Systems And Methods For Metrology Beam Stabilization

A method comprising:

• projecting a measurement beam to a specimen under measurement along an illumination beam path;
• collecting an amount of collected light from the surface of the specimen in response to the incident measurement beam;
• projecting the measurement beam from the specimen to a detector along a collection beam path;
• generating a plurality of output signals indicative of a response of the specimen to the incident illumination measurement beam;
• varying a rotational polarization of the measurement beam at a first location in an optical path of the measurement beam;
• measuring a variation of position of the measurement beam at a second location in the optical path, wherein the second location in the optical path is located after the first location as the measurement beam propagates in the optical path;
• adjusting a position of the measurement beam to reduce the variation of position of the measurement beam at a third location in the optical path, wherein the third location is located between the first location and the second location as the measurement beam propagates in the optical path.

A measurement system comprising:

• a rotating optical polarizer element located in a beam path of the measurement system;
• a beam position sensor located in the beam path after the rotating optical polarizer element, wherein the beam position sensor generates output signals indicative of a location of the beam;
• an active beam compensating element located in the beam path between the rotating optical polarizer element and the beam position sensor;
• and
• a computing system configured to:
• receive the output signals indicative of the location of the beam;
• and
• communicate a command signal to the active beam compensating element that causes the active beam compensating element to adjust a location of the beam toward a desired location as measured by the beam position sensor.