AVS 46th International Symposium
    Nanometer-scale Science and Technology Division Tuesday Sessions
       Session NS-TuA

Paper NS-TuA9
Measuring Average Tip-sample Forces in Intermittent-contact (Tapping) Force Microscopy in Air

Tuesday, October 26, 1999, 4:40 pm, Room 612

Session: Innovative Nanoscale Measurements
Presenter: S.C. Fain, Jr., University of Washington
Authors: S.C. Fain, Jr., University of Washington
K.A. Barry, University of Washington
M.G. Bush, University of Washington
B. Pittenger, University of Washington
R.N. Louie, Pacific Lutheran University
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Many soft substances such as polymers and biological molecules show much less damage when imaged with intermittent-contact (tapping) force microscopy; however, the tip-sample forces involved have never been directly measured. The mathematical solutions to the non-linear differential equations needed to calculate these forces can be extremely complicated even for mass-on-a-spring models. In these experiments piezoresistive cantilevers (from Park Scientific/ ThermoMicroscopes) are used to measure the average force on a silicon surface produced by the silicon tip of a separate mechanically-driven (active) cantilever. The procedure consists of slowly moving the two cantilevers toward and away from each other while measuring as a function of the change in distance between the two support points: the average deflection of the piezoresistive cantilever, the amplitude of the active cantilever, the change in phase of the active cantilever relative to the mechanical driver, and the change in the average deflection of the active cantilever. Recent simulations by Garcia and San Paulo (Phys. Rev. B., in press) predict the force exerted on a model substrate averaged over a complete cycle, the amplitude, and the phase shift as a function of distance; they find that the force starts to rise when the phase shift indicates repulsive tapping. These predictions are compared with our measurements. The limit on the sensitivity of our average force measurements is set by the 1/f noise of the piezoresistive cantilever. This work was supported by NSF DMR 96-23590 and the UW Royalty Research Fund.