Method and apparatus for reticle inspection using aerial imaging
A reticle inspection system for inspecting reticles can be used as an incoming inspection tool, and as a periodic and pre-exposure inspection tool. Mask shops can use it as an inspection tool compatible to their customers, and as a printable error detection tool. The inventive system detects two kinds of defects: (1) line width errors in the printed image; (2) surface defects. The line width errors are detected on the die area. The detection is performed by acquiring the image of the reticle under the same optical conditions as the exposure conditions, (i.e. wavelength, numerical aperture, sigma, and illumination aperture type) and by comparing multiple dies to find errors in the line width. Surface defects are detected all over the reticle. The detection of surface defects is performed by acquiring transmission and dark-field reflection images of the reticle and using the combined information to detect particles, and other surface defects.
1. An apparatus for inspecting a multiple die reticle that is used with an optical exposure system under a set of exposure conditions, said multiple die reticle including at least a first die and a second die, said apparatus comprising:
an optical system simulating said set of exposure conditions of said optical exposure system;
a scanner for acquiring a plurality of aerial images of said multiple die reticle under said set of exposure conditions; said plurality of aerial images of said reticle comprising a first plurality of aerial images of said first die and a second plurality of aerial images of said second die; and
an image processing module for detecting variations in line width of said first die of said reticle using said first plurality of aerial images of said first die and said second plurality of aerial images of said second die of said multiple die reticle.
2. The apparatus according to claim 1 , wherein said scanner comprises a plurality of cameras for acquiring said plurality of aerial images of said multiple die reticle.
3. The apparatus according to claim 2 , wherein said plurality of cameras comprises:
a first camera for acquiring a first image of said plurality of aerial images of said multiple die reticle;
a second camera for acquiring a second image of said plurality of aerial images of said multiple die reticle; and
a third camera for acquiring a third image of said plurality of aerial images of said multiple die reticle.
4. The apparatus according to claim 3 , wherein:
said first camera is in focus;
said second camera is out of focus in a positive direction; and
said third camera is out of focus in a negative direction.
5. The apparatus according to claim 3 , wherein said scanner further comprises:
a transmission light illumination system for illuminating said reticle;
a dark field illumination system for illuminating said reticle; and
an image acquisition module for collecting light emerging from said reticle and creating aerial images of said reticle in said first, said second, and said third cameras.
6. The apparatus according to claim 5 , wherein said optical system comprises a numerical aperture diaphragm for reproducing said set of exposure conditions.
7. The apparatus according to claim 5 , wherein said dark field illumination system is positioned adjacent to said image acquisition module.
8. The apparatus according to claim 5 , wherein said dark field illumination system is coaxial with said optical system.
9. The apparatus according to claim 5 , wherein said dark field illumination system and said transmission light illumination system are positioned on opposite sides of said reticle.
10. The apparatus according to claim 2 , wherein:
said scanner further comprises a light source for illuminating said reticle with an illuminating light; and
said plurality of cameras are sensitive to said illuminating light.
11. The apparatus according to claim 10 , wherein said light source is a pulsating light source.
12. The apparatus according to claim 11 , wherein said pulsating light source is a pulsating laser.
13. The apparatus according to claim 11 , further comprising a stage on which said reticle is placed, and means for moving said stage in a continuous manner.
14. The apparatus according to claim 1 , further comprising a stage on which said reticle is placed, and means for moving said stage in a continuous manner.
15. An apparatus for inspecting a reticle that is used with an optical exposure system under a set of exposure conditions, said apparatus comprising:
an optical system simulating said set of exposure conditions of said optical exposure system;
a scanner for acquiring a first plurality of aerial images of said reticle in a transmitted light under said set of exposure conditions and a second plurality of dark field images of said reticle in a reflected light; and
an image processing module for detecting defects in said reticle using said first plurality of aerial images of said reticle and said second plurality of dark field images of said reticle.
16. The apparatus according to claim 15 , wherein said scanner further comprises:
a transmission light illumination system for illuminating a first face of said reticle;
a dark field illumination system for illuminating a second face of said reticle; and
an imaging module for collecting light emerging from said reticle and acquiring said first and said second pluralities of aerial images of said reticle.
17. An apparatus for inspecting a multiple die reticle that is used with an optical exposure system under a set of exposure conditions, said multiple die reticle including at least a first die and a second die, said apparatus comprising:
a light source;
transmission light illumination means for illuminating said reticle;
optical means for producing a plurality of magnified aerial images of said reticle under said set of exposure conditions, said optical means having a numerical aperture diaphragm for reproducing said set of exposure conditions;
imaging means for acquiring said plurality of magnified aerial images of said reticle; said plurality of magnified aerial images of said reticle comprising a first plurality of aerial images of said first die and a second plurality of aerial images of said second die; and
image processing means for analyzing a condition of said reticle using said first plurality of aerial images of said first die and said second plurality of aerial images of said second die.
18. The apparatus according to claim 17 , wherein said light source is a pulsating light source.
19. The apparatus according to claim 18 , further comprising a stage on which said reticle is placed, and means for moving said stage in a continuous manner.
20. The apparatus according to claim 18 , wherein said pulsating light source is a pulsating laser.
21. The apparatus according to claim 17 , further comprising a stage on which said reticle is placed, and means for moving said stage in a continuous manner.
22. The apparatus according to claim 17 , further comprising a dark field illumination means for illuminating said reticle.
23. The apparatus according to claim 17 , wherein said transmission light illumination means and said dark field illumination means are positioned on opposite sides of said reticle.
24. The apparatus according to claim 17 , wherein said imaging means further comprises a plurality of cameras for acquiring said plurality of magnified aerial images of said reticle when the reticle is illuminated by said transmission light illumination means.
25. The apparatus according to claim 24 , wherein said plurality of cameras comprises:
a first camera for acquiring a first image of said plurality of magnified aerial images of said reticle;
a second camera for acquiring a second image of said plurality of magnified aerial images of said reticle; and
a third camera for acquiring a third image of said plurality of magnified aerial images of said reticle; said first, said second and said third aerial images of said reticle being respectively acquired by said first, said second and said third cameras when the reticle is illuminated by said transmission light illumination means.
26. The apparatus according to claim 25 , wherein:
said first camera is in focus;
said second camera is out of focus in a positive direction; and
said third camera is out of focus in a negative direction.
27. The apparatus according to claim 26 , wherein:
said first camera acquires a fourth image of said plurality of dark field images of said reticle, said fourth image being acquired when said reticle is illuminated by said dark field illumination system.
28. The apparatus according to claim 27 , wherein said image processing means uses said fourth image to identify defects in said reticle.
29. The apparatus according to claim 25 , wherein:
a wavelength of the light source is identical to the wavelength of the exposure system; and
said first, said second, and said third cameras are sensitive to said wavelength of said laser light source.
30. The apparatus according to claim 17 , further comprising a post process and review means for displaying said condition of said reticle in a graphical form.
31. The apparatus according to claim 17 , further comprising a homogenizer disposed in the vicinity of said transmission light illumination means for reducing speckle resulting from use of said light source.