Low-temperature Radiofrequency Research Articles

Atomic structure of the nanocrystalline Si particles appearing in nanostructured Si thin films produced in low-temperature radiofrequency plasmas

Nanostructured Si thin films, also referred as polymorphous, were grown by plasma-enhanced chemical vapor deposition. The term “polymorphous” is used to define silicon material that consists of a two-phase mixture of amorphous and ordered Si. The plasma conditions were set to obtain Si thin films from the simultaneous deposition of radical and ordered nanoparticles. Here, a careful analysis by electron transmission microscopy and electron diffraction is reported with the aim to clarify the specific atomic structure of the nanocrystalline particles embedded in the films. Whatever the plasma conditions, the electron diffraction images always revealed the existence of a well-defined crystalline structure different from the diamondlike structure of Si. The formation of nanocrystallinelike films at low temperature is discussed. A Si face-cubic-centered structure is demonstrated here in nanocrystalline particles produced in low-pressure silane plasma at room temperature.

Radiofrequency tonsil and adenoid ablation

We describe our experience with 2 new techniques for tonsil and adenoid reduction after experience with 150 procedures. Both techniques are based on low-temperature radiofrequency energy to reduce tonsil and adenoid size rather than excision of tonsil or adenoid tissue. The advantages are almost complete elimination of pain and postoperative bleeding risk. Tonsil ablation is performed by applying radiofrequency energy to the tonsil tissue, which causes posttreatment shrinkage and scarring. Tonsil coblation is based on tonsil reduction by surface application radiofrequency until 90% of the tonsil is eliminated. The disadvantage is that both procedures do not accomplish complete tonsillectomy and some residual tonsillar tissue is left. With tonsil ablation, approximately 40% of the tonsil remains, whereas with tonsil coblation, only 10% of the tonsil remains. Tonsil coblation is more predictable because the surgeon controls the amount of reduction at the time of surgery. Both procedures result in minimal pain, and over 90% of the patients resume a solid diet within 48 hours.

Structural and electroacoustic studies of AlN thin films during low temperature radio frequency sputter deposition

AlN is a material used in a wide variety of applications such as electroacoustic devices, blue diodes, IR windows, thermal conductors, metal–insulator–semiconductor structures, integrated circuit packaging, etc. In this work thin piezoelectric AlN polycrystalline films have been grown on Si and SiO2 using rf magnetron sputter deposition in an Ar/N2 gas mixture. The structural properties of the films have been optimized by varying the deposition parameters, such as process pressure, gas mixture, substrate temperature, discharge power, etc. [K. Tominaga et al., Jpn. J. Appl. Phys., Part 1 35, 4972 (1996); H. Okana et al., ibid. 31, 3446 (1992); K. Kazuya, T. Hanabusa, and K. Tominaga, Thin Solid Films 281-282, 340 (1996)]. It was found that the best film texture was obtained for a particular set of parameters, namely process pressure of 4 mTorr, substrate temperature 350 °C, discharge power 350 W, and a gas mixture of 25% Ar and 75% N2. The films as examined by x-ray diffraction exhibited a columnar structure with a strong (001) texture, and a full width at half maximum (FWHM) rocking curve of 1.6°. Atomic force microscopy measurements indicated a surface roughness with a rms value of 8 Å. Classical nonapodized transversal surface acoustic wave filters operating at a frequency of 534 MHz were fabricated to characterize the electroacoustic properties of the films. The measurements indicated a coupling coefficient of 0.37% and a phase velocity of 4900 m/s. Further, thin epitaxial films were grown on (001)α-Al2O3 (sapphire) under the same deposition conditions except the substrate temperature. The films exhibited a (001)AlN//(001)α-Al2O3 plane orientation with a (002) rocking curve FWHM value of about 0.4°, showing a relatively good alignment of the c axis. The in-plane orientation was [110]AlN//[120]α-Al2O3 corresponding to a rotation of the AlN film of 30° with respect to the (001)α-Al2O3 surface. Cross-sectional transmission electron microscopy studies indicated a population of both thread and edge dislocations with decreasing concentrations with film thickness.

Chemical and biological characteristics of low-temperature plasma treated ultra-high molecular weight polyethylene for biomedical applications.

Several low-temperature radio-frequency (RF) plasma surface treatments were performed on ultra-high molecular weight polyethylene (UHMWPE) used in biomedical applications. Process gases included Ar, C3F6, CH4, hexamethyldisiloxane (HMDSO), and NH4. These treatments were carried out at pressures in the range of 64-400 mTorr, RF powers of 240-1200 W, and temperatures well below the melting point of UHMWPE. X-ray photoelectron spectroscopy (XPS) was used to obtain information about the surface characteristics of UHMWPE treated with the HMDSO, C3F6, and CH4 gases as a function of treatment conditions. XPS spectra of UHMWPE treated with C3F6 and CH4 and exposed to a laboratory environment for different time periods were examined in order to assess the stability of these treatments. It was found that for the C3F6 process gas the amount of fluorine at the surface decreased over time, whereas the oxygen content of the CH4 treated samples increased as a function of time. In vitro cytotoxicity of Ar, C3F6, CH4, and NH4 plasma treated samples was studied in light of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) test results. The hemolytic nature of the various plasma treatments was evaluated using standard hemolysis tests. All of the samples tested in this study exhibited no cytotoxic and negligible hemolytic effects. The process parameters for several low-temperature plasma treatments demonstrating chemical and structural stability and good biocompatibility are discussed in conjunction with the broad applicability to other biomedical polymers.

Surface modification of poly(tetrafluoroethylene) by a low-temperature cascade arc torch and radio-frequency plasmas

Poly(tetrafluoroethylene) (PTFE) films were treated with a low-temperature cascade arc torch (LTCAT) and radio-frequency (RF) plasmas of argon and hydrogen. The plasma-treatment effect on the PTFE surface was studied with contact-angle measurement and scanning electron spectroscopy (SEM). LTCAT argon plasma, which is recognized as a beam of excited argon neutrals, was very efficient at improving the surface hydrophilicity of PTFE. For both the LTCAT and RF operation, argon plasma was more effective at modifying the surface wettability of PTFE films than hydrogen plasma was. Furthermore, the sample positions (inside or beyond the glow region) had a strong impact on the efficiency of the plasma treatment. SEM surface images indicated that no significant morphology change was induced on the PTFE films exposed to a LTCAT and RF argon plasmas.

High nucleation rate in pure SiC nanometric powder by a combination of room temperature plasmas and post-thermal treatments

Abstract High structural purity nanocrystalline SiC nanopowder can be produced in low pressure and low temperature radiofrequency (RF) plasmas by using silane and methane diluted in Ar. The main drawback is the low powder yield thus obtained. Powder yield can reach competitive levels by using pure silane and methane and increasing RF power. The as-formed powder shows local arrangement in silicon carbide units but it is highly hydrogenated and undergoes spontaneous oxidation when exposed to the atmosphere. In order to obtain both the high nucleation rate of SiC particles and the high structural purity, post-thermal treatments up to 1500 °C were performed on this as-formed SiC powder. Ex situ characterization techniques, such as high-resolution transmission electron microscopy, electron diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and elemental analysis, were used to analyse the structural changes induced by annealing.

SiCN nanometric powder produced in square-wave modulated RF glow discharges

Abstract Square-wave modulation of the electrical power supplied to low pressure and temperature radio frequency (RF) plasmas has been revealed as a suitable method to produce nanometric powder of high purity and nanometric size (10–100 nm). Here, we present a study involving the production and characterisation of silicon carbon nitride (SiCN) nanometric powder, obtained from square-wave modulated RF plasmas of SiH4+CH4+NH3 and SiH4 + CH4 + N2 gas mixtures. Electron diffraction patterns and transmission electron micrographs showed that the SiCN powder particles are amorphous and have a size distribution in the range 25–45 nm. X-ray photoelectron spectroscopy, elemental analysis and Fourier-transform infrared spectroscopy showed important differences in the structure, hydrogenation, chemical composition and bond distribution (SiC, SiN) of the samples owing to the use of different gas mixture. This powder could be used as a raw material to produce advanced ceramics and composites with higher thermal and chemical stability and improved mechanical properties.

Elemental Composition of Reactively Sputtered Indium Nitride Thin Films

Indium nitride (InN) thin films have been grown on a variety of substrates using low-temperature radio frequency reactive sputtering of indium metal in pure nitrogen plasma. Quantitative compositional analyses of the films, carried out using X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS), suggest that large amounts of oxygen are present in them. The high concentration of oxygen in our films is attributed to the voided microstructure as revealed by cross-sectional scanning electron microscopy. The XPS studies also suggest that the oxygen incorporated into the films is bonded to nitrogen.

Low-Temperature Growth and Measurement of Oxygen in Reactively Sputtered AlN Thin Films

Aluminium nitride (AlN) thin films have been grown on a variety of substrates using low-temperature radio frequency reactive sputtering of aluminium in pure nitrogen plasma. Quantitative analyses of the film composition, carried out using X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS), suggest the resulting films to be near stoichiometric with high amounts of oxygen present in them. The high concentration of oxygen in our films is attributed to the voided microstructure as revealed by cross-sectional scanning electron microscopy.

Anistropic electron-electron and electron-phonon scattering rates in aluminium from high-precision RFSE measurements between 0.2 and 10 K

High-precision low-temperature radio frequency size effect (RFSE) experiments have been carried out to extract local values of the electron-phonon (e-p) and electron-electron (e-e) scattering rates (SRs) in aluminium. The RF was varied to help separate these two contributions. For the first time an unexpected large anisotropy of the e-e SR has been observed, suggesting that the phonon mediated e-e SR dominates the Coulomb contribution. Comparison of the experimental data with recent calculated values of the e-e SR confirm the importance of this mechanism.

Temperature dependent electron scattering rates in aluminium from high precision RFSE measurements between 0.2 and 10K.

High precision low temperature Radio Frequency Size Effect (RFSE) experiments have been made to extract local values of the electron-phonon and electron-electron scattering rates (SR) in Aluminium. The RF was varied to help separate these contributions. Comparison with calculated values reported in another contribution 1 suggests that the phonon mediated e-e SR dominates the Coulomb interaction. A very sensitive detection system including a digital lock-in amplifier is described.

Low temperature radio frequency shielding of exfoliated graphite

The results of experiments using exfoliated graphite as a low temperature substrate for NMR studies of physisorbed molecules at ultra-high frequencies (100–300 MHz) are reported. The radio frequency shielding was observed to be anomalously high at very high frequencies for the graphite samples studied and precluded the use of this material for NMR studies requiring high sensitivity in this frequency range at liquid helium temperatures. It is shown that this behaviour can be understood in terms of the in- and out-of-plane conductivities and the capacitive coupling of the microcrystalline structure of the graphite.

Low temperature radio frequency sputter deposition of TiN thin films using optical emission spectroscopy as process monitor

TiN films were deposited onto various substrates including InP by rf sputtering in an N2/Ar ambient at room temperature. The rf power, the ratio of gas flows, and the total pressure were systematically varied. To optimize the deposition conditions, the plasma excitation processes were examined by optical emission spectroscopy using a calibrated crystal thickness monitor to determine the corresponding deposition rates. At pressures below 15×10−3 mbar, the deposition rate is linearly proportional to the intensity of the optical emission at 364.2 nm, I(Ti), associated with excited Ti. Although I(Ti) increases with the total pressure, at a given rf power, the resulting deposition rate decreases at pressures above 20×10−3 mbar due to greater gas-phase scattering. The [N]/[Ti] ratio in the deposited films, as determined by Rutherford backscattering and Auger electron spectroscopy, is found to be linearly correlated with the ratio of the optical emission intensities of excited N+2(391.4 nm) and Ti at 364.2 nm, I(N+2)/I(Ti).

ESCA investigation of low-temperature ammonia plasma-treated polyethylene substrate for immobilization of protein

A low-temperature radiofrequency plasma excited in anhydrous ammonia was used to modify polyethylene substrate surfaces for covalent immobilization of proteins. Electron spectroscopy for chemical application (ESCA) was used for surface characterization of polyethylene to a depth scale of 7 nm. The data revealed that surface modification is extensive and occurs in seconds at low discharge power. Primary amino functionalities were detected on the polyethylene surface and the level is dependent on plasma parameters. 125l-labelled antibodies covalently attached to amino groups via glutaraldehyde allowed the conditions for optimum level of primary amine to be established. Both ESCA data and protein loadings are in excellent agreement.

Low-Temperature PACVD Silicon Carbide Coatings

AbstractPlasma-assisted chemical vapor deposition (PACVD) is used extensively to coat planar (2-dimensional) substrates. In principle, the technique can be used to deposit coatings on 3-dinensional objects. However, extending PACVD to coat 3-dimensional objects uniformly requires careful control of the plasma, substrate temperature, and reactant concentrations over a large volume. A novel low-temperature radio frequency PACVD reactor design was developed to deposit coatings uniformly and reproducibly on 3-dimensional metallic substrates. The design features a temperature-controlled reaction chamber fitted with one or more rf-driven electrodes to generate uniform, large-volume plasma. The reactor was used to develop a series of silicon carbide coatings, which were deposited at or below 500°C. The coatings contain SiC and varying amounts of free silicon and/or amorphous carbon (diamond-like carbon), depending on reagent gas composition and reactor operating parameters. The coatings significantly reduced wear on stainless steel samples in ball-on-disk and abrasive wear tests and provided oxidation protection to molybdenum and titanium alloy.

Low-temperature oxygen-fluorine radiofrequency ashing of biological materials in poly(tetrafluoroethylene) dishes prior to the determination of tin, iron, lead and chromium by atomic-absorption spectroscopy.

Low-temperature radiofrequency ashing utilising plasma-excited oxygen alone requires long ashing times and for this reason has not been generally accepted. In this paper it is shown how ashing time can be reduced considerably by introducing fluorine into the oxygen plasma and a novel and efficient method of achieving this is described. Food products and National Bureau of Standards Standard Reference Materials, both of known trace metal content, are prepared by the method described. Analysis of the ash by atomic-absorption spectroscopy shows good recovery of added tin and agreement with certified values for iron, lead and chromium.

Gas plasma effects on polymer surfaces

Low-temperature radiofrequency excited gas plasma was applied to the surfaces of a number of polymers. Polymers that are known to crosslink as well as those that only degrade under irradiation were included in the investigation. Surface changes were studied by viscosity, gel content, and contact-angle measurements. Changes in adhesive bond strength were used as a measure of overall practical effects of plasma treatment. In each case the response of the polymer surface to an oxidizing (oxygen) and a nonoxidizing (helium) plasma environment is discussed. Further indications of the nature of the surface changes were suggested by statistical treatment of the bond-strength data.

Comparison of low temperature radiofrequency ashing with other methods of organic sample oxidation for the determination of sodium in an acrylic fibre

Acrylic fibre samples can be prepared for sodium determination by low temperature radiofrequency ashing, by ignition in air and by wet oxidation with a mixture of sulphuric and nitric acids or sulphuric acid and hydrogen peroxide. The sodium is determined by flame photometry. A minimum of eleven determinations by each technique allows a statistical examination of the results. It is demonstrated that low temperature radiofrequency ashing offers advantages in both precision of analysis and simplicity of sample handling.

Helium Cooled Radio Frequency Preamplifier for Use in NMR

A low temperature (4.2 K) radio frequency amplifier has been constructed using metal oxide semiconductor field effect transistors (MOSFET). Noise characteristics of the amplifier are compared to a model that includes two noise sources: shot noise and induced gate noise. The model is similar to models used to represent noise in vacuum tubes. Amplifier use for observation of low temperature nuclear magnetic resonance (NMR) signals is considered. Conditions for optimum signal-to-noise (S/N) are expressed in terms of the noise figure and are compared to those for room temperature vacuum tube amplifiers. Improvement of S/N by factors of 3 to 10 are readily obtainable. The S/N improvement with the low temperature MOSFET amplifier is demonstrated by observation of the NMR signal of 205Tl at 4.2 K at a frequency of 20 MHz.