碳化矽晶背Cr/Ni/Ag與Cr/Ag奈米孿晶薄膜分析 及其與DBC陶瓷基板固晶接合

Abstract

半導體技術在二十世紀中葉開始萌芽後，隨著人工智能(AI)、5G 通訊、電動車及物聯網等發展，對功率半導體提出更嚴苛的要求，不但要求輕薄短小外，也要求要有高功率、良導熱及高可靠性等特性；現今以寬能隙的第三代半導體為主，其中以碳化矽最為矚目，因其高強度的機械性質、寬能隙可承受高功率、高電壓的工作環境及良好的光學性質等，使得被廣泛的應用在先進的半導體中，因此本研究將以碳化矽做為基板做為研究，並為了滿足其高工作溫度的環境，後續選擇銀燒結接合，實現低溫接合高溫應用。 另外，可以透過晶背研磨晶背金屬化(Backside Grinding Backside Metallization, BGBM)的技術來改善基板的電性及機械性質，其中奈米孿晶(nanotwin)擁有非常好的熱穩定性及機械性質，並且在強化的同時亦不失去延展性，因此本研究在濺鍍晶背金屬化層時，透過在基板施加150V的偏壓，能夠得到最優的奈米孿晶銀(nanotwin Ag, nt-Ag)結構。由於施加偏壓的關係，原子將呈現最密堆積，加上金屬銀是面心立方(FCC)結構，因此表面的平面族{111}多寡則被視為奈米孿晶結構的指標，本研究透過聚焦離子束顯微鏡(FIB)，直接以離子束影像觀察其橫截面微結構；利用X射線繞射分析儀(XRD)及電子背向式散射繞射技術(EBSD)來觀察銀薄膜的晶格取向，在偏壓為150V時的表面(111)比例為95.2%。 由於銀奈米孿晶表面高比例的{111}平面族，使表面原子在擇優取向<111>擴散速率快，進而可增加接合時的速率，同時降低接合溫度，而本研究將BGBM後的碳化矽晶面與DBC基板做銀燒結接合，一組樣品是傳統鍍層(Cr/Ni/Ag)結構；另一組是銀奈米孿晶(Cr/nt-Ag)結構，比較兩者在銀燒結接合時的差別，先是利用掃描式電子顯微鏡(SEM)觀察其接合的橫截面；同時透過推力測試來評估其接合附著力，最終發現在相同接合條件下，銀奈米孿晶的樣品相較於傳統鍍層的樣品，皆有較好的接合強度；在納美仕銀膏實驗中，輔助壓力為10MPa、燒結溫度150°C、真空接合10min下與DBC基板銀燒結接合後的最大推力強度為33.1MPa，遠大於傳統鍍層樣品在同樣參數下的11.9MPa。昇貿銀膏中，其FIB及EBSD的分析結果進一步證實了銀奈米孿晶表面的高擴散性。綜合以上研究，銀奈米孿晶結構的晶背金屬化層，在銀燒接合上較傳統鍍層更有優勢。

The emergence of semiconductor technology in the mid-20th century has encountered escalating demands on power semiconductors due to advancements in artificial intelligence (AI), 5G communication, electric vehicles, and the Internet of Things (IoT). These demands surpass conventional compact size requirements, now encompassing high power, efficient heat conduction, and high reliability. Presently, wide bandgap third-generation semiconductors, especially silicon carbide (SiC), have gained prominence owing to their exceptional mechanical properties, wide bandgap for high-power tolerance, and excellent optical characteristics. This study centers on SiC as a substrate, emphasizing its application in advanced semiconductors. To cater to high operating temperatures, silver sintering bonding is chosen as a low-temperature bonding method. Furthermore, the utilization of Backside Grinding Backside Metallization (BGBM) employing nanotwinned structures enhances the electrical and mechanical properties of the substrate. Nanotwins exhibit outstanding thermal stability and mechanical properties while preserving ductility. When a bias voltage of 150V is applied during the sputtering of the backside metallization layer, superior silver nanotwinned (nt-Ag) structures are observed, characterized by the most densely packed arrangement due to the applied bias voltage, as observed using focused ion beam microscopy (FIB). X-ray diffraction analysis (XRD) and electron backscatter diffraction (EBSD) techniques are utilized to examine the orientation of the thin film silver, revealing a surface (111) proportion of 95.2% under a bias voltage of 150V. The prevalence of {111} planes on the nanotwin silver surface facilitates rapid diffusion of surface atoms in the preferred <111> direction, thereby potentially increasing bonding rates or reducing bonding temperatures. The study compares traditional coatings (Cr/Ni/Ag) with nanotwin silver coatings (Cr/nt-Ag) in silver sintering bonding between SiC crystal faces and direct bond copper (DBC) substrates. Scanning electron microscopy (SEM) is employed to observe bond cross-sections, while push tests evaluate bonding adhesion. Results indicate that, under identical bonding conditions, nanotwin silver samples exhibit superior bonding strength compared to traditional layered samples. In experiments with Sumitomo Electric Industries (SEI) silver paste, the maximum push strength after silver sintering bonding with a DBC substrate at 10MPa auxiliary pressure, 150°C sintering temperature, and 10 minutes of vacuum bonding is 33.1MPa, significantly higher than the 11.9MPa achieved by traditional coatings under the same parameters. FIB and EBSD analyses further corroborate the high diffusivity of nanotwin silver surfaces. In conclusion, the backside metallization layer with nanotwin silver structures demonstrates advantages in silver sintering bonding over traditional coatings.