Thermocompression bonding

[3] The most established materials for thermocompression bonding are copper (Cu), gold (Au) and aluminium (Al)[1] because of their high diffusion rates.

The diffusion of these metals requires good knowledge of the CTE differences between the two wafers to prevent resulting stress.

Therefore, clean deposition practices and bonding with oxide removal and re-oxidation prevention steps are applied.

[5] Using the CMP process, which is especially for Cu and Al required, creates a planarized surface with micro roughness around several nanometres and enables the achievement of void-free diffusion bonds.

[2] 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.

Evaporation and sputtering, producing high quality films with limited impurities, are slow and hence used for micrometre and sub-micrometre layer thicknesses.

[6] To enable the diffusion process, a high force is applied to plastically deform the surface asperities in the film, i.e. reducing bow and warp of the metal.

[10] The pressure atmosphere supports the heat conduction and prevents thermal gradients vertically across the wafer and re-oxidation.

[2] Based on the difficult control of thermal expansion differences between the two wafers, precision alignment and high quality fixtures are used.

Thermocompression bonding is well established in the CMOS industry and realizes vertical integrated devices and production of wafer level packages with smaller form factors.

The use of the compliant medium ensures the physical integrity of the lead by controlling the extent of wire deformation.