Low Pressure Plasma Processing Research Articles

Spectroscopic in situ diagnostics of boron nitride film growth in plasma-enhanced deposition

The development of in situ diagnostics of the most important species and reactions in the plasma and/or on the surface during thin-film growth is one of the current topics in plasma-enhanced vapor deposition. In situ thin film diagnostic methods which could be used in plasma processing are restricted due to the presence of electrons and ions. The advantages and disadvantages of different applicable methods will be discussed. The spectroscopic in situ control of boron nitride film growth is presented as an example of surface modification in low-temperature, low-pressure plasma processing. The growth of cubic and hexagonal boron nitride is observed by polarized infrared reflection spectroscopy in absorption and ellipsometric configurations as well as by single-wavelength ellipsometry in the visible spectral range. Modeling of the experimental results gives detailed information on growth conditions and internal stress of the films.

Space- and surface-charge analysis of plasma-preprocessed PTFE thin films

The space- and surface-charge behavior of corona-charged polytetrafluoroethylene thin films after plasma processing were studied experimentally by space-charge density distribution measurement and thermally stimulated discharge current (TSDC) measurement. The effect of the antistatic process using low-pressure discharge plasma and charge elimination process dipping in tap water was examined. The surface composition of the samples which were plasma- processed was analyzed with X-ray photoelectron spectroscopy (XPS) to study the mechanisms of antistatic process. It was found that charge elimination of plasma-processed samples was enhanced, independently of the kind of processing gas during the plasma processing. Oxygen and nitrogen atoms were observed from the XPS measurement of plasma-processed samples. In particular, a large amount of nitrogen atoms was found on the surface of the samples plasma processed in pure nitrogen gas. The samples plasma processed in pure nitrogen gas showed a large hetero TSDC peak at room temperature before the charge-elimination process. This might be due to nitrogen atoms on the sample surface that were generated during plasma processing.

Plasma surface treatment of poly(acrylonitrile) films by fluorocarbon compounds

This paper concerns the use of the low-pressure plasma process to confer an hydrophobic character to poly(acrylonitrile) films, without altering their bulk properties. Plasma based fluorination processes using saturated fluorine compounds such as CF 4, C 6F 14 and mixtures CF 4/H 2 were used. Such treated polyacrylonitrile films were characterized by XPS analysis, infrared (ATR) spectroscopy and contact angle measurements. The use of CF 4 led to the fluorination of the film surfaces. Indeed, XPS spectra showed the presence of -CHF-, -CF 2- and -CF 3 groups. Moreover, the contact angle θ which was 63° for untreated PAN films increased to 115°. Fluorine incorporation in the PAN disappeared when hydrogen is added to CF 4. In the case of treatment by C 6F 14, fluorination was accompanied by a polymerization, as evidenced by XPS with the disappearance of the N 1s photopeak corresponding to the C≡N functions in PAN. By considering the deposition of F- containing layer thickness, it was possible in this latter case to follow the surface modifications by ATR-i.r. spectroscopy.

Surface cleaning by plasma-enhanced desorption of contaminants (PEDC)

Low-pressure plasma processes were applied for cleaning the surface of glass, metals, carbon and polymers from contaminants such as air pollutants, fingerprints, coupling agents, oxide layers, weakly bonded surface layers, slip agents, light stabilizer or additive enrichments at the surface. It is possible to remove contaminations by simple plasma sputtering (particle bombardment) in plasmas of noble gases, by oxidation of organic contaminants with oxygen plasma or reduction of oxides or sulphides by hydrogen plasma. Examples were mentioned for purification of glass or glass fibre surface and the consequences for optical properties or adhesion in polymer–fibre composites. Degreasing of oil-contaminated sheet metals and removal of weakly bonded oxides were performed by plasma cleaning. The consequences of surface contamination of polymers in gluing and metallization are demonstrated.

Consequences of asymmetric pumping in low pressure plasma processing reactors: A three-dimensional modeling study

Low pressure (<10s mTorr), high plasma density (>1011 cm−3) reactors are rapidly becoming the tool of choice for etching during microelectronics fabrication. Although gas injection and pumping are well characterized process parameters in higher pressure systems, the impact of the symmetry of gas injection and pumping on the uniformity of reactant fluxes to the substrate are less well known in low pressure systems. In this article, results from a three-dimensional plasma equipment model are used to investigate the consequences of asymmetric gas pumping on reactive fluxes in inductively coupled plasma reactors operating in Cl2. We find that for typical conditions (10 mTorr, 150 sccm, 400 W) the azimuthal symmetry of ion density and ion fluxes are little affected by, for example, side pumping. This results from the dominance of ambipolar electric fields which are largely determined by the uniformity of ion sources and shape of the chamber. Neutral reactant fluxes do suffer azimuthal asymmetries due to pumping, however the variations are typically <10%–15% over a pressure range of 5–20 mTorr. The magnitude of the side-to-side variations is largely determined by the relative rates of loss of reactant species to surfaces compared to pumping.

NSERC (Natural Sciences and Engineering Research Council of Canada) industrial research chair on “Low pressure plasma processing of materials”

NSERC (Natural Sciences and Engineering Research Council of Canada) industrial research chair on “Low pressure plasma processing of materials”

The design and application of electron cyclotron resonance discharges

During the past ten years electron cyclotron resonance (ECR) plasma-processing technology has matured into a diverse assortment of ECR plasma reactor and plasma source design concepts and has been extensively applied to numerous low-pressure plasma processing applications. This paper reviews the substantial progress made in the design and application of ECR plasma technology in recent years. Five representative ECR reactor/source designs from large-area 450-cm/sup 2/ discharges to compact plasma sources inserted into molecular-beam epitaxy (MBE) machines are described in detail. The performance of these ECR devices is evaluated by computing performance figures of merit from the available experimental data. These calculations are then compared with the behavior as predicted from a global model of the discharge. This comparison suggests that global plasma models can be employed as an approximate method for ECR reactor design. More extensive diagnostics and numerical models that investigate the spatial variation of ion density and ion energy distributions are also presented. Several illustrative ECR plasma-processing applications are discussed. These include submicron etching of silicon, etching of III-V and II-VI electronic and photonic devices, and the epitaxial growth of GaN. The variety and the sophistication of these applications demonstrate that low-pressure high-density ECR plasma processing technology has evolved into a very useful, versatile group of plasma-processing machines.

Linear arc discharge (LAD): a new type of hollow cathode plasma source

A novel linearly scalable source for low pressure plasma processing is described. The source is based on a parallel plate hot hollow cathode in a focusing magnetic field which allows generation of a linearly uniform plasma in a gas admitted into the slit between the plates. Results of experiments with the laboratory prototype of 10 cm long linear arc discharge (LAD) source supplied by a 13.56 MHz max. 2 kW radio frequency (r.f.) generator are presented. An r.f. impedance probe was used to characterize the discharge. The optical emission from Ar discharge along the slit between Ti cathode plates in the LAD was monitored to study production of Ti and Ti + ions at different experimental characteristics, e.g. varying the r.f. power , gas flows and gas pressure. First results on Ti films depositions are presented. Experiments show that the discharge performance of the LAD source is similar to the r.f. hollow cathode plasma jet (RHCPJ). The LAD source can favorably extend RHCPJ abilities to large area processing.

Plasma-enhanced CVD of TiN and Ti using low-pressure and high-density helicon plasma

Titanium nitride (TiN) and titanium (Ti) were deposited using a low-pressure and high-density helicon plasma process with titanium tetrachloride (TiCl 4). This study aimed to clarify the characteristics of the TiN and Ti deposition processes and the properties of films. The rate-determining process of the TiN film-deposition reaction was changed with the TiCl 4 partial pressure. High-deposition rate and excellent bottom coverage was achieved by using high TiCl 4 partial pressure, but low film resistivity was not achieved simultaneously. By increasing the RF biased power, an improvement of the bottom coverage and the film resistivity was possible. The Ti film was deposited successfully by helicon plasma with a deposition rate of 4 nm min −1 and a film resistivity of 300 μΩ cm. It was found that synthesizing Ti films by plasma-enhanced chemical vapor deposition (CVD) required higher plasma density. The use of helicon plasma is promising for the deposition of TiN and Ti films. However, the deposited Ti film was an amorphous structure; this would be caused by impurities in this film (30 at% oxygen and 15 at% silicon). These impurities would be suspected to be induced by sputter etching of the quartz glass belljar used for the plasma generation chamber and by desorbing of H 2O on the chamber wall.

Experimental validation of a direct simulation by Monte Carlo molecular gas flow model

The Sandia direct simulation Monte Carlo (DSMC) molecular/transition gas flow simulation code has significant potential as a computer-aided design tool for the design of vacuum systems in low pressure plasma processing equipment. The purpose of this work was to verify the accuracy of this code through direct comparison to experiment. To test the DSMC model, a fully instrumented, axisymmetric vacuum test cell was constructed, and spatially resolved pressure measurements made in N2 at flows from 50 to 500 sccm. In a ‘‘blind’’ test, the DSMC code was used to model the experimental conditions directly, and the results compared to the measurements. It was found that the model predicted all the experimental findings to a high degree of accuracy. Only one modeling issue was uncovered. The axisymmetric model showed localized low pressure spots along the axis next to surfaces. Although this artifact did not significantly alter the accuracy of the results, it did add noise to the axial data.

Surface pretreatment of plastics for adhesive bonding

Abstraet-Many plastics have a poor tendency to bond to other materials because of their inherent inert chemical structure and thus require a pretreatment. Wet chemical methods are expensive because of the disposal of the waste liquids. In this study, the corona treatment (Ional process), the low-pressure plasma process, and the fluorination process were tested and compared with each other. The following plastics were tested: PP (polypropylene), PBT (polybutyleneterephthalate), PBT blends, and a high-temperature thermoplastic, PEEK (polyetheretherketone). In particular, the low-temperature plasma process results in excellent adhesion strength. In addition, we have shown that the stability of freshly plasma-treated surfaces could be maintained for time periods of at least several days.

Investigation of selective SiO2-to-Si etching in an inductively coupled high-density plasma using fluorocarbon gases

A recent, important development in low-pressure plasma processing is the radio frequency inductively (RFI) coupled high density discharge. Its ability to create high densities of excited and charged species at low pressures (<10−3 Torr) makes it an attractive etching tool. In this work we have examined selective etching of SiO2 over Si using a home-built RFI source. CHF3, C2F4, C3F6 and their mixtures with hydrogen were examined. Without biasing of the substrate strong fluorocarbon deposition occurs over the investigated pressure range from 5 to 20 mTorr. As the pressure increases the ion current density decreases, whereas the fluorocarbon deposition rate increases. Both parameters increase roughly linearly with inductive rf power from 500 up to 1250 W. Etching was achieved by rf biasing. When the pressure is reduced from 20 to 6 mTorr, the oxide and silicon etch rates decrease less than 20% for all gases. The highest oxide etch rate of 830 nm/min at 350 W rf bias power is achieved for C3F6. Adding H2 decreases the etch rates for oxide and silicon for all gases. The drop of the silicon etch rate is considerably higher than for the oxide etch rate resulting in a better selectivity. The best selectivity of 45 is achieved for C2F4 when 30% H2 is added into the discharge. The results obtained with the RFI source are compared to results with a microwave electron cyclotron resonance discharge.

Low temperature plasma diffusion treatment of stainless steels for improved wear resistance

Abstract The use of a thermionically enhanced low pressure plasma process to provide nitriding and carburizing treatments on a precipitation hardening martensitic stainless steel (AISI 17/4PH) at low temperature (420°C or less) is reported. The resulting diffusion layers are analysed by glow discharge optical spectroscopy and X-ray diffraction depth profiling to provide information on the changes in diffused-species concentration and metallurgical structure with treatment depth. It is shown that there exists the possibility to synthesize a variety of layered structures under different plasma conditions using the low pressure enhanced plasma process—with particular emphasis on the production of hard, yet “precipitate-free” surface layers based on an expanded austenite lattice. These types of layered diffusion treatments may provide considerable improvements in the wear resistance of stainless steels without significantly compromising their desirable corrosion-resistant properties.

A plasma reactor based on the forced constricted type dc plasma jet and its application to thermal plasma processing in low pressure

A reactor consisting of a forced constricted type dc plasma jet generator and a feed ring has been developed. In order to demonstrate the application feasibility of this reactor to low pressure plasma processing, the characteristics of the forced constricted arc in low pressure, and production of ultrafine particles of refractory materials (Al2O3, ZrC, TiC) are studied.

Harmonics Analysis of the Electron Component of rf Bulk Plasmas. I. Theoretical Base

Abstractrf discharges are increasingly used in low pressure plasma processing, i.e. for etching, film deposition and sputtering. The modelling of such discharges is a very complex task, especially dependent on discharge conditions, however of large importance for the insight into the main physical processes and thus for their control to improve the final results. One main important aspect is the determination of the electron velocity distribution function and of relevant rate and transport coefficients. The paper contributes to the treatment of this problem. In the first part a systematic Fourier expansion of the kinetic equation and of the consistent particle, energy and momentum balance equation is described. Then, a mathematical analysis of the resulting ordinary differential equation system for the coefficients of the Fourier expansion is performed. Based upon this we succeeded to develop a numerical approach to calculate the physical relevant solution of this system. By this approach in addition to the harmonics of the distribution function that of relevant macroscopic quantities, as transport coefficients and collision frequencies, can be determined. In the second part of this paper this method will be applied to investigate the bulk plasma of a rf discharge in molecular hydrogen.

Plasma processes under vacuum conditions

In the first part the basics of low pressure plasma processes are briefly reviewed considering excitations by gas phase collisions and electrical potential near surfaces in contact with the plasma (sheat formation). In the second part we discuss the special situation given by the combination of vacuum technology and low pressure plasma technology. The impact of plasma technology on pumps and vacuum vessels is discussed taking into account in particular degassing, impurity situation in the gas phase and gas flow. This is done with a special emphasis on large area industrial machines. In the third part we present a new type of design for a machine for plasma enhanced CVD. This so-called plasma box type reactor is a good example for vacuum technology adapted to a plasma process. The plasma box reactor is designed for large area industrial applications, ensures high thin film uniformity and a uhv-type gas phase purity. The plasma box reactor was tested by deposition of amorphous silicon (a-Si) (for solar cells or thin film transistors), first results of these a-Si depositions are presented.

Electron cyclotron resonance microwave discharges for etching and thin-film deposition

A recent, important development in low-pressure and low-temperature plasma processing is the microwave electron cyclotron resonance (ECR) discharge. Its lack of electrodes and its ability to create high densities of charged and excited species at low pressures (≲10−4 Torr) make it an attractive processing discharge in etching and thin-film deposition applications. This article reviews the basic physics of ECR discharges and reviews the associated microwave system and applicator technologies. Waveguide and cavity ECR applicators are compared and are described in detail. Several ECR plasma processing reactors are also described. Methods of processing large surfaces are outlined, and typical experimentally measured ECR discharge characteristics are presented.

高エネルギー特集 １ プラズマ・レーザー利用 （１） プラズマ・放電化学

There are two different categolies in Plasma Chemical Process: one using electron energy in Non-Equilibrium plasma to produce radicals and other chemically active species, and other using extremely high temperature thermal energy possible to generate by electrical discharges. The former one “Non-Equilibrium Plasma Chemistry”is widely in use in semiconductor processing in a form of “High-Frequency Low-Pressure Plasma Processing”, as well as “Pulse Corona Plasma Chemical Processing (PPCP) ”and “Surface Discharge Plasma Chemical Process (SPCP) ”. Discussions are made on the basic nature of the non-equilibrium plasma as a whole, and on PPCP and SPCP which enabled use of this process under ordinally pressure range.

Application of Plasma to Processing for Ceramics

The thermal plasma processing for ceramics was surveyed with a stress on plasma spraying, plasma CVD and plasma sintering. The development of the low pressure plasma process and RF plasma process in plasma spraying is worth of special mention. These processes will be applied to ceramics coating in near future. Researches on thermal CVD are being carried out very actively for synthesis of high purity ultra fine ceramic powders such as silicon nitride and silicon carbide. This process has a very high potential as a means for rapid growth of ceramic films on various substrates. Low pressure RF inductive plasma and high power microwave plasma have characteristics quite different from RF glow discharge plasma. It is very interesting that these plasmas are applied to synthesize diamond and cubic boron nitride films. Rapid sintering of ceramic powders in plasma atmosphere is one of the most important applications of plasma to materials processing, but many problems remain to be solved in the sintering process.

Application of plasma to processing for ceramics

The thermal plasma processing for ceramics was surveyed with a stress on plasma spraying, plasma CVD and plasma sintering. The development of the low pressure plasma process and RF plasma process in plasma spraying is worth of special mention. These processes will be applied to ceramics coating in near future. Researches on thermal CVD are being carried out very actively for synthesis of high purity ultra fine ceramic powders such as silicon nitride and silicon carbide. This process has a very high potential as a means for rapid growth of ceramic films on various substrates. Low pressure RF inductive plasma and high power microwave plasma have characteristics quite different from RF glow discharge plasma. It is very interesting that these plasmas are applied to synthesize diamond and cubic boron nitride films. Rapid sintering of ceramic powders in plasma atmosphere is one of the most important applications of plasma to materials processing, but many problems remain to be solved in the sintering process.