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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Broadband NbTiN based SIS mixers for HIFI Band 2

S. Glenz, M. Justen, P. Pütz, R. Teipen, Thomas Tils, Cornelia E. Honingh, Karl Jacobs

Published Jan 1, 2002• 3 citations
Publisher

Abstract

We present SIS waveguide mixer results in the frequency range 638-802 GHz at an IF frequency of 4-8 GHz. We use Nb/Al/sub 2/O/sub 3//Nb junctions with areas down to 0.6 /spl mu/m/sup 2/ defined by e-beam lithography and chemical mechanical polishing (CMP), embedded in low loss NbTiN/SiO/sub 2//Nb or NbTiN/SiO/sub 2//Al microstrip tuning circuits.

DOI
10.1109/icimw.2002.1076182

Related papers

NbTiN Based Tuning Structures for Broadband Nb-Al 2 O >3 -Nb SIS Mixers from 640 GHz-800 GHz

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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.

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Waveguide SIS mixers in which Nb/Al-AlO/sub x//Nb tunnel junctions are integrated with NbTiN/SiO//sub 2//Al tuning circuits are shown to yield receiver noise temperatures as low as 565 K at 970 GHz. Analyzing the noise and gain of one such receiver, it is shown that the NbTiN ground plane is low-loss (<0.6 dB) at 970 GHz. These results are in good agreement with results obtained previously with a quasi-optical mixer incorporating a similar tuning circuit. A decrease in sensitivity above 1 THz is attributed to increasing loss in the NbTiN.

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MIXER DEVELOPMENT FOR HIFI BAND 2 (640 — 800 GHz)

Cornelia E. Honingh, M. Justen, R. Teipen +2 more

We present a fixed tuned SIS waveguide mixer for 640-800 GHz. The waveguide and filter design has been done using 3D EM-Simulation to improve the accuracy of the design for the required large fractional bandwidth. In this frequency range we use standard Nb—Al20 3—Nb SIS junctions with an R NA product of approximately 15 ohm!Am 2 , and areas between 1 gm 2 and 0.4 firll 2 . The choice of tuning structure material is not directly obvious because the frequency range includes the niobium gap frequency at 700 Gliz. Therefore devices with a NbTiN ground plane and as a choice with a niobium or an aluminum top electrode are used Current best noise temperatures are 300 K at the lower frequency end of the band and 450 K at the high end of the band, measured with the prototype HIFI mixer at 4-8 GHz IF frequency, for a device with a niobium top electrode., Measurements and analysis of devices with different designs, and both integrated tuning structure top electrode materials will be presented. The accuracy of the RF mixer design will be evaluated.

•Mar 1, 2002•0 citations
We present SIS waveguide mixer results in the frequency range 638-802 GHz at an IF frequency of 4-8 GHz. We use Nb/Al/sub 2/O/sub 3//Nb junctions with areas down to 0.6 /spl mu/m/sup 2/ defined by e-beam lithography and chemical mechanical polishing (CMP), embedded in low loss NbTiN/SiO/sub 2//Nb or NbTiN/SiO/sub 2//Al microstrip tuning circuits.
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.
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.
Abstract—As a building block for an 8-pixel 800 GHz array receiver at KOSMA we developed a low noise SIS mixer. The mixerblock is tunerless and similar to the "stamped waveguide cavity " mixers we have used from 345 GHz to 800 GHz before [1]. We embedded the SIS junction into a standard broadband tuning circuit, commonly used at lower frequencies. The circuit consists of a short end-loaded stub in series with two impedance transforming sections. For the 780-880 GHz band, we fabricated the electrodes of the tuning circuit as a combination of niobium and aluminum layers in different sequences. First heterodyne tests were performed on devices with a tuning microstrip circuit ground plane consisting of 40nm Nb with 200nm of Al on top and a top conductor consisting of a 70 nm thick Nb layer with a 280 nm thick Al layer on top. The Nb-Al2O3-Nb junction has an area of about 0.5 µm 2 and is fabricated with optical lithography. For anisotropic Nb etching, we use a RIE gas mixture of CCl2F2 and NF3. The Al2O3 barrier is removed with Ar sputter etching. Fabrication yield improved considerably when we introduced a light CMP-step after self-aligned SiO dielectric liftoff. For a SIS junction with RnA equal to 14 Ωµm 2, the RSubgap/Rn value of 15.4 is remarkably good. We measured uncorrected receiver noise temperatures around 400 K over the whole band from 780 to 880 GHz, with a best noise temperature of 370 K at 804 GHz. 1
We present measurements and simulations of mixer performance around 660 GHz and around 800 GHz. We use Nb-Al/sub 2/O/sub 3/-Nb tunnel junctions with areas of 0.9 /spl mu/m/sup 2/ and 0.7 /spl mu/m/sup 2/, and RA-products of 14.5 /spl Omega/(/spl mu/m)/sup 2/ and 13 /spl Omega/(/spl mu/m)/sup 2/ for 660 GHz and 800 GHz. Both junctions have an integrated tuning structure made of niobium that consists of a series resonant stub and a quarter lambda transformer. The waveguide mixerblock has no additional adjustable tuning elements. It contains just a waveguide cavity and a substrate channel across it. A horn is carefully adjusted to the cavity and flanged to the block. The measured receiver noise temperatures from 630-690 GHz are below 190 K with a best value of 120 K at 655 GHz. From 780-820 GHz they are below 950 K with a best value of 780 K at 792 GHz. When the operating temperature is reduced from 4.2 K to 2.5 K, a reduction in noise temperature from 830 K to 660 K is observed at 810 GHz. The mixer performance is simulated using the quantum theory of mixing. The simulated performance shows a fairly good agreement with the measured one.
Waveguide SIS mixers in which Nb/Al-AlO/sub x//Nb tunnel junctions are integrated with NbTiN/SiO//sub 2//Al tuning circuits are shown to yield receiver noise temperatures as low as 565 K at 970 GHz. Analyzing the noise and gain of one such receiver, it is shown that the NbTiN ground plane is low-loss (<0.6 dB) at 970 GHz. These results are in good agreement with results obtained previously with a quasi-optical mixer incorporating a similar tuning circuit. A decrease in sensitivity above 1 THz is attributed to increasing loss in the NbTiN.
We present a fixed tuned SIS waveguide mixer for 640-800 GHz. The waveguide and filter design has been done using 3D EM-Simulation to improve the accuracy of the design for the required large fractional bandwidth. In this frequency range we use standard Nb—Al20 3—Nb SIS junctions with an R NA product of approximately 15 ohm!Am 2 , and areas between 1 gm 2 and 0.4 firll 2 . The choice of tuning structure material is not directly obvious because the frequency range includes the niobium gap frequency at 700 Gliz. Therefore devices with a NbTiN ground plane and as a choice with a niobium or an aluminum top electrode are used Current best noise temperatures are 300 K at the lower frequency end of the band and 450 K at the high end of the band, measured with the prototype HIFI mixer at 4-8 GHz IF frequency, for a device with a niobium top electrode., Measurements and analysis of devices with different designs, and both integrated tuning structure top electrode materials will be presented. The accuracy of the RF mixer design will be evaluated.