Diffractive optical element for extreme ultraviolet wavefront control
View Patent ↗A wavefront modulating optical element device employs two or more materials lithographically patterned with programmed thickness profiles. The spatially-varying thickness profiles are chosen to yield arbitrary relative phase-shift and attenuation upon transmission. The device can be designed to create arbitrary diffractive optical elements with specific applications including diffusers, arbitrary holographic optical elements, null-elements for wavefront compensation and control in interferometry.
1. An optical element for extreme ultraviolet (EUV) radiation wavefront control that comprises:
a pattern of at least two bilayer structures with a first bilayer structure comprising a first layer of a first material and a second layer of a second material and a second bilayer structure comprising a third layer of the first material and a fourth layer of the second material, wherein the thickness of the first layer is different than that of the third layer and the thickness of the second layer is different than that of the fourth layer and wherein the pattern is formed on the planar surface of a substrate or membrane and wherein the first material has a different index of refraction than that of the second material and characterized in that incident EUV radiation passing through a first bilayer structure is phase shifted relative to EUV radiation passing through a second bilayer structure by a desired value and characterized in that radiation passing through the first bilayer structure exhibits an amplitude transmission reduction of a desired first value and that radiation passing through the second bilayer structure exhibits an amplitude transmission reduction of a desired second value.
2. The optical element of claim 1 wherein the radiation is phase shifted by approximately an odd multiple off π radians.
3. The optical element of claim 1 wherein the first material has approximately the same absorptive to radiation as that of the second material and the thicknesses of the bilayer structures are approximately the same.
4. The optical element of claim 1 wherein the radiation is phase-shifted and the amplitude transmission ranges from 100% to less than 1%.
5. The optical element of claim 1 wherein the first and second materials are selected from the following pairs of materials: (i) beryllium and silicon, (ii) molybdenum and carbon, and (iii) ruthenium and nickel.
6. The optical element of claim 1 wherein the pattern is formed on a membrane that has one or more apertures.
7. The optical element of claim 6 wherein the membrane is substantially transmissive to the radiation.
8. The optical element of claim 1 wherein the substrate or membrane is made of silicon nitride or silicon.
9. A method of fabricating an optical element for extreme ultraviolet (EUV) radiation wavefront control that comprises:
forming a pattern of at least two bilayer structures with a first bilayer structure comprising a first layer of a first material of a first thickness and a second layer of a second material of a second thickness, and a second bilayer structure comprising a third layer of the first material of a third thickness and a fourth layer of the second material of a fourth thickness, wherein the first thickness is different than the third thickness and the second thickness is different than the fourth thickness and wherein the pattern is formed on the planar surface of a substrate or membrane that is made of a third material and wherein the first material has a different index of refraction than that of the second material such that incident EUV radiation passing through a first bilayer structure is phase shifted relative to EUV radiation passing through a second bilayer structure by a desired value and characterized in that radiation passing through the first bilayer structure exhibits an amplitude transmission reduction of a desired first value and that radiation passing through the second bilayer structure exhibits an amplitude transmission reduction of a desired second value.
10. The method of claim 9 wherein for a given bilayer structure, said bilayer structure is formed by:
(i) calculating the combination of thicknesses of a layer of the first material and a layer of the second material required to achieve the desired phase shift relative to the incident radiation; and
(ii) depositing the layer of the first material and the layer of the second material to create the bilayer structure.
11. The method of claim 9 wherein for a given bilayer structure, the thicknesses are selected so that the structure exhibits a spatially varying amplitude transmission reduction of a desired value.
12. The method of claim 9 wherein the radiation is phase shifted by approximately an odd multiple of π radians.
13. The method of claim 12 wherein the radiation is phase-shifted and the amplitude transmission ranges from 100% to 1%.
14. The method of claim 9 wherein the first material has approximately the same absorptivity to radiation as that of the second material and the thicknesses of the bilayer structures are approximately the same.
15. The method of claim 9 wherein the pattern defines contiguous bilayer structures and is characterized in that the radiation passing through each bilayer structure does not all exhibit the same phase shift.
16. The method of claim 9 wherein the first and second materials are selected from the following pairs of materials: (i) beryllium and silicon, (ii) molybdenum and carbon, and (iii) ruthenium and nickel.
17. The method of claim 9 wherein the pattern is formed on a substrate that has one or more apertures.
18. The method of claim 9 wherein the substrate is substantially transmissive to the radiation.
19. The method of claim 9 wherein the substrate or membrane is made of silicon nitride or silicon.