chapter 6A, Optics Contamination

Excerpt

6A.1 Introduction

Extreme ultraviolet lithography (EUVL), using a 13.5-nm wavelength, is likely to be the most promising next-generation lithographic (NGL) technique. This technology, which is a natural extension of optical lithography, seems superficially very similar to that technique. However, shorter wavelengths, which enable printing of much smaller features, also create new technological challenges. Radiation at these short wavelengths is strongly absorbed by any matter. For this reason, reflective rather than transmissive optics must be used, and the entire EUVL system must be maintained in a vacuum environment. Reflective optics consist of precisely figured substrates coated with alternating layers of molybdenum (Mo) and silicon (Si) (Fig. 6A.1). Such multilayer (ML) coatings, invented by Spiller, enable high reflectivity and wavelength selectivity of EUV mirrors. Residual water and other contaminants in the presence of EUV photons oxidize and degrade the optics surface. Both oxidation and carbon deposition on the optical surfaces reduce the reflectivity of the optics and can introduce wavefront aberrations. The overall effect not only decreases the throughput of the exposure tool but also the printing uniformity.

Optics lifetime is one of the critical issues for the success of EUVL technology. Despite recent progress obtained by combining oxidation-resistant capping layers, and in-situ cleaning strategies that attempt to leverage chamber gas-surface interactions to impede oxidation and contamination, optics lifetimes still fall short of specifications for high-volume manufacturing (HVM) by nearly two orders of magnitude. This is not the first instance when the lifetime and stability of Mo∕Si ML-coated optics has come under scrutiny. For example, astrophysicists have been using the reflective properties of Mo∕Si MLs for imaging the solar corona for the last 20 years. The solar and heliospheric observatory (SOHO) was designed to study the dynamics and structure of the solar interior and to image the solar corona. This telescope was launched into space in 1995 and designed nominally to perform for two years, but because of spectacular results, its mission was extended to December 2009. Among other instruments, SOHO has on board an Extreme Ultraviolet Imaging Telescope (EIT) that utilizes Mo∕Si coatings. Another solar telescope, the Transition Region and Coronal Explorer (TRACE), whose primary mission is to study magnetic fields and the associated plasma structures on the sun, was launched in 1998 and also uses normal-incidence mirrors with EUV coatings. Both SOHO's EIT and TRACE's instruments are still working after 13 and 10 years in space, respectively.