Investigation of electroless cobalt-phosphorous layer and its diffusion barrier properties of Pb-Sn solder

Abstract

The capability of a cobalt-phosphorous [Co(P)] layer, which was grown via the electroless plating process, to serve as the diffusion barrier of lead-tin (PbSn) solder was investigated in this work. The Auger electron spectroscopy (AES) and energy dispersive spectrometry (EDX) indicated that the phosphorous contents in Co(P) films decrease with increasing film thickness and that the average contents are no less than 8.7 at.% for the specimens prepared in this work. X-ray diffraction in conjunction with composition analyses revealed that the electroless Co(P) layer was a mixture of amorphous and nanocrystalline structures; however, the AES depth profile and subsequent analyses indicated that the first-formed Co(P) layer should be amorphous because it contains as much as 18 at.% P. This implied a good barrier capability for electroless Co(P) because, as revealed by EDX line scan, the Sn and Cu atoms could not penetrate the Co(P) layer after the PbSn/Cu/Co(P)/Cu/Ti/Si sample was subjected to annealing at 250°C in a forming gas ambient for 24 h. The fact that Sn and Cu underlayers could not penetrate the Co layer after such a liquid-state annealing step was evidence that the Co(P) layer may simultaneously serve as a diffusion-barrier interlayer dielectric and as an under-bump metallization for flip-chip copper (Cu) ICs.

Co-W as an advanced barrier for intermetallics and electromigration in fine-pitch flipchip interconnections

“…Tungsten (W) being a refractory metal, has superior diffusion barrier capabilities at higher temperatures. Incorporation of tungsten in cobalt clogs up the grain boundaries [14]. By removing these fast diffusion paths, the ability to retard diffusion is enhanced.…”

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.…”

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.…”