Optical: systems and elements – Light interference – Produced by coating or lamina
Reexamination Certificate
1998-11-25
2001-05-08
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Light interference
Produced by coating or lamina
C359S359000, C359S360000, C359S585000
Reexamination Certificate
active
06229652
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to multilayer reflective films, particularly to high reflectance and low stress multilayer films, and more particularly to Mo
2
C/Be multilayers having low stress and high reflectance, and which may include a carbon capping layer.
Multilayer reflective mirrors for soft x-ray to ultraviolet wavelength have been under development for several decades. These early multilayer efforts involved layers of a transition metal and layers of a compound of carbon and silicon, boron, beryllium, or aluminum, and resulted in a reflectance of about 55%. Such a multilayer system is exemplified by U.S. Pat. No. 5,310,603, issued May 10, 1994 to Y. Fukuda et al.
In recent years, extensive efforts have been directed to the development of extreme ultraviolet (EUV) lithography tools or systems which operate in the 11-13 nm wavelength regime. Such systems require several precisely figured, low roughness optics coated with highly reflective multilayers. To obtain sufficient throughput and image quality, these multilayer coatings should simultaneously have high reflectance, preferably near 70%, and low stress (less than 100 MPa), and have high thermal and radiation stability.
Until recently, the only multilayer material pair that would reflect near 11 nm and appear to have the potential to satisfy the requirement for high reflectance, low stress, and long term stability, was Mo/Be. However, this material combination had problems relative to long term stability and stress. The Mo/Be multilayer development efforts are exemplified by: K. M. Skulina et al., Molybdenum/beryllium multilayer mirrors for normal incidence in the extreme ultraviolet, Applied Optics 34, 3727 (1995); D. G. Stearns et al., “Beryllium-based multilayer structures, Mat. Res. Soc. Symp. Proc. 382, 329 (1995) Materials Research Society; T. W. Barbee Jr. et al., Interfacial Effects in Multilayers, SPIE 1997; and C. Montcalm et al., Multilayer reflective coatings for extreme-ultraviolet lithography, Emerging Lithography Technologies II, Y. Vladimirsky, ed., Proc. Soc. Opt. Instrum. Eng. 3331, paper 331-05 (1998).
The present invention involves a new material pair (Mo
2
C and Be) to make EUV multilayer coatings. The Mo
2
C layers are nano-crystalline (amorphous). The formation of a beryllium carbide at the Mo
2
C/Be interfaces both stabilizes and has a smoothing effect on the interface. Tests have indicated that the interfaces appear to be smoother than the interfaces in Mo/Be multilayers, and such interfaces are expected to be more stable, based on the Mo
2
C/Si study by T. W. Barbee Jr. et al., referenced above. To reduce the reduction in reflectance due to oxidation of the outer layer, a carbon capping layer is deposited, which has been shown to increase the reflectivity by about 7%, as compared to the multilayer where the outer layer has been oxidized. The reflectance of the carbon capped Mo
2
C/Be multilayers is above 65% at 11.25 nm with a tensile stress of +88 MPa, compared to a stress of +330 MPa of Mo/Be multilayers of the same thickness. The Mo
2
C/Be multilayer system of the present invention can be deposited using first and last Be layers, or a first layer of Be and last Mo
2
C layer and capping the Mo
2
C layer with a thin carbon layer.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a high reflectance, low stress multilayer optic.
A further object of the invention is to provide multilayers composed Of Mo
2
C and Be.
A further object of the invention is to provide high reflectance, low stress multilayers that reflect at about 11.1 to 11.5 nm composed of Mo
2
C/Be.
Another object of the invention is to provide Mo
2
C/Be multilayer with a carbon capping layer.
Another object of the invention is to provide high reflectance, low stress multilayers composed of Mo
2
C and Be, wherein the Be is deposited first and last.
Another object of the invention is to provide multilayers having a stress of less than 100 MPa composed of layers of Mo
2
C and Be, wherein a Be layer is deposited first and a Mo
2
C layer is deposited last, with a thin carbon capping layer whereby reflectance is above 65%.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawing. The present invention provides high reflectivity and low stress multilayers that reflect in the EUV region at wavelengths, just above the BeK absorption edge. The invention involves a new material pair combination that reflects at about 11.1-11.5 nm, measured at 5 degrees off normal incidence angle with a stress of only +88 MPa, and which may include a carbon capping layer for increasing the reflectivity to above 65%. Mo
2
C/Be multilayers can have a reflectivity in the 65-70% range depending on the bilayer composition and if a capping layer is utilized. The material pair combination of this invention may be deposited with the Be first and last, or with the Be first and the Mo
2
C last, followed by a thin carbon capping layer.
REFERENCES:
patent: 5022726 (1991-06-01), Austin et al.
patent: 5052003 (1991-09-01), Ikeda et al.
Bajt Sasa
Barbee, Jr. Troy W.
Carnahan L. E.
Spyrou Cassandra
The Regents of the University of California
Thompson Alan H.
Winstedt Jennifer
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