Noncontact Method for Calibration of Lateral Forces in Scanning Force Microscopy

This paper describes a noncontact calibration procedure for lateral force microscopy in air and liquids. The procedure is based on the observation that the sensitivity of a force microscope may be calibrated using the raw thermal noise spectrum of the cantilever and its known spring constant, which can be found from the same uncalibrated thermal noise spectrum using Sader's method (Rev. Sci. Instrum. 1999, 70, 3967-3969). In addition to the power spectrum of the cantilever thermal noise, this noncontact calibration method only requires knowledge of the plan view dimensions of the cantilever that could be measured using an optical microscope. This method is suitable for in situ force calibration even in viscous fluids through a two-step calibration procedure, where the cantilever thermal spectra are captured both in air and in the desired liquid. The lateral calibration performed with the thermal noise technique agrees well with sensitivity values obtained by the wedge calibration procedure. The approach examined in this paper allows for complete calibration of normal and lateral forces without contacting the surface, eliminating the possibility for any tip damage or contamination during calibration.