Pre-conditioning 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 nanometres 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.
Deposition The metal films can be deposited by
evaporation,
sputtering or
electroplating. Evaporation and sputtering, producing high quality films with limited impurities, are slow and hence used for micrometre and sub-micrometre layer thicknesses. The electroplating is commonly used for thicker films and 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, i.e.
oxide or
nitride. Also, an additional nano crystalline metal film, e.g. Ta, Cr, W, or Ti, can enhance the adhesion strength of the diffusion bond at decreased applied pressure and bonding temperature.
Bonding The factors of the chosen temperature and applied pressure depend on the diffusion rate. The diffusion occurs between the crystal lattices by lattice vibration. Atoms can not leap over free space, i.e. contamination or vacancies. Beside the most rapid diffusion process (surface diffusion), the grain boundary and the bulk diffusion exist.
Surface diffusion, also referred to as atomic diffusion, describes the process along the surface interface, when atoms move from surface to surface to free energy. The
grain boundary diffusion terms the free
migration of atoms in free atomic lattice spaces. This is based on polycrystalline layers and its boundaries of incomplete matching of the atomic lattice and grains. The
diffusion through bulk crystal is the exchange of atoms or vacancies within the lattice that enables the mixing. The bulk diffusion starts at 30 to 50% of the materials melting point increasing exponentially with the temperature. 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. Further, the applied force and its uniformity is important and depends on the
wafer diameter and the metal
density features. The high degree of force uniformity diminish the total force needed and alleviate the stress gradients and sensitivity to
fragility. The bonding temperature can be lowered using a higher applied pressure and vice versa, considering that high pressure increases the chances of damage to the structural material or the films. The bonding process itself takes place in a
vacuum or
forming gas environment, e.g. N2. The pressure atmosphere supports the
heat conduction and prevents thermal gradients vertically across the wafer and re-oxidation. Based on the difficult control of
thermal expansion differences between the two wafers, precision alignment and high quality
fixtures are used. The bonding settings for the most established metals are following (for 200 mm wafers): ; Aluminium (Al): bonding temperature can be from 400 to 450 °C with an applied force above 70 kN for 20 to 45 min ; Gold (Au): bonding temperature is between 260 and 450 °C with an applied force above 40 kN for 20 to 45 min ; Copper (Cu):bonding temperature lies around 380 to 450 °C with an applied force between 20 and 80 kN for 20 to 60 min == Examples ==