Target mode transition during the DC reactive sputter deposition process has been studied for several metal oxides, focused on the effect of the transport process of sputtered metal atoms. Si, Ti and Nb targets were sputtered in a gas mixture of argon and oxygen. These targets were chosen to elucidate the effect of the atomic mass on the reactive sputtering through the transport process. At a fixed argon gas flow rate of 20 sccm, evacuation valve was throttled to achieve a gas pressure between 0.5~2 Pa. At this fixed pumping speed, oxygen flow rate was modified and the transition between the metal and oxide modes was monitored by the cathode voltage. In the case of Si sputtering, the mode transition occurred at distinctly lower oxygen flow rate as the pumping speed was decreased and argon gas pressure increased. On the contrary, in the case of heavy Nb, the transition flow rate was not dependent on the ambient Ar gas pressure.
According to the Berg's reactive sputtering model, metal to oxide mode transition is not explicitly related to the gas pressure, and not strongly dependent on the pumping speed. The reason of the latter is that the large part of introduced oxygen is gettered by the substrate/chamber wall where target metal atoms are deposited and absorb oxygen. We propose that the discrepancy between the result of Si case and the Berg's model is due to the transport behavior of silicon sputtered atoms. Since silicon atoms are lighter than argon atoms, they are easily backscattered by them, and thermalize at relatively lower gas pressure. Therefore, Si sputtered atoms are apt to redeposit on the target surface, which results in the reduction of effetive sputtering yield, and hence the gettering capacity by the Si deposited on the chamber wall. Actually, in pure Ar gas sputtering, the deposition rate of Si clearly decreased as the gas pressure increases between 0.5~2 Pa, while that of Nb was almost constant in this pressure region. We are trying to apply the Monte-Carlo particle transport simulation to the experimental condition. The comparison of the effective emission of Si atoms with the reduction of mode-transition gas flow rate will be discussed. |