功率模組晶背金屬層及銀奈米孿晶鍍膜開發與優化

Abstract

功率半導體產業因應高功率應用及高可靠度需求，發展出特殊元件結構及製程，並隨著第三類寬能隙半導體之採用，進一步提升可靠度的門檻與製程的挑戰，晶圓背面金屬化製程(Backside Metallization)即為因應此趨勢發展而來，作為中段製程，晶圓背面金屬層品質優劣牽一髮而動全身，是影響功率半導體元件及模組可靠度的關鍵因素。 本博士論文與樂鑫材料科技公司(Ag materials technology co., Ltd)合作，以傳統Ti/Ni/Ag為基礎，開發新型晶背金屬層，並透過材料科學製程－微結構－性質間關係，優化製程與提高可靠度。藉由晶圓前處理、鍍膜環境、製程溫度、材料預融時間和電子束掃描頻率等控制及優化，製備出具良好結構、電性、附著性與熱穩定性的晶背金屬層，研究同時指出Ti/Ni/Sn/Ag與400 ℃高溫下之Ti/Ni/Cu/Ag會出現潛在風險，由此篩選出具產業應用性晶背金屬層，並建立最佳製備方法。 功率模組固晶製程(Die Attach)接續前道晶圓背面金屬化製程，正面臨接合材料轉型的挑戰。因此本博士論文亦研究具高度擴散速率之(111)奈米孿晶銀薄膜(Ag nanotwinned thin films)，開發製備方法、優化結構並探討孿晶生成機制，最終應用於功率模組固晶直接接合(Direct Bonding)。 利用習知濺鍍法製備銀奈米孿晶，鈦層除提升銀膜附著性，Ti (002)亦有助Ag (111)沉積並降低過渡層(transition layer)厚度；負基板偏壓的施加，大幅提升銀奈米孿晶膜(111)擇優取向、降低過渡層厚度且促進奈米孿晶生成，使其更具應用性。搭配離子輔助沉積系統，本論文進一步使用蒸鍍方法，製備出過去以濺鍍或電鍍法才可製備出之(111)奈米孿晶結構，並透過離子槍電壓及電流優化結構品質。本論文指出負基板偏壓或離子槍之離子轟擊引入足夠應力，得以使孿晶大量成核，於銀薄膜沉積同時生成高密度奈米孿晶結構。 本博士論文最後展示銀(111)奈米孿晶膜之應用潛力，除了可製備於各種基板上、可量產於8吋矽晶圓，亦可透過施加適當負基板偏壓，使銀奈米孿晶層厚度減至700 nm，仍保有96% (111)比例之優異品質。進一步應用至晶背金屬層以及固晶直接接合製程，本論文指出影響接合品質的因素有：直接接合銅基板(direct bonded copper, DBC)表面粗糙度、表面材料、表面曲率及晶背金屬層附著性等。於DBC基板表面銅粗糙度0.09 µm時，可於大氣中260 ℃, 20 MPa, 10 min接合條件下達42.2 MPa接合強度，與傳統焊錫或金屬粒子燒結技術相比具有應用優勢。

Due to the increasing demand for high-power applications and high reliability, the power semiconductor industry has developed novel transistor structures and processes. Moreover, the utilization of wide-bandgap semiconductors has enhanced the threshold for reliability and presented challenges for manufacturing processes. In response to the trend, wafer backside metallization was developed. As a mid-stage process, the quality of the backside metal is the critical factor affecting the reliability of power devices and modules. This research cooperates with Ag materials technology company (Amtc) to develop new types of backside metal films based on traditional Ti/Ni/Ag, and through the processing–structure–properties relationship in materials science, it optimizes manufacturing processes and increases reliability. By pretreating the wafer backside surface, controlling the environment and optimizing the deposition temperature, source pre-melting time and electron beam scanning frequency, backside metal films with good structures, electrical properties, adhesion and thermal stabilities can be produced. On the other hand, reliability issues appeared in Ti/Ni/Sn/Ag and Ti/Ni/Cu/Ag at under 400 ℃. Therefore, backside metal films with good industrial applicability and the best process to produce backside metal films were established. The die attach process of power modules, following the previous backside metallization process, faces the transformation challenges of die attach materials. Therefore, this research also studies Ag (111) nanotwinned films, which possess high diffusivity, and it also develops production methods, optimizes the structures, discusses the twinning mechanism, and finally applies the films to the direct die attach process. The use of the conventional sputtering method to produce Ag nanotwinned films improves the adhesion of the Ti layers, and Ti (002) also enhances the deposition of Ag (111) and reduces the thickness of the transition layers. Furthermore, the application of negative substrate bias significantly improves the deposition of Ag with (111) preferred orientation, reduces the thickness of the transition layers, and promotes the formation of nanotwins, making the films more applicable. With the ion beam assisted deposition system, this research further utilized evaporation methods to produce Ag (111) nanotwinned films, which could be produced only by sputtering or electroplating methods in the past, and optimized the structures through the ion gun voltage and current parameters. This research concludes that by either negative substrate bias or assisted ion beam, ion bombardment induced sufficient stress to the Ag films and generated twin nucleation, therefore forming high-density nanotwinned structures in the deposited films. Finally, this research demonstrates the application potential of Ag (111) nanotwinned films. In addition to being capable of being fabricated on various substrates and mass-produced on 8-inch Si wafers, Ag nanotwinned films can also be deposited with excellent quality of 96% (111) and the film thickness reduced to 700 nm by the application of appropriate negative substrate bias. Furthermore, Ag (111) nanotwinned films have been applied to backside metal films and the direct die attach process. This research points out that the factors affecting the bonding quality include the following: DBC substrate surface roughness, surface materials, surface curvature and adhesion of the backside metal films. With a DBC copper surface roughness of 0.09 µm, a bonding strength of 42.2 MPa can be reached at under 260 ℃, 20 MPa and 10 min. This result demonstrates that the application of Ag (111) nanotwinned films is advantageous compared to the traditional solder or sintering of metal particles methods of die attach.