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Packaging and Integration Technologies
Published in Shiban Kishen Koul, Sukomal Dey, Radio Frequency Micromachined Switches, Switching Networks, and Phase Shifters, 2019
Shiban Kishen Koul, Sukomal Dey
Readers may note that many well-established technologies are also available and widely implemented for die bonding for MEMS packaging, including solder bonding, gold–gold thermocompression bonding, low temperature glass frit or seals [17,18]. In addition, anodic bonding is one of the reliable and strong bonds for high-vacuum packaging (~10−4 mbar) [18,19]. Fusion bonding or silicon direct bonding (SDB) is used to bond two silicon wafers with chemical force at high temperature (~1000°C) [18,19]. A diffusion-based method is used for joining two materials to form eutectic bonding. A few examples of this bonding are Si and Au, Si and Al, Si and Pd, and Sn and Au, and all these bonds are formed at the eutectic point that is the lowest fusion temperature of the alloy. One similar category of eutectic bonding is solder bonding. In this type, an alloy is formed after bonding at the melting point of the solder material. A few examples are 157°C for indium (In), 232°C for Sn and 280°C for eutectic Au80%–Sn20%. A few other types of bonding are polymer bonding, glass-frit bonding and metal-to-metal bonding. Polymer bonding recently gained more attention in the MEMS community for its ease of implementation where it can act as a glue and bond many wafers together in spite of having large deviations in the thermal coefficient of expansion. Glass-frit bonding is another type of polymer bonding and it is formed by inorganic spin on glass layer. Metal–metal bonding needs extremely high bonding force to make proper contact between two wafers through plastic deformation. For example, nearly 200 N force is required to contact between gold–gold followed by 360°C for 30 min [33].
Progress in wafer bonding technology towards MEMS, high-power electronics, optoelectronics, and optofluidics
Published in International Journal of Optomechatronics, 2020
Jikai Xu, Yu Du, Yanhong Tian, Chenxi Wang
Although the alloy solder can join different materials with ultrahigh bonding strength, they usually need high heating temperatures.[30–36] On the contrary, the transient liquid phase (TLP) technology which utilizes the principle of eutectic bonding can be operated at low temperatures. Metal layers that can satisfy the eutectic conditions are usually used for the realization of eutectic bonding. Based on the reaction principle of phase equilibrium, at least two kinds of metals are needed. One has a low melting point (e.g., In and Sn), while the other one possesses a high melting point (e.g., Au, Ag, and Cu). The metal layers can be formed by thermal evaporation, sputtering, and electroplating. The bonding temperature is more than 20 °C than the melting temperature of the solder. During the reaction process, the solder transforms to be liquid, and then the solid-liquid interdiffusion begins to conduct. The entire eutectic bonding process is considered to be a complex process combined with a chemical reaction and physical interdiffusion. Due to the joint has excellent properties in the sealing and electrical interconnection, eutectic bonding is popular in MEMS packaging. Because of the excellent wetting behavior and good thermal conductivity, the Au-Sn eutectic system has been widely applied in optoelectronics and radio-frequency device packaging.[37] Among the four stable intermetallic compounds (i.e., Au5Sn, AuSn, AuSn2, and AuSn4), Au5Sn has a melting point of 280 °C and is suitable for temperature-sensitive devices. Au/Cu metalized substrates have been used for the studies of Au-Sn eutectic bonding, as shown in Figure 2(a). Thin buffer layers are deposited to prevent the solder consumption. A strong interface with a stable phase can be generated with sufficient heating time. Besides, the contaminants absorbed on surfaces also influence the joint quality. Compared with surfaces without cleaning, a clean process can lead to a void-free interface, as shown in SEM images from Figure 2(a). Au-In alloys belong to hard solders with a high melting temperature (>450 °C). However, In is a kind of widely used soft solder with a low melting temperature (157 °C). According to the Au-In equilibrium phase diagram, eutectic bonding between Au and In can be realized at low temperatures. To demonstrate the Au-In eutectic bonding, multilayers of Au and In thin films have been deposited on surfaces to be bonded, as shown in Figure 2(b). The X-ray image and interfacial SEM image show that void-free bonding is achieved. Due to Au-In alloys possess a higher melting temperature than the Au-Sn system, this kind of eutectic system can be used at low temperatures and working at high temperatures.