Abstract Body

Superconducting nanostrip/nanowire single photon detectors (SNSPDs) show superior performances over semiconductor-based detectors in terms of detection efficiency, detection speed, time resolution, and noisiness [1]. Therefore, SNSPDs have become key devices in a wide range of fields such as quantum communication, quantum optics, sensing, and medical biotechnology. To achieve high performances, superconducting nanostrips with a line width of about 100 nm are generally used for SNSPDs. However, such nanofabrication tends to result in serious degradation of the superconducting properties, and therefore, the operating temperature of current SNSPDs using low critical temperature (T_c) superconductors such as NbN (T_c of the bulk is 16 K) is very low, typically 0.8 ~ 2 K.

 Using a high- T_c superconductor would increase the operating temperature if SNSPDs can be successfully fabricated. We have been focusing on NdFeAs(O,H), which exhibits the highest T_c among the iron-based superconductors. Previously, we reported the fabrication of NdFeAs(O,H) microstrips by conventional photolithography and Ar-ion dry etching using a reactive ion etcher (Ar-RIE) [2]. By adjusting the etching conditions, NdFeAs(O,H) microstrips maintained high T_c’s of about 35 K and high critical current densities (J_c’s) of about 4 MA/cm² at 4 K even with a line width of 0.88 µm. This result indicates that NdFeAs(O,H) has a high tolerance against microfabrication. However, the width of these microstrips was still too large for photon detector applications. Therefore, we adopted the electron beam (EB) lithography method in this study and investigated the patterning conditions to fabricate narrower strip lines of NdFeAs(O,H).

The parent NdFeAsO thin film was grown on a MgO (001) substrate by molecular beam epitaxy [3]. After the growth, the film was sealed in a quartz tube with CaH₂ powders and was heated at 350 °C for 36 h to substitute H for O [4]. Nanostrips with line widths down to 100 nm were then patterned on the obtained NdFeAs(O,H) thin film by EB lithography and Ar-RIE. The positive EB resist, ZEP520A (Zeon), was used for EB lithography. The fabricated nanostrips were examined by scanning electron microscopy (SEM).

The nanostrips were patterned with different exposure doses ranging from 100 to 200 µC/cm². The acceleration voltage and the current were set to 100 kV and 2 nA, respectively. Irradiating the EB resist with doses less than 120 µC/ cm² resulted in under-exposed patterns, while nanostrips with line widths of 200 nm and wider were successfully obtained at all exposure doses of 130 µC/ cm² or higher. On the other hand, nanostrips with a line width of 100 nm were obtained only at very limited exposure doses. A 100 nm sized pattern could not be formed at exposure doses of 160 µC/ cm² or higher and were obtained only at exposure doses of 130 ~ 150 µC/ cm², with the sharpest pattern boundary formed at 130 µC/ cm² as shown in Figure.

References

[1] C. M. Natarajan et al., Supercond. Sci. Technol. 25, 063001 (2012).
[2] A. Yoshikawa et al., The 35th International Symposium on Superconductivity PC8-3 (2022).
[3] T. Kawaguchi et al., Appl. Phys. Express 4, 083102 (2011).
[4] K. Kondo et al., Supercond. Sci. Technol. 33, 09LT01 (2020).