Optical behavior of multilayer DLC films produced by periodically altered RF voltage: Experimental and theoretical model

Diamond-like carbon (DLC) films are generally used as protective layers for environmental effects and optic filters in laser seekers and infrared sensors. The sp²/sp³ bond ratio of DLC films affects their optical properties. Therefore, if the DLC films are formed as a stack of layers with different sp²/sp³ bond structures, the reflectance and transmittance of the film can be tuned to a specific wavelength in the infrared region of the electromagnetic spectrum.

In this study, DLC thin films were deposited on silicon and glass substrates with ultra-low thermal expansion coefficient by using RF PE-CVD technique. In the first step of the study, single-layer DLC films were produced with different RF voltages (from −150V to −450V). The bond characteristics of the produced films with different bias voltages were characterized by Raman Spectroscopy and XPS techniques. The sp²/sp³ ratios and hydrogen contents were measured by using XPS and Raman Spectroscopy techniques. The refractive index ($n$) and extinction coefficient ($k$) were investigated in the visible and NIR regions of the electromagnetic spectrum. An optical model based on the “Herzberger Dispersion Equation” approach was also utilized. The experimental and simulation results based on the model were compared to justify the theoretical approach. The number of layers had a critical role in the reflection intensity of the tuned center wavelength of the multilayer films. The thickness of the layers had shifted the center wavelength in the spectrum.