Based on the record for reasonable throughput, 19x nm wavelength inspection is one of the strongest candidates available today for the initial EUV (Extreme Ultraviolet) mask inspection approach until high-throughput E-Beam or actinic inspection is ready. However, there are several key challenges with 19x nm optical inspection of EUV masks. In the previous study, it was demonstrated that a 19x nm inspection system was capable of detecting programmed 15nm edge defects and 7nm CD errors on the programmed defect mask (PDM) containing EUV device designs, and inspected at maximum sensitivity. However, in that study, the inspectability on the product mask was not considered. In this study, EUV product mask inspection with a 19x nm inspection system is demonstrated, with special attention paid to defect sensitivity and inspectability on the product mask. In our results, we discuss whether inspection conditions, such as focus, can be employed to create a trade-off between defect sensitivity and inspectability. In addition, we discuss how defect measurement definitions affect the programmed defect size and the printability on EUV AIMS.
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.
19x nm defect inspection is the strongest candidate for initial EUV production until high-throughput E-Beam or Actinic inspection is ready. However, EUV mask inspection on an optical, 19x nm wavelength tool has some difficulties compared to optical masks. The issue of varying base pattern contrast is an example of one such difficulty. This paper explores the defect sensitivity differences among the base pattern sizes, as well as the relationship between base pattern contrast and defect sensitivity. Focus offset and polarization adjustments on programmed defect test masks are used to create new inspection recipes.
Tight control of intra-field CD variations becomes more and more important as the pattern sizes on wafer shrink. For
intra-field CD uniformity improvement several techniques have been developed. A very effective method is changing the
local mask blank transmittance according to measured Intra Field (IF) CD variations using Pixer's CDC<sup>TM</sup> technique.
This process is irreversible. For various practical reasons it would be helpful to have the opportunity for a second or
more mask blank treatments. A first application could be to improve an unsatisfying CDU post first treatment. A second
application can be the switch of the mask usage to another tool group. Furthermore, the opportunity to use multiple CDC
treatments would allow the splitting of the correction process for the mask and the tool separately, whereas in a first
correction only the mask CDU errors will be corrected and after the mask is supplied to the customer another correction
may be required to reduce the exposure tool contributions to the CDU budget.
Therefore the intention of the paper is to evaluate the opportunities of a Multiple CDC (MCDC) correction process, to
determine its accuracy and the corresponding limits.
To do this two CDC tool projection lenses have been characterized, which have been developed for different focus
positions. We will characterize their transmittance transfer performance, stability and sensitivities. The required multiple
layer distances will be determined. The linearity of the multiple CDC treatment will be analyzed using AIMS<sup>TM</sup>
measurements and wafer prints. We will present results of successful multiple CDC corrections for production masks.