Thickness-related variation in the structure, morphology and optical characteristics of nickel thin films for neutral density filtering

Abstract In this paper we fabricate neutral density filter (NDF) for the visible region on the Borosilicate glass substrate at room tempreature. E-Beam coating unit is used for fabrication of Nickel thin film on the Borosilicate glass substrate under high vacuum. XRD measurement examined the amorphous growth of thin film at low thickness (t = 10 nm) while higher thickness supported crystalline growth. The Crystallite size (D) and lattice strain ( ϵ ) is decreasing with increasing the thickness. Microstructural investigation by atomic force microscope (AFM) revealed that surface roughness is decreasing with increasing of thickness i.e. From 0.018 to 0.008 nm. The decreasing of roughness prevents scattering loss in neutral density filter. Optical transmittance spectra are obtained using UV–visible spectrophotometer. Nickel 70 nm thickness is an optimum thickness to achieve high optical density (OD = 2.5) but neutrality is poor for higher thickness of filter. Hence the spectral variation of thin filters in our case 10 nm has least spectral variation (ΔOD = 0.11) for stable and durable NDF.