無電鍍鈷鎢磷薄膜對銲錫之擴散阻障特性研究

Abstract

本論文研究以無電鍍（Electroless Plating）技術製備非晶態（Amorphous）與複晶態（Polycrystalline）之鈷鎢磷（Co(W,P)）薄膜，探討其對銲錫的擴散阻障（Diffusion Barrier）特性。研究的銲錫種類包括共晶錫鉛（Eutectic PbSn）、錫鉍（SnBi）與錫銀銅（SnAgCu，SAC），試片經液態時效（Liquid-state Aging）與固態時效（Solid-state Aging）後，分析Co(W,P)與銲錫的合金反應及擴散阻擋能力。PbSn與SAC系統並計算其介金屬化合物（Intermetallic Compound，IMC）成長之活化能（Activation Energy，Ea），並以推球試驗（Ball Shear Test）評估界面接合強度與破壞模式。 時效測試結果顯示，經液態時效之非晶態Co(W,P)與銲錫反應生成之IMC，在PbSn與SnBi系統中IMC主要為CoSn2及CoSn3相，SAC系統中IMC則為CoSn3+(Co,Cu)Sn3相；隨著時效時間的增加，IMC會因球化（Spallation）而進入銲錫中，同時反應界面生成一由奈米尺度之IMC晶粒組成的複晶態富磷層（P-rich Layer），未反應的Co(W,P)則有再結晶且析出Co2P相等現象。固態時效試驗則觀察到層狀IMC生成於富磷層之上，且無球化行為，且未反應之Co(W,P)薄膜仍維持非晶態。 經液態與固態時效後，複晶態Co(W,P)與銲錫反應生成一較厚的IMC層於非晶態之富鎢層（Amorphous W-rich Layer）上，且無球化行為；此一非晶態富鎢層無法阻擋銲錫與Co(W,P)後續的合金反應，故阻障之能力應肇因於原子鍵結之特性，材料之結晶結構為何不宜做為擴散阻障機制的分類依據。 擴散阻障特性之研究結果顯示，非晶態Co(W,P)為複合式擴散阻障層，即犧牲型擴散阻障層（Sacrificial-type Barrier）與填塞型阻障層（Stuffed-type Barrier）之組合；複晶態Co(W,P)則主要為犧牲型擴散阻障層。IMC成長之Ea計算顯示，PbSn/非晶態Co(W,P)系統之Ea = 338.6kJ/mole，PbSn/複晶態Co(W,P)系統之Ea = 167.5 kJ/mole；SAC/非晶態Co(W,P)系統之Ea = 110.7 kJ/mole，SAC/複晶態Co(W,P)系統之Ea = 81.8 kJ/mol。 推球測試結果顯示PbSn系統及SAC/poly-Co(W,P)試片之破壞模式以延性破壞（Ductile Mode）為主，而SAC/複晶態Co(W,P)試片在短時間熱處理下主要是界面破斷（Interfacial Break Mode），若經長時間熱處理則轉為銲墊舉離模式（Pad Lift Mode）。分析結果顯示銲墊舉離模式肇因於高磷成分的非晶態Co(W,P) 有礙於銲錫之潤濕性（Wettability）而降低界面接合強度，且Co2P相析出及非晶態Co(W,P)再結晶現象將導致未反應之Co(W,P)脆化及熱膨脹係數改變進而產生熱應力。故無電鍍Co(W,P)薄膜之磷含量不僅影響界面合金反應之行為，也是影響其擴散阻障能力與銲錫接點可靠度的重要因素。

Diffusion barrier characteristics of electroless amorphous and polycrystalline Co(W,P) (termed a-Co(W,P) and poly-Co(W,P) hereafter) to eutectic PbSn, SnBi and SnAgCu (SAC) solders are investigated in this study. The samples were treated by liquid- and solid-state aging tests and the alloy reactions and diffusion barrier capabilities were evaluated. For PbSn and SAC systems, the activation energy of intermetallic compound (IMC) growth (Ea), interfacial bonding strength and failure modes were also analyzed. In all solder/a-Co(W,P) samples subjected to liquid-state aging, spallation of IMC into solder, formation of a nano-crystalline P-rich layer at reacting interface, and the recrystallization of Co(W,P) containing Co2P precipitates were observed. The IMCs observed in PbSn and SnBi samples were mainly CoSn2 and CoSn3, whereas that in SAC sample were mixture of CoSn3 and (Co,Cu)Sn3. As to solders/a-Co(W,P) samples subjected to solid-state aging, IMCs resided on the P-rich layer without spallation. In the samples containing poly-Co(W,P), thick IMC neighboring to an amorphous W-rich layer was seen regardless of the solder and aging types. It was found that the amorphous W-rich layers could not inhibit subsequent alloy reactions. Hence, diffusion barrier capability should be correlated to the nature of chemical bonds, rather than the amorphism of microstructure. Moreover, a-Co(W,P) was a composite-type barrier, i.e., sacrificial- plus stuffed-type barrier, while poly-Co(W,P) is mainly a sacrificial-type barrier. Analytical results indicated that the P content in Co(W,P) is a crucial factor affecting the structural evolution at the solder/electroless Co(W,P) interface. The values of Ea’s for IMC growth in PbSn/a-Co(W,P) and PbSn/poly-Co(W,P) samples were separately equal to 338.6 and 167.5 kJ/mol, whereas the Ea’s of IMC growth were 110.7 and 81.8 kJ/mol for SAC/a-Co(W,P) and SAC/poly-Co(W,P) samples, respectively. Ball shear test revealed the ductile mode dominates the failure in PbSn/a-Co(W,P), PbSn/poly-Co(W,P) and SAC/poly-Co(W,P) samples in most aging conditions, whereas interfacial break dominates at short-time aged samples and pad lift dominates when aging time was long in SAC/a-Co(W,P) case. Analytical results indicated that the decrement of bonding strength due to pad lift failure was ascribed to the deterioration of adhesion due to high P content, loss of toughness due to the formation of Co2P precipitates and the thermal stress induced by the change of CTE due to the recrystallization in aged Co(W,P) layer although high P content might enhance barrier capability. The P content of electroless plating layer affects not only the alloy reactions at solder/Co(W,P) interface, but also the diffusion barrier characteristics and reliability of solder joints.