Reflective projection lens for EUV-photolithography
View Patent ↗A projection lens for imaging a pattern arranged in an object plane onto an image plane using electromagnetic radiation from the extreme-ultraviolet (EUV) spectral region has several imaging mirrors between its object plane and image plane that define an optical axis of the projection lens and have reflective coatings. At least one of those mirrors has a graded reflective coating that has a film-thickness gradient that is rotationally symmetric with respect to a coating axis, where that coating axis is acentrically arranged with respect to the optical axis of the projection lens. Providing at least one acentric, graded, reflective coating allows designing projection lenses that allow highly uniform field illumination, combined with high total transmittance.
1. A projection lens for imaging a pattern arranged in an object plane onto an image plane employing electromagnetic radiation from the extreme-ultraviolet (EUV) spectral region,
the projection lens comprising a plurality of imaging mirrors having reflective coatings and defining an optical axis of the projection lens,
wherein the mirrors are arranged between the object plane and the image plane and are aligned for projection of an off-axis field from the object plane to the image plane,
wherein the mirrors are configured and arranged with respect to one another such that every mirror is irradiated over a range of angles of incidence that is characteristic of that particular mirror,
wherein at least one of the mirrors, having the largest range of angles of incidence, has a centric, graded, reflective coating that has a film-thickness gradient that is rotationally symmetric with respect to a coating axis, and
wherein the coating axis is substantially coincident with the optical axis of the projection lens.
2. A projection lens according to claim 1 , wherein the film-thickness gradient of the graded, reflective coating is optimized to yield a pupil-irradiance distribution that is substantially rotationally symmetric.
3. A projection lens according to claim 1 , wherein at least two of the mirrors have centric, graded, reflective coatings, wherein the gradients in the film thicknesses of the reflective coatings of the at least two of the mirrors are adapted to suit one another such that the coatings yield a pupil-irradiance distribution that is substantially rotationally symmetric.
4. A projection lens according to claim 1 , wherein the projection lens has an image-side numerical aperture, NA, given by NA≧0.15.
5. A projection lens according to claim 1 , wherein the projection lens has an image-side numerical aperture, NA, given by NA≧0.2.
6. A method for fabricating microdevices, comprising the following steps:
providing a mask having a prescribed pattern;
illuminating the mask with electromagnetic radiation from the extreme-ultraviolet (EUV) spectral region; and
projecting via a projection lens an image of the pattern onto a photosensitive substrate arranged in an image plane of the projection lens,
wherein the projection lens comprises a plurality of imaging mirrors having reflective coatings and defining an optical axis of the projection lens,
wherein the mirrors are arranged between the object plane and the image plane and are aligned for projection of an off-axis field from the object plane to the image plane,
wherein the mirrors are configured and arranged with respect to one another such that every mirror is irradiated over a range of angles of incidence that is characteristic of that particular mirror,
wherein at least one of the mirrors, having the largest range of angles of incidence, has a centric, graded, reflective coating that has a film-thickness gradient that is rotationally symmetric with respect to a coating axis, and
wherein the coating axis is substantially coincident with the optical axis of the projection lens.