The main purpose for depositing metal layers onto Bi2Te3-Sb2Te3 semiconductor dices
1) Creation of the diffusion barrier preventing solder components (especially copper, silver and tin) from penetrating semiconductors dices.
2) Creation of solderable layer over the diffusion barrier in order to protect the diffusion layer from oxidation and to provide an good quality of soldering.
Nickel-based alloys with 6-9 micron thickness are used as diffusion layer and 0.1-0.2 micron electroless deposited gold flash or approx. 5 micron layer of Sn (+1 % Bi) are used as solderable layer.
Such coating ensures the protection of modules assembled using tin (Sn) solder (for 4,000-5,000 h) and lead (Pb) solder-based modules for 6,000-8,000 h at hot side temperature of 230 °С (power generating modules).
Metal layers are deposited onto slices cut from ingots; the slices are then cut into dices.