Patent ID: 11899371
Assignee: NIKON CORPORATION
Field: nan (nan)
Classification: CPC nan | IPC G

Claim 40:
41. A catoptric system having a reference axis and comprising:
a primary mirror unit made of a first material and having a first area centered on the reference axis, the first area being devoid of the first material;
a secondary mirror unit made of a second material and having a second area centered on the reference axis, the second area being devoid of the second material; and
a reflective illumination element situated on the reference axis between the primary mirror unit and the secondary mirror unit,
the catoptric system having a length defined by distance separating the primary mirror unit and the secondary mirror unit, the length equal to approximately 0.3 m,
the catoptric system configured to image a substantially one-dimensional (1D) diffractive structure through the first and second areas onto an image plane in extreme UV light with a reduction ratio between 4 and 6,
wherein the substantially 1D diffractive structure is characterized by a pattern period between 40 nm and 60 nm and is associated with a curved surface of an object, and
wherein the image plane is separated from the primary mirror unit by about 25 mm and is separated from the secondary mirror unit by the primary mirror unit,
the primary mirror unit having a concave reflecting surface,
the secondary mirror unit having a convex reflecting surface,
the system configured to:
receive, at the convex reflecting surface of the secondary mirror unit, optical radiation in a form of two diffracted beams of radiation delivered thereto through the first area of the primary mirror unit, and
form an image of an object by irradiating an image plane with the optical radiation after the optical radiation has been reflected from the concave reflecting surface of the primary mirror unit and after the optical radiation has passed through the second area of the secondary mirror unit,
wherein the reflective illumination element is situated to reflect illumination radiation to the object, the two diffracted beams of radiation being generated at the object in response to the illumination radiation received from the reflective illumination element, and to block light propagating from the object between the two diffracted beams of radiation.