The electroplating area has 2 components: Anisotropic wet etching, and electrolytic metal growth.
Anisotropic wet etching
The anisotropic chemical etching is to etch Silicon chemically on depths of a few microns to several hundred microns with an alkaline chemical solution. The redox reaction that occurs between silicon and the solution lead the dissolution of the preferential directions. The realization of a prior nitride or Silicon oxide mask can locate this etching and achieve grounds (membrane, cavities, and trenches).
This technique is to turn the metal ions in a solution in metal deposits. The deposit can be full substrate or localized in thicknesses ranging from a few µm to several hundred µm. The silicon substrate is immersed in a solution containing metal ions to be deposited. A current is imposed between the substrate is counter electrode. This current will allow the transformation of the ions in metal atoms. The composition of the bath, the density of the current, and the agitation influences the characteristics of the deposit.
Means of fabrication
Anisotropic wet etching
- KOH: the rate for the <100> plans rangs from 0.5 to 2.5 µm/min for concentrations of 20 to 50% by weight and temperatures of 50 to 90 °. This solution of attack requires a mask in Silicon nitride:
La vitesse de gravure des plans 100 variant de 0.5 à 2.5 µm /min pour des concentrations de 20 à 50 % en poids et des températures de 50 à 90°. Cette solution d'attaque nécessite un masque en nitrure de silicium
- TMAH : the rate for the <100> plans rangs from 0.2 to 1 µm/min for concentrations of 20 to 50% by weight and temperatures of 50 to 90 °. This solution does not etch oxide and therefore allows membranes.
IPA influence on TMAH etching
TMAH 20% - 2 min
TMAH 20% + 17 % IPA - 2 min
Gold :Continuous current full plate or in a resist mould for thicknesses up to 10 µm. Can be deposited on 4 or 6 inches substrates. This deposit is with low mechanical stress< 10 MPa and homogeneous (inhomogeneity < 5%)
- Copper : Continuous or pulsed current, full plate or in a resist mould for depths up to 100 µm
- Co-Ni-Fe: Continuous current, full plate or in a resist mould for depths up to a few tenth of µm
Micro converter for power device
- Platinum : Continuous current, in a resist or alumina mould for depths up to a few µm
- SAC : Continuous current, in a resist mould for depths up to 100 of µm
SAC bumps before et after reflow
- Gold : By immersion for the protection of deposited copper.