Degree of planarization and the contact diameter

Before the blanket fill process can be applied, two important parameters need to be considered. These are the variation in the diameter of the contact/via openings and the degree of planarization of the dielectric layers. 

Degree of planarization (DOP) The blanket tungsten process is in fact a planarization method for contacts and vias, Therefore, if a topography exists before the tungsten deposition is done there will be a risk of tungsten residues or stringers after the etch back step (see figure 2.22). This cannot be tolerated since shorts between aluminum lines will occur. 

In a detailed study [de Graaf et al/,s], three different planarization schemes were compared, namely, BPSG flow anneal, Spin On Glass (SOG) 

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