Growth and Structure of alpha-Ta films for Quantum Circuit Integration

TL;DR

This study explores the growth, structure, and properties of alpha-Ta films on various substrates, aiming to optimize their use in superconducting quantum circuits by reducing defects and improving quality factors.

Contribution

It presents new procedures for growing fully-oriented alpha-Ta films on sapphire and demonstrates their integration into quantum devices with high quality factors.

Findings

Alpha-Ta films on sapphire achieve high residual resistivity ratios (~60).

Resonators with alpha-Ta films exhibit TLS-limited quality factors of ~1.3 million at 10 mK.

Structural analysis links impurity levels and dislocation density to processing conditions.

Abstract

Tantalum films incorporated into superconducting circuits have exhibited low surface losses, resulting in long-lived qubit states. Remaining loss pathways originate in microscopic defects which manifest as two level systems (TLS) at low temperature. These defects limit performance, so careful attention to tantalum film structures is critical for optimal use in quantum devices. In this work, we investigate the growth of tantalum using magnetron sputtering on sapphire, Si, and photoresist substrates. In the case of sapphire, we present procedures for growth of fully-oriented films with alpha-Ta [1 1 1] // Al2O3 [0 0 0 1] and alpha-Ta [1 -1 0] // Al2O3 [1 0 -1 0] orientational relationships, and having residual resistivity ratios (RRR) ~ 60 for 220 nm thick films. On Si, we find a complex grain texturing with Ta [1 1 0] normal to the substrate and RRR ~ 30. We further demonstrate airbridge fabrication using Nb to nucleate alpha-Ta on photoresist surfaces. For the films on sapphire, resonators show TLS-limited quality factors of 1.3 +/- 0.3 x 10^6 at 10 mK (for waveguide gap and conductor widths of 3 microns and 6 microns, respectively). Structural characterization using scanning electron microscopy, X-ray diffraction, low temperature transport, secondary ion mass spectrometry, and transmission electron microscopy reveal the dependence of residual impurities and screw dislocation density on processing conditions. The results provide practical insights for fabrication of advanced superconducting devices including qubit arrays, and guide future work on crystallographically deterministic qubit fabrication.