Projection objective for a microlithographic projection exposure apparatus

A method for converting a projection objective of a microlithographic projection exposure apparatus from dry operation to immersed operation, the method comprising:

• a) providing a projection objective comprising a last optical element on an image side of the objective, wherein the last optical element is plane on the image side and confines, together with the image plane, an immersion space which is configured to be filled with an immersion liquid;
• b) removing at least one liquid or solid volume from a location in the beam path of the objective that is different from the immersion space, wherein the at least one liquid or solid volume has plane-parallel interfaces, and has an optical thickness that is substantially equal to the optical thickness of the immersion space when the immersion space is filled with the immersion liquid;
• and
• c) filling the immersion space with an immersion liquid which has the same refractive index as the at least one volume removed in step b).

A projection objective configured so that during use the projection objective can direct light from an object plane to an image plane, the projection objective compnsing:

• an optical axis,
• a space which
• is configured to be filled with a liquid and
• is confined by two at least substantially plane-parallel surfaces extending perpendicularly to the optical axis, and
• at least one liquid or solid volume, which
• has plane-parallel interfaces,
• is configured to be introduced into a beam path of the projection objective and
• has an optical thickness that is substantially equal to the optical thickness of the space when it is filled with the liquid,
• wherein the projection objective is configured to be used in a microlithographic projection exposure apparatus.

A method for operating a projection objective of a microlithographic projection exposure apparatus, the method comprising:

• a) providing a projection objective comprising an optical axis and an empty space which is configured to be filled with a liquid and is confined by two at least substantially plane- parallel surfaces extending perpendicularly to the optical axis;
• b) carrying out exposures;
• c) removing at least one liquid or solid volume from a location in the beam path of the objective that is different from the space, wherein the at least one liquid or solid volume has plane-parallel interfaces, and has an optical thickness that is substantially equal to the optical thickness of the space when the space is filled with the liquid;
• and
• d) filling the space with a liquid.

A method for operating a projection objective of a microlithographic projection exposure apparatus, the method comprising:

• a) providing a projection objective comprising an optical axis and a space which is filled with a liquid and is confined by two at least substantially plane-parallel surfaces extending perpendicularly to the optical axis;
• b) carrying out exposures;
• c) removing the liquid from the space and
• d) introducing at least one liquid or solid volume into a location in the beam path of the objective that is different from the space, wherein the at least one liquid or solid volume has plane-parallel interfaces, and has an optical thickness that is substantially equal to the optical thickness of the space when the space is filled with the liquid.