1. Field of the Invention
This invention relates to a photolithography aligner system, and more particularly relates to alignment marks designed for aligner systems.
2. Description of the Related Art
For the manufacturing process of integrated circuits, the lithography process is one of the most important processes. The quality of the lithography process is evaluated by critical dimensions and alignment accuracy. Besides the performance of the alignment system and the accuracy of metrology, the alignment marks on the mask and wafer is another factor that affects the alignment accuracy. Circuit accuracy is profoundly affected by the alignment marks, especially when process error happens.
FIGS. 1A-1C show an alignment procedure using a conventional aligner system. First, referring to FIG. 1A, charge coupled devices (CCD, not shown) of the aligner system search the cross shaped alignment marks 104 on a mask and are aligned thereto. Referring to FIG. 1B, charge coupled devices of the aligner system search the alignment mark 102 on a wafer and are aligned thereto. After the charge coupled devices find positions of the alignment marks 102 and 104 on the mask and the wafer, as shown in FIG. 1C, the aligner system aligns the cross shaped alignment mark 104 on the mask to the alignment mark 102 on the wafer and then performs an exposure step.
FIGS. 2A-2C show a general error occurring on a conventional aligner system. Since fabrication processes for a product typically includes multiple steps, sometimes previously used exposed patterns are left on the wafers. For example, the patterns formed by exposing a negative resist may be left on the wafer. Specifically, referring to FIG. 2A, the cross patterns 202 formed by previous steps are left on the wafer. Therefore, when the charge coupled devices try to find the alignment marks 206 on the mask, some errors occur because the charge coupled devices mistakenly assume the cross shaped alignment mark 202 on the wafer as the alignment mark 206 on the mask, but the charge coupled devices indeed are not aligned to the alignment mark 206 on the mask. Referring to FIG. 2B, charge coupled devices of the aligner system search the alignment mark 204 on a wafer and are aligned thereto. Next, as shown in FIG. 2C, the aligner system aligns the cross shaped alignment mark 206 on the mask to the alignment mark 204 on the wafer and then performs an exposure step. However, since the charge coupled devices did not find the real positions of the alignment mark 206 on the mask, there is deviation in the exposure. Therefore, a method to eliminate the above mentioned deficiencies is needed.