Krypton-sputtered tantalum films for scalable high-performance quantum devices

TL;DR

This work addresses the need for BEOL-compatible deposition of high-performance Ta films for superconducting quantum devices. By switching the sputter gas from Ar to Kr, the authors stabilize the BCC Ta phase on Si at temperatures as low as 200 C, achieving cleaner, more conductive films and a broadened process window. Microwave and qubit measurements show Kr-based Ta films reach state-of-the-art performance, with CPW resonators delivering LP Q ~ 4e6 and HP Q ~ 25e6, and transmon qubits with 20 μm capacitor gaps attaining a median Q1 up to 14e6, illustrating strong scalability potential. Collectively, these results establish Kr-based Ta deposition as a scalable, BEOL-friendly route for high-performance Ta-based superconducting devices, enabling industrial adoption in large-scale quantum computing fabrication.

Abstract

Superconducting qubits based on tantalum (Ta) thin films have demonstrated the highest-performing microwave resonators and qubits. This makes Ta an attractive material for superconducting quantum computing applications, but, so far, direct deposition has largely relied on high substrate temperatures exceeding \SI{400}{\celsius} to achieve the body-centered cubic phase, BCC (\textalpha-Ta). This leads to compatibility issues for scalable fabrication leveraging standard semiconductor fabrication lines. Here, we show that changing the sputter gas from argon (Ar) to krypton (Kr) promotes BCC Ta synthesis on silicon (Si) at temperatures as low as \SI{200}{\celsius}, providing a wide process window compatible with back-end-of-the-line fabrication standards. Furthermore, we find these films to have substantially higher electronic conductivity, consistent with clean-limit superconductivity. We validated the microwave performance through coplanar waveguide resonator measurements, finding that films deposited at \SI{250}{\celsius} and \SI{350}{\celsius} exhibit a tight performance distribution at the state of the art. Higher temperature-grown films exhibit higher losses, in correlation with the degree of Ta/Si intermixing revealed by cross-sectional transmission electron microscopy. Finally, with these films, we demonstrate transmon qubits with a relatively compact, \SI{20}{\micro\meter} capacitor gap, achieving a median quality factor up to 14 million.

Figures (23)

• Figure 1: (a) Schematic for the temperature deposition windows of -Ta on Si wafers with Ar and Kr sputter gases and their comparison with BEOL process windows. (b) Schematic of the cross-section profile of the films. (c) Residual resistivity ratio (RRR) 100nm-thick -Ta films as a function of substrate temperature during deposition for the two process gases. (d) The determined Ginzburg-Landau coherence length of superconductivity as a function of the effective transport mean free path ($\ell_{\rm 5K}$) for films of different thickness. The dashed line represents a 1:1 line.
• Figure 2: Atomic force microscopy (AFM, row a) and scanning transmission electron microscopy (STEM, rows b and c) of Ta films deposited with Kr and Ar between room temperature (RT) to 600. AFM and STEM of tantalum films grown with Ar at RT are visually indistinguishable from those grown with Kr at RT. a) AFM scans with equal-range height colormaps that are zeroed at the mean height of each image. b) High-angle annular dark-field-STEM (HAADF-STEM) images of the films at low magnification and c) zoomed into the metal-substrate interface, showing the amorphous interlayer between Si and the Ta film. A false-color map is applied to improve the perceptibility of the silicon lattice, which is weakly visible in the bottom layer.
• Figure 3: (a) Optical microscope image of a 100nm thick Ta CPW resonator deposited with Kr at 350. The inconsistency of the color in the trenched region reflects the inhomogeneous depth due to grain orientation-dependent etch rates (see also App. \ref{['sec:roughness']}). (b) Resonator quality factor measurement at single photon powers for 100nm thick Ta CPW resonator deposited with Kr at 350. Black indicates magnitude, while pink is the phase. (c) Power dependence of losses for three different types of resonators, from top to bottom: 600 Ar, RT Nb-seeded Ar, and 350 Kr. (d) Resonator losses for samples with post-BOE treatment. Each box represents $\delta_\mathrm{LP}$ for one chip: the red lines marks the median LP loss, the black box marks the 25% and 75% quartiles. Stars indicate median HP losses. Plus signs indicate median difference between LP and HP losses.
• Figure 4: (a) Optical microscope image of the transmon device with a 20µm gap between the capacitor pads. The inconsistency of the color in the exposed Si reflects the inhomogeneous depth due to grain orientation-dependent etch rates (see also App. \ref{['sec:roughness']}). (b) False-colored scanning electron microscopy images of the region of the Josephson junction. The first and second Al electrodes of the junction are show in purple and blue, respectively. The tantalum film is shown in yellow. (c) Violin plots of transmon quality factor for seven qubits measured; the distributions come from measuring $T_1$ repeatedly over about twelve hours. Values top to bottom represent maximum, median, and minimum lifetimes measured. Transmons 1-3 were fabricated without an oxygen descum, while transmons 4-7 had an additional oxygen descum. Insert has two example plots of lifetimes near the median value for both sets of devices, Norm. I/Q is the normalized I/Q signal and t is the measurement time.
• Figure A1: Measurements of resistivity and superconducting transition temperature $\rm T_{\rm c}$ for Ta films deposited at 350 with krypton with various thicknesses. The insert shows a zoomed in portion of the plot near $\rm T_{\rm c}$ for a 100nm film. The $\rm T_{\rm c}$ for this film was 4.32K.