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Journal de Physique IV (Proceedings)

New developments in textured and epitaxial NbN superconducting layers for ultimate sensors and RSFQ digital circuits

J.C. Villégier, N. Hadacek, Corentin Jorel, Jean-Luc Thomassin, Vincent Bouchiat, Marc Faucher, Pascal Febvre, A. Rousy, G. Lamura

Published May 1, 2002• 4 citations
Publisher

Abstract

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.

DOI
10.1051/jp420020051

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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.
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.
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.
We have fabricated both NbN/Ta x N/NbN SN*S-type, NbN/MgO/NbN SIS-type Josephson junctions and microwave band-pass filters on different substrates (Si, Sapphire, MgO). NbN films have been deposited on both sides of (100) oriented, 250 µm thick, MgO substrates with a high crystalline texture quality. The aim was to investigate the performances and the maximum achievable operating frequency in an NbN based RSFQ modulator front-end of an ADC in the 4 K-10 K temperature range. We observed that Ta x N thin films can be tuned from an insulating phase to a superconducting phase (Tc ∼4K) by varying the nitrogen content during sputter deposition while the barrier height of MgO can also be controlled by deposition conditions and by tri-layer postdeposition annealing. Junction properties (Jc∼10-25 kA/cm 2 ), Mac Cumber parameter and RnIc product measured up to 1 mV are shown to be controlled by the reactive sputtering conditions. We have designed three pole band-pass filters and resonators in a micro-strip configuration and studied the junction coupling with the filters. We will show that a sigma-delta NbN technology is a suitable solution for analogue-to-digital conversion in the future generations of telecommunication satellites to achieve high sampling frequency and large bandwidth at high carrier frequency signal.
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.
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).