Various alignment methods for a semiconductor exposure tool have been proposed and developed. Especially, the TTR (through the reticle) alignment technique has been expected as the ideal system since the direct measure between a reticle and a wafer through the projection lens has no baseline error. However, it requires that an alignment illumination be a single wavelength of the exposure light because of the chromatic aberration of the projection lens. The strong absorption by the resist and the BARC (bottom anti reflective coating) weakens the alignment signal intensity, and the interference fringe in the resist by the single wavelength sacrifices the precise position detection. Such difficulty in signal detection has blocked the TTR system from becoming realized. We tried to address this problem by peeling the resist and BARC on alignment marks. To peel the resist and BARC, we performed elective ablation using a laser ablation method with the Q-switch Nd YAG laser. The laser-ablated alignment marks on some process wafers were measured by the TTR alignment system. The signal waves with enough contrast were measured over all wafers and the satisfied alignment accuracy was examined.
We have successfully achieved accurate alignment to remove stacked TiN/Ti/Al/TiN/Ti films on damascene W marks by using laser ablation technology. Because, the Al films deposited on the damascene W marks lead to poor quality of alignment accuracy due to the asymmetric topography of the deposited Al surfaces. In the case that the TiN/Ti film is not formed on the Al surface, high-density plasma is formed above the Al surface during laser irradiation. This plasma screens off the surface from laser irradiation. Therefore, it is difficult that the naked Al films are removed completely by the laser irradiation. From the results of thermal analysis of the TiN/Ti/Al films during laser irradiation, it is concluded that the irradiation fluence should be controlled as abrupt evaporation occurs at the Al surfaces without evaporation of the top TiN/Ti films. In this condition, the plasma cannot be formed above the TiN/Ti surfaces during laser irradiation. Therefore, the irradiation energy is absorbed efficiently in the TiN/Ti films and the TiN/Ti/Al/TiN/Ti films could be removed completely by laser irradiation.
The global alignment random of wafer alignment after the TiN/Ti/Al/TiN/Ti film ablation on the W marks is equal to that of the ideal W marks before the Al film deposition. These results mean that the laser ablation is the most effective technology for locally removing thin metal films on alignment marks to achieve accurate alignment.
In the tri-level resist process, it is sometimes difficult to detect the alignment mark because of the anti-reflection performance of the organic thick anti-reflective (ARL). Laser ablation in running water was one of the most effective techniques for removing the organic thick ARL on the alignment mark. Generally, the ablation process produces many particles. The results of our experiment indicate that the particle distribution area greatly depends on the dome-shape bubble on the ablation area. The particle distribution area could be minimized by optimizing some ablation conditions according to the estimated size of the dome-shape bubble. By optimizing a shift of the narrow slit-laser-beam and its energy so as to keep the ablation/initial thickness ratio to less than 20%, fine ablation area could be obtained. This novel ablation technique is very useful for particle-free selective removal of the organic thick ARL film.