Tantalum thin films sputtered on silicon and on different seed layers: material characterization and coplanar waveguide resonator performance

TL;DR

This study explores the deposition and characterization of tantalum thin films on silicon and seed layers, demonstrating high-quality superconducting properties suitable for quantum computing components.

Contribution

It presents a detailed analysis of tantalum thin films deposited on various substrates and seed layers, highlighting optimal conditions for high-performance superconducting resonators.

Findings

High-quality alpha-Ta films achieved at 600°C on silicon and seed layers.

Ta films on heated silicon exhibit internal quality factors up to 10^6.

Seed layers influence the phase and superconducting properties of Ta films.

Abstract

Superconducting qubits are a promising platform for large-scale quantum computing. Besides the Josephson junction, most parts of a superconducting qubit are made of planar, patterned superconducting thin films. In the past, most qubit architectures have relied on niobium (Nb) as the material of choice for the superconducting layer. However, there is also a variety of alternative materials with potentially less losses, which may thereby result in increased qubit performance. One such material is tantalum (Ta), for which high-performance qubit components have already been demonstrated. In this study, we report the sputter-deposition of Ta thin films directly on heated and unheated silicon (Si) substrates as well as onto different, nanometer-thin seed layers from tantalum nitride (TaN), titanium nitride (TiN) or aluminum nitride (AlN) that were deposited first. The thin films are characterized in terms of surface morphology, crystal structure, phase composition, critical temperature, residual resistance ratio (RRR) and RF-performance. We obtain thin films indicative of pure alpha-Ta for high temperature (600{\deg}C) sputtering directly on silicon and for Ta deposited on TaN or TiN seed layers. Coplanar waveguide (CPW) resonator measurements show that the Ta deposited directly on the heated silicon substrate performs best with internal quality factors $Q_i$ reaching 1 x $10^6$ in the single-photon regime, measured at $T=100 {\space \rm mK}$.