Systems and methods for fluid flow control in an immersion lithography system

A method for controlling fluid flow in an immersion lithography unit having a fluid retention hood and a fluid flow path therethrough, the system comprising:

• sensing a pressure drop in the fluid flow path across the fluid retention hood;
• determining whether the sensed pressure drop is equal to a desired pressure drop;
• and
• controlling a flow control valve positioned in the fluid flow path downstream from the fluid retention hood to drive the sensed pressure drop to the desired pressure drop.

A software program product comprising a storage medium readable by a data processor in a flow control system that contains one or more instructions configured to cause the data processor to perform the method comprising:

• determining a pressure drop in a fluid flow path across a fluid retention hood;
• determining whether the sensed pressure drop is equal to a desired pressure drop;
• and
• generating control signals for a flow control valve positioned in the fluid flow path downstream from the fluid retention hood to drive the pressure drop across the fluid retention hood to the desired pressure drop.