A Thin Film Metallization Process Development for Silicon Nitride Ceramic Substrates in Power Electronics Packaging

In the power electronics packaging, the substrate is not only responsible for the electrical connection and mechanical support but also significant to the thermal management and reliability. Recently, silicon nitride (Si₃N₄) ceramic has been receiving much attention, for having excellent mechanical properties, such as high bending strength and high fracture toughness. It can significantly improve the mechanical reliability of the substrate for power electronics, under the circumstances of power cycling and thermal impact. This opens up a new research direction of high-performance power electronics substrate for silicon carbide (SiC) devices. However, the surface metallization technique of Si₃N₄ ceramic substrates is not as mature as its counterpart, such as alumina (Al₂O₃) or aluminum nitride (AlN) ceramics, in direct-bond copper (DBC) and direct-bond alumina (DBA) technologies. Therefore, how to implement metallization on Si₃N₄ ceramic surfaces and improve the interfacial strength of the metallization layers has become interesting topics for many researchers.In this paper, we proposed a novel thin-film process for Si₃N₄ ceramic substrate metallization, which deposited titanium as a transition layer and silver as a conductive layer by magnetron sputtering technique. Titanium was selected due to its high chemical reactivity and good diffusion performance. The titanium reaction with Si₃N₄ substrate is beneficial to the increase of bonding strength between metal film layers and ceramic substrate. With low electrical resistivity and high temperature stabilities, silver is preferably selected as the conductive layer, which can be further used as a seeding layer in the electroplating process or a high-temperature bonding layer in transient liquid phase (TLP) technology. Therefore, the current work is inherently compatible with direct plated copper (DPC), silver nano-sintering and TLP technologies. In addition, various materials characterization methods have been utilized to evaluate the quality of the Si₃N₄ ceramic substrate metallization. The surface morphology and resistivity of the films were characterized by atomic force microscopy (AFM) and dual electrometry four-probe method, respectively. The interfacial strength of the thin films to Si₃N₄ ceramics was analyzed using the nano-scratch test method. By studying the effects of the variations of magnetron sputtering parameters, this work provides useful guidance for the process development of Si₃N₄ ceramic substrate metallization in the field of power electronics packaging.