Multilayer reflective mirrors for EUV, wavefront-aberration-correction methods for same, and EUV optical systems comprising same

In a method for making a multilayer mirror, wherein a stack of alternating layers of first and second materials is formed on a surface of a mirror substrate, and the first and second materials have different respective refractive indices with respect to EUV radiation, a method for reducing wavefront aberrations of EUV radiation reflected from a surface of the multilayer mirror, comprising:

• placing the multilayer mirror in an EUV optical system transmissive to EUV radiation at a wavelength at which the multilayer mirror is to be used;
• at the EUV wavelength at which the multilayer is to be used, measuring a profile of a wavefront transmitted through the EUV optical system to obtain a map of the surface indicating regions targeted for surficial removal of one or more layers of the multilayer film necessary to reduce wavefront aberrations of EUV light reflected from the surface;
• and
• based on the map, removing one or more surficial layers in the indicated regions.

A method for making a multilayer mirror for use in an EUV optical system, comprising:

• forming a stack of alternating layers of superposed first and second materials on a surface of a mirror substrate, the first and second materials having different respective refractive indices with respect to EUV radiation;
• at an EUV wavelength at which the multilayer mirror is to be used, measuring a profile of a reflected wavefront from the surface to obtain a map of the surface indicating regions targeted for surficial removal of one or more layers of the multilayer film necessary to reduce wavefront aberrations of EUV light reflected from the surface;
• and
• based on the map, removing one or more surficial layers in the indicated regions.

A multilayer mirror that is reflective to incident EUV radiation, comprising:

• a mirror substrate;
• and
• a thin-film layer stack formed on a surface of the mirror substrate, the stack including multiple thin-film first layer groups and multiple thin-film second layer groups alternatingly superposed relative to each other in a periodically repeating manner, each first layer group including at least one sublayer of a first material having a refractive index to EUV light substantially equal to the refractive index of a vacuum, and each second layer group including at least one sublayer of a second material and at least one sublayer of a third material, the first and second layer groups being alternatingly superposed relative to each other in a periodically repeating configuration, the second and third materials having respective refractive indices that are substantially similar to each other but different from the refractive index of the first material sufficiently such that the stack is reflective to incident EUV light, and the second and third materials having differential reactivities to sublayer-removal conditions such that a first sublayer-removal condition will remove a sublayer of the second material preferentially without substantial removal of an underlying sublayer of the third material, and a second sublayer-removal condition will remove a sublayer of the third material preferentially without substantial removal of an underlying sublayer of the second material.

A method for making a multilayer mirror for use in an EUV optical system, comprising:

• on a surface of a mirror substrate, forming a thin-film layer stack including multiple thin-film first layer groups and multiple thin-film second layer groups alternatingly superposed relative to each other in a periodically repeating configuration, each first layer group including at least one sublayer of a first material having a refractive index to EUV light substantially equal to the refractive index of a vacuum, and each second layer group including at least one sublayer of a second material and at least one sublayer of a third material, the first and second layer groups being alternatingly superposed relative to each other in a periodically repeating configuration, the second and third materials having respective refractive indices that are substantially similar to each other but different from the refractive index of the first material sufficiently such that the stack is reflective to incident EUV light, and the second and third materials having differential reactivities to sublayer-removal conditions such that a first sublayer-removal condition will preferentially remove a sublayer of the second material without substantial removal of an underlying sublayer of the third material, and a second sublayer-removal condition will preferentially remove a sublayer of the third material without substantial removal of an underlying sublayer of the second material;
• and
• in selected regions of a surficial second layer group, removing one or more sublayers of the surficial second layer group so as to reduce wavefront aberrations of EUV radiation reflected from the surface.

A multilayer mirror, comprising:

• a mirror substrate;
• a stack of alternatingly superposed layers of first and second materials formed on a surface of the mirror substrate, the first and second materials having different respective refractive indices with respect to EUV radiation, wherein selected regions of the multilayer mirror have been subjected to surficial-layer shaving so as to correct a reflected-wavefront profile from the mirror;
• and
• a cover layer formed on a surface of the stack, the cover layer being of a material exhibiting a persistent and consistently high transmissivity to electromagnetic radiation of a specified wavelength, the cover layer extending over regions of the surface of the stack including the selected regions and having a substantially uniform thickness.

A method for making a multilayer mirror for use in an EUV optical system, comprising:

• on a surface of a mirror substrate, forming a thin-film layer stack including multiple layers of a first material and multiple layers of a second material alternating superposed relative to one another in a periodically repeating manner, the first and second materials having different respective refractive indices with respect to EUV radiation;
• removing one or more surficial layers from selected surficial regions of the multilayer mirror so as to correct a reflected-wavefront profile from the mirror;
• and
• forming a cover layer on a surface of the stack, the cover layer being of a material exhibiting a persistent and consistently high transmissivity to electromagnetic radiation of a specified wavelength, the cover layer extending over regions of the surface of the stack including the selected surficial regions and having a substantially uniform thickness.