Extreme Ultraviolet (EUV) sources rely on droplet laser plasmas for EUV generation. These sources consist of a small (30 µm diameter) droplet which is excited into plasma emitting EUV around 13.5 nm, the industry’s chosen wavelength for EUV lithography (EUVL). These sources are the best candidates for the commercialization of EUVL allowing mass production of computer chips with 32 nm or even smaller feature size. However, the biggest challenges which EUV source developers encounter today are the issues of conversion efficiency (CE) and debris. In order to satisfy the technology requirements, the source will need to meet high levels of stability, performance, and lifetime. Our tin-doped droplet plasma has demonstrated high CE and low debris resulting in long lifetime. Long term stability is obtained through the use of novel tracking techniques and active feedback. The laser plasma targeting system combines optical illumination and imaging, droplet technology innovation, advanced electronics, and custom software which act in harmony to provide complete stabilization of the droplets. Thus, a stable, debris-free light source combined with suitable collection optics can provide useful EUV radiation power. Detailed description of the targeting system and the evaluation of the system will be presented.

With the availability of high reflectivity multilayer mirrors and zone plate lenses, the EUV region (5nm - 40nm) of the electromagnetic spectrum is currently being explored for applications of nanoscale printing and imaging. Advances made in this area have consequences for many areas of science. Research for producing a compact, bright EUV source for laboratory use has gained momentum in recent years. For this study, EUV radiation is produced by irradiating target materials using a focused laser beam. Focused laser beam ionizes the target to create a hot, dense, pulsed plasma source, where emission is a result of the relaxation of excited levels. Spectroscopy is used as the main diagnostic to obtain the spectral signature of the plasma. Spectral characteristics are used to deduce the physical state of plasma, thus enabling the tuning of laser irradiance conditions to maximize the needed emission bandwidth. Various target materials are studied, as well as different target geometries, with spectroscopy below 200 nm on pulsed micro-plasmas being a particularly daunting task. Total range spectroscopy from 1 nm to greater than 1 micron is completed for tin-doped spherical droplet plasma source. Reliable plasma diagnostics require both accurate measurements and solid theoretical support in order to interpret the experimental results. Using existing 1D-hydrocode, temperature and density characteristics of the expanding plasma is simulated for any set of experimental conditions. Existing atomic codes written for calculating one-electron radial wavefunctions with LS-coupling scheme via Hartree-Fock method is used in order to gain details of the ion stages, populations, transitions, etc, contributing to the spectral data.

<div>Laser-produced plasmas (LPP) have been used in a wide range of applications such as in pulsed laser deposition (PLD), extreme ultraviolet lithography (EUVL), laser-induced breakdown spectroscopy (LIBS), and many more. In the collision of two laser-produced plasmas, the two counter-streaming plasmas may face a degree of stagnation which influences the subsequent development of the compound plasma plume. The plume development of the stagnation layer can deviate quite noticeably from typical laser-plasma behavior. For instance, an enhanced degree of collisionality is expected, especially when the plasma collision transpires in a low pressure ambient. Colliding plasma can be intentionally implemented or conversely may occur naturally. In EUV lithography colliding plasma could service as an efficient EUV source with inherent debris mitigation. Conversely, colliding plasma could manifest in an inertial fusion energy (IFE) chamber leading to contamination, disrupting successful device operation.</div><div><br></div><div>Various techniques such as optical emission spectroscopy (OES), CCD plume imaging, laser-induced fluorescence (LIF), laser-induced incandescence (LII), and scanning electron microscopy (SEM) may be used to study laser-produced plasmas and their associated byproducts. These techniques will be used extensively throughout this work to aid in developing an understanding of the various physical and chemical phenomena occurring in these plasmas.</div><div><br></div><div><div>Chapter 1 provides introductory knowledge regarding LPPs with a specific exploration into colliding plasma and its relevance to a broad body of scientific knowledge. Additionally, the principles behind the various experimental techniques are capitulated.</div><div><br></div><div>Chapter 2 presents the laboratory facilities available at our Center for Materials Under eXtreme Environment (CMUXE) which can be used to study LPP. The various equipment (chambers, lasers, spectrograph, etc.) are discussed in detail.</div><div><br></div><div>Chapter 3 begins the series of substantive chapters which comprise the original research of this thesis. Here, the early formation (< 1 μs) of colliding carbon plasmas produced from the ablation of graphite is explored. The influence of plume hydrodynamics on the temporary lateral confinement of the stagnation layer is discussed with attention to the three different laser intensities studied. Additionally, species in the plasma were identified using OES and monochromatic plume imaging. A large increase in Swan emission from C2 dimers is observed in the stagnation layer, suggesting formation of C2 and/or re-excitation of C2 produced ab initio during laser ablation. Results were compared with HEIGHTS computational modeling to verify observations and to validate the code package for a new plasma regime.</div><div><br></div><div>Chapter 4 functions as a continuation from Chapter 3, looking into the intermediate time (1-10 μs) dynamics of colliding carbon plasma. To observe transient molecular species of carbon, C2 and C3, LIF was employed. By acquiring plume images through LIF, the various mechanisms by which C2 and C3 appear at different times in the plasma lifetime may be discerned. Using optical time-of-flight (OTOF), more information of carbon species populations may be determined to construct space-time contours which offer corroborative information regarding the spatiotemporal development of the stagnation layer.</div></div><div><br></div><div><div>Chapter 5 presents work on colliding Sn plasma for application as a EUV light source. The accumulation of material along the stagnation layer makes colliding plasmas a suitable preplasma in a dual pulse laser scheme. Dual-pulse EUV concepts call for the formation of a preplasma from the stagnation of two Sn plasmas. This preformed plasma is then subject to a second, pumping laser purposed to optimize the conversion efficiency (CE) of laser energy into EUV output. Characterization of the stagnation layer was obtained through optical emission spectroscopy while CE data is obtained using an absolutely calibrated EUV photodiode. HEIGHTS computational modeling then provides prediction of EUV emission upon using a CO2 laser for preplasma reheat.</div><div><br></div><div>Chapter 6 explores the collision between two dissimilar plasmas. Laser-produced plasma of Si and C are created in a manner which enables the two plasmas to collide. The ensuing development of the colliding plasma regime is then discussed in terms of relevant plume hydrodynamics. Analysis of the colliding regime is accomplished using fast-gated plume imaging and optical time-of-flight.</div><div><br></div><div>The final chapter, Chapter 7, provides a concise summary of the results presented in the preceding chapters. Additionally, recommended research directives are presented which are designed with consideration for the current facilities and capabilities at CMUXE.</div></div>