Investigation On High-density Plasma Excited By High-power Millimeter-wave

Microwave plasma has the characteristics of high electron temperature and high electron density.Thus the microwave plasma is widely used in surface etching,material preparation,and gas treatment.The plasma discharged by millimeter waves can achieve higher electron temperature and electron density.This makes the growth rate of diamond film in millimeter wave CVD faster.And the plasma generated by millimeter waves has been widely studied for applications including beamed energy propulsion,CO₂ treatment,and ultraviolet(UV)radiation sources.Therefore,the plasma generated by millimeter-wave has great application prospects and research value.In this dissertation,the diagnosis,control and physics mechanism of plasma have been systematically studied.And in the experiment,millimeter waves were used to discharge the gas to generate the plasma.The main research contents of this dissertation are as follows:(1)Based on the study of Langmuir probe theory,the design of Langmuir probe structure,circuit and data acquisition for microwave plasma was completed.Based on the study of Langmuir probe theory,the design of Langmuir probe structure,circuit,data acquisition and data processing for microwave plasma was completed.In the Ar plasma experiment,the result shows the Langmuir probe is a feasible method to measure the plasma.However,as the reaction progresses in plasma discharging,the contamination would be attached to the probe surface and leads to higher incorrect electron temperature.Then the electron density cannot be obtained.A simply approach to combining the Langmuir probe and the optical emission spectrometry(OES),which can be used to obtain the electron temperature to solve this problem,was reported in this dissertation.The optical emission spectrometry(OES)can be used to obtain the electron temperature.And electron temperature obtained by OES can be used to calculate the electron density in the contaminative Langmuir probe I-V curve.A home-made coaxial line microwave plasma source driven by 2.45GHz magnetron is adopted to verify this method,and the electron temperature and density in different pressure(40-80pa)and microwave power(400-800W)are measured to verify that it is feasible.Therefore,the contaminative Langmuir probe with the OES can give the approximately electron temperature and density,and the cleaning interval of the Langmuir probe could be significantly extended.(2)In this dissertation,the effect of electromagnetic wave frequency on plasma has been studied.And through COMSOL simulation,it is found that under the same power and pressure,the maximum electron density in the reaction chamber is increased with the electromagnetic wave frequency increased.Compared with 2.45GHz,the electron density of the plasma excited by the 20GHz EM wave is increased by an order of magnitude.A comparative experiment of 915MHz and 2.45GHz was carried out.It found that when lower than the electron plasma frequency(915 MHz)was inputted,the plasma electron density did not change sufficiently as the 915 MHz microwave power increased.But whenhigher than the electron plasma frequency(2450 MHz)was inputted,the plasma electron density gradually increased as the 2450 MHz microwave power increased.This exploratory research would improve the operation frequency of the dual frequency microwave plasma source from RF to microwave,and it proves only the higher frequency electromagnetic waves(millimeter waves)can excite higher electron density plasma.(3)In this dissertation,the millimeter-wave plasma has been studied to obtain.In order to obtain higher electron temperature and higher density,it is necessary to increase the EM frequency.A long pulse(500ms)220GHz high-power gyrotron designed by the University of Electronic Science and Technology was used to discharge the gas in the experiment.The power,frequency and electromagnetic distribution of this gyrotron was measured.The results show that when the gyrotron is working at V₀=38 k V,B₀=8.3 T,?_m=500ms,the output power is 200W and the frequency is 225.12GHz,and the EM energy is concentrated in the lower right corner of the output window.The discharge of argon at 20 Pa was observed by the gyrotron.The characteristic peaks of Ar I(794.8176 nm and 810.3692 nm)were measured by an optical emission spectrometry(OES).It makes the plasma excited by high-power millimeter-wave realized.