The paper describes recent improvements of Physikalisch-Technische Bundesanstalt's (PTB) reference measuring instrument for length graduations, the so-called nanometer comparator, intended to achieve a measurement uncertainty in the domain of 1 nm for a length up to 300 mm. The improvements are based on the design and realization of a new sample carriage, integrated into the existing structure and the optimization of coupling this new device to the vacuum interferometer, by which the length measuring range of approximately 540 mm with sub-nm resolution is given. First, measuring results of the enhanced nanometer comparator are presented and discussed, which show the improvements of the measuring capabilities and verify the step toward the sub-nm accuracy level.
The Nanometer Comparator is the PTB reference length measuring machine for high precision calibrations of line scales
and encoder systems. Up to now the Nanometer Comparator allows to measure the position of line structures in one
dimension only. For high precision characterisations of masks, scales and incremental encoders, the measurement of the
straightness of graduations is a requirement from emerging lithography techniques. Therefore the Nanometer
Comparator will be equipped with an additional short range measurement system in the Y-direction, realized as a single
path plane mirror interferometer and supposed to achieve sub-nm uncertainties.
To compensate the topography of the Y-mirror, the Traceable Multi Sensor (TMS) method will be implemented to
achieve a reference-free straightness measurement. Virtual experiments are used to estimate the lower accuracy limit and
to determine the sensitive parameters. The virtual experiments contain the influence of the positioning devices,
interferometer errors as well as non-perfect adjustment and fabrication of the machine geometry. The whole dynamic
measurement process of the Nanometer Comparator including its influence on the TMS analysis, e.g. non-equally spaced
measurement points, is simulated.
We will present the results of these virtual experiments as well as the most relevant error sources for straightness
measurement, incorporating the low uncertainties of the existing and planned measurement systems.
To minimize the measurement uncertainty of one dimensional length measurements on line scales, linear encoders and
interferometers the PTB in cooperation with the Dr. Johannes Heidenhain GmbH had built up a new length comparator.
The Nanometer Comparator [1,2] has already proven its performance during the measurements of incremental encoders
and line scales with an expanded measurement uncertainty of below 5 nm [3,4,5]. Due to the introduction of double and
multiple exposure in advanced lithography techniques the overlay and registration metrology requirements will
drastically increase so that reference metrology tools need to be developed further to be able to follow the resulting
decrease of the specifications. Therefore, the PTB further develops the new 1D vacuum comparator to add a
measurement possibility for straightness and to reach a measurement accuracy in the sub nanometer range . One key
development will be the interferometric measurement of all six degrees of freedom of the measurement slide of the
comparator. A new multi axis heterodyne interferometer electronics and optical interferometer designs minimizing
nonlinearities by spatially separated beams are under development.
The so-called Nanometer Comparator is the PTB vacuum length comparator which has been developed for high precision
length metrology on measurement objects with micro- and nanostructured graduations, like e.g. line scales, incremental
encoders or photomasks. The Nanometer Comparator allows to achieve smallest measurement uncertainties in the
nm-range by use of vacuum laser interferometry for the displacement measurement. We will report on the achieved
measurement performance of this high precision vacuum length comparator and the already started developments to substantially
enhance its measurement capabilities by additional straightness measurement capabilities. The enhanced
Nanometer Comparator will provide traceability for photomask pattern placement measurements in industry, also facing
the challenges due to the increased requirements on registration metrology as set by the introduction of new lithography
techniques like double patterning methods.