Scanner Artifacts
by Paul West and Natalia Starostina Scanners that move the probe in an atomic force microscope in the X, Y and Z directions are typically made from piezoelectric ceramics. As electromechanical transducers, piezoelectric ceramics are capable of moving a probe very small distances. However, when a linear voltage ramp is applied to piezoelectric ceramics, the ceramics move in a nonlinear motion. Further, the piezoelectric ceramics exhibit hysteresis effects caused by self-heating. Artifacts can also be introduced into images because of the geometry of the scanner. The positioning of the scanner relative to the sample can also create artifacts. Probe/Sample Angle
If the features that are being imaged by the AFM are much larger in profile than the probe, and the image does not seem “correct”, the artifact may be caused by a non-perpendicular probe surface angle. Ideally, the probe of the microscope should be perpendicular to the surface.  Figure 8: In this example the probe is much sharper than the feature it is scanning across and should give a correct image. However, because of the extreme probe sample angle, the line profile will show an artifact at the left edge of the feature. Solving this problem is achieved by adjusting the angle between the probe and the sample so that they are perpendicular. In some microscopes the probe is designed to be at a 12 degree angle with respect to the sample. Also some AFM microscopes do not have mechanical adjustments to control the probe/sample angle. X-Y Calibration/Linearity
All atomic force microscopes must be calibrated in the X-Y axis so that the images presented on the computer screen are accurate. Also the motion of the scanners must be linear so that the distances measured from the images are accurate. With no correction, the features on an image will typically appear smaller on one side of the image than on the other.  Figure 9A-B: A test pattern with squares, shown in figure 9A, will appear severely distorted if the piezoelectric scanner in the AFM is not linear as in 9B. Once the scanner is properly linearized, it is also critical that the scanner be calibrated. For example it is possible for the scanner to be linear but not calibrated. If the calibration is incorrect, then the X-Y values measured from line profiles will be incorrect.  Figure 10: This AFM image of a test pattern is very linear. The spacing of the squares at the top, bottom, left and right sides are all the same distance apart. It appears as it should. A common method for correcting the problems of X-Y non-linearity and calibration is to add calibration sensors to the X-Y piezoelectric scanners. These sensors can be used to correct the linearity and the calibration in real time. Z Calibration/Linearity
Height measurements in an AFM require that the piezoelectric ceramics in the Z axis of the microscope be both linear and calibrated. Often the microscope is calibrated at only one height. However, if the relationship between the measured Z height and the actual Z height is not linear, then the height measurements will not be correct.  Figure 11: This graph shows the relationship between an actual Z height and a measured Z height in an atomic force microscope. Often only one calibration point is measured as shown by the grey circle, and the Z ceramic is assumed to be linear, as shown by the blue line. However, as is often the case, the ceramic is nonlinear, as shown by the red line. In such cases incorrect Z heights are measured with the microscope unless the feature being measured is close to the calibration measurement. Background Bow/Tilt
The piezoelectric scanners that move the probe in an atomic force microscope typically move the probe in a curved motion over the surface. The curved motion results in a “Bow” in the AFM image. Also, a large planar background or “Tilt” can be observed if the probe/sample angle is not perpendicular. Often the images measured by the AFM include a background “Bow” and a background “Tilt” that are larger than the features of interest. In such cases the background must be subtracted from the image. This is often called “leveling” or “flattening” the image. After “leveling” the desired.  Figure 12: An AFM piezoelectric scanner is often supported at the top by a mechanical assembly. Thus the motion of the probe is nonlinear in the Z axis as it is scanned across a surface. The motion can be spherical or even parabolic depending on the type of piezoelectric scanner.   Figure 13A-B: Image (A) is an 85 × 85 micron image of a flat piece of silicon. The bow introduced into the image is seen at the edges. (B) A line profile across this image shows the magnitude of the bow. Z Edge Overshoot
Hysteresis in the piezoelectric ceramic that moves the cantilever in the perpendicular motion to the surface can cause edge overshoot. This problem is most often observed when imaging micro-fabricated structures such as patterned Si wafers or compact discs. The effect can cause the images to be visually better because the edges appear sharper. However, a line profile of the structure shows errors.  Figure 14A-B: (A) The probe is scanned from left to right across a feature on a surface. (B) Overshoot may be observed in the line profile at the leading and trailing edge of the structure.   Figure 15 A-B: (A) The AFM image of a test pattern appears to have no artifacts. (B) However, a line profile of the test pattern shows overshoot at the top of each of the lines. Scanner Drift Drift in AFM images can occur because of "creep" in the piezoelectric scanner and because an AFM can be susceptible to external temperature changes. The most common type of drift occurs at the beginning of a scan of a zoomed-in region of an image. This artifact causes the initial part of a scan range to appear distorted. Drift artifacts are most easily observed when imaging test patterns. Drift will cause lines that should appear straight to have curvature.  Figure 16: After a region of a sample is scanned with the AFM it is common to “zoom” into a small section of the image to get a higher magnification of an image. Scanner drift will cause the image to appear distorted at the beginning of the scan.  Figure 17: Zoomed image showing a distortion at the beginning of the scan. The scan angle is 45 degrees. X-Y Angle Measurements
If the motion generated by the X-Y scanner is not orthogonal, then there can be errors in the horizontal measurements in an image. This error, or artifact, can best be seen when imaging a test pattern with squares. The error in orthogonality can be measured by using a straight edge to measure “orthogonal” lines in the images.  Figure 18: The blue lines drawn on this image show that the scanner has no measurable cross-talk between the X and the Y axis. The lines are orthogonal. Z Angle Measurements
Mechanical coupling between the piezoelectric ceramics that move the probe in the X or Y directions and the Z direction can cause substantial errors when trying to measure side wall angles with the AFM. This error can best be measured with a sample that has repeating triangular structures.  Figure 19 A-B: (A) This cross section is an ideal sample for demonstrating the ability of an AFM to measure angles. The sample has a series of repeating triangles at its surface. (B) A line profile of the sample shows that the triangles do not appear symmetric.   Figure 20 A-B: (A) The AFM image of a sample having a triangle pattern at its surface. (B) A line profile extracted from the AFM image.
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