Study On Low-pressure Tubular Plasma And Fabrication Of Amorphous Silicon Nitride Films

Low-temperature plasma is very useful in material modification and film deposition, thus has received much attention. A novel plasma technology may improve the technologies for material modification and film synthesis. For example, atmospheric pressure plasma jet (APPJ) is capable of producing a high active plasma jet, thus having potential in applications of material synthesis, material surface modification, and medical sterilization. APPJ had been used as a source for plasma enhanced chemical vapor deposition (PECVD), but the film quality needs to be further improved considerably, because the films are too poor to meet the requirement in practical application. In this thesis, we developed and studied a low-pressure tubular plasma source similar to APPJ, and further designed a tubular PECVD system. Using the novel PECVD, low H-content amorphous silicon nitride (a-SiNx:H) films were deposited in a relatively high rate at room temperature, and silicon nitride quantum dot (QD) films were prepared successfully. The main achievements are summarized below.(1) Study of low-pressure tubular plasma dischargeUsing the specially designed low-pressure tubular plasma source powered with low frequencies (20?80 kHz), stable discharge in N2, O2 and Ar was realized at low pressure (0.1-200 Pa). The tubular plasma source can be maintained at two different states, which are referred to as the G-mode and SP-mode discharges in this study. The G-mode discharge produces plasmas that were confined in the tube around the electrode. With an increase in the input power, the plasma column gradually expand along the tube. When the input power is loaded to a critical value, the discharge immediately change from the G-mode to the SP-mode, and the plasma fill the entire volume of the tube above the electrode. At the same time, a plasma plume is observed like a flame burning at the tube nozzle. Electric characterization revealed that the G-mode discharge exhibits a capacitive nature, and the SP mode can be assigned to a resistive discharge that is sustained by a continue discharge comprising a periodically pulsing discharge, in which the instantaneous power of the pulsed discharge could be larger than 1 kW and strongly depends on the input power, the working pressure, and the gas species. In 12 Pa nitrogen, a input power of 115 W is capable of producing a plasma with a density as high as 0.5×1012cm-3, and the discharge current is as high as 27 A/cm-2. The tubular plasma source has been used for PECVD, plasma etching, plasma nitriding, and plasma enhanced pulsed laser depositon.(2) Characterization of nitrogen plasma created by the low-pressure tubular dischargeBased on least squares optimization theory, a software was designed to fit the second positive system (2PS) of N2 for study of the vibrational and rotational temperatures (Tvib and Trot) in nitrogen plasma. The software is capable of fast processing the raw data of the optical emission spectra (OES), and is easier operated. More importantly, the software can automatically reach to the best fit with the help from operators. Using the software, the vibrational and rotational temperatures of N plasma were systematically studied, such as the input-power dependence and the axial distributions, which further proved the G-mode distinguishing from the SP-mode. In the G-mode discharge, Tvib increases from 3000 to 3500 K with the increase in the input power, but Trot keeps at a constant of-320 K. In the SP-mode discharge, Tvib is maintained at-5000 K and slowly decreases with the input power, while Trot is monotonically increased from 350 to 700 K. On the other hand, atomic N lines at 746.83,821.63, and 868.03 nm, corresponding to the transitions of 4S3/2°?4P5/2,4P5/2°?4P5/2 and 4D7/2°?4P5/2, respectively, have been used for the self-consistent determination of the electron temperature responsible for the excitation by electron direct impact from the ground state, and then a self-consistent OES method was established for quantitative determination of nitrogen dissociation in the highly constricted plasma flow. The dissociation degree of N2 molecules in the tubular source is determined to be a nearly linear function of input power for the discharges driven at different frequencies. A input power of 115 W is able to produce a dissociation degree of nitrogen as high as 15%, evidencing the tube plasma enabling generation high activity of nitrigon plasma at low pressure.(3) Low-pressure tubular PECVD and synthesis of low H-content a-SiNx:H films.Based on the low-pressure tubular plasma source, a novel PECVD system with separately feeding N2 and SiH4 was built, thus maintaining the generation of high-activity nitrigon plasma. Using pure nitrogen and Ar-diluted 5% silane, low H-content a-SiNx:H films were successfully deposited at room temperature, with a sufficient deposition rate higher than 100 nm/min. The hydrogen density in the films was estimated to be 1.0±0.2×1022 cm-3, and the film roughness was in sub-nanometer scale. The deposition rate is revealed to be linearly dependent on the dissociation degree of nitrogen, thus evidencing atomic N plays a crucial role in depositing a-SiNx:H films. The PECVD system also has the advantages in coatings on the inner wall of containers with small inlets and large-scale coatings on workpieces with complicated shapes by integrating them into an arraw, thus is potential in industrial applications.(4) Study of a new kind of a-SiNx:H quantum dot (QD) films.A new kind of a-SiNx:H quantum dots embedded in a-SiOx:H matrix were prepared using the PECVD system. The a-SiNx:H QD films exhibit bright photoluminescence. Using the quantum confinement effect, the exciton radius in a-SiNx:H is estimated to be 4.8 nm and the effective mass of it is-0.05 me (me is the mass of electron). Using the fluorescence theory of amorphous semiconductor, the photoluminescence was found originating from the radiative recombination of the excitons in the thermal relaxation process in the band tails.