Spring Constant Calibration of Microcantilever by Astigmatic Detection System

For calibrating the spring constant of a microcantilever, the thermal fluctuation method has been widely used in atomic force microscope (AFM). For this method, the force curve must be first acquired to correlate the voltage response of the photodiode with the known cantilever deflection. However, it usually takes a long time to move the cantilever tip close to the sample, and is not adapted to calibrate a tipless cantilever. To realize an efficient way of detecting photodiode responses, we developed a calibration routine employing an astigmatic detection system (ADS). In comparison with the technique of optical beam deflection (or optical lever), the ADS is more sensitive to the vertical and less sensitive to the angular displacement of a cantilever. Therefore, its photodiode response can be directly associated with a predetermined vertical movement of the cantilever, without the need of any subtract. Our experiments have verified that the derived spring constants by the ADS method are close to the typical values of the cantilevers, and this method can be applied to tipless cantilevers. (C) 2012 The Japan Society of Applied Physics