Experimental Research On DC/RF Combined And Dual-Frequency Inductively Coupled Plasmas Driven With Cylindrical Antenna

Inductively coupled plasma(ICP)has been widely used in high technology fields,such as large-scale integrated circuits,wafer etching,neutral beam injection,and material processing etc.because of its advantages of high density,simple discharge device and good uniformity.In order to meet the requirements of large-area chip processing,the size of plasma discharge chamber continues to increase,the ICP source suffers from the difficulty of achieving large-area,high-density,and uniform plasma at the same time.Therefore,how to obtain the high-density plasma with a uniform spatial distribution on the basis of large-area has become a research hotspot in the field of radio frequency(RF)discharge.Based on the above,this paper deals with the investigation and development of large-area ICP source with the purpose of improving the property of spatial discharge parameters of cylindrical ICP source and exploring the underlying physical mechanism.The related work is as following:1.A large-area direct current(DC)enhanced cylindrical ICP source is built,mainly including the fabrication of some key parts,such as cusp magnetic field,DC plate electrodes and matching network,etc.Two discharge types of the DC enhanced ICP source,i.e.,single DC discharge and DC+RF hybrid discharge are debugged and optimized to obtain the longterm stable operation,and then the discharge characteristics of the DC enhanced ICP source are comprehensively studied on the basis of the stable discharge,mainly including:Plasma characteristics of single RF and DC+RF hybrid discharges are preliminarily studied at a fixed gas pressure of 50 mTorr.It is found that DC+RF hybrid discharge has a higher ionization efficiency than single RF discharge and can produce higher density plasma when the total power is fixed.In the DC+RF hybrid discharge,since the asymmetric DC electrode is located at the radial center of RF chamber,DC discharge has a significant enhancement effect on the plasma density at the radial center of RF chamber,which leads to the variation of radial distribution of plasma density.When the DC power is relatively low,the radial distribution of plasma density of RF discharge is half saddle-shape profile with poor uniformity.As the DC power increases,the plasma density increases obviously,and the radial plasma uniformity improves gradually.However,when the DC power is increased excessively,the plasma uniformity is deteriorated,and the radial distribution of plasma density evolves to the convex shape.Effects of DC discharge power on RF discharge at different gas pressures are compared experimentally.The experimental results show that:the radial distribution of the plasma density shows the bell shape profile due to the nonlocal electron kinetics,and in this situation DC discharge can increase the plasma density while cannot improve the plasma uniformity at low gas pressure.However,as the gas pressure increases,the electron kinetics changes from nonlocal electron kinetics to local kinetics accompanied by the radial distribution profile transition of plasma density from bell shape to saddle shape.In this situation,DC discharge can not only increase the plasma density but also improve the plasma uniformity.Moreover,the DC+RF hybrid discharge can reduce the electron temperature effectively.2.Aiming at the shortcomings of chamber pollution and limited enhancement effect of DC power on RF discharge when the RF power is relatively high,another large-area dualfrequency(DF)ICP source device driven by cylindrical antenna is built and its discharge characteristics are also studied systematically,the contents of the study are listed as follows:Design of DF(high-frequency fH=13.56 MHz,low-frequency fL=2 MHz)cylindricalantenna ICP source is introduced,which mainly includes discharge debugging of device,calculation of interference power between high-frequency(HF)and low-frequency(LF)coils,effect of discharge coil turn on plasma parameters and comparison of plasma parameters generated by different coil arrangement topology,i.e.interleaved and non-interleaved antenna type.By analyzing the effect of coil turns on HF plasma parameters(electron density ne,electron temperature Te,plasma potential Vs,power transfer efficiency ?,and electron probability distribution function EEPF),it is found that n_e?Te?Vs?? and the high-energy population group of EEPF increase with the number of coil turns N when power is constant,which is due to the obvious increase of inductive coupling efficiency.Electron density generated by interleaved and non-interleaved antenna type is compared,it is found that the interleaved antenna structure can generate higher and more uniform plasma.The reason leading to this result is that when the antennas are arranged in a interleaved way,the superposition of the HF and LF discharge areas is conducive to generating a small-volume and high-density plasma;while when the antennas are arranged in a non-interleaved way,the separation of the HF and LF discharge areas and the axial transportation of charged particles between HF and LF discharge areas is beneficial for generating large-volume,low-density plasma.In terms of the number of coil turns and antenna arrangement topology,an optimized DF-ICP source consisting of 2-turns 13.56 MHz HF antenna and 6-turns 2 MHz LF antenna in a close interleaved way is obtained,and the discharge property and underlying physical mechanism of the DF-ICP source are diagnosed and analyzed.Effect of different HF and LF power combinations on the modulation behavior of ne?Te?Vs and EEPF are investigated at different gas pressures.The results of the study indicate that both the populations of high-energy electron and low-energy electron group increase with HF and LF power,and the increase rate of high-energy electron group population is slightly faster than that of low-energy electron at gas pressure of 1 mTorr.At gas pressure of 15 mTorr,HF power mainly increases the population of high-energy electron group,while LF power mainly increases the population of low-energy electron group.They both increase the population of low-energy electron group at gas pressure of 60 mTorr.EEPF exhibits different evolutionary trends with HF and LF power is related to different electron heating mechanisms of HF and LF discharge.At gas pressure of 1 mTorr,the electron heating mechanism is collision-less dominated for both HF and LF discharge,while collision-dominated for LF discharge and collision-less dominated for HF discharge at 15 mTorr,but collision-dominated for both HF and LF discharge at 60 mTorr.In the case of DF discharge,ne increases with HF and LF power,and the increase rate increases with the gas pressure.Te and Vs show a consistent trend with HF and LF power,namely,they increase with both HF and LF power at 1 mTorr and,increase with HF power and decrease with LF power at 15 mTorr,while decrease with both HF and LF power at 60 mTorr.Besides,according to the measurement of radial distribution of n_e,it is found that DF discharge is favorable to achieve the radial uniform plasma at intermediate gas pressure.