近年來因新能源科技如電動車、再生能源與智慧城市電網的快速發展，功率元件的需求量快速增加。然而目前對於高溫應用的電子構裝材料相當稀少。利用奈米銅膠燒結接合的銅對銅直接接合構裝技術為相當具有潛力的電子構裝技術，其歸因於銅優良的導熱、導電與抗電遷移能力及適當的價格。然而奈米銅膠燒結接合製程中仍有許多可改善之處。為了合成出奈米尺寸的銅粒子，合成過程中經常使用具有毒性的還原劑。為了增加奈米銅粒子的燒結程度以及加強銅對銅的接合強度，接合過程中時常使用較高的溫度與外加壓力，易產生熱應力及損害電子元件。為了防止銅氧化，接合過程需通入具還原能力的氣體，此舉增加製程危險性與設備成本。 有鑑於上述製程缺點，本研究開發出在大氣環境下，以無毒還原劑且不添加保護劑的條件下合成出奈米銅粒子的化學合成法。奈米銅粒子的粒徑大小與粒徑分佈可透過調整還原溶液的酸鹼值控制。不同還原溶液酸鹼值合成出的奈米銅粒子與聚乙二醇溶劑混合形成奈米銅膠，並在300度一小時，氮氣環境及無外加壓力的條件下實現銅對銅低溫無壓直接接合。實驗結果顯示燒結前擁有較高隨機堆積密度的奈米銅粒子燒結後會有較緻密的微結構、較高的機械強度以及較低的孔洞率。燒結接合過程中聚乙二醇與奈米銅粒子表面氧化層的還原機制也由奈米銅膠熱分析結果證實。奈米銅粒子表面氧化層可作為觸媒使聚乙二醇在低溫下裂解，而裂解產物接著將氧化層還原。還原銅粒子表面氧化層後，殘留的聚乙二醇可防止奈米銅粒子氧化直到到達接合溫度。使用較低分子量的聚乙二醇作為溶劑可降低接合溫度，接合溫度最低可降至聚乙二醇的裂解溫度。而透過奈米銅粒子表面氧化層的催化，不同分子量的聚乙二醇皆在220度下裂解。此外，在低溫接合條件下，透過混合一微米的銅粒子與奈米銅膠，接點之接合強度可進一步提升。 ;The demand for power devices increases due to the development of new technologies, such as electric cars, renewable energy, or smart electric grids. However, the packaging materials for high-temperature applications are rare. Cu-to-Cu direct bonding by Cu nanoparticle (NP) paste is a promising choice given its superior thermal and electrical properties, high electromigration resistance, and appropriate cost. However, this method still needs improvement. High-toxicity chemicals are usually used as reducing agents to synthesize nanosized Cu particles, which hinders the wide application of Cu NP paste. High temperature and pressure in the bonding process can cause thermal stress and damage to the device. A reducing atmosphere is usually applied to prevent oxidation during bonding, but it increases the risk of processing and equipment cost. In this study, a green chemical reduction method was proposed to prepare Cu NPs without a capping agent in ambient atmosphere. The particle size and size distribution of Cu NPs can be easily controlled by the pH value of the reducing solution. The synthesized Cu NPs with different sizes were mixed with polyethylene glycol (PEG) as the solvent to form Cu NP paste, and the paste was bonded with the Cu substrate and pillar at 300°C for 1 h under nitrogen atmosphere. The results showed that pre-sintered Cu NPs with high random close packing density resulted in dense sintered microstructure and low void ratio after bonding. The reduction mechanism of PEG and Cu oxide was proposed, and it was verified by thermal analyses. The oxide layer on the surface of Cu NP catalyzed the decomposition of PEG; then, the decomposed products reduced Cu oxide. After reduction, the remaining PEG protected Cu from oxidation until the bonding temperature was reached. Sintering temperature can be further reduced by mixing PEG of lower molecular weight. The lowest bonding temperature can reach 220 °C as the decomposition temperature of PEG. The shear strength of the joints can be further increased by mixing 1 ?m Cu particles into Cu NP paste when bonded at low temperatures.
