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arXiv:1401.2292

Development of a NbN Deposition Process for Superconducting Quantum Sensors

D. Glowacka, D. J. Goldie, S. Withington, Muhammad, H., G. Yassin, Tan, B. K.

Published Jan 10, 2014• 8 citations
arXivHTMLPDF

Abstract

We have carried out a detailed programme to explore the superconducting characteristics of reactive DC-magnetron sputtered NbN. The basic principle is to ignite a plasma using argon, and then to introduce a small additional nitrogen flow to achieve the nitridation of a Nb target. Subsequent sputtering leads to the deposition of NbN onto the host substrate. The characteristics of a sputtered film depend on a number of parameters: argon pressure, nitrogen flow rate and time-evolution profile, substrate material, etc. Crucially, the hysteresis in the target voltage as a function of the nitrogen flow can be used to provide a highly effective monitor of nitrogen consumption during the reactive process. By studying these dependencies we have been able to achieve highly reproducible film characteristics on sapphire, silicon dioxide on silicon, and silicon nitride on silicon. Intrinsic film stress was minimised by optimising the argon pressure, giving NbN films having Tc = 14.65 K. In the paper, we report characteristics such as deposition rate, Residual Resistance Ratio (RRR), film resistivity, transition temperature, and stress, as a function of deposition conditions. In order to enhance our understanding of the microwave properties of the films, we fabricated a wide range of microstrip NbN resonators (half wavelength, quarter wavelength, ring resonators). In the paper, we provide an illustrative result from this work showing a 2.1097 GHz resonator having a Q of 15,962 at 3.3 K.

DOI
10.48550/arxiv.1401.2292

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We have carried out a detailed programme to explore the superconducting characteristics of reactive DC-magnetron sputtered NbN. The basic principle is to ignite a plasma using argon, and then to introduce a small additional nitrogen flow to achieve the nitridation of a Nb target. Subsequent sputtering leads to the deposition of NbN onto the host substrate. The characteristics of a sputtered film depend on a number of parameters: argon pressure, nitrogen flow rate and time-evolution profile, substrate material, etc. Crucially, the hysteresis in the target voltage as a function of the nitrogen flow can be used to provide a highly effective monitor of nitrogen consumption during the reactive process. By studying these dependencies we have been able to achieve highly reproducible film characteristics on sapphire, silicon dioxide on silicon, and silicon nitride on silicon. Intrinsic film stress was minimised by optimising the argon pressure, giving NbN films having Tc = 14.65 K. In the paper, we report characteristics such as deposition rate, Residual Resistance Ratio (RRR), film resistivity, transition temperature, and stress, as a function of deposition conditions. In order to enhance our understanding of the microwave properties of the films, we fabricated a wide range of microstrip NbN resonators (half wavelength, quarter wavelength, ring resonators). In the paper, we provide an illustrative result from this work showing a 2.1097 GHz resonator having a Q of 15,962 at 3.3 K.
At the present time, ultrahigh performance superconducting nanowire single-photon detectors are the key elements in a variety of devices from biological research to quantum communications and computing. Accurate tuning of superconducting material properties is a powerful resource for fabricating single-photon detectors with desired properties. Here, we report on the major theoretical relations between ultrathin niobium nitride (NbN) film properties and superconducting nanowire single-photon detector characteristics, as well as the dependence of ultrathin NbN film properties on reactive magnetron sputtering recipes. Based on this study, we formulate the exact requirements for ultrathin NbN films for ultrahigh performance superconducting nanowire single-photon detectors. Then, we experimentally studied the properties of ultrathin NbN films (morphology, crystalline structure, critical temperature, and sheet resistance) on silicon, sapphire, silicon dioxide, and silicon nitride substrates sputtered with various recipes. We demonstrate ultrathin NbN films (obtained with more than 100 films deposition) with a wide range of critical temperature from 2.5 to 12.1 K and sheet resistance from 285 to 2000 Ω/sq and report a sheet resistance evolution of more than 40% within two years. Finally, we found out that one should use ultrathin NbN films with a specific critical temperature near 9.5 K and a sheet resistance of about 350 Ω/sq for ultrahigh performance state-of-the-art superconducting nanowire single-photon detectors at 1550 nm wavelength.
The key of improving hot-electron bolometer (HEB) mixer performance lies inevitably in the quality of ultra-thin NbN films itself. This work presents a thorough investigation of crucial process parameters of NbN films deposited by means of reactive DC-sputtering on Si-substrates at elevated temperatures up to 750°C.The polycrystalline NbN films with thickness of 4 to 10nm were characterized by DC resistivity measurements, ellipsometry and high resolution transmission electron microscopy (HRTEM) in order to confirm thickness and film structure. Since the macroscopic properties such as critical temperature, thickness as well as the transition width to the superconducting state are directly linked to HEB mixer noise temperature and IF bandwidth, a set of experiments were conducted to enhance aforementioned properties. We considered deposition temperature, RF biasing, nitrogen and argon partial and total pressure during deposition as major process variable parameters. Careful optimization of the deposition conditions allowed setting up a process resulting in high-quality NbN ultra-thin films with thickness of 5.5nm exhibiting Tc of 10.5K. Moreover, the transition width could be kept as low as 1.4K. The produced films were stored at ambient conditions and re-characterized over a period of 4 month without measurable degradation.
The main focus of this work is the development of ultrathin NbN and NbTiN films. A reproducible and reliable deposition process for ultrathin NbN and NbTiN films for the use in phonon-cooled HEB devices was established. The ultrathin films were deposited on silicon (Si) substrates and on 2æm Si3N4 membranes by DC reactive magnetron sputtering. A method for the precise control of the nitrogen partial pressure by monitoring the target voltage has been introduced to deposit high quality, ultrathin NbN (3-4nm, Tc=8.5 K) and NbTiN (4-5nm, Tc=8K) films. Substrate heating of at least 600ʿC during the deposition is essential for the fabrication of ultrathin NbN and NbTiN films on Si substrates and Si3N4 membranes. The fabrication process required for HEB devices to be used in a quasi-optical mixer was developed. The ultrathin film was patterned by electron beam lithography (EBL), resulting in bolometer devices that measure areas of about 0.4 æm x 4 æm. The nature of the contact determines the interface transparency between the bolometer and the contact structure. Different cleaning processes have been performed and the influence on the contact resistance has been instigated. A better interface transparency gives less RF losses and could improve the HEB sensitivity and local oscillator (LO) requirement ...
Nano-meter(nm)-bridges with high-Tcmaterials hold great technological interest because of their smaller capacitance and expected higher IoRn-product. They are promising especially for applications such as high speed logic and high frequency radiation detectors. Also they can operate over a wide temperature range and are stable against thermal cycles as are incorporating the refractory high-Tcsuperconductors. One of the essential ingredient for high quality bridges is to have NbN films. Ultra-thin NbN films have been prepared by rf reactive-sputtering. NbN films deposited epitaxially on rf sputtered MgO films have high superconducting transition temperature Tc. Tcvalue of the film with thickness of about 5nm was about 14K, and is much higher than those deposited on Al2O3films and Si substrates. Two types of nm-bridges were reproducibly fabricated with NbN films deposited epitaxially on MgO films. The obtained IoRn-products were in a range of 0.5 to 3.6mV. The microwave-induced voltage steps were observed up to the voltage comparable to IoRn- product. The dc and ac quantum effects of the dc SQUIDs were observed in quite wide range of temperature, 4.2 to 11.4K.
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).
Abstract. We describe the collective fabrication and characterization of superconducting single photon detectors (SSPD) made of a superconducting NbN meander, suitable for near-infrared applications. The NbN layer acting as the photon absorber is deposited epitaxially on a three inch diameter R-plane Sapphire heated substrate by DC-Magnetron sputtering from a Nb target in a reactive mixture of nitrogen and argon gas. Very thin NbN (~4 nm) films superconducting up to 12 K with a rather large critical current density are in-situ covered by a 1.4 nm thick room temperature sputtered AlN layer protecting NbN from oxidation. Two reliable patterning processes has been developed successfully, one based on electron beam lithography, the second based on selective NbN film anodization under an AFM tip. The filling factor obtained with 150 nm wide stripes in the meander pixel is about 0.5 with e-beam lithography and improved up to 0.8 with AFM patterning. In order to achieve an improved optical coupling of the SSPD by using wafers integrating ion implanted IR waveguides, growth studies of NbN layers have been focused on R-plane Sapphire and silicon substrates. We present optical and superconducting properties of NbN layers by FTIR, ellipsometry and STM.