Nickel plating on p+ silicon : a characterization of contact resistivity and line resistance

Abstract

Nickel plating on p+ Si is a promising approach for the metallization of n-type Si solar cells. Ni acts as diffusion barrier for copper, which is used for thickening of the Ni contacts. In this work the adhesion and the contact resistivities of Ni plated lines on different boron emitters as well as the line resistances are evaluated. For that purpose, boron emitters with different sheet resistances on Czochralski n-type Si wafers are used. The dielectric passivation layer (SiNx) on top of the emitter is locally opened by photolithography or by laser ablation to perform a line structure, which is afterwards electroless Ni plated. During a sintering step, nickel silicide is formed to achieve the required adherence and contact resistivity to the Si. To improve the adherence of the Ni plated layer and therefore to decrease the contact resistivity, a plating process with two separated Ni plating steps, named “two-step Ni plating”, is introduced. With this process narrow and sharp lines (15-80 μm) with contact resistivities of about 0.6 mΩcm² are demonstrated. Using electrodeposition of Cu, line resistances of 0.45 Ω/cm are measured with line widths below 50 μm. This work demonstrates that the introduced plating technique is well suited for high efficient solar cell.