Reflective reduction imaging optical system for X-ray lithography

• US 5,917,879 A
• Filed: 03/17/1997
• Issued: 06/29/1999
• Est. Priority Date: 03/15/1996
• Status: Expired due to Fees

• Current Assignee: Nikon Corporation
• Original Assignee: Nikon Corporation
• Inventors: Mashima, Kiyohito
• Primary Examiner(s): Church, Craig E.
• Application Number:
  US08/822,139
• Time in Patent Office: 834 Days
• Field of Search: 378/34, 378/35
• US Class Current: 378/34
• CPC Class Codes: G02B 17/0657 off-axis or unobscured systems in which all of the mirrors share a common axis of rotational symmetry
• CPC Class Codes: G03F 7/70233 Optical aspects of catoptric systems, i.e. comprising only reflective elements, e.g. extreme ultraviolet [EUV] projection systems
• CPC Class Codes: G21K 1/06 using diffraction, refraction or reflection, e.g. monochromators G21K1/10, G21K7/00 take precedence

Abstract

A reflective reduction imaging optical system for X-ray lithography achieves good imaging performance using a simple construction. The system includes, from the side of mask M, first concave mirror G₄, plane mirror G₃, convex mirror G₂, and second concave mirror G₁ are placed, in that sequence, from the object side. The concave mirrors G₄ and G₁ and the convex mirror G₂ are formed in aspherical shapes. The convex mirror G₂ is placed at the pupil plane and the system becomes telecentric on the image side of wafer W.

Claims

1. A reflective reduction imaging optical system for X-ray lithography, for transferring patterns from a mask to a wafer, the system employing an X-ray source and comprised by coaxially placing, in the following sequence from said source and the mask, a first concave mirror, a plane mirror, a convex mirror, and a second concave mirror, with said concave mirrors and said convex mirror formed with mirror surfaces having aspherical shapes, said convex mirror being placed at a pupil plane, and wherein the system becomes telecentric on an image side of said wafer.

2. The reflective reduction imaging optical system for X-ray lithography according to claim 1, wherein in said concave mirrors and said convex mirror, at least two of said mirror surfaces are formed in different aspherical shapes.

3. The reflective reduction imaging optical system for X-ray lithography according to claim 1, wherein at least one said mirror surface is formed in a shape selected from the group comprising ellipsoid, parabolic, and hyperbolic shapes.

4. The reflective reduction imaging optical system for X-ray lithography according to claim 2, wherein at least one said mirror surface is formed in a shape selected from the group comprising ellipsoid, parabolic and hyperbolic shapes.

5. The reflective reduction imaging optical system for X-ray lithography according to claim 3, wherein at least one said mirror surface is formed with an oblate spheroidal surface.

6. The reflective reduction imaging optical system for X-ray lithography according to claim 4, wherein at least one said mirror surface is formed with an oblate spheroidal surface.

7. The reflective reduction imaging optical system for X-ray lithography according to claim 1, wherein the mask and wafer are adapted for being scanned in synchronization with each other in a ring field.

8. The reflective reduction imaging optical system for X-ray lithography according to claim 1, wherein the aspherical shapes are formed as secondary aspherical surfaces chosen from the group comprising ellipsoidal and oblate spherical surfaces, parabolic surfaces, and hyperbolic surfaces.

9. The reflective reduction imaging optical system for X-ray lithography according to claim 8, wherein the first concave mirror has an ellipsoidal surface, and ##EQU2## wherein y is the height in the direction perpendicular to the optical axis;

S(y) is the displacement in the direction of the optical axis at height y;

r is the apex radius of curvature;

κ

is the conical coefficient; and

C_n is the nth aspherical coefficient.

10. The reflective reduction imaging optical system for X-ray lithography according to claim 1, wherein the X-rays are focused onto said wafer and a side of said wafer becomes a telecentric ring field.

11. The reflective reduction imaging optical system for X-ray lithography according to claim 2, wherein the X-ray beam is focused on said wafer and one side of said wafer becomes telecentric.

12. The reflective reduction imaging optical system for X-ray lithography according to claim 1, wherein an aperture stop is located on the convex mirror surface.

13. The reflective reduction imaging optical system for X-ray lithography according to claim 1, wherein the X-ray beam is focused on said wafer and one side of said wafer becomes telecentric.

14. The reflective reduction imaging optical system for X-ray lithography according to claim 1, wherein in an optical path in the system between a mask and a wafer, there is no mechanical interference in the optical path by the wafer.

15. The reflective reduction imaging optical system for X-ray lithography according to claim 1, wherein incident angles of X-rays on reflective surfaces of said mirrors are nearly 0°

.