Just saw some info in
TSMC Reference Flow 5.0
Extract below
DFM Flow
Router-based dummy metal insertion
Current status:
Able to add specified rectangular patterns in router. Faster dummy insertion and RC extraction run time than utility. Different rectangular patterns for .13u and N90 designs.
ECO flow: Should maintain previous inserted dummy metals during ECO and only removes those touch ECO wires to minimize timing impact.
Redundant via insertion
Current status: Scan all via in the insertion process
ECO flow: Scan only via in ECO nets for insertion to save run time An option to avoid redundant via insertion causing DRC violations in order to avoid ECO route and maintain timing closure.
May the above might help