Reducing TLS loss in tantalum CPW resonators using titanium sacrificial layers

TL;DR

TLS loss at the metal–air interface limits coherence in Ta-on-Si superconducting resonators. The authors introduce an ultrathin Ti sacrificial layer (~2 Å) deposited on pre-sputtered α-Ta that acts as an oxygen getter to modify the native Ta oxide; after lithography, the Ti layer is removed with buffered oxide etchant (BOE), and high-temperature annealing further suppresses TLS loss. This yields $Q_i$ values exceeding $1.5\times10^{6}$ in the single-photon regime across ten devices, about a $>3\times$ improvement over identical devices without Ti and up to $\sim4\times$ compared to baseline conditions. The method is compatible with standard Ta fabrication workflows, highlights interfacial oxide chemistry as a key loss mechanism, and provides a practical route toward longer $T_1$ lifetimes in Ta-based quantum circuits, with potential applicability to other planar qubit architectures.

Abstract

We demonstrate a substantial reduction in two-level system loss in tantalum coplanar waveguide resonators fabricated on high-resistivity silicon substrates through the use of an ultrathin titanium sacrificial layer. A 0.2nm titanium film, deposited atop pre-sputtered α-tantalum, acts as a solid-state oxygen getter that chemically modifies the native Ta oxide at the metal-air interface. After device fabrication, the titanium layer is removed using buffered oxide etchant, leaving behind a chemically reduced Ta oxide surface. Subsequent high-vacuum annealing further suppresses two-level system loss. Resonators treated with this process exhibit internal quality factors Qi exceeding an average of 1.5 million in the single-photon regime across ten devices, over three times higher than otherwise identical devices lacking the titanium layer. These results highlight the critical role of interfacial oxide chemistry in superconducting loss and reinforce atomic-scale surface engineering as an effective approach to improving coherence in tantalum-based quantum circuits. The method is compatible with existing fabrication workflows applicable to tantalum films, offering a practical route to further extending T1 lifetimes in superconducting qubits.

Figures (9)

• Figure 1: (a) Schematic of CPW fabrication with a sacrificial Ti layer. Native Ta oxide was removed by Ar$^+$ ion beam milling, followed by in situ deposition of a 2 Å Ti layer. Circuit patterning was performed using positive-tone optical lithography and CF$_4$-based reactive ion etching. Post-fabrication treatments consisted of buffered oxide etching (BOE) and annealing. (b) Surface atomic concentrations from XPS survey scans at stages marked in (a). The Ti layer is retained during lithography and etching, then removed by long-BOE prior to annealing.
• Figure 2: (a) Schematic of the fabricated circuit with five $\lambda/4$ resonators side-coupled to a transmission line. The resonator CPW has a 5 µm gap and 9 µm width. (b) Complex $S_{21}$ data (points) and circle fit (line) over a 100 kHz span around the centre frequency $f_c$. The scattering data traces a circle in the complex plane (left) and produces a Lorentzian dip in amplitude (right) rieger_fano_2023.
• Figure 3: Internal quality factor measurements for bare Ta reference samples, and variants of Ti/Ta devices. (a) Measured $Q_i$ data (points) as a function of mean intracavity photon number $\langle n \rangle$ with a saturable TLS model fit according to Eq. \ref{['eq:TLS_model']} (solid lines), yielding $F\delta_\text{TLS}$ values given in Table \ref{['tab:QR_results']}. (b) Box plot showing the distribution of $Q_i$ values at single photon power ($\langle n \rangle\approx1$) across resonators on each sample. The box spans the interquartile range (IQR) from the 25th (Q1) to the 75th (Q3) percentile, with a white dot at the median value. Whiskers extend to the most extreme data points within 1.5$\times$IQR from the box.
• Figure 4: Temperature dependence of $Q_i$ for a single resonator at $f_0=6.34$ GHz on a Ti-treated and annealed sample (T-LA2). Experimental data is shown as points, with a fitted temperature- and power-dependent loss model (Eq. \ref{['eq:total_losses']}) displayed as a solid line. The measurement was repeated at six evenly-spaced power levels between $P_\text{app}=-92$ dBm (dark blue) and $P_\text{app}=-142$ dBm (light blue), corresponding to intracavity photon numbers of $\langle n \rangle \approx 10^6$ and $\langle n \rangle \approx 10^1$ respectively.
• Figure 5: Wiring diagram for two-sample CPW resonator measurements.