The optical beam deflection sensor is generally used to detect cantilever deflection in atomic force microscope (AFM). In the method, a cantilever is irradiated with laser beam from just above and the laser is reflected to a segmented photo detector. Thus, the cantilever deflection is detected as the displacement of the laser spot on the detector. In some AFM apparatuses, a cantilever is mounted on a vertical scanner which controls the cantilever displacement in the direction vertical to sample surface (Z displacement) for topographic imaging. This setup is useful for imaging large-scale objects. However, the cantilever deflection and the Z displacement move the laser spot on the detector along the same vertical axis. This crosstalk often causes instability of the system. In an optical beam deflection sensor developed by the inventors employs oblique incidence setup. Figure shows the relation between the position of a laser diode and a segmented photodetector. Here a laser beam is brought diagonally to the cantilever and is reflected diagonally so that a difference is made between the moving direction of the laser spot caused by the Z displacement and that caused by the deflection. In this method, the moving direction by the deflection and the Z displacement can be separated.  

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