Development of Supersonic Ion-Beam Apparatus and its Application to Low-Energy Ion-Molecule Reactions

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A new type of supersonic ion-beam apparatus has been developed and applied to low-energy ion-molecule reactions of plasma processing gases. Rate constants and product ion distributions have been determined by using mass spectroscopy, whereas reaction dynamics has been clarified by using emission spectroscopy. Thermal ion-molecule reactions of Ar^+ with halogenated methanes and simple aliphatic hydrocarbons were investigated systematically. It was demonstrated that product ion distributions reported by using a selected ion flow tube method have often deviated from the initially distributions due to high operating pressures. Most of all charge-reactions were found to proceed through near-resonant processes, being independent of the magnitude of Franck-Condon factors for ionization. On the basis of emission spectroscopic studies, it was found that the formation rate constants of excited states occupy below 5% of the total reaction rate constant.Ion-molecule reaction of CO_2^+ with chloromethanes have been studied in order to examine the different reactivity between atomic ions and molecular ions. Not only near-resonant charge-transfer but also non -resonant one occurs in the CO_2^+ reactions due to a larger degrees of freedom in the products. The supersonic ion-beam apparatus has successfully applied to reactions of simple cluster ions with hydrocarbons. The total reaction rate constants of Ar_2^+,ArN_2^+,and N_4^+ with hydrocarbons have been determined. The product ion distributions led us to conclude that the reactivity of ArN_2^+ is similar to that of Ar_2^+. Since a high density of reactive halogen atoms can be generated by using the present ion-beam apparatus, it will be very useful for the reactive etching of a wafer.