The Fluid Simulation Of N₂-O₂ Dual-Frequency Capacitively Coupled Plasma

Dual-frequency capacitively coupled plasma(DF-CCP) source is one of most important equipments for etching and film deposition in semiconductor industry,due to its ability to produce large-area uniform plasma,together with its excellent controllability and low-cost advantage.Plasma enhanced chemical vapor deposition(PECVD) technology is an important method for deposition of thin film,since PECVD technology is possessed of advantages such as forming film under low temperature(＜500℃),low power loss,and so on.Since oxygen(O₂) and nitrogen(N₂) gases can be obtained from air easily,they are the two most commonly used gases in PECVD technology.Moreover,in the process of DF-CCPECVD,macro-physical parameters of plasma,particularly density of plasma,velocity of ion,electron temperature, density of power deposition,potential,as well as chemical reactions in the discharge,have direct and substantial relationships with the performances of PECVD technology.To get these relationships can help to improve the applications and enhance the performances of PECVD technology.The traditional global model has simplified the discharge too much,thus it could not accurately grasp the effects of external conditions on macro-physical parameters of plasma. This dissertation uses a self-consistent fluid model to simulate N₂-5%O₂ dual-frequency capacitively coupled discharge with the typical condition parameters of PECVD.This work is a preliminary process of trying to make a comprehensive simulation of the N₂-O₂-SiH₄ mixed gases dual-frequency capacitively coupled discharge,which is used for deposition of SiO_xN_y film by PECVD method.By the way,the role of the work in this dissertation is not limited to this application.Chapterâ… briefly introduces the concept of plasma,the development of silicon-based films,and the research progress of oxygen and nitrogen discharges.Chapterâ…¡demonstrates the fluid model in the simulation of capacitively coupled discharge in detail.It is also presented here the comprehensive summary of the selection and determination of parameters,as well as the main numerical algorithms in the simulation.Chapterâ…¢,based on the results of one dimensional fluid simulation of N₂-5%O₂ DF capacitively coupled discharge,focuses on the effects of gas pressure,high-frequency(HF) voltage,low-frequency(LF) voltage,high frequency,low frequency,plate spacing on the macro-characteristics of DF-CCP.The results of simulation are as follows.The plasma density is mainly governed by high-frequency source.The effective ways to increase the density of plasma include increasing HF voltage amplitude,high frequency,and pressure. And the efficiencies of such three variables follow the order:high frequency＞HF voltage amplitude＞pressure.Comparatively,the effect of low-frequency source on the plasma density is slight.The density of plasma is inversely proportional to voltage amplitude and frequency of low-frequency source.However,when the low-frequency voltage is higher than 200V,this inverse relationship becomes very weak.The velocity of ion near the lower plate is proportional to the amplitude of both HF voltage and LF voltage.The ion velocity is inversely proportional to the cross section for momentum transfer among different ions.In the plasma zone,the total density of positive ions and the total density of negative ions and electron are almost the same,thus the spatial distributions of electric field and electron temperature are smooth.In contrast,the density of negative ion is almost zero and the density of electron is very small compared to the positive ions in the sheath,thus there is a considerable discrepancy between densities of negative charges and positive charges.Such a discrepancy can give rise to the significant enhancement of electric field in the sheath.At the same time, with the acceleration of electron by the strong electric field in the sheath,the electron temperature there is several times larger than that in the plasma zone.The electronegativity of plasma is in inverse ratio to pressure,high frequency and high-frequency voltage.More precisely,the electronegativity is reversely proportional to the density of electron and power deposition.It is noteworthy that the z-axial profiles of electronegativity and the density of power deposition are similar.In addition,the intensity of resonant electron transfer between N₂⁺ ion and O₂ molecule is mainly affected by the pressure proportionally.Chapterâ…£uses the two dimensional model to simulate the N₂-5%O₂ discharge.The main discussion of this chapter is the radial distribution of plasma density.The results show a uniform radial distribution within the center region and there is a peak value in the vicinity of where the radius is a little bigger than the electrode radius.In addition,through the comparison of discharge of pure N₂ gas and that of N₂-5%O₂ mixture,we can see that the addition of 5%oxygen to the nitrogen has effectively increased the efficiency of ionization, thus the densities of particles.