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IEEE Transactions on Applied Superconductivity

NbN multilayer technology on R-plane sapphire

J.-C. Villegirr, N. Hadacek, S. Monso, B. Delnet, Agnès Roussy, Pascal Febvre, G. Lamura, J.Y. Laval

Published Mar 1, 2001• 41 citations
Publisher

Abstract

A new NbN multilayer technology has been developed on 3 inch diameter R-plane sapphire substrates, for combining on-chip fast RSFQ circuits with GHz bandwidth optical links, The circuits take advantage of two high quality [110] NbN layers sputtered epitaxially on sapphire at 600/spl deg/C and selectively patterned: a 400 nm thick layer (/spl lambda//sub L//spl sim/250 nm at 6 K) acts for the ground-plane and microbridge photodetectors are made of a 3.5-8 nm thick NbN epilayer with T/sub c/ above 11 K. Innovative dielectrics formed of 10 nm thick MgO sputtered on top of 200 mm SiO/sub 2/ layers are found to improve significantly the superconductivity of NbN junction electrode lines deposited below 300/spl deg/C. Good quality, hysteretic 2 /spl mu/m/sup 2/ area, NbN/MgO/NbN junctions with high J/sub c/ (up to 50 kA/cm/sup 2/) are obtained with very large gap voltage (6.20 mV) and low sub-gap leakage current (V/sub m/>15 mV) at 4.2 K. At 11 K such junctions are found self-shunted (J/sub c//spl sim/10 k4/cm/sup 2/) with R/sub n/I/sub c/ above 0.5 mV and with low J/sub c/ spread in arrays. J/sub c/ can be adjusted (reduced) without any detrimental effect on the junction quality or spread by annealing at 250/spl deg/C.

DOI
10.1109/77.919286

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IEEE Transactions on Applied Superconductivity•Mar 1, 2001•30 citations
A new NbN multilayer technology has been developed on 3 inch diameter R-plane sapphire substrates, for combining on-chip fast RSFQ circuits with GHz bandwidth optical links, The circuits take advantage of two high quality [110] NbN layers sputtered epitaxially on sapphire at 600/spl deg/C and selectively patterned: a 400 nm thick layer (/spl lambda//sub L//spl sim/250 nm at 6 K) acts for the ground-plane and microbridge photodetectors are made of a 3.5-8 nm thick NbN epilayer with T/sub c/ above 11 K. Innovative dielectrics formed of 10 nm thick MgO sputtered on top of 200 mm SiO/sub 2/ layers are found to improve significantly the superconductivity of NbN junction electrode lines deposited below 300/spl deg/C. Good quality, hysteretic 2 /spl mu/m/sup 2/ area, NbN/MgO/NbN junctions with high J/sub c/ (up to 50 kA/cm/sup 2/) are obtained with very large gap voltage (6.20 mV) and low sub-gap leakage current (V/sub m/>15 mV) at 4.2 K. At 11 K such junctions are found self-shunted (J/sub c//spl sim/10 k4/cm/sup 2/) with R/sub n/I/sub c/ above 0.5 mV and with low J/sub c/ spread in arrays. J/sub c/ can be adjusted (reduced) without any detrimental effect on the junction quality or spread by annealing at 250/spl deg/C.
A generic NbN Superconducting Tunnel Junctions (STJ) technology has been developed using conventional substrates (Si and SOI-SIMOX) for making THz spectrometers including SIS receivers and RSFQ logic gates. NbN/MgO/NbN junctions with area of 1 /spl mu/m/sup 2/, Jc of 10 kA/cm/sup 2/ and low sub-gap leakage current (Vm>25 mV) are currently obtained from room temperature sputtered multilayers followed by a post-annealing at 250/spl deg/C. Using a thin MgO buffer layer deposited underneath the NbN electrodes, ensures lower NbN surface resistance values (Rs=7 /spl mu//spl Omega/) at 10 GHz and 4 K. Epitaxial NbN [100] films on MgO [100] with high gap frequency (1.4 THz) have also been achieved under the same deposition conditions at room temperature. The NbN SIS has shown good I-V photon induced steps when LO pumped at 300 GHz. We have developed an 8 levels Al/NbN multilayer process for making 1.5 THz SIS mixers (including Al antennas) on Si membranes patterned in SOI-SIMOX. Using the planarization techniques developed at the Si-MOS CEA-LETI Facility, we have also demonstrated on the possibility of extending our NbN technology to high level RSFQ circuit integration with 0.5 /spl mu/m/sup 2/ junction area, made on large area substrates (up to 8 inches).
We have evaluated the electrical characteristics of NbN/Al-AlNx/NbN tunnel junctions fabricated on sapphire-C or fused-silica substrates. NbN/Al-AlNx/NbN multilayers were prepared by reactive dc-magnetron sputtering deposition of lower and upper NbN layers, and rf-magnetron sputtering deposition of the Al layer, followed by radical-nitridation. The junctions showed low sub-gap leakage currents in the current-voltage characteristics at 4.2 K. The quality parameter, Rsg/Rn, where respective Rsgand Rnare the sub-gap resistance at 2 mV and junction resistance at 5 mV, was above 15. The gap voltage (Vg) was around 3.3 mV, which was reduced in comparison with Vgvalues of ~4.3 mV for junctions on MgO substrates. The reduced Vgwas caused by superconducting properties of lower NbN layers deposited on the sapphire or fused-silica substrates. Uniformity in critical current (Ic) was relatively good, and the maximum-to-minimum spread in Icwas ±1.7% for 200 junctions on a sapphire substrate and ±2.0% for the junctions on a fused-silica substrate.
Nitride superconductors and specially niobium nitride are key materials for developing high performance optoelectronic and digital circuits. We are presenting a way to achieve such high frequency devices on R-plane sapphire or MgO substrates. Deposition of thin and flat NbN films with Te above 10 K, low and reproducible penetration depth ( L 250 nm) and surface resistance (Rs) values up to I THz, is required and obtained by sputtering on a substrate heated in the 300-600 °C range. Simple sub-micrometer size HEB bridge structures where patterned even in a very thin (2-5 nm thick) NbN layers offering relaxation times below 30 ps. It is then possible to achieve fast optoelectronic data links and sensors on-chip with high clock frequency NbN RSFQ digital circuits.
We report on the development of a 10 K, NbN superconductive integrated circuit (IC) technology that utilizes an improved SiO/sub 2/ interlevel dielectric (ILD) deposition process and a thick NbN ground plane layer to reduce parasitic circuit inductances. The ILD process uses a novel low frequency (40 kHz) substrate bias during sputter deposition of SiO/sub 2/, which produces very smooth oxide films having a roughness less than 0.1 nm (rms) as measured by atomic force microscopy (AFM). Bias-sputtered SiO/sub 2/ is used to planarize and to smooth the surface of the NbN ground plane layer in preparation for fabrication of NbN/MgO/NbN tunnel junctions. High current density tunnel junctions ranging from 1000 A/cm/sup 2/ to 5000 A/cm/sup 2/, fabricated over NbN ground planes up to 1 /spl mu/m thick, exhibit low subgap leakage (V/sub m//spl sim/15 mV at 10 K) and high subgap voltage (V/sub g/=4.4 mV at 10 K). Typical wiring inductance over ground plane has been reduced by 25% compared to our present NbN foundry process.
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.
To improve the performance of Josephson oscillators and SIS mixers over 700 GHz, we have developed a fabrication process to grow epitaxial NbN/MgO/NbN trilayers by reactive dc-sputtering. Trilayers were fabricated on a single-crystal MgO substrate at ambient temperature. NbN and MgO films were deposited by reactive dc-sputtering with an Nb and Mg target, respectively. The MgO inter-layer thickness was changed up to 480 nm, but, T/sub C/ and 20-K resistivity of the NbN upper-layer showed no remarkable dependency and they were 15.7 K and about 60 /spl mu//spl Omega/cm, respectively. To evaluate the RF performance of the epitaxial NbN films, we fabricated Josephson junctions with a microstrip resonator constructed from an epitaxial NbN/MgO/NbN trilayer. The I-V characteristics of the junction exhibited resonance steps up to 2.5 mV, which suggests that the epitaxial NbN films have low loss up to 1.2 THz.