Ellipsometer focusing system

An ellipsometer comprising:

• a source that emits radiation;
• a polarization state generator for polarizing the radiation to produce a sample beam that is incident on and reflected by a sample;
• a polarization state analyzer positioned to receive reflected radiation from the sample;
• a detector positioned to receive the reflected radiation after the reflected radiation passes through the polarization state analyzer;
• and
• a focusing system positioned to receive a portion of the reflected radiation, the focusing system comprising:
• a beam splitter in a beam path between the sample and the detector, the beam splitter positioned to receive the reflected radiation and to provide the portion of the reflected radiation;
• a lens system that receives the portion of the reflected radiation and magnifies by at least 2× any deviation from a best focus position of the ellipsometer;
• and
• a two-dimensional sensor, the lens system focusing the portion of the reflected radiation on the two-dimensional sensor in a spot that is 50 percent or smaller than the two-dimensional sensor, the two-dimensional sensor generates an image signal of the spot;
• a processor that receives the image signal, filters noise from the image signal and that is configured to find a location of the spot on the two-dimensional sensor after noise is filtered to determine the deviation from the best focus position of the ellipsometer.

A method of focusing an ellipsometer, the method comprising:

• generating radiation;
• polarizing the radiation to produce a sample beam that is incident on and reflected by a sample;
• analyzing reflected radiation from the sample;
• and
• focusing the sample beam on the sample comprising:
• providing a portion of the reflected radiation, the portion of the reflected radiation including an entire cross-section of the reflected radiation;
• focusing the portion of the reflected radiation into a spot on a two-dimensional sensor;
• determining a location of the spot on the two-dimensional sensor;
• determining a deviation from a best focus position using the location of the spot on the two-dimensional sensor;
• and
• adjusting a focal position of the ellipsometer based on the deviation from the best focus position.

An ellipsometer comprising:

• a source that emits radiation;
• a polarization state generator for polarizing the radiation to produce a sample beam that is incident on and reflected by a sample;
• a polarization state analyzer positioned to receive reflected radiation from the sample;
• a detector positioned to receive the reflected radiation after the reflected radiation passes through the polarization state analyzer;
• and
• a focusing system positioned to receive a portion of the reflected radiation, the focusing system comprising:
• a beam splitter in a beam path between the sample and the detector, the beam splitter positioned to receive the reflected radiation and to provide the portion of the reflected radiation;
• a lens system that receives the portion of the reflected radiation and magnifies any deviation from a best focus position of the ellipsometer;
• a camera comprising a two-dimensional sensor, the lens system focusing the portion of the reflected radiation on the two-dimensional sensor in a spot, the two-dimensional sensor generates an image signal of the spot;
• a processor that receives the image signal, filters noise from the image signal and that is configured to find a location of the spot on the two-dimensional sensor to determine the deviation from the best focus position of the ellipsometer;
• and
• a proportional-integral-derivative controller coupled to the camera, the proportional-integral-derivative controller controlling at least one of exposure time and gain for the camera.

An ellipsometer comprising:

• a source that emits radiation;
• a polarization state generator for polarizing the radiation to produce a sample beam that is incident on and reflected by a sample;
• a polarization state analyzer positioned to receive reflected radiation from the sample;
• a detector positioned to receive the reflected radiation after the reflected radiation passes through the polarization state analyzer;
• and
• a focusing system positioned to receive a portion of the reflected radiation, the focusing system comprising:
• a beam splitter in a beam path between the sample and the detector, the beam splitter positioned to receive the reflected radiation and to provide the portion of the reflected radiation;
• a lens system that receives the portion of the reflected radiation and magnifies any deviation from a best focus position of the ellipsometer;
• a two-dimensional sensor, the lens system focusing the portion of the reflected radiation on the two-dimensional sensor in a spot, the two-dimensional sensor generates an image signal of the spot;
• a processor that receives the image signal, filters noise from the image signal and that is configured to find a location of the spot on the two-dimensional sensor to determine the deviation from the best focus position of the ellipsometer;
• wherein the polarization state generator and/or the polarization state analyzer comprise a rotating optic that causes the location of the spot to move on the two-dimensional sensor as the rotating optic rotates, wherein the processor is further configured to compensate for movement of the location of the spot on the two-dimensional sensor caused by the rotating optic.