EUV (Extreme Ultraviolet) lithography is one of the most promising techniques for imaging 5-nm node and beyond wafer features. Mask defects that matter are the ones that print during exposure at 13.5 nm wavelength. To support EUV development and production schedules, mask defectivity must be reduced to be at or near the optical defect levels. This task is complicated by the fact that actinic EUV mask inspectors are not currently available. In the absence of an actinic EUV inspection tool, all available methods for detecting and characterizing defects must be deployed.
Based on extensive deployment and experience in the industry with optical masks, and on its record for reasonable throughput, 19x nm wavelength inspection is one of the strongest candidates available today, for the initial EUV mask inspection approach. However, there are several key challenges with 19x nm optical inspection of EUV masks. One such challenge is defect sensitivity. Another challenge is that EUV mask pattern image contrast changes as a function of pattern size and pitch. This is often referred to as “Tone Reversal”, and it is a phenomenon that occurs for specific features. It is essential to understand the impact of tone reversal on defect sensitivity and overall inspectability, specifically for image sizes and pitches at the point of tone reversal, and for those immediately on either side of the tone reversal.
In this study, the relationship between base pattern contrast and absorber defect sensitivity will be discussed through the analysis of programmed defect macros (PDMs). We will also discuss whether we can influence the point at which tone reversal occurs and furthermore, whether that reversal point can be tailored to specific patterns sizes or pitches. We will demonstrate how inspection parameter optimization can be done to tailor 19x inspection to specific layer and specific groundrules to maximize both sensitivity and inspectability.