Combined method of focused ion beam milling and ...

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May 25, 2016 - aElectronic mail: [email protected]. bAlso at: Department of Electrical Engineering, University of Maryland,. College Park, MD 20742. 2898.
Combined method of focused ion beam milling and ion implantation techniques for the fabrication of high temperature superconductor Josephson junctions C.-H. Chen, I. Jin, S. P. Pai, Z. W. Dong, R. P. Sharma, C. J. Lobb, T. Venkatesan, K. Edinger, J. Orloff, J. Melngailis, Z. Zhang, and W. K. Chu Citation: Journal of Vacuum Science & Technology B 16, 2898 (1998); doi: 10.1116/1.590291 View online: http://dx.doi.org/10.1116/1.590291 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/16/5?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Tailoring of high- T c Josephson junctions by doping their electrodes Appl. Phys. Lett. 75, 850 (1999); 10.1063/1.124534 A potential method to correlate electrical properties and microstructure of a unique high- T c superconducting Josephson junction Appl. Phys. Lett. 74, 1024 (1999); 10.1063/1.123443 Fabrication of high-temperature superconducting Josephson junctions on substrates patterned by focused ion beam Appl. Phys. Lett. 73, 1730 (1998); 10.1063/1.122259 Electron-beam damaged high-temperature superconductor Josephson junctions J. Appl. Phys. 82, 5612 (1997); 10.1063/1.366422 Fabrication of high-temperature superconductor Josephson junctions by focused ion beam milling J. Vac. Sci. Technol. B 15, 2379 (1997); 10.1116/1.589651

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Combined method of focused ion beam milling and ion implantation techniques for the fabrication of high temperature superconductor Josephson junctions C.-H. Chen, I. Jin,a) S. P. Pai, Z. W. Dong, R. P. Sharma, C. J. Lobb, and T. Venkatesanb) Center for Superconductivity Research, University of Maryland, College Park, Maryland 20742

K. Edinger, J. Orloff, and J. Melngailis Institute for Plasma Research, University of Maryland, College Park, Maryland 20742

Z. Zhang and W. K. Chu Texas Center for Superconductivity, University of Houston, Houston, Texas 77204

~Received 26 March 1998; accepted 3 July 1998! We have studied the T c degradation of epitaxial YB2Cu3 O72x ~YBCO! films on ~100! LaAlO3 substrates implanted with 100 keV O1 ions at different conditions. The influence of Au mask thickness and the implantation doses on the film characteristics have been investigated systematically. YBCO bridges have been modified by local oxygen ion implantation at optimal condition through a narrow trench in an Au/photoresist mask, which was fabricated formed by focused ion beam milling and reactive ion etching. The critical current and normal resistance of the modified bridges were found to be characteristic of superconductor/normal/superconductor Josephson junction behavior. Microwave irradiation of the junctions resulted in Shapiro steps in the I – V characteristics. © 1998 American Vacuum Society. @S0734-211X~98!02705-X#

Josephson junctions are the key elements in superconducting electronics. In order to develop devices using high temperature superconductors ~HTS!, the fabrication of reliable and reproducible Josephson junctions is necessary. Many types of HTS junctions have been developed such as bicrystal grain-boundary junctions,1 step-edge junctions2,3 and ramp junctions.4 Due to the inherent microstructural nonuniformity of the interfaces and barrier layers formed during the growth, the desired properties of the junctions ~better reproducibility, higher I c R n ) are yet to be fully realized. Another type of junction that has been reported is the e-beam written junction,5,6 which takes advantage of the fact that the beam spot size is about 1–2 nm. Using electron radiation, Josephson coupling can be obtained via a damaged region. The junctions fabricated by this technique show good superconductor/normal/superconductor ~SNS!. But this kind of junction has serious drawbacks, namely, low throughput and poor long-term stability.7 Ion implantation, a well-established semiconductor process technology can be used to achieve chemical and structural alternations in a controllable manner. The local modification of the electrical properties of the HTS thin films using ion implantation is an alternative for junction fabrication. HTS Josephson junctions and superconducting quantum interference devices ~SQUIDs! have been made of YBa2 Cu3 O7 ~YBCO! by focused ion beam ~FIB! writing,8 or by ion implantation using masking techniques9,10 with limited success. Here we will report a novel approach of YBCO junction fabrication by local oxygen ion implantation a!

Electronic mail: [email protected] Also at: Department of Electrical Engineering, University of Maryland, College Park, MD 20742.

b!

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through a narrow trench in an Au/photoresist mask, which was prepared by FIB milling and reactive ion etching. The YBCO thin films, with thickness of about 100 nm, were prepared on ~100!LaAlO3 ~LAO! single-crystal substrates by pulsed laser deposition. The average laser energy density was 1 J/cm2 . The substrate temperature was maintained at 780 °C, and the oxygen pressure was kept at 100 mTorr during deposition. The deposited films were cooled to room temperature in 500 mTorr of oxygen. Transport measurements showed that the as-deposited YBCO films had a zero-resistance transition temperature of ;90 K and a critical current density of >106 A/cm2 at 77 K. X-ray diffraction data including F-scans and rocking curves, indicated that the YBCO films were epitaxially grown on LAO with the c axis perpendicular to the substrate. From a transport of ions in matter ~TRIM! simulation, for 100 keV oxygen ions, the longitudinal projected range is 121 nm with a lateral straggle of 42 nm. For 100 nm YBCO, the 100 keV oxygen ion can damage the entire film. In order to study the film degradation with these ions, the implantation was done on YBCO single layer by using with 100 keV oxygen ions with doses from 1013 to 1014 ions/cm2 . As can be seen in Fig. 1~a!, the zero-resistance transition temperature (T C0 ) of YBCO films decreases as the dose increases. After implantation with a dose of 631013 ions/cm2 , the T C0 has been completely suppressed. This experiment give one an idea of what dose of implanted ions is necessary for producing a weak link, the cause of weak-link region in YBCO film, and the optimum mask layer in terms of materials and thickness needed for the protection of the underlying YBCO film without any degradation during the subsequent ion implantation. The implanted samples were all characterized by Ruther-

0734-211X/98/16„5…/2898/4/$15.00

©1998 American Vacuum Society

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ford backscattering spectroscopy ~RBS! for examining the film structural quality. RBS has been carried out using a very well collimated beam of 1.5 MeV 4 He1 ions. It is found that the ion implantation creates not only lattice displacements of oxygen but also of other elements. The effect of ion implantation is more stable compared to that of the electron implantation, where only O displacement is induced.11 To further demonstrate the stability of the ion implantation damage, the implanted film implanted with a dose of 331013 ions/cm2 was annealed at 100 °C for 1 h, with no changes in the channeling spectrum. It is also known that ion implantation can reduce the critical current.12 Before making any devices, we have studied what the minimum thickness of metallic film, served as a mask on top of YBCO, is required for effective protection of the superconducting YBCO electrodes. To minimize the mask thickness as required for a narrow milling trench, a high-density material had to be selected. Au and W were chosen as candidates for the film-mask material. Figure 1~b! shows the dependence of T c of YBCO on various Au thickness under 100 keV, 1014 ions/cm2 oxygen ion implantation. Using an Au mask thinner than 200 nm, T c of YBCO films is greatly decreased. Thus, the minimum Au thickness required is 200 nm to prevent the degradation of YBCO films. To fabricate the junction, the YBCO films were patterned into dog-bone shaped mesas by photolithography and broad beam Ar1 ion milling. The width of the mesas between two contacts was 10 mm. W has was first considered to be the film-mask material. Using FIB milling and SF6 reactive ion etching, we achieved the trench with a width of ;20 nm. Due to the reaction of W with YBCO, resulting in degraded T c ’s, we changed over to Au as the film-mask instead. In order to make high aspect ratio trench, we have coated the patterned films with the diluted photoresist ~S1805!. The photoresist was baked for 1 h at 90 °C. The resist thickness was measured to be 200 nm. Then an Au ~200 nm!/Cr ~50 nm! bilayer was evaporated on top of the resist. The trench of the film mask was first patterned using FIB milling. By controlling the doses of Ga1 ions, we can stop the milling when the photoresist is reached. Then the trench above YBCO was obtained by O1 reactive ion etching. Figure 2 shows the schematic and side view of the trench. The scanning electron microscopy ~SEM! micrography was taken along a 52° tilted angle with the cross section prepared by FIB milling. Typically, the width of the trench was ;40 nm and the minimum can be ;20 nm. The depth of the trench is all the way down to the surface of YBCO film. Ac susceptibility measurements indicated that the T c of the patterned YBCO remained unchanged. To make a Josephson junction, the patterned films with a trenched mask were was then implanted with 100 keV oxygen ions. The electrical properties of the as-deposited film and the implanted junction and as grown film were measured using a four-terminal technique. Their typical R – T curves were shown in Fig. 3. The T C0 of the implanted junction at a dose of 331013 ions/cm2 is decreased to be ;80 K, while

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FIG. 1. ~a! Effect of 100 keV P1 ion implantation in YBCO ~100 nm! at ~i! 131013 ions/cm2, ~ii! 331013 ions/cm2, ~iii! 631013 ions/cm2, and ~iv! 131014 ions/cm2. ~b! Effect of 100 keV, 1014/cm2 O1 ion implantation in Au/YBCO ~100 nm! with thickness of 0, 50, 100, 200, 300, 400, and 600 nm for ~i!–~v!.

the normal state resistivity is increased by a factor of 2. It indicates that the implanted region of YBCO is greatly degenerated and most likely is a normal conductor or a lowerT C ’s material. As shown in Fig. 1, with such dose of oxygen implantation the YBCO will become a 60 K superconductor ~ S 8 ! @Fig. 1~a!#. Therefore, we expected to see a Josephson junction effect of S-S 8 -S kind in these devices. Figures 4~a! and 4~b! show two I – V characteristics at 65 K with and without microwave irradiation at 60 K, respectively. The junction showed a typical flux-flow like I – V characteristic @Fig. 4~a!#. Under the microwave irradiation with frequency ~f! of ;14.5 GHz, a number of current steps in the I – V characteristics can be seen at voltage interval of V n 5n f h/2e, as well known the Shapiro steps @Fig. 4~b!#. We have not seen a good magnetic diffraction pattern in such junctions since the supercurrent distribution could be nonuniform across the junctions. Figure 5 shows a typical temperature dependence of I c and R n . Their temperature dependence resembles that of typical S-N-S junctions. The I c R n product is about 50 mV at

JVST B - Microelectronics and Nanometer Structures

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FIG. 2. ~a! Schematic of the ion implantation mask. Mask was fabricated by FIB milling and O2 reactive ion etching. ~b! SEM image shows the side view of the mask.

77 K and about 4.2 mV at 4.2 K. It should be noted that the junction resistance is monotonically decreased from T C0 to about 60 K and keeps more or less constant down to 4.2 K. In other words, the areal junction resistance is decreased from about 231028 V cm2 at temperature close to T C0 to 531029 V cm2 at about 60 K. This decreasing junction resistance is characteristic of the N layer, 60 K T C0 YBCO conducting barrier, since the 60 K is exactly the same T C0 of

FIG. 3. Resistance vs temperature curves for the junctions implanted with 100 keV oxygen ions at various doses, ~a! 1013 ions/cm2 , ~b! 331013 ions/cm2 , ~c! 631013 ions/cm2 .

FIG. 4. ~a! I – V for the junction with 100 keV, 331013 ions/cm2 oxygen ion implantation at 60 K, ~b! under microwave modulation ( f ;14.5 GHz!.

the YBCO film under the ion implantation of same dose @Fig. 1~b!#. This again implies that the region of implanted junction is indeed a lower T C0 YBCO conductor. At temperature below 60 K, the junction is changed from S-N-S to S-S 8 -S type and shows a temperature independent normal resistance, as could be understood by a direct tunneling for the conduc-

FIG. 5. I c and R n for the junction with 100 keV, 331013 ions/cm2 O1 ion implantation.

J. Vac. Sci. Technol. B, Vol. 16, No. 5, Sep/Oct 1998

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tion mechanism. Compared with the S-S 8 -S junction in which one usually uses Pr-doped YBCO as conducting barrier, FIB milling is an easier approach to making high-T c junctions. In summary, we demonstrated junction fabrication using oxygen ion implantation through the Cr/Au/photoresist mask. Combined FIB milling and reactive ion etching, the trench of the W mask has been fabricated down to 20 nm in the W mask and ;40 nm in the Au/photoresist mask. The junctions exhibit I c R n products of 50 mV at 77 K and 4.2 mV at 4.2 K. The Junctions also exhibit flux-flow like I – V characteristics and Shapiro steps under microwave irradiation. With optimization of the irradiation dose, this process may provide a novel approach for the fabrication of high-T c junctions with reasonable reproducibility. The authors gratefully acknowledge Z. Tranjanovic for helpful discussions. This work was supported by the NRL Contract No. N0001496C2008, Center for Superconductivity Research, and Material Research Science and Engineering Center.

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JVST B - Microelectronics and Nanometer Structures

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