Fast wavelength correction technique for a laser

• US 6,529,531 B1
• Filed: 06/19/2000
• Issued: 03/04/2003
• Est. Priority Date: 07/22/1997
• Status: Expired due to Term

• Current Assignee: Cymer Incorporated (ASML Holding NV)
• Original Assignee: Cymer Incorporated (ASML Holding NV)
• Inventors: Everage, George J., Sandstrom, Richard L., Das, Palash P., Fomenkov, Igor V., Algots, John M., Erie, Frederick G.
• Primary Examiner(s): Ip, Paul
• Assistant Examiner(s): MONBLEAU, DAVIENNE N
• Application Number:
  US09/597,812
• Time in Patent Office: 988 Days
• Field of Search: 372/20, 372/29.011, 372/57-59, 372/102, 372/38.01
• US Class Current: 372/20
• 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 3/036 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 3/104 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 3/13 Stabilisation of laser output parameters, e.g. frequency or amplitude
• CPC Class Codes: H01S 3/134 in gas lasers
• 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 3/225 comprising an excimer or exciplex

Abstract

Electric discharge laser with fast chirp correction. Fast wavelength chirp correction equipment includes at least one piezoelectric drive and a fast wavelength detection means and has a feedback response time of less than 1.0 millisecond. In a preferred embodiment a simple learning algorithm is described to allow advance tuning mirror adjustment in anticipation of the learned chirp pattern. Techniques include a combination of a relatively slow stepper motor and a very fast piezoelectric driver. In another preferred embodiment chirp correction is made on a pulse-to-pulse basis where the wavelength of one pulse is measured and the wavelength of the next pulse is corrected based on the measurement. This correction technique is able to function at repetition rates as rapid as 2000 Hz and greater.

Claims

1. An electric discharge laser with active wavelength chirp mitigation comprising:

A) a laser chamber,

B) an elongated electrode structure enclosed within said chamber comprising an elongated anode and an elongated cathode separated by comprising an elongated anode and an elongated cathode separated by a distance defining a discharge region, said discharge region defining a long dimension in a beam direction,

C) a laser gas contained in said chamber,

D) a fan for circulating said laser gas within said chamber and through said discharge region, and

E) an active chirp mitigation means, for mitigating wavelength chirp comprising;

1) a fast wavelength detection means,

2) a fast wavelength control means including a tuning mirror driven by a piezoelectric drive,

3) a feedback circuit means providing feedback control having a response time of less than 10 millisecond, and

4) a tuning mirror adjusting means for adjusting positions of said tuning mirror in advance of a burst of pulses said adjusting means comprising a processor programmed with a learning algorithm for learning shapes of chirps occurring early in a burst of pulses.

2. A laser as in claim 1 wherein said wavelength control means comprises a stepper motor.

3. A laser as in claim 1 wherein said active chirp mitigation means also comprises a stepper motor having an external spindle.

4. A laser as in claim 3 wherein said active chirp mitigation means also comprises a lever arm pivoted about a pivot axis to provide a de-magnification of linear movements of said external spindle.

5. A laser as in claim 1 wherein said active chirp mitigation means comprises a stepper motor for coarse wavelength control and a piezoelectric device for fine wavelength control.

6. An electric discharge laser with active wavelength chirp mitigation comprising:

A) a laser chamber,

B) an elongated electrode structure enclosed within said chamber comprising an elongated anode and an elongated cathode separated by a distance defining a discharge region, said discharge region defining a long dimension in a beam direction,

C) a laser gas contained in said chamber,

D) a fan for circulating said laser gas within said chamber and through said discharge region,

E) a wavemeter for measuring centerline wavelength,

F) a wavelength tuning mechanism including a tuning mirror and at least one piezoelectric drive for driving said tuning mirror,

G) a feedback control system for controlling said tuning mechanism using measurement information from said wavemeter in order to actively control wavelength chirp, and

H) an adjusting mechanism for adjusting position of the tuning mirror in advance of a burst of pulses to mitigate wavelength chirps occurring in early parts of the burst.

7. A laser as in claim 6 wherein the early occurring chirp has a duration of less than one millisecond.

8. A laser as in claim 6 wherein said adjusting mechanism comprises a stepper motor.

9. A laser as in claim 6 wherein said adjusting mechanism comprises a processor programmed with a learning algorithm for learning the shape of the early occupying chirps.

10. A laser as in claim 6 wherein said tuning mechanism also comprises a stepper motor having an external spindle.

11. A laser as in claim 10 wherein said tuning mechanism also comprises a lever arm pivoted about a pivot axis to provide a de-magnification of linear movements of said external spindle.

12. A laser as in claim 6 wherein said tuning mechanism comprises a stepper motor for coarse tuning and a piezoelectric device for fine tuning.