Achievement of sub nanometer reproducibility in line scale measurements with the nanometer comparator

Abstract

Due to the discussed introduction of double patterning techniques the overlay and registration metrology requirements in advanced lithography will drastically increase so that reference metrology tools need to be developed further to be able to follow the resulting tightening of the specifications. Therefore, the PTB further develops the Nanometer Comparator, a 1D vacuum comparator, which already has proven its performance during the measurements of incremental encoders. The implementation of the angular control loops decreases the angle deviations of the measurement carriages down to 0.15 µrad, which led to a reduction of the contribution of the Abbe errors to the measurement uncertainty to insignificant levels. Changes in the illumination and alignment of its optical microscope resulted in an improved defocus behaviour, which consecutively reduced the reproducibility (1 σ) of measurements of high quality scales in the order of 1 nm. Therefore an expanded measurement uncertainty of below 5 nm has been achieved.

Improved measurement performance of the Physikalisch-Technische Bundesanstalt nanometer comparator by integration of a new Zerodur sample carriage

“…5,6,7 Measurement uncertainties in the nm range and with a reproducibility in the sub-nm range were realized. Consequently, the PTB developed a one dimensional (1-D) vacuum length reference comparator, the so-called nanometer comparator, in cooperation with partners from industry.…”

Development of a 1.5D reference comparator for position and straightness metrology on photomasks

“…The details of the slit microscope currently used for line edge detection and localization on the Nanometer Comparator were already described before [6]. A cold light source with a green interference filter is used for a Köhler type illumination of the reflection mode microscope.…”

“…These uncertainty values are specified for a coverage factor of k = 2, which corresponds to a confidence level of 95%. Due to latest improvements in the line position detection by optical microscopy, the uncertainties for line scale position measurements could recently be further improved to below 5 nm on high quality line scales of 280 mm length [6], see section 3.2.…”

Length Metrology and Calibration Systems

“…In practice the lower limit for optical interferometry is in the range of 10 -7 and mainly restrained by variations of air refractive index which in turn can be computed by Edlen's formula contributing to the length measurement error in amount of 5 ×10 -8 and might be considered as ultimate limit for measurements in air. On the other hand, vacuum interferometry might be an option where currently achievable expanded measurement uncertainties are below 5 nm measuring high quality artifacts in lengths below 1 m (Köning et al, 2007).…”