New reference standards and artifacts for nanoscale physical property characterization Jon R. Pratt, John A. Kramar, Gordon Shaw, Richard Gates, Paul Rice, and John Moreland NIST, MEL, Bldg 217, Rm B137, Gaithersburg, MD 20899, jon.pratt@nist.gov NIST, MEL, Gaithersburg, MD, john.kramar@nist.gov , gordon.shaw@nist.gov NIST, MSEL, Gaithersburg, MD, richard.gates@nist.gov NIST, MSEL, Boulder, CO, paulrice@boulder.nist.gov NIST, EEEL, Boulder, CO, moreland@boulder.nist.gov ABSTRACT This paper provides an overview of calibration artifacts being developed at the National Institute of Standards and Technology (NIST) that are intended to aid the accurate determination of nanoscale physical properties across a broad range of applications. We focus on three proposed reference standards: an SI traceable spring constant artifact for calibration of atomic force microscope cantilever stiffness in the nominal range between 0.02 N/m and 0.2 N/m, a piezoresistive force sensor for SI calibration of micronewton level contact forces, and a torsional oscillator for the absolute measurement of thin-film magnetic moments on the order of 1 μA m 2 . Keywords: atomic force microscopy, cantilever spring constant measurement, magnetic moment measurement, standard references and practices 1 INTRODUCTION Products incorporating “new nanotechnology” are projected to generate one trillion dollars in revenues within the next decade [1]. Length [2] and force metrologies [3] are being developed at the National Institute of Standards and Technology (NIST) in anticipation of this trend. These metrologies are the basis for reference standards traceable to the International System of Units (SI) that can help calibrate scanning probe microscopes to measure physical properties in a traceable fashion. Artifacts traceable to well-defined length standards are already available from commercial sources to assist atomic force microscope (AFM) users with calibrating length measurements [4], and techniques to characterize probe tip geometry have been demonstrated to broad acceptance [5]. But accurate artifacts and techniques for calibrating AFM force sensitivity are still emerging [3]. We have shown that the force sensitivity of an AFM can be calibrated in a traceable fashion using a piezoresistive cantilever sensor that is first calibrated using an electrostatic force balance (EFB) [6]. In section 2, we give results using a new version of our EFB to calibrate a similar sensor, while in sections 3, 4, and 5 we report our efforts to create reference materials with calibrated physical properties. 2 FORCE SENSOR CALIBRATION The force sensitivity and stiffness of a Kleindiek force measurement system * (FMS-MS, Kleindiek nanotechnik, Reutlingen, Germany) were calibrated using an EFB in a force versus displacement measurement mode like that proposed in Ref. 3. The goal was to calibrate a force sensor to verify the mean contact force applied during mechanical property measurements using atomic force acoustic microscopy (AFAM [7]). 2.1 Experiment Figure 1 shows the force sensor in its holder, positioned near the latest NIST EFB. The sensing element is a 400 μm long, piezoresistive micromachined cantilever (too small to be visible in photo). The complete force measurement system includes an external bridge amplifier that measures changes in the electrical resistance of the piezoresistive element when forces are applied at the cantilever’s free end. A long standoff microscope was employed to observe the free end of the cantilever as it made contact with a 2 μm radius Hysitron cono-spherical indenter tip that was mounted on the balance’s weighing pan. A three-axis, fine motion stage was used for the gross manipulation, and then the balance null-control setpoint was varied to bring the indenter tip into contact. Contact was determined visually and also by monitoring the output of the force system’s measurement bridge, which was nulled before contact. Experiments were conducted in air, with the chamber sealed. The initial contact force was approximately 2.5 μN. A computer automated data acquisition and control routine incremented the balance displacement setpoint while maintaining feedback control. The resulting force, * Certain commercial equipment, instruments, or materials are identified in this article in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose. 764 NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-6-5 Vol. 1, 2006