Using Piezo Flexure Stages to Correct Orthogonality for AFM Experiments

Table of Contents

Introduction
Measuring Orthogonality Error
nPoint DSP Controller

Introduction

nPoint offers an easy method for correcting the XY scanners’ orthogonality error for the most demanding scanning applications.

This capability is primarily used in applications such as Atomic Force Microscopy (AFM) by OEM customers. An external method for measuring the orthogonality error is needed before applying such corrections. The AFM application is used as an example in order to demonstrate this capability, whereas an nPoint XY scanner is used as the AFM scanner. It should be noted that some AFM software may have orthogonality correction capabilities. No external capabilities were used in this case.

Measuring Orthogonality Error

Orthogonality error measurement can be done by acquiring an image of a calibration grid. Figure 1 shows a 50 µm scan of a 10 µm calibration grid. The scanner has excellent orthogonality but can still be employed to demonstrate that the orthogonality behavior can be easily changed through nPoint GUI.

Measuring Orthogonality Error

Figure 1. 50 x 50 µm scan using an nPoint XY scanner without orthogonality correction.

Users can enter the number of degrees they want to change the orthogonality by, using the nPoint application GUI. The interface is simple and is shown in Figure 2. The user can select either axis (X or Y; Ch1 or Ch2) in order to “sheer” by a certain amount. The rotation capability will be included in the upcoming versions.

Measuring Orthogonality Error

Figure 2. Sheer correction GUI. The user can pick either Ch1 or Ch2 and apply a certain “sheer” which will change the angle of one axis with respect to the other.

The same grid was used and a 4° sheer (Figure 3a) and a 9° sheer (Figure 3b) were applied. This is shown in the images below.

AFM images of a calibration grid

Figure 3a. AFM images of a calibration grid after a 4° sheer has been applied using the nPoint GUI.

AFM images of a calibration grid

Figure 3b. AFM images of a calibration grid after a 9° sheer has been applied using the nPoint GUI.

nPoint DSP Controller

The orthogonality correction illustrated above is just one example of what would generally be known as a co-ordinate transformation function. This type of function can be used to sheer one axis with regard to the other (as shown), to rotate the coordinate system by a certain angle in order to map three planar actuators into an X, Y, theta configuration or three Z actuators into a tilt, tip and Z movement combination.

The nPoint DSP controller does not treat each piezo driver and sensor circuit as part of a single physical axis to achieve such co-ordinate transformations, but instead it performs a linear mapping using multiple channels. The output of the control loop is changed back into the real world system in order to run the individual physical axes.

This information has been sourced, reviewed and adapted from materials provided by nPoint | Nanopositioning and Motion Control.

For more information on this source, please visit nPoint | Nanopositioning and Motion Control.

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