Surface cleaning of optics by plasma (scope) with atomic hydrogen

A major obstacle in the implementation of extreme ultraviolet (EUV) light photolithography in production tools is the accumulation of fuel debris on the collector optics near the pinch region. Specifically, removal of deposited tin from the source onto the collector optics is needed to improve the lifetime of these optics and lower the cost of ownership. Most cleaning processes investigated thus far have had trouble with selectivity; they require highly reactive gases that will degrade the optics in the removal process. In addition, the current cleaning gases have low transmission for the EUV light, eliminating in-situ operation as an option. An investigation into a new approach is researched by the Center for Plasma Material???s Interactions (CPMI) at University of Illinois in Urbana, Illinois. This unique non-invasive background process selectively etches tin from the collector optics surface at room temperature without damage to the underlying collector optics composition. A computer model of the plasma system used and of the etching process of the tin film is shown in correlation to experimental results. Created using collisional cross sections of input gas particles in a helical-resonated plasma and assumed surface interactions at a sample substrate, a clear indication of how the plasma system behaves with the desired substrate is found from the model (approximate etch rates of 80 nm/min). The cross sections are developed from past research data on particle-particle interactions and are numerically integrated into the computer model. Surface interactions are initially set to related material specifications as this process is relatively new and unexplored. Comparison to experimental data allows for changes to the surface interaction inputs in the model. The developed procedure has selected etching of tin ranging from 20-160 nm/min experimentally. A variation in inputs causes alterations in the etch rate. Mainly, gas pressure and plasma input creates a difference in the reactive species created in the plasma system. Gas flowrate and surface temperature affects the surface interactions. These variances are explored as to optimize the cleaning process. With these results, the hope is to input this new process as an in-situ cleaning process or a quick cleaning cycle in the EUV lithography process