Wafer-level direct bonding of optimized superconducting NbN for 3D chip integration

Ye Li, Amir Mirza Gheytaghi*, Miki Trifunovic, Yuanxing Xu, Guo Qi Zhang, Ryoichi Ishihara

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

9 Citations (Scopus)
267 Downloads (Pure)


3D integration has well-developed for traditional CMOS technology operating at room temperature, but few studies have been performed for cryogenic applications such as quantum computers. In this paper, a wafer-to-wafer bonding of superconductive joints based on niobium nitride (NbN) is performed to demonstrate the possibility of 3D integration of superconducting chips. The NbN thin films are deposited by magnetron sputtering. Its high critical temperature (15.2 K) is achieved by optimizing the sputtering recipe in terms of N2 flow rate and discharge voltage. Wafer-level bumping is bonded by the thermo-compression method. The sheet resistance of the thin film and the contact resistance of the joints are measured by the Greek-cross (4-point Kelvin method) and daisy chain structures at cryogenic temperature, respectively. Direct-bonding wafers with NbN superconductive joints avoid using adhesive layers and the bonding interface could still present superconducting electrical connections in a cryogenic environment above 4 K, which will allow us to use a smaller and high-cooling power cryostat. The contribution of this work could lead to the fabrication of multi-layered superconducting chip that operates beneficially in cryogenic temperature, which is essential in building scalable quantum processors.

Original languageEnglish
Article number1353823
Number of pages6
JournalPhysica C: Superconductivity and its Applications
Publication statusPublished - 2021


  • NbN
  • Sputtering
  • Superconductivity
  • Thermo-compression
  • Wafer bonding


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