The plating characteristics of an electroless cobalt-tungsten-phosphorus ͓Co͑W,P͔͒ layer and its capability to serve as a diffusion barrier of underbump metallurgy ͑UBM͒ for flip-chip Cu-IC are investigated. Increasing the pH of the plating solution in a range of 7.6-9.0 decreased the rate of deposition, increased the phosphorus content, altered the structure from polycrystalline to amorphous, and decreased the surface roughness of Co͑W,P͒ films. The barrier capability of Co͑W,P͒ to eutectic PbSn solder was evaluated by means of liquid-and solid-state aging tests. In specimens subjected to tests of these two types, a P-rich layer presented in between solder and unreacted Co͑W,P͒, demonstrating the sacrificial barrier behavior of Co͑W,P͒. Transmission electron microscopy ͑TEM͒ analysis of specimens subjected to liquid-state aging indicated the supersaturated P atoms, and the Co 2 P consequently formed, might aggregate at grain boundaries of the P-rich layer, thus enabling the stuffed-type barrier capability of the Co͑W,P͒ layer. TEM, scanning electron microscopy, and energy-dispersive spectrometer characterizations of specimens subjected to solid-state aging revealed that the reaction of solder and Co͑W,P͒ implies a formation of CoSn 2 and CoSn 3 intermetallic compounds ͑IMCs͒ and a mixture of various IMCs in P-rich layer. Our experimental results demonstrated a satisfactory barrier property of electroless Co͑W,P͒, heralding promising applications in UBM structures for flip-chip Cu-IC.

Diffusion barrier and bonding characteristics of amorphous and polycrystalline electroless Co(W,P) layers (α-Co(W,P) and polyCo(W,P)) to SnAgCu (SAC) solder were investigated. In the SAC/α-Co(W,P) sample subjected to liquid-state aging at 250 • C, the spallation of intermetallic compound (IMC) into solder, a nano-crystalline P-rich layer at SAC/Co(W,P) interface, and the recrystallized Co(W,P) containing Co 2 P precipitates were observed. As to the SAC/α-Co(W,P) sample subjected to solid-state aging at 150 • C, a thick IMC layer neighboring to the P-rich layer formed at the solder/Co(W,P) interface. Liquid-state aging resulted in an IMC mixture without spallation whereas solid-state aging induced a layer-like IMC in SAC/poly-Co(W,P) samples. Amorphous W-rich layers were also observed in SAC/poly-Co(W,P) samples and it could not inhibit subsequent alloy reactions in the samples. Analytical results indicated the α-Co(W,P) is a sacrificial-plus stuffed-type barrier whereas the poly-Co(W,P) is mainly a sacrificial barrier. The activation energies of IMC growth were 110.7 and 81.8 kJ/mol for SAC/α-Co(W,P) and SAC/poly-Co(W,P) samples, respectively. High P content in α-Co(W,P) was found to degrade the bonding strength to the solder as revealed by the shear test and the control of P content would be a key issue for electroless plating layer applied to under bump metallurgy as the diffusion barrier.

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Diffusion barrier characteristics of amorphous and polycrystalline electroless Co(W,P) layers (α-Co(W,P) and poly-Co(W,P)) to lead-free SnAgCu (SAC) solder were investigated via the liquid- and solid-state aging tests. In the sample containing α-Co(W,P) subjected to liquid-state aging at 250°C for 1 hr, the spallation of (Co,Cu)Sn3 intermetallic compound (IMC) into the solder and formation of a polycrystalline P-rich layer in between SAC and Co(W,P) were found. Further, the α-Co(W,P) transforms into polycrystalline structure embedded with tiny Co2P precipitates As to the sample containing α-Co(W,P) subjected to solid-state aging at 150°C up to 1000 hrs, a thick (Cu,Co)6Sn5 IMC resided in between SAC and Co(W,P) and the P-rich layer beneath IMCs was similarly seen. In the samples containing poly-Co(W,P) subjected to liquid-state aging, a mixture of (Co,Cu)Sn3 and (Co,Ag)Sn3 IMCs formed in between SAC and Co(W,P). An amorphous W-rich layer formed in between SAC and poly-Co(W,P). Similar interfacial morphology was observed in the samples subjected to the solid-state aging test. Analytical results indicated the electroless Co(W,P) is in essential a combined-type, i.e., sacrificial-type plus stuffed-type, diffusion barrier. However, the α-Co(W,P) is a better diffusion barrier for under bump metallurgy (UBM) applications in flip-chip (FC) bonding since it exhibits a lower Co consumption rate in comparison with poly-Co(W,P).