Effects of the ratio of sphere size to laser focal spot on the dominant in-band EUV emitting region

Previous experiments on Nd:YAG laser produced Sn plasmas showed little dependence of the location of the dominant in-band extreme ultraviolet emitting region on target geometry. Modeling and experiments were preformed in order to better understand this phenomenon. A two-dimensional radiation hydrodynamic code, h2d, was benchmarked with experiments using Sn microspheres. Numerically obtained densities above the diagnostic limitation and temperature profile maps, which cannot be measured experimentally, were used to explain the effect, which comes from a relatively small change in the location of the critical density surface. This is important to EUV lithography because both the density and temperature play significant roles in the generation and transport of in-band EUV light. It is desired to have the highest possible density at the dominant emitting surface and the smallest possible surrounding plasma to produce high 13.5 nm conversion efficiency (2% bandwidth). The target geometry impacts the amount of lateral expansion leading to dramatically different density profiles and therefore can be optimized to meet these conditions.