矽晶片成長奈米孿晶銀薄膜及其低溫固晶接合應用

Abstract

本研究旨在探討於矽晶片上成長奈米孿晶銀薄膜之低溫固晶接合技術及其封裝應用，以提升元件之可靠度與性能。隨著電動車、再生能源轉換系統與高頻通訊等領域對高功率與高頻操作之需求增加，封裝材料與製程需朝向高可靠度與低製程溫度發展。目前業界多採銲錫或銀燒結作為固晶接合方式，然而傳統銲錫之工作溫度較低，限制其於高溫環境下之可靠度，而銀燒結製程雖具備優異的導熱與導電性，但仍可能因孔洞率高導致界面不穩定，進而影響元件的長期電性表現。此外，固晶過程若採用過高的溫度，易造成晶片功能劣化，若於晶片與基板間引入中間層，亦可能增加熱應力與界面脆弱區，提升失效風險。因此，本研究聚焦於低溫直接接合技術，以兼顧低溫製程需求與高溫操作環境下之熱穩定性。 本研究採用濺鍍與電子束蒸鍍兩種乾式製程沉積奈米孿晶銀薄膜。於濺鍍製程中，透過調整濺鍍功率、基板偏壓以及薄膜厚度等參數，可製備具有良好附著性之銀薄膜，並在適當條件下形成柱狀結構奈米孿晶銀薄膜。於蒸鍍製程中，藉由離子束輔助可提升能量輸入，進而形成高度<111>擇優取向的銀奈米孿晶薄膜。由於面心立方 (FCC) 金屬之(111)面具有快速擴散的特性，且奈米孿晶結構同時具備高機械強度及良好的熱穩定性，使其成為低溫固晶接合之潛力材料。 實驗結果顯示，濺鍍製程所製備之銀奈米孿晶銀薄膜能有效提升接合品質，並在不同DBC基板與接合參數下，於低溫 (<250 °C) 接合後可使接合界面孔洞顯著降低或消除，且平均剪切強度最高可達到38 MPa。當銀薄膜具有較高比例之<111>取向奈米孿晶時，可提升原子擴散速率並加速孔洞消除，使界面緻密化。此外，接合前之電漿清潔可有效提升表面潔淨度與初始接觸活化，有助於在較低溫條件下達成更完整之接合。 在電性驗證方面，透過晶圓針測比較接合前後晶片之表現，結果顯示其閾值電壓 (Vth)、汲極–源極崩潰電壓 (BVDSS) 以及汲極漏電流 (IDSS) 之平均值皆維持於產品規格範圍內，顯示低溫固晶接合製程未造成晶片電性造成劣化，亦確保了元件的可靠性。綜合而言，本研究證實銀奈米孿晶薄膜可成功應用於低溫直接固晶接合，展現其於功率模組封裝之應用潛力，並為未來低溫、高可靠度封裝材料與製程提供可行方案。

This study investigates low-temperature die bonding utilizing nanotwinned Ag thin films grown on Si chips, and evaluates its potential for enhancing device reliability and performance. With the continuous rise in demand for high-power and high-frequency electronic devices, particularly in electric vehicles, renewable energy conversion systems, and high-frequency communication, innovations in packaging materials and processes and are increasingly important. Conventional die-attach approaches commonly rely on soldering or Ag sintering. However, the relatively low service temperature of solder limits the reliability under high-temperature operation, whereas Ag sintering, despite its excellent thermal and electrical conductivity, may exhibit interfacial porosity that compromises interfacial integrity and long-term electrical stability. In addition, excessive bonding temperatures can degrade device functionality, and introducing intermediate layers between the chip and the substrate may increase thermomechanical stress and create interfacial weak regions, thereby elevating the risk of failure. To address these issues, this study focuses on low-temperature direct bonding while aiming to maintain the thermal stability of the bonded structure for reliable operation under harsh conditions. Nanotwinned Ag thin films were deposited via two dry processes: magnetron sputtering and electron-beam evaporation. In the sputtering process, the effects of sputtering power, substrate bias, and film thickness were systematically investigated to obtain Ag films with strong adhesion and columnar nanotwinned microstructures. In the evaporation process, ion-beam assistance was introduced to facilitate the formation of highly <111>-oriented nanotwinned Ag films. Leveraging the fast diffusion characteristics associated with the FCC (111) plane, together with the high strength and thermal stability of the nanotwinned structure, these films were employed as candidate metallizations for low-temperature direct bonding. The experimental results demonstrate that nanotwinned Ag films significantly improved bonding quality. Under various DBC substrates and bonding conditions, low-temperature bonding (<250 °C) resulted in interfaces with drastically reduced or nearly eliminated voids, achieving an average shear strength of 38 MPa. In particular, Ag films with a high fraction of <111>-oriented nanotwins exhibited enhanced atomic diffusion, which promoted void closure and resulted in a denser bonding interface. Moreover, plasma cleaning prior to bonding improved surface activation during the early bonding stage, enabling high-quality bonding at reduced temperatures. Electrical characterization using chip probe (CP) tests confirmed that the threshold voltage (Vth), drain–source breakdown voltage (BVDSS), and drain leakage current (IDSS) remained within product specifications before and after bonding. These results indicate that the low-temperature die bonding process does not deteriorate the device electrical performance and supports reliable operation. Overall, this study demonstrates that nanotwinned Ag thin films are a promising route for low-temperature direct die bonding in power module packaging and provide a feasible pathway for future materials and process development.