Superconducting qubits have emerged as a potentially foundational platform technology for addressing complex computational problems deemed intractable with classical computing. Despite recent advances enabling multiqubit designs that exhibit coherence lifetimes on the order of hundreds of μs, material quality and interfacial structures continue to curb device performance. Two-level system defects in the thin superconducting film and adjacent dielectric regions introduce stochastic noise and dissipate electromagnetic energy at the cryogenic operating temperatures. In this study, we utilize time-of-flight secondary ion mass spectrometry to understand the role specific fabrication procedures play in introducing such dissipation mechanisms in these complex systems. We interrogated Nb thin films and transmon qubit structures fabricated through slight modifications in the processing and vacuum conditions. We find that when the Nb film is sputtered onto the Si substrate, oxide and silicide regions are generated at various interfaces. We also observe that impurity species, such as niobium hydrides and carbides, are incorporated within the niobium layer during the subsequent lithographic patterning steps. The formation of these resistive compounds likely impacts the superconducting properties of the Nb thin film. Additionally, we observe the presence of halogen species distributed throughout the patterned thin films. We conclude by hypothesizing the source of such impurities in these structures in an effort to intelligently fabricate superconducting qubits and extend coherence times moving forward.

Superconducting qubits represent one of the most mature platforms of emerging quantum information technologies.1–3 This platform has enabled computers that can outperform the world's largest classical computers with respect to calculating random circuits4 with processors consisting of 65 superconducting qubits with gate and readout fidelities of 99.97% and 99.8%, respectively.5 Nonetheless, the presence of surfaces, interfaces, and defects in the constituent materials has proven to curtail coherence times well below the millisecond and second timescales necessary for scalable quantum devices.6–8 In particular, these structures introduce two-level systems (TLSs), which contribute heavily to microwave loss at the cryogenic operating temperatures and the single photon level operating powers.9 As such, this has accelerated worldwide efforts to identify and eliminate these sources of decoherence in superconducting qubit systems.10–13 

In terms of materials, Nb thin films have been employed extensively as the superconducting component in such qubit architectures. These films can be deposited via a variety of techniques, including high power impulse magnetron sputtering (HiPIMS),14 electron cyclotron resonance,15 and cathodic arcs.16 Their low kinetic inductance17 and compatibility with industrial level processes, particularly when deposited on silicon wafers, make them quite attractive for superconducting qubit applications.18 Furthermore, the considerable development in niobium metal refining over the past few decades has enabled the extraction of ultra-high purity niobium sources at a fraction of the cost associated with alternative superconducting metals, such as Mo, Re, and Ta.19 Nevertheless, the experimentally derived microwave loss associated with Nb thin films is typically orders of magnitude larger than that expected for the pure elemental compound, as demonstrated by extremely high quality factor Nb cavity resonators.20 This has generally been attributed to the presence of native amorphous oxides with varying stoichiometries (NbO, NbO2, and Nb2O5) that are known to host TLS defects.21–24 Although comparatively under-investigated, it is likely that additional chemical impurities in these films impact the superconducting properties and help explain the measured loss in these films.25,26 As such, a systematic investigation of the chemical constituents present within these thin films combined with an identification of processing protocols that give rise to these defect structures is critical for enabling a more intelligent design of superconducting qubit architectures.

To this end, we employ time-of-flight secondary ion mass spectrometry (TOF-SIMS) to thoroughly understand the chemical constituents present within Nb thin films deposited on Si wafers through sputter deposition. This technique combines high mass resolution and sensitivity to light elements with <100 nm spatial resolution. By performing this detailed characterization on samples taken from various steps throughout the superconducting qubit fabrication process, we are able to readily identify the role that different processes play in terms of introducing structural deviations and contamination in such systems. These samples include a blanket film of Nb sputter deposited on a Si substrate, along with samples that have undergone lithographic patterning steps and a fully fabricated superconducting qubit architecture. We find that oxide and silicide regions are formed at the metal/air and metal/substrate interfaces following deposition of Nb, respectively. We also find that the subsequent lithographic patterning steps lead to an inward diffusion of hydrogen and carbon throughout the thin film, which can generate niobium hydrides and carbides. Additionally, we observe that the etching procedures lead to a similar inward diffusion of halogen species throughout the patterned thin films, which, thereby, generates a final architecture composed of a multitude of sources potentially leading to quantum decoherence.

Nb films were prepared on the Si (001) wafers (float-zone >10 000 Ω cm) via HiPIMS with a base pressure less than 1 × 10−8 Torr at room temperature. Nb films were also prepared on similar Si (001) wafers via DC magnetron sputtering at a pressure of 3 × 10−3 Torr with a 15 sccm Ar flow from a 3 in. diameter Nb target having a metal basis purity of 99.95%. A 300 W sputtering power was used and resulted in a deposition rate of 1.3 Å/s at room temperature.

Prior to deposition, the wafer was prepared with an RCA surface treatment detailed previously.8,27 An annular dark field (ADF) scanning/transmission electron microscopy image and energy dispersive spectroscopy (EDS) elemental maps of the Nb/Si interface are provided in Figs. 1(a)–1(c). Following Nb deposition, transmon qubits were fabricated following the procedures detailed by Nersisyan et al.8 TOF-SIMS measurements were performed following each processing step using a dual beam IONTOF 5 system to analyze the concentration and depth distribution of impurities in the Nb film. Secondary ion measurements were performed using a liquid bismuth ion beam (Bi+). A cesium ion gun with an energy of 500 eV was used for sputtering the surface for depth profile measurements to detect anions, and an oxygen ion gun with an energy of 500 eV was used for sputtering the surface for depth profile measurements to enhance the detection of metallic impurities.28 

FIG. 1.

Nb/Si interface. (a) STEM image taken at the film/substrate interface of the Nb thin film deposited on a Si (001) wafer with HiPIMS. (b) and (c) Associated EDS maps constructed from this interface using characteristic Nb Lα and Si Kα x-ray emission. (d) TOF-SIMS depth profile taken from this interface. Alloyed region is indicated by dotted lines.

FIG. 1.

Nb/Si interface. (a) STEM image taken at the film/substrate interface of the Nb thin film deposited on a Si (001) wafer with HiPIMS. (b) and (c) Associated EDS maps constructed from this interface using characteristic Nb Lα and Si Kα x-ray emission. (d) TOF-SIMS depth profile taken from this interface. Alloyed region is indicated by dotted lines.

Close modal

TOF-SIMS measurements were taken following Nb deposition, and a depth profile taken from the film is provided in Fig. 1(d). In this figure, the Nb film region and Si substrate are clearly indicated. At the surface of the film, we observe the presence of an oxide layer. As previously reported by various groups, this surface oxide is known to host TLS defects and can reduce coherence times.21–24 Additionally, we observe the presence of Si signal at the surface, but we expect that this signal is a result of surface contamination associated with wafer processing and/or handling. With respect to the metal/substrate interface, the TOF-SIMS data are suggestive of a gradual interface between Nb and Si where Nb atoms are implanted within the Si substrate. Although collisional cascade effects including atomic mixing during TOF-SIMS depth profiling can preclude direct quantification of this graded interface,29 STEM EDS results corroborate the presence of an alloyed interface on the order of 8 nm. Based on previous findings, it is likely that this region is composed of alloys such as the Nb5Si3 and NbSi2 phases.30 As the signal intensity in ADF images is proportional to Zα, where Z represents atomic number and α lies between 1.2 and 1.8,31 the presence of this alloyed region is supported by Fig. 1(a) as well. It is possible that this alloyed region could appreciably reduce the superconducting transition temperature or serve as an additional scattering source leading to the breaking of Cooper pairs.32 

In addition to the surface oxide and the interfacial amorphous alloys, we examined the distribution of oxides and hydrocarbons in the blanket Nb film. Both of these species are known to introduce loss in superconducting niobium systems and are presented in Fig. 2.20–24,33 In addition to the appreciable content of oxygen in the immediate vicinity of the surface, from this figure, we observe that this oxygen signal decays to a baseline level of 0.1 O/Nb counts within the film. Moreover, as the propensity for Nb to form hydrides and carbides has been studied extensively,26,34,35 it is no surprise to observe enhanced signals corresponding to carbon and hydrogen at the surface.

FIG. 2.

TOF-SIMS depth profiles of hydrocarbon species in Nb thin films. (a) Oxygen, (b) hydrogen, and (c) carbon signals are plotted as a function of depth for the three samples investigated. These include the Nb thin film deposited with HiPIMS, the Nb thin film deposited with DC magnetron sputtering, and the Nb contact pad from patterned transmon qubit.

FIG. 2.

TOF-SIMS depth profiles of hydrocarbon species in Nb thin films. (a) Oxygen, (b) hydrogen, and (c) carbon signals are plotted as a function of depth for the three samples investigated. These include the Nb thin film deposited with HiPIMS, the Nb thin film deposited with DC magnetron sputtering, and the Nb contact pad from patterned transmon qubit.

Close modal

Similar measurements taken from Nb deposited via DC magnetron sputter deposition are provided in Fig. 2 as well. In this case, we observe similar H and C profiles to that observed for Nb films deposited with HiPIMS. The normalized oxygen counts, however, are roughly one order of magnitude higher than the Nb films deposited with HiPIMS deposition. This can likely be attributed to differences in vacuum conditions. Based on previous findings, it is likely that this oxygen is mainly localized at grain boundaries in the Nb film.23 

Following the patterning of the qubit architecture, TOF-SIMS depth profiles are taken from large area Nb contact pads deposited with HiPIMS to once again probe the distribution of oxides and hydrocarbons in the film. We find that the transmon device exhibits increased concentrations of hydrogen, carbon, and oxygen relative to the blanket films, likely as a result of the lithography and fabrication techniques. Specifically, we find that in addition to observing a large intensity of these species at the surface, the signal associated with these species decays much more gradually before asymptotically approaching a baseline value, implying that the contaminant species are driven into the Nb layer. In all three cases, this decay length extends through the upper 30 nm of the Nb film. Additionally, the baseline signal of these species is roughly one order of magnitude larger than the respective signals measured in the blanket Nb film. Three-dimensional images representing the distribution of these species are provided in Fig. 3. The relative in-plane uniformity present in these profiles confirms that depth profiles are taken from representative regions free of surface features and the artifacts that they can introduce.36,37 Furthermore, it is again apparent that the lithography process introduces increased oxygen and hydrocarbon species throughout the film.

FIG. 3.

3D rendering representing TOF-SIMS depth profiles of hydrocarbon species in Nb thin films. (a) Oxygen, (b) hydrogen, and (c) carbon signals are plotted as a function of depth for the Nb thin film deposited with HiPIMS. (d) Oxygen, (e) hydrogen, and (f) carbon signals are plotted as a function of depth for the Nb contact pad from patterned transmon qubit.

FIG. 3.

3D rendering representing TOF-SIMS depth profiles of hydrocarbon species in Nb thin films. (a) Oxygen, (b) hydrogen, and (c) carbon signals are plotted as a function of depth for the Nb thin film deposited with HiPIMS. (d) Oxygen, (e) hydrogen, and (f) carbon signals are plotted as a function of depth for the Nb contact pad from patterned transmon qubit.

Close modal

Based on the fabrication process, we hypothesize that these contaminant species are likely introduced through the organic photoresist used during the lithography process. We suspect that photoresist residue may remain on the thin film surface following liftoff procedures and can diffuse into the film during subsequent baking steps with the help of grain boundaries.23 One potential solution to reduce the presence of these species would be to define all relevant patterns using hard, physical masks.38,39

In addition to the aforementioned contaminants, we also probe the distribution of other anionic and cationic contaminants in the Nb film. In the case of hydroxides, fluorides, and chlorides, we observe a similar behavior in the blanket films compared to the lithographically defined contact pads. Specifically, we observe that whereas all samples exhibit >101 normalized counts of OH, F, and Cl at the surface, in the blanket film sample, the intensity of these contaminants decays to a stable baseline value within a few nanometers. For the lithographically defined devices, we observe a much more gradual decay over a distance of 30 nm although collisional cascade effects can again preclude direct quantification of this decay length. Additionally, the baseline value of these signals is an order of magnitude larger than the patterned films compared to the blanket films and potentially impact the conductivity of the superconducting film. We hypothesize that these halogen species are introduced through the reactive ion etching and wet etching processes, which further highlight the value in potentially employing physical masks to define relevant features in the qubit structure.38–40 

Finally, a distribution of cationic contaminants is provided in Fig. 4. In this case, as well, the concentration of species such as K+, Na+, and Ca+ is amplified in the thin film region of the pattern sample. While the specific source introducing these species is unknown, these species, in particular K+ and Na+ that have unpaired electrons, can potentially facilitate noise at low temperatures.9 

FIG. 4.

TOF-SIMS depth profiles of various anionic and cationic species in Nb thin films. (a) OH, (b) F, (c) Cl, (d) Ca+, (e) K+, and (f) Na+ signals are plotted as a function of depth for the Nb thin film deposited with HiPIMS and the Nb contact pad from patterned transmon qubit.

FIG. 4.

TOF-SIMS depth profiles of various anionic and cationic species in Nb thin films. (a) OH, (b) F, (c) Cl, (d) Ca+, (e) K+, and (f) Na+ signals are plotted as a function of depth for the Nb thin film deposited with HiPIMS and the Nb contact pad from patterned transmon qubit.

Close modal

Here, we investigate the role that various fabrication procedures play in introducing dissipation mechanisms in quantum systems using TOF-SIMS. In the case of an optimized deposition process for 2D transmon qubit (comparison to a previous generation of a 3D transmon qubit and a standard Nb RF cavity is provided in Figs. S1 and S2), we find that following deposition of Nb thin films, an oxide layer is present at the surface and an alloyed Nb/Si region is generated at the film/substrate interface. We also find that the lithography steps employed lead to the incorporation of impurity species, including O, H, C, Cl, F, Na+, Mg+, and Ca+ within the Nb thin film. As these species can appreciably alter the superconducting properties and/or introduce stochastic charge noise in these systems, extending coherence times will require future designs that employ strategies to diminish these contaminants.

See the supplementary material for depth profile comparisons of 2D and 3D transmon structures and of transmon qubit and standard Nb RF cavity.

This material is based upon work supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under the Contract No. DE-AC02-07CH11359. We thank Rigetti Computing for supporting the development of these devices and the Rigetti chip design and fabrication teams for the development and manufacturing of the qubit devices used in the reported experimental study. This work made use of instruments in the Electron Microscopy Core of UIC's Research Resources Center. The authors thank members of the Superconducting Quantum Materials and Systems (SQMS) Center for a valuable discussion.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Supplementary Material