Plasma-induced interface reaction is a key of plasma processing, which is an inevitable tool of contemporary science and technology. Studies on “Plasma-induced surface and interface reaction” are one of important fields of surface science, as well as that of vacuum science.
As feature sizes of semiconductor devices diminish to the atomic scale, their fabrication also requires an atomic-scale accuracy. Atomic layer etching (ALE) is a dry etching process having an atomic-scale resolution. In ALE, a thin layer of a material surface is removed sequentially by a cycle of two reaction steps, i.e., the formation step of surface modification and the removal step of a surface layer affected by the surface modification. In plasma-assisted ALE (PA-ALE), plasmas are used to generate reactive species that modify the surface and/or low-energy ions that assist the removal of the modified surface layer. In this review article, the interactions of halogen atoms and hyper-thermal halogen molecules with a silicon (Si) surface are discussed, which correspond to typical surface reactions in the formation step of PA-ALE of Si. Similarly, the interactions of halogenated Si surfaces with low-energy inert-gas incident ions are also examined as typical surface reactions of the removal step of PA-ALE of Si.
Atomic layer etchings (ALE) with gas cluster ion beam (GCIB) were reviewed. Gas cluster ions are aggregates of thousands of atoms or molecules. Thus, energy/atom of gas cluster ions are easily reduced to several eV/atom. In addition, gas cluster ions realize transient high-temperature and high-pressure conditions on the bombard area, enhancement of chemical reaction and removal of chemically altered surface layer are expected. These irradiation effects are beneficial for ALE. In this report, ALE of metal films (Cu or Ni) by oxygen GCIB (O2-GCIB) with acetic acid or acetylacetone (acac) vapor were investigated. Metal oxide layer was removed owing to chemical reactions with acetic acid or acac included by 5 kV O2-GCIB irradiation. Since there was no physical sputtering by 5 kV O2-GCIB, etching terminated after removal of surface layer.
Recently, we have revealed that the vibrational energy of CO2 drives the hydrogenation reaction of CO2 on Cu single crystal surfaces, Cu(111) and Cu(100), (CO2＋Ha → HCOOa, where ‘a’ denotes adsorbate) via Eley–Rideal-type mechanism. The finding of the reaction dynamics of CO2 may provide deeper insights into the mechanism and kinetics of all the CO2-related surface chemical reactions and a possible application in the ‘state-to-state chemistry’ of methanol synthesis. Here we review the experimental results obtained with supersonic molecular beam.
We investigated the rate coefficient of CO2 splitting via vibrational excited states using recombining H2 and He plasmas with ultralow electron temperatures between 0.1 and 0.3 eV. The ultralow-temperature plasmas were useful for investigating the CO2 splitting via vibrational excited states, since the rate coefficients of dissociation of CO2 via electronic excited states are negligible. The rate coefficient of the CO2 splitting decreased clearly with the electron temperature. In addition, the rate coefficient observed in the ionizing H2 plasma with an electron temperature of 4 eV was one order of magnitude smaller than that observed in the recombining plasmas. It has been shown that the CO2 splitting via vibrational excited states has a larger rate coefficient than that via electronic excited states.
Plasma-assisted heterogeneous catalysis for CH4 conversion attracts keen attention because the strong C-H bond breaking is possible via vibrational excitation of CH4 at low temperature. Similarly, vibrational excitation of CO2 possesses unique reactivity in heterogeneous catalysis. This paper provides a kinetic analysis of nonthermal plasma-assisted CH4 dry reforming to elucidate the drastic reaction promotion mechanism. Lanthanum-modified Ni/Al2O3 catalyst was combined with dielectric barrier discharge at 5 kPa and 400–700℃. Activation energy decreases from 91.0 kJ/mol to 44.7 kJ/mol which was well correlated with the state-specific gas-surface reactivity of vibrationally excited CH4 on Ni surface. Reactivity of plasma-excited CO2 is discussed by the in situ DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) under the influence of CO2 plasma, showing the plasma-excited CO2 promotes the formation of surface carbonate species (CO32－) : co-operative activation of CO2 is the key to enhancing overall reforming performance.
Plasma catalysis is gathering attentions for its unique characters in chemical reactivity and product selectivity. Various applications have been suggested and demonstrated. When catalysts are located inside the plasma zone, bilateral interactions occur which are generally complicated to get full understanding of the detailed steps. Nevertheless, a lot of experimental results have been filed up for various chemical reactions (decomposition, synthesis, partial redox). Notwithstanding the progress during the last decades, however, the understanding of the working mechanism is still in early stage. As a representative model reaction, room temperature oxidation of CO was compared for the conventional thermal catalysis, plasma-driven catalysis, and ozone-assisted catalysis. The effect of different catalyst, reaction mechanism, and catalyst regeneration was discussed. In this review, the current progress to understand the bilateral interactions of plasma and catalyst, literature survey, and some of future perspective will be presented.