Low-cost EUV collector development: design, process, and fabrication

Abstract

Cost of ownership (COO) is an area of concern that may limit the adoption and usage of Extreme Ultraviolet Lithography (EUVL). One of the key optical components that contribute to the COO budget is the collector. The collectors being fabricated today are based on existing x-ray optic design and fabrication processes. The main contributors to collector COO are fabrication cost and lifetime. We present experimental data and optical modeling to demonstrate a roadmap for optimized efficiency and a possible approach for significant reduction in collector COO. Current state of the art collectors are based on a Wolter type-1 design and have been adapted from x-ray telescopes. The Wolter collector design uses a long format that is suitable for imaging distant light sources such as stars. As applied to industrial equipment and very bright nearby sources, however, a Wolter collector tends to be expensive and requires significant debris shielding and integrated cooling solutions due to the source proximity and length of the collector shells.Three collector concepts are discussed in this work. The elliptical collector that has been used as a test bed to demonstrate alternative cost effective fabrication method has been optimized for collection efficiency. However, this fabrication method can be applied to other optical designs as well. The number of shells and their design may be modified to increase the collection efficiency and to accommodate different EUV sourcesThe fabrication process used in this work starts with a glass mandrel, which is elliptical on the inside. A seed layer is coated on the inside of the glass mandrel, which is then followed by electroplating nickel or nickel alloys. The inside/exposed surface of the electroformed nickel is then diamond turned and/or polished to meet the figure and finish requirements for the particular shell and finally coated with Ru or a multilayer film depending on the angle of incidence of EUV light. Finally the collector shell is released from the inside surface of the mandrel.There are several potential cost and fabrication advantages to this process. There is flexibility in the choice of material for producing the mandrel -this allows for optimizing the cost of fabrication of the mandrel. Moreover, since the final surface and figure of the collector optic can be modified, after electroforming the optic, the mandrel, in principle does not have a limited lifetime. Finally, the mandrel provides mechanical support to the electroformed optic throughout the fabrication process, thereby reducing deformation of the optic during diamond turning and/or polishing and coating.The optical design, optimization of collection efficiency, fabrication and characterization results is discussed in this work.