Thermocompression bonding

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: 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: This method enables internal structure protecting device packages and direct electrical interconnect structures without additional steps beside the surface mounting process. 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. Oxidation and impurities in the metal films affect the diffusion reactions by reducing the diffusion rates. Therefore, clean deposition practices and bonding with oxide removal and re-oxidation prevention steps are applied. The oxide layer removal can be realized by various oxide etch chemistry methods. Dry etching processes, i.e. formic acid vapor cleaning, are preferred based on the minimization of the immersion in fluids and the resulting etching of the passivation or the adhesion layer. Using the CMP process, which is especially for Cu and Al required, creates a planarized surface with micro roughness around several nanometer and enables the achievement of void-free diffusion bonds. Further, a surface treatment for organic removal, e.g. UV-ozone exposure, is possible. Methods, i.e. plasma surface pretreatment, provide an accelerated diffusion rate based on an increased surface contact. Also the use of an ultra planarization step is considered to improve the bonding due to a reduction of material transport required for the diffusion. This improvement is based on a defined height Cu, Au and Sn.