A new method for improving the accuracy of SPM and its application to AFM in liquids

In this paper presents an accurate AFM used in liquid that is free from the Z-directional distortion of a servo actuator is described. Two correction methods are employed in this AFM. One is the external-monitoring method, an d the other is the in situ self-calibration method. The external-monitoring method is based on the fact that PZT actuators from the same class show si milar hysteresis patterns. The same voltage signal used to control the serv o actuator is applied to another PZT actuator which is termed the reference actuator. The displacement of the reference actuator is measured by a disp lacement sensor. As a result, the distortion of the AFM image can be correc ted satisfactorily. In the in situ self-calibration method, the derivative of the calibration curve function of the PZT actuator is calculated from th e profile measurement data sets which are obtained by repeating measurement s after a small Z-directional shift. Input displacement at each sampling po int is approximately estimated first by using a straight calibration line. The derivative is integrated with reference to the approximate input to obt ain the approximate calibration curve. Then the approximation of the input value of each sampling point is improved using the obtained calibration cur ve. Next the integral of the derivative is improved using the newly estimat ed input values. As a result of repeating these improving process, the cali bration curve converges to the correct one, and the distortion of the AFM i mage can be corrected. Principles and experimental results of the two metho ds are presented.