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IEEE Transactions on Applied Superconductivity

Using Surface Engineering to Modulate Superconducting Coplanar Microwave Resonator Performance

Evgeniya H. Lock, Peng Xu, Timothy Kohler, Lizmarie Camacho, Joseph Prestigiacomo, Yaniv Jacob Rosen, Kevin D. Osborn

Published Jan 9, 2019• 14 citations
Publisher

Abstract

Superconducting microwave resonators are important components of superconducting quantum information and astronomy detector systems. In this paper, we show how to modify the microwave resonator performance after fabrication through surface engineering. In particular, we focus on titanium nitride (TiN)/silicon (Si) resonators because they have shown potential for achieving high-quality factors (Qis). Depending on the type of surface treatment, chemicalor plasma-based, we found Qis that vary by approximately a factor of 18. We used inductively coupled plasma (ICP) combined with reactive ion etching (RIE) for the plasma surface treatment. We found that the microwave resonator performance depends on the type of plasma environment, such as single gas (oxygen) or gas mixtures [argon/hydrogen (Ar/H2), argon/octafluorocyclobutane (Ar/C4F8), and argon/sulfur hexafluoride (Ar/SF6)], and the plasma processing conditions, such as treatment time, ICP power, and RIE power. Of the plasma surface treatments, the Ar/SF6environments with no or low ICP power showed the highest potential to improve Qi. The processing conditions determined the chemistry and roughness of the Si and TiN surfaces, TiN film thickness, and the overall TiN/Si resonator structure (edge and sidewall). Our results can be used as a guideline for optimizing the microwave resonator performance using surface treatments.

DOI
10.1109/tasc.2019.2891883

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•Nov 11, 2019•0 citations
Superconducting microwave resonators are important components of superconducting quantum information and astronomy detector systems. In this paper, we show how to modify the microwave resonator performance after fabrication through surface engineering. In particular, we focus on titanium nitride (TiN)/silicon (Si) resonators because they have shown potential for achieving high-quality factors (Qis). Depending on the type of surface treatment, chemicalor plasma-based, we found Qis that vary by approximately a factor of 18. We used inductively coupled plasma (ICP) combined with reactive ion etching (RIE) for the plasma surface treatment. We found that the microwave resonator performance depends on the type of plasma environment, such as single gas (oxygen) or gas mixtures [argon/hydrogen (Ar/H2), argon/octafluorocyclobutane (Ar/C4F8), and argon/sulfur hexafluoride (Ar/SF6)], and the plasma processing conditions, such as treatment time, ICP power, and RIE power. Of the plasma surface treatments, the Ar/SF6environments with no or low ICP power showed the highest potential to improve Qi. The processing conditions determined the chemistry and roughness of the Si and TiN surfaces, TiN film thickness, and the overall TiN/Si resonator structure (edge and sidewall). Our results can be used as a guideline for optimizing the microwave resonator performance using surface treatments.
We report on the fabrication and characterization of superconducting coplanar waveguide (CPW) resonators based on titanium nitride (TiN) thin films deposited on 200\,mm diameter high-resistivity Si(100) substrates. We systematically investigate how deposition conditions, dry-etch power and in-situ resist strip temperature affect morphology, superconducting properties and dielectric losses. By tuning reactive sputtering conditions, three distinct preferred out-of-plane crystal orientations - (111), (200), and a mix of both are achieved. Our results demonstrate that all films exhibit similar minimal two-level system (TLS) losses, with TiN111 exhibiting the lowest median TLS losses δ~TLS\tildeδ_\mathrm{TLS}δ~TLS​, and greater robustness against reoxidation. The applied structuring process, in contrast, has a far greater influence on the TLS loss than the crystal orientation of the TiN film and, consequently, the intrinsic material properties of the superconducting layer. The lowest TLS losses for all TiN depositions were achieved with a low power etch and low temperature resist strip. An additional buffered oxide etch (BOE) treatment could remove high-loss interfacial oxides at the metal-air (MA) and substrate-air (SA) interface and recover the etch-induced TLS losses. Consequently, TiN resonators exhibiting δ~TLS\tildeδ_\mathrm{TLS}δ~TLS​ values as low as 9.67×10−79.67 \times 10^{-7}9.67×10−7 were realized. The corresponding median low-power loss, δ~LP\tildeδ_\mathrm{LP}δ~LP​, amounts to 11.04×10−711.04 \times 10^{-7}11.04×10−7, which translates to an internal quality factor approaching one million. These findings highlight the critical role of process induced oxide formation at the MA and SA interfaces in limiting the performance of TiN resonators and provide a scalable, low-loss process compatible with industry-grade 200\,mm CMOS qubit fabrication workflows.
Titanium nitride is an attractive material for a range of superconducting quantum-circuit applications owing to its low microwave losses, high surface inductance, and chemical stability. The physical properties and device performance, nevertheless, depend strongly on the quality of the materials. In this paper, we focus on the highly crystalline and epitaxial titanium nitride thin films deposited on sapphire substrates using magnetron sputtering at an intermediate temperature (${300}^{\ensuremath{\circ}}\mathrm{C}$). We performed a set of systematic and comprehensive material characterizations to thoroughly understand the structural, chemical, and transport properties. Microwave losses at low temperatures were studied using patterned microwave resonators, where the best internal quality factor in the single-photon regime is measured to be $3.3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ and that in the high-power regime $>1.0\ifmmode\times\else\texttimes\fi{}{10}^{7}$. Adjusted with the material filling factor of the resonators, the microwave loss-tangent here compares well with the previously reported best values for superconducting resonators. This work lays the foundation of using epitaxial titanium nitride for low-loss superconducting quantum circuits.
We have investigated the correlation between the microwave loss and patterning method for coplanar waveguide titanium nitride resonators fabricated on silicon wafers. Three different methods were investigated: fluorine- and chlorine-based reactive ion etches and an argon-ion mill. At high microwave probe powers, the reactive etched resonators showed low internal loss, whereas the ion-milled samples showed dramatically higher loss. At single-photon powers, we found that the fluorine-etched resonators exhibited substantially lower loss than the chlorine-etched ones. We interpret the results by use of numerically calculated filling factors and find that the silicon surface exhibits a higher loss when chlorine-etched than when fluorine-etched. We also find from microscopy that re-deposition of silicon onto the photoresist and side walls is the probable cause for the high loss observed for the ion-milled resonators.
We fabricated TiN coplanar waveguides using standard lithography techniques followed by ICP etch. In order to achieve high quality factor, we investigated the film growth by choosing different deposition conditions for various substrates. Quality factors of waveguide resonators were measured at 20 mK in both high and low microwave power limits. An inner quality factor of several million was achieved at high power limit for a predominantly (200)-oriented TiN film which was grown on HF cleaned silicon wafer. A quality factor of larger than one million was achieved at high power limit for TiN film grown on sapphire.
Tantalum (Ta) has recently received considerable attention in manufacturing robust superconducting quantum circuits. Ta offers low microwave loss, high kinetic inductance compared to aluminium (Al) and niobium (Nb), and good compatibility with complementary metal-oxide-semiconductor (CMOS) technology, which is essential for quantum computing applications. Here, we demonstrate the fabrication engineering of thickness-dependent high quality factor (high-Q_i) Ta superconducting microwave coplanar waveguide resonators. All films are deposited on high-resistivity silicon substrates at room temperature without additional substrate heating. Before Ta deposition, a niobium (Nb) seed layer is used to ensure a body-centred cubic lattice (α-Ta) formation. We further engineer the kinetic inductance (L_K) resonators by varying Ta film thicknesses. High L_K is a key advantage for applications because it facilitates the realisation of high-impedance, compact quantum circuits with enhanced coupling to qubits. The maximum internal quality factor Q_i of ~ 3.6 * 10^6 is achieved at the high power regime for 100 nm Ta, while the highest kinetic inductance is obtained to be 0.6 pH/sq for the thinnest film, which is 40 nm. This combination of high Q_i and high L_K highlights the potential of Ta microwave circuits for high-fidelity operations of compact quantum circuits.
Superconducting coplanar waveguide (CPW) microwave resonators are crucial elements in Photon detectors, Quantum-limited parametric amplifiers, Narrow-band filters, Read-out, interconnect in quantum processors and Hybrid devices, connecting solid-state spins with superconducting circuits [1-3]. In the quantum regime, the dominant loss mechanism for high-Q superconducting resonators can be attributed to parasitic two-level systems (TLSs) in the dielectrics. Interface TLSs are common by products of the fabrication process, often introduced by impurities associated with Si surfaces [3]. To reduce intrinsic losses, we employ isotropic deep reactive-ion etching (DRIE) of Si substrate to create suspended NbN superconducting resonators (SSR).In this study, thin films of niobium (Nb) and niobium nitride (NbN) are deposited on Si substrate by a DC magnetron sputtering system. The influence of the N2/Ar gas ratio, the deposition current, the substrate bias potential on the superconducting critical temperature of the films are investigated. Plasma etching of Nb and NbN in a SF6 and Cl2-BCl3 gas plasma is studied using an inductively coupled plasma (icp) reactor. Parametric studies on the effects of total gas flow rate and chamber pressure on the edge angles and etch rates are reported. Finally, the suspended NbN superconducting resonator is fabricated and will be tested. This could be applied to the fabrication of superconducting qubits in integrated circuits, offering a path towards longer qubit coherence times.Reference:[1] Landig et al. Coherent spin–photon coupling using a resonant exchange qubit. Nature 560, 179–184 (2018)[2] Tosi et al. Silicon quantum processor with robust long-distance qubit couplings. Nat Commun 8, 450 (2017)[3] Bruno et al. Reducing intrinsic loss in superconducting resonators by surface treatment and deep etching of silicon substrates, Appl. Phys. Lett. 106, 182601 (2015)[4] Kennedy et al. Tunable Nb Superconducting Resonator Based on a Constriction Nano-SQUID Fabricated with a Ne Focused Ion Beam, Phys. Rev. Applied 11, 014006 (2019).