Characterization and fabrication of high internal quality factor superconducting aluminum microwave resonators grown by molecular beam epitaxy

Lawrence Boyu Young^1*, Y. H. G. Lin¹, C. K. Cheng¹, L. S. Chiang¹, W. S. Chen¹, H. W. Wan¹, Y. T. Cheng¹, C. H. Hsu², Y. H. Lin³, J. Kwo³, M. Hong¹

¹Graduate Institute of Applied Physics and Department of Physics, National Taiwan University, Taipei, Taiwan
²National Synchrotron Radiation Research Center, Hsinchu, Taiwan
³Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Lawrence Boyu Young, email:f03222019@ntu.edu.tw

High-coherence superconducting quantum circuits are essential elements for quantum information science. Higher internal quality factor (Qi) values give longer decoherence time in quantum devices and qubits with hundreds of microseconds decoherence time correspond to overall Qi values greater than 1 million. [1] Currently, materials for superconducting quantum circuits are mostly prepared by physical vapor deposition. [2-4] The morphology of such films is polycrystalline with amorphous interfaces and surfaces, which leads to unwanted decoherence due to dielectric loss. Therefore, single-crystal films with well-controlled interfaces are demanded for material perfection to overcome the challenges of decoherence in quantum devices.
In this work, we have fabricated superconducting resonators using single-crystal Aluminum films grown on sapphire substrates by molecular beam epitaxy (MBE). [5] These Al films exhibited high crystallinity and atomically flat surface as confirmed by in-situ reflection high-energy electron diffraction, synchrotron radiation X-ray diffraction and atomic force microscopy, and all exhibited superconductivity below 1.2K. We have patterned Al films with photolithography and dry etching into half-wavelength 10-micrometer wide microstrip microwave resonators and measured them in a wide 3.5GHz to 10GHz passband rectangular waveguide. During the fabrication process, excess Al films were removed by using inductively coupled plasma reactive ion etching (ICP-RIE) with chlorine-based etching gas. After the etching process, the photoresist was removed by ultrasonic cleaning in organic solvent solutions, such as NMP or Acetone. We compared the microwave properties of the resonators made by different process conditions and found that resonators made by the optimized process showed Qi values orders of magnitude higher than others. These resonators reveal Qi higher than 1M around 7 GHz in the low power limit at 10 mK, regularly. Our work demonstrates that well-controlled material growth, fabrication process, and measurement are critical conditions to achieve highly coherent superconducting quantum circuits.

[1] de Leon et al., Science 372, eabb2823 (2021).
[2] H. Wang et al., Appl. Phys. Lett. 95, 233508 (2009).
[3] A. Megrant et al., Appl. Phys. Lett. 100, 113510 (2012).
[4] C. T. Earnest et al., Supercond. Sci. Technol. 31, 125013 (2018).
[5] Y. H. G. Lin et al., 2022 International Conference on Molecular-Beam Epitaxy


The authors would like to thank the support from the Ministry of Science and Technology (MOST) in Taiwan through Nos. MOST 110-2112-M-002-036-, 111-2119-M-007-005-, and the National Science and Technology Council (NSTC) in Taiwan through No. NSTC 111-2622-8-002-001-, 111-2112-M-007-043-. Y. H. G. Lin would like to thank the support from the National Science and Technology Council through No. NSTC 111-2811-M-002-123-. The authors acknowledge the support from YuShan Fellowship Program from the Ministry of Education, Taiwan, and the Center for Quantum Technology, Taiwan


The corresponding authors: yhlin@phys.nthu.edu.tw (Y. H. Lin), raynien@phys.nthu.edu.tw (J. Kwo) and mhong@phys.ntu.edu.tw (M. Hong).

Keywords: quantum computing, superconducting devices, single-crystal Al films