Metrology development for extreme ultraviolet lithography: Flare and out-of-band qualification

Abstract

Extreme ultraviolet lithography (EUVL) is the leading candidate for lithography beyond the 22 nm half-pitch device manufacturing node. These geometries impose tighter requirements for standard critical dimension metrology and call for new strategies able to quantify and monitor extreme ultraviolet (EUV) specific parameters. In this paper, the approaches to measure two key EUV imaging parameters, namely flare and out-of-band (OoB) radiation, are discussed. EUV sources are known to emit a broad spectrum of wavelengths ranging from EUV to deep ultraviolet (DUV) and beyond. As the DUV can contribute to the photoresist exposure and degrade imaging performance, it is critical to accurately determine the amount of DUV OoB in EUVL exposure tools at the wafer level. In this paper, a methodology using an aluminum-coated reticle to measure the DUV/EUV ratio in resist is discussed. Such a mask is able to provide quantitative in situ information on the scanner DUV content thanks to its ability to transmit DUV and absorb EUV. The experimental OoB results for two EUVL tools are reported and compared with modeling predictions. Flare in EUVL is caused by light scattered by the surface roughness of the optical elements and has a larger impact as compared to optical lithography. As a consequence, a precise and accurate flare metrology is essential to guarantee a proper qualification of the effect, as well as to implement an effective compensation strategy. However, the flare level estimate has been historically based on operator and tool-dependent procedures that are unable to meet the requirements for accuracy and precision dictated by EUVL. A robust in-line approach to flare metrology is developed and qualified. As in the case of OoB, experimental flare results for two EUVL tools are reported. The experimental data are compared to full-chip simulations using the point spread function of the tool’s optical system.