Apparatus for optical system coherence testing

• US 7,161,684 B2
• Filed: 05/31/2005
• Issued: 01/09/2007
• Est. Priority Date: 02/15/2000
• Status: Expired due to Fees

• Current Assignee: ASML Holding NV
• Original Assignee: ASML Holding NV
• Inventors: Hansen, Matthew E.
• Primary Examiner(s): CONNOLLY, PATRICK J
• Application Number:
  US11/139,479
• Publication Number:
  US 20050219550A1
• Time in Patent Office: 588 Days
• Field of Search: 356/450, 356/521
• US Class Current: 356/521
• CPC Class Codes: G01J 2009/0211 for measuring coherence
• CPC Class Codes: G01J 9/02 by interferometric methods using interferometers for measuring optically the linear dimensions of objects G01B9/02
• CPC Class Codes: G03F 1/44 Testing or measuring features, e.g. grid patterns, focus monitors, sawtooth scales or notched scales
• CPC Class Codes: G03F 7/70025 by lasers
• CPC Class Codes: G03F 7/70583 Speckle reduction, e.g. coherence control or amplitude/wavefront splitting
• CPC Class Codes: G03F 7/70591 Testing optical components

Abstract

The present invention is directed at a coherence test reticle or lithographic plate, and a method for testing the coherence of a laser beam using the test reticle. The quality or coherence of the laser beam is measured by illuminating the test reticle and recording and/or analyzing the optical patterns generated by the illumination. The technique was designed for the characterization of laser-based systems via the detection of optical radiation modulated by transmissive, reflective and diffractive patterns printed on a reticle or lithographic plate designed specifically for this purpose. The novelty and advantages over the prior art are insensitivity to vibration, alignment, and multi-path differences associated with classical interferometric coherence measurement techniques. The technique is focus error insensitive. The robustness and convenience of the technique is driven by the use of a single plate with no optical alignment, making the technique easily implemented in the field.

Claims

1. An apparatus for optical system coherence testing comprising a transparent plate, wherein said transparent plate is made to be opaque on a surface in all areas except for an area of a pattern, wherein said pattern comprises two elongated areas, wherein said two elongated areas have a width of a dimension that would cause coherent light from the optical system to diffract upon transmitting through said area of said pattern, wherein said two elongated areas are joined at a common point, wherein said two elongated areas diverge from said common point to form an angle, and wherein an interior of said two elongated areas includes a diffraction grating pattern.

2. The apparatus of claim 1, wherein said diffraction grating pattern is arranged to diffract light in both a horizontal and a vertical direction.

3. The apparatus of claim 1, wherein said interior of said two elongated areas includes a second diffraction grating pattern, wherein said diffraction grating pattern has a first measure of pitch and wherein said second diffraction grating pattern has a second measure of pitch.

4. The apparatus of claim 3, wherein said diffraction grating pattern with said first measure of pitch is located within a first of said two elongated areas and wherein said second diffraction grating pattern with said second measure of pitch is located within a second of said two elongated areas.

5. The apparatus of claim 1, wherein said width includes a first width and a second width, wherein a first of said two elongated areas has said first width, and wherein a second of said two elongated areas has said second width.

6. The apparatus of claim 1, wherein said two elongated areas have a straight shape.

7. The apparatus of claim 1, wherein said two elongated areas have a curved shape.

8. The apparatus of claim 1, wherein said two elongated areas are aligned symmetrically with respect to an orientation of light from the optical system.

9. The apparatus of claim 1, wherein said two elongated areas are aligned asymmetrically with respect to an orientation of light from the optical system.

10. The apparatus of claim 1, wherein said pattern is repeated at other locations on said transparent plate.

11. The apparatus of claim 1, wherein said pattern further comprises another two elongated areas, wherein said four elongated areas form a diamond shape.

12. The apparatus of claim 1, wherein the apparatus is a reticle.

13. The apparatus of claim 12, further comprising a spacing device attached to a surface of said reticle.

14. The apparatus of claim 13, wherein said spacing device is a piezoelectric spacer.

15. The apparatus of claim 13, wherein said spacing device is a transmissive crystal.

16. The apparatus of claim 15, wherein said transmissive crystal is wedge-shaped.

17. The apparatus of claim 15, wherein said transmissive crystal is piezoelectric.

18. A reticle for optical system coherence testing, comprising:

a transparent plate, wherein said transparent plate is made to be opaque on a first surface in all areas except for an area of a pattern, wherein said pattern comprises two elongated areas, wherein said two elongated areas have a width of a dimension that would cause coherent light from the optical system to diffract upon transmitting through said area of said pattern, wherein said two elongated areas are joined at a common point, and wherein said two elongated areas diverge from said common point to form an angle;

a spacing device attached to one of said first surface and a second surface of said transparent plate; and

a recording medium attached to an opposite surface of said spacing device.

19. The apparatus of claim 18, wherein said recording medium is photographic.

20. The apparatus of claim 18, wherein said recording medium is electronic.

21. The apparatus of claim 18, further comprising a phosphorus film placed between said recording medium and said spacing device.

22. The apparatus of claim 18, wherein the apparatus is mounted within a tube, wherein said tube is designed to replace a section of tube in the optical system.

23. The apparatus of claim 18, further comprising a demodulator reticle attached between said spacing device and said recording medium.

24. The apparatus of claim 23, further comprising a phosphorus film placed between said recording medium and said demodulator reticle.

25. The apparatus of claim 23, wherein the apparatus is mounted within a tube, wherein said tube is designed to replace a section of tube in the optical system.

26. A reticle for optical system coherence testing, comprising:

a transparent plate, wherein said transparent plate is made to be opaque on a first surface in all areas except for an area of a pattern, wherein said pattern comprises two elongated areas, wherein said two elongated areas have a width of a dimension that would cause coherent light from the optical system to diffract upon transmitting through said area of said pattern, wherein said two elongated areas are joined at a common point, and wherein said two elongated areas diverge from said common point to form an angle;

a spacing device attached to one of said first surface and a second surface of said transparent plate; and

a demodulator reticle attached to an opposite surface of said spacing device.

27. The apparatus of claim 26, further comprising a phosphorus film placed between said recording medium and said demodulator reticle.

28. The apparatus of claim 26, wherein the apparatus is mounted within a tube, wherein said tube is designed to replace a section of tube in the optical system.