Al/AlOx/Al junctions with the characteristic linewidth ranging from 130 nm to 200 nm have been fabricated on 2-inch c-plane sapphire substrates. The area of the traditional Dolan style junction17 is dependent on the thickness of the resist and the deposition angle of the bottom and top electrodes, which can affect the uniformity of the junctions. The bridgeless ‘Manhattan Style’ junctions18,19 were used in this report. Additionally, in order to avoid introducing two level systems (TLS) and other unstable factors in the ‘parasitic’ junctions that can cause parameter fluctuations20, the technique called ‘Patch integrated cross-type (PITC)’ was used9. To prepare submicron Al/AlOx/Al junctions, layouts were generated and exposed by using a 50 kV Electron Beam Pattern Generator. After the pattern transfer is completed by photolithography, junctions are deposited in Plassys MEB550SL3 with base pressure of 3×10− 8 mbar.
The bi-layer electron beam resist used 500 nm MMA EL9 as the bottom resist and 300 nm PMMA A4 as the top resist. In order to spin MMA more uniformly, the small hole in the top of the spin coater was covered when spin MMA 8. In the process of electron beam exposure, poor conductivity of the sapphire substrate can lead to charge accumulation. The accumulated charges induce electric fields on the surface of the sample, causing deflection of primary and secondary electrons, which can reduce the pattern resolution and positioning precision21, resulting in poor uniformity of junctions. To reduce the charging effect, covering the photoresist with charge dissipaters22 is a good solution, but it may cause contamination. To avoid subsequent contamination, we chose Al, which is easily removable. However, a thick conductive layer increases electron scattering volume, resulting in decreased resolution. Therefore, the thickness of the conductive layer should be as thin as possible. Insufficient conductivity of a thin conductive layer still causes significant charging effects, as shown in Fig. 1(c) (top), leading to obvious distortion and poor edges of the Josephson junctions produced. By optimizing the thickness of the Al conductive layer, we obtained Josephson junctions with steep edges (Fig. 1(c) (bottom)). In this experiment, both MMA and PMMA are exposed by 50 kV electron beam, and the optimal exposure doses were 200 and 1100 µC/cm2 respectively. To remove the Al conductive layer after the exposure, a two-step method was developed. Firstly, a diluent of TMAH with minimal attack on the electron beam resist was used to etch most of the Al layer, and then deionized water was used to react and rinse off the remaining Al. The sample was developed at room temperature with IPA: MIBK = 3: 1. Oxygen plasma with an optimal condition (60 W, 100 s) was used to ash the sample after development for removing the residual organics which have an effect on stability of the Al/AlOx/Al junctions23,24 .
The subsequent evaporation steps are shown in Fig. 1(a), where the planetary and tilt angles (from the z-axis) of the sample holder are denoted by θ and φ, respectively. The final layout of the SQUID in this experiment is shown in Fig. 1(b), and the yellow rectangle marks one of the junctions. To mitigate the impact of transverse incident angle effect, which is discussed in literature25 and leads to variation in the junctions area, the sample and sample holder were aligned under the microscope prior to introducing the sample into the UHV system. After full degassing, the first Al electrode of the junctions was deposited at θ = 0° and φ = 45° to reduce the shading effects10. 1 nm/s deposition rate was used. Both of the deposition angle and growth rate were optimized to achieve the best grain uniformity for the bottom electrode, which would improve the uniformity of the oxide layer in the next step10. After static oxidation at 5 mbar for 30 mins, the second Al electrode was also deposited with 1 nm/s deposition rate at the angle of θ = 90° and φ = 45°. After removing the surface oxide layer from the sample using Ar+ ion milling, aluminum was deposited at an angle of θ = 45° and φ = 60° for patching. The final step in the fabrication process is the passivation process, which involves static oxidation of these junctions at 100 mbar for 30 minutes. The barrier region is observed using transmission electron microscopy (TEM) (see Fig. 2), revealing a very small roughness and a steep interface between Al and O.