[19a-PB2-1~39] 11 Superconductivity (Poster)

▲ [19a-PB2-2] Fabrication of ultra-small NbN tunnel junctions on Si-substrate

〇Wei Qiu¹, Hirotaka Terai¹ (1.NICT)

Keywords:Josephson juntions, NbN, e-beam lithography

In this reported research, we have succeeded in developing a new fabrication process for full-epitaxial NbN tunnel junction by introducing e-beam lithography and chemical mechanical planarization (CMP) process for superconducting qubit circuit fabrication. A thin (200)-oriented TiN film has been deposited on low-loss Si substrate by dc magnetron sputtering method to serve as a buffer layer prior to the base electrode NbN film deposition, followed by AlN barrier material and counter electrode NbN deposition in the same manner. The junctions were patterned by e-beam lithography on ZEP-520A positive resist. The e-beam exposure conditions have been optimized to ensure the best resolution (Figure 1). An extra thin Al film to serve as an etching stop layer for junction reactive ion etch (RIE) in CF₄ plasma post the junction patterning by e-beam. SiO₂ deposited by sputtering to provide isolation and later was chemical-mechanical polished (CMP) down and etched back by CHF₃ plasma to open contact hole on the base electrode for final NbTiN wiring layer deposition. The I-V characteristics (Figure 2) of the tunnel junctions on the initial wafer showed very promising junction properties in terms of the junctions’ gap voltage ~ 5.7 mV and junction’s quality factor R_sg/R_n>50 from junction’s I-V characteristics at 4.2 K.