Developing a viable multilayer coating process for extreme ultraviolet lithography reticles

Reticle blanks for extreme ultraviolet lithography (EUVL) are fabricated by depositing reflective Mo/Si multilayer films on superpolished substrates. To obtain a reasonable cost of ownership for EUVL, the multilayer films must be nearly defect free, have excellent reflectance/thickness uniformity, and have a high EUV reflectance. Small particle contaminants on the substrate that can nucleate printable Mo/Si phase defects are a serious concern. We develop an ion-beam thin film planarization process for mitigating the effect of small substrate contaminants that relies on enhancing the smoothing capability of Mo/Si multilayer films; we observe that etching of the Si layers in between deposition steps can yield a significant improvement in smoothing. Using this process substrate particles as large as ~50 nm in diameter are smoothed to ~1 nm in height, rendering them harmless. We further develop this process so that it retains these particle-smoothing capabilities while also achieving a high EUV reflectance and excellent uniformity.