Microwave Plasma Source Research Articles

Microwave PECVD of Micro-Crystalline Silicon

AbstractThe deposition of micro-crystalline silicon by means of PECVD with a new linear microwave plasma source is investigated. This plasma source has successfully been introduced in the large scale production of multi-crystalline Si solar cells for the deposition of passivating silicon nitride layers. Advantages of this linear plasma source are the high deposition rates and the large area (up to 80 cm width, no length limitations) on which a homogeneous deposition can be achieved. Since this source has not been applied for deposition of micro-crystalline silicon before, we explored a large parameter space (substrate temperature, pressure, MW-power, gas flow rates), in order to find optimum growth conditions. It is observed that with this microwave source it is possible to grow micro-crystalline layers at significantly higher silane/hydrogen ratios and higher deposition rates than for conventional RF PECVD. In this paper, structural properties of the silicon layers, as investigated by Raman and FTIR spectroscopy, XRD and SEM measurements are discussed.

Microwave Plasma Assisted VHF-PECVD of Micro-Crystalline Silicon

AbstractGrowth of intrinsic micro-crystalline silicon layers by means of VHF-PECVD, assisted by remote microwave (MW) plasma has been investigated. The aim of the MW plasma is to enhance the deposition rate by introducing excited hydrogen and Ar atoms from the MW plasma in the VHF deposition zone. For this purpose a remote microwave plasma source was constructed in which a H2/Ar plasma is generated in a 20 mm diameter quartz tube. A gasshower has been constructed for homogeneous distribution of the flow of excited gas species from the microwave source into the deposition zone of the VHF-PECVD reactor where the dissociation of silane takes place. In a first series of experiments we applied high microwave power (< 500 W) and pure hydrogen in the MW source. This resulted in a larger deposition rate, but all layers – even grown at low silane concentrations – were amorphous and had a high oxygen content. The oxygen contamination was partly due to reduction of the quartz tube by the hydrogen plasma. In a second series of experiments Ar dilution and reduced MW power were used to eliminate the effect of etching of the tube by the microwave hydrogen plasma. In this series of experiments an increase of the growth rate of micro-crystalline silicon by about 15 % due to assistance of the microwave plasma was found. Optical emission spectroscopy indicates that – in these experiments – the main mechanism for the increased dissociation of silane is through molecular quenching reaction of Ar* metastables.

Deposition of diamond films at low pressure in a planar large-area microwave surface wave plasma source

In this study a planar large-area microwave plasma source is used to grow diamond films at low gas pressure. This plasma source is based on the excitation of plasma surface waves so that overdense plasmas can be generated. Above all, this plasma source is easy to scale up. For admixture of CH4/H2 gas, radical information and characteristics of the plasma are carefully characterized at low pressure. Some features different from those at high pressure are observed. A three-step process for diamond growth in the planar microwave plasma chemical vapor deposition system has been developed. High nucleation density can be achieved as a result. At a low pressure of 0.2 Torr, diamond films can be successfully deposited on a 4-in. Si(100) wafer, exhibiting a large amount of non-sp3 bonding. The effects of plasma properties on the diamond film are addressed.

Plasma-Induced Surface Functionalization of Polymeric Biomaterials in Ammonia Plasma

Plasma-chemical modification is frequently used to improve the adaption of polymer surfaces to biological environments. In this regard amino functional groups play a key role. They provide an excellent basis for subsequent modifications with specific biomolecules. It would be of great value to get an amino functionalization independent of the specific material in use. The paper reports on an investigation concerning the feasibility of such an universal plasma functionalization procedure. Two different downstream microwave plasma sources were taken to apply a procedure, which was developed for high-grade modification of polystyrene (PS), to a number of other polymers including polyetheretherketone (PEEK), polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN), polycarbonate (PC), polyethylene (PE), polymethylmethacrylate (PMMA) and fluorinated polymers. In many cases, very similar results were obtained. At maximum 5% of the surface were covered by nitrogen functional groups. In some cases, about 50% of total nitrogen functional groups were amino groups. The results suggest that a downstream ammonia plasma treatment indeed is a fairly universal method for high performance amino functionalization of polymeric biomaterials.

Plasma-Induced Surface Functionalization of Polymeric Biomaterials in Ammonia Plasma

Plasma-chemical modification is frequently used to improve the adaption of polymer surfaces to biological environments. In this regard amino functional groups play a key role. They provide an excellent basis for subsequent modifications with specific biomolecules. It would be of great value to get an amino functionalization independent of the specific material in use. The paper reports on an investigation concerning the feasibility of such an universal plasma functionalization procedure. Two different downstream microwave plasma sources were taken to apply a procedure, which was developed for high-grade modification of polystyrene (PS), to a number of other polymers including polyetheretherketone (PEEK), polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN), polycarbonate (PC), polyethylene (PE), polymethylmethacrylate (PMMA) and fluorinated polymers. In many cases, very similar results were obtained. At maximum 5% of the surface were covered by nitrogen functional groups. In some cases, about 50% of total nitrogen functional groups were amino groups. The results suggest that a downstream ammonia plasma treatment indeed is a fairly universal method for high performance amino functionalization of polymeric biomaterials.

Transition from polymer-like to diamond-like carbon coatings synthesized by a hybrid radiofrequency–microwave plasma source

Diamond-like-carbon (DLC) coatings synthesized by microwave (MW) plasma enhanced chemical vapor deposition (MPECVD) with radio-frequency (RF) bias are studied. A comparative study is performed on films grown with the same processing parameters using both MW and RF or only RF for plasma generation. An improvement of the DLC quality when methane is diluted with an increasing quantity of helium is demonstrated with both deposition systems. A gradual and controllable change from polymer-like to diamond-like character of the coating can be achieved in the depositions by RF–MW plasma varying the process parameters in different ways: increasing the substrate bias and/or lowering the pressure, hence augmenting the ion energy.

Microwave plasma characteristics of a circulating fluidized bed-plasma reactor for coating of powders

Coating of powders by Plasma-CVD is an adequate way to protect particles from environmental attack or to provide a large catalytic surface on carrier particles. The μ-SLAN® microwave plasma source is placed into the riser of a circulating fluidized bed, and its performance as a particle-coating reactor is evaluated for silicon oxide deposition on glass beads. Fluctuations of the particle concentration reduce the efficiency of MW coupling. The minimum MW input power for stable plasma generation is 4 times higher in the presence of particles and increases with oxygen partial pressure. The temperature of the entrained particles can be adjusted to very low values, but higher temperatures are achieved near the MW-coupling slots. The conversion ratio of HMDSO only depends on the specific power ( P/ F). It is slightly lower with particles and 5 W/sccm are required for 80% conversion.

Optical emission patterns and microwave field distributions in a cylindrical microwave plasma source

A cylindrical high density (∼10 12 cm −3) large volume (32 cm in diameter and 50 cm in length) homogeneous argon plasma has been produced by a microwave with a frequency of 2.45 GHz and a power of 900 W without a magnetic field. The plasma source is based on a ring shaped rectangular waveguide with eight equally spaced slots in its inside wall. Several optical emission patterns are observed on different conditions and the microwave field is measured by a movable antenna, which showed a clear relationship between the optical emission patterns and the electron field distributions. A mode transition, from a TE 8j mode to a TE 16j mode, occurs when the gas pressure increases from 660 to 1000 Pa. And there is an optical emission pattern when the microwave power decreases from 900 to 300 W. All these phenomena are described in detail and analyzed according to the interactional theory of electrons in plasma with microwave.

Comparison of diamond-like carbon films synthesized by 2.45 GHz microwave and 13.56 MHz multi-jet radiofrequency plasma sources

Diamond-like carbon films (DLC) were grown by two different plasma deposition systems: the RF–MW system employing a radio frequency (RF) powered substrate holder with an additional 2.45 GHz slot antenna (SLAN) microwave (MW) plasma source and the RF–RF system employing a RF powered substrate holder with an additional RF (13.56 MHz) jet matrix plasma source (JeMPS). Helium and methane were used as carrier gas and carbon source, respectively. When operating the RF–MW system, ion densities approached 4×10 10 cm −3. Because of the large working distance used in our experiments the MW plasma did not contribute significantly to the charged particle density. In the RF–RF system substantially higher ion concentrations of up to 1.6×10 11 cm −3 were measured. Optical and structural properties of the coatings deposited were compared using ellipsometry, Raman spectroscopy and Fourier transform infrared (FTIR). When depositing with the RF–MW system a gradual and controllable change from polymer-like to DLC-based films with increasing substrate RF-power was obtained. This is reflected by a refractive index variation from 1.6 to 2.1. Furthermore, the film growth rates decreased with increasing RF-bias. Typical growth rates were 30 nm min −1 at −400 to −500 V bias. A similar change of the films deposited in the RF–RF system was not observed. Instead, DLC films were produced in all cases. The growth rate peaked at 70–80 nm min −1 when using −350 to −450 V bias. The refractive index (at λ=632 nm) and Vickers hardness were approximately 2.3 and 30 GPa, respectively.

Characteristics of a large diameter reactive ion beam generated by an electron cyclotron resonance microwave plasma source

A new ion source with a large beam diameter generated by an electron cyclotron resonance microwave plasma source is presented. Ion beams with a beam diameter of >160 mm and a maximum argon-ion current density of 2.1 mA/cm2 are obtained. For this large two-electrode silicon-ion extraction optics are mounted into an electron cyclotron resonance microwave plasma source. With ion extraction optics installed an electron temperature up to three times higher than without optics was observed. Due to this high electron temperature (up to 12 eV) the O+ concentration as compared to the O2+ concentration is enhanced in an oxygen plasma. Therefore, a very reactive oxygen ion beam with an ion current density of 1.1 mA/cm2 can be generated. Ion energies of 1 keV corresponding to the screen grid voltage are obtained with a small energy spread of 10 eV full width at half maximum or less. The low working pressure (about 10−4 mbar) is well suited for low-pressure ion-beam processes.

Characterization of the slot antenna microwave plasma source

An annular ring slot antenna is used as the generation of large-volume 2.45-GHz microwave plasma. With argon, a surface wave at the quartz tube can be excited above 250 W. Optical emissions testify the existence of azimuthal modes of m=4 at a pressure of 10 Pa and m=6 at a pressure of 230 Pa. An important conclusion that the uniformity of electromagnetic field in plasma has a dependence on the value of plasma density has been acquired based on theoretical calculations under low pressures. Meanwhile, the efficiency of the plasma source is investigated experimentally. Especially, the spatial distribution of plasma parameter show that homogeneous plasma cylinder with a diameter of 160 mm has been successfully produced at the pressure of 10-1000 Pa.

Development of hybrid pulse plasma coating system

A new plasma coating system is developed, which incorporates plasma-based ion implantation (PBII) with pulse-gas introduction, as well as pulse-plasma generation. This system operates by combining plasma chemical vapor deposition (plasma CVD) with ion beam mixing. For to create a deposited layer with a uniform thickness, both gas introduction and plasma generation are performed in pulses. Gas is introduced into the chamber while the plasma is extinguished. Then a negative high voltage is supplied to the workpiece to perform atomic mixing by PBII. In this system a microwave-plasma source, which completely eliminates static magnetic field, is adopted. The plasma is ignited with the help of inductively coupled RF power of 13.56 MHz, and the density is then increased by absorption of 2.45 GHz of microwave power. In this paper, the principle of the new hybrid pulse-plasma coating system and some of the fundamental performances are described.

Highly sensitive beryllium detection with microwave plasma source atomic emission spectrometry

A highly sensitive technique for beryllium determination using microwave induced plasma atomic emission spectrometry (MIP-AES) is explored in this work based on a self-assembled instrumental system. The analytical performance of this system for beryllium determination was examined using argon as working gas and an ultrasonic nebulization–desolvation system for solution sample introduction. Experimental operating parameters, such as working gas flow rate, microwave power, and observation height were optimized during the system characterization. Influence of matrix elements on beryllium determination was studied. The matrix elements, such as Cu and Ni, were found to have little influence on Be(I) 265.06 nm spectral line even at a concentration of 50 μg ml −1, which is 1000 times higher than that of beryllium. Short-term stability of the MIP-AES system was evaluated and a relative standard deviation of 1.8% is achieved. A calibration range of five orders of magnitude for beryllium determination could be obtained with its multiple spectral lines. The detection limits of Be(II) 313.04/313.11, and Be(I) 234.86 nm were calculated based on 3 σ using 1% HNO 3 blank solution and shown to be in parts per trillion level.

Plasma-based low-energy ion implantation for low-temperature surface engineering

This paper summarizes the plasma-based low-energy ion implantation technique, including plasma source ion nitriding/carburizing and plasma source low-energy ion enhanced deposition of thin films, developed from a combination of two techniques based on conventional plasma-based ion implantation and low-energy ion beam implantation for improvement in wear resistance and corrosion resistance for metals and alloys. An electron cyclotron resonance (ECR) microwave plasma source is used to produce the plasma with the high plasma density, electron temperature and ionization degree. The ions are accelerated from the plasma by a low pulsed negative bias of −0.4–−3 kV, which is similar to the cathode potential of conventional plasma thermo-chemical diffusion processing. The low process temperature is in the range from 150°C to 500°C, which corresponds to the upper limit of conventional ion beam implantation and to the lower limit of plasma thermo-chemical diffusion processing, respectively. Low-energy ion implantation and simultaneous indiffusion is the main mass transfer mechanism, and direct thermo-chemical diffusion absorption is an additional mass transfer mechanism for formation of the nitrided/carburized layer and thin film. It has been proved that plasma-based low-energy ion implantation technique has the potential for applications in industry for surface modification of metals and alloys.

Study of the surface-modified Teflon/ceramics complex material treated by microwave plasma with XPS analysis

A large volume microwave plasma source has been developed in our laboratory. The plasma is generated in a Pyrex glass cylinder with an inner diameter of 30 cm and a height of 50 cm, and the microwave power is coupled into the cylinder by eight equally spaced slot antennae on the inside wall of annular waveguide. The apparatus has been applied to surface modification of the material of PTFE/ceramic complex dielectrics (PTFEC). The chemical composition of the modified PTFEC was investigated by X-ray photoelectron spectroscopy (XPS). Under plasma treatment, the oxygen content of the surface is found to increase and fluorine content to decrease. XPS analysis proved that interaction between different plasma and PTFEC surface layer exists in etching, cross-linking, oxidizing simultaneously, and especially oxygen plasma etching.

Mass spectrometry with plasma sources at atmospheric pressure--state-of-the-art and some developmental trends.

The state-of-the art and trends of development in ICP-MS are presented. Special reference is made to progress in plasma sources, which can be used as ion sources and, especially, the inductively coupled plasma and new microwave plasma sources. Further, progress in sample introduction for plasma mass spectrometry is discussed with special reference to new pneumatic nebulizers, enabling, e.g., work with microflows of solutions at high efficiency, new developments in ultrasonic nebulization, in electrothermal evaporation, laser ablation and hydride generation. Innovation in the types of mass spectrometers used is discussed with special reference to the use of reaction and collision cells, sector field mass spectrometry, time-of-flight mass spectrometry and multicollector mass spectrometers. Challenging applications in the field of speciation, especially in environmental samples with the example of the chromium speciation, and in materials analysis, with special reference to the analysis of ceramic powders with and without on-line matrix removal, are also discussed.

Plasma source low-energy ion-enhanced deposition of thin films

Plasma source low-energy ion-enhanced deposition is a new approach for the deposition of thin films on steel and alloys with specific advantages over ion beam-enhanced deposition (IBED) and the complex combined plasma source ion implantation (PSII) and IBED (PSII-IBED). It may be used to develop a thin film process using the plasma-based low-energy ion implantation including plasma source ion nitriding and plasma source ion carburizing. A deposition apparatus based on an electron cyclotron resonance (ECR) microwave plasma source, a magnetron sputtering target and a pulsed low-voltage power supply from 0.5 to 2 kV has been developed. The magnetron sputter deposition of Ti metal films and plasma source low-energy nitrogen ion implantation under a high nitrogen ion implantation dose rate of 0.65–1.20 mA/cm 2 are used alternately in order to synthesize the stoichiometric TiN films on A3 mild steel and M2 high-speed steel at a deposition rate of 50–110 nm/min. The Knoop microhardness of the 1.2–1.6 μm thick TiN films on the M2 steel is in the range of 18.0–20.0 GPa (0.05 N load). The critical loads of the TiN films on the M2 steel measured by the scratch test are 22–35 N corresponding to the formation of an intermixed layer of about 40 nm thick between the film and the steel substrate. The corrosion resistance of the TiN films in 0.5 mol/l H 2SO 4 solution is evaluated by performing electrochemical polarization measurements. Their superior corrosion resistance is a result of the dense fine-grained microstructure and is independent of the preferential orientation of the films.

Large-diameter microwave plasma source excited by azimuthally symmetric surface waves

This article describes a large-diameter, surface-wave excited plasma (SWP) source designed for materials processing. The plasma reactor employs a launcher of 2.45 GHz azimuthally symmetric surface waves in the field-free region of 24-pole line-cusp magnetic fields, generated by a set of permanent magnets surrounding the reactor chamber walls; the magnets also provide an electron cyclotron resonance (ECR) magnetic field of 875 G near the chamber wall surfaces. Langmuir probe and optical emission measurements were made for characterizing the plasma produced in Ar. After the microwave power was turned on, the discharge was observed to start near the ECR region and then propagate toward the field-free region in the central area of the chamber. Moreover, the discharge was also observed to be excited by ECR at low microwave-power levels, and by surface waves in the field-free region at above a critical power strongly depending on the gas pressure. Such a transition of plasma excitation from ECR to SWP was found to occur under conditions where the plasma electron density exceeds a 2.45 GHz microwave cutoff value of ≈7.4×1010 cm−3. As a result, overdense plasmas with large diameters could be successfully produced with moderate microwave input powers ≲1 kW; the electron density was measured to be Ne∼1−3.5×1011 cm−3 in Ar plasmas, with its spatial variation being ≲5% over a 26-cm-diam area. Furthermore, preliminary experiments of SiO2 etching in pure CF4 plasmas showed that the SiO2 etch rate was typically ∼300 nm/min with a good uniformity ≲3% over an 8-in.-diam wafer.

Chromatic monitoring of downstream microwave plasma source

The application of the chromatic sensing for monitoring of a microwave plasma source is described. The emitted radiation from the plasma excited in the argon, oxygen and CF4 mixture was measured with three PIN-diodes with integrated optical filters. The response of the chromatic signals on variation of power and gas composition was investigated. Whereas a good sensitivity of the integrated optical signal to the power was confirmed, only a limited sensitivity to the working gas mixture was found.

Design and development of a highly sensitive, field portable plasma source instrument for on-line liquid stream monitoring and real-time sample analysis

The development of a highly sensitive, field portable, low-powered instrument for on-site, real-time liquid waste stream monitoring is described in this article. A series of factors such as system sensitivity and portability, plasma source, sample introduction, desolvation system, power supply, and the instrument configuration, were carefully considered in the design of the portable instrument. A newly designed, miniature, modified microwave plasma source was selected as the emission source for spectroscopy measurement, and an integrated small spectrometer with a charge-coupled device detector was installed for signal processing and detection. An innovative beam collection system with optical fibers was designed and used for emission signal collection. Microwave plasma can be sustained with various gases at relatively low power, and it possesses high detection capabilities for both metal and nonmetal pollutants, making it desirable to use for on-site, real-time, liquid waste stream monitoring. An effective in situ sampling system was coupled with a high efficiency desolvation device for direct-sampling liquid samples into the plasma. A portable computer control system is used for data processing. The new, integrated instrument can be easily used for on-site, real-time monitoring in the field. The system possesses a series of advantages, including high sensitivity for metal and nonmetal elements; in situ sampling; compact structure; low cost; and ease of operation and handling. These advantages will significantly overcome the limitations of previous monitoring techniques and make great contributions to environmental restoration and monitoring.