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. I n particular, we could observe in some circumstances, a new defect gen eration owing to a pitting corrosion of the polysilicon substrate duri ng wet processing. Polysilicon pitting corrosion has been investigated in hvdrofluoric solutions (DHF and BHF). The mechanism of corrosion c an proceed by two different modes. First, a purely electroless mechani sm can appear on a nearly homogeneous surface of silicon where a few s ites play the role of micro-cathodes; in that case, surface metallic c ontamination by metal impurities from hydrofluoric solutions is a sour ce of silicon corrosion. A study to characterize all intervening param eters is made using total reflectance x-ray fluorescence (TXRF). Secon d, a spontaneous difference of potential between Pt-Si plots and polys ilicon electrode enhances the short-circuit current. A procedure is de veloped to study this corrosion in relation to real conditions existin g during the wet process. This corrosion is analogous to the anodic di ssolution very near the thermodynamic equilibrium value; then, a very small defect on the structure results in a local variation of potentia l, favorable to pitting or intergranular corrosion. Such a mechanism, at a low value of the overpotential and low current density is known t o be responsible for a preferential etching on crystal lattice defects and leads to surface roughening and pinhole formation.