Lithographic apparatus, device manufacturing method and device manufactured thereby

A lithographic projection apparatus cleaning method comprising:

• establishing an electrical field near a surface of an optical element that forms part of a radiation system, a patterning structure, or a projection system of a lithographic projection apparatus;
• charging reactive gas molecules;
• and
• supplying said charged reactive gas molecules to the vicinity of said optical element.

A device manufacturing method comprising:

• providing a beam of radiation using a radiation system;
• patterning the beam of radiation with a patterning structure;
• projecting the patterned beam of radiation onto a target portion of a substrate using a projection system;
• and
• cleaning an individual optical element or a subset of optical elements that form part of the radiation system, the patterning structure, or the projection system,
• wherein said cleaning said optical element comprises
• establishing an electrical field near a surface of said optical element;
• charging reactive gas molecules;
• and
• supplying said charged reactive gas molecules to the vicinity of said optical element.