Electron beam writing in fabricating planar high-Tc Josephson junctions

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Physlca C 209 (1993) 211-214 North-Holland

Electron beam writing in fabricating planar High -T c Josephson junctions S Tolpygoa, B.Nadgomya, S.Sholdaora, F.Tafuric, Y Lma, A.Bourdillonb and M.Gurvitcha

aDepartment of Physics, SUNY at Stony Brook, Stony Brook ,NY I 1794 bDepartment of Matertals Science, SUNY at Stony Brook, Stony Brook, NY 11794 CDepartment of Physics, Universtty of Napoli, 80125, Napoli, Italy Electron beam irradiation was utilized to fabricate planar Josephson junctions in ~Ba2Cu307thin films. After the nucron-size bridges had been fabricated by way of standard optical lithography, they were modified using a well focused nanometer STEM probe with beam energy within the range of 80 - 120 keV. Modified junctions exhibit a two-step normal/superconductor transition We attribute the lower transition temperature, which is of the order of 77 K, to the transition of the damaged region. Shapiro steps under applied microwave radiation of the frequency 10-15 GHz, as well as oscillation of the critical current in a magnetic field were observed up to 70 K. A comparison with the RSJ model is made and a possible damage mechanism is discussed. 1. I N T R O D U C T I O N Recent years have witnessed considerable progress in preparation of High-Temperature Superconductor (HTS) Josephson junclaons. SNS junctions with noble metal normal links and grain boundary junctions display both de and ac losephson effects, operating at 77 K. However, the problems of reproducibility, uniformity, non optimal IcRn and especially technological complexity still persist, even for the best available junctions. These problems are rooted in the intrinsic properties of I-ITS materials, such as short coherence length, structural complexity, high sensitivity of critical parameters to the structural or stoichiometric imperfections and high chemical reactivity.

1.1. Concept Our idea was to apply a completely different approach to the formation of planar Josephson weak links in HTSs, utilizing their high sensitivity to structural disordering and oxygen content [1]. The simplest arrangement, consistent with this idea is a c&-oriented film of good quality , in which a narrow, micron-size bridge has been developed. Across the bridge we produce a damaged (nonsuperconducting or lower T o region comparable to ~,~ ( of the order of 10 nm or less) (see Fig .1). At the required scale this can only be done with a well focused intense electron beam, such as a beam m a good electron microscope.

Fig 1. SEM micrograph of a typical bridge, with the wtute line outlining the damaged area

2. EXPERIMENT 2.1. Electron beam writing For producing a weak link we used a Phillips CM-12 STEM with accelerating voltage up to 120 kV, beam current about 20 pA and nnnimum probe size of 1 nm. From previous experiments we had found that electron beam at energies from 5 to 30 keV usually did not influence c-oriented YBCO film of high quality [2], so in this work we used 80-120 keV energies. A bridge was placed into the electron microscope and the beam was scanned

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S Tolpygo et al / Electron beam writing m fabrwatmg planar hzgh-Tc Josephson juncttons

across the bridge once, with over 103 equally spaced stop-points per scan. The dwell time for each point varied from 1 to 5 seconds. To control the degree of damage we monitored the resistance of the bridge m - s i t u during the beam writing, using a specially designed sample holder with electrical leads. Electron beam spreading inside I-ITS, estimated from Monte-Carlo simulations [3] is about 10 nm for a 50 nm film at 30 keV and still comparable with ~ ; at higher beam energies it is even smaller.

2.2. S a m p l e s We experimented with different I-ITS films (YBCO, TBCCO, BKBO) and some others. In this paper we only report the results obtained for YBCO films. These films were prepared at AT&T Bell Labs [4] using the BaF 2 e x - s t t u annealing process. To reduce beam spreading we used the thinnest films available, with a thickness of 25 nm, with T c = 90 K and resistivity 200-300 # [2.can at room temperature. The film was patterned into 2-3 # m wide and 4/~ m long bridges, using standard photolithography with PMMA resist and then etohed in Br-methanol etchant. The bridges typically have almost the same T c as the original film (within 1 K), a critical current density Jc '~ 2 x 106 A/cm2 at 77 K, and approximately 10 times higher at 4.2 K. 3. E X P E R I M E N T A L

RESULTS

3.1 C h a n g e in R ( T ) d e p e n d e n c e The effect of the beam writing on a typical YBCO bridge is shown in Fig.2. The curve for an irradiated bridge develops a "tail" from the original transition temperature Tco to a lower temperature Tcl. The value of Tcl and the tail resistance depend upon the irradiation dose and the beam energy. We attribute the tail resistance to the normal resistance of the damaged region and the second transition at Tcl to the supeconducting transition of the damaged region. Between these temperatures the damaged region showed linear I-V dependence. We were able to change Tcl continuously from 87 K down to helium temperature and even make it completely non-superconducting by changing the dwell time of the line scan. We found that for a given dwell time there is a distance between the stop-points, shorter

than which the tail begins to form. Obviously this happens when the damaged areas around the two adjacent points begin to overlap. We measured 20 nm for the size of the damaged region, which is close to the result of Monte Carlo simulations (see sec.2.1). Using this number and the typical value of the tail resistance (about 3~), one can get 1.5 f~.cm for the resistivity of the damaged region, which is about 10 times higher than the resistivity of the original non-irradiated film.

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Fig.2 Temperature dependence of the resistance for the two typical YBCO bridges after e-beam writing at 120 keV 1 - 1000 stop-points, 5s dwell tune, 2 - 2000 stoppoints, ls dwell time

3.2 Shapiro steps To investigate the nature of the produced weak link, I-V curves were taken at various temperatures with and without a microwave field. At typical writing conditions the critical current of a bridge at 4.2 K reduces by a factor of 10 in comparison with the original bridge, which implies that the critical current density of a weak link is still very high. Nevertheless, we were able to observe microwave induced current steps and other features of Josephson-like behavior, which were totally absent before e-beam writing. Fig.3 demonstrates Shapiro steps for different levels of microwave power for a typical irradiated bridge. Four steps, oscillating with microwave power can clearly be seen. For such weak

213

S Tolpygo et al I Electron beam writing m fabricating planar h+gh-Tc Josephson juncnons

links we were able to observe Shapiro steps to high order for temperatures from 4.2 K to Tc1. The shape of the I-V curves, the amplitude of the steps, as well as the value of IcRn depend on writing conditions and on the temperature. The typical values of IcRn found for the e-beam irradiated bridges are in the range of 0.1-0.5 mV at 65 K.

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3.4. Stability The properties of the bridge do not change if it is stored in liquid nitrogen . When stored at room temperature the critical current of the bridge kept increasing, reaching saturation after a few weeks. We attribute this effect to the annealing of the damaged area. We still observed Shapiro steps at 77 K even after the saturation had been reached. We note that the study of this process at room and elevated temperatures may give some insight into diffusion mechanisms in HTS on a nanometer scale.

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Fig. 4 shows the microwave power dependence of the amplitude of the Shapiro steps and the critical current at T = 63 K. Dashed lines represent the results of RSJ model at f~==0.06, where f2 =hf/2eIcRn is the dimensionless frequency (see for instance [5]). The agreement is quite reasonable and can be improved even further if thermal fluctuations are taken into account.

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Another important test for the Josephson nature of a weak link is the magnetic field dependence of the critical current. It is shown in Fig. 5 for a typical irradiated bridge. A magnetic field was applied perpendicular to the film plane. The observed behavior is characteristic for long Josephson junctions where the Ic(I-1) dependence is an envelope of different It(H) curves, corresponding to a different number of magnetic flux quanta accumulated in the junction, e.g. [5]. For our bridges the Josephson

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214

S Tolpygo et al / Electron beam writing m fabrTcatmg planar high- Tc Josephson junctions

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Fig 5 Cntteal current vs magnetic field for 3 /z m w~de bndge irradiated at 100keV, 1500 stop-points, 1 5 s dwell tune Someflux was trapped before the measurements 4. D I S C U S S I O N A N D S U M M A R Y It has been recently shown that for all HTS the decrease of T c under irradiation is a universal function of nonionizing energy loss [6], i.e. the T c is a function of the total number of atoms displaced. This knock-on mechanism of damage implies the existence of some threshold energy for incident electrons, Eg. The lowest Eg corresponds to the oxygen knock-on. If one take~ the oxygen binding energy of 20 eV, Eg will be about 150 keV Note here a successful attefnpt in producing the Josephson weak link using the electron beam at energies above Eg [7]. However we observed considerable damage at-the energies below Eg Two possible explanations may be suggested. The first is based on a two-stage mechanism of defect formation, involving excitation (ionization) of an oxygen atom in a low binding energy state with subsequent knock-on. The second possibility is that the binding energy might be significantly lower than 20 eV, e.g. for chain oxygen in 1-2-3 structure As we know from oxygen diffusion and desorption experiments the activatmn energy for displacement of chain oxygen is about 0.5 eV. Eg corresponding to this energy is about 4 keV. Not6 that the second explanation is

specific for 1-2-3 compounds. Studying all of these effects is of interest in its own right. We have reported a new approach to the preparation of high-Tc Josephson junctions utilizing direct electron beam writing. The junctions exhibit all the characteristic properties of the Josephson effect, including Shapiro steps, microwave power dependence and critical current modulation in a magnetic field. This approach opens up new technological perspectives, as well as furnishing some insight into the fundamental properties of HTS on a nanometer scale. We would like to thank S.Hou and J.Phillips for providing YBCO film, Z.Bao and Li Ji for helping with some of the experiments, K.Likharev and J.Lukens for useful discussions. This work was supported by the DARPA grant N0014-90-J-4032.

REFERENCES 1. M.Gurvitch, J.Macaulay, Z Bao, B.B], S.Han, J.Lin, J. Lukens, B.Nadgorny, Julia M Phillips, M.P.Siegal, E.S.Hellman, Extended Abstracts of 3-d FED Workshop on High-Temperature Superconductmg Electron Devices, May 15-17, 1991, Kumamoto, Japan, p.162. 2. M.Gurvitch, S.Tolpygo, B.Nadgorny, S.Shokhor, J.Lin, F.Tafuri, J.Macaulay, A.Bourdlllon, S.Hou and J.Phillips, Proceedings of the Third lnternatmnal Workshop on Tunnehng Phenomena m Hagh &Low Tc Superconductors, October 1-4, 1991. 3. J.M.Macaulay, Z Bao, B.BI, M.Gurvitch, S Han, J Lin, J.Lukens, and B.Nadgorny, Superlat. and Microstruct. 11, 211 (1992). 4. M.P.Siegal, J.M Phillips, Y -F Hsieh, and J.H.Marshall, Physica C172, 282 (1990). 5. A.Barone and G.Paterno, Physws and Applications of the Josephson effect, Wiley, New York, 1982. 6. B.D.Weaver, E.M. Jackson, G.P.Summers, and E.A.Burke, Phys.Rev.B, 46, 44-47 (1992). 7. A.LPanT~ Z.Barber, A.M.Campbell, J E.Evetts, R.E.Somekh, D.F.Moore, and A N.Broers, Extended Abstracts, Thtrd International Superconductive Electronics Conference, 25-27 June 1991, Glasgow, Scotland, p. 352.