Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-01-13
2001-10-30
Beck, Shrive (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S162000, C427S397700, C427S376700, C427S383100, C427S404000, C427S419100
Reexamination Certificate
active
06309705
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to multilayer coatings, particularly to Mo—Si multilayer reflective coatings, and more particularly to Mo—Si multilayer coatings having a near normal incidence reflectance of ≧65% in the extreme ultraviolet wavelength region and a magnitude of residual stress of not greater than about 100 MPa and to the method for fabricating same.
Multilayer structures composed of alternating thin layers of materials with vastly different optical properties have proven effective as high-reflectance coatings for various applications. The Mo—Si system which gives a high reflectance (~60%) for certain wavelengths, such as in the 13.0-13.5 wavelength region, is of particular technological importance due to its application to high-resolution, multiple-reflection imaging systems now being developed for projection lithography.
Extreme-ultraviolet (EUV) lithography systems require several high precision optics coated with reflective multilayers. To obtain sufficient throughput and image quality, these multilayer coatings must simultaneously have high reflectance (R≧65%) and low magnitude of residual stress (less than 100 MPa) The multilayer coatings generally consist of alternating layers of molybdenum (Mo) and silicon (Si).
There is a strong commercial driving force for increased miniaturization in electronic devices and, hence, an EUV lithography tool has significant commercial potential. The performance of an EUV lithography tool is key to its implementation over other competing technologies, and high film stresses and low EUV reflectances degrade the performance of a EUV lithography tool.
Over the past decade or so numerous publications described the dependency of EUV reflectance of Mo—Si multilayer mirrors or optics on their fabrication parameters. However, the number of publications addressing the problem of stress in Mo—Si multilayers designed for high reflectance in the EUV wavelength is relatively small. The later publications describe how the stress of Mo—Si multilayer coatings can be reduced to low levels (<100 MPa) by: 1) post-deposition annealing (see Kola et al, Appl. Phys. Lett. 60,3120 (1992) and Kassner et al, J. Mat. Sci. 31, 2291 (1996); 2) variation of the Mo to Si layer thickness or ratio (see Nguyen et al, in Physics of X-Ray Multilayer Structures, Optical Society of America, Washington, D.C., 1994, Vol. 6, P. 103; Windt et al, J. Appl. Phys. 78,2423 (1995); and Tinone et al, J. Electron Specrosc. Relat. Phenom. 80,461 (1996); and 3) adjustment of the sputter deposition process such as base pressure or target power (see Windt et al and Tinone et al above). Although Kola et al and Windt et al discuss low stress coatings, none of these prior efforts involve the fabrication of a high reflectance (≧65%) Mo—Si multilayer coating with a low magnitude of stress (<100 MPa), and no EUV reflection data was shown, except in Kola et al where reflectances around 58% were measured, which is significantly lower than 65%, which is necessary for use in EUV lithography.
Recently, Mo—Si multilayer coatings using a buffer layer between the substrate and the Mo—Si coatings have provided high reflectance and low stress. This non-thermal or a thermal method of producing the high reflectance-low stress Mo—Si multilayer coatings is described and claimed in copending U.S. application Ser. No. 09/027,309, filed Feb. 20, 1998, now U.S. Pat. No. 6,011,646, entitled “A Method To Adjust Multilayer Film Stress Induced Deformation Of Optics”, assigned to the same assignee.
The present invention provides a thermal approach to producing Mo—Si multilayer reflective coatings with high reflectance and low stress, which are particularly applicable for use in an EUV lithography tool. The present invention provides a high reflectance-low stress Mo—Si multilayer coating produced by heating the multilayer coating to a given temperature during a given time period after deposition to induce structural changes in the multilayer coating. Mo—Si multilayer coatings of this invention produced using this method have a high reflectance (R≧65%) and a low magnitude of residual stress (≦100 MPG). It has been verified by experimentation that using the thermal method, low stresses are obtained in Mo—Si multilayer coatings with minimal (~1%-3%) loss in reflectance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a high reflectance-low stress Mo—Si multilayer reflective coating.
A further object of the invention is to reduce multilayer stress by at least 75% while maintaining reflectance of at least 65%.
A further object of the invention is to provide Mo—Si multilayer coatings for use in extreme-ultraviolet (EUV) lithography systems.
A further object of the invention is to utilize slow or rapid post-deposition annealing to reduce stress in multilayers.
A further object of the invention is to enable the production of reflective coatings for EUN lithography tools having both high (>65%) reflectance and low (<100 MPa) magnitude of residual stress.
Another object of the invention is to provide a method for substantially reducing the stress in Mo—Si multilayer coatings.
Another object of the invention is to provide Mo—Si multilayers using a method which results in reducing stress in the Mo—Si multilayers with minimal loss of reflectance thereof.
Another object of the invention is to provide Mo—Si multilayer films by thermal processing of the deposited Mo—Si multilayers thereby reducing the stress of the multilayers with minimal (~1%-3%) reflectance loss.
Another object of the invention is to produce high reflectance-low stress Mo—Si multilayer coatings for EUV lithography tools by a method that involves heating the multilayer coatings to a given temperature during a given time after deposition which reduces the magnitude of stress with minimal loss in reflectance of the multilayer coating.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawing. The invention provides high reflectance-low stress Mo—Si multilayer reflective coatings for EUV lithography.
The Mo—Si multilayer reflective coatings of the present invention may be produced by a method which involves thermally processing the Mo—Si multilayer coatings after deposition for reducing the residual stress therein. The thermal processing, for example, is carried out by either slow or rapid annealing by heating the deposited Mo—Si multi-layers to a given temperature (e.g. 220° C. or 300° C.) for a given time period (e.g. 6 hrs. or 30 secs.) which reduces the magnitude of stress to less than 100 MPG, with a 1%-3% reflectance loss, whereby the reflectance of the multilayers remains above 65% thus enabling use thereof in EUV lithography. The stress can be reduced by at least 75%. While the Mo—Si multilayer reflective coatings of the present invention are particularly applicable for EUV lithography, the reflective coatings may also be used for x-ray laser cavities and optics, optics for sources including laser-produced plasmas and synchrotrons, and in optics for x-ray microscopy and astronomy.
REFERENCES:
patent: 5500312 (1996-03-01), Harriott et al.
patent: 5911858 (1998-06-01), Ruffner
patent: 6011646 (2000-01-01), Mirkarimi et al.
Mirkarimi Paul B.
Montcalm Claude
Barr Michael
Beck Shrive
Carnahan L. E.
The Regents of the University of California
Thompson Alan H.
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