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IEEE Transactions on Applied Superconductivity

Mixing Properties of NbN-Based SIS Mixers With NbTiN Wirings

Mahoto Takeda, Wenlei Shan, Takafumi Kojima, Yoshinori Uzawa, M. Kroug, Jing Li, Sheng‐Cai Shi, Zhen Wang

Published May 27, 2009• 10 citations
Publisher

Abstract

We report on the mixing properties of waveguide SIS mixers with epitaxial NbN/AlN/NbN junctions and NbN/SiO2/NbTiN microstrips on MgO substrates. The superconducting transition temperature and 20-K resistivity of NbTiN film on the SiO2dielectric were 14.7 K and 175 muOmega cm, respectively. The junction tuning circuit was composed of an NbN/SiO2/NbTiN microstrip and two parallel-connected NbN/AlN/NbN junctions 0.9 mum in diameter. The critical current density and normal-state resistance of the fabricated NbN junction were 13 kA/cm2and 22 Omega, respectively. The heterodyne response was measured using 295- and 77-K blackbody sources, a local oscillator (LO) source, and a 25-mum-thick Kapton film as a beam splitter. An intermediate frequency (IF) amplifier with a bandwidth of 4-12 GHz was used in the measurements. The receiver noise temperature corrected for losses in the beam splitter and in the vacuum window of the cryocooler was 370 K at 860 GHz and was comparable to that of all-NbN mixes with NbN/MgO/NbN microstrips.

DOI
10.1109/tasc.2009.2017886

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IEEE Transactions on Applied Superconductivity•Mar 1, 2001•21 citations
We report on the mixing properties of waveguide SIS mixers with epitaxial NbN/AlN/NbN junctions and NbN/SiO2/NbTiN microstrips on MgO substrates. The superconducting transition temperature and 20-K resistivity of NbTiN film on the SiO2dielectric were 14.7 K and 175 muOmega cm, respectively. The junction tuning circuit was composed of an NbN/SiO2/NbTiN microstrip and two parallel-connected NbN/AlN/NbN junctions 0.9 mum in diameter. The critical current density and normal-state resistance of the fabricated NbN junction were 13 kA/cm2and 22 Omega, respectively. The heterodyne response was measured using 295- and 77-K blackbody sources, a local oscillator (LO) source, and a 25-mum-thick Kapton film as a beam splitter. An intermediate frequency (IF) amplifier with a bandwidth of 4-12 GHz was used in the measurements. The receiver noise temperature corrected for losses in the beam splitter and in the vacuum window of the cryocooler was 370 K at 860 GHz and was comparable to that of all-NbN mixes with NbN/MgO/NbN microstrips.
The low-noise performance of a waveguide SIS mixer with NbN/AlN/NbN tunnel junctions and an NbN/MgO/NbTiN tuning circuit on an MgO substrate was demonstrated at frequencies above the gap frequency of Nb. To design the mixer, the complex refractive index of the MgO substrate was measured by using a THz time-domain spectrometer. The superconductive properties of NbTiN films on MgO were examined in detail. An uncorrected receiver noise temperature of 262 K (DSB) at 820 GHz was achieved in an IF bandwidth of 4-12 GHz at a bath temperature of 4.2 K. By comparing the measured and calculated conversion gains, the loss in the NbN/MgO/NbTiN tuning circuit was estimated to be 0.98 dB at 810 GHz.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlOx/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.
We have developed a low-noise quasioptical NbN/AlN/NbN superconductor-insulator-superconductor (SIS) mixer that operates at terahertz frequencies. The mixer uses a MgO hyperhemispherical lens with an antireflection cap, a single-crystal NbN log-periodic antenna, and two-junction tuning circuits which employ Al/SiO/NbN microstriplines. The NbN/AlN/NbN junction size was about 0.9 μm in diameter, and the current density was about 45 kA/cm2. The frequency dependence of the receiver noise temperature was investigated by using an optically pumped far-infrared laser and a backward-wave oscillator as a local oscillator at several frequencies from 670 to 1082 GHz. The experimental results showed that the center frequency was around 800 GHz, and the receiver noise temperature measured by the standard Y-factor method was 457 K (DSB) at 783 GHz including a 9-μm-thick Mylar beam splitter loss and other optical losses at the physical bath temperature of 4.2 K. This is the first SIS mixer based on NbN with low-noise performance (12hν/kB) above the gap frequency of Nb.
We have designed and fabricated waveguide superconductor-insulator-superconductor (SIS) receivers at 870 GHz with NbN/AlN/NbN tunnel junctions on an MgO substrate. A waveguide probe was incorporated into the mixer chip for a waveguide-to-microstrip transition. As the tuning circuit for the SIS mixer, we designed a resonant distributed junction whose length is one wavelength and critical current density is 20 kA/cm/sup 2/. The mixing properties were calculated based on the quantum theory of mixing. The SSB mixer noise temperature should be less than 100 K and conversion gain larger than -8.0 dB in the frequency range from 840 to 920 GHz. We also observed the resonance steps of the fabricated SIS mixer. The tuning frequency was slightly higher than the designed frequency.
We present measurements with a prototype waveguide SIS mixer for band 2 (640 GHz – 800 GHz) of HIFI (Heterodyne Instrument for FIRST). The Nb/Al2O3/Nb junctions have an RnA product from 12–18 Ωμm. These junctions are embedded in microstrip circuits tuning out the device capacitance using a NbTiN ground layer and various top conductor materials such as NbTiN, Nb and Al. The NbTiN films are fabricated by DC magnetron sputtering which is optimized for a minimum value of the DC resistivity and for a high critical temperature by varying the sputter pressure, sputter power and Ar:N2 flow. Typical NbTiN film parameters for device fabrication are ρ = 120 μΩcm and Tc = 14.5 K. The best uncorrected noise temperatures we measured so far are for devices with NbTiN/SiO2/Nb tuning. At 4.2 K bath temperature, a standard Yfactor measurement yields uncorrected receiver noise temperatures of 350 K at 642 GHz and 500 K at 810 GHz for 1.5 GHz intermediate frequency. We discuss the effect of the various tuning circuit materials on the mixer performance. In addition, measurements at IF’s between 4 GHz and 8 GHz will be presented.
We have designed, fabricated, and tested terahertz superconductor-insulator-superconductor (SIS) mixers having a self-compensated NbN/AlN/NbN tunnel junction and an epitaxially grown NbN/MgO/NbN microstripline. The junction as a distributed element was 0.8-/spl mu/m wide and 2.4-/spl mu/m long, and its current density was 50 kA/cm/sup 2/. The microstripline consisted of a 200-nm-thick NbN ground plane, a 180-nm-thick MgO insulator, and a 350-nm-thick NbN wiring layer. An investigation of the mixer in our receiver configuration showed flat noise characteristics from 870 to 960 GHz. The lowest receiver noise temperature of about 550 K was obtained at 909 GHz, including a 9-/spl mu/m-thick Mylar beam splitter loss and other optical losses. These characteristics suggest that SIS mixers with self-compensated NbN/AlN/NbN junctions and NbN/MgO/NbN microstriplines are appropriate for used in wideband and low-noise operations at terahertz frequencies.