Wettability modification of polysilicon for stiction reduction in silicon based micro-electromechanical structures

Abstract

Surface micromachining using deposited polysilicon films is a technology that is widely used for the fabrication of micro-electromechanical structures. One of the biggest yield and reliability problems in the fabrication of such structures is "stiction" or adhesion to the substrate. This may occur during the drying step that is required after wet processing and/or during use of a device. Deposition of self-assembled monolayer coatings is one of the most successful approaches to chemical modification of silicon surfaces to reduce stiction. This approach involves making the surfaces of pre-oxidized polysilicon highly hydrophobic. As a result, microstructures come out of the final water rinse extremely dry without being broken or adhered to the substrate. Available technology requires that these coatings are applied from organic media . However, increasing pressure on semiconductor companies to reduce the generation of organic wastes has sparked interest in the feasibility of applying these coatings from aqueous media. The objective of this research was to develop the chemistry and techniques for the application of hydrophobic coatings on polysilicon from aqueous media. The results obtained from three commercially available water dispersible silanes and cationic alkoxysilanes are discussed. Key experimental variables that were investigated are concentration of reactive silane, type of oxidation pretreatment of polysilicon, pH and temperature of the silane dispersion and curing temperature of the coating. The stability of the dispersions was characterized by viscosity measurements. The formation and quality of the films were studied using atomic force microscopy (AFM), ellipsometry, dynamic contact angle measurements and electrochemical impedance spectroscopy (EIS). The coatings showed contact angles greater than 100°. It was found using AFM that the structure of these films is a continuous film with some particulates attributed to bulk polymerization of the precursor molecule in water. EIS results indicated that the coatings had low porosity as well as high charge transfer resistance across the silicon/HF interface. Ellipsometric analysis showed that thickness of these coatings is roughly a (statistical) monolayer. The stability improvement of the dispersions by the addition of quaternary ammonium cationic surfactants is also discussed.