Electrochemical Coupling Effects on the Corrosion of Silicon Samples in HF Solutions

Multilevel metallization commonly used in semiconductor manufacturing for VLSI and ULSI circuits production requires low defect densities. In particular, we could observe in some circumstances, a new defect generation owing to a pitting corrosion of the polysilicon substrate during wet processing. Polysilicon pitting corrosion has been investigated in hydrofluoric solutions (DHF and BHF). The mechanism of corrosion can proceed by two different modes. First, a purely electroless mechanism can appear on a nearly homogeneous surface of silicon where a few sites play the role of microcathodes; in that case, surface metallic contamination by metal impurities from hydrofluoric solutions is a source of silicon corrosion. A study to characterize all intervening parameters is made using total reflectance x-ray fluorescence (TXRF). Second, a spontaneous difference of potential between Pt-Si plots and polysilicon electrode enhances the short-circuit current. A procedure is developed to study this corrosion in relation to real conditions existing during the wet process. This corrosion is analogous to the anodic dissolution very near the thermodynamic equilibrium value ; then, a very small defect on the structure results in a local variation of potential, favorable to pitting or intergranular corrosion. Such a mechanism, at a low value of the overpotential and low current density, is known to be responsible for a preferential etching on crystal lattice defects and leads to surface roughening and pinhole formation.