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Thermo-compression Bonding

Thermo-compression Bonding. Mohammadreza Soleymaniha. 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

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Thermo-compression Bonding

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  1. Thermo-compression Bonding MohammadrezaSoleymaniha

  2. 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)

  3. 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 vaporcleaning. high diffusion rates - soft metals - good ductile properties

  4. 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.

  5. 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

  6. 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 vacuumor forming gasenvironment, e.g. N2 • Due to difficulty of control of thermal expansiondifferences between the two wafers, precision alignment and high quality fixturesare needed

  7. 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.

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