External Alignment Marks Technique for Front-to-Back Side Alignment Using Single-Side Mask Aligner

Some sensors based on Micro-Electro-Mechanical System (MEMS) require lithographic process on both sides of the wafer. In these cases, it is important to provide patterns alignment carefully on both sides of a substrate. To achieve such purpose, a Double-Sided mask (DSM) aligner system is needed which is not found in research centers with limited equipment. In this work, a simple technique for Front-to-Back Side (FBS) alignment is presented using standard Single Side Mask (SSM) aligner system, which is accessible even in laboratories with limited facilities. The essential component of the proposed technique is to utilize external alignment marks (EAM) on the transparent substrate. EAM technique is an uncomplicated and low cost approach that requires no specialized technical knowledge. The transparent substance is attached to a specimen and expands it to get EAM features. A particular process is planned that leads to experimental results. The results indicate that the alignment offset in the center of the specimen is less than the sides. After applying statistical analysis on obtained experimental data and ensuring of normal data distribution, the following values are calculated respectively, along x and y axes: (1) accuracy (10.96 μm and 7.96 μm); (2) precision (2.68 μm and 2.77 μm); (3) standard deviation of repeatability (2.78 μm and 2.69 μm); and (4) reproducibility along (3.09 μm and 4.61 μm). The results demonstrate that EAM is practically reliable to minimize the alignment offset. It has an acceptable precision for MEMS applications as well.