Method and apparatus for improving critical dimension variation

1. A method, comprising:

obtaining a relationship between thicknesses of a contamination layer formed on a mask and amounts of compensation energy to remove the contamination layer;

measuring a first thickness of a first contamination layer formed on the mask; and

increasing an energy applied to a first light from a first light source to the mask multiple times to remove the first contamination layer, wherein each time an amount of increase in the energy is less than an amount of increase in the energy in the previous time.

2. The method of claim 1 , wherein the first thickness is measured by directing a second light from a second light source to the mask.

3. The method of claim 1 , wherein the measuring the first thickness of the first contamination layer is performed in a lithography system.

4. The method of claim 1 , wherein the measuring the first thickness of the first contamination layer is performed in a mask storage.

5. The method of claim 1 , further comprising forming a second contamination layer on the mask after the first contamination layer is removed, wherein the second contamination layer is formed while the mask is idle for a period of time.

6. The method of claim 5 , wherein the period of time is greater than about eight hours.

7. The method of claim 5 , wherein the mask is disposed in a lithography system while being idle.

8. The method of claim 7 , further comprising removing the second contamination layer by increasing the energy applied to the first light multiple times.

9. The method of claim 1 , wherein the first light is utilized to pattern a substrate.

10. A lithography process, comprising:

performing the lithography process in a lithography system, comprising:

directing a first light from a first light source to a mask, wherein the first light is configured to pattern a substrate;

measuring a thickness of a contamination layer formed on the mask; and

controlling the first light source so the first light includes gradually decreasing amounts of compensation energy to remove the contamination layer.

11. The lithography process of claim 10 , wherein the first thickness is measured by directing a second light from a second light source to the mask and reflecting the second light from the mask to a detector.

12. The lithography process of claim 10 , further comprising reflecting the first light from the mask to a projection optics module.

13. The lithography process of claim 12 , further comprising directing the first light to an illuminator prior to directing the first light to the mask.

14. The lithography process of claim 13 , wherein the illuminator comprises mirrors that are switchable to reflect the first light to different illumination positions.

15. The lithography process of claim 10 , wherein the first light is an extreme ultraviolet light.

16. The lithography process of claim 15 , wherein the lithography process is an extreme ultraviolet lithography process.

17. A lithography system, comprising:

a light source configured to expose a photoresist layer;

a mask stage;

a mask disposed on the mask stage;

a substrate stage;

a thickness measuring device configured to measure a thickness of a contamination layer formed on the mask; and

a controller configured to control the light source to emit a light with gradually decreasing amounts of compensation energy to remove the contamination layer.

18. The lithography system of claim 17 , further comprising:

an illuminator disposed between the light source and the mask stage; and

a projection optics box disposed between the mask stage and the substrate stage.

19. The lithography system of claim 17 , wherein the mask stage and the substrate stage are disposed in a lithography chamber, and the light source is disposed in a device chamber adjacent the lithography chamber.

20. The lithography system of claim 19 , wherein the thickness measuring device is disposed in the device chamber.