Nozomi Tanaka

Negative carbon (C−) and oxygen (O−) ions are expected to find wide applications for ion implantation processes. However, systematic experimental data required for their efficient generation and the elucidation of their underlying atomic processes are lacking. In this study, we systematically measured the angular and incident beam energy dependencies of the reflected carbon and oxygen ion properties from Mo, W, and low-work function material 12CaO⋅7Al2O3 electride targets, for low-energy (<1 keV) carbon (C+) and oxygen (O+) ion injections. For all targets, both C+ and C− ions were reflected for the C+ ion injection, whereas only the reflected O− ions were observed for O+ ion injection. The energies of C+, C−, and O− ions reflected from the electride and Mo decreased significantly with increasing reflection angle. The effect of the lower work function was an increase in the intensity of the O− ions reflected at larger-incident and smaller-reflection angles by comparing the reflected ions from Mo and electride targets. On the other hand, the effect of the work function on the production of C− ions was small. When 1 keV-O+ ions were incident on W, the fraction of the reflected O− ions was 10%–30%, and the rest were neutral oxygen particles. Good agreement between the measured and calculated incident angular dependency of reflected O neutral particles was observed.

To efficiently generate H− ions from positive atomic or molecular hydrogen beams injected onto a solid surface, it has been suggested to use a material with a low-work function as the target material. However, it is not clear under what conditions the most efficient H− production is realized for incident beam parameters or reflection angles. Therefore, we studied the interaction between low-energy atomic and molecular hydrogen beams (less than 1 keV/nucleon) with a low-work function electride 12CaO⋅7Al2O3 (C12A7). The production ratio of H− to H+ ions from the C12A7 electride was much higher than Mo targets with higher work functions, especially at smaller incident and smaller reflection angles. The H− to H+ production ratio slightly increased as the incident energies were decreased, but there was no significant difference between the electride and Mo targets. These results indicate that smaller incident angles and lower beam energies of the incident hydrogen beam are favorable for the enhancement of the production ratio of H− to H+ ions in C12A7. The higher production ratio appeared at the vertical beam energies less than on the order of 100 eV, where quantum mechanical processes may become important.

Effective cleaning of tin contamination on the collecting mirrors in extreme ultraviolet source is one of the key techniques to improve throughput and cost performance of extreme ultraviolet lithography. Hydrogen radicals produced in hydrogen plasma that is induced by wideband extreme ultraviolet radiation are expected to be utilized for in situ tin contamination cleaning in extreme ultraviolet sources. In this Letter, we clarified absolute density and cleaning ability of the hydrogen radicals produced by intense extreme ultraviolet pulse through ground state population density measurement by laser-induced fluorescence technique. The experimentally obtained radical parameters coincided well with simulation results and collisional radiative model. It was found that the extreme ultraviolet induced plasma was in quasi-steady state with abundant amount of hydrogen radicals in ground state. Further, it was found that the in situ tin contamination cleaning in extreme ultraviolet lithography source would become more practical with increase in operational parameters, such as extreme ultraviolet emission intensity, gas pressure, and radical production cross section.

One of the critical issues in lithography using extreme ultraviolet (EUV) light is tin contamination of the EUV collector mirrors in the tin-based LPP-EUV light source. The contamination can be removed by the reaction of tin atoms with hydrogen radicals producing stannane (SnH4), which is gaseous at the normal temperature. Hydrogen radicals can be produced from hydrogen molecules through photo-dissociation and photo-ionization induced by broadband radiation emitted from the EUV light source. In this work, an efficient production of hydrogen radical using vacuum ultraviolet (VUV) light emitted from laser-produced high-Z plasma is experimentally demonstrated. A two-dimensional radiation hydrodynamic simulation, coupled with photoionization and photo-dissociation cross sections, also shows the efficient hydrogen radical production by increased VUV light emission, as observed in the experiment.

We have investigated hydrogen plasma induced by intense extreme ultraviolet (EUV) radiation by measuring the plasma parameters and population density of radicals, which is expected to clean the tin contamination on the optics in EUV sources, using a time-resolved optical emission spectroscopy. The obtained electron density was n _e = (2 ± 0.4) × 10¹³ cm⁻³, and the electron temperature was T _e = 1 ± 0.2 eV. The electron density was five orders of magnitude higher than that of previous study. The radical population density determined by the experiment was consistent with that calculated with a collisional radiative model, showing excitation and recombination are dominant production processes.

Recent developments of extreme ultraviolet (EUV or XUV) sources for lithography technology enabled us to seek for further advanced materials processing using intense EUV radiations. The interaction of the lights in EUV wavelength region and matter is expected to be completely different from that of lights or lasers with longer wavelength, ranging from ultraviolet to infrared. Unlike well-known inverse-bremsstrahlung photo absorption, the photon energy of EUV light is directly absorbed by the orbital electrons. The critical density for the EUV wavelength range exceeds even solid density >10²³ cm^-3, so that the light is not absorbed in the ablation plasma even with a nanosecond pulse. As a consequent of the photon absorption and heating mechanism, the EUV induced ablation plasma is expected to be low temperature and high density compared to the ordinal laser ablation at the same irradiation condition. Further, one of the possible applications of such intense EUV light in materials processing is utilization of the expanding particles from the ablation plasma. For example, pulsed laser deposition (PLD) is a well know laser thin film deposition method that utilizes expanding particles from ablation plasma. EUV ablation would expand possibilities new parameters and materials to process. Thus, understandings of the mechanism of materials ablation by intense EUV radiation is important issue. However, to date, not so many studies on EUV-matter interaction have been conducted [1-3]. One of the approaches to study the EUV ablation is to investigate the particles expanding from the ablation plasma, because they preserve information of the ablation plasma itself and comparably easy to measure. The aim of this study is to understand the characteristics of expanding ions from EUV ablation plasma, and EUV plasma itself by analyzing the charge distribution and each energy spectrum. The results were compared with ordinal laser ablation to clarify noticeable difference between EUV and laser ablations.

Positive and negative hydrogen ion reflections from surfaces by injecting singly charged hydrogen ion beams show a clear difference between atomic and molecular ion injections at low energy and grazing incidence. The intensity ratio of reflected negative to positive ions H⁻/H⁺ increased as the incident beam energy per nucleon decreased only when molecular ion beams are injected. It implies that negative ions are more produced upon beam-surface interaction when molecules are injected. A possible reason was discussed in terms of difference in the negative ion production processes between atomic and molecular ions.