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Applied Physics Letters

Reduced microwave loss in trenched superconducting coplanar waveguides

Michael R. Vissers, Jeffrey S. Kline, Jiansong Gao, David Wisbey, David P. Pappas

Published Feb 20, 2012• 35 citations
Publisher

Abstract

Reducing the contribution of all sources of microwave loss is important for increasing coherence times in superconducting qubits. In this paper we investigate reducing the loss by systematically removing Si substrate material from the gap region in titanium nitride coplanar waveguides fabricated on intrinsic Si substrates. By exploiting the radial dependence of the etch rate in a parallel plate reactive ion etcher, otherwise identical coplanar waveguides with only the Si gaps etched to varying depth, i.e., trenched, were created in a single TiN film within a single processing step. Measurements at these multiple depths permit the study of the loss reduction in isolation to the unintentional effects caused by any single processing step. When comparing the loss from all trench depths we found that the high power loss was similar, but in the single photon limit the loss was reduced by a factor of two for deeper trenches in agreement with predictions from finite element analysis.

DOI
10.1063/1.3683552

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Reducing the contribution of all sources of microwave loss is important for increasing coherence times in superconducting qubits. In this paper we investigate reducing the loss by systematically removing Si substrate material from the gap region in titanium nitride coplanar waveguides fabricated on intrinsic Si substrates. By exploiting the radial dependence of the etch rate in a parallel plate reactive ion etcher, otherwise identical coplanar waveguides with only the Si gaps etched to varying depth, i.e., trenched, were created in a single TiN film within a single processing step. Measurements at these multiple depths permit the study of the loss reduction in isolation to the unintentional effects caused by any single processing step. When comparing the loss from all trench depths we found that the high power loss was similar, but in the single photon limit the loss was reduced by a factor of two for deeper trenches in agreement with predictions from finite element analysis.
Improving the performance of superconducting qubits and resonators generally results from a combination of materials and fabrication process improvements and design modifications that reduce device sensitivity to residual losses. One instance of this approach is to use trenching into the device substrate in combination with superconductors and dielectrics with low intrinsic losses to improve quality factors and coherence times. Here, we demonstrate titanium nitride coplanar waveguide resonators with mean quality factors exceeding two million and controlled trenching reaching 2.2 μm in the silicon substrate. Additionally, we measure sets of resonators with a range of sizes and trench depths and compare these results with finite-element simulations to demonstrate quantitative agreement with a model of interface dielectric loss. We then apply this analysis to determine the extent to which trenching can improve resonator performance.
© 2020 Author(s). Lossy dielectrics are a significant source of decoherence in superconducting quantum circuits. In this report, we model and compare the dielectric loss in bulk and interfacial dielectrics in titanium nitride (TiN) and aluminum (Al) superconducting coplanar waveguide resonators. We fabricate isotropically trenched resonators to produce a series of device geometries that accentuate a specific dielectric region's contribution to the resonator quality factor. While each dielectric region contributes significantly to loss in TiN devices, the metal-air interface dominates the loss in the Al devices. Furthermore, we evaluate the quality factor of each TiN resonator geometry with and without a post-process hydrofluoric etch and find that it reduced losses from the substrate-air interface, thereby improving the quality factor.
Lossy dielectrics are a significant source of decoherence in superconducting quantum circuits. In this report, we model and compare the dielectric loss in bulk and interfacial dielectrics in titanium nitride (TiN) and aluminum (Al) superconducting coplanar waveguide resonators. We fabricate isotropically trenched resonators to produce a series of device geometries that accentuate a specific dielectric region's contribution to the resonator quality factor. While each dielectric region contributes significantly to loss in TiN devices, the metal–air interface dominates the loss in the Al devices. Furthermore, we evaluate the quality factor of each TiN resonator geometry with and without a post-process hydrofluoric etch and find that it reduced losses from the substrate–air interface, thereby improving the quality factor.
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.
In current superconducting qubit circuit development, coherence time is limited by low-frequency flux noise introduced from localized two-level systems (TLSs) in the fabricated superconducting circuit, including superconducting materials, the interface between thin films, and the substrate. As a part of the effect to tackle this issue from a substrate standpoint, we succeed in fabricating NbN (200) oriented coplanar waveguide resonator (CPWR) on a thin titanium nitride (TiN) (200) buffered Si (100) substrate. The internal loss is close to 3 × 10-5nearphoton> ~1, improved from ~2 × 10-4in NbN (200) CPWR fabricated on MgO (100). For NbN (200) resonators grown on Si (100), the internal loss shows microwave energy independent. To further clarity if this energy independence is due to the change of substrate material, we also fabricated TiN (200) CPWR on Si (100). In contrast to NbN (200) CPWR, the loss of TiN (200) resonator shows a strong microwave dependent, an indication of TLSs originated from the interface and the surface. We suggest TLSs in NbN (200) CPWR are dominated from different mechanism other than arising from the interface and surface.
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.