Logo of the Physikalisch-Technische Bundesanstalt

Straightness measurements at the nanometer comparator with 2 nm uncertainty


The increasing demands placed on the positioning accuracy of multiaxial manufacturing and measuring machines lead to the requirement that the coordinate and straightness references have to be calibrated with small uncertainties. For this purpose, the nanometer comparator has been further improved in order to determine, in addition to the position deviations of length graduations, their straightness deviations with uncertainties in the single- digit nanometer range. The nanometer comparator is the national standard for the dissemination of the unit “meter” by the calibration of line scales, photo masks and encoder systems with a length of up to 550 mm. During the calibration, the graduation structures of the measurement objects are optically probed, while the measurement objects themselves are moved with constant speed on a carriage with air bearings, and the position of the carriage is precisely determined by means of a vacuum displacement interferometer.

Figure 1: Straightness measurement of an encoder system at the nanometer comparator on the basis of the additional three displacement interferometers

Figure 2: The straightness deviations of the grid scale were determined by means of two different error separation methods (TMS method and reversal method).

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.