Electrical characterization of across-field lithographic performance for 256-Mbit DRAM technologies

Lithographic performance has typically been evaluated at a single point within the stepper field. However, this evaluation method does not completely provide the total lithographic performance on a chip because of variations introduced by the stepper as well as the reticle. In this paper, the evaluation method and characteristics of across-field performance are shown through the use of electrical line width measurements and exposure-defocus (ED) analysis. The across-field performance is analyzed by both the average process window and the common process window for two resolution enhanced photolithography techniques: phase-shifting mask (PSM) and off-axis illumination (OAI). The average process window corresponds to a single-point evaluation while the common process window includes all lithographic fluctuations across the field. Consequently, the common process window is much smaller than the average process window. Moreover, to consider the effect of mask critical dimension (CD) deviation on lithographic performance, a mask CD deviation enhancement factor (MEF) is introduced. By MEF correction, the contribution of mask CD deviation to common window degradation is obtained.