Quantum gate computers are expected to find solutions faster than Neumann-style computers in some fields of logical calculation using quantum superposition. Al is generally used for superconducting material for quantum computing devices because of chemically and mechanically stable characteristic. Nb is also stable superconducting material. However, Nb is not as popular as Al. One of the reasons of it is complicated composition of Nb surface oxide which consists of Nb₂O₅ and suboxides of NbO and NbO₂. Nb₂O₅ has good insulation property, but the suboxides shows metallic property [1]. It is known that the suboxides affect bad influences to Nb/NbO_x/Nb Josephson junction properties [2]. It has also been pointed out that the proportion of suboxide on the Nb surface is related to the plasma etching state before Nb oxidation, which also leads to deterioration of the resonance characteristics of the Nb superconducting resonator used for data reading [3]. Resonators have three types of interfaces that can introduce surface losses: metal-air, metal-substrate, and substrate-air. Dominant surface loss is found to arise from the metal-substrate and substrate-air interfaces [4]. The surface oxide layer on Si can be removed by HF solution or NF₃ gas treatment with H radical process. However, the relationship between Nb oxide layer component and Q-value at 1 photon level is not clear. A high precisely oxidation technique is necessary to discuss the effect of the thin Nb oxide layer. Neutral beam oxidation (NBO) technique has achieved to control the high-quality oxidation thickness at nm order [5,6]. In this study, we investigated the effect of the Q-value of the superconducting resonator with changing the Nb surface oxide layer using NBO.

Figure 1 shows the schematic image of the neutral beam equipment. The neutral beam takes away the plasma damage by a high-aspect-ratio bottom conductive aperture electrode, which neutralized and collimated the positive and negative ions discharged by inductively coupled plasma. The aperture electrode also prevents ultra-violet irradiation from plasma. Moreover, when the bias power to the aperture electrode is applied, neutral beam irradiation energy can control from a few eV to a few hundred eV. Thus, NB achieves the defect-less etching interface and high controllable oxidation synthesis.

The superconducting resonator to measure the Q-value was formed deposited 50-nm-thick Nb by DC magnetron sputtering and etched by an SF₆ neutral beam etching (NBE) method, and then, oxidized by an NBO. Resonator etching was carried out of very weak NB irradiation whose bias power is 0 W. The NBO was carried out by changing the plasma source power (500 and 1000 W) and process time (2 and 20 min). To confirm the surface Nb oxidation condition, flat surface Nb deposited film was prepared and apply the same NBO process, and x-ray photoelectron spectroscopy (XPS) measurement was carried out. The Nb3d signal was separated by Gaussian peak fitting into four kinds of components that are Nb metal, Nb₂O₅, NbO₂, and NbO and component ratios were determined by their area intensity.

The relationship between source power and irradiation time is shown in Fig. 2. Surface oxidation is promoted at 500 W more than at 1000 W. We were able to control the component ratio by changing the source power and irradiation time. The relationship between the Q-value and the component ratio of Nb surface with NBO is shown in Fig. 3. For 500 W, the Q-value increased due to the increase in Nb₂O₅ and the decrease in suboxide. For 1000 W, Q-value increased with decreasing Nb₂O₅ component ratio. It is the opposite trend of the previous result, but it is considered that the Q factor was also influenced by decreasing Nb oxide film thickness. Consequently, we found that the thickness and oxide components of Nb affects the Q-value.

<Reference>
[1] G. V. Chandrashekhar, et al., J. Solid State Chem. 2, 528 (1970).
[2] M. Gurvitch, et al., IEEE Trans. Magn., 19, 791 (1983).
[3] T. Konno, D. Ohori, M. Hidaka, K. Endo, H. Mukai, J.S. Tsai, S. Samukawa, The 68th JSAP Spring Meeting, 18p-Z27-7 (2021).
[4] J. Wenner, et al., Appl. Phys. Lett. 99, 113513 (2011).
[5] A. Wada, et al., Appl. Phys. Lett. 98, 203111 (2011).
[6] T. Ohno, et al., Appl. Phys. Lett. 106, 173110 (2015).

Keywords: Niobium, Resonator, Neutral Beam Oxidation