Keiichiro Tounai

Resolution improvement with annular illumination

In order to improve resolution and Depth Of Focus (DOF) in reduction projection aligner, authors investigated an annular illumination method in which the center portion of light source is screened. This paper describes the improved resist pattern resolution and DOF with annular illumination. The pattern deformation induced by annular illumination was also investigated. Furthermore, an optimization method of light source shape and its results were described. First, basic annular illumination characteristics were analyzed using simulation, and then its effect was confirmed experimentally in i-line exposure

Practical 3D lithography simulation system

We have developed a 3D practical lithography simulation system and realized the practical parameter optimization. A very fast 3D profile computation is performed by using the FFT algorithm in aerial images and the post exposure bake. The optimization system is developed by using object- oriented technology in order to reduce the computation time. The resulted computational time is less than one tenth of the conventional system. The total parameter optimization time is also reduced.

Optimization of modified illumination for 0.25-um resist patterning

In order to achieve wide focus latitude in various 0.25 micrometers patterns, the optimization of optical parameters in KrF excimer laser lithography has been investigated by means of optical intensity simulation. The accuracy of simulated DOF values was confirmed experimentally, using a chemically amplified negative resist and a KrF excimer laser stepper with NA equals 0.5 and sigma equals 0.7. The optical parameters, such as NA, sigma, and annular shield ratio in an annular illumination, were optimized for 1:1 L&S. Our results indicate that the DOF value in the conventional illumination is insufficient even under the optimum condition, but that in the annular illumination it is wide enough. To investigate the DOF value for sparse patterns the minimum contrast value, as well as the optimum optical parameters, was estimated for each defocal position. The optimum parameters set for sparse patterns was very different from that for 1:1 L&S. Moreover, the DOF value of sparse pattern was relatively small compared to that of 1:1 L&S.

Resolution improvement of isolated line pattern in quarter-micrometer level by layout-optimized assistant pattern method

In the previous report, we showed the optimized results of off-axis illumination (OAI) for 0.25 micrometers resist patterning. Wide DOF was obtained for dense patterns like 1:1 L and S pattern, however, DOF was small for the sparse patterns. In order to widen the DOF for isolated line patterns under the OAI, we have examined the assistant pattern method, which needs the only unresolved additional patterns beside the isolated pattern. Optimization has been carried out by light intensity simulation under the following criteria; CD variation within plus or minus 10 percent, resist thickness loss of 10 percent and unresolved assistant patterns. CD value has been defined by the threshold method of aerial images. The evaluation method by aerial image is useful for rough estimate because the calculation is very rapid. However, the difference from the experimental results cannot be ignored in some cases, especially, in important CD- focus characteristics. For more accurate evaluation, we also investigated the result difference using a simplified resist development model. This model is useful for rapid calculation as light intensity calculation. From the evaluation of the exposure dose and focus latitude (EFL), the optimal layout must be decided considering exposure dose latitude decrease by resist development process. We have additionally investigated the combination effect of assistant pattern and halftone phase-shifting under OAI. EFL was improved a little by halftone phase- shifting mask without assistant patterns, on the other hand, EFL was improved by combination of HPSM and assistant patterns.