- Email: [email protected]
Patterning of YBCO thin films by ion implantation and magneto-optical investigations
Physica C 294 Ž1998. 1–6
Patterning of YBCO thin films by ion implantation and magneto-optical investigations M. Kuhn
a,)
a b
, B. Schey a , R. Klarmann a , W. Biegel a , B. Stritzker a , J. Eisenmenger b, P. Leiderer b UniÕersitat ¨ Augsburg, Institut fur ¨ Physik, D-86135 Augsburg, Germany UniÕersitat ¨ Konstanz, Fakultat ¨ fur ¨ Physik, D-78434 Konstanz, Germany
Received 13 August 1997; revised 2 October 1997; accepted 6 October 1997
Abstract YBa 2 Cu 3 O 7y d thin films deposited by pulsed laser deposition were patterned by ion implantation. The patterning technique is based on the local disordering of the crystal structure due to elastic collisions of the implanted Oq-ions and thus on a strong breakdown of superconductivity in the irradiated areas. With this patterning method circular and meandering structures with a size of 1 mm were prepared for investigations regarding high current applications. Oq-ions with an energy of 180 keV were used to avoid a contamination of the film. The achieved structures were investigated concerning a dependence on the dose by means of RBS, XRD, AFM and magneto-optical characterization. Using a fluence of 5 = 10 14 Oqrcm2 a fully planar and stable structure can be obtained. The patterning depth can be adjusted by varying the implantation energy. So this technique was also used successfully on YBCO thin films covered already with a 200 nm Au top layer. q 1998 Elsevier Science B.V. PACS: 74.60.Mj; 74.75.q t; 85.70.Sq Keywords: Applications of high-Tc superconductors; Thin films; Irradiation effects; Ion implantation; Magneto-optical
1. Introduction There are different methods of patterning high-Tc superconductors for device fabrication where the most common ones are based on chemical etching techniques. A more unconventional technique is the patterning with ion beam implantation. This method is based on a local deterioration of the superconducting properties depending on the copper-oxide planes )
Corresponding author. Tel.: q49 821 598 3403; fax: q49 821 598 3405; e-mail: [email protected]
and are very sensitive to the oxygen content in the copper-oxide chains. Thus Ži. a reduction of the oxygen content, Žii. a disordering of the copper-oxide planes or Žiii. a total damage of the YBCO lattice leads to a vanishing of superconductivity. By means of the ion implantation process such a disturbance of the superconducting properties is achieved because of the elastic nuclear scattering of the projectile at the target atoms of the HTS film. Therefore this process itself causes just a physical damage. Furthermore, the incorporated ions can also induce a chemical damage due to a reaction with the target atoms
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 7 7 9 - 6
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M. Kuhn et al.r Physica C 294 (1998) 1–6
w1x. In our case we use oxygen ions for implantation to avoid such a chemical contamination of the YBCO thin films. Depending on the grade of destruction ŽŽi. – Žiii.. a subsequent annealing step in an oxygen atmosphere can reorder the disordered regions and reload the oxygen content. Concerning the long time stability of the pattern a reload with oxygen in the irradiated areas and thus a recovery of the superconductivity should be avoided. The aims of the described experiments are the realization of a stable pattern of the YBCO thin film without producing steps on the surface allowing a further epitaxial growth on top as well as the structuring through an already deposited top layer. In the case of high current applications of large area HTS films a structure size in the range of mm to cm is demanded w2x.
Fig. 1. TRIM calculation of the ion profile after a 360 keV Oq implantation into the layer system 200 nm Aur300 nm YBCO. The oxygen ions penetrate the whole depth of the YBCO layer.
2. Experimental The epitaxial YBCO thin films used for patterning were prepared by pulsed laser deposition ŽPLD. w3x. The film thickness is typically about 250 nm and the crystalline structure is fully c-axis-oriented on SrTiO 3 substrates with a FWHM of the rocking curves of D v Ž005. F 0.408. The critical temperature Tc of the different films ranges from 88.0 to 91.9 K and the critical current density jc is about Ž1.0–3.0. = 10 6 Arcm2 at 77 K and zero field. Some of the YBCO films were covered in situ by PLD with a 200 nm thick gold layer resulting in a AurYBCO bilayer. The deposition of the gold layer does not affect the superconducting properties of the YBCO film. This follows from inductive measurements of the critical current density jc and critical temperature Tc . These two different kinds of samples were patterned by ion implantation at room temperature with a Ta mask. Concerning high current applications the dimensions of the pattern were chosen to be ; 1 mm. In the case of the single layer sample oxygen ions with an energy of 180 keV were used for patterning. The fluences range from 1 = 10 13 to 1 = 10 16 Oqrcm2 . At this energy the projected ion range, longitudinal straggling and lateral straggling were calculated by TRIM w4x to be 1981, 663 and ˚ TRIM is a computer program simulating the 330 A. ion implantation process by means of the Monte-
Carlo simulation. The AurYBCO multilayers were irradiated with 360 keV Oq-ions and a constant fluence of 5 = 10 15 Oqrcm2 . In this case the TRIM ˚ calculations ŽFig. 1. result in 2825, 1226 and 542 A, respectively. The energy is sufficient for the ions to penetrate the whole depth of the YBCO layer. Because of ion implantation being a strong directional process the lateral spreading for this energy range is only 30–60 nm. This induces sharp structure edges just affected by the mask. In other cases, this technique has been used for mm- w1,5x and sub-mm-patterning. The obtained, structured YBCO films were studied by Rutherford backscattering spectroscopy ŽRBS., atomic force microscopy ŽAFM., optical reflection microscopy, X-ray diffractometry ŽXRD. as well as magneto-optically ŽMO.. The magneto-optical measurements were carried out with an iron garnet film as magneto-optical indicator in a homogeneous magnetic field at a temperature of 4.2 K. With this technique the flux penetration into the superconductor can be visualized by the magneto-optical indicator illuminated with polarized light perpendicular to the film surface. The flux pattern results in a greyscale picture recorded by a CCD camera on a video tape. Moreover, the superconducting properties of the YBCO films were also investigated by inductive jc and Tc measurements before and after the irradiation.
M. Kuhn et al.r Physica C 294 (1998) 1–6
3. Results and discussion To study the influence of the implantation on the crystalline structure the irradiated sample areas were investigated by RBS Žchanneling.. The YBCO layers were analysed with a 1.5 MeV Heq-beam in random and aligned crystal orientation before and after irradiation with oxygen ions. The RBS spectra of a YBCO sample which was implanted with a dose of 5 = 10 14 Oqrcm2 is shown in Fig. 2. The minimum yield xmin of the untreated YBCO layer, which describes the proportion of backscattered particles in the case of an aligned crystal to a random oriented crystal, amounts to 3.6% near the surface ŽFig. 2Ža... This value indicates the perfect crystallinity of the as deposited YBCO thin film. After the 180 keV Oq implantation with a fluence of 5 = 10 14 Oqrcm2 the value for xmin has risen up to 92% ŽFig. 2Žb... Thus this fluence is sufficient to destroy the crystal structure almost completely. Channeling measurements show that a fluence of 1 = 10 15 Oqrcm2 causes a xmin of 100%. By contrast a fluence of 1 = 10 13 Oqrcm2 leads only to a disordering of the crystal lattice Ž xmin s 60–70%.. To check if the irradiated YBCO is still polycrystalline or already amorphous the samples were studied by XRD. The XRD analyses show a vanishing of the YBCO reflexes already
3
after an irradiation with a fluence of 5 = 10 14 Oqrcm2 . Thus this irradiation leads to a complete amorphization of the YBCO thin films. This seems to be remarkable because the calculated energy loss Žcalculated by TRIM. for this system is only 22 ˚ . in maximum. In contrast to this no eVrŽion A indications of an amorphization could be observed for Si w6x or Al w7x implantation into YBCO despite of a higher calculated energy loss in that case Ž65 ˚ . and 60 eVrŽion A˚ ., respectively.. The eVrŽion A reason for the different results in the two experiments is not clear at the moment. The fact that the pattern is caused by an amorphization of the YBCO crystal guarantees the long time stability of the patterning at room temperature. Fig. 3 shows two atomic force micrographs of the YBCO film surface in the area of the pattern edge. The micrographs are shaded by the software to make the edge more evidently. In Fig. 3Ža. the YBCO surface is shown after an 1 = 10 16 Oqrcm2 implantation. The irradiated area is located in the left part of the picture, the structure edge runs vertically. A patterning with this fluence results in a structure edge with a step height of 20–25 nm. This is caused by a sputtering of the YBCO surface by the incident oxygen ions. Otherwise the surface roughness is less than 10 nm except for some particles on the surface
Fig. 2. RBS spectra of a YBCO thin film in random and aligned orientation Ža. before and Žb. after a 180 keV Oq implantation with a fluence of 5 = 10 14 Oqrcm2 . This fluence is sufficient to destroy the crystal structure almost completely, which is indicated by xmin .
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M. Kuhn et al.r Physica C 294 (1998) 1–6
Fig. 3. Two atomic force micrographs of the YBCO surface in the area of the pattern edge. Ža. The surface after an irradiation with 1 = 10 16 Oqrcm2 in the left part of the micrograph. By this fluence an edge step of about 20 nm was generated. Žb. After an irradiation with 5 = 10 14 Oqrcm2 , here no step could be observed.
with diameters of up to a few microns. Decreasing the fluence to 1 = 10 15 Oqrcm2 a step with a height of only about 15 nm is obtained. A further reduction of the fluence to 5 = 10 14 Oqrcm2 leads to a fully planar structure without any steps ŽFig. 3Žb... Magneto-optical analyses were used to investigate the suitability of the structuring method. In Fig. 4 a
Fig. 4. Magneto-optical image of a meandered YBCO thin film. The pattern Žsee inserted graphic: white lines correspond to irradiated areas. was generated by a fluence of 5=10 15 Oqrcm2 . The image was recorded at a temperature of 4.2 K and a magnetic field of 33 mT. An iron garnet film was used as magneto-optical indicator.
magneto-optical image is presented showing the flux distribution in the YBCO thin film meandered with a fluence of 5 = 10 15 Oqrcm2 . The image was taken at 4.2 K and a magnetic field of 33 mT. Bright regions indicate areas with high magnetic flux density, the regions remaining in the Meissner phase stay dark. Applying a small homogeneous magnetic field Ž5–10 mT. perpendicular to the sample surface after zero field cooling ŽZFC. flux penetrates immediately and completely into the whole irradiated area. This indicates that there is no magnetic screening due to superconducting shielding currents any more. This could be confirmed by means of inductive jc measurement, where no critical current density could be determined in the irradiated area. Thus patterning with this fluence is working well. By increasing the magnetic field the flux starts to enter also the superconducting paths from the pattern edges. This penetration happens in most parts of the paths in a very symmetrical manner, except a region at the upper image edge Žarrow in Fig. 4.. In this area there is a faster penetration of the flux front from the right path edge than from the left one. This is probably caused by a lower critical current density of the YBCO thin film in the described area. The sequence of magneto-optical images in Fig. 5 shows a YBCO thin film patterned circularly with a fluence of 5 = 10 14 Oqrcm2 . Even at this low fluence the flux enters immediately and homogeneously into the whole implanted regions when a magnetic field is applied ŽFig. 5Ža... A sickle-like defect can
M. Kuhn et al.r Physica C 294 (1998) 1–6
be observed, where flux can even enter the superconducting path at a magnetic field of 14 mT Žarrow 1.. Increasing the magnetic field results in ŽFig. 5Žb. is taken at 35 mT. a flux penetration simultaneously from all edges into the conducting paths. Along the small defect the flux plume opens up and the flux front can already enter the center of the superconducting path. A second defect can also be observed in the path Žarrow 2.. This is more evident in Fig. 5Žc., which shows the film at the saturation magnetization of 70 mT. The dark line, a so-called dq-line Ždiscontinuity line of the current w8x., following the conducting path points rather exactly to the center of
5
it. In most parts of the path the flux distribution is highly symmetrically oriented around the dq-line indicating a good homogeneity of the superconductor. Except for the two small line defects and for a region on the right side, where the dq-line has a parabolic course ŽFig. 5Žc.: dotted line at arrow 3., because the flux front from the left pattern edge could enter the superconductor at even lower magnetic fields. This seems again to be caused by a lower critical current density in this region. The circular course of the dq-line with the three defective areas can also clearly be seen in Fig. 5Žd. Žbright line. showing the remanent state of the film with
Fig. 5. Sequence of magneto-optical images of a circular structured YBCO thin film. The fluence of the irradiation was 5 = 10 14 Oqrcm2 . Ža. B s 14 mT, Žb. B s 35 mT, Žc. saturation field B s 70 mT and Žd. remanent state.
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M. Kuhn et al.r Physica C 294 (1998) 1–6
trapped flux. The investigation of the sample in the area of the two line defects by optical microscopy shows that these reductions of the screening currents are caused by two small scratches in the substrate influencing the growth of the YBCO layer. In contrast to a fluence of 5 = 10 14 Oqrcm2 leading to an amorphization of the irradiated areas and thus to a complete breakdown of superconductivity a lower fluence of 1 = 10 13 Oqrcm2 only reduces Tc from 91.3 to 80.0 K and jc by two orders of magnitude. A part of the YBCO thin films was in situ covered with a 200 nm gold layer. The irradiation of these double layer systems was performed with 380 keV Oq-ions with a fluence of 5 = 10 15 Oqrcm2 . At this energy and fluence the enlarged sputter yield of the gold top layer leads to a step height of 50 nm, which is higher than that of the YBCO surface in the first experiments. The RBS analysis of the AurYBCO system shows no intermixing of the two layers at the interface. The performance of the YBCO structuring through the gold top layer could be proved magneto-optically. The same flux penetration behaviour as in the single YBCO film was observed.
4. Conclusion We have presented a method of patterning YBCO thin films with and without an Au top layer by ion implantation of oxygen. For the single YBCO layer a 180 keV oxygen irradiation with a fluence of 5 = 10 14 Oqrcm2 is enough to amorphize the YBCO film forming an effective and stable pattern without any steps on the surface. A lower fluence of 1 = 10 13 Oqrcm2 leads only to a decrease of the superconducting properties, but not to an effective patterning, whereas the irradiation with a higher fluence of
1 = 10 15 Oqrcm2 already causes a 15 nm step at the surface. Using 360 keV oxygen with a fluence of 5 = 10 15 q O rcm2 a patterning of the YBCO film through a 200 nm gold film could also be realized. The quality of the YBCO patterns could be analysed magneto-optically. Moreover, with this method small defects in the remaining superconducting path and their influence on the penetration of magnetic flux could be observed. This indicates that magnetooptical analyses are one of the most suitable techniques for a complete investigation of the quality of HTS thin films as well as for a testing of HTS patterns for device fabrication. Acknowledgements The authors would like to thank M. Wallenhorst and H. Dotsch for providing iron garnet films for ¨ magneto-optical investigations. The work was supported by the Bayerische Forschungsstiftung ŽFORSUPRA.. References w1x D.C. DeGroot, D.A. Rudman, K. Zhang, Q.Y. Ma, H. Kato, N.A.F. Jaeger, Appl. Phys. Lett. 69 Ž14. Ž1996. 2119. w2x B. Gromoll, G. Ries, W. Schmid, H.-P. Kramer, B. Seebacher, ¨ P. Kummeth, S. Fischer, H.-W. Neumuller, presented at EU¨ CAS 1997. ¨ w3x B. Schey, T. Bollmeier, M. Kuhn, W. Biegel, G. Ostreicher, B. Stritzker, Rev. Sci. Instrum., submitted. w4x J.F. Ziegler, J. Biersack, V.L. Littmark, in: The Stopping and Range of Ions in Solids, vol. 1, Pergamon, Oxford, 1985. w5x S.S. Tinchev, Physica C 256 Ž1996. 191. w6x Q.Y. Ma, P. Dosanjh, J.F. Carolan, W.N. Hardy, Appl. Phys. Lett. 63 Ž26. Ž1993. 3633. w7x S.H. Hong, J.R. Miller, Q.Y. Ma, E.S. Yang, Appl. Phys. Lett. 67 Ž18. Ž1995. 2717. w8x Th. Schuster, M.V. Indenbom, M.R. Koblischka, H. Kuhn, H. Kronmuller, Phys. Rev. B 49 Ž1994. 3443. ¨
Patterning of YBCO thin films by ion implantation and magneto-optical investigations M. Kuhn
a,)
a b
, B. Schey a , R. Klarmann a , W. Biegel a , B. Stritzker a , J. Eisenmenger b, P. Leiderer b UniÕersitat ¨ Augsburg, Institut fur ¨ Physik, D-86135 Augsburg, Germany UniÕersitat ¨ Konstanz, Fakultat ¨ fur ¨ Physik, D-78434 Konstanz, Germany
Received 13 August 1997; revised 2 October 1997; accepted 6 October 1997
Abstract YBa 2 Cu 3 O 7y d thin films deposited by pulsed laser deposition were patterned by ion implantation. The patterning technique is based on the local disordering of the crystal structure due to elastic collisions of the implanted Oq-ions and thus on a strong breakdown of superconductivity in the irradiated areas. With this patterning method circular and meandering structures with a size of 1 mm were prepared for investigations regarding high current applications. Oq-ions with an energy of 180 keV were used to avoid a contamination of the film. The achieved structures were investigated concerning a dependence on the dose by means of RBS, XRD, AFM and magneto-optical characterization. Using a fluence of 5 = 10 14 Oqrcm2 a fully planar and stable structure can be obtained. The patterning depth can be adjusted by varying the implantation energy. So this technique was also used successfully on YBCO thin films covered already with a 200 nm Au top layer. q 1998 Elsevier Science B.V. PACS: 74.60.Mj; 74.75.q t; 85.70.Sq Keywords: Applications of high-Tc superconductors; Thin films; Irradiation effects; Ion implantation; Magneto-optical
1. Introduction There are different methods of patterning high-Tc superconductors for device fabrication where the most common ones are based on chemical etching techniques. A more unconventional technique is the patterning with ion beam implantation. This method is based on a local deterioration of the superconducting properties depending on the copper-oxide planes )
Corresponding author. Tel.: q49 821 598 3403; fax: q49 821 598 3405; e-mail: [email protected]
and are very sensitive to the oxygen content in the copper-oxide chains. Thus Ži. a reduction of the oxygen content, Žii. a disordering of the copper-oxide planes or Žiii. a total damage of the YBCO lattice leads to a vanishing of superconductivity. By means of the ion implantation process such a disturbance of the superconducting properties is achieved because of the elastic nuclear scattering of the projectile at the target atoms of the HTS film. Therefore this process itself causes just a physical damage. Furthermore, the incorporated ions can also induce a chemical damage due to a reaction with the target atoms
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 7 7 9 - 6
2
M. Kuhn et al.r Physica C 294 (1998) 1–6
w1x. In our case we use oxygen ions for implantation to avoid such a chemical contamination of the YBCO thin films. Depending on the grade of destruction ŽŽi. – Žiii.. a subsequent annealing step in an oxygen atmosphere can reorder the disordered regions and reload the oxygen content. Concerning the long time stability of the pattern a reload with oxygen in the irradiated areas and thus a recovery of the superconductivity should be avoided. The aims of the described experiments are the realization of a stable pattern of the YBCO thin film without producing steps on the surface allowing a further epitaxial growth on top as well as the structuring through an already deposited top layer. In the case of high current applications of large area HTS films a structure size in the range of mm to cm is demanded w2x.
Fig. 1. TRIM calculation of the ion profile after a 360 keV Oq implantation into the layer system 200 nm Aur300 nm YBCO. The oxygen ions penetrate the whole depth of the YBCO layer.
2. Experimental The epitaxial YBCO thin films used for patterning were prepared by pulsed laser deposition ŽPLD. w3x. The film thickness is typically about 250 nm and the crystalline structure is fully c-axis-oriented on SrTiO 3 substrates with a FWHM of the rocking curves of D v Ž005. F 0.408. The critical temperature Tc of the different films ranges from 88.0 to 91.9 K and the critical current density jc is about Ž1.0–3.0. = 10 6 Arcm2 at 77 K and zero field. Some of the YBCO films were covered in situ by PLD with a 200 nm thick gold layer resulting in a AurYBCO bilayer. The deposition of the gold layer does not affect the superconducting properties of the YBCO film. This follows from inductive measurements of the critical current density jc and critical temperature Tc . These two different kinds of samples were patterned by ion implantation at room temperature with a Ta mask. Concerning high current applications the dimensions of the pattern were chosen to be ; 1 mm. In the case of the single layer sample oxygen ions with an energy of 180 keV were used for patterning. The fluences range from 1 = 10 13 to 1 = 10 16 Oqrcm2 . At this energy the projected ion range, longitudinal straggling and lateral straggling were calculated by TRIM w4x to be 1981, 663 and ˚ TRIM is a computer program simulating the 330 A. ion implantation process by means of the Monte-
Carlo simulation. The AurYBCO multilayers were irradiated with 360 keV Oq-ions and a constant fluence of 5 = 10 15 Oqrcm2 . In this case the TRIM ˚ calculations ŽFig. 1. result in 2825, 1226 and 542 A, respectively. The energy is sufficient for the ions to penetrate the whole depth of the YBCO layer. Because of ion implantation being a strong directional process the lateral spreading for this energy range is only 30–60 nm. This induces sharp structure edges just affected by the mask. In other cases, this technique has been used for mm- w1,5x and sub-mm-patterning. The obtained, structured YBCO films were studied by Rutherford backscattering spectroscopy ŽRBS., atomic force microscopy ŽAFM., optical reflection microscopy, X-ray diffractometry ŽXRD. as well as magneto-optically ŽMO.. The magneto-optical measurements were carried out with an iron garnet film as magneto-optical indicator in a homogeneous magnetic field at a temperature of 4.2 K. With this technique the flux penetration into the superconductor can be visualized by the magneto-optical indicator illuminated with polarized light perpendicular to the film surface. The flux pattern results in a greyscale picture recorded by a CCD camera on a video tape. Moreover, the superconducting properties of the YBCO films were also investigated by inductive jc and Tc measurements before and after the irradiation.
M. Kuhn et al.r Physica C 294 (1998) 1–6
3. Results and discussion To study the influence of the implantation on the crystalline structure the irradiated sample areas were investigated by RBS Žchanneling.. The YBCO layers were analysed with a 1.5 MeV Heq-beam in random and aligned crystal orientation before and after irradiation with oxygen ions. The RBS spectra of a YBCO sample which was implanted with a dose of 5 = 10 14 Oqrcm2 is shown in Fig. 2. The minimum yield xmin of the untreated YBCO layer, which describes the proportion of backscattered particles in the case of an aligned crystal to a random oriented crystal, amounts to 3.6% near the surface ŽFig. 2Ža... This value indicates the perfect crystallinity of the as deposited YBCO thin film. After the 180 keV Oq implantation with a fluence of 5 = 10 14 Oqrcm2 the value for xmin has risen up to 92% ŽFig. 2Žb... Thus this fluence is sufficient to destroy the crystal structure almost completely. Channeling measurements show that a fluence of 1 = 10 15 Oqrcm2 causes a xmin of 100%. By contrast a fluence of 1 = 10 13 Oqrcm2 leads only to a disordering of the crystal lattice Ž xmin s 60–70%.. To check if the irradiated YBCO is still polycrystalline or already amorphous the samples were studied by XRD. The XRD analyses show a vanishing of the YBCO reflexes already
3
after an irradiation with a fluence of 5 = 10 14 Oqrcm2 . Thus this irradiation leads to a complete amorphization of the YBCO thin films. This seems to be remarkable because the calculated energy loss Žcalculated by TRIM. for this system is only 22 ˚ . in maximum. In contrast to this no eVrŽion A indications of an amorphization could be observed for Si w6x or Al w7x implantation into YBCO despite of a higher calculated energy loss in that case Ž65 ˚ . and 60 eVrŽion A˚ ., respectively.. The eVrŽion A reason for the different results in the two experiments is not clear at the moment. The fact that the pattern is caused by an amorphization of the YBCO crystal guarantees the long time stability of the patterning at room temperature. Fig. 3 shows two atomic force micrographs of the YBCO film surface in the area of the pattern edge. The micrographs are shaded by the software to make the edge more evidently. In Fig. 3Ža. the YBCO surface is shown after an 1 = 10 16 Oqrcm2 implantation. The irradiated area is located in the left part of the picture, the structure edge runs vertically. A patterning with this fluence results in a structure edge with a step height of 20–25 nm. This is caused by a sputtering of the YBCO surface by the incident oxygen ions. Otherwise the surface roughness is less than 10 nm except for some particles on the surface
Fig. 2. RBS spectra of a YBCO thin film in random and aligned orientation Ža. before and Žb. after a 180 keV Oq implantation with a fluence of 5 = 10 14 Oqrcm2 . This fluence is sufficient to destroy the crystal structure almost completely, which is indicated by xmin .
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M. Kuhn et al.r Physica C 294 (1998) 1–6
Fig. 3. Two atomic force micrographs of the YBCO surface in the area of the pattern edge. Ža. The surface after an irradiation with 1 = 10 16 Oqrcm2 in the left part of the micrograph. By this fluence an edge step of about 20 nm was generated. Žb. After an irradiation with 5 = 10 14 Oqrcm2 , here no step could be observed.
with diameters of up to a few microns. Decreasing the fluence to 1 = 10 15 Oqrcm2 a step with a height of only about 15 nm is obtained. A further reduction of the fluence to 5 = 10 14 Oqrcm2 leads to a fully planar structure without any steps ŽFig. 3Žb... Magneto-optical analyses were used to investigate the suitability of the structuring method. In Fig. 4 a
Fig. 4. Magneto-optical image of a meandered YBCO thin film. The pattern Žsee inserted graphic: white lines correspond to irradiated areas. was generated by a fluence of 5=10 15 Oqrcm2 . The image was recorded at a temperature of 4.2 K and a magnetic field of 33 mT. An iron garnet film was used as magneto-optical indicator.
magneto-optical image is presented showing the flux distribution in the YBCO thin film meandered with a fluence of 5 = 10 15 Oqrcm2 . The image was taken at 4.2 K and a magnetic field of 33 mT. Bright regions indicate areas with high magnetic flux density, the regions remaining in the Meissner phase stay dark. Applying a small homogeneous magnetic field Ž5–10 mT. perpendicular to the sample surface after zero field cooling ŽZFC. flux penetrates immediately and completely into the whole irradiated area. This indicates that there is no magnetic screening due to superconducting shielding currents any more. This could be confirmed by means of inductive jc measurement, where no critical current density could be determined in the irradiated area. Thus patterning with this fluence is working well. By increasing the magnetic field the flux starts to enter also the superconducting paths from the pattern edges. This penetration happens in most parts of the paths in a very symmetrical manner, except a region at the upper image edge Žarrow in Fig. 4.. In this area there is a faster penetration of the flux front from the right path edge than from the left one. This is probably caused by a lower critical current density of the YBCO thin film in the described area. The sequence of magneto-optical images in Fig. 5 shows a YBCO thin film patterned circularly with a fluence of 5 = 10 14 Oqrcm2 . Even at this low fluence the flux enters immediately and homogeneously into the whole implanted regions when a magnetic field is applied ŽFig. 5Ža... A sickle-like defect can
M. Kuhn et al.r Physica C 294 (1998) 1–6
be observed, where flux can even enter the superconducting path at a magnetic field of 14 mT Žarrow 1.. Increasing the magnetic field results in ŽFig. 5Žb. is taken at 35 mT. a flux penetration simultaneously from all edges into the conducting paths. Along the small defect the flux plume opens up and the flux front can already enter the center of the superconducting path. A second defect can also be observed in the path Žarrow 2.. This is more evident in Fig. 5Žc., which shows the film at the saturation magnetization of 70 mT. The dark line, a so-called dq-line Ždiscontinuity line of the current w8x., following the conducting path points rather exactly to the center of
5
it. In most parts of the path the flux distribution is highly symmetrically oriented around the dq-line indicating a good homogeneity of the superconductor. Except for the two small line defects and for a region on the right side, where the dq-line has a parabolic course ŽFig. 5Žc.: dotted line at arrow 3., because the flux front from the left pattern edge could enter the superconductor at even lower magnetic fields. This seems again to be caused by a lower critical current density in this region. The circular course of the dq-line with the three defective areas can also clearly be seen in Fig. 5Žd. Žbright line. showing the remanent state of the film with
Fig. 5. Sequence of magneto-optical images of a circular structured YBCO thin film. The fluence of the irradiation was 5 = 10 14 Oqrcm2 . Ža. B s 14 mT, Žb. B s 35 mT, Žc. saturation field B s 70 mT and Žd. remanent state.
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M. Kuhn et al.r Physica C 294 (1998) 1–6
trapped flux. The investigation of the sample in the area of the two line defects by optical microscopy shows that these reductions of the screening currents are caused by two small scratches in the substrate influencing the growth of the YBCO layer. In contrast to a fluence of 5 = 10 14 Oqrcm2 leading to an amorphization of the irradiated areas and thus to a complete breakdown of superconductivity a lower fluence of 1 = 10 13 Oqrcm2 only reduces Tc from 91.3 to 80.0 K and jc by two orders of magnitude. A part of the YBCO thin films was in situ covered with a 200 nm gold layer. The irradiation of these double layer systems was performed with 380 keV Oq-ions with a fluence of 5 = 10 15 Oqrcm2 . At this energy and fluence the enlarged sputter yield of the gold top layer leads to a step height of 50 nm, which is higher than that of the YBCO surface in the first experiments. The RBS analysis of the AurYBCO system shows no intermixing of the two layers at the interface. The performance of the YBCO structuring through the gold top layer could be proved magneto-optically. The same flux penetration behaviour as in the single YBCO film was observed.
4. Conclusion We have presented a method of patterning YBCO thin films with and without an Au top layer by ion implantation of oxygen. For the single YBCO layer a 180 keV oxygen irradiation with a fluence of 5 = 10 14 Oqrcm2 is enough to amorphize the YBCO film forming an effective and stable pattern without any steps on the surface. A lower fluence of 1 = 10 13 Oqrcm2 leads only to a decrease of the superconducting properties, but not to an effective patterning, whereas the irradiation with a higher fluence of
1 = 10 15 Oqrcm2 already causes a 15 nm step at the surface. Using 360 keV oxygen with a fluence of 5 = 10 15 q O rcm2 a patterning of the YBCO film through a 200 nm gold film could also be realized. The quality of the YBCO patterns could be analysed magneto-optically. Moreover, with this method small defects in the remaining superconducting path and their influence on the penetration of magnetic flux could be observed. This indicates that magnetooptical analyses are one of the most suitable techniques for a complete investigation of the quality of HTS thin films as well as for a testing of HTS patterns for device fabrication. Acknowledgements The authors would like to thank M. Wallenhorst and H. Dotsch for providing iron garnet films for ¨ magneto-optical investigations. The work was supported by the Bayerische Forschungsstiftung ŽFORSUPRA.. References w1x D.C. DeGroot, D.A. Rudman, K. Zhang, Q.Y. Ma, H. Kato, N.A.F. Jaeger, Appl. Phys. Lett. 69 Ž14. Ž1996. 2119. w2x B. Gromoll, G. Ries, W. Schmid, H.-P. Kramer, B. Seebacher, ¨ P. Kummeth, S. Fischer, H.-W. Neumuller, presented at EU¨ CAS 1997. ¨ w3x B. Schey, T. Bollmeier, M. Kuhn, W. Biegel, G. Ostreicher, B. Stritzker, Rev. Sci. Instrum., submitted. w4x J.F. Ziegler, J. Biersack, V.L. Littmark, in: The Stopping and Range of Ions in Solids, vol. 1, Pergamon, Oxford, 1985. w5x S.S. Tinchev, Physica C 256 Ž1996. 191. w6x Q.Y. Ma, P. Dosanjh, J.F. Carolan, W.N. Hardy, Appl. Phys. Lett. 63 Ž26. Ž1993. 3633. w7x S.H. Hong, J.R. Miller, Q.Y. Ma, E.S. Yang, Appl. Phys. Lett. 67 Ž18. Ž1995. 2717. w8x Th. Schuster, M.V. Indenbom, M.R. Koblischka, H. Kuhn, H. Kronmuller, Phys. Rev. B 49 Ž1994. 3443. ¨