The ion transport through In-rich oxide and Pb-rich oxide during rf-plasma oxidation is studied. The structures of the oxides formed are investigated by Auger electron spectroscopy accompanying Ar sputtering. It is shown that metal ion direct transport through the In-rich oxide occurs and metal ions combine with oxygen at the oxide-oxidation atmosphere interface during rf-plasma oxidation. Furthermore, it is clarified that oxides grow at the oxide-metal interface during rfplasma oxidation through the Pb-rich oxide. Oxygen transport seems to be the most likely transport process in this case.The oxide barriers for Josephson junctions have frequently been formed by the rf-plasma oxidation of PbIn-Au alloy thin film. During this process, the samples are mounted on an electrode which is capacitively coupled to a source of 13.56 MHz rf-power (1-4). The characteristics of such junctions have been investigated extensively. However, only a few studies have been made on the ion transport during rf-plasma oxidation.Ion transport during rf-plasma oxidation through the previously existing thermal oxide [combined thermal and rf-plasma oxidation (2)] has been discussed by the present authors in a previous paper. They used an oxygen isotope followed by secondary ion mass spectroscopic (SIMS) analysis (5). Through the analysis, it was clarified that oxygen arriving at the oxygen-oxide interface remains and forms additional oxide in the outer portion of the oxide during rf-plasma oxidation in the combined oxidation process. Thus, direct transport of oxygen ions was ruled out. Therefore, direct or substitutional transport of metal ions or substitutional transport of oxygen ions must occur (6) through the previously formed thermal oxide, i.e., In-rich oxide (2, 7).In the present paper, an effort to clarify which of the above ion transport processes occurs through the In-rich oxide during rf-plasma oxidation is described. In addition, ion transport through Pb-rich oxide during rfplasma oxidation is discussed.
ExperimentalIon transport through In-rich oxide.--Thin oxide film formed by the thermal oxidation of Pb-In [12 weight percent (w/o)]-Au (4 w/o) alloy film consists mainly of Inoxide with a very small amount of overlying Pb-oxide (8). This is because In is selectively oxidized in spite of its small concentration in the alloy. Furthermore, such overlying Pb-oxide was observed to dissolve in water very easily. Pb is said to be etched off and dissolved in deionized water as lead hydroxide (9). Taking these facts into consideration, In-rich oxide on the alloy film with no overlying Pb-oxide was obtained in the following way.A 200 nm thick Pb-alloy film was formed by sequential electron beam deposition of Au (4 w/o), Pb, and In (12 w/o) onto thermally oxidized silicon wafers in that order. An hour after the deposition, but before removing the samples, thermal oxidation of the Pb-alloy film was carried out in the deposition chamber by exposing the samples to oxygen; first, at a pressure of 27 mPa for 15 min, then at 2.7 Pa for 30 rain ...
