Particle cleaning of optical elements for microlithography

Optical assembly mounted in a projection exposure apparatus for microlithography, comprising:

• at least one optical element having an optical surface arranged to be impinged upon by a useful beam bundle of the projection exposure apparatus;
• deposition device arranged to deposit a carbon containing layer onto the optical surface;
• a cleaning device arranged to clean the optical surface;
• a vacuum chamber accommodating said optical element;
• a pressure sensor configured to detect a gas pressure in
• said vacuum chamber;
• and,
• at least one access opening to the vacuum chamber lockable by door or by a valve, with a door signal device which is configured to provide a door signal depending on a present position of the door with a valve signal device which is configured to provide a valve signal depending on a present position of the valve.

A method for cleaning of an optical surface of an optical element arranged in a. vacuum chamber Of a projection exposure apparatus for extreme-ultraviolet lithography, the method comprising the steps of:

• performing particle cleaning of the optical surface at a gas pressure (pc) within the vacuum chamber which is higher than a vacuum pressure (po) at which an exposure operation is performed with projection exposure apparatus;
• pumping out the vacuum chamber accommodating the optical element during or after cleaning;
• and,
• depositing a carbon containing layer onto the optical layer of the optical element prior to cleaning the optical element wherein said depositing is performed directing after detecting that a vacuum chamber accommodating the optical element is open, the detecting being based on a door signal depending on a present position of a door for locking the vacuum chamber or being based on a valve signal depending on a present position of a valve for locking the vacuum chamber.

Method for cleaning an optical element, comprising:

• providing the opitcal element with a carbon containing layer on an optical surface of the optical element;
• depositing a carbon containing layer onto the optical layer of the optical element prior to the optical element with said depositing being in accordance with a gas pressure value measured in a vacuum chamber accommodating the optical element;
• cleaning the optical element;
• and,
• wherein said depositing is performed directly after detecting that a vacuum chamber accommodating the optical element is open, the detecting being based on a door signal depending on a present position of a door for locking the vacuum chamber or being based on a valve signal depending on a present position of a valve for locking the vacuum chamber.