Improved Double-Probe Technique for Spatially Resolved Diagnosis of Dual-Frequency Capacitive Plasmas

The conventional double-probe technique was improved with a combination of selfpowering and radio-frequency（RF） choking.RF perturbations in dual-frequency capacitively coupled discharge were effectively eliminated,as judged by the disappearance of self-bias on the probes.The improved technique was tested by spatially resolved measurements of the electron temperature and ion density in both the axial and radial directions of a dual-frequency capacitive plasma.The measured data in the axial direction were compared with simulation results,and they were excellently consistent with each other.The measured radial distributions of the ion density and electron temperature were influenced significantly by the lower frequency（LF） power.It was shown that superposition of the lower frequency to the higher frequency（HF） power shifted the maximum ion density from the radial center to the edge region,while the trend for the electron temperature profile was the opposite.The changing feature of the ion density distribution is qualitatively consistent with that of the optical emission intensity reported.

Effect of a negative DC bias on a capacitively coupled Ar plasma operated at different radiofrequency voltages and gas pressures

The effect of a negative DC bias,|V_(dc)|,on the electrical parameters and discharge mode is investigated experimentally in a radiofrequency(RF)capacitively coupled Ar plasma operated at different RF voltage amplitudes and gas pressures.The electron density is measured using a hairpin probe and the spatio-temporal distribution of the electron-impact excitation rate is determined by phase-resolved optical emission spectroscopy.The electrical parameters are obtained based on the waveforms of the electrode voltage and plasma current measured by a voltage probe and a current probe.It was found that at a low|V_(dc)|,i.e.inα-mode,the electron density and RF current decline with increasing|V_(dc)|;meanwhile,the plasma impedance becomes more capacitive due to a widened sheath.Therefore,RF power deposition is suppressed.When|V_(dc)|exceeds a certain value,the plasma changes toα–γhybrid mode(or the discharge becomes dominated by theγ-mode),manifesting a drastically growing electron density and a moderately increasing RF current.Meanwhile,the plasma impedance becomes more resistive,so RF power deposition is enhanced with|V_(dc)|.We also found that the electrical parameters show similar dependence on|V_(dc)|at different RF voltages,andα–γmode transition occurs at a lower|V_(dc)|at a higher RF voltage.By increasing the pressure,plasma impedance becomes more resistive,so RF power deposition and electron density are enhanced.In particular,theα–γmode transition tends to occur at a lower|V_(dc)|with increase in pressure.

Effect of Low-Frequency Power on Etching Characteristics of 6H-SiC in C4F8/Ar Dual-Frequency Capacitively Coupled Plasma

Dry etching of 6H silicon carbide （6H-SiC） wafers in a C4Fs/Ar dual-frequency capacitively coupled plasma （DF-CCP） was investigated. Atomic force microscopy （AFM） and X-ray photoelectron spectroscopy （XPS） were used to measure the SiC surface structure and compositions, respectively. Optical emission spectroscopy （OES） was used to measure the relative concentration of F radicals in the plasma. It was found that the roughness of the etched SiC surface and the etching rate are directly related to the power of low-frequency （LF） source. At lower LF power, a smaller surface roughness and a lower etching rate are obtained due to weak bombardment of low energy ions on the SiC wafers. At higher LF power the etching rate can be efficiently increased, but the surface roughness increases too. Compared with other plasma dry etching methods, the DF-CCP can effectively inhibit CχFγ films＇ deposition, and reduce surface residues.

Characteristics of dual-frequency capacitively coupled SF_6/O_2 plasma and plasma texturing of multi-crystalline silicon

Due to it being environmentally friendly, much attention has been paid to the dry plasma texturing technique serving as an alternative candidate for multicrystalline silicon （mc-Si） surface texturing. In this paper, capacitively coupled plasma （CCP） driven by a dual frequency （DF） of 40.68 MHz and 13.56 MHz is first used for plasma texturing of mc-Si with SF6/O2 gas mixture. Using a hairpin resonant probe and optical emission techniques, DF-CCP characteristics and their influence on mc-silicon surface plasma texturing are investigated at different flow rate ratios, pressures, and radio-frequency （RF） input powers. Experimental results show that suitable plasma texturing of mc-silicon occurs only in a narrow range of plasma parameters, where electron density ne must be larger than 6.3 x 109 cm-3 and the spectral intensity ratio of the F atom to that of the O atom （[F]/[O]） in the plasma must be between 0.8 and 0.3. Out of this range, no cone-like structure is formed on the mc-silicon surface. In our experiments, the lowest reflectance of about 7.3% for mc-silicon surface texturing is obtained at an [F]/[O] of 0.5 and ne of 6.9 × 109 cm-3.

Investigation of Capacitively Coupled Argon Plasma Driven by Dual-Frequency with Different Frequency Configurations

Low pressure argon dual-frequency （DF） capacitively coupled plasma （CCP） is generated by using different frequency configurations, such as 13.56/2, 27/2, 41/2, and 60/2 MHz. Characteristics of the plasma are investigated by using a floating double electrical probe and optical emission spectroscopy （OES）. It is shown that in the DF-CCPs, the electron temperature Te decreases with the increase in exciting frequency, while the onset of 2 MHz induces a sudden increase in Te and the electron density increases basically with the increase in low frequency （LF） power. The intensity of 750.4 nm emission line increases with the LF power in the case of 13.56/2 MHz, while different tendencies of line intensity with the LF power appear for other configurations. The reason for this is also discussed.

Frequency Matching Effects on Characteristics of Bulk Plasmas and Sheaths for Dual-Frequency Capacitively Coupled Argon Discharges: One-Dimensional Fluid Simulation

A one-dimensional fluid model is proposed to simulate the dual-frequency capacitively coupled plasma for Ar discharges. The influences of the low frequency on the plasma density, electron temperature, sheath voltage drop, and ion energy distribution at the powered electrode are investigated. The decoupling effect of the two radio-frequency sources on the plasma parameters, especially in the sheath region, is discussed in detail.

Effect of Discharge Parameters on Properties of Diamond-Like Carbon Films Prepared by Dual-Frequency Capacitively Coupled Plasma Source

Diamond-like carbon （DLC） films were prepared with CH4-Ar using a capacitively coupled plasma enhanced chemical vapor deposition （CCP-CVD） method driven by dual-frequency of 41 MHz and 13.56 MHz in combination. Due to a coupling via bulk plasma, the self-bias voltage depended not only on the radiofrequency （RF） power of the corresponding electrode but also on another RF power of the counter electrode. The influence of the discharge parameters on the deposition rate, optical and Raman properties of the deposited films was investigated. The optical band decreased basically with the increase in the input power of both the low frequency and high frequency. Raman measurements show that the deposited films have a maximal sp3 content with an applied negative self-bias voltage of -150 V, while high frequency power causes a continuous increase in the sp3 content. The measurement of atomic force microscope （AFM） shows that the surface of the deposited films under ion-bombardment becomes smoother than those with non-intended self-bias voltage.

Determination of Plasma Parameters in a Dual-Frequency Capacitively Coupled CF_4 Plasma Using Optical Emission Spectroscopy

Optical emission spectroscopy measurements of dual-frequency capacitively coupled CF4 plasmas were carried out. The gas temperature （Tg） was acquired by fitting the optical emission spectra of a CF B-X system in 201～206 nm. The atomic fluorine concentration and the electron temperature （Te） were obtained by trace rare gas optical emission spectroscopy and a modified Boltzmann plot technique, respectively. It was found that the gas temperature was about 620±30 K at 50 mTorr and the atomic fluorine concentration increased while the electron temperature decreased with increasing gas pressure and power of high frequency （60 MHz）. With increasing low frequency （2 MHz） power, the electron temperature also increased, but the atomic fluorine concentration was insensitive to this change. The generation and disappearance mecha- nisms of F atoms are discussed.

Abnormal Enhancement of N_2^+ Emission Induced by Lower Frequency in N_2 Dual-Frequency Capacitively Coupled Plasmas

Nitrogen dual-frequency capacitively coupled plasmas （DF-CCPs） with different fre- quency configurations, i.e., 60/2 MHz and 60/13.56 MHz, are investigated by means of opticM emission spectroscopy （OES） and a floating double probe. The excited nitrogen molecule ion N＋（B） is monitored by measuring the emission intensity of the （0,0） bandhead of the first neg- ative system （FNS） at 391.44 nm. It is shown that in the discharge with 60/13.56 MHz, the N＋ emission intensity decreases with the increase in pressure. In the discharge with 60/2 MHz, however, an abnormal enhancement of N＋ emission at higher pressure is observed when a higher power of 2 MHz is added. Variation in the ion density shows a similar dependence on the gas pressure. This indicates that in the discharge with 60/2 MHz there is a mode transition from the alpha to gamma type when a higher power of 2 MHz is added at high pressures. Combining the measurements using OES and double probe, the influence of low frequency on the discharge is investigated and the excitation route of the N＋（B） state in the discharge of 60/2 MHz is also discussed.

Experimental Characterization of Dual-Frequency Capacitively Coupled Plasma with Inductive Enhancement in Argon

The dual-frequency capacitively coupled plasma （DF-CCP） with inductive enhancement system is a newly designed plasma reactor. Different from the conventional inductively coupled plasma （ICP） reactors, now a radio frequency （rf） power is connected to an antenna placed outside the chamber with a one-turn bare coil placed between two electrodes in DF-CCP. This paper gives a detailed description of its structure of discharges in this apparatus were made via a Moreover, investigations on some characteristics Langmuir probe.

Global simulation of plasma series resonance effect in radio frequency capacitively coupled Ar/O_(2) plasma

Radio frequency capacitively coupled plasmas(RF CCPs)play a pivotal role in various applications in etching and deposition processes on a microscopic scale in semiconductor manufacturing.In the discharge process,the plasma series resonance(PSR)effect is easily observed in electrically asymmetric and geometrically asymmetric discharges,which could largely influence the power absorption,ionization rate,etc.In this work,the PSR effect arising from geometrically and electrically asymmetric discharge in argon-oxygen mixture gas is mainly investigated by using a plasma equivalent circuit model coupled with a global model.At relatively low pressures,as Ar content(α)increases,the inductance of the bulk is weakened,which leads to a more obvious PSR phenomenon and a higher resonance frequency(ω_(psr)).When the Ar content is fixed,varying the pressure and gap distance could also have different effects on the PSR effect.With the increase of the pressure,the PSR frequency shifts towards the higher order,but in the case of much higher pressure,the PSR oscillation would be strongly damped by frequent electron-neutral collisions.With the increase of the gap distance,the PSR frequency becomes lower.In addition,electrically asymmetric waveforms applied to a geometrically asymmetric chamber may weaken or enhance the asymmetry of the discharge and regulate the PSR effect.In this work,the Ar/O_(2) electronegative mixture gas is introduced in a capacitive discharge to study the PSR effect under geometric asymmetry effect and electrical asymmetry effect,which can provide necessary guidance in laboratory research and current applications.

Effect of parallel resonance on the electron energy distribution function in a 60 MHz capacitively coupled plasma

In general,as the radio frequency(RF)power increases in a capacitively coupled plasma(CCP),the power transfer efficiency decreases because the resistance of the CCP decreases.In this work,a parallel resonance circuit is applied to improve the power transfer efficiency at high RF power,and the effect of the parallel resonance on the electron energy distribution function(EEDF)is investigated in a 60 MHz CCP.The CCP consists of a power feed line,the electrodes,and plasma.The reactance of the CCP is positive at 60 MHz and acts like an inductive load.A vacuum variable capacitor(VVC)is connected in parallel with the inductive load,and then the parallel resonance between the VVC and the inductive load can be achieved.As the capacitance of the VVC approaches the parallel resonance condition,the equivalent resistance of the parallel circuit is considerably larger than that without the VVC,and the current flowing through the matching network is greatly reduced.Therefore,the power transfer efficiency of the discharge is improved from 76%,70%,and 68%to 81%,77%,and 76%at RF powers of 100 W,150 W,and 200 W,respectively.At parallel resonance conditions,the electron heating in bulk plasma is enhanced,which cannot be achieved without the VVC even at the higher RF powers.This enhancement of electron heating results in the evolution of the shape of the EEDF from a biMaxwellian distribution to a distribution with the smaller temperature difference between high-energy electrons and low-energy electrons.Due to the parallel resonance effect,the electron density increases by approximately 4%,18%,and 21%at RF powers of 100 W,150 W,and 200 W,respectively.

Enhancing surface adhesion of polytetrafluoroethylene induced by two-step in-situ treatment with radiofrequency capacitively coupled Ar/Ar+CH_(4)+NH_(3) plasma

Although some progress in plasma modification of the polytetrafluoroethylene(PTFE) surface has been made recently,its adhesion strength still needs to be further improved.In this work,the surface of a PTFE sample was treated with a two-step in-situ method.Firstly,the PTFE surface was treated with capacitively coupled Ar plasma to improve its mechanical interlocking performance;then,Ar+NH_(3)+CH_(4) plasma was used to deposit an a-CNx:H cross-linking layer on the PTFE surface to improve the molecular bonding ability.After treatment,a high specific surface area of 2.20 and a low F/C ratio of 0.32 were achieved on the PTFE surface.Its surface free energy was increased significantly and its maximum adhesion strength reached77.1 N·10 mm^(-1),which is 56% higher than that of the single-step Ar plasma-treated sample and32% higher than that of the single-step Ar+CH_(4)+NH_(3) plasma-treated sample.

Effects of Low-Frequency Source on a Dual-Frequency Capacitive Sheath near a Concave Electrode

A self-consistent two-dimensional （2D） collisionless fluid model is developed to sim- ulate the effects of the low-frequency （LF） power on a dual frequency （DF） capacitive sheath over an electrode with a cylindrical hole. In this paper, the time-averaged potential, electric field （E- field）, ion density in the sheath, and ion energy distributions （IEDs） at the center of the cylindrical hole＇s bottom are calculated and compared for different LF powers. The results show that the LF power is crucial for determining the sheath structure. As the LF power decreases, the potential drop decreases, the sheath becomes thinner, and the plasma molding effect seems to be more significant. The existence of a radial E-field near the sidewalls of a hole may cause a significant portion of ions to strike the sidewall and lead to the phenomenon of notching.

Plasma Uniformity in a Dual Frequency Capacitively Coupled Plasma Reactor Measured by Optical Emission Spectroscopy

Local measurement of plasma radial uniformity was performed in a dual frequency capacitively coupled argon plasma （DF-CCP） reactor using an optical probe. The optical probe collects the light emission from a small separate volume in plasma, thus enabling to diagnose the plasma uniformity for different experimental parameters. Both the gas pressure and the low- frequency （LF） power have apparent effects on the radial uniformity of argon plasma. With the increase in either pressure or LF power, the emission profiles changed from a bell-shaped to a double-peak distribution. The influence of a fused-silica ring around the electrodes on the plasma uniformity was also studied using the optical probe. Possible reasons that result in nonuniform plasmas in our experiments are discussed.

Analytical Expressions of Dual-Frequency Plasma Diagnostic Theory

Based on the fundamental ideas concerning microwave attenuation in plasma, we obtain a new expression of transmission attenuation of microwaves as a function of the incident wave frequency. And with reasonable hypothesis, analytical forms of the electron density and the electron-neutral collision frequency are derived from the equations of the transmission attenuation of microwaves at two near frequencies. This method gives an effective and easy approach to diagnose the unmagnetized plasma.

A One-Dimensional Hybrid Simulation of DC/RF Combined Driven Capacitive Plasma

We developed a one-dimensional hybrid model to simulate the DC/RF combined driven capacitively coupled plasma for argon discharges. The numerical results are used to analyze the influence of the DC source on the plasma density distribution, ion energy distributions （IEDs） and ion angle distributions （IADs） on both the RF and DC electrodes. The increase in DC voltage drives more high-energy ions to the electrode applied to the DC source, which makes the IEDs at the DC electrode shift towards higher energy, and the peaks in the IADs shift towards small angle regions. At the same time, it also decreases the ion energy at the RF electrode and enlarges the incident angles of the ions, which strike the RF electrode.

Effect of Low-frequency Power on F, CF_2 Relative Density and F/CF_2 Ratio in Fluorocarbon Dual-Frequency Plasmas

Effect of low-frequency power on F, CF2 relative density and F/CF2 ratio, in C2F6, C4F8 and CHF3 dual-frequency capacitively couple discharge driven by the power of 13.56 MHz/2 MHz, was investigated by using optical emission spectroscopy. High F, CF2 relative density and high F/CF2 ratio were obtained in a CHF3 plasma. But for C2F6 and C4Fs plasmas, the F, CF2 relative density and F/CF2 ratio all decreased significantly due to the difference in both reactive paths and reactive energy. The increase of LF power caused simultaneous increase of F and CF2 radical relative densities in C4Fs and CHF3 plasmas, but led to increase of F with the decrease in CF2 relative densities in C2F6 plasma due to the increase of lower energy electrons and the decrease of higher energy electrons in electron energy distribution function （EEDF）.

Experimental investigation of the electromagnetic effect and improvement of the plasma radial uniformity in a large-area,very-high frequency capacitive argon discharge

We performed an experimental investigation on the electromagnetic effect and the plasma radial uniformity in a larger-area, cylindrical capacitively coupled plasma reactor. By utilizing a floating hairpin probe, dependences of the plasma radial density on the driving frequency and the radio-frequency power over a wide pressure range of 5-40 Pa were presented. At a relatively low frequency(LF, e.g. 27 MHz), an evident peak generally appears near the electrode edge for all pressures investigated here due to the edge field effect, while at a very high frequency(VHF, e.g.60 or 100 MHz), the plasma density shows a sharp peak at the discharge center at lower pressures, indicating a strong standing wave effect. As the RF power increases, the center-peak structure of plasma density becomes more evident. With increasing the pressure, the standing wave effect is gradually overwhelmed by the ‘stop band’ effect, resulting in a transition in the plasma density profile from a central peak to an edge peak. To improve the plasma radial uniformity, a LF source is introduced into the VHF plasma by balancing the standing wave effect with the edge effect. A much better plasma uniformity can be obtained if one chooses appropriate LF powers, pressures and other corresponding discharge parameters.

Surface modification of silicone rubber by CF4 radio frequency capacitively coupled plasma for improvement of flashover

The flashover performance of insulating materials plays an important role in the development of high-voltage insulation systems.In this paper,silicone rubber(SIR)is modified by CF4 radio frequency capacitively coupled plasma(CCP)for the improvement of surface insulation performance.The discharge mode and active particles of CCP are diagnosed by the digital single-lens reflex and the spectrometer.Scanning electron microscopy and x-ray photoelectron spectroscopy are used for the surface physicochemical properties of samples,while the surface charge dissipation,charge accumulation measurement,and flashover test are applied for the surface electrical characteristics.Experimental results show that the fluorocarbon groups can be grafted and the surface roughness increases after plasma treatment.Besides,the surface charge dissipation is decelerated and the positive charge accumulation is inhibited obviously for the treated samples.Furthermore,the surface flashover voltage can be increased by 26.67%after 10 min of treatment.It is considered that strong electron affinity of C–F and increased surface roughness can contribute to deepening surface traps,which not only inhibits the development of secondary electron emission avalanche but also alleviates the surface charge accumulation and finally improves the surface flashover voltage of SIR.