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Resputtering of the superconducting BiSrCaCuO thin films
PHYSICA ELSEVIER
Physica C 254 (1995)167-174
Resputtering of the superconducting Bi-Sr-Ca-Cu-O thin films S.K. Park, Jung Ho Je * Department of Materials Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja Dong, Nam Ku, Pohang, Kyungbuk, 790-784, South Korea
Received 1 August 1995
Abstract
Oxygen is normally used as a sputtering gas with or without argon for the sputter deposition of the B i - S r - C a - C u - O superconductor. Then the grown films are frequently found to have poor qualities by severe resputtering. The purpose of this study is to identify the resputtering particle during the sputter deposition and to investigate its effect on the film growth. The experimental results support the assertion that the resputtering particle is the oxygen anion. We speculate that the oxygen anions pass through the plasma and arrive on the substrate surface nearly in a straight line. The bombardment by the oxygen caused a decrease in the crystallinity and the increase in the surface roughness of the film, and caused the a-axis orientation. A method to prevent the resputtering of the film by the oxygen anions was developed and the very smooth film of the 2212 phase with the c-axis orientation could be grown.
1. Introduction
RF magnetron sputtering process is frequently used for the deposition of the B i - S r - C a - C u - O superconducting thin films [1-3]. The films deposited at relatively low temperatures have to be annealed at higher temperatures for their crystallization. However, in the process of the crystallization, they may react with the substrates specially near the interface, resulting in a very rough surface [4]. In addition, considering their application to electronic devices, the in-situ deposition process is recently very often carried out [2,5].
* Corresponding author.
It is well known that oxygen has to be added besides argon as sputtering gas for the in-situ deposition of oxide superconductors [6-9]. When oxygen gas or a mixture of high ratio of oxygen-to-argon gas is used as sputtering gas, the growing film is known to be severely resputtered [10-15]. The degrees of resputtering are variable according to the elements within the film. In case of the B i - S r - C a - C u - O system, the resputtering, in particular, can make the concentration of the Bi element very deficient [11]. To solve the problem of the resputtering, the in-situ deposition of the oxide superconductor was frequently carried out by off-axis sputtering [7,16]. However the problem of the resputtering could not be completely removed by this method. The method to increase the sputtering pressure above 0.1 Torr was also tried so that resputtering particles could be
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00538-2
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scattered before they arrive on the growing film [1,5,17]. However, the increase in the oxygen partial pressure caused the increase in the thermodynamic temperature for the formation of the superconducting phases [18] and the control of the film composition became more difficult. Therefore, for the fabrication of the superconducting film with good qualities, the oxygen partial pressure should be reduced to usual sputtering pressures. In addition, the problem of the resputtering should be minimized. For this purpose, the particle to involve in the resputtering has to be at first identified. So far, the resputtering particle has been asserted to be oxygen anion [16,19] or oxygen neutral with high energy [18]. The purpose of this study is to identify the resputtering particle and to investigate its effect on the growth of the B i - S r - C a - C u - O superconducting film by RF magnetron sputtering. The experimental results support the assertion that the resputtering particle is the oxygen anion. A method to prevent the resputtering of the film was developed and a film of the 2212 phase with the c-axis orientation could be grown.
2. Experimental procedure RF magnetron sputtering was used to deposit the B i - S r - C a - C u - O superconducting film. The sputtering target was fabricated as a disc of 4" diameter and its composition was Bi : Sr : Ca : Cu = 2.0 : 2.1 : 1.0 : 1.9. The MgO (100) single crystal (10 mm X 10
Table 1 Summary of the sputtering conditions Target Bi-Sr-Ca-Cu-O (2.0:2.1 : 1.0 : 1.9) Substrate MgO (100) 3 X 10- 6 Torr Base pressure Ar, O2 Sputtering gas Flow rate 10-15 sccm 1 × 10-2-4.5 X 10-2 Torr Working pressure RF Power 120-150 W Substrate temperature 200°C, 520-720°C Substrate-to-targetdistance 7.5 cm (on-axis) 5.0 cm (off-axis) Deposition rate 700 ,~/h (for 150 W) 600 A/h (for 120 W)
mm x 0.5 mm) was used as a substrate and it was located at on-axis or off-axis position to the target. The temperature of the substrate was controlled by the thermocouple put in the heating block. The temperature was, beforehand, calibrated by a thermocoupie in contact with the surface of the heating block. Table 1 shows the sputtering parameters. The sputtering pressure was controlled by the exhaust throttle valve and the mass flow controller. The crystal structure of the film was analyzed by an X-ray diffractometer (Rigaku D/Max-38) with monochromatic CuK~x radiation and the resistance of the films was measured by the DC four-probe method.
3. Results and discussion 3.1. Resputtering particles during sputter deposition
There are not only neutrals but also ions within the sputtering plasma. The ions can be divided into the positive ions like Ar + or 02 + and the negative ions like electron or oxygen anions. Most of the particles within the plasma are, however, neutrals such as Ar and 0 2, including various sputtered species. Therefore it is necessary to make clear whether the particles causing the resputterir~g of the growing film are ions or neutrals, and whether they are positive or negative if they are ions. For this purpose, we tried to apply various electrical potentials on the substrate during sputtering. The electrical potential applied on the substrate may influence the effect of the bombarding particles on the degree of the resputtering that is dependent on their charge. The schematic diagram applying the electrical potential to the substrate was shown in Fig. 1. The MgO substrate was held by the stainless steel holder and its middle area was overlapped by the A1 foil as shown in Fig. l(a). It is, then, expected that the electrons will be more accumulated on the exposed area of the insulating surface rather than on the grounded middle area by the A1 foil. This setup results in different electrical potentials depending on the area of the substrate. The substrate was on on-axis position and 7.5 cm from the target. The substrate temperature was kept to 200°C. The B i -
S.K. Park, J.H. Je /Physica C 254 (1995) 167-174
ate
(a)
MgO surface l'he deposited film
169
In Figs. 2(c) and (d), the dark areas show those where the films still remain, while the white ones those where the films were completely removed. Of course, the areas masked by the AI foil or the stainless steel holder were white because they had not been deposited even at the first deposition by using only Ar gas. It is very interesting that the middle area, which had not been masked by the AI foil as in Fig. 2(b), was also white as in Fig. 2(d). It indicates that any deposition did not take place on that area. The black stripes indicated by the arrows A in Fig. 2(c) are due to the over-masking by the AI foil at the second deposition. Meanwhile, white areas were also found even on the areas of the films near the AI foil or the holder. This means that the films were resputtered on those areas rather than deposited, when only O 2 was used as a sputtering gas at the second deposition.
(b) Fig. 1. (a) The schematic diagram for applying various electrical potentials to the substrate during sputter deposition by Ar plasma. (b) The schematic of the deposited film for 1.5 h at a pressure of 1 × 10 -2 Torr, at the substrate temperature of 200°C, and at an RF power of 150 W.
S r - C a - C u - O film was first deposited on this substrate by using only the Ar as a sputtering gas for 1.5 h at a pressure of 1 × 10 -2 Torr, and at a RF power of 150 W. Fig. l(b) shows the schematic of the deposited film. The deposited film was amorphous and uniform in thickness on the exposed area. However, the deposition was completely blocked on the areas masked by the AI foil and b y the stainless steel holder. For two substrates on which the amorphous B i S r - C a - C u - O film had been deposited like Fig. l(b) by using only Ar as in Fig. 1, the second deposition was tried again by using only 0 2 gas at a pressure of 1 X 10 -2 Torr, at a RF power of 120 W, and at a substrate temperature of 200°C. At this time, one of the substrates was also masked at the middle area by the AI foil as shown in Fig. 2(a), while the other was not as shown in Fig. 2(b). Figs. 2(c) and (d) show the optical micrographs of Figs. 2(a) and (b), respectively, after deposition by using only 0 2 gas for 40 min.
-~
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B
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Fig. 2. The schematic diagrams for the substrate designs during the sputter-deposition by the 0 2 plasma ((a) and (b)) and the optical micrographs of the resputtered films ((c) and (d)): (a) masked by the AI foil in the middle area, (b) non-masked,(c) the resputtered film of the design (a), (d) the resputtered film of the design (b).
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S.K. Park, J.H. Je /Physica C 254 (1995) 167-174
The exposed amorphous film is an electrical insulator, while the AI foil or the substrate holder which was grounded is the electrical conductor. Therefore the electrons can be accumulated on the insulating film and repel the negative ions arriving on the substrate. However, the A1 foil or the holder which has higher potentials than the insulating areas attracts negative ions from the plasma. Therefore, as the negative ions are attracted by the conducting areas with high potential, it may be possible that they also bombard the insulating areas in the immediate neighborhood of the conducting areas, resulting in the resputtering of the film on those areas as shown in Figs. 2(c) or (d). It is remarkable that any resputtering was not observed on the areas of the films near the middle area that had not been masked as shown in Fig. 2(d). This may also be related with the negative potential on the insulating area by the accumulation of electrons. As the two specimens of Figs. 2(c) and (d) were deposited for two more hours by using only the 02 gas for the third time, the films that had been still left were completely removed in both specimens. From the above results, we speculate that the resputtering particles are not neutral but charged particles. In the first deposition where the only Ar was used as a sputtering gas, the positive Ar + ions did much toward the deposition of the film rather than toward the resputtering of the film. Then it is considered that the resputtering particles are negative ions. Here the oxygen anion and electron are mainly considered as negative ions in the sputter deposition of the oxide film by Ar and/or O 2 gases. However, the mass of electrons is extremely small and they cannot contribute to the resputtering because it takes place by momentum transfer. Therefore, it is believed that the resputtering particle is the oxygen anion. Of course, the oxygen anion could be generated even during sputtering only by argon gas, because the target in itself contains the oxygen element. However, the amount of oxygen anion may not be sufficient that the amount of resputtering on the film could not exceed that of the deposition. In Figs. 2(c) and (d), the black bands were observed even on the areas which were masked by the holder, as indicated by the arrows B. They correspond to the films deposited during sputtering only by the Ar gas in the first deposition. The holder
could not make contact with the substrate as tightly as the AI foil. The black bands are just the deposited films by the diffuse scattering of the sputtered species in the gap between the holder and the substrate. It is surprising that these areas of the films were not resputtered during sputtering by the 0 2 gas, even if the areas of the films immediately adjacent to the holder were resputtered by the oxygen anions. From this result we speculate that the oxygen anions may arrive on the growing film in a straight line. Then the incoming oxygen anions can be blocked by the holder and the areas of the films indicated by the arrow B can be protected from the resputtering by the oxygen anions in the second deposition. It is also interesting that the black bands are observed at the top and the bottom of the middle area, which was not masked, as indicated by the arrow C in Fig. 2(d) unlike in Fig. 2(c). The films of these black bands were deposited not during the first, but during the second deposition, because the middle area had been masked by the AI foil in the first deposition. In fact, the sputtered fluxes are discharged from the target even during sputtering by only the 0 2 gas. They can be resputtered on the exposed surface by the oxygen anions as soon as they arrive on the substrate. However, the films of these black bands that had been also deposited by the diffuse scattering in the gap between the substrate and the holder could be kept from the resputtering by the oxygen anions. This result also indicates the linearity of the oxygen anions. The energy of the accelerated particle needed to resputter the film deposited on a substrate is known to be greater than 200 eV [20]. Therefore, it is expected that the average energy of the oxygen anions in this experiment could be greater than 200 eV during sputtering by the 0 2 gas. As the energy of the particle increases, its collision cross-section usually decreases [21]. Therefore, we speculate that the high energies of the oxygen anions which are greater than 200 eV can make it possible for them to pass through the plasma in a straight line.
3.2. Study on the prevention of the resputtering The resputtering of the growing film can make its composition different from that of the target in BiS r - C a - C u - O [11] and it should be avoided if possi-
171
S.K. Park, J.H. Je / Physica C 254 (1995) 167-174
ble. The clue for the solution of this problem is given by the result that the oxygen anions pass through the plasma in almost a straight line and arrive to the substrate. The idea for the prevention of the resputtering is
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to set up a shield near the substrate that can block the oxygen anions flying linearly to the substrate. Fig. 3(a) shows the schematic diagram for the off-axis sputtering. The substrate is located 5 cm above from the target. Fig. 3(b) shows the design of the shield that is parallel to the target surface and normal to the surface of the substrate. The square shield was set up on the bottom of the substrate and its size was 20 × 20 mm. After set up the shield, the B i - S r - C a - C u - O film was deposited on the MgO substrate even by using only the 02 for 5 h at the substrate temperature of 620°C, at the sputtering pressure of 2.5 × 10 -2 Torr, and at an RF power of 140 W. Just by using the shield, the resputtering could be prevented. Fig. 4
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172
S.K. Park, J.H. Je /Physica C 254 (1995) 167-174
shows the X-ray diffraction pattern of the deposited film. The 2212 phase with the c-axis orientation was observed. While the 2212 phase with the c-axis orientation is usually obtained at a substrate temperature above 720°C [1,5,17], the phase could be obtained at the lower temperature of 620°C in this experiment. Fig. 5 shows the temperature versus resistance curve for this sample. The resistance began to drop at 80 K and became zero at 65 K. The critical temperature is of course relatively low [22]. And yet it shows the typical result for the film of the 2212 phase that was not annealed after deposition. Fig. 6 shows the SEM image for the surface of the film. Extending over the whole area of the surface, it was very difficult to recognize any growth features. The surface was very glassy and smooth, compared with the other films fabricated by the in-situ deposition [2,23]. The composition of the film was Bi : Sr : Ca : Cu = 1.9 : 2.1 : 1.2 : 1.8 and was not greatly different
(a)
(b)
Fig. 7. Schematic diagrams of various shield designs: (a)the substrate fully exposed to the plasma, (b) the partially shielded substrate.
from that of the target (2.0 : 2.1 : 1.0 : 1.9), even if the film was deposited at the high temperature of 620°C. To sum up, a shield that can block the oxygen anions flying linearly to the growing film was designed. By using the shield, a very smooth film of the 2212 phase with the c-axis orientation could be grown. 3.3. Effect of the resputtering on the microstructure of the 2212 phase
ltxm Fig. 6. SEM image of the B i - S r - C a - C u - O thin film grown on
the shielded substrate.
The effect of the resputtering by the oxygen anions on the microstructure of the superconducting film was investigated. The idea for this study is that
S.K. Park, J.H. Je/Physica C 254 (1995) 167-174 ila) , •
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173
the preferred orientation was not the c-axis but the a-axis orientation as shown in Fig. 8(a). The surface became very rough due to the bombardment by the oxygen anions• When the substrate was fully blocked from the bombardment of the oxygen anion as in Fig. 3(b), the film showed the best crystallinity with a very smooth surface and the c-axis preferred orientation as in Fig. 4. However, when the substrate was half shielded from the bombardment of the oxygen anions, the film was divided into two regions• The upper region, which had been exposed to the bombardment of the oxygen anions, showed a very rough surface like the specimen of Fig. 7(a). On the other hand, the lower region, which had been shielded from the bombardment of the oxygen anions, showed a very smooth and shiny surface like that of Fig. 6. The X-ray diffraction pattern also shows a mixture of c-axis and a-axis orientations as shown in Fig. 8(b). To sum up, the bombardment by oxygen anions during sputtering decreased the crystallinity of the film and increased the surface roughness, and caused the a-axis orientation. 4. Conclusion
the extent of the resputtering might be dependent on the size of the shield. The substrate was in the off-axis position as in Fig. 3(a) and the exposing extent of the substrate to the oxygen anions was controlled with the size of the shield as shown in Figs. 7(a) and (b). In Fig. 7(a), there was no shield in the substrate holder for much more bombardment by the oxygen anions. In Fig. 7(b), the half-shield (20 X 7 mm) was set up. Meanwhile, the shield size in Fig. 3(b) was relatively large (20 X 20 mm) compared to that of the substrate. The sputtering was done by using only 02 as a sputtering gas for 5 h, at the substrate temperature of 620°C, at a sputtering pressure of 2.5 x 10 -2 Torr, and at a RF power of 140 W. Figs. 8(a) and (b) show the X-ray diffraction patterns for the films of Figs. 7 (a) and (b), respectively. When the substrate was fully exposed to the oxygen anions as in Fig. 7(a), the crystaUinity of the film was greatly degraded even if the depositing film was not resputtered completely unlike the on-axis sputtering as described in Section 3.1. In addition,
The B i - S r - C a - C u - O thin films were deposited by in-situ RF magnetron sputtering. When oxygen was used as a sputtering gas, the growing films were severely resputtered. The experimental results support the assertion that the resputtering particle is the oxygen anion• We speculate that the oxygen anions pass through the plasma and arrive on the substrate surface nearly in a straight line. The bombardment by the oxygen anions during sputtering decreased the crystallinity of the film and increased the surface roughness, and caused the a-axis orientation. By using the shield that was designed for the blocking of the oxygen anions flying linearly to the growing film, a very smooth film of the 2212 phase with the c-axis orientation could be grown. References [1] Y. Hakuraku, S. Higo, D. Miyagi and T. Ogushi, Jpn. J. Appl. Phys. 29 (1990) L600. [2] J.S. Moodera, A.M. Rao, A. Kussmaul and P.M. Tedrow, Appl. Phys. Lett. 57 (1990) 2498.
174
S.K. Park, J.H. Je /Physica C 254 (1995) 167-174
[3] K. Kuroda, K. Kojima, K. Yokohama and K. Hamanaka, Jpn. J. Appl. Phys. 29 (1990) L2033. [4] C.G. Kim and J.H. Je, Physica C 203 (1992) 385. [5] Y. Hakuraku, S. Higo and T. Ognshi, Appl. Phys. Left. 57 (1990) 925. [6] R.H. Hammond and R. Borman, Physica C 162-164 (1989) 703. [7] A.C. Westerheim, L.S. Yu-Jahes and A.C. Anderson, IEEE Trans. Magn. 27 (1991) 1001. [8] D.E. Morris, C.T. Hultgren, A.M. Markelz, J.Y.T. Wei, N.G. Asmar and J.H. Nickle, Phys. Rev. B 39 (1989) 6612. [9] G. Triscone, J.Y. Genoud, T. Graf, A. Junod and J. Muller, Physica C 176 (1991) 247. [10] E.E. Inameti, M.S. Raven, Y.M. Wan and B.G. Murray, Vacuum 43 (1992) 61. [11] J.M. Grace, D.B. Mcdonard, M.T. Reiten, J. Olson, R.T. Kampwirth and K.E. Gray, J. Vac. Sci. Technol. A 10 (1992) 1600. [12] J.J. Cuomo, R.J. Gamhino, J.M.E. Harper and J.D. Kuptisis, IBM J. Res. Dev. 21 (1977) 580.
[13] J.J. Cuomo, R.J. Gambino, J.M.E. Harper, J.D. Kuptisis and J.C. Webber, J. Vac. Sci. Technol. 15 (1978) 281. [14] S.I. Shah, Thin Solid Films 181 (1989) 157. [15] J.M. Grace, D.B. McDonald, M.T. Reiten, J. Olson, R.T. Kampwirth and K.E. Gray, J. Appl. Phys. 70 (1991) 3867. [16] Q.X. Jia and W.A. Anderson, Appl. Phys. Lett. 60 (1992) 2689. [17] P. Wagner, H. Adrain and C. Tom-Rosa, Physica C 19 (1992) 258. [18] S.N. Ermolov, V.A. Marchenko, V.Z. Rosenflantz and A.G. Znamenski, Thin Solid Films 204 (1991) 229. [19] D.J. Kester and R. Messier, J. Mater. Res. 8 (1993) 1928. [20] S.M. Rossnagel, in: Thin Film Processes II, eds. J.L. Vossen and W. Kern (Academic Press, San Diego, 1991) p. 30. [21] L.L. Chang, and R. Ludeke, in: Epitaxial Growth Part A, ed. J.W. Matthews (Academic Press, New York, 1975) p. 54. [22] V.V. Metlushko and G. Guntherodt, Appl. Phys. Left. 63 (1993) 2821. [23] D. Jedamzik, B.R. Barnard, M.R. Harrison, W.G. Freeman and P.J. Howard, Appl. Phys. Lett. 56 (1990) 1371.
Physica C 254 (1995)167-174
Resputtering of the superconducting Bi-Sr-Ca-Cu-O thin films S.K. Park, Jung Ho Je * Department of Materials Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja Dong, Nam Ku, Pohang, Kyungbuk, 790-784, South Korea
Received 1 August 1995
Abstract
Oxygen is normally used as a sputtering gas with or without argon for the sputter deposition of the B i - S r - C a - C u - O superconductor. Then the grown films are frequently found to have poor qualities by severe resputtering. The purpose of this study is to identify the resputtering particle during the sputter deposition and to investigate its effect on the film growth. The experimental results support the assertion that the resputtering particle is the oxygen anion. We speculate that the oxygen anions pass through the plasma and arrive on the substrate surface nearly in a straight line. The bombardment by the oxygen caused a decrease in the crystallinity and the increase in the surface roughness of the film, and caused the a-axis orientation. A method to prevent the resputtering of the film by the oxygen anions was developed and the very smooth film of the 2212 phase with the c-axis orientation could be grown.
1. Introduction
RF magnetron sputtering process is frequently used for the deposition of the B i - S r - C a - C u - O superconducting thin films [1-3]. The films deposited at relatively low temperatures have to be annealed at higher temperatures for their crystallization. However, in the process of the crystallization, they may react with the substrates specially near the interface, resulting in a very rough surface [4]. In addition, considering their application to electronic devices, the in-situ deposition process is recently very often carried out [2,5].
* Corresponding author.
It is well known that oxygen has to be added besides argon as sputtering gas for the in-situ deposition of oxide superconductors [6-9]. When oxygen gas or a mixture of high ratio of oxygen-to-argon gas is used as sputtering gas, the growing film is known to be severely resputtered [10-15]. The degrees of resputtering are variable according to the elements within the film. In case of the B i - S r - C a - C u - O system, the resputtering, in particular, can make the concentration of the Bi element very deficient [11]. To solve the problem of the resputtering, the in-situ deposition of the oxide superconductor was frequently carried out by off-axis sputtering [7,16]. However the problem of the resputtering could not be completely removed by this method. The method to increase the sputtering pressure above 0.1 Torr was also tried so that resputtering particles could be
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00538-2
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scattered before they arrive on the growing film [1,5,17]. However, the increase in the oxygen partial pressure caused the increase in the thermodynamic temperature for the formation of the superconducting phases [18] and the control of the film composition became more difficult. Therefore, for the fabrication of the superconducting film with good qualities, the oxygen partial pressure should be reduced to usual sputtering pressures. In addition, the problem of the resputtering should be minimized. For this purpose, the particle to involve in the resputtering has to be at first identified. So far, the resputtering particle has been asserted to be oxygen anion [16,19] or oxygen neutral with high energy [18]. The purpose of this study is to identify the resputtering particle and to investigate its effect on the growth of the B i - S r - C a - C u - O superconducting film by RF magnetron sputtering. The experimental results support the assertion that the resputtering particle is the oxygen anion. A method to prevent the resputtering of the film was developed and a film of the 2212 phase with the c-axis orientation could be grown.
2. Experimental procedure RF magnetron sputtering was used to deposit the B i - S r - C a - C u - O superconducting film. The sputtering target was fabricated as a disc of 4" diameter and its composition was Bi : Sr : Ca : Cu = 2.0 : 2.1 : 1.0 : 1.9. The MgO (100) single crystal (10 mm X 10
Table 1 Summary of the sputtering conditions Target Bi-Sr-Ca-Cu-O (2.0:2.1 : 1.0 : 1.9) Substrate MgO (100) 3 X 10- 6 Torr Base pressure Ar, O2 Sputtering gas Flow rate 10-15 sccm 1 × 10-2-4.5 X 10-2 Torr Working pressure RF Power 120-150 W Substrate temperature 200°C, 520-720°C Substrate-to-targetdistance 7.5 cm (on-axis) 5.0 cm (off-axis) Deposition rate 700 ,~/h (for 150 W) 600 A/h (for 120 W)
mm x 0.5 mm) was used as a substrate and it was located at on-axis or off-axis position to the target. The temperature of the substrate was controlled by the thermocouple put in the heating block. The temperature was, beforehand, calibrated by a thermocoupie in contact with the surface of the heating block. Table 1 shows the sputtering parameters. The sputtering pressure was controlled by the exhaust throttle valve and the mass flow controller. The crystal structure of the film was analyzed by an X-ray diffractometer (Rigaku D/Max-38) with monochromatic CuK~x radiation and the resistance of the films was measured by the DC four-probe method.
3. Results and discussion 3.1. Resputtering particles during sputter deposition
There are not only neutrals but also ions within the sputtering plasma. The ions can be divided into the positive ions like Ar + or 02 + and the negative ions like electron or oxygen anions. Most of the particles within the plasma are, however, neutrals such as Ar and 0 2, including various sputtered species. Therefore it is necessary to make clear whether the particles causing the resputterir~g of the growing film are ions or neutrals, and whether they are positive or negative if they are ions. For this purpose, we tried to apply various electrical potentials on the substrate during sputtering. The electrical potential applied on the substrate may influence the effect of the bombarding particles on the degree of the resputtering that is dependent on their charge. The schematic diagram applying the electrical potential to the substrate was shown in Fig. 1. The MgO substrate was held by the stainless steel holder and its middle area was overlapped by the A1 foil as shown in Fig. l(a). It is, then, expected that the electrons will be more accumulated on the exposed area of the insulating surface rather than on the grounded middle area by the A1 foil. This setup results in different electrical potentials depending on the area of the substrate. The substrate was on on-axis position and 7.5 cm from the target. The substrate temperature was kept to 200°C. The B i -
S.K. Park, J.H. Je /Physica C 254 (1995) 167-174
ate
(a)
MgO surface l'he deposited film
169
In Figs. 2(c) and (d), the dark areas show those where the films still remain, while the white ones those where the films were completely removed. Of course, the areas masked by the AI foil or the stainless steel holder were white because they had not been deposited even at the first deposition by using only Ar gas. It is very interesting that the middle area, which had not been masked by the AI foil as in Fig. 2(b), was also white as in Fig. 2(d). It indicates that any deposition did not take place on that area. The black stripes indicated by the arrows A in Fig. 2(c) are due to the over-masking by the AI foil at the second deposition. Meanwhile, white areas were also found even on the areas of the films near the AI foil or the holder. This means that the films were resputtered on those areas rather than deposited, when only O 2 was used as a sputtering gas at the second deposition.
(b) Fig. 1. (a) The schematic diagram for applying various electrical potentials to the substrate during sputter deposition by Ar plasma. (b) The schematic of the deposited film for 1.5 h at a pressure of 1 × 10 -2 Torr, at the substrate temperature of 200°C, and at an RF power of 150 W.
S r - C a - C u - O film was first deposited on this substrate by using only the Ar as a sputtering gas for 1.5 h at a pressure of 1 × 10 -2 Torr, and at a RF power of 150 W. Fig. l(b) shows the schematic of the deposited film. The deposited film was amorphous and uniform in thickness on the exposed area. However, the deposition was completely blocked on the areas masked by the AI foil and b y the stainless steel holder. For two substrates on which the amorphous B i S r - C a - C u - O film had been deposited like Fig. l(b) by using only Ar as in Fig. 1, the second deposition was tried again by using only 0 2 gas at a pressure of 1 X 10 -2 Torr, at a RF power of 120 W, and at a substrate temperature of 200°C. At this time, one of the substrates was also masked at the middle area by the AI foil as shown in Fig. 2(a), while the other was not as shown in Fig. 2(b). Figs. 2(c) and (d) show the optical micrographs of Figs. 2(a) and (b), respectively, after deposition by using only 0 2 gas for 40 min.
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S.K. Park, J.H. Je /Physica C 254 (1995) 167-174
The exposed amorphous film is an electrical insulator, while the AI foil or the substrate holder which was grounded is the electrical conductor. Therefore the electrons can be accumulated on the insulating film and repel the negative ions arriving on the substrate. However, the A1 foil or the holder which has higher potentials than the insulating areas attracts negative ions from the plasma. Therefore, as the negative ions are attracted by the conducting areas with high potential, it may be possible that they also bombard the insulating areas in the immediate neighborhood of the conducting areas, resulting in the resputtering of the film on those areas as shown in Figs. 2(c) or (d). It is remarkable that any resputtering was not observed on the areas of the films near the middle area that had not been masked as shown in Fig. 2(d). This may also be related with the negative potential on the insulating area by the accumulation of electrons. As the two specimens of Figs. 2(c) and (d) were deposited for two more hours by using only the 02 gas for the third time, the films that had been still left were completely removed in both specimens. From the above results, we speculate that the resputtering particles are not neutral but charged particles. In the first deposition where the only Ar was used as a sputtering gas, the positive Ar + ions did much toward the deposition of the film rather than toward the resputtering of the film. Then it is considered that the resputtering particles are negative ions. Here the oxygen anion and electron are mainly considered as negative ions in the sputter deposition of the oxide film by Ar and/or O 2 gases. However, the mass of electrons is extremely small and they cannot contribute to the resputtering because it takes place by momentum transfer. Therefore, it is believed that the resputtering particle is the oxygen anion. Of course, the oxygen anion could be generated even during sputtering only by argon gas, because the target in itself contains the oxygen element. However, the amount of oxygen anion may not be sufficient that the amount of resputtering on the film could not exceed that of the deposition. In Figs. 2(c) and (d), the black bands were observed even on the areas which were masked by the holder, as indicated by the arrows B. They correspond to the films deposited during sputtering only by the Ar gas in the first deposition. The holder
could not make contact with the substrate as tightly as the AI foil. The black bands are just the deposited films by the diffuse scattering of the sputtered species in the gap between the holder and the substrate. It is surprising that these areas of the films were not resputtered during sputtering by the 0 2 gas, even if the areas of the films immediately adjacent to the holder were resputtered by the oxygen anions. From this result we speculate that the oxygen anions may arrive on the growing film in a straight line. Then the incoming oxygen anions can be blocked by the holder and the areas of the films indicated by the arrow B can be protected from the resputtering by the oxygen anions in the second deposition. It is also interesting that the black bands are observed at the top and the bottom of the middle area, which was not masked, as indicated by the arrow C in Fig. 2(d) unlike in Fig. 2(c). The films of these black bands were deposited not during the first, but during the second deposition, because the middle area had been masked by the AI foil in the first deposition. In fact, the sputtered fluxes are discharged from the target even during sputtering by only the 0 2 gas. They can be resputtered on the exposed surface by the oxygen anions as soon as they arrive on the substrate. However, the films of these black bands that had been also deposited by the diffuse scattering in the gap between the substrate and the holder could be kept from the resputtering by the oxygen anions. This result also indicates the linearity of the oxygen anions. The energy of the accelerated particle needed to resputter the film deposited on a substrate is known to be greater than 200 eV [20]. Therefore, it is expected that the average energy of the oxygen anions in this experiment could be greater than 200 eV during sputtering by the 0 2 gas. As the energy of the particle increases, its collision cross-section usually decreases [21]. Therefore, we speculate that the high energies of the oxygen anions which are greater than 200 eV can make it possible for them to pass through the plasma in a straight line.
3.2. Study on the prevention of the resputtering The resputtering of the growing film can make its composition different from that of the target in BiS r - C a - C u - O [11] and it should be avoided if possi-
171
S.K. Park, J.H. Je / Physica C 254 (1995) 167-174
ble. The clue for the solution of this problem is given by the result that the oxygen anions pass through the plasma in almost a straight line and arrive to the substrate. The idea for the prevention of the resputtering is
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to set up a shield near the substrate that can block the oxygen anions flying linearly to the substrate. Fig. 3(a) shows the schematic diagram for the off-axis sputtering. The substrate is located 5 cm above from the target. Fig. 3(b) shows the design of the shield that is parallel to the target surface and normal to the surface of the substrate. The square shield was set up on the bottom of the substrate and its size was 20 × 20 mm. After set up the shield, the B i - S r - C a - C u - O film was deposited on the MgO substrate even by using only the 02 for 5 h at the substrate temperature of 620°C, at the sputtering pressure of 2.5 × 10 -2 Torr, and at an RF power of 140 W. Just by using the shield, the resputtering could be prevented. Fig. 4
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172
S.K. Park, J.H. Je /Physica C 254 (1995) 167-174
shows the X-ray diffraction pattern of the deposited film. The 2212 phase with the c-axis orientation was observed. While the 2212 phase with the c-axis orientation is usually obtained at a substrate temperature above 720°C [1,5,17], the phase could be obtained at the lower temperature of 620°C in this experiment. Fig. 5 shows the temperature versus resistance curve for this sample. The resistance began to drop at 80 K and became zero at 65 K. The critical temperature is of course relatively low [22]. And yet it shows the typical result for the film of the 2212 phase that was not annealed after deposition. Fig. 6 shows the SEM image for the surface of the film. Extending over the whole area of the surface, it was very difficult to recognize any growth features. The surface was very glassy and smooth, compared with the other films fabricated by the in-situ deposition [2,23]. The composition of the film was Bi : Sr : Ca : Cu = 1.9 : 2.1 : 1.2 : 1.8 and was not greatly different
(a)
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from that of the target (2.0 : 2.1 : 1.0 : 1.9), even if the film was deposited at the high temperature of 620°C. To sum up, a shield that can block the oxygen anions flying linearly to the growing film was designed. By using the shield, a very smooth film of the 2212 phase with the c-axis orientation could be grown. 3.3. Effect of the resputtering on the microstructure of the 2212 phase
ltxm Fig. 6. SEM image of the B i - S r - C a - C u - O thin film grown on
the shielded substrate.
The effect of the resputtering by the oxygen anions on the microstructure of the superconducting film was investigated. The idea for this study is that
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173
the preferred orientation was not the c-axis but the a-axis orientation as shown in Fig. 8(a). The surface became very rough due to the bombardment by the oxygen anions• When the substrate was fully blocked from the bombardment of the oxygen anion as in Fig. 3(b), the film showed the best crystallinity with a very smooth surface and the c-axis preferred orientation as in Fig. 4. However, when the substrate was half shielded from the bombardment of the oxygen anions, the film was divided into two regions• The upper region, which had been exposed to the bombardment of the oxygen anions, showed a very rough surface like the specimen of Fig. 7(a). On the other hand, the lower region, which had been shielded from the bombardment of the oxygen anions, showed a very smooth and shiny surface like that of Fig. 6. The X-ray diffraction pattern also shows a mixture of c-axis and a-axis orientations as shown in Fig. 8(b). To sum up, the bombardment by oxygen anions during sputtering decreased the crystallinity of the film and increased the surface roughness, and caused the a-axis orientation. 4. Conclusion
the extent of the resputtering might be dependent on the size of the shield. The substrate was in the off-axis position as in Fig. 3(a) and the exposing extent of the substrate to the oxygen anions was controlled with the size of the shield as shown in Figs. 7(a) and (b). In Fig. 7(a), there was no shield in the substrate holder for much more bombardment by the oxygen anions. In Fig. 7(b), the half-shield (20 X 7 mm) was set up. Meanwhile, the shield size in Fig. 3(b) was relatively large (20 X 20 mm) compared to that of the substrate. The sputtering was done by using only 02 as a sputtering gas for 5 h, at the substrate temperature of 620°C, at a sputtering pressure of 2.5 x 10 -2 Torr, and at a RF power of 140 W. Figs. 8(a) and (b) show the X-ray diffraction patterns for the films of Figs. 7 (a) and (b), respectively. When the substrate was fully exposed to the oxygen anions as in Fig. 7(a), the crystaUinity of the film was greatly degraded even if the depositing film was not resputtered completely unlike the on-axis sputtering as described in Section 3.1. In addition,
The B i - S r - C a - C u - O thin films were deposited by in-situ RF magnetron sputtering. When oxygen was used as a sputtering gas, the growing films were severely resputtered. The experimental results support the assertion that the resputtering particle is the oxygen anion• We speculate that the oxygen anions pass through the plasma and arrive on the substrate surface nearly in a straight line. The bombardment by the oxygen anions during sputtering decreased the crystallinity of the film and increased the surface roughness, and caused the a-axis orientation. By using the shield that was designed for the blocking of the oxygen anions flying linearly to the growing film, a very smooth film of the 2212 phase with the c-axis orientation could be grown. References [1] Y. Hakuraku, S. Higo, D. Miyagi and T. Ogushi, Jpn. J. Appl. Phys. 29 (1990) L600. [2] J.S. Moodera, A.M. Rao, A. Kussmaul and P.M. Tedrow, Appl. Phys. Lett. 57 (1990) 2498.
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[3] K. Kuroda, K. Kojima, K. Yokohama and K. Hamanaka, Jpn. J. Appl. Phys. 29 (1990) L2033. [4] C.G. Kim and J.H. Je, Physica C 203 (1992) 385. [5] Y. Hakuraku, S. Higo and T. Ognshi, Appl. Phys. Left. 57 (1990) 925. [6] R.H. Hammond and R. Borman, Physica C 162-164 (1989) 703. [7] A.C. Westerheim, L.S. Yu-Jahes and A.C. Anderson, IEEE Trans. Magn. 27 (1991) 1001. [8] D.E. Morris, C.T. Hultgren, A.M. Markelz, J.Y.T. Wei, N.G. Asmar and J.H. Nickle, Phys. Rev. B 39 (1989) 6612. [9] G. Triscone, J.Y. Genoud, T. Graf, A. Junod and J. Muller, Physica C 176 (1991) 247. [10] E.E. Inameti, M.S. Raven, Y.M. Wan and B.G. Murray, Vacuum 43 (1992) 61. [11] J.M. Grace, D.B. Mcdonard, M.T. Reiten, J. Olson, R.T. Kampwirth and K.E. Gray, J. Vac. Sci. Technol. A 10 (1992) 1600. [12] J.J. Cuomo, R.J. Gamhino, J.M.E. Harper and J.D. Kuptisis, IBM J. Res. Dev. 21 (1977) 580.
[13] J.J. Cuomo, R.J. Gambino, J.M.E. Harper, J.D. Kuptisis and J.C. Webber, J. Vac. Sci. Technol. 15 (1978) 281. [14] S.I. Shah, Thin Solid Films 181 (1989) 157. [15] J.M. Grace, D.B. McDonald, M.T. Reiten, J. Olson, R.T. Kampwirth and K.E. Gray, J. Appl. Phys. 70 (1991) 3867. [16] Q.X. Jia and W.A. Anderson, Appl. Phys. Lett. 60 (1992) 2689. [17] P. Wagner, H. Adrain and C. Tom-Rosa, Physica C 19 (1992) 258. [18] S.N. Ermolov, V.A. Marchenko, V.Z. Rosenflantz and A.G. Znamenski, Thin Solid Films 204 (1991) 229. [19] D.J. Kester and R. Messier, J. Mater. Res. 8 (1993) 1928. [20] S.M. Rossnagel, in: Thin Film Processes II, eds. J.L. Vossen and W. Kern (Academic Press, San Diego, 1991) p. 30. [21] L.L. Chang, and R. Ludeke, in: Epitaxial Growth Part A, ed. J.W. Matthews (Academic Press, New York, 1975) p. 54. [22] V.V. Metlushko and G. Guntherodt, Appl. Phys. Left. 63 (1993) 2821. [23] D. Jedamzik, B.R. Barnard, M.R. Harrison, W.G. Freeman and P.J. Howard, Appl. Phys. Lett. 56 (1990) 1371.