DUV and VUV laser with on-line pulse energy monitor

• US 20030219057A1
• Filed: 05/14/2003
• Published: 11/27/2003
• Est. Priority Date: 05/17/2002
• Status: Active Grant

• Current Assignee: Coherent GmbH (Coherent Corp.)
• Original Assignee: Coherent GmbH (Coherent Corp.)
• Inventors: Hua, Gongxue, Govorkov, Sergei V.
• Granted Patent: US 6,970,492 B2
• Time in Patent Office: Days
• US Class Current: 372/57
• CPC Class Codes: H01S 3/0014 Monitoring arrangements not otherwise provided for photometry G01J1/00, e.g. G01J1/4257; radiation pyrometry G01J5/00; measuring coherence of light G01J9/00; measuring wavelength of light G01J9/00, e.g. G01J9/0246; measuring optical pulses G01J11/00; calorimetrically measuring power of laser beams G01K17/003
• CPC Class Codes: H01S 3/034 Optical devices within, or forming part of, the tube, e.g. windows, mirrors reflectors having variable properties or positions for initial adjustment of the resonator H01S3/086
• CPC Class Codes: H01S 3/225 comprising an excimer or exciplex
• CPC Class Codes: H01S 3/2258 F2, i.e. molecular fluoride is comprised for lasing around 157 nm

Abstract

A beam parameter monitoring unit for coupling with an excimer or molecular fluorine (F₂) laser resonator that produces an output beam having a wavelength below 200 nm includes an on-line laser pulse energy detector. This, in turn, allows output pulse energy stabilization to the same degree of accuracy, which is crucial for stability of exposure dose and other process parameters in microlithography and industrial applications.

Claims

1. An excimer or molecular fluorine laser system, comprising:

a laser tube containing a gas mixture at least including a halogen-containing species and a buffer gas, and including a heat exchanger and a gas circulation fan therein;

multiple electrodes within the laser tube connected to a pulsed electrical discharge circuit for energizing the gas mixture at a repetition rate greater than 1 kHz;

a resonator for generating a sub-250 nm output laser beam; and

a substantially transmissive energy monitor window in the beam path of the output laser beam.

2. The system of claim 1, the energy monitor window comprising a material that fluoresces upon absorption of a portion of the incident light of the sub-250 nm output laser beam.

3. The system of claim 2, wherein the energy monitor window comprises one or more materials selected from group of materials consisting of CaF₂, BaF₂, MgF₂, sapphire, LiSrAIF₆, LiCaAIF₆, crystalline quartz, and at least substantially OH-free fused silica.

4. The system of claim 3, wherein the group of materials further consists of SrF₂ and LiF.

5. The system of claim 2, further comprising one or more photodetectors for detecting fluorescence intensities emanating from the energy monitor window.

6. The system of claim 2, further comprising one or more spectral filters for filtering wavelengths from the fluorescence to be incident on the one or more photodetectors.

7. The system of claim 6, the wavelengths filtered exhibiting greater thermal fluorescence variation than wavelengths not filtered by the one or more spectral filters.

8. The system of claim 7, the wavelengths filtered exhibiting faster thermal fluorescence responses than wavelengths not filtered by the one or more spectral filters.

9. The system of claim 6, the wavelengths filtered exhibiting faster thermal fluorescence responses than wavelengths not filtered by the one or more spectral filters.

10. The system of claim 6, the laser system being a molecular fluorine laser system, and the wavelengths filtered by the one or more spectral filters include a red atomic fluorine emission.

11. The system of claim 1, the resonator comprising an output coupler which seals the laser tube and output couples the output laser beam to be incident on the energy monitor window.

12. The system of claim 1, further comprising at least one Brewster window sealing the laser tube, and a partially reflecting output coupler for output coupling the output laser beam to be incident on the energy monitor window.

13. The system of claim 1, further comprising at least one Brewster window sealing the laser tube, and the energy monitor window further for output coupling the output laser beam.