Target reflectance study for the laser-fuse redundancy

A powerful and versatile program was written to study the target reflectance in connection with laser-fuse redundancy. The target is a piece of polysilicon sandwiched by SiO2and is used for the alignment of the laser on the wafer. In order to be detected consistently, the target should not change its reflectance with respect to the background in spite of process variations. The theory was verified against experiments that involved SEM cross sections, oxide etchings, and laser target scannings. A new target is described that is always reflective throughout the process specifications.