Christophe MICHEL told us more precise informations about spin coater cleaning.

They always clean with pur water and cotton tipped sticks.
First a spray of water, then they put the wet cotton tipped sitck onto the mirror from the center to the edge ~x3 while continuing to spread water. They finish by putting more water without the cotton tipped stick.

WARNING: WATER HAS TO BE CHANGED EVERY DAY 

They said they only use clean water but in case of grease/oil onto the mirror they would use first acetone then clean water.