Photospeed considerations for extreme ultraviolet lithography resists

Abstract

Photospeed is a prime consideration for wafer throughput of extreme ultraviolet (EUV) lithography. Faster photoresists additionally provide system advantages such as less thermal management of the mirrors and mask, and potentially increased component lifetimes. However, there are some predicted detrimental considerations when using fast photoresists such as shot noise. In this article, we report details of the formulation of photoresists exposed at 248 nm and identical formulations exposed at 13.4 nm. Compositions typically contained co- or terpolymers of poly-4-hydroxystyrene, t-butyl acrylate and as an option, styrene, a photoacid generator of bis-t-butylphenyl iodonium camphorsulfonate or perfluoroxbenzensulfonate and tetrabutyalummonium or triphenylsulphonium hydroxide base. With these formulations, the EUV photospeed was varied from 34 to 2.7 mJ/cm2. Scanning electron microscope analysis was done for all wafers at Sandia using GORA software to determine the line-edge roughness (LER). Identical formulations were exposed at photon dense deep UV (DUV) wavelengths and comparatively photon-sparse EUV wavelengths. Therefore, ratioing the LER of identical formulations exposed at DUV minimizes the confounding effects of formulation changes that affect dissolution, resolution, etc. A plot of the line-edge roughness ratio of LEREUV/LERDUV as a function of the photospeed was used to gain insight into the effect of shot noise on the LER of dense lines. If shot noise effects were dominating the LER for EUV, then the ratio would increase above the nominal value as the dose to size was decreased. It is clear that the current generation of DUV resists, with the invention of no new material required, can be reformulated to satisfy the photospeed specification of 5 mJ/cm2 for EUV lithography and no shot noise effects have been observed.