True Topography AFM Scanning Using A Low Noise Z-Position Sensor

Initially, the scanner was controlled in open-loop scanning configuration with no feedback. The application of calibrated voltage would cause the expansion and contraction of the piezoelectric actuator causing the scanner to move accordingly. Very soon, the closed-loop scanning configuration was introduced to compensate for creep and hysteresis errors affecting the open-loop configuration. Additional position detectors were used in the closed-loop scanning to each scanner axis to further improve the performances, and voltage was applied to the scanner in order to rectify and maintain its position at the particular location.

The closed-loop scanning method became crucial for the accurate control of the scanner. However, the closed-loop feedback was restricted to the (horizontal) X- and Y-directions and not on the (vertical, height) Z-direction because of the significant noise level on the corresponding detector.

When the applied voltage to the Z-scanner was used as the measuring signal to profile the surface topography, the hysteresis of piezoelectric actuators resulted in errors.

The position error was known as edge overshot at the leading and trailing edges (Fig. 1a). Furthermore, as the piezoelectric actuator grew older, the voltage required regular calibrations. Piezo-creep was caused by the intrinsic materials property of a piezoelectric actuator that drove the Z-scanner. The only method of correcting this artefact was to use an independent position sensor that directly measured the topography.

Figure 1. a) Topography obtained from the voltage signal applied to the Z-scanner. The nonlinear nature of the piezoelectric scanner with creep and hysteresis introduced errors to the real topography. b) Topography from the low noise Z-position detector. Note, the noise level of the Z-position detector has to be insignificant to be used as the true sample topography.

Figure 2. a) A typical example of an AFM image with edge over-shoot phenomena. b) The line profile shows Z-scanner overshoots when it meets steep height changes.

Latest AFM Developments

The Park NX10 AFM features a very low-noise Z-position detector with 0.02 nm noise level (Fig. 3). The noise level is low enough to replace the applied voltage to the Z-scanner as the topography signal. The topography images (Fig. 4) obtained by Park XE-100 and Park NX10, show the same sapphire wafer surface. Both the topography images obtained from the Z-voltage signal (Figs. 4a and c) show the atomic steps of the material, while the image from the Z-position detector of the NX10 (Fig. 4d) shows the atomic steps with the same quality. The image obtained from the Park XE-100 (Fig. 4b) picked noise from the Z-position detector.

The benefits of the topography obtained by the low-noise Z-position detector is evident when a sample has a clear steep height change as shown in Figure 5. The 1-µm step-height Z calibration sample (TGZ04, Mikromasch) was determined using Park NX10. The obtained image by the applied voltage (Fig. 5a) shows the overshoot, making a flat top area look slanted. The overshoot is observed only at abrupt height changes, something that is not trivial to correct by software even with an increased order correction. The image acquired with the Z-position detector (Fig. 5b) on the other hand, has no such artefact, resulting in a true topography of the sample.

As the research demand for an increase in precision and resolution, technological advances are made. The closed-loop scan method, and the decoupled XY-Z scanner structure contribute to this need by improving the positioning accuracy of the scanner. The low-noise Z-position detector of Park NX10 gives a more accurate profile for the topography of the sample surface.

Figure 3. The noise level of the Z position detectors of (a) the Park XE-100 and of the (b) Park NX10, respectively.

Figure 4. The topography images of a sapphire wafer obtained from XE-100 and NX10.

From figure 4 it can be seen that the noise level of the Z-position detector of Park XE-100 is not enough to differentiate between the atomic steps of the sapphire wafer. However, the topography images of the same sapphire wafer obtained from the applied voltage to the Z-scanner (c) and the Z-position detector of Park NX10 (d), show the atomic steps. The height information from the Z-position detector shows identical noise level with the Z-voltage signal. The scan size of the all the images is 3 µm × 3 µm.

Figure 5. a) Topography of the 1-µm step-height grating obtained from the driving bias signal to the Z-scanner. b) Topography from the Z-position detector.

This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.

For more information on this source, please visit Park Systems Inc.