In this paper we measure the evolution of adhesion between two polycrystalline silicon sidewalls of a microelectromechanical adhesion sensor during three million contact cycles. We execute a series of AFM-like contact force measurements with comparable force resolution, but using real MEMS multi-asperity sidewall contacts mimicking conditions in real devices. Adhesion forces are measured with a very high sub-nanonewton resolution using a recently developed optical displacement measurement method. Measurements are performed under well-defined, but different, low relative humidity conditions. We found three regimes in the evolution of the adhesion force. (I) Initial run-in with a large of cycle-to-cycle variability, (II) Stability with low variability, and (III) device-dependent long term drift. The results obtained demonstrate that although a short run-in measurement shows stabilization, this is no guarantee for long-term stable behavior. Devices performing similarly in region II, can drift very differently afterwards. The adhesion force drift during millions of cycles is comparable in magnitude to the adhesion force drift during initial run-in. The boundaries of the drifting adhesion forces are reasonably well described by an empirical model based on random walk statistics. This is useful knowledge when designing polycrystalline silicon MEMS with contacting surfaces.

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The strange and unpredictable behavior of meso-scale adhesion and friction forces is a practical problem for the development of microelectromechanical systems (MEMS) with contacting surfaces. To overcome the associated limitations when designingMEMS devices, the first obstacle to remove is the fact that it is hard to measure displacements and forces in MEMS with sufficient resolution to discern atomic scale details from these meso-scale measurements. In this PhD thesis we show how an non-invasive, optical method can be used to measure forces and displacements inMEMS with sub-nanometer resolution. It is fundamentally impossible to opticallymeasure topological details below 500 nmin size, due to the wavelike nature of light. However, the location of a moving feature can be tracked with a much higher resolution, by curve-fitting a mathematical function to its shape. @en

In this paper, we report on the influence of capillary condensation on the sliding friction of sidewall surfaces in polycrystalline silicon micro-electromechanical systems (MEMS). We developed a polycrystalline silicon MEMS tribometer, which is a microscale test device with two components subject to sliding contact. One of the components can be heated in situ by Joule heating to set the temperature of the contact and thereby control the capillary kinetics at the MEMS sidewalls. We used an optical displacement measurement technique to record the stick–slip motion of the slider with sub-nanometer resolution, and we assessed the friction force with nanonewton resolution. All friction measurements were performed under controlled ambient conditions while sweeping the contact temperature from room temperature to 300 C, and from 300 C to room temperature. We were able to distinguish the two ways in which energy is dissipated during sliding: the ‘semi-statically’ dissipated energy attributed to asperity deformation and contact yield, and the dynamically dissipated energy ascribed to the release of the tension in the slider during slip events. We observed an increase in the dynamically dissipated energy at 80 C while sweeping down in temperature. This increase is caused by higher adhesion due to capillary condensation between the conformal surfaces. Our study highlights how energy is dissipated during the sliding contact of MEMS sidewalls, and it is helpful in overcoming friction in multi-asperity systems. @en

In this paper we demonstrate a new method for analyzing and visualizing friction force measurements of meso-scale stick–slip motion, and introduce a method for extracting two separate dissipative energy components. Using a microelectromechanical system tribometer, we execute 2 million reciprocating sliding cycles, during which we measure the static friction force with a resolution of 0.6 nN and the displacement with a resolution of 0.2 nm. We plot the lateral force as a function of the real contact position by compensating for the values of the spring constants of the system. This allows all friction loops to be combined in a single hexagonal bin plot, which clearly shows the evolution of the friction force magnitude and its distribution across the sliding track. We identify all individual slip events in the entire experiment using a thresholding algorithm. This allows us to show the evolution of the slip event count, the static friction force, and the coefficient of friction. Crucially, it allows us to disentangle the dissipated energy into two components: the dynamically dissipated energy, which is associated with slip motions, and the semi-statically dissipated energy, which is related to small contact deformations, plastic yield and other non-elastic behavior. Our technique provides new insight into the mechanics of stick–slip motion in multi-asperity contact systems, and paves the way towards a better understanding of the physics of meso-scale friction.@en

In this work, we have incorporated heaters in a MEMS device, which allow the in situ local heating of its contacting surfaces. This design offers a promising solution for MEMS devices with contacting components by preventing capillary-induced adhesion. The force of adhesion was assessed by optically measuring in-plane snap-off displacements. We were able to decrease adhesion from 500 nN to 200 nN with just one heated surface of which the temperature was set above 300 °C. The temperature should not be set too high: we observed increased adhesion due to a direct bonding process once the temperature was increased above 750 °C. Remarkably, adhesion increased by heating from room temperature to 75 °C, which is attributed to more water being transferred to the contact area due to faster kinetics. We observed the same effect in the cases where both surfaces were heated, although at slightly different temperatures. We demonstrated that heating only one surface to between 300 °C and 750 °C is sufficient to significantly lower adhesion, due to the removal of capillary menisci. The required heater is typically most easily implemented in a stationary part of the device.@en

We report a novel investigation of the tribological properties of aluminum oxide (Al2O3) when it is used as protective coating on the sidewalls of microelectromechanical systems (MEMS). By using an in-house built optical displacement measurement system, we were able to measure the on-chip displacements with an unprecedented resolution of 2 nm. This corresponds to 2 nN and 9 nN force resolution, respectively, depending on whether an adhesion or a friction sensor MEMS device was used for the measurement. Al2O3 was deposited on the vertical etched sidewalls using atomic layer deposition (ALD). All tests were carried out in ambient conditions. The same tests carried out on uncoated polysilicon devices were not reproducible due to stiction, which sometimes prevented the interacting surfaces from moving once contact was made. The higher adhesion of silicon was also found to hinder the mobility of the slider. In the ALD-coated devices, we observed increasing adhesion after 50000 repeated contacts. We attribute this increase to the accumulation of aluminum hydroxide debris produced by the reaction with moisture in the environment. We also investigated the long-term effect of friction on the coated silicon sidewalls. The dissipated energy decreases, with a minimum lateral force occurring around the 1000th cycle. After 1000 cycles, the lateral displacement decreases, suggesting an additional lateral dragging force caused by the interaction between a mixture of aluminum hydroxides and water. However, the small overall amount of debris produced during the friction test indicates the outstanding characteristic of Al2O3 as a protective coating for MEMS that use contacting or sliding interfaces.@en