Lithographic flare measurements of EUV full-field projection optics

Abstract

We demonstrate direct flare measurements of 4-mirror projection optics in the Engineering Test Stand (ETS) using a conventional resist clearing method (the Kirk method). Two extreme ultraviolet (EUV) lithographic projection optics, one with higher flare than the other, have been characterized and the results compared. The measured results have also been compared to analytical calculations based on measured mirror roughness and the extended point spread function (PSF). Full-field flare across the 24 mm field width has been measured, and we have verified that flare is constant across the field for EUV lithography as predicted. Horizontal (H) and vertical (V) flare bias has been observed and the cause of the H-V flare bias has been investigated. The main cause has been identified to be anisotropic mirror polishing. Simulations with the 2 dimensional Power Spectral density (PSD) function have confirmed the experimental results.

Anisotropic EUV flare measured in the engineering test stand (ETS)

“…The calculated intrinsic flare or open frame flare is based on the average PSD measured by scatterometry and AFM. 1 Since the PSD is azimuthally averaged, there are no variations in azimuthal directions.…”

“…1 Flare in the full field projection optics has been verified to be constant across the field, however, different flare values have been observed in the vertical and horizontal directions. The H-V biased flare was first reported last year, and the possible causes for the directional flare variations were investigated in the study 1 . The H-V flare bias is believed to be due to anisotropic mirror polishing.…”

“…1,6 Flare for a given feature size is determined by dividing the dose to clear the bright region, E 0 , by the dose to clear the dark feature, E D . By using this technique, flare of a full-field projection optic, POB2, has been measured in the ETS.…”

“…Thus, flare is less likely a concern for EUV lithography as long as the amount of open frame flare is below 10%. [1][2][3][4]14 However, being proportional to 1/λ 2 , flare in EUV systems can be difficult to control. Any deviation from the constant flare can complicate the flare variation compensation (FVC) methods.…”

“…Since EUV flare is primarily generated by surface scattering from each mirror, the EUV flare is easier to predict than DUV flare if surface roughness of the mirrors is known. [1][2][3][4] Since flare is proportional to the inverse square of the actinic wavelength, the amount of EUV flare can be large. However, due to the shorter scattering range at the EUV wavelengths, EUV flare is essentially constant over the field.…”