Polishing method for extreme ultraviolet optical elements and elements produced using the method

Abstract

The invention is directed to polished glass substrates suitable for extreme ultraviolet lithography. The elements are silica-titania glass elements having a coefficient of thermal expansion of 0±30×10−9 / ° C. or less, and containing 5-10 wt. % titania. The polished elements have a mid-spatial frequency peak-to-valley roughness of <10 nm and a high-spatial frequency roughness of <0.20 nm average roughness. The invention is further directed to a method of for producing optical elements suitable for extreme ultraviolet lithography (“EUVL”), the method having at least the steps of providing a glass substrate in the shape of the desired optical element and polishing the shaped substrate using a high abrasive slurry flow rate of >2.0 ml / cm2 / min. Generally the flow rates are in the range of 2.0-10 ml / cm2 / min. Glass substrates suitable for extreme ultraviolet lithography element have a coefficient of thermal expansion of 0±30×10−9 / ° C. or less. A particular glass suitable for EUVL use is silica-titania glass containing 5-10 wt. % titania.

Description

PRIORITY[0001]This application claims the priority of U.S. Provisional Application No. 60 / 861,922, filed Nov. 30, 2006, and titled “A POLISHING METHOD FOR EXTREME ULTRAVIOLET OPTICAL ELEMENTS AND ELEMENTS PRODUCED USING THE METHOD.”FIELD OF THE INVENTION[0002]The invention is directed to polished silica-titania glass elements, and in particular to polished silica-titania glass elements suitable for use in extreme ultraviolet lithography, and to a polishing method for making such elements.BACKGROUND OF THE INVENTION[0003]Glass materials such as silica-titania (SiO2—TiO2) glasses that are used for extreme ultraviolet lithography (“EUVL”) elements have been produced by various methods. In some of these methods, for example the flame hydrolysis method, striae can occur during the laydown process, resulting in discrete layers of silica-titania glass that can have a variation of ±0.1 wt. % TiO2. During cool-down from glass forming temperatures differential stress relaxation from the sligh...

AI Technical Summary

Problems solved by technology

Although these small, localized glass chemistry deviations do not affect bulk CTE, the uniformity of material removal during polishing can be affected due to the fact that these areas of localized stress can be susceptible to differential reaction in aqueous environments.

However, materials with differential stress (for example, silica-titania and other “doped” glasses) can be sensitive to ceria's combined chemical-mechanical material removal mechanism.

As a result, high surface roughness can occur.

However, it is nearly impossible to extract or form such pieces without having at least some striae break the surface.

When standard polishing processes are used to polish the piece into the desired optical element it has been found that these standard methods result in finished pieces that have mid-spatial frequency (“MSF”) topographical defects that have been found to perfectly coincide with varying TiO2 level in the glass (the difference in TiO2 between different layers), and thus the stress between discrete striae layers.

If the pieces are intended as for use as a mask materials for EUVL, these striae-induced MSF defects are a limiting factor for the qualification of any polished part.

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