Microwave Plasma Source Research Articles

Hydrogen Balmer-α line broadening in a microwave plasma source

Emission spectroscopy was used to investigate the Balmer-α line broadening in a slot-antenna excited microwave plasma source operating in helium–hydrogen and argon–hydrogen mixtures, and in pure hydrogen at low pressure (p = 0.3 mbar). No evidence was found for excessive broadening of the Hα line under the present operating conditions, even though this line was found to be broader than the helium singlet line at 667.8 nm. The Doppler temperatures corresponding to this helium line (∼650 K) are close to the rotational temperatures (∼500 K) determined from the Q-branch of the Fulcher-α band under the same conditions. The Doppler temperatures corresponding to the Balmer-α broadening are about 3000 K. The Hα line shape does not change as the amount of hydrogen (up to 50%) in the He–H2 mixture is changed. The results indicate selective heating of H atoms in low-pressure He–H2, Ar–H2 and H2 microwave plasmas.

High density distributed microwave plasma sources in a matrix configuration: concept, design and performance

Plasma scaling up can be achieved by distributing elementary microwave plasma sources over two or tri-dimensional networks. This concept is applied to a planar reactor comprising 4 × 3 microwave plasma sources distributed according to a square lattice matrix configuration. In each elementary plasma source, the plasma is produced at the end of a coaxial applicator implemented perpendicularly to the planar source. An argon plasma can be sustained in the medium pressure range from 7.5 to 750 Pa. The sheet of plasma thus obtained becomes uniform at a distance from the source plane of 15 mm, i.e. less than half the 40 mm lattice mesh. Using a cylindrical Langmuir probe, plasma density and electron temperature have been investigated as functions of pressure and microwave power. Results show that the plasma can reach densities between 1012 and 1013 cm−3 with a uniformity better than ±3.5%.

Effect of substrate bias on high-rate synthesis of microcrystalline silicon films using a high-density microwave SiH4/H2 plasma

In this paper, we demonstrate the effect of dc substrate bias on high-rate deposition of microcrystalline silicon (μc-Si) films by using a high-density microwave plasma source. Film depositions are performed at a high silane concentration of 67% (deposition rate of ∼ 60 Å s−1) with a low substrate temperature of 250 °C. The μc-Si films deposited under appropriate negative dc substrate bias exhibit improved film crystallinity, mass density and reduced defect density along with thinner amorphous silicon incubation layer at the initial growth stage. These can be attributed to the beneficial effect of moderate ion bombardment as well as the less contribution of harmful short-lifetime radicals to film growth.

Influence of substrate temperature on morphology of SiO x films deposited on particles by PECVD

Thin SiO x films were deposited on silica gel particles by plasma enhanced chemical vapor deposition (PECVD), using hexamethyldisiloxane (HMDSO) as a monomer. The deposition process was conducted in a circulating fluidized bed (CFB) reactor with a microwave plasma source integrated in the riser tube. Thin films were deposited at different substrate particle temperatures to investigate the influence on the growth structure. After the deposition process the fracture cross sections of the coated particles were analyzed by means of scanning electron microscopy (SEM). According to the well-known structure zone model (SZM) the deposition conditions correspond to the zone 1 growth regime where geometric shadowing effects are dominant. In addition, the primary particles on the surface of the silica gel particles acted as nucleation sites. Thus no dense and coherent film was deposited, but a pronounced nodular growth structure could be observed. However, increasing the surface diffusion rate reduced the effect of the geometric shadowing. Therefore, the number of nodules was decreased at elevated substrate temperatures.

Large-scale Ar and N2–Ar microwave plasma sources

The spatial structure, including a discharge and a remote plasma zone, of a large-scale, slot antenna excited microwave plasma source operating in Ar and N2–Ar mixtures at 2.45 GHz has been investigated. The discharge source is sustained by the electric field of pure TM surface modes. Optical emission spectroscopy, probe diagnostic techniques and radiophysics methods are applied to measure the emission intensity of radiative states, the electron density and the microwave field intensity, respectively. The spatial distribution of the electron density and the intensity of emission lines belonging to the 2nd positive and 1st negative systems of N2 follow the electric field pattern in the active-discharge zone of the plasma source. On the contrary, the populations in vibrational and rotational levels of the N2(C 3Πu) state are homogeneous irrespective of the azimuthal position.

Numerical analysis and optimization of the microwave plasma source for ion beam neutralization

A microwave plasma source operated at a frequency of 2.45 GHz has been used for ion beam neutralization. It could provide a sufficient number of electrons to neutralize a high current ion beam. However, it was operated with high reflection powers. Experimentally changing waveguide dimensions to get an optimum operating condition is difficult, time-consuming and cumbersome. The key approach for a good and stable working plasma source is to get the right dimensions, by three-dimensional simulation tools. Therefore, a three-dimensional computer simulation program was used to calculate the microwave parameters, and to reveal the behaviour inside the source. It utilizes a single block, non-self-consistent model with Drude dispersion to represent the plasma. For this model the plasma angular frequency, ωp, and the electron-heavy particle collision frequency, νe, have to be investigated. This simple yet powerful model allows the analysis and optimization of the source to identify the best dimensions.

Plasma parameters of an oxygen microwave discharge obtained by probe diagnostics: I. Axial distributions

Oxygen plasmas generated by the slot antenna type microwave plasma source are studied using the single Langmuir probe technique. Microwave power in the range from 600 to 1200 W at frequency 2.45 GHz is applied at neutral gas pressure 100 Pa. Axial distributions of floating potential, plasma potential, concentrations of electrons and negative ions and electron energy probability function are obtained. The influence of the electromagnetic field on the plasma parameter distribution is discussed on the basis of analytical formulas and numerical field computation. The experimentally obtained electronegativity α ≃ 2 is compared with the theoretical results from a simple model based on the balance equation for the negative ions.

Fast Deposition of Highly Crystallized Microcrystalline Si Films Utilizing a High-Density Microwave Plasma Source for Si Thin Film Solar Cells

AbstractIn this work, we investigate the influences of source gas supply method and dc substrate bias on the deposition rate as well as structural properties of μc-Si films and demonstrate the high rate synthesis of highly crystallized μc-Si films with improved film density by using a high-density and low-temperature microwave plasma source. A high flux of SiH3 and atomic hydrogen on film growing surface achieved through adjusting SiH4 gas supply configuration is effective for the fast deposition of highly crystallized μc-Si films and for the improvement of film mass density. Appropriate ion bombardment under moderate negative dc substrate bias along with the suppression of contributions of detrimental species to film growth also account for the improvement in properties of resulting films. As a consequence, a very fast deposition rate of ~65 Å/s was achieved with a high Raman crystallinity Ic/Ia>3 and a low defect density of (1-2)×1016 cm-3 despite a high silane concentration of 67%.

A New Design and Computer Simulation of a 5-Electrode Ion Extraction/Focusing System

A 13.56 MHz radio-frequency (rf) driven multicusp ion source has been constructed [1] to produce a high argon ion current density. Milliampere-range argon ion current can be extracted from the source. An in-waveguide microwave plasma source has been utilized as the ion beam neutralizer [2]. The neutralization source was placed 20 cm downstream from the extraction system. With the former extraction system, comprised of extraction electrodes and an Einzel lens, the electrons from the neutralizer were attracted to the high positive potential of the lens. Consequently, the potential of the lens drops and the beam is diverged. To suppress electrons from being accelerated to the Einzel lens a negatively biased electrode was placed before the last electrode, which is grounded, to produce a retarding electric field for electrons. The hole of the electrode was made small to make sure that the potential at the center is negative enough to suppress electrons. All simulations have been performed with the KOBRA3-INP simulation software. The results of the beam shape from the simulation will be presented.

Low-temperature fabrication of silicon films by large-area microwave plasma enhanced chemical vapor deposition

A novel designed microwave PECVD system with large-area microwave plasma source is employed for the feasibility study to grow thin silicon films at low temperature. With the total area being 25 cm × 30 cm, three sets of array antenna composed by many copper rods with adjustable length are employed to couple the microwave power into the deposition chamber. A RF power source is employed to induce a negative bias on the substrates. High purity (99.99%) SiH 4, H 2, and Ar are introduced as reaction gases, and ultrasonically cleaned glasses are used as substrates. Silicon films with grain sizes about 80∼100 nm are successfully deposited at temperatures below 82 °C. Further studies will focus on the modification of the system design so as to reach the goals to deposit silicon films with improved crystallinity, larger grain sizes, and more uniformity over the large area.

On the Parameters of Inductively Coupled and Microwave BF3 Plasmas Used for Plasma Immersion Ion Implantation

AbstractSummary: A comparison of inductively coupled plasma (ICP) and microwave plasma sources was carried out under the same discharge conditions, in the same discharge chamber, and using the same diagnostics method. Investigations were fulfilled over a wide range of external discharge parameters (at pressures 0.5–20 mTorr and for powers deposited in the plasma at 400–1 500 W) in boron trifluoride and in argon discharges. A variety of plasma parameters (Te, ne, n+, EEDF) and their radial profiles at a 2 cm distance above a wafer holder were determined by using a single Langmuir probe technique. Analysis of measurements has shown that the charged particle concentrations in the ICP are higher than are obtainable in a microwave discharge, for a deposited power at 1.2 kW the ICP source produced an ion number density of ∼1012 cm−3. The required plasma uniformity can be maintained in ICP over a wider range of external discharge parameters than in microwave plasma. The use of microwave plasma source gives a bi‐Maxwellian type EEDF, whereas the EEDF of ICP is close to the single Maxwellian distribution with an electron temperature higher than the temperature of cold electrons in microwave discharge. BF3 plasma is electronegative, with a degree of electronegativity of ∼0.3–0.5 for both plasma sources.Electron currents as functions of potential applied to a cylindrical Langmuir probe in inductively coupled and microwave BF3 plasmas.magnified imageElectron currents as functions of potential applied to a cylindrical Langmuir probe in inductively coupled and microwave BF3 plasmas.

Linear radio frequency plasma sources for large scale industrial applications in photovoltaics

Linear plasma sources can be used both in PECVD-processes for deposition of dielectric layers like oxides or nitrides and for plasma etching of substrates at dimensions of more than 1×1 m2. Furthermore effective plasma surface modification or simple surface plasma cleaning can be carried out. A high substrate throughput is achieved by using inline vacuum processes with typical process times of 60 s for 1 m2 substrate area.Besides well developed linear microwave plasma sources applied to industrial large scale silicon nitride deposition for photovoltaics, linear radio frequency plasma sources become more and more important to generate linear plasmas for applications like deposition of amorphous and microcrystalline silicon or reactive ion etching.The principle of linear radio frequency plasma sources is shown. An additional magnetic field enhances both the process pressure range (down to 10−3 mbar) and the plasma density.Results of silicon nitride deposition and of hydrogen passivation at multicrystalline silicon wafers are presented.

Investigation of the practicability of low-pressure microwave plasmas in the sterilisation of food packaging materials at industrial level

Experiments on laboratory scale carried out by use of different microwave-driven plasma sources proved that low-pressure plasmas represent an excellent means to efficiently sterilise materials for all kind of applications. Based on experimental results obtained for four different test spores, more detailed experiments on Bacillus subtilis with initial contaminations between 106 and 107 spores and varying spore densities were carried out showing the adaptability of the process parameters microwave power, continuous wave and pulsed power supply, and distance between plasma source and substrate to the individual requirements of the aspired sterilisation process. Spore reduction kinetics reproduced by a low-pressure microwave plasma source designed for industrial applications demonstrated the easy scalability of this plasma sterilisation technique.

Detection and identification of pinholes in plasma-polymerised thin film barrier coatings on metal foils

Investigations of barrier coatings on metal foils are presented. The coating materials are plasma-polymerised quartz-like silicon oxide and silicon nitride deposited with the low-pressure microwave plasma source Duo-Plasmaline.In order to detect and identify pinholes, the new test equipment based on a chemical electrolysis reaction was developed.

Characterization of a microwave discharge by thermography

The slot antenna (SLAN) type microwave plasma source is characterized by use of thermography. The influence of microwave power (2.45 GHz, 600–1400 W), gas pressure (5–10 mbar) and oxygen flow (500–1500 sccm) on symmetry of the temperature profile along the axis of the cylindrical plasma chamber is investigated. The thermography results are compared with axial Langmuir probe measurements and with results of the electricfield simulations, for which the plasma is modelled by use of Drude model.

Microwave PECVD for large area coating

Microwave Plasma Enhanced Chemical Vapour Deposition (PECVD) of thin films is the method of choice when highest deposition rates and/or high fragmentation of precursor material is desirable. However, large area applications have always suffered from poor film thickness uniformity and unacceptable variations of thin film properties. Coaxial plasma line sources in various arrangements have proven their ability to generally overcome all former technological limitations inherent in many other types of microwave plasma sources, which prevented microwave PECVD from becoming a mainstream technology in the field of large area coatings. In this article, the advantages and the potential of the coaxial plasma line sources and suitable vacuum processes are described. Improvements of the coaxial plasma lines are introduced which are indispensable for large area plasma applications and which have been investigated in several laboratory and production prototype vacuum process systems. A variety of thin films have been deposited, ranging from organic and ceramic silicon dioxide to nanocrystalline diamond and microcrystalline silicon for photovoltaic applications.

Growth structure of SiOx films deposited on various substrate particles by PECVD in a circulating fluidized bed reactor

Thin SiOx films were deposited on salt and silica gel particles by plasma enhanced chemical vapor deposition (PECVD). A microwave plasma source was integrated in the riser tube of a circulating fluidized bed (CFB) reactor. The CFB reactor provided high deposition rates and good precursor conversion efficiency. The cross-section of the coated particles was analyzed by means of scanning electron microscopy (SEM). The films deposited on smooth salt crystals revealed a dense and coherent film morphology. However, dust particles on the substrate surface caused nodular defects. In contrast on silica gel particles no continuous film could be deposited, but a columnar growth structure was observed. Both growth structures are caused by geometric shadowing effects, which is the dominant growth mechanism at the given deposition conditions.

Uranium emission spectra with a low power microwave plasma source

This work presents the first report of uranium spectra generated by a low power microwave plasma source to conduct emission measurements. Distinct uranium peaks in the wavelength range examined from 320 to 430 nm have been successfully obtained with a 200 W plasma utilizing low gas flow rates. The influence of temperature on the uranium behavior in the plasma source is discussed, and the intensity of the spectral lines obtained with this low power source is systematically compared with the results reported in literature, in which high power plasma sources were employed. Concentration effects are studied and the calibration curves are made for some strong spectral lines. The detection limits of uranium are also estimated to be at the 0.4 ppm level, with linear dynamic range at least two and half orders of magnitude. This research establishes a method to effectively generate uranium atoms and ions at a low power and low gas flow rate, which should be useful for uranium fundamental characteristic studies and on-site uranium monitoring work.

Radical-Enhanced Atomic Layer Deposition of Metallic Copper Thin Films

Radical-enhanced atomic layer deposition (REALD) of metallic copper films from copper(II)acetylacetonate and hydrogen radicals was studied. For this work, a new kind of REALD reactor was developed by adding a surface-wave launcher type of microwave plasma source to an inert gas-valved flow-type ALD reactor. The copper films, grown at 140°C, were polycrystalline, exhibited low resistivity, 15 μΩ cm for a 25 nm thick film, and had relatively low impurity levels. The films had excellent adhesion, and they grew conformally. The successful incorporation of the radical source to the ALD reactor encourages the study of other challenging ALD processes. © 2004 The Electrochemical Society. All rights reserved.

Experimental electron energy distribution functions in argon, nitrogen and oxygen high-density and low-pressure reflex and microwave plasma sources

The Langmuir probe method and Druyvesteyn procedure coupled with suitable software are used to measure the electron energy distribution functions (EEDFs) in atomic and molecular gases. Two complementary high-density and low-temperature plasma sources are used, i.e. a low-pressure direct current reflex discharge plasma source and a low-pressure electron cyclotron resonance (ECR) microwave discharge plasma source. The electron energy is restrained within the range 0–15 eV. The electron density and electron temperature are obtained directly from the EEDF data. The experiment is run within the pressure range 1.0–10−2 Pa with a gas flow rate of 1.0–3.0 sccm. The discharge mechanism and the kinetic and dynamic processes in reflex discharge and ECR plasmas play a key role in determining the shape of the EEDFs. The EEDF is bi-Maxwellian in the reflex discharge plasma and is Maxwellian in the ECR plasma, and is independent of the gas used. The distinctive feature of all the EEPDs in nitrogen and oxygen reflex plasmas and in the nitrogen ECR plasma is the presence of a complex structure independent of the plasma source, discharge mode, and/or radial position. The complex dip/shoulder structures obtained for the EEDFs in molecular plasmas are discussed by considering different inelastic interaction channels between electrons and heavy particles.