Determination of optical properties of thin films and surfaces in 157-nm lithography

Photolithography using 157-nm pulsed F-2 lasers has emerged as the leading candidate technology for the 0.1 mu m lithography node for the post-193-nm generation. The extension of operating wavelength to the VUV range presents new challenges for thin film metrology tools, such as ellipsometers and spectrophotometers, most of which have not yet shown robust performance at high accuracy at wavelengths below 193 nm. Knowledge of material optical properties near 157 nm is essential for several areas of microlithography, such as 1) optimization of resist and bottom antireflectance coating (BARC) and lithographic performance modeling; 2) development of thin dielectric layers for lens coatings, including antireflectance, beamsplitter and high reflectance designs; and 3) development of resolution enhancement techniques, such as attenuating phase shifting masks. In this work we review our experience with VUV spectrophotometers, as well as techniques for obtaining stable reflection and transmission measurements necessary for deriving optical constants of thin films. In particular, we find that reliably accurate reflection data can be obtained only using absolute reflectance methods. Extraction of optical constants is performed utilizing global optimization methods with a commercially available software package. Kramers-Kronig-consistent dispersion relations are used to describe the material dielectric constants. We will present real and imaginary refractive index values of various thin films, as determined from reflection/transmission data into the deep UV wavelengths to as low as 140 nm. A separate study designed to understand scatter losses of materials at 157 nm will also be described. We have constructed a 157-nm laser-based scatterometer for obtaining bidirectional reflection distribution function (BRDF) measurements. By correlating scatter signals with total transmission losses, we are able to separate absorption from scatter effects.