Origin of defects on targets used to make extreme ultraviolet mask blanks

Particle formation is a major problem in extreme ultraviolet masks, and one source of these particles has been identified to be the targets used to produce the mask surfaces. In particular, the silicon (Si) and ruthenium (Ru) target appear to produce more particles, especially silicon. The evidence of this is seen as a rough region on the edges of the silicon target. The features in the region were found to be triangular mesas pointing in the direction of the incident beam. The aim of this research is to prevent the mesa formation features on the target and thus reduce particle formation on the target. Both Si and Ru targets were sputtered using different ion beam conditions to understand the mesa formation mechanisms on the target and explore the ion beam conditions that can mitigate mesas. A simple 2D Monte-Carlo computer model (Illinois surface analysis model) was used to understand the formation of mesas with different incident angles of ion beam (0°, 35°, 54°, 75°) that agrees with the shapes of mesas seen in the experiments. Additionally, srim was used to calculate sputtering yields to better understand the different mechanisms between Si and Ru. It is concluded from both experiment and calculation results that an effective way to stop mesas formation is to have a sample oscillating between 0° and the desired angle during sputtering.