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Thermocompression bonding describes a wafer bonding technique and is also referred to as diffusion bonding, pressure joining, thermocompression welding or solid-state welding. Two metals, e.g. gold (Au)-gold (Au), are brought into atomic contact applying force and heat simultaneously.〔 The diffusion requires atomic contact between the surfaces due to the atomic motion. The atoms migrate from one crystal lattice to the other one based on crystal lattice vibration.〔 This atomic interaction sticks the interface together.〔 The diffusion process is described by the following three processes: * surface diffusion * grain boundary diffusion * bulk diffusion This method enables internal structure protecting device packages and direct electrical interconnect structures without additional steps beside the surface mounting process.〔 == Overview == The most established materials for thermocompression bonding are copper (Cu), gold (Au) and aluminium (Al)〔 because of their high diffusion rates.〔 In addition, aluminium and copper as relatively soft metals have good ductile properties. The bonding with Al or Cu requires temperatures ≥ 400 °C to ensure sufficient hermetical sealing. Furthermore, ''aluminium'' needs extensive deposition and requires a high applied force to crack the surface oxide, as it is not able to penetrate through the oxide. Using ''gold'' for diffusion, a temperature around 300 °C is needed to achieve a successful bond. Compared to Al or Cu, it does not form an oxide. This allows to skip a surface cleaning procedure before bonding.〔 ''Copper'' has the disadvantage that the damascene process is very extensive.〔 Also it forms immediately a surface oxide that can be removed by formic acid vapor cleaning. The oxide removal conduces also as surface passivation. The metal diffusion requires a good control of the CTE differences between the two wafers to prevent resulting stress.〔 Therefore, the temperature of both heaters needs to be matched and center-to-edge uniform. This results in a synchronized wafer expansion.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Thermocompression bonding」の詳細全文を読む スポンサード リンク
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