FORCE-DISPLACEMENT MEASUREMENTS IN A BEAM-REFLECTION SCANNING FORCE MICROSCOPE: CALIBRATION ISSUES

G. Haugstad and W. L. Gladfelter
Department of Chemistry, University of Minnesota
Minneapolis, MN 55455

Abstract

Analysis of force-displacement measurements obtained with a commercial beam-reflection scanning force microscope on rigid samples in the contact regime reveals the need for previously-unpublished calibration procedures. The measured cantilever deflection is linearly proportional to the sample displacement to lowest order as expected, but the proportionality coefficient is dependent on the cantilever/sample immersion medium via its refractive index. An additional term which is cubic in the sample displacement becomes significant at large cantilever deflections, amounting to about 8% of the total signal at the largest measurable deflections. This nonlinearity apparently derives from a cross-sectional nonuniformity in the intensity of laser light used to monitor cantilever deflections. The importance of these considerations to studies in fluid media and on biological and other soft materials is illustrated employing gelatin films in air and in water. Appropriate calibration measures are discussed.