Creep and Bow

The qualities of Piezoelectric Scanners that make them useful for nanoscale positioning also lead to certain functional characteristics that must be appropriately managed or compensated for when collecting data.

Creep

Creep is the drift of the piezo displacement after a DC offset voltage is applied to the piezo. This may occur with large changes in X and Y offsets or when using the frame up and frame down commands when the piezo travels over most of the scan area to restart the scan.

When a large offset is performed, the scanner stops scanning and a DC voltage is applied to the scanner to move the requested offset distance. However, the scanner does not move the full offset distance all at once. It initially moves the majority of the offset distance quickly, and then slowly moves over the remainder. The scanning resumes after a majority of the offset distance has been moved although the scanner is still slowly moving in the direction of the offset. Creep is the result of this slow movement of the piezo over the remainder of the offset distance once scanning has resumed.

Creep appears in the image as an elongation and stretching of features in the direction of the offset for a short period of time after the offset:

An example of creep is shown in the above image of a calibration grating. The tip was scanning from top to bottom and an offset of 10 μm in the X direction was performed near the beginning of the scan (indicated by the arrow). The slight bending of the lines that occurs directly after the offset is due to creep. You can see that the creep settles out by the end of the scan.

When creep appears in the image, it will often settle out by the end of the scan and the next image can be captured. For very large offsets (>50 μm), it may take longer than 1 scan for the creep to settle out. Creep can be reduced by offsetting beyond the desired point and then offsetting back to the desired point.

Bow

Bow is scanner motion out of the ideal plane. Piezo scanners—whether tube scanners like the Dimension Icon scanner, or flexure scanners—do not move in an entirely level XY plane. Instead, the motion of the scanner often results in 2nd order or 3rd order curvatures (as shown below). The specific bow characteristic displayed in the image depends on the scanner type. Bow increases with scan size and can be easily removed from a captured image by using Modify Planefit, Manual or Auto.

Figure 1: An example of 3rd order bow

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