Balance between the diffraction efficiency and process robustness for plasmonic lithographic alignment technology considering the Fabry–Perot resonator effect

Different from traditional lithography, metal material with high absorptivity and high reflectivity is introduced into plasmonic lithography technology. In particular, a silver/photo resist/silver film stack can form a Fabry-Perot (F-P) resonator structure, which can greatly change the behavior of the light reflection and transmission. Since the silver layer has a strong absorption ability to the alignment probe light with a wavelength of 532 or 633 nm, the quality of the alignment signal is seriously affected. In this paper, a thin film Fourier transfer model is established to quantitatively calculate the amplitude and phase information of the diffraction light with different orders. The results show that the diffraction optical power can be enhanced by the thickness optimization of all film stacks, and the maximum wafer quality (normalized diffraction efficiency) can be increased to 25.7%. The mechanism analysis of alignment signal enhancement is based on the F-P resonator phase oscillation amplification effect. However, it can also bring the reverse of both the power and phase for the alignment probe signal when the thickness fluctuation of the F-P resonator exists, which will be a great challenge for through-the-mask moiré fringe alignment technology. To obtain the optical power distribution of the structure surface and image of moiré fringes, a transfer matrix method is given to point-by-point calculate the incidence and reflection of the probe light in the vertical direction. The finite-difference time-domain method is also used to demonstrate alignment performance. It is proved that the subtle fluctuation of the photoresist thickness can make a huge difference to moiré fringes. A balance between the diffraction efficiency and process robustness can be achieved for plasmonic lithographic alignment technology by controlling the thickness range of the F-P resonator structure. In addition, the metal-insulator-metal structure has excellent thickness sensitivity and is applicable to optical signal detection and material property monitoring.