Substrate surface engineering for high-quality silicon/aluminum superconducting resonators

Abstract

Quantum bits (qubits) with long coherence times are an important element for the implementation of medium-and large-scale quantum computers. In the case of superconducting planar qubits, understanding and improving qubits' quality can be achieved by studying superconducting planar resonators. In this Paper, we fabricate and characterize coplanar waveguide resonators made from aluminum thin films deposited on silicon substrates. We perform three different substrate treatments prior to aluminum deposition: One chemical treatment based on a hydrofluoric acid clean; one physical treatment consisting of a thermal annealing at 880 • C in high vacuum; one combined treatment comprising both the chemical and the physical treatments. The aim of these treatments is to remove the two-level state defects hosted by the native oxides residing at the various sample's interfaces. We first characterize the fabricated samples through cross-sectional tunneling electron microscopy acquiring electron energy loss spectroscopy maps of the samples' cross sections. These measurements show that both the chemical and the physical treatments almost entirely remove native silicon oxide from the substrate surface and that their combination results in the cleanest interface. We then study the quality of the resonators by means of microwave measurements in the "quantum regime", i.e., at a temperature T ∼ 10 mK and at a mean microwave photon number n ph ∼ 1. In this regime, we find that both surface treatments independently improve the resonator's intrinsic quality factor and that the highest quality factor is obtained for the combined treatment, Q i ∼ 0.8 million. Finally, we find that the TLS quality factor averaged over a time period of 3 h is ∼ 3 million at n ph ∼ 10, indicating that substrate surface engineering can potentially reduce the TLS loss below other losses such as quasiparticle and vortex loss.

Improving the Time Stability of Superconducting Planar Resonators

“…In our work of Ref. [11] (similar to the works in Refs. [12][13][14][15]), we have fabricated and characterized loss in CPW resonators made from thin-film Al on Si substrates.…”

“…We fabricate two samples on high-resistivity ( 10 kΩ cm) 500 μm-thick undoped Si substrates (for more details on sample preparation see our work of Ref. [11]): 1. One control sample without any Si substrate surface cleaning ("Unprocessed"); 2.…”

“…The samples have an equal layout, where one feed CPW transmission line is capacitively coupled to ten meandered quarter-wave resonators; each resonator has a different in a multiplexed design (see the work of Ref. [11] for more details). We measure through the feed line in the frequency range between 4 and 8 GHz.…”

“…Following Ref. [11], , * , and are obtained by fitting the complex function where is an offset angle (also fitted) and 1. In our work of Ref.…”

Geometric scaling of two-level-system loss in superconducting resonators

“…This method only probes TLS effects and has the benefit of being sensitive to a wide frequency distribution of TLS. Consequently, the intrinsic loss tangent is robust against spectrally unstable TLS that produce time variations in the quality factor [39]. This allows us to independently determine the intrinsic loss tangent (times the filling factor) F T LS δ i T LS .…”