Plasma Beam Source Research Articles

Optimization of film thickness profiles using a magneticcusp homogenizer

In this paper we report on our investigations of the effects of a magnetic cusp plasma homogenizer on thin film deposition profiles produced by cathodic vacuum arcs as a function of distance between the homogenizer and the plasma beam source. Both filtered and unfiltered arc plasma sources were examined. The thickness profiles of titanium films deposited onto transparent media were mapped using a quantitative optical technique based on depositing the films onto transparence foils and analysing the resulting image. The thickness homogeneity of the films was found to improve as the distance between the plasma source and homogenizer increased; however the deposition rate decreased with distance. The optimal distance that produced good homogeneity and maintained the highest rate of deposition was found to vary depending on the divergence of the plasma beam.

Characterization of a linear venetian-blind macroparticle filter for cathodic vacuum arcs

The cathodic vacuum are is a simple means of creating a highly ionized metal or carbon plasma, which can be used to produce a wide variety of metal and metal compound thin films. In order to produce smooth, defect-free films suitable for optical and electronic applications, the plasma needs to be filtered of micrometer-sized molten and solid cathode debris known as macroparticles. To date, most of the research effort on plasma filtering has been focused on development and optimization of magnetic filter designs based on curved solenoids. Ryabchikov et al. (1998) proposed and employed novel filter designs for a combined plasma and ion beam source. The new designs are based on a nested set of conical plates in one version and a set of angled plates (venetian blind) in another. We report on a detailed characterization of the operation of a filter employing a linear venetian blind geometry. The effects of parameters such as plate and arc currents, as well as plate bias on the throughput and deposition profile are reported.

A comparative analysis of a-C:H by infrared spectroscopy and mass selected thermal effusion

A comparative analysis of quantitative infrared absorption spectra and mass selected thermal effusion transients measured for a large number of hydrogenated amorphous carbon (a-C:H) films is presented in order to establish reliable IR absorption cross sections for the different hydrogen bonding configurations in the material. The structural character of the material and the hydrogen concentration were varied over a wide range from soft and wide band gap polymerlike a-C:H, via so-called diamondlike carbon, to the tetrahedral form of a-C:H deposited from a plasma beam source. The results show that the IR absorption cross sections from molecular hydrocarbons can well be transferred to the solid state as long as the material is of polymeric character, but that this procedure fails as soon as the material converts into a three-dimensional cross-linked and mechanically hard structure. For the latter material an empirical average IR absorption cross section for the C–H stretch band can nevertheless be determined from a comparison between hydrogen effusion and IR spectra.

Nitrogen doping of amorphous carbon thin films

Nitrogenated and hydrogenated amorphous carbon (a-C:H:N) films have been deposited by a plasma beam source using a gas mixture of C2H2, Ar and N2. The Ar/C2H2 ratio is kept constant at a ratio of 3, with the nitrogen flow allowed to vary. Nonnitrogenated films, with Ar/C2H2 ratios of 3 and 6 were also deposited and analyzed before attempting to identify the modifications to the microstructural properties due to nitrogen doping. The nitrogenated and hydrogenated a-C (a-C:H:N) films deposited in this study reveal interesting properties with regard to their optical gap, electrical conductivity, and mobility of the charge carriers. The optical E04 gap passes through a maximum of 2.7 eV as a function of incorporated nitrogen. The electrical conductivity, too, reaches a peak value of 10−3(Ω cm)−1 with increasing optical gap and remains constant for higher N2 flows. The electrical conductivity process is thermally activated with activation energies in the range 0.1–0.3 eV. This is discussed in terms of the mobility of the charge carriers (determined by Hall measurements) and electronic doping. The defect density (measured by electron spin resonance) is found to decrease with increasing nitrogen incorporation. The films have also been characterized by infrared spectroscopy, photo thermal deflection, and Raman spectroscopy. The microstructure of the deposited a-C:H:N films is discussed in terms of the electronic density of states.

Electronic properties and doping of hydrogenated tetrahedral amorphous carbon films

Abstract Hydrogenated tetrahedral amorphous carbon (ta-C:H) films were deposited over a range of ion energies using a plasma beam source. The optimum ion energy for maximum sp3 content was found to be 92 eV per carbon ion, and films were grown at this energy using both acetylene and methane as the source gas. The conductivity of the acetylene films was found to be 3.9 × 10−8S cm−1. Upon addition of nitrogen, the conductivity increased, while the optical band gap remained constant up to a nitrogen partial pressure of 10−5 mbar, which suggested n-type doping. At higher partial pressures, the band gap rapidly decreased, ta-C:H/Si heterojunction measurements using both p and n-type silicon showed that the films were n-type in character. However, there was found to be a large amount of nitrogen present in the acetylene gas, so a truly undoped film could not be produced. Thus, further films were made using high-purity methane as the source gas. The undoped films were then found to be p-type and could be doped n-type using nitrogen. Intrinsic behaviour was observed at a nitrogen partial pressure of 4 × 10−6 mbar.

Temperature dependence of the formation of highly tetrahedral a-C:H

Deposition from a low pressure plasma beam source creates a highly tetrahedral form of hydrogenated amorphous carbon (ta-C:H)) which is analogous to the ta-C formed by deposition from a filtered cathodic arc or mass-selected ion beam. The properties of ta-C:H have been studied as a function of the substrate deposition temperatureTs and ion energy using electron energy loss spectroscopy, X-ray diffraction and atomic force microscopy. The density decreases suddenly for deposition temperatures above a threshold value, which is found to decrease with increasing ion energy. The films above the threshold are mainly sp2 bonded, with graphitic layering and high roughness. The variation of the film density with the ion energy andTs is consistent with deposition occurring by subplantation.

Ion Induced Nucleation of Diamond

The process of bias enhanced nucleation of microwave chemical vapour deposited diamond on silicon is characterised using plasma diagnostics, scanning and transmission electron microscopy (TEM), Raman spectroscopy, and X-ray diffraction. The nucleation density is measured as a function of bias voltage, methane gas flow rate, and temperature. The nucleation density is found to be increased above 650°C and reach a maximum at around −250 V which corresponds to an ion energy of 70 to 90 eV. This is close to the optimum energy for ion subplantation, responsible for sp3 bonding in diamond-like carbon. This is taken as strong evidence that nucleation enhancement involves ion subplantation. To prove the influence of ion subplantation on the nucleation process and to reduce the description of the diamond nucleation process to physical parameters such as ion energy, substrate temperature, and film forming particle flux, films are prepared with a low pressure plasma beam source (BPS). The PBS films are prepared at an energy per C atom of 83 eV and in a temperature range of 30 to 800°C. Two characteristic temperature thresholds have been found: above 260°C the highly tetrahedrally bonded amorphous material, which is very compact and smooth with densities above 2.7 g/cm3 and internal stresses above 8 GPa changes immediately into a sp2 bonded, stress reduced (<2 GPa) structure, with densities below 2.2 g/cm3 and a very rough surface. Above 450°C, the hydrogen content is mainly evolved and the films turn into a more ordered structure with crystal impacts and a less rough surface. These films are very similar to bias-MW-CVD deposited two phase structures and lead to comparable nucleation densities when they are used as substrates in a standard microwave diamond deposition process.

Preparation and properties of highly tetrahedral hydrogenated amorphous carbon.

A highly tetrahedrally bonded, hydrogenated amorphous carbon (ta-C:H) has been deposited from an acetylene-fed plasma beam source. The plasma beam source is operated to provide a highly ionized, monoenergetic plasma beam of ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}^{+}$ ions. The resulting films are characterized in terms of their ${\mathit{sp}}^{3}$ content, mass density, intrinsic stress, surface roughness, radial distribution function, C---H bonding, Raman spectra, optical gap, electrical conductivity, gap states, Youngs modulus, and hardness. The ${\mathit{sp}}^{3}$ content reaches a maximum value of 75% at an ion energy of 200 eV, or 92 eV per C ion. The density and stress also reach a maximum at this ion energy. The formation of ta-C:H is interpreted in terms of the subplantation of ${\mathrm{C}}^{+}$ ions, which produces a densified, ${\mathit{sp}}^{3}$ bonded network. The C-H vibration spectra suggest that C ${\mathit{sp}}^{2}$ sites form C=C alkene groups. The optical gap ${\mathit{E}}_{04}$ reaches a maximum of 2.85 eV and increases with the ${\mathit{sp}}^{3}$ fraction. The spin density due to defects is high and decreases with increasing ion energy. The Youngs modulus and hardness measured by microindenter reach maximum values of 290 and 61 GPa, consistent with the highly ${\mathit{sp}}^{3}$ bonding. The variation of hardness with ${\mathit{sp}}^{3}$ fraction is consistent with the constraint model of network rigidity. \textcopyright{} 1996 The American Physical Society.

Properties of carbon nitride layers generated by direct plasma beam deposition

A capacitively coupled r.f. plasma beam source was applied for direct plasma beam deposition of thin carbon nitride (C x N y ) layers. For N 2-Ar mixtures and pure N 2 as working gases and by sputter injection of carbon into the source plasma, mixed plasma beams containing N 2 +, N +, Ar + ions and post-ionized C + are obtained. The CN films were deposited on Si(100) substrates with an ion energy of 100 eV in the electrically neutral plasma beam. A distinct influence of the nitrogen fraction in the working gas on both the growth rate and the film composition was found. Nitrogen concentrations in the films up to 41 at.% are achieved. IR spectra of the deposited films corroborate that nitrogen is chemically bonded to carbon in the C-N network. The formation of small β-C 3N 4 agglomerates under specific deposition conditions is surmised by several results from scanning tunnelling microscopy imaging, ellipsometric studies and hardness measurements.

Ion energy determination for r.f. plasma and ion beam using a multigrid retarding field analyser

The ion energy distribution of a capacitively coupled r.f. (13.56 MHz) plasma and ion beam source has been measured. The extraction optics consists of two stainless steel grids. The source has been operated in two different configurations, either with or without extraction optics. In the latter case the discharge serves only as a plasma source to be used directly or as remote plasma. For measurements of ion energy in the plasma mode a four-grid retarding field analyser (RFA) was used. For analysis of ion energy in the ion beam mode a slightly different RFA was used, because the retarding voltage here is higher, corresponding to extraction voltages of several hundred volts. In the plasma mode, all distributions exhibit a single-peak structure with a low-energy tail. With increasing pressure from 4 × 10 −2 to 4 Pa the distribution width shows an increase from 1.5 to 4.5 eV, whereas the average ion energy decreases from 48 to 17.4 eV. In the ion beam mode, the ion energy distribution is studied as a function of the r.f. power and pressure.

Nucleation during deposition of hydrocarbon ions as a function of substrate temperature

Abstract Hydrogenated carbon films were prepared from a highly ionized plasma beam source using acetylene. The films deposited at low temperatures have a very high sp 3 fraction, density, compressive stress and hardness. The sp 3 fraction, density and stress are found to fall sharply at about 530 K to values suggesting sp 2 bonding. The transition is believed to be associated with a change in mobility of carbon atoms in the surface area during the deposition process. At a higher temperature of about 700 K, the films possess a high nucleation density for diamond growth. X-ray diffraction and Raman spectra do not reveal diamond crystallites in these films, but further growth in an etching, hydrogen-rich plasma produces films which do show features of diamond.

Formation of highly tetrahedral amorphous hydrogenated carbon, ta-C:H

Abstract A highly tetrahedral form of hydrogenated amorphous carbon (ta-C:H) with maximum density of 2.9 g cm−3, sp3 fraction of 0.75 and hardness of 61 GPa has been deposited from acetylene using a low pressure plasma beam source. The ion energy dependence of its properties suggests that the subplantation deposition model of a-C also describes the deposition of a-C:H.

Highly tetrahedral, diamond-like amorphous hydrogenated carbon prepared from a plasma beam source

A highly tetrahedral, diamond-like form of hydrogenated amorphous carbon (a-C:H) with density of 2.9 g cm−3, sp3 fraction of 0.75, and hardness of 61 GPa, the highest values so far attained from a hydrocarbon source gas, has been deposited from acetylene in a novel plasma beam source. The ion energy for highest sp3 fraction is measured to be about 100 eV per C atom, similar to that for a-C, indicating that the subplantation deposition model of a-C also describes a-C:H.

Direct plasma beam deposition of TiNx layers with an r.f. plasma beam source

An r.f. plasma beam source which was modified for the production of monoenergetic mixed gas-metal plasma beams has been used for the direct beam deposition of TiNx layers. Ti was sputter injected into the source plasma maintained in different N2-Ar gas mixtures. The energy Eion of the ion component in the electrically neutral plasma beam was varied between 30 and 200 eV. A strong influence of the nitrogen fraction in the working gas was found on both the growth rate and the composition of the TiNx layers with x varying from 1 to 1.8. Structural analysis with grazing-incidence X-ray diffraction revealed an average grain size of 160 A and a transition from a random crystallite orientation at Eion = 30 eV to a strong (111) fibre texture for Eion above 130 eV.

Investigation of hard a-C:H layers generated by a novel r.f. plasma beam source

A novel r.f. plasma beam source being operated with CH 4 as the working gas has been employed for the formation of a-C:H films on (100)Si substrates. The energy E i of the ion component in the extracted electrically neutral plasma beam was varied between 120 and 600 eV. The mechanical and electrical film properties are determined and discussed in dependence of E i, which in turn is shown to control the hydrogen content of the films in a definite manner.

Plasma beam deposition of diamond-like films

A novel rf plasma beam source operated with CH 4 and CH 4+H 2 mixtures is used for the formation of hydrogenated hard carbon layers. The film composition, particularly the H content, is determined by secondary neutral mass spectrometry (SNMS) and the binding structure by XPS utilizing the characteristic plasmon losses of the C(1s) photoelectrons. As demonstrated by the characteristic XPS structures, diamond-like films are obtained for plasma beam energies around 120 eV with the substrates being held at room temperature. The microhardness and the electrical resistance of the films compare with the best values reported in the literature.

Generation of surface layers and microstructures with a low enery plasma beam source

The potentialities of a novel plasma beam source are demonstrated by the production of a thin insulating oxide layer on silicon and a conducting copper layer on an insulating glass substrate. The source is also applied for reactive plasma beam etching of micrometre-scale structures for microelectronics applications using SF 6 and O 2 as etching species.

Molecular beam epitaxy of layered Dy-Ba-Cu-O compounds

Heteroepitaxial Dy-Ba-Cu-O films have been grown in situ on SrTiO3 substrates using an oxygen plasma beam and elemental source beams in a modified molecular beam epitaxy machine. By periodically shuttering the Dy and Ba beams during growth, flat surfaces of layered Dy-Ba-Cu-O compounds have been obtained. Periodic oscillations in the intensity of the in situ reflection high-energy electron diffraction pattern were observed during shuttered growths. Depending on growth conditions, the as-grown layers have ranged from insulating to superconducting with onset temperatures above 60 K.

Si and Al Etching and Product Detection in a Plasma Beam under Ultrahigh Vacuum

The study of plasma etching mechanisms is made difficult by the inaccessability of the wafer surface to analysis during etching. In the present work, a “plasma beam” is extracted from a miniaturized plasma etching chamber through an array of molecular‐flow orifices in an Mg‐foil electrode. The beam effuses into ultrahigh vacuum and onto either a mass spectrometer for ionic and neutral plasma species analysis or onto an Si or Al film on a quartz crystal thickness monitor for etching studies. Desorbing etch product can be mass‐analyzed before encountering gas or wall collisions. The plasma beam source and its rf power supply can be d‐c biased relative to the mass spectrometer and the etching film to accelerate or retard the ionic component without affecting neutral plasma species, thus allowing energy analysis of effusing ions and measurement of ion energy effect on etching. For plasma maximum ion energy was ≅100 eV at 27 MHz and ≅400V at 100 kHz. Beam etching of Si using and plasma (27 MHz) and of Al using plasma has been carried out, and , ,, and desorption has been observed. Ion bombardment is required to initiate etching in all cases. Subsequently, the effect of ion energy variation from zero (ions reflected) to 500 eV on etching rate and product desorption rate is large (≅x18 enhancement) for Si in , much less (x4) for Si in and zero for Al in . These results correlate with the degree of anisotropy obtained for these etching systems in conventional plasma etching, assuming that the anisotropy is produced by vertical etching rate enhancement by the normally incident ions. They also explain the much higher etching rate observed for Si and at the lower frequency, where ion energy is higher.