Paper
19 May 2003 High accuracy interferometric measurements of EUVL mask blank substrates
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Abstract
Mask substrates for advanced semiconductor microlithography at 157 nm and 13 nm (EUV) require a surface flatness of 50 nm P-V. Production polishing methods require the surface metrology accuracy to be about 1110th this tolerance, or 5nm. Interferometric measurement accuracy is affected by the following factors: fringe pattern from the backside of the plane-parallel substrate, accuracy of the reference flat surface, retrace error of the interferometer optics, and coherent noise (speckle) associated with the coherent source. This paper describes the performance of a new wavelength-shifting Fizeau interferometer that is capable of simultaneously measuring the surface height of the front and back surfaces of an EUVL mask blank substrate to an absolute accuracy of 5 nm and an RMS repeatability of <0.01 nm. Unlike conventional Fizeau interferometers, it can simultaneously measure front surface height, back surface height, and relative thickness of a plane-parallel transparent glass plate while minimizing multi-surface fringe print-through artifacts. Surfaces with substrates as thin as 2 mm can be measured by this instrument, although the technology is readily extendable to thinner substrates. Back side fringe print-through is shown to be in the range of 0.1-0.5 nm P-V. Additional features of this new interferometer include a 300 mm diameter aperture, a set-up configuration suitable for automated part handling, an optical design optimized for low retrace error, a wavelength-tunable AlGaAs laser diode operating at 850 nm, a fiducialized transmission flat surface that is calibrated to 3 nm absolute accuracy, and a spatially incoherent source configuration for low speckle noise performance. Measurement data as well as an accuracy budget are presented.
© (2003) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Richard E. Bills "High accuracy interferometric measurements of EUVL mask blank substrates", Proc. SPIE 10314, Optifab 2003: Technical Digest, 1031418 (19 May 2003); https://doi.org/10.1117/12.2284033
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