Researchers in the Nanomechanical Properties Group, Ceramics Division, MSEL and the Mass and Force Group, Manufacturing Metrology Division, MEL are collaborating to produce a new reference artifact that can be used to calibrate the spring constant (the stiffness = force/displacement ratio) of atomic force microscope (AFM) cantilevers with SI-traceable accuracy.
AFM is a popular, versatile, research technique that uses a small probe (a cantilever supporting a tip) to interrogate and measure the topography and mechanical properties of surfaces down to the nanoscale. One limitation in interpreting the data and images generated by AFM probes at these very sensitive scales is the lack of accurate determination of the spring constant of the cantilever used to manipulate and control the tip. Manufacturers’ specifications for commercial AFM cantilevers provide a stiffness range of a factor of two to four. Techniques are available to narrow the precision of the stiffness but with unknown accuracy as the techniques are not SI traceable.
The NIST research team has microfabricated a prototype cantilever array that could provide an SI-traceable calibration pathway for AFM probe stiffness. The device consists of a series of uniform cantilevers of different stiffness in the range 0.02 N m-1 to 0.3 N m-1. Calibration using an electrostatic force balance (EFB), traceable to SI base units of length, time, and electrical current was achieved with an uncertainty of better than 2 % at 0.02 N m-1. Uniformity of a number of devices processed as a batch on a Si wafer was determined by measuring the cantilevers’ first flexural mode resonance frequency. Variation in the resonance frequency for cantilevers on different devices was found to be less than 1 % (relative standard deviation). These results indicate the feasibility of microfabricating large numbers of extremely uniform cantilevers and calibrating a statistical subset of the population using EFB. The paper “Prototype cantilevers for SI-traceable nanonewton force calibration,” Meas. Sci. Technol. 17 (2006) 2852–2860 was awarded the 2006 “Best Paper of the Year Award in Precision Measurement” in the journal Measurement Science and Technology.
Work is progressing in the development and production of a forthcoming cantilever array standard reference material (SRM 3461) that will be calibrated using the SI traceability of the EFB and will provide a pathway for accurate and precise calibration of AFM cantilever stiffness.