Plasmon-assisted optical critical dimension measurement for three-layer orthogonal metallic gratings

In this work, a 3-layer cross copper grating structure, which is the basic structure in semiconductor processing technology, is simulated by the rigorous coupled wave analysis (RCWA) and the finite element method (FEM). The reflection spectrum shows a reflection dip in the infrared frequencies. The electric field distributions indicate that the dip is caused by the 1st order bonding localized surface plasmon mode excited in the 2nd Cu gratings. Such mode is excited by the part of incident light which evanescently passes through the 1st Cu gratings. Another anti-bonding mode can be observed in the visible light frequencies. As the plasmons excited in the 2nd Cu gratings are screened by the 1st and 3rd Cu gratings, they are very sensitive to the change in the dielectric environment between the 1st and 3rd Cu gratings. Therefore, the plasmonic resonance peaks in the reflection spectrum can be useful for the Optical Critical Dimension (OCD) metrology because the depth and the frequency of the resonance peak are highly sensitive to the structural parameters.