Wen-Qi Lu

Modulating effects of the low-frequency source on ion energy distributions in a dual frequency capacitively coupled plasma

With the energy resolved quadrupole mass spectrometer and hybrid simulation, the influence of low-frequency (LF) source parameters on the ion energy distributions (IEDs) of argon ions impinging on the grounded electrode was studied, both experimentally and numerically, in a dual frequency capacitively coupled plasma. It was shown that for decreasing LF or increasing LF power, the high energy peak in IEDs shifts toward the high energy region significantly. The simulation results were in general agreement with the experimental data.

Spatially resolved measurements of ion density and electron temperature in a dual-frequency capacitively coupled plasma by complete floating double probe technique

Spatially resolved measurements of the ion density and electron temperature in a dual-frequency capacitively coupled Ar discharge plasma are performed with a newly developed complete floating double probe. Axial and radial distributions of the ion density and electron temperature under various high-frequency (HF) power and gas pressure were studied in detail. Both the ion density and the electron temperature increased with increasing HF power. With increasing gas pressure from 1.3 to 9.3 Pa, the radial profile of ion density below the driven electrode experienced a change from “bimodal” to “unimodal” shape, with better uniformity being achieved at the optimal pressure of about 5 Pa. In addition, changing the axial profile of ion density was also observed with the peak shift toward the powered electrode at higher pressures. The measured results showed satisfying consistency with that of improved two dimensional fluid simulations.

Experimental investigation of ion energy distributions in a dual frequency capacitively coupled Ar/CF4 plasma

An energy resolved quadrupole mass spectrometer was adopted to determine ion energy distributions (IEDs) impinging on the ground electrode in a dual frequency (DF) capacitively coupled Ar/CF4 (95%/5%) plasma. The influences of discharge parameters, such as power of low frequency (LF power) source, frequency of LF source (LF frequency), and gas pressure, on IEDs of Ar+ and CF3+ were investigated. The enhancement in LF power, which hence means the increase in sheath potential, results in a significant shift in the IEDs of Ar+ and CF3+ toward higher energy area and then a broader energy width. However, the increase in LF frequency leads to narrow and unimodal IEDs, which is probably because the regime of DF CCP has changed during the process. The pressure has a remarkable effect on IEDs structure, i.e., the exhibited saddle-shaped structure of IEDs is obvious in a collisionless sheath at lower pressure but becomes eliminated in a collision sheath at higher pressure. The Ar+ IEDs have low energy regions becau...

The effect of N2 flow rate on discharge characteristics of microwave electron cyclotron resonance plasma

The properties of plasma in Ar/N2 microwave electron cyclotron resonance discharge with a percentage of N2 flow rate ranging from 5% to 50% have been studied in order to understand the effect of N2 flow rate on the mechanical properties of silicon nitride films. N2+ radicals as well as N2, N+ are found by optical emission spectroscopy analysis. The evolution of plasma density, electron kinetic energy, N2+, N2, and N+ emission lines from mixed Ar/N2 plasma on changing mixture ratio has been studied. The mechanisms of their variations have been discussed. Moreover, an Ar/N2 flow ratio of 2/20 is considered to be the best condition for synthesizing a-Si3N4, which has been confirmed in the as-deposited silicon nitride films with quite good mechanical properties by nanoindentation analyses.

Experimental validation and simulation of collisionless bounce-resonance heating in capacitively coupled radio-frequency discharges

In low-pressure capacitively coupled radio-frequency discharges, when the driving frequency and discharge gap satisfy certain resonant conditions, the high-energy beam-like electrons generated by fast sheath expansion are bounced back and forth between two sheath edges, during which they can gain energy in each of the collisions with either of the expanding sheaths, and can consequently be heated by the two sheaths coherently. This is the so-called collisionless electron bounce-resonance heating (BRH). The first experimental evidence of BRH was reported by Liu et al (2011 Phys. Rev. Lett. 107 055002). Using a combined measurement of floating double probe and optical-emission spectroscopy, we further demonstrate the effect of BRH on plasma properties, such as plasma density and light emission. It is found that plasma density and excitation are enhanced due to BRH and have a significant dependence on the gap length, pressure, low frequency, high-frequency power and driving frequency, which are presented and discussed in detail. These observations can be explained satisfactorily by a self-consistent 1D3v particle-in-cell/Monte Carlo collision simulation in more detail.

Experimental and numerical investigations of electron density in low-pressure dual-frequency capacitively coupled oxygen discharges

The electron density is measured in low-pressure dual-frequency (2/60 MHz) capacitively coupled oxygen discharges by utilizing a floating hairpin probe. The dependence of electron density at the discharge center on the high frequency (HF) power, low frequency (LF) power, and gas pressure are investigated in detail. A (1D) particle-in-cell/Monte Carlo method is developed to calculate the time-averaged electron density at the discharge center and the simulation results are compared with the experimental ones, and general agreements are achieved. With increasing HF power, the electron density linearly increases. The electron density exhibits different changes with the LF power at different HF powers. At low HF powers (e.g., 30 W in our experiment), the electron density increases with increasing LF power while the electron density decreases with increasing LF power at relatively high HF powers (e.g., 120 W in our experiment). With increasing gas pressure the electron density first increases rapidly to reach a maximum value and then decreases slowly due to the combined effect of the production process by the ionization and the loss processes including the surface and volume losses.

EXPERIMENTAL STUDY OF SPATIAL NON-UNIFORMITIES IN A DUAL FREQUENCY CAPACITIVELY COUPLED PLASMA

Radial non-uniformities of dual frequency capacitively coupled argon plasma were experimentally studied, using an optical emission spectroscopy probe, for different discharge pressures, discharge gaps, and frequencies and powers of the low frequency source. It was shown that the plasma emission profiles strongly depend on the working gas pressure and the lower frequency (LF) power. With the increasing LF power the emission profiles changed from a bell-shaped distribution at low LF power to a double-peak distribution at high LF power.

Improved inflection point method of emissive probe for accurate measurement of plasma potential

The potential of the inflection point of emissive probe characteristics ( V i p) in the limit of zero emission is considered to be an accurate indication of the plasma potential. Previous method for this limit is linearly extrapolating the change of V i p with respect to the electron emission current I e m i s to the limit I e m i s ≈ 0, which may result in inaccurate results since evidences showed that V i p does not change well linearly with I e m i s. The authors found in this study that, instead of I e m i s, V i p changes linearly with the probe heating current ( I h t) which is a function of the probe temperature ( T p), and the phenomenon is reasonably interpreted by the space charge voltage increasing linearly with T p. An improved inflection point method of emissive probe, which utilizes linear extrapolating the V i p − I h t relation to the heating current for initial emission, is proposed for accurate measurement of plasma potential.

Automatic emissive probe apparatus for efficient plasma potential measurements

The improved inflection point method of emissive probe is the most accurate method for plasma potential measurements, but its manual operation is quite cumbersome and time-consuming. This paper describes the design and test of an automatic emissive probe apparatus for efficient plasma potential measurements. The apparatus consists of a computer controlled data acquisition (DAQ) card, a working circuit composed of a biasing unit and a heating unit, as well as the emissive probe. The main feature of the apparatus is that both the biasing scan and the heating scan of the probe are controlled by the computer program through analog outputs of the DAQ card, which easily realizes the required timing between the biasing and heating scans of the probe. The apparatus can automatically execute the improved inflection point method of emissive probe and give the plasma potential result. The advantages of high-accuracy, high-efficiency, and durability of probe filament make the apparatus promising for extensive use in plasma potential measurements.

Influence of the bias signal amplitude and frequency on the harmonic probe measurements in plasma diagnostics

The harmonic probe technique may be used for the diagnostics of the plasma in insulative film deposition circumstances where the conventional Langmuir probe cannot work. In this study, we investigated the influence of the bias signal amplitude V0 and frequency f of the harmonic probe on the diagnostic results. While the measured electron temperature Te and ion density ni change little with f within the frequency range of 1–10 kHz, both of them show a considerable increase with V0. The reasons for the results were analyzed, and based on the understanding, an improved harmonic probe technique was proposed. The validity of the improved technique was verified by comparing its results with those of a conventional Langmuir probe in Ar plasmas. The improved harmonic probe technique was applied in diagnostics of the plasma circumstance for microwave electron cyclotron resonance plasma enhanced radio frequency magnetron sputtering deposition of SiNx films.