We report a milestone in achieving large-scale, ultrathin (~5 nm) superconducting NbN thin films on 300 mm Si wafers using a high-volume manufacturing (HVM) industrial physical vapor deposition (PVD) system. The NbN thin films possess remarkable structural uniformity and consistently high superconducting quality across the entire 300 mm Si wafer, by incorporating an AlN buffer layer. High-resolution X-ray diffraction and transmission electron microscopy analyses unveiled enhanced crystallinity of (111)-oriented δ-phase NbN with the AlN buffer layer. Notably, NbN films deposited on AlN-buffered Si substrates exhibited a significantly elevated superconducting critical temperature (~2 K higher for the 10 nm NbN) and a higher upper critical magnetic field or Hc2 (34.06 T boost in Hc2 for the 50 nm NbN) in comparison with those without AlN. These findings present a promising pathway for the integration of quantum-grade superconducting NbN films with the existing 300 mm CMOS Si platform for quantum information applications.
We report a milestone in achieving large-scale, ultrathin (~5 nm) superconducting NbN thin films on 300 mm Si wafers using a high-volume manufacturing (HVM) industrial physical vapor deposition (PVD) system. The NbN thin films possess remarkable structural uniformity and consistently high superconducting quality across the entire 300 mm Si wafer, by incorporating an AlN buffer layer. High-resolution X-ray diffraction and transmission electron microscopy analyses unveiled enhanced crystallinity of (111)-oriented δ-phase NbN with the AlN buffer layer. Notably, NbN films deposited on AlN-buffered Si substrates exhibited a significantly elevated superconducting critical temperature (~2 K higher for the 10 nm NbN) and a higher upper critical magnetic field or Hc2 (34.06 T boost in Hc2 for the 50 nm NbN) in comparison with those without AlN. These findings present a promising pathway for the integration of quantum-grade superconducting NbN films with the existing 300 mm CMOS Si platform for quantum information applications.
Abstract In this paper, we study the crystalline properties and superconducting critical temperature of ultra-thin (5–9 nm) NbN films deposited on 8-inch silicon wafers by reactive sputtering. We show that the deposition of NbN on a thin (10–20 nm) AlN buffer layer, also synthesized by reactive sputtering, improves the critical temperature by several Kelvin, up to 10 K for 9 nm NbN on 20 nm AlN. We correlate this improvement to the higher-crystalline quality of NbN on AlN. While NbN deposited directly on silicon is polycrystalline with randomly oriented grains, NbN on AlN(0001) is textured along (111), due to the close lattice match. The superconducting properties of the NbN/AlN stack are validated by the demonstration of fibre-coupled normal-incidence superconducting nanowire single photon detectors. The whole fabrication process is CMOS compatible, with a thermal budget compatible with the integration of other passive and active components on silicon. These results pave the way for the integration of a large number of surface or waveguide-integrated detectors on large-scale silicon wafers. Furthermore, as AlN is transparent over a broad wavelength range from the visible to the near-infrared, the optimized superconducting NbN/AlN stack can be used for a wide variety of applications, from imaging to quantum communications and quantum computing.
Abstract In this paper, we investigate the superconducting critical temperature, morphology and structural properties of NbN ultra-thin films (5–7 nm) deposited by reactive sputtering on top of silicon nitride (SiN) over 200 mm silicon wafers. We demonstrate the effectiveness of a 10 nm thick spacer layer of AlN in raising the superconducting critical temperature from 5.0 K to 7.2 K and from 7.3 K to 9.3 K for 5 nm and 7 nm NbN thickness respectively. By combining investigations with x-ray diffraction and transmission electron microscopy, we highlight significant modifications of the crystalline properties of the NbN film on top of the AlN spacer layer. When directly deposited on SiN, the grains of the NbN polycrystalline layer are randomly oriented. In contrast, when deposited on top of the AlN (0001) spacer layer, the NbN layer is textured along the (111) direction, leading to increased critical temperatures. As AlN is a CMOS-compatible material, these findings are particularly relevant in view of the future integration of high-performance superconducting nanowire single photon detectors on large-scale quantum photonic chips.
We present our research on fabrication and structural and transport characterization of ultrathin superconducting NbN layers deposited on both single-crystal Al2O3 and Si wafers, and SiO2 and Si3N4 buffer layers grown directly on Si wafers.The thicknesses of our films varied from 6 nm to 50 nm and they were grown using reactive RF magnetron sputtering on substrates maintained at the temperature 850 • C. We have performed extensive morphology characterization of our films using the X-ray diffraction method and atomic force microscopy, and related the results to the type of the substrate used for the film deposition.Our transport measurements showed that even the thinnest, 6 nm thick NbN films had the superconducting critical temperature of 10-12 K, which was increased to 14 K for thicker films.
Abstract Atomic layer deposition (ALD) has been identified as a promising growth method for high-uniformity superconducting thin films for superconducting quantum photonic applications, offering superior uniformity, thickness control and conformality to techniques such as reactive sputtering. The potential scalability of ALD makes this method especially appealing for fabrication of superconducting nanowires and resonators across large areas. We report on the growth of highly uniform superconducting NbN thin films via plasma-enhanced atomic layer deposition (PEALD) with radio frequency substrate biasing, on a 200 mm (8 inch) Si wafer, specifically for superconducting nanowire single-photon detector applications. Niobium nitride films were grown using (tert-butylimido)-tris(diethylamido)-niobium(V) precursor and an H 2 /Ar plasma. The superconducting properties of a variable thickness series of films (5.9–29.8 nm) show critical temperature ( T c ) of 13.5 K approaching bulk thickness (28.8 nm) with low suppression down to the ultrathin regime (5.9 nm), with T c = 10.2 K. T c across the 200 mm wafer with 8 nm thick NbN, measured in 15 mm intervals, exhibits minimal variation (<7%). Microbridge structures fabricated on 8 nm thick NbN films also exhibit high critical current densities ( J c ), > 10 MA cm −2 at 2.6 K. PEALD could therefore be a pivotal technique in enabling large-scale fabrication of integrated quantum photonic devices across a variety of applications.
Superconducting nanowire single photon detectors stand today as the best technology, due to their near-unity detection efficiency, low dark count rates, and low timing jitter. In this work, we demonstrate the improvement of the superconducting properties of NbN thin films on 8” silicon-on-insulator wafers by using an ultra-thin (10-15 nm) sputtered AlN buffer layer. The higher crystalline quality of NbN, leads to an increase of the superconducting critical temperature up to 10 K for 9-nm-thick NbN films. The material was validated for single photon detection using a fiber-coupled vertical SNSPD with half-cavity architecture. This results opens the way for the development of CMOS compatible waveguide-integrated detectors. The implementation of such guided devices is a keystone for the development of a fully integrated quantum photonics platform able to generate, manipulate and detect a large number of photonic qubits for secure communications and quantum computing applications.
Ultra-thin epitaxial NbN layers are a key component of Superconducting Single Photon infrared Detectors. Efforts devoted to the layer growth aim at improving their critical temperature and critical current density, while keeping their thickness close to 5 nm and Tc above 10 K, which insure a large bandwidth, large SNR detection at 4K. Choice of substrate is critical: for both applications, MgO wafers and R-plane sapphire are usually considered as best substrates to grown onto. However, growing NbN on M-plane orientation of sapphire wafer, 3 inch in diameter, can help improving the film quality and fabrication yield. NbN thin films were grown by reactive DC magnetron sputtering at about 600°C and passivated by an AlN layer 1.5nm thick deposited in-situ at room temperature. Growth on M-plane is shown to be better than on other sapphire orientations, including R-plane: NbN layer critical temperature reaches 13.3 K, uniform on the wafer, for a film thickness of 4.4nm measured by X-ray reflectivity. Transport properties of NbN grown on those various substrates have been correlated to their crystallographic microstructure, examined by both symmetric and asymmetric X ray diffraction. Observation of diffraction peaks has given insight on the disorientation of the NbN film.