- Email: [email protected]
Reactive sputtering of Ti target with nonneutralized Ar beam
Solid State Communications, Vol. 99, No. 3, pp. 201-204, 1996 Copyright 0 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 003%1098/96 $12.00 + .OO
Pergamon
PI1SOO38-1098(96)00231-l REACTIVE SPUTTERING OF Ti TARGET WITH NONNEUTRALIZED W. B. Peng.
E. V. Sviridov*.
M.
Chen.
Department of Materials Science. Sichuan University. (Accepted
Ion-beam reactive co-sputtering used to study peculiarities
3 March
People’s Republic of China
1996 by Z. Gun)
system with separate metal targets of Pb, La and Ti was
of film growth under different sputtering energies.
Despite
the Ti target, the deposited films were
in the form of a uniform Pb film and drop-like islands
of Pb and La. The contamination increasing
J. G. Zhu. and D. Q. Xiao
Chengdu 610064,
the use of a single ion gun to sputter exclusively polluted with foreign elements
Ar BEAM
of uniform film with Pb film could be eliminated
by
the energy of sputtering ions, whereas the presence of foreign metal islands
remained.
The roles of both spreading the incident beam and arc-effect
Copyright
0 1996 Published by Elsevier Science Ltd
Keywords:
A. thin films, C. scanning electron microscopy,
are discussed.
E. atom, molecule and ion
impact.
Recently multi-ion beam reactive co-sputtering technique has been designed and involved in oxide ferroelectric tilm processing.l-4 In the technique, ions Corn independent guns sputter separate targets of desired components to deposit a multicomponent tihu on a nearby substrate. The most widely used ion gun is Kaufman-type ion source providing broad ion beam. To prevent ion beam spreading by space charge repulsion, and charging the insulating targets (if any) with positive ions, neutralization of the beam is recommended.6 This is usually realized by adding electrons emitted from thermionic filament or generated within hollow cathode. The use of emitter neutralizer is accompanied with a f&n contamination by the material of the filament, whereas hollow cathode neutralizer requires a high purity of inert gas for operation that hinders its application for the reactive sputtering. As we used metal grounded targets, we tried to avoid the drawbacks associated with utilization of neutralizer by using nonneutralized ion beams. This paper is to elucidate the intluence of reactive sputtering with nonneutralized ion beam on
the result of fihu deposition, and its dependence on ion energy. Thin film deposition was carried out using multiion-beam reactive co-sputtering technique described in detail elsewhere.t.4 The system contains separate targets of Pb, La and Ti located symmetrically around the system axis. Each target can be sputtered by independent Ar ion-beam source, however, in this work the Hrn deposition by sputtering exclusively the Ti target is discussed. The deposition was performed on unheated bare Si substrates in an atmosphere of Ar/OZ = 2.6/1.0 at a total gas pressure of 4.7x10-2 Pa. The energy of sputtering ions varied within 250-800 eV, the time of deposition was 40 minutes for each tilm. Film surface morphology was studied by SEM, and the local chemical composition of the fihns was estimated by EPMA with e-probe diameter of about 1 Pm. Figure 1 is a typical SEM micrograph of the films. At the magnification shown, one can see that the deposit is represented by isolated drop-like islands of about 0.5-10 um in diameter located occasionally on the smooth surface. The island sizes and the density of island population seemed to vary
*Permanent address: Institute of Physics, Rostov-onDon, 344104, Russia 201
202
REACTIVE
SPUTTERING
OF Ti TARGET
Vol. 99, No. 3
Pb content decreased), however, that for Ti was more noticeable, so that the fraction of Ti in the film Ti/(Ti+Pb) gradually icreased from 58% to 100%. Due to the equality of deposition times for different films, the increase in Pb and Ti contents in the film was indicative of the increase in the corresponding deposition rates with sputtering energy. It is noteworthy here, that the increase in deposition rate of Ti with Ei seemed to contradict the data on independence of sputtering rate of partially oxidized Ti from incident energy of Ar ions.2 Figure 1. Typical SEM micrograph of the deposit. Drop-like islands are melted Pb and La debris integrated into uniform sputtered f&n.
with the sputtering energy slightly. No relief peculiarities of the film among the islands could be observed up to magnification of 10 000. Semiquantitative analysis of film chemical composition was carried out in several points for each deposit. Besides sputtered Ti and sputtering Ar, Si of the underlying substrate was always detected due to a small film thickness. Moreover, Pb was present in any point of some films, and La appeared in some islands. According to EPMA, the chemical composition of the islands differed strongly from that of the area free of islands. Hereinafter we use the term “smooth film” to mean the deposit among the islands. Despite sputtering the Ti target exclusively, smooth film chemical composition corresponded to a mixture of Ti and Pb, and seemed to be uniform within each deposit. The averaged data on the chemical composition of smooth fihns sputtered at different energies of Ar ions Ei, are presented in Table 1. The oxygen content was not analyzed. The fraction of captured Ar was less than 1 at% and was not dependent on sputtering energy within the accuracy of analysis. One can see from the Table 1 that the fraction of Si of the substrate decreased with increasing the sputtering energy, demonstrating the increase in smooth tihn thickness with Ei. One can also see that this increase in film thickness was associated with the increase in both Ti and Pb quantities (except for the films #5 and #6, where the
Tablel. Film # Ei, eV
1. 2. 3. 4. 5. 6.
250 350 500 650 750 800
Smooth film chemical composition Si, at%
99 98 94 93 92 92
Ti, at%
0.6 1.2 4.4 5.0 7.5 8.0
Pb, at%
0.4 0.8 1.6 2.0 0.5 0.0
Ti/(Ti+Pb)
0.58 0.61 0.73 0.71 0.94 1.00
As for the islands, their composition strongly differed Corn island to island even within one deposit, and showed no correlation with Ei, The islands were greatly enriched by either Pb, as compared with smooth film, or La, which was totally absent in both smooth films and Pb-enriched islands, the number of Pb-enriched islands being much higher than that of Laenriched ones. To illustrate these peculiarities the chemical composition of some islands with sizes exceeding the diameter of the electron probe, is shown in Tables 2 and 3. The tirst figure of the island number in these Tables corresponds to the number of the deposit according to the Table 1, and the second figure shows the number of tested point. Both Ti and Pb contents of the smooth film of the respective deposits (Ti sf and Pb +” are also given in the Tables for convenience of comparison. The presence of Pb in all the points of the deposits ###l-5, and its uniform distribution within the smooth fihn indicate that a condensation of Pb from a gaseous phase took place, as well as that of Ti. As the targets were cooled well and, consequently, Pb could not be evaporated, the only possible reason of Pb appearance in such a phase might be the sputtering of
Vol. 99, No. 3
REACTIVE
SPUTTERING
the Pb target. This sputtering was caused, probably, by Ar ions from the ion beam directed onto Ti target and spreaded due to a space charge repulsion. Indeed, increasing the ion energy leads to decreasing ion beam spreading, that would result in decreasing the part of Pb target exposed to the ion beam bombardment and increase the number of ions bombarding the Ti target. The latter could be responsible for the increase in the rate of Ti with IQ observed in our deposition experiment, even at a constant sputtering yeild reported in Ref. 2 for almost the same deposition conditions. On the other hand, the increase in deposition rate of Pb with J?i taking place in smooth fihns ## l-4 was not so obvious. Actually, the decrease in ion beam spreading would cause the decrease in number of ions bombarding Pb target. However, this decrease could be initially compensated by the substantial increase in sputtering yeild with Ei, characteristic of the partly oxidized Pb at low sputtering energies.2 By further increasing Ei, the sputtering and deposition rates of Pb finally decreased (ti # 5) due to a strong reduction of number of ions bombarding Pb target and, at Ei = 800 eV, the beam spreading was overcome well enough to prevent the pollution of the smooth fihn with sputtered Pb (fihn # 6). The absense of La in smooth tihns indicated that noticeable sputtering of La did not occur. The contamination of smooth tihn with Pb only could be associated with a lower threshold sputtering energy and much higher sputtering yeild of Pb with respect to La or Ti.4,6 The chemical composition of La-enriched islands showed that the islands were scraps of La covered with (or, lying on) the uniform sputtered fihn (smooth tihn). As shown in Table 2, the La content in these islands is as much as 70-80 %, and the fractions of Pb and Ti in La-enriched islands are almost equal to those
Table 2. dhemical composition of La-enriched islands I&ud# Si @ W(Ti+Pb+La)
2.1 2.2 6.1
Ti @ Ti sf@
Pb@ Pb sf@
89
0.77
1.5
1.2
1.0
0.8
91 58
0.68 0.77
1.8 9.3
1.2 8.0
1.2 0.0
0.8 0.0
Q - in atom %
OF Ti TARGET
203
in smooth f&n of the respective deposit. The spherical shape of the islands seemed to indicate that La scraps appeared on the substrate as melted droplets. The Pb-enriched islands seemed to be scraps of melted Pb integrated into the uniform sputtered film, as well as La islands. The approximate equality of Ti content in the islands to that in smooth Chn of the respective deposit, shown in Table 3, confirmed the conclusion. This equality seemed to be violated for the “huge and thick” (according to SEM and EPMA data) islands # 2.3 and # 3.2. This apparent violation could be attributed to a low tilting angle of an electron probe (-15y. Indeed, if the island is covered with a smooth f?hn, electron probe penetrating through such island will cross the covering film twice. In this case, the measured concentration of Ti in the covering Chn must be approximately two times higher than real one, that is in a good agreement with the data for the islands # 2.3 and # 3.2. Table 3. Chemical composition of Pb-enriched islands Island # Ei, eV Si, at% Pb/(Ti+Pb)
1.1 2.3 3.1 3.2 4.1
250 350 500 500 650
98 10 88 49 80
0.72 0.97 0.59 0.78 0.62
Ti, at%
Ti sx a@6
0.6 2.9 5.0 11 5.6
0.6 1.2 4.4 4.4 5.0
The appearence of Pb and La islands in the Urns could be associated with sparking on the target surfaces always observed in the process of deposition. The occurence of spark or short duration arcs during ion beam sputtering could be attributed to the high electric potential of nonneutralized ion beam with respect to grounded metal parts of hardware. On the other hand, during reactive sputtering, oxygen oxidizes metal surfaces partly,7 so that they may become insulators. Being exposed to irradiation of charged particles produced during sputtering, such surfaces are able to keep charge, so that at certain electric potential, break-down of a nearby gas occurs also resulting in sparks appearance. Those charged particles could be the low-energetic ions sputtered or scattered from Ti target, or generated within the ion beam by highenergetic ions passing near background atoms or molecules. They might not be Ar ions of the spreaded incident beam, as the degree of spreading, according to the data on the degree of smooth Chn contamination
204
REiACTIVE SPUTTE~NG OF Ti TARGET
with sputtered Pb) varied strongly with sputtering energy, wheras the degree of fihn pollution with the splashed metal islands was ahnosrt independent from Ei. Just these low energetic ions could case charging the extraneous targets without their sputtering. Irrespective of what was the reason of arc development, the process must be accompanied with microexplosions on the target surfaces, which cause splashing the material from the target and transfer of the melted debris onto the s~s~ate, splashing the target material being responsible for the appearance of Pb and La drops on the substrate. The difference between the population densities of Pb and La islands (the number of Pb islands is much higher) and the absence of Ti islands can be attributed to the difference between the magnitudes of the melting temperatures. Pb possessing low melting point (T, = 327 ‘C),* would provide melted splashes easily, wheras tbe appearance of those of La (T,, = 920 “C) is more acult. Despite the sparks on Ti target surface were also observed, high melting point of Ti (T, = 1668 “C) suppressed probably splashing the metal and thus, the appearance of Ti melted debris on the substrate. To summarize, reactive sputtering of Ti target
Vol. 99, No. 3
exclusively in multi-ion co-sputtering system with nonneutralized ion beam is accompanied with the pollution of depositing Ti film with the material of extraneous targets in the form of uniform film of Pb is associated with the sputtering of Pb target due to nonneutralized ion beam spreading, and can be eliminated by the increase in ion energy. The Pb and La droplet appearance is attributed to the occurrence of sparks due to either difference between ion-beam and grounded metal target potentials or, charging the oxidized target surfaces (if it took place) with low-energetic ions produced during the process. Irrespective of the mechanism, it was caused by using nonneutralized ion beams. To overcome the problem, the use of the recently proposed neutralizer of cold cathode type, providing minimum film contamination and compatible with a reactive sputtering in an atmosphere containing oxygen,’ could be helpful. Current neutralization mode with an excess of electrons to compensate the possible charging of oxidized targets with low-energetic ions generated during the sputtering, is recommended. Acknowledgement - This project was supported by the National Advanced Materials Committee of China (NAMCC) and Natural Science Foundation of China (NSFC).
References
f 1]
D. Xiao, Z. Xiao, 3. Zhu, Y, Li, and H.Guo,
Ferroelectrics 108,59 (1990). [2] S. B. Krupanidhi, H. Hu, and V. Kumar, J. Appl. Phys. 71,376 (1992). [3] I. Kanno, T. Kamada, S. Hayashi, M. Kitagawa, and T. Hiraa, Jpn. J. Appl. Phys. 32-2, IL950 (1993). [4] D. Xiao, Z. Xiao, J. Zhu, D. Wang, H. Guo, B. Xie, and H. Yuan, Appl. Phys. Letters 58,36 (1991). [5] J. M. E. Harper, in: Thin Film Process, ed. by J.L.Vossen and W.Kern (Academic Press, New York, 1978) p.186.
[6] H. H. Anderson and H. L. Bay, in: ~~~erj~g by auricle bombardment I, ed. R. Behrisch (Springer Verlag, Berlin, 198 1) ch.4. [7] D.Q. Xiao, J.G. Zhu, Z.H. Qian, W.B. Peng, L. F. Wei, and Z.S. Li, in: 1994 MRS Fall A4eeting Procedings, v.354 Alfa Catalog. Research Chemicals and PI Accessories. 1990, Johnson Mattey Catalog Co., Inc. USA. [9] J. J. Cuomo, H. R. Kau&an, and S. M. U. S. Patent “Hollow Cathode”, Rossnagel, 4,633,129, Dec. 1986.
Pergamon
PI1SOO38-1098(96)00231-l REACTIVE SPUTTERING OF Ti TARGET WITH NONNEUTRALIZED W. B. Peng.
E. V. Sviridov*.
M.
Chen.
Department of Materials Science. Sichuan University. (Accepted
Ion-beam reactive co-sputtering used to study peculiarities
3 March
People’s Republic of China
1996 by Z. Gun)
system with separate metal targets of Pb, La and Ti was
of film growth under different sputtering energies.
Despite
the Ti target, the deposited films were
in the form of a uniform Pb film and drop-like islands
of Pb and La. The contamination increasing
J. G. Zhu. and D. Q. Xiao
Chengdu 610064,
the use of a single ion gun to sputter exclusively polluted with foreign elements
Ar BEAM
of uniform film with Pb film could be eliminated
by
the energy of sputtering ions, whereas the presence of foreign metal islands
remained.
The roles of both spreading the incident beam and arc-effect
Copyright
0 1996 Published by Elsevier Science Ltd
Keywords:
A. thin films, C. scanning electron microscopy,
are discussed.
E. atom, molecule and ion
impact.
Recently multi-ion beam reactive co-sputtering technique has been designed and involved in oxide ferroelectric tilm processing.l-4 In the technique, ions Corn independent guns sputter separate targets of desired components to deposit a multicomponent tihu on a nearby substrate. The most widely used ion gun is Kaufman-type ion source providing broad ion beam. To prevent ion beam spreading by space charge repulsion, and charging the insulating targets (if any) with positive ions, neutralization of the beam is recommended.6 This is usually realized by adding electrons emitted from thermionic filament or generated within hollow cathode. The use of emitter neutralizer is accompanied with a f&n contamination by the material of the filament, whereas hollow cathode neutralizer requires a high purity of inert gas for operation that hinders its application for the reactive sputtering. As we used metal grounded targets, we tried to avoid the drawbacks associated with utilization of neutralizer by using nonneutralized ion beams. This paper is to elucidate the intluence of reactive sputtering with nonneutralized ion beam on
the result of fihu deposition, and its dependence on ion energy. Thin film deposition was carried out using multiion-beam reactive co-sputtering technique described in detail elsewhere.t.4 The system contains separate targets of Pb, La and Ti located symmetrically around the system axis. Each target can be sputtered by independent Ar ion-beam source, however, in this work the Hrn deposition by sputtering exclusively the Ti target is discussed. The deposition was performed on unheated bare Si substrates in an atmosphere of Ar/OZ = 2.6/1.0 at a total gas pressure of 4.7x10-2 Pa. The energy of sputtering ions varied within 250-800 eV, the time of deposition was 40 minutes for each tilm. Film surface morphology was studied by SEM, and the local chemical composition of the fihns was estimated by EPMA with e-probe diameter of about 1 Pm. Figure 1 is a typical SEM micrograph of the films. At the magnification shown, one can see that the deposit is represented by isolated drop-like islands of about 0.5-10 um in diameter located occasionally on the smooth surface. The island sizes and the density of island population seemed to vary
*Permanent address: Institute of Physics, Rostov-onDon, 344104, Russia 201
202
REACTIVE
SPUTTERING
OF Ti TARGET
Vol. 99, No. 3
Pb content decreased), however, that for Ti was more noticeable, so that the fraction of Ti in the film Ti/(Ti+Pb) gradually icreased from 58% to 100%. Due to the equality of deposition times for different films, the increase in Pb and Ti contents in the film was indicative of the increase in the corresponding deposition rates with sputtering energy. It is noteworthy here, that the increase in deposition rate of Ti with Ei seemed to contradict the data on independence of sputtering rate of partially oxidized Ti from incident energy of Ar ions.2 Figure 1. Typical SEM micrograph of the deposit. Drop-like islands are melted Pb and La debris integrated into uniform sputtered f&n.
with the sputtering energy slightly. No relief peculiarities of the film among the islands could be observed up to magnification of 10 000. Semiquantitative analysis of film chemical composition was carried out in several points for each deposit. Besides sputtered Ti and sputtering Ar, Si of the underlying substrate was always detected due to a small film thickness. Moreover, Pb was present in any point of some films, and La appeared in some islands. According to EPMA, the chemical composition of the islands differed strongly from that of the area free of islands. Hereinafter we use the term “smooth film” to mean the deposit among the islands. Despite sputtering the Ti target exclusively, smooth film chemical composition corresponded to a mixture of Ti and Pb, and seemed to be uniform within each deposit. The averaged data on the chemical composition of smooth fihns sputtered at different energies of Ar ions Ei, are presented in Table 1. The oxygen content was not analyzed. The fraction of captured Ar was less than 1 at% and was not dependent on sputtering energy within the accuracy of analysis. One can see from the Table 1 that the fraction of Si of the substrate decreased with increasing the sputtering energy, demonstrating the increase in smooth tihn thickness with Ei. One can also see that this increase in film thickness was associated with the increase in both Ti and Pb quantities (except for the films #5 and #6, where the
Tablel. Film # Ei, eV
1. 2. 3. 4. 5. 6.
250 350 500 650 750 800
Smooth film chemical composition Si, at%
99 98 94 93 92 92
Ti, at%
0.6 1.2 4.4 5.0 7.5 8.0
Pb, at%
0.4 0.8 1.6 2.0 0.5 0.0
Ti/(Ti+Pb)
0.58 0.61 0.73 0.71 0.94 1.00
As for the islands, their composition strongly differed Corn island to island even within one deposit, and showed no correlation with Ei, The islands were greatly enriched by either Pb, as compared with smooth film, or La, which was totally absent in both smooth films and Pb-enriched islands, the number of Pb-enriched islands being much higher than that of Laenriched ones. To illustrate these peculiarities the chemical composition of some islands with sizes exceeding the diameter of the electron probe, is shown in Tables 2 and 3. The tirst figure of the island number in these Tables corresponds to the number of the deposit according to the Table 1, and the second figure shows the number of tested point. Both Ti and Pb contents of the smooth film of the respective deposits (Ti sf and Pb +” are also given in the Tables for convenience of comparison. The presence of Pb in all the points of the deposits ###l-5, and its uniform distribution within the smooth fihn indicate that a condensation of Pb from a gaseous phase took place, as well as that of Ti. As the targets were cooled well and, consequently, Pb could not be evaporated, the only possible reason of Pb appearance in such a phase might be the sputtering of
Vol. 99, No. 3
REACTIVE
SPUTTERING
the Pb target. This sputtering was caused, probably, by Ar ions from the ion beam directed onto Ti target and spreaded due to a space charge repulsion. Indeed, increasing the ion energy leads to decreasing ion beam spreading, that would result in decreasing the part of Pb target exposed to the ion beam bombardment and increase the number of ions bombarding the Ti target. The latter could be responsible for the increase in the rate of Ti with IQ observed in our deposition experiment, even at a constant sputtering yeild reported in Ref. 2 for almost the same deposition conditions. On the other hand, the increase in deposition rate of Pb with J?i taking place in smooth fihns ## l-4 was not so obvious. Actually, the decrease in ion beam spreading would cause the decrease in number of ions bombarding Pb target. However, this decrease could be initially compensated by the substantial increase in sputtering yeild with Ei, characteristic of the partly oxidized Pb at low sputtering energies.2 By further increasing Ei, the sputtering and deposition rates of Pb finally decreased (ti # 5) due to a strong reduction of number of ions bombarding Pb target and, at Ei = 800 eV, the beam spreading was overcome well enough to prevent the pollution of the smooth fihn with sputtered Pb (fihn # 6). The absense of La in smooth tihns indicated that noticeable sputtering of La did not occur. The contamination of smooth tihn with Pb only could be associated with a lower threshold sputtering energy and much higher sputtering yeild of Pb with respect to La or Ti.4,6 The chemical composition of La-enriched islands showed that the islands were scraps of La covered with (or, lying on) the uniform sputtered fihn (smooth tihn). As shown in Table 2, the La content in these islands is as much as 70-80 %, and the fractions of Pb and Ti in La-enriched islands are almost equal to those
Table 2. dhemical composition of La-enriched islands I&ud# Si @ W(Ti+Pb+La)
2.1 2.2 6.1
Ti @ Ti sf@
Pb@ Pb sf@
89
0.77
1.5
1.2
1.0
0.8
91 58
0.68 0.77
1.8 9.3
1.2 8.0
1.2 0.0
0.8 0.0
Q - in atom %
OF Ti TARGET
203
in smooth f&n of the respective deposit. The spherical shape of the islands seemed to indicate that La scraps appeared on the substrate as melted droplets. The Pb-enriched islands seemed to be scraps of melted Pb integrated into the uniform sputtered film, as well as La islands. The approximate equality of Ti content in the islands to that in smooth Chn of the respective deposit, shown in Table 3, confirmed the conclusion. This equality seemed to be violated for the “huge and thick” (according to SEM and EPMA data) islands # 2.3 and # 3.2. This apparent violation could be attributed to a low tilting angle of an electron probe (-15y. Indeed, if the island is covered with a smooth f?hn, electron probe penetrating through such island will cross the covering film twice. In this case, the measured concentration of Ti in the covering Chn must be approximately two times higher than real one, that is in a good agreement with the data for the islands # 2.3 and # 3.2. Table 3. Chemical composition of Pb-enriched islands Island # Ei, eV Si, at% Pb/(Ti+Pb)
1.1 2.3 3.1 3.2 4.1
250 350 500 500 650
98 10 88 49 80
0.72 0.97 0.59 0.78 0.62
Ti, at%
Ti sx a@6
0.6 2.9 5.0 11 5.6
0.6 1.2 4.4 4.4 5.0
The appearence of Pb and La islands in the Urns could be associated with sparking on the target surfaces always observed in the process of deposition. The occurence of spark or short duration arcs during ion beam sputtering could be attributed to the high electric potential of nonneutralized ion beam with respect to grounded metal parts of hardware. On the other hand, during reactive sputtering, oxygen oxidizes metal surfaces partly,7 so that they may become insulators. Being exposed to irradiation of charged particles produced during sputtering, such surfaces are able to keep charge, so that at certain electric potential, break-down of a nearby gas occurs also resulting in sparks appearance. Those charged particles could be the low-energetic ions sputtered or scattered from Ti target, or generated within the ion beam by highenergetic ions passing near background atoms or molecules. They might not be Ar ions of the spreaded incident beam, as the degree of spreading, according to the data on the degree of smooth Chn contamination
204
REiACTIVE SPUTTE~NG OF Ti TARGET
with sputtered Pb) varied strongly with sputtering energy, wheras the degree of fihn pollution with the splashed metal islands was ahnosrt independent from Ei. Just these low energetic ions could case charging the extraneous targets without their sputtering. Irrespective of what was the reason of arc development, the process must be accompanied with microexplosions on the target surfaces, which cause splashing the material from the target and transfer of the melted debris onto the s~s~ate, splashing the target material being responsible for the appearance of Pb and La drops on the substrate. The difference between the population densities of Pb and La islands (the number of Pb islands is much higher) and the absence of Ti islands can be attributed to the difference between the magnitudes of the melting temperatures. Pb possessing low melting point (T, = 327 ‘C),* would provide melted splashes easily, wheras tbe appearance of those of La (T,, = 920 “C) is more acult. Despite the sparks on Ti target surface were also observed, high melting point of Ti (T, = 1668 “C) suppressed probably splashing the metal and thus, the appearance of Ti melted debris on the substrate. To summarize, reactive sputtering of Ti target
Vol. 99, No. 3
exclusively in multi-ion co-sputtering system with nonneutralized ion beam is accompanied with the pollution of depositing Ti film with the material of extraneous targets in the form of uniform film of Pb is associated with the sputtering of Pb target due to nonneutralized ion beam spreading, and can be eliminated by the increase in ion energy. The Pb and La droplet appearance is attributed to the occurrence of sparks due to either difference between ion-beam and grounded metal target potentials or, charging the oxidized target surfaces (if it took place) with low-energetic ions produced during the process. Irrespective of the mechanism, it was caused by using nonneutralized ion beams. To overcome the problem, the use of the recently proposed neutralizer of cold cathode type, providing minimum film contamination and compatible with a reactive sputtering in an atmosphere containing oxygen,’ could be helpful. Current neutralization mode with an excess of electrons to compensate the possible charging of oxidized targets with low-energetic ions generated during the sputtering, is recommended. Acknowledgement - This project was supported by the National Advanced Materials Committee of China (NAMCC) and Natural Science Foundation of China (NSFC).
References
f 1]
D. Xiao, Z. Xiao, 3. Zhu, Y, Li, and H.Guo,
Ferroelectrics 108,59 (1990). [2] S. B. Krupanidhi, H. Hu, and V. Kumar, J. Appl. Phys. 71,376 (1992). [3] I. Kanno, T. Kamada, S. Hayashi, M. Kitagawa, and T. Hiraa, Jpn. J. Appl. Phys. 32-2, IL950 (1993). [4] D. Xiao, Z. Xiao, J. Zhu, D. Wang, H. Guo, B. Xie, and H. Yuan, Appl. Phys. Letters 58,36 (1991). [5] J. M. E. Harper, in: Thin Film Process, ed. by J.L.Vossen and W.Kern (Academic Press, New York, 1978) p.186.
[6] H. H. Anderson and H. L. Bay, in: ~~~erj~g by auricle bombardment I, ed. R. Behrisch (Springer Verlag, Berlin, 198 1) ch.4. [7] D.Q. Xiao, J.G. Zhu, Z.H. Qian, W.B. Peng, L. F. Wei, and Z.S. Li, in: 1994 MRS Fall A4eeting Procedings, v.354 Alfa Catalog. Research Chemicals and PI Accessories. 1990, Johnson Mattey Catalog Co., Inc. USA. [9] J. J. Cuomo, H. R. Kau&an, and S. M. U. S. Patent “Hollow Cathode”, Rossnagel, 4,633,129, Dec. 1986.