Patent Number: 
Section: claims

1. A reflective mirror comprising:a base; anda multilayer film configured to reflect at least a portion of incident light, the multilayer film including a first layer and a second layer that are laminated alternately on the base, the multilayer film having a periodic length defined as a sum of the thickness of the first layer and the thickness of the second layer,wherein the base includes a first surface and a second surface, the first surface and the second surface formed with first and second height levels, respectively, relative to a surface of the multilayer film, wherein the first and second height levels are different from each other, andwherein the multilayer film includes a first portion formed on the first surface and a second portion formed on the second surface, and further wherein the periodic length at the first portion is different from the periodic length at the second portion, and the periodic length at the first portion and the periodic length at the second portion are set such that a difference in reflectance between the first portion and the second portion is reduced, while the multilayer film is configured to reflect a portion of the incident light that is incident at a first angle on the first portion of the multilayer film, and reflect a portion of the incident light that is incident at a second angle on the second portion of the multilayer film, the second angle being different from the first angle. 2. The reflective mirror according to claim 1, wherein the first portion of the multilayer film is provided at a position rotationally symmetric to a position of the second portion of the multilayer film about an optical axis of the reflective mirror. 3. The reflective mirror according to claim 1, wherein at least one of the first and second height levels is determined based on an aberration caused by the thickness distribution of the multilayer film. 4. The reflective mirror according to claim 1, wherein the multilayer film has a thickness distribution, the thickness distribution changing in an azimuthal direction of an optical axis of the reflective mirror. 5. The reflective mirror according to claim 1, wherein the multilayer film has a thickness distribution, the thickness distribution not being rotationally symmetric about an optical axis of the reflective mirror. 6. The reflective mirror according to claim 1, wherein the multilayer film is line symmetric with respect to a line passing through an optical axis of the reflective mirror. 7. A projection optical system comprising a plurality of optical elements, the projection optical system projecting an image of a first face onto a second face, and at least one of the optical elements being the reflective mirror of claim 1. 8. The projection optical system according to claim 7, wherein light from the first face is irradiated onto the second face via the plurality of optical elements, and at least an optical element where an incident angle range of the light is largest out of the plurality of optical elements is the reflective mirror. 9. An exposure apparatus for exposing a substrate to exposure light comprising the reflective mirror of claim 1. 10. A device manufacturing method comprising steps of:exposing a substrate using the exposure apparatus of claim 9; anddeveloping the exposed substrate. 11. The reflective mirror according to claim 1, wherein the light is extreme ultraviolet light. 12. The reflective mirror according to claim 1, wherein the multilayer film has a thickness distribution, which is non-rotationally symmetric.