||The Material Interaction Behaviors in a Cu/Sn/Cu Interconnect Induced by Room Temperature Electromigration
||Department of Materials Science and Engineering
Room temperature electromigration
Meta-stable amorphous interphase
Intermetallic compound (IMC)
本研究的目的是為了釐清電遷移(Electromigration)的基礎非熱(Athermal)效應對銅/錫/銅接點的材料反應行為。電遷移實驗在室溫環境下進行(約29.3oC)，並透過薄膜接點結構設計以為達到最佳了解非熱效應之方法，除此之外，本研究透過磁控濺鍍(Magnetron sputtering)技術製作出無介金屬化合物(Intermetallic compound, IMC)的金屬界面以觀察銅/錫界面的原子級材料反應行為，室溫電遷移所產生的材料反應行為則利用球面像差修正掃描穿透式電子顯微鏡進行觀察(Spherical aberration corrected scanning transmission electron microscope, Cs-corrected STEM)。
本研究發現室溫電遷移初期(1 h)會於無介金屬化合物的銅/錫界面產生材料反應並生成非晶質銅錫界面層(Amorphous CuSn interphase)，其中內部含有奈米級銅結晶組織。這個非晶質界面層是由銅/錫界面原生的界面扭曲層(Interfacial distortion zone)轉變而成，此介穩態(Meta-stable)非晶質銅錫界面層會進一步於長時間室溫電遷移(81 h)後轉換成結晶態高溫相Cu6Sn5介金屬化合物作為一個相對穩定的銅/錫材料反應生成物。除了介金屬化合物生成外，本研究亦於銅/錫界面觀察到一特殊再結晶(Recrystallization)行為，本研究認為此再結晶行為是除了生成介金屬化合物外另一種發生於介穩態非晶質界面層的應力釋放(Stress relaxation)機制，且再結晶錫的界面層則被視為另一種室溫電遷移下穩定的銅/錫材料反應生成物。室溫電遷移除了對銅/錫界面產生材料反應行為外，亦對錫基地相產生銅過飽和(Supersaturation)現象，此現象使錫基地相成為一介穩態組織，本研究發現過飽和的程度受電遷移時間與試片位置(陰陽極)影響。本研究所觀察到的非晶質銅錫界面層生成、高溫相Cu6Sn5介金屬化合物生成、銅/錫界面再結晶、以及銅在錫中的過飽和現象均由電遷移的非熱效應所產生，而非電遷移伴隨的焦耳熱效應所致，本研究詳細探討與說明這些電遷移的基礎非熱效應與其機制中被。
The aim of the present study is to clarify the fundamental athermal effects of electromigration on the material interaction behaviors in a Cu/Sn/Cu interconnect. The electromigration experiment was conducted under a room temperature ambient condition, approximately at 29.3oC, through a thin film interconnect structure design to best reveal the athermal behaviors. In addition, an intermetallic compound (IMC)-free metallic interface was fabricated using a magneton sputtering technique to visualize the atomic-scale Cu/Sn interaction behavior. The material interaction behaviors under room temperature electromigration were investigated using a spherical aberration corrected scanning transmission electron microscope (Cs-corrected STEM).
The early stage room temperature electromigration at the IMC-free Cu/Sn interface within 1 h was found to trigger the Cu/Sn interaction and formed an amorphous CuSn interphase embedded with nano-scale Cu crystalline cells. This amorphous interphase was derived from the original interfacial distortion zone across the as-annealed Cu/Sn interface. The meta-stable amorphous CuSn interphase will further transform into a crystalline high-temperature η-Cu6Sn5 IMC as a relatively stable material reaction product under a long-term room temperature electromigration for up to 81 h. Besides the IMC formation, a particular athermal recrystallization behavior was found at the Cu/Sn interface and was proposed to be another stress relaxation mechanism that may occur within the meta-stable amorphous interphase to relief the accumulated high strain energy. The recrystallized Sn interphase is considered as another stable material reaction product formed under room temperature electromigration. The room temperature electromigration was also found to result in the accumulation of Cu atoms in the Sn matrix and thus the formation of a meta-stable Cu-in-Sn solid-solution, revealing the occurrence of non-equilibrium supersaturation behavior. The extent of supersaturation in the metal matrix was controlled by electromigration periods and geometrical (cathode/anode side) parameters. The formation of the meta-stable amorphous CuSn interphase, high-temperature η-Cu6Sn5 IMC, recrystallized Sn interphase, as well as the occurrence of the non-equilibrium supersaturation behavior resulted from the athermal behavior induced by electromigration, rather than the thermal Joule heating effect. The fundamental athermal behaviors induced by electromigration were established in the present study, and their governing mechanisms were also disclosed.
Table of Contents
Table of Contents IV
List of Figures VI
List of Tables XIV
Chapter 1 Introduction 1
1.1 Solder interconnects in microelectronic packaging 1
1.2 Cu/Sn interaction behavior during liquid-state soldering reaction 3
1.2.1 IMC formation and growth 3
1.2.2 Early stage interdiffusion and IMC nucleation 6
1.3 Material interaction behavior induced by electromigration 9
1.3.1 In bulk metals 11
1.3.2 At metallic interfaces 18
1.4 Research motivation and purpose 24
Chapter 2 Experimental procedures 25
2.1 Experimental design 25
2.2 Cu/Sn/Cu interconnect fabrication 27
2.3 Room temperature electromigration experiment 32
2.4 STEM analyses of the Cu/Sn/Cu interconnect 34
Chapter 3 Results and discussion 35
3.1 The formation of amorphous CuSn interphase at an IMC-free Cu/Sn interface during early stage room temperature electromigration: A meta-stable material state 35
3.1.1 The as-annealed Cu/Sn interface: A thermal benchmark 37
3.1.2 The meta-stable amorphous interphase formed during early stage room temperature electromigration 44
3.1.3 The mechanism of the meta-stable amorphous interphase formation 69
3.1.4 Summary 73
3.2 The formation of high-temperature Cu6Sn5 IMC at the Cu/Sn interface during room temperature electromigration: A stable material state 74
3.2.1 The high-temperature IMC formed during room temperature electromigration 74
3.2.2 The mechanism of the high-temperature IMC formation 89
3.2.3 Summary 91
3.3 The formation of recrystallized Sn interphase at the Cu/Sn interface during room temperature electromigration: A stable material state 93
3.3.1 The recrystallized interphase formed during room temperature electromigration 93
3.3.2 The mechanism of the recrystallized interphase formation 101
3.3.3 Summary 104
3.4 The non-equilibrium supersaturation behavior in the Cu/Sn/Cu interconnect induced by room temperature electromigration 105
3.4.1 The Cu concentration in the Sn matrix of the as-annealed Cu/Sn/Cu interconnect: A thermal benchmark 105
3.4.2 The non-equilibrium supersaturation behavior induced by room temperature electromigration 106
3.4.3 The mechanism of the non-equilibrium supersaturation behavior 116
3.4.4 Summary 124
Chapter 4 Conclusions 125
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