How to Optimize the Structure and Quality of Thin Film Nobium for Superconducting Circuits

The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, researchers investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. They analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that they measure have quality factors of over 1 million at single-photon powers, among the best ever recorded using the Nb on sapphire platform.

They discovered the optimum temperature for deposition for the best circuits and other optimizations of the fabrication process.

Niobium is commonly used in the superconducting circuits community because it has the highest critical temperature and critical magnetic field of any elemental superconductor. Furthermore, mono-crystalline Nb growth is possible at sufficiently high temperature and optimised growth conditions. Sapphire substrates offer good thermal as well as chemical stability and have a small lattice mismatch with the Nb lattice, which allows for epitaxial growth. These key properties make Nb on sapphire suitable material platform to study the effect of crystallinity changes on superconducting resonator performance.

They have found that the crystallographic structure of Nb on sapphire films is drastically affected by changing the deposition temperature. The films ranging from a poly-crystalline structure at room temperature to a completely mono-crystalline character at the highest temperature of 975 K. With increasing temperature, the films have fewer grain boundaries and the crystal domains undergo an increasing degree of ordering in the
preferred orientation. These differences in crystallinity also lead to variation in surface roughness.

Losses in Nb qubits on sapphire are reduced by increasing the substrate temperature by only 250 K above room temperature. A similar optimum temperature might also exist for other material systems and are a subject for further investigation. The loss rate could be further reduced by optimizing the fabrication process by including steps like a HF etch to remove the processing oxide, etching trenches, encapsulating the resonator, or optimizing the CPW geometry for low electric field participation at material interfaces.

The highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, they analyze the temperature-dependent behavior of the resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. The results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and demonstrate that even a moderate change in temperature during thin film deposition can significantly affect the resulting
quality factors.

The effort to build large-scale quantum processors has emphasized how studying materials can improve the lifetime and coherence of quantum systems. Superconducting microwave resonators are often used to investigate loss in superconducting qubits because they are generally simpler to fabricate and measure while all sources of relaxation and decoherence affecting the performance of a resonator will also impact the performance of a qubit
made from the same material. Additionally, there are other applications that benefit from superconducting resonators with low levels of microwave dissipation, such as parametric amplifiers, quantum sensors, and microwave kinetic inductance detectors (MKIDs) used for astronomy and particle physics.