| CPC G03F 7/70975 (2013.01) | 22 Claims |

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1. A method of maintaining a projection exposure apparatus comprising first and second modules and a reference element to which the first and second modules are referenced, the method comprising:
removing the first module;
attaching a service module to or in a vicinity of the projection exposure apparatus;
referencing the service module to the reference element; and
using the service module to implement a maintenance measure of the projection exposure apparatus.
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