Method for inspecting a reticle

• US 6,614,520 B1
• Filed: 12/18/1997
• Issued: 09/02/2003
• Est. Priority Date: 12/18/1997
• Status: Expired due to Term

• Current Assignee: KLA-Tencor Corporation (KLA Corporation)
• Original Assignee: KLA-Tencor Corporation (KLA Corporation)
• Inventors: Desplat, Christian G., Bareket, Noah, Glasser, Lance A.
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US08/993,107
• Time in Patent Office: 2,084 Days
• Field of Search: 356/394, 356/398, 356/237.3, 356/237.4, 356/239.8
• US Class Current: 356/237.3
• CPC Class Codes: G01N 21/95607 using a comparative method

Abstract

Disclosed is a method of inspecting a reticle for defects that occur over time. The invention accomplishes this by generating and storing a “baseline” image of the reticle and then periodically generating a “current” image of the reticle and comparing the current and baseline images. The baseline image is taken at a time when the reticle is known to be acceptable. This may be when the reticle has been “qualified” by an optical test or when a die fabricated by reticle has passed an electrical test. Also disclosed in a method for compacting the baseline image before storage.

Claims

1. A method for identifying a defect in a reticle containing features that scatter light, which features together define a pattern to be transferred a substrate surface, the method comprising:

providing a baseline image of said reticle which baseline image was created while the reticle was qualified as being of acceptable quality;

generating a current image of the reticle; and

comparing the baseline and current images wherein the differences between these images identify defects that may have arisen in the time between when the baseline image was created and when the current image is generated.

2. The method of claim 1, wherein said features that scatter light include features that modulate light transmission.

3. The method of claim 2, wherein said features that scatter light include features that are opaque or transparent to light.

4. The method of claim 1, wherein the current image is generated by scanning said reticle to develop an image of said reticle.

5. The process of claim 4, wherein each pixel of the current image is generated by a method comprising:

illuminating a region of said reticle by an incident beam generated by an illuminating source;

detecting a scattered energy distribution from said region of said reticle by a detector; and

recording said scattered energy distribution from said region of said reticle.

6. The process of claim 1, wherein comparing the baseline and current images includes:

determining whether intensity of scattered radiation at a first location of the current image is greater than a defined threshold;

if the scattered radiation from the first location is greater than the defined threshold, determining whether an absolute value of a difference between the intensity of scattered light at the first location and the intensity of scattered light in a corresponding region of the baseline image of the region exceeds a predefined deviation value; and

if the absolute value of the difference between the intensity of scattered light at the first location and the intensity of scattered light in the corresponding region of the baseline image of the region exceeds a predefined deviation value, flagging a defect at the first location.

7. A method of inspecting an object, comprising:

(a) providing an object to be inspected;

(b) storing a baseline image of the object at a time when the object is known to be of acceptable quality;

(c) inspecting the object at a time when the object is not known to be of acceptable quality;

(d) comparing data derived from the inspection with data derived from the stored baseline image; and

(e) identifying defects on the object based on the comparison.

8. The method of claim 7, wherein the object is a reticle.

9. The method of claim 7, wherein the stored baseline image contains data correlating the intensity of radiation scattered from the object with locations of the object.

10. The method of claim 9, wherein storing the baseline image includes compacting the baseline image to reduce the quantity of stored data for portions of the image where the intensity of the scattered radiation does not exceed a defined threshold.

11. The method of claim 10, wherein compacting includes the following:

segmenting the baseline image into regions of the object; and

removing data from the baseline image for those regions of said object where the intensity of the scattered radiation does not exceed the defined threshold.

12. The method of claim 7, wherein the object is known to be of acceptable quality by subjecting it to an optical qualification technique.

13. The method of claim 8, wherein the object is known to be of acceptable quality by subjecting it to a method comprising:

electronically testing at least one die fabricated on al substrate by process employing and reticle; and

determining whether said at least one die passes the testing, in which case the reticle is qualified.

14. The method of claim 7, wherein inspection is performed by generating a darkfield image of the object.

15. The method of claim 8, wherein inspection of the object includes measuring and recording scattered light from the object as a function of position on the reticle.

16. The method of claim 7, wherein defects are identified at locations wherein the inspection data and the baseline image data deviate from one another.

17. A method of providing a baseline image of a reticle for use in subsequent inspections of the reticle, comprising:

(a) providing the reticle to be inspected;

(b) generating data specifying intensity of radiation scattered from the reticle as a function of location on the reticle;

(c) defining a first portion of the data which was derived from a first region on the reticle;

(d) applying a plurality of imaging algorithms to the first portion of the data;

(e) selecting a first imaging algorithm from among the plurality of imaging algorithms based upon ability to suppress scattered radiation from valid features on the first region of the reticle; and

(f) associating the first imaging algorithm with the first portion of data in the baseline image, such that during the subsequent inspections of the reticle the first imaging algorithm is applied to the first portion of the data to provide an image of the first region of the reticle.

18. The method of claim 17, further comprising ranking the plurality of imaging algorithms according to ability to suppress scattered radiation from valid features on the reticle.

19. The method of claim 17, further comprising storing in memory an association of the first imaging algorithm with the first region of the reticle.

20. The method of claim 17, wherein generating data specifying intensity of radiation comprises:

(i) carrying out a first scan of the reticle under a first apparatus setting for determining intensity of radiation scattered from the reticle as a function of location on the reticle;

(ii) carrying out a second scan of the reticle under a second apparatus setting which are different from the first apparatus setting;

(iii) selecting an apparatus setting based upon ability to suppress scattered radiation from valid features on the reticle.

21. The method of claim 20, wherein (iii) is performed for each of a plurality of regions of the reticle.

22. The method of claim 21, further comprising associating a selected apparatus setting with one or more regions of the reticle, such that during the subsequent inspections of the reticle data from the selected apparatus setting may be used to image the one or more regions.

23. A system for identifying a defect in a reticle containing transparent regions and opaque regions defining a pattern to be transferred a substrate surface, the system comprising:

a baseline image stored in a memory, the baseline image being an image of the reticle taken while the reticle was known to be of acceptable quality;

an imaging system arranged to generate a current image of the reticle, which current image corresponds to the baseline image; and

a processing unit configured to compare the baseline and current images and thereby identify any new defects that may have arisen in the time between when the baseline image was created and when the current image is generated.

24. The system of claim 23, wherein the baseline image contains data correlating the intensity of radiation scattered from the object with locations of the object.

25. The system of claim 23, wherein the baseline image is compacted.

26. The system of claim 23, wherein the imaging system includes:

a source of illumination oriented to direct light onto a specified location of the reticle; and

one or more detectors oriented to detect light from the source which has been scattered by the reticle.