Simulations of EUV Induced Hydrogen Plasmas and In-Situ Tin Cleaning

Extreme ultraviolet (EUV) lithography tools enable microelectronic processing to produce devices having critical dimensions of a few nanometers. The EUV photons in one widely adopted process are produced from the emission of tin ions, where the tin plasma is generated by ablating tin droplets with a pulsed laser. The EUV photons are collected by multilayer mirrors and directed toward the intermediate focus. The mirrors' reflectivity decreases due to contamination resulting from the deposition of tin atoms and, in some cases, clusters. Filling the EUV chamber with low-pressure hydrogen gas can reduce this contamination. Since EUV photons from this plasma source (13.5 nm, 92 eV) have enough energy to ionize the hydrogen gas, a hydrogen plasma will be formed. Contamination of the mirrors is reduced by reactions of tin ions and atoms in the gas phase with tin atoms on the mirror, etching the tin deposited on the mirror. In this work, we investigate the consequences of EUV-generated hydrogen plasmas in an idealized EUV tool, including etching tin from mirror surfaces and the redeposition of stannane. These investigations were performed with the Hybrid Plasma Equipment Model (HPEM). The fluxes of hydrogen radicals and ions to the collector mirror are critical factors for in-situ cleaning, and their energy and angular distributions will be discussed. A small bias applied to the substrate under the collector mirror to regulate the ion flux will be discussed.