EDFAAO (2014) 3:14-19 1537-0755/$19.00 ©ASM International® How to Recognize Electromigration

Electromigration History and Failure Analysis David Burgess, Accelerated Analysis [email protected]

lectromigration is a wearout mechanism recog- Figure 2 shows additional details. Specifically, nized as an important cause of failure in semi- Fig. 1 shows dark spots in the wide collector contact. Econductor integrated circuits (ICs). Modern In Fig. 2, the dark spots are clearly voids or missing design rules necessarily push the limits for current aluminum. current flow in the collector is density. Nevertheless, analysis of field failures rarely from the silicon into the metal. Voids in this loca- identifies electromigration as a cause of failure. This tion are consistent with electromigration; that is, the may be evidence that designers and reliability engi- electron wind tends to push metal away from the neers have achieved near perfection. Alternatively, it contact. Voids must occur because no aluminum is may be that electromigration in field failures is hard to available to replenish the vacated metal. Voids have recognize and even harder to prove. The mechanism also developed in the narrow collector metal at the remains, but its failure modes and visible evidence left in the photo. have changed. A contrasting situation exists in the emitter contacts Field Failure Example of where flow from the metal into the silicon. Metal molecules pushed toward the emitter reach a Electromigration dead end. Aluminum piled up and caused mechani- Figure 1 is an optical microscope photo of electro- cal pressure on the transistor. Pressure on the emitter migration damage in a 1980s field failure. Irrefutable base junction caused a decrease in gain. The emitter evidence of electromigration is clearly visible. dark spots in Fig. 1 are bright spots in the second- ary electron microscopy (SEM) image in Fig. 2. The emitter defects are extrusions of aluminum pushed through the ruptured interlayer oxide. Another characteristic of electromigration is illu­- strated in the lower right corner of Fig. 2. Electromi-

Fig. 1 A large NPN output transistor is in the center of the photo. The failure mode was low gain of the transistor. The large “W” shape is the collector metal (aluminum) of the NPN. The two two-prong structures entering from the top are the emit- ter metal of the NPN. The narrow metal line snaking in from the right is the base of the NPN. Evidence of electromigration Fig. 2 Secondary electron image taken after top was can be seen as dark spots in the collector and emitter metal. removed

14 Electronic Device Failure Analysis gration is driven by both and exposed to high current. Copious data showed the temperature. Note the large block of second-layer electromigration phenomenon to be a predictable and metal, which is connected to the collector through well-behaved mechanism. Measurements were made a large rectangular via. The bright spots in the via of time to failure of metal lines under controlled cur- are aluminum hillocks caused by electromigration. rent density and temperature conditions. Plots show Electromigration moved metal from the contact to- that lifetime is inversely proportional to the square ward the via. Metal entering the via was stuck there. of current density. Further, data show that lifetime Current density was lower in the thicker, wider metal as a function of temperature can be characterized by 2. Additionally, the massive block of metal acted as a a parameter called activation energy. Both variables heat sink and decreased the temperature, or, simply are included in one equation, now known as Black’s put, differences in current density and temperature equation: caused metal to enter the via at a faster rate than t = (A/J2) exp(E /kT) metal moved out. Electromigration caused metal to 50 a accumulate. where t50 is the median time to failure, A is a constant, E is the activation energy, and T is the temperature in This example provides a vivid picture of the ele- a Kelvin. Black’s equation has survived the years with ments of electromigration. These characteristics are as only minor adjustments. valid today as they were 30 years ago, and these basic elements arguably are sufficient for failure analysis. A very limited sampling of electromigration papers The elements are: is discussed here. The selected papers contain insight of value to failure analysts. Electromigration was a • Above a minimum current density, electron flow major obstacle to design. Many papers can cause aluminum or copper to move in the direc- focused on material changes to improve electromigra- tion of electron flow. transferred from tion performance. Improvements of a factor of 2 re- many electrons creates a on metal . sulted from using large-grain rather than small-grain • Thermal energy causes metallic molecules to diffuse aluminum. Similar improvement was measured for or move within their crystal lattice. Electromigration metal with a glass or nitride overcoat. By suppressing occurs when momentum from a very heavy flow of the growth of hillocks, the transport of aluminum was electrons is transferred to thermally energized metal hindered. More significant improvement was gained atoms. The combination of heat and momentum by adding a few percent of copper in the alumi- [3] results in a tendency for the metal to move, or drift, num. Copper made grains smaller and clogged up in the direction of electron flow. the grain boundaries where aluminum electromigra- tion occurred. • “Failure is not caused by the movement of metal. From the above information, it may be expected Failure is caused by the unequal transfer of metal that missing passivation over a portion of a metal line into an area compared with metal transferred out would have an effect on electromigration of that line. of that same area.” This is loosely quoted from a The question is: Would a missing passivation defect video presented in 1985.[1] cause premature failure due to localized increased In the example cited here, metal moved away from rate of electromigration? An electromigration study the collector, creating voids and increasing resistance. of samples with intentional passivation “defects” Accumulation of aluminum in emitter contacts caused confirms that expectation. The study also shows that mechanical pressure on underlying silicon, causing the location of the defect is important. A passivation junction leakage and diminished transistor gain. defect located at the negative end of a metal strip re- After pressure in the metal caused rupture of the en- duces lifetime significantly more than a defect located [4] capsulating oxide, metal protrusions resulted. In this farther downstream. case, shorting did not occur. However, with different A life test of microwave power transistors[5] attaches geometry, electrical shorting is clearly a possibility. the electromigration mechanism to shorted junction failure, not just open metal. Nevertheless, test data fit Early History: Aluminum Black’s equation, with a calculated activation energy of 1.01 eV. Current density in affected metal was near Electromigration 1 × 105 A/cm2. The subject device has wide metal Electromigration has a fascinating, well-document- lines to limit current density. Obviously, a metal with ed history stemming from the problem of “cracked superior electromigration performance is required. strips” in 1967.[2] James Black demonstrated that However, even with a 10× improvement in lifetime, the cracks and electrical opens were the result of electromigration will impose a limiting factor for aluminum material moving within a metal strip aluminum in semiconductor design.

Volume 16 No. 3 15 Electromigration History and Failure Analysis

Reference 6 is invaluable for its visual documenta- width. Further, data show that time to failure reaches tion of how electromigration progresses and ends a minimum as line widths narrow to 1.5 to 2.0 mm. with an open failure. Time-lapsed SEM images show The reason for improved lifetime is that very narrow growth and movement of voids and hillocks. The metal lines are less likely to include any triple points paper also documents behavior of a thin oxide layer at all. When aluminum grain size exceeds line width, on the metal. For failure analysts, it provides a picture grain boundaries span all the way from one side to of the angular break associated with electromigration. the other. This geometry is known as a “bamboo” (This is in contrast to straighter breaks associated structure. Reference 8 addresses the effect of line with stress cracks.) Failure analysts should be aware width and line length for several aluminum alloys. that this break characteristic may be destroyed if a constant current caused electromigration. With con- Current History: Copper stant current, the current density will increase as the Electromigration conductor area decreases. The final open will come from melted metal. Copper metallization replaced aluminum as the metal of choice for multilayer metallization in By the late 1980s, aluminum electromigration was 130 nm and smaller technologies. Copper lines can be reasonably well understood. Perhaps more accurately, thinner and better suited for multiple layers. Lower the limitations of aluminum were defined and many resistivity and better electromigration performance aspects, such as line width and thickness, substrate are major advantages. The threshold current density roughness, and surrounding passivation, were well is approximately 10 times higher for copper than studied. for aluminum. Figure 3 shows a cross section of Simple lines with Kelvin contacts were no longer electromigration failures in copper applied by the adequate to study the effects of multiple-layer met- dual-damascene process. In this process, copper is allization. The National Institute of Standards and electroplated into trenches lined with a barrier metal. Technology proposed new test structures to study Figure 3(a) shows an electrical open in a via. The the effects of interlayer connections, and Reference 7 barrier prohibits second-layer copper from replenish- offers test structures specifically designed to isolate ing copper that moved away from the interconnect. contributions of interlayer connections to electro- Figure 3(b) shows a large void within a metal line. migration performance. Included results provide This void would cause an increase in resistance, not valuable insight about how interconnections effect an open. The liner metal maintains continuity through changes in stress level. the bottom and sides. Aluminum electromigration occurs in the bound- All the noted indicators of electromigration in alu- aries between crystals of different orientation. Triple minum apply to copper. However, the readily detect- points are points where three aluminum crystals able voids and hillocks of large-scale aluminum do meet. Voids tend to occur where one grain bound- not exist in copper. Voids can still be detected in cross ary carries metal in, while two grain boundaries sections at defect sites located electrically. carry metal out. Hillocks occur where the opposite Copper electromigration occurs more predomi- is true. This makes perfect sense. Think of the way nantly between the copper and the barrier liner, not cars pile up where two lanes of a highway narrow at triple points as for aluminum. Small electromigra- to one, and traffic jams clear after one lane branches tion voids in copper will be located along a surface. to two. Most of the electromigration investigations References 9 and 10 provide thoughtful discussions intentionally used relatively wide metal lines to en- of different types of voids in copper, both with cross- sure triple points were included in the test structure. sectional images. (continued on page 18) With the effect of triple points clearly in mind, one might guess that very wide lines will perform better than narrow lines. Wide lines will include triple points in both orientations. To fail open, a crack must propagate across multiple grains. Narrow lines, on the other hand, become open where one void spans the whole line width. As expected, data confirm that time to (a) (b) failure increases linearly with line Fig. 3 Circles show voids caused by electromigration in copper. Courtesy of Tony Oates

16 Electronic Device Failure Analysis Volume 16 No. 3 17 Electromigration History and Failure Analysis (continued from page 16)

Electromigration shorts may be visible in well- tions. Specific design and process details should be placed cross sections. In nanoscale technologies, carefully considered. copper lines are very close together, and the dielec- • Voids may be visible with backscattered SEM, but tric is likely to be low-k dielectric, not more rugged voids may not be evident except in cross section SiO . Mechanically weak dielectric is less capable 2 after the defect location is otherwise determined. of containing metal under compressive stress due to metal accumulation. Obviously, extrusions due • Electromigration voids, if they exist, will occur at to electromigration can directly cause an electrical the upstream end (negative potential) of a long short. Some thought about this process leads to the metal line. In short metal lines, pressure opposes idea that failure does not require a bridge across the metal movement and prevents voiding. dielectric. Metal simply has to encroach into the soft • Electrical shorting to nearby conductors is an insulation, such that an increased electric field will expected failure mode at the downstream (posi- cause complete failure of the dielectric. Reference 11 tive) end of metal lines (above, below, or beside). recommends time-dependent dielectric breakdown Insulation surrounding metal suppresses hillock (TDDB) testing to fully evaluate material changes growth; however, accumulation of metal causes made to improve electromigration. pressure against the surrounding insulation. When pressure exceeds the strength of the insulation, Failure Analysis metal breaks through the insulation. Multilayer Failure analysis begins with any or all diagnostic conductors do not prevent accumulation due to tests to locate, isolate, and characterize the defect electromigration. Minimal spacing between metal site. Even if the actual cause of an IC failure is elec- lines and weak low-k dielectrics make shorting tromigration, irrefutable evidence, such as in Fig. 3, more, not less, likely. may be too much to expect. These cross sections were • Long metal lines are most susceptible to damage made on test structures with known stress history, due to accumulation. Not enough metal is available simple electrical access, and documented layout. in shorter lines to build damaging pressure.[12, 13] Nevertheless, cross sections of voids or protruding metal can be sufficient to conclude electromigration In summary, electromigration defects in a modern where the shape and location of voids are not consis- IC can be practically invisible. The only physical in- tent with simple processing defects. The location of dication may be a fault located in the metal system at voids is consistent with the direction of electron flow. a location consistent with the above observations. At In Fig. 3(a), barrier metal prevented copper from re- the risk of sounding glib, electromigration cannot be plenishing migrated metal. An analyst can only hope dismissed as a possible mechanism if electromigration for such clear evidence. is a possible failure mechanism. Lacking a single observation perfectly explained Electromigration Checklist No. 1 by electromigration and by nothing else, an analyst must rely on the weight of all the evidence. A failure • Does the failure history indicate a change from analysis is not complete just by locating a failure site. good to failing? That is, did the device function An analysis must include the cause of the defect, satisfactorily and fail after use or operating stress? whatever that may be. • Is the device design, process, or application new or different? Electromigration: Failure • Has a defect site been localized within the metal- Analysis Considerations lization system? The limited history outlined here has introduced • Does high resistance or electrical shorting explain some characteristics of electromigration and some the observed failure mode? pertinent factors. A few simple observations are • Does the defect location correspond to likely ac- listed below: cumulation or voiding sites? Consider both layout • An electrical open is not the most expected failure and circuit factors. Consider current density and mode in a metal span. Conductors typically include temperature implications. Consider high voltage one or more redundant layers that bridge across an between an accumulation site and adjacent metal. electromigration void. Increased resistance is more Accumulation will mechanically stress the dielec- likely. tric. Final failure may be TDDB of the degraded dielectric. Electromigration is still the main failure • An electrical open can occur at interlayer connec- mechanism.

18 Electronic Device Failure Analysis Affirmative responses to these questions suggest 2. J.R. Black: “ Transport of Aluminum by Momentum that closer scrutiny is required. Obviously, one path Exchange with Conducting Electrons,” Proc. 1967 Ann. of action is to define the nature of the actual electrical Symp. Reliab. Phys., pp. 148-59. failure mode. Another path is to define the local stress 3. J.R. Black: “Electromigration of Al-Si Alloy Films,” 16th conditions in the defect area. Ann. Proc. Reliab. Phys., 1978, pp. 233-40. After a conscious consideration of whether electro- 4. H. Schaft, C. Younkin, and T. Grant: “Effect of Passivation migration could be involved, another checklist must and Passivation Defects on Electromigration,” Proc. 22nd Int. Reliab. Phys. Symp. (IRPS), 1984, pp. 250-55. be considered. 5. S. Gottesfeld: “A Life Test Study of Electromigration in More Questions (Sometimes Microwave Power Transistors,” Proc. 12th Int. Reliab. Phys. Symp. (IRPS), Called a Sanity Check) 1974, pp. 94-100. 6. R.W. Thomas: “Phenomenological Observations on An erroneous diagnosis of electromigration is an Electromigration,” Proc. 21st Int. Reliab. Phys. Symp. (IRPS), expensive mistake. If electromigration is not the 1983, pp. 1-9. cause, a false alarm is costly. If the correct conclusion 7. L.M. Ting and C.D. Graas: “Impact of Test Structure Design is electromigration, any other diagnosis, or lack of any on Electromigration Lifetime Measurements,” Proc. 33rd Int. diagnosis, delays corrective action while the problem Reliab. Phys. Symp. (IRPS), 1995, pp. 326-31. grows larger. More questions are prudent to avoid 8. T. Kwok: “Effect of Metal Line Geometry on Electromigration either mistake. A diagnosis of electromigration must Lifetime in Al-Cu Submicron Interconnects,” Proc. 26th Int. make sense in the total world, not only in the case at Reliab. Phys. Symp. (IRPS), 1988, pp. 185-89. hand. Everything must fit. 9. A.S. Oates and M.H. Lin: “Analysis and Modeling of • Why now, not earlier? Are there qualification life-test Critical Current Density Effects on Electromigration Failure results that should have produced similar failures? Distributions of Cu Dual-Damascene Vias,” Proc. 46th Int. Reliab. Phys. Symp. (IRPS), 2008, pp. 385-91. Have this design and package been tested, or were they “qualified” on the basis of similar designs from 10. B. Li, T.D. Sullivan, and T.C. Lee: “Line Depletion the same process? Were the conditions of qualifying Electromigration Characteristics of Cu Interconnects,” Proc. 41st Int. Reliab. Phys. Symp. (IRPS), 2003, pp. 140-45. tests correct to detect electromigration failure? 11. T. Kane and Y.Y. Wang: “22 nm BEOL TDDB Defect • Does the failure count? If this is a life-test failure, is it Localization and Root Cause Analysis,” Proc. 39th Int. Symp. pertinent? Highly accelerated stress conditions may Test. Fail. Anal. (ISTFA), 2013, p 33-39. alter current flow in the circuit, such that some ele- 12. R.S. Hemmert and M. Costa: “Electromigration-Induced ments see higher current density than they do under Compressive Stresses in Encapsulated Thin-Film field operating conditions. Electromigration occur- Conductors,” Proc. 29th Int. Reliab. Phys. Symp. (IRPS), ring under runaway conditions is really electrical 1991, pp. 64-69. overstress. (The stress condition requires corrective 13. J. Lloyd: “Electromigration-Induced Extrusions in Multi- action, not the design.) Level Technologies,” Proc. 21st Int. Reliab. Phys. Symp. (IRPS), 1983, pp. 208-10. • Why now, not earlier? How long has the product been in service? Was the application unique? Have About the Author there been other failures? What percentage of field failures is returned for analysis? David Burgess is a failure analyst and reliability engineer. He devel- • Other failures? The failure rate of any wearout oped techniques and taught in those mechanism, by definition, increases with time. areas at Fairchild Semiconductor Electromigration is a wearout mechanism, and an and Hewlett-Packard. He is the increasing failure rate is expected. Is the overall founder of Accelerated Analysis, situation compatible with this expectation? a manufacturer and distributor • Should something else be failing? If so, is it? Unless of specialty failure analysis tools. there is a unique feature of the design or application, David is the co-author of Wafer Failure Analysis for Yield other products designed under the same design Enhancement. A graduate of Rensselaer Polytechnic rules could share the same design problem. Institute and San Jose State University, he is a member of EDFAS and has served on various ISTFA committees. References David is a Senior Life Member of IEEE and was General 1. B.J. Root and T. Turner: “Wafer Level Electromigration Tests Chairman of the 1983 International Reliability Physics for Production Monitoring,” Proc. 23rd Int. Reliab. Phys. Symposium (IRPS). Symp. (IRPS), 1985, pp. 100-07.

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