Inspection system with non-circular pupil

A photomask inspection system comprising:

• an illumination source configured to generate an illumination beam;
• one or more illumination optics configured to direct the illumination beam to a photomask within a range of incidence angles corresponding to an illumination pupil distribution in a pupil plane of the one or more illumination optics, wherein a center of the illumination pupil distribution corresponds to an off-axis incidence angle along a first direction on the photomask;
• one or more collection optics configured to collect light from the photomask in response to the incident illumination beam within a range of collection angles corresponding to an imaging pupil distribution in a pupil plane of the one or more collection optics, wherein a center of the imaging pupil distribution corresponds to an off-axis collection angle along the first direction on the photomask, wherein the range of incidence angles and the range of collection angles are non-overlapping, wherein the range of incidence angles is smaller than a selected cut-off angle, wherein at least a portion of the range of collection angles is larger than the selected cut-off angle, wherein at least one of the illumination pupil distribution or the imaging pupil distribution is non-circular with a size along the first direction smaller than a size along a second direction perpendicular to the first direction, wherein at least one of the illumination pupil distribution or the imaging pupil distribution is smaller in directions in a plane of incidence than directions out of the plane of incidence;
• and
• a detector configured to receive at least a portion of the light collected by the one or more collection optics and generate one or more detection signals associated with the received light.

An inspection method comprising:

• directing extreme ultraviolet (EUV) light to a sample within a range of incidence angles corresponding to an illumination pupil distribution in a pupil plane of one or more illumination optics, wherein a center of the illumination pupil distribution corresponds to an off-axis incidence angle along a first direction on the sample;
• collecting light from the sample in response to the incident EUV light within a range of collection angles corresponding to an imaging pupil distribution in a pupil plane of one or more collection optics, wherein a center of the imaging pupil distribution corresponds to an off-axis collection angle along the first direction on the sample, wherein the range of incidence angles and the range of collection angles are non-overlapping, wherein the range of incidence angles is smaller than a selected cut-off angle, wherein at least a portion of the range of collection angles is larger than the selected cut-off angle, wherein at least one of the illumination pupil distribution or the imaging pupil distribution is non-circular with a size along a first direction shorter than a size along a second direction perpendicular to the first direction, wherein at least one of the illumination pupil distribution or the imaging pupil distribution is smaller in directions in a plane of incidence than directions out of the plane of incidence;
• and
• generating, with a detector, an image of the sample based on at least a portion of the light collected by the one or more collection optics.