Validation of CMOS-compatible fabrication of capped Niobium coplanar resonators for transmon qubit integration  

A promising route to improving the coherence time of Josephson-junction-based transmon qubits lies in minimizing dielectric losses originating from both the junction and the surrounding resonator interconnects. While substrates such as sapphire offer superior intrinsic coherence [1,2], silicon enables CMOS-compatible fabrication that is crucial for scalable quantum processor integration. Niobium remains one of the most practical superconductors for implementing transmon circuitry on silicon substrates, including microwave resonators [3] and superconducting seed layers [4]. However, it often suffers from two-level-system losses, primarily arising from amorphous native oxides at the metal–air interface of thin films [5,6].

In this work, we enhanced the performance of coplanar waveguide niobium resonators—fabricated through a purely subtractive process on a 200 mm silicon line—by two approaches. First, we explored different superconducting capping layers (Ta, TaN, TiN) to passivate niobium surfaces and improve the internal quality factor (Qi) [7,8]. Cryogenic RF measurements combined with HAXPES analyses revealed that stronger plasma pre-cleaning leads to higher Qi values, reaching up to 1.8 × 10⁶ in the single-photon regime for Ta-capped resonators. Second, fabrication splits on various silicon substrates showed that increasing substrate resistivity significantly enhances Qi and reduces its within-wafer dispersion, consistent with the extracted loss tangent trends. These results demonstrate that high-resistivity intrinsic silicon wafers yield improved resonator performance. Although the Qi values remain below those achieved on sapphire substrates or optimized designs, silicon demonstrates adequate performance while preserving full CMOS compatibility. We thus established the first step towards realizing, within a fully subtractive CMOS-compatible process, a transmon qubit based on niobium resonators that will be co-integrated with Al/AlOx/Al Josephson junctions.

1. Nat. Commun. 15.1 (2024): 3687

2. Npj Quantum Inf. 8.1 (2022): 3

3. arXiv:2508.09577 (2025)

4. Scientific Reports 15.1 (2025): 27113

5. Phys. Rev. Appl. 22.2 (2024): 024035

6. Phys. Rev. Appl. 23.4 (2025): 044023

7. arXiv:2508.15957 (2025)

8. ACS Appl. Electronic Materials 6.10 (2024): 7372-7379