Filter
Top Products
6.4.3
6 - 65
(b) Methods for Improving Measurement Reproducibility
It is recommended to use the SEM under fixed operating conditions (magnification,
accelerating voltage, probe current, raster rotation angle, working distance, etc.) in CD
measurement. If measurement under varied conditions is unavoidable, then
calibration should be made under each of the conditions. A number of calibration
conditions are registered in the CD measurement function, and these can be selected
either automatically or by specifying a registration No.
It is also recommended to measure dimensions in the X (horizontal) direction for
images acquired by slow scan. Although measurement is also possible in the Y
(vertical) direction, accuracy is lower than that in the X direction for the reasons given
below.
Since image acquisition time is longer in Y direction measurement, external magnetic
field may fluctuate during the acquisition and cause specimen drift, resulting in
measurement error. This error is more conspicuous as the measured dimension
becomes smaller.
To measure dimensions in the Y direction accurately, it is recommended to rotate the
image 90° by means of raster rotation and measure the dimensions as X-direction
ones. Note that since the magnification accuracy of the instrument is also dependent
on the raster rotation angle, it is important to calibrate the magnification at each raster
rotation angle to permit high-accuracy measurement.
In order to minimize the effect of magnetic hysteresis of the objective lens and improve
the focus reproducibility (working distance), it is essential to set the sample Z position
and focus in the following procedure.
• Set the working distance to be used for measurement on the Operation panel -
Cond. tab.
• Adjust Z-axis control of the stage to obtain an approximate focus (so that the
image profile can be identified at a few thousand times magnification).
• After focus adjustment, click the DeGauss button on Setup dialog window -
Optics tab and focus again.
It is recommended to set magnification once after making final focus
adjustment. It is because focusing operation will cause small fraction to
magnification value even when indication do not have fraction. It may cause
discrepancy with calibration condition.
The abovementioned procedure allows measurement reproducibility (3σ) of 2% or on
the order of 10 nm or less when the effects of sample damage, charge-up and
installation conditions are not serious. Note that the 3σ value depends on
measurement dimensions and image magnification.