Quantitative analytical atomic force microscopy: a cantilever reference device for easy and accurate AFM spring-constant calibration

Calibration of atomic force microscope (AFM) cantilevers is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules. We have developed a compact and easy-to-use reference artefact for this calibration by bulk micromachining of silicon, which we call a cantilever microfabricated array of reference springs (C-MARS). Two separate reference cantilever structures, each nominally 3 μm thick, are fabricated from a single crystal silicon membrane. A binary code of surface oxide squares (easily visible in light, electron and atomic force microscopy) makes it easy to locate the position of the AFM tip along the length of the cantilevers. Uncertainty in location is the main source of error when calibrating an AFM using reference cantilevers, especially for those having spring constants greater than around 10 N m−1. This error is effectively eliminated in our new design. The C-MARS device spans the range of spring constants from 25 N m−1 down to 0.03 N m−1 important in AFM, allowing almost any contact-mode AFM cantilever to be calibrated easily and rapidly.