Short wavelength sources for microlithography

Summary form only given. The evolution of microlithography tools to generate smaller features on microchips has involved using shorter ultraviolet wavelength sources and larger numerical aperture transmissive optics for imaging the complex mask patterns onto the wafers. During this evolution, the imaging source went from the mercury g-line at 436 nm to the i-line at 365 nm and then to the 248 nm KrF laser. At the 248 nm wavelength a laser source was necessary because the imaging optics required a very narrow linewidth at high power levels in order to obtain diffraction-limited imaging over a large imaging field. Presently, manufacturing tools are beginning to use the 193 nm wavelength of the ArF excimer laser. Operating at this wavelength places even more stringent requirements on the illuminating linewidth than at 248 nm. Looking ahead, it is expected that an EUV stepper operating at a wavelength of 13.5 nm will become available that could eventually print features as small as 30 nm. Fortunately there is a very strong group of emission lines from transitions in tenth ionized xenon that radiate in the region of 13.5 nm. These emission lines can be produced with significant flux from either a laser-produced plasma or a plasma discharge source. At this short wavelength, there are no transmissive optical materials available to provide imaging from the mask to the wafer. Hence reflective optics must be used for the imaging process. The scaling of radiative transition probabilities at shorter wavelengths in atoms and ions allows for a very significantly increased power emitted from an incoherent plasma source when compared to a similar source in the UV region. Hence, laser sources are not required in this wavelength region to achieve the necessary flux needed for the high wafer throughputs of a microlithographic manufacturing system.