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Formation and detection of molecules in laser induced plasmas (LIP) is a hot topic. In analytical plasmas like LIBS, the detection of molecules is important for identification of geological and other materials, analysis of isotopes and difficult elements (Cl, F, etc.) via molecular emission. In chemical plasmas, like PECVD (plasma enhanced chemical vapor deposition) or PLD (pulsed laser deposition), molecules formed in the plasma determine a composition and a thickness of deposits. Similarly, molecules play an important role in microstructuring and oxidizing metal surfaces by laser ablation. It is unfortunate that different communities, which utilize plasma methods and seek for solutions of similar problems, do not strongly overlap, and do not fully use knowledge accumulated by each other.
In this presentation, mechanisms of formation of molecules will be analyzed on the example of LIPs used for chemical vapor deposition and metal microstructuring. Theoretical analysis includes equilibrium chemistry calculations combined with plasma hydrodynamics. First, LIP excited in a gas mixture of BCl3 or BF3 with H2 or CH4 will be analyzed; this chemical system is used for obtaining deposits of refractory solid boron and boron carbide. Second, a breakdown in the SiF4 + SiCl4 gas mixtures will be described; this method allows synthesis of fluorochlorosilanes SiFxCl4-x (x = 1, 2, 3), the good etching agents (Figure). Third, solid ablation of Mo in BF3+H2 and Ti in air will be considered aimed at obtaining deposits of high hardness MoxBy and films of TixOy on textured Ti surfaces, correspondingly.
In experiment, reaction gases before and after laser illumination, and solid deposits are analyzed by optical emission spectroscopy (OES), IR and mass spectrometry (MS), SEM, X-ray, and AFM. It will be shown that the hydrodynamic-chemical model adequately predicts the composition of LIPs, zones of molecular formation, dependence on reactant stoichiometry, plasma temperature and pressure.
Superhard materials with a Vickers hardness in the range of 30-40 GPa are of great interest, both from a fundamental and a practical viewpoints, since they have outstanding mechanical, thermal and chemical properties. Molybdenum borides belong to this group of materials. A review of publications on synthesis of molybdenum boride indicates a) great interest in this superhard material and b) the need for new effective methods of its synthesis, especially in a nanocrystalline form.
Very promising are the plasma-chemical methods based on laser induced breakdown. The breakdown can be created either in reactive gases containing volatile compounds of boron and molybdenum or on solid samples. For gas breakdown, molybdenum hexafluoride MoF6 and boron trifluoride BF3 were used in the mixture with hydrogen and argon; for solid breakdown, the pure molybdenum sample was ablated into the mixture of H2 and BF3.
The plasma-chemical synthesis of MoxBy structures was carried out in the reactor shown in Figure. Laser breakdown was created by a pulsed Nd: YAG laser operating at 1064 nm with a 15 ns pulse duration, 5 Hz repetition rate, and 800 mJ pulse energy. The laser was focused by a 5 cm focal length lens to produce 26 J/cm3 energy density in the focal point. The ratios H2:BF3: MoF6 = 5:2:1 and H2:BF3 = 3:1 were used in a pressure range 30 - 760 Torr.
After ablation in the mixture H2 + BF3 + MoF6, the deposit contained an amorphous phase with a small impurity of crystalline molybdenum and no boride phase. After ablation of metallic Mo into H2 + BF3, the main phase was MoB2 in the form of nano dispersed powder with an average grain size of 100 nm. The degree of conversion of boron trifluoride and the yield of molybdenum boride were studied as a function of pressure. It was established that 30 Torr is optimal for the formation of MoB2. This work was supported by the Russian Science Foundation grant No. 20-13-00035.