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Surface topology changes during electromigration in metallic thin film stripes
Thin Solid Films -
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
SURFACE TOPOLOGY CHANGES DURING ELECTROMIGRATION IN M E T A L L I C T H I N F I L M STRIPES L. BERENBAUMAND R. ROSENBERG I B M Watson Research Center, Yorktown Heights, N.Y. (U.S.A.)
(Received July 14, 1969)
SUMMARY Electromigration damage in silver and aluminum thin film stripes was observed by scanning electron microscopy. Early formation of voids and surface growths occurred in regions of maximum temperature and in the same crosssection. Once formed, void damage was comparatively stable, and failure ultimately occurred at the cathode end of the stripes. The growths in the aluminum stripe were nucleated at grain boundaries and were restricted to the boundary. The flux of atoms into one growth was approximated to be 2 × 10 s atoms h-1. Growths in the silver stripe were not restricted by the grain size but had the appearance of surface undulations. Large protrusions appeared in coincidence with a catastrophic upswing in stripe resistance. It is felt that local failure of a thin oxide and subsequent "extrusion" may be responsible for this behavior.
INTRODUCTION Recently, attention has been drawn to mass transport in thin films as it is influenced by a d.c. current, or "electron wind ' ' 1 - 7. The interest in films stems primarily from the high current densities ( ~ 10 6 amp/cm 2) that can be attained with comparatively little joule heating, the wide interest in integrated circuit metallization, the large range of temperatures over which the tests can be performed, and the creation of holes or growths in the test stripe by local mass divergencies. Blech and Meieran 1 and Rosenberg and Berenbaum z have illustrated local hole formation in grain boundaries and gross film thinning by transmission electron microscopy, suggesting the transport mechanism to be mainly boundary controlled, at least at temperatures lower than 175 °C. Rosenberg and Berenbaum have substantiated this by measuring an activation energy of about 0.5-0.6 eV for the migration by a resistance-temperature technique, although this may prove Thin Solid Films, 4
(1969) 187-204
1 88
L. BERENBAUM, R. ROSENBERG
to be limited for detailed study in that it averaged the effects of all local disturbances rather than a single occurrence. The objectives of the work reported here are to follow the localized changes in surface topology in detail during a test by in situ scanning electron microscopy, to determine the feasilibity of using these observations to obtain kinetic data for electromigration, and to assist in developing mechanisms by which local damage takes place. With this technique, both holes and growths can be analyzed simultaneously in detail. Of importance is consideration of the relative contributions of the different driving forces for mass divergence, namely gross and local temperature gradients and grain-boundary triple-point structural configurations. Damage in both silver and aluminum stripes was characterized to illustrate differences in transport and changes in surface topology in the two systems.
EXPERIMENTAL PROCEDURE
Sample preparation A silver film, 7100 A. thick, was deposited at a pressure of 10 -6 torr on a heated (200 °C), thermally oxidized silicon wafer in a conventional vacuum system utilizing an oil diffusion pump and cooling baffles. The silver was photoprocessed into stripes 10 rail×0.3 mil with 20 mil square contacts on each end by well-known techniques using K T F R photoresist. The wafer was cut into chips, the chips were mounted on a header, and one-rail gold wire leads were attached. A four-point probe method was used for monitoring the resistance. The aluminum film was deposited in a high-vacuum system by electron beam techniques. The pressure within the system prior to deposition was 4.2 ~ 10 ~ tort and rose to 1.7x10 -7 torr during evaporation. The film was deposited to a thickness of 3700 A. at a rate of 10-20 A/sec on a thermally oxidized silicon substrate heated to 400 °C. The film was photoprocessed and handled as described earlier for silver, except that one-mil aluminum wires were used for bonding.
Method of testing The experiment was conducted & situ in a "Stereoscan" Scanning Electron Microscope (SEM) manufactured by the Cambridge Instrument Co. This instrument utilizes a 20 kV electron beam which scans the specimen surface with a variable frequency between 3.3 and 100 c/s. It differs from conventional electron microscopy in that it looks at the secondary electron emission, and permits direct visual non-destructive examination of the stripe surface, i.e. it does not require surface replication. The high magnification (50,000 ×), large depth of focus, and Thin Solid Films, 4 (1969) 187-204
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a resolution of 200-500 A at a working distance of 10 mm at 20 kV provides the necessary versatility. The header was placed in a holder having three degrees of freedom and mounted in the SEM vacuum chamber, which was held at 2 x 10- 5 torr. Electrical leads were connected from the header to the outside world, and a constant current power supply was used to power the stripe. Electron micrographs were taken periodically, the elapsed time interval being based on the change in slope of resistance vs. time curves. Observation of the surface was made at angles of 45 ° and 82 ° from the specimen normal, the latter to provide a profile perspective of surface growths.
EXPERIMENTAL RESULTS
Silver
The power input to the silver test stripe was increased in steps of 1 x l 0 6 amp/cm 2 until a time-dependent resistance change was detected, allowing observation of damage within a reasonable period of time (i.e. days to failure). Restriction of the ambient temperature by in situ testing necessitated the use of a current density of 9 x 10 6 amp/cm 2 to produce a resistance change of about 0.3 ~/h. At this power level, joule heating raised the average stripe temperature to about 135 °C. Figure l(a) represents the surface topology of a section of the stripe after a 1 ~o change in resistance had taken place. Figures l(b)-(d) were taken after a resistance change of about 10%. Clearly, the surface structure changes drastically and continuously with time and can be related to the corresponding resistance change observed. Initially the growths are homogeneously distributed over the stripe length, although slightly more dense in the central region, and are about equivalent to the grain size. With continued powering the average growth diameter does not appear to be restricted by grain size. A rapid increase in resistance occurred after 40 h of stressing at 9 x l 0 6 amp/cm 2, and lasted for about 1.5 h until failure. During this final stage, the surface structure changed from that of Figs. l(b)-(d) to that of Figs. 2(a)-(d). Such a catastrophic short time effect, which was characteristic of most of the stripe, can be a result of a self-accelerating damage mechanism or local breakdown of damage resistance (e.g. the fuse-type of failure found at the cathode (see Fig. 2(e)) is a strong indication of a temperature run-away concept). At least two different types of excessive surface growths were found. The first, illustrated in Figs. 2(a), (b) and (d), was the more common and was characterized by growth facets and a "hollowness" (see in particular Fig. 2(b)). The second gross change in surface structure occurred at the anode end of the stripe and was more of a Thin Solid Films, 4 (1969) 187-204
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Fig. 1 (a) en (b). Thin Solid Fihns, 4 (1969) 187 204
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Fig. l. Scanning electron micrographs o f the surface o f the silver stripe. (a) A n o d e end after powering to 1 ~ change in resistance. (b)-(d) Sections o f the stripe after a 10 ~ change in resistance. ( × 10,000, reduced x 4) Thin Solid Films, 4 (1969) 187-204
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Fig. 2 (a) en (b). (For caption see pa~ze 194). Thin Solid Films, 4 (1969) 187-204
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SURFACE TOPOLOGY CHANGES DURING ELECTROMIGRATION
Fig. 2 (c) en (d). (For caption see page 194). Thin Solid Films, 4 (1969) 187-204
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Fig. 2. Surface of silver stripe after failure. (a)-(d) Surface growths shinning faccting and a "'hollowness" and (e) " f u s e - t y p e " failure region. ( I 0 , 0 0 0 1
broad undulation rather than the more crystallographic projections just described. This is what normally would be expected for the effects of a gross temperature gradient and is just the antithesis of the mass depletion at the cathode end. The interesting feature of this growth is the preservation of the initial surface topology throughout the test cycle. In fact, as shown in Fig. 2(d), some of the original surface irregularities (Fig. l(b)) have rotated with respect to the stripe surface, even though they are unchanged in appearance and are located on the growths. In addition to the surface growths, a uniform distribution of holes (or "'voids") was found in the stripe. Figure 3 shows these holes to be located in grain boundaries and to be similar in appearance to those seen by transmission microscopy ~'2. Once having formed, further growth of the holes was drastically reduced. The question of hole stability or instability is significant and will be considered in a later section. An observation of interest was the formation of holes at the anode end where temperature gradients would be expected to produce accumulation of mass rather than vacancies. One such configuration is the opening shown in Fig. 4. Without changing the direction (),/'electron .[tow, continued powering reversed the trend and caused healing, or closing of the hole. Thin Solid Films, 4 (1969) 187-204
SURFACE TOPOLOGY CHANGES DURING ELECTROMIGRATION
Fig. 3. Scanning electron m i c r o g r a p h s m a d e at 45 ° angles showing hole formation.
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Fig. 4. (a) en (b).
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Fig. 4. Hole near positive end of stripe. (a) 82°, (b) 45" and (c) same region at a later time showing how the hole has been partially filled.
Aluminum
The test stripe was powered in air at 7 × l 0 6 amp/cm 2 and average temperature of 100 °C. Figure 5 (a) illustrates the surface topograph before test. The initial as-deposited structure was quite rough with deep grain-boundary grooves. On powering, a uniform distribution of holes was produced, the holes appearing to be caused by deepening of the grooves. These holes were found very early in the specimen life but had no apparent effect on the current-carrying capacity; that is, once the initial rash of hole formation was over, further growth was more localized in the test stripe and occurred much later in the power cycle. This behavior was very similar to that found in the silver stripe where, in both instances, there was no striking connection between the initial hole distribution and location of the failure. The surface growths were of a different nature than those shown for silver, although they also were uniformly distributed. (It is important to note that both holes and growths were found simultaneously in the same location of the stripe, casting doubt on the effectiveness of the macroscopic temperature gradient as the driving force for the mass flux divergencies needed for the damage observed at this stage of the test.) The growths in the aluminum stripe were related to the Thin Solid Films, 4 (1969) 187-204
198
Fig. 5 ('a) en (b). (F~r captioJt .see pu~,e 200). Thin Solid Films, 4 (1969) 187-204
I. BERI!NBAUM, R. ROSt!NBERG
SURFACE TOPOLOGY CHANGES DURING ELECTROMIGRATION
Fig. 5 (a) en (b). (For caption see page 200). Thin Solid Films, 4 (1969) 187-204
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Fig. 5. High magnification o f a region o f the aluminum stripe, !a) betk)re powering, (circle locates grain boundary from which growth emanated), (b) alter 152 h, (c) after 298 h, ~d) detail o f large growth shown in (c), (e) fuse-type failure region.
grain structure much more so than were the growths in silver. Figures 5 (b)-(d) trace the nucleation and extension of the growths in a typical area of the stripe. Significantly, several peaks (including the highest) emanate from grain boundaries, as can be seen by close inspection of Figs. 5 (a) and (b). This represents the case of an atomic flux divergence in combination with grain boundary diffusion whereby mass is accumulated. Also the unidirectional nature of the growths and the approximate breadths of 1 /~m indicate the possibility that these could be whisker nuclei.
DISCUSSION
The observations generally confirm the grain-boundary nature of the electromigration. The direction of motion of both aluminum and silver ions in boundaries is towards the anode, which is similar to the results of Penney 8 and Ho and Huntington 9 for lattice migration in bulk samples. The following discussion will be directed more toward understanding possible divergencies in the atomic flux than in obtaining numerical values for the parameters that would appear in the flux itself. Thin Solid l~Thl~s 4 (1969) 187-204
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The general appearance and topographical changes that take place during the electromigration of the stripes are self-evident from the photographs. There are several aspects of the observed pre-failure damage that require comment and relate to possible mechanisms. Transient vs. static fields
The location of initial damage sites in both specimens was in the region of highest temperature. This, in combination with the observation that holes and growths are found in the same cross-section, removes the gross temperature gradient as the major driving force for early damage. A uniform distribution of local driving forces for atom flux divergencies must exist and must be superimposed onto the macroscopic temperature profile. These could be structural irregularities, grain-boundary triple points, hot spots, etc. The important point is the stability of the defects once produced. Although some link-up of holes by current crowding was noted, in general laterial extension was not observed. Growths, on the other hand, extended continuously. Once holes are formed, the local gradients that produced the driving forces disappear (e.g., recent work 2 showed non-adhering spots in A1 stripes on NaC1 to be nucleation centers for early hole formation). (On formation of the holes, the hot spots dissipated and the holes did not propagate.) In the case of growth extensions, where material does not leave the area of the gradient, the driving force is maintained. Healing of the holes at the anode end illustrates an important phase of the test cycle, namely, the onset of the effects of the macroscopic temperature profile. This necessarily takes over later in the test cycle because of the lower nominal temperature involved and the spreadout gradient. The "stable" holes that were previously formed and are located in the region of maximum temperature gradient then become the most probable failure sites. The implication is that early damage produced by transient fields act to reduce failure time by providing nuclei for the gross damage that results from the static field. It is likely that the transient effects are most susceptible to the presence of damage nucleating sites caused by local structural irregularities such as grain boundary grooving and hillocks, impurity centers, triple points, etc., while the static effect is related more to gross parameters that affect transport rates, such as grain size and specimen configuration. Both are related to general diffusion parameters. Surface growths
Although the holes appeared to be similar in form and distribution in the silver and aluminum stripes, the surface growths were not. Silver was characterized by growths seemingly unrelated to grain size, uniformly covering the entire stripe surface, and in the final stage of test, much surface faceting. The final stage where Thin Solid Films, 4 (1969) 187-204
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catastrophic growth appeared may well be associated with local mechanical breakdown of a thin silver oxide, Ag20, by increased growth-induced stress fields, and "extrusion" of the silver by free surface diffusion and progressively higher temperatures. The likelihood of the presence of such an oxide is indicated by results of Kubaschewski and Hopkins ~°. In general, the electromigration behavior of silver appears to be controlled by surface active diffusion, which is reasonable, based on the low-activation energy indicative of surface mobility of silver ions. Close inspection of individual growths in the aluminum stripe reveals the following: (1) the growths were much more localized, leaving most of the surface area unchanged, (2) nucleation sites were individual grain boundaries somewhat parallel to the specimen axis, (3) the configuration at the tip of a growth remained constant (see Fig. 5, for example), indicating extension from the base rather than addition of the atoms to the tip by surface diffusion, and (4) there does not seem to be a uniform relationship to mixed grain structures; that is, as often as not, growths occur where holes would be expected and where an equiaxed structure exists. These observations and the fact that holes and growths form together strongly indicate a local boundary interaction model in which positive or negative divergencies can be produced. Such a possibility is the grain-boundary triple-point configuration (junction of three grain boundaries to form a line perpendicular to electron flow) in which mass flow in and out of the boundary intersection must be balanced (one boundary path in, two paths out). It should be pointed out that the divergence at a triple point can result from either the relative angles between adjacent boundaries or by a discontinuity of diffusivity at the junction (for example, change in boundary energies). An approximate rate of atomic buildup at a particular location can be determined by the present technique. This was done for one of the peaks in Fig. 5 (the highest) with the results shown in Fig. 6. Although the rate of total accumulation appears to increase with time, a correction should be made for the area through which the atoms are being supplied. Since the base of the peak can r
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SURFACE TOPOLOGYCHANGESDURING ELECTROMIGRATION
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be considered to be a grain boundary (the growth, being a single crystal, cannot grow epitaxially on both sides of the boundary); it follows that the boundary area supplying atoms increases with time. Replotting the data as (N/A), where A is the total boundary area, results in a constant flux of atoms through the peak base of the order of 2 × 1019 atoms cm-2 h-~. Considering an average boundary area of about 10- ~ cm 2 for the peak base plus the original boundary, the number of atoms piled up per hour is about 2 × 108, which is of the same order as the total number of atoms in the boundary region. Extrapolation of the results to zero N indicates the existence of a relatively short incubation period of about 20 h before the growth began.
CONCLUSIONS 1. Stripes have been powered in situ in the scanning electron microscope. Several observations were made at an angle of 82 ° from the specimen normals to accentuate surface growths. 2. In both silver and aluminum stripes, holes and growths were found in the same specimen cross-section at positions of maximum temperature of the stripe. It is proposed that "transient" fields are superimposed onto gross temperature profiles to initiate early damage. This damage appears to be relatively stable, and only when the gross effects take over does the actual failure mode come into evidence. The distribution of these early damage sites will influence the overall lifetimes by providing failure nuclei. 3. In silver, growths extend over many grains and have the appearance of surface undulations. The original surface microstructure remains unchanged in the undulated regions. A catastrophic resistance increase appeared at the end of the test which corresponded to the production of large faceted surface protrusions. These are thought to originate at ruptures in a thin surface oxide. 4. In aluminum, growths were localized and extended from individual grain boundaries. The rate of atomic buildup in one growth was approximated to be 2 × 1019 atoms cm -2 h - I by measuring ( N / A ) vs. time, where N is the number of atoms in the peak and A is the area of the peak base plus adjacent boundary.
ACKNOWLEDGMENTS The authors would like to thank P. Totta for supplying the silver specimens, C. Bremer and C. Aliotta for taking the SEM photographs, and K. Asai for technical assistance.
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REFERENCES
1 2 3 4
1. A. BLECH AND E. S. MEIERAN,Appl. Phys. Letters, I1 (1967) 263. R. ROSENBERG AND L. BERENBAUM, Appl. Phys. Letters, 12 (1968). P . B . GHATE, Appl. Phys. Letters, 11 (1967) 14. D. CHAABRA, N. AINSLIE AND D. JEPSEN, The Electrochemical Society Meetinq, Dallas,
5 6
W . E . MUTTER, The Electrochemical Society Meeting, Dallas, Texas, May, 1967. R . G . SHEPHEARD AND R. P. SOPHER, The Electrochemical Society Meetinq, Dallas, Texas,
7 8 9 10
J . K . HOWARD AND R. F. ROSS, Appl. Phys. Letters, 11 (1967) 85. R . V . PENNEY, J. Phys. Cheat. Solids, 25 (1964) 335. P . S . Ho AND H. B. HUNTINGTON,J. Phys. Cheat. Solids, 27 (1966) 1319. O. KUBACHEWSKI AND B. Z. HOPKINS, Oxidation o f Metals and Alloys, A c a d e m i c Press, New Y o r k , 1962.
Texas, Mar', 1967.
May, 1967.
Thin Solid Films, 4 (1969) 187-204
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
SURFACE TOPOLOGY CHANGES DURING ELECTROMIGRATION IN M E T A L L I C T H I N F I L M STRIPES L. BERENBAUMAND R. ROSENBERG I B M Watson Research Center, Yorktown Heights, N.Y. (U.S.A.)
(Received July 14, 1969)
SUMMARY Electromigration damage in silver and aluminum thin film stripes was observed by scanning electron microscopy. Early formation of voids and surface growths occurred in regions of maximum temperature and in the same crosssection. Once formed, void damage was comparatively stable, and failure ultimately occurred at the cathode end of the stripes. The growths in the aluminum stripe were nucleated at grain boundaries and were restricted to the boundary. The flux of atoms into one growth was approximated to be 2 × 10 s atoms h-1. Growths in the silver stripe were not restricted by the grain size but had the appearance of surface undulations. Large protrusions appeared in coincidence with a catastrophic upswing in stripe resistance. It is felt that local failure of a thin oxide and subsequent "extrusion" may be responsible for this behavior.
INTRODUCTION Recently, attention has been drawn to mass transport in thin films as it is influenced by a d.c. current, or "electron wind ' ' 1 - 7. The interest in films stems primarily from the high current densities ( ~ 10 6 amp/cm 2) that can be attained with comparatively little joule heating, the wide interest in integrated circuit metallization, the large range of temperatures over which the tests can be performed, and the creation of holes or growths in the test stripe by local mass divergencies. Blech and Meieran 1 and Rosenberg and Berenbaum z have illustrated local hole formation in grain boundaries and gross film thinning by transmission electron microscopy, suggesting the transport mechanism to be mainly boundary controlled, at least at temperatures lower than 175 °C. Rosenberg and Berenbaum have substantiated this by measuring an activation energy of about 0.5-0.6 eV for the migration by a resistance-temperature technique, although this may prove Thin Solid Films, 4
(1969) 187-204
1 88
L. BERENBAUM, R. ROSENBERG
to be limited for detailed study in that it averaged the effects of all local disturbances rather than a single occurrence. The objectives of the work reported here are to follow the localized changes in surface topology in detail during a test by in situ scanning electron microscopy, to determine the feasilibity of using these observations to obtain kinetic data for electromigration, and to assist in developing mechanisms by which local damage takes place. With this technique, both holes and growths can be analyzed simultaneously in detail. Of importance is consideration of the relative contributions of the different driving forces for mass divergence, namely gross and local temperature gradients and grain-boundary triple-point structural configurations. Damage in both silver and aluminum stripes was characterized to illustrate differences in transport and changes in surface topology in the two systems.
EXPERIMENTAL PROCEDURE
Sample preparation A silver film, 7100 A. thick, was deposited at a pressure of 10 -6 torr on a heated (200 °C), thermally oxidized silicon wafer in a conventional vacuum system utilizing an oil diffusion pump and cooling baffles. The silver was photoprocessed into stripes 10 rail×0.3 mil with 20 mil square contacts on each end by well-known techniques using K T F R photoresist. The wafer was cut into chips, the chips were mounted on a header, and one-rail gold wire leads were attached. A four-point probe method was used for monitoring the resistance. The aluminum film was deposited in a high-vacuum system by electron beam techniques. The pressure within the system prior to deposition was 4.2 ~ 10 ~ tort and rose to 1.7x10 -7 torr during evaporation. The film was deposited to a thickness of 3700 A. at a rate of 10-20 A/sec on a thermally oxidized silicon substrate heated to 400 °C. The film was photoprocessed and handled as described earlier for silver, except that one-mil aluminum wires were used for bonding.
Method of testing The experiment was conducted & situ in a "Stereoscan" Scanning Electron Microscope (SEM) manufactured by the Cambridge Instrument Co. This instrument utilizes a 20 kV electron beam which scans the specimen surface with a variable frequency between 3.3 and 100 c/s. It differs from conventional electron microscopy in that it looks at the secondary electron emission, and permits direct visual non-destructive examination of the stripe surface, i.e. it does not require surface replication. The high magnification (50,000 ×), large depth of focus, and Thin Solid Films, 4 (1969) 187-204
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a resolution of 200-500 A at a working distance of 10 mm at 20 kV provides the necessary versatility. The header was placed in a holder having three degrees of freedom and mounted in the SEM vacuum chamber, which was held at 2 x 10- 5 torr. Electrical leads were connected from the header to the outside world, and a constant current power supply was used to power the stripe. Electron micrographs were taken periodically, the elapsed time interval being based on the change in slope of resistance vs. time curves. Observation of the surface was made at angles of 45 ° and 82 ° from the specimen normal, the latter to provide a profile perspective of surface growths.
EXPERIMENTAL RESULTS
Silver
The power input to the silver test stripe was increased in steps of 1 x l 0 6 amp/cm 2 until a time-dependent resistance change was detected, allowing observation of damage within a reasonable period of time (i.e. days to failure). Restriction of the ambient temperature by in situ testing necessitated the use of a current density of 9 x 10 6 amp/cm 2 to produce a resistance change of about 0.3 ~/h. At this power level, joule heating raised the average stripe temperature to about 135 °C. Figure l(a) represents the surface topology of a section of the stripe after a 1 ~o change in resistance had taken place. Figures l(b)-(d) were taken after a resistance change of about 10%. Clearly, the surface structure changes drastically and continuously with time and can be related to the corresponding resistance change observed. Initially the growths are homogeneously distributed over the stripe length, although slightly more dense in the central region, and are about equivalent to the grain size. With continued powering the average growth diameter does not appear to be restricted by grain size. A rapid increase in resistance occurred after 40 h of stressing at 9 x l 0 6 amp/cm 2, and lasted for about 1.5 h until failure. During this final stage, the surface structure changed from that of Figs. l(b)-(d) to that of Figs. 2(a)-(d). Such a catastrophic short time effect, which was characteristic of most of the stripe, can be a result of a self-accelerating damage mechanism or local breakdown of damage resistance (e.g. the fuse-type of failure found at the cathode (see Fig. 2(e)) is a strong indication of a temperature run-away concept). At least two different types of excessive surface growths were found. The first, illustrated in Figs. 2(a), (b) and (d), was the more common and was characterized by growth facets and a "hollowness" (see in particular Fig. 2(b)). The second gross change in surface structure occurred at the anode end of the stripe and was more of a Thin Solid Films, 4 (1969) 187-204
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Fig. 1 (a) en (b). Thin Solid Fihns, 4 (1969) 187 204
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Fig. l. Scanning electron micrographs o f the surface o f the silver stripe. (a) A n o d e end after powering to 1 ~ change in resistance. (b)-(d) Sections o f the stripe after a 10 ~ change in resistance. ( × 10,000, reduced x 4) Thin Solid Films, 4 (1969) 187-204
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Fig. 2 (a) en (b). (For caption see pa~ze 194). Thin Solid Films, 4 (1969) 187-204
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SURFACE TOPOLOGY CHANGES DURING ELECTROMIGRATION
Fig. 2 (c) en (d). (For caption see page 194). Thin Solid Films, 4 (1969) 187-204
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Fig. 2. Surface of silver stripe after failure. (a)-(d) Surface growths shinning faccting and a "'hollowness" and (e) " f u s e - t y p e " failure region. ( I 0 , 0 0 0 1
broad undulation rather than the more crystallographic projections just described. This is what normally would be expected for the effects of a gross temperature gradient and is just the antithesis of the mass depletion at the cathode end. The interesting feature of this growth is the preservation of the initial surface topology throughout the test cycle. In fact, as shown in Fig. 2(d), some of the original surface irregularities (Fig. l(b)) have rotated with respect to the stripe surface, even though they are unchanged in appearance and are located on the growths. In addition to the surface growths, a uniform distribution of holes (or "'voids") was found in the stripe. Figure 3 shows these holes to be located in grain boundaries and to be similar in appearance to those seen by transmission microscopy ~'2. Once having formed, further growth of the holes was drastically reduced. The question of hole stability or instability is significant and will be considered in a later section. An observation of interest was the formation of holes at the anode end where temperature gradients would be expected to produce accumulation of mass rather than vacancies. One such configuration is the opening shown in Fig. 4. Without changing the direction (),/'electron .[tow, continued powering reversed the trend and caused healing, or closing of the hole. Thin Solid Films, 4 (1969) 187-204
SURFACE TOPOLOGY CHANGES DURING ELECTROMIGRATION
Fig. 3. Scanning electron m i c r o g r a p h s m a d e at 45 ° angles showing hole formation.
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Fig. 4. (a) en (b).
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Fig. 4. Hole near positive end of stripe. (a) 82°, (b) 45" and (c) same region at a later time showing how the hole has been partially filled.
Aluminum
The test stripe was powered in air at 7 × l 0 6 amp/cm 2 and average temperature of 100 °C. Figure 5 (a) illustrates the surface topograph before test. The initial as-deposited structure was quite rough with deep grain-boundary grooves. On powering, a uniform distribution of holes was produced, the holes appearing to be caused by deepening of the grooves. These holes were found very early in the specimen life but had no apparent effect on the current-carrying capacity; that is, once the initial rash of hole formation was over, further growth was more localized in the test stripe and occurred much later in the power cycle. This behavior was very similar to that found in the silver stripe where, in both instances, there was no striking connection between the initial hole distribution and location of the failure. The surface growths were of a different nature than those shown for silver, although they also were uniformly distributed. (It is important to note that both holes and growths were found simultaneously in the same location of the stripe, casting doubt on the effectiveness of the macroscopic temperature gradient as the driving force for the mass flux divergencies needed for the damage observed at this stage of the test.) The growths in the aluminum stripe were related to the Thin Solid Films, 4 (1969) 187-204
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Fig. 5 ('a) en (b). (F~r captioJt .see pu~,e 200). Thin Solid Films, 4 (1969) 187-204
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SURFACE TOPOLOGY CHANGES DURING ELECTROMIGRATION
Fig. 5 (a) en (b). (For caption see page 200). Thin Solid Films, 4 (1969) 187-204
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Fig. 5. High magnification o f a region o f the aluminum stripe, !a) betk)re powering, (circle locates grain boundary from which growth emanated), (b) alter 152 h, (c) after 298 h, ~d) detail o f large growth shown in (c), (e) fuse-type failure region.
grain structure much more so than were the growths in silver. Figures 5 (b)-(d) trace the nucleation and extension of the growths in a typical area of the stripe. Significantly, several peaks (including the highest) emanate from grain boundaries, as can be seen by close inspection of Figs. 5 (a) and (b). This represents the case of an atomic flux divergence in combination with grain boundary diffusion whereby mass is accumulated. Also the unidirectional nature of the growths and the approximate breadths of 1 /~m indicate the possibility that these could be whisker nuclei.
DISCUSSION
The observations generally confirm the grain-boundary nature of the electromigration. The direction of motion of both aluminum and silver ions in boundaries is towards the anode, which is similar to the results of Penney 8 and Ho and Huntington 9 for lattice migration in bulk samples. The following discussion will be directed more toward understanding possible divergencies in the atomic flux than in obtaining numerical values for the parameters that would appear in the flux itself. Thin Solid l~Thl~s 4 (1969) 187-204
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The general appearance and topographical changes that take place during the electromigration of the stripes are self-evident from the photographs. There are several aspects of the observed pre-failure damage that require comment and relate to possible mechanisms. Transient vs. static fields
The location of initial damage sites in both specimens was in the region of highest temperature. This, in combination with the observation that holes and growths are found in the same cross-section, removes the gross temperature gradient as the major driving force for early damage. A uniform distribution of local driving forces for atom flux divergencies must exist and must be superimposed onto the macroscopic temperature profile. These could be structural irregularities, grain-boundary triple points, hot spots, etc. The important point is the stability of the defects once produced. Although some link-up of holes by current crowding was noted, in general laterial extension was not observed. Growths, on the other hand, extended continuously. Once holes are formed, the local gradients that produced the driving forces disappear (e.g., recent work 2 showed non-adhering spots in A1 stripes on NaC1 to be nucleation centers for early hole formation). (On formation of the holes, the hot spots dissipated and the holes did not propagate.) In the case of growth extensions, where material does not leave the area of the gradient, the driving force is maintained. Healing of the holes at the anode end illustrates an important phase of the test cycle, namely, the onset of the effects of the macroscopic temperature profile. This necessarily takes over later in the test cycle because of the lower nominal temperature involved and the spreadout gradient. The "stable" holes that were previously formed and are located in the region of maximum temperature gradient then become the most probable failure sites. The implication is that early damage produced by transient fields act to reduce failure time by providing nuclei for the gross damage that results from the static field. It is likely that the transient effects are most susceptible to the presence of damage nucleating sites caused by local structural irregularities such as grain boundary grooving and hillocks, impurity centers, triple points, etc., while the static effect is related more to gross parameters that affect transport rates, such as grain size and specimen configuration. Both are related to general diffusion parameters. Surface growths
Although the holes appeared to be similar in form and distribution in the silver and aluminum stripes, the surface growths were not. Silver was characterized by growths seemingly unrelated to grain size, uniformly covering the entire stripe surface, and in the final stage of test, much surface faceting. The final stage where Thin Solid Films, 4 (1969) 187-204
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L. BERENBAUM, R. ROSENBERG
catastrophic growth appeared may well be associated with local mechanical breakdown of a thin silver oxide, Ag20, by increased growth-induced stress fields, and "extrusion" of the silver by free surface diffusion and progressively higher temperatures. The likelihood of the presence of such an oxide is indicated by results of Kubaschewski and Hopkins ~°. In general, the electromigration behavior of silver appears to be controlled by surface active diffusion, which is reasonable, based on the low-activation energy indicative of surface mobility of silver ions. Close inspection of individual growths in the aluminum stripe reveals the following: (1) the growths were much more localized, leaving most of the surface area unchanged, (2) nucleation sites were individual grain boundaries somewhat parallel to the specimen axis, (3) the configuration at the tip of a growth remained constant (see Fig. 5, for example), indicating extension from the base rather than addition of the atoms to the tip by surface diffusion, and (4) there does not seem to be a uniform relationship to mixed grain structures; that is, as often as not, growths occur where holes would be expected and where an equiaxed structure exists. These observations and the fact that holes and growths form together strongly indicate a local boundary interaction model in which positive or negative divergencies can be produced. Such a possibility is the grain-boundary triple-point configuration (junction of three grain boundaries to form a line perpendicular to electron flow) in which mass flow in and out of the boundary intersection must be balanced (one boundary path in, two paths out). It should be pointed out that the divergence at a triple point can result from either the relative angles between adjacent boundaries or by a discontinuity of diffusivity at the junction (for example, change in boundary energies). An approximate rate of atomic buildup at a particular location can be determined by the present technique. This was done for one of the peaks in Fig. 5 (the highest) with the results shown in Fig. 6. Although the rate of total accumulation appears to increase with time, a correction should be made for the area through which the atoms are being supplied. Since the base of the peak can r
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SURFACE TOPOLOGYCHANGESDURING ELECTROMIGRATION
203
be considered to be a grain boundary (the growth, being a single crystal, cannot grow epitaxially on both sides of the boundary); it follows that the boundary area supplying atoms increases with time. Replotting the data as (N/A), where A is the total boundary area, results in a constant flux of atoms through the peak base of the order of 2 × 1019 atoms cm-2 h-~. Considering an average boundary area of about 10- ~ cm 2 for the peak base plus the original boundary, the number of atoms piled up per hour is about 2 × 108, which is of the same order as the total number of atoms in the boundary region. Extrapolation of the results to zero N indicates the existence of a relatively short incubation period of about 20 h before the growth began.
CONCLUSIONS 1. Stripes have been powered in situ in the scanning electron microscope. Several observations were made at an angle of 82 ° from the specimen normals to accentuate surface growths. 2. In both silver and aluminum stripes, holes and growths were found in the same specimen cross-section at positions of maximum temperature of the stripe. It is proposed that "transient" fields are superimposed onto gross temperature profiles to initiate early damage. This damage appears to be relatively stable, and only when the gross effects take over does the actual failure mode come into evidence. The distribution of these early damage sites will influence the overall lifetimes by providing failure nuclei. 3. In silver, growths extend over many grains and have the appearance of surface undulations. The original surface microstructure remains unchanged in the undulated regions. A catastrophic resistance increase appeared at the end of the test which corresponded to the production of large faceted surface protrusions. These are thought to originate at ruptures in a thin surface oxide. 4. In aluminum, growths were localized and extended from individual grain boundaries. The rate of atomic buildup in one growth was approximated to be 2 × 1019 atoms cm -2 h - I by measuring ( N / A ) vs. time, where N is the number of atoms in the peak and A is the area of the peak base plus adjacent boundary.
ACKNOWLEDGMENTS The authors would like to thank P. Totta for supplying the silver specimens, C. Bremer and C. Aliotta for taking the SEM photographs, and K. Asai for technical assistance.
Thin Solid Films, 4 (1969) 187-204
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L. BERENBAUM, R. ROSENBERG
REFERENCES
1 2 3 4
1. A. BLECH AND E. S. MEIERAN,Appl. Phys. Letters, I1 (1967) 263. R. ROSENBERG AND L. BERENBAUM, Appl. Phys. Letters, 12 (1968). P . B . GHATE, Appl. Phys. Letters, 11 (1967) 14. D. CHAABRA, N. AINSLIE AND D. JEPSEN, The Electrochemical Society Meetinq, Dallas,
5 6
W . E . MUTTER, The Electrochemical Society Meeting, Dallas, Texas, May, 1967. R . G . SHEPHEARD AND R. P. SOPHER, The Electrochemical Society Meetinq, Dallas, Texas,
7 8 9 10
J . K . HOWARD AND R. F. ROSS, Appl. Phys. Letters, 11 (1967) 85. R . V . PENNEY, J. Phys. Cheat. Solids, 25 (1964) 335. P . S . Ho AND H. B. HUNTINGTON,J. Phys. Cheat. Solids, 27 (1966) 1319. O. KUBACHEWSKI AND B. Z. HOPKINS, Oxidation o f Metals and Alloys, A c a d e m i c Press, New Y o r k , 1962.
Texas, Mar', 1967.
May, 1967.
Thin Solid Films, 4 (1969) 187-204