Hydrogenated amorphous silicon (a-Si:H) grown by PECVD has a lower loss tangent (tan _) among conventional dielectrics (such as SiO2 and SiNx) and hence is considered as the best amorphous dielectric material for superconducting qubit application. The incorporation of PECVD a-Si:H into the Nb technology requires attention due to the possible degradation of the superconductivity of the Nb films. Superconducting transition temperature (Tc) and residual resistivity (_0) of 20 nm, 50 nm and 100 nm thick Nb films were measured before and after a-Si:H deposition. The penetration of oxygen and hydrogen inside the Nb films was evaluated from the variation of the lattice parameter obtained by X-ray di_raction. The high process temperature (250_ C) and the presence of energetic hydrogen ions during the a-Si:H layer growth caused a decrease of Tc and increase of _0 through two physical processes: 1) oxygen di_usion from the surface Nb oxides and 2) hydrogen di_usion inside the Nb films. The degradation of Tc was reduced with the increase of the film thickness. Nitridation of Nb films and deposition of a sputtered thin amorphous silicon layer (a-Si) on the Nb films (in both cases made in situ after the Nb film deposition) were investigated a s surface treatments to protect the Nb filmsduring PECVD. It was demonstrated that both methods markedly reduce oxygen and hydrogen di_usion into Nb films during a-Si:H deposition, but the a-Si layer was more e_ective to protect the Nb films.
Hydrogenated amorphous silicon (a-Si:H) grown by PECVD has a lower loss tangent (tan _) among conventional dielectrics (such as SiO2 and SiNx) and hence is considered as the best amorphous dielectric material for superconducting qubit application. The incorporation of PECVD a-Si:H into the Nb technology requires attention due to the possible degradation of the superconductivity of the Nb films. Superconducting transition temperature (Tc) and residual resistivity (0) of 20 nm, 50 nm and 100 nm thick Nb films were measured before and after a-Si:H deposition. The penetration of oxygen and hydrogen inside the Nb films was evaluated from the variation of the lattice parameter obtained by X-ray di_raction. The high process temperature (250 C) and the presence of energetic hydrogen ions during the a-Si:H layer growth caused a decrease of Tc and increase of _0 through two physical processes: 1) oxygen di_usion from the surface Nb oxides and 2) hydrogen di_usion inside the Nb films. The degradation of Tc was reduced with the increase of the film thickness. Nitridation of Nb films and deposition of a sputtered thin amorphous silicon layer (a-Si) on the Nb films (in both cases made in situ after the Nb film deposition) were investigated a s surface treatments to protect the Nb filmsduring PECVD. It was demonstrated that both methods markedly reduce oxygen and hydrogen di_usion into Nb films during a-Si:H deposition, but the a-Si layer was more e_ective to protect the Nb films.
With a view to the fabrication of superconducting qubits with low decoherence time, we have investigated the influence of a-Si:H deposition by the plasma enhanced chemical vapor deposition method at 250◦C on the superconducting and structural properties of a 20 nm thick Nb film treated by two surface protection methods: plasma nitridation and deposition of a thin unhydrogenated Si layer. A suppression of theTc and an increase of the residual resistivity are observed due to hydrogen diffusion and decomposition of the native surface oxide, with subsequent oxygen diffusion caused by sample heating. The unhydrogenated Si layer is found to efficiently protect the Nb films against both diffusion processes.
The improvement of the coherence times of superconducting qubits depends on the reduction of the dielectric losses in the insulating materials implemented for the device fabrication. These losses depend on the density of spurious dipoles of different nature (two-level systems, TLSs) which couple to phase qubits and, hence, limit their coherence times. Hydrogenated amorphous silicon (a-Si:H), because of its lower loss tangent (tan δ) among conventional dielectrics (a-SiO, a-SiO2, a-SiNx:H), is considered as the best amorphous dielectric for superconducting qubit application. We have developed a reliable method for the direct measurement of dielectric losses in amorphous dielectric thin films using a novel design based on four lumped superconducting LC resonators connected in series without coupling capacitors. The losses are obtained as tan δ = 1/Q0, where Q0 is the intrinsic quality factor of the resonator, measured at 3 dB above the resonance frequency without any fitting procedure. The series type LC resonators with a-Si:H as dielectric were fabricated by the Nb technology. The measurements were done at the conditions of a qubit application (0.5 -10 GHz frequency range and low temperatures). The low values of the loss tangent of a-Si:H (up to 2.5 x 10−5 at 4.2 K) have required the development of superconducting housing for the resonators in order to eliminate a spurious dependence of tan δ on the microwave power, by reducing losses which were not originated in the dielectric itself. The results of the simulations agree well with the experiments.
Amorphous hydrogenated silicon (a-Si:H) based lumped elements are one of the promising ways for large-scale integration of superconducting quantum devices. We present a detailed study devoted to the plasma-enhanced chemical vapor deposition process parameters' influence on the structure and morphology of a-Si:H dielectric thin films. We experimentally demonstrate a microcrystalline-to-amorphous growth transition by changing the H2/SiH4 ratio and establish a relationship between the film's structure, thickness, and dielectric loss tangent (tan δ). It is shown that the best dielectric loss tangents of tan δLP = 4.5 × 10−5 (average 9.2 × 10−5) and tan δHP = 1.0 × 10−6 (average 1.2 × 10−6) for superconducting lumped elements with 350 nm-thick amorphous films can be achieved in single- and multi-photon regimes at sub-20 mK, correspondingly. These values are extracted by characterizing superconducting resonators with integrated parallel-plate capacitors (C = 0.2…6.0 pF) for a frequency range from 4.0 to 7.0 GHz. The presented low-loss a-Si:H films can be useful for various superconducting quantum applications, which is illustrated by the fabrication of lumped-element bandpass filters with a well-predicted amplitude–frequency response characteristic.
The loss of amorphous hydrogenated silicon nitride (a-SiNx:H) is measured at 30 mK and 5 GHz using a superconducting LC resonator down to energies where a single-photon is stored, and analyzed with an independent two-level system defect model. Each a-SiNx:H film was deposited with different concentrations of hydrogen impurities. We find that quantum-regime dielectric loss tangent tan δ0 in a-SiNx:H is strongly correlated with N–H impurities, including NH2. By slightly reducing x we are able to reduce tan δ0 by approximately a factor of 50, where the best films show tan δ0≃3×10−5.
To avoid degradation of circuit performance, fabrication processes for niobium-based superconductive electronics are typically limited to temperatures below 150°C. In this study, we investigated protective dielectric capping layers that preserve the superconducting properties of Niobium (Nb) wiring at processing temperatures as high as 400°C. To assess the thermal stability of Nb films, 400 nm thick Nb layers were deposited on oxidized silicon wafers and were either left uncapped or were intentionally capped with selected dielectric materials. The samples were subjected to post-deposition annealing in an argon atmosphere for comparison. Samples were annealed up to 450°C, and changes in room temperature sheet resistance were used as a proxy for film degradation and checked with cryogenic measurements of samples annealed up to 400°C. A sharp increase in room temperature sheet resistance, and a resulting decrease in residual resistivity ratio was observed in pristine Nb films above 300°C. We found that theex-situdeposition of insulating covering (“cap”) reduces the onset of degradation. Notably, silicon oxide (SiO_x) capping shows significant improvement and silicon nitride (Si_xN_y) capped samples exhibit minimal changes in resistance across the full annealing range. These results suggest that Si_xN_yencapsulation as a dielectric “cap” preserves superconductive properties and offers an expanded thermal budget for Nb-based superconductive electronics, with implications for multilayer integration and scalable fabrication.
Superconducting qubits are among the most advanced platforms for quantum information, offering potential scalability and compatibility with standard device fabrication techniques [1-5]. Among the materials investigated for superconducting qubits, Tantalum (Ta) has emerged as a compelling candidate due to its self-terminating stoichiometric surface oxide and chemical resistance to harsh post-fabrication processes. Efforts to mitigate surface loss by treating the surface oxides and utilizing high-resistivity silicon substrates have led to Ta-based qubits with lifetime and coherence times exceeding 1 ms [3]. Guided by these successes in reducing dielectric loss in Ta-based devices, we are interested in exploring material engineering approaches that enhance the superconducting gap. Alloying Ta with another element to enhance the superconducting performance expands the range of viable material candidates available to tackle different loss channels in superconducting circuits [1]. Recently, we explore superconducting gap engineering in Ta-based devices as a powerful strategy for expanding the range of suitable host materials. By alloying 20 at.% hafnium (Hf) into Ta thin films, we achieve a superconducting transition temperature (Tc) of 6.09 K as observed in direct current (DC) transport measurements, reflecting an increase in the superconducting gap. Here we demonstrate an approach for controlling varying deposition conditions to control film orientation and transport properties of Ta and Ta-Hf alloy thin films. We investigate the microstructure and composition distribution of the thin films by using high-resolution scanning transmission electron microscopy (STEM) with super-X EDS system (no serious beam damage issue [6-8]). Our structural analysis using TEM diffraction and STEM verifies the BCC structure across all the films (Figure 1), and direct current (DC) transport measurements reveal a 40% enhancement in Tc relative to undoped Ta (Tc=4.3 K). We monitored oxide growth over an 18-day period following film deposition to better understand how Hf incorporation influences surface oxides. In both Ta and Hf spectra, the fraction of fully oxidized (Ta2O5 and HfO2) states gradually increased during the 18-day exposure, indicating progressive surface oxidation. To estimate the surface oxide thickness, we use cross-sectional TEM/EDS mapping (Figure 1c). The analysis reveals that the oxide thickness for the 1 ks Ta-Hf alloy film is approximately 3 nm in thickness, comparable to the 3 nm oxide observed on native Ta [2]. For the 2 ks Ta-Hf film, the oxide thickness increases slightly to about 4 nm (Figure 1). The measured surface loss tangent of the Ta-Hf alloy remains comparable to that of native Ta. This result suggests that the surface oxide of the Ta-Hf alloy, which contains a measurable fraction of HfOx, does not introduce additional dielectric loss. By engineering the alloy composition and film deposition, this study achieves both improved superconducting properties and compatibility with fabrication processes. These findings show that material doping and alloying can strategically expand the suite of viable materials for superconducting quantum devices. This approach provides a pathway for tailoring material properties to support the next generation of quantum technologies and offers broad relevance to researchers in materials science, quantum information, and device engineering [9]. STEM images on cross sections of 1 ks (a) and 2 ks (b) Ta-Hf thin films, both films were sputtered at 750 ◦C. (c) Cross-sectional EDS elemental mapping on Ta-Hf. Cross-sectional EDS mapping Ta (blue) and Hf (orange) confirmed a uniform Ta-Hf alloy with the Ta to Hf atomic ratios for the bulk to be (83% : 17%). The oxygen (red) map reveals a surface oxide layer with the estimated surface oxide thickness of around 4 nm. The 1 ks Ta-Hf film exhibits an oxide thickness of approximately 3 nm, similar to that of native Ta. The EDS resolution limit is 0.5 atomic percent.