[17] S.L. Zhang, T. Liu, C.J. Li, S.W. Yao, C.X. Li, G.J. Yang, and M.L. Liu, Atmospheric plasma-sprayed La0.8Sr0.2Ga0.8Mg0.2O3 electrolyte membranes for intermediate-temperature solid oxide fuel cells, J. Mater. Chem. A, 2015. 3(14): 7535-7553. [18] Y. Li, Z. Sun, C. Sun, H.B. Jin, and Y.J. Zhao, Exploring the origin of ZrO2 phase in Na3Zr2Si2PO12 ceramic electrolyte, Ceram. Int., 2023. 49(2): 3094-3098. 16 <strong>DVS</strong> 393
Cold Atmospheric Plasma Metallization of Power Semiconductor Devices with CuSn Pseudo-Alloys for Diffusion Soldering Manuela Ockel, Aleyna Gökçen, Bettina Ottinger, Matthias Petersen, Christian Voigt, Jörg Franke Cold atmospheric plasma spraying is used to produce thin coatings of copper and tin between 20-80 µm thickness for use in diffusion soldering. This study presents an alternative process to apply composite solders directly onto power electronic bare dies. The formation of intermetallic phases may be promoted by the homogeneous distribution of the Cu and Sn particles as they are presented not in a layered structure but as a pseudo alloy within the coating. The Cu and Sn powder is mixed in situ using two powder conveyors, enabling adjustable mix ratios. The presented approach has been shown to produce a homogeneous particle distribution within the coating. Furthermore, preliminary experiments indicate the feasibility of the technology for applications in diffusion soldering. 1 Introduction Efficient modules are crucial for the conversion of energy in electronic systems. The reliability of power modules directly impacts functionality, cost effectiveness and lifetime. To realize the full potential of Wide-Band- Gap semiconductors, the adjacent assembly and interconnection technologies need to have high-temperature stability and must support efficient cooling. While silver sintering with nano-particles is an established hightemperature die attach technology (> 300 °C), it uses high pressure and expensive materials to establish the connection. For working at high temperatures (> 250 °C) gold-added solder pastes can be used. The interconnecting materials are either provided as a paste or preform, or are deposited on one or both joined surfaces for example by electroplating [1]. Diffusion soldering, Trans-Liquid-Phase Soldering (TLPS) or Solid Liquid Inter-Diffusion technology (SLID) is used to create intermetallic phases (IMP) between a higher (e.g. copper) and lower (e.g tin) melting material where die interconnection has a higher remelting temperature than the material of the joining surfaces or the processing temperatures [2]. Cu atoms dissolve into molten Sn and form Cu6Sn5 and Cu3Sn. Syed-Khaja [2] used the Cold Atmospheric Plasma Metallization (CAPM) to apply Cu particles or Cu layers onto a printed Sn-based solder layer. In this study, we aim to eliminate the solder paste printing by directly spraying lead free “dry-solder” in combination with Cu particles onto the substrate, using CAPM. Thereby evaluating the feasibility of the technology to produce homogeneous copper tin composites by combing two powders in situ. The resulting coating is a pseudo alloy where the two materials are mechanically joined, yet the materials maintain their interfaces. The alloying is only promoted during diffusion soldering. Additionally, the preforms introduced in Ottinger et. Al. [3] – a layered structure of Cu and Sn (see Figure 1, b) – are combined with a direct Cu metallization of the backside of the bare die. The metallization in this paper is similar to the front side metallization summarized in Ockel et. Al [4], where pure Cu coatings between 20-100 µm are applied via cold atmospheric plasma metallization to apply thermo-mechanical bond buffers to enable Cu-based top-side interconnections, e.g. ultrasonic heavy Cu wire bonding. The diffusion soldering process can be divided into four steps, as depicted in Figure 1. Firstly, the interlayer materials have to be set up (1). Secondly, the materials are heated to a specified bonding temperature to liquefy the low-melting layers (2) under mechanical pressure. Subsequently, the assembly is held at temperature until the liquid has isothermally solidified due to interdiffusion (3). Lastly, the bond can be homogenized by thermally treating the substrate several hours (4). The latter is dependent on the technical and economical requirements of the bond. There are several approaches to set up the interlayer materials (Figure 1, a-c)). Usually the materials are layered vertically and exist as a combined preform or solder and metallization combination. The preform can have several layers of Cu and Sn. Horizontal approaches where Cu and Sn pastes were applied side by side are also possible. However, a composite solder – a homogenous distribution of Cu and Sn –is beneficial, since the formation of IMPs is dependent on the distances between the two materials and can be minimized with this approach. The composite production can differ from using solder pastes with Cu and Sn particles, coating Cu particles into a Sn-based paste or the direct deposition of Cu and Sn particles via thermal spraying as introduced in this paper. <strong>DVS</strong> 393 17
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