Actually, I was asked by layout person to do this.
This is the first time I heard about spare cells. So, I'm
still learning. And so far, I only use DC, not physical compiler.
I check the manual, the command does exist in physical compiler,
which is psyn_shell-t> insert_spare_cells -lib_cell .....
So, maybe, it can be done both at front end and backend. In my
case, I was asked to do it, which is need to be done at front end.
So, I need answers for the questions that I posted in my first post.