Specific Process Knowledge/Characterization/Stylus Profiler Measurement Uncertainty: Difference between revisions
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==Total Uncertainty for steps < 1 µm== | ==Total Uncertainty for steps < 1 µm== | ||
The underlying sample curvature can influence the measurement accuracy for steps < 1 µm. We have in June 2025 found both random and repeatable scan bowing or scan 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, especially steps up to 500 nm. | |||
We therefore cannot recommend using the stylus profilers for measuring steps <100 nm. One must expect relatively large uncertainty on small steps < 1 µm. | |||
If measuring steps <100 nm it is important to check a measurement of a "flat" area nearby and to play with positioning the measured step in different locations along the scan length (to show variation in the underlying scan noise right under the step). | |||
Note that the step determination will usually be better than the noise of the underlying scan. 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 our measurements with the DektakXT of a 500 µm wide step compared to XRR measurements of films of the same thickness. Probably this is because we level the scan close to the step, so only the curvature in an 800 µm range in x is important, and since we average the thickness across approx. 400 µm across the top of the step compared to 50 µm intervals nearby where the average height is set to zero, the variation comes out much smaller than what is seen across a full 2000 µm. | |||
In practice we have found the step heights around 100 nm can be measured with higher accuracy in the DektakXT than the P17: We tend to get a measurement with the DektakXT within a few nm of the XRR measurement while the measurement with the P17 is off by around 10 We don't yet have a good explanation for this - nor do we have enough data to prove it's a general trend. | |||
===How flat is a "flat" scan?=== | |||
KLA, the manufacturer of the P17 profiler, provides some information on the flatness one can expect from the scans, some available in [https://www.kla.com/wp-content/uploads/KLA_AppNote_Stylus_2D_Stress.pdf this document]. From this we can expect a variation of up to 40 nm across a flat scan of 3 cm, which means the variation we have measured is not far beyond the presumably best-case scenario painted by the manufacturer (our tool could use some lubrication of the scan axes at the time of measurement). | |||
The variation depends partly on the location of the leveling cursors during data analysis. We have generally found the worst bowing far from the center of the chuck. | |||
For the P17 the underlying scan noise or bow is consistent 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 scan derives from some underlying structure on the optical flat or the scan rails. | |||
For the Dektaks, the scan noise is also in some cases reproducible for scans in the same location, but in other cases 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. | |||
A lot of "flat" scans for the Dektak and P17 can be seen here: [[:File:Dektak XT and P17 scan flatness comparison summer 2025.pptx]]. | |||