面向空间应用的太赫兹频段全NbN超导SIS混频器特性研究

For terahertz astronomical observation, the space platform overcomes the absorption and disturbance of the Earth's own atmosphere to the observed target signal, so it can observe the weak signal emitted from the target farther from the Earth. Therefore, in recent years, the international terahertz astronomy programs have been vigorously developed. Superconducting SIS mixers have become one of the most commonly detectors used in these space terahertz detection programs due to their high conversion gain, ultra-high sensitivity, and ultra-low requirement for local oscillator power. At present, China is developing a superconducting SIS detection module based on the manned space station in the 500 GHz frequency band to realize the dual scientific tasks of astronomical observation and atmospheric observation. In view of the particularity of space applications, this project intends to study all-NbN superconducting SIS mixers, which have higher operating temperature and better temperature stability (compared with common superconducting material Nb). The research contents include the study of their DC characteristics, the study of their noise characteristics and mechanism, and the study of their characteristics of high-energy particle irradiation. This project will eventually help to develop an all-NbN superconducting SIS mixer with high critical temperature, high quality factor, high critical current density and fully meeting the needs of space applications.

对于太赫兹天文观测而言，因为空间平台克服了地球自身大气层对观测目标信号的吸收和扰动，可以观察距离地球更远目标发射的微弱信号，所以近年来国际上一直在大力发展空间太赫兹天文计划。而超导隧道结（SIS）混频器以其高增益、超高灵敏度、极低本振功率需求等优点，则成为了这些空间太赫兹探测计划中最常搭载的探测器之一。目前，我国正在开展基于载人空间站的500 GHz频段超导SIS探测模块的研制，以实现天文观测和大气观测双重科学任务。鉴于空间应用的特殊性，本项目拟开展针对具有更高工作温度、更好温度稳定度（与常用超导材料铌相比）的全氮化铌（NbN）超导SIS混频器的研究，包括对其直流特性的研究、噪声特性及机理的研究以及高能粒子辐照特性的研究。本项目将最终将协助研制出具有高临界温度、高品质因子、高临界电流密度、完全能满足空间应用需求的全NbN超导SIS混频器。

本项目针对载人空间站工程巡天空间望远镜高灵敏度太赫兹探测模块的应用需求，对500 GHz频段全NbN超导SIS混频器开展了一系列研究。通过对其直流特性的研究，不仅得到了全NbN超导SIS结的能隙电压以及漏电流随温度变化的理论公式，还得到了NbN这种超导材料中准粒子的寿命。进一步将500 GHz频段全NbN超导SIS结封装成混频器，通过对其噪声特性的研究，成功实现了在8 K温区、系统噪声小于130 K混频性能。除此之外，针对全NbN超导SIS结混频器空间应用的特点，有针对性地对混频器进行了一些改进。一是改进了NbN超导SIS结混频器芯片的安装及加固方式。采用热熔蜡对芯片的键合线进行加固，并使用拉力测试仪对加固前后的键合线强度进行了测试。结果表明，经过加固后的键合线强度平均增加50%，证明加固方式有效。二是改进了NbN超导SIS结混频器的Bias-T。针对原NbN超导SIS结混频器中的集成Bias-T可靠性较差，不符合航空规范的问题，研制了新型分布式Bias-T，在实现相同性能的同时大大提高了混频器的可靠性。最终，对500 GHz频段全NbN超导SIS结混频器空间环境适应性研究结果表明，本混频器不仅能满足我国目前空间高灵敏度太赫兹探测的需求，还能应用在未来我国极端环境下的太赫兹探测计划（如我国天文界正在积极推动的南极DATE5 太赫兹望远镜计划）。