Lithographic apparatus, alignment system, and device manufacturing method

An alignment system for aligning a substrate comprising:

• a spot source configured to illuminate an alignment mark on the substrate with an illumination spot;
• a combiner system configured to transfer two images of the illuminated alignment mark without spatial filtering of the images, rotate the images 180° relatively to each other, and combine the two images;
• and
• a detection system configured to detect an alignment signal from the combined images and to determine a unique alignment position by selecting one of a plurality of extreme values in the detected alignment signal.

A device manufacturing method comprising:

• projecting a patterned beam of radiation onto a substrate;
• illuminating an alignment mark in a scribe lane on the substrate with an illumination spot, the alignment mark comprising a plurality of lines and spaces;
• transferring the images of the illuminated alignment mark without spatial filtering of the images;
• combining two images of the alignment mark that are rotated by 180° relative to each other;
• and
• detecting an alignment signal from the combined images and determining a unique alignment position by selecting an extreme value in the detected alignment signal.

A method of aligning a substrate, said method comprising:

• illuminating an alignment mark on the substrate with a spot, said alignment mark including an alternating plurality of lines and spaces;
• rotating at least one of two images of the illuminated mark with respect to the other such that a rotation of one of the images with respect to the other is substantially equal to one-hundred-eighty degrees to produce a combined image, said combined image being based on said two images subsequent to said rotating;
• detecting an alignment signal based on the combined image, said alignment signal including a plurality of locally extreme values having substantially equal amplitudes;
• and selecting one of a plurality of locally extreme values in the detected alignment signal.

An alignment system comprising:

• a spot source configured to illuminate an alignment mark on a substrate with an illumination spot;
• a plurality of optical elements configured to rotate at least one of two images of the illuminated mark with respect to the other such that a rotation of one of the images with respect to the other is substantially equal to one-hundred-eighty degrees to produce a combined image, said combined image being based on said two images subsequent to said rotating;
• a detector configured to detect an alignment signal from the combined image, said detected alignment signal including a plurality of locally extreme values having substantially equal amplitudes;
• and
• a signal analyzer configured to determine a unique alignment position by selecting one of a plurality of locally extreme values in the detected alignment signal.