Transmission electron-microscope observations of the structure of electrolytically deposited copper and its annealing behaviour

Electrochimica Acta. 1964. Vol. 9. pp. 925 to 928. Pergamon Press Ltd. Printed in Northern Ireland

TRANSMISSION ELECTRON-MICROSCOPE OBSERVATIONS OF THE STRUCTURE OF ELECTROLYTICALLY DEPOSITED COPPER AND ITS ANNEALING BEHAVIOUR* T. G. STOBBE, F. H. HAMMADand M. L. RUDEE Department

of Materials Science, Stanford University, Stanford, California, U.S.A.

Abstract-Transmission electron-microscope studies have been made of high purity copper samples electrolytically deposited at room temperature from an acid copper electrolyte at current densities of l-1, 2.7 and 5.4A/dma. Samples were deposited in the bulk and thinned for observation. The microstructure was dominated by very fine growth twins at the low deposition rates; the higher deposition rates reduced the number of twins, and increased the number of dislocation tangles and grown-in stacking faults. Annealing the 2.7 A/dmB samples at 150°C increased the dislocation density, while at higher temperatures recrystallization occurred. Samples deposited in cyanide and pyrophosphate copper baths showed a very fine grain size. R&m&Des khantillons de cuivre de haute puretd obtenus par d&p& 6lectrolytique rl temperature ordinaire B partir de bains cuivriques acides et B des densites de courant de l,l, 2,7 et 5,4 A/drn’ ont Les khantillons depost% ont 6th amincis dti &udi& par microscopic tlectronique de transmission. pour les observations. La microstructure est domin& par la t&s fine croissance de jumeaux aux basses vitesses de dep&; les vitesses plus blev&s rkduisent le nombre de jumeaux et augmentent le nombre d’enchev&e.ments de dislocations et de fissures d’empilement. Le recuit & 150°C des tihantillons obtenus B 2,7 A/dms augmente la densiti des dislocations, tandis qu’il de plus hautes temperatures une recristallisation a lieu. Les khantillons depoti & partir de bains cuivriques de cyanure et de pyrophosphate exhibent une t&s fine dimension de grains. Zusammmfw-Hochreines elektrolytisches Kupfer wurde durch Transmission-Elektronenrnikroskopie untersucht. Das Metal1 wurde aus sauren Btidern bei Stromdichten von 1,l 2,7 und $4 A/drng bei Zimmertemperatur abgeschieden. Fiir die mikroskopische Untersuchung wurden die abgeschiedenen Schichten diinner gemacht. Bei geringen Abscheidungsgeschwindigkeiten besteht die Mikrostruktur haupts&hlich aus sehr kleinen Wachstumszwillingen. Erhijhung der Abscheidungsgeschwindigkeit verkleinert die Anzahl Zwillinge und vergriissert die Zahl von VersetzungsGliihen der mit 2,7 A/dmB abgeschiedenen Proben bei knaueln und eingewachsenen Stapelfehlem. 150°C erhiiht die Versetzungsdichte, wiihrend bei hiiheren Temperaturen Rekristallisation auftritt. Metallschichten, die aus Cyanid- und Pyrophosphat-Bldern abgeschieden wurden, hatten eine sehr kleine Komgriisse. INTRODUCTION

of an electrolytically deposited metal is controlled by several variables, such as the substrate structure, the deposition rate, and the composition and temperature of the solution. Control of these variables can yield information on the mechanisms involved in the electrodeposition process. Transmission electron microscopy allows direct observation of structural defects in crystals1 and can provide information about the effect of experimental variables. Transmission-electron-microscopy studies of copper have been performed by Steinemann and Hintemanq2 who studied the detailed effects of the substrate structure and the concentration of inhibitor in the solution on the structure of the electrodeposit. Copper was electrolytically deposited on rolled and annealed, mechanically polished, or vapour-deposited copper substrates at 2 A/dm2 from an acid copper solution. The specimens examined were about 30 pm from the substrate/deposit * Manuscript received 16 October 1963. THE structure

^^_

T. G.

926

STOEBE,

F. H. HAMMADand M. L. RUDEE

interface. For an inhibitor-free solution, the substrate grain and dislocation structure was reproduced in the electrodeposit. We now describe observations, made by transmission electron microscopy, of the structure produced in copper electrodeposited from an acid copper solution at three current densities, and from cyanide and pyrophosphate baths at one current density; and that of specimens heated at several temperatures after deposition. The samples were prepared by a technique which minimized the influence of the substrate on the nature of the deposit. EXPERIMENTAL

PROCEDURE

AND

RESULTS

To minimize substrate influence on the structure of the deposited layer, an aluminum substrate was used. Reproduction of the substrate is unlikely in this case because of the large difference in the lattice parameters (Cu, 3.62 A; Al, 4.05 A). The twin boundary energy of aluminum is the highest of fee metals and twins are rarely observed.s Stacking faults have never been observed.4 As will be shown, both twins and stacking faults dominate the structure of the copper electrodeposits. Since neither were present in the substrate, they were assumed to be a product of the deposition process. Samples were deposited on a zincated aluminum substrate. The acid copper electrolyte consisted of an aqueous solution of 15 M C&O, and 0.77 M HsSO,. Samples were deposited at current densities of l-1,2*7 and 5.4 A/dma at 21°C with moderate bath agitation, to a thickness of about 80 pm. To improve adhesion to the substrate, a “strike” layer of thickness less than O-5pm was plated in the pyrophosphate bath described below before plating in the acid copper bath. This layer was later removed during electropolishing. Spectroscopic analysis of the deposits showed purities in excess of 99.998 per cent. Samples were also prepared in cyanide and pyrophosphate copper baths at 3.2 A/dm” current density. The former was a DuPont Coppralyte number 661 bath, containing 1.5 weight per cent sodium thiocyanate brightener; the latter was a Unichrome pyrophosphate bath containing O-125 volume per cent PC-l brightener. The aluminum substrate was dissolved away from the copper layer in a sodium hydroxide solution which was maintained below room temperature to avoid annealing effects. The bulk samples were thinned by electropolishing at 10°C and at 5 to 6 V in a solution of 60 volume per cent orthophosphoric acid and 40 volume per cent distilled water. The position of the thinned samples was approximately in the centre of the deposited layer, about 40 ,um from the substrate. The thinned samples, with thicknesses of about 2000 A, were observed in a Hitachi HU-11 electron microscope operated at 100 KV. As-deposited

samples

The microstructure of copper electrodeposited in the acid copper bath at 1-I A/dm2 current density was characterized by the presence of a large number of fine growth twins, illustrated in Fig. 1. These twins were found in almost every grain and range from 100 to 10,000 A in thickness. Dislocations and stacking faults were rarely observed at this deposition rate. Increasing the current density to 5.4 A/dm* produced many stacking faults and greatly reduced the number of twins observed. As shown in Fig. 2, large areas of

Electron microscopy of electrolytically deposited copper

927

high dislocation density were also seen at this high deposition rate. The dislocations seen in the as-deposited samples usually occurred in tangles near grain boundaries and were rarely randomly dispersed inside a grain. The stacking faults (the fringed areas in Fig. 2) are a result of the deposition process. They were present on first viewing the samples, and almost always extended from one side of a grain to the other, which eliminated the possibility that they were formed in the electron microscope as a result of bombardment by the electron beam. As previously observed in pure copper,5 dislocations have been seen to dissociate and form stacking faults if one particular area was illuminated for several minutes. Those shown in the figures were neither created nor altered in any way by lengthy observation in the microscope and were, therefore, grown-in. Figure 3 shows the profusion of grown-in stacking faults seen in certain areas of the 5.4 A/dm2 sample. Samples deposited at 2.7 A/dm2, a current density typical of commercial practice, from the acid copper solution exhibited characteristics of both the 1.1 and the 5.4 A/dm2 specimens. As shown in Fig. 4, many fine twins were present, but regions of high dislocation density and some grown-in stacking faults were also observed. Selected area electron-diffraction patterns indicated that no crystallographic texture existed in any of the ,acid bath deposits. In addition, it was observed that the grain size decreased with increasing current density. At 1.1 A/dm2, grain sizes were typically 1 to 10 pm, whereas at 5.4 A/dm2, they were O-2to 2 ,um. These observations are consistent with previous work.* Microstructures of samples plated in the cyanide and pyrophosphate baths at 3.2 A/dm2 showed a much smaller grain size, about 0.1 pm, as shown in Fig. 5 for the cyanide bath samples. Stacking faults were present, although none are in the figure, while twins were absent. The microstructure of the pyrophosphate bath samples was similar to those of the cyanide bath samples except that about 15 per cent of the microstructure consisted of grains which were ten to twenty times larger than the rest. Annealed structure

The annealing behaviour of samples plated from the acid copper solution at 2~7 A/dm2 was investigated by observing microstructures of samples separated from the substrate and heated for one hour in hydrogen at temperatures between 100 and 600°C. Annealing at low temperatures significantly increased the dislocation density over the whole specimen. Furthermore, in samples annealed at 100, 150 and 200°C it was observed that dislocation tangles usually appeared near twin boundaries, which probably demonstrated a mode of stress relief. This is illustrated in Fig. 6. Microstructures observed after annealing at 200 and 300°C showed the presence of large, dislocation-free grains that were recrystallized since the fine twins were no longer present. Figure 7 shows a typical area containing some of the recrystallized grains and some of the original grains, which still contained large numbers of dislocations. Numerous stacking faults still existed after annealing at 200 and 3OO”C,generally occurring in grains with high dislocation densities. Occasional annealing twins, much larger than the fine growth twins, were observed in the recrystallized grains. Annealing at 400°C produced an almost completely recrystallized structure, Fig. 8. These specimens still contained occasional strained areas, which disappeared on annealing at 600°C.

928

T. G. STOEBE,F. H. HAMMADand M. L. RUDEE DISCUSSION

The observation of twins was expected in electrodeposited metals, where the rate of addition of atoms at the growth surface was relatively high. Buerger’s theory of growth twinning7 predicts that if the rate of addition of atoms to the surface is rapid enough, there is a high probability that one atom which arrives at the growth in a twin position will remain in that position until it is joined by other atoms. Clusters of atoms in twin positions will be relatively stable, allowing the twin to grow. Twin formation will be enhanced if the boundary between twinned and untwinned regions is of low energy, as in copper.s Buerger’s theory, in apparent contradiction to the present research, predicts that high current densities should result in a high number of twins. This discrepancy can be explained by the increased number of stacking faults produced at the higher current densities. Both stacking faults and twins are caused by mistakes in the stacking of (111) planes. Since a stacking fault is two successive twin faults, it can be postulated that at high deposition rates both types of stacking defects occur, while at low deposition rates the growth process is slow enough that one mistake in stacking will not be followed by another. This would result in an increase in the number of stacking faults and a decrease in the number of twins as the current density is raised, as observed. The density of twins and stacking faults observed in the present research was much greater than that observed by Steinemann and Hintemann.2 Their deposits of copper on copper duplicated the structure of the substrate. The twins and stacking faults observed in our investigation could not have been formed by direct reproduction of the aluminum substrates used since these defects are not present in aluminum. Although the twin and stacking faults could have been caused by the differing lattice parameters of copper and aluminum, the dependence of the nature of the defects on the deposition rate indicate that in this system the deposition rate is more important than the nature of the substrate in determining the type of structure produced. It is interesting to note that, on annealing, recrystallization occurred without prior plastic deformation, which indicated that stresses were present in the samples as a result of the electrodeposition process. The increase in dislocation density observed on annealing at low temperatures is also interesting, since the opposite would be expected in a normal recovery process. Acknowledgements-The authors wish to thank Mr. J. A. Pope of the Stanford Linear Accelerator Center for preparing the electrodeposits, and Dr. J. B. Kushner of Evansville College for valuable discussions. One author (T. G. S.) wishes to thank the United States Atomic Energy Commission, and another author (F. H. H.) the Government of the U.A.R., for fellowship support. This work was partially supported by the Office of Naval Research as well as the Advanced Research Projects Agency, through the Center for Materials Research at Stanford University.

REFERENCES 1. P. B. HIRSCH,A. HOWIEand M. J. WHELAN,Phil. Trans. Roy. Sot. A 252,499 (1960). 2. S. STE~~+~ANNand H. E. -MANN, Schweiz. Arch. Angew. Wissen u. Techn. 26,202 (1960). 3. R. L. FULLMAN, quoted by J. C. FISHERand C. G. DUNN, Imperfections in Nearly Perfect Crystals, p. 343. Wiley, New York (1952). 4. G. THOMAS,Transmission Electron Microscopy of Metals, p. 197. Wiley, New York (1962). 5. J. T. Fouars and R. J. MURPHY,Phil. Mag. 6, 1069 (1961). 6. W. H. SAFRANEK and J. H. Wumm~, Acid Copper Electroplating and Electroforming in Modern Electroplating, ed. A. J. GRAY. Wiley, New York (1953). 7. M. J. BUIZRGER, Amer. Min. 30, 469 (1945). 8. M. C. INMANand A. R. KHAN, Phil. Mag. 6,937 (1961).