Max.Neg., Max. Pos. FPD (GFLD), TIR (GFLR)
\(FPD = Focal plane deviation\)
\(TIR = total indicator reading\)
By representing all thickness values overhead a common plane, a vacuum chuck is
simulated mathematically. By application of the „last squares fit“-algorithm to the
given thickness values, a theoretical plane is computed, which is the best
approximation to these values. It is called Focal Plane, at every sensor position,
there is a local deviation between the plane and the local wafer thickness.
The extrema found in the given set of Focal Plane Deviation values are called
Maxima Negative and Maximum Positive Focal Plane Deviation. Total Indicator
Rating (TIR) is the sum of the absolute values of those two extrema.
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