Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1999-05-26
2001-05-08
Speer, Timothy M. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S627000, C428S634000, C428S641000, C428S649000, C428S633000, C428S663000
Reexamination Certificate
active
06228512
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to multilayer mirrors, particularly to beryllium based multilayers for extreme ultraviolet (EUV) lithography optics, and more particularly to a high reflectance, low intrinsic roughness and low stress multilayer composed of MoRu/Be, which may include a diffusion barrier on the interfaces and/or a capping layer.
2. Background
Beryllium (Be) based multilayers reflect at wavelengths above 11.1 nm. At these wavelengths, the spectral match with a laser-plasma EUV source improves the optical throughput by a factor of five, as compared to the currently used Mo/Si wavelength (13.4 nm). Higher throughput is crucial for commercial success of EUV lithography because it reduces the costs.
Molybdenum/beryllium (Mo/Be) multilayers have been studied as potential candidates for EUV applications, as discussed in Skulina et al., “Molybdenum/beryllium multilayer mirrors for normal incidence in the extreme ultraviolet,”
Appl. Opt.
34:327-3730 (1995); and Stearns et al., “Beryllium-based multilayer structures,”
Mat. Res. Soc. Symp. Proc.,
Vol. 382, 1995 Materials Research Society. A 70% reflectance using Mo/Be coatings with 70 periods (bilayer pairs) at 11.3 nm and 0.28 nm peak width has been demonstrated. Even though Mo/Be multilayers have higher reflectance than Mo/Si multilayers, they have a disadvantage: the effect of a rough (>0.2 nm RMS) substrate on their reflectance. A rougher substrate induces a substantial increase in the roughness of the Mo/Be multilayer. See D. G. Stearns, “Intrinsic defects in multilayer coatings: A statistical analysis,” (1998). The reason for this is the crystallinity of both Mo and Be layers. Because both layers are crystalline, there is no mechanism to heal imperfections on the substrate or defects occurring during the growth of the multilayer. This leads to a much higher intrinsic roughness in Mo/Be multilayers as compared to Mo/Si multilayers.
Also, a typical Mo/Be multilayer consists of 70 bilayers (or periods). A peak width is dependent on the number of bilayers and on the optical properties of the two materials in the multilayer. A smaller number of bilayers is preferable because the peak width is then larger. For example, a typical Mo/Si multilayer coating consists of only 40 bilayers and therefore the peak width is about 0.5 nm. The peak width of a 70 bilayer Mo/Be coating is 0.28 nm. Thus, there is a need for a high reflectance, low intrinsic roughness and low stress multilayer system for EUV lithography optics which has a low number of bilayers and a high peak width.
The present invention addresses this problem by providing a multilayer system with a reflectivity of above 70% and a peak width 0.35 to 0.45 nm with 50 bilayers. The multilayer system of this invention is composed of amorphous layers of an alloy composed of molybdenum and ruthenium, and crystalline beryllium layers, thus resulting in a multilayer of molybdenum ruthenium/beryllium (MoRu/Be), with an ideal theoretical peak width of 0.45 nm and reflectivity of over 75%. The period (bilayer) thickness is typically 5.85 nm and the multilayer reflects at 11.4 nm. This multilayer is much thinner than the 70 bilayer Mo/Be and is of comparable thickness to 40 bilayer Mo/Si multilayer. Therefore, the image quality due to thickness uniformity will be comparable to Mo/Si multilayer optics. The MoRu/Be multilayer system has a very low intrinsic stress (21 MPa tensile stress for a 50-bilayer system). To increase the peak width and improve the throughput, a diffusion barrier can be added to make the interfaces sharper. Also, a capping layer can be utilized to improve the long term reflectance stability of the multilayer.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved multilayer system for EUV lithography.
A further object of the invention is to provide a high reflective, low intrinsic roughness and low stress multilayer system for EUV lithography optics.
A further object of the invention is to provide a multilayer system which includes amorphous layers of MoRu and crystalline Be layers.
A further object of the invention is to provide a multilayer system having a reflectance of above 70% which reflects at 11.4 nm, with a peak width in the 0.35-0.45 nm range.
Another object of the invention is to provide low intrinsic roughness, high reflectance MoRu/Be multilayer mirrors for EUV lithography, which has a higher throughput than a Mo/Be multilayer system.
Another object of the invention is to provide a MoRu/Be multilayer with a diffusion barrier on the Be—MoRu interface to sharpen the interfaces and to increase the throughput.
Another object of the invention is to provide a MoRu/Be multilayer with a capping layer to improve long term reflectance stability of the multilayer.
Another object of the invention is to provide a MoRu/Be multilayer system having about 50 bilayers with a peak width of up to 0.45 nm.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically, the present invention involves low intrinsic roughness, low stress, and high reflectance MoRu/Be multilayer mirrors for EUV lithography. The multilayer comprises amorphous layers of an alloy composed of molybdenum and ruthenium, and crystalline layers of beryllium. A MoRu/Be multilayer system with 50 bilayers has a very low, tensile stress. A tensile stress of only 21 MPa has been measured. An amorphous silicon layer may be used as a first layer in the multilayer stack to smooth the substrate.
Also, a diffusion barrier layer made of a low Z material such as carbon or carbides (e.g., Be
2
C or B
4
C) may be used at the MoRu/Be interfaces of the stack to provide higher reflectance and larger peak width for improving the optical throughput, which can be potentially 30-40% higher than the Mo/Be multilayer coatings. In addition, a thin capping layer of ZrO
2
, SiO
2
(4 nm) or ZnO (0.6 nm) may be utilized to improve the long term reflectance stability of the multilayer. These capping materials form native oxides and are water resistant, an important property for the EUV system where residual water vapor is anticipated. The capping layer materials must have optical transparency at EUV wavelengths. The optimum number of bilayers in the MoRu/Be multilayer is 50, with a typical period (bilayer) thickness of 5.85 nm. The maximum peak reflectivity wavelength is 11.4 nm, with a peak width of 0.35 to 0.45 nm.
REFERENCES:
patent: 5199055 (1993-03-01), Noguchi et al.
patent: 5399448 (1995-03-01), Nagata et al.
patent: 5572564 (1996-11-01), Murakami
patent: 5958629 (1999-09-01), Yan et al.
patent: 6013399 (2000-01-01), Nguyen
patent: 01094300 (1989-12-01), None
Mandeep, Singh, et al., “Design of Multilayer Extreme-Ultraviolet Mirrors for Enhanced Reflectivity,” Applied OPtics, vol. 39, No. 13, 1 May 2000, XP-002145180
Bajt, Sasa, “Molybdenum-Ruthenium/Beryllium Multilayer Coatings, ” Journal of Vacuum Science & Tech., vol. 18, No. 2, Mar/Apr 2000, XP-00900490
Mirkarimi, P.B., et al., “Advances in the Reduction and Compensation of Film Stress in High-Refectance Multilayer Coatings for Extreme Ultraviolet Lithography, ” SPIE, vol. 3331,23-25 Feb. 1998, XP-000900531
Skulina, K.M., et al., “Molybdenum/Beryllium Multilayer Mirrors for Normal Incidence in the Extreme Ultraviolet,” Applied Optics, vol. 34, No. 19, 1 July 1995, XP-000537295
Stearns, D.G., et al., “Beryllium-Based Multilayer Structures,”Mat. Res. Soc. Symp. Proc. (17-20 Apr. 1995), vol. 382, XP-000900472
Bajt Sasa C.
Wall Mark A.
Carnahan L. E.
Speer Timothy M.
The Regents of the University of California
Thompson Alan H.
Young Bryant
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