Active bandwidth control for a laser

1. A lithography apparatus for exposing a resist with a pre-selected pattern, said apparatus comprising:

a mask designed using optical proximity correction to produce said pre-selected pattern on said resist;

an instrument measuring laser bandwidth of at least one pulse in a burst of pulses; and

a laser source for generating a burst of laser light pulses for transmission through said mask to expose said resist, said source comprising;

an active bandwidth control system responsive to a bandwidth measurement control signal to cause the source to produce a pulse in the burst having a pulse bandwidth larger than a previous pulse in the burst.

2. An apparatus as recited in claim 1 wherein said laser source defines a resonant laser cavity and said active bandwidth control system comprises an optic for altering a wavefront of a laser beam in said cavity and a dispersive element to selectively adjust an output laser bandwidth.

3. An apparatus as recited in claim 2 wherein said optic alters coma.

4. An apparatus as recited in claim 3 wherein said optic comprises a positive lens and a negative lens.

5. An apparatus as recited in claim 3 wherein said optic comprises a positive cylindrical lens and a negative cylindrical lens.

6. An apparatus as recited in claim 5 wherein said positive cylindrical lens and said negative cylindrical lens comprise an afocal pair.

7. An apparatus as recited in claim 3 wherein one of said cylindrical lenses is moveable relative to the other cylindrical lens to alter coma.

8. An apparatus as recited in claim 5 wherein said laser beam defines a laser axis at said moveable cylindrical lens and said lens movement is in a direction non-parallel to said laser axis.

9. An apparatus as recited in claim 5 wherein said laser beam defines a laser axis at said cylindrical lenses and said lenses are rotated about an axis normal to said laser axis to alter coma.

10. An apparatus as recited in claim 1 wherein said instrument comprises an etalon and a photodetector array.

11. An apparatus as recited in claim 1 wherein said laser source produces a laser having a wavelength variation across an aperture and said active bandwidth control system comprises:

an instrument measuring laser bandwidth and producing a control signal proportional thereto; and

an aperture blocking element moveable to adjust a size of the aperture to selectively vary an output laser bandwidth in response to said control signal.

12. An apparatus as recited in claim 11 wherein said laser source comprises a gas discharge laser having two spaced apart discharge electrodes defining an arc direction and said element is moveable in a direction normal to said arc direction.

13. An apparatus as recited in claim 11 wherein said element comprises a first plate and a second plate with said second plate simultaneously moveable with said first plate to adjust said aperture size.

14. An apparatus as recited in claim 1 wherein said measured pulse and said subsequent pulse are both in a same burst of pulses.

15. An apparatus as recited in claim 1 wherein said measured pulse and said subsequent pulse are both in a same pulse window, said pulse window having between 3 and 50 pulses.

16. An apparatus as recited in claim 2 wherein said dispersive element comprises a grating.

17. An active bandwidth control system operable within a burst of pulses from a pulsed laser source, said laser source defining a resonant laser cavity, said system comprising:

an instrument to measure laser bandwidth and produce a control signal proportional thereto; and

an optic for altering a wavefront of a laser beam in said cavity to selectively increase an output laser pulse bandwidth relative to a previous pulse in the burst in response to said control signal.

18. A system as recited in claim 17 wherein said optic alters coma.

19. A system as recited in claim 18 wherein said optic comprises a positive cylindrical lens and a negative cylindrical lens.

20. A system as recited in claim 19 wherein said positive cylindrical lens and said negative cylindrical lens comprise an afocal pair.

21. A system as recited in claim 19 wherein one of said cylindrical lenses is moved relative to the other cylindrical lens to alter coma.

22. A system as recited in claim 21 wherein said laser beam defines a laser axis at said moveable cylindrical lens and said lens movement is in a direction non-parallel to said laser axis.

23. A system as recited in claim 19 wherein said laser beam defines a laser axis at said cylindrical lenses and said lenses are rotated about an axis normal to said laser axis to alter coma.

24. A system as recited in claim 19 wherein said instrument comprises an etalon and a photodetector array.

25. An active bandwidth control system operable within a burst of pulses from a pulsed laser source, said laser source defining a resonant laser cavity and producing a laser having a wavelength variation across an aperture, said system comprising:

an instrument measuring laser bandwidth and producing a control signal proportional thereto; and

an aperture blocking element in said cavity and moveable to adjust a size of the aperture to increase an output laser pulse bandwidth relative to a previous pulse in the burst in response to said control signal.

26. A system as recited in claim 25 wherein said laser source comprises a gas discharge laser having two spaced apart discharge electrodes defining an arc direction and said element is moveable in a direction normal to said arc direction.

27. A system as recited in claim 25 wherein said element comprises a first plate and a second plate with said second plate simultaneously moveable with said first plate to adjust said aperture size.

28. A system as recited in claim 25 wherein said instrument comprises an etalon and a photodetector array.