Thomas Mayer

Optimizing CMOS-compatible, superconducting Titanium Nitride Resonators: Deposition Conditions and Structuring Processes

  1. Simon J. K. Lang,
  2. Alexandra Schewski,
  3. Ignaz Eisele,
  4. Johannes Weber,
  5. Carla Moran-Guizan,
  6. Zhen Lou,
  7. Moritz Singer,
  8. Benedikt Schoof,
  9. Marc Tornow,
  10. Thomas Mayer,
  11. Daniela Zahn,
  12. Rui N. Pereira,
  13. and Christoph Kutter
We report on the fabrication and characterization of superconducting coplanar waveguide (CPW) resonators based on titanium nitride (TiN) thin films deposited on 200,mm diameter high-resistivity
Si(100) substrates. We systematically investigate how deposition conditions, dry-etch power and in-situ resist strip temperature affect morphology, superconducting properties and dielectric losses. By tuning reactive sputtering conditions, three distinct preferred crystal orientations – (111), (200), and mixed are achieved. Our results demonstrate that all films exhibiting similar minimal two-level system (TLS) losses, with TiN111 exhibit the lowest median TLS losses δ̃ TLS, and greater robustness against reoxidation. The applied structuring process, in contrast, had a far greater influence on the TLS loss than the crystal orientation of the TiN film and, consequently, the intrinsic material properties of the superconducting layer. The lowest TLS losses for all TiN depositons were achieved with a low power etch and low temperature resist strip. An additional buffered oxide etch (BOE) treatment could remove high-loss interfacial oxides at the metal-air (MA) and substrate-air (SA) interface and recover the etch-induced TLS losses. Consequently, TiN resonators exhibiting δ̃ TLS values as low as 9.67×10−7 were realized. The corresponding median low-power loss, δ̃ LP, amounts to 11.04×10−7, which translates to an internal quality factor approaching one million. These findings highlight the critical role of process induced oxide formation at the MA and SA interfaces in limiting the performance of TiN resonators and provide a scalable, low-loss process compatible with industry-grade 200 mm CMOS qubit fabrication workflows.
arxiv pdf

Surface Optimization of Aluminum Resonators for Robust Quantum Device Fabrication

  1. Simon J. K. Lang,
  2. Ignaz Eisele,
  3. Alwin Maiwald,
  4. Emir Music,
  5. Luis Schwarzenbach,
  6. Carla Morán-Guizán,
  7. Johannes Weber,
  8. Daniela Zahn,
  9. Thomas Mayer,
  10. Rui N. Pereira,
  11. and Christoph Kutter
Aluminum remains the central material for superconducting qubits, and considerable effort has been devoted to optimizing its deposition and patterning for quantum devices. However,
while post-processing of Nb- and Ta-based resonators has been widely explored, primarily focusing on oxide removal using buffered oxide etch (BOE), post-treatment strategies for Al resonators remain underdeveloped. This challenge becomes particularly relevant for industry-scale fabrication with multichip bonding, where delays between sample preparation and cooldown require surface treatments that preserve low dielectric loss during extended exposure to ambient conditions. In this work, we investigate surface modification approaches for Al resonators subjected to a 24-hour delay prior to cryogenic measurement. Passivation using self-limiting oxygen and fluorine chemistries was evaluated utilizing different plasma processes. Remote oxygen plasma treatment reduced dielectric losses, in contrast to direct plasma, likely due to additional ashing of residual resist despite the formation of a thicker oxide layer on both Si and Al surfaces. A fluorine-based plasma process was developed that passivated the Al surface with fluorine for subsequent BOE treatment. However, increasing fluorine incorporation in the aluminum oxide correlated with higher loss, identifying fluorine as an unsuitable passivation material for Al resonators. Finally, selective oxide removal using HF vapor and phosphoric acid was assessed for surface preparation. HF vapor selectively etched SiO2 while preserving Al2O3, whereas phosphoric acid exhibited the opposite selectivity. Sequential application of both etches yielded dielectric losses as low as δLP=5.2×10−7 (Qi≈1.9M) in the single photon regime, demonstrating a promising pathway for robust Al-based resonator fabrication.
arxiv pdf