The interfacial structure and strength of solder joints between Sn-9 mass%Zn solder and plated Au/Ni-P alloy film on a Cu substrate have been investigated. Three reaction layers with 0.2 to 0.5 µm thickness were formed along the interface between the plated Ni-P alloy films and Sn-9 mass%Zn solder. The outermost layer contains a Ni-Sn intermetallic compound. The middle layer contains approximately 40 mass%Au, 35 mass%Zn, 20 mass%Ni and 5 mass%Sn. The thickness of the Au layer is 0.1 µm, so the Au layer does not dissolve. The innermost layer contains about 63 mass%Zn, 25 mass%Ni, 10 mass%Au and 2 mass%Sn. The strength of the Sn-9 mass%Zn solder joints take almost the same values independent of P concentration. The strength of Sn-Zn solder joints with Sn-Zn/Ni-2 mass%P, Sn-Zn/Ni-4 mass%P and Sn-Zn/Ni-8 mass%P joints were found to be almost constantly independent of reflow cycles. Therefore, Sn-9 mass%Zn solder is considered to be an excellent solder material for plated Ni-P alloy films.

One of the critical issues which needs to be solved in the packaging technology of high speed and high density semiconductor devices is the enhancement of micro-solder joint reliability and strength. The reliability and strength of the solder joints depend on the interfacial structures between metallization and lead free solder. Both the interfacial structures and the strengths of the solder joints between plated Ni-P alloy films with various P concentrations and various solder materials have been investigated. The places where intermetallic compounds crystallized were found to vary according to the P concentration in plated Ni-P alloy films and the composition of the solder. Pyramidal intermetallic compounds that formed on plated Ni-P alloy films had the following compositions: Sn-3.5 mass%Ag/Ni-2 mass%P, Sn-3.5Ag-0.7 mass%Cu/Ni-P(2, 8 mass%) and Sn-50 mass%Pb/Ni-P(2, 8 mass%). Whereas intermetallic compounds were crystallized in the solder of the Sn-3.5 mass%Ag/Ni-8 mass%P sample. A P-enriched layer was formed between the plated Ni-P alloy films and the intermetallic compounds. The thickness of the P-enriched layers of each sample increased with the reaction time. In experiments using the same solder material, the P-enriched layer of the solder/Ni-8 mass%P sample was much thicker than that of the solder/Ni-2 mass%P sample. In experiments with plated Ni-8 mass%P alloy films, the P-enriched layers became thicker in this order: Sn-50 mass%Pb/Ni-8 mass%P; Sn-3.5Ag-0.7Cu/Ni-8 mass%P; Sn-3.5 mass%Ag/Ni-8 mass%P. The strengths of the solder joints decreased with the P concentration in plated Ni-P alloy films for all solder materials. However, it was found that the strength degradation ratio varied with the solder materials and they increased in the following order: Sn-50 mass%Pb; Sn-3.5Ag-0.7 mass%Cu; Sn-3.5 mass%Ag. Therefore, it was found that the solder joint strength is very sensitive to the thickness of the P-enriched layer at the solder joint and the solder joint strength decreased with the thickness of the P-enriched layer independent of the solder materials. Therefore, research into the interfacial structures between electroless plated Ni-P alloy film and solder is very important. It has been reported that reliability degradation occurs at the interface between plated electroless Ni-P alloy film and solder, when a P-enriched layer is formed at the interface during the soldering process.8) However, no-one has yet confirmed the relationship between the interfacial structure, including intermetallic compound and the P-enriched layer, and the mechanical behavior of the solder joints.The first purpose of the present paper is to investigate the interfacial structures formed during the soldering of plated Ni-P alloy films having various P concentrations with solder materials. Next, solder joint strength was investigated as a function of the solder materials and P concentrations in plated Ni-P alloy films. Finally, the relationship between the interfacial structure and the solder joint strength was investigated.

Resistivity difference between Cu wires made with plating using high purity ͑new plating process͒ and conventional purity ͑conventional process͒ materials has been evaluated in order to develop the process for the realization of high performance LSIs. This resistivity difference is relatively small, i.e., 8% when line width is wide ͑200 nm͒. However, it increases with the decrease in line width, and it reaches about 20%, i.e., 2.8 ⍀ cm for the former and 3.5 ⍀ cm for the latter at 50 nm line width. A 50 nm wide Cu wire formed with the new plating process had more uniform and larger grain sizes and lower impurity concentrations than the wire formed with the conventional process.Copper has been used as an interconnect material for high performance ultralarge scale integrations ͑ULSIs͒ due to its low electrical resistivity and high reliability. However, the resistivity of Cu interconnects increases significantly with a decreasing line width of less than 100 nm. 1-4 This is becoming a critical issue for the realization of high speed ULSIs, and it is mainly because the line widths are comparable to the mean free path of the electron ͑40 nm͒; hence, electron scattering occurs at the grain boundaries, resulting in the higher resistivity of very narrow Cu wires. 5 To lower the resistivity, both the coarsening of the grain sizes and reduction of the thickness of high resistivity barrier metals in Cu wires are very important. Self-forming barriers using Cu-Mn or Cu-Ti alloys and atomic layer deposition are possible candidates to promote the formation of thinner barriers. 6,7 The most effective method to reduce Cu wire resistivity is to lower the resistivity of the Cu wires themselves by coarsening the grain sizes. It has been recently reported that impurities such as oxygen, sulfur, and nitrogen concentrate on the grain boundaries of Cu wires and depress their grain growth during annealing. 8,9 These results imply that low resistivity Cu wires can be formed if high purity, very narrow Cu wires can be formed.Hence, we focused our attention on the purification of Cu wires using a newly developed nominal high purity 9N anode and nominal high purity 6N-CuSO 4 ·5H 2 O electrolyte ͑new plating process͒. Resistivities of Cu wires formed with the new plating process were measured and compared to those of Cu wires formed with a conventional purity 4N anode and 3N-CuSO 4 ·5H 2 O electrolyte ͑conven-tional process͒.Using the new plating process, we achieved 50 nm Cu wires with ϳ20% lower resistivity than those made by the conventional process. In this paper, we first investigated grain sizes and textures of plated films obtained using the new plating and conventional processes. Then, the resistivities of Cu wires made with the new plating process were evaluated as a function of wire width in comparison with those of Cu wires formed with the conventional process. Finally, we considered the mechanism for achieving low resistivity by evaluating the grain sizes, impurities, and lattice images of 50 nm wide Cu wires made by both the new p...

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