Method of measuring asymmetry in a scatterometer, a method of measuring an overlay error in a substrate and a metrology apparatus

1. A method of measuring asymmetry in a scatterometer, the method comprising:

first illuminating a target pattern with radiation and measuring a first two-dimensional angular scatter spectrum of radiation reflected or scattered by the target pattern by using the scatterometer to measure intensity at a plurality of pre-determined angular positions relative to the target pattern;

relatively rotating the target pattern and the scatterometer by about 180° in a plane substantially perpendicular to the optical axis of the scatterometer;

second illuminating the target pattern with radiation and measuring a second two-dimensional angular scatter spectrum by using the scatterometer to measure intensity at the plurality of pre-determined positions relative to the target pattern and rotating the second angular scatter spectrum by about 180° to produce a third two-dimensional angular scatter spectrum; and

producing a sensor correction value for each of the plurality of pre-determined angular positions by, for each of the plurality of pre-determined positions, subtracting the intensity at the respective pre-determined position of the first two-dimensional angular scatter spectrum from the intensity at the respective pre-determined position of the third two-dimensional angular scatter spectrum.

2. The method of claim 1 , wherein each intensity at the pre-determined positions is measured over an integration time of at least about 200 milliseconds.

3. The method of claim 2 , wherein each intensity at the pre-determined positions is measured over an integration time of at least about 400 milliseconds.

4. The method of claim 1 , wherein the first illuminating, relatively rotating, second illuminating and producing are first carried out with radiation polarized in a first direction to obtain any asymmetry correction value for each of the plurality of pre-determined angular positions for radiation polarized in the first direction and then carried out with radiation polarized in a second direction to obtain a sensor correction value for each of the plurality of pre-determined angular positions for radiation polarized in the second direction.

5. The method of claim 1 , further comprising updating the sensor correction value for each pixel by dividing it by two.

6. A method of measuring asymmetry in a scatterometer due to illumination beam asymmetry, the method comprising:

first measuring with a detector of the scatterometer the intensity of radiation of an illumination beam at a plurality of pre-determined positions relative to the detector in a cross-section of the illumination beam to produce a first map of intensities;

relatively rotating the illumination beam and the scatterometer by about 180° in a plane substantially perpendicular to an axis of the illumination beam;

second measuring with the detector the intensity of radiation of the illumination beam at the plurality of pre-determined positions relative to the detector to produce a second map of intensities and rotating the second map of intensities by about 180° to produce a third map of intensities; and

producing an illumination asymmetry correction value for each of the plurality of pre-determined positions by, for each of the plurality of pre-determined positions, subtracting the intensity at the respective pre-determined position of the first map of intensities from the intensity at the respective pre-determined position of the third map of intensities.

7. A method of measuring asymmetry in a scatterometer due to asymmetries in a microscope objective of the scatterometer, the method comprising:

(1) obtaining sensor correction values using a method including

first illuminating a target pattern with radiation and measuring a first two-dimensional angular scatter spectrum of radiation reflected or scattered by the target pattern by using the scatterometer to measure intensity at a plurality of pre-determined angular positions relative to the target pattern;

relatively rotating the target pattern and the scatterometer by about 180° in a plane substantially perpendicular to an optical axis of the scatterometer;

second illuminating the target pattern with radiation and measuring a second two-dimensional angular scatter spectrum by using the scatterometer to measure intensity at the plurality of pre-determined positions relative to the target pattern and rotating the second angular scatter spectrum by about 180° to produce a third two-dimensional angular scatter spectrum; and

producing a sensor correction value for each of the plurality of pre-determined angular positions by, for each of the plurality of pre-determined positions, subtracting the intensity at the respective pre-determined position of the first two-dimensional angular scatter spectrum from the intensity at the respective pre-determined position of the third two-dimensional angular scatter spectrum; and

(2) obtaining illumination asymmetry correction values using a method including

first measuring with a detector of the scatterometer the intensity of radiation of an illumination beam at the plurality of pre-determined positions in a cross-section of the illumination beam to produce a first map of intensities;

relatively rotating the illumination beam and the scatterometer by about 180° in a plane substantially perpendicular to the axis of the illumination beam;

second measuring with the detector the intensity of radiation of the illumination beam at the plurality of pre-determined positions to produce a second map of intensities and rotating the second map of intensities by about 180° to produce a third map of intensities; and

producing an illumination asymmetry correction value for each of the plurality of pre-determined positions by, for each of the plurality of pre-determined positions, subtracting the intensity at the respective pre-determined position of the first map of intensities from the intensity at the respective pre-determined position of the third map of intensities; and

(3) for each pre-determined position, subtracting the respective illumination asymmetry correction value from the sensor correction value to obtain a microscope objective asymmetry value for each respective pre-determined position.

8. A method of measuring an overlay error in a substrate comprising:

projecting a beam of radiation onto a target pattern on the substrate;

measuring a first overlay two-dimensional angular scatter spectrum of radiation reflected or scattered by the target pattern by using a scatterometer to measure intensity at a plurality of pre-determined angular positions relative to the target pattern;

relatively rotating the target pattern and the scatterometer by about 180° in a plane substantially perpendicular to the optical axis of the scatterometer;

projecting a second beam of radiation onto the target pattern on the substrate and measuring a second two-dimensional angular scatter spectrum by using the scatterometer to measure intensity at the plurality of pre-determined positions relative to the target pattern;

rotating the second angular scatter spectrum by about 180° to produce a third two-dimensional angular scatter spectrum;

producing a sensor correction value for each of the plurality of pre-determined angular positions by, for each of the plurality of pre-determined positions, subtracting the intensity at the respective pre-determined position of the first two-dimensional angular scatter spectrum from the intensity at the respective pre-determined position of the third two-dimensional angular scatter spectrum;

computing a corrected two-dimensional angular scatter spectrum by subtracting, for each pre-determined position, the sensor correction value for each of the plurality of pre-determined angular positions from the measured intensity at the respective position of the overlay two-dimensional angular spectrum; and

determining the overlay error from the corrected two-dimensional angular scatter spectrum.

9. A metrology apparatus configured to measure a property of a substrate, the apparatus comprising:

a radiation projector configured to illuminate a target pattern of a substrate with radiation;

a detector configured to measure an intensity of reflected or scattered radiation from the target pattern at a plurality of pre-determined angular positions relative to the target pattern;

a system configured to relatively rotate the target pattern and the scatterometer by about 180° in a plane substantially perpendicular to the target portion; and

a controller adapted to control illumination of the target pattern with radiation,

determine a first two-dimensional angular scatter spectrum of radiation reflected or scattered by the target pattern by using the detector to measure intensity at a plurality of pre-determined angular positions relative to the target pattern,

relatively rotate the target pattern and the scatterometer by about 180° in a plane substantially perpendicular to an optical axis of the scatterometer,

control illumination of the target pattern with radiation and determine a second two-dimensional angular scatter spectrum by using the detector to measure intensity at the plurality of pre-determined portions relative to the target pattern and rotate the second angular scatter spectrum by about 180° to produce a third two-dimensional angular scatter spectrum, and

produce a sensor correction value for each of the plurality of pre-determined angular positions by, for each of the plurality of pre-determined positions, subtracting the intensity at the respective pre-determined position of the first two-dimensional angular scatter spectrum from the intensity at the respective pre-determined position of the third two-dimensional angular scatter spectrum.

10. The metrology apparatus of claim 9 , further comprising a system to store the sensor correction value for each of the plurality of pre-determined angular positions.

11. The metrology apparatus of claim 9 , wherein the controller is further adapted to project a beam of radiation onto a target pattern on a substrate;

measure an overlay two-dimensional angular scatter spectrum of radiation reflected or scattered by the target pattern by using the detector to measure intensity at the plurality of pre-determined angular positions relative to the target pattern,

compute a corrected two-dimensional angular scatter spectrum by subtraction, for each pre-determined position, a respective the sensor correction value for a respective pre-determined position from the measured intensity at the respective position of the overlay two-dimensional angular spectrum, and

determine the overlay error from the corrected two-dimensional angular scatter spectrum.