Analysis and control of Cu6Sn5 preferred nucleation on single crystal (0 0 1)Cu
Introduction
3D packaging is one of the most popular and advanced connecting technologies in electronic industry [1], [2], [3]. In multiple reflow process of 3D packaging, solder joints of high melting point are demanded since solders formed earlier are required not to melt again in the later reflows [4], [5]. Normally, this problem can be solved by the formation of fully intermetallic interconnects [6], [7], [8]. Especially for fully oriented-intermetallic interconnect (FOI), its physical linking property can be strengthened controllably and therefore has quite considerable potential application in 3D packaging [9], [10].
Simultaneously, the number of grains in under bump metallurgy is decreasing rapidly following the downsizing of interconnects [11]. For a limit case, intermetallic compound (IMC) formed on a single crystal substrate inclines to exhibit textured morphology of highly uniform orientation [11], [12], [13], [14], [15]. Since continually epitaxial growth of textured IMC can be easily realized [16], fabricating of FOI based on single crystal substrates becomes a practical method.
The primary process of FOI fabrication is the preferred nucleation of interfacial Cu6Sn5 (dominant phase of IMC) grains. Not all grains on Cu single crystals can form textured microstructure perfectly. For instance, textured Cu6Sn5 can hardly form on (0 0 1)Cu at low reflow temperature and high Cu concentration [17]. Therefore, the primary task of FOI preparation should be the control of Cu6Sn5 preferred nucleation on Cu single crystals.
Section snippets
Experimental methods
Sn-3.0Ag solder ball in diameter of 100 μm was reflowed to single crystal (0 0 1) Cu in dimensions of 5 × 3 × 0.2 mm3 at 523.15 K and as soon as it started to melt, the liquid solder was immediately removed by high pressure air blowing [18] and then the left formed interconnect was put into distilled water of 273.15 K to cool down rapidly. Sketched in Fig. 1(a), the (0 0 1)Cu substrate was heated to 523.15 K by a heating plate with ceramics on under the protection of rosin flux
Results and discussion
The band contrast (BC) and related inverse pole figures (IPFs) of interfacial nucleated Cu6Sn5 phase are respectively demonstrated in Fig. 1(b) and (c), where 59 successive Cu6Sn5 grains along the radius of reaction interface are marked out exactly. Obviously, orientations of Cu6Sn5 grains in the edge are much different with that in the center. Euler angles and accurate ND indices of crystal directions of these 59 grains are exhibited in “PART II” of the Supplementary material, through which
Conclusion
In summary, decrease of ∂μ/∂x can degrade the level of Cu6Sn5 regularly texturing and therefore a practical model analyzing and controlling the preferred nucleation of Cu6Sn5 on single crystal (0 0 1)Cu through chemical potential gradient was proposed herein. This result is quite significant for preparation of orientation controllable fully intermetallic interconnects in 3D packaging.
CRediT authorship contribution statement
H.R. Ma: Data curation, Formal analysis, Methodology, Writing - original draft. C. Dong: Data curation, Software, Methodology. N. Zhao: Writing - review & editing. Y.P. Wang: Writing - review & editing. X.G. Li: Writing - review & editing. H.T. Ma: Funding acquisition, Writing - review & editing. J. Chen: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (Grant No. 2017YFA0403804) and the National Natural Science Foundation of China (Grant Nos. 51871040).
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