Electromigration Immortality of Purely Intermetallic Micro -bump for 3D Integration

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1 Electromigration Immortality of Purely Intermetallic Micro -bump for 3D Integration Hsiao-Yun Chen,, Chih-Hang Tung, Yi-Li Hsiao, Jyun-lin Wu, Tung-Ching Yeh, Larry Liang-Chen Lin, and Chih Chen 1 Douglas Cheng-Hua Yu, TSMC R & D, Taiwan Semiconductor Manufacturing Company, Hsinchu 30844, Taiwan, R.O.C. 1 Department of Materials Science and Engineering, National Chiao Tung University, Taiwan, R.O.C. chtungc@tsmc.com, Abstract The progress of three-dimensional integrated circuit (3D IC) micro-bump joining technology has led to an increased volume fraction of intermetallics (IMC) in the post reflow joints, to an extent that a solder micro-bump may consist almost entirely of IMCs. Therefore, the current carrying capability and electromigration (EM) life time of the purely IMC micro-joint needs to be understood as functions of stressing conditions and degradation mechanisms. Superior EM performance and robustness of IMC joints is demonstrated with no resistance fluctuation under ultra-high stressing condition for over 9000 hrs while solder microbumps led to an open failure within 500 hrs. At least an order of magnitude greater current carrying capability of IMC micro-joint compared with solder micro-joint is observed experimentally. The observed degradation mechanism is void formation within Al trace rather than damage inside IMC joint. IMC joint is not the EM reliability bottle neck of the test circuit. Introduction 3D integrated circuit (IC) is evolving to become a leading technology in many applications that require small form factor, high performance and low power [1-3]. Researches relevant to bump EM behaviors for fine pitch 3D interconnects are relatively abundant [1-12]. The effect of UBMs, including ENEPIG (electroless Ni/electroless Pd/immersion Au) and electroplated Ni and Cu were reported [1-12]. Reduction of solder volume eventually leads to a microbump consisting only IMCs. For IMCs, due to their relatively high melting point compared to parent solder phase, they are more robust for high temperature applications. Besides improving in thermal stability, inter-diffusion and EM is also expected to perform better for purely IMCs interconnects compared to the usual solder micro-bumps. The role of IMC on EM behavior becomes increasingly more important as shrinkage in bump size continues. Very limited information exists on the EM behavior of IMC, especially in the 3D bump arena [4, 13]. The critical product of Cu-Sn based IMCs is reported by Wei et al [3]. by using short Blech strips fabricated by lithography and FIB etching. It is found that Cu 6 Sn 5 IMCs have a much higher EM rate than the Cu 3 Sn IMCs and the lower melting point and higher resistivity of Cu 6 Sn 5 are attributed for its higher EM rate. A very first result of EM experiments on solder-based IMCs interconnects was reported by Labie et al[8]. Both Co- Sn and Cu-Sn IMC joints showed highly resistant to EM tests under 150 C, 4.7~ A/cm 2 for over 1000 hr with no EM induced damage was found inside joints. Lin et al [7] also claimed that IMC micro-joints possess a better EM performance than the normal solder joint by using preannealing treatment. However, the reported damages were mostly located at the Al wiring that masked the true IMCs EM performance. Both of them showed the superior EM performance on IMC joints compared to traditional solder micro-joints. Generally speaking, IMCs have higher resistance and higher melting temperatures than solder. For Cu based UBM, Cu 6 Sn 5 and Cu 3 Sn are the final IMCs with respective melting temperatures of 415 C and 676 C [14]. For Ni based UBM, the melting point of Ni 3 Sn 4 is close to 800 C. Experimental IMC micro-joint EM samples were prepared with Kelvin structures. Lead-free SnAg solder was utilized in the μbumps for joining between the top and bottom dies. Thick and wide Al wiring traces were fabricated on the Kelvin structure in order to avoid any non-imc micro-joint EM caused damages. Test chip, was fabricated, assembled, and stressed under a constant current power supply in an oven. To obtain a fully IMC micro-joint, a two step thermal pretreatment was applied. The fisrt thermal pre-treatment was carried under 180 C for 150hr, and followed up with a thermal compression bonding (TCB) process. After TCB process, second step thermal annealing was then applied under 200 C for 100 hr. Since solder height was reduced, IMC can connect and bridge. A fully IMC micro-joints were then obtained as shown in Fig. 1(b), compared to the usual micro-joint in Fig 1(a). After a fully IMC joint was obtained, underfill dispensing and assembling was then applied. Fig. 1. Cross-sectional SEM image of (a) as fabricated microbumps, and (b) fully IMC joint after two step thermal treatments /15/$ IEEE Electronic Components & Technology Conference

2 Three ultra-high stressing conditions were applied. The current was chose from 0.45A, 0.55A to 0.65A with its corresponding current density from 1.4 to 2.1 x10 5 A/cm 2. And the with joule heating stressing temperature is in the range of 170 C to 180 C. During stressing, circuit resistances, including two IMC micro-joints and one Al trace, were monitored in-situ by 4-point resistance with an Agilent multiplexer data acquisition system. Fig. 2 showed schematic representation of test structures during EM test. Current was applied through N3 and N4. The time-dependent resistance plots were correlated with the degradation mechanisms which were characterized by using cross-sectional scanning electron microscopy (SEM), Focus ion beam microscopy (FIB), electron probe x-ray micro-analysis (EPMA) and electron back-scatter diffraction (EBSD) analyses. Detailed results and mechanism discussion are summarized in the following sections. (a) Microstructural evolutions vs. the resistance plots To understand the EM behavior of IMC micro-joint, the microstructure evolution with corresponding resistance curves were analyzed. Fig 3(a) shows an example of stable resistance curve maintaining for 800 hrs with no variation, was observed at the stressing condition of A/cm 2, 170 C. And the corresponding cross-sectional SEM images are shown in Fig 3 (b). Compared with time-zero IMC micro-joint from other samples, there is no obvious difference of IMC microstructure was observed after 800 hr stressing. Regardless of the electron flow direction, two powered joints showed no difference. Fig. 2. Schematic drawings of the Kelvin test structure. Current was applied through N3 and N4.The resistance variation of two micro-bumps and one Al trace was monitored using N1 and N6. Results and discussion Void free, high quality bump samples were fabricated on Kelvin structures and stressed under various current density and temperature combinations. Table I shows the temperature difference with applied current, from resistance measurement calculation under these accelerated tests. The corresponding stressing temperature is then estimated around 170 C to 180 C. Fig. 3. Typical resistance as a function of stressing time under 170 C with A/cm 2 was shown in (a), and (b) showed the corresponding SEM images after 800 hrs current stressing. Elemental distribution within IMCs were carried out by using EDX mapping. From results shown in Fig. 4(a) and 4(b) with electron flow upwards and downwards, respectively, there is no non-uniform elemental distribution inside the IMCs. No evidence shows the distribution of Cu, Ni or Sn correlating to the electron wind direction either. IMC microjoint, showed no observable changes after 800 hr of ultra-high current stressing. Table I. Stressing temperature and Joule heating resulted from ultra-high applied current. For SnAg solder joint, when stressed under the low to moderate conditions, all samples survived without failureexceeded 550 days (>13,000 hrs) of stressing and exhibited only resistance increases. It was observed that after prolonged stressing, the resistance plots reached a plateau of steady state, and the entire solder joint transformed into IMCs. Immortality of bumps EM lifetime is confirmed under the low to moderate stressing conditions for a prolonged time. Almost all the solder had transformed into IMCs with very little or no voids in the IMC joint. Fig. 4. EDX mapping on test sample after 800 hrs current stressing with (a) electron flow upwards, and (b) electron flow downwards. Sample with different stressing condition of A/cm 2, 180 C was analyzed after 1900 hrs stressing, as shown in Fig. 5(a), and the corresponding cross-sectional SEM images of stressed IMC joints in Fig 5(b). IMC joint maintained its integrity. 621

3 marked in red arrow. Same as obtained before, even after 5000 hr stressing, no EM induced damage was found in two stressed IMC micro-joints. Fig. 7. EM induced damages were shown as marked in red, both Al extrusion and void formation. Fig. 5. A resistance curve as a function of stressing time under 180 C with A/cm 2 for 1900 hr, as shown in (a), and also the corresponding powered bumps as shown in (b). No any electromigration induced damage was observed. From above results, IMC joints show a promising EM performance under ultra-high stressing conditions. Below shows two ultra-high stressing conditions stressed for 5000 hr with negligible resistance increase. Fig. 6(a) shows the resistance curve under stressing condition of A/cm 2, 170 C, and 6(b) the resistance evolution under the condition of A/cm 2, 170 C, respectively. In Fig. 6(b), resistances maintained well and showed no fluctuations within 5000 hr; however, a slight increase in resistance, which is about 4% increases, was observed in Fig 6(a). Besides 800 hr, 1900 hr and 5000 hr, a prolonged stressing has been extended to hrs. Cross-sectional analysis was carried out on sample stressed for 6250 hr under the condition of A/cm 2, 170 C. Similar to previous results, as presented in Fig. 6(b), no resistance change was observed. Since the brittleness IMC is well-known, FIB (focus ion beam) mircoscopy was ultilized to execute for the purpose of eliminating any artifacts during smaple preparation. Fig 8 showed cross-sectional results on both stressed and unstressed neighboring IMC micro-joints after 6250 hr stressing under A/cm 2, 170 C. A BEI (backscattered electron image) image and a SEI (secondary electron image) image were both shown in Fig. 8(a) and 8(b), respectively. No EM induced damage was observed in powered bumps. From BEI images, in Fig. 8(a), different IMCs can be observed clearly. Tiny voids were observed inside IMC joint and was resulted from metallurgical reaction during the initailly IMC formaion. Compared with powered and un-powered bumps, there was no difference observed and. Again, IMC micro-joints showed superior EM performance. Fig. 6. Resistance plots of the EM stressed IMC micro-bumps under two ultra-high stressing conditions. (a) A/cm 2, 170 C, and (b) A/cm 2, 170 C, respectively. Cross-sectional analysis was carried out for identifying the 4% resistance increase shown above in Fig 6(a). From SEM image, as shown in Fig. 7, it was quite clear that the resistance increase was resulted from the damages inside Al trace. Beside void formation at the cathode side of Al trace (red dotted line rectangle), extrusion of Al was also observed as Fig. 8. Cross-sectional analysis was shown in (a) BEI images and (b) SEI images. Larger contrast can be obtained in BEI image and therefore different IMCs can be observed clearly. No trend of IMC distribution was observed on powered bumps. To compare with IMC micro-joint and solder joint EM performance, the Black s equation is taken into consideration. Black s equation, as shown below has been modified for predicting solder joint EM lifetime [16, 17]: 1 Q MTTF A exp (1) n ( cj) k( T T ) 622

4 where c accounts for current crowding and ΔT is Joule heating. Increasing c andδt shortens the lifetime of bumps. For solder joints, it is more complicated because of the change of the dominant diffusion species and the change of the dominant failure modes. For solder joint, the user level for passing 10 year is set as 40 ma under 110 C and the corresponding activation energy and n value is taken as 1eV and 2eV, respectively. Table II showed the corresponding estimated lifetime of solder joint under those accelerated stressing condition. For accelerated condition I, solder joint can only survive hr while IMC joint can survive more than 5000 hr and still maintain its integrity. Table II. A summary table of IMC predicted lifetime under ultra-high stressing conditions. Compared with solder joint, solder can only survive less than 15 hr. Fig. 9 summarizes the various stressing conditions and correlates them with their respective degradation modes. Solder jointsup to 13,000 hr current stressing can be obtained without failure at low to moderate stressing conditions. Void formation dominated the failure mode are marked in red circles under highly accelerated stressing conditions. The stressing time of failed samples is in the range of 160 hr to 380 hr with severe void formation inside joints [ref your ECTC paper]. A boundary, marked in a dash line, is obtained in between void formation and IMC formation in solder joint. The black diamond represents user level. Test conditions of IMC micro-joints were marked as yellow circle, which were on the upper right corner in Fig. 9(a). More than 9,000 hr stressing time is obtained without failure in IMC joint. The current carrying capability is about 2 orders greater than that of solder micro-bumps. Fig. 9. A summary correlates the degradation mechanisms with stressing conditions. In solder joints, IMC formation is the dominant mechanism for the low to moderate stressing conditions, which is represented in green. The excessive stressing conditions are dominated by void formation and are marked in red. IMC micro-joint, marked in yellow circle. (b) Superior EM performance of IMC micro-joint According to the microstructural analysis, no EM induced damage was observed at current density from A/cm 2 to A/cm 2 under 170 C~180 C. When the electrical current drives atoms to move from the cathode to the anode, a built-in stress gradient is developed across the solder joint [18, 19]. Hence, a built-up vacancy flux from cathode to anode occurs based on the Nabarro-Herring model. The vacancy flux is strongly dependent on the length of the materials considered, in this case solder bump height, and the shorter bump height would exhibit greater vacancy flux. Since this vacancy flux gradient is in the opposite direction of EM induced flux, the driving force of EM is hindered. The total atomic flux caused by current stressing and back stress can be expressed as D d D * J em C C Z ee (2) kt dx kt where J em is atomic flux (atom/cm 2 s), C is the concentration of atoms per unit volume, D/kT is atomic mobility, σ is stress (dyn/cm2), Ω is atomic volume, Z* is the effective charge number of EM, and E is the electric field (E=ρj, where ρ is resistivity and j is current density). If no EM induced damage occurs (J em =0), the expression for the critical current density can be obtained as j c (3) * Z e x Under a fixed bump height, once the applied current density is greater than the critical current density, EM induced flux unbalance would occur and result in damage, such as void formation or protrusion. One can also notice that from equation (3), the critical current density depends greatly on the intrinsic properties, such as resistivity, effective charge number and also Young s modulus of the stressed materials. In other words, a material which possesses a higher Young s modulus, larger atomic volume, or a relatively smaller effective charge number and resistivity, can result in a greater critical current density. For Young s modulus of IMC joint, firstly take Cu 6 Sn 5 into consideration, the reported Young s modulus is 52 Gpa at 150 C [20]. At the elastic limit, the strain is 0.2%. If we take ρ = 17.5 µω.cm, Z*=26 [19] and x was taken as 10 µm for IMC micro-joint used in our experiment, the threshold current density to trigger EM would be A/cm 2. In other words, unless the applied current density is greater than A/cm 2, it is not possible to observe EM induced damage in a Cu 6 Sn 5 IMC micro-joint. This confirms why there is no observable EM induced damages in the IMC EM tests in the present study. 623

5 (c) Potential risks on fully IMC micro-joint IMCs have high modulus, but they are also very brittle. As shrinkage in bump size continues, the role of IMC on mechanical reliability becomes more important. The brittleness of IMCs can cause undesirable cracks in joint and result in a failure. For a fully IMC micro-joint, it is important to avoid cracks formation. Few approaches are provided to protect IMC micro-joint, such as underfill, or molding compound. Cracks were reported inside IMC joint after thermal aging [21], both on powered and un-powered microbumps. The length of cracks increased with the duration of aging. However, cracks formation and propagation mechanisms were still unclear. Similar phenomenon was also observed in other s work. Cracks inside a fully transformed IMC joint were observed in Ouyang s work [22] after hr current stressing under 150 C, A/cm 2. Severe cracks formation inside IMC joint was observed as well in some cases in the present study after current stressing, at 180 C, A/cm 2. It is highly likely that the observed cracks could be resulted from sample preparation. Samples, which were tested under similar conditions showed no microcracks with careful stress-free sample preparation using FIB. More study is needed in this area. Conclusions The EM behavior of full IMC joint is demonstrated for 3D IC bump interconnects. The EM behavior of IMC joint was evaluated at 170 C and 180 C under the current stressing from 1.4 to 2.1 x10 5 A/cm 2. The results show that after over 9000 hr ultra-high stressing condition, there were no resistance variations. The full IMC joint found to be much more robust in EM reliability when compared to the solder-based microbumps. Acknowledgements The authors would like to thank Prof. J. G. Duh for his valuable suggestions and comments in EPMA analysis. References [1] T. Y. Chen, H. R. Wang, I. C. 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