Report on the first international comparison of small force facilities: a pilot study at the micronewton level

Abstract

Measurements of forces less than a micronewton are critical when examining the mechanical behaviour of materials and devices at characteristic length scales below a micrometre. As a result, specification standards for nanomechanical tests and test equipment are being proposed by international standards organizations, and an infrastructure for traceable small force calibration is developing. In this context, results are reported from the first interlaboratory comparison of micronewton-level force metrology. The basis of the comparison was the calibration of a set of five piezoresistive cantilever force sensors similar to those used for atomic force microscopes but employed here as transfer artefacts. The artefacts were circulated among four national metrology institutes with each using their own force balance to calibrate the stiffness (force change per unit displacement) and sensitivity (signal output change per unit force) of the artefacts. By considering the weighted mean of the stiffness and sensitivity values reported for a given artefact, reference values were obtained. The largest contributing uncertainty components were due to the transfer artefacts themselves, rather than from the measurements of the physical quantities of force, voltage and displacement. The results imply that it should be possible to determine cantilever stiffness using force balance techniques with an accuracy of better than 1% if necessary, but that improvements in the ability to orient the transfer artefacts, to characterize the non-linearity of their output, and to compensate for the stiffness of the associated fixtures and load frames are required if the resolution of future comparisons is to improve.