Scanner bow is the relevant artefact. A piezoelectric tube scanner bends to produce lateral displacement; its moving end also follows a curved vertical trajectory. Cross-coupling of lateral and vertical motion therefore gives a roughly quadratic height background even over a flat reference. Over a wide scan it can mimic or conceal a genuine parabolic sample profile; its sign depends on scanner geometry and height convention.
Common processing fits and subtracts a low-order polynomial background, often a quadratic surface or a polynomial for each scan line. This AFM image flattening removes scanner curvature on an assumed flat sample. Here the desired specimen signal is itself roughly quadratic, so such subtraction can remove the actual bulge along with the artefact. The two contributions cannot be separated by polynomial fitting to this image alone. A plane subtraction removes tilt but does not independently calibrate curvature.
Measure a genuinely flat calibration reference over the same scan range, scanner position and operating conditions, then subtract its reproducible bow background from the specimen data. Retain reference flat regions outside the feature if available, fitting only those regions rather than the bulge. A scanner with independently measured vertical motion or suitable closed-loop correction reduces the instrumental uncertainty. Translating or rotating the specimen checks whether a feature follows the specimen or the scanner coordinates; independent optical or stylus profilometry can corroborate its height. For a rotationally symmetric parabola, rotation alone is not sufficient. Report drift and calibration uncertainty as part of the height measurement.
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