Study of the absorption spectra of fluorocarbon coatings obtained with the aid of a LF-plasmatron at atmospheric pressure
The paper presents the results of studying the absorption spectra of fluorocarbon coatings obtained with the aid of a low-frequency plasmatron of a low-temperature atmospheric pressure plasma. The amplitudes of the absorption peaks are considered and the chemical composition of the coatings is established. The band gap is determined by the Tauz method.
Research and development in modification of promising materials and the creation of new ones are designed to accelerate integration of opto- and microelectronic products into all spheres of the domestic economy [1–3]. Among the promising materials, a special place is occupied by polymers having such qualities as low density, high flexibility, chemical resistance and high dielectric characteristics. However, as a result of the influence of negative environmental factors (high humidity, UV radiation), the dielectric, mechanical and optical properties of polymeric materials undergo significant degradation, which affects operation of an optoelectronic product [4, 5].
The use of low-energy methods of processing and modifying the materials surface is increasingly in demand in modern industry and science. These methods include the use of low-temperature atmospheric pressure plasma. Due to absence of the need to create and maintain vacuum, as well as simplicity of process control, atmospheric pressure plasma has found its application in a number of technological processes [6–9]. The most attractive method for generating low-temperature plasma is a low-frequency (LF) arc gas discharge, and the devices based on it are called plasmatrons. It allows of making spot processing of the product using a supplied gas mixture with a minimum energy consumption not exceeding 100 W .
The aim of this work is to study the absorption spectra of fluorocarbon coatings obtained using an atmospheric pressure LF-plasmatron.
To achieve the set task, an experimental setup was developed, consisting of an LF-plasmatron, a CNC-base and a gas block.
The computer-controlled installation made use of the NC-Studio software installed. Deposition of fluorocarbon coatings from the gas phase is ensured by supplying a mixture of several gas streams according to the scheme shown in Fig.1.
The coatings were formed under the following technological parameters: plasmatron-substrate distance (15–24 mm), deposition time (10–20 s) and C6H12 concentration (2–3%). The total gas flow rate was 7.1 ± 0.1 l / min. The gas discharge frequency was fixed at 113 kHz. The coatings were formed on polyethylene terephthalate (PET) substrates for the subsequent study of the absorption spectra obtained on a Photolab 6600 spectrophotometer.
The study of the optical properties of fluorocarbon coatings makes it possible to determine the features of their application in optoelectronic products, such as diode emitters, photoresistors or photosensitive matrices. The absorption spectra are able to demonstrate influence of the technological parameters of vapor deposition on the chemical composition of the resulting coatings.
In order to exclude the influence of PET on the obtained spectrum, the spectrophotometer was calibrated against a pure PET sample, which was assumed to be an absorption unit, thus, all the obtained spectra were formed relative to unity in the wavelength range from 190 to 600 nm. A number of other peaks were recorded for the study where the absorption value changed sharply in a small spectral range, which includes carbon. Negative absorbance values relative to unity is an indication of the increase in transmittance relative to the pure PET sample.
Figures 2–4 show the fluorocarbon coatings absorption spectra with different concentrations of C6H12 at different plasmatron-substrate distances obtained in the point mode of vapor deposition.
The huge differences in the absorption intensity relative to unity for C6H12 concentrations of 2–2.5% and 3–3.5% show the effect of the limited energy input of the LF-plasmatron at atmospheric pressure where the overall rate of coating formation decreases with an increase in the particles of the film-forming gas. As a consequence, the coatings obtained with a higher C6H12 content are thinner and, naturally, have a lower absorption index, which is more or less better than in pure PET. At the same time, there is an increase in the amplitude of the absorption peak and the peaks at wavelengths of 217 nm and 223 nm, which corresponds to the types of bonds C = C ‒ C = C and C≡C. As the concentration increases, the difference between the spectra obtained at different plasmatron-substrate distances decreases, since the number of C-C bonds on the surface of the PET substrate increases.
The dependencies of the absorption spectra peaks shown in Fig.5, demonstrate a slight increase in absorption relative to a unit of the absorption peak, which characterizes the C–C bonds due to reduction of other bonds in the obtained coating.
The influence of the plasmatron-substrate technological parameter largely affects the spectra up to 300 nm wavelength (UV range), as shown in Fig.2–5.
This effect is due to production of dissociation products of the film-forming and transport mixture, which is affected by the plasmatron-substrate parameter, and it is associated with the concentration of C=C compounds; C≡C; C=O; C=C‒C=C, having peaks in the spectral range from 160–300 nm, as well as with the presence of unpaired electrons, which lead to appearance of additional peaks and a change in the slope of the absorption peak lying in the range of 306–310 nm. The dynamics of absorption relative to unity in fluorocarbon coatings is associated with the use of CF4 and the chemical nature of fluorine, which, after the ionization process of C6H12, leads to substitution of C–H bonds thereby minimizing a possibility of recombination of the charged particles and dissociation products with atmospheric air, allowing to reduce the influence of the environment on the ongoing processes.
As the distance of the plasmatron-substrate increases, the number of recombined charged particles and products of dissociation with atmospheric air increases as well, where, first of all, the compounds with a lower ionization threshold and a lower charge undergo recombination. Such compounds include H, C–H, N and NO. With an increase in the distance of the plasmatron-substrate, the number of oxygen groups participating in the recombination of ionized compounds increases, predominantly entering into a bond with C–O and C–H and bringing them out of the ionized state. At the same time, the C–C and C=C compounds undergo, to a various degree, lesser recombination, which allows them to reach the substrate so as to form a coating , as a result, it is possible to obtain thin coatings with an increased carbon concentration.
The influence of the deposition time on the absorption relative to unity is noticeable in the spectral range of 190–300 nm and is associated with the inhomogeneity of the combustion processes of NPs of the arc gas discharge over time. In the spectral range from 310 to 600 nm the nature of the spectra in time is almost identical due to the properties of fluorocarbon coatings, which include increased optical transmission .
The band gap was calculated by the Tauz method, which averaged 4.18 ± 0.04 eV. This band gap corresponds to the dielectric material. The influence of technological parameters on the band gap is negligible.
As a result of this study the influence of technological parameters on the absorption relative to the control PET sample, as well as on the change in the chemical composition of the resulting coatings was established. In particular, it was shown that the plasmatron-substrate distance significantly affects the number of C–C bonds formed on the substrate. The influence of the C6H12 concentration on the relative absorption was determined. The band gap was determined by the Tauz method, which averaged 4.18 ± 0.04 eV.