Confocal chromatic metrology for EUV source condition monitoring

Patent metadata

Field	Value
Publication number	US-12418972-B2
Application number	US-202217944715-A
Country	US
Kind code	B2
Filing date	Sep 14, 2022
Priority date	Sep 14, 2022
Publication date	Sep 16, 2025
Grant date	Sep 16, 2025

Abstract

A light source includes a rotatable drum to be coated with xenon ice and illuminated by a laser beam to produce a plasma. The drum may also be translatable. The light source further includes a confocal chromatic sensor to measure distances from the confocal chromatic sensor to the rotatable drum. The confocal chromatic sensor may include a sensor head to focus light onto the rotatable drum and to detect reflected light from the rotatable drum. The sensor head and the rotatable drum may be disposed within a vacuum chamber.

First claim

What is claimed is:

1. A light source, comprising:
a rotatable drum to be coated with xenon (Xe) ice and illuminated by a laser beam to produce a plasma;
a confocal chromatic sensor to measure distances from the confocal chromatic sensor to the rotatable drum;
one or more processors; and
memory storing one or more programs for execution by the one or more processors, the one or more programs comprising instructions for:
detecting defects in the Xe ice on the rotatable drum using the confocal chromatic sensor when the rotatable drum is coated with the Xe ice; and
shutting off the laser beam in response to detecting a defect in the Xe ice on the rotatable drum using the confocal chromatic sensor.

2. The light source of claim 1 , further comprising a vacuum chamber, wherein:
the rotatable drum is disposed within the vacuum chamber; and
the confocal chromatic sensor comprises a sensor head, disposed within the vacuum chamber, to focus light onto the rotatable drum and to detect reflected light from the rotatable drum.

3. The light source of claim 2 , wherein the confocal chromatic sensor further comprises a controller, disposed outside the vacuum chamber, to control operation of the confocal chromatic sensor and to generate broadband light to be provided to the sensor head.

4. The light source of claim 3 , wherein the confocal chromatic sensor further comprises an optical fiber, coupled between the sensor head and the controller, to provide the broadband light to the sensor head, wherein:
the vacuum chamber comprises a wall having a feed-through; and
the optical fiber passes through the feed-through.

5. The light source of claim 2 , further comprising a casing within the vacuum chamber, wherein:
the rotatable drum is disposed within the casing;
the sensor head is disposed outside the casing; and
the casing comprises a window situated between the rotatable drum and the sensor head.

6. The light source of claim 2 , wherein the distances from the confocal chromatic sensor to the rotatable drum to be measured by the confocal chromatic sensor comprise:
first distances from the sensor head to respective portions of a bare outer surface of the rotatable drum before the rotatable drum is coated with the Xe ice; and
second distances from the sensor head to respective portions of an outer surface of the Xe ice when the rotatable drum is coated with the Xe ice.

7. The light source of claim 1 , wherein:
the instructions for detecting defects comprise instructions for:
measuring thicknesses of respective portions of the Xe ice on the rotatable drum using the confocal chromatic sensor, and
determining whether the thicknesses of the respective portions of the Xe ice on the rotatable drum satisfy a threshold; and

the instructions for shutting off the laser beam comprise instructions for shutting off the laser beam in response to determining that one or more thicknesses of one or more respective portions of the Xe ice on the rotatable drum do not satisfy the threshold.

8. The light source of claim 7 , further comprising a vacuum chamber, wherein:
the rotatable drum is disposed within the vacuum chamber;
the confocal chromatic sensor comprises a sensor head, disposed within the vacuum chamber, to focus light onto the rotatable drum and to detect reflected light from the rotatable drum; and
the instructions for measuring thicknesses of the respective portions of the Xe ice on the rotatable drum comprise instructions for:
measuring first distances from the sensor head to respective portions of a bare outer surface of the rotatable drum using the confocal chromatic sensor before the rotatable drum is coated with the Xe ice,
measuring second distances from the sensor head to respective portions of an outer surface of the Xe ice using the confocal chromatic sensor when the rotatable drum is coated with the Xe ice, and
subtracting respective first distances from respective second distances.

9. The light source of claim 1 , wherein:
the instructions for detecting defects comprise instructions for:
measuring roughness of the Xe ice on the rotatable drum using the confocal chromatic sensor, and
determining whether the roughness satisfies a threshold; and

the instructions for shutting off the laser beam comprise instructions for shutting off the laser beam in response to determining that the roughness satisfies the threshold.

10. The light source of claim 9 , wherein:
the instructions for determining whether the roughness satisfies the threshold comprise instructions for identifying a crater in the Xe ice on the rotatable drum using the confocal chromatic sensor; and
the instructions for shutting off the laser beam in response to determining that the roughness satisfies the threshold comprise instructions for shutting off the laser beam in response to identifying the crater.

11. The light source of claim 1 , wherein:
the instructions for detecting defects comprise instructions for:
measuring reflectivities of respective portions of the Xe ice on the rotatable drum using the confocal chromatic sensor, and
determining whether the reflectivities of the respective portions of the Xe ice on the rotatable drum are within a specified range; and

the instructions for shutting off the laser beam comprise instructions for shutting off the laser beam in response to determining that one or more reflectivities of one or more respective portions of the Xe ice on the rotatable drum are not within the specified range.

12. The light source of claim 1 , wherein the one or more programs further comprise instructions for:
monitoring the rotatable drum for defects in the Xe ice using the confocal chromatic sensor after shutting off the laser beam in response to detecting the defect; and
reactivating the laser beam in response to identifying an absence of defects in the Xe ice on the rotatable drum.

13. The light source of claim 12 , wherein:
the instructions for monitoring the rotatable drum comprise instructions for:
measuring thicknesses of respective portions of the Xe ice on the rotatable drum using the confocal chromatic sensor, and
determining whether the thicknesses of the respective portions of the Xe ice on the rotatable drum satisfy a threshold; and

the instructions for reactivating the laser beam comprise instructions for reactivating the laser beam based at least in part on a determination that the thicknesses satisfy the threshold.

14. The light source of claim 12 , wherein:
the instructions for monitoring the rotatable drum comprise instructions for:
measuring roughness of the Xe ice on the rotatable drum using the confocal chromatic sensor, and
determining whether the roughness satisfies a threshold; and

the instructions for reactivating the laser beam comprise instructions for reactivating the laser beam based at least in part on a determination that the roughness does not satisfy a threshold.

15. The light source of claim 12 , wherein:
the instructions for monitoring the rotatable drum comprise instructions for:
measuring reflectivities of respective portions of the Xe ice on the rotatable drum using the confocal chromatic sensor, and
determining whether the reflectivities of the respective portions of the Xe ice on the rotatable drum are within a specified range; and

the instructions for reactivating the laser beam comprise instructions for reactivating the laser beam based at least in part on a determination that the reflectivities of the respective portions of the Xe ice on the rotatable drum are within the specified range.

16. The light sour

Assignees

• Kla Corp

Inventors

• Tae Patrick
• Su Caijun
• Jagannath Ravichandra
• Ahr Brian

Classifications

• H05G2/0084

  Control of the laser beam · CPC title

• H05H1/46

  using applied electromagnetic fields, e.g. high frequency or microwave energy (H05H1/26 takes precedence) · CPC title

• H05H2240/20

  Non-thermal plasma · CPC title

• H05G2/0027 Primary

  Arrangements for controlling the supply; Arrangements for measurements · CPC title

• G01B2210/50

  Using chromatic effects to achieve wavelength-dependent depth resolution · CPC title