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A transmission electron microscope study of electrodeposited nickel films on (001) copper substrates
Thin Solid Films, 15 (1973) 163-172 © Elsevier Sequoia S.A., Lausanne---Printed in Switzerland
163
A TRANSMISSION ELECTRON MICROSCOPE STUDY OF ELECTRODEPOSITED NICKEL FILMS ON (001) COPPER SUBSTRATES
H. L. GAIGHER AND G. N. VAN WYK
Department of Physics, University of Pretoria, Pretoria (South Africa) (Received September 12, 1972)
Nickel was electrodeposited on to (001) single-crystal copper films. Deposit thicknesses were in the range 10 A to 500 A. Bicrystals and stripped deposits were examined by transmission electron microscopy and high-resolution electron diffraction. Misfit, as a function of thickness, was determined from moir6 fringe spacings, splitting of diffraction spots and the spacing of misfit dislocations. The misfit was found to be less than that of vapour-deposited films at corresponding thicknesses. The misfit as calculated from the spacing of misfit dislocations was not in agreement with the misfit as determined from moir6 fringes or electron diffraction. These discrepancies might be due to foreign material incorporated in the deposit.
I. INTRODUCTION
Electroplated metal films are being used to an increasing extent for industrial applications. To understand how the mechanism of film formation, structure and properties are interrelated, basic studies of the nucleation and growth of electrodeposited films are essential. It is especially the very early stages of the formation and growth of electroplated films which, compared with evaporated films, have been neglected. The bulk of the experimental work on the structural characterization of plated films has been concentrated on the later stages of growth, and includes for example (a) surface topography and structurC -3, (b) origin of stresses 4' s and (c) structure of stripped films6' v. The structure of deposits in the very early stages of film formation (thickness < 100 A) was investigated many years ago by Cochrane s. His interpretations in terms of pseudomorphic growth were however contradicted by Newman 9. More recent studies by Thompson and Lawless 1o, Lawless 11, Dickson e t al. 1z and Ives e t al.13 indicate, at least qualitatively, structural similarities between electroplated and vapour-deposited films. The electrodeposition process is most complex and almost always takes place in the presence of a variety of foreign particles which often compete with the metal atoms for favourable lattice positions on the cathode. Considering that the epitaxial growth of vapour-deposited films can be adversely affected by small amounts of impurities 14, the presence of foreign particles during
164
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electrolytic growth might be expected to influence growth details and film properties significantly. In the present study the early stages of the electrolytic growth of nickel on to (001) single-crystal copper films were investigated. Transmission electron microscopy and high-resolution electron diffraction techniques were used to study the structure and misfit of the nickel/copper duplex crystals. 2.
EXPERIMENTAL PROCEDURE
Single-crystal copper substrates (1 cm x 1 cm) were prepared by evaporating high purity ( > 9 9 . 9 % ) copper in a vacuum of 10 -5 torr on to (001) faces of air-cleaved sodium chloride held at 320 °C during deposition. The copper substrates were about 1500 A thick, and were well oriented with (001) parallel to their surface planes. The copper films were stripped from the salt by immersion in water and transferred to the plating bath. The substrates floated on the surface o f the electrolyte such that subsequent deposition occurred on the " c l e a n " copper side, i.e. the side which had been adjacent to the salt. Electrical contact was made to the floating film with a strip of aluminium foil 1°. Nickel was plated at a current density of I mA/cm 2 from a pH 3.3 Watts bath at 25 ~C. The bath was prepared from " A n a l a R " chemicals and contained 240 g/1 NiSO4.6H20, 45 g/l NiC12.6H20 and 30 g/1 HaBO 3. High purity nickel was used as anode. The average thickness of the electrodeposit was calculated from Faraday's law, taking 92% for current efficiency15. As a further check the thickness was determined using an X-ray microanalysis attachment for electron microscopes 16. The average thickness determined in this way agreed within ,-~7 % (for thicknesses >~200 A) to ,-~ 16 % (for thicknesses ~< 100 A) with those determined by Faraday's law. Following deposition the floating bicrystals were transferred to water, washed, mounted on copper grids and examined by transmission electron microscopy and diffraction. Stripped nickel deposits were examined after dissolving the copper away in trichloracetic acid, ammonia and water ~7. Films with thicknesses < 100 A were backed with evaporated carbon before dissolving the substrate. 3. RESULTS (a) Substrates The single-crystal copper substrates contained a grown-in dislocation density of ,-~5 x 10 a cm-2 as well as a low density of microtwins. The substrates, furthermore, had a grainy structure, Fig. 1. Grains, 1500 to 2500 A in diameter, all with a common orientation, were separated by shallow surface grooves which were from a few ~ngstrrms to ~ 200 A deep. Replicas of the copper surfaces showed these grooves to be on that side of the copper film which had been in contact with the salt, and on to which electrodeposition subsequently occurred, Fig. l(b). The free surface of the copper, i.e. the surface on to which deposition
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Fig. 1. Structure of copper substrate. (x 40500). (a) Transmission image showing grainy features, microtwin and dislocations. (b) Shadowed carbon replica of cathode surface.
did not take place, was, apart from isolated holes, essentially smooth. The observed surface relief is probably a direct consequence of the shape xs of the initial three-dimensional nuclei. It should be remarked that these results differ from the surface relief which has been observed for epitaxial (001) copper films on rocksalt, using however different evaporation conditions, notably evaporation rate x9.
(b) Bicrystal films For average deposit thicknesses of <20 /~ electron microscopy of the bicrystals did not reveal any features which could, beyond any doubt, be ascribed to the deposit. As the deposit thickness increased to ,~ 30 A, pronounced contrast effects were observed: dark or light bands were associated with the edges of the grooves, and within the grains short, irregular dark bands and spots were observed, Fig. 2. These contrast effects depended critically on the operating reflection. Segments of the grooves which were approximately parallel to the diffraction vector, g, showed very weak, if any, contrast, whereas segments perpendicular to g showed up very strongly. Likewise the short contrast bands within the grains were always observed to be oriented roughly perpendicular to OThe contrast phenomena were most pronounced with the bicrystal in, or very close to, the exact reflecting position. Under these conditions many of the short bands and spots exhibited a dark-light type contrast. When the bicrystal was mounted such that the electron beam was incident on the nickel deposit (nickel facing up) both bright and dark field images were dark in the direction of positive O, Figs. 2(a) and (b). Upon flipping the bicrystal (nickel facing down) the bright field images became light in the direction of positive O, Fig. 2(c), and remained dark along positive 0 for dark field, Fig. 2(d). For a given reflection the bright and dark field images of the contrast bands at the groove edges were similar for the nickel facing up, and complementary for the nickel facing down.
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Fig. 2. Dark-light type contrast, examples of which are marked, and contrast at substrate grooves for Ni/Cu bicrystal; deposit thickness 50 A. (a) Bright field. Nickel facing up. g = 002. ( x 66000). (b) Dark field of (a). g = 00~. ( x 66000). (c) Bright field. Nickel facing down. g = 002. ( x 87000). (d) Dark field of (c). g = 00~. ( x 87000).
The contrast effects described above were the distinctive feature for bicrystals with deposit thicknesses up to ~100 A, but became progressively less pronounced with further increase in thickness. For deposits with thickness > 100 A long misfit dislocation lines were usually observed. In agreement with previous work10, 16 the dislocation lines were parallel to both < 110> directions in the (001) film plane, and the Burgers vectors were found to be primarily of type ½ a < l l 0 > , inclined at 45 ° to the film plane. Dislocations with Burgers vector in the plane o f the film were also observed however. For deposits with thicknesses between 50 and 100/~ a very low density of scattered misfit dislocations was erratically observed. Apart from typical long, straight dislocations, a low density of short ( ~ 1000 /~) dislocation lines often occurred, Fig. 3. Dislocations were very rarely observed for a deposit thickness < 50 A. In contrast, for nickel grown by evaporation on to copper single crystals,
(001)
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Fig. 3. Bicrystal, deposit thickness 60 A, showing short dislocation lines, g = 002. ( x 87000).
a relatively high density of long straight misfit dislocations are characteristic for thin (15-30 A) deposits 16. As the thickness of the electrodeposited nickel increased, the average linear density of misfit dislocations increased. The average spacing between misfit dislocations decreased continuously from ~600 A, for a deposit 100 A in thickness, to --~300 A, for a deposit thickness of 500 /~, Fig. 4. These spacings are generally larger than those observed for vapour-deposited films of corresponding thicknesses 16. 600
,~ 5oo 0
8
400
r~ o
300 0
I 100
I 200 Thickness
I 300
I 400
I, 500
(~,)
Fig. 4. Average spacing of misfit dislocations
vs.
deposit thickness.
At a deposit thickness of ~ 100 A, moir6 fringes became visible. The fringes were badly distorted and discontinuous. Only for rather thick films, 500 A, did the fringes become more straight and continuous. The separation of the fringes had wide variations for a given film. For example, at the edges of the grooves the fringe spacing was noticeably smaller than elsewhere. The average separation of fringes decreased as the nickel thickness increased (see section 3(d)).
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(c) Stripped films The most noticeable characteristic of stripped nickel films was the occurrence of a high density of small, almost circular, light markings as shown in Fig. 5. Stereo electron micrographs showed that these markings were not surface artefacts but were distributed throughout the volume of the deposit.
Fig. 5. Transmission micrograph of stripped nickel film, average thickness 150 A, showing small light markings. ( x 77000).
Even the very thin (30/~) nickel films were found to be essentially continuous except for local holes and the above mentioned markings. There was in no case any evidence for individual nuclei of any appreciable height. The deposit thickness within the substrate grooves was markedly less than the average thickness, Fig. 5. The deposit consequently had a grain-like structure in correspondence with that of the substrate.
(d) Measurement of misfit The misfit, M, at any particular thickness is defined as the difference between the overgrowth and substrate lattice parameters divided by the lattice parameter of the overgrowth. In the present investigation M could be measured by the following techniques: (1) Measurement of the spacing between moir6 fringes. (2) Measurement of the spacing between misfit dislocations. (3) Transmission high-resolution electron diffraction2°. A difference between deposit and substrate lattice parameters causes a splitting in the corresponding diffraction spots for a given reflection. The misfit follows as the ratio of this splitting to the distance between the undiffracted spot and the appropriate substrate spot. Figure 6 shows the misfit as a function of deposit thickness as determined from (a) moir6 fringe spacing and (b) the splitting of low-index diffraction spots. The two methods Of measurement gave values for M which are in reasonable agreement with each other. The spacing of misfit dislocations yielded values for M which were at any given thickness much lower than the values in Fig. 6. This discrepancy is further discussed in section 4.
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169
0.02
0.015
U
0.01
0.005 0
I 100
I 200 Thickness
I 300
I 400
I 500
(~)
Fig. 6. Misfit as determined from spacing of moir~ fringes (O) and splitting of diffraction spots (/X) as a function of deposit thickness.
A separation of nickel and copper diffraction spots was generally not observed for deposit thicknesses < 100 A. The thickness at which splitting first occurred coincided with the thickness at which moir~ fringes became clearly visible. 4. DISCUSSION Contrast calculations by Ashby and Brown21 show that prismatic loops which intersect a foil surface give rise to asymmetric (anomalous) images. Interstitial loops give dark-field images that are black in the direction of positive g, provided the remaining segment of the loop lies within half an extinction distance from either foil surface. In bright field the sense of the asymmetry depends on whether the loop segment is near the top or bottom surface of the foil: for a loop segment near the top surface bright and dark field images are nearly similar, but near the bottom surface bright and dark field images are almost complementary for the same g. For a vacancy loop the contrast in dark field is light on the side of positive g. Since the unstrained lattice parameter of nickel (3.52377/~) is less than that of copper (3.61496 A) a coherent or semi-coherent deposit of nickel on copper will be in tension. It was furthermore shown that although the very thin deposits were essentially continuous they contained holes which extended through the thickness of the film. For such a coherent or semi-coherent deposit the substrate will of course also be elastically strained, particularly at the periphery of holes, as is schematically illustrated in Fig. 7(a). This strain field will result in a strain contrast which might, at least qualitatively, be similar to that of an interstitial prismatic loop which intersects the foil surface, Fig. 7(b). The contrast effects previously described (section 3(b)) are indeed consistent with the contrast rules for an interstitial loop which intersect the foil surface. The contrast phenomena at the substrate grooves (section 3(b)) can in principle also be explained by
170
H . L . GAIGHER, G. N. VAN WYK
(a)
(b)
Fig. 7. Schematic diagram to show that the elastic strain field at (a) the periphery of a hole in an otherwise continuous deposit might be expected to result in strain contrast qualitatively similar to that of (b) an interstitial loop which intersects the foil surface.
considering the elastic strain fields which exist at these grooves due to the grainlike structure (section 3(a)) of the deposit, the latter being in a state of tension. The fact that no moird fringes or splitting of diffraction spots were observed for deposit thicknesses < 100 /~ does not necessarily imply pseudomorphism up to thicknesses of ~ 100 A. P r o o f of this statement lies in the fact that application of the high resolution electron diffraction technique to duplex films consisting of nickel evaporated (vacuum 10-s torr) on to copper substrates failed to show a splitting of nickel and copper spots for very thin (say -~ 25/~,) deposits, although transmission micrographs showed the overgrowths to be non-coherent as evidenced by networks of interfacial dislocations 22. It is suggested that for these films the strain fields associated with the relatively widely spaced dislocations are so highly localized, i.e. the proportion of non-coherent material is so very low, that diffraction patterns give rise to no measurable effect. The absence of dislocations and moird fringes in the very thin (--~30 /~,) films strongly suggests a coherent deposit. For thicker films the different contrast effects discussed complicate the observations to the extent that unambiguous interpretations become difficult. The misfit measured for the electroplated films, Fig. 6, was found to be significantly smaller than the misfit as measured, at corresponding thicknesses, for vapour-deposited nickel on copper. For a vapour-deposited nickel film with a thickness of ~ 100 A for example, Matthews and Crawford 16 as well as Gradmann 23 observed misfits which are about twice as large as the misfit measured for 100 A thick electroplated nickel (Fig. 6). This disagreement could probably be due to the incorporation of foreign material into the deposit. These foreign substances could be hydrogen, coordinated water, basic material (Ni(OH)2 e.g.,) etc. s Small quantities of copper could dissolve from the substrate 9 in which case copper will also initially be codeposited with nickel. Incorporation of these substances into the deposit could cause an expansion of the nickel lattice as a result of which part of the misfit between the nickel and copper is relieved. Small structural features revealed by transmission micrographs of stripped films, Fig. 5, could be pores due to hydrogen evolution and perhaps also amorphous, basic inclusions.
ELECTRODEPOSITED NICKEL FILMS ON (001) COPPER SUBSTRATES
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These structural features will furthermore act as obstacles to the passage of dislocations in the deposit, and might inhibit the formation of misfit dislocations by mechanisms involving glide of pre-existing dislocations 24. As compared with vapour-deposited films the above-mentioned factors will contribute to a lower misfit and will also increase the thickness at which coherency breaks down. This could then also explain the absence of misfit dislocations and moir6 fringes from very thin (say, < 50 A) electrodeposits of nickel on copper. According to Frank and van der Merwe 25 the misfit at the interface of epitaxial bicrystals is accommodated by elastic strain and misfit dislocations. All the misfit in excess of that eliminated by elastic strain is accommodated by misfit dislocations. Quantitative experimental support for this theory has been obtained for vapour-deposited films 16' 26 In the present study the density of misfit dislocations increased, and the separation of moir6 fringes decreased as the nickel thickness increased. This observation does suggest that misfit dislocations were generated to accommodate part of the difference between the lattice parameters of nickel and copper. However, the observed dislocation density was generally much too low to account for the measured misfit. For a deposit thickness of 300 A for example, application of the B r o o k s 27 formula shows the observed spacing of misfit dislocations (,-~ 380 A, Fig. 4) to be about 4.5 times too large to account for the measured misfit of ,-~0.015 (Fig. 6). A lack of definite correspondence between moir6 fringes and misfit dislocations has already been pointed out by Yagi e t aL 26 for the case of evaporated bicrystals where local places of incoherency occurred. Such a local loss of incoherency is even more conceivable for electroplated films due to interference of gas bubbles and other foreign substances at the cathode. Furthermore, imperfect alignment of the deposit and other imperfections due to the incorporation of foreign substances will certainly have an important effect on the actual misfit to be accommodated by dislocations. These effects will be further discussed in a future publication. ACKNOWLEDGEMENTS The authors wish to thank Prof. Dr. J. H. van der Merwe for his interest and encouragement, the University o f Pretoria for financial support, and the Council for Scientific and Industrial Research as well as the Atomic Energy Board for the use o f X-ray microanalysis attachments and scanning electron microscope facilities. REFERENCES 1 2 3 4 5 6 7
B.C. Banerjee and P. U Walker, J. Electrochem. Soc., 109 (1962) 436. R. Weil and H. C. Cook, J. Electrochem. Soc., 109 (1962) 295. J.A. Crossley, P. A. Brook and J. W. Cuthbertson, Electrochim. Acta, 11 (1966) 1153. T.P. Hoar and D. J. Arrowsmith, Trans. Inst. Met. Finishing, 36 (1958-59) I. R. Weil, Plating, 58 (1971) 137. L. Reimer, Z. Metallkunde, 47(1956) 631. E.J. Suoninen and T. Hakkarainen, J. Mater. Sci., 3 (1968) 446.
i72 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
n . L. GAIGHER, G. N. VAN WYK W. Cochrane, Proc. Phys. Soc., 48 (1963) 723. R.C. Newman, Proc. Phys. Soc. B, 69 (1956) 432. E.R. Thompson and K. R. Lawless, AppL Phys. Letters, 9 (1966) 138. K . R . Lawless, J. Vac. Sci. TechnoL, 2 (1965) 24. E.W. Dickson, M. H. Jacobs and D. W. Pashley, Phil. Mag., 11 (1965) 575. A . G . Ives, J. W. Edington and G. P. Rothwell, Electrochim. Acta, 15 (1970) 1797. L.E. Murr and M. C. Inman, Phil. Mag., 14 (1966) 135. H. Fischer, Elektrolytische Abscheidung und Elektrokristallisation yon Metallen, Springer, G6ttingen (1954) 634. J.W. Matthews and J. L. Crawford, Thin Solid Films, 5 (1970) 187. D . L . Carr, Rev. Sci. Instr., 40 (1969) 965. J.W. Matthews, Phil. Mag., 12(1965) 1143. M.J. Stowell and T. J. Law, Phil. Mag., 19 (1969) 1257. R. Kuntze, A. Chambers and M. Prutton, Thin Solid Films, 4 (1969) 47. M . F . Ashby and L. M. Brown, Phil Mag., 8 (1963) 1649. G . N . Van Wyk, Thesis, University of Pretoria (1972) (unpublished). U. Gradmann, Ann. Phys. Lpz., 13 (1964) 213. J.W. Matthews, Phil. Mag., 13 (1966) 1207. F.C. Frank and J. H. van der Merwe, Proc. Roy. Soc. (London), A198 (1949) 216. K. Yagi, K. Takayanagi, K. Kobayashi and G. Honjo, J. Crystal Growth, 9 (1971) 84. H. Brooks, Metal Interfaces, Am. Soc. Metals, Metals Park, Ohio, 1952, p. 20.
163
A TRANSMISSION ELECTRON MICROSCOPE STUDY OF ELECTRODEPOSITED NICKEL FILMS ON (001) COPPER SUBSTRATES
H. L. GAIGHER AND G. N. VAN WYK
Department of Physics, University of Pretoria, Pretoria (South Africa) (Received September 12, 1972)
Nickel was electrodeposited on to (001) single-crystal copper films. Deposit thicknesses were in the range 10 A to 500 A. Bicrystals and stripped deposits were examined by transmission electron microscopy and high-resolution electron diffraction. Misfit, as a function of thickness, was determined from moir6 fringe spacings, splitting of diffraction spots and the spacing of misfit dislocations. The misfit was found to be less than that of vapour-deposited films at corresponding thicknesses. The misfit as calculated from the spacing of misfit dislocations was not in agreement with the misfit as determined from moir6 fringes or electron diffraction. These discrepancies might be due to foreign material incorporated in the deposit.
I. INTRODUCTION
Electroplated metal films are being used to an increasing extent for industrial applications. To understand how the mechanism of film formation, structure and properties are interrelated, basic studies of the nucleation and growth of electrodeposited films are essential. It is especially the very early stages of the formation and growth of electroplated films which, compared with evaporated films, have been neglected. The bulk of the experimental work on the structural characterization of plated films has been concentrated on the later stages of growth, and includes for example (a) surface topography and structurC -3, (b) origin of stresses 4' s and (c) structure of stripped films6' v. The structure of deposits in the very early stages of film formation (thickness < 100 A) was investigated many years ago by Cochrane s. His interpretations in terms of pseudomorphic growth were however contradicted by Newman 9. More recent studies by Thompson and Lawless 1o, Lawless 11, Dickson e t al. 1z and Ives e t al.13 indicate, at least qualitatively, structural similarities between electroplated and vapour-deposited films. The electrodeposition process is most complex and almost always takes place in the presence of a variety of foreign particles which often compete with the metal atoms for favourable lattice positions on the cathode. Considering that the epitaxial growth of vapour-deposited films can be adversely affected by small amounts of impurities 14, the presence of foreign particles during
164
H.L.
GAIGHER, G. N. VAN WYK
electrolytic growth might be expected to influence growth details and film properties significantly. In the present study the early stages of the electrolytic growth of nickel on to (001) single-crystal copper films were investigated. Transmission electron microscopy and high-resolution electron diffraction techniques were used to study the structure and misfit of the nickel/copper duplex crystals. 2.
EXPERIMENTAL PROCEDURE
Single-crystal copper substrates (1 cm x 1 cm) were prepared by evaporating high purity ( > 9 9 . 9 % ) copper in a vacuum of 10 -5 torr on to (001) faces of air-cleaved sodium chloride held at 320 °C during deposition. The copper substrates were about 1500 A thick, and were well oriented with (001) parallel to their surface planes. The copper films were stripped from the salt by immersion in water and transferred to the plating bath. The substrates floated on the surface o f the electrolyte such that subsequent deposition occurred on the " c l e a n " copper side, i.e. the side which had been adjacent to the salt. Electrical contact was made to the floating film with a strip of aluminium foil 1°. Nickel was plated at a current density of I mA/cm 2 from a pH 3.3 Watts bath at 25 ~C. The bath was prepared from " A n a l a R " chemicals and contained 240 g/1 NiSO4.6H20, 45 g/l NiC12.6H20 and 30 g/1 HaBO 3. High purity nickel was used as anode. The average thickness of the electrodeposit was calculated from Faraday's law, taking 92% for current efficiency15. As a further check the thickness was determined using an X-ray microanalysis attachment for electron microscopes 16. The average thickness determined in this way agreed within ,-~7 % (for thicknesses >~200 A) to ,-~ 16 % (for thicknesses ~< 100 A) with those determined by Faraday's law. Following deposition the floating bicrystals were transferred to water, washed, mounted on copper grids and examined by transmission electron microscopy and diffraction. Stripped nickel deposits were examined after dissolving the copper away in trichloracetic acid, ammonia and water ~7. Films with thicknesses < 100 A were backed with evaporated carbon before dissolving the substrate. 3. RESULTS (a) Substrates The single-crystal copper substrates contained a grown-in dislocation density of ,-~5 x 10 a cm-2 as well as a low density of microtwins. The substrates, furthermore, had a grainy structure, Fig. 1. Grains, 1500 to 2500 A in diameter, all with a common orientation, were separated by shallow surface grooves which were from a few ~ngstrrms to ~ 200 A deep. Replicas of the copper surfaces showed these grooves to be on that side of the copper film which had been in contact with the salt, and on to which electrodeposition subsequently occurred, Fig. l(b). The free surface of the copper, i.e. the surface on to which deposition
ELECTRODEPOSITI~ NICKEL FILMS ON
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Fig. 1. Structure of copper substrate. (x 40500). (a) Transmission image showing grainy features, microtwin and dislocations. (b) Shadowed carbon replica of cathode surface.
did not take place, was, apart from isolated holes, essentially smooth. The observed surface relief is probably a direct consequence of the shape xs of the initial three-dimensional nuclei. It should be remarked that these results differ from the surface relief which has been observed for epitaxial (001) copper films on rocksalt, using however different evaporation conditions, notably evaporation rate x9.
(b) Bicrystal films For average deposit thicknesses of <20 /~ electron microscopy of the bicrystals did not reveal any features which could, beyond any doubt, be ascribed to the deposit. As the deposit thickness increased to ,~ 30 A, pronounced contrast effects were observed: dark or light bands were associated with the edges of the grooves, and within the grains short, irregular dark bands and spots were observed, Fig. 2. These contrast effects depended critically on the operating reflection. Segments of the grooves which were approximately parallel to the diffraction vector, g, showed very weak, if any, contrast, whereas segments perpendicular to g showed up very strongly. Likewise the short contrast bands within the grains were always observed to be oriented roughly perpendicular to OThe contrast phenomena were most pronounced with the bicrystal in, or very close to, the exact reflecting position. Under these conditions many of the short bands and spots exhibited a dark-light type contrast. When the bicrystal was mounted such that the electron beam was incident on the nickel deposit (nickel facing up) both bright and dark field images were dark in the direction of positive O, Figs. 2(a) and (b). Upon flipping the bicrystal (nickel facing down) the bright field images became light in the direction of positive O, Fig. 2(c), and remained dark along positive 0 for dark field, Fig. 2(d). For a given reflection the bright and dark field images of the contrast bands at the groove edges were similar for the nickel facing up, and complementary for the nickel facing down.
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Fig. 2. Dark-light type contrast, examples of which are marked, and contrast at substrate grooves for Ni/Cu bicrystal; deposit thickness 50 A. (a) Bright field. Nickel facing up. g = 002. ( x 66000). (b) Dark field of (a). g = 00~. ( x 66000). (c) Bright field. Nickel facing down. g = 002. ( x 87000). (d) Dark field of (c). g = 00~. ( x 87000).
The contrast effects described above were the distinctive feature for bicrystals with deposit thicknesses up to ~100 A, but became progressively less pronounced with further increase in thickness. For deposits with thickness > 100 A long misfit dislocation lines were usually observed. In agreement with previous work10, 16 the dislocation lines were parallel to both < 110> directions in the (001) film plane, and the Burgers vectors were found to be primarily of type ½ a < l l 0 > , inclined at 45 ° to the film plane. Dislocations with Burgers vector in the plane o f the film were also observed however. For deposits with thicknesses between 50 and 100/~ a very low density of scattered misfit dislocations was erratically observed. Apart from typical long, straight dislocations, a low density of short ( ~ 1000 /~) dislocation lines often occurred, Fig. 3. Dislocations were very rarely observed for a deposit thickness < 50 A. In contrast, for nickel grown by evaporation on to copper single crystals,
(001)
ELECTRODEPOSITED NICKEL FILMS ON
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Fig. 3. Bicrystal, deposit thickness 60 A, showing short dislocation lines, g = 002. ( x 87000).
a relatively high density of long straight misfit dislocations are characteristic for thin (15-30 A) deposits 16. As the thickness of the electrodeposited nickel increased, the average linear density of misfit dislocations increased. The average spacing between misfit dislocations decreased continuously from ~600 A, for a deposit 100 A in thickness, to --~300 A, for a deposit thickness of 500 /~, Fig. 4. These spacings are generally larger than those observed for vapour-deposited films of corresponding thicknesses 16. 600
,~ 5oo 0
8
400
r~ o
300 0
I 100
I 200 Thickness
I 300
I 400
I, 500
(~,)
Fig. 4. Average spacing of misfit dislocations
vs.
deposit thickness.
At a deposit thickness of ~ 100 A, moir6 fringes became visible. The fringes were badly distorted and discontinuous. Only for rather thick films, 500 A, did the fringes become more straight and continuous. The separation of the fringes had wide variations for a given film. For example, at the edges of the grooves the fringe spacing was noticeably smaller than elsewhere. The average separation of fringes decreased as the nickel thickness increased (see section 3(d)).
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(c) Stripped films The most noticeable characteristic of stripped nickel films was the occurrence of a high density of small, almost circular, light markings as shown in Fig. 5. Stereo electron micrographs showed that these markings were not surface artefacts but were distributed throughout the volume of the deposit.
Fig. 5. Transmission micrograph of stripped nickel film, average thickness 150 A, showing small light markings. ( x 77000).
Even the very thin (30/~) nickel films were found to be essentially continuous except for local holes and the above mentioned markings. There was in no case any evidence for individual nuclei of any appreciable height. The deposit thickness within the substrate grooves was markedly less than the average thickness, Fig. 5. The deposit consequently had a grain-like structure in correspondence with that of the substrate.
(d) Measurement of misfit The misfit, M, at any particular thickness is defined as the difference between the overgrowth and substrate lattice parameters divided by the lattice parameter of the overgrowth. In the present investigation M could be measured by the following techniques: (1) Measurement of the spacing between moir6 fringes. (2) Measurement of the spacing between misfit dislocations. (3) Transmission high-resolution electron diffraction2°. A difference between deposit and substrate lattice parameters causes a splitting in the corresponding diffraction spots for a given reflection. The misfit follows as the ratio of this splitting to the distance between the undiffracted spot and the appropriate substrate spot. Figure 6 shows the misfit as a function of deposit thickness as determined from (a) moir6 fringe spacing and (b) the splitting of low-index diffraction spots. The two methods Of measurement gave values for M which are in reasonable agreement with each other. The spacing of misfit dislocations yielded values for M which were at any given thickness much lower than the values in Fig. 6. This discrepancy is further discussed in section 4.
ELECTRODEPOSITED NICKEL FILMS ON
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169
0.02
0.015
U
0.01
0.005 0
I 100
I 200 Thickness
I 300
I 400
I 500
(~)
Fig. 6. Misfit as determined from spacing of moir~ fringes (O) and splitting of diffraction spots (/X) as a function of deposit thickness.
A separation of nickel and copper diffraction spots was generally not observed for deposit thicknesses < 100 A. The thickness at which splitting first occurred coincided with the thickness at which moir~ fringes became clearly visible. 4. DISCUSSION Contrast calculations by Ashby and Brown21 show that prismatic loops which intersect a foil surface give rise to asymmetric (anomalous) images. Interstitial loops give dark-field images that are black in the direction of positive g, provided the remaining segment of the loop lies within half an extinction distance from either foil surface. In bright field the sense of the asymmetry depends on whether the loop segment is near the top or bottom surface of the foil: for a loop segment near the top surface bright and dark field images are nearly similar, but near the bottom surface bright and dark field images are almost complementary for the same g. For a vacancy loop the contrast in dark field is light on the side of positive g. Since the unstrained lattice parameter of nickel (3.52377/~) is less than that of copper (3.61496 A) a coherent or semi-coherent deposit of nickel on copper will be in tension. It was furthermore shown that although the very thin deposits were essentially continuous they contained holes which extended through the thickness of the film. For such a coherent or semi-coherent deposit the substrate will of course also be elastically strained, particularly at the periphery of holes, as is schematically illustrated in Fig. 7(a). This strain field will result in a strain contrast which might, at least qualitatively, be similar to that of an interstitial prismatic loop which intersects the foil surface, Fig. 7(b). The contrast effects previously described (section 3(b)) are indeed consistent with the contrast rules for an interstitial loop which intersect the foil surface. The contrast phenomena at the substrate grooves (section 3(b)) can in principle also be explained by
170
H . L . GAIGHER, G. N. VAN WYK
(a)
(b)
Fig. 7. Schematic diagram to show that the elastic strain field at (a) the periphery of a hole in an otherwise continuous deposit might be expected to result in strain contrast qualitatively similar to that of (b) an interstitial loop which intersects the foil surface.
considering the elastic strain fields which exist at these grooves due to the grainlike structure (section 3(a)) of the deposit, the latter being in a state of tension. The fact that no moird fringes or splitting of diffraction spots were observed for deposit thicknesses < 100 /~ does not necessarily imply pseudomorphism up to thicknesses of ~ 100 A. P r o o f of this statement lies in the fact that application of the high resolution electron diffraction technique to duplex films consisting of nickel evaporated (vacuum 10-s torr) on to copper substrates failed to show a splitting of nickel and copper spots for very thin (say -~ 25/~,) deposits, although transmission micrographs showed the overgrowths to be non-coherent as evidenced by networks of interfacial dislocations 22. It is suggested that for these films the strain fields associated with the relatively widely spaced dislocations are so highly localized, i.e. the proportion of non-coherent material is so very low, that diffraction patterns give rise to no measurable effect. The absence of dislocations and moird fringes in the very thin (--~30 /~,) films strongly suggests a coherent deposit. For thicker films the different contrast effects discussed complicate the observations to the extent that unambiguous interpretations become difficult. The misfit measured for the electroplated films, Fig. 6, was found to be significantly smaller than the misfit as measured, at corresponding thicknesses, for vapour-deposited nickel on copper. For a vapour-deposited nickel film with a thickness of ~ 100 A for example, Matthews and Crawford 16 as well as Gradmann 23 observed misfits which are about twice as large as the misfit measured for 100 A thick electroplated nickel (Fig. 6). This disagreement could probably be due to the incorporation of foreign material into the deposit. These foreign substances could be hydrogen, coordinated water, basic material (Ni(OH)2 e.g.,) etc. s Small quantities of copper could dissolve from the substrate 9 in which case copper will also initially be codeposited with nickel. Incorporation of these substances into the deposit could cause an expansion of the nickel lattice as a result of which part of the misfit between the nickel and copper is relieved. Small structural features revealed by transmission micrographs of stripped films, Fig. 5, could be pores due to hydrogen evolution and perhaps also amorphous, basic inclusions.
ELECTRODEPOSITED NICKEL FILMS ON (001) COPPER SUBSTRATES
171
These structural features will furthermore act as obstacles to the passage of dislocations in the deposit, and might inhibit the formation of misfit dislocations by mechanisms involving glide of pre-existing dislocations 24. As compared with vapour-deposited films the above-mentioned factors will contribute to a lower misfit and will also increase the thickness at which coherency breaks down. This could then also explain the absence of misfit dislocations and moir6 fringes from very thin (say, < 50 A) electrodeposits of nickel on copper. According to Frank and van der Merwe 25 the misfit at the interface of epitaxial bicrystals is accommodated by elastic strain and misfit dislocations. All the misfit in excess of that eliminated by elastic strain is accommodated by misfit dislocations. Quantitative experimental support for this theory has been obtained for vapour-deposited films 16' 26 In the present study the density of misfit dislocations increased, and the separation of moir6 fringes decreased as the nickel thickness increased. This observation does suggest that misfit dislocations were generated to accommodate part of the difference between the lattice parameters of nickel and copper. However, the observed dislocation density was generally much too low to account for the measured misfit. For a deposit thickness of 300 A for example, application of the B r o o k s 27 formula shows the observed spacing of misfit dislocations (,-~ 380 A, Fig. 4) to be about 4.5 times too large to account for the measured misfit of ,-~0.015 (Fig. 6). A lack of definite correspondence between moir6 fringes and misfit dislocations has already been pointed out by Yagi e t aL 26 for the case of evaporated bicrystals where local places of incoherency occurred. Such a local loss of incoherency is even more conceivable for electroplated films due to interference of gas bubbles and other foreign substances at the cathode. Furthermore, imperfect alignment of the deposit and other imperfections due to the incorporation of foreign substances will certainly have an important effect on the actual misfit to be accommodated by dislocations. These effects will be further discussed in a future publication. ACKNOWLEDGEMENTS The authors wish to thank Prof. Dr. J. H. van der Merwe for his interest and encouragement, the University o f Pretoria for financial support, and the Council for Scientific and Industrial Research as well as the Atomic Energy Board for the use o f X-ray microanalysis attachments and scanning electron microscope facilities. REFERENCES 1 2 3 4 5 6 7
B.C. Banerjee and P. U Walker, J. Electrochem. Soc., 109 (1962) 436. R. Weil and H. C. Cook, J. Electrochem. Soc., 109 (1962) 295. J.A. Crossley, P. A. Brook and J. W. Cuthbertson, Electrochim. Acta, 11 (1966) 1153. T.P. Hoar and D. J. Arrowsmith, Trans. Inst. Met. Finishing, 36 (1958-59) I. R. Weil, Plating, 58 (1971) 137. L. Reimer, Z. Metallkunde, 47(1956) 631. E.J. Suoninen and T. Hakkarainen, J. Mater. Sci., 3 (1968) 446.
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n . L. GAIGHER, G. N. VAN WYK W. Cochrane, Proc. Phys. Soc., 48 (1963) 723. R.C. Newman, Proc. Phys. Soc. B, 69 (1956) 432. E.R. Thompson and K. R. Lawless, AppL Phys. Letters, 9 (1966) 138. K . R . Lawless, J. Vac. Sci. TechnoL, 2 (1965) 24. E.W. Dickson, M. H. Jacobs and D. W. Pashley, Phil. Mag., 11 (1965) 575. A . G . Ives, J. W. Edington and G. P. Rothwell, Electrochim. Acta, 15 (1970) 1797. L.E. Murr and M. C. Inman, Phil. Mag., 14 (1966) 135. H. Fischer, Elektrolytische Abscheidung und Elektrokristallisation yon Metallen, Springer, G6ttingen (1954) 634. J.W. Matthews and J. L. Crawford, Thin Solid Films, 5 (1970) 187. D . L . Carr, Rev. Sci. Instr., 40 (1969) 965. J.W. Matthews, Phil. Mag., 12(1965) 1143. M.J. Stowell and T. J. Law, Phil. Mag., 19 (1969) 1257. R. Kuntze, A. Chambers and M. Prutton, Thin Solid Films, 4 (1969) 47. M . F . Ashby and L. M. Brown, Phil Mag., 8 (1963) 1649. G . N . Van Wyk, Thesis, University of Pretoria (1972) (unpublished). U. Gradmann, Ann. Phys. Lpz., 13 (1964) 213. J.W. Matthews, Phil. Mag., 13 (1966) 1207. F.C. Frank and J. H. van der Merwe, Proc. Roy. Soc. (London), A198 (1949) 216. K. Yagi, K. Takayanagi, K. Kobayashi and G. Honjo, J. Crystal Growth, 9 (1971) 84. H. Brooks, Metal Interfaces, Am. Soc. Metals, Metals Park, Ohio, 1952, p. 20.