Finding substrate notch on substrate in a chemical mechanical polishing system

A polishing apparatus, comprising:

• a polishing station;
• a notch finding station;
• a substrate handling system to move a substrate between the polishing station and the notch finding station and including a carrier hold a substrate at the notch finding station;
• wherein the notch finding station includes
• a housing including floor and sidewalls that form an open-topped reservoir,
• a passage to introduce a liquid into the open-topped reservoir to form a liquid column extending to the substrate, and
• an optical fiber extending through a floor of the housing and positioned to direct a light beam through the liquid column onto the substrate when held by the carrier at the notch finding station and to receive reflections of the light beam from the substrate;
• a liquid source to supply the liquid;
• and
• a controller configured to cause the liquid source to flow the liquid at a first flow rate when the notching finding station is operating to detect a notch and at a second reduced non-zero flow rate when the notching finding station is idling.