A Piezoresistive Cantilever Force Sensor for Direct AFM Force Calibration
- PDF / 11,957,420 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 80 Downloads / 257 Views
1021-HH02-03
A Piezoresistive Cantilever Force Sensor for Direct AFM Force Calibration Jon R. Pratt1, John A. Kramar1, Gordon A. Shaw1, Douglas T. Smith1, and John M. Moreland2 1 National Institute of Standards and Technology, Gaithersburg, MD, 20899 2 National Institute of Standards and Technology, Boulder, CO, 80305
ABSTRACT We describe the design, fabrication, and calibration testing of a new piezoresistive cantilever force sensor suitable for the force calibration of atomic force microscopes in a range between tens of nanonewtons to hundreds of micronewtons. The sensor is calibrated using the NIST Electrostatic Force Balance (EFB) and functions either as a force reference or stiffness artifact that is traceable to the International System of Units. The cantilever has evenly spaced fiducial marks along its length. We report stiffnesses that vary quadratically with location, from a high of 12.1 N/m at the first fiducial to a low of 0.394 N/m at the last; with force sensitivities that vary linearly, ranging from 18.1 Ω/mN to 106 Ω/mN. We also test the device to transfer the unit of force to an atomic force microscope, finding that force and stiffness based approaches yield independent estimates of the contact force consistent within 2 % of each other. INTRODUCTION The desire among researchers and industry to measure forces that are traceable to the International System of Units (SI) using an atomic force microscope (AFM) has prompted national metrology institutes to explore new metrologies, including sub-micronewton force realizations [1,2], a quantum-based piconewton [3], a microelectromechanical nano-balance [4], and a variety of reference cantilevers with techniques and apparatuses for calibrating their stiffnesses and/or force sensitivities [5-9]. NIST has shown that one method for obtaining a traceable force calibration is to simply press the AFM probe against a calibrated piezoresistive cantilever force sensor [9]. This approach bypasses the problems associated with calibrating the optical lever as a displacement sensor [10]. Previously, we were constrained by the size and mechanical stiffness of the piezoresistive cantilevers that were commercially available. Here, we seek to address this limitation, and present the design, calibration, and performance testing of a new piezoresistive cantilever force transducer, or PFT, fabricated at NIST. DESIGN AND FABRICATION OF THE PIEZORESISTIVE FORCE TRANSDUCER Piezoresistive cantilever force sensors for AFM were first demonstrated by Tortonese, et al. [11] and a low frequency force sensitivity of 8 fN was achieved by Harley and Kenny [12]. Typically, two legs support a beam, with the sensors on the surface of the legs. If the legs are thinner than the beam, they form a hinge where the bending occurs, increasing sensitivity [13]. The NIST PFT shown in Figure 1 employs this last approach, and is akin to devices proposed by the Physikalisch Technische Bundesanstalt (PTB) in Germany [6], and by the National Physical Laboratory in the UK [8] for AFM force and stiffness c
Data Loading...
