Due to the fact that several process steps which modify the topography of the wafer surface (etching, deposition, oxidation) are necessary to generate an IC, the surface becomes severely non-planar. This creates several problems especially for the lithography process, because non-planar surfaces require a high depth of focus which is incompatible with the requirement of a high numerical aperture of the imaging systems, which is necessary to guarantee a small minimum feature size.
Chemical mechanical polishing CMP is used as a planarization technique. A chemical slurry with etchant agents and abrasives is applied to the wafer surface.
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