Generic rules to achieve bump electromigration immortality for 3D IC integration

Abstract

An immortal solder micro-bump (bump) electromigration (EM) lifetime has been demonstrated for 3D IC integration. This ultimate goal was achieved under strictly controlled conditions, including the optimal design of bump metallurgy, geometry, optimized processes, along with welldefined stressing conditions and manufacturing window. The current carrying capability and EM lifetime of bump have been investigated as functions of stressing conditions which are correlated with the degradation mechanisms. When stressed under the appropriate g conditions, all bump test samples survived prolonged stressing, some over 13,000 hours, without a failure. Cross-sectional analyses revealed that the entire solder joint almost all transformed into intermetallic compounds (IMCs) with very minor or no voids. The resistance plots showed an initial fast rise in resistance due to IMC formation, then gradually leveled off and eventually reached a steady state. The observed degradation mechanism is dominated by IMC formation, which is the same as that of the user conditions. On the other hand, void formation that eventually led to open failure was the dominant degradation mechanism when samples were aggressively stressed. In other words, when all other conditions were the same, the stressing conditions make a huge difference in determining between an almost immortal EM lifetime vs. a short lifetime using the same high quality bumps. The boundary that separates the stressing conditions is roughly defined and will be discussed.In addition, since full IMC bump will become inevitable in the future miniaturized solder interconnect structure, the EM behavior of IMC dominated bump has also been evaluated in this study. Under highly accelerated stressing conditions of 174°C, at 1.6×10 5 A/cm 2 current density, the IMC dominated bumps were able to survive more than 600 hours and are still going strong. In comparison, solder bump failed quickly after just 107 hours when stressed under the same condition. This comparison study clearly demonstrated that IMC dominated joint has significantly higher current carrying capability than that of the solder joint. After reviewing all the data, we have concluded that the failure criteria for solder bump should be raised significantly higher than the 20% criteria traditionally used for the much larger C4 bumps.

Experimental and simulation analysis of concave-down resistance curve during electromigration in solder joints

“…27 Lin et al stressed microbumps at 1.23 Â 10 5 A/cm 2 at an ambient temperature of 130 C and they detected concave-down resistance curves. 28 Similar results were also found in 5 lm-thick-solder microbumps with 30 lm pitch stressed at 10 4 -10 5 A/cm 2 at 150 C. 29 Chen et al also reported a fast initial rise in resistance, then gradually taper-off and reaches a steady state after prolonged stressing in microbumps stressed under 1.0 Â 10 5 A/cm 2 at 140 C. 30 However, there have been no studies addressing this interesting phenomenon and the discrepancy in results.…”

“…This behavior was also reported in several previous studies. [27][28][29][30] To investigate the reason behind the different behaviors of the resistance curve, FEA modeling was carried out. The structure of the solder joint was almost identical to the one in Fig.…”

“…It is noteworthy that in order to observe the concavedown resistance curve, the current density should be large enough to trigger IMC formation at the initial stage. In general, the current density should exceed 1 Â 10 4 A/cm 2 and the temperature should be above 130 C. 30 The stressing conditions for microbumps are usually more stringent than these conditions. Therefore, an abrupt increase in resistance was observed in the literature.…”

“…26 However, for electromigration in microbumps with solder thickness less than 20 lm under high current densities, the resistance curve behaves completely different. [27][28][29][30] The resistance increases abruptly in the beginning, followed by more gradual increase, thus causing the resistance curve to behave concave-down. Wei et al conducted electromigration tests in microbumps by 9.6 Â 10 4 A/cm 2 at 168 C and they observed concave-down resistance curves.…”

Study of electro-migration resistivity of micro bump using SnBi solder

“…And several studies have reported that the bump geometry of fine pitch micro joining using Sn-Ag solder after electromigration test [6,7]. But fine pitch micro bump using a SnBi solder has been little investigated [8].…”