Electromigration in Sn Pb solder strips as a function of alloy composition

Transcription

1 JOURNAL OF APPLIED PHYSICS VOLUME 88, NUMBER NOVEMBER 2000 Electromigration in Sn Pb solder strips as a function of alloy composition C. Y. Liu, a) Chih Chen, b) and K. N. Tu c) Department of Materials Science and Engineering, UCLA, Los Angeles, California Received 5 June 2000; accepted for publication 24 August 2000 Using thin film solder strips, we have investigated the electromigration of six different compositions of Sn Pb solders at current density of 10 5 A/cm 2 near ambient temperature. The six compositions are pure Sn, Sn 80 Pb 20,Sn 70 Pb 30,Sn 62 Pb 38 eutectic,sn 40 Pb 60, and Sn 5 Pb 95. The eutectic alloy, with the lowest melting point and a high density of lamella interfaces, was found to have the fastest hillock growth. As composition moving toward the two terminal phases, the hillock growth rate decreases; but it increases again in pure Sn. The interface between Sn and Pb, being the fastest kinetic path of mass transport, also serves as the place to initiate hillock and void formation American Institute of Physics. S I. INTRODUCTION The current trend in electronic packaging is a wider application of flip chip technology to microprocessors and consumer products. The technology uses area array of solder bumps to attach chips directly to organic modules. 1,2 The size of these solder bumps is quite small because a large number is needed as chip-to-package interconnects. As the wireless hand-held devices become more functional, smaller chip-to-package interconnects are necessary. Serious reliability problems arise with respect to thermal-mechanical fatigue, heat dissipation, and electromigration in these small solder bumps. Concerning electromigration, at the moment the device design rule requires each solder bump to carry 0.2 A and to extend to 0.4 A in the near future. For solder bumps of 50 m in diameter, the current density will reach 10 4 A/cm 2. Since solder is a low melting point alloy having a fast atomic diffusion at ambient temperature, electromigration is a serious concern. Indeed, it has been shown that electromigration occurred within a few hundred hours in eutectic Sn Pb solder joints kept at 150 C under a current density of A/cm 2. 3 Besides the eutectic alloy, electromigration in the Pb- and Sn-rich solders are also of interest. The Pb-rich solder of Sn 5 Pb 95 is of interest since it is used in the first level packaging in high-end computers. Then all the Pb-free solders such as Sn 96 Ag 4 are Sn-rich alloys. Unlike the Al or Cu interconnect lines, 4 6 the Sn Pb solder is a two-phase alloy. The microstructure of the alloy plays a major role in affecting the electromigration behavior, however, the microstructure depends on alloy composition. We need to conduct a systematic study of the effect of alloy composition on electromigration. In this research, electromigration behavior of six Sn Pb solders has been investigated. They are pure Sn, Sn 80 Pb 20, Sn 70 Pb 30, eutectic Sn 63 Pb 37, Sn 40 Pb 60, and Sn 5 Pb 95, i.e., besides the pure Sn and eutectic Sn Pb alloy, we have studied two Sn rich and two Pb-rich alloys. a Present address: Intel, Chandler, AZ. b Present address: National Chiao Tung University, Hsinchu, Taiwan, ROC. c Electronic mail: kntu@ucla.edu II. EXPERIMENT The sample preparation is the same as in the previous work reported by Liu and Tu. 7 A bilayer Cu/Cr thin film was deposited on an oxidized Si wafer by electron-beam evaporation. The thickness of Cu and Cr were 2 m and 50 nm, respectively. Using lithographic methods, a thin film Cu/Cr line of 100 m wide, having a 100 m gap, was patterned on the oxidized Si wafer as depicted in Fig. 1 a. Then a thin, mildly activated resin flux layer was applied over the whole sample to protect the Cu film from oxidation, and the sample was placed onto a hot plate. A tiny drop of solder was applied across the gap to join the two Cu ends. Six different compositions of solder mentioned above were used. The reflow temperatures were 10 C above their melting temperatures and the reflow time was less than 30 s. After the reflow, the solder bump over the Cu line was polished with an ultrafine SiC sandpaper to nearly the same height as the Cu, followed by another polishing with 0.05 mal 2 O 3 powder on a polishing cloth. To perform the electromigration study, the solder thin strips were stressed by a constant current of 0.2 A current density was about 10 5 A/cm 2 at room temperature. Amray 1830 scanning electron microscope SEM and energy dispersive x-ray EDX were used to examine and identify hillock and void formation. Figure 1 b shows a tilt view of a thin strip of pure Sn after the current stressing of 80 h. We can see hillocks and voids formed at the anode and cathode, respectively. Since the room temperature is a relative high temperature for the Sn Pb alloys, solid state reactions and grain growth are expected to take place during the current stressing. To minimize the effect of grain growth, a set of three samples of the pure Sn, the eutectic, and the Pb-rich alloy of Sn 5 Pb 95 were annealed to stabilize the microstructure prior to current stressing. The annealing conditions were: 48 h at 120 C for the eutectic alloy, 150 C for the pure Sn, and 200 C for the Pb-rich alloy. Joule heating has always been a concern in electromigration experiments. In our previous study of electromigration in eutectic Sn Pb strips, we used a low melting point crayon of 56 C to detect joule heating. 7 We found that the crayon /2000/88(10)/5703/7/$ American Institute of Physics

2 5704 J. Appl. Phys., Vol. 88, No. 10, 15 November 2000 Liu, Chen, and Tu FIG. 1. a Schematic diagram of the thin stripe sample, and b tilt view of a thin strip of pure Sn after current stressing at 10 5 A/cm 2 of 80 h at ambient temperature. did not melt, so the joule heating was not serious at all. III. RESULTS A. Microstructures Depending on composition, different microstructures are developed in different binary Sn Pb alloys in the asprepared samples without annealing, as shown by the backscattering mode of SEM images in Fig. 2. The light color phase represents the Pb-rich phase, and the dark color phase, the Sn-rich phase. The alternating lamellas of the eutectic structure shown in Fig. 2 d is the key feature of this binary system. When the composition deviates from the eutectic and towards more Sn as in the Sn 80 Pb 20 and Sn 70 Pb 30 alloys, larger Sn primary phases start to appear together with the eutectic microstructure, as shown in Figs. 2 b and 2 c. If the volume fraction of eutectic becomes very small, it forms the so-called divorced eutectic, or isolated eutectic regions as shown in Fig. 2 b, where the eutectic phase appears as isolated islands embraced by large Sn primary phases. 8 On the other hand, as the composition moves away from the eutectic composition and towards higher Pb as in the Sn 40 Pb 60 alloy, the microstructure consists of the eutectic and the Pb primary phase, as shown in Fig. 2 e. In the Pb primary phase, there are small Sn precipitates. Also, we note in Fig. 2 e that the Sn in the eutectic phase forms a continuous matrix. In the Pb-rich alloy of Sn 5 Pb 95, as shown in Fig. 2 f, the Sn precipitated out as small particles in the Pb matrix. A common feature of the four Sn Pb alloys, other than the Sn 5 Pb 95, is that the Sn matrix is continuous. Since grain boundary diffusion is known to be the main kinetic path of atomic transport in electromigration in thin films, we should know the grain size. We used etching to delineate the morphology of Sn grains in the pure Sn, and the average grain size was estimated to be about 3 m, as shown in Fig. 2 a. FIG. 2. Microstructure of the as-prepared Sn Pb alloys. a Pure Sn, b Sn 80 Pb 20, c Sn 70 Pb 30, d Sn 63 Pb 37, e Sn 40 Pb 60, and f Sn 5 Pb 95. B. Hillock formation We reported that the hillock growth driven by electromigration starts from the Pb grains in strips of the eutectic Sn Pb alloy. 7 The same mechanism has been observed here in both the as-prepared and the annealed eutectic Sn Pb strips. In an annealed strip, the hillock growth also starts at the Pb grains, as shown in Fig. 3 a. With a prolonged current stressing, Pb grains were extruded out as hillocks, as seen in Fig. 3 b. To identify what is the extruded material under the Pb grain, we polished the hillocks away. The exposed bases of the hillocks were identified to be the Sn phase by EDX, as indicated by an arrow in Fig. 3 c. The base appears to be the same as the surrounding Sn matrix. This result agrees with the earlier finding 7 that the Sn is the dominant diffusing species during electromigration and these Sn atoms push some of the Pb grains out at the anode. After the polishing, we again passed the same current density through the sample. We found no hillocks grew from their original sites, rather that they restarted from other Pb grains. The new hillocks are shown in Fig. 3 d. The morphologies of hillock formation of the six compositions after 40 h of current stressing are shown in Fig. 4. The hillocks in the pure Sn and eutectic Sn Pb alloy are shown in Figs. 4 a and 4 d, respectively. For the two Snrich alloys Sn 80 Pb 20 and Sn 70 Pb 30, shown in Figs. 4 b and 4 c, the hillock formation tends to occur in the eutectic phase region rather than in the primary phase region as indicated by the arrows. This might be because the eutectic mi-

3 J. Appl. Phys., Vol. 88, No. 10, 15 November 2000 Liu, Chen, and Tu 5705 FIG. 4. Hillock formation of as-prepared Sn Pb alloys after 40 h current stressing at ambient temperature: a pure Sn; b Sn 80 Pb 20 ; c Sn 70 Pb 30 ; d eutectic Sn 63 Pb 37 ; e Sn 40 Pb 60 ;and f Sn 5 Pb 95. crostructure has a higher density of interfacial boundaries and the eutectic phase is the lowest melting point phase. Again the protrusions originated from Pb grains. In the two Pb-rich alloys, Sn 40 Pb 60 and Sn 5 Pb 95 the hillock formation rate is much slower Figs. 4 e and 4 f. Besides, the hillocks start from Pb grains as shown in Fig. 4 e, although here Sn is the minor phase and Sn is the dominant diffusing species. Even in the Sn 5 Pb 95 alloy, most hillocks occur in the Pb phase as shown in Fig. 4 f. We believe that in this case the Sn precipitates tend to nucleate within a Pb grain, so they lack the connecting interphase boundaries to serve as fast kinetic paths. A common feature among the five Pb-containing alloys is that the hillocks of electromigration start from Pb grains. For the pure Sn, whiskers were observed in a prolonged stressing as shown in Fig. 5. Comparing Fig. 5 to Figs. 2 a and 4 a, we see that the diameter of the Sn whisker is the same as the average grain size of Sn. It implies that the Sn whisker originated from a Sn grain and was pushed up from below by mass transport. 9 Figures 5 a 5 d show the growth evolution of a Sn whisker. The rate was calculated to be m/s, which is of the same order as the observed whisker growth rate driven by a compressive stress. 9 FIG. 3. Hillock formation of eutectic Sn Pb which has been preannealed at the temperature of 120 C after 48 h. a After 20 h current stressing; b after 40 h current stressing; c hillocks were polished away; and d hillock formed at a new Pb grain after 20 h current restressing. C. Void formation Void formation in electromigration is the key concern of interconnect reliability. To improve the reliability, we need to find out how and where voids form. For the Al interconnect, the voids were reported to form at the divergence of diffusion paths, such as the triple points of grain boundaries, 10 or they form in no or low current density regions Here, in the two-phase alloys of the Sn Pb system, we found that void formation occurs at grain boundaries in the pure Sn and at interphase boundaries in the other al-

4 5706 J. Appl. Phys., Vol. 88, No. 10, 15 November 2000 Liu, Chen, and Tu FIG. 7. Void formation of Sn Pb alloys after 40 h of current stressing: a pure Sn, b Sn 80 Pb 20, c Sn 70 Pb 30, and d Sn 5 Pb 95. FIG. 5. Whisker growth of pure Sn after a 20 h, b 40 h, c 60 h, and d 80 h of current stressing. loys. Figure 6 shows the evolution of a void in the eutectic Sn Pb solder strip. The void started at the interface between the Sn and Pb phases. Both Sn and Pb atoms were transported away during the void growth. In the initial stage of electromigration, the Pb was depleted faster than the Sn, so the void was located in the Pb, as shown in Fig. 6. Figures 7 a 7 d show the morphology of void formation of the other compositions at the cathode side after 40 h of current stressing. The effect of grooving was found at grain boundaries of pure Sn, as indicated by an arrow in Fig. FIG. 6. Evolution of void formation in the eutectic Sn Pb after a 20 h, b 40 h, c 60 h, and d 80 h of current stressing. 7 a. The depletion region did not expand evenly into all the surrounding Sn grains; instead a faster depletion occurs at certain Sn grains. The very straight and sharp edges of the voids in the Sn imply that they are faceted. For the Sn-rich alloys of Sn 80 Pb 20 and Sn 70 Pb 30, the Pb-rich phase between the primary grains was caved down, shown in Figs. 7 b and 7 c, respectively, as indicated by an arrow in the former. We found that the Pb atoms were removed faster than the Sn atoms around the voids. In other words, the voids tend to favor the Pb phase. This seems contrary to the observation at the anode that Sn is the dominant diffusing species and forms hillocks there. For the Pb-rich alloy of Sn 5 Pb 95, the Sn precipitates tend to be transported away first and then the void expanded into the neighboring Pb regions, as shown in Fig. 7 d. The arrows in Fig. 7 d indicate the depleted Sn precipitates. But a few of the undepleted Sn precipitates, the darker phases, can still be seen in the figure. A common feature of void formation in all the samples is that it starts from an interphase boundary, except in the case of pure Sn, it starts from a grain boundary. D. Measurement of mass transportation by electromigration Combining the top and side view of SEM images of hillocks, we can measure the total volume of hillocks extruded at the anode side, and in turn we can measure the rate of mass transport by electromigration as a function of time. Figures 8 a and 8 b show the total volume of hillocks in the as-prepared and annealed samples as a function of current stressing time. The fastest electromigration rate occurs at the eutectic composition, then it decreases when the composition moves toward the two terminal solid solutions. We believe that this is mainly because the eutectic structure has the highest density of the interphase boundaries and the lowest melting point. This can also explain why the annealed samples with a larger grain size showed a slower hillock growth rate. The average grain diameter, d, for the as-prepared eutectic sample and the annealed eutectic sample are about 0.3 and 2 m, respectively. Therefore, it gives us a ratio of 2/0.3

5 J. Appl. Phys., Vol. 88, No. 10, 15 November 2000 Liu, Chen, and Tu , which is close to 5, the measured ratio of mass transported by electromigration in the as-prepared sample to that of the annealed see Figs. 8 a and 8 b. The difference in the hillock growth rate between the as-prepared and the annealed sample is particularly obvious in the eutectic Sn Pb solder owing to the larger reduction of the interphase boundaries resulted from the annealing. On the other hand, the differences in the pure Sn- and Pb-rich alloy are not so apparent. The hillock growth rates in terms of total volume per unit time are found to be constant due to the constant force of electric current. However, the hillock growth rate of the as-prepared eutectic alloy deviated from linearity in the initial current stressing but reaches a constant rate in the later stressing. We believe that this is due to the rapid microstructural change induced by current stressing in the initial stage. The mass transport by electromigration in a polycrystalline metal film is governed by the flux equation: 10 J em D b b C b KTd Z b *ee, 1 where C b is the grain boundary concentration, D b is the grain boundary diffusion coefficient, is the effective width of grain boundaries, d is the diameter of the grains, Z b * is the effective charge number of grain boundary electromigration, e is the charge of electron, T is temperature, K is Boltzmann constant, and E is the electric field. The parameter, Z b *, which represents the effective charge number of moving atoms driven by electrical current, can be obtained if the flux, the grain size, the temperature, the electric field, and the diffusivity are known. Assuming that V J em b A b t, 2 where V is the total volume of hillocks extruded, A b is the cross-sectional area of interphase boundaries per unit area, t is the time of electromigration, and is the average atomic volume, we can calculate the flux J em b by measuring V from Fig. 8. The grain boundary diffusivities of Sn in pure Sn, Pb in Sn 5 Pb 95, and Sn in eutectic Sn Pb alloy are available in the literature We have determined Z b * of pure Sn, Sn 5 Pb 95, and the eutectic Sn Pb alloy to be 7, 3, and 50, respectively. IV. DISCUSSIONS A. Effect of microstructure on electromigration in the two-phase Sn Pb alloys In a two-phase microstructure, there are multiple diffusional paths, i.e., Sn and Pb can diffuse in the Sn-rich phase, in the Pb-rich phase, and also in their interphase boundaries. That makes the electromigration analysis quite complicated, therefore we try to simplify it. We may do so based on the fact that often one phase is dispersed or embedded within the other phase. For example, in eutectic Sn Pb alloy, the Pb phase is physically surrounded by the Sn phase, in other words, the Sn forms the matrix with the Pb particles dispersed in it. This is especially true in an annealed sample that the Sn matrix is continuous, but the Pb particles are isolated. Therefore, both Pb and Sn atoms driven by the electric current from the cathode to the anode must go through the Sn phases. Therefore, we should first consider the diffusion of Sn and Pb in the Sn phase. According to the measurement by Gupta et al., 14 the diffusivity of Sn is larger than that of Pb in the eutectic Sn Pb alloy at room temperature. Thus, in our study here the diffusion of Sn in the Sn-rich phase should be the dominant diffusion process in the eutectic, and the diffusion occurs mainly along the interphase boundaries. Furthermore, the resistivity of pure Sn and pure Pb are 11 and 21 cm, respectively. In the eutectic structure, the solubility of Sn in the Pb phase is much higher than that of Pb in the Sn phase. Hence the Sn is a better conductor than the Pb, in turn the current density in the Sn is expected to be higher than that in the Pb during current stressing. Combining the faster diffusivity, the higher current density, and the matrix phase, we expect Sn to be the dominant diffusing species in room temperature electromigration in the Sn Pb alloys. Of course, the Sn 5 Pb 95 alloy is different. We should point out that in the measurements, 14 there is a reversal between the diffusivities of Pb and Sn; Pb becomes the faster diffuser at temperatures about 100 C. Thus, the electromigration behavior at temperatures above 100 C may be different. Indeed, the Pb rather than the Sn in the eutectic alloy was found to accumulate at the anode during the electromigration at 150 C. 3 Actually, at 150 C the lattice diffusion rather than interphase boundary diffusion is dominant and the grain size is bigger too. B. Hillock and void formation Both hillock and void formation are notorious to interconnect reliability. It was reported that the bump-to-bump shorts caused by extrusions is one of the key reliability issues in electronic packaging of microprocessors. 18,19 In our study, we have argued that Sn is the dominant diffusing species, and we have observed the growth of Sn hillocks. However, they start from Pb grains. Why not from Sn grains? This is because hillock formation is a surface relief phenomenon of thin films under a compressive stress. The relief requires vacancies, and the surface serves as the vacancy source. However, both Sn and Pb form surface oxides; their oxidized surfaces are not good vacancy sources, especially the Sn oxide which is known to be very protective. Thus, the surface oxide must be broken, and where it breaks, it becomes a free surface and can supply vacancies. 9 The anode is under compressive stress induced by electromigration. 4,20 There the surface oxide under tension will break at weak points. We postulate that the interfaces between the Pb oxide and the Sn oxide are the weak points, in other words, the oxide boundary formed at the interphase boundaries between the Pb and the Sn are the weak points. The interphase boundaries are also the fastest diffusion paths for Sn. Hence, the Pb grains are pushed up by the atomic flux of Sn, which exchanges with the flux of vacancies supplied from the surface crack around the Pb grains. As a consequence, the Sn hillock starts from a Pb grain.

6 5708 J. Appl. Phys., Vol. 88, No. 10, 15 November 2000 Liu, Chen, and Tu FIG. 9. Schematic diagram of the Pn Sn system. The dotted curve is to indicate the tendency of change of the rate of electromigration as a function of alloy composition. FIG. 8. Total volume of hillock vs the current stressing time: a asprepared, and b annealed. We recall in Fig. 3, where we have shown that when the surface is polished, the hillock restarts at new Pb grains. This is in agreement with the above picture that the weak points in the surface oxide play an important role in selecting the sites of hillock formation. At the cathode, voids start at the interphase boundaries. This is expected due to the nature of heterogeneous nucleation of a void. What is intriguing is that the voids tend to grow into the Pb grains, as shown in Fig. 6, although Pb is not the dominant diffusing species. One plausible explanation is that the current density in the Pb is lower than that in the surrounding Sn, and vacancies prefer to go to the low current density region Then where do these Pb atoms go? We speculate that they will take part in the microstructure evolution which occurs in the middle of the strip between the cathode and the anode. During electromigration, besides the hillock and void formation, there is substantial grain growth and ripening in the two-phase microstructure, which is a subject that needs to be carefully studied. C. Effect of alloy composition on electromigration of SnPb alloys The dependence of the electromigration rate on alloy composition is shown in Fig. 8. We have translated the results to Fig. 9, where a schematic binary phase diagram of Sn and Pb is shown together with a dotted curve. The curve indicates the tendency of change of the rate of electromigration as a function of alloy composition. Starting from the pure Sn, the rate goes down when Pb is alloyed yet it increases as the alloy approaches the eutectic point, and the eutectic alloy has the highest rate. Then it decreases again when the alloy composition moves towards Pb. Since we have not studied pure Pb, we do not know if the rate of pure Pb is greater or smaller than that of the Sn 5 Pb 95 alloy. While it is not a surprise to find that the eutectic solder has the highest electromigration rate, it is important to show that a preannealing of the sample before electromigration has reduced the rate by a factor of 5 as shown in Figs. 8 a and 8 b. Clearly it is due to a change of the microstructure but it indicates that electromigration behavior depends strongly on the history of the sample. For a two-phase alloy, we must first have a detailed study of the microstructure before we study its electromigration, for example, the calculation of the effective charge number of the eutectic alloy depends on knowing the diffusion species and paths in the microstructure. Although we have calculated the value of Z b * 7, 3, and 50 for the pure Sn, the Sn 5 Pb 95, and eutectic Sn Pb, respectively, we are not certain how accurate they are. Why the value of the eutectic alloy is much larger than those of the other two alloys is also unclear. V. SUMMARY We have studied room temperature electromigration of six alloys in the Sn Pb system. The applied current density was 10 5 A/cm 2. Owing to the high density of interfacial boundary and the low melting point, the eutectic alloy was found to have the highest electromigration rate. By preannealing the samples, the electromigration rate can be reduced by a factor of 5 due to grain growth.

7 J. Appl. Phys., Vol. 88, No. 10, 15 November 2000 Liu, Chen, and Tu 5709 The electromigration rate was determined by measuring the total volume of hillocks at the anode. These are Sn hillocks, yet they are capped by a Pb grain, except those in the pure Sn and the Sn 5 Pb 95 alloy. At the cathode, the void formation occurs in the interphase boundaries. However, they tend to grow initially into the Pb grain before consuming the Sn grain. Whisker growth of pure Sn was observed under electromigration. The whisker growth rate is found to be similar to that driven by a mechanical force. ACKNOWLEDGMENT This project has been supported by NSF Contract No. DMR and SRC Contract No. NJ International Technology Roadmap for Semiconductors, Semiconductor Industry Association, San Jose, CA, J. H. Lau, Flip Chip Technologies McGraw-Hill, New York, S. Brandenberg and S. Yeh, Surface Mount International Conference and Exposition, SMI 98 Proceedings 1998, p I. A. Blech, J. Appl. Phys. 47, J. R. Lloyd, Semicond. Sci. Technol. 12, C. K. Hu and J. M. E. Harper, Mater. Chem. Phys. 52, C. Y. Liu, C. Chen, C. N. Liao, and K. N. Tu, Appl. Phys. Lett. 75, D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys Chapman and Hall, New York, K. N. Tu, Phys. Rev. B 49, K. N. Tu, J. W. Mayer, and L. C. Feldman, Electronic Thin Film Science Macmillan, New York, 1992, Chap K. N. Tu, C. C. Yeh, C. Y. Liu, and C. Chen, Appl. Phys. Lett. 76, H. Okabayashi, H. Kitamura, M. Komatsu, and H. Mori, Proceedings of Third International Workshop on Stress Induced Phenomena in Metallization, edited by P. S. Ho, J. Bravman, C.-Y. Li, and J. Sanchez 1995, p S. Shingubara, T. Osaka, S. Abdeslam, H. Sakue, and T. Takahagi, Proceedings of Fourth International Workshop on Stress Induced Phenomena in Metallization, edited by H. Okabayashi, S. Shingubara, and P. S. Ho 1997, p D. Gupta, K. Viergge, and W. Gust, Acta Mater. 47, F. H. Huang and H. B. Huntington, Phys. Rev. B 9, D. L. Decker, J. D. Weiss, and H. B. Van Fleet, Phys. Rev. B 16, D. Gupta and K. K. Kim, J. Appl. Phys. 51, R. Shukla, V. Murali, and A. Bhansali, Proceedings 49th Electronic Components and Technology Conference, IEEE 1999, p V. K. Nagesh, R. Peddada, S. Ramalingam, B. Sur, and A. Tai, Proceedings 49th Electronic Components and Technology Conference, IEEE 1999, p M. A. Korhonen, P. Borgesen, K. N. Tu, and C. Y. Li, J. Appl. Phys. 73,