Effects of adsorbed water and sample aging in air on the μN level adhesion force between Si(100) and silicon nitride

The microscale adhesion force of single crystal silicon (100) to silicon nitride was studied using a newly developed micro/nanoadhesion measurement apparatus. The effect of water adsorption on adhesion between clean surfaces and surfaces aged in air was determined. During experiments, varying relative humidity (RH) controlled water adsorption. Clean and aged samples showed measurably changed water adsorption and adhesion above 60% RH only, and aged samples showed more water adsorption and adhesion than clean ones above 90% RH. A contaminant film grew to a saturated thickness of about 6 إ over 50 days of aging, and the film changed surface chemistry which resulted in higher contact angle. The contaminated surface with higher contact angle exhibited more affinity for water and microadhesion on the molecular scale, which is contrary to the common interpretation of contact angle data. The divergence of water adsorption and adhesion force behavior between clean and aged samples above 90% RH may indicate that the Kelvin radius became comparable to a characteristic length on the contaminated surface at high RH. A molecular model is proposed in which the contaminant film provides porosity, which allows capillary condensation and the growth of a multilayer water film.