Methods and scatterometers, lithographic systems, and lithographic processing cells

An angularly resolved scatterometer configured to determine a focus of a lithographic apparatus used in a lithographic process on a substrate, wherein the lithographic process is used to form a structure on the substrate, the structure comprising at least one feature having a profile which has an asymmetry, the asymmetry depending on the focus of the lithographic apparatus on the substrate, the scatterometer comprising:

• an illumination arrangement configured to deliver first and second beams of radiation to the substrate for use in first and second measurements;
• a detection arrangement operable during the first and second measurements to form and detect respective first and second images of the structure using radiation diffracted from the substrate;
• and
• a stop arrangement within the detection arrangement, wherein the illumination arrangement and stop arrangement together are effective to stop zero order diffracted radiation contributing to the first and second images, while the first and second images are formed using first and second parts respectively of non-zero order diffracted radiation, the first and second parts being symmetrically opposite one another in a diffraction spectrum of the diffracted radiation;
• and
• a computational arrangement configured to determine the asymmetry for the feature from the first and second images and/or to use the determined asymmetry and a relationship between the focus and the asymmetry for each feature to provide an indication of the focus on the substrate.

A lithographic system comprising:

• a lithographic apparatus comprising:
• an illumination optical system arranged, to illuminate a pattern;
• a projection optical system arranged to project an image of the pattern on to a substrate;
• and
• an angularly resolved scatterometer configured to determine a focus of the lithographic system used in a lithographic process on the substrate, wherein the lithographic process is used to form a structure on the substrate, the structure comprising at least one feature having a profile which has an asymmetry which depends on the focus of the lithographic system on the substrate, the scatterometer comprising:
• an illumination arrangement configured to deliver first and second beams of radiation to the substrate for use in first and second measurements;
• a detection arrangement operable during the first and second measurements to form and detect respective first and second images of the structure using radiation diffracted from the substrate;
• and
• a stop arrangement within the detection arrangement,
• wherein the illumination arrangement and stop arrangement together are configured to stop zero order diffracted radiation contributing to the first and second images, while the first and second images are formed using first and second parts respectively of non-zero order diffracted radiation, the first and second parts being symmetrically opposite one another in a diffraction spectrum of the diffracted radiation;
• and
• a computational arrangement configured to determine the asymmetry for the feature from the first and second images and/or to use the determined asymmetry and a relationship between the focus and the asymmetry or each feature to provide an indication of the focus on the substrate.

A lithographic cell comprising:

• a coater arranged to coat a substrate with a radiation sensitive layer;
• a lithographic apparatus arranged to expose images onto the radiation sensitive layer of the substrate;
• a developer arranged to develop images exposed by the lithographic apparatus;
• and
• an angularly resolved scatterometer configured to determine a focus of the lithographic apparatus used in a lithographic process on the substrate, wherein the lithographic process is used to form a structure on the substrate, the structure comprising at least one feature having a profile which has an asymmetry, the asymmetry depending on the focus of the lithographic apparatus on the substrate, the scatterometer comprising:
• an illumination arrangement operable to deliver first and second beams of radiation to the substrate for use in first and second measurements;
• a detection arrangement operable during the first and second measurements to form and detect respective first and second images of the structure using radiation diffracted from the substrate;
• and
• a stop arrangement within the detection arrangement,
• wherein the illumination arrangement and stop arrangement together are configured to stop zero order diffracted radiation contributing to the first and second images, while the first and second images are formed using first and second parts respectively of non-zero order diffracted radiation, the first and second parts being symmetrically opposite one another in a diffraction spectrum of the diffracted radiation;
• and
• a computational arrangement configured to determine the asymmetry for the feature from the first and second images and/or to use the determined asymmetry and a relationship between the focus and the asymmetry for each feature to provide an indication of the focus on the substrate.

An angularly resolved scatterometer configured to determine a focus of a lithographic apparatus used in a lithographic process on a substrate, wherein the lithographic process is used to form a structure on the substrate, the structure comprising at least one feature having a profile which has an asymmetry, the asymmetry depending on the focus of the lithographic apparatus on the substrate, the scatterometer comprising:

• an illumination device configured to produce a beam of radiation;
• a masking device arranged such that the beam of radiation produced by the illumination device and directed onto the substrate through the masking device will illuminate the structure within a first range of angles;
• a detection device configured to detect an image produced by radiation diffracted from the structure;
• a stop device between the substrate and the detection device, the masking device and stop device together being effective to stop zero order diffracted radiation and a part of higher order diffracted radiation from being detected,
• wherein the masking device and the substrate are configured to rotate relative to each other about their respective optical axes such that after the relative rotation the structure is illuminated within a second range of angles, the second range being diametrically opposed to the first range within a cross section of the beam of radiation, and
• wherein the detection device is configured to measure different portions of the image produced by the diffracted radiation produced by directing the beam of radiation onto the structure through the stop device before and after the rotation of the masking device and the substrate relative to each other about their respective optical axes;
• and
• a computational device configured to determine the asymmetry for the feature from the different portions of the image measured before and after the rotation of the device and the substrate relative to each other or to use the determined asymmetry and a relationship between the focus and the asymmetry for each feature to provide an indication of the focus on the substrate.