The intrinsic nonlinearity of a piezoelectric material is the ratio of the maximum deviation y from the linear behavior to the ideal linear extension y at that voltage. In other words, the nonlinearity is the change in y divided by y, expressed as a percentage. Intrinsic nonlinearity typically ranges from 2% to 25% in the piezoelectric materials used in SPM systems.
In the plane of the sample surface, the effect of intrinsic nonlinearity is distortion of the measurement grid of Figure 2-1. Because the scanner does not move linearly with applied voltage, the measurement points are not equally spaced. As a result, an SPM image of a surface with periodic structures will show non-uniform spacings and curvature of linear structures. Less regular surfaces may not show recognizable distortion, even though the distortion is present.
Perpendicular to the plane of the sample surface (in the z direction), intrinsic nonlinearity causes errors in height measurements. Height is usually calibrated for SPMs by scanning a sample with a known step height. By reading the voltage applied to the z electrode as it traverses the step, the z component of the strain coefficient can be calculated. However, if that strain coefficient is directly applied to measure a feature different in height from the calibrated height, the intrinsic nonlinearity of the scanner will generate errors.
