SIS Mixers Based on NbN Techniques for ALMA Band 10

Abstract

We designed, fabricated, and tested SIS mixers based on NbN techniques and estimated the specific capacitance of NbN/AlN/NbN tunnel junctions, which were fabricated by DC-magnetron sputtering, by measuring DC-SQUID resonance steps. In the mixers, NbN/AlN/NbN junctions and NbN SiO 2 Al tuning circuits were used for investigating receiver performance at frequencies greater than 900 GHz. The specific capacitance of the junction was estimated at 120 fF m 2 with a critical current density of 12 kA cm 2 . The resonance frequencies generated in the tuning circuits linearly changed with respect to the tuning lengths up to at least 1.1 THz. We found that an input-circuit structure, which we used, is a factor that degrades receiver noise at frequencies greater than 900 GHz.Index Terms-Fourier transform spectrometer, niobium nitride, SIS mixers, submillimeter wave.

Low-noise waveguide SIS mixer with NbN/AlN/NbN tunnel junctions tuned by an NbN/MgO/NbTiN microstrip circuit

“…The junction quality is substantially superior to other high gap junctions, such as NbTiN/MgO/NbTiN and Nb/AlN/NbTiN [9,10,14]. A relatively low sub-gap leakage current and a high gap voltage above 5.5 mV have been achieved in NbN/AlN/NbN junctions [15]. The use of NbN junctions allows all-superconducting structures in SIS mixers to work up to the gap frequency without heating effects, which increase the deterioration of the mixing properties by a depression of the energy gap, cause backbending of the quasiparticle current jump and cause an increase in the sub-gap current [16,17].…”

“…The length and width of the NbN/MgO/NbTiN microstrip between the two junctions were determined to be 3.0 μm and 4.0 μm, respectively, assuming a junction-specific capacitance of 140 fF μm −2 . The specific capacitance was estimated by measuring dc-SQUID resonance steps [15]. The tuning circuit was directly positioned at a feed point without impedance transformers.…”

Frequency Bandwidth Adjustment by Precision Post-Processing for a 0.78–0.95 THz Band SIS Mixer

“…Therefore, an ALMA Band 10 SIS mixer must use a different superconducting material with a higher gap frequency for its mixer chip superconducting circuitry. We have previously considered NbN and NbTiN for this material, eventually developing ALMA Band 10 SIS mixers using the latter [8,9]. All ALMA specifications require high reliability and state-of-the-art performance.…”

A sensitive ALMA Band 10 SIS receiver engineering model

“…In order to reduce the total receiver noise temperature, it is important to minimize the noise contributions of each component by reducing their loss by as much as possible. We have investigated and developed receiver components such as waveguide SIS mixers at terahertz frequencies, and have demonstrated their great potential for use in the ALMA Band 10 receivers [16][17][18][19][20]. In this paper, we present for the first time the design, assembly and testing of a terahertz receiver that demonstrates the low-noise characteristics specified for ALMA.…”