One of the most important characteristics of an EUV light source for microlithography is the available inband EUV power. A power of at least 80 W, measured at the second focus of the collector, will be needed for a production tool, taking into account the optical requirements—expressed by the system etendue—of the whole imaging system. The available power at the second focus is given by the electrical input power Pel, the conversion efficiency (CE) of electrical input to EUV radiation, the system transmission, Tsys, taking into account collector reflectivity and possible gas transmission, and—to be considered here—the collection efficiency, ηcol: P EUV =P el ⋅CE⋅T sys ⋅η col .
The collection efficiency is one of the most important characteristics of a radiation source in an EUVL system. It is a geometrical quantity defined as the fraction of the total inband EUV radiation power that can actually be picked up and used in the subsequent total optical path of an EUV projection system down to the wafer. It is not a property of the source alone, but is determined both by the details of the optical system and by the geometrical intensity distribution of the source plasma.
The collection efficiency of an extended source in an optical system can be analyzed with the etendue formalism. Under the assumption of an ellipsoidal source intensity distribution, an analytical expression for the source etendue is derived. This can be used to analyze the collection efficiency for various source geometries as a function of collection angle.
The design of efficient illumination systems and detailed simulation analysis of the collectible source power is complex and time-consuming. The goal of this chapter is therefore to provide an estimate of the collectible source power by a simplified analysis. In a first step, the etendue of an idealized source in its dependence on the collection aperture is evaluated. This leads to a collectible source volume, from which in a second step the collected power of a real source can be estimated by projecting and integrating the collected radiance of the source.
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