Six to ten KHz, or greater gas discharge laser system

• US 6,914,919 B2
• Filed: 06/28/2002
• Issued: 07/05/2005
• Est. Priority Date: 06/19/2000
• Status: Expired due to Term

• Current Assignee: Cymer Incorporated (ASML Holding NV)
• Original Assignee: Cymer Incorporated (ASML Holding NV)
• Inventors: Palenschat, Frederick A., Sandstrom, Richard L., Hofmann, Thomas, Ershov, Alexander I., Ujazdowski, Richard C., Ness, Richard M., Melcher, Paul C., Watson, Tom A., Rettig, Curtis L., Shannon, Robert A., Webb, R. Kyle, Ivaschenko, Alex P.
• Primary Examiner(s): Harvey, Minsun Oh
• Assistant Examiner(s): MENEFEE, JAMES A
• Application Number:
  US10/187,336
• Publication Number:
  US 20030012234A1
• Time in Patent Office: 1,103 Days
• Field of Search: 372/25, 372 55- 58
• US Class Current: 372/25
• CPC Class Codes: G03F 7/70025 by lasers
• CPC Class Codes: G03F 7/70041 by pulsed sources, e.g. multiplexing, pulse duration, interval control or intensity control
• CPC Class Codes: G03F 7/70333 Focus drilling, i.e. increase in depth of focus for exposure by modulating focus during exposure [FLEX]
• CPC Class Codes: G03F 7/70575 Wavelength control, e.g. control of bandwidth, multiple wavelength, selection of wavelength or matching of optical components to wavelength
• CPC Class Codes: G03F 7/70933 Purge, e.g. exchanging fluid or gas to remove pollutants
• CPC Class Codes: H01S 3/0057 Temporal shaping, e.g. pulse compression, frequency chirping soliton generation and propagation G02F1/3513, H01S3/063 and H01S3/108
• CPC Class Codes: H01S 3/0071 Beam steering, e.g. whereby a mirror outside the cavity is present to change the beam direction
• CPC Class Codes: H01S 3/02 Constructional details housings or packages of fibre lasers H01S3/06704
• 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 3/038 Electrodes, e.g. special shape, configuration or composition
• CPC Class Codes: H01S 3/0385 Shape
• CPC Class Codes: H01S 3/0387 Helical shape
• CPC Class Codes: H01S 3/0404 Air- or gas cooling, e.g. by dry nitrogen
• CPC Class Codes: H01S 3/041 for gas lasers H01S3/0401 takes precedence
• CPC Class Codes: H01S 3/08009 using a diffraction grating
• CPC Class Codes: H01S 3/08036 using intracavity dispersive, polarising or birefringent elements
• CPC Class Codes: H01S 3/097 by gas discharge of a gas laser
• CPC Class Codes: H01S 3/09702 Details of the driver electronics and electric discharge circuits
• CPC Class Codes: H01S 3/0971 transversely excited H01S3/0975 takes precedence
• CPC Class Codes: H01S 3/0975 using inductive or capacitive excitation
• CPC Class Codes: H01S 3/1024 : for pulse generation
• CPC Class Codes: H01S 3/104 : in gas lasers
• CPC Class Codes: H01S 3/105 : by controlling the mutual position or the reflecting properties of the reflectors of the cavity , e.g. by controlling the cavity lengthH01S3/10076, H01S3/13 take precedence
• CPC Class Codes: H01S 3/1055 : one of the reflectors being constituted by a diffraction grating
• CPC Class Codes: H01S 3/1305 : Feedback control systems
• CPC Class Codes: H01S 3/134 : in gas lasers
• CPC Class Codes: H01S 3/137 : for stabilising of frequency
• CPC Class Codes: H01S 3/139 : by controlling the mutual position or the reflecting properties of the reflectors of the cavity , e.g. by controlling the cavity length
• CPC Class Codes: H01S 3/22 : Gases
• CPC Class Codes: H01S 3/2207 : Noble gas ions, e.g. Ar+>, Kr+>
• CPC Class Codes: H01S 3/223 : the active gas being polyatomic, i.e. containing two or more atoms H01S3/227 takes precedence
• CPC Class Codes: H01S 3/225 : comprising an excimer or exciplex
• CPC Class Codes: H01S 3/2251 : ArF, i.e. argon fluoride is comprised for lasing around 193 nm
• CPC Class Codes: H01S 3/2256 : KrF, i.e. krypton fluoride is comprised for lasing around 248 nm
• CPC Class Codes: H01S 3/2258 : F2, i.e. molecular fluoride is comprised for lasing around 157 nm
• CPC Class Codes: H01S 3/2333 : Double-pass amplifiers
• CPC Class Codes: H01S 3/2366 : comprising a gas as the active medium H01S3/10092, H01S3/2383 take precedence

Abstract

The present invention provides gas discharge laser systems capable of reliable long-term operation in a production line capacity at repetition rates in the range of 6,000 to 10,0000 pulses power second. Preferred embodiments are configured as KrF, ArF and F₂ lasers used for light sources for integrated circuit lithography. Improvements include a modified high voltage power supply capable for charging an initial capacitor of a magnetic compression pulse power system to precise target voltages 6,000 to 10,0000 times per second and a feedback control for monitoring pulse energy and determining the target voltages on a pulse-by-pulse basis. Several techniques are disclosed for removing discharge created debris from the discharge region between the laser electrodes during the intervals between discharges. In one embodiment the width of the discharge region is reduced from about 3 mm to about 1 mm so that a gas circulation system designed for 4,000 Hz operation could be utilized for 10,000 Hz operation. In other embodiments the gas flow between the electrodes is increased sufficiently to permit 10,000 Hz operation with a discharge region width of 3 mm. To provide these substantial increased gas flow rates, Applicants have disclosed preferred embodiments utilize tangential forms of the prior art but with improved and more powerful motors and novel bearing designs. New bearing designs include both ceramic bearings and magnetic bearings. In other embodiments, some or all of the gas circulation power is provided with a blower located outside the laser chamber. The outside blower can be located in the laser cabinet or in separate location.

Claims

1. An extreme repetition rate gas discharge laser system capable of operating at pulse repetition rates in excess of 4,000 pulses per second, and providing light in the form of a laser output light pulses said laser comprising:

A) a laser discharge chamber containing a laser gas and having two elongated electrodes, defining a discharge region,

B) a blower system producing sufficient gas movement of said laser gas in said discharge region to clear from said discharge region, following each gas discharge, substantially all discharge produced ions prior to a next gas discharge when operating at a laser output light pulse repetition rate in the range of 6,000 laser output light pulses per second or greater,

C) a heat exchanger system removing heat energy from said laser gas,

D) a pulse power system providing electrical pulses to said electrodes suffcient to produce laser output light pulses at rates of about 6,000 laser output light pulses per second with precisely controlled laser output light pulse energies in the range of about 5 mJ and above, and

E) a laser output light pulse beam measurement and control system measuring laser output light pulse energy, wavelength and bandwidth of every laser output light pulse or substantially every laser output light pulse with feedback control of laser output light pulse energy and wavelength.

2. A laser as in claim 1 wherein said discharge laser system is a KrF gas discharge laser system and the laser gas is comprised of krypton, fluorine and neon.

3. A laser as in claim 1 wherein said discharge laser system is a ArF gas discharge laser system and the laser gas is comprised of argon, fluorine and neon.

4. A laser as in claim 1 wherein said discharge laser system is a F₂ gas discharge laser and the laser gas is comprised of fluorine and a buffer gas.

5. A laser as in claim 1 and wherein said chamber also comprises a vane structure upstream of said discharge region for normalizing gas velocity upstream of said discharge region.

6. A laser as in claim 1 where said blower system comprises a tangential fan mounted on a shaft driven by two brushless DC motors.

7. A laser as in claim 6 wherein said motors are water-cooled motors.

8. A laser as in claim 6 wherein each of said motors comprise a stator and each of said motors comprise a magnetic rotor contained in a pressure cup separating said stator from said laser gas.

9. A laser as in claim 6 wherein said tangential fan comprises a blade structure machined from a single block of aluminum stock.

10. A laser as in claim 9 wherein said blade structure has an outside diameter of about five inches.

11. A laser as in claim 6 wherein said motors are sensorless motors and further comprising a master motor controller for controlling one of said motors and a slave motor controller for controlling the other motor.

12. A laser as in claim 1 wherein said heat exchanger system is water-cooled.

13. A laser as in claim 12 wherein said heat exchanger system comprises at least four separate water-cooled heat exchangers.

14. A laser as in claim 13 wherein each of said four heat exchangers comprise a tubular water flow passage containing a turbulator.

15. A laser as in claim 12 wherein heat exchanger system comprises at least one heat exchanger having a tubular water flow passage wherein at least one turbulator is located in said passage.

16. A laser as in claim 1 wherein said pulse power system comprise water-cooled electrical components.

17. A laser as in claim 16 wherein at least one of said water-cooled components is a component operated at high voltages in excess of 12,000 volts.

18. A laser as in claim 17 wherein said high voltage is isolated from ground using an inductor through which cooling water flows.

19. A laser as in claim 1 wherein said pulse power system comprises a resonant charging system to charge a charging capacitor to a precisely controlled voltage.

20. A laser as in claim 19 wherein said resonance charging system comprises a De-Qing circuit.

21. A laser as in claim 19 wherein said resonance charging system comprises a bleed circuit.

22. A laser as in claim 19 wherein said resonant charging system comprises a De-Qing circuit and a bleed circuit.

23. A laser as in claim 1 wherein said pulse power system comprises a charging system comprised of at least three power supplies arranged in parallel.

24. A laser as in claim 1 wherein said laser beam measurement and control system comprises an etalon unit, a photo diode array, a programmable logic device, and optics to focus laser light from said etalon unit on to said photo diode array wherein said programmable logic device is programmed to analyze data from said photodiode array to determine locations on said photo diode array of etalon fringes.

25. A laser as in claim 24 wherein said measurement an control system also comprises a microprocessor programmed to calculate wavelength and bandwidth from fringe data located by said programmable logic device.

26. A laser as in claim 24 wherein said programmable logic device is programmed with an algorithm for calculating wavelength and bandwidth based on measurement of said fringes.

27. A laser as in claim 26 wherein said programmable logic device is configured to make calculations of wavelength and bandwidth faster than {fraction (1/10,000)} of a second.

28. A laser as in claim 24 wherein said etalon unit comprises a defractive diffusing element.

29. A laser as in claim 1 and further comprising a line narrowing unit comprising a tuning mirror driven at least in part by a PZT drive.

30. A laser as in claim 29 wherein said tuning mirror is also driven in part by a stepper motor.

31. A laser as in claim 29 and further comprising a pretuning means.

32. A laser as in claim 29 and further comprising an active tuning means comprising a learning algorithm.

33. A laser as in claim 29 and further comprising an adaptive feed forward algorithm.

34. A laser as in claim 29 wherein said line narrowing unit comprises a grating defining a grating face and a purge means for forcing purge gas adjacent to said grating face.

35. A laser as in claim 34 wherein said purge gas is nitrogen.

36. A laser as in claim 34 wherein said purge gas is helium.

37. A laser as in claim 29 and further comprising a beam seal system providing a first beam seal between a first window of said chamber and line narrowing unit and a second beam seal between a second window of said chamber and an output coupler unit, each of said beam seals comprising a metal bellows.

38. A laser as in claim 37 wherein each of said first and second beam seals permit easy replacement of said laser chamber.

39. A laser as in claim 37 wherein each of said beam seals contain no elastomer, provide vibration isolation from said chamber, provide beam train isolation from atmospheric gases and permit unrestricted replacement of said laser chamber without disturbance of said LNP or said output coupler unit.

40. A laser as in claim 1 and further comprising a nitrogen purge system comprising a nitrogen purge system comprising a nitrogen filter.

41. A laser as in claim 1 and further comprising a nitrogen purge module comprising flow monitors said laser also comprising purge exhaust tubes for transporting exhaust purge gas from said laser.

42. A laser as in claim 1 and further comprising a shutter unit comprising an electrically operated shutter and a power meter which can be positioned in a laser output beam path with a command signal.

43. A laser as in claim 1 wherein said measurement and control system comprises a primary beam splitter for splitting off a small percentage of each of the output pulses from said laser, a second beam splitter for directing a portion of said small percentage to said pulse energy detector and a means isolating a volume bounded said primary beam splitter, said secondary beam splitter and a window of said pulse energy detector from other portions of said measurement and control system to define an isolated region.

44. A laser as in claim 43 and further comprising a purge means for purging said isolated region with a purge gas.

45. A laser as in claim 44 wherein said laser further comprises an output coupler unit and an output window unit said purge means exhausting from said isolated region and at the same time purging said output coupler unit and said output window unit.

46. A laser as in claim 1 wherein said chamber also comprises a current return having ribs with a generally rectangular cross section with a long direction in a direction of laser gas flow.

47. A laser as in claim 1 wherein said chamber comprises an anode and dielectric spacers positioned on two sides of said anode to improve laser gas flow in regions between the two electrodes.

48. A laser as in claim 1 wherein said pulse power system comprises a corona plate electrically connected to capacitors of a peaking capacitor bank and one of said electrodes by metal rods having lengths chosen to provide increased inductance.

49. A laser as in claim 1 wherein said discharge region defines an average width and said electrodes are configured limit said average width to less than 2 mm.

50. A laser as in claim 49 wherein each of said electrodes define a discharge surface and at least one of said electrode is comprised of a conductor having its discharge surface covered by insulator material with a plurality of discharge holes.

51. A laser as in claim 50 wherein said holes are approximately round with diameters in the range of 50 to 200 microns.

52. A laser as in claim 51 wherein said diameters are about 100 microns.

53. A laser as in claim 50 where both electrodes have their discharge surfaces covered with insulator material with a plurality of discharge holes.

54. A laser as in claim 50 wherein said insulator material is alumina.

55. A laser as in claim 1 wherein said heat exchanger system comprises at least one finned water-cooled heat exchanger having du al pass cooling water flow direction in each tube in the opposite direction as compared to the other tube.

56. A laser as in claim 1 wherein said heat exchanger system comprises at least one water-cooled finned heat exchanger said a water flow control system comprising a proportional valve and a fast acting digital on-off valve.

57. A laser as in claim 1 wherein said heat exchanger system comprises a monolithic heat exchanger created by machining cooling fins in a portion of said chamber.

58. A laser as in claim 57 wherein said monolithic heat exchanger comprises a plurality of cooling tubes in thermal contact with said cooling fins.

59. A laser as in claim 1 and further comprising a preionizer comprised of an elongated conductor integrated within a high voltage insulator, said conductor being positioned in close proximity and parallel to one of said elongated electrodes.

60. A laser as in claim 1 and further comprising a suction means downstream of at least one of said elongated electrodes improving laser gas flow in said discharge region.

61. A laser as in claim 1 and further comprising ceramic bearings supporting a shaft of said blower.

62. A laser as in claim 61 wherein said ceramic bearings comprise ceramic cages.

63. A laser as in claim 61 wherein said ceramic bearings comprise a cageless race way, ceramic ball bearings and separation balls having smaller diameters than said ceramic ball bearings.

64. A laser as in claim 1 wherein said blower system comprises an injection circulator wherein a portion of the laser gas flow in said chamber is extracted from said chamber, pressurized and injected back into said chamber to produce a circulating laser gas flow.

65. A laser as in claim 1 wherein said blower system comprises a blower external to said laser discharge chamber.

66. A laser as in claim 65 wherein said heat exchanger system comprises a heat exchangers co-located with said blower.

67. A laser as in claim 65 wherein said heat exchanger system comprises a heat exchanger within said laser discharge chamber.

68. A laser as in claim 1 wherein said blower system comprises a blower driven by hydraulic turbine drive.

69. A laser as in claim 68 wherein said hydraulic turbine drive comprises a magnetic coupling, to couple hydraulic drive force into a shaft of a laser gas blower.