Specific Process Knowledge/Characterization/Profiler: Difference between revisions
→Stylus Profiler (Tencor P17): added acceptance tests |
→Dektak XTA stylus profiler: corrected and hopefully clarified |
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|style="background:LightGrey; color:black"|Resolution x y | |style="background:LightGrey; color:black"|Resolution x y | ||
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Down to 0.003 µm | Down to 0.003 µm in theory, but in practice limited by the 5 µm radius of the tip | ||
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|style="background:LightGrey; color:black"|Resolution z | |style="background:LightGrey; color:black"|Resolution z | ||
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|style="background:LightGrey; color:black"|Height accuracy z (95 % confidence) | |style="background:LightGrey; color:black"|Height accuracy z (95 % confidence) | ||
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~ | For very well defined steps ~ 2 % for a 1 µm step and ~ 1 % for a 25 µm step ([[#Height measurement accuracy for the DektakXT|see below]]) | ||
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|style="background:LightGrey; color:black"|Max scan depth as a function of trench width W | |style="background:LightGrey; color:black"|Max scan depth as a function of trench width W | ||
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====Use the right measurement settings for your sample==== | ====Use the right measurement settings for your sample==== | ||
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. | 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 lots of noise while too high scan speed like low force may mean that the stylus tip does not have time to reach the bottom of the features you are measuring (see the [http://labmanager.dtu.dk/d4Show.php?id=2346&mach=304 DektakXT manual], Figure 3 for details). | ||
A sharp vertical step is easiest to measure. If the step is gradual or the surface is very rough, it can be difficult to determine where to measure and how the scan should be leveled. | A sharp vertical step is easiest to measure. If the step is gradual or the surface is very rough, it can be difficult to determine where to measure and how the scan should be leveled. | ||
====Influence of calibration standard uncertainty==== | ====Influence of calibration standard uncertainty==== | ||
Nanolab staff check the instrument's measurement accuracy with a standard step height of 917 nm for the | Nanolab staff check the instrument's measurement accuracy with a standard step height of 917 nm for the three smaller ranges and 24.925 µm for the three ranges, so that the two middle ranges are checked with both standards. The 95 % confidence intervals for the standards are 17 nm for the 917 nm standard and 0.072 µm for the 24.925 µm standard. If the control measurement is beyond the limit set in our Quality Control procedure, the instrument is calibrated and the users informed (see LabManager for details on the [http://labmanager.dtu.dk/d4Show.php?id=2493&mach=304 control instruction] and the [https://labmanager.dtu.dk/view_binary.php?type=data&mach=304 control measurement data]). | ||
All this means that the 95 % confidence interval of a 1 µm step measured with the 6.5 kÅ range is at least the 1.8 % error of the standard step while the 95 % confidence interval of a 25 µm step measured with the largest range is at least on the order of the 0.29 % error of the standard step. Steps between 1 and 25 µm measured with the intermediate ranges will presumably have an intermediate error ''just due to the calibration uncertainty'' while the calibration-related uncertainty is presumably larger percentagewise for smaller or larger steps. | |||
====Total uncertainty==== | ====Total uncertainty==== | ||
[[File:Error probability distributions sep 2020 update big.png|upright=2|alt=Four different probability distributions that contribute to the total error on the Dektak measurement for the 6.5 micron range. By far the widest distribution is the one from the error on the calibration standard, which is a Gaussian. The others are the non-Gaussian spread of the average measurement of the calibration standard height, which cuts off at the QC limits, the resolution, which is a very narrow uniform distribution, and the spread of measurement values for a given step being measured, which is a Gaussian whose width depends on the step in question.|right|thumb|The probability distributions of the main sources of error that are convoluted to create the total error on a Dektak measurement.]] | [[File:Error probability distributions sep 2020 update big.png|upright=2|alt=Four different probability distributions that contribute to the total error on the Dektak measurement for the 6.5 micron range. By far the widest distribution is the one from the error on the calibration standard, which is a Gaussian. The others are the non-Gaussian spread of the average measurement of the calibration standard height, which cuts off at the QC limits, the resolution, which is a very narrow uniform distribution, and the spread of measurement values for a given step being measured, which is a Gaussian whose width depends on the step in question.|right|thumb|The probability distributions of the main sources of error that are convoluted to create the total error on a Dektak measurement.]] | ||
Apart from the error due to the calibration standard's uncertainty, there will be random noise in any measurement, which we have found for many repeated measurements of the standard step height (a rigid, well defined vertical step) is on the order of ± 5 nm. There is also a tiny contribution to the error from the instrument's resolution and a small but significant contribution from the spread of values that the Dektak actually measures compared to the theoretical height of the standard step height. | |||
The error | To estimate the overall accuracy of the Dektak's measurements you can convolute the various sources of error. This is shown graphically on the right. You can see an uncertainty budget for the Dektak measurements here (made by Rebecca Ettlinger): [[Media:uncertainty budget Dektak rev.xlsx]]. It is based on the assumption that all the error sources are independent and can therefore be added by the sum of squares method. | ||
The resulting error calculation for a 1 micron very well defined standard step is about 2 % (as the uncertainty on the calibration standard dominates), while for a very well defined step of 25 microns the cumulative error is about 0.7-1 %. These are the uncertainties listed in the table. However, in real devices the random error will often be much larger than for our standard samples and so the real confidence interval will be larger. | |||
To improve the accuracy of your particular measurement, you should repeat the measurement several times and estimate the standard deviation. If the scatter is quite small you can try to include the calibration error as a percentage of the step height in your estimate of the total error. If the scatter of your measurements is large that will probably be the dominant source of error in your measurement. | |||
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