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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Modeling and measurement of IF gain bandwidth and intrinsic noise temperature for NbTiN hot electron bolometer mixers at Terahertz frequencies

Ling Jiang, Fei Liu Yun, Satoshi Yamamoto

Published Nov 3, 2010• 0 citations
Publisher

Abstract

In this paper, we present the modeling and measurement results of niobium titanium nitride hot electron bolometer mixers with different sizes at terahertz frequencies. A distributed hot spot model which is based on solving a heat balance equation is employed to characterize current-voltage curve, electron and phonon temperature distribution along superconducting microbridge, intermediate frequency gain bandwidth and intrinsic noise temperature of the mixers. The simulated intermediate frequency gain bandwidth decreases with the increase of microbridge length, which is in good agreement with measured results. A difference of factor of several tens exists between the calculated and measured mixer noise temperature, which is probably due to the disfigurement of waveguide structure and unknown noise from the mixer.

DOI
10.1117/12.870090

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In this paper, we present the modeling and measurement results of niobium titanium nitride hot electron bolometer mixers with different sizes at terahertz frequencies. A distributed hot spot model which is based on solving a heat balance equation is employed to characterize current-voltage curve, electron and phonon temperature distribution along superconducting microbridge, intermediate frequency gain bandwidth and intrinsic noise temperature of the mixers. The simulated intermediate frequency gain bandwidth decreases with the increase of microbridge length, which is in good agreement with measured results. A difference of factor of several tens exists between the calculated and measured mixer noise temperature, which is probably due to the disfigurement of waveguide structure and unknown noise from the mixer.
In this paper, we present the intrinsic mixing behavior of superconducting niobium titanium nitride hot electron bolometer mixer based on in-situ technique. We make detailed comparison between simulated and measured performance of the HEB mixers at 0.8 and 1.5 THz. A distributed hot spot model which is based on solving a heat balance equation is employed to characterize current-voltage curve, electron and phonon temperature distribution along superconducting microbridge, intermediate frequency gain bandwidth and intrinsic noise temperature of the mixers. The simulated intermediate frequency gain bandwidth decreases with the increase of microbridge length, which is in good agreement with measured results. For the microbridge with 0.15 µm in length, the IF gain bandwidth is simulated to be 2.6 GHz at 0.8 and 1.5 THz. When the microbridge length is increased to 0.3 µm, the calculated IF gain bandwidth is 1.8 GHz. The simulated results demonstrate the IF gain bandwidth is only related with microbridge length, independent of RF frequency. It's very useful to find the critical parameters which mainly affects the mixer noise temperature and IF gain bandwidth. In this paper we will give the critical factors as the reference for fabrication and measurement of HEB mixers. We have found that when the ratio of length to width of the microbridge is set to be around 0.15, the simulated and measured mixer noise temperature show the lowest value. More details will be presented in the paper.
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.
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 investigate the critical temperature dependence of the performance of niobium nitride superconducting hot electron bolometer (HEB) mixers. The input noise temperature and intermediate frequency (IF) noise bandwidth of superconducting HEB mixers of different critical temperatures Tc(between 6 and 12 K) are simulated with a hot spot model. Simulation results show that the mixer input noise temperature is the lowest at Tcapproximately equal to 7-9.5 K and the IF noise bandwidth increases with Tc. We also measure three superconducting HEB mixers of different critical temperatures (i.e., 7.5, 8.8, and 10.3 K) at 0.85 and 1.3 THz. An agreement between measurement and prediction is observed.
This paper presents the modeling and measurement of a quasioptical niobium nitride superconducting hot electron bolometer mixer at submillimeter wavelengths. The modeling is performed with a distributed hot spot model which is based on solving a heat balance equation for electron temperature along the superconducting microbridge. Particular care has been taken during the modeling concerning the temperature-dependent resistance and the bias current dependence of the critical temperature of the device. The dc and mixing characteristics of this mixer have been computed and we have observed a quite good match between the predicted and the measured results for both dc characteristics and mixing performances at submillimeter wavelengths.
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.