Effects of vertical sidewall roughness on near-UV biosensing nanoresonant gratings

The effects of vertical sidewall roughness (VSR) were investigated for a nanoresonant grating designed to have enhanced surface-to-bulk sensitivity for biosensing when illuminated by a near-ultraviolet source. Rigorous coupled wave analysis (RCWA) was applied to design a grating with an ultra-narrow spectral full width at half maximum (FWHM) through the examination of virtual biosamples. We assumed a power spectral density function consisting of only the correlation length ξ and roughness deviation σ to describe the VSR superimposed on the grating. For the designed wavelength, the reflectance on the ξ–σ diagram revealed a deviation from the peak value that formed bandgap-like striped areas resembling the bandgaps in the band diagram of a photonic crystal. We defined the concept of a process window in the ξ–σ diagram and the parameter η to quantify the probability of the bandgap-like peak-value stripes that appeared in the process window. η was only >50% when σ and ξ were less than 3 and 100 nm, respectively, which means that there is a >50% probability of bandgap-like stripes for which the roughness can be tolerated under severe nanofabrication limitations compared to the near-IR case. On the other hand, within the process window but outside the bandgap-like stripes, even a small σ of less than a nanometer necessitates accurate calibration of the peak wavelength value (PWV) shift due to the VSR effect on the nanoresonant biosensor. When η = 90%, the process window is narrower and almost the same for near-UV and near-IR cases. However, there is less need to calibrate the PWV shift at the expense of a more severe fabrication limit.