Principles Thermo-compression bonding can be used for wire bonding or Flip Chip bonding and wafer bonding as well. Other names: – diffusion bonding, pressure joining, thermo-compression welding or solid- state welding Two metals, e.g. gold (Au)-gold (Au), are brought into atomic contact applying force and heat simultaneously(above 400 c)
Used Material Used material: – gold (Au) – copper (Cu) – aluminum (Al) Al: – needs extensive deposition, – requires a high applied force to crack the surface oxide. gold : – Lower Temperature(around 300 °C) – does not form an oxide. – skip a surface cleaning procedure before bonding copper : – disadvantage that the damascene process is very extensive.(patterning technique) – forms immediately a surface oxide that can be removed by formic acid vapor cleaning. high diffusion rates - soft metals - good ductile properties
TC steps(Pre-conditioning) Oxidation and impurities in the metal films affect the diffusion reactions by reducing the diffusion rates. Methods: oxide etch chemistry methods Dry etching processes (formic acid vapor cleaning) Chemical Mechanical Polishing/Planarization(CMP) is a process of smoothing surfaces with the combination of chemical and mechanical forces.( For Cu and Al) surface treatment for organic removal (UV-ozone exposure) Plasma surface pretreatment, provide an accelerated diffusion rate based on an increased surface contact.
TC steps(Deposition of metals) Methods : – Evaporation – Sputtering – Electroplating Evaporation and sputtering: – producing high quality films with limited impurities, – slow (used for micrometre and sub-micrometre layer thicknesses) Electroplating : – commonly used for thicker films – needs careful monitoring and control of the film roughness and the layer purity The gold film can also be deposited on a diffusion barrier film like oxide or nitride
TC steps(Bonding) Diffusion rate defines the temperature and the force The atoms migration based on crystal lattice vibration diffusion process – surface diffusion(the most rapid diffusion process) – grain boundary diffusion – bulk diffusion bonding temperature can be lowered using a higher applied pressure and vice versa The bonding process itself takes place in a vacuum or forming gas environment, e.g. N 2 Due to difficulty of control of thermal expansion differences between the two wafers, precision alignment and high quality fixtures are needed
challenges Apply force and heat to the component simultaneously Provide thermally and mechanically stable component heater. Minimize substrate heater's compliance to absorb bonding force without deformation Provide stable, fine resolution bonding force control to bond even the thinnest and most brittle materials as well as large components. Therefore, the temperature of both heaters needs to be matched. This results in a synchronized wafer expansion.