Stage apparatus, lithographic apparatus and method of positioning an object table

A lithographic apparatus comprising:

• an illumination system configured to condition a radiation beam;
• a patterning device support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;
• a substrate table constructed to hold a substrate;
• a projection system configured to project the patterned radiation beam onto a target portion of the substrate;
• a positioning device configured to position the substrate table or patterning device support relative to a substantially stationary frame to which the projection system is mounted;
• a measurement system configured to provide a first position dependent signal of the substrate table or patterning device support, said measurement system comprising:
• a first measurement system part mounted on the substrate table or patterning device support;
• a second measurement system part mounted on the substantially stationary frame, wherein, in use, the first and second measurement system parts co-operate with each other to provide the first position dependent signal;
• a controller configured to generate a control signal to control the positioning device;
• a mounting device configured to mount said second measurement system part on said substantially stationary frame,
• a position sensor configured to measure a displacement between the mounting device and the substantially stationary frame and to provide a second position dependent signal representing a position of the second measurement system part relative to the substantially stationary frame based on the measured displacement between the mounting device and the substantially stationary frame,
• wherein the controller is constructed and arranged to receive, at an input terminal thereof, the first and second position dependent signals, and wherein the control signal is based on both the first and second position dependent signal.

A method of positioning an object table relative to a substantially stationary frame using a positioning device configured to position the object table and a measurement system comprising a first measurement system part and a second measurement system part that are configured to co-operate with each other to provide a first position dependent signal of the object table, the method comprising:

• providing a first position dependent signal representing a position of the first measurement system part mounted on said object table relative to the second measurement system part mounted, with a mounting device, on the substantially stationary frame;
• measuring, with a position sensor, a displacement between the mounting device and the substantially stationary frame and providing a second position dependent signal representing a position of the second measurement system part relative to the substantially stationary frame based on the measured displacement between the mounting device and the substantially stationary frame;
• providing the first and second position dependent signals at an input terminal of a controller to control the positioning device;
• and
• providing, by the controller, a control signal for controlling the positioning device,
• wherein the control signal is based on both the first and second position dependent signals.