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IEEE Transactions on Applied Superconductivity

Performance of the NbTiN Hot Electron Bolometer Mixer with AlN Buffer Layer at Terahertz Frequency Range

Denis Loudkov, Edward Tong, R. Blundell, Koko G. Megerian, J. A. Stern

Published Jun 1, 2005• 16 citations
Publisher

Abstract

We present recent measurements of receiver noise temperature and intermediate frequency (IF) bandwidth in the frequency range 0.8-1.3 THz for waveguide NbTiN HEB mixers of various dimensions. These devices are fabricated from an NbTiN film deposited on crystalline quartz substrates with AlN buffer layer. The lengths of the mixer elements vary from 0.3 to 0.5 /spl mu/m and their widths vary from 3 to 10 /spl mu/m. Critical temperatures are typically at 8.5 K, and the measured normal state resistance of the devices is about 1000 ohms per square. All the device DC parameters demonstrate a high degree of uniformity. A double side band noise temperature at 0.8 THz as low as 550 K has been measured at an IF frequency of 1.8 GHz, with a conversion loss of around 14 dB. At an IF of 3 GHz, the noise temperature increases to 750 K. We have also made extensive measurements of the IF bandwidth as a function of bias voltages and currents. At the optimal low-noise operation point, a 3-dB IF bandwidth of 1.2 GHz is obtained for a wide variety of device dimensions and bath temperature.

DOI
10.1109/tasc.2005.849881

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We present recent measurements of receiver noise temperature and intermediate frequency (IF) bandwidth in the frequency range 0.8-1.3 THz for waveguide NbTiN HEB mixers of various dimensions. These devices are fabricated from an NbTiN film deposited on crystalline quartz substrates with AlN buffer layer. The lengths of the mixer elements vary from 0.3 to 0.5 /spl mu/m and their widths vary from 3 to 10 /spl mu/m. Critical temperatures are typically at 8.5 K, and the measured normal state resistance of the devices is about 1000 ohms per square. All the device DC parameters demonstrate a high degree of uniformity. A double side band noise temperature at 0.8 THz as low as 550 K has been measured at an IF frequency of 1.8 GHz, with a conversion loss of around 14 dB. At an IF of 3 GHz, the noise temperature increases to 750 K. We have also made extensive measurements of the IF bandwidth as a function of bias voltages and currents. At the optimal low-noise operation point, a 3-dB IF bandwidth of 1.2 GHz is obtained for a wide variety of device dimensions and bath temperature.
The paper reports the latest development and the measured results of NbTiN hot electron bolometer mixers at THz frequencies. The devices are based on 4-7 nm thin NbTiN films, which were deposited by reactive magnetron sputtering of NbTi target in an Ar/N2 atmosphere, on heated high-resistivity Si substrates over a 20 nm thick AlN buffer layer. The quality parameters of the film are transition temperature and normal-state resistivity, which were optimized by varying the sputtering parameters. The resistivity and the critical temperature of the films are about 400 μΩcm and 9 K, respectively. Bolometers (4 μm wide and 0.7 μm long), integrated with logarithmic spiral antennas are fabricated. Mixer noise performance is tested in a quasi-optical receiver in 1.5-4 GHz IF band. The DSB receiver noise temperature of 700 K, 1100 K and 3000 K is obtained at 0.7, 1.6 and 2.6 THz LO frequencies, respectively.
We have developed and characterized waveguide phonon-cooled NbN Hot Electron Bolometer (FMB) mixers fabricated from a 3-4 nrn thick NbN film deposited on a 200nm thick MgO buffer layer over crystalline quartz. Double side band receiver noise temperatures of 900-1050 K at 1.035 THz, and 1300-1400 K at 1.26 THz have been measured at an intermediate frequency of 1.5 GHz. The intermediate frequency bandwidth, measured at 0.8 THz LO frequency, is 3.2 GHz at the optimal bias point for low noise receiver operation.
In this paper, we present the results of IF bandwidth measurements on 3-4 nm thick NbN hot electron bolometer waveguide mixers, which have been fabricated on a 200-nm thick MgO buffer layer deposited on a crystalline quartz substrate. The 3-dB IF bandwidth, measured at an LO frequency of 0.81 THz, is 3.7 GHz at the optimal bias point for low noise receiver operation. We have also made measurements of the IF dynamic impedance, which allow us to evaluate the intrinsic electron temperature relaxation time and self-heating parameters at different bias conditions.
We present a characterization of a 1.5 THz waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixer which can be pumped by a commercial solid state tunable local oscillator (LO) source. The NbTiN HEB mixer is made from a 12 nm thick NbTiN thin film deposited on a quartz substrate at room temperature. A gold electrode is formed in situ on the NbTiN thin film without breaking vacuum to ensure good contact. The uncorrected DSB receiver noise temperature is measured to be 1700 K at 1.5 THz, whereas the mixer noise temperature is derived to be 1000 K after corrections for losses of the input optics and the intermediate frequency (IF) amplifier chain. The required LO power absorbed in the HEB mixer is evaluated to be 340 nW by using an isothermal technique. The IF gain bandwidth is supposed to be about 1.3 GHz or higher. The present results show that good performance can be obtained at 1.5 THz even with a relatively thick NbTiN film (12 nm), as in the case of 0.8 THz. In order to investigate the cooling mechanism of our HEB mixers, we have conducted performance measurements for a few HEB mixers with different microbridge sizes both at 1.5 and 0.8 THz. The noise performance of the NbTiN HEB mixers is found to depend on the length of the NbTiN microbridge. The shorter the microbridge is, the lower the receiver noise temperature is. This may imply a contribution of the diffusion cooling in addition to the phonon cooling.
In this paper, we present measured results and theoretical modeling of a 1.5 THz waveguide NbTiN HEB mixer, which is fabricated by in situ process. The NbTiN HEB mixer with dimensions of 1 μm in width and 0.15 μm in length is measured to be as low as 570 K in noise temperature (7hν/k) even with a relatively thick NbTiN film (10.8 nm). The intrinsic mixer noise temperature is 300 K after the correction of the losses of quasi-optical and IF amplifier chain. At the same time, we adopt an optimized hot spot model to simulate the DC and RF behaviors of the NbTiN HEB mixer by combining phonon-cooling and diffusion-cooling mechanisms together. The measured and calculated current-voltage curves are in good agreement. The calculated intrinsic mixer noise temperature is 100 K and IF gain bandwidth is 2.7 GHz at the optimum bias point. The difference between the measured and simulated mixer noise temperature is factor of 3 times, which is probably due to misalignment of quasi-optical RF coupling system and air loss. The calculated and measured results show the two cooling mechanism work together to lower the noise temperature of the waveguide NbTiN HEB mixers. The details will be presented in this paper.
In this paper, we present the development of the waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixers, cryogenically cooled by a 4 K close-cycled refrigerator. The NbTiN thin film is formed on a crystalline quartz substrate by sputtering an NbTi target with the Ar and N2gas at room temperature. The HEB mixer element is fabricated by using the 12 nm NbTiN film, and is mounted on a waveguide block. Measurement of a Fourier transform spectrometer shows that the response of the mixer is centered near 810 GHz with a bandwidth of about 500 GHz. The uncorrected DSB receiver noise temperature is measured to be 500 K, and the noise bandwidth is to be 1.4 GHz at 810 GHz. The present result shows that a good noise performance can be obtained for the NbTiN HEB mixer even with a relatively thick film (12 nm) fabricated at the room temperature.