The application of atomic force microscopy to go beyond topographic imaging and accurately measure nanomechanical properties of materials often depends on being able to accurately calibrate the stiffness of the cantilevers used. There are techniques available to provide these calibrations in the field but their uncertainties are often in the double digits (i.e., ± 10% - 30%). The laser Doppler vibrometer instrument in the Nanomechanical Properties Group has been optimized to provide very accurate measurements with uncertainties closer to ± 1% - 2%.
The laser Doppler vibrometer (LDV) system used for calibrating the spring constant of AFM cantilevers is housed 12 m underground in the Nanomechanical Properties Group’s clean room lab in the Advanced Measurement Laboratory (AML) at NIST. The lab affords a clean, low vibration environment with good temperature and humidity control. The LDV is additionally vibration isolated through an air table. The instrument itself consists of a dual beam (sample and reference laser spot) microscope with the capability to measure a frequency spectrum for a cantilever with sub-picometer deflection sensitivity. By measuring the vibrational spectrum under pure thermal excitation, the equipartition theorem can be invoked and used to estimate the stiffness of the cantilever by measuring the area under the resonance frequency peak of the vibrational spectrum.
The utility of the NIST implementation of the LDV Thermal method has evolved over the years as refinements were made to tailor the measurements to specific cantilevers. It was first demonstrated for ideal rectangular cantilevers (1), then triangular cantilevers, and tipped cantilevers (2) and even colloid probe cantilevers which have spheres attached (3). A significant breakthrough in accurately measuring the stiffness of non-ideal (i.e., nor rectangular and tipped) cantilevers was achieved in 2013 with the discovery of an experimental means of measuring the mode correction factor which is needed to convert the dynamic stiffness (measured by the LDV) to the static stiffness needed for static force measurements in the AFM. As a result, the technique, as implemented at NIST, has achieved superb accuracy as demonstrated by comparison of cantilever calibration values to an SI traceable independent measurement using the NIST Electrostatic Force Balance (EFB). Over the years, LDV and EFB measurements have been compared on over a dozen cantilevers spanning three decades of stiffness (nominally 0.01 N/m through 10 N/m) with discrepancies averaging about a percent.
Most recently, the NIST LDV Thermal method was used to experimentally confirm the form of the cantilever stiffness tilt correction as:
where ke is the effective stiffness of a cantilever, ki is the intrinsic stiffness of a cantilever and Θ is the inclined angle of the cantilever (4).
The LDV Thermal calibration method is also being used to calibrate the stiffness of a new NIST SRM (3461 – Standard Reference Cantilevers for AFM Spring Constant Calibration). Initial devices have been successfully fabricated and are in the process of being accurately calibrated.