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Physica C 372­376 (2002) 76­79 www.elsevier.com/locate/physc

Submicron YBCO Josephson junctions on sapphire bicrystal substrates for microwave devices
E. Stepantsov
a

a,b,*

, M. Tarasov a,c, A. Kalabukhov Z. Ivanov a, T. Claeson a

a,d

, T. Lindstro a, om

MINA, Chalmers University of Technology and Gteborg University, Gteborg, SE-41296, Sweden o o b Institute of Crystallography, Leninsky Prosp. 59, Moscow 117333, Russia c Institute of Radio Engineering and Electronics, Mokhovaya 11, Moscow 101999, Russia d Department of Physics, Moscow State University, 119899 Moscow, Russia

Abstract Submicron YBCO bicrystal Josephson junctions (JJ) and devices for high frequency applications were designed, fabricated and experimentally studied. The key elements of these devices are bicrystal sapphire substrates with symmetric misorientation angles 14.2°, 18.4° and 24° which were fabricated by a solid-state intergrowth method. A technological process based on deep ultraviolet photolithography using a hard carbon mask was developed for the fabrication of 0.4­0.6 lm wide JJ. The junctions have normal resistances up to 2 kX, Ic Rn > 2 mV and a 1 mV/U0 SQUID response at 4.2 K. The junctions were used as Josephson detectors and spectrometers at frequencies up to 1.5 THz, dc SQUID RF amplifiers operating at 1.5 GHz and submillimeter wave array oscillators. ñ 2002 Elsevier Science B.V. All rights reserved.
Keywords: High critical temperature superconductors; Sapphire bicrystal substrates; Josephson junctions; Microwave sensors

1. Introduction Terahertz applications of Josephson junctions (JJ) have gained much attention lately. For optimal operation of Josephson microwave detectors (see Ref. [1] for theory) the characteristic voltage should be above the normalised frequency Vc > f U0 , where Vc ¼ Ic Rn is the characteristic voltage, Ic the critical current, Rn the junction normal re-

* Corresponding author. Address: MINA, Chalmers University of Technology and Gteborg University, Gteborg, SEo o 41296, Sweden. Tel.: +46-031-7723117. E-mail address: evgueni@fy.chalmers.se (E. Stepantsov).

sistance, f the signal frequency, and U0 ¼ h=2e ¼ 2 á 10þ15 Tm2 is a flux quantum. The Vc values of the order of 2­4 mV have been achieved in high temperature superconductor JJ of different type e.g. bicrystal, step edge or ramp junctions. It should be noted that in most cases the substrates were SrTiO3 or (Y)ZrO2 with high microwave losses. Sapphire substrates are attractive for THz devices because of the low losses and low dielectric constant. A number of papers have addressed JJ on sapphire bicrystal substrates with high misorientation angles [2­4]. Here we report on the development of a technology for submicron JJ on low angle sapphire bicrystal substrate. Junctions integrated with log-periodic antennas were tested

0921-4534/02/$ - see front matter ñ 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 7 8 3 - 9


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in the frequency range 63­500 GHz. Junctions parameters were also evaluated for SQUID amplifiers.

2. Fabrication Sapphire bicrystals are formed by solid-state intergrowth of two monocrystals oriented so that the plane of contact (grain boundary plane) is orthogonal to the (1 0 1 2) planes of the crystals and makes a 33° angle with their (1 1 2 0) planes. The bulk bicrystal was diced along the common (1 0 1 2) plane. The final stage included a chemical­ mechanical polishing of the bicrystals and dicing them into 5 á 5 mm2 substrates with the grain boundary in the centre. The CeO2 and YBa2 Cu3 O7þd (YBCO) films were grown by pulse deposition with a KrF excimer laser at a pulse rate of 10 Hz. The distance between target and substrate was 50 mm in all cases. A thin CeO2 buffer layer was initially deposited to prevent the chemical interaction of the YBCO with sapphire. To minimize the YBCO/CeO2 interaction a 20 nm YBCO film was initially grown at reduced temperature, 750 °C, which was followed by deposition of 250 nm YBCO film at 790 °C. Deposition parameters for all layers are listed in Table 1. Finally, a 20 nm gold thin film was deposited in situ at 100 °C. Atomic force microscope (AFM) images show that the films are free of a-axis particles and the density of outgrowths, presumably CuO and Y2 O3 does not exceed 1 per 200 lm2 . The critical temperature of the films was above 89 K. Microbridges with width varying between 300 nm and 1.5 lm were pattern across the grain boundary to form JJ. A deep ultraviolet lithogra-

phy, with k ¼ 248 nm, was used to define submicron patterns in the Microposit UV-5 photoresist, that was later transferred into a three layer hard mask, consisting of Au (50 nm)/a-carbon (100 nm)/ Au (50 nm). Through the resist pattern the top layer of gold was etched by Ar× ion beam with an energy of 250 eV and current density of 0.2 mA/ cm2 , afterwards the residual resist and carbon were removed through the opened areas in an oxygen plasma. The required pattern was formed in the YBCO film by using Ar× beam etching through the carbon mask. The rest of the carbon was removed in oxygen plasma (5 min, 250 W, 10 mbar). Finally, the 20 nm in situ gold layer was removed by Ar× beam etching. The described technique allows us to prepare bicrystal bridge junctions crossing the grain boundary that are aligned with an accuracy of about 2 lm (see Fig. 1). The typical bridge has a

Fig. 1. Submicron YBCO JJ at the bicrystal boundary. The width at the top is 0.4 lm and at the bottom 0.6 lm.

Table 1 Process parameters for pulsed laser deposition Layer CeO2 YBCO YBCO Au Thickness (nm) 15 20 250 50 Deposition rate (nm/min) 20 35 35 18 Oxygen pressure (mbar) 0.15 0.6 0.6 ­ Substrate temperature (°C) 770 750 790 20 Pulse energy density (J/cm2 ) 1.1 1.9 1.9 Thermal evaporation Pulse energy (mJ) 55 95 95 ­


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Fig. 4. I­V curves of a Josephson receiving structure under irradiation. The frequency is 113, 62 and 500 GHz, the later at two different levels of power. Fig. 2. A dc SQUID on a sapphire bicrystal substrate. The junction width is less than 0.5 lm.

trapezoidal shape with a 0.6 lm bottom and 0.4 lm top width. The junctions were integrated in planar log-periodic antennas and SQUID loops (Fig. 2) in order to investigate their properties.

3. Measurements JJ on bicrystal substrates with misorientation angles of 24°, 18.4° and 14.2°, were studied. The

current­voltage characteristics can be well described by the resistively shunted junction model. It is worth noting that all submicron junctions had almost no excess current. The junctions have normal resistances up to 2 kX and characteristic voltages over 2 mV. We do see a decrease of the critical current density for submicron junctions (Fig. 3) that can be explained by damage during processing or by the faceting of the grain boundary. It should however be noted that even 300 nm wide junctions had a Jc of 104 A/cm2 . Junctions integrated with log-periodic antennas were tested in the frequency range 63­500 GHz. Well pronounced Shapiro steps were observed in the I­V curve at all frequencies (Fig. 4). Similar junctions and arrays have also been used for

Fig. 3. Critical current vs. junction width for several submicron junctions.

Fig. 5. I­V curves from a dc SQUID at two levels of external magnetic flux.


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Hilbert spectroscopy junctions can also be cillators [7]. SQUIDs with 0.5 were fabricated and as high as 1 mV was These SQUIDs were at a signal frequency

[5,6]. Parallel arrays of such used as electrically tuned oslm wide bicrystal junctions tested. A voltage response measured at 4.2 K. (Fig. 5). also tested as RF amplifiers of 1.5 GHz [8].

Acknowledgements The authors acknowledge the financial support from the Swedish Royal Academy of sciences, the Swedish Institute, EU-INTA-01-686, ISTC and the Swedish OXIDE program.

References 4. Conclusion High-quality YBCO JJ have been fabricated on sapphire bicrystal substrates. By using sapphire as a substrate material and making submicrometer size junctions the required parameters for microwave devices were achieved: a characteristic voltage over 2 mV, a SQUID voltage response over 1 mV and a normal resistance up to 2 kX. These parameters together with very low losses at RF and submillimeter waves in sapphire substrate make such junctions perfect candidates for high frequency applications e.g. Josephson detectors and spectrometers, SQUID amplifiers and oscillators.
[1] K. Likharev, Dynamics of Josephson junctions and circuits, Gordon and Breach Science Publishers, New York, 1986. [2] D.E. Oates et al., IEEE Trans. Appl. Supercond. 9 (1999) 2446. [3] L.R. Vale et al., IEEE Trans. Appl. Supercond. 7 (1997) 3193. [4] G. Kunkel et al., IEEE Trans. Appl. Supercond. 7 (1997) 3339. [5] M. Tarasov et al., Supercond. Sci. Technol. 12 (1999) 995­997. [6] M. Tarasov et al., Proceeding of EUCAS-1999, Applied Superconductivity 1999, Vol. 2, Institute of Physics Conference Series 167, 1999, pp. 611­614. [7] M. Tarasov et al., Physica C; these Proceedings. [8] M. Tarasov et al., Proc. 85th Int. Supercond. Electronics Conf., June 19­22, 2001 Osaka, Japan, pp. 149­150.