銀銅合金奈米孿晶薄膜特性與銀燒結固晶及光電化學產氫應用研究

Abstract

隨著高效能電子、電動化與能源系統等應用推動功率半導體元件的發展，功率模組逐漸面臨更高電壓、功率密度與長期高溫操作之可靠度挑戰。傳統錫基銲料因熔點偏低、導熱導電受限及長期高溫下易產生界面孔洞等問題，已難以滿足寬能隙 SiC 功率元件之可靠度需求。奈米銀燒結固晶接合具高熔點、高導電與高強度，被視為取代銲錫的關鍵技術，然而實際應用仍面臨燒結孔洞粗大與界面緻密化不足等瓶頸。奈米孿晶金屬薄膜兼具高強度、良好導電性與高熱穩定性，且其高密度 (111) 晶面提供快速擴散通道，作為晶背金屬層在強化銀燒結接合上具有一定潛力。在奈米孿晶優異性質的基礎上，為了進一步強化機械強度與界面可靠度，調控奈米孿晶密度和擴散等性質，進一步導入合金化奈米孿晶，同時具備奈米孿晶結構與合金可調控性，成為兼具應用潛力與研究價值的材料系統。 本研究以磁控共濺鍍製備Ag–Cu合金奈米孿晶薄膜，系統性調整合金成分、濺鍍功率、基板偏壓與基板種類變因，並結合 FIB、EPMA、XRD、EBSD 及 TEM 分析其微結構與形成機制，進一步量測薄膜粗糙度、硬度、電阻率與熱穩定性，並與純金屬奈米孿晶薄膜比較。本研究結果顯示微量固溶可有效降低疊差能、增加奈米孿晶結構比例、細化柱狀晶粒與縮短孿晶間距，使 (111) 取向比例與 Σ3 孿晶界密度顯著提升，薄膜硬度可達 3 GPa 以上且表面粗糙度顯著下降至低於9 奈米；然而過量溶質將造成晶格扭曲與雙相形成，反而抑制奈米孿晶生成並降低熱穩定性，因此成分與偏壓皆存在最佳參數範圍。整體而言，適當條件之銀銅合金奈米孿晶薄膜兼具高密度孿晶、高(111) 取向、低粗糙度與低電阻率，熱穩定性亦優於純金屬奈米孿晶薄膜。 第二部分進一步將高密度奈米孿晶銀銅合金薄膜作為 SiC 晶背金屬層，並採用 Namics 與昇貿銀燒結膏，系統性探討燒結溫度、時間、輔助壓力與預成型銀燒結片等製程參數對銀燒結接合層之孔隙率、界面結構與剪切強度之影響。本研究結果顯示在 200至250 °C 之低溫條件下，即可形成低孔隙率之緻密接合層，平均剪切強度可達約 35至60 MPa，且破斷多沿銀燒結層本體及偏向 DBC 一側發生，晶片端奈米孿晶界面未見剝離。高密度奈米孿晶鍍層相較於一般細小等軸晶鍍層可顯著降低界面孔隙率並提升接合強度，其表面 (111) 快速擴散面可視為促進原子填補孔洞的綠色通道，有效加速燒結頸生成與成長。HTS 與 TCT 可靠度測試後，接合層仍維持低孔隙率、高密度孿晶與穩定大晶粒燒結銀，剪切強度僅輕微下降，顯示本系統具長期熱應力下之機械可靠度。 此外，本研究亦初步將奈米孿晶薄膜應用於光電化學催化，作為延伸性探討，高對稱性結構之Σ3 孿晶界可作為活性缺陷位點，為催化劑與材料選擇提供新的設計方向。

With the rapid development of high-performance electronics, electrification, and energy systems, power semiconductor devices are being driven to operate at higher voltages, higher power densities, and prolonged high-temperature conditions, posing increasingly critical reliability challenges for power modules. Conventional Sn-based solders have become increasingly inadequate for wide-bandgap SiC power devices due to their low melting point, limited thermal and electrical conductivity, and the formation of interfacial voids under long-term high-temperature operation. Nanosilver sintering die-bonding, offering a high melting point, excellent electrical conductivity, and high joint strength, has therefore emerged as a promising replacement for solder. However, its practical implementation is still limited by coarse sintering pores, insufficient strength and interfacial densification. Nanotwinned thin films exhibit a unique combination of high strength, good conductivity, and superior thermal stability, while their high-density (111) planes provide fast diffusion pathways, indicating their potential as backside metallization for enhancing bonding joints. Furthermore, alloying enables additional tunability while preserving the intrinsic advantages of pure silver or copper nanotwins, improving mechanical properties and long-term reliability. In this study, a series of Ag-Cu alloy nanotwinned thin films were deposited by magnetron co-sputtering. Alloy composition, sputtering power, substrate bias, and substrate type were systematically varied, and the resulting microstructures and formation mechanisms were characterized by FIB, EPMA, XRD, EBSD, and TEM. Film surface roughness, hardness, resistivity, and thermal stability were further evaluated and compared with those of pure Ag and pure Cu nanotwinned films. The results show that minor solid solution effectively lowers the stacking fault energy, increases the proportion of nanotwinned structures, refines columnar grains, and shortens twin spacing, thereby significantly enhancing the (111) texture fraction and Σ3 twin boundary density. Under optimized conditions, the film hardness exceeds 3 GPa and the surface roughness is markedly reduced to below 9 nm. In contrast, excessive solute leads to lattice distortion and dual-phase formation, suppressing nanotwinned formation and degrading thermal stability, indicating that both composition and bias must be controlled within an optimal range. Overall, properly designed Ag-Cu nanotwinned films combine high twin density, strong (111) texture, low surface roughness, low resistivity, and thermal stability superior to that of pure nanotwinned metals. In the second part, high-density nanotwinned Ag-Cu alloy films were further employed as backside metallization on SiC chips, in conjunction with silver sintering pastes from Namics and Shenmao. The effects of processing parameters, including sintering temperature, sintering time, applied pressure, and the use of preformed silver sintering sheets, on the porosity, interfacial microstructure, and shear strength of the silver-sintered joints were systematically investigated. The results demonstrate that, under low-temperature conditions of 200-250 °C, dense joints with low porosity and average shear strengths of 35-60 MPa can be obtained. Fracture predominantly occurs within the Ag sintered layer and near the DBC side, with no debonding observed at the nanotwinned film or chip interface. Compared with fine equiaxed coatings, high-density nanotwinned films significantly reduce interfacial porosity and enhance joint strength; their (111)-rich surfaces act as a “green channel” for accelerated atomic diffusion, promoting neck formation and growth and effectively eliminating voids. After HTS and TCT reliability tests, the joints still retain low porosity, high-density nanotwinned structures, and stable large sintered Ag grains, with only minor reductions in shear strength, confirming robust mechanical reliability under long-term thermal stress. In addition, this study preliminarily explores the application of nanotwinned thin films in photoelectrochemical catalysis as an extension of this work. The high-symmetry Σ3 twin boundaries are suggested to act as potential catalytically active defect sites, providing a new design perspective for catalyst and material selection.