Correlation between Optical Properties and Hardness of Diamond-Like Carbon Films

Abstract

For this study, four kinds of DLC films were prepared using the ionized evaporation method. The substrate voltage was controlled, from 1 to 3 kV. Three kinds of films were prepared with benzene, and one with cyclohexane as the source gas. The DLC structure could be controlled using these parameters. This is related to hardness and optical properties. Various types of DLC films, which were used in round-robin testing for standardization were also analyzed in this study. These DLC films were deposited on Si substrates using the ionized evaporation method, the sputtering method, the arc method, and the plasma CVD method. The hardness of DLC films was determined using the nano-indentation technique. The refractive index and the extinction coefficient of DLC films were evaluated using a spectroscopic ellipsometer. The refractive index of tetrahedral amorphous carbon (ta-C) was found to be relatively high, from 2.50 to 2.74, whereas the extinction coefficient was low, from 0.04 to 0.50. For hydrogenated tetrahedral amorphous carbon (ta-C:H), the refractive index was lower than ta-C. Hydrogenated amorphous carbon (a-C:H) showed its own unique extinction coefficient and refractive index range, as did amorphous carbon (a-C). The refractive index was found to be proportional to the hardness of DLC films. The refractive index and the extinction coefficient were different depending on the kind of DLC, which were in turn related to the deposition method and DLC structure. Using optical properties, such as the refractive index and the extinction coefficient, we can distinguish the type of DLC film.

Modification Processes for Highly Hydrogenated Diamond-Like Carbon Thin Films by Soft X-ray Irradiation

“…It was reported that the refractive index increased with increasing sp 3 content and decreasing H content, and the extinction coefficient increased with sp 2 content. (35) We investigated the SR dosedependence of the optical properties of H-DLC film using SE. Ellipsometric measurements were conducted with a spectroscopic ellipsometer incorporating non-rotating phase modulation and liquid crystal modulation technologies (Horiba, UVISEL).…”

“…Optical constants, such as the refractive index (n) and extinction coefficient (k) of DLC film, have attracted attention as convenient indices of DLC structure. (12) In addition, the dependence of the local structure of carbon on the SR dose was investigated to clarify the reason for the changes in film properties. Furthermore, the species desorbed from H-DLC film due to SR exposure were detected to understand the reactions caused by exposure to the soft-X rays.…”

Spectroscopic Ellipsometry - Application on the Classification of Diamond-Like Carbon Films

“…The existence of the gaps should Figure 10. The relationship of (a) n and k over the classification of the deposited DLC films modified from past works [11,12,34] with wavelength at 550 nm; (b) Tauc-gap E g and E k or E n-max ; (c) hydrogen contents and E k or E n-max ; (d) sp be an important evidence and root cause that why the DLC films can be classified, and also corresponds to the results of the analysis in Figure 10(a). Thus, the hydrogen content should be the decisive factor affecting the optical properties of DLC films.…”

“…Especially, the optical constants in terms of the refractive indices (n) and extinction coefficients (k) (at λ = 550 nm) measured by spectroscopic ellipsometry (SE) have been proved to be a simple and effective method to classify the DLC films. It was proposed in 2013 [11], for which a lot of considerable discussion in the past ISO/TC-107 meetings in 2016 (BSI, UK) and 2017 (Tokyo, Japan) has been made [12]. Since this method needs to use only an SE without the largescale equipment (the hydrogen content was measured by the RBS/ERDA using an electrostatic accelerator [13], and the sp 2 /(sp 2 + sp 3 ) ratio was obtained by NEXAFS based on synchrotron radiation [2,7,14]), it is more conducive to simplify the classification of DLC films.…”

“…According to our previous classification scheme, the present films can be well classified into three types: ta-C, a-C:H, and PLC films [14], although the boundary of hydrogen content between a-C: H and PLC films is still controversial. Figure 8(a) shows the results of spectra of n in the photon energy range of 0.6-4.8 eV which is evaluated by using a three-layer optical model based on the Tauc-Lorenz and effective medium approximation model theory [11]. These films can be categorized into four groups based on the n at the energy of 2.25 eV (λ = 550 nm).…”