Study On Surface Contamination Of EUV Multilayer Optics

Extreme ultraviolet (EUV) lithography is the most promising candidate for thenext generation lithography target at the22-11nm features. One of the most criticalissues delaying the delivery of EUV lithography into high volume manufacturing isexposure-induced EUV optics contamination and optics lifetime. When the optic isexposed to EUV radiation the residual hydrocarbons and water vapor will causecarbon deposition and oxidation on the mirror surface which will reduce the EUVreflectivity and the overall system throughput, and also affect the imagingperformances.In order to understand the relevant mechanisms of each of these processeswhich would be used to estimate the reflectively loss in any given EUV environment,this paper model the EUV-induced carbon contamination and oxidation on mirrorsurface, and also design and optimize the capping layers(CL) of the multilayermirror to minimize the surface contamination.1. A model of EUV-induced carbon contamination of optics is presented in thepresence of residual hydrocarbons. A description of the relevant physical andchemical processes is developed including vapor-phase adsorption, surfacedesorption, surface diffusion, and molecular dissociation by direct photoabsorptionand by secondary electron processes. The model provides a quantitative account ofexperimental data and suggests that the predominant cause of hydrocarbon dissociation is bond breaking by direct photon absorption. Detailed predictions forcarbon deposition for a variety of conditions of EUV power and hydrocarbonpressure are reported. The model predicts that light hydrocarbons (~100amu) posea negligible risk to EUV optics and modest increases in substrate temperature(~30°C) will substantially reduce optic contamination.2. Amodel of EUV-induced oxidation of a Ru-coated EUV optic is presented inthe presence of water vapor. The model describes the key processes including theadsorption and thermal desorption of water to and from the Ru surface, moleculardiffusion across the optic surface, and the dissociation of the water by both directEUV ionization and secondary electron excitation. The model predicts oxidethickness over time, which may later be used to estimate the reflectively lossattributable to the oxide in any given EUV environment. Model predictions provide agood description observed in available electron-beam experiments. The model is alsoused to estimate oxygen penetration into the Ru coating under various conditions ofwater partial pressure, EUV power, and temperature. The model predicts reducedoxidation with higher temperatures and for substrates that bind water less tightlythan ruthenium.3. This paper focuses on properties and surface chemistry of different materials,which as thin films could be used as capping layers to protect and extend the lifetimeof multilayer mirror optics. The most promising candidates include ruthenium,rhodium, TiO2and ZrO2. Then the thickness of the capping layer and standard MLare optimized and analyzed from the view of normalized electric-field intensitydistribution within the ML. The reflectivity of the standard Mo/Si ML is highest(75.59%) with a1.72-nm Ru CL, while with a given2.0-nm Ru CL, the reflectivityreach the maximum(75.55%) when the final Si layer’s thickness is optimized to4.01nm. When the thickness of top Si layer is in the range of3.25-5.35nm, the valueof Ru CL’s thickness is not controlled precisely for maintaining a relatively highpeak R. For optimized Mo/Si ML, the node of the standing wave is located withinthe absorptive Ru capping layer with the lowest E-field intensity which will minimize the surface contamination.The research contents of this paper provide theoretical basis for the relevantstudy of contamination on EUV multilayer mirror surface, whcih can be used toestimate the reflectively loss in any given EUV environment, and provide valuableinsight into possible methods for remediation.