Thank you for your answer, dick_freebird
I have read and searched many with google and design rule manual but I am not clear about this.
I made layout for my cell and in final phase of this process, I used a tool of calibre to auto fill metal into my layout.
I see tool add dummy layers into layout, ex: metal 1 has 4 "special" layers, in higher metal as metal 4,5,6, they also add dummy metal and dummy OPC. I know OPC is a process to correct mask in fabrication.
My question is why we need OPC dummy if we had dummy metal?
Pls help me with this.
Thanks so much