Niobium nitride Josephson junction process development (original) (raw)
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New fabrication technique and electronic tunneling studies of NbN
We have fabricated NbN films using a new dual ion-beam sputter deposition technique. In this technique, one ion beam sputters Nb from a target, while the second ion beam directly bombards the growing film. The T c of these NbN films is typically 11-12 K when deposited on near-room temperature substrates. Preliminary results on tunneling through a native oxide barrier yield a superconducting energy gap for NbN of 1.9 meV, for which 2A/kT c = 3.9. We have also studied oxidized overlayers of AI and Ta to produce an artificial oxide tunneling barrier. Conductance data indicate that the native oxide barrier is broad with a low barrier height (0,2-0.3 eV), whereas the oxidized AI overlayer has a higher (0.5-1.2 eV) and narrower barrier. The quality of the I-V curves for native oxide and for artificial oxide barriers is comparable.
Superconducting Characteristics of NbN Films Deposited by Atomic Layer Deposition
IEEE Transactions on Applied Superconductivity, 2017
A fabrication process for NbN films with a high transition temperature (TC) of about 17 K was developed to realize NbN/AlN/NbN superconducting-tunnel-junction (STJ) array particle detectors with a high operation temperature of >1 K and a high sensitivity by using atomic layer deposition (ALD). The source (precursor) of the NbN films was (tertbutylimido)bis(diethylamino)niobium (TBTDEN) [C16H39N4Nb]. NbN films were deposited on M-plane sapphire wafers. A 50-nmthick NbN film was deposited with a substrate temperature of about 570 K and exhibited a TC of 12.35 K. To improve the superconducting properties of the NbN formed by ALD, a postdeposition rapid thermal annealing process was performed. The dependence of TC, the surface morphology, and the crystallinity of the film on thermal annealing temperatures was evaluated.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1986
Reactive dc magnetron sputtering is used to deposit hard, refractory, superconducting thin films of niobium nitride on amorphous (glass) as wen as crystalline (sapphire) substrates held at room temperature. Stoichiometric NbN films so obtained possess the desired B 1 crystal structure with a distinct (Ill) texture, and exhibit high (-16 K) superconductingtransition temperature. Room temperature deposition of the high Tc films is achieved primarily by optimizing the reactant fluxes of niobium and nitrogen reaching the substrate, through a systematic study of the nitrogen consumption versus injection characteristics which are found to be an absolute indicator of the quality of NbN formed. Superconductor-insulator-superconductor (SIS) junctions are fabricated by using high Tc NbN as the base electrode, a thin layer of native oxide grown in oxygen plasma or in room air as the insulator, and vacuum deposited Pb as the counterelectrode. The current versus voltage (I-V) characteristics of the junctions exhibit high (-110) nonlinearity parameter and the superconducting gap parameter for NbN, a ~ 2. 8 me V. The transition temperature, superconducting gap parameter, and nonlinearity parameter of the junctions are identical for the films deposited at room temperature with predominant (111) texture as well as the films deposited at elevated temperatures possessing (200) texture.
Fabrication and characterization of epitaxial NbN films and tunnel junctions
We report fabrication and characterization of epitaxial NbN/TaN/NbN Josephson junctions grown by pulsed laser ablation. These SNS junctions can be used as elements of rapid-single-flux-quantum (RSFQ) logic, which is a promising technology for high speed digital electronic devices. The NbN/TaN/NbN trilayer films were prepared on a single crystal MgO substrate by pulsed laser ablation, and patterned into junctions using a novel process utilizing e-beam lithography, chemical vapor deposition and e-beam evaporation. The quality of junctions was tested by measuring the temperature dependence of the junctions' I c R n values, observed to be quite close to theoretical values.
Refractory Niobium Nitride NbN Josephson Junctions and Applications
The Refractory Nitride Josephson Tunnel Junctions with NbN superconducting thin-film base-electrode (Tc~16.5K) were studied since 1982 using thermal or plasma oxide tunnel barriers, showing attracting I-V non-linear characteristics only when a superconducting soft-metal (Pb-In) was used as counter-electrode. Few years later, introduction of in-situ deposited tri-layers including MgO or AlN tunnel barrier permits a large improvement in the junction quality, yield and reproducibility, opening new application fields. Since, NbN (or one of its parent compounds such as Nb1-xTixN) was established as a possible alternative to refractory Nb Josephson devices (operating at or below 4.2K) for building Superconductive Electronics circuits achieving higher frequencies, up to the THz range for SIS mixers or oscillators as well as operating temperature up to 10K. “Self-shunted” nitride SNS and SS’S junctions, such as NbN-TaNx-NbN JJs, are taking advantage of large RNIC products induced by efficient quasi-particles diffusion and good nitride barriers matching at interfaces with nitride electrodes which solve frequency limitation problems induced by the capacitance of SIS junctions. RF frontends, MUX, ADC and digital RSFQ nitrides circuits have been demonstrated, some of them optimized to operate near 10K inside closed cycle refrigerators for minimizing energy dissipation in a space satellite applications. While NbN circuits are today still less mature than the established Nb-Al-AlOx-Nb JJ state-of-art technology (~20k JJs with JJ ~8kA/cm2, ~1µm2) suitable to achieve prototypes of low dissipation integrated circuit SFQ processors described in [1] and references inside. However important properties of NbN technology seems more economically relevant for the future: a conventional Si-CMOS foundry factories should be used to establish a NbN-SiO2 stack-layered process, stable up to ~350°C, not sensitive to hydride species diffusion, while present Nb trilayers are degraded above 180°C and hydrogen diffusion is detrimental of junctions yield. A recent NbN IC processing demonstration (0.5µm linewidth) has been done on 8-inch silicon wafers at the CEA-LETI CMOS facility, making possible to integrate further NbN ADC and processors chips reliably designed and fabricated with high yield at reasonable cost in any CMOS foundry. Another advantage of NbN thin and very thin (few nm thick) films and nanowires with very short electron-phonon relaxation time leads to fast, light or current sensitive self-resetting disjunctions switches with few ps jitter and fast heat dissipation in the substrate. On-chip integration of nitride front-ends circuits such as resonators, microwave filters, registers, highly sensitive and fast photo-sensors (KIDs, SNSPD, HEB,..) and superconducting-semiconducting interfaces circuits widen the field of superconductive electronics applications from mK up to 10K. Moreover recently, deep submicron size (~0.01µm2), very low capacitance NbN-MgO-NbN SIS and SQUID circuits were achieved using a self-aligned process. Such junctions, beside their attracting properties for nitride IC, are able to emit or detect in a controllable way single microwave photons and could be a new building block of quantum information circuits.
Electron spectroscopy analysis on NbN to grow and characterize NbN/AlN/NbN Josephson junction
Superlattices and Microstructures, 2008
Three layers, NbN based Josephson junction, has been growth by RF and by DC sputtering within the constrain required by the photolithography technology. An interesting superconducting film with critical temperature of Tc = 14 K, well above the temperature of the commercial cryocooler, has been obtained reducing sputtering power and finding a proper N 2 concentration in the gas mixture. The search of the new sputtering parameters has been obtained with the help of electron spectroscopy and X-ray diffraction analysis.
Anodization-based process for the fabrication of all niobium nitride Josephson junction structures
Beilstein Journal of Nanotechnology, 2017
We studied the growth and oxidation of niobium nitride (NbN) films that we used to fabricate superconductive tunnel junctions. The thin films were deposited by dc reactive magnetron sputtering using a mixture of argon and nitrogen. The process parameters were optimized by monitoring the plasma with an optical spectroscopy technique. This technique allowed us to obtain NbN as well as good quality AlN films and both were used to obtain NbN/AlN/NbN trilayers. Lift-off lithography and selective anodization of the NbN films were used, respectively, to define the main trilayer geometry and/or to separate electrically, different areas of the trilayers. The anodized films were characterized by using Auger spectroscopy to analyze compounds formed on the surface and by means of a nano-indenter in order to investigate its mechanical and adhesion properties. The transport properties of NbN/AlN/NbN Josephson junctions obtained as a result of the above described fabrication process were measured ...
Ambient temperature growth and characterization of stoichiometric NbN thin films
DAE SOLID STATE PHYSICS SYMPOSIUM 2018, 2019
In this work, we report preparation and characterization of NbN x thin films deposited using a reactive dc magnetron sputtering process at ambient temperature (without any intentional substrate heating). We found that formation of a stoichiometric NbN film is closely related to the response of a Nb target under reactive nitrogen environment. Although appearance of NbN phase is visible within the metallic state of Nb target, fully stoichiometric NbN phase can be obtained at the onset of target poisoning state.