Interferometer system and litographic step-and-scan apparatus provided with such a system

• US 6,795,197 B2
• Filed: 06/19/2002
• Issued: 09/21/2004
• Est. Priority Date: 06/21/2001
• Status: Expired due to Fees

• Current Assignee: Zygo Corporation (Ametek Incorporated)
• Original Assignee: Zygo Corporation (Ametek Incorporated)
• Inventors: Hill, Henry Allen
• Primary Examiner(s): Turner, Samuel A.
• Assistant Examiner(s): Lyons, Michael A.
• Application Number:
  US10/175,166
• Publication Number:
  US 20020196448A1
• Time in Patent Office: 825 Days
• Field of Search: 356/28.5, 356/485, 356/486, 356/500, 356/508, 356/509
• US Class Current: 356/486
• CPC Class Codes: G01B 9/02003 using beat frequencies
• CPC Class Codes: G01B 9/02045 using the Doppler effect
• CPC Class Codes: G01B 9/02083 characterised by particular signal processing and presentation
• CPC Class Codes: G03F 7/70775 Position control, e.g. interferometers or encoders for determining the stage position

Abstract

Interferometric apparatus and methodology for monitoring the relative motion among objects, preferably that of mask and wafer stages in photolithographic processes. The apparatus comprises a plurality of interferometers with each operating to provide a mixed optical interference signal containing phase information indicative of the motion of a corresponding object. Electrical interference signals are generated from the optical interference signals, and one of these is modified to compensate for any Doppler shift differences among the electrical interference signals caused by differences in preferred relative rates of motion in the objects. A mixer receives the electrical interference signals and the modified electrical interference signal and generates an output electrical interference signal containing information about the relative motion between objects. In another aspect of the invention the plurality of interferometers are configured to optically compensate for any Doppler shift differences among the mixed optical interference signals.

Claims

1. Interferometric apparatus for monitoring the relative motion among objects, said apparatus comprising:

a plurality of interferometers for monitoring the motion of objects, each of said plurlity of interferometers operating to provide a mixed optical interference signal containing phase information indicative of the motion of a corresponding object;

detector means for receiving said first mixed optical interference signal and generating electrical interference signal containing information corresponding to the motion of each object;

electronic processing means for receiving at least one select one said electrical interference signal and generating a corresponding modified electrical interference signal that is compensated for any Doppler shift differences among said electrical interference signals caused by differences in preselected relative rates of motion in said object; and

electronic mixing means for receiving said electrical interference signal and said modified electrical interference signal and generating an output electrical interference signal containing information about the relative motion between object.

2. Interferometric apparatus for monitoring the relative motion among objects, said apparatus comprising:

a plurality of interferometers for monitoring the motion of objects, each of said plurality of interferometers having a measurement beam that travels over a predetermined optical path and operating to provide a mixed optical interference signal containing phase information indicative of the motion of a corresponding object, the measurement beam of at least one of said interferometers traveling over its corresponding predetermined optical path a different number of passes than that traveled by said measurement beams of said other interferometers to compensate for any Doppler shift differences among said mixed optical interference signals caused by differences in preselected relative rates of motion between said objects;

detector means for receiving said first mixed optical interference signal and generating electrical interference signal containing information corresponding to the motion of each object;

electronic mixing means for receiving said electrical interference signals and said modified electrical interference signal and generating an output electrical interference signal containing, information about the relative motion between objects.

3. Interferometric apparatus for monitoring the relative motion among objects, said apparatus comprising:

a first interferometer for providing a first mixed optical interference signal containing phase information indicative of the motion of a first object;

first detector means for receiving said first mixed optical interference signal and generating a first electrical interference signal containing information corresponding to the motion of said first object;

at least one other interferometer for providing another mixed optical interference signal containing phase information indicative of the motion of another object;

at least one other detector means for receiving said other mixed optical interference signal and generating another electrical interference signal containing information corresponding to motion of said other object;

electronic processing means for receiving said other electrical interference signal and generating a modified electrical interference signal that is compensated for any Doppler shift differences in said first and other electrical interference signals caused by differences in preselected relative rates of motion in said first and other object; and

electronic mixing means for receiving said first electrical interference signal and said modified electrical interference signal and generating an output electrical interference signal containing information about the relative motion between said first and other object.

4. The interferometric apparatus of claim 3 wherein said electronic processing means is configured and arranged to multiply the phase of said other electrical interference signal by a multiplier corresponding to the ratio of the preselected relative rates of motion of said first and other object to generate said modified electrical interference signal.

5. The interferometric apparatus of claim 3 wherein first object comprises a photolithographic reticle stage with a first rate of motion and wherein said other object comprises a photolithographic wafer stage having another rate of motion no faster than that of said first rate of motion.

6. The interferometric apparatus of claim 3 wherein said first interferometer comprises a source having a split frequency of f₂ and a Doppler frequency shift of _D2, wherein said other interferometer comprises a source having a split frequency of f₁ and a Doppler frequency shift of _D1, wherein the ratio of said preselected relative rate of motion of said first object to said other object is 1/n, and wherein said differences in Doppler shifts caused by differences in said preselected relative rates of motion are compensated in accordance with the following:

(nf₁−

f₂)+(n_D1−

_D2)

where |nf₁−

f₂|>

|n_D1−

_D2|.

7. The interferometric apparatus of claim 6 wherein n is an integer having a value over the range including 1 and at least 4.

8. The interferometric apparatus of claim 3 wherein said first, other, and modified electrical interference signals comprise heterodyne signals.

9. The interferometric apparatus of claim 8 wherein said electronic mixing means is configured to multiply said first and said modified heterodyne electrical interference signals to generate said output signal as a superheterodyne signal.

10. Interferometric apparatus for monitoring the relative motion among objects, said apparatus comprising:

a first interferometer having a measurement beam that travels over a predetermined optical path for providing a first mixed optical interference signal containing phase information indicative of the motion of a first object;

first detector means for receiving said first mixed optical interference signal and generating a first electrical interference signal containing information corresponding to the motion of said first object;

at least one other interferometer having a measurement beam that travels over a predetermined optical path a different number of passes than that traveled by said measurement beam of said first interferometer for providing another mixed optical interference signal containing phase information indicative of the motion of another object, said other mixed optical interference signal compensating for any Doppler shift differences in said first and other mixed optical interference signals caused by differences in preselected relative rates of motion of said first and other object;

at least one other detector means for receiving said other mixed optical interference signal and generating another electrical interference signal containing information corresponding to motion of said other object; and

electronic mixing means for receiving said first electrical interference signal and said other electrical interference signal and generating an output electrical interference signal containing information about the relative motion between said first and other object.

11. The interferometric apparatus of claim 10 wherein first object comprises a photolithographic reticle stage with a first rate of motion and wherein said other object comprises a photolithographic wafer stage having another rate of motion no faster than that of said first rate of motion.

12. The interferometric apparatus of claim 10 wherein said first interferometer comprises a source having a split frequency of f₂ and a Doppler frequency shift of _D2, wherein said other interferometer comprises a source having a split frequency of f₁ and a Doppler frequency shift of _D2, and wherein differences in said Doppler shifts caused by differences in the preselected relative rates of motion are compensated in accordance with the following:

(f₁−

f₂)+(_D1−

_D2)

where |f₁−

f₂|>

|_D1−

_D1|.

13. The interferometric apparatus of claim 10 wherein the ratio of the rate of motion of said first object to said other object is 1/n where n is an integer having a value between the range including 1 and at least 4.

14. The interferometric apparatus of claim 10 wherein the ratio of the number of passes of the respective measurement beams of said first and other interferometer corresponds to the ratio of their respective preferred rates of motion.

15. The interferometric apparatus of claim 10 wherein said first and other, electrical interference signals comprise heterodyne signals.

16. The interferometric apparatus of claim 15 wherein said output electrical interference signal comprises a superheterodyne signal.