Method for adjusting lithographic mask flatness using thermally induced pellicle stress

1. A method for adjusting the flatness of a lithographic mask, the method comprising:

determining an initial mask flatness of the mask, the mask at an ambient temperature;

determining an applied stress for bringing the mask to a desired mask flatness;

determining a mounting temperature of a pellicle frame to be mounted to the mask, said mounting temperature corresponding to said applied stress, wherein at least a portion of said pellicle frame has a different coefficient of thermal expansion with respect to the mask;

adjusting the actual temperature of said pellicle frame to said determined mounting temperature; and

mounting said pellicle frame, being at said mounting temperature, to the mask, being at said ambient temperature;

wherein the pellicle frame provides one of an expansive force or a contractive force to the mask as a result of a difference in pellicle frame mounting temperature and mask ambient temperature, and the difference in coefficient of thermal expansion between the pellicle frame and the mask.

2. The method of claim 1 , further comprising applying a mathematical model to determine said applied stress and said mounting temperature.

3. The method of claim 2 , wherein inputs to said mathematical model include: a stepper temperature, said initial mask flatness, and materials properties of the mask, said pellicle frame and an adhesive used to mount said pellicle frame to said mask.

4. The method of claim 3 , wherein said materials properties include: elastic modulus, Poisson's ratio, coefficient of thermal expansion (CTE) and dimensions of the mask, said pellicle frame and said adhesive.

5. The method of claim 1 , further comprising heating said pellicle frame whenever said determined mounting temperature exceeds the room temperature of said pellicle frame.

6. The method of claim 5 , wherein said pellicle frame is heated by passing an electrical current therethrough.

7. The method of claim 5 , further comprising increasing the thickness of said pellicle frame whenever said determined mounting temperature exceeds a threshold temperature value.

8. The method of claim 1 , further comprising cooling said pellicle frame whenever said determined mounting temperature is less than the room temperature of said pellicle frame.

9. The method of claim 8 , wherein said pellicle frame is cooled by thermal conduction between said pellicle frame and a colder object brought into contact therewith.

10. The method of claim 1 , wherein sides of said pellicle frame disposed in a first direction have a first thickness and sides of said pellicle frame disposed in a second direction have a second thickness.

11. The method of claim 1 , wherein sides of said pellicle frame disposed in a first direction comprise first material and sides of said pellicle frame disposed in a second direction comprise a second material having a different coefficient of thermal expansion than said first material.

12. The method of claim 11 , wherein said first material is aluminum and said second material is quartz.

13. The method of claim 1 , wherein sides of said pellicle frame comprise an electrically conductive material and corners of said pellicle frame comprise a thermally insulating material.

14. The method of claim 1 , further comprising mapping the initially determined flatness of the mask prior to mounting the pellicle frame.