Structure-in-Void Quasi-Bound State in the Continuum Metasurface for Deeply Subwavelength Nanostructure Metrology

Fano lineshapes associated with quasi bound-state-in-the-continuum resonances, that are supported by dielectric metasurfaces, have the advantageous properties of being extremely sensitive to minute geometrical changes in the meta-atoms. We show an approach to determine deep subwavelength feature sizes, comparable to semiconductor critical dimension metrology, by structurally infilling a void of a dielectric disk-hole metasurface design. Our simulated results show a sensitivity of 40.5 nm resonant wavelength shift for a 1 nm feature width (i.e., critical dimension) change, at an optical line width of 1.8 nm. We present both experimental and simulated results of different void infillings and attribute the spectral change of the resonance to the sensitivity to an effective index in the void of the meta-atom, which arises from the filled volume fraction and the material boundaries orientation relative to the local polarization. Treating our metasurface as an effective index sensor, the sensitivity is 262 nm·RIU^-1 and the figure of merit is 146 RIU^-1, which underlies the pronounced resonant wavelength shift driven by similarly large changes in the effective index caused by extremely tiny critical dimension variations. This approach could impact critical dimension measurements in semiconductor metrology, as it works at the high throughput of optical measurements while performing at the high resolution of scanning electron microscopy.