Radiation analysis system

1. A radiation analysis system comprising:

a target comprising two marks which are separated from each other, the target being configured to undergo thermal expansion when illuminated with radiation;

a position measurement system configured to measure a change in the separation of the marks; and

a processor configured to determine a power of the radiation using the measured change in separation of the marks.

2. The radiation analysis system of claim 1 , wherein the target is thermally isolated from its surroundings.

3. The radiation analysis system of claim 1 , wherein the target is in a vacuum environment.

4. The radiation analysis system of claim 1 , wherein the target comprises a measurement plate.

5. The radiation analysis system of claim 4 , wherein the measurement plate comprises a metal or a semiconductor.

6. The radiation analysis system of claim 1 , wherein the target is one of multiple targets having different radiation absorption properties.

7. The radiation analysis system of claim 6 , wherein different targets comprise different coatings having different radiation absorption properties.

8. The radiation analysis system of claim 1 , wherein the separation between the marks is between approximately 20 mm and 110 mm.

9. The radiation analysis system of claim 1 , wherein a coefficient of thermal expansion of the target is between approximately 2 ppm K-1 to 30 ppm K-1.

10. The radiation analysis system of claim 1 , wherein a specific heat capacity of the target is between approximately 1 J cm-3K-1 to 3 J cm-3K-1.

11. The radiation analysis system of claim 1 , wherein a thickness of the target is between approximately 0.1 mm to 1.0 mm.

12. The radiation analysis system of claim 1 , wherein the radiation analysis system forms part of a lithographic apparatus, the lithographic apparatus comprising:

an illumination system configured to condition a radiation beam;

a support structure 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; and

a projection system configured to project the patterned radiation beam onto the substrate, wherein the radiation analysis system is configured to analyze the radiation beam.

13. The radiation analysis system of claim 12 , wherein the target is a measurement substrate.

14. The radiation analysis system of claim 13 , wherein the measurement substrate is mounted on the substrate table and supported by burls, the burls forming a burl area across the measurement substrate, wherein the marks are located outside of the burl area.

15. The radiation analysis system of claim 13 , wherein the measurement substrate is mounted on the substrate table and supported by burls, the burls forming a burl area across the measurement substrate, wherein the marks are located within the burl area and the separation of the marks is less than a pitch of the burls.

16. The radiation analysis system of claim 12 , wherein the patterning device comprises the target.

17. The radiation analysis system of claim 12 , wherein the target is a measurement plate mounted on the support structure via flexures.

18. The radiation analysis system of claim 12 wherein the target is a measurement plate mounted on the substrate table via flexures.

19. The radiation analysis system of claim 17 , wherein the flexures comprise leaf springs.

20. A method of analyzing radiation, the method comprising:

illuminating a target, the target comprising two marks separated from each other, the illuminating inducing thermal expansion of the target;

measuring a change in the separation of the marks; and

determining a power of the radiation using the measured change in separation of the marks.

21. The method of claim 20 , wherein the target is thermally isolated from its surroundings.

22. The method of claim 20 , wherein:

multiple targets are provided, and

the multiple targets have different radiation absorption properties.

23. The method of 20 , wherein the target is illuminated with radiation until a temperature of the target has increased by a value between approximately 1K to 10K.

24. The method of claim 20 , wherein heating and/or cooling effects acting on the target that are not due to the radiation are modeled and accounted for when determining the power of the radiation.

25. The method of claim 20 , wherein a coefficient of thermal expansion of the target is between approximately 2 ppm K-1 to 30 ppm K-1.

26. The method of claim 20 , wherein a specific heat capacity of the target is between approximately 1 J cm-3K-1 to 3 J cm-3K-1.

27. The method of claim 20 , wherein a thickness of the target is between approximately 0.1 mm to 1.0 mm.

28. The method of claim 20 , wherein a delay between a time at which the target is no longer illuminated with radiation and a time at which the change in separation of the marks is measured is between approximately 0.5 seconds to 5.0 seconds.

29. The method of claim 20 , wherein a cooling of the target is interrupted when the radiation analysis system is in use.

30. A computer readable medium for storing computer readable code wherein the code causes a lithographic apparatus to perform the method of analyzing radiation, the method comprising:

illuminating a target, the target comprising two marks separated from each other, the illuminating induces thermal expansion of the target;

measuring a change in the separation of the marks; and

determining a power of the radiation using the measured change in separation of the marks.