Optical: systems and elements – Light interference – Produced by coating or lamina
Patent
1998-09-25
2000-10-17
Chang, Audrey
Optical: systems and elements
Light interference
Produced by coating or lamina
359584, 359586, 359587, 359588, 359589, 359359, G02B 528, G02B 110, F21V 904
Patent
active
061340499
ABSTRACT:
Stress compensating systems that reduces/compensates stress in a multilayer without loss in reflectivity, while reducing total film thickness compared to the earlier buffer-layer approach. The stress free multilayer systems contain multilayer systems with two different material combinations of opposite stress, where both systems give good reflectivity at the design wavelengths. The main advantage of the multilayer system design is that stress reduction does not require the deposition of any additional layers, as in the buffer layer approach. If the optical performance of the two systems at the design wavelength differ, the system with the poorer performance is deposited first, and then the system with better performance last, thus forming the top of the multilayer system. The components for the stress reducing layer are chosen among materials that have opposite stress to that of the preferred multilayer reflecting stack and simultaneously have optical constants that allow one to get good reflectivity at the design wavelength. For a wavelength of 13.4 nm, the wavelength presently used for extreme ultraviolet (EUV) lithography, Si and Be have practically the same optical constants, but the Mo/Si multilayer has opposite stress than the Mo/Be multilayer. Multilayer systems of these materials have practically identical reflectivity curves. For example, stress free multilayers can be formed on a substrate using Mo/Be multilayers in the bottom of the stack and Mo/Si multilayers at the top of the stack, with the switch-over point selected to obtain zero stress. In this multilayer system, the switch-over point is at about the half point of the total thickness of the stack, and for the Mo/Be--Mo/Si system, there may be 25 deposition periods Mo/Be to 20 deposition periods Mo/Si.
REFERENCES:
patent: 4969175 (1990-11-01), Nelson et al.
patent: 5841584 (1998-11-01), Takatani et al.
Bajt Sasa
Folta James A.
Mirkarimi Paul B.
Montcalm Claude
Spiller Eberhard A.
Carnahan L. E.
Chang Audrey
The Regents of the University of California
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