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Fabrication of MgB2 nanobridge dc SQUIDs by focused ion beam
Physica C 470 (2010) S1036–S1037
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Fabrication of MgB2 nanobridge dc SQUIDs by focused ion beam Sung-Hak Hong a, Soon-Gul Lee a,*, Won Kyung Seong b, Won Nam Kang b a b
Korea University, Department of Display and Semiconductor Physics, Jochiwon, Chungnam 339-800, Republic of Korea Sungkyunkwan University, Department of Physics, Suwon, Gyeonggi-do 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Accepted 7 February 2010 Available online 10 February 2010 Keywords: MgB2 superconductor Intragrain nanobridge dc SQUID
a b s t r a c t We have studied fabrication of MgB2 intragrain nanobridge dc SQUIDs by a focused ion beam (FIB) patterning technique. Not only the nanobridges as Josephson elements but the SQUID loop was patterned by FIB. The beam voltage was 30 kV and the beam current was 0.9 nA for the SQUID loop and 1.5 pA for the nanobridges. Each bridge had a nominal width and length of about 100 nm and a thickness of 650 nm. The SQUID loop had a 3 lm 3 lm hole with a 2 lm average linewidth. The zero-field superconducting transition temperature (Tc) of the SQUID was 37 K. Current–voltage (I–V) characteristics of the SQUID showed large excess currents at all temperatures with a small portion of a resistively-shunted-junction (RSJ) component which increases as temperature approaches Tc. At low temperatures, the I–V curves exhibit a large heating effect with a second transition step, which is believed to be due to the transition of a grain boundary near the nanobridge. The SQUID showed well-behaving modulation properties at all temperatures with a modulation depth of more than 30 lV at 33.5 K and 110 lV at 15 K. These results together with our previous results on the intergrain nanobridge dc SQUID suggest that fabrication of dc SQUIDs based on FIB-patterned MgB2 nanobridges is highly tolerant of fabrication conditions. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction A Josephson junction is the key requirement for the fabrication of superconducting quantum interference devices (SQUIDs). In spite of a relatively long coherence length of the MgB2 superconductor, the material properties, for example, an extremely high vapor pressure of Mg, did not allow a reliable conventional junction fabrication technique. Alternatively, weak link type Josephson elements have been developed by various methods and some of them have been successfully adopted to the fabrication of MgB2 SQUIDs. They include bulk point-contact tunnel junction dc SQUIDs [1] and FIB-etched nanobridge dc SQUIDs [2,3]. We have reported MgB2 dc SQUIDs based on FIB-patterned intergrain nanobridges [4,5]. In this work, we studied fabrication of intragrain MgB2 nanobridge dc SQUIDs to demonstrate a wide tolerance and feasibility of FIB-patterned MgB2 nanobridges in the SQUID fabrication whether they contain grain boundaries or not. 2. Experimental results and discussion The SQUID was fabricated on an 8 lm wide film line of MgB2 by FIB with a beam voltage of 30 kV and a beam current of 9 nA for the SQUID ring structure and 1.5 pA for the nanobridges. To compare with the intergrain nanobridge SQUIDs [5] in which * Corresponding author. Tel.: +82 41 860 1327; fax: +82 41 865 0939. E-mail address: [email protected] (S.-G. Lee). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.02.014
the nanobridges were patterned across the grain boundaries, we devised the nanobridges within single crystalline grains. Fig. 1 shows SEM images of the SQUID. The bridge is 120 nm wide and long, and 650 nm thick. The SQUID loop has a 3 lm 3 lm hole with an average linewidth of 2 lm, corresponding to an inductance of 5 pH. In the viewpoint of fabrication only, the intragrain nanobridge is advantageous because it is less susceptible to ion beam damage in comparison with the intergrain counterpart. Fig. 2 shows current–voltage (I–V) curves of the SQUID measured at different temperatures in ambient field. The curves show large excess currents at all temperatures with a small portion of the resistively-shunted-junction (RSJ) component, which increases as temperatures approaches Tc. The RSJ component is associated with the Josephson coupling across the bridge and contributes to the SQUID modulation. The basic structure of the I–V curves with excess currents is interpreted within the frame of the flux flow theory [4]. At low temperatures (below 33 K in Fig. 2) the curves exhibit downward curvature in the first quadrant, which is due to Joule heating by the bridge current. The second transition (marked by an arrow on the curve) is believed to be due to the superconducting transition of a grain boundary near the nanobridge. The intragrain Jc of the SQUID nanobridges is calculated to be 2.5 MA/cm2 at 35 K and 51 MA/cm2 at 16 K. In contrast to intragrain nanobridges, intergrain nanobridges had a wide spectrum of strength in coupling, ranging from extremely weak coupling showing an ideal RSJ behavior [6] to very strong coupling with large flux-flow effect, depending on the fabrication conditions [7].
S.-H. Hong et al. / Physica C 470 (2010) S1036–S1037
S1037
Fig. 1. SEM images of the SQUID: (left) bird-eye view, (right top) top view, and (right bottom) bridge area. The bridge is 120 nm wide and long and 650 nm thick. The SQUID loop has a 3 lm 3 lm hole and an average linewidth of 2 lm. Patterned are whitened mainly due to redeposition.
Fig. 2. I–V characteristics measured at different temperatures. All the curves show large excess currents. Low-temperature curves show large heating effect with a second transition step (indicated by arrows) which is believed to be due to the transition of a grain boundary near the bridge.
Fig. 3. Modulations of the SQUID in response to applied fields. The magnetic field was generated by a small wire-wound coil placed on top of the SQUID. Ib is the bias current.
Acknowledgment Fig. 3 shows modulation of the SQUID voltage at various bias currents in response to applied fields measured at 33.5 K. The magnetic field was generated by a small wire-wound coil placed on top of the device. The data showed well-behaving modulation properties at all measured temperatures up to just below Tc (37 K). The relative modulation depth with respect to IcRN decreased with decreasing T, in agreement with decreasing relative RSJ component as T decreases. The maximum modulation depth was 30 lV at 33.5 K and 110 lV at 15 K. 3. Conclusion We fabricated MgB2 intragrain nanobridge dc SQUIDs by FIB and studied their properties. The device showed well-behaving modulation properties with large modulation voltages. The results demonstrate that the FIB-patterned nanobridge is highly tolerant as a Josephson element and thus feasible in SQUID fabrication.
This work was supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2008-313C00240). References [1] Y. Zhang, D. Kinion, J. Chen, J. Clarke, D.G. Hinks, G.W. Crabtree, Appl. Phys. Lett. 79 (2001) 3995. [2] A. Brinkman, D. Veldhuis, D. Mijatovic, G. Rijnders, D.H.A. Blank, H. Hilgenkamp, H. Rogalla, Appl. Phys. Lett. 79 (2001) 2420. [3] D. Mijatovic, A. Brinkman, D. Veldhuis, H. Hilgenkamp, H. Rogalla, G. Rijnders, D.H.A. Blank, A.V. Pogrebnyakov, J.M. Redwing, S.Y. Xu, Q. Li, X.X. Xi, Appl. Phys. Lett. 87 (2005) 192505. [4] S.G. Lee, S.H. Hong, W.N. Kang, D.H. Kim, J. Appl. Phys. 105 (2009) 013924. [5] S.G. Lee, S.H. Hong, W.K. Seong, W.N. Kang, Supercond. Sci. Technol. 22 (2009) 064009. [6] S.G. Lee, S.H. Hong, W.K. Seong, W.N. Kang, Appl. Phys. Lett. 95 (2009) 202504. [7] S.H. Hong, S.G. Lee, W.K. Seong, W.N. Kang, Physica C, in press.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Fabrication of MgB2 nanobridge dc SQUIDs by focused ion beam Sung-Hak Hong a, Soon-Gul Lee a,*, Won Kyung Seong b, Won Nam Kang b a b
Korea University, Department of Display and Semiconductor Physics, Jochiwon, Chungnam 339-800, Republic of Korea Sungkyunkwan University, Department of Physics, Suwon, Gyeonggi-do 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Accepted 7 February 2010 Available online 10 February 2010 Keywords: MgB2 superconductor Intragrain nanobridge dc SQUID
a b s t r a c t We have studied fabrication of MgB2 intragrain nanobridge dc SQUIDs by a focused ion beam (FIB) patterning technique. Not only the nanobridges as Josephson elements but the SQUID loop was patterned by FIB. The beam voltage was 30 kV and the beam current was 0.9 nA for the SQUID loop and 1.5 pA for the nanobridges. Each bridge had a nominal width and length of about 100 nm and a thickness of 650 nm. The SQUID loop had a 3 lm 3 lm hole with a 2 lm average linewidth. The zero-field superconducting transition temperature (Tc) of the SQUID was 37 K. Current–voltage (I–V) characteristics of the SQUID showed large excess currents at all temperatures with a small portion of a resistively-shunted-junction (RSJ) component which increases as temperature approaches Tc. At low temperatures, the I–V curves exhibit a large heating effect with a second transition step, which is believed to be due to the transition of a grain boundary near the nanobridge. The SQUID showed well-behaving modulation properties at all temperatures with a modulation depth of more than 30 lV at 33.5 K and 110 lV at 15 K. These results together with our previous results on the intergrain nanobridge dc SQUID suggest that fabrication of dc SQUIDs based on FIB-patterned MgB2 nanobridges is highly tolerant of fabrication conditions. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction A Josephson junction is the key requirement for the fabrication of superconducting quantum interference devices (SQUIDs). In spite of a relatively long coherence length of the MgB2 superconductor, the material properties, for example, an extremely high vapor pressure of Mg, did not allow a reliable conventional junction fabrication technique. Alternatively, weak link type Josephson elements have been developed by various methods and some of them have been successfully adopted to the fabrication of MgB2 SQUIDs. They include bulk point-contact tunnel junction dc SQUIDs [1] and FIB-etched nanobridge dc SQUIDs [2,3]. We have reported MgB2 dc SQUIDs based on FIB-patterned intergrain nanobridges [4,5]. In this work, we studied fabrication of intragrain MgB2 nanobridge dc SQUIDs to demonstrate a wide tolerance and feasibility of FIB-patterned MgB2 nanobridges in the SQUID fabrication whether they contain grain boundaries or not. 2. Experimental results and discussion The SQUID was fabricated on an 8 lm wide film line of MgB2 by FIB with a beam voltage of 30 kV and a beam current of 9 nA for the SQUID ring structure and 1.5 pA for the nanobridges. To compare with the intergrain nanobridge SQUIDs [5] in which * Corresponding author. Tel.: +82 41 860 1327; fax: +82 41 865 0939. E-mail address: [email protected] (S.-G. Lee). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.02.014
the nanobridges were patterned across the grain boundaries, we devised the nanobridges within single crystalline grains. Fig. 1 shows SEM images of the SQUID. The bridge is 120 nm wide and long, and 650 nm thick. The SQUID loop has a 3 lm 3 lm hole with an average linewidth of 2 lm, corresponding to an inductance of 5 pH. In the viewpoint of fabrication only, the intragrain nanobridge is advantageous because it is less susceptible to ion beam damage in comparison with the intergrain counterpart. Fig. 2 shows current–voltage (I–V) curves of the SQUID measured at different temperatures in ambient field. The curves show large excess currents at all temperatures with a small portion of the resistively-shunted-junction (RSJ) component, which increases as temperatures approaches Tc. The RSJ component is associated with the Josephson coupling across the bridge and contributes to the SQUID modulation. The basic structure of the I–V curves with excess currents is interpreted within the frame of the flux flow theory [4]. At low temperatures (below 33 K in Fig. 2) the curves exhibit downward curvature in the first quadrant, which is due to Joule heating by the bridge current. The second transition (marked by an arrow on the curve) is believed to be due to the superconducting transition of a grain boundary near the nanobridge. The intragrain Jc of the SQUID nanobridges is calculated to be 2.5 MA/cm2 at 35 K and 51 MA/cm2 at 16 K. In contrast to intragrain nanobridges, intergrain nanobridges had a wide spectrum of strength in coupling, ranging from extremely weak coupling showing an ideal RSJ behavior [6] to very strong coupling with large flux-flow effect, depending on the fabrication conditions [7].
S.-H. Hong et al. / Physica C 470 (2010) S1036–S1037
S1037
Fig. 1. SEM images of the SQUID: (left) bird-eye view, (right top) top view, and (right bottom) bridge area. The bridge is 120 nm wide and long and 650 nm thick. The SQUID loop has a 3 lm 3 lm hole and an average linewidth of 2 lm. Patterned are whitened mainly due to redeposition.
Fig. 2. I–V characteristics measured at different temperatures. All the curves show large excess currents. Low-temperature curves show large heating effect with a second transition step (indicated by arrows) which is believed to be due to the transition of a grain boundary near the bridge.
Fig. 3. Modulations of the SQUID in response to applied fields. The magnetic field was generated by a small wire-wound coil placed on top of the SQUID. Ib is the bias current.
Acknowledgment Fig. 3 shows modulation of the SQUID voltage at various bias currents in response to applied fields measured at 33.5 K. The magnetic field was generated by a small wire-wound coil placed on top of the device. The data showed well-behaving modulation properties at all measured temperatures up to just below Tc (37 K). The relative modulation depth with respect to IcRN decreased with decreasing T, in agreement with decreasing relative RSJ component as T decreases. The maximum modulation depth was 30 lV at 33.5 K and 110 lV at 15 K. 3. Conclusion We fabricated MgB2 intragrain nanobridge dc SQUIDs by FIB and studied their properties. The device showed well-behaving modulation properties with large modulation voltages. The results demonstrate that the FIB-patterned nanobridge is highly tolerant as a Josephson element and thus feasible in SQUID fabrication.
This work was supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2008-313C00240). References [1] Y. Zhang, D. Kinion, J. Chen, J. Clarke, D.G. Hinks, G.W. Crabtree, Appl. Phys. Lett. 79 (2001) 3995. [2] A. Brinkman, D. Veldhuis, D. Mijatovic, G. Rijnders, D.H.A. Blank, H. Hilgenkamp, H. Rogalla, Appl. Phys. Lett. 79 (2001) 2420. [3] D. Mijatovic, A. Brinkman, D. Veldhuis, H. Hilgenkamp, H. Rogalla, G. Rijnders, D.H.A. Blank, A.V. Pogrebnyakov, J.M. Redwing, S.Y. Xu, Q. Li, X.X. Xi, Appl. Phys. Lett. 87 (2005) 192505. [4] S.G. Lee, S.H. Hong, W.N. Kang, D.H. Kim, J. Appl. Phys. 105 (2009) 013924. [5] S.G. Lee, S.H. Hong, W.K. Seong, W.N. Kang, Supercond. Sci. Technol. 22 (2009) 064009. [6] S.G. Lee, S.H. Hong, W.K. Seong, W.N. Kang, Appl. Phys. Lett. 95 (2009) 202504. [7] S.H. Hong, S.G. Lee, W.K. Seong, W.N. Kang, Physica C, in press.