Hot Hollow Cathode Research Articles

Plasma-assisted deposition of indium tin oxide thin films by sublimation using an anodic vacuum arc discharge

With the anodic vacuum arc discharge as a method of physical vapor deposition (PVD) excellent properties of thin transparent conductive oxide films can be achieved, but the relations between the process parameters and the coating properties have been insufficiently described in the literature. This paper is intended to narrow this gap by investigations on indium tin oxide (ITO) thin films deposited on borosilicate glass sheets by sublimation of indium tin oxide (90:10) using an anodic vacuum arc. The arc discharge is based on a hot hollow cathode electron source and a special evaporator connected as anode. The concentration of the discharge onto the sublimating material is achieved by combining the magnetic fields of an axially symmetrical permanent magnet system and a solenoid coil. Coating rates of about 15 nm/s were achieved with an arc current of 60 A. Measurements with a planar Langmuir probe have shown that the vapor is highly ionized (about 95%). The discharge voltages are relatively low (< 60 V), so it can be assumed that the particle energies are correspondingly low. Substrate temperature (30–300 °C), oxygen gas flow (0–100 sccm) and gas pressure (0.04–0.4 Pa) were varied to study their influence on the thin film properties. Specific electrical conductivity, density and mobility of charge carriers, as well as optical properties were determined and compared with those of other PVD processes. Low specific electrical resistance of 1.8–2.0 x 10–4 Ω cm was reached at a substrate temperature of 200–300 °C independently of the pressure. The mean optical transmission (in the spectral range between 400 nm and 1100 nm) of 100 nm thick ITO films deposited at a temperature of 200 °C and at an optimized oxygen flow was determined to 84%. The roughness of the layers determined by atomic force microscopy is significantly lower than typically for magnetron sputtered films. At a pressure of 0.04 Pa, the crystalline bixbyite phase can already be detected at a coating temperature of approx. 30 °C, whereby the layer growth begins with the formation of an amorphous structure and changes to the crystalline phase with increasing layer thickness. At a temperature above 100 °C and low coating pressure the layers are completely crystalline. At a higher pressure the transition from amorphous to crystalline phase is shifted towards higher temperature. For lower oxygen flows the crystallites exhibit mainly a preferred (211) orientation. For higher oxygen flows the texture of crystallites is changed and the intensity of (440) and (622) is increased. The formation of these special crystallographic textures can probably explain the very low roughness values of the ITO layers measured by atomic force microscopy. The investigations have shown how the film properties can be adjusted by the process parameters and that the ITO sublimation using an anodic vacuum arc is suited very well for plasma assisted ITO thin film deposition.

High rate deposition of photoactive TiO2 films by hot hollow cathode

In this paper we present a plasma deposition technique that allows the reactive deposition of oxide layers with extremely high deposition rate. The new approach combines reactive sputtering by DC hollow cathode discharge with thermal evaporation from the hot surface of the hollow cathode. As an example of successful fast deposition, photoactive films of titanium dioxide (TiO2) with various thicknesses were deposited using this technique. The uncooled titanium nozzle served as a hot hollow cathode and simultaneously as an inert gas (Ar) inlet. The reactive gas (O2) was introduced into the vacuum chamber through a separate inlet. During deposition, the temperature of the titanium hollow cathode reached up to 1600 °C, depending on the discharge parameters. This made it possible to combine the ion sputtering of hot titanium cathode with its thermal surface evaporation, which significantly increased the TiO2 deposition rate. The highest achieved deposition rate was 567 nm/min (34 μm/h), which (with respect to the geometry of this process) corresponds to total volume of the deposited TiO2 material 1.2 mm3/min per 1 kW of absorbed power. Despite extremely high thermal flux to the substrate, TiO2 films were successfully deposited even on temperature-sensitive PET foil. The as-deposited and post-annealed TiO2 films prepared on fluorine doped tin oxide (FTO) substrates and glass were subject to further analyses including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and photoelectrochemical (PEC) measurements. Whereas the as-deposited TiO2 films had an amorphous (or nearly amorphous) structure, which exhibited only weak photoactivity, after annealing their PEC activity increased by an order of magnitude.

Features of the operating modes of a hot hollow cathode in a vacuum arc discharge

The design features of the hot hollow cathode, its thermionic characteristics and current-voltage characteristics of a vacuum arc discharge with plasma-forming gases of helium and argon are considered. The rationale for these characteristics based on the theory of vacuum-arc discharge is given. The article discusses the use of a hollow cathode discharge for processing tantalum powders.

AISI 5140 steel nitriding in a plasma of a non-self-sustaining arc discharge with a thermionic cathode under the pulse action of ions

Steel AISI 5140 was exposed to pulsed (50 kHz) low-energy (up to 1 kV) nitriding in the plasma of a non-self-sustained arc discharge with a hot hollow cathode in a pure nitrogen atmosphere. The aim was to investigate the effect of the ion energy (pulse amplitude) and ion current density on the nitride layer thickness and on the depth of diffusion saturation with nitrogen. Experiments were also performed to study the effect of the ion energy and ion current density on the efficiency of nitriding at a constant specimen temperature and constant process time.

Plasma characterization and technological application of a hollow cathode plasma source with an axial magnetic field

An efficient plasma source has been established by arranging a hot hollow cathode electron emitter in a strong axial magnetic field, allowing for a reduction of working gas flow by one order of magnitude without loss of discharge stability. Moreover, with the reduction of gas flow not only an increase of the discharge impedance was observed, but also a multiplication of ion current density together with a highly expanded volume of the plasma plume. By means of spatially resolved Langmuir probe measurements, combined with the usage of an energy-resolved mass-spectrometer, plasma density profiles and energy distribution functions of electrons and ions have been measured. Generally, with an increase of the magnetic field and with the reduction of the working gas flow ion energy distribution functions shift from mean values of a few eV to 10 eV and more, while charge carrier densities increase from 10 9 cm − 3 to more than 10 12 cm − 3 . A strongly increased ability to dissociate and ionize reactive gases was observed. Two promising applications related to the coating of tools and components are discussed: the sputter etching with argon ions and the reactive pulse magnetron sputter deposition of wear-resistant chromium nitride layers. Whereas the first mentioned process provides pre-heating and etching rates higher than all actually used in tool coating industry, the second one offers advantages for film growth kinetics leading to significant improvements in composition, structure, surface morphology, and hardness of the deposited layers.

High-rate hot hollow cathode arc deposition of chromium and chromium nitride films

The hollow cathode in the diffuse arc regime (hot cathode arc) was used for deposition of chromium (Cr) and chromium nitride (CrN) films. The chromium hollow cathode serving as a gas inlet was connected to a radio frequency (rf) generator with the rf power up to 300 W. The temperature of the hollow cathode outlet reached 1400 °C. The microcrystalline structure and properties of both Cr and CrN films deposited at different rf powers were studied in more detail. Films were deposited on unheated silicon and steel substrates. Hard, highly textured crystalline CrN films with thickness of 45 μm were deposited at rates as high as 2.5 μm/min. These deposition rates exceed rates of TiN films deposited at comparable conditions. The process parameters and film properties are very promising for industrial applications of the hot hollow cathode arcs (HCAs) generated by the linear magnetized hollow cathodes.

Linear arc discharge source for large area plasma processing

A linearly scalable plasma source based on the radio frequency generated hot hollow cathode discharge between two parallel plates with a magnetic field perpendicular to the plates near the outlet of the cathode is introduced. The magnetic field facilitates and confines the hollow cathode discharge which leads to a high power density and a high cathode wall temperature. The geometry and location of hot zones is directly controlled by magnetic field. The linear arc discharge (LAD) source exhibits similar features as the cylindrical radio frequency hollow cathode plasma jet. Experiments indicate a metastable assisted growth of TiN films. LAD source extends abilities of the radio frequency hollow cathode plasma jet to the large area processing.

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.

Corrosion resistance, structure and mechanical properties of PVD, TiC xN 1 − x coatings

Corrosion protection coefficient and porosity of TiC, TiCN and TiN coatings obtained by three PVD methods: Cathodic Arc Plasma Deposition (CAPD), Hot Hollow Cathode Deposition (HHCD) and High Rate Reactive Sputtering (HRRS) were investigated. Structure of the layers and the interphases were observed by means of SEM and optical microscope within the rings formed at the Calotte grinding. The microhardness and adhesion of all the coatings were measured previously. The HSS substrates and the gas mixture of C 2H 2, N 2 and Ar as the atmosphere of PVD processes were applied. The chemical composition of the coatings was analysed by means of RBS and NRA. The corrosion protection degree of the coatings were established by the electrochemical measurements in the H 3PO 4 solution. The highest corrosion resistance of coated specimens was found for the CAPD method and the lowest one for TiC 0.5N 0.5 HRRS coatings. The influence of structure, cracks and microporosity of coatings, obtained by different PVD methods, on corrosion resistance was discussed.

Determination of trace metals by laser-excited fluorescence with hot hollow cathode atomization

The possibilities of hot hollow cathode atomization combined with laser-excited fluorescence detection for analysis of solutions and powdered samples are discussed. Optimum analytical conditions (discharge gas, its pressure and cathode temperature) were chosen. Significant improvement of signal-to-background ratio was provided by switching off the discharge current with excitation of fluorescence in the resulting afterglow. Detection limits of Co and Ni in dry residues of aqueous solutions were 15 pg ml and 5 ng ml , respectively. The possibility of Co trace determination in powdered Al 2O 3 and SiO 2 was shown.

Current industrial practices-technical note: Ion behavior in the hot hollow cathode coating of silver

Experimental data are presented on two aspects of the copious ions produced in the vacuum coating chamber in a hot hollow cathode (HHC) device. First, the general behavior of both the ions of the silver evaporant and those ions of the process gas argon is described in terms of the current collected by the biased substrate as a function of various operating parameters. This substrate current is used in conducting a specific ion bombardment step, prior to the deposition step, that etches and modifies the substrate surface to ensure extremely good coating adhesion. Second, a threshold arc or cathode current was pinpointed as a vital parameter for the design scale-up of the HHC coating device. The threshold value was determined by estimating an effective sputter yield and comparing it with the respective trends of the silver deposition rate and the ion current collected from the substrate holder as a function of the arc or cathode current. Above the threshold value, net deposition of silver was demonstrated to occur as predicted. It is concluded that necessary precuations need to be taken for a scale-up, in addition to the substrate current density requirement.

Laser-fluorescent determination of traces of cobalt in dry residues of solutions with atomization in a hot hollow cathode

Laser-fluorescent determination of traces of cobalt in dry residues of solutions with atomization in a hot hollow cathode

Diffusion welding of silver interlayers coated onto base metals by planar-magnetron sputtering

Silver has been coated onto various base metals by planar-magnetron (PM) sputtering to provide interlayers for diffusion welding. The vacuum-coating process consisted of two stages: (1) sputter etching of the base metals to remove surface oxide layers, followed by (2) vapor deposition of high-purity silver onto the clean surfaces. The silvers surfaces were diffusion welded at elevated pressure (207 MPa) and temperatures (483–673 K). The structures of the diffusion-welded-silver interlayer and the ‘‘as-deposited’’ coatings were determined using optical metallography and electron microscopy. The as-deposited structure consists of fine columnar grains ∼0.25 μm in diameter, perpendicular to the base-metal surface with the axes of the columns oriented along the [111] crystallographic direction. These grains contain a high density of growth twins ∼15 nm thick. The diffusion-welded-silver interlayer consists mostly of large recrystallized grains (&amp;gt;1 mm in diameter) containing a high density of annealing twins. However, a significant amount of the interlayer has not recrystallized. Additionally, the diffusion-welded-silver (silver-silver) interface consists of a high-angle grain boundary often formed between recrystallized and nonrecrystallized regions. For various base metals, the tensile strengths of diffusion-welded-silver joints fabricated using PM sputter deposition were found to be equal to or greater than previously reported strengths for those fabricated using brazing, electroplating, or other vapor-deposition methods. Tensile properties of diffusion-welded-silver joints fabricated using PM sputter-deposition were also found to be more reproducible than properties previously reported for joints fabricated using hot-hollow cathode (HHC) evaporation. Torsion tests of the diffusion-welded-silver joints revealed that the yield stress and strain-hardening rates in the interlayer are much higher than corresponding values for high-purity annealed bulk silver, although the maximum (steady-state) stresses are nearly identical. These results may be at least partially explained by the high twin density in the recrystallized silver and also the very fine microstructure of the nonrecrystallized regions.

Activated reactive ion plating (ARIP)

Summary Activated reactive ion plating (ARIP) processes are characterized by the following features: (1) the substrate is subjected to a flux of ions before and during film formation, (2) a reactive gas is incorporated into the growing film, and (3) the evaporating material is activated because it is either the cathode or the anode of an arc discharge such as in thermionic arc evaporators, hot hollow cathode discharge evaporators or cathode spot arc evaporators. ARIP processes are used to produce well adhering, dense nitride, carbide and oxide films at relatively low substrate temperatures. They are in commercial use to produce films for both wear protection and optical applications.

High heat flux test by high current and low energy electron beam

High thermal shock resistance is required for plasma facing materials in nuclear fusion devices. Graphite and carbon based composites are the primary candidates for high heat flux materials to be used for limiter or armor plates at present. In this study, thermal shock tests of commercially available isotropic graphite were conducted using a hot hollow cathode type electron gun with a high current (200 A) and low voltage (50 V). Heat flux was 1–2 kW/cm 2 , flux diameter was 18 mm, and specimen size was 25 mm×25 mm×1 mm. Specimens fractured into 2 or 3 pieces within l s of heat load duration. A magnitude of flux when a specimen fractured for l s of heat load duration was used as a measure of thermal shock resistance. The thermal shock resistance of the most anisotropic sample was the highest in this experiment. Correlations between thermal shock resistance and various material properties were examined among samples, except the anisotropic samples. The thermal shock resistance was higher for samples with lower fracture strength and lower apparent density and larger pore size. This result is not in accordance with a simple model based on thermo-elastic theory. It may suggest that thermal shock resistance is controlled by a crack propagation process rather than a crack initiation process.

An investigation on the bonding of hot-hollow cathode deposited silver layers to type 304 stainless steel

The conditions prevailing during the deposition of thick metallic films by the hot-hollow cathode method were investigated. The spacial distribution of temperature, surface roughness, and depth of etch during the pre-deposition etching cycle was investigated for two different sample holder designs. The influence of substrate temperature and the angle of incidence of the vapour on the structure and adhesion of thick silver films to type 304 stainless steel was determined. The nature and strength of adhesion were evaluated on the basis of microscopic examinations and tensile test determinations. Similar evaluations were made on tensile test samples formed through electroplating copper and nickel onto the silver films instead of the joining of two silver-coated samples by hot-isostatic pressing.

Hot hollow cathode and its applications in vacuum coating: A concise review

A concise description of the hollow cathode (HC) effect is given to clarify the confusing terminology and their diverse applications. The cold and hot types of HC’s are then discussed. It is followed by a summary of the behavior of the hot HC as an arc generating device, with emphasis on its use for vacuum coating purposes. Two major coating devices, one developed by Ulvac Corp. (Japan) and the other at the Rocky Flats Plant (RFP) of Rockwell International (USA), are then reviewed with their respective applications. Review of the latter includes recent results of plume effect upon both the unusual deposition rate distribution and the extremely high substrate (ion) current. The coating morphology and microstructure can be best explained using the Krikorian concept and, when optimized, can help to render very high bonding strength (as high as 90 ksi for 304 stainless steel substrates), with the coatings used as the bonding interlayer. Finally, comments are made concerning the directions of future technology development based on the system features reviewed.

Characterization of the BeAg interfacial region of silver films deposited onto beryllium using a hot hollow cathode discharge

Silver films are physically vapor deposited onto beryllium substrates using a hot hollow cathode discharge. To obtain high BeAg adhesion strengths, an atomically “clean” surface is obtained by ion bombardment cleaning. In this investigation, the relationship of the ion cleaning parameters to contaminants in the BeAg interfacial region and their effect on adhesion strength were evaluated. Specimens were ion cleaned at various bombardment parameters and then flash coated with silver. In-depth film profiles were taken by sputter etching in argon and monitoring the Auger electron peak-to-peak heights. The interface was also analyzed by taking a complete spectrum at the edge of the sputter crater. Impurities found at the interface were tantalum, copper and oxygen. The source of tantalum is evaporation from the hot hollow cathode tube. Copper is deposited by sputtering of the specimen holder. If the copper holder had been predeposited with silver, no copper would have been found in the interface. The results for oxygen show that oxygen can be left in the interface if the ion cleaning period is not long enough to remove completely the oxide from the beryllium surface. Oxygen was also found in the interface when the substrate ion current was below a threshold value. Ion cleaning below a threshold current density allowed oxygen to readsorb on the surface. To produce an oxygen-free interface on a machined beryllium surface, the bombardment cleaning time should be at least 10 min with a current density 1.3 mA cm −2 or more. The results for adhesion strengths showed that asmall amount of oxygen (about 2 at.%) left in the BeAg interface will reduce the adhesion strength of the coating. Silver films deposited in an air leak that was greater than a leak which is easily detectable by residual gas analysis contained only about 0.5 at.% O with no reduction in film adherence strengths.

Vacuum Plasma Electron Beam Melting of Reactive and Refractory Metals and Their Alloys: One Step Melting of Low Oxygen Content Titanium Scrap by Using VPEB

Vacuum plasma electron beam (VPEB) melting with a hot hollow cathode gives satisfactory results for recovery of low oxygen content titanium scraps and Zircaloy‐2 scraps. The yield of relatively high vapor pressure elements, such as Cr, can be controlled easily to meet with the tolerance. The VPEB has a high energy density and purification effect which is similar to that of high voltage electron beam melting. Power input of the VPEB to a target of a molten pool is by far more stable and the yield of materials throughout the melting of reactive and refractory metals and their alloys is quite satisfactory compared to those of high voltage EB or the vacuum arc melting process. The VPEB process is also safer, easier to operate, and suitable for automation. Three examples of VPEB application at VMC‐ULVAC are described in this paper.

The investigation of two sample preparation techniques applied to the determination of rare earths in solutions with the aid of hollow cathode excitation

Different types of hollow cathodes (HC), i.e. a cooled HC, a transitional HC and a hot HC, have been applied to the determination of rare earths (RE) in solution using graphite electrodes. In a first sample preparation technique, the graphite electrodes were pretreated with a solution of Apiezon N in benzene (5% w:v) and after drying, 200 μl of the sample solution was introduced. After evaporating until dry, the solution residue covered the inner electrode surface. In a second technique, the electrodes were placed in a teflon cup with close-fitting dimensions and overfilled with 1 ml sample solution. After evaporating until dry, the sample was reproducibly distributed in the graphite electrodes. The first technique could be used in the case of a cooled HC operated at 200 mA and 3.3 mbar argon; absolute detection limits for RE ranged from 0.04 to 0.3 μg. The second techique gave excellent results when using a hot HC; here detection limits for RE were 0.01 to 3 μg.