Observation of nanoscale adhesion, friction and wear between ALD  Al₂O₃ coated silicon MEMS sidewalls

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.