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An atomic force microscope (AFM) can be used to monitor stiction (by measuring the work of adhesion) at different stages of manufacture, which then might be correlated to device performance. The four main challenges to using the AFM as an in-fab stiction monitor are calibration, effects from the angle of repose of the cantilever, surface roughness, and material properties. We measured the work of adhesion between different AFM tips and samples. There were 17 tips of four different types, with radii from 200 nm to 60, covering the range of typical microelectromechanical systems (MEMS) contacts. The samples were unpatterned amorphous silicon dioxide MEMS die with two types of surface conditions (untreated and treated with a few angstroms of vapor-deposited diphenylsiloxane). A simple correction for the surface roughness resulted in the expected linear dependence for work of adhesion on radius, but the magnitudes were higher than expected. Commercial and heat-treated AFM tips have minimal surface roughness and result in magnitudes that were more reliable. In this paper, we review the four main challenges and show their influence on the measured work of adhesion, from which we develop a set of recommendations for the use of an AFM as an in-fab stiction monitor.
Date of Publication: June 2007