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6.4 CD Measurement Function (Option)
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6.4.3.4 Measurement Accuracy and Error Factors
Following is a description on measurement accuracy and error factors.
(1) Measurement Reproducibility and Absolute Dimensional Accuracy
In CD (critical dimension) measurement, the reliability of measured results can be evaluated in
terms of variation of measured values and accuracy of mean value obtained by measuring the
same dimension repeatedly. The variation of measured values is represented by measurement
reproducibility defined by 3 σ (dispersion). On the other hand, absolute dimensional accuracy
can be evaluated in terms of difference in measurements between mean values (statistical mean
values) and true values.
CD measurement function of the S-3400N provides a calibration function and the measured
result can be calibrated so that measured values (mean values) of a standard sample such as a
microscale agree with known dimensions.
The dimensional accuracy of measured values after calibration is determined by the dimensional
accuracy of the standard sample itself. Therefore, as long as calibration is made using a
standard sample of sufficiently high reliability, the reliability of measurement data finally depends
on measurement reproducibility.
Although the measurement reproducibility depends on a number of factors described in the next
section, even if all the factors are temporarily ideal, one must be careful about measurement
errors caused by pixel size or resolution when the measured object is extremely small (a thin film
of a few 10 nm thickness, for instance).
(2) Factors Determining Measurement Reproducibility, and Methods of Improvement
(a) Factors Determining Measurement Reproducibility
Measurement reproducibility is affected by magnetic hysteresis of the objective lens,
reproducibility of focus, stability of image contrast (for instance, a stable contrast is
difficult to obtain when there is charge-up or specimen damage), floor vibration or
external magnetic field variation at the installation site, and other factors. One must
also be careful about dimensional change of samples due to contamination or sample
damage in measurement of very fine pattern widths. Also, upon changing the
instrument operating parameters (viewing parameters) such as accelerating voltage or
working distance during a measurement, the magnification accuracy may vary and
measurement reproducibility may decrease.