Analysis of electromigration effects within various types of aluminum test structures

Reliability Engineering and System Safety 37 (1992) 57-64

Analysis of electromigration effects within various types of aluminum test structures E. A. Weis*, E. Kinsbron, M. Snyder & N. Croitoru Faculty of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel (Received 30 October 1990; accepted 15 March 1991)

Advances in integrated circuit fabrication technology over the past two decades have resulted in integrated circuits with smaller device dimensions and larger area and complexity. This evolution of technology highlights electromigration as a major reliability problem in silicon VLSI circuits. Emphasis is placed on the scope and detail of the electromigration test structures themselves, and on the analysis of electromigration effects within various types of aluminum test structures. silver did not significantly affect the electromigration characteristics, which led to the hypothesis that the copper atoms are preferentially adsorbed at the grain boundaries. 6 However, the AI-Cu alloy is difficult to dry-etch because of the lack of a volatile copper halide, and corrosion often occurs after etching. Long-term corrosion and bonding problems are also a concern. Alloying aluminum with nickel, chromium, and magnesium also enhances the lifetime, 7 but nickel (as copper) has a very low solid-solubility limit at room temperature, alloying aluminum with chromium causes a substantial resistivity increase, and the difficulties with magnesium are that it reacts with silicon dioxide and increases resistivity. Silicon is normally added to aluminum to reduce junction spiking, and basically it improves aluminum lifetimes. 9'1° More recent investigations '1,12 indicated that the addition of 0.1 wt % to 0.2 wt % titanium and 1-2 wt % silicon to aluminum, resulted in a factor of 25 increase in lifetime over that of AI-Si. The problem introduced by adding titanium to aluminum is the increased resistivity. When silicon is added, resistivity drops, but remains rather high. This paper describes electromigration test structures that were designed to address the issues of metal interconnect lifetimes, with current M2CMOS process. It allows 'on product' measurement of metal quality, and time to failure, and is designed to be tested in volume with autoprobing equipment as well as within a packaged test chip. This presentation deals with the test structures philosophy. In the problem of failures due to electromigration in thin metallic films, a knowledge of the statistical distribution of the times to failure of the films is of

1 INTRODUCTION Interconnections in integrated circuits are normally located above the surface of the substrate, with less emphasis placed on them, compared to the transistors below the surface. However, they are beginning to set limits on performance and scaling. Aluminum has been the most widely used metal for interconnection, because of its process compatibility and cost effectiveness. Reliability studies in the mid sixties led to the identification of electromigration in AI film conductors as one of the primary failure mechanisms limiting the reliability of IC film interconnections. 1-5 This phenomenon is a wearout mechanism and appears as a result of momentum exchange between electrons and the material atoms. Mass transport of the material atoms can take place through either point defects, such as vacancies or interstitials, or gross defects, such as grain boundaries or surfaces. Nevertheless, aluminum continues to be the preferred metal for interconnection structures because of its low resistivity and silicon compatibility. The electromigration lifetimes of aluminum films can be extended, by homogeneously alloying AI with other elements. D'Heurle reported that the addition of 4% copper lengthens the lifetime by a factor of 70 and that this increase is proportional to the copper concentration below 4%. The addition of gold or *To whom correspondence should be addressed at: 2 Haneveim St., Ramat-Hasharon, Israel, 47279.

Reliability Engineering and System Safety 0951-8320/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. 57

E . A . Weis et al.

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interest, since it plays an essential role in determining the reliability levels of the films. The various research works reported in the electromigration fieldt-t2 are mainly based upon 'classical' (non-Bayesian) methods of estimation and hypothesis testing. A publication by Schafft et al.13 presented some theoretical applications of implementing Maximum Likelihood Estimators (the underlying assumption used in that paper was that the failure times are log normally distributed). In the present research, a large number of electromigration life tests on AI, A1 + Cu, and AI + Cu + Si film conductors has been carried out at a current density of 2 x 106 A/cm 2 in the 175-200°C ambience.

2 BASIC PHYSICS OF THE ELECTROMIGRATION FAILURE MECHANISM

Electromigration in thin film conductors has been studied intensively for nearly 25 years since it was first recognized as a potential failure mechanism in microelectronic circuits. 1-12 Most of the electromigration studies have been carried out with AI and Al-alloy films, as they happen to be most widely used as IC interconnections. In a lattice, the atomic flux Ja due to electromigration can be expressed as

Ja

=

( N D / k T ) Z * E = (ND/kT)Z*pj

(1)

where N is the density of ions, D is the self-diffusion coefficient, k is the Boltzmann's constant, T is the absolute temperature, Z~* is the effective charge, E is the electric field, p is the resistivity of the conductor, and j is the current density. In polycrystalline thin films, the expression given in eqn (1) for the atomic flux, due to EM in bulk samples, has been modified as follows: 14 t~ Db Jb = Nb -d - ~ Z~ eE

(2)

where 6 is the effective width of the grain boundary, d is the average grain size, p is the resistivity of the film, j is the current density and the subscript b refers to boundary parameters: Jb is the flux of metal ions, Nb is the local density of ions in the grain boundary, Db is the grain boundary diffusion coefficient, and Z~ is the effective charge. Here it is assumed that mass transport proceeds with the same characteristics in all the grain boundaries. Hence, the grain boundary diffusion coefficient in eqn (2) is a suffably averaged quantity for the film; other quantities related to the grain boundaries are appropriate averages. It is assumed that the atomic flux in thin films due to electromigra-

tion can be qualitatively expressed by an expression of the type in eqn (2). For real films, eqn (2) should be modified to take into account variations in the transport parameters of individual grain boundaries, both as a function of the intrinsic structural properties of the boundaries and as a function of their geometrically random orientation. The grain boundary flux, given by eqn (2), is not as well defined as the lattice flux given by eqn (1), because of the imprecision in the available knowledge of grain boundary parameters, even if one assumed that the diffusivity and the effective charge for the grain boundaries are known. Precise values cannot be ascribed to Nb in the absence of a proper understanding of the grain boundary structures. For impurity atoms, this is further complicated by grain boundary segregation. For lattice electromigration, the flux of atoms is directly proportional to the flux of charge carriers. For grain boundary transport, the problem is considerably complicated by uncertainties relating to the different resistivities in the boundaries and in the bulk, and the way these factors affect the local current density at the grain boundaries. In most thin films, transport along grain boundaries usually dominates. For example, at 175°C the ratio Jb/Ja has been estimated to be 106 for Al in Al films. Hence, for practical purposes in considering electromigration phenomena in thin films, one may generally neglect the lattice mechanism of diffusion and pay attention mainly to the grain boundary transport. It should be highlighted that in polycrystalline Al films, there may be different vacancy contributions to the mass flow. In addition to the mass flow that can occur in the metal lattice, or in the grain boundaries of polycrystalline films, where one vacancy adjoins to the other, there may be a vacancy injection from grain boundaries into the lattice or a surface contribution of oxide-free surfaces, for instance at voids or holes, which had been formed inside the metal film by electromigration. The different mass flow mechanisms or contributions, are marked by characteristic activation energies.15 In order to investigate stripe interruptions, accelerated lifetime tests are performed with increased current densities at elevated temperatures. The interruption failure times or the median time to failure (MTFF), are the measurements normally obtained to determine quantitatively the resistance to an electromigration failure, and are generally assumed to be lognormally distributed. The simplified empirical formula 1-3 A

Ea

where n is the current-density exponent, Ea is the

Electromigration effects in aluminum test structures activation energy, k is Boltzmann's constant, and T is temperature, can be used for a rough estimation of operating failure times. In this formula Ea and n depend upon various fabrication and test parameters. Conventional accelerated electromigration testing is regularly carded out at current densities of the order of 1 x 106 - 2 x 106 A cm 2 and temperatures from 150 to 250°C. Based on these life test data, the failure rates of these film conductors for prescribed current densities and operating temperatures can be calculated.

3 THE E L E C T R O M I G R A T I O N TEST CELL STRUCTURES The test vehicles used for evaluation were designed to emulate current technology metallization variations. Within the test chips, the test structures were designed to address film lifetime evaluation of a current M2CMOS process, AI-0.8%Si metal interconnect. It allows 'on product' measurement of metal quality, and time to failure. They are designed to be tested in volume with autoprobing equipment as well 48E

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59

as within a packaged test chip. The metal reliability is sensitive to any defect which reduces the crosssectional area of the metal line. Electromigration is a power function of current density. Thus, this can be a very significant consideration for metal reliability. BasicaUy, 16-1s electromigration is the root cause of most metallization reliability failures. All aluminum metal lines will experience electromigration under high current stress. The test structures to test for the sensitivity of the electromigration limited life of a metal line must then consider all possible geometric combinations. For metal lines this is primarily those geometries which might produce or simulate step-coverage thinning, and those which might promote reflective notching. Structures aiming to test for geometric-related problems do not have to be exceptionally large. Their higher probability of failure is due to the layout of the structure rather than to any probability distribution. Also, the probability of metal line failure is a function of the rate of void accumulation, rather than simply the rate of electromigration. Therefore, these structures should be designed so that voids generated

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by a specific test are not allowed to migrate randomly before a failure can be detected. Three different electromigration test chips were developed and are presented: EM1A (Fig. 1), EM2A (Fig. 2), and CHIP 15 (Fig. 3). Important test structures are duplicated with the various test chips. The EM1A and EM2A electromigration test chips, are part of a bigger, general purpose, test chip, and are 4576 ~um x 3289 ~m. Each has forty-eight 'external' pads, arranged in its peripheral perimeter, and another group of forty 'internal' pads arranged as around the peripheral perimeter of an internal test chip. In general, EM1A and EM2A have continuity structures. Those test structures connected to the 'external' pads are principally oriented towards traditionally (packaged) accelerated electromigration testing, while those structures connected to the 'internal' pads are principally oriented towards wafer level accelerated electromigration testing (probing). Structures were designed according to minimum design rules guidelines, while in some specific test structures anticipated next-generation design rules were implemented. Attention should be paid to the various versions of the basic SWEAT structures. 19 The term was coined by Brian Root at Sperry and stands

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for Standard Wafer Level Electromigration Acceleration Test. The design allows a very high and yet controlled, acceleration factor of electromigration. The variation in metal width at the oxide steps results in discontinuities that contribute to the rapid formation of hillocks and voids. Current density is the most obvious factor that changes abruptly, but there is a temperature gradient as well, which aids the process. If a current is passed through these lines, the current density will change dramatically, at each location where a wide line connects to a narrow segment, and again when the narrow segment reconnects to the wide line. This creates a significant current-density discontinuity at each of these areas. These discontinuities are known to be traps for voids or hillocks. When sufficient current is passed through the line, causing appreciable Joule heating of the metal line, a thermal discontinuity will also occur where there is a major change in the line width. The wide areas also serve as heat sinks and reduce the overall resistance of the conductor, providing a better power-consumption test vehicle. Figures 4 and 5 present test structures within EM1A and EM2A, respectively.

Electromigration effects in aluminum test structures

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The Chip 15 is part of a bigger, general purpose, test chip. Out of the ten test structures, three are different types of runners, three are based upon the National Bureau of Standards test structure, and the other four are SWEAT structures.

4 DESIGN OF THE CONVENTIONAL ELECTROMIGRATION ACCELERATED LIFETIME TESTS The aim of the conventional electromigration accelerated lifetime experiment was to study the form of the failure distribution and to estimate the lifetime of the tested structures by conducting long-term accelerated electromigration aging tests. The accelerated electromigration lifetime tests were carried out using a conventional electromigration tester (configured around an IBM/AT computer, an oven, and dedicated current sources), implementing 175°C and 30mA (2 x 106A/era 2) as the electromigration test envelope. This method was deduced after a long preliminary study of the identification of the common

(d) Fig. 5. Test structures within EM2A test chip. (a) Structure no 9; (b) structure no 10; (c) structure no 14; (d) structure no 16.

electromigration failure mechanisms, and more than two years of working at the specified conditions. The statistical analysis determining the optimal number of components out of each lot and wafer, depends upon the spread of the measured parameters. The aim of good selection is to increase, as much as possible, the control over the similarity between the real distributions of measured parameters within the whole population and the distribution of measured parameters within the sampled population. The formula for selecting the number of components for an accelerated lifetime test is: n -> (as/A) 2

(4)

where n is the minimal number of test structures in a sample for the accelerated lifetime test, S is the standard deviation of the measured population, and A is the maximum allowed difference between the average of the measured parameter in the sample and the nominal mean of the measured population. The experience gained 19-2° emphasized the effect of the metal structure on the electromigration failure mechanism. In order to be able to compare the results with other laboratories, the test structure chosen was a metal meander designed to minimum line-width specification over flat oxide. A maximum resistance of

62

E . A . Weis et al. EM Log Norrnol ProbabiLity PLot

70 g2 was designed, as an outcome of a power supply issue. The metals chosen, AI, AI-Si, AI-Cu and AI-Cu-Si films, have been deposited using various deposition techniques and environments. The selection of the film composition was influenced by several pragmatic considerations such as resistivity, microstructure, patterning, process controls, growth of precipitates, and earlier-life test data.

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In order to induce or reproduce electromigration failures that would ordinarily occur under field operation stresses, but within an operation time that is shorter than required to produce the same electromigration failures under field usage stresses, many accelerated stress tests were conducted. 2° The tests were carried out using the conventional electromigration tester and implementing the electromigration lifetime tests presented above. As explained, this method was deduced after a long preliminary study of the identification of the common electromigration failure mechanisms, and more than two years of working at the specified conditions. Scanning electron microscopy has been used to examine the microstructure, in comparative mode tests. The analysis of the Conventional Accelerated Lifetime Test (CALT) results, have been carried out using a four-step process. The process included Bayesian procedures, using plots of inspected interval data based on the maximum likelihood technique, and implementing the software package CENSOR. 21 (Censor works with two-parameter distributions which are of the location-scale type, or which can be transformed into a location-scale distribution. Censor assumes that the scale parameter is fixed, and that the location parameter is a linear function of independent variables. Censor recognizes four types of observation: failure times, left-censored observations, rightcensored observations, and interval observations). The following outlines the method:

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Fig. 6. Electromigration lognormal probability plot. superior estimates of parameters with their associated confidence intervals. In the present study, by using plots of inspected interval data, based on the maximum likelihood technique, it has been found 22 that in most of the experiments, the lognormal distribution fits the lifetime test data. This is in accordance with the model developed by Attardo et al., 23 and the statistical models presented by Venables and Lye, 24 Bobbio and Saracco a5 and Towner et al. 26 In some rare cases, data present a poor fitting to the lognormal distribution (Fig. 6, upper curve), and the Weibull distribution fits better. For example, a better fit to the data in Fig. 6 (upper curve), was achieved by the application of a two-stage Weibull distribution (Fig. 7). Based on the scale and shape parameters that were estimated, the cumulative distribution function was calculated. From the physical point of view, it seems that this abnormal fitting is an outcome of some process instabilities developed during the specific process. However, a study should focus on determining the exact relationship between the process EM WeibulL Probobitity PLot 2 x

A. Attempt to fit test results to conventional electromigration failure models (lognormal distribution) using linear least-squares procedures. B. Visual inspection of the fit of the data to the estimated regression line. For the case of a reasonable fit, maximum likelihood methods are utilized to optimize numerical results. C. In the case of a poor fit, alternative models are investigated. D. Maximum likelihood methods are then implemented on the revised model to produce

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Electromigration effects in aluminum test structures parameters and the electromigration cumulative distribution factor. From eqn (3), one can calculate the acceleration factor (AF):

(Jo~ -n Ea 1 1 AF=kZ ' exp[-~- (-~oo- ~ ) ]

(5)

where Jo is the operating current density, Js is the stress current density, n is an index factor, Ea is the activation energy, k is the Boltzmann's constant, To is the operating temperature and T~ is the stress temperature. The current dependency of the ~ (eqn (3)) has been investigated, and n = 2 was selected for extrapolation of M T r F s at lower current densities. This approach is consistent with many of the reported values of the exponent n ;27-29 preliminary experiments showed that the activation energy for electromigration, referring to the metals chosen, is low, in the range of 0.45-0.55 eV. This matches published data (Refs 3, 15, 20, 28, 29) referring to the grain boundary electromigration, which plays an important role ~5'29 among the following contributions to electromigration: bulk electromigration ( E b = l - 4 e V ) , vacancy motion from grain boundaries into the bulk (Esb ----0.62 eV), motion of vacancies originating from defects in the bulk ( E d > 0 . 6 2 e V ) , surface electromigration ( E s = 0 . 2 8 e V ) , only to be observed at oxide-free surfaces, for instance at voids or holes inside the aluminum), and the grain boundary electromigration. Table 1 presents the Mean Time To Failure (M'ITF, tso) for a specific alloy (AI-0-8% Si) and structure (metal meander), and under the chosen electromigration test envelope (175°C and 2 x 106 A/cm2), within numerous tests, utilizing different process parameters. As can be seen, the sputtering technology has a

63

tremendous effect on the electromigration resistance of the metal conductors. The best electromigration resistance has been achieved through implementing a 450°C anneal temperature. This heating for several minutes stabilizes the microstructure, and the degree of (100) orientation texture increases. Temperatures of approximately two-thirds the melting point are required in order to maximize the annealing effectiveness. 6 CONCLUSIONS Electromigration life tests on aluminum alloy metallizations, have demonstrated the sensitivity of electromigration to the sputtering technology and to the microstructure of the metallization, confirming the need for an electromigration specification test. The life tests have also demonstrated the existence of an annealing effect on the aluminum tracks, which increases the lifetime. From the present results, where particular care was taken to ensure good control of test conditions, it is found that a value of n = 2 applies over the practical range for testing down to a current density of I x 105A/cm 2. A test procedure, with conditions during which only a specified number of failures are allowed, has been evaluated. It is concluded that specific high-temperature and highcurrent test conditions, referring to specific metals, are based on an activation energy of 0-5 eV, so that failures corresponding with the usual range of activation energies can be detected. In the present study it has been found that the lognormal distribution is an excellent model of the electromigration failure mechanism. The implementation of the maximum likelihood (ML) methods with the electromigration CALT results is extremely important. The electromigration experiments are rather long (hundreds of hours) and the data is

Table 1. Eleetromigralion summary

Experiment 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

6 kA tube cap 2.5 kA ETS cap Varian process I Standard dielectric ETS sputtering 300°C, slow rotation Double smoothing, before cap Standard dielectric, TiW, standard ETS sputtering 9.9 kW 1 mirr--10 rotations, 1.5 min-2 rotations, 3.5 mitt--7 rotations ETS 9-9 kW for 6 min, 5 rotations per minute 9.9 kW applying 80W RF bias Varian process II 450*(2sputtering annealing temperature 390"C sputtering annealing temperature

Sputtering System

tso[h]

Sigma

([°C]

ETS ETS VAR ETS

61-98 29-40 4.56 53.84

1.29 0.98 0.71 1.37

200 200 200 200

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200 200

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0.87

190

ETS ETS VAR ETS ETS

13.37 70.15 3-28 96-162 13-32

0.41 0.78 190 0.66 1.06

190 190

a ts = t.... + A T where torch= 175°C; A T = internal heating of the conductor due to high current.

190 190

64

E. A. Weis et al.

regularly censored. Small sample size is a desired goal and most important. The tso estimation derived from the presumed distribution will be multiplied thousands of times by the accelerating factor (AF) achieved in EM C A L T experiments. The ML methods enable us to handle multicensored data, while keeping the sample size to reasonable proportions.

ACKNOWLEDGEMENT

The authors are grateful to National Semiconductor Israel for its continued support, and to the company employees for their valuable advice and constructive criticism.

REFERENCES

1. Black, J. R., Mass transport of aluminum by momentum exchange with conducting electrons. In 5th Annual Proc. Reliability Physics Symposium, IEEE, 1967, pp. 148-59. 2. Black, J. R., Electromigration modes in aluminum metallization for semiconductor devices. Proc. IEEE, 57 (1969) 1587-94. 3. Black, J. R., Physics of electromigration. In 12th Annual Proc. Reliability Physics Symposium, IEEE, 1974, pp. 142-9. 4. Blech, I. A. & Meieran, E. S., Direct transmission electron microscope observation of electrotransport in aluminum thin films. Appl. Phys. Lett., 11(8) (1967) 263-6. 5. Blech, I. A., A study of failure mechanisms in silicon planar epitaxial transistors. Technical Report Contract No. AF30(602)-3776, Rome Air Development Center, 1965. 6. d'Heurle, F. M., The effect of copper additions on electromigration in aluminum thin films. Metallurgical Trans., 2 (1971) 683-9. 7. Gangulee, A. & d'Heurle, F. M., Effect of alloy additions on electromigration failures in thin aluminum films. Appl. Phys. Lett., 19(3) (1971) 76-7. 8. d'Heurle, F. M. & Gangulee, A., Solute effects on grain boundary electromigration and diffusion. In The Nature and Behavior of Grain Boundaries, ed. J. M. Poate, K. N. Tu, J. W. Mayer. Plenum Press, New York, 1972, pp. 339-70. 9. van Gurp, G. J., Electromigration in Al films containing Si. Appl. Phys. Left., 19(11) (1971) 476-8. 10. Black, J. R., Electromigration of AI-Si alloy films. 16th Annual Proc. Intl. Reliability Physics Symp., IEEE, 1978, pp. 233-40. 11. Gardner, D. S., Michalka, T. L., Flinn, P. A., Barbee, T. W., Saraswat, K. C. & Meindl, J. D., Homogeneous and layered films of aluminum/silicon with titanium for multilevel interconnects. Proc. 2nd International IEEE VLSI Multilevel Interconnection Conference, IEEE, 1985, pp. 102-13. 12. Gardner, D. S., Michalka, T. L., Saraswat, K. C., Barnee, T. W. & Meindl, J. D., Layered and

homogeneous films of aluminum and aluminum-silicon with titanium, zirconium, and tungsten for multilevel interconnects. International Symposium on VLSI Technology, Systems and Applications, IEEE, 1985, pp. 157-61. 13. Schaft, H., Lechner, J. A., Sabi, B., Mahaney, M. & Smith, R. C., Statistics for electromigration testing. 26th Annual Proc. International Reliability Physics Symposium, IEEE, 1988, pp. 192-202. 14. d'Heurle, F. M. & Gangulee, A., Effects of complex alloy additions on electromigration in aluminum thin films, lOth Annual Proc. Reliability Physics Symposium, IEEE, 1972, pp. 165-70. 15. Schreiber, H. U., Activation energies for the different electromigration mechanisms in aluminum. Solid-State Electronics, 24 (1981) 583-9, 16. Ghate, P. B., Failure mechanism studies on multilevel metallization systems for LSI. Technical Report Contract No. F30602-70-C-0214, Rome Air Development Center, 1971. 17. Sigsbee, R. A., Electromigration and metallization lifetimes. J. Applied Physics, 44(6), (1973) 2533-40. 18. Black, J., Electromigration. Wafer Level Reliability Assessment Workshop, IEEE, 1982, pp. 59-78. 19. Root, B. J. & Turner, T., Wafer Level Electromigration Tests for Production Monitoring. 23rd Annual Proc. International Reliability Physics Symposium, IEEE, 1985, pp. 100-7. 20. Weis, E. A., Kinsbron, E., Vogel, B. & Croitoru, N., Emprical electromigration data analysis using maximum likelihood methods. 8th International Conference of the Israel Society for Quality Assurance, Jerusalem, Israel. IEEE, 1990, pp. 243-6. 21. Meeker, W. Q. & Duke, S. D., CENSOR--A user oriented computer program for life data analysis. Iowa State University, Ames, Iowa, 1980. 22. Weis, E. A., Kinsbron, E., Vogel, B. & Croitoru, N., Electromigration behavior analysis of aluminum alloys thin film conductors using maximum likelihood methods. Microelectronics and Reliability Journal, in press. 23. Attardo, M. J., Rutledge, R. & Jack, C., Statistical metallurgical model for electromigration failure in aluminum thin film conductors. Journal of Applied Physics, 42(11) (1971) 4343-9. 24. Venables, J. D. & Lye, R. G., A statistical model for electromigration induced failure in thin film conductors. lOth Annual Proc. International Reliability Physics Symposium, IEEE, (1972) 159-64. 25. Bobbio, A. & Saracco, O., O'n the spread of time to failure measurements in thin metallic films. Thin Solid Films, 17 (1973) $13-$16. 26. Towner, J. M., Are electromigration failures lognorreally distributed? 28th Annual Proc. International Reliability Physics Symposium, IEEE, (1990) 100-5. 27. Ghate, P. B., Electromigration testing of AI-Alloy films. 19th Annual Reliability Physics Symposium, 1981, pp. 243-52. 28. Sim, S. P., Procurement specification requirements for protection against electromigration failures in aluminum metallizations. Microelectronics and Reliability, 19 (1991) 207-18. 29. Schreiber, H. U., Characteristics of electromigration in aluminum interconnect lines for integrated circuits. Materials Research Society Symposium Proc., 71 (1986) 249-60.