Determination of pull-off forces of textured silicon surfaces by AFM force curve analysis

This paper presents a novel methodology to measure the pull-off force of rigid bodies by recording force curves with an atomic force microscope (AFM). With the presented method, structured or modified surfaces can be inspected under close to target device conditions. Furthermore, large sample arrays can be inspected using the AFM as a sensing element. The inspected structures in this study are rigid silicon discs with a diameter of 200 ?m, supported by soft silicon oxide springs. The discs are pushed down by the AFM tip on a bottom substrate. When retracting the AFM tip and the disc, a discontinuity in the force curve of the AFM occurs due to the pull-off. The surface interaction of silicon oxide surfaces provided with hemispherical tips and flat silicon surfaces is determined. Therefore, various surface configurations are inspected, with varying tip radii and varying number of tips. The lowest adhesion force measured for a single device is 240 nN. In terms of total work of adhesion per area this value equals 9 ? 10?7 mJ m?2. The measured adhesion is compared to calculated data based on a Van der Waals surface interaction model. The method features a relative error of 0.26 which is comparable to the corresponding value of cantilever beam detachment (CBD) measurements from the literature. The resolution of the presented method is only limited by the force resolution of the AFM which in this work is 20 nN.