I have doubt here : what is the relationship between metal density rule and yield ? I heard some one saying that metal density rules enhances the yield. But am not sure how .. I request you guys to comment on this ..
I do not know exact relation for that, but If we do not have unifrom metal density across ur die then yield will be impacted. To make uniform density we use a metal fill.
It is because metal layer connections are formed by etching process, for that we need a uniform metal density for better accuracy. Or it will etch more metal from area where density is low and hense chanse of failure.
Besides the etching process effect, some optical effect during fabrication has more impact, specially for deep sub-micron technology. More uniform metal density helps the lithography.