The Microstructure, Thermal Conductivity and Joining Strength of Eutectic Bonded Cu/Al2O3 Laminates

本研究利用銅-氧共晶接合法，又通稱直接覆銅法，接合金屬銅與氧化鋁。藉此方法接合金屬與陶瓷時，氧在接合性質上扮演了關鍵性角色，因而研究陶瓷與不同氧含量之金屬接合之界面。為得含不同氧含量的銅金屬，進行改變溫度、時間以及氣氛搭配成多樣的預氧化處理。因預氧化處理改變了銅的表面氧含量，本研究同時也探討了銅之氧化機制。氧化過程中，於銅表面發現了氧化亞銅(Cu2O)和氧化銅(CuO)，且氧化機制由擴散控制。透過氧化機制之研究，可有效調整預氧化處理參數(溫度、時間和氣氛)而得含不同氧含量之銅金屬。本實驗於氮氣下1075℃ 進行共晶接合，共晶液相於銅和氧化鋁的接觸面形成，潤濕氧化鋁表面，於冷卻後獲得接合良好之金屬-陶瓷界面。利用高解析度穿透式顯微鏡(HRTEM)觀察接合界面之微結構。微結構分析觀察到於銅/氧化鋁和氧化亞銅/氧化鋁界面上有界面反應相CuAlO2，本研究提出兩程式反應機制，並分別使用熱力學與動力學對界面反應進行探討，並探討反應相的熱穩定性，及維持界面上CuAlO2之臨界所需之氧含量。因此接合前之銅預氧化處理可確保界面上有足夠氧含量。對接合界面的性質，氧化亞銅(Cu2O)與銅鋁氧化合物界面相(CuAlO2)有著不同的影響。CuAlO2在界面上的存在，可阻止界面裂痕的成長，有效的提升了界面強度。然而，Cu2O的生成降低了界面強度同時也造成大量的殘留應力，對於得到最佳的銅-氧化鋁接合界面性質來說，適當氧含量的選擇相當重要。本研究利用閃光法以及彎曲強度法來評估接合之性質，本研究製備之雙層銅/氧化鋁試片之熱導係數可達30 Wm-1K-1以上，且擁有23 Jm-2 的界面穩態能量釋放率。因有著高熱導性和高界面強度，本實驗所得之銅-氧化鋁試片可應用散熱基板。The present study applies the eutectic bonding, so called direct bonding, to join copper to alumina. The oxygen plays an important role on the joining of the metal to ceramic, and the ceramic/metal interfaces with various oxygen concentrations are investigated. In order to introduce various oxygen concentrations into copper, several pre-oxidation treatments were adopted. The oxidation behavior of Cu is also investigated in this work. Cu2O and CuO form at the copper surface during oxidation. The oxidation is controlled by diffusion. The eutectic bonding of copper to alumina is achieved at 1075℃ in nitrogen atmosphere. Eutectic melt forms and wets both copper and alumina, therefore intimate bonding is obtained after cooling. HRTEM is used to observe the microstructure at the interface. CuAlO2 is found at the Cu/Al2O3 and Cu2O/Al2O3 interfaces. Two chemical reactions for the formation of CuAlO2 are proposed. Thermodynamics and kinetics are used to elucidate the formation of CuAlO2 at the interface. Since the processing window for the stable CuAlO2 is narrow, to maintain a sufficient oxygen concentration at the Cu/Al2O3 interface is critical. The interfacial phases, Cu2O and CuAlO2, have contradictory influences on the joint properties. The presence of CuAlO2 improves the interfacial strength by crack pinning mechanism whereas Cu2O reduces the interfacial strength and introduces large residual stress. An adequate oxygen content should be used to join Cu to Al2O3 for optimum properties. Flash method and flexure bending test were used to characterize the laminate properties. The bilayer Cu/Al2O3 laminates fabricated in the present study exhibit a thermal conductivity above 30 Wm-1K-1 and a steady-state energy release rate of 23 Jm-2. Since the thermal conductivity and interfacial strength of the Cu-Al2O3 laminates are high, the use of the laminate as thermal dissipation substrate is high.Contents Chapter 1 1 Chapter 2 12 2-1 Basic of direct bonding 12 2-1.1 Thermodynamic assessment 12 2-1.2 Kinetic consideration 16 2-2 Effect of interfacial phases on mechanical properties 18 2-2.1 Cu2O precipitates at interface 19 2-2.2 CuAlO2 at the interface 21 2-2.3 Oxide/glass after oxidation on nitrides 24 2-3 Critical issues 27 2-3.1 Voids at the interface 28 2-3.2 Thermal cycling reliability 31 Chapter 3 33 3-1 Starting materials 33 3-2 Pre-oxidation of copper 34 3-3 Direct Bonding 37 3-4 Bond strength 40 3-5 Thermal conductivity 41 3-6 Reliability 42 3-7 Effect of porosity of alumina on bonding interface 45 Chapter 4 46 4-1 Raw materials 46 4-2 Microstructure observation on the joint 51 4-2.1 Integrity examination 51 4-2.2 Cross-section microstructure 56 4-2.3 Peeled surface examination 62 4-2.4 Interfacial analysis 88 4-3 Thermal diffusivity measurement 94 4-4 Mechanical properties 97 4-5 Temperature cycling test 101 4-6 Infrared imaging 105 4-7 Effect of the porosity of alumina on the interfacial condition 108 Chapter 5 111 5-1 Mechanism of void formation 111 5-2 Effect of interfacial phases on thermal properties 118 5-3 The influence of Ni on direct bonded copper/alumina interface 123 5-4 Thermal cycling reliability 130 Chapter 6 132 References 13