無電鍍鈷鎢磷薄膜應用於凸塊底層金屬化之擴散阻障層之研究

Abstract

本論文探討以無電鍍法（Electroless Planting）所沉積的鈷鎢磷（Co(W,P)）薄膜應用在銲錫底層金屬化（Under Bump Metallurgy，UBM）中之擴散阻障層（Diffusion Barrier）的可行性。實驗以預鍍鈦、銅兩層金屬薄膜的矽晶片基板模擬銅製程晶片，之後在基板表面沉積鈷鎢磷薄膜。實驗首先改變鍍液的pH值、離子濃度以及析鍍時間等變因，研究不同的鍍膜條件對鈷鎢磷薄膜成分、結構之影響；再將各種不同條件長成的無電鍍鈷鎢磷薄膜與銲錫施予液態時效（Liquid-state Aging）與固態時效（Solid-state Aging）測試，研究無電鍍鈷鎢磷薄膜應用在UBM層時，其與銲錫的合金化反應以及擴散阻擋之能力。 實驗結果顯示，當鍍液之pH值在8.0至9.0範圍內，鍍率有隨著pH值上升而降低的趨勢，但此同時增加鈷鎢磷薄膜中的磷含量、減少結晶程度與提升表面平整度。當鍍液pH值達到8.6以上，薄膜結構逐漸轉變為非晶質態（Amorphous），但在長時間析鍍後，由於鍍液中離子濃度的消耗造成磷含量降低，薄膜的結構終將轉變為多晶。與銲錫之液態與固態時效試驗顯示，鈷鎢磷薄膜與銲錫會生成針狀CoSn2介金屬相，鈷鎢磷薄膜之消耗顯示其阻障行為類似犧牲型擴散阻障層（Sacrificial Barrier），而磷、鎢等元素填塞在奈米微晶晶界所發揮之阻障效果亦使鈷鎢磷薄膜具備填塞型阻障層（Stuffed Barrier）之行為；1至2 at.%鎢的加入對於合金化反應的速率、擴散阻擋能力等有明顯助益，再加上它使薄膜對於銅原子的阻擋能力的提升，使用無電鍍鈷鎢磷薄膜為阻障層有助於提升銅製程晶片凸塊接點之長時間可靠度。

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.