Observation of nanoscale adhesion, friction and wear between ALD Al₂O₃coated silicon MEMS sidewalls

Abstract

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.

Review—Beyond the Highs and Lows: A Perspective on the Future of Dielectrics Research for Nanoelectronic Devices

“…The high elastic modulus 7 and corrosion resistance 408 of high-k oxide and nitrides have additionally made them of interest as stiction prevention, frequency tuning, and movable/resonating materials in micro/nanoelectromechanical devices. 51,52,[409][410][411] Further, the high hardness 7 and chemical inertness 412 of high-k oxides makes them of interest for numerous tribological 413,414 and protective antibacterial 55,56,415 and moisture encapsulation 416,417 coating applications. The high refractive index and optical transparency 7 of high-k oxides also makes them of interest in waveguide and optical coating applications.…”

Surface activated bonding of ALD Al2O3 films

“…Due to the high dielectric constant, proper electrical property, large band gap, good thermal stability and thermal conductivity, Al 2 O 3 films are widely used as gate dielectrics [1][2][3] or protective layers 4,5) for functional layers transferred to different substrates in the semiconductor industry. In recent years, their applications have been extended to optics, 6) micro-electromechanical systems 7) and protective coatings 8) thanks to its optical, mechanical and barrier properties.…”

Research on Surface Modification of Screen Printing Electronics

“…Achieving controllable adhesion is an effective way to solve ink-blocking mesh in the electronic field of screen printing and improve the ink transferring rate. In order to regulate the surface adhesion independently according to the performance requirements, domestic and foreign researchers defined the pulling force needed to separate two contact objects as adhesion, and used adhesion as an index to measure the size of adhesion [31], and put forward a variety of methods [32] to regulate the surface adhesion from the perspectives of changing the contact area [33][34][35][36][37] and changing the surface energy [38][39][40][41], which is mainly divided into "reshaping" and "modification." Wu et al [42] prepared nano-convex-patterned polyimide surface and nano-concave patterned polyimide surface, which effectively reduced the adhesion on the surface of the samples.…”