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Magnetic shielding performance of superconducting YBCO thin film in a multilayer device structure
Physica C 507 (2014) 90–94
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Magnetic shielding performance of superconducting YBCO thin film in a multilayer device structure Y. Uzun ⇑, I. Avci Ege University, Faculty of Science, Department of Physics, 35100 Bornova, Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 14 April 2014 Received in revised form 14 August 2014 Accepted 19 October 2014 Available online 28 October 2014 Keywords: YBCO thin films Multilayer devices Magnetic shielding
a b s t r a c t Magnetic shielding performance of superconducting YBaCu2O7x (YBCO) thin film on an YBCO microbridge was analyzed in a multilayer structure. A sandwich type multilayer structure was fabricated onto a single crystal (1 0 0) SrTiO3 (STO) substrate in the form of YBCO/STO/YBCO by depositing a thin STO interlayer in between two YBCO layers. The top YBCO was patterned as 20 lm width meander-type microbridges and the bottom layer YBCO was used as magnetic shield. YBCO and STO thin films were deposited by dc and rf magnetron sputtering respectively, and the patterning was performed by using standard photolithography and wet etching. In order to enhance long-term stability of the final device, an additional STO thin film was deposited onto the device as an encapsulation layer. Electrical and magnetic characterizations of the YBCO thin film layers were carried out by means of ac magnetic susceptibility (v–T) and resistance vs. temperature (R–T) measurements. The current–voltage (I–V) measurements were performed on the microbridges at 77 K by observing the shielding performance of the bottom YBCO layer under various applied magnetic fields. The results were compared with that of a same-type single layer YBCO device without a shielding layer. The zero field critical current value of the single layer 20 lm wide YBCO device was measured as 30 mA and decreased down to 20 mA as the field increased up to 100 mT. The same measurements on the multilayer device showed that the critical current values remained almost constant around 27 mA as the applied field increased. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Magnetic field dependence of the critical currents of Josephson junctions and superconducting strip lines are the main characteristic properties of the superconducting devices such as superconducting quantum interference devices (SQUIDs) [1], single flux quantum circuits (SFQ) [2] and superconducting filters [3]. Since the critical current values of such devices are very sensitive to the background magnetic fields, shielding procedure has to be applied in almost all applications in order to enhance the signal quality and reduce the field noise contribution [4–6]. The most common sources are the Earth’s magnetic field, power lines and electronic equipments that could likely generate noises and must be shielded during the superconducting device operation [7]. In most cases, these external fields are shielded by using l-metal cages during the operation, becoming one of the main parts of the system [8,9]. However, these kinds of shielding ‘rooms’ most of time become impractical in many applications, particularly in the mobile systems such as non-destructive evaluation (NDE) ⇑ Corresponding author. Tel.: +90 505 395 9661. E-mail address: [email protected] (Y. Uzun). http://dx.doi.org/10.1016/j.physc.2014.10.009 0921-4534/Ó 2014 Elsevier B.V. All rights reserved.
[10,11]. Moreover, the use of l-metal shield mostly limits the testing capability of the system by reducing the room for the sample under test [12,13]. The use of self-shielding mechanisms in the devices can overcome such limitations. For example, in the niobium based low temperature superconducting devices, the use of compensation coils integrated to the devices significantly improves the shielding performance [14]. Since the multilayer fabrication process has been successfully implemented to the niobium based devices [15,16], these types of self-shielding mechanisms could easily be performed [14]. However, there has been a lack of YBCO based high temperature superconducting devices because the multilayer processing has not been able to be implemented to this material yet [17,18]. One of the solutions has been the use of gradiometric configuration for the YBCO SQUIDs [19]. On the other hand, many attempts have been performed to integrate compensation coils to the YBCO devices by using on-chip configurations [20,21]. However, most of them require complex fabrication process and do not provide long term stability. Another solution could be the use of superconducting YBCO thin film itself as a magnetic shield by making use of the diamagnetic behavior of the superconducting YBCO thin film [22]. In this case, a separate YBCO thin film is generally integrated to the device in flip-chip configuration [23].
Y. Uzun, I. Avci / Physica C 507 (2014) 90–94
Although, flip-chip configuration highly eliminates external magnetic field sources, integration of superconducting thin film and superconducting device onto a single substrate as multilayer structure is more practical in the device application than using two individual superconducting samples. Therefore, the multilayer device processing should be evaluated as a considerable development for self-shielding mechanism of superconducting YBCO device fabrication. With this motivation, a multilayer fabrication process was developed in this study for superconducting YBCO devices in order to integrate a shielding layer to the device in the form of YBCOshield/STO/YBCO-microbridge. The electronic properties of individual layers were characterized depending on the fabrication process parameters and the shielding performance of the bottom YBCO layer was tested on the I–V characteristics of the microbridge under various applied magnetic fields.
2. Experimental procedure Before processing the multilayer structure on a single substrate, deposition parameters of YBCO and STO thin films were studied individually in order to obtain optimum thin film process parameters. The process steps during the thin film growth are summarized as follows: (1 0 0) single crystal SrTiO3 (STO) substrates were first ultrasonically cleaned in acetone and subsequently in propanol for 10 min, and rinsed with ultra-pure de-ionized water. The substrates were then silver-glued onto the heater/substrate holder of the sputtering system and the chamber was vacuumed down to 1 107 mbar. After the ultimate vacuum was reached, the substrates were heated up to the deposition temperatures listed in Table 1. YBCO thin films were deposited at 750 °C under the total gas pressure of 0.4 mbar with Ar:O2 ratio of 4:1. The deposition rate for the YBCO thin film was adjusted to be 1.6 nm/min by applying the dc power of 70 W to the 100 diameter cylindrical hallow type YBCO target. Deposition time was chosen as 2 h in order to obtain 200 nm thick YBCO thin films. STO thin films were deposited at 700 °C under the total gas pressure of 0.04 mbar with Ar:O2 ratio of 4:1. The deposition rate for the STO thin film was adjusted to be 1.25 nm/min by applying the rf power of 75 W to the 200 diameter planar STO target. Deposition time was chosen as 2 h in order to obtain 150 nm thick STO thin films. An additional oxygenation process was used for both films after the deposition. YBCO thin films were kept at 750 °C for additional 10 min under 1 mbar flowing O2 atmosphere, then the O2 pressure increased up to 700 mbar and the substrates were allowed to cool down to the room temperature by turning off the heater power supply. Same oxygenation profile was applied to the STO thin films as well. We prepared two types of YBCO thin film samples: (1) the YBCO thin films were deposited onto bare-STO substrates and tested in order to check the superconducting properties depending on the Table 1 Thin film deposition parameters for YBCO and STO. Parameters
Base vacuum Substrate temperature Deposition power PAr:PO2 Total deposition pressure Deposition time Thickness Oxygenation at deposition temperature Oxygenation during the cooling
Materials YBCO
STO
1 107 mbar 750 °C 70 W DC 4:1 0.4 mbar 2h 200 nm 10 min with 1 mbar flowing O2 at 750 °C 700 mbar O2
1 107 mbar 700 °C 75 W RF 4:1 0.04 mbar 2h 150 nm 10 min with 1 mbar flowing O2 at 700 °C 700 mbar O2
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Fig. 1. Schematic of the layers of YBCO/STO/YBCO multilayer structure and microbridge.
process parameters. (2) Another YBCO thin film samples were prepared by using STO thin film deposited substrates in order to make sure that the rf deposited STO thin films can provide good buffer for YBCO thin film growth. Both types of YBCO thin films were tested and the properties were compared. YBCO thin films were analyzed by performing dc resistance vs. temperature (R–T) and ac magnetic susceptibility vs. temperature (v–T) measurements. By preparing multiple samples, reproducible YBCO thin films with and without STO buffer were obtained. The multilayer YBCO/STO/YBCO structure was then fabricated by sequentially depositing the YBCO and STO layers with the same parameters given in Table 1 without taking the samples out of the deposition chamber in order to eliminate any possible contamination at the film interfaces. The heater of our multi-target sputtering system is 360° tiltable; therefore, we easily adjusted the substrate surface to the desired target during the full deposition. After depositing the 3-layer thin film structure, the top YBCO layer was patterned as microbridges as shown in Fig. 1. The pattern was designed as meander-type with the line width of 20 lm. We applied standard photolithography and wet etching for the patterning process. In order to compare the properties of the multilayer device with a standard single layer YBCO device, another microbridge was also fabricated onto a bare STO substrate by using exactly the same process parameters used for multilayer. After the patterning process, v–T measurement was performed on the full structure in order to test the diamagnetic behavior of the bottom YBCO thin film. The measurement was carried out in a vacummable liquid nitrogen cryostat equipped with a two-coil system by using mutual inductance method. The excitation coil was fed with 1 kHz ac signal and the response was read out from the secondary coil by a lock-in amplifier (Stanford Research System SR530) while reducing the temperature with the liquid nitrogen (LN). The temperature was read from Pt100 sensor by LakeShore LS331 temperature controller. In-phase (v0 ) and out-of-phase (v00 ) signals along with the temperature data were picked up by a computer. The superconducting transition of the top layer YBCO microbridge was then tested by R–T measurement. DC 4-point measurement technique was used by bonding the contact pads of the devices onto a gold plated printed circuit board (PCB) that was glued onto a Cu chip carrier. The sample was placed into the vacummable LN cryostat and the measurement was carried out by applying the current with Keithley-224 current source and measuring the voltage with Keithley-199 System DMM. The temperature was controlled by Lakeshore 331 temperature controller and the data was collected via computerized data acquisition system. Magnetic field dependence of the critical currents of the devices was measured over the I–V curves. For this, neodymium permanent magnets as dc magnetic field sources were attached to the LN cryostat and I–V measurements were carried out by adjusting the field values as 0 mT, 30 mT, 60 mT, and 100 mT. The I–V mea-
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surements were performed for both single layer and multilayer YBCO microbridges, and the shielding effect of the bottom YBCO layer was clearly observed in the multilayer structure. The whole
structure was then covered with a room temperature deposited STO layer to protect the device and thin films from the environmental effects arise from the thermal cycles.
3. Results and discussion
Fig. 2. Ac magnetic susceptibility vs. temperature measurements of YBCO thin films. Data with squares (h) show the single layer YBCO thin film, and circles (s) show the bottom layer YBCO thin film in the multilayer structure.
Fig. 3. Dc resistance vs. temperature measurements of single and multilayer YBCO microbridges.
Ac magnetic susceptibility vs. temperature (v–T) measurements of the YBCO thin films were performed for both type of samples. In Fig. 2, diamagnetic responses of single layer YBCO thin film and bottom layer YBCO of multilayer structure are shown together for comparison. These measurements were done mainly to test the superconducting transition of the YBCO thin films without disturbing the film surfaces by contact wiring. The magnetic susceptibility response of the top layer does not have a significant contribution to the measurement because the top layer was already patterned as micrometer wide strip lines. The comparison in Fig. 2 was aimed to show any possible degradation or enhancement on the superconducting transition profile of the film after the multilayer processing. As shown in Fig. 2, the standard single layer YBCO thin film on STO substrate has a quite sharp diamagnetic transition at around 90 K (square data points) which is the most common transition profile of YBCO thin films. However, the transition temperature of the bottom layer YBCO thin film in the multilayer structure seemed to be enhanced with the Tc of 92 K. This enhancement could be linked to the additional heat treatments during the upper layers depositions. The oxygen deficiency of the first layer YBCO thin film could be increased up to an acceptable amount which is generally concluded to be the reason for the Tc enhancement [24,25]. Besides the enhancement on the Tc of bottom layer YBCO in the multilayer structure, which is out of the scope of this work, we confirmed that the multilayer processing of the YBCO thin films with STO interlayers and lithographic process on the top layer did not degrade the superconducting properties of the first layer YBCO thin film. Therefore, this layer could be used as a magnetic shield for the top layer device. After confirming the superconductivity in the bottom layer YBCO, we performed a dc resistance vs. temperature (R–T) measurement on the top layer patterned-YBCO microbridge. As shown in Fig. 3, the multilayer processed YBCO thin film as the top layer device have a similar superconducting transition with that of the single layer YBCO. The Tc of the upper layer device was observed as 92 K as in the single layer YBCO device. This result confirmed the superconductivity on the upper layer YBCO that can be considered as superconducting multilayer device integrated to a superconducting YBCO thin film.
Fig. 4. Current vs. voltage characteristics of (a) single layer and (b) multilayer YBCO microbridges under different applied magnetic fields.
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field environment. This could be considered as an alternative method to the flip-chip or l-metal type shielding of the YBCO devices. The magnetic field could also be applied parallel to the current flow direction by making some modifications on the sample stage of the cryostat, and we believe, the shielding layer of our configuration could also act as a magnetic shield in such a field direction. In this study, we aimed to check the success of the multilayer type fabrication of the shielding layer in high-Tc devices. Further study has been planned to test the effect and shielding of various magnetic field sources in any direction.
Acknowledgment This work was supported by the TUBITAK under the project number of 112T075.
Fig. 5. Magnetic field dependence of the critical currents of shielded and unshielded superconducting YBCO microbridges.
In order to test the critical current profile, and furthermore, the magnetic field dependence of the critical current of upper layer device integrated to a bottom layer superconducting YBCO thin film, a series of I–V measurements were performed under various applied magnetic fields. In Fig. 4, I–V measurements of single layer and multilayer YBCO microbridges under different applied magnetic fields are shown. As in the case of single layer YBCO device, the critical current values gathered from the I–V curves decreased with increasing applied field. The zero field critical current value of the single layer YBCO device was measured as almost 30 mA for the 20 lm meander type superconducting line and this value decreased down to 20 mA as the field increased up to 100 mT (Figs. 4a and 5). The same measurements on the multilayer device showed that the critical current values remained almost constant around 27 mA as the applied field increased (Figs. 4b and 5). The sample was glued onto the copper cold finger of the vacuum LN cryostat and the permanent magnet was attached to the system as to be perpendicular to the sample surface. Because of the limitations in wiring directions and sample attachment onto the cold finger of the cryostat, the field was applied only in perpendicular alignment to the sample. The superconducting layer of YBCO thin film acted as a magnetic shield and significantly cancelled the applied field. Therefore, it can be concluded that the multilayer type fabrication of shielding layers could be successfully applied to the superconducting YBCO devices and considered as an alternative technique to enhance the operational flexibility of the high-Tc superconducting devices. 4. Conclusion Performance of the superconducting YBCO thin film as a magnetic shielding layer in a multilayer processed superconducting YBCO device was tested. A multilayer YBCO-shield/STO/YBCOmicrobridge was fabricated on a (1 0 0) single crystal STO substrate and the superconducting transitions were measured by performing ac magnetic susceptibility and R–T measurements. The dependence of the critical current values to the applied magnetic fields was analyzed by measuring the I–V curves of shielded and unshielded devices under various applied fields which were applied perpendicular to the current flow direction. The results showed that the multilayer fabrication of superconducting shielding layer integrated to the YBCO device successfully worked and the critical current values remained almost constant under the
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Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Magnetic shielding performance of superconducting YBCO thin film in a multilayer device structure Y. Uzun ⇑, I. Avci Ege University, Faculty of Science, Department of Physics, 35100 Bornova, Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 14 April 2014 Received in revised form 14 August 2014 Accepted 19 October 2014 Available online 28 October 2014 Keywords: YBCO thin films Multilayer devices Magnetic shielding
a b s t r a c t Magnetic shielding performance of superconducting YBaCu2O7x (YBCO) thin film on an YBCO microbridge was analyzed in a multilayer structure. A sandwich type multilayer structure was fabricated onto a single crystal (1 0 0) SrTiO3 (STO) substrate in the form of YBCO/STO/YBCO by depositing a thin STO interlayer in between two YBCO layers. The top YBCO was patterned as 20 lm width meander-type microbridges and the bottom layer YBCO was used as magnetic shield. YBCO and STO thin films were deposited by dc and rf magnetron sputtering respectively, and the patterning was performed by using standard photolithography and wet etching. In order to enhance long-term stability of the final device, an additional STO thin film was deposited onto the device as an encapsulation layer. Electrical and magnetic characterizations of the YBCO thin film layers were carried out by means of ac magnetic susceptibility (v–T) and resistance vs. temperature (R–T) measurements. The current–voltage (I–V) measurements were performed on the microbridges at 77 K by observing the shielding performance of the bottom YBCO layer under various applied magnetic fields. The results were compared with that of a same-type single layer YBCO device without a shielding layer. The zero field critical current value of the single layer 20 lm wide YBCO device was measured as 30 mA and decreased down to 20 mA as the field increased up to 100 mT. The same measurements on the multilayer device showed that the critical current values remained almost constant around 27 mA as the applied field increased. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Magnetic field dependence of the critical currents of Josephson junctions and superconducting strip lines are the main characteristic properties of the superconducting devices such as superconducting quantum interference devices (SQUIDs) [1], single flux quantum circuits (SFQ) [2] and superconducting filters [3]. Since the critical current values of such devices are very sensitive to the background magnetic fields, shielding procedure has to be applied in almost all applications in order to enhance the signal quality and reduce the field noise contribution [4–6]. The most common sources are the Earth’s magnetic field, power lines and electronic equipments that could likely generate noises and must be shielded during the superconducting device operation [7]. In most cases, these external fields are shielded by using l-metal cages during the operation, becoming one of the main parts of the system [8,9]. However, these kinds of shielding ‘rooms’ most of time become impractical in many applications, particularly in the mobile systems such as non-destructive evaluation (NDE) ⇑ Corresponding author. Tel.: +90 505 395 9661. E-mail address: [email protected] (Y. Uzun). http://dx.doi.org/10.1016/j.physc.2014.10.009 0921-4534/Ó 2014 Elsevier B.V. All rights reserved.
[10,11]. Moreover, the use of l-metal shield mostly limits the testing capability of the system by reducing the room for the sample under test [12,13]. The use of self-shielding mechanisms in the devices can overcome such limitations. For example, in the niobium based low temperature superconducting devices, the use of compensation coils integrated to the devices significantly improves the shielding performance [14]. Since the multilayer fabrication process has been successfully implemented to the niobium based devices [15,16], these types of self-shielding mechanisms could easily be performed [14]. However, there has been a lack of YBCO based high temperature superconducting devices because the multilayer processing has not been able to be implemented to this material yet [17,18]. One of the solutions has been the use of gradiometric configuration for the YBCO SQUIDs [19]. On the other hand, many attempts have been performed to integrate compensation coils to the YBCO devices by using on-chip configurations [20,21]. However, most of them require complex fabrication process and do not provide long term stability. Another solution could be the use of superconducting YBCO thin film itself as a magnetic shield by making use of the diamagnetic behavior of the superconducting YBCO thin film [22]. In this case, a separate YBCO thin film is generally integrated to the device in flip-chip configuration [23].
Y. Uzun, I. Avci / Physica C 507 (2014) 90–94
Although, flip-chip configuration highly eliminates external magnetic field sources, integration of superconducting thin film and superconducting device onto a single substrate as multilayer structure is more practical in the device application than using two individual superconducting samples. Therefore, the multilayer device processing should be evaluated as a considerable development for self-shielding mechanism of superconducting YBCO device fabrication. With this motivation, a multilayer fabrication process was developed in this study for superconducting YBCO devices in order to integrate a shielding layer to the device in the form of YBCOshield/STO/YBCO-microbridge. The electronic properties of individual layers were characterized depending on the fabrication process parameters and the shielding performance of the bottom YBCO layer was tested on the I–V characteristics of the microbridge under various applied magnetic fields.
2. Experimental procedure Before processing the multilayer structure on a single substrate, deposition parameters of YBCO and STO thin films were studied individually in order to obtain optimum thin film process parameters. The process steps during the thin film growth are summarized as follows: (1 0 0) single crystal SrTiO3 (STO) substrates were first ultrasonically cleaned in acetone and subsequently in propanol for 10 min, and rinsed with ultra-pure de-ionized water. The substrates were then silver-glued onto the heater/substrate holder of the sputtering system and the chamber was vacuumed down to 1 107 mbar. After the ultimate vacuum was reached, the substrates were heated up to the deposition temperatures listed in Table 1. YBCO thin films were deposited at 750 °C under the total gas pressure of 0.4 mbar with Ar:O2 ratio of 4:1. The deposition rate for the YBCO thin film was adjusted to be 1.6 nm/min by applying the dc power of 70 W to the 100 diameter cylindrical hallow type YBCO target. Deposition time was chosen as 2 h in order to obtain 200 nm thick YBCO thin films. STO thin films were deposited at 700 °C under the total gas pressure of 0.04 mbar with Ar:O2 ratio of 4:1. The deposition rate for the STO thin film was adjusted to be 1.25 nm/min by applying the rf power of 75 W to the 200 diameter planar STO target. Deposition time was chosen as 2 h in order to obtain 150 nm thick STO thin films. An additional oxygenation process was used for both films after the deposition. YBCO thin films were kept at 750 °C for additional 10 min under 1 mbar flowing O2 atmosphere, then the O2 pressure increased up to 700 mbar and the substrates were allowed to cool down to the room temperature by turning off the heater power supply. Same oxygenation profile was applied to the STO thin films as well. We prepared two types of YBCO thin film samples: (1) the YBCO thin films were deposited onto bare-STO substrates and tested in order to check the superconducting properties depending on the Table 1 Thin film deposition parameters for YBCO and STO. Parameters
Base vacuum Substrate temperature Deposition power PAr:PO2 Total deposition pressure Deposition time Thickness Oxygenation at deposition temperature Oxygenation during the cooling
Materials YBCO
STO
1 107 mbar 750 °C 70 W DC 4:1 0.4 mbar 2h 200 nm 10 min with 1 mbar flowing O2 at 750 °C 700 mbar O2
1 107 mbar 700 °C 75 W RF 4:1 0.04 mbar 2h 150 nm 10 min with 1 mbar flowing O2 at 700 °C 700 mbar O2
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Fig. 1. Schematic of the layers of YBCO/STO/YBCO multilayer structure and microbridge.
process parameters. (2) Another YBCO thin film samples were prepared by using STO thin film deposited substrates in order to make sure that the rf deposited STO thin films can provide good buffer for YBCO thin film growth. Both types of YBCO thin films were tested and the properties were compared. YBCO thin films were analyzed by performing dc resistance vs. temperature (R–T) and ac magnetic susceptibility vs. temperature (v–T) measurements. By preparing multiple samples, reproducible YBCO thin films with and without STO buffer were obtained. The multilayer YBCO/STO/YBCO structure was then fabricated by sequentially depositing the YBCO and STO layers with the same parameters given in Table 1 without taking the samples out of the deposition chamber in order to eliminate any possible contamination at the film interfaces. The heater of our multi-target sputtering system is 360° tiltable; therefore, we easily adjusted the substrate surface to the desired target during the full deposition. After depositing the 3-layer thin film structure, the top YBCO layer was patterned as microbridges as shown in Fig. 1. The pattern was designed as meander-type with the line width of 20 lm. We applied standard photolithography and wet etching for the patterning process. In order to compare the properties of the multilayer device with a standard single layer YBCO device, another microbridge was also fabricated onto a bare STO substrate by using exactly the same process parameters used for multilayer. After the patterning process, v–T measurement was performed on the full structure in order to test the diamagnetic behavior of the bottom YBCO thin film. The measurement was carried out in a vacummable liquid nitrogen cryostat equipped with a two-coil system by using mutual inductance method. The excitation coil was fed with 1 kHz ac signal and the response was read out from the secondary coil by a lock-in amplifier (Stanford Research System SR530) while reducing the temperature with the liquid nitrogen (LN). The temperature was read from Pt100 sensor by LakeShore LS331 temperature controller. In-phase (v0 ) and out-of-phase (v00 ) signals along with the temperature data were picked up by a computer. The superconducting transition of the top layer YBCO microbridge was then tested by R–T measurement. DC 4-point measurement technique was used by bonding the contact pads of the devices onto a gold plated printed circuit board (PCB) that was glued onto a Cu chip carrier. The sample was placed into the vacummable LN cryostat and the measurement was carried out by applying the current with Keithley-224 current source and measuring the voltage with Keithley-199 System DMM. The temperature was controlled by Lakeshore 331 temperature controller and the data was collected via computerized data acquisition system. Magnetic field dependence of the critical currents of the devices was measured over the I–V curves. For this, neodymium permanent magnets as dc magnetic field sources were attached to the LN cryostat and I–V measurements were carried out by adjusting the field values as 0 mT, 30 mT, 60 mT, and 100 mT. The I–V mea-
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surements were performed for both single layer and multilayer YBCO microbridges, and the shielding effect of the bottom YBCO layer was clearly observed in the multilayer structure. The whole
structure was then covered with a room temperature deposited STO layer to protect the device and thin films from the environmental effects arise from the thermal cycles.
3. Results and discussion
Fig. 2. Ac magnetic susceptibility vs. temperature measurements of YBCO thin films. Data with squares (h) show the single layer YBCO thin film, and circles (s) show the bottom layer YBCO thin film in the multilayer structure.
Fig. 3. Dc resistance vs. temperature measurements of single and multilayer YBCO microbridges.
Ac magnetic susceptibility vs. temperature (v–T) measurements of the YBCO thin films were performed for both type of samples. In Fig. 2, diamagnetic responses of single layer YBCO thin film and bottom layer YBCO of multilayer structure are shown together for comparison. These measurements were done mainly to test the superconducting transition of the YBCO thin films without disturbing the film surfaces by contact wiring. The magnetic susceptibility response of the top layer does not have a significant contribution to the measurement because the top layer was already patterned as micrometer wide strip lines. The comparison in Fig. 2 was aimed to show any possible degradation or enhancement on the superconducting transition profile of the film after the multilayer processing. As shown in Fig. 2, the standard single layer YBCO thin film on STO substrate has a quite sharp diamagnetic transition at around 90 K (square data points) which is the most common transition profile of YBCO thin films. However, the transition temperature of the bottom layer YBCO thin film in the multilayer structure seemed to be enhanced with the Tc of 92 K. This enhancement could be linked to the additional heat treatments during the upper layers depositions. The oxygen deficiency of the first layer YBCO thin film could be increased up to an acceptable amount which is generally concluded to be the reason for the Tc enhancement [24,25]. Besides the enhancement on the Tc of bottom layer YBCO in the multilayer structure, which is out of the scope of this work, we confirmed that the multilayer processing of the YBCO thin films with STO interlayers and lithographic process on the top layer did not degrade the superconducting properties of the first layer YBCO thin film. Therefore, this layer could be used as a magnetic shield for the top layer device. After confirming the superconductivity in the bottom layer YBCO, we performed a dc resistance vs. temperature (R–T) measurement on the top layer patterned-YBCO microbridge. As shown in Fig. 3, the multilayer processed YBCO thin film as the top layer device have a similar superconducting transition with that of the single layer YBCO. The Tc of the upper layer device was observed as 92 K as in the single layer YBCO device. This result confirmed the superconductivity on the upper layer YBCO that can be considered as superconducting multilayer device integrated to a superconducting YBCO thin film.
Fig. 4. Current vs. voltage characteristics of (a) single layer and (b) multilayer YBCO microbridges under different applied magnetic fields.
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field environment. This could be considered as an alternative method to the flip-chip or l-metal type shielding of the YBCO devices. The magnetic field could also be applied parallel to the current flow direction by making some modifications on the sample stage of the cryostat, and we believe, the shielding layer of our configuration could also act as a magnetic shield in such a field direction. In this study, we aimed to check the success of the multilayer type fabrication of the shielding layer in high-Tc devices. Further study has been planned to test the effect and shielding of various magnetic field sources in any direction.
Acknowledgment This work was supported by the TUBITAK under the project number of 112T075.
Fig. 5. Magnetic field dependence of the critical currents of shielded and unshielded superconducting YBCO microbridges.
In order to test the critical current profile, and furthermore, the magnetic field dependence of the critical current of upper layer device integrated to a bottom layer superconducting YBCO thin film, a series of I–V measurements were performed under various applied magnetic fields. In Fig. 4, I–V measurements of single layer and multilayer YBCO microbridges under different applied magnetic fields are shown. As in the case of single layer YBCO device, the critical current values gathered from the I–V curves decreased with increasing applied field. The zero field critical current value of the single layer YBCO device was measured as almost 30 mA for the 20 lm meander type superconducting line and this value decreased down to 20 mA as the field increased up to 100 mT (Figs. 4a and 5). The same measurements on the multilayer device showed that the critical current values remained almost constant around 27 mA as the applied field increased (Figs. 4b and 5). The sample was glued onto the copper cold finger of the vacuum LN cryostat and the permanent magnet was attached to the system as to be perpendicular to the sample surface. Because of the limitations in wiring directions and sample attachment onto the cold finger of the cryostat, the field was applied only in perpendicular alignment to the sample. The superconducting layer of YBCO thin film acted as a magnetic shield and significantly cancelled the applied field. Therefore, it can be concluded that the multilayer type fabrication of shielding layers could be successfully applied to the superconducting YBCO devices and considered as an alternative technique to enhance the operational flexibility of the high-Tc superconducting devices. 4. Conclusion Performance of the superconducting YBCO thin film as a magnetic shielding layer in a multilayer processed superconducting YBCO device was tested. A multilayer YBCO-shield/STO/YBCOmicrobridge was fabricated on a (1 0 0) single crystal STO substrate and the superconducting transitions were measured by performing ac magnetic susceptibility and R–T measurements. The dependence of the critical current values to the applied magnetic fields was analyzed by measuring the I–V curves of shielded and unshielded devices under various applied fields which were applied perpendicular to the current flow direction. The results showed that the multilayer fabrication of superconducting shielding layer integrated to the YBCO device successfully worked and the critical current values remained almost constant under the
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