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IEEE Transactions on Applied Superconductivity

Design and Fabrication for the Construction of MIR HEB Mixers

Akira Kawakami, Hisashi Shimakage, Junsei Horikawa, Masaharu Hyodo, Shingo Saito, Shukichi Tanaka, Yoshinori Uzawa

Published Nov 30, 2016• 7 citations

Abstract

In order to design antennas for mid-infrared (MIR) hot electron bolometers (HEBs), the surface impedance of Au thin films at cryogenic temperatures was evaluated using Fourier transform infrared spectroscopy with a sample cooling system. We fabricated resonator arrays that were constructed by gold thin film strips. The length, width, and thickness of the resonators were set at 1300, 200, and 55 nm, respectively. As the temperature was lowered, the resonant frequency was shifted to the low-frequency side. By fitting the resonant frequency to simulated results, the corrected surface reactance was established for designing MIR nano-antennas. MIR HEBs with a twin-slot antenna were designed and fabricated using the corrected surface impedance. The design frequency was 61 THz. The HEBs were formed as a superconducting strip with Au antennas and electrodes. Here, the superconductivity under the Au electrodes was suppressed by a Ni thin film. For evaluating the mixer properties of the MIR HEBs, measurement setup without beam splitter was constructed. In this setup, local oscillator power was applied directly to the HEB, and we confirmed that the critical current of the HEB can be reduced to be almost zero by the LO irradiation. The HEB responses with clear polarization dependencies were also observed.

DOI: 10.1109/tasc.2016.2631898

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Hot-electron bolometer (HEB) mixers are a complex multi-layer thin film structure containing an ultra-thin superconducting film of NbN as a detecting element and a thick normal metal layer as an antenna structure. We have optimized the fabrication process starting with ultra-thin NbN films, Au films for antenna structures and their patterning using e-beam lithography and lift-off. The coupling between normal conducting antenna and NbN detector has been improved by introducing an intermediate NbN film to reduce proximity suppression of superconductivity in the detecting element. A critical temperature of about 9.5 K is reached for NbN films with a thickness between 5 nm and 6 nm. A twofold increase of the film thickness increases the critical temperature to 12 K. We have shown that a 20 nm thick buffer layer of NbN under a much thicker Au layer is sufficient to ensure a critical temperature of the bi-layer of 9 K. This value is close to the critical temperature of 5.5 nm thick HEB devices. The noise temperature of HEB mixer made using improved technology is about 800 K and was measured in a liquid cryogen free system with a quantum cascade laser as 2.5 THz local oscillator.

In this study, the use of nano-antennas to improve the response performance of infrared detectors is proposed, and the design and fabrication of antennas that operate in the infrared range is demonstrated. Prototypes of a mid-infrared superconducting hot-electron bolometer (HEB) formed by a twin-slot antenna with a niobium nitride strip were fabricated. When the bolometer was irradiated with mid-infrared (λ = 4.89 μm) pulsed light, responses with clear polarization dependency were observed. When the HEB was biased close to the critical current under mid-infrared pulsed light irradiation, the detector output synchronized with the trigger signal was observed. The output waveforms comprised voltage pulse trains, and the full width at half maximum of the pulse was evaluated to be approximately 0.3 ns.

Hot-electron bolometer (HEB) mixers are used in many low noise heterodyne radio astronomical receivers. Their noise temperature is at the level of 10–15 times the quantum limit, and their gain bandwidth is from 2 GHz to slightly more than 5 GHz depending on the type of substrate material used for a device fabrication. The HEB mixers presented in this Letter are a complex multilayer thin-film structure containing an ultra-thin superconducting film of NbN as a detecting element, and a thick normal metal layer as an antenna structure. The overall performance of the device has been improved by optimisation of the HEB mixer fabrication steps starting with ultra-thin NbN films, followed by NbN/Au bi-layers for antenna structures. Patterning was achieved using electron-beam lithography and lift-off processes. The noise temperature of the HEB mixer fabricated with the improved technology is about 800 K, as measured in a cryogenic-free system with a quantum cascade laser as a local oscillator.

To expand the intermediate frequency (IF) band and improve the sensitivity of a hot-electron bolometer mixer (HEBM), we proposed and examined a new HEBM structure using a magnetic thin film. We found that it was possible to suppress the superconductivity of the 5-nm-thick niobium nitride (NbN) thin film by the addition of a 1.8-nm-thick nickel (Ni) thin film. It was also confirmed that the superconductivity disappeared in the Au (70 nm)/Ni (1.8 nm)/NbN (5 nm) trilayer for forming the electrodes of the HEBM. By using the magnetic thin film for the electrodes, we suggested that the superconductivity of the HEBM strip would be affected and that hot spots would be formed near the electrodes. This approach is effective for shortening the hot-electron drift length and will lead to the expansion of the IF bandwidth. We think that the new structure lowers the required local oscillator power and improves the HEBM sensitivity by suppressing the proximity effect under the electrode. The IF bandwidth of the fabricated Ni-HEBMs was evaluated at 1.9 THz. We confirmed that the IF bandwidth expands, and the evaluated bandwidths were in the range of 5.1-5.7 GHz at 4 K. Ni-HEBMs with 0.1-μm strip length were also fabricated and evaluated. The IF bandwidth was about 6.9 GHz at 4 K.

We have proposed using nano-antennas to improve the response performance of infrared detectors. Prototypes of mid-infrared superconducting hot electron bolometer formed by a twin-slot-antenna with an NbN strip were fabricated. By the irradiation of mid-infrared (λ=4.89 μm) pulsed light, clear polarization dependency of responses was observed. When the HEB was biased close to the critical current, the detector output synchronized with the trigger signal was observed. The output waveforms were composed of voltage pulse trains, and the half width of the pulse was evaluated about 0.3 nsec.

As a high-speed and sensitive mid-infrared (MIR) photon detector, we have studied an MIR superconducting hot electron bolometer (MIR-HEB) with a twin slot-antenna. MIR HEBs have been fabricated; it has been demonstrated that HEBs respond to MIR photons with a response time of a few nanoseconds. However, the effective area of the twin slot-antenna was measured as only approximately 6.3 µm2at 66 THz. Therefore, we attempted to expand the area by making it a phased array. The fabricated antenna consists of multiple slot-antennas, MSL circuits, and a load resistor of niobium nitride (NbN) thin film, and the antenna effective areas were evaluated from the absorption rate in the reflection spectrum using an FTIR microscope. The effective area of the antenna consisting of 4 × 4 slot-antennas was estimated to be approximately 48 µm2at 66 THz. We also fabricated prototype MIR-HEBs equipped with a phased array antenna and evaluated its dc-characteristics.