Improved CD-SEM optics with retarding and boosting electric fields

Because of rapidly decreasing line-width of integrated circuits, it is necessary to measure and control their critical dimensions with high accuracy. Hitachi has developed a new critical-dimension-measurement scanning electron microscope S-9000 series, which has a new electron optics with retarding and boosting electric fields. The upper pole piece of the objective lens is kept at a high positive voltage with respect to the ground so as to reduce aberration of the objective lens drastically. To optimize the boosting voltage we have developed optics simulators that is capable of computing aberration coefficients in electric and magnetic mixed fields. At the optimized boosting voltage of around 5kV, 3nm resolution is achieved for a final accelerating voltage of 800V. The high boosting voltage is effective in imaging bottoms of contact holes having high aspect ratios.