Friction and pull-off forces on submicron-size asperities

We measured the friction and pull-off forces between an atomic force microscope (AFM) probe and submicron-size asperities of various radii of curvature on a silicon wafer. First, we used a focused ion beam to produce two types of asperities: two-dimensional arrays in which asperities were arranged at uniform intervals of 240 nm, and independent asperities in which the distance between adjacent peaks was approximately 3 mu m. The arrays were made by milling grooves of various depths (3.2 to 49.2 nm) in two perpendicular directions at uniform intervals. Areas in contact with the probe had radii of curvature from 86 to 792 nm, depending on the asperity. The independent asperities were made by milling an area around a point in a step-like form and had radii of curvature from 71 to 941 nm. The tip of the AFM probe had a flat square surface measuring 0.7 X 0.7 mu m(2). On all asperities measured, both the friction and pull-off forces were proportional to the radius of curvature of the tip of the asperity, irrespective of the micro-surface roughness of the asperity. The friction force was proportional to the pull-off force. The radius of curvature of the tip of an independent asperity was determined by approximating the tip as a spherical surface. When this spherical surface was set greater than the contact area, the friction and pull-off forces were proportional to the radius of curvature. This verified that the surface tension due to capillary force around the contact area was the predominant component of the adhesion force. (C) 1998 Elsevier Science S.A. All rights reserved.