Velocity Characterization of Target Debris from a Laser Produced Plasma Utilizing a “Time-of-Flight” Technique

Abstract

The generation of significant amounts of target debris from laser produced plasmas is well known and has been characterized in terms of quantity, particle sizes, rate of deposition on surfaces, and other parameters for conditions relevant to soft x-ray projection lithography (SXPL).1 This debris will be detrimental to the condenser and/or illumination optics in an SXPL system. Of the three categories of debris, high velocity ions, neutral atoms, and particulates (or clusters), it is the latter that is quite difficult to control and potentially the most damaging. It has been shown that atomic and ionic flux, as well as very small particulate (<0.3 μm) emission can be minimized by He background gas while the effect on larger clusters (>0.5 μm) requires He pressures that are significantly higher than that allowed for SXPL.2,3 The damage due to collisional impact by many large clusters on a multilayer coated optic would be difficult if not impossible to repair. It is the purpose of this paper to report on the velocity distribution, along with size and mass distribution measurements of particulate emission, in order to understand the mass ejection process in more detail. In turn, this may provide answers as to potential interdiction techniques to stop debris from reaching expensive optical components.