A New Microdevice for SI-Traceable Forces in Atomic Force Microscopy | NIST

TLDR: In this paper, a self-excited micro-oscillator is proposed as a velocity standard for dissemination of nanonewton-level forces that are traceable to the International System of Units (SI).

Abstract: A new self-excited micro-oscillator is proposed as a velocity standard for dissemination of nanonewton-level forces that are traceable to the International System of Units (SI). The microfabricated oscillator is top-coated with magnetic thin films and closely surrounded with conductive microwires to enable both magnetic sensing and actuation. An analog control system will keep the actuation side of the device oscillating sinusoidally with a frequency up to 200 kHz and a nanometerlevel amplitude that is fairly insensitive to the quality factor. Consequently, the device can be calibrated as a velocity standard in air and used in ultra-high vacuum with a velocity shift of less than one percent. Because of the nanometer-level oscillation amplitude, the microdevice could be used to probe capacitance gradients near tips of cantilevers used for atomic force microscopy (AFM). Hence, the calibrated micro-oscillator could be used with electrostatic forces to calibrate AFM cantilevers as SI-traceable force transducers for fundamental metrology of electrical and mechanical nanoscale quantities. Introduction Since the development of AFM [1], atomic resolution was achieved over a decade ago [2,3] and the method has been used to characterize electrical, magnetic, and mechanical properties of materials. However, commercial AFM suffers from a lack of accurate force measurements because there is presently no method to disseminate SI-traceable nanonewton-level forces to most AFM users. This situation concerns AFM users who need to measure and control the small forces between an AFM cantilever tip and the substrate surface, e.g., in single-molecule force spectroscopy [4,5]. Traceability is also needed to compare AFM force measurements to those made by optical tweezers and other methods. Otherwise, the reliable fabrication and testing of future microscale devices may be in doubt. Currently, no method exists for the SI-traceable calibration of AFM cantilevers in various AFM environments. Tip-sample forces are usually calculated by Hooke’s law (F = kx) with an estimated cantilever stiffness (k) and a measured tip deflection (x). For example, dynamic methods for estimation of the stiffness usually rely on the thermal noise spectrum [6, 7], the resonant frequency shift with added mass [8], or the resonant frequency with knowledge of the cantilever density and dimensions [9]. While possibly being efficient or available for in situ calibration, these methods do not yield SI-traceable cantilever stiffnesses because of the lack of SI-traceable forces. Alternatively, traceable forces from calibrated masses [10] or reference cantilevers calibrated with an electrostatic force balance [11] may be used for static cantilever calibrations, but these methods are usually not efficient or available for in situ calibration, particularly in extreme cryogenic environments. Proposed Device A new self-excited oscillator is proposed to allow SI-traceable calibrations of AFM cantilevers in several environments. The proposed micro-oscillator is composed of a sensing side and an actuation side, as outlined in Fig. 1(a). The actuation side is attached to a rigid substrate (not shown) through two flexures, while the sensing side is attached to the actuation side by a thin flexure. Magnetic sensing and actuation are possible because both sides of the device are top-coated with magnetic thin films of nickel and closely surrounded by chrome/gold microwires. Accordingly, the rotational velocity of the sensing side is observed in the sensing current according to Faraday’s law of magnetic induction, while the interaction of the actuation current with the magnetic thin film produces a torque on the actuation side. Proceedings of the XIth International Congress and Exposition June 2-5, 2008 Orlando, Florida USA ©2008 Society for Experimental Mechanics Inc.