Piezoelectric material has been widely used as the actuator of the positioning scanner in an atomic force microscope (AFM) since the invention of the AFM [1]. It has been used not only in academic instrumentation, but in commercial AFM products because of its simplicity in structure, and the possibility to move it continuously on the nanometer scale. However, it has also been associated with hysteresis and creep errors that prevented conventional AFMs from providing an accurate topography of a sample. Park has now developed the Park NX10 featuring a very low noise Z sensor that enables the true topography AFM scanning of a sample. In the early stage of AFM development, the open-loop scanning method was used to control the scanner. Here, the scanner moves as the piezoelectric actuator expands and contracts in response to the calibrated applied voltage. This was soon replaced by the closed-loop scanning method [2] because the open-loop method introduced nonlinear errors of hysteresis and creep errors to the AFM image. The closed-loop scanning method employed additional position detectors to each axis of the scanner, and applied a voltage to the scanner to correct and keep its position at the desired location. Subsequently, the closed-loop scanning method became indispensible for accurate scanner control. However, the closed loop feedback was limited to the X and Y directions of a sample surface and not on Z (height) direction due to the significant noise level of its Z position detector.
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