Recent progress in source development for extreme UV lithography

The continuation of Moore's law for semiconductor fabrication envisages the introduction of extreme ultraviolet lithography (EUVL) based on a source wavelength of 13.5nm for high-volume manufacturing within the next few years. While exposure tools have already been developed and the feasibility of the technology well demonstrated, the key source requirement in terms of power output remains to be achieved. Currently, sources based on laser-produced plasmas from tin droplet targets appear to be the most promising and are being deployed in manufacturing tools. Progress in CO ₂ laser design aimed at increasing conversion efficiency to close to 5% should make possible the attainment of greater than 200W of in-band optical power. Recently, research has commenced on the development of sources operating at a wavelength near 6.7nm for beyond 13.5nm lithography and gadolinium has been identified as the fuel of choice. The results of these experiments are described and show many similarities to the behavior of tin plasmas as essentially the same atomic and plasma processes are involved, albeit at an electron temperature close to a factor of three higher.