EUV mask blank defect mitigation
View Patent ↗A solution for mitigating the effects of EUV substrate surface defects is disclosed. In one embodiment, a layer of polyimide material is formed upon a mask substrate surface, resulting in a substantially defect free surface adjacent to which a reflective multilayer may be positioned for EUV lithography. To reduce the possibility of polyimide outgassing and resultant added roughness to adjacently positioned layers, the layer of polyimide may be cured in a vacuum at an elevated temperature before other layers are adjacently positioned.
1. An EUV mask structure comprising:
a substrate layer;
a reflective multilayer;
a polyimide layer between the substrate layer and reflective multilayer.
2. The EUV mask structure of claim 1 further comprising a mask absorber layer adjacent the reflective multilayer.
3. The EUV mask structure of claim 1 wherein the substrate layer comprises a silicate glass or glass ceramic.
4. The EUV mask structure of claim 3 wherein the substrate layer comprises positive and negative defects, wherein the positive defects protrude away from the substrate layer by less than about 100 nanometers, and the negative defects are defined into the substrate layer by less than about 100 nanometers.
5. The EUV mask structure of claim 1 wherein the reflective multilayer comprises about 40 pairs of alternating layers of molybdenum and silicon having thicknesses of about 2.8 nanometers and about 4.2 nanometers, respectively.
6. The EUV mask structure of claim 1 wherein the reflective multilayer comprises about 70 pairs of alternating layers of molybdenum and beryllium, each pair having a thickness of about 5.8 nanometers, wherein 40% of said thickness is molybdenum and about 60% of said thickness is beryllium.
7. The EUV mask structure of claim 4 wherein the polyimide layer has a thickness sufficient to cover the highest positive defects.
8. The EUV mask structure of claim 7 wherein the polyimide layer substantially completely infills each of the negative defects.
9. The EUY mask structure of claim 2 wherein the mask absorber layer comprises a material selected from the group consisting of tantalum nitride, tantalum oxynitride, and chromium.
10. The EUV mask structure of claim 2 further comprising a protective layer positioned between the reflective multilayer and the mask absorber layer.
11. The EUV mask structure of claim 10 wherein the protective layer comprises a material selected from the group consisting of ruthenium, silicon dioxide, and silicon oxynitride.
12. A method to form an EUV mask structure comprising:
minimizing defect geometry defined by a substrate surface;
depositing a layer of polyimide precursor material upon the substrate surface;
curing the polyimide precursor material to form a cured polyimide layer;
forming a reflective multilayer adjacent the cured polyimide layer.
13. The method of claim 12 wherein minimizing defect geometry comprises polishing the substrate surface.
14. The method of claim 12 wherein depositing a layer of polyimide precursor material comprises spin coating the polyimide precursor material upon the substrate surface.
15. The method of claim 12 wherein curing the polyimide precursor material comprises heating the polyimide precursor material to a temperature between about 250 and about 400 degrees Celsius.
16. The method of claim 12 further comprising applying a vacuum before and during curing the polyimide precursor material.
17. The method of claim 13 wherein the substrate surface defines positive defects after polishing extending away from the substrate surface by less than about 100 nanometers.
18. The method of claim 17 wherein the cured polyimide layer covers the positive defects.
19. The method of claim 12 further comprising forming a protective layer adjacent the reflective multilayer.
20. The method of claim 12 further comprising forming a mask absorber layer adjacent the reflective multilayer.