Method and device for measuring actuators in a projection exposure apparatus for semiconductor lithography

A method of using a semiconductor lithography projection exposure apparatus, the apparatus comprising a first actuator, a second actuator mechanically coupled with the first actuator, and an optical element mechanically coupled to the first and second actuators, the first and second actuators configured to be deflected to deform the optical element, the method comprising:

• a) using a first control signal to drive and deflect the first actuator to:
• i) deform the optical element so that the optical element is in a first deformed position;
• and
• ii) deflect the second actuator due to the mechanical coupling between the first and second actuators so that the second actuator moves from a first position to a deflected position;
• and
• b) after a), determining a change in the capacitance of the second actuator due to the second actuator moving from the first position to the deflected position;
• c) based on the change in the capacitance of the second actuator, determining the first deformed position of the optical element;
• and
• d) using a second control signal to drive and deflect the second actuator to deform the optical element to be in a second deformed position.