Since the resolution capability of lithography can be extended by using shortwavelength light, at least in principle, a number of concepts involving light with wavelengths much shorter than 193 nm have been proposed. Considerable effort has been applied to the development of one of these approaches, referred to as extreme ultraviolet (EUV) lithography. In this chapter, the basic concepts underlying EUV technology are discussed.
12.1 Background and Multilayer Reflectors
As the wavelength of light decreases significantly below 193 nm, all known materials become too absorbing to be used for fabricating effectual refractive optical elements (see Fig. 12.1). Moreover, at such short wavelengths the reflectivity of all homogenous materials becomes very small, at least at the near-normal angles of incidence relevant to high-resolution imaging optics (see Fig. 12.2). However, in the 1980s layered coatings were developed to provide practical reflectivities at wavelengths <15 nm. This development led to proposals for lenses with all-reflecting optics that could be used for projection lithography.
Reflection occurs at interfaces between materials that have different indices of refraction. The larger the difference between the refractive indices of the materials, the greater the reflectivity. At wavelengths <50 nm, all materials have indices of refraction ≈1. Thus, it is difficult to create high reflectance from a single interface, except at grazing angles of incidence. At EUV wavelengths, it has proven possible to make mirrors with moderate reflectivity at near-normal angles of incidence, in the range of 60-70%, by the use of coatings comprised of multiple layers. Multilayer reflectors are made by depositing alternating layers of high-Z (Z is atomic number) and low-Z materials, giving a small but effective difference between refractive indices at each interface. The net effect of small reflectivity at each interface can lead to moderately high reflectivity overall when the stack has a sufficient number of layers (Fig. 12.3), provided the layer stacking satisfies, at least approximately, the Bragg condition: