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“In-situ” sputter deposition of superconducting BiSrCaCuO thin films
Physica C 162-164 (1989) 645-646 North-Holland
"IN-SITU" SPUTTER DEPOSITION OF SUPERCONDUCTING Bi-Sr-Ca-Cu-O THIN FILMS T.P. THORPE, M.S. OSOFSKY, E.F. SKELTON, S.B. QADRI, AND C.R. GOSSE'I-I" NAVAL RESEARCH LABORATORY, WASHINGTON, D.C. 20375 Thin films of Bi-Sr-Ca-Cu-O, which are superconducting as-deposited, have been sputtered onto MgO substrates using a single Bi2Sr2Ca2Cu3Ox cast target. An rf-magnetron source was used, with substrates held at temperatures up to 850 °C in an argon-oxygen (50%-50%) atmosphere. Evidence of the higher transition temperature "2223" phase is present in both x-ray diffraction data and resistivity vs. temperature measurements. Necessary conditions for "in-situ" growth are have been explored.
The purpose of this study is to investigate the possibility of sputter depositing thin films of Bi-SrCa-Cu-O which are superconducting as-deposited, and which contain a high proportion of the higher transition temperature "2223" phase. "ln-situ" films are desirable because they generally possess a lesser density of material defects than films requiring a post-deposition anneal, and are a necessity for the fabrication of several types of superconducting devices. All films were deposited onto MgO(100) substrates by rf-magnetron sputtering using a single three inch diameter target. The target composition was stoichiometric Bi2Sr2Ca2Cu3Ox, and was prepared by casting from the melt in a copper mold. The sputtering gas was a 50%-50% mixture of argon and oxygen. Gas pressure during deposition was held constant between 100 and 200 mTorr. The rf input power ranged from 50 to 150 watts. Substrate temperatures ranged from 600 °C to 850 °C. Source to substrate distance varied from 21 mm to 60 mm, dependent on obtaining a sufficient deposition rate at a given pressure. Deposition rates ranged from 1000 to 2500 A/hr. Following deposition the sputtering chamber was vented slowly (10 min.) to air with the substrate table held at the deposition temperature. For"in-situ" films the substrate was then allowed to cool to ambient. Other films were subsequently post-annealed for 5 min. in air at 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
860 °C. Film composition was determined by RBS measurements. The structure of the films was studied using standard x-ray diffraction techniques. Resistivity vs. temperature measurements were made by a conventional 4-pt. probe using pressed indium contacts. The best "in-situ" film properties achieved currently were obtained at a substrate temperature of 800 °C with a sputtering pressure of 100 mTorr. Source to substrate distance was approximately 60 ram, with 25' Bi-Sr-Ca-Cu-O
FILMS
20' A
15'
==
=. n-
10"
•
5"
•
•e
•
•
• 0
--
50
1753 1771
:
100
300
T
(K)
FIGURE 1 Temperature dependence of resistivity for Bi-Sr-CaCu-O thin films-1753,.as-deposited; 1771, annealed in air at 860 °C.
T.P. Thorpe et aL / "'In-situ" sputter deposition
646
an rf input power of 100 watts. Fig. 1 illustrates temperature vs. resistivity for both an "in-situ" film and one which received a postanneal in air at 860 °C. As can be seen, both are metallic and exhibit onset temperatures above 100 °K; indicating the presence of the "2223" phase. The "in-situ" film achieves zero resistance at approximately 62 °K. The postannealed film exhibits a clear drop in resistivity above 100 °K, and has a zero resistance temperature value of approximately 75 °K. RBS measurements showed both films to be somewhat Bi deficient, but relatively close to stoichiometric values. Fig. 2 displays the x-ray diffraction data of the "insitu" film represented in Fig. 1. In addition to the MgO substrate peaks, two sets of single crystal-type diffraction peaks appear, one relatively sharp and the second rather broad. Both are aligned with the MgO substrate in that the w-peak positions are relatively 6 1 0 ~
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FIGURE 2 X-ray diffraction scan of as-deposited Bi-Sr-Ca-Cu-O thin film. sharp and equivalent. The sharp set of diffraction peaks are associated with the 85 °K transition temperature "2212" tetragonal phase, indicating its' predominance. Based on the peak positions assigned in the figure, a c-axis lattice parameter of about 30.65 A is calculated, which is slightly higher than published bulk values 1. Other peaks could be associated with a slightly ;distorted "2223" phase.
"ln-situ" films deposited at temperatures 40 to 50 degrees above or below the optimum value resulted in greatly increased as-deposited resistivities, with broader superconducting transitions. Onset and zero resistance temperature values were generally lower. X-ray diffraction scans produced much less welldefined "2212" peaks for those films. It can be concluded that 800 °C is close to an optimum substrate temperature to produce "in-situ" formation of the "2212" phase in our system, with some formation of the "2223" phase. It has been found in previous research that the "2223" phase forms preferentially over the "2212" phase in only a very narrow temperature band near the materials' melting point 2. It has also been observed that oxygen is desorbed in all of the Bi-containing high temperature superconductors as their melting point is approached, with an accompanying large increase in electrical resistivity, and destruction of the supercOnducting phases 3. The exact temperature range at which this desorption occurs decreases with decreasing ambient oxygen partial pressure. Our results would indicate that, in the reduced oxygen partial pressure under which films are sputter deposited, the lower limit of the temperature regime where significant oxygen desorption occurs falls below the temperature required for preferential formation of the "2223" phase. It is known that the partial substitution of Pb for Bi in these materials promotes the formation of the "2223" phase, possibly broadening and lowering the temperature band where it forms. Current literature indicates, however, extreme difficulty with incorporating the optimum amount of Pb into films deposited onto heated substrates. Our future research effort will be along those lines. REFERENCES 1.) J.M. Liang et al., Appl. Phys. Lett. 53 (1988) 913. 2.) T. Hatano et. al., Jpn. J. Appl. Phys., 27 (1988) L2055. 3.) M.Z. Harford et al., J. Super. 1 (1988) 407.
"IN-SITU" SPUTTER DEPOSITION OF SUPERCONDUCTING Bi-Sr-Ca-Cu-O THIN FILMS T.P. THORPE, M.S. OSOFSKY, E.F. SKELTON, S.B. QADRI, AND C.R. GOSSE'I-I" NAVAL RESEARCH LABORATORY, WASHINGTON, D.C. 20375 Thin films of Bi-Sr-Ca-Cu-O, which are superconducting as-deposited, have been sputtered onto MgO substrates using a single Bi2Sr2Ca2Cu3Ox cast target. An rf-magnetron source was used, with substrates held at temperatures up to 850 °C in an argon-oxygen (50%-50%) atmosphere. Evidence of the higher transition temperature "2223" phase is present in both x-ray diffraction data and resistivity vs. temperature measurements. Necessary conditions for "in-situ" growth are have been explored.
The purpose of this study is to investigate the possibility of sputter depositing thin films of Bi-SrCa-Cu-O which are superconducting as-deposited, and which contain a high proportion of the higher transition temperature "2223" phase. "ln-situ" films are desirable because they generally possess a lesser density of material defects than films requiring a post-deposition anneal, and are a necessity for the fabrication of several types of superconducting devices. All films were deposited onto MgO(100) substrates by rf-magnetron sputtering using a single three inch diameter target. The target composition was stoichiometric Bi2Sr2Ca2Cu3Ox, and was prepared by casting from the melt in a copper mold. The sputtering gas was a 50%-50% mixture of argon and oxygen. Gas pressure during deposition was held constant between 100 and 200 mTorr. The rf input power ranged from 50 to 150 watts. Substrate temperatures ranged from 600 °C to 850 °C. Source to substrate distance varied from 21 mm to 60 mm, dependent on obtaining a sufficient deposition rate at a given pressure. Deposition rates ranged from 1000 to 2500 A/hr. Following deposition the sputtering chamber was vented slowly (10 min.) to air with the substrate table held at the deposition temperature. For"in-situ" films the substrate was then allowed to cool to ambient. Other films were subsequently post-annealed for 5 min. in air at 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
860 °C. Film composition was determined by RBS measurements. The structure of the films was studied using standard x-ray diffraction techniques. Resistivity vs. temperature measurements were made by a conventional 4-pt. probe using pressed indium contacts. The best "in-situ" film properties achieved currently were obtained at a substrate temperature of 800 °C with a sputtering pressure of 100 mTorr. Source to substrate distance was approximately 60 ram, with 25' Bi-Sr-Ca-Cu-O
FILMS
20' A
15'
==
=. n-
10"
•
5"
•
•e
•
•
• 0
--
50
1753 1771
:
100
300
T
(K)
FIGURE 1 Temperature dependence of resistivity for Bi-Sr-CaCu-O thin films-1753,.as-deposited; 1771, annealed in air at 860 °C.
T.P. Thorpe et aL / "'In-situ" sputter deposition
646
an rf input power of 100 watts. Fig. 1 illustrates temperature vs. resistivity for both an "in-situ" film and one which received a postanneal in air at 860 °C. As can be seen, both are metallic and exhibit onset temperatures above 100 °K; indicating the presence of the "2223" phase. The "in-situ" film achieves zero resistance at approximately 62 °K. The postannealed film exhibits a clear drop in resistivity above 100 °K, and has a zero resistance temperature value of approximately 75 °K. RBS measurements showed both films to be somewhat Bi deficient, but relatively close to stoichiometric values. Fig. 2 displays the x-ray diffraction data of the "insitu" film represented in Fig. 1. In addition to the MgO substrate peaks, two sets of single crystal-type diffraction peaks appear, one relatively sharp and the second rather broad. Both are aligned with the MgO substrate in that the w-peak positions are relatively 6 1 0 ~
,
,
F
I
los i
10 0
~
E
,
E
~
~
20
i
~
I
'
~
E~...........
i
L
40 e~GLE IBEG I
60
80
FIGURE 2 X-ray diffraction scan of as-deposited Bi-Sr-Ca-Cu-O thin film. sharp and equivalent. The sharp set of diffraction peaks are associated with the 85 °K transition temperature "2212" tetragonal phase, indicating its' predominance. Based on the peak positions assigned in the figure, a c-axis lattice parameter of about 30.65 A is calculated, which is slightly higher than published bulk values 1. Other peaks could be associated with a slightly ;distorted "2223" phase.
"ln-situ" films deposited at temperatures 40 to 50 degrees above or below the optimum value resulted in greatly increased as-deposited resistivities, with broader superconducting transitions. Onset and zero resistance temperature values were generally lower. X-ray diffraction scans produced much less welldefined "2212" peaks for those films. It can be concluded that 800 °C is close to an optimum substrate temperature to produce "in-situ" formation of the "2212" phase in our system, with some formation of the "2223" phase. It has been found in previous research that the "2223" phase forms preferentially over the "2212" phase in only a very narrow temperature band near the materials' melting point 2. It has also been observed that oxygen is desorbed in all of the Bi-containing high temperature superconductors as their melting point is approached, with an accompanying large increase in electrical resistivity, and destruction of the supercOnducting phases 3. The exact temperature range at which this desorption occurs decreases with decreasing ambient oxygen partial pressure. Our results would indicate that, in the reduced oxygen partial pressure under which films are sputter deposited, the lower limit of the temperature regime where significant oxygen desorption occurs falls below the temperature required for preferential formation of the "2223" phase. It is known that the partial substitution of Pb for Bi in these materials promotes the formation of the "2223" phase, possibly broadening and lowering the temperature band where it forms. Current literature indicates, however, extreme difficulty with incorporating the optimum amount of Pb into films deposited onto heated substrates. Our future research effort will be along those lines. REFERENCES 1.) J.M. Liang et al., Appl. Phys. Lett. 53 (1988) 913. 2.) T. Hatano et. al., Jpn. J. Appl. Phys., 27 (1988) L2055. 3.) M.Z. Harford et al., J. Super. 1 (1988) 407.