A Study of Electroless Co(W,P) Thin Film as the Diffusion Barrier of Under Bump Metallurgy

• Main Authors: Wen-Chen Wu, 吳文成
• Other Authors: T.-E. Hsieh
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/90461366986860038532

碩士 === 國立交通大學 === 材料科學與工程系所 === 94 === This work prepares electroless cobalt-tungsten-phosphorus (Co(W,P)) thin films to serve as the diffusion barrier of under bump metallurgy (UBM) for flip-chip Cu-ICs. The Si wafer pre-deposited with a Ti/Cu bi-layer to simulate the Cu-IC chips was adopted as the substrate for electroless Co(W,P) deposition. In the first part of the study, we varied the parameters of electroless deposition such as pH values, plating solution concentrations and deposition times so as to investigate their effects on the composition and crystal structure of Co(W,P) films. Liquid- and solid-state aging tests of eutectic PbSn solder/Co(W,P) samples were also carried out in order to characterize the evolution of intermetallic compounds (IMCs) at the interface and the diffusion barrier capability of the electroless Co(W,P) films. Experimental results found that the deposited rate decreases with the pH values in the range of 8.0 to 9.0, however, it led to the increase of phosphorus content, the decrease of crystallinity and the improvement of surface flatness of Co(W,P) films. The Co(W,P) films became amorphous when pH values exceeded 8.6, but the film crystallinity increased when the plating solution was subjected to long-time usage due to the consumption of ionic components in the solution. The liquid- and solid-state aging tests revealed the formation of needle-like CoSn2 phase at solder/Co(W,P) interface. The consumption of Co(W,P) films revealed they essentially served as the sacrificial diffusion barrier; however, the diffusion retardation by P and W elements residing in the grain boundaries of nanocrystals indicated the electroless Co(W,P) films are also stuffed barriers. About 1 to 2 at.% tungsten (W) in the Co(W,P) film further retarded the diffusion of Cu into Co(W,P) films and suppress the reaction rate with solder. For these reasons, the addition of W benefited the reliability of solder joints and electroless Co(W,P) films were one of ideal barrier layers in UBM of flip-chip Cu-ICs.