hi lightcloud,
Are you talking about metal slotting violations, in DRC run? if it is so then, metal slotting goes like this.
In most manufacturing processes, a metal layer cannot be layered in uniform thickness. Wide wires typically result in high metal densities. Metal applied over a wide area is thin in the middle and thick on the edges, causing yielding and current density problems in that metal segment. Use metal slotting to control metal density in your design.
Candidates for slotting are wires with a width that is greater than the specified maximum width. The Magma tool performs slotting on a segment−by−segment (or wire−by−wire) basis−cutting the segments that are wider than maximum width into multiple, thinner segments of the same type. The tool ensures that each thin segment is narrower than the specified maximum width