Investigation of Electroless Co(W,P) Thin Film as the Diffusion Barrier of Underbump Metallurgy

Abstract

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.

An Investigation of Diffusion Barrier Characteristics of an Electroless Co(W,P) Layer to Lead-Free SnBi Solder

“…[8][9][10][11][12][13][14][15][16][17][18] Recently, electroless cobalt (Co) has attracted a lot of research interest due to its promising application as a diffusion barrier in Cu-ICs 19 and UBMs. [20][21][22][23] However, studies regarding the diffusion barrier characteristics of electroless Co layer/SnBi solder couples are lacking. In this work, we prepared electroless cobalt-tungstenphosphorus [Co(W,P)] layers, either amorphous or polycrystalline, and studied their diffusion barrier properties relative to deposited eutectic SnBi solder.…”

Wettability and interfacial reactions between the molten Sn–3.0 wt%Ag–0.5 wt%Cu solder (SAC305) and Ni–Co alloys

“…As the integrated circuits move from Al to Cu metallization, UBM materials need to be re-evaluated [7]. Ni-Co alloys are good diffusion barriers for Cu up to 500 • C [8], and Co is another diffusion barrier layer material of UBM [9][10][11][12][13]. Ni-Co alloys are potential materials as an alternative diffusion barrier layer in flip chip packaging for Cu/low k integrated circuits.…”

Sn/Ni–Co solid/solid interfacial reactions

“…Nickel is the most frequently used diffusion barrier layer material of UBM because of its low reactivity with solders [4][5][6]. Co has been considered as the alternative diffusion barrier layer material [7][8][9][10][11][12][13]. Thus, Ni-Co alloys are the potential alternative diffusion barrier layer material of UBM in flip chip packaging.…”

“…There have been extensive investigations upon Sn/Ni [14][15][16][17][18][19][20][21][22][23][24][25] and Sn/Co [7][8][9][10][11][12][13]26,27] interfacial reactions. However, solid/solid Sn/Ni-Co interfacial reactions have not been examined.…”