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Experimental studies of metal film nucleation and growth on amorphous substrates
Thin Solid Films, 32 (1976) 233-236 © Elsevier Sequoia S.A., Lausanne-Printed in Switzerland
233
EXPERIMENTAL STUDIES OF METAL FILM NUCLEATION AND GROWTH ON AMORPHOUS SUBSTRATES* J. F. HAMILTON, P. C. LOGEL AND R. C. BAETZOLD Research Laboratories, Eastman Kodak Company, Rochester, N. Y. 14650 (U.S.A.)
(Received August 25, 1975)
The nuclei formed in the early stages of thin film deposition are useful in the study of size effects in heterogeneous catalysis. Apparatus to produce a fairly large number of reproducibly controlled samples with modulated coverages over a wide range has been described I'2. A quartz-crystal-type controller is used to set and to maintain the deposition rate, and a shutter in the form of a slotted belt is moved past a substrate at a constant speed to regulate the time of deposition. The apparatus is ion pumped but not bakeable, and pressures in the 10-7 Torr range are normal. Substrates used have been primarily either evaporated carbon or evaporated SiO 2, made just prior to depositing the nuclei, but a few deposits have been made on glass or polyester film. Nuclei of silver, gold, copper and palladium have been studied. Most of the interesting size effects are in the range below that which can be directly characterized by electron microscopy. However, a possible method for selectively growing these subvisible nuclei to resolvable size depends upon the prediction a-~ that, if homogeneous nucleation is the governing mechanism, there is a direct dependence of nucleation rate on deposit rate. Therefore it should be possible to form nuclei at one rate, and then to grow them to visible size at a much lower rate, for which the nucleation probability is negligible. Owing to the potential usefulness of this technique, it has been explored extensively in our laboratory. Unfortunately, it is not successful in any of the combinations of substrate and nuclei that we have studied, because the saturation density is in fact totally independent of deposition rate. The lack of a rate dependence is a significant result, for it rules out homogeneous two-dimensional nucleation as an important process, and attests to the presence of a limited number of sites with some particular activity for capturing the atoms of the incident metal. Thus the saturation nucleus density gives essentially the density of the active sites for the particular metal being deposited. Determination of an apparent sticking coefficient from electron micrographs of nuclei at or above the coverage for saturation density gives values of the order 0.1 or less. Thus the deposits fit into the category termed by Stowell and Hutchinson 7 and Venables s as "extreme incomplete condensation", during which essentially all growth occurs by direct impingement; surface diffusion of adatoms to existing clusters is unimportant. This condition should be characterized by a linear increase in mean particle radius with coverages; experimental results confirm this relationship for all of the metal-substrate combinations in question. The intercepts of these graphs are related to * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applicalions and Trends", Budapest, Hungary, August 25-29, 1975; Paper 2-29.
234
J. F. HAMILTON,P. C. LOGEL, R. C. BAETZOLD
the initial capture radii for the active sites involved, and these are found to be in the range of atomic dimensions. Thus the nucleation and growth appears to be a purely statistical process of single and then multiple occupancy of active substrate sites. It is governed by Poisson statistics, except as affected by the increased capture probability as a particle grows in area. Given this dependence, the particle size distribution may be specified for any coverage lower than that at which coalescence becomes important. The density of nuclei of size i or larger increases with the ith power of the incident coverage. A confirmation of this predicted relation could be obtained if it were possible to achieve selective growth by lowering the deposit rate as discussed earlier. Although this technique is not successful, it is possible to achieve selective growth of existing nuclei by using a second metal with a lower saturation nucleus density. Zinc, which self-nucleates with extreme difficulty on these substrates 8'9, will grow on nuclei of any of the metals studied. When the densities of these Zn-grown nuclei are plotted against the incident coverage of the nucleating metal, the dependence is found to be linear, indicating that any active site that is occupied by at least one atom of the nucleating metal will serve as a growth center for the Zn. The fit to the theoretical curves gives a value for the active site capture area which is in satisfactory agreement with the values determined from visible particles. Analytical determinations of the sticking coefficients were made by atomic absorption spectroscopy and neutron-activated radioactive decay. Results show that essentially all of the incident metal atoms are quantitatively retained on the sample. The actual sticking coefficients are near unity, although the apparent values determined from electron micrographs are lower by an order of magnitude. Thus the incident metal atoms that are not captured by active sites are not simply re-evaporated, but are retained on the substrate and in some way deactivated so that they do not participate in forming the nuclei. We have speculated I that they might diffuse into the substrates, whose amorphous structure we view as being rather open. However, other proposals have been made l°. Depth profiles determined by Auger spectroscopy and sputter etching indicate that the metal is distributed within the thickness of the substrate. However, there are questions about the uniformity of the sputter-etch process, and also the posssibility that surface atoms are driven into the film during etching, by inelastic knock-on collisions with the sputtering gas ions. Electrochemical replacement of the nuclei can be used to advantage in these experiments. Vacuum-deposited Pd nuclei are immersed for a few minutes in a solution containing Au + ions, and the samples and the solution are then analyzed for the two elements. Results show that the amount of Pd replaced agrees with the amount included in the nuclei, as determined from the micrographs. Electron diffraction shows that the crystalline phase is essentially all Au, and the conclusion therefore is that the gold replaces only the Pd that is included in the nuclei, leaving the extranuclear Pd atoms unaffected. Sputter etching and Auger analysis of these samples give a comparison profile of the Au and the Pd, and remove part of the uncertainty of the technique. An example of the results is shown in Fig. 1. The Au signal decreases much more sharply with sputtering time than does the Pd signal.
GROWTH OF METAL FILMS ON AMORPHOUS SUBSTRATES
235
O4
O2
~ 02 Pd O~ Au 0
02 04 Froctlonol cocb~ lhickness
OS
Fig. 1. Relative depth profiles of Au (o) and Pd (o) by Auger analysis ofa Pd deposit on carbon, gold replaced. Still another experimental approach is to sputter-etch from the rear of the coating. If the carbon film is deposited on mica, it can easily be stripped from the mica and mounted in an inverted configuration. This m e t h o d completely eliminates the possibility of deeper implantation o f the metal atoms by the sputtering process itself. Results show that the Pd peak is detected throughout the substrate film. In conclusion, brief mention might be made of some o f the studies of these nuclei that are underway in our laboratories. Their use I 1,12 in studying size effects in catalysis has already been pointed out. Baetzold 1a has been engaged for the past few years in a program o f calculating their physical and electronic properties using semi-empirical quantum-mechanical methods. Recently, Mason and Baetzold 14 have been successful in characterizing the valence electronic states from the same type o f nuclei, using photoelectron spectroscopy. There is a shift of the valence s level and a splitting of the d band, as predicted theoretically, in the range of a few atoms. Optical and ESR studies are underway. REFERENCES 1 J. F. Hamilton and P. C. Logel, Thin Solid Films, 16 (1973) 49. 2 J. F. Hamilton and P. C. Logel, Thin Solid Films, 23 (1974) 89. 3 G. Zinsmeister, Vacuum, 16 (1966) 529; Thin Solid Films, 2 (1968) 497; 4 (1969) 363; 7 (1971)51. 4 D. Frankel and J. A. Venables, Adv. Phys., 19 (1970) 409. 5 J. A. Venables, Philos. Mag., 27 (1973) 697. 6 M.J. Stowell, J. Cryst. Growth, 24/25 (1974) 45. See also the original references given in these reviews (refs. 3-6). 7 M. J. StoweU and T. E. Hutchinson, Thin Solid Films, 8 (1971) 41. 8 A. G. Kaspaul and E. E. Kaspaul, in G. H. Bancroft (ed.), Tranz lOth Nat. Vac. Syrup. Am. Vac. Soc., Macmillan, New York, 1963, p. 422. 9 H. G. Wehe, in R. Bakish (¢d.), Electron andIon Beam Science and Technology, Vol. 2, Am. Inst. Min., Metall. Pet. Eng., New York, 1966, p. 813. 10 J. A. Venables, Thin Solid Films, 18 (1973) 511. 11 J. F. Hamilton and P. C. Logel, J. Catal., 29 (1973) 253.
236
J. F. HAMILTON, P. C. LOGEL, R. C. BAETZOLD
12 J. F. Hamilton and P. C. Logel, Photogr. Sci. Eng., 18 (1974) 507. 13 R.C. Baetzold, J. Chem. Phys., 55 (1971) 4355, 4363; Comments Solid State Phys., 4 (1972) 62; Surf. Sci., 36 (1972) 123; J. Solid State Chem., 6 (1973) 352; Photogr. Sei. Eng., 17 (1973) 78;J. CataL, 29 (1973) 129. 14 M.G. Mason and R. C. Baetzold, ESCA and molecular orbital studies of small silver particles, J. Chem. Phys., in the press.
233
EXPERIMENTAL STUDIES OF METAL FILM NUCLEATION AND GROWTH ON AMORPHOUS SUBSTRATES* J. F. HAMILTON, P. C. LOGEL AND R. C. BAETZOLD Research Laboratories, Eastman Kodak Company, Rochester, N. Y. 14650 (U.S.A.)
(Received August 25, 1975)
The nuclei formed in the early stages of thin film deposition are useful in the study of size effects in heterogeneous catalysis. Apparatus to produce a fairly large number of reproducibly controlled samples with modulated coverages over a wide range has been described I'2. A quartz-crystal-type controller is used to set and to maintain the deposition rate, and a shutter in the form of a slotted belt is moved past a substrate at a constant speed to regulate the time of deposition. The apparatus is ion pumped but not bakeable, and pressures in the 10-7 Torr range are normal. Substrates used have been primarily either evaporated carbon or evaporated SiO 2, made just prior to depositing the nuclei, but a few deposits have been made on glass or polyester film. Nuclei of silver, gold, copper and palladium have been studied. Most of the interesting size effects are in the range below that which can be directly characterized by electron microscopy. However, a possible method for selectively growing these subvisible nuclei to resolvable size depends upon the prediction a-~ that, if homogeneous nucleation is the governing mechanism, there is a direct dependence of nucleation rate on deposit rate. Therefore it should be possible to form nuclei at one rate, and then to grow them to visible size at a much lower rate, for which the nucleation probability is negligible. Owing to the potential usefulness of this technique, it has been explored extensively in our laboratory. Unfortunately, it is not successful in any of the combinations of substrate and nuclei that we have studied, because the saturation density is in fact totally independent of deposition rate. The lack of a rate dependence is a significant result, for it rules out homogeneous two-dimensional nucleation as an important process, and attests to the presence of a limited number of sites with some particular activity for capturing the atoms of the incident metal. Thus the saturation nucleus density gives essentially the density of the active sites for the particular metal being deposited. Determination of an apparent sticking coefficient from electron micrographs of nuclei at or above the coverage for saturation density gives values of the order 0.1 or less. Thus the deposits fit into the category termed by Stowell and Hutchinson 7 and Venables s as "extreme incomplete condensation", during which essentially all growth occurs by direct impingement; surface diffusion of adatoms to existing clusters is unimportant. This condition should be characterized by a linear increase in mean particle radius with coverages; experimental results confirm this relationship for all of the metal-substrate combinations in question. The intercepts of these graphs are related to * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applicalions and Trends", Budapest, Hungary, August 25-29, 1975; Paper 2-29.
234
J. F. HAMILTON,P. C. LOGEL, R. C. BAETZOLD
the initial capture radii for the active sites involved, and these are found to be in the range of atomic dimensions. Thus the nucleation and growth appears to be a purely statistical process of single and then multiple occupancy of active substrate sites. It is governed by Poisson statistics, except as affected by the increased capture probability as a particle grows in area. Given this dependence, the particle size distribution may be specified for any coverage lower than that at which coalescence becomes important. The density of nuclei of size i or larger increases with the ith power of the incident coverage. A confirmation of this predicted relation could be obtained if it were possible to achieve selective growth by lowering the deposit rate as discussed earlier. Although this technique is not successful, it is possible to achieve selective growth of existing nuclei by using a second metal with a lower saturation nucleus density. Zinc, which self-nucleates with extreme difficulty on these substrates 8'9, will grow on nuclei of any of the metals studied. When the densities of these Zn-grown nuclei are plotted against the incident coverage of the nucleating metal, the dependence is found to be linear, indicating that any active site that is occupied by at least one atom of the nucleating metal will serve as a growth center for the Zn. The fit to the theoretical curves gives a value for the active site capture area which is in satisfactory agreement with the values determined from visible particles. Analytical determinations of the sticking coefficients were made by atomic absorption spectroscopy and neutron-activated radioactive decay. Results show that essentially all of the incident metal atoms are quantitatively retained on the sample. The actual sticking coefficients are near unity, although the apparent values determined from electron micrographs are lower by an order of magnitude. Thus the incident metal atoms that are not captured by active sites are not simply re-evaporated, but are retained on the substrate and in some way deactivated so that they do not participate in forming the nuclei. We have speculated I that they might diffuse into the substrates, whose amorphous structure we view as being rather open. However, other proposals have been made l°. Depth profiles determined by Auger spectroscopy and sputter etching indicate that the metal is distributed within the thickness of the substrate. However, there are questions about the uniformity of the sputter-etch process, and also the posssibility that surface atoms are driven into the film during etching, by inelastic knock-on collisions with the sputtering gas ions. Electrochemical replacement of the nuclei can be used to advantage in these experiments. Vacuum-deposited Pd nuclei are immersed for a few minutes in a solution containing Au + ions, and the samples and the solution are then analyzed for the two elements. Results show that the amount of Pd replaced agrees with the amount included in the nuclei, as determined from the micrographs. Electron diffraction shows that the crystalline phase is essentially all Au, and the conclusion therefore is that the gold replaces only the Pd that is included in the nuclei, leaving the extranuclear Pd atoms unaffected. Sputter etching and Auger analysis of these samples give a comparison profile of the Au and the Pd, and remove part of the uncertainty of the technique. An example of the results is shown in Fig. 1. The Au signal decreases much more sharply with sputtering time than does the Pd signal.
GROWTH OF METAL FILMS ON AMORPHOUS SUBSTRATES
235
O4
O2
~ 02 Pd O~ Au 0
02 04 Froctlonol cocb~ lhickness
OS
Fig. 1. Relative depth profiles of Au (o) and Pd (o) by Auger analysis ofa Pd deposit on carbon, gold replaced. Still another experimental approach is to sputter-etch from the rear of the coating. If the carbon film is deposited on mica, it can easily be stripped from the mica and mounted in an inverted configuration. This m e t h o d completely eliminates the possibility of deeper implantation o f the metal atoms by the sputtering process itself. Results show that the Pd peak is detected throughout the substrate film. In conclusion, brief mention might be made of some o f the studies of these nuclei that are underway in our laboratories. Their use I 1,12 in studying size effects in catalysis has already been pointed out. Baetzold 1a has been engaged for the past few years in a program o f calculating their physical and electronic properties using semi-empirical quantum-mechanical methods. Recently, Mason and Baetzold 14 have been successful in characterizing the valence electronic states from the same type o f nuclei, using photoelectron spectroscopy. There is a shift of the valence s level and a splitting of the d band, as predicted theoretically, in the range of a few atoms. Optical and ESR studies are underway. REFERENCES 1 J. F. Hamilton and P. C. Logel, Thin Solid Films, 16 (1973) 49. 2 J. F. Hamilton and P. C. Logel, Thin Solid Films, 23 (1974) 89. 3 G. Zinsmeister, Vacuum, 16 (1966) 529; Thin Solid Films, 2 (1968) 497; 4 (1969) 363; 7 (1971)51. 4 D. Frankel and J. A. Venables, Adv. Phys., 19 (1970) 409. 5 J. A. Venables, Philos. Mag., 27 (1973) 697. 6 M.J. Stowell, J. Cryst. Growth, 24/25 (1974) 45. See also the original references given in these reviews (refs. 3-6). 7 M. J. StoweU and T. E. Hutchinson, Thin Solid Films, 8 (1971) 41. 8 A. G. Kaspaul and E. E. Kaspaul, in G. H. Bancroft (ed.), Tranz lOth Nat. Vac. Syrup. Am. Vac. Soc., Macmillan, New York, 1963, p. 422. 9 H. G. Wehe, in R. Bakish (¢d.), Electron andIon Beam Science and Technology, Vol. 2, Am. Inst. Min., Metall. Pet. Eng., New York, 1966, p. 813. 10 J. A. Venables, Thin Solid Films, 18 (1973) 511. 11 J. F. Hamilton and P. C. Logel, J. Catal., 29 (1973) 253.
236
J. F. HAMILTON, P. C. LOGEL, R. C. BAETZOLD
12 J. F. Hamilton and P. C. Logel, Photogr. Sci. Eng., 18 (1974) 507. 13 R.C. Baetzold, J. Chem. Phys., 55 (1971) 4355, 4363; Comments Solid State Phys., 4 (1972) 62; Surf. Sci., 36 (1972) 123; J. Solid State Chem., 6 (1973) 352; Photogr. Sei. Eng., 17 (1973) 78;J. CataL, 29 (1973) 129. 14 M.G. Mason and R. C. Baetzold, ESCA and molecular orbital studies of small silver particles, J. Chem. Phys., in the press.