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Both the force setting and the scan speed are important: Too high force may compress a soft material like Al, Au or some polymers, while too low force may lead to the stylus "jumping" over features, especially if the scan speed is high. Too low scan speed may result in drift of the measurement and noise from vibrations while too high scan speed may mean that the stylus tip does not have time to reach the bottom of the features you are measuring and can also give rise to increased noise levels (see the [http://labmanager.dtu.dk/d4Show.php?id=2346&mach=304 Dektak XTA manual] on LabManager, Figure 3 for details).
Both the force setting and the scan speed are important: Too high force may compress a soft material like Al, Au or some polymers, while too low force may lead to the stylus "jumping" over features, especially if the scan speed is high. Too low scan speed may result in drift of the measurement and noise from vibrations while too high scan speed may mean that the stylus tip does not have time to reach the bottom of the features you are measuring and can also give rise to increased noise levels (see the [http://labmanager.dtu.dk/d4Show.php?id=2346&mach=304 Dektak XTA manual] on LabManager, Figure 3 for details).


A sharp vertical step is easiest to measure. If the step is gradual or the surface is rough, it can be difficult to determine where to measure and how the scan should be leveled. Underlying sample curvature can also make it hard to level the scan. See further below under '''Total Uncertainty for steps < 1 µm'''.
A sharp vertical step is easiest to measure. If the step is gradual or the surface is rough, it can be difficult to determine where to measure and how the scan should be leveled. For very small steps, underlying scan noise can also make it hard to level properly and will convolute with the shape of the step. See further below under '''Total Uncertainty for steps < 1 µm'''.
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Apart from the uncertainty described above, the underlying "shape" of the scan due to the instrument itself will influence the measurement accuracy for steps <1 µm. Although the stylus profilers use an optically flat surface as a reference, the basic underlying scan is never truly completely flat. We have in June 2025 found both random and repeatable scan bowing / noise in the range of 10-60 nm across a 2 cm (2000 µm) scan. This is enough to influence the measurement accuracy of steps <1 µm and especially <500 nm.
Apart from the uncertainty described above, the underlying "shape" of the scan due to the instrument itself will influence the measurement accuracy for steps <1 µm. Although the stylus profilers use an optically flat surface as a reference, the basic underlying scan is never truly completely flat. We have in June 2025 found both random and repeatable scan bowing / noise in the range of 10-60 nm across a 2 cm (2000 µm) scan. This is enough to influence the measurement accuracy of steps <1 µm and especially <500 nm.


We therefore cannot recommend using the stylus profilers for measuring steps <100 nm and one must expect relatively large uncertainty on small steps < 1 µm.
We therefore cannot recommend using the stylus profilers for measuring steps <100 nm and one must expect relatively large uncertainty on steps in the range of hundreds of nm. See further discussion below.


[[image:Si 6 in left_50umprsec_aveof2.png||right|thumb|Sample average of 2 scans of a blank, new 6" Si wafer made with the P17 Stylus Profiler.]]
[[image:Si 6 in left_50umprsec_aveof2.png||right|thumb|Sample average of 2 scans of a blank, new 6" Si wafer made with the P17 Stylus Profiler.]]
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KLA, the manufacturer of the P17 profiler, provides some information on the flatness one can expect from the scans, some of which is freely available in [https://www.kla.com/wp-content/uploads/KLA_AppNote_Stylus_2D_Stress.pdf this document]. Here we see that we can expect a variation of up to 40 nm across a flat scan of 3 cm. Thus the variation of up to 40 nm across 2 cm that we have seen in practice is not far beyond the presumably best-case scenario stated by the manufacturer (our tool could use some lubrication of the scan axes at the time of measurement).
KLA, the manufacturer of the P17 profiler, provides some information on the flatness one can expect from the scans, some of which is freely available in [https://www.kla.com/wp-content/uploads/KLA_AppNote_Stylus_2D_Stress.pdf this document]. Here we see that we can expect a variation of up to 40 nm across a flat scan of 3 cm. Thus the variation of up to 40 nm across 2 cm that we have seen in practice is not far beyond the presumably best-case scenario stated by the manufacturer (our tool could use some lubrication of the scan axes at the time of measurement).


For the P17 the underlying scan noise or bow is extremely reproducible for many scans in the same position whether or not the vacuum hold is turned on. Therefore it seems that the non-flatness of the scans derive from some underlying structure on the optically flat surface or the scan rails.  
For the P17 the underlying scan noise or bow is extremely reproducible for many scans in the same position whether or not the vacuum hold is turned on. However the underlying noise varies when measuring different locations on a blank wafer. Therefore it seems that the non-flatness of the scans derive from some underlying structure on the optically flat surface or the scan rails.  


For the Dektaks, we have in some cases seen that the scan noise is reproducible not only for scans in the same location, but even for scans in different locations on the stage, which must be due to some kind of dirt on the "feet" of the stage as they move across the optical flat. However in other cases, as in all the scans shown in the document above, the bowing of the scan varies even though the scan coordinates and sample are the same. The latter might be because the stage positioning is a little less accurate than for the P17 or because there is more environmental noise influencing the measurement.
For the Dektaks, we have in some cases seen that the scan noise is reproducible not only for scans in the same location, but even for scans in different locations on the stage, which must be due to dust or bumps on the "feet" of the stage as they move across the optical flat. However in other cases, as in all scans shown in the document above, the bowing/noise of the scan varies even though the scan coordinates and sample are the same. The latter might be because the stage positioning is a little less accurate than for the P17 or because there is more environmental noise influencing the measurement.


===How does the underlying scan noise affect the measurement accuracy?===
===How does the underlying scan noise affect the measurement accuracy?===
The step determination will usually be better than the noise of the underlying scan - we don't expect an error in the measured step sizes as big as 30-60 nm. This comes down to removing some of the noise due to averaging parts of the scan during leveling and measuring.
The step determination will usually be better than the noise of the underlying scan - we don't expect an error in the measured step sizes as big as 30-60 nm. This comes down to removing some of the noise due to averaging parts of the scan during leveling and measuring. Also, usually it is not necessary (or recommended) to measure such steps across more than 1 cm. The closer together the measurement points, the "flatter" the underlying scan will be.


For example, in the DektakXT we regularly measure shadow masked metal films made by e-beam evaporation that are around 100 nm thick. We have found good agreement between the DektakXT measurements of a 500 µm wide step and XRR measurements of films of the same thickness, so we see no reason not to trust these measurements to within around +/- 5 nm. We level the scan with the leveling cursors close to the step before and after it, so only the scan noise in an ~700 µm range is important. Additionally, since we average the thickness across approx. 400 µm at the top of the step compared to the ~40 µm leveling intervals nearby (where the height is also averaged), the variation comes out much smaller than what is seen across a full 2000 µm scan.  
For example, in the DektakXT we regularly measure shadow masked metal films made by e-beam evaporation that are around 100 nm thick. We have found good agreement between the DektakXT measurements of a 500 µm wide step and XRR measurements of films of the same thickness, so we see no reason not to trust these measurements to within around +/- 5 nm. We level the scan with the leveling cursors close to the step before and after it, so only the scan noise in an ~700 µm range is important. Additionally, since we average the thickness across approx. 400 µm at the top of the step compared to the ~40 µm leveling intervals nearby (where the height is also averaged), the variation comes out much smaller than what is seen across a full 2000 µm scan.  
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