底电极应力对Nb/Al-AlOx/Nb结退火稳定性的影响研究

Effect of bottom electrode stress on the annealing stability of Nb/Al-AlOx/Nb junctions

  • 摘要: Nb/Al-AlOx/Nb约瑟夫森结是当前主流低温超导器件的核心元件。随着超导器件规模提升需求的不断增加,提高工艺温度成为关键技术方向之一。然而,在较高温度工艺过程中,铌与衬底之间的热膨胀系数差异会引入显著热应力,从而影响结的质量。为抑制热应力对结质量的不利影响,本文提出一种应力预补偿方法通过预先调控底电极铌膜的残余应力,抵消后续高温工艺中产生的热应力,从而将底电极总应力维持在较低水平。实验中,在4.2 K下对一系列具有不同底电极应力的约瑟夫森结进行退火前后的电学性能测试。结果表明,将底电极残余应力控制在一定的张应力范围内,可提高结在退火后的良率。经过250 ℃、30 min的退火处理后,初始底电极Nb膜为张应力的结相比压应力的结,良率提升约14%。该规律在Nb/Al-AlOx-Al*/Nb 约瑟夫森结的退火过程中也得到验证。

     

    Abstract: The Nb/Al-AlOx/Nb Josephson junction serves as the core component of mainstream low-temperature superconducting devices. With the growing demand for scaling up superconducting circuits, increasing the process temperature has become a key technical direction. However, during higher-temperature processing, the difference in the coefficient of thermal expansion between niobium and the substrate introduces significant thermal stress, which degrades junction quality. To mitigate the adverse effects of thermal stress, this paper proposes a stress pre-compensation method: by pre-tuning the residual stress of the bottom niobium electrode, the thermal stress generated in subsequent high-temperature steps is counteracted, thereby maintaining the total stress in the bottom electrode at a low level. In experiments, a series of Josephson junctions with different bottom-electrode stresses were electrically characterized at 4.2 K before and after annealing. The results show that controlling the residual stress of the bottom electrode within a specific tensile range can improve the post-annealing yield. After annealing at 250 ℃ for 30 min, junctions with an initially tensile-stressed bottom Nb electrode exhibited approximately a 14% higher yield compared to those with compressive stress. This trend was also verified in the annealing process of Nb/Al-AlOx-Al*/Nb Josephson junctions.

     

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