Reflective multilayer coatings, an enabling component of Extreme Ultraviolet Lithography and beyond

EUV lithography requires, because of the wavelength of 13.5 nm, all reflective optics which can be realized by using multilayer Bragg reflectors for normal incidence. The research required to develop this class of optics is ongoing for several decades already but experienced an enormous boost from the extremely stringent requirements of EUV Lithography. In this presentation we will discuss the path from fundamental research on the deposition of layers of a few nm thickness only towards a fully developed process, matured to deposit optics for prototype lithography machines. Topics like multilayer deposition, smoothing of interfaces, thermal stability, interface engineering, multilayer induced stress, and lateral uniformity will be discussed and examples of multilayer coated optics that fulfil the extremely tight specifications of EUV lithography machines will be discussed. Yet, while the first EUV litho tools are being shipped to semiconductor manufacturers, research on multilayers for an even shorter wavelength of 6.x nm already takes place. This means other multilayer materials and, because of its increasing importance, even more focus on issues like smooth layer growth and prevention of intermixing.