Definition: | plane used as a reference for spatial measurements Note 1 to entry: The reference plane may be one of the following types: - for clamped measurements, the flat chuck surface that contacts the back surface of the wafer;
- three points at specified locations on the front surface within the fixed quality area;
- the least-squares fit to the front surface using all measured points within the fixed quality area;
- the least-squares fit to the front surface using all measured points within one site.
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