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Design and fabrication of superconducting HEB mixer WANG JinPing, LI YangBin, KANG Lin†, WANG Yu, ZHONG YangYin, LIANG Min, CHEN Jian, CAO ChunHai, XU WeiWei & WU PeiHeng Research Institute of Superconductor Electronics (RISE), Deptartment of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China

This paper describes the design and fabrication of superconducting hot electron bolometer (HEB) mixer based on ultra-thin superconducting NbN films. The high quality films were epitaxially grown on high resistance Si substrates. The device was fabricated by magnetron sputtering, electron beam lithography (EBL), reactive ion etching (RIE), lithography, and so on. The device’s resistance-temperature (R-T) curves and current-voltage (I-V) curves were studied. The results of THz response of the device are presented. Y-factor technique was used to measure the device’s noise temperature. When the device was irradiated with a laser radiation of 2.5 THz, the obtained lowest noise temperature of the device was 2213 K.

THz wave is important for the astronomical and atmospheric observations, especially for the ozone layer investigation, and its potential applications include nondestructive testing for organisms, wireless networks, etc. For the band of 0.1 ― 1 THz, superconductorinsulator-superconductor (SIS) mixers are considered as one of the most sensitive detectors[1]. As is well known, electrons exist as the cooper pairs in superconducting materials. When radiation energy is absorbed by the cooper pairs, with hν=2Δ (ν is frequency, Δ is gap energy), the pairs are separated into quasi-particles[2]. Hence, the SIS mixers’ working frequency is limited by ν=2Δ/h. Even for the SIS mixers based on high gap energy materials, such as NbN, NbTiN, devices’ working frequency is still below 1.4 THz[3]. Here we describe a device, whose frequency is not limited by the material’s energy gap. In the past decade, a great deal of progress has been made for the hot electron bolometer (HEB)[4]. When a superconducting HEB mixer absorbs radiation, nonequilibrium electrons are generated from its ultra-thin superconducting film, and thereby the radiation signal can be detected. The mixer consists of two essential elements, one is the superconducting nano-bridge which is used to detect the RF sig-

nal, and the other is the planar antenna, made of normal metal thin film and used to couple the RF signal from free space to the superconducting bridge. The HEB mixers can be categorized into two large types. One is the diffusion-cooled type of devices, whose bridge dimension must be less than the electrons’ diffusioncooled region. The other is the lattice-cooled type of devices, also known as phonon-cooled devices. For the phonon-cooled devices, when they absorb RF signals, hot electrons are activated with temperature higher than the phonons. The electrons and phonons interact strongly, and they exchange energy. Then the energy is transferred from phonons to the substrates[5]. Thus, the ultra-thin film is a critical component for the mixer. The thinner the films are, the wider bandwidth the HEB mixers have[5,6]. The phonon-cooled NbN HEB mixers can offer a high spectral resolution, and the highest sensitivity data reported have reached 10×hν/k [7], in which h is the Planck’s constant and k the Boltzman’s constant. Received January 22, 2009; accepted March 25, 2009 doi: 10.1007/s11434-009-0311-3 † Corresponding author (email: [email protected]) Supported by the National Basic Research Program of China (Grant Nos. 2006CB601006, 2007CB310404) and High-Tech Research and Development Program of China (Grant No. 2006AA12Z120)

Citation: Wang J P, Li Y B, Kang L, et al. Design and fabrication of superconducting HEB mixer. Chinese Sci Bull, 2009, 54: 2013-2017, doi: 10.1007/s11434-0090311-3

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HEB, ultra-thin superconducting NbN film, planar equiangular spiral antenna, noise temperature

Today many developed countries have invested a great deal of resources dedicating to this research direction, and the big projects include the High Elevation Antarctic Terahertz Telescope (HEAT) on Antarctica[8], the SAFIR of NASA, USA[9], the ESPRIT of Europe[10]. In this paper, we report the design and fabrication of the superconducting NbN HEB mixers fabricated on the high resistance Si substrates (resistivity, ρ: 1―3 kΩ· cm). We also report the measurements of the mixers’ THz responses.

1 Design and fabrication of the mixer 1.1 Ultra-thin NbN films As mentioned above, the characteristics of the ultra-thin films are very important for the mixers, including the superconducting critical temperature Tc, critical current density Jc, and so on. Theoretical calculations[11] and experimental results[12] have shown that larger gain bandwidth required by mixers could be achieved with thinner films. The thinner the film, the shorter the time of phonon escape. The growth of such thin films with a high critical temperature and critical current density is extremely challenging. One should be aware of the technological limitations if the targeted thickness is very small; meanwhile the high Tc and Jc need to be retained[13]. Experiments have proved that if the thickness of NbN film on Si substrate was thinner than 3 nm, the Tc of this film dropped to lower than 4.2 K[14]. The high quality NbN films were epitaxially grown with a magnetron sputtering system on the high resistance Si substrates (ρ: 1―3 kΩ·cm), and the thickness was 5－6 nm. Terahertz time-domain spectroscopy (THz-TDS) system has been used to test the transmission loss of the high resistance Si. When the substrate thickness was 0.3 mm, the transmission efficiency of the THz signals was up to 80%. Ar ion milling was used to remove the contaminations on the substrates. Then the substrates were put into the chamber of the magnetron sputtering system. The sputtering conditions are shown in Table 1. The 5 nm NbN film was grown on a Si substrate. A Tc of about 8.0 K and a ΔT (critical temperature bandwidth) of 0.3 K have been achieved. For microwave devices, Table 1

the roughness of the film surface will cause signal loss. Atomic force microscope (AFM) was used to study the 5 nm NbN films. For a 1 μm×1 μm region, the roughness was about 0.340 nm (RMS). 1.2 Fabrication of the device Figures 1 and 2 show the top view of the NbN HEB mixer and a cross-sectional view of the nano-bridge with contact structure. This HEB consists of a superconducting bridge and a metallic antenna. The NbN film thickness of the nano-bridge was about 5 nm. The antenna was connected to the nano-bridge by contact pads. These pads consist of 6 nm thick NbN and 50 nm thick Au on top. This NbN layer reduced the contact resistance. At the same time, it reduced the superconducting proximity effect between the bridge and pads. The bridge was precisely defined by electron beam lithography (EBL) and reactive ion etching (RIE), and its dimension was 0.4 μm×4 μm. The antenna was prepared by lithography and sputtering, using the 250 nm Au on the substrate. 1.3 Antenna design and impedance matching For this HEB mixer, the planar antenna consists of one and a half laps equiangular spiral. The relation between the external diameter (R) and internal diameter (r0) of the device is as follows:

R = r0 × eϕ a |ϕ =3π = r0 × e3πa ,

(1)

where typically a = 0.221, R = 8.03r0, and the wave length is determined by the inner diameter and external diameter: r0 =

λmin 4

, R=

λmax 4

(2)

.

When R = 200 μm, r0 = 25 μm, the frequency range of the antenna that we have adopted was from 0.4 to 3 THz. In order to improve the coupling efficiency, it is necessary to maintain resistance match between antenna and nano-bridge. A planar equiangular spiral antenna is a non-resonant antenna. By the Babinet’s principle, the antenna and the gap form a self-compensation structure. The impedance of the antenna Za is

The sputtering conditions

Blackground vacuum

N2:Ar (mass)

Sputtering pressure

Sputtering current

Sputtering voltage

Growth rate

< 2.5×10−2 Pa

6:60

0.27 Pa

0.9 A

410 V

60 nm/min

2014

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ARTICLES Figure 3 The simulated coupling efficiency when the antenna was on Si substrate and its impedance was 75 Ω.

The basic operating principle of HEBs utilizes the fact that the terahertz radiation heats the electrons in the superconducting bridge. This induces a local resistance due to the dependence of the resistive transition intrinsic to the NbN bridge on the temperature and bias current. The antenna is used to collect and feed signals to the bridge. If Zb ranges from 29  to 192 Ω, the antenna coupling efficiency will be higher than 80%. Thus, we will focus on this bridge scale. Figure 2

2 Performance of the mixer

Cross-sectional view of the HEB structure.

η0

μ0 Za = , Z0 = = 2, ε0 2 (1 + ε r ) / 2 Z0

2.1 DC characterization and analysis (3) 

where η0 is the wave impedance in free space, μ0 is the permeability of vacuum, ε0 is the permittivity of vacuum, and εr is the dielectric constant, namely, Za just depends on the material’s intrinsic properties. In free space, Za = Z0 = 60 πΩ, and εr of high resistance Si is 11.9, so the impedance of the antenna on Si substrate is about 75 Ω. In order to evaluate the coupling efficiency η, assume that the impedance of the antenna Za is 75 Ω, and the impedance of the bridge Zb is in the range of 1 to 400 Ω. As 2

⎛ Z − Zb ⎞ η =1− ⎜ a (4) ⎟ , ⎝ Za + Z b ⎠ we calculated the simulation curve (Figure 3), simulating the relationship between the coupling efficiency and the bridge impedance. When Za = Zb, the device has the best efficiency.

The bridge dimension of the mixer was 0.4 μm × 4 μm. And the resistance-temperature (R-T) curve of the device is shown in Figure 4 by curve (b). From curves (a) and (b) in Figure 4, one can see that the Tc of the device was 7.7 K. Compared with the original film, Tc decreased about 0.3 K, whereas the ΔT increased up to 1.1 K. The reason for these degradations may relate to the lithography, RIE and other processes during device fabrication. There are three steps on curve (b). The first step reflects the intrinsic character of the 5 nm NbN film. The second step, at about 13 Ω, reflects the superconducting proximity effect between the NbN bridge and the Au pads.  The third step appears at 4.75 K, and it is caused by mixer’s residual resistance, about 3 Ω. It resulted mainly from the antenna and the contact resistance between Au layer and NbN layer. The resistance of bridge Rb=Rd−Rm =91 Ω−13 Ω=78 Ω, close to the best coupling efficiency condition.

Wang J P et al. Chinese Science Bulletin | June 2009 | vol. 54 | no. 12

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Figure 1 Micrograph of the NbN HEB mixer with a planar equiangular spiral antenna on Si substrate.

2.2 Measurement of noise temperature

Figure 4 R-T curves measured with four-probe technique. Curve (a) refers to the 5 nm NbN film and curve (b) refers to the device.

I-V characteristics of the device were studied with a voltage source. The I-V curves (see Figure 5) were measured at different temperatures. At 4.5 K, the superconducting critical current Ic of the mixer was 135 μA. It means that the superconducting critical current density Jc was 6.75×109 A/m2. When the temperature was elevated, Ics (the critical current in the superconducting state) descended obviously. At 4.5 K, when the bias voltage ranged from 2.95 to 4.15 mV, the bridge was in the metastable hotspot state. Khosropanah et al.[15] used the non-liner hot spot model to calculate this curve, and they considered this region as the optimum operating area. This metastable region expands or shrinks periodically when the device is heated or cooled.

The mixer’s noise temperature was measured using the traditional Y-factor technique by alternatively placing a hot/cold (300/77 K) load[16,17]. The THz signals were generated by a CO2 FIR laser. The mixing noise of the system was studied at 4.5 K with 1.6 and 2.5 THz signals. When the signal loss of incident window was not considered, the bias voltage was 1.5 mV, and the lowest noise temperature was 1710 K at 1.6 THz. When the bias voltage was 1.8 mV, the lowest noise temperature was 2213 K. The noise temperature at different bias voltages and at 2.5 THz is shown in Figure 6. Curve (a) corresponds to the mixer’s I-V curve without irradiation. I-V curve (b) was measured when the mixer mixed with 2.5 THz signal.

Figure 6 The device’s I-V character without radiation (a) and I-V character with the radiation of 2.5 THz (b). The stars mean that different noise temperatures are measured at different bias voltages, when the device is used for mixing experiences with 2.5 THz radiation.

3 Conclusion

Figure 5 I-V curves of the device measured at different temperatures ranging from 4.5 to 7.5 K.

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We have demonstrated a heterodyne terahertz mixer based on the ultra-thin NbN HEB on Si substrate. The analysis of the mixer revealed some parameters related to the performance, especially the design and fabrication of the device. With a suitable bridge and a planar equiangular spiral antenna, the mixer demonstrated an excellent coupling efficiency. The device was used for mixing experiment with 1.6 and 2.5 THz signals. The HEB mixer was quite sensitive, and the lowest noise temperatures obtained was 1710 K at 1.6 THz and 2213 K at 2.5 THz, respectively. 2

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