Improved mask-based CD uniformity for gridded-DesignRule Lithography

The difficulties encountered during lithography of state-of-the-art 2D patterns are formidable, and originate from the fact that deep sub-wavelength features are being printed. This results in a practical limit of k₁, ≥ 0.4 as well as a multitude of complex restrictive design rules, in order to mitigate or minimize lithographic hot spots. An alternative approach, that is gradually attracting the lithographic community's attention, restricts the design of critical layers to straight, dense lines (a ID grid), that can be relatively easily printed using current lithographic technology. This is then followed by subsequent, less critical trimming stages to obtain circuit functionality. Thus, the ID gridded approach allows hotspot-free, proximity-effect free lithography of ultra low-k₁ features. These advantages must be supported by a stable CD control mechanism. One of the overriding parameters impacting CDU performance is photo mask quality. Previous publications have demonstrated that IntenCD™ - a novel, mask-based CDU mapping technology running on Applied Materials' Aera2™ aerial imaging mask inspection tool - is ideally fit for detecting mask-based CDU issues in ID (L&S) patterned masks for memory production. Owing to the aerial nature of image formation, IntenCD directly probes the CD as it is printed on the wafer. In this paper we suggest that IntenCD is naturally fit for detecting mask-based CDU issues in ID GDR masks. We then study a novel method of recovering and quantifying the physical source of printed CDU, using a novel implementation of the IntenCD technology. We demonstrate that additional, simple measurements, which can be readily performed on board the Aera2™ platform with minimal throughput penalty, may complement IntenCD and allow a robust estimation of the specific nature and strength of mask error source, such as pattern width variation or phase variation, which leads to CDU issues on the printed wafer. We finally discuss the roles played by IntenCD in advanced GDR mask production, starting with tight control over mask production process, continuing to mask qualification at mask shop and ending at in-line wafer CDU correction in fabs.