Cleaning of tin debris by reactive ion etching in a discharge-produced EUV plasma source
Extreme ultraviolet lithography (EUVL) is a promising candidate for the next generation of lithography technique to continue Moore's law. Moore's law has been upheld by shrinking the feature size on devices and driving the development of faster and denser integrated chips. The short wavelength (13.5±0.2 nm) of EUV light enables EUVL to improve the resolution (the minimum feature size) far beyond the conventional limit in current high volume manufacturing. EUV light can be generated by making hot (Te ∼ 30 eV) plasma, and collected by optical mirrors in vacuum. During the light generation process, energetic ions or neutral debris are emitted from the plasma in addition to the desired light. The contamination by such debris will critically affect the life time of the mirror as well as the total light output. The contamination by debris is more serious when making an EUV-emitting plasma with tin (Sn). Sn has a high conversion efficiency leading to higher EUV power, but easily condenses on the surface of all the components in an EUV source system. In this work, a method was investigated of cleaning Sn contamination by reactive ion etching (RIE) using chlorine plasma. The primary investigation revealed details of etch rate selectivity between Sn and Ru, the mirror surface material. The optimal condition, to remove Sn several hundred times faster than Ru with minimum influence on Ru, was identified and other important findings having effects on etching were revealed. By cleaning the Sn-contaminated Ru surface using a plasma etching method, it was shown that Sn can be removed from Ru surface without damaging the Ru surface. A simple surface reaction model of Sn reactive ion etching using chlorine plasma was proposed and validated through the measured etch rate. The model successfully shows that etch rate depends on plasma density and the bias potential in reactive ion etching. Based on the promising preliminary etch rate study, Sn samples were cleaned in a mock-up collector optics in an XTS 13-35 EUV source system to investigate the proposed cleaning technique method. The mock-up is made of two shells with different gap widths (4 cm, 7.5 cm and 10 cm) and with a length of 30 cm. Using a chlorine plasma with its density and temperature around ∼ 9 × 109 /cm3 and ∼ 3 eV respectively, it was found that the cleaning rates in the mock-up vary as a function of distance from the chlorine plasma in the range of 20 -100 nm/min mainly due to the plasma density uniformity. Starting from the electronegative plasma diffusion, the plasma transport between the collector shells with different gap widths was theoretically investigated. As a result, a simple analytical model of the plasma transport in the gap was developed. Through the combination of the plasma transport model and the surface reaction model including the effects of chlorine radicals uniformity and pumping flow on the Sn removal, the Sn removal rates is shown to be predictable at the different locations in the collector in accordance with the experimental measurements. In addition, the effectiveness of the cleaning method was investigated by measuring the EUV reflectivity of a Ru sample during the cycles of contamination and cleaning processes. It was found that the reflectivity gradually degraded along the cleaning cycles due to the carbon contamination in the EUV source system investigated. An analytical model was developed to describe the effect on the carbon contamination of the EUV flux, the energetic ion flux and the cleanliness in the source system. Despite the limited success of recovering the EUV reflectivity of the mirror after cleaning in the system investigated, all the knowledge of plasma transport and plasma-material interactions obtained in the study will help use the plasma-based method to clean Sn contamination fast and in situ. The analytical models validated throughout the experiments will also provide a support to design an EUV collector with the integrated cleaning system.
Extreme Ultraviolet (EUV) source, Plasma, Chlorine, Tin, Cleaning, Etching, Contamination, Mirror, Model
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign