System for Traceable Calibration of Nanonewton Forces and Force vs. Deformation Curves

Abstract

The paper discusses a system for the measurement and calibration of nanonewton forces and force vs. deformation curves of micro force sensors such as AFM Cantilevers. The system bases on an electromagnetic compensated microbalance whereas its control loop was modified to generate and measure the force at the same time without the need of an additional actuator. Thus, the concept is similar to the NIST Electrostatic Force Balance where the force is generated and measured based on a capacitor. To probe the micro force sensor under calibration, the balance pan is moved by feeding a defined current through the balance internal coil. Thereby the current is proportional to the acting force which was calibrated with milligram mass artifacts before. The movement of the balance pan as well as the position of the micro force sensor under test is measured with a special triple beam interferometer. Hence, the calibration of forces and force vs. deformation curves is traceable to the SI units mass and length. To compare our concept of force generation to the concept of PTB and KRISS an additional piezo actuator was integrated in the setup. Thus, we can alternatively push the micro force sensor under test onto the balance pan using the piezo. Measurement and calibration results will be presented and discussed. A standard deviation of < 1 nN is achieved for the measurement of constant forces. The stiffness (force vs. deformation curve) of a contact mode AFM cantilever was calibrated to be 0.26 N/m with a repeatability of 0.25 %. Thereby linearity deviations of < 1 nN were observed for this measurement. Due to the uncertainty of the contact parameters (contact angle and position as well as friction) between the AFM tip and the stylus mounted on the balance pan the combined uncertainty of the calibration is in the range 3 %.