Considering the above, it is an object of the invention to provide a method to calibrate spot positions on a mirror of an interferometer system. According to an embodiment of the invention, there is provided a method for calibrating spot positions on a mirror of an interferometer system including at least two pairs of interferometers to provide a redundant measurement of a rotational position of an object about a rotation axis, wherein the method comprises the following steps: a. rotating the object; b. measuring a change of the rotational position of the object about the rotation axis with each of the at least two pairs of interferometers; c. determining a deviation in the change of the rotational position measured by one of the at least two pairs of interferometers compared to the change of the rotational position measured by another one of the at least two pairs of interferometers or compared to an average change of the rotational position measured by two or more pairs of interferometers; and d. obtaining information about the spot positions on the mirror of said one of the at least two pairs of interferometer based on the determined deviation. According to another embodiment of the invention, there is provided a lithographic apparatus comprising: 1. an object to be positioned; 2. an actuator system to position the object; 3. a measurement system including an interferometer system, which interferometer system includes at least two pairs of interferometers to provide a redundant measurement of a rotational position of the object about a rotation axis; and 4. a control system to drive the actuator system based on an output of the measurement system, 5. wherein the control system is configured to carry out the method according to the invention. According to a further embodiment of the invention, there is provided a device manufacturing method wherein use is made of a lithographic apparatus according to the invention. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: 1.Figure 1 depicts a schematic overview of a lithographic apparatus; 2.Figure 2 depicts a detailed view of a part of the lithographic apparatus of Figure 1; 3.Figure 3 schematically depicts a position control system; 4.Figure 4 schematically depicts a top view ofan interferometer system; 5.Figure 5 schematically depict spot positions on a mirror of the interferometer system of Fig. 4; 6.Figure 6 schematically depicts a side view of the object of Fig. 4 after rotating the object about an Y-axis; 7.Figure 7A schematically depicts a simplified top view of the interferometer system of Fig. 4 in a first rotational position about an axis parallel 1o the Z-axis; and 8.Figure 7B schematically depicts a simplified top view of the interferometer system of Fig. 4 in a second rotational position about an axis parallel to the Z-axis.
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