In the continuous drive for smaller feature sizes, process monitoring becomes increasingly important to compensate for the smaller lithography process window and to assure that Critical Dimensions (CD) remain within the required specifications. Moreover, the higher level of automation in manufacturing enables almost real-time correction of lithography cluster machine parameters, resulting in a more efficient and controlled use of the tools. Therefore, fast and precise in-line lithography metrology using Advanced Process Control (APC) rules are becoming crucial, in order to guarantee that critical dimensions stay correctly targeted. In this paper, the feasibility of improving the CD control of a 193nm lithography cluster has been investigated by using integrated scatterometry. The target of the work was to identify if a dose correction on field and wafer level, based on precise in-line measurements, could improve the overall CD control. Firstly, the integrated metrology has been evaluated extensively towards precision and sensitivity in order to prove its benefits for this kind of control. Having a long-term repeatability of significantly better than 0.75nm 3σ, this was very promising towards the requirements for subnanometer CD correction. Moreover, based on an extensive evaluation of the process window on the lithography cluster, it has been shown that the focus variation is minimal and that CD control can be improved using dose correction only. In addition, systematic variations in across-wafer uniformity and across-lot uniformity have been determined during this monitoring period, in order to identify correctable fingerprints. Finally, the dose correction model has been applied to compensate for these systematic CD variations and improved CD control was demonstrated. Using a simple dose correction rule, a forty percent improvement in CD control was obtained.
The challenging metrology application for scatterometry and CD-SEM is to accurately measure both CD and profile. To apply this metrology specifically to dual-damascene hole structures is critical for the back-end processing, in order to control both the CD and the process overall. This paper discusses applications of Optical Digital Profilometry-based scatterometry to the advanced 90nm node dual-damascene process. The application includes contact ADI, via AEI, via etch, and via fill. The results show that scatterometry can measure CD, as well as provide sidewall angle and profile information that is unavailable by CD-SEM. Correlations to CD-SEM and cross-sectional SEM are also presented. For future applications, scatterometry is a viable solution for 3D structures, and provides higher precision, and more metrology information than current metrology methods for critical dual-damascene processes. INTRODUCTIONWith the demand for faster devices, the implementation of the damascene process is essential in order to build smaller metal lines, while maintaining acceptably low electrical resistance. The dual-damascene copper process requires the etching of vias and trenches, which are subsequently filled with copper simultaneously. The profile and film thickness determined using ODP gives much more information about the process than previous metrology using CD-SEM 1 , and the objective was to demonstrate ODP capability for damascene. In this process, the via has been etched first, with the trench etch following, and ODP scatterometry has been used to measure the via profile and layer thicknesses, at the following four process steps: (1) Via After-Develop Inspect (ADI); (2) Via After-Etch Inspect (AEI), with resist removed; (3) Via plug, following a resist fill; and (4) following the etch-back of the vias. Figure 1. ODP scatterometry is used to measure damascene via profiles and film thicknesses. From left to right is shown (1) Via ADI; (2) Via AEI; (3) Via fill; and (4) Via etch-back.
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