A conventional Optical Pickup Unit (OPU) based atomic force microscope (AFM) suffered signal drift during imaging due to structural problems of OPU. The drift is caused by thermal deformation occurring in the OPU or Cantilever tip. So, when we image specimens with an OPU-based AFM, we have to wait until the drift is stabilized. Nevertheless, measurement results may be distorted due to signal drift during imaging. Therefore, we set the goal to solve this problem.
To solve the problem, we utilized the voice coil motor (VCM) included in OPU. When the drift occurs, the position of the objective lens of the OPU is controlled by the VCM to compensate for the signal change. The input signal to drift compensation controller was filtered by a low pass filter(LPF) to prevent the drift controller from responding to the height change of the specimen. As usual the topography signal changes faster than the signal drift and it is filtered by LPF. As a result, the drift compensation controller will not affect the specimen height. We applied this controller to OPU based AFM and could get drift free images.