Four KHz gas discharge laser

• US 6,757,316 B2
• Filed: 05/11/2001
• Issued: 06/29/2004
• Est. Priority Date: 12/27/1999
• Status: Expired due to Term

• Current Assignee: Cymer Incorporated (ASML Holding NV)
• Original Assignee: Cymer Incorporated (ASML Holding NV)
• Inventors: Duffey, Thomas P., Fleurov, Vladimir B., Johns, David M., Anderson, Stuart L., Saethre, Robert B., Rettig, Curtis L., Shannon, Robert A., Pan, Xiaojiang J., Kulgeyko, Vladimir, Sandstrom, Richard L., Fomenkov, Igor V., Partlo, William N., Ujazdowski, Richard C., Ness, Richard M., Smith, Scott T., Melcher, Paul C., Algots, John M., Newman, Peter C., Spangler, Ronald L., Rokni, Shahryar
• Primary Examiner(s): Porta, David
• Assistant Examiner(s): MONBLEAU, DAVIENNE N
• Application Number:
  US09/854,097
• Publication Number:
  US 20020021728A1
• Time in Patent Office: 1,145 Days
• Field of Search: 372/20, 372/29.011, 372/57-59, 372/38.02, 372/38.04, 372/38.07, 372/25
• US Class Current: 372/57
• CPC Class Codes: G01J 1/4257 applied to monitoring the characteristics of a beam, e.g. laser beam, headlamp beam monitoring arrangements for lasers in general H01S3/0014
• CPC Class Codes: G01J 3/027 Control of working procedures of a spectrometer; Failure detection; Bandwidth calculation
• CPC Class Codes: G01J 9/0246 Measuring optical wavelength
• 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 7/70483 Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
• 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 3/02 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 3/2308 : Amplifier arrangements, e.g. MOPA

Abstract

The present invention provides an excimer laser capable of producing a high quality pulsed laser beam at pulse rates of about 4,000 Hz at pulse energies of about 5 mJ or greater. A preferred embodiment is an ArF excimer laser specifically designed as a light source for integrated circuit lithography. An improved wavemeter with special software monitors output beam parameters and controls a very fast PZT driven tuning mirror and the pulse power charging voltage to maintain wavelength and pulse energy within desired limits. In a preferred embodiment two fan motors drive a single tangential fan which provides sufficient gas flow to clear discharge debris from the discharge region during the approximately 0.25 milliseconds between pulses.

Claims

1. An extreme repetition rate gas discharge laser capable of producing laser light pulses operating at pulse repetition rates in the range of 4,000 pulses per second, said laser comprising:

A) a laser chamber containing a laser gas and having a first and a second elongated electrode, including a first electrode support defining a discharge region having a gas flow path with a gradually increasing cross section downstream of said first and second electrodes to permit recovery of a large percentage of static pressure drop occurring in the discharge region,

B) a tangential type fan for producing sufficient gas velocities of said laser gas in said discharge region to clear from said discharge region, following each laser light pulse, substantially all discharge produced ions prior to a next laser light pulse when operating at a repetition rate in the range of 4,000 laser light pulses per second or greater,

C) a heat exchanger system removing at least 16 kw of heat energy from said laser gas,

D) a pulse power system providing electrical pulses to said electrodes sufficient to produce laser light pulses at rates of about 4,000 pulses per second with precisely controlled laser light pulse energies in the range of about 5 mJ, and

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

2. A laser as in claim 1 wherein said gradually increasing cross section down stream of said discharge region increases at about 20 degrees.

3. A laser as in claim 2 and wherein said chamber also comprises a vane structure upstream of said discharge region for normalizing gas velocity upstream of said discharge region.

4. A laser as in claim 1 wherein said fan comprises a shaft driven by two brushless DC motors.

5. A laser as in claim 4 wherein said motors are water cooled motors.

6. A laser as in claim 4 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.

7. A laser as in claim 4 wherein said tangential fan comprise a blade structure machined from aluminum stock.

8. A laser as in claim 7 wherein said blade structure has an outside diameter of about five inches.

9. A laser as in claim 4 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.

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

11. A laser as in claim 10 wherein said heat exchanger system comprises at least four separate water cooled heat exchangers.

12. A laser as in claim 10 wherein heat exchanger system comprises at least one heat exchanger comprising a tubular water flow passage containing a turbulator.

13. A laser as in claim 11 wherein each of said four water cooled heat exchangers comprises a tubular water flow passage containing a turbulator.

14. A laser as in claim 1 wherein said pulse power system comprises water cooled electrical components.

15. A laser as in claim 14 wherein at least one of said water cooled electrical components is a component operated at high voltages in excess of 12,000 volts.

16. A laser as in claim 15 wherein said high voltage is isolated from ground using an inductor through which cooling water flows.

17. 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.

18. A laser as in claim 17 wherein said resonance charging system comprises a De-Qing circuit.

19. A laser as in claim 17 wherein said resonance charging system comprises a bleed circuit.

20. A laser as in claim 17 wherein said resonant charging system comprises a De-Qing circuit and a bleed circuit.

21. 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.

22. 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.

23. A laser as in claim 22 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.

24. A laser as in claim 22 wherein said programmable logic device is programmed with an algorithm for calculating wavelength and bandwidth based on measurement of said fringes.

25. A laser as in claim 24 wherein said programmable logic device makes calculations of wavelength and bandwidth faster than {fraction (1/4,000)} of a second.

26. A laser as in claim 22 wherein said etalon unit comprises a defractive diffusing element.

27. 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.

28. A laser as in claim 27 wherein said tuning mirror is also driven in part by a stepper motor.

29. A laser as in claim 27 and further comprising a pretuning means.

30. A laser as in claim 27 and further comprising an active tuning means comprising a learning algorithm.

31. A laser as in claim 27 and further comprising an adaptive feed forward algorithm.

32. A laser as in claim 27 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.

33. A laser as in claim 32 wherein said purge gas is nitrogen.

34. A laser as in claim 32 wherein said purge gas is helium.

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

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

37. 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.

38. A laser as in claim 27 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.

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

40. A laser as in claim 38 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.

41. A laser as in claim 1 wherein said measurement and control system comprises a primary beam splitter for splitting off a small percentage of the light from 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 by 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.

42. A laser as in claim 41 and further comprising a purge means for purging said isolated region with a purge gas.

43. A laser as in claim 42 wherein said laser further comprises an output coupler unit and an output window unit, said purge means exhausts from said isolated region and such exhaust from said isolated region also purges said output coupler unit and said output window unit.