The insufficient accuracy of the reported scanner focus using the existing methods originates from:
a) Focus measurement quality, which is due to low sensitivity of measured targets, especially around the nominal production focus.
b) The scanner focus is estimated using special targets, e.g. large pitch target and not using the device-like structures (irremovable aberration impact).
Both of these factors are eliminated using KLA-Tencor proprietary “Focus Offset” technology.