Formation mechanism of Sn-patch between SnAgCu solder and Ti/Ni(V)/Cu under bump metallization
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Toung-Yi Shih Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan; and United Microelectronics Corporation, Hsinchu 300, Taiwan (Received 7 January 2009; accepted 13 April 2009)
An Sn-patch formed in Ni(V)-based under bump metallization during reflow and aging. To elucidate the evolution of the Sn-patch, the detailed compositions and microstructure in Sn–Ag–Cu and Ti/Ni(V)/Cu joints were analyzed by a field emission electron probe microanalyzer (EPMA) and transmission electron microscope (TEM), respectively. There existed a concentration redistribution in the Sn-patch, and its microstructure also varied with aging. The Sn-patch consisted of crystalline Ni and an amorphous Sn-rich phase after reflow, whereas V2Sn3 formed with amorphous an Sn-rich phase during aging. A possible formation mechanism of the Sn-patch was proposed. I. INTRODUCTION
In flip chip technology, under bump metallization (UBM) composed of multimetallic thin films is usually used to improve the bonding between the solder and the pad on the chip.1–11 One of the commercial UBMs is a sputtered Ti/Ni(V)/Cu layer, because doping 7 wt% V into the Ni target can eliminate the magnetism of Ni during sputtering. Thus, the Ni(V) layer is widely used as a sputtered UBM material. It was noted that V in the Ni(V) layer did not react with solders and intermetallic compounds (IMCs) during the reflow and aging process, yet an Sn-rich phase, the so-called “Sn-patch,” would form in the Ni(V) layer.5 The possible reason for Sn-patch formation may be the fast Sn diffusion from the solder matrix to the Ni(V) layer. In the literature, the mechanical properties of solder joints with the Ni(V)-based UBM6,7 or the composition and structure of IMCs were reported.8–13 However, the formation mechanism of the Sn-patch and the detailed composition variation and structure evolution in the Sn-patch have not been fully discussed. In this study, the Snpatch was analyzed by a field emission electron probe x-ray microanalyzer (FE-EPMA) and a transmission electron microscope (TEM) to elucidate composition redistribution and microstructure evolution. Furthermore, the possible formation mechanism of the Snpatch was proposed. In addition, the composition and
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0315
http://journals.cambridge.org
II. EXPERIMENTAL PROCEDURE
The sputtered Ti/Ni(V)/Cu thin film was used as the UBM on the Si wafer. The thickness was 0.3 and 0.5 mm for the sputtered Cu and Ni(V) layers, respectively. Afterward, Sn3.0Ag0.5Cu solder was jointed with the Ti/ Ni(V)/Cu UBM at 250 C for 90 s. The height of the solder joints was 120 mm, and the diameter of the pads was 80 mm. Finally, the specimens were treated at 125 and 200 C, respectively, for 1000 and 2000 h. The cross-section samples were prepared using a crosssection polisher (SM-09011; JEOL, Japan) to avoid damage from the mechanical polishing process. The annealed samples were mounted by the G2 resin and polished b
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