Gas discharge laser with gas temperature control

• US 6,034,978 A
• Filed: 05/12/1999
• Issued: 03/07/2000
• Est. Priority Date: 05/12/1999
• Status: Active Grant

• Current Assignee: Cymer Incorporated (ASML Holding NV)
• Original Assignee: Cymer Incorporated (ASML Holding NV)
• Inventors: Harrington, Steven M., Thompson, Paul S., Tedesco, Carl E. II, Ishihara, Toshihiko, Fomenkov, Igor V., Partlo, William N., Ujazdowski, Richard C., Duffey, Thomas P., Berger, Dmitry, Oliver, I. Roger, Morton, Richard G., Watson, Tom A., Shannon, Robert A., Azzola, James H., Larson, Donald G.
• Primary Examiner(s): Scott, Jr., Leon
• Application Number:
  US09/310,872
• Time in Patent Office: 300 Days
• Field of Search: 372/34, 372/57, 372/59, 372/58
• US Class Current: 372/34
• CPC Class Codes: G03F 7/70025 by lasers
• CPC Class Codes: H01S 3/036 Means for obtaining or maintaining the desired gas pressure within the tube, e.g. by gettering, replenishing; Means for circulating the gas, e.g. for equalising the pressure within the tube H01S3/031 takes precedence
• CPC Class Codes: H01S 3/134 in gas lasers

Abstract

A gas discharge laser with fast response gas temperature control to maintain laser gas temperature within desired limits during burst mode operation. Preferred embodiments include a passive temperature stabilizer having fins with surface areas exposed to flowing laser gas at least equal to the surface area of the cooling fins of a laser gas heat exchanger. Preferred embodiments utilize heating elements and coolant flow control to regulate laser gas temperatures using processors programmed to anticipate idle periods.

Claims

1. A gas discharge laser system with fast response temperature control for controlling gas temperature within desired limits during burst mode operation in which:

1) heat is added to the laser gas by electric discharge in bursts of pulses with a pulse rate of about 2000 Hz or greater and heat rates in excess of 4 kw/sec for short time periods such as about 1/3 second followed by a short time idle period such as about 1/3 second during which no heat is added to the laser gas by electric discharge, and

2) following a large number of such bursts, such as about 85 bursts, there follows a longer idle period such as about 9 seconds during which time no heat is added to the laser gas by electric discharge;

said laser comprising,

A. a laser chamber

B. at least two elongated electrodes disposed in said laser chamber;

C. a high voltage pulse power source for producing electric discharges between said elongated electrodes;

D. a laser gas contained in said laser chamber;

E. a fan for circulating said laser gas between said two elongated electrodes;

said electric discharges and said fan adding heat to said laser gas over several minutes of such burst mode operation at a heat addition rate defining an average heat addition rate,

F. a laser gas temperature control system for maintaining laser gas temperatures within desired limits when operating in a burst mode, said temperature control system comprising;

1) a heat exchanger for removing heat from said laser gas said heat exchanger having a heat removal capacity exceeding said average heat addition rate,

2) a fast acting temperature control means providing temperature responses at time periods of less than a fraction of one second,

wherein said fast response temperature control is capable of controlling laser gas temperature to within a desired temperature range of less than 8°

C. during such burst mode operation.

2. A laser as in claim 1 wherein said fast acting temperature control means comprises a passive temperature stabilizer having a surface area exposed to said laser gas greater than 1 m².

3. A laser as in claim 1 wherein said fast acting temperature control means comprises an actively cooled heat exchanger defining a heat exchanger surface area exposed to said laser gas said passive temperature stabilizer having a surface area equal to said cooled heat exchanger surface area.

4. A laser system as in claim 3 wherein said temperature stabilizer comprises a cavity at least partially filed with a phase change material.

5. A laser system as in claim 4 wherein said phrase change material has a phase change temperature within a few degrees of a desired gas temperature.

6. A laser as in claim 5 wherein said phase change material is chosen from a group consisting of nickel, nitrate, silicon tetradicloride and azobenzene.

7. A laser system as in claim 1 said heat exchanger is a water cooled heat exchanger with water flow control wherein said temperature control system comprises a processor programmed to control said laser in a continuous burst mode over periods of at least one hour with said water flow control maintained constant, wherein said gas temperature is maintained stable within a desired range by regulating electrical heat input into said laser gas through said electrodes.

8. A laser system as in claim 7 and further comprising a beam switch system wherein said laser is configured to provide illumination, alternatively to at least two lithography devices.

9. A laser system as in claim 7 wherein said beam switch system comprises at least one Pockels cell and at least one polarizing beam splitter.

10. A laser as in claim 1 wherein said fast acting temperature control means comprises electrodes wherein said high voltage pulse power source is programmed to provide high voltage pulses to each of two pairs of electrodes so as to maintain relatively constant, discharge heat into said laser chamber.

11. A laser system as in claim 1 wherein said fast acting temperature control means comprises a coolant cooled heat exchanger comprising a fast acting heater and a heater control system programmed to control said heater to maintain said laser gas temperature within a desired range during burst mode operation.

12. A laser system as in claim 11 wherein said heater control system is programmed to activate said heater in advance of idle periods so as to maintain said gas temperature within a desired range during said idle period.

13. A laser system as in claim 11 wherein said heater control system comprises a processor programmed to detect repetitive laser operation, to predict, based on said detected repetitive operation, in advance laser down time periods and to activate said heater in advance of idle periods.

14. A laser as in claim 11 wherein said heater is configured to directly heat said laser gas.

15. A laser as in claim 11 wherein said heater is configured to heat said coolant.

16. A laser system as in claim 1 wherein said heat exchanger comprises a coolant cooled heat exchanger and a fast flow control system to control coolant flow to said heat exchanger.

17. A laser system as in claim 16 wherein said fast flow control system is programmed to reduce coolant flow in advance of idle periods so as to maintain gas temperature within a desired range during said idle periods.

18. A laser system as in claim 1 wherein said fast flow control system comprises a processor programmed to detect repetitive laser operation and to predict, based on said detected repetitive operations, in advance laser idle periods and to reduce coolant flow in advance of said idle periods.

19. A laser system as in claim 1 and further comprising a fan control system programmed to increase fan speed during idle periods so as to increase fan heating of said laser gas.

20. A laser system as in claim 1 wherein said heat exchanger is a water cooled heat exchanger.

21. A laser system as in claim 20 wherein said water cooled heat exchanger is a two-tube heat exchanger comprising a first tube and a second tube wherein said heat exchanger is configured so that water flows in the first tube in a direction opposite to water flow in the second tube.

22. A laser system as in claim 21 wherein flow in said first tube can be stopped so as to convert a portion of said heat exchanger into a passive temperature stabilizer.

23. A laser as in claim 21 and further comprising an electric motor driving said fan wherein water flow exiting each of said two tubes is utilized to cool said motor.

24. A laser as in claim 20 and further comprising a PID controller.

25. A laser as in claim 1 wherein said heat exchanger is a refrigerant cooled heat exchanger.

26. A laser as in claim 25 and further a refrigerant control system having reverse features to permit the heat exchanger to add heat to the laser chamber.

27. A laser as in claim 25 wherein said refrigerant cooled heat exchanger comprises a cold finger.

28. A laser as in claim 1 and further comprising a heat exchanger and an adjustable heat exchanger vane for directing flow toward or away from a said heat exchanger.