The effect of relative humidity on friction and pull-off forces measured on submicron-size asperity arrays

The friction and pull-off forces were measured between atomic force microscope (AFM) probe and submicron-size asperity arrays on a silicon wafer in various relative humidities. First, a focused ion beam (FIB) was used to produce two-dimensional arrays in which asperities were arranged at uniform intervals of 240 nm. Independent asperities were also processed for comparison with the results for asperity arrays. The two kinds of arrays were made by milling grooves in two perpendicular directions at uniform intervals. Each asperity peak in contact with the probe had a radius of curvature of 168 and 452 nm, respectively. The tip of the AFM probe had a flat, square surface measuring 0.7×0.7 μm2. The pull-off forces were measured by using force curve mode in the AFM system. Measurements were carried out 256 or 1024 times in various humidities and on each array. The average pull-off force increased with relative humidity on each asperity array. On independent asperities, however, the pull-off force did not change with relative humidity. Histograms of pull-off force measured on the asperity arrays showed that the position of the maximum peak along the horizontal axis, i.e., the mode of pull-off force, did not change with relative humidity and that the other peaks at higher pull-off forces grew with relative humidity. The intervals between the adjacent peaks were almost the same. These findings suggest that the pull-off force on one asperity was the same but the number of contacting points between the flat probe and asperity array increased with relative humidity. The friction force was measured on the asperity arrays while changing the sliding speed in various relative humidities. The friction force increased with higher relative humidity and lower sliding speed. These findings can be explained by the viscous resistance and lubricating effect of condensed water.