Patent Number: 
Section: claims

1. A multilayer mirror configured to reflect extreme ultraviolet (EUV) radiation, the mirror comprising:a substrate; anda stack of layers formed on the substrate,wherein the stack of layers comprises layers comprising a low index material and a high index material, the low index material having a lower real part of the refractive index than the high index material at a given operating wavelength λ, andwherein the mirror provides a first peak of reflectivity of at least 20% at a first wavelength λ1 in a first wavelength band extending from 6 nm to 7 nm and a second peak of reflectivity of at least 20% at a second wavelength λ2 in a second wavelength band extending from 12.5 nm to 15 nm. 2. The multilayer mirror according to claim 1, wherein a first peak reflectivity at the first wavelength is greater than 30% and a second peak reflectivity at the second wavelength is greater than 40%. 3. The multilayer mirror according to claim 1, wherein the first wavelength is between 40% and 60% of the second wavelength. 4. The multilayer mirror according to claim 1, wherein the stack of layers comprises:a first layer group formed on the substrate, the first layer group providing higher reflectivity at the first wavelength than at the second wavelength, anda second layer group formed on the first layer group, the second layer group providing higher reflectivity at the second wavelength than at the first wavelength. 5. The multilayer mirror according to claim 4, wherein materials of the layers of the second layer group are selected from a group of materials having low absorption with extinction coefficient β less than 0.05 at the first wavelength. 6. The multilayer mirror according to claim 4, wherein alternating layers of a first material and a second material form a number of consecutive two-layer repetitive units, each two-layer repetitive unit comprising a single layer of the first material and an adjacent single layer of the second material, the first layer group comprising a first number of two-layer repetitive units and the second layer group comprising a second number of two-layer repetitive units, wherein at least one of the following conditions is met:the first number is greater than the second number;the first number is more than ten times the second number;the first number is greater than 100 and the second number is less than 20. 7. The multilayer mirror according to claim 1, wherein the stack of layers comprises a plurality of four-layer repetitive units, each comprising:a first layer of a first material,a second layer of a second material on the first layer,a third layer of a third material on the second layer, anda fourth layer of a fourth material on the third layer, whereina first real part of the refractive index (1−δ)1 of layer materials of the first to fourth material at the first wavelength λ1 varies periodically in a depth direction of the stack of layers according to a first refractive index period length,a second real part of the refractive index (1−δ)2 of layer materials at the second wavelength λ2 varies periodically in the depth direction of the stack of layers according to a second refractive index period length; andthe second refractive index period length is about twice the first refractive index period length. 8. The multilayer mirror according to claim 7, wherein the respective refractive index period length corresponds to about half the respective wavelength so that RIP1=(0.5 λ1)±10% and RIP2=(0.5 λ2)±10%. 9. The multilayer mirror according to claim 7, wherein the first material and the third material each has a second real part of refractive index at the second wavelength which is intermediate between a second real part of refractive index of the second material and a second real part of refractive index of the fourth material. 10. The multilayer mirror according to claim 7, wherein a sequence of consecutive four-layer repetitive units comprises a sequence of three consecutive layers of a high index material, an intermediate index material and a low index material, wherein the low index material has a lower real part of the refractive index than do the high index material and the intermediate index material, and the intermediate index material has a lower real part of the refractive index than does the high index material. 11. The multilayer mirror according to claim 10, wherein the intermediate index material has a real part of the refractive index, (1−δ) in a range from 0.94 to 0.98 at the second wavelength. 12. The multilayer mirror according to claim 7, wherein the first material and the third material is selected from a group consisting of boron (B), boron carbide (B4C), zirconium (Zr) and boron nitride (BN). 13. The multilayer mirror according to claim 7, wherein the first material is the same as the third material. 14. The multilayer mirror according to claim 1, wherein the stack of layers comprises:a first layer group formed on the substrate, the first layer group comprising a plurality of first four-layer repetitive units, anda second layer group formed on the first layer group, the second layer group comprising a plurality of second four-layer repetitive units different from the first four-layer repetitive units, wherein each of the first and second four-layer repetitive units comprises:a first layer of a first material,a second layer of a second material on the first layer,a third layer of a third material on the second layer, anda fourth layer of a fourth material on the third layer, wherein:a first real part of the refractive index (1−δ)1 of layer materials of the first to fourth material at the first wavelength λ1 varies periodically in a depth direction of the stack of layers according to a first refractive index period length,a second real part of the refractive index (1−δ)2 of layer materials at the second wavelength λ2 varies periodically in the depth direction of the stack of layers according to a second refractive index period length; andthe second refractive index period length is about twice the first refractive index period length. 15. An EUV optical system comprising at least one multilayer mirror according to claim 1. 16. The EUV optical system according to claim 15, wherein the optical system is selected from the group consisting of:a projection objective of a microlithography projection exposure apparatus;an illumination system of a microlithography projection exposure apparatus;a microscopy optical system;an inspection system.