以碳化硅（SiC）为代表的宽禁带功率器件可在高温、高频、高压等极端环境下工作，实现功率芯片与基板连接接头高温可靠性对于SiC器件应用至关重要，也是当前研究的重点。烧结纳米银具有良好的导热导电性能，是目前最有希望实现芯片低温连接高温服役的互连材料，然而纳米银烧结连接的SiC芯片接头的高温可靠性，尤其是300°C以上的可靠性，尚未有系统研究。本文基于SiC功率芯片，配制了具有梯度分解特性的混合粒径纳米银焊膏，实现了SiC芯片和直接覆铜氧化铝陶瓷（DBC）基板低温烧结连接。采用高温存储试验评估了SiC烧结接头在350°C下的高温可靠性，建立了“芯片/连接层/基板”界面扩散模型并揭示了接头的失效机理，提出并论证了高温存储中连接界面和烧结组织稳定的解决方案。 配制了具有梯度分解特性的混合粒径纳米银焊膏。焊膏具有良好的导电导热性。焊膏的梯度分解特性与低压力辅助烧结工艺相匹配，具有较宽的工艺窗口，实现了SiC芯片和DBC基板低温烧结连接。在烧结温度300°C、烧结时间15min和辅助压力3MPa的工艺参数下，连接接头的平均剪切强度达到26MPa。接头连接层为多孔结构，孔洞呈不规则岛状不均匀分布，平均孔隙率为19%。连接接头薄弱区位于靠近DBC侧的连接层内。 研究了SiC烧结连接接头在350°C下的高温可靠性及其失效机理。高温存储试验中，随保温时间的延长，接头剪切强度先增大后减小；烧结连接层内依次发生组织致密化、孔洞长大和孔洞偏聚；失效位置从连接层内部向连接界面演化。DBC基板Ni层氧化是高保温接头界面失效的根本原因，且连接接头高温存储中Ni层氧化难以避免。现有的化学镀镍浸金（EING）表面因不能阻碍氧扩散无法满足纳米焊膏烧结接头的高温服役要求。 提出并论证了高温存储中连接界面和烧结组织稳定的解决方案。基于接头高温失效机理，设计镀Ag层作为氧扩散阻挡层延缓Ni层氧化，结果表明DBC侧镀Ag层有效地阻碍了氧的扩散，19μm厚的镀Ag层可使接头在350°C高温存储中可靠性提高至1200h以上，且接头可靠性随着镀Ag层厚度增加而提高。烧结连接层内残留的有机物降低了表面能并阻碍了高温存储中烧结颈的长大，起到稳定组织的作用；在真空环境下高温存储，因更多有机物的参与使得连接层组织始终处于致密化状态，有效地保持了烧结组织的高温稳定性。
SiC-based power devices can operate under extreme conditions such as high temperature, frequency and voltage.The thermal stability of die attachment is very critical for the SiC devices at high temperature applications and has become an important issue. Silver sintering technique is one of the most promising approaches for low-temperature interconnection and high-temperature application due to the excellent thermal and electrical properties of bondline. However, the high-temperature reliability of SiC sintered joint, espectially above 300 °C, has not been systematically studied yet. In this study, mixed-size Ag paste with gradient decomposition characteristics have been prepared and bonded the SiC chip and DBC substrate. The reliability of die attachemt was evaluated after high temperature storage (HTS) at 350 °C, and the failure mechanism during HTS have been also discussed. Based on the failure mechanism, the solution to improve the interface reliability and microstructure stability of die attachment have been demonstrated. A mixed-size Ag paste with gradient decomposition was prepared. The paste has excellent electrical conductivity and thermal conductivity. The characteristic of gradient decomposition was suitable for pressure-assisted sintering, and the sintering process window was wide. The SiC chip and DBC substrate could be boned at 300 °C and pressure 5 MPa for 15 min, and the average shear strength of sintered joints was 26 MPa. The bondline was the porous sintered structure, and the pore distribution was nonuniform and the average porosity was about 19 % in the bondline. The weak region of die attachment was in the bondline near to the DBC side. The high-temperature reliability and failure mechanism of SiC die attachment during HTS at 350°C were studied. The bondline experienced the sintered microstructure densification, pore growth and pore segregation with the increasing of the storage time. The failure position of die attachment transtered from bondline to bonding interface. The oxidation of nickel layer on DBC substrate resulted in the interface failure, and the nickel oxidation during HTS was inevitable because the EING structure does not support the high temperature application for the sintered die attachment. In order to improve the reliability and meet the requirements of high temperature application, the electroplated Ag layer was chosen as the diffusion barrier in this study. The results showed that the electroplated Ag layer on the DBC side could effectively hindered the diffusion of oxygen, and the die attachment on the 19 μm electroplated Ag substrate could withstand more than 1200 h 350 °C in air. The reliability of die attachment improved with the increasing of the thickness of electroplated Ag. The residual organics can stabilize the sintered microstructure during HTS at 350°C because the residual organics decreased their surface free energy and prevented the neck growth. When the die attachments were stored in vacuum, more residual organics prevented the neck growth and maintained the thermal stability of bondline.
