TEM Observation of the Ni(V) UBM Consumption Induced by High Current Density in Flip-Chip Solder Joints
Date Issued
2010
Date
2010
Author(s)
Tsai, Ming-Yen
Abstract
In our previous studies, a two-stage failure mechanism of flip-chip solder joints during electromigration was pointed out. When the test vehicles were under current stressing and solid-state aging, the combination of current crowding and resulting Joule heating would make the original Ni(V) UBM be replaced by the so-called “consumed Ni(V)” at the edge of the passivation where electron flows entered into the solder joints. With increasing the reaction time, the “consumed Ni(V)” would transform to the so-called “porous structure” by comparing the morphology change in scanning electron microscopy (SEM) back-scattered electron (BSE) images. The formation of porous structure would block electron flows. Hence, the electron flows would be diverted to the neighboring Ni(V) and subsequently this area would also be attacked to form consumed Ni(V). The above processes would proceed and the range of porous structure would expand. Once porous structure propagated all the Ni(V) layer, the device would fail.
All the above processes were observed by SEM. In this study, transmission electron microscopy (TEM) was used to observe the microstructures of “consumed Ni(V)” and “porous structure” in detail. The sputtered Ni(V) UBM after one reflow was with columnar structures in TEM bright field (BF) images. However, consumed Ni(V) was a solid layer without crystalline grains. Besides, the original Ni(V)/(Cu,Ni)6Sn5 interface became blurred. These results were due to the Ni-Cu-Sn interdiffusion at the original Ni(V)/(Cu,Ni)6Sn5 interface. Confirmation was done by scanning TEM (STEM) compo- sitional analyses. The consumed Ni(V) became a V-Cu-Sn-Ni region. The selected area diffraction patterns (SADP) of consumed Ni(V) showed a broad incident beam with faint diffraction rings and spots, which indicated that it was composed of an amorphous phase and crystalline phases with ultra-fine grains. The BF images of porous structure showed that there were many voids near the original Ni(V)/(Cu,Ni)6Sn5 interface. This result corresponded to that porous structure was shown as a two-phase structure by SEM in our previous studies. For the original Ni(V) layer, Cu and Sn in-fluxes could not balance the serious Ni out-flux, which resulted in the void formation. It was confirmed by the fact that almost no Ni signal was detected in porous structure by STEM-EDX analyses. V atoms were immobile and trapped in Ni(V) layer without agglomeration, which was beneficial to form an amorphous matrix. The SADP of porous structure implied that it was composed of an amorphous matrix and fine-crystalline Cu6Sn5 and VSn2 intermetallic compounds (IMC). The following HRTEM observation supported this implication. Compared to consumed Ni(V), the contrast of porous structure was lighter in TEM BF images. This implied that porous structure had lower atomic weight. The V content could be a critical value since V was immobile during electromigration. From consumed Ni(V) to porous structure, the related V content increased from 30-40 at.% to over 40 at.%. Since the volume of Ni(V) layer was unchanged, it implied that porous structure had higher degree of porosity. In addition, by comparing the SADP types of consumed Ni(V) and porous structure, porous structure was convinced to have higher degree of crystallization. Finally, the current applying was considered to play an important role on the void formation. The electron flows enhanced the Ni out-flux and retarded the Cu and Sn in-fluxes, which resulted in more serious unbalanced fluxes. Porous structure with higher degree of crystallization also provided more fast diffusion paths for vacancies. These phenomenons would accelerate the void formation.
It was pointed out in the past literature that a NiAl3 phase was presumably formed at the original Al/Ni(V) interface. This phase formation followed by the oxide formation should be correlated to the abrupt increase of resistance of the interconnect after Ni(V) consumption. Based on our TEM BF/DF images, STEM line scan analyses and HRTEM observation, a thin continuous layer was exactly observed between porous structure and Al. After identification, this continuous layer was convinced to be NiAl3. In addition, according to STEM high angle angular dark field (HAADF) images, an oxide layer was likely to form between NiAl3 and porous structure. The slight increase of electrical resistance when electron flows mainly passed through consumed Ni(V) was due to the amorphization of Ni(V) matrix. The fast increase of resistance when electron flows mainly passed through porous structure was due to the combination of amorphization of Ni(V) matrix, formation of crystalline fine grains and continuous oxide layer.
Subjects
Transmission Electron Microscopy(TEM)
Flip Chip
Solder
Electromigration
Type
thesis
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