Efficient fabrication technique for single photon detectors
20.02.2025 - New devices are based on fractal superconducting nanowires.
From high-speed communication to quantum computing and sensing, the detection, transmission, and manipulation of photons have transformed modern electronics. Central to these systems are photon detectors, which detect and measure photons. One notable type is the superconducting nanowire single-photon detector (SNSPD). SNSPDs utilize ultra-thin superconducting wires that quickly transition from a superconducting state to a resistive state when a photon strikes, allowing for ultra-fast detection. The wires in these detectors are arranged in a Peano arced-fractal pattern, which remains consistent across various scales. This unique design enables the detector to detect photons regardless of their direction or polarization. Due to these advantages, arced-fractal SNSPDs (AF SNSPDs) are crucial in applications such as light detection and ranging, quantum computing, and quantum communication.
Xiaolong Hu and Kai Zou from Tianjin University provide a comprehensive guide to fabricating high-quality AF SNSPDs. They outline the necessary materials and techniques for constructing these detectors and addresses challenges associated with their complex fractal design. “This paper aims to present the nano- and microfabrication developments of high-performance fractal SNSPDs, with particular emphasis on the important experimental details that are key to the success of these devices,” says Hu.
AF SNSPDs consist of three components: nanowires for photon detection, optical microcavities to capture photons, and keyhole-shaped chips that house and align the detector with the optical fiber. The fabrication process begins with creating the optical microcavity by coating a silicon wafer with six or eight alternating layers of silicon dioxide and tantalum oxide using ion-beam-assisted deposition (IBD) to form a bottom-distributed Bragg reflector, followed by the addition of a SiO2 defect layer. Next, a 9-nm niobium-titanium nitride (NbTiN) superconducting film is deposited on the defect layer using reactive magnetron sputtering, creating the photon-sensitive surface. Titanium-gold electrodes are then fabricated on this surface using optical lithography and lift-off processes.
The nanowires are patterned into a fractal design using scanning-electron-beam lithography and then transferred to the NbTiN layer through reactive-ion etching. The microcavity is completed 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 its keyhole form using optical lithography, inductively coupled plasma etching, and the Bosch etching process, and packaged for optical fiber connections.
The scientists also provided suggestions for optimizing the fabrication processes of nanowires, optical microcavities, and keyhole-shaped chips. Some of their recommendations include: Applying a 5-nm silicon or 3-nm SiO2 layer as an adhesion promoter to improve bonding between the resist patterned into nanowires and the NbTiN material, using auxiliary AF nanowire patterns to ensure consistent nanowire widths, and a careful design of the layout and spacing for optical microcavities to minimize photoresist deformation. They also suggested using accurate alignment markers for keyhole-shaped chips and gradually applying heat during the curing process to enhance photoresist stability and minimize etching defects.
In conclusion, the researchers were able to develop SNSPDs with impressive 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,” says Hu. Such steady improvements in SNSPD design and fabrication can pave the way to breakthroughs in quantum computing, telecommunications, and optical sensing. (Source: IEEE)
Link: School of Precision Instrument and Optoelectronic Engineering, Tianjin University, Tianjin, China