This work investigates the interactions between polysilicon surfaces for the purpose of alleviating the strong interfacial adhesion present between contacting microstructure sufaces, a problem referred to as stiction. Without specifically-designed surface treatments, most microeIectromechanical systems @EMS) possess hydrophilic oxide surfaces which are susceptible to strong capillary forces that act over much of the nominal contact area. This is evidenced by a work of adhesion of 140 mJ/m^2 (approximately twice the surface energy of water) observed between oxide surfaces in a humid environment. The approach therefore taken in this work to minimize adhesion in microstructures is the chemical modification of the polysilicon surface to create a hydrophobic, low energy surface.
For this purpose, hydrogen-terminated silicon surfaces, diamond-like carbon coatings, and self-assembled monoliiyer films have been investigated, all of which show promise for stiction reduction. To illustrate, pull-off forces measured by atomic force microscopy decrease by about a factor of ten on hydrogen-terminated silicon and diamond-Iike carbon surfaces compared to hydrophilic oxide surfaces. In addition, micromechanical test structures designed to quantify stiction show that hydrogen terminated surfaces and self-assembled monolayer films reduce adhesion by three to four orders of magnitude in polysilicon devices, where apparent work of adhesion values have been measured as low as 3 uJ/m^2. Existing theories are not capable of quantitatively predicting adhesion between hydrophobic polysilicon surfaces, but the magnitude of the work of adhesion values measured are consistent with van der Waals interactions between rough surfaces.
The utility of anti-stiction surface treatments also depends on their compatibility with the fabrication, release, drying, assembly, and packaging of the entire microsystem. For example, the meta-stable nature of hydrogen-terminated surfaces and the high compressive stress of diamond-like carbon films may render these surface treatments unsuitable for many commercial devices. Self-assembled monolayer films, on the other hand, can be easily integrated with microstructures and exhibit promising long-term and thermal stability properties. Thus, this work explores the anti-stiction properties and compatibility issues with current MEMS manufacturing processes for a variety of surface treatments, and the results indicate that surface treatments can be rationally designed to alleviate stiction in many types of micromechanical structures.
December 31, 1996
Houston, M. R. (1996). Surface Treatments for Adhesion Reduction in Polysilicon Micromechanical Devices. United States: University of California, Berkeley.