Enhancing extreme ultraviolet photons emission in laser produced plasmas for advanced lithography

Abstract

Current challenges in the development of efficient laser produced plasma sources for the next generation extreme ultraviolet lithography (EUVL) are increasing EUV power and maximizing lifetime and therefore, reducing cost of devices. Mass-limited targets such as small tin droplets are considered among the best choices for cleaner operation of the optical system because of lower mass of atomic debris produced by the laser beam. The small diameter of droplets, however, decreases the conversion efficiency (CE) of EUV photons emission, especially in the case of CO 2 laser, where laser wavelength has high reflectivity from the tin surface. We investigated ways of improving CE in mass-limited targets. We considered in our modeling various possible target phases and lasers configurations: from solid/liquid droplets subjected to laser beam energy with different intensities and laser wavelength to dual-beam lasers, i.e., a pre-pulse followed by a main pulse with adjusted delay time in between. We studied the dependence of vapor expansion rate, which can be produced as a result of droplet heating by pre-pulse laser energy, on target configuration, size, and laser beam parameters. As a consequence, we studied the influence of these conditions and parameters on the CE and debris mass accumulation. For better understanding and more accurate modeling of all physical processes occurred during various phases of laser beam/ target interactions, plasma plume formation and evolution, EUV photons emission and collection, we have implemented in our HEIGHTS package state-of-the art models and methods, verified, and benchmarked against laboratory experiments in our CMUXE center as well as various worldwide experimental results. V C 2012 American Institute of Physics. [http://dx.

Laser produced plasma sources for nanolithography—Recent integrated simulation and benchmarking

“…11 Similar values were predicted using our HEIGHTS simulations. 3 Variations in parameters of the CO 2 laser, i.e., increase of pulse intensity or spot size, do not significantly increase CE of such single pulse laser devices using small spherical targets. This is mainly due to the high reflection of 10.6-lm wavelength from solid/liquid tin, low critical density, and therefore lower vaporization of droplet by CO 2 laser, and less confinement of the evolved plasma plume to efficiently absorb and reabsorb (after reflection) laser photons due to the hydrodynamic motion of plasma around the droplet.…”

“…HEIGHTS Monte Carlo models for laser energy deposition take into account laser photons interaction with solid/liquid target, with vapor and plasma including photons absorption, reflection, and reabsorption after reflection. 3,6 Detailed model for target vaporization are based on target thermophysical properties as well as the kinetics of evaporation and takes into account possibility of recondensation. 8 The developed algorithms in HEIGHTS for unstructured mesh generation allow, for example, fine resolution of most important regions in domain, e.g., around 0.1 lm cell size for the target surface.…”

“…2 For further optimization, from the point of view of the minimum EUV power requirements needed for high volume production manufacturing, increasing droplet size to 30 lm or larger is deemed necessary. 3 This is also important for the desired potential increase in the main laser energy for higher throughput, without loss of source efficiency, which is restricted in the case of the 20 lm droplets or smaller. 2 We extended and enhanced our modeling and analysis of LPP sources with small droplets using Nd:YAG laser for the pre-pulse followed by CO 2 laser as the main pulse.…”

“…Our previous simulations showed an increase of the CE using pre-pulse, up to 1.3% for 10-lm droplet and up to 3% for the 30-lm droplet. 3 The smaller droplets, i.e., 10 lm and 20 lm were almost entirely vaporized; this means we used the full mass of these targets for mist (vapor-plasma) creation. Vapor density was near uniformly distributed in the case of smaller droplets before the main CO 2 laser pulse at the predicted optimum delay time between these two pulses.…”

The role of plasma evolution and photon transport in optimizing future advanced lithography sources

“…[4][5][6] We evaluated using our HEIGHTS simulation package the CE of dual-beam systems with various droplet sizes, starting from 10 lm, and our results indicated that the smallest efficient droplets should be in the range of 20-30 lm. 7,8 The purpose of this study is to investigate the differences in EUV photons production for different geometries and target conditions that can explain recent experimental results and then demonstrate ways for future target optimizations in these regards. We also studied the influence of different processes on EUV source characteristics for various preplasma conditions and main laser parameters.…”