Fabrication of superconducting niobium nitride nanowire with high aspect ratio for X-ray photon detection

Abstract

The niobium nitride (NbN) nanowires fabricated with the high-quality ultra-thin NbN film with a thickness of 3 nm-6 nm were widely used for single photon detectors. These nanowires had a low aspect ratio, less than 1:20. However, increasing the thickness and the aspect ratio of highly-uniformed NbN nanowires without reducing the superconductivity is crucial for the device in detecting high-energy photons. In this paper, a high-quality superconducting nanowire with aspect ratio of 1:1 was fabricated with optimized process, which produced a superconducting critical current of 550 μA and a hysteresis of 36 μA at 2.2 K. With the optimization of the electron beam lithography process of AR-P6200.13 and the adjustion of the chamber pressure, the discharge power, as well as the auxiliary gas in the process of reactive ion etching (RIE), the meandered NbN nanowire structure with the minimum width of 80 nm, the duty cycle of 1:1 and the depth of 100 nm were finally obtained on the silicon nitride substrate. Simultaneously, the sidewall of nanowire was vertical and smooth, and the corresponding depth-width ratio was more than 1:1. The fabricated NbN nanowire will be applied to the detection of soft X-ray photon emitted from pulsars with a sub-10 ps time resolution. Niobium nitride (NbN) has been widely studied in superconductor electronics for high superconducting transition temperature (up to 16 K), critical current density and critical magnetic field 1. Moreover, the corresponding physical properties were stable, and it could have large scale with high quality. Also, it was the most common materials to fabricate the superconducting nanowire single-photon detectors (SNSPDs) 2,3 , the mixer of Terahertz wave detection hot-electron-bolometer (HEB) 4 , and the superconducting quantum interference device (SQUID). At present, the preparation of SNSPDs mainly use the epitaxial method to grow the high-quality ultra-thin NbN film with a thickness of 3-6 nm. Then, the film is prepared to the meandered nanowire structure by means of micro-nano processing. The width of NbN nanowire is about 100 nm, and the aspect ratio is generally lower than 1:20. Considering the advantages of SNSPDs, such as the low dark count, the wide response spectrum, the short recovery time and the high time precision, SNSPDs were widely studied and applied to the detection of visible/ infrared single photon 5. However, the application of SNSPDs in the direction of high-energy photon (UV, X-ray, and gamma ray) was rarely studied for low absorption of ultra-thin NbN films with the high-energy photons 6-8. Through Geant4 simulations, we obtained that the X-ray photon absorption of the 10 nm thick NbN film for 1 keV and 6 keV was only 3.69% and 0.23%, respectively 9 , and the SNSPD prepared with the ultra-thin NbN film was unable to effectively detect high-energy single photons. The simulation results show that the absorption of 1 keV and 6 keV by NbN with a 100 nm film thickness are 31.31% and 2.25%, respectively. To improve the absorption of superconducting NbN...

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