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reliability wise, u need a uniform metal distribution among the die so that during CMP all parts on the die is equally affected .
i guess from yield point of view u need a small gradient of metal change (i.e. the metal concentration should change with a small rate among the die.
planarization of layers is required to ensure a flat surface. if a certain layer doesn't have a flat surface, there is a tendency of the succeeding layers to have a curvature leading to breaks(microfracture). this is the reason why there is a minimum density of a certain layer.
hope it helps...
According to all messages I understand that at the low density areas oxide layer over the metals gets bend down and succedding layers are effected ,
Please explain me how exactly it gets bend or give me any pdf on this.