Probe-force calibration experiments using the NIST electrostatic force balance Jon R. Pratt, David Newell, John Kramar, Jonathan Mulholland  , and Eric Whitenton National Institute of Standards and Technology, Gaithersburg, MD 20899 The sensitivity of a piezoresistive cantilever force sensor has been determined by probing the weighing pan of the National Institute of Standards and Technology’s prototype electrostatic force balance. In this experiment, micronewton contact forces between a force probe and the balance’s weighing pan are determined from the voltage required to maintain the null position of the force balance. The force balance incorporates length and electrical metrology so that an electrical realization of force consistent with the International System of Units may be computed from measurements of the voltage and capacitance gradient. Here, the balance serves as a primary realization of force in a continuous range between 0 and 6 x 10 -5 N, allowing calibration of the sensor against a traceable standard. Introduction Originally conceived as a means to image insulators [1], the atomic force microscope (AFM) has quickly distinguished itself as a useful tool for the measurement of surface forces [2] and, in more recent studies, of intermolecular forces of single macromolecules, such as DNA and proteins [3,4]. The problem of calibrating the force sensitivity of these devices is well known, and a variety of approaches have been considered in the literature [5,6]. Recently, NIST developed an electronic null balance for comparing weak forces derived from mechanical, electrical, optical, or molecular sources as part of our work to link small force measurement to the International System of Units [7]. The goal is to realize and disseminate small force through an electrostatic force balance similar to, but on a smaller scale than, voltage balances used in fundamental electrical metrology [8]. Such a balance might serve as a primary standard for probe-type force measuring instruments in the regime below 10 -5 N. Eventually, the plan is to design transfer artifacts, i.e., calibrated load cells or force generators, through which we can disseminate this realized force to users in industry or academia. In this paper, we briefly review the working principles of the electrostatic force balance and describe how it can be employed to provide a force calibration of a piezoresistive AFM cantilever. We suggest that such a cantilever can be employed directly to achieve calibrated force measurements in AFM style instruments, or indirectly by acting as a calibration specimen, or transfer artifact, that can be probed with another cantilever. As a first step in our effort to explore these possibilities, we attempt to calibrate the force sensitivity of a commercially available cantilever. The results of this proof-of-concept experiment are presented and discussed. We find encouraging repeatability in the determination of force sensitivity, with deviations below a percent for a specific contact condition; however, our ability to reproduce the measurement is limited at the level of a few percent, presumably due to inconsistencies at the contact interface. Finally, we conclude with suggestions for improvements in the procedure and apparatus that might result in higher levels of reproducibility. Force balance and load cell calibration We have designed and built a prototype electrostatic force balance for realizing forces in the micronewton range, as illustrated schematically in Figure 1(a). The active electrodes are concentric cylinders, the outer serving as the reference and the inner suspended and guided by a rectilinear flexure mechanism. The geometry has been designed such that a near-linear capacitance gradient of 1pF/mm is achieved at a working overlap of 5 mm. Thus, an electrical potential across the cylinders produces an electronic realization of force along the suspension axis 2 2 1 V dz dC F  (1)  NSF/NIST Summer Undergraduate Research Fellow, currently at Physics Department, St. Marys College, MD 2003 Winter Topical Meeting - Volume 28 64