Comparison of various lithography strategies

At present, the question of the move from 193 to 157nm lithography is under discussion. There are still several major issues such as the development of 157nm photo-resists and pellicles, as well as calcium-fluoride lens material availability. The extension of the 193nm lithography down to the 65and 45-nm half pitch technologies is now considered as a serious alternative. This requires several technical challenges with the use of phase shift masks (PSM), optical proximity effects corrections or liquid immersion. Simulation gives information on expected process latitudes and is an important tool to help this technical choice. Previous works [1,2] have shown that the "Diffused Aerial Image Model" (DAIM) [3] is accurate for CD prediction. Reliable process latitudes can be extracted from the simulated focus-exposure matrices (FEM). The model is used for the process latitudes evaluation of the different lithography approaches possibly used to print the 65and 45-nm half pitches. 193nm illumination in addition to PSM is compared to 157nm lithography associated with conventional or optimized illumination schemes. This work shows that PSM at 193nm gives generally better exposure latitude for all pitches and CD, and confirms that 193nm lithography is a possible alternative to achieve 45nm and 70nm half pitches patterning. The process windows are nevertheless very small, and huge mask error factors (MEEF) are another sign that printing such small features is close to the physical limit (k1 factor close to the quarter).