Accurate analytical measurements in the atomic force microscope: a microfabricated springconstant standard potentially traceable to the SI

Abstract

Calibration of atomic force microscope (AFM) cantilevers is necessary for the measurementof nanonewton and piconewton forces, which are critical to analytical applications of AFMin the analysis of polymer surfaces, biological structures and organic molecules at nanoscalelateral resolution.We have developed a compact and easy-to-use reference artefact for this calibration, usinga method that allows traceability to the SI (Système International). Traceability is crucial toensure that force measurements by AFM are comparable to those made by optical tweezersand other methods. The new non-contact calibration method measures the springconstant of these artefacts, by a combination of electrical measurements andDoppler velocimetry. The device was fabricated by silicon surface micromachining.The device allows AFM cantilevers to be calibrated quite easily by the ‘cantilever-on-reference’method, with our reference device having a spring constant uncertainty of around ± 5% at one standard deviation. A simple substitution of the analogue velocimeterused in this work with a digital model should reduce this uncertainty to around ± 2%.Both are significant improvements on current practice, and allow traceability to the SI forthe first time at these nanonewton levels.