A highly stable noncontact SPM for surface profile measurement and its application to insulating samples

A noncontact scanning electrostatic force microscope (SEFM) for surface profile measurement is introduced, and applied to the profile measurement of insulating samples. The SEFM calculates the tip-sample distance with an algorithm named 'dual-height method', and compensates for the fluctuation of the tip-sample distance to obtain accurate surface profile image. Glass and acrylic, which are commonly applied to optical components, are prepared as the insulating samples, and bias voltage is applied to the samples with an electrode clamp. On the glass sample, electrostatic force is observed in the case that the bias voltage of 50 V is on. On the acrylic sample, however, electrostatic force is not observed regardless of the bias voltage. Surface profile measurement of the glass sample is demonstrated. The tip-sample distance image is created with the dual-height method from the data of the tip trajectories and the frequency shift signals obtained through the constant tip-sample interaction scan. The results show that the tip-sample distance is kept larger than 100 nm, which is more than ten times larger than that of conventional noncontact SPMs. The large tip-sample distance brings the SEFM a high robustness against the disturbances from environment, which would cause tip-sample collision in the conventional noncontact SPMs. The same area on the same sample is measured also in an atomic force microscope (AFM) mode, which is realized by setting the bias voltage 0 V. The SEFM image agrees with the AFM image quantitatively, showing that the SEFM with the dual-height method is applicable to noncontact profile measurement of the glass. On the other hand, it is difficult to utilize the SEFM in the profile measurement of the acrylic sample because the electrostatic force on the acrylic is not controllable with the voltage of the electrode clamp.