Report on the first international comparison of small force facilities: A pilot study at the micronewton level
Jon R. Pratt, Min-Seok Kim, Uwe Brand, Christopher Jones
Measurements of forces less than a micronewton are critical when examining the mechanical behaviors of materials and devices at characteristic length scales below a micrometer. As a result, standards for nanomechanical tests and test equipment are being proposed within international standards organizations, and an infrastructure for traceable small force calibration is developing. In this context, we report results from the first inter-laboratory comparison of micronewton-level force metrology. The basis of the comparison was a set of five piezoresistive cantilever force sensors similar to ones used for atomic force microscopy (AFM) but employed here as transfer artifacts. The artifacts were circulated among four National Metrology Institutes (NMIs) with each NMI using their own force balance to calibrate the stiffness (force change per unit displacement) and sensitivity (signal output change per unit force) of the artifacts. By comparing the measurements reported by the NMIs, consensus values were obtained with a relative standard deviation well below a percent in most cases. The largest contributing uncertainty sources were due to the transfer artifacts themselves, rather than from the measurements of the physical quantities of force, voltage, and displacement. The results imply that cantilever stiffness might be determined using force balance techniques with accuracy better than a percent if necessary, but that more advanced transfer artifacts are required if the resolution of future comparisons is to improve.