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1. (WO2019032730) VARIABLE APERTURE MASK
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WHAT IS CLAIMED IS:

1. An aperture mask for use in a semiconductor metrology tool, comprising:

a frame having a hole to allow transmission of an optical beam; and

a plurality of opaque plates mechanically coupled to the frame and having adjustable positions with respect to the hole, to block respective portions of the hole and corresponding parts of the optical beam.

2. The aperture mask of claim 1, wherein:

the plurality of opaque plates comprises a first opaque plate and a second opaque plate; the aperture mask further comprises:

a first translation stage, on which the first opaque plate is mounted, to adjust a linear position of the first opaque plate; and

a second translation stage, on which the second opaque plate is mounted, to adjust a linear position of the second opaque plate;

the first and second translation stages are mechanically coupled to the frame; and the first and second translation stages are independently operable to adjust the linear positions of the first and second opaque plates.

3. The aperture mask of claim 2, wherein the first and second translation stages are motorized.

4. The aperture mask of claim 2, wherein the first and second translation stages are substantially aligned along a common axis and allow independent adjust of the linear positions of the first and second opaque plates along the common axis.

5. The aperture mask of claim 2, further comprising a rotation stage mounted on the frame and having a hole to allow transmission of the optical beam, the rotation stage being rotatable with respect to the frame,

wherein the first and second translation stages are mounted on the rotation stage.

6. The aperture mask of claim 1, further comprising a plurality of translation stages, on which respective opaque plates of the plurality of opaque plates are mounted, to adjust respective positions of the plurality of opaque plates,

wherein the plurality of translation stages is mechanically coupled to the frame.

7. The aperture mask of claim 6, wherein each translation stage of the plurality of translation stages is independently operable to adjust the positions of the plurality of opaque plates.

8. The aperture mask of claim 7, further comprising a rotation stage mounted on the frame and having a hole to allow transmission of the optical beam, wherein:

the rotation stage is rotatable with respect to the frame; and

the plurality of translation stages is mounted on the rotation stage.

9. The aperture mask of claim 1, wherein:

the plurality of opaque plates comprises a first opaque plate and a second opaque plate; the aperture mask further comprises a rotation stage mounted on the frame and having a hole to allow transmission of the optical beam; and

the first and second opaque plates are mechanically coupled to the rotation stage to allow rotation of the first and second opaque plates with respect to the hole.

10. A collection system of a semiconductor metrology tool, comprising:

a chuck to support a target from which an optical beam is reflected;

an aperture mask to provide an adjustable aperture for the reflected optical beam, the aperture mask comprising a plurality of opaque plates having adjustable positions; and

a spectrometer to receive the reflected optical beam;

wherein the aperture mask is situated between the chuck and the spectrometer along the optical axis.

11. The collection system of claim 10, further comprising:

a collection lens, situated between the chuck and the aperture mask along the optical axis, to collimate the reflected optical beam, wherein the reflected optical beam is substantially collimated at the aperture mask; and

a focusing lens, situated between the aperture mask and the spectrometer along the optical axis, to focus the reflected optical beam onto the spectrometer.

12. The collection system of claim 11, further comprising an analyzer, situated between the aperture mask and the focusing lens along the optical axis, to change the polarization of the reflected optical beam.

13. The collection system of claim 10, wherein:

the plurality of opaque plates comprises a first opaque plate and a second opaque plate; the aperture mask further comprises:

a first translation stage, on which the first opaque plate is mounted, to adjust a linear position of the first opaque plate; and

a second translation stage, on which the second opaque plate is mounted, to adjust a linear position of the second opaque plate; and

the first and second translation stages are independently operable to adjust the linear positions of the first and second opaque plates.

14. The collection system of claim 13, wherein:

the aperture mask further comprises a rotation stage on which the first and second translation stages are mounted and having a hole to allow transmission of the optical beam, the rotation stage being rotatable about the optical axis or an axis parallel to the optical axis; and the first and second opaque plates are configurable to block respective adjustable portions of the hole.

15. The collection system of claim 14, wherein the chuck is rotatable to vary an angle of azimuth of the target with respect to the optical beam, the rotation stage being rotatable in accordance with rotation of the chuck.

16. The collection system of claim 10, wherein:

the plurality of opaque plates comprises a first opaque plate and a second opaque plate; the aperture mask further comprises a rotation stage mounted on the frame and having a hole to allow transmission of the optical beam; and

the first and second opaque plates are mechanically coupled to the rotation stage to allow rotation of the first and second opaque plates with respect to the hole.

17. A method of operating a semiconductor metrology tool, comprising:

loading a target on a chuck;

illuminating the target with an optical beam, wherein the target reflects the optical beam; adjusting positions of one or more opaque plates on an aperture mask that masks the reflected optical beam, to achieve at least one of a selected central angle of incidence for the optical beam with respect to the target, a selected range of angles of incidence for the optical beam with respect to the target, or a selected degree of rotation of the one or more opaque plates that corresponds to an angle of azimuth of the target with respect to the optical beam; and

with the positions of the one or more opaque plates adjusted, receiving the optical beam at a spectrometer.

18. The method of claim 17, further comprising rotating the chuck to set the angle of azimuth of the target with respect to the optical beam;

wherein adjusting the positions of the one or more opaque plates comprises rotating the positions of the one or more opaque plates to achieve the selected degree of rotation.

19. The method of claim 17, wherein adjusting the positions of the one or more opaque plates comprises adjusting linear positions of first and second opaque plates on the aperture mask to achieve at least one of the selected central angle of incidence or the selected range of angles of incidence.

20. The method of claim 19, wherein adjusting the linear position of the first opaque plate is performed independently of adjusting the linear position of the second opaque plate.

21. The method of claim 17, further comprising:

collimating the reflected optical beam, wherein the aperture mask masks the collimated optical beam; and

focusing the collimated optical beam onto the spectrometer.

22. A collection system of a semiconductor metrology tool, comprising:

a chuck to support a target from which an optical beam is reflected;

a spectrometer to receive the reflected optical beam; and

a plurality of aperture masks arranged in a rotatable sequence about an axis parallel to an optical axis, wherein each aperture mask of the plurality of aperture masks is rotatable into and out of the reflected optical beam between the chuck and the spectrometer to selectively mask the reflected optical beam.

23. The collection system of claim 22, further comprising a turret on which the plurality of aperture masks is mounted.