Failure analysis and elimination of poly residues contamination in wafer fabrication

The poly residue problem in wafer fabrication was investigated in this paper. Surface and cross sectional SEM (scanning electron microscopy) was used to identify the root cause. After poly etching, particle contamination was found at the N-well and field oxide overlap region. Some wafers were scrapped due to this issue. To identify the root cause and solution, some affected wafers were subjected to surface and cross sectional SEM. Surface SEM inspection found the particles at the edge of field oxide. Cross sectional SEM and EDX confirmed that it was poly residue. The residue was due to the high topography at the edge of the field oxide, thus causing higher poly thickness. The difference in height resulted in the vertical thickness of the slope of ONO and poly layers to be thicker than that of the planar layer. During the poly etch process, which was anisotropic, the planar poly could be etched away completely but the poly at the slope might not be etched away as it was thicker than the planar layer. Hence some poly residue was left behind. After investigation, the solutions used are to optimize the poly etching recipe by removing He clamp flow at the break through step, and to increase the isotropic etch and etching time from 80 s to 100 s. The poly residue is then eliminated after implementing the new etch recipe.