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International Journal of Infrared and Millimeter Waves

Low-Loss NbTiN Films for THz SIS Mixer Tuning Circuits

J. Kooi, Jeff A. Stern, Goutam Chattopadhyay, Henry George LeDuc, B. Bumble, Jonas Stasys Zmuidzinas

Published Mar 1, 1998• 37 citations
Publisher
DOI
10.1023/a:1022595223782

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SIS heterodyne mixer technology based on niobium tunnel junctions has now been pushed to frequencies over 1 THz, clearly demonstrating that the SIS junctions are capable of mixing at frequencies up to twice the energy gap frequency (4(Delta) /h). However, the performance degrades rapidly above the gap frequency of niobium (2(Delta) /h approximately equals 700 GHz) due to substantial ohmic losses in the on-chip tuning circuit. To solve this problem, the tuning circuit should be fabricated using a superconducting film with a larger energy gap, such as NbN; unfortunately, NbN films often have a substantial excess surface resistance in the submillimeter band. In contrast, the SIS mixer measurements we present in this paper indicate that the losses for NbTiN thin films can be quite low.

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Waveguide SIS mixers are presented in which Nb/Al-AlOx/Nb tunnel junctions are integrated with both NbTiN/SiO2/NbTiN and NbTiN/SiO2/Al tuning circuits. The mixers with a NbTiN/SiO2/NbTiN tuning circuit yield relatively low receiver noise temperatures between 700 and 830 GHz, but their sensitivity drops significantly above ~ 850 GHz. In contrast, low-noise heterodyne mixing near 1 THz is observed in mixers incorporating a NbTiN/SiO2/Al tuning circuit. In particular, an uncorrected receiver noise temperature of 585 K is measured at 970 GHz. From an analysis of the receiver sensitivity and gain, it is determined that the loss in the NbTiN ground plane is low (≤ 0.6 dB) below 970 GHz. A decrease in receiver sensitivity above 1 THz is attributed to an onset of rf loss in the NbTiN.

•May 1, 2000•1 citation
SIS heterodyne mixer technology based on niobium tunnel junctions has now been pushed to frequencies over 1 THz, clearly demonstrating that the SIS junctions are capable of mixing at frequencies up to twice the energy gap frequency (4(Delta) /h). However, the performance degrades rapidly above the gap frequency of niobium (2(Delta) /h approximately equals 700 GHz) due to substantial ohmic losses in the on-chip tuning circuit. To solve this problem, the tuning circuit should be fabricated using a superconducting film with a larger energy gap, such as NbN; unfortunately, NbN films often have a substantial excess surface resistance in the submillimeter band. In contrast, the SIS mixer measurements we present in this paper indicate that the losses for NbTiN thin films can be quite low.
Waveguide SIS mixers are presented in which Nb/Al-AlOx/Nb tunnel junctions are integrated with both NbTiN/SiO2/NbTiN and NbTiN/SiO2/Al tuning circuits. The mixers with a NbTiN/SiO2/NbTiN tuning circuit yield relatively low receiver noise temperatures between 700 and 830 GHz, but their sensitivity drops significantly above ~ 850 GHz. In contrast, low-noise heterodyne mixing near 1 THz is observed in mixers incorporating a NbTiN/SiO2/Al tuning circuit. In particular, an uncorrected receiver noise temperature of 585 K is measured at 970 GHz. From an analysis of the receiver sensitivity and gain, it is determined that the loss in the NbTiN ground plane is low (≤ 0.6 dB) below 970 GHz. A decrease in receiver sensitivity above 1 THz is attributed to an onset of rf loss in the NbTiN.