Reflective mask useful for transferring a pattern using extreme ultraviolet (EUV) radiation and method of making the same
View Patent ↗An EUV mask ( 10 ) includes an opening ( 26 ) that helps to attenuate and phase shift extreme ultraviolet radiation using a subtractive rather than additive method. An etch stop layer ( 20 ) may be provided between a lower multilayer reflective stack ( 14 ) and an upper multilayer reflective stack ( 22 ) to ensure an appropriate and accurate depth of the opening. An absorber layer ( 32 ) may be deposited within the opening to sufficiently reduce the amount of reflection within dark region ( 30 ). Optimal thicknesses and locations of the various layers are described.
1. A reflective mask useful for transferring a pattern to a semiconductor substrate comprising:
a mask substrate;
a reflective stack formed directly on the mask substrate, wherein an uppermost surface of the reflective stack defines a top plane of the mask;
an embedded etch stop layer embedded within the reflective stack, and wherein a bottom of the opening is defined by the embedded etch stop layer; and
an opening formed partially through the reflective stack from the top plane, wherein the opening location corresponds to a region where lower reflectivity than that provided by the reflective stack is desired.
2. The reflective mask of claim 1 wherein the reflective stack comprises a stack of alternating first and second layers, wherein a material of the first layer is a transmissive material and a material of the second layer has an index of refraction substantially different than an index of refraction of the material of the first layer.
3. The reflective mask of claim 2 wherein the stack of alternating layers comprises alternating layers of molybdenum and silicon.
4. The reflective mask of claim 1 further comprising an absorber layer formed over the embedded etch stop layer within the opening.
5. The reflective mask of claim 4 wherein the embedded etch stop layer and absorber layer each comprise chromium.
6. A reflective mask useful for transferring a pattern to a semiconductor substrate comprising:
a mask substrate;
a lower multilayer reflective stack formed directly on the mask substrate;
an etch stop layer formed on the tower multilayer reflective stack;
an upper multilayer reflective stack formed over the etch stop layer;
an opening formed through the upper multilayer reflective stack and exposing the etch stop layer; and
an absorber layer formed over the etch stop layer within the opening.
7. The reflective mask of claim 6 , wherein the etch stop layer has a thickness of approximately λcos θ/(2n), wherein λ is a wavelength of EUV radiation to be used; n is an index of refraction of the etch stop layer; and θ is an angle of incidence of EUV radiation on the reflective mask to be used.
8. The reflective mask of claim 6 wherein:
the opening corresponds to a dark region;
areas of the upper multilayer reflective stack adjacent the opening correspond to bright regions;
the upper multilayer reflective stack defines a top plane of the reflective mask; and
the thickness of the absorber layer is chosen so that reflection of EUV radiation at the top plane within the opening is 3-20 percent of reflection of EUV radiation at the top plane within the upper multilayer reflective stack.
9. The reflective mask of claim 8 wherein the lower and upper multilayer reflective stacks each comprise periods of alternating layers of different materials, and wherein a number of periods for each of the lower and upper multilayer reflective stack is determined, at least in part, by a requirement that EUV radiation reflected in the dark region is approximately 180° out of phase with respect to EUV radiation reflected in the bright region.
10. The reflective mask of claim 6 wherein the absorber layer comprises chromium.
11. The reflective mask of claim 6 wherein the absorber layer has a thickness of between approximately 10 to 50 nanometers.
12. The reflective mask of claim 6 wherein the etch stop layer comprises a material selected from a group consisting of chromium, ruthenium, chrome oxide, chrome nitride, boron carbide, zirconium, tantalum oxide, tantalum nitride, tantalum silicon nitride.
13. The reflective mask of claim 6 wherein the etch stop layer comprises chromium.
14. The reflective mask of claim 6 wherein the etch stop layer has a thickness of between approximately 5 to 20 nanometers.
15. The reflective mask of claim 6 wherein the lower and upper multilayer reflective stacks each comprise periods of alternating layers of different materials, and wherein a number of periods in the lower multilayer reflective stack is between 10 and 20.
16. The reflective mask of claim 15 wherein a number of periods in the upper multilayer reflective stack is between 20 and 30.
17. A method for making a reflective mask useful for transferring a pattern to a semiconductor substrate using extreme ultraviolet (EUV) radiation comprising:
providing a mask substrate;
forming a lower multilayer reflective stack directly on the mask substrate;
forming an etch stop layer over the lower multilayer reflective stack;
forming an upper multilayer reflective stack on the etch stop layer;
etching an opening through the upper multilayer reflective stack to expose the etch stop layer; and
forming an absorber layer formed over the etch stop layer within the opening.
18. The method of claim 17 wherein the absorber layer comprises chromium.
19. The method of claim 17 wherein forming a lower and an upper multilayer reflective stack each comprise forming a series of alternating first and second layers, wherein the first layer comprises a transmissive material and the second layer has an index of refraction substantially different than an index of refraction of the first layer.
20. The method of claim 19 wherein the series of alternating layers in each of the lower and upper multilayer reflective stacks comprises alternating layers of molybdenum and silicon.
21. The method of claim 19 wherein a combination of one first layer and one second layer constitutes a period, and a number of periods in the lower multilayer reflective stack is between 10 and 20.
22. The method of 19 wherein the etch stop layer is formed to have a thickness of approximately λcos θ/(2n), wherein λ is an wavelength of EUV radiation to be used; n is an index of refraction of the etch stop layer; and θ is an angle of incidence of EUV radiation on the reflective mask.
23. The method of claim 17 wherein the absorber layer is formed to have a thickness of between approximately 10 to 50 nanometers.
24. The method of claim 17 wherein the etch stop layer comprises a material selected from a group consisting of chromium, ruthenium, chrome oxide, chrome nitride, boron carbide, zirconium, tantalum oxide, tantalum nitride, tantalum silicon nitride.
25. The method of claim 24 wherein the etch stop layer comprises chromium.
26. The method of claim 24 wherein the etch stop layer has a thickness of between approximately 5 to 20 nanometers.
27. The method of claim 17 further comprising the step of forming a hardmask on the upper multilayer reflective stack, and wherein the step of etching an opening comprises etching an opening using the hardmask as an etch mask.
28. The method of claim 27 wherein the hardmask comprises chromium.
29. A method for patterning a photoresist layer on a semiconductor substrate using a reflective mask comprising
providing a semiconductor substrate;
forming a photoresist layer over the semiconductor substrate;
providing a reflective mask, the reflective mask comprising:
a mask substrate;
a lower multilayer reflective stack formed directly on the mask substrate;
an etch stop layer formed on the lower multilayer reflective stack;
an upper multilayer reflective stack formed over the etch stop layer;
an opening formed through the upper multilayer reflective stack and exposing the etch stop layer;
an absorber layer formed over the etch stop layer within the opening projecting incident radiation on the reflective mask;
reflecting the incident radiation from the reflective mask as reflected radiation; and illuminating the photoresist layer with the reflected radiation.