Large area infrared frequency selective surface with dimensions reproducible by optical lithography

Abstract

An infrared frequency selective surface (FSS) with absorptive resonance near 6.5 μm was fabricated by electron-beam lithography using a patch design with dimensions reproducible by optical-projection lithography. By selective wet etching along with reactive-ion etching, the sample was divided into miniature FSS particles, which were released from the substrate. A large number of such particles could be implemented as a large area, conformal coating. Spectral reflectivity of the full FSS array as well as the FSS particles was measured and compared to electromagnetic simulations. To show the feasibility of this approach, the full array FSS design was fabricated using a g-line (λ = 436 nm) 5× projection lithography stepper and compared to the array fabricated by electron-beam lithography using scanning electron microscopy and Fourier transform infrared spectroscopy. Even though the resolution of the g-line stepper led to a poor fabrication output, the optical resonance was found to be robust, with only slight detuning attributed to the changes in unit cell geometry. This work highlights the utility of optical-projection lithography, coupled with the releasable particle fabrication procedure, to create a large area, conformal coating with specific infrared spectral properties.