A nondestructive calibration method for maximizing the range and accuracy of AFM force measurement

In this paper, a nondestructive method for the normal and lateral sensitivity calibrations of the optical lever in atomic force microscope (AFM) is presented. The practical application of this method in a dual-probe AFM is discussed in detail. To calibrate the conversion factors between photodiode responses and probe's deflection angles accurately without applying forces to the probe, a two-degrees-of-freedom flexure-hinge-based calibration device (FHCD) is developed. The device, which mainly consists of two mutually perpendicular flexure-hinge levers that share the same rotational center, serves as a switching mechanism for precise translation-to-rotation conversions both in the normal and lateral directions. During the calibration, a probe is attached to the FHCD at the meeting of the rotational axes of two levers. The FHCD is mounted on an AFM sample platform. The probe in this method acts as a mirror to be normally and laterally tilted at nanoscale angles to deflect the reflected laser beam by twisting the corresponding flexure-hinge levers, rather than the force-inducing probe deflection in traditional AFM calibration methods. With this method, the nondestructive calibration of the local and full-range sensitivities of the optical levers can be completed without destroying the probe tip or modifying the actual system setup of an AFM. Moreover, the nonlinearities of the optical levers are accurately compensated. Experimental results show that the linear ranges (with a deviation of 5% in the full range) of the force measurement are extended to 3.6 and 4.5 times in the normal and lateral directions, respectively, increasing to over 90% of the full range of the force measurement.