In a study published in the IEEE Journal of Selected Topics in Quantum Electronics, Professor Xiaolong Hu and Dr. Kai Zou from Tianjin University, China, provided a complete guide on fabricating high-quality AFSNSPDs. The research describes the materials and procedures required to build these detectors, as well as the obstacles posed by their intricate fractal architecture.
This study published in the IEEE Journal of Selected Topics in Quantum Electronics offers a detailed guide for fabricating fractal SNSPDs, while addressing key challenges in the fabrication process. Its goal is to assist researchers in developing highly sensitive and reliable detectors. Image Credit: "Optical fiber" by brixendk via Creative Commons Search Repository
The evolution of modern electronics has been significantly influenced by the detection, transmission, and manipulation of light (photons), impacting areas such as high-speed communication, quantum computing, and sensing. Photon detectors play a pivotal role in these systems, with the superconducting nanowire single-photon detector (SNSPD) being a prominent example.
SNSPDs employ ultra-thin superconducting wires that transition rapidly from a superconducting state to a resistive state upon photon interaction, facilitating ultra-fast detection. The arrangement of these wires in a Peano arced-fractal pattern is consistent across different scales, allowing for the detection of photons irrespective of their direction or polarization. This design makes arced-fractal SNSPDs (AF SNSPDs) essential for applications in light detection and ranging, quantum computing, and quantum communication.
This paper aims to present the nano- and micro-fabrication developments of high-performance fractal SNSPDs, with particular emphasis on the important experimental details that are key to the success of these devices.
Xiaolong Hu, Professor, Tianjin University
The construction of AF SNSPDs involves three main components: nanowires for photon detection, optical microcavities for photon capture, and keyhole-shaped chips that align the detector with optical fibers. The fabrication process initiates with the creation of the optical microcavity, which involves coating a silicon wafer with alternating layers of silicon dioxide (SiO2) and tantalum oxide (Ta2O5) through ion-beam-assisted deposition (IBD) to form a bottom-distributed Bragg reflector, followed by a SiO2 defect layer.
A 9-nm niobium-titanium nitride (NbTiN) superconducting film is then deposited on the defect layer using reactive magnetron sputtering, forming the photon-sensitive surface. Subsequently, titanium-gold electrodes are fabricated on this surface via optical lithography and lift-off techniques.
The nanowires are patterned into a fractal design using scanning-electron-beam lithography and transferred to the NbTiN layer through reactive-ion etching. The microcavity is finalized by depositing a top SiO2 defect layer and additional alternating layers of Ta2O5/SiO2 using aligned optical lithography and IBD. The chip is shaped into a keyhole form through optical lithography, inductively coupled plasma etching, and the Bosch etching process, followed by packaging for optical fiber connections.
The authors also offer recommendations for optimizing the fabrication processes of nanowires, optical microcavities, and keyhole-shaped chips. Suggestions include applying a 5-nm silicon or 3-nm SiO2 layer as an adhesion promoter to enhance bonding between the resist patterned into nanowires and the NbTiN material, utilizing auxiliary AF nanowire patterns for consistent widths, and carefully designing the layout and spacing of optical microcavities to reduce photoresist deformation.
They also recommend using precise alignment markers for keyhole-shaped chips and gradually applying heat during the curing process to improve photoresist stability and minimize etching defects.
In conclusion, the researchers successfully developed SNSPDs with remarkable sensitivity and system detection efficiency.
“These advancements will help simplify the fabrication of fractal SNSPDs enabling the development of more advanced devices with additional functionalities,” remarked Hu.
The ongoing enhancements in SNSPD design and fabrication hold the potential to drive significant advancements in quantum computing, telecommunications, and optical sensing, indicating a promising future for photonics.
Journal Reference:
Zou, K. et al. (2025) Fabrication Development of High-Performance Fractal Superconducting Nanowire Single-Photon Detectors. IEEE Journal of Selected Topics in Quantum Electronics. doi.org/10.1109/JSTQE.2024.3522176