By Muhammad OsamaReviewed by Lexie CornerJun 23 2025A recent study published in Advanced Materials Interfaces examined how surface roughness impacts the performance of superconducting resonators—key components in quantum computing systems.
The research focused on niobium (Nb) thin-film resonators on silicon substrates and analyzed how microscopic surface features influence microwave losses and the internal quality factor (Qi).
The findings offer useful insights into improving the consistency and performance of superconducting circuits, which is important for building scalable quantum devices.
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Superconducting Materials and Their Role
Superconducting circuits use materials such as aluminum, niobium, and tantalum to build resonators, waveguides, and Josephson junctions. Niobium is commonly used due to its relatively high critical temperature (Tc ≈ 9.25 K), large superconducting energy gap (1.41 meV), low kinetic inductance, and smooth surface finish (approximately 1 nm RMS). These properties help increase coherence times and improve gate fidelity.
Despite these advantages, achieving a consistently high internal Qi remains difficult. Loss mechanisms such as two-level system (TLS) defects, vortex trapping at grain boundaries, and quasiparticle losses from pinholes or edge roughness can degrade performance.
Although surface cleaning and passivation have reduced TLS-related losses, variability in Qi across devices on the same chip continues to be a challenge for scaling quantum technologies.
Investigating the Effects of Surface Roughness
The researchers studied superconducting resonators operating under medium photon numbers (108 and 1010) and moderately elevated temperatures (~1.2 K), which are relevant to practical quantum computing applications.
They fabricated coplanar waveguide (CPW) resonators on high-resistivity silicon wafers (>5 kΩ·cm) using physical vapor deposition (PVD) to deposit Nb films with a preferred crystalline orientation.
Two surface treatments were compared:
- Ozone exposure, which created rougher niobium surfaces (NbR) with RMS roughness of ~0.98 nm.
- Oxygen plasma treatment, which produced smoother surfaces (NbS) with RMS roughness of ~0.31 nm.
Atomic force microscopy (AFM) showed that oxygen plasma reduced surface roughness by etching away features and decreasing pinhole depths from 4 nm to 1.5 nm. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) confirmed a ~2.0 nm increase in native oxide thickness with oxygen plasma, without damaging the crystal structure.
The critical temperature Tc decreased from 9.02 K (NbR) to 8.98 K (NbS), consistent with the thicker oxide layer. The smoother surfaces also showed lower resistance at low temperatures, suggesting reduced surface scattering.
The resonators were measured at 1.2 K using vector network analyzers to assess microwave transmission and determine Qi. Multiple chips from different wafers were tested to verify the consistency of the results.
Results: Quality Factor and Loss Mechanisms
The experiments showed that at lower frequencies (4-5 GHz), the smoother NbS resonators achieved Qi values nearly five times higher than those of the rougher NbR devices. However, this difference diminished at higher frequencies (5-6 GHz) with smaller areas, where Qi values became similar regardless of surface roughness.
This frequency- and area-dependent behavior indicates that microwave-induced quasiparticle losses are significantly affected by surface roughness. The total microwave loss was modeled as a combination of temperature-dependent TLS resistance RTLS, quasiparticle resistance RQP, and residual resistance Rres due to surface imperfections. While TLS losses dominate at millikelvin temperatures and low power levels, quasiparticle losses become more significant at the 1.2 K operating temperature.
The study also emphasized the importance of fabrication cleanliness and process control. Dust particles and process-induced defects could lower the yield of functional resonators. By optimizing room-temperature deposition, researchers achieved yields exceeding 90 %, surpassing prior results that required high-temperature growth.
Implications for Quantum Device Fabrication
The findings highlight the critical role of surface roughness and oxide composition in minimizing microwave-induced losses and ensuring consistent resonator performance. Smoother surfaces with fewer pinholes reduce electric field interaction at lossy interfaces, improving Qi and reducing variation among resonators on the same chip.
These improvements are important for scaling superconducting quantum circuits, where large arrays of qubits and resonators must maintain uniform coherence and coupling. Oxygen plasma treatment enhances surface smoothness and oxide quality without compromising the crystal structure or superconducting critical temperature.
Achieving uniform resonator quality across a chip enables the integration of more qubits, boosts system reliability, and enhances performance, key requirements for advancing quantum processors. This research supports the development of reliable and uniform-quality resonators, which are necessary for quantum error correction and the implementation of complex algorithms in fields such as cryptography, optimization, and simulation.
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Conclusion: Advancing Quantum Computing
This study shows that surface roughness influences the internal quality factor of niobium-based superconducting resonators at relevant temperatures for quantum computing. Comparing ozone-treated and oxygen plasma-treated Nb films, the researchers found that smoother surfaces reduce quasiparticle losses and improve resonator performance.
The findings support further efforts to optimize surface treatments, evaluate alternative passivation methods, and explore different substrate materials. A deeper understanding of how surface defects interact with other loss mechanisms will help improve device consistency and performance.
These insights offer practical guidance for refining fabrication processes and designing high-performance superconducting circuits for use in quantum computing applications such as cryptography, optimization, and simulation.
Journal Reference
Karuppannan, S, K., et al. (2025). Impact of Surface Roughness on Consistent Resonator Performance. Advanced Materials Interfaces. DOI: 10.1002/admi.202500020, https://advanced.onlinelibrary.wiley.com/doi/10.1002/admi.202500020
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