Remote plasma cleaning of Sn from an EUV collector mirror

Abstract

Despite a higher conversion efficiency of Sn for extreme ultra violet (EUV) light generation at 13.5 nm, Sn contamination on collector optics in EUV source systems must be overcome before adopting Sn as EUV fuel. Considerable portion of debris from Sn source can be suppressed by various debris mitigation techniques. However, debris mitigation technique alone will not be sufficient for high volume manufacturing (HVM) scale light production. Sn contamination affects not only the light output but also cost of ownership because of costly and time-consuming cleaning or replacing. In order to solve this contamination issue, Center for Plasma Material Interactions (CPMI) at University of Illinois at Urbana-Champaign(UIUC) had been working on cleaning Sn from EUV collector mirror surface using inductively coupled plasma-reactive ion etching (ICP-RIE) method. Previously, our group showed the fast cleaning rate of >100±10 nm/min and the dependence of cleaning on plasma-source location. Atomic force microscopy (AFM) surface roughness scan after cleaning showed almost 95% recovery in root-mean-square roughness compared to beforecleaning. Sn debris contamination can also be cleaned by halogen gas at high pressure (several hundreds mTorr). However, cleaning rate is much slower so that longer cleaning time is needed and other components in the system can be harmed by high pressure of corrosive gas. In this study, a remote plasma cleaning method is newly investigated. We designed and fabricated a remote plasma cleaning system which operates with 13.56MHz RF. A residual gas analyzer is used to quantify the chlorine radicals generated in a remote plasma system. A comparative study on the chlorine radicals generated in ICP and remote plasma is carried out. The initial result with gas temperature control shows that more chlorine radicals generate by remote plasma than ICP. It is also reported that high power can produce more chlorine radicals as expected.

Investigation of a facile plasma-driven method for in situ cleaning of metal-based contamination

“…Multilayer mirrors (MLMs) applied to collect extreme ultraviolet light also face the serious problem of reflectance reduction caused by deposition of tin, which comes from a light source in a laser-produced plasma (LPP) lithography machine [14,15]. Shin et al [16] and Sporre et al [17] utilized an ICP source and radiofrequency (RF)-driven helicon plasma source, respectively, to study the cleaning effect of hydrogen plasma on tin contaminants. In the accelerator field, a method based on ICP cleaning was developed by our laboratory to clean hydrocarbon contamination on the internal surface of the superconducting niobium cavity in the accelerator to improve the work function and weaken the secondary electron emission [18].…”