Excellent Oxidation Resistance and Solder Wettability of (111)-Oriented Nanotwinned Cu
XU Zengguang1,2,3, ZHOU Shiqi1,3, LI Xiao1,3, LIU Zhiquan1,2,3()
1 Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China 2 Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences, Shenzhen 518055, China 3 Shenzhen Institute of Advanced Electronic Materials, Shenzhen 518103, China
Cite this article:
XU Zengguang, ZHOU Shiqi, LI Xiao, LIU Zhiquan. Excellent Oxidation Resistance and Solder Wettability of (111)-Oriented Nanotwinned Cu. Acta Metall Sin, 2024, 60(7): 957-967.
In the advanced-packaging industry, (111)-oriented nanotwinned copper ((111)nt-Cu) offers several advantages over common randomly oriented equiaxed polycrystalline Cu (C-Cu), including high strength, excellent elongation, and promising electrical conductivity. In recent years, (111)nt-Cu has shown potential for becoming a C-Cu replacement in under-bump metallization and redistribution layer applications. This shift is attributed to the escalating demand for thermal stability and enhanced electrical and mechanical performances of Cu materials amid the rapid transition toward three-dimensional electronic packaging. This study proposed that (111)nt-Cu exhibits better oxidation resistance than C-Cu after aging in air at 250oC. The thickness and composition of (111)nt-Cu and C-Cu oxide layers were analyzed respectively via TEM and XPS. Various grain boundaries on the surface of the prepared (111)nt-Cu and C-Cu substrates were evaluated using EBSD and FIB techniques. The morphology at the interface between the two Cu substrates and their oxide layers was characterized using SEM. In addition, the solderability of the oxidized layers was assessed by measuring the wetting angles and spreading areas of the involved Sn-Cu joint structures. Results show that when the oxidation time was 9 min, the thickness of the (111)nt-Cu oxide layer was 43.2% lesser than that of the C-Cu oxide layer. After 12 min of oxidation, the contact angle between Sn and oxidized (111)nt-Cu was 26.7% smaller than that between Sn and oxidized C-Cu, while the spreading area of Sn on (111)nt-Cu was 24.6% larger than that of Sn on C-Cu. After oxidation, the surface layers of both Cu substrates comprised CuO and Cu2O nanocrystals coexisting within the same layer. Because oxidation is closely related to the diffusion of Cu atoms through grain boundaries, the grain boundaries of both Cu substrates were investigated in the natural-growth direction. The results show that compared to C-Cu, (111)nt-Cu has a lower surface energy and smaller area fraction of high angle grain boundaries, effectively limiting the outward-diffusion rate of Cu atoms.
Fig.1 Electroplating process for the deposition of (111)-oriented nanotwinned copper ((111)nt-Cu)
Fig.2 Color changes of the surfaces of (111)nt-Cu (a1-a5) and common randomly oriented equiaxed polycrystalline Cu (C-Cu) (b1-b5) samples after oxidation at 250oC for 0 min (a1, b1), 3 min (a2, b2), 6 min (a3, b3), 9 min (a4, b4), and 12 min (a5, b5) (The dimension of the individule sample is 1 cm × 1 cm)
Fig.3 XPS analyses of the surface of (111)nt-Cu and C-Cu samples after oxidation (a) 2p orbital spectrum of (111)nt-Cu (b) LM2 orbital spectrum of (111)nt-Cu (c) depth profiles of the content of O in (111)nt-Cu and C-Cu samples after oxidation
Fig.4 SEM (a, c) and FIB (b, d) images of the cross sections of (111)nt-Cu (a, b) and C-Cu (c, d) samples after oxidation
Fig.5 TEM bright field images of the cross sections of (111)nt-Cu (a) and C-Cu (b) samples after oxidation, and the corresponding HRTEM images (c, d) as well as selected area electron diffraction (SAED) patterns (e, f) of the oxide layer
Fig.6 Contact angles of solder bumps on (111)nt-Cu and C-Cu samples with oxidation time at 250oC
Fig.7 Spreading cloud images of the solder bump on the surfaces of (111)nt-Cu (a-c) and C-Cu (d-f) samples oxidized at 250oC for 0 min (a, d), 3 min (b, e), and 12 min (c, f)
Sample
0 min
3 min
12 min
(111)nt-Cu
2.639
2.276
1.896
C-Cu
2.132
2.015
1.522
Table 1 Spreading areas of the solder bump on the surfaces of (111)nt-Cu and C-Cu samples oxidized at 250oC for different time
Fig.8 EBSD images of the original surfaces of (111)nt-Cu (a) and C-Cu (b) samples showing the grain boundry and twin boundary in the normal direction, as well as those of FIB surface images (c, d) after oxidation
Sample
Area fraction of LAGB %
Area fraction of HAGB %
Grain boundary length μm
Twin boundary length μm
(111)nt-Cu
11.1
88.9
5621
13814
C-Cu
0.9
99.1
2536
7109
Table 2 Statistical data of grain boundaries of (111)nt-Cu and C-Cu samples
Fig.9 Grain misorientation distributions (a, b) and grain size distributions (c, d) of (111)nt-Cu (a, c) and C-Cu (b, d) samples
Fig.10 Cross section FIB images of electroplated (111)nt-Cu (a) and C-Cu (b) samples; diffution illustrations of Cu atom on grain boundaries in (111)nt-Cu (c) and C-Cu (d) samples (JCu1—Cu diffusion flux in (111)nt-Cu along the grain boundary; JCu2—Cu diffusion flux in C-Cu along the grain bounadry)
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