| Definition: | plane used as a reference for flatness measurements
Note 1 to entry: The reference plane can be one of the following types:
- for measurements in which the wafer is clamped, the reference plane is the flat chuck surface that is identical with the back surface of the wafer;
- for measurements in which the wafer is not clamped, the reference plane is defined by the surface height at three points on the front surface of the wafer within the FQA;
- for measurements in which the wafer is not clamped, the reference plane is defined by the least-squares fit to the front surface of the wafer using the surface height at all measured points within the FQA.
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