SiO₂-like Films Deposited By Non-equilibrium Atmospheric Pressure Plasma Technology

SiO2 has been used in many fields, since it is excellent in the properties such as hardness, anti-resistance, optical transparency, corrosion resistance, dielectric etc. Many different deposition technologies were developed for different purposes. Recently, non-equilibrium atmospheric pressure plasma deposition technology was developed for film deposition. It has attracted much attention, due to its advantages such as atmospheric-pressure, low temperature and high deposition rate.In this work, the preparation methods of silicon dioxide films were introduced and the progress of non-equilibrium atmospheric pressure plasma technology was overviewed. A non-equilibrium atmospheric pressure plasma jet discharging by gliding arc was developed. FLUENT software was used to perform the numerical simulation of the nozzle flow field. By the atmospheric plasma jet, SiO2-like films were deposited. The influences of process parameters on deposition rate were investigated, including deposition time, substrate temperature, monomer flow rate, electrical power and N2/H2/O2 flow rate. The influences of deposition time and substrate temperature on film surface morphology, chemical configuration and hardness were discussed.Numerical simulation of the nozzle flow fields with jet free expanding and jet impinging substrate was performed by FLUENT software. The distributions of temperature, velocity, volume-fraction etc. were investigated. The results are as follow:1) while plasma jet is free-expansion, the shape of the iso-volume-fraction line, iso-temperature line and iso-volecity line vary gradually from cone to ellipse down along the axial direction. Both of expanding speed and region become larger with increasing distance to nozzle.2) While jet impinge substrate, there is a temperature interface near the substrate with a thickness of 0.25mm, where temperature decreases sharply from 500K to 35 OK. As the radial position varies from Omm to 15mm on the substrate surface, the ejected N2 volume fraction decreases from 1 to 0.4, while the temperature and gas density are almost the same.The films were deposited with precursor tetraethyl-orthosilicate (TEOS), and characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), ellipsometer and nano-indenter. The results of FTIR and XPS indicate that a SiO2-like film is deposited. There are a majority of Si-O-Si bonds and a minority of Si-OH bonds in the deposited films. The O/Si ratio is 1.84, which is close to that of SiO2. Some hemispheric protrusions are found on the film surface, with a size from tens of nanometer to hundreds of nanometer.The influences of process parameters on the film deposition rate were investigated in this paper. The results show that substrate temperature, TEOS flow rate, O2 and H2 flow rate are the main influencing factors on the deposition rate. With lowering of substrate temperature, increasing of TEOS flow rate or adding of O2 or H2 gas, the deposition rate would rise a lot. In addition, increase in N2 flow rate would lead to a slight decrease in the deposition rate. And no significant differences were observed with variation in deposition time and electric power (170-310W).The influence of deposition time on surface morphology, chemical configuration and hardness of the deposited films was investigated in this paper. The results show that with increasing deposition time, protrusions on the film surface would be enlarged, the content of Si-OH would increase and the hardness would decrease. Due to the concentration gradient of active intermediates between top and bottom of the protrusions, the size of protrusions would increase as deposition continues. The coarsening of the depositing surface would lead to more cavities during the following deposition. Therefore, the hardness of thin films declines, and the content of Si-OH increases which are tend to exist in the inner surface of cavity.The influence of substrate temperature on surface morphology, chemical configuration and hardness of the deposited films was investigated. The results show that with increasing substrate temperature, the size of protrusions and the content of Si-OH would decrease, and the hardness would increase. A higher temperature would produce some effect:1) it can accelerate polymenrization between Si-OH groups, which would decrease the content of Si-OH; 2) it can raise diffusion capacity of active intermediates, which makes the intermediates diffuse easier to interstitial site; 3) it can reduce the adsorption rate of active intermediates on substrate surface and slow down surface coarsening. Consequently, the film deposited in higher temperature tends to obtain a smoother surface, a lower Si-OH content and cavity density, and a higher hardness.