillocks to form in air. We expected that the alcohol vapor would stop the wear process and allow the formation of hillocks, but our results in Figure 1 (c) indicate that we were only half correct. The alcohol vapor did indeed prevent the surface from wearing, but it also prevented the formation of a hillock. Until this point, the group hypothesized that the hillocks formed in response to a shear force from rubbing the surfaces together, but any chemical interactions would cause surface wear. But the lack of any wear or hillock in the alcohol vapor environment indicates that the mechanism is more complex: both shear force and some chemical interaction is required to produce the hillocks. Prof. Qian’s group in Chengdu had studied the hillock formation phenomenon for a few years, and published a handful of papers on the subject, but the research we performed together over the summer helped to highlight a large gap in their hypothesis which may have lead them astray.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Carter Kimsey
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Marchand David J
University Park
United States
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