Patent Number: 
Section: claims

1. A reflective mirror for reflecting incident light comprising:a base; anda multilayer film configured to reflect at least a portion of the incident light and including a first layer and a second layer that are laminated alternately on the base,the multilayer film including a first portion having a first thickness and a second portion having a second thickness different from the first thickness, the second portion being provided at a position rotationally symmetric to a position of the first portion about an optical axis of the reflective mirror, andwherein the base comprises a first surface height corresponding to the first portion and a second surface height that is different from the first surface height corresponding to the second portion. 2. The reflective mirror according to claim 1, wherein the second surface is formed at a position further from a surface of the multilayer film than the first surface. 3. The reflective mirror according to claim 1, wherein, in the multilayer film, the first thickness and the second thickness are each determined such that light incident at a first angle on the first portion is reflected, light incident at a second angle on the second portion is reflected, and a difference in reflectance of the incident light between the first portion and the second portion is reduced. 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 the 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 the 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 a center of a region to which the light is incident on a surface of the multilayer film and the optical axis of the reflective mirror. 7. The reflective mirror according to claim 1, wherein the multilayer film has a thickness distribution, the thickness distribution having a finite number of axes of rotational symmetry on the surface of the multilayer film. 8. The reflective mirror according to claim 1, wherein a shape of a surface of the base where the multilayer film is formed is determined such that an aberration caused by the thickness distribution of the multilayer film is reduced. 9. A projection optical system comprising a plurality of optical elements, the projection optical system projecting an image of a first surface onto a second surface, and at least one of the optical elements being the reflective mirror described in claim 1. 10. The projection optical system according to claim 9, wherein light from the first surface is irradiated onto the second surface 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. 11. An exposure apparatus for exposing a substrate to exposure light comprising the reflective mirror described in claim 1. 12. A device manufacturing method comprising steps of:exposing a substrate using the exposure apparatus described in claim 11; anddeveloping the exposed substrate.