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Iron diffusion from pure Fe substrate into TiN buffer layers
ELSEVIER
PhysicaC 241 (1995) 397-400
Iron diffusion from pure Fe substrate into TiN buffer layers G.I. Grigorov a,., K.G. Grigorov a, M. Stojanova a, J.L. Vignes b, j.p. Langeron b, P. Denjean b, L. Ranno c a Institute of Electronics, Bulgarian Academy of Sciences, 72, Blvd. Tzarigradsko Chaussee, BG- 1784 Sofia, Bulgaria b CECM CNRS, 15, rue G. Urbain, F-94407 Vitry/Seine Cedex, France ¢ Univ. Paris VII (URA 17), Tour 23, 2, PI. Jussieu, 75251 Paris, France
Received 10 October 1994
Abstract
The diffusivity of iron in TiN films has been determined in samples prepared by reactive evaporation of Ti in N2 atmosphere on silicon substrates followed by evaporation of pure iron. The iron diffusion profiles have been investigated by 2 MeV 4He+ Rutherford backscattering spectroscopy (RBS) after annealing at temperatures up to 600°C. The diffusivity from 200°C to 600°C, D [m2/s ] -- 1.4 X 10-~5 exp [ -46/(RT) ] is rather high when compared to the diffusivity of other atom species, as for example Si and Al, in TiN films.
1. Introduction
There has been considerable interest, since the early eighties, in refractory-metal nitride films, especially in titanium nitride, which has been investigated for applications as a conducting film and passive diffusion barrier in semiconductor technology. This is clue to favourable characteristics of this material: high temperature stability, low resistivity and stable contact resistance, and low diffusivity of different species through the barrier layer [ 1-9 ]. Immediately after the advent of high-To superconductivity (HTS), superconducting thin films because the subject of many investigations inasmuch as a major impact of HTS is expected in the area ofmicroelectronics. Deposition of HTS films with good properties on practical substrates-Si, SiO2, GaAsneeds, in general, the use of barrier layers preventing substrate and film material interdiffusion. In the lat* Correspondingauthor.
ter case, as shown by the authors [ 10 ], TiN barrier layers are also efficient. In addition to device applications, thin film technologies of HTS may have many other potential applications, including power applications-wires and cables, high-field superconducting magnets, power electronic devices, electro-magnetic shields, etc.. For these purposes the HTS films have to be deposited onto flexible metallic substrates. However, YBa2Cu307_6 (YBCO) thin films on metallic substrates show poor superconducting properties [ 11,12 ], due to extensive interdiffusion between the film and substrate material. Hence the need for barrier layers. Stainless steel (SS) and Ni-based oxidation-resistant alloys (hastelloy, inconel ) are mainly envisaged as practical flexible substrates. YBCO films with good properties have been deposited on such substrates using various barrier layers-Ag [ 13,14 ], amorphous MgO [ 15 ], ytria-stabilized zirconia [ 1619], as well as TiN [20]. A pulsed ablation technique is usually used and deposition temperatures for both superconducting and barrier films are typically
0921-4534/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSD10921-4534 ( 94 )02360-3
398
G.1. Grigorov et al. / Physica C 241 (1995) 397-400
550-650"C, while the barrier thickness ranges between 200 and 500 nm. The common problem which arises when depositing YBCO films on such kinds of substrates, as revealed by the investigations of Witanachchi et al. [ 14 ], is the problem of Fe diffusion from the substrate, detrimental to the superconducting properties of YBCO films. That is why a given barrier layer should be thick enough to prevent Fe penetration in the superconducting film. The minimum, efficient enough barrier thickness can be estimated if the Fe diffusivity through the barrier is known. This paper reports results on iron diffusivity, determined by RBS depth profiling of the diffused iron in TiN layers when Fe atoms come out from pure iron. In the case when Fe atoms originate from an iron-containing material like hastelloy or SS, a lower diffusivity than that in the former case could be expected (see Section 3). Hence, an efficient enough barrier thickness determined on the basis of the diffusivity reported here should reasonably well prevent iron penetration in the YBCO film when practical substrates are used.
2. Titanium nitride and iron layers Samples for RBS depth profiling were prepared by sequential deposition of 150 nm TiN, 150 nm Fe and 20 nm TiN on (100) silicon in the same chamber, carried out by evaporation of Ti and Fe. TiN layers have been deposited by reactive evaporation of Ti in an N2 atmosphere. The deposition conditions are as follows: substrate temperature 60-70°C, background pressure 5X 10 -6 Pa, N2 pressure 5X 10 - 4 Pa and deposition rate of some 10- 9 g c m - 2 s- 1. Iron layers are deposited in vacuo (p< 10 -s Pa during the evaporation) at a rate of 20 A min -~ (few 10 -s g cm - : s-~ ). The nitrogen-to-titanium ratio in the TiN films is 1.0, as determined by the AES procedure proposed by the authors [ 21 ]. X-ray diffraction is used to study the phase composition and crystalline structure of the TiN films. They consist of ~-TiN, ~t-Ti and a phase composed of a rather disordered Ti-N compound. Films are polycristaUine; the average grain size, as determined from halfwidth of the most pronounced (200) TiN and (011) Ti peaks by using Scherrer's equation, is respectively 25 nm and 20 nm. SEM measurements revealed a columnar growth, the
columns being separated by voids which occupy about a quarter of the layer volume. The structure of the iron films has not been investigated since iron reacts quickly with atmospheric oxygen and all measurements must be carried out in deep vacuum without exposing the films to atmosphere. The top layer of 20 nm TiN is deposited to ensure some protection of the iron film during annealing and RBS analysis. Information on iron film structure can be found in Ref. [22] where the films are deposited under similar conditions. The TiN/Fe/TiN/Si structures obtained have been annealed at 200, 300, 450 and 600°C in flowing argon at atmospheric pressure.
3. Experimental results and discussion Profiling of the T i N / F e / T i N / S i structures is performed by RBS analysis with 2 MeV 4He+ ions. The RBS spectra are evaluated using RUMP simulation [23]. The iron diffusion is calculated from the penetration profiles, as already described in a previous paper [8]. A problem which arises during sample processing is the frequent cracking and peeling of the iron layer when annealing at temperatures higher than about 500°C; at 700°C all the samples are virtually compromised. Strains in the layer together with a weakened adhesion explain this phenomenon. During the warm-up period the layer is subjected to compressive stress, while cooling the sample generates tensile stress. When the stress exceeds the film fracture strength, cracks will develop. Minute defects such as voids or chemical impurities act as sites where any applied stress is concentrated. Thus the fracture threshold can easily be exceeded at the sites of defects even though the bulk of the material remains below the threshold, leading to crack formation at these sites. In the case under consideration, according to the above mentioned, the iron layer cracking and peeling off can be attributed to the presence of a certain quantity (5-8 at.%) of oxygen at the Fe/TiN interface as proven by the RBS and AES elemental depth profiling. This oxygen has been captured by the TiN layer during the brief vacuum break-down in the deposition chamber, preceding the subsequent iron layer deposition. Oxygen, either as 02 or water vapour, is accommodated in the TiN film voids. During an-
G.L Grigorov et al. / Physica C 241 (1995) 397-400
nealing this oxygen stock enables the formation of a thin oxide layer, probably nonuniformly distributed (on a micro-scale) all over the interface. Samples which show visual cracks following an annealing cycle have not been RBS-analyzed. Table 1 displays the resuits obtained. To the extent that the TiN layers are multiphase, the determined diffusivity corresponds to a combined effect of diffusion, described by the effective diffusivity D. The Arrhenius relationship for D is then calculated by least squares fit to the experimental results. Hence, for the iron diffusivity in TiN layers, we obtain: D = 1.4× 10 - ~ exp[ -46/(kT) ]m2s -t . Generally, the possible mechanisms of diffusion are: bulk diffusion within the material grains, diffusion along grain boundaries, surface diffusion in voids and pinholes and diffusion along dislocations in grains. The low value of the activation energy of iron diffusion in TiN films (46 kJ mol -~ ) suggests that we encounter grain boundary diffusion together with a certain presence of surface diffusion in the TiN layer voids. It is worth noting that the diffusivity found is rather high when compared with the diffusivities of other atom species, e.g. silicon or aluminum in TiN films [8,9]. The determined diffusivity allows an estimate of an efficient enough TiN barrier-thickness. So, if we assume, arbitrarily, a deposition and processing time for both TiN and YBCO layers of 100 min at 600"C, the mean penetration depth d= 2 [Dt] ~/2 for iron atoms would be about 250 nm, i.e. a barrier thickness of 300 nm should be good enough. Table 1 Diffusivity results for Fe diffusion in TiN layers Annealing
Time
Diffusivity
temperature
( X 10 3 S)
[m2s-I1
3.6 3.6 1.8 1.8 3.6 1.8 1.8 1.8 1.8 1.8
1.5 X 10 -2° 2.0× 10 -20 5.0X 10 -2° 6.0× 10 -20 1.0X 10 -19
399
As already mentioned, Ni-based oxidation-resistant alloys and SS are envisaged as flexible substrates for practical purposes. In these cases we have to consider a rather complex barrier structure-the TiN layer itself together with an interposed oxygen- and chromium-rich layer on the substrate surface [ 24,14,18,25 ] which should result in lower iron diffusivity, hence, a relatively thinner barrier layer should be efficient enough. Preliminary results on SS/ TiN/Si structures indicate an iron diffusivity of about an order of magnitude lower than that mentioned above. Final results will be published after completion of the study in due course.
4. Conclusion The diffusion of iron in titanium nitride layers has been investigated in the temperature range of 200600°C. The iron diffusivity found is relatively high as established by RBS elemental depth profiling of multilayered structures. The iron diffusion takes place mainly along the grain boundaries. Considerably lower iron diffusivities could be expected when iron atoms originate from stainless steel or corrosion-resistant superalloys.
Acknowledgement This work was partially supported by the Bulgarian National Fund for Scientific Investigations (NFNI No F-64).
References
[K] 473 573
723
873
4 . 0 X 10 -19
5.5× 10 -19 7.0X 10 -19 3.0X10 -18 5.0X 10 -Is
[ 1 ] M.A. Nicolet, Thin Solid Films 52 (1978) 415. [2] C.Y. Ting, J. Vac. Sci. Technol. 21 (1978) 14. [3] C.Y. Ting, Thin Solid Films 119 (1984) 11. [4] M. Witmer, J. Vac. Sci. Technol. A 3 (1985) 1797. [ 5 ] S. Kanamori, Thin Solid Films 136 ( 1986 ) 195. 16] R.C. Ellwanger and J.M. Towner, Thin Solid Films 161 (1988) 289. [7] N. Fujimura, N. Nishida, T. Ito and Y. Nakayama, Mater. Sci. Eng. A 108 (1989) 153. [8] K.G. Grigorov, G.I. Grigorov, M. Stojanova, J.L. Vignes, J.P. Langeron, P. Denjean and J. Perriere, Appl. Phys. A 55 ( 1992 ) 502.
400
G.L Grigorov et al. / Physica C 241 (1995) 397-400
[9] G.I. Grigorov, K.G. Grigorov, M. Stojanova, J.L. Vignes, J.P. Langeron and P. Denjean, Appl. Phys. A 57 (1993) 195. [10]K.G. Grigorov, G.I. Grigorov, M.V. Stojanova, R.A. Chakalov, J.L. Vignes, J.P. Langeron, J. Perriere and P. Denjean, Vacuum 44 ( 1993 ) 1119. I11 ] T. Yamaguchi, S. Aoki, N. Sadakata, O. Kohno and H. Osanai, Appl. Phys. Lett. 55 (1989) 1581. [12] M. Ozaki, N. Harada, S. Akashita and J. Chang, in: Proc. Science and Technology of Thin Film Superconductors, Colorado Springs (Plenum, New York, 1988) p. 363. [ 13] R.E. Russo, R.P. Reade, J.M. Mc Millan and B.L. Olsen, J. Appl. Phys. 68 (1990) 1354. [14] S. Witanachchi, S. Patel, Y.Z. Zhu, H.S. Kwok and D.T. Shaw, J. Mater. Res. 5 (1990) 717. [ 15] J. Saitoch, M. Fukutomi, Y. Tanaka, T. Asano, H. Maeda and H. Takahara, Jpn. J. Appl. Phys. 29 (1990) L 1117. [ 16 ] E. Narumi, L.W. Song, E Yang, S. Patel, Y.H. Kao and D.T. Shaw, Appl. Phys. Lett. 56 (1990) 2684.
[17] A. Kumar, L. Ganapathi, S.M. Kanetkar and J. Narayan, Appl. Phys. Lett. 57 (1990) 2594. [ 18] A. Kumar, L. Ganapathi, S.M. Kanetkar and J. Narayan, J. Appl. Phys. 69 (1991) 2410. [ 19 ] R.P. Reade, P. Berdhal, R.E. Russo and S.M. Garrison, Appl. Phys. Lett. 61 (1992) 2231. [20] A. Kumar, J. Narayan and X. Chen, Appl. Phys. Lett. 61 (1992) 976. [21 ] J.L. Vignes, J.P. Langeron, G.I. Grigorov, I.N. Martev and M. Stoyanova, Vacuum 42 ( 1991 ) 151. [ 22 ] P. WiBmann and H. Zitzmann, Thin Solid Films 90 (1982) 329. [23 ] L.R. Doolitle, Nucl. Instrum. Methods B 9 (1985) 344. [24] A. Mosser and J. Werckmann, in: Proc. IV Int. Conf. Solid Surfaces and III Eur. Conf. Surface Science (Cannes, France, 1980) Suppl. Vide, Couches Minces, No 201, V.I, p. 559. [25 ] K. Baba, S. Nagata, R. Hatada, T. Daikoku and M. Hasaka, Nucl. Instrum. Methods B 80/81 (1993) 297.
PhysicaC 241 (1995) 397-400
Iron diffusion from pure Fe substrate into TiN buffer layers G.I. Grigorov a,., K.G. Grigorov a, M. Stojanova a, J.L. Vignes b, j.p. Langeron b, P. Denjean b, L. Ranno c a Institute of Electronics, Bulgarian Academy of Sciences, 72, Blvd. Tzarigradsko Chaussee, BG- 1784 Sofia, Bulgaria b CECM CNRS, 15, rue G. Urbain, F-94407 Vitry/Seine Cedex, France ¢ Univ. Paris VII (URA 17), Tour 23, 2, PI. Jussieu, 75251 Paris, France
Received 10 October 1994
Abstract
The diffusivity of iron in TiN films has been determined in samples prepared by reactive evaporation of Ti in N2 atmosphere on silicon substrates followed by evaporation of pure iron. The iron diffusion profiles have been investigated by 2 MeV 4He+ Rutherford backscattering spectroscopy (RBS) after annealing at temperatures up to 600°C. The diffusivity from 200°C to 600°C, D [m2/s ] -- 1.4 X 10-~5 exp [ -46/(RT) ] is rather high when compared to the diffusivity of other atom species, as for example Si and Al, in TiN films.
1. Introduction
There has been considerable interest, since the early eighties, in refractory-metal nitride films, especially in titanium nitride, which has been investigated for applications as a conducting film and passive diffusion barrier in semiconductor technology. This is clue to favourable characteristics of this material: high temperature stability, low resistivity and stable contact resistance, and low diffusivity of different species through the barrier layer [ 1-9 ]. Immediately after the advent of high-To superconductivity (HTS), superconducting thin films because the subject of many investigations inasmuch as a major impact of HTS is expected in the area ofmicroelectronics. Deposition of HTS films with good properties on practical substrates-Si, SiO2, GaAsneeds, in general, the use of barrier layers preventing substrate and film material interdiffusion. In the lat* Correspondingauthor.
ter case, as shown by the authors [ 10 ], TiN barrier layers are also efficient. In addition to device applications, thin film technologies of HTS may have many other potential applications, including power applications-wires and cables, high-field superconducting magnets, power electronic devices, electro-magnetic shields, etc.. For these purposes the HTS films have to be deposited onto flexible metallic substrates. However, YBa2Cu307_6 (YBCO) thin films on metallic substrates show poor superconducting properties [ 11,12 ], due to extensive interdiffusion between the film and substrate material. Hence the need for barrier layers. Stainless steel (SS) and Ni-based oxidation-resistant alloys (hastelloy, inconel ) are mainly envisaged as practical flexible substrates. YBCO films with good properties have been deposited on such substrates using various barrier layers-Ag [ 13,14 ], amorphous MgO [ 15 ], ytria-stabilized zirconia [ 1619], as well as TiN [20]. A pulsed ablation technique is usually used and deposition temperatures for both superconducting and barrier films are typically
0921-4534/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSD10921-4534 ( 94 )02360-3
398
G.1. Grigorov et al. / Physica C 241 (1995) 397-400
550-650"C, while the barrier thickness ranges between 200 and 500 nm. The common problem which arises when depositing YBCO films on such kinds of substrates, as revealed by the investigations of Witanachchi et al. [ 14 ], is the problem of Fe diffusion from the substrate, detrimental to the superconducting properties of YBCO films. That is why a given barrier layer should be thick enough to prevent Fe penetration in the superconducting film. The minimum, efficient enough barrier thickness can be estimated if the Fe diffusivity through the barrier is known. This paper reports results on iron diffusivity, determined by RBS depth profiling of the diffused iron in TiN layers when Fe atoms come out from pure iron. In the case when Fe atoms originate from an iron-containing material like hastelloy or SS, a lower diffusivity than that in the former case could be expected (see Section 3). Hence, an efficient enough barrier thickness determined on the basis of the diffusivity reported here should reasonably well prevent iron penetration in the YBCO film when practical substrates are used.
2. Titanium nitride and iron layers Samples for RBS depth profiling were prepared by sequential deposition of 150 nm TiN, 150 nm Fe and 20 nm TiN on (100) silicon in the same chamber, carried out by evaporation of Ti and Fe. TiN layers have been deposited by reactive evaporation of Ti in an N2 atmosphere. The deposition conditions are as follows: substrate temperature 60-70°C, background pressure 5X 10 -6 Pa, N2 pressure 5X 10 - 4 Pa and deposition rate of some 10- 9 g c m - 2 s- 1. Iron layers are deposited in vacuo (p< 10 -s Pa during the evaporation) at a rate of 20 A min -~ (few 10 -s g cm - : s-~ ). The nitrogen-to-titanium ratio in the TiN films is 1.0, as determined by the AES procedure proposed by the authors [ 21 ]. X-ray diffraction is used to study the phase composition and crystalline structure of the TiN films. They consist of ~-TiN, ~t-Ti and a phase composed of a rather disordered Ti-N compound. Films are polycristaUine; the average grain size, as determined from halfwidth of the most pronounced (200) TiN and (011) Ti peaks by using Scherrer's equation, is respectively 25 nm and 20 nm. SEM measurements revealed a columnar growth, the
columns being separated by voids which occupy about a quarter of the layer volume. The structure of the iron films has not been investigated since iron reacts quickly with atmospheric oxygen and all measurements must be carried out in deep vacuum without exposing the films to atmosphere. The top layer of 20 nm TiN is deposited to ensure some protection of the iron film during annealing and RBS analysis. Information on iron film structure can be found in Ref. [22] where the films are deposited under similar conditions. The TiN/Fe/TiN/Si structures obtained have been annealed at 200, 300, 450 and 600°C in flowing argon at atmospheric pressure.
3. Experimental results and discussion Profiling of the T i N / F e / T i N / S i structures is performed by RBS analysis with 2 MeV 4He+ ions. The RBS spectra are evaluated using RUMP simulation [23]. The iron diffusion is calculated from the penetration profiles, as already described in a previous paper [8]. A problem which arises during sample processing is the frequent cracking and peeling of the iron layer when annealing at temperatures higher than about 500°C; at 700°C all the samples are virtually compromised. Strains in the layer together with a weakened adhesion explain this phenomenon. During the warm-up period the layer is subjected to compressive stress, while cooling the sample generates tensile stress. When the stress exceeds the film fracture strength, cracks will develop. Minute defects such as voids or chemical impurities act as sites where any applied stress is concentrated. Thus the fracture threshold can easily be exceeded at the sites of defects even though the bulk of the material remains below the threshold, leading to crack formation at these sites. In the case under consideration, according to the above mentioned, the iron layer cracking and peeling off can be attributed to the presence of a certain quantity (5-8 at.%) of oxygen at the Fe/TiN interface as proven by the RBS and AES elemental depth profiling. This oxygen has been captured by the TiN layer during the brief vacuum break-down in the deposition chamber, preceding the subsequent iron layer deposition. Oxygen, either as 02 or water vapour, is accommodated in the TiN film voids. During an-
G.L Grigorov et al. / Physica C 241 (1995) 397-400
nealing this oxygen stock enables the formation of a thin oxide layer, probably nonuniformly distributed (on a micro-scale) all over the interface. Samples which show visual cracks following an annealing cycle have not been RBS-analyzed. Table 1 displays the resuits obtained. To the extent that the TiN layers are multiphase, the determined diffusivity corresponds to a combined effect of diffusion, described by the effective diffusivity D. The Arrhenius relationship for D is then calculated by least squares fit to the experimental results. Hence, for the iron diffusivity in TiN layers, we obtain: D = 1.4× 10 - ~ exp[ -46/(kT) ]m2s -t . Generally, the possible mechanisms of diffusion are: bulk diffusion within the material grains, diffusion along grain boundaries, surface diffusion in voids and pinholes and diffusion along dislocations in grains. The low value of the activation energy of iron diffusion in TiN films (46 kJ mol -~ ) suggests that we encounter grain boundary diffusion together with a certain presence of surface diffusion in the TiN layer voids. It is worth noting that the diffusivity found is rather high when compared with the diffusivities of other atom species, e.g. silicon or aluminum in TiN films [8,9]. The determined diffusivity allows an estimate of an efficient enough TiN barrier-thickness. So, if we assume, arbitrarily, a deposition and processing time for both TiN and YBCO layers of 100 min at 600"C, the mean penetration depth d= 2 [Dt] ~/2 for iron atoms would be about 250 nm, i.e. a barrier thickness of 300 nm should be good enough. Table 1 Diffusivity results for Fe diffusion in TiN layers Annealing
Time
Diffusivity
temperature
( X 10 3 S)
[m2s-I1
3.6 3.6 1.8 1.8 3.6 1.8 1.8 1.8 1.8 1.8
1.5 X 10 -2° 2.0× 10 -20 5.0X 10 -2° 6.0× 10 -20 1.0X 10 -19
399
As already mentioned, Ni-based oxidation-resistant alloys and SS are envisaged as flexible substrates for practical purposes. In these cases we have to consider a rather complex barrier structure-the TiN layer itself together with an interposed oxygen- and chromium-rich layer on the substrate surface [ 24,14,18,25 ] which should result in lower iron diffusivity, hence, a relatively thinner barrier layer should be efficient enough. Preliminary results on SS/ TiN/Si structures indicate an iron diffusivity of about an order of magnitude lower than that mentioned above. Final results will be published after completion of the study in due course.
4. Conclusion The diffusion of iron in titanium nitride layers has been investigated in the temperature range of 200600°C. The iron diffusivity found is relatively high as established by RBS elemental depth profiling of multilayered structures. The iron diffusion takes place mainly along the grain boundaries. Considerably lower iron diffusivities could be expected when iron atoms originate from stainless steel or corrosion-resistant superalloys.
Acknowledgement This work was partially supported by the Bulgarian National Fund for Scientific Investigations (NFNI No F-64).
References
[K] 473 573
723
873
4 . 0 X 10 -19
5.5× 10 -19 7.0X 10 -19 3.0X10 -18 5.0X 10 -Is
[ 1 ] M.A. Nicolet, Thin Solid Films 52 (1978) 415. [2] C.Y. Ting, J. Vac. Sci. Technol. 21 (1978) 14. [3] C.Y. Ting, Thin Solid Films 119 (1984) 11. [4] M. Witmer, J. Vac. Sci. Technol. A 3 (1985) 1797. [ 5 ] S. Kanamori, Thin Solid Films 136 ( 1986 ) 195. 16] R.C. Ellwanger and J.M. Towner, Thin Solid Films 161 (1988) 289. [7] N. Fujimura, N. Nishida, T. Ito and Y. Nakayama, Mater. Sci. Eng. A 108 (1989) 153. [8] K.G. Grigorov, G.I. Grigorov, M. Stojanova, J.L. Vignes, J.P. Langeron, P. Denjean and J. Perriere, Appl. Phys. A 55 ( 1992 ) 502.
400
G.L Grigorov et al. / Physica C 241 (1995) 397-400
[9] G.I. Grigorov, K.G. Grigorov, M. Stojanova, J.L. Vignes, J.P. Langeron and P. Denjean, Appl. Phys. A 57 (1993) 195. [10]K.G. Grigorov, G.I. Grigorov, M.V. Stojanova, R.A. Chakalov, J.L. Vignes, J.P. Langeron, J. Perriere and P. Denjean, Vacuum 44 ( 1993 ) 1119. I11 ] T. Yamaguchi, S. Aoki, N. Sadakata, O. Kohno and H. Osanai, Appl. Phys. Lett. 55 (1989) 1581. [12] M. Ozaki, N. Harada, S. Akashita and J. Chang, in: Proc. Science and Technology of Thin Film Superconductors, Colorado Springs (Plenum, New York, 1988) p. 363. [ 13] R.E. Russo, R.P. Reade, J.M. Mc Millan and B.L. Olsen, J. Appl. Phys. 68 (1990) 1354. [14] S. Witanachchi, S. Patel, Y.Z. Zhu, H.S. Kwok and D.T. Shaw, J. Mater. Res. 5 (1990) 717. [ 15] J. Saitoch, M. Fukutomi, Y. Tanaka, T. Asano, H. Maeda and H. Takahara, Jpn. J. Appl. Phys. 29 (1990) L 1117. [ 16 ] E. Narumi, L.W. Song, E Yang, S. Patel, Y.H. Kao and D.T. Shaw, Appl. Phys. Lett. 56 (1990) 2684.
[17] A. Kumar, L. Ganapathi, S.M. Kanetkar and J. Narayan, Appl. Phys. Lett. 57 (1990) 2594. [ 18] A. Kumar, L. Ganapathi, S.M. Kanetkar and J. Narayan, J. Appl. Phys. 69 (1991) 2410. [ 19 ] R.P. Reade, P. Berdhal, R.E. Russo and S.M. Garrison, Appl. Phys. Lett. 61 (1992) 2231. [20] A. Kumar, J. Narayan and X. Chen, Appl. Phys. Lett. 61 (1992) 976. [21 ] J.L. Vignes, J.P. Langeron, G.I. Grigorov, I.N. Martev and M. Stoyanova, Vacuum 42 ( 1991 ) 151. [ 22 ] P. WiBmann and H. Zitzmann, Thin Solid Films 90 (1982) 329. [23 ] L.R. Doolitle, Nucl. Instrum. Methods B 9 (1985) 344. [24] A. Mosser and J. Werckmann, in: Proc. IV Int. Conf. Solid Surfaces and III Eur. Conf. Surface Science (Cannes, France, 1980) Suppl. Vide, Couches Minces, No 201, V.I, p. 559. [25 ] K. Baba, S. Nagata, R. Hatada, T. Daikoku and M. Hasaka, Nucl. Instrum. Methods B 80/81 (1993) 297.