The Electronic State at Excited States and the Plasma Chemical Reactivity of Vinylsilane

Abstract

The correlation between the plasma reactivity and the electronic state at excited states of vinylsilane is examined. A quantum chemical analysis is based on the two-center bond energy and the localized orbital energy obtained by using MNDO molecular orbital method. As shown in Table 1, the two-center bond energy of the C=C bond at the lowest excited state is more positive than that of the ground state. The π bond character disappears at the lowest excited triplet state (T₁) as shown in Table 2. Thus, the vinyl π bond is expected, to be activated at the lowest excited state. On the other hand, the Si-C bond is calculated to be weak at the second lowest excited state as shown in Tables 1 and 2. This tendency is more pronounced at the triplet state than at the singlet state. The excitation energy to the T, state is much lower than that to the other excited states (Fig.1). Thus, the reaction via T₁ state is expected to be predominant when the excitation energy is low. The experimenta l results of the rf plasma reaction of vinylsilane is consistent with this prediction. XPS and IR spectra of the film produced by the plasma reaction of vinylsilane (Fig.4 and 5) sh ow that a carbon-carbon skeletal network is formed via the cleavage of the vinyl π bond when the supplied power is low. The excitation with a higher power facilitates the cleavage of Si-C bond. Thus, the C/Si ratio of the film produced can be controlled by the supplied power. The quantum chemical calculation can be a powerful tool for designing the reaction mode of such a complex reaction as the plasma chemical decomposition of vinylsilane.