Method of and preventing focal plane anomalies in the focal plane of a projection system

• US 20040263811A1
• Filed: 04/23/2004
• Published: 12/30/2004
• Est. Priority Date: 04/24/2003
• Status: Active Grant

• Current Assignee: ASML Netherlands BV (ASML Holding NV)
• Original Assignee: ASML Netherlands BV (ASML Holding NV)
• Inventors: Wielen, Petrus Augustinus Marie Van Der, Oorschot, Peter Van, Baselmans, Johannes Jacobus Matheus, Wagner, Christian, Van Hout, Frits Jurgen, Van Zwol, Adriaan Roelof, Minnaert-Janssen, Ingrid, Van Ginneken, Hubrecht Lodewijk, Van Es, Roderik Willem
• Granted Patent: US 7,068,349 B2
• Time in Patent Office: Days
• US Class Current: 355/52
• CPC Class Codes: G03B 27/68 Introducing or correcting distortion, e.g. in connection with oblique projection
• CPC Class Codes: G03F 7/706 Aberration measurement

Abstract

To reduce and prevent local field anomalies created localized thinning of the coating of optical elements in the apparatus. The projection system is flood exposed to an intense beam of radiation for a substantial period of time. As the rate of thinning of the coating decreases with time, a uniform coating results.

Claims

1. A method of decreasing focal plane anomalies of a focal plane of a projection system of a lithographic projection apparatus comprising:

exposing the projection system to an intense beam of radiation for a substantial period of time.

2. A method according to claim 1 comprising exposing the projection system to said radiation for at least 40 hours.

3. A method according to claim 1 comprising exposing the projection system to radiation of an intensity in the range of 500 to 6000 mW/cm².

4. A method according to claim 1 comprising exposing the projection system to electromagnetic radiation with a wavelength in the range 1 to 500 nm.

11. A method according to claim 1 wherein the focal plane anomaly is a non-flatness of the focal plane.

12. A method according to claim 11 wherein the non-flatness of the focal plane is a field curvature.

13. A method according to claim 12 wherein the field curvature is a negative field curvature.

14. A method according to claim 12 wherein the field curvature is an astigmatism field curvature.

5. A method of decreasing focal plane anomalies of the focal plane of a projection system of a lithographic projection apparatus comprising:

exposing the projection system to an intense beam of radiation for a substantial period of time, wherein the method is carried out in a lithographic projection apparatus comprising a radiation system for providing a projection beam of radiation.

6. A method according to claim 5 wherein the lithographic projection apparatus further comprises a substrate table for holding a substrate which, during exposure continuously moves in directions substantially perpendicular to the direction of propagation of the radiation.

7. A method according to claim 5 comprising exposing the projection system to radiation of an intensity in the range of 500 to 6000 mW/cm².

8. A method according to claim 5 comprising exposing the projection system to electromagnetic radiation with a wavelength in the range 1 to 500 nm.

9. A method according to claim 5 wherein the lithographic projection apparatus is a mercury I-line apparatus.

10. A method of pre-treating a projection system to substantially prevent focal plane anomalies of the focal plane of a projection system of a lithographic projection apparatus due to optical element coating thinning comprising:

exposing the projection system to an intense beam of radiation for a substantial period of time.

15. A method of decreasing optical aberrations of a projection system of a lithographic projection apparatus comprising:

exposing the projection system to an intense beam of radiation for a substantial period of time.

16. A method according to claim 15 whereby the optical aberration is distortion.

17. A method of decreasing focal plane anomalies of the focal plane of a projection system of a lithographic projection apparatus comprising:

exposing the projection system to an intense beam of radiation for a substantial period of time wherein the method is carried out in a lithographic projection apparatus comprising a radiation system for providing a projection beam of radiation and a substrate table for holding a substrate whereby the substrate is a reflective device arranged to reflect light back into the projection system during exposure.

18. A method according to claim 17 wherein the reflective device is a wafer with a reflective coating.

19. A method according to claim 18 whereby the wafer is patterned with a structure arranged to increase the spatial uniformity of the radiation at the optical elements of the projection system during exposure, said structure comprising one or more structures from the group consisting of:

scattering structures, diffracting structures or refracting structures.

20. A method according to claim 17 whereby the reflective device comprises an aluminum substrate.

21. A dummy mask comprising a plane glass plate for diverging radiation to fill a projection system.

22. A lithographic apparatus comprising a projection system and a machine readable medium comprising machine executable instructions for:

measuring focal plane anomalies of the focal plane in the projection system, and

exposing the projection system to an intense beam of radiation for a substantial period of time when the measured focal plane anomalies exceed a preselected value.

23. An apparatus according to claim 22 whereby the focal plane anomaly is a non-flatness of the focal plane, or a field curvature, or a negative field curvature, or an astigmatism field curvature.

24. A method comprising:

measuring at least one focal plane anomaly of a focal plane in a projection optical system of a lithographic apparatus;

comparing the measured anomaly to a preselected threshold value; and

when the measured anomaly exceeds the preselected threshold value, exposing the projection system to a beam of radiation so that a rate of thinning of a coating of an optical element of the projection optical system is substantially reduced.