Straightness measurements at the nanometer comparator with 2 nm uncertainty
To separate the horizontal guiding errors of the carriage from the desired straightness deviation of the measurement object, an error separation method is required. At the nanometer comparator, the extended “Traceable Multi-Sensor (TMS)” method as well as a reversal method have been realized. For this purpose, the comparator was equipped with three additional interferometers and with a new scale carrier made of Zerodur, with a long mirror applied to it. The two realized methods of straightness metrology were evaluated by comparing measurements of a straightness encoder system having a graduation period of 512 nm and a length of the grating of 322 mm. Within the tolerance of their uncertainties, the two different error separation methods provided the same results which, in the case of the determined straightness deviation of the scale of ±72 nm, deviated from each other by less than ±1.3 nm.
Thus, it was possible to calibrate the grid-based straightness scale with an expanded uncertainty of less than 2 nm.
The TMS method developed at PTB allows the required measurement time to be clearly reduced as – in contrast to the reversal method – only one single application of the scale is necessary. The extended functionality of the nanometer comparator now allows PTB to calibrate also the straightness deviations of structures on scales and photo masks with uncertainties in the single-digit nanometer range and to investigate, in addition, other straightness measuring systems such as straightness interferometers. The uncertainty achieved was limited by the stability of the references required for the error separation procedures and can, in future, be reduced down to the sub-nanometer range.