Flatness is an important surface tolerance requirement during the manufacturing of a part of an ASML machine. Flatness can be measured by either measuring the variation in height of the surface in one measurement (a plane-wise measurement) or by measuring the height on multiple xy-coordinates on this surface (point-wise measurement). Point-wise distance measurement sensors tend to have small physical dimensions, while having good specifications on resolution perpendicular to the measured surface. During a separate research, a gap is found in performing a point-wise flatness measurement while accounting for undesired stage deviations. The stage deviations are in the order of
micrometers, while the sensor accuracy is in the order of nanometers. The performance of a point-wise flatness measurement is therefore in a negative sense dominated by the stage inaccuracies rather than sensor performance. During this research the gap is filled by designing, building and testing a feasibility demonstrator which uses reference sensors to account for stage inaccuracies. By performing a live correction with either 1 or 3 reference sensors, the stage inaccuracy (in z, Rx and Ry) can be compensated for. The measured stage wobble for this specific setup is 3.2 µm, which is reduced to 0.13 µm when using three reference sensors. The method of using reference sensors for flatness measurements is shown to work, mainly when using 3 reference sensors.