J. Laimer

Plasma‐assisted chemical vapor deposition of titanium nitride in a capacitively coupled radio‐frequency discharge

A new type of plasma reactor for the plasma‐assisted chemical vapor deposition (PACVD) of titanium nitride (TiN) was designed and built. The reactor uses a rf discharge operating at a pressure ∼1 mbar and at a frequency of 13.56 MHz. The dependence of various properties of the coating produced on the deposition parameters is studied in detail. Besides a determination of the thickness of the coating different analytical techniques such as Auger electron spectroscopy (AES), x‐ray diffraction (XRD), and scanning electron microscopy (SEM) have been used to characterize the coating. In order to investigate mechanical properties of the deposit, Vickers hardness and critical load have been determined. TiN coatings of excellent quality have been deposited on all surfaces of objects of complex geometry. The quality of these coatings is equivalent to the quality obtained by classical CVD and physical vapor deposition (PVD) techniques. The deposition temperature of 500 °C permits the coating of hardened steel tools.

Study of the ignition behavior of a pulsed dc discharge used for plasma-assisted chemical-vapor deposition

An investigation of a pulsed dc discharge used for plasma-assisted chemical-vapor deposition of titanium nitride has been performed in order to understand the nonuniformity of quality and thickness of the deposited films. The experiments have been performed using a gateable image intensifier to study the temporal and spatial light intensity of the discharge. Additionally, a single Langmuir probe has been used to study the temporal evolution of the plasma potential and the charged particle density. The influence of varying parameters like geometry, gas composition, voltage, pressure, duration of the pulses, etc., on the spreading of the plasma has been investigated. Our experiments reveal that in the presence of electronegative species like TiCl4, which is a source gas for the production of titanium nitride, the spreading of the discharge along the substrates is slow, reaching some parts of the reactor with substantial delay. The result is a nonuniform plasma power density in front of the cathode as well as a spatially varying exposure time of the surface to the plasma. These effects are the reason for the inhomogeneity of the deposited films. The experiments revealed that the slow spreading of the discharge is accompanied with low local cathode fall voltages. The problems with the slow spreading of the discharge can be solved by measures which increase the conductivity of the plasma at the beginning of the pulses. The effectiveness of such measures has been studied.An investigation of a pulsed dc discharge used for plasma-assisted chemical-vapor deposition of titanium nitride has been performed in order to understand the nonuniformity of quality and thickness of the deposited films. The experiments have been performed using a gateable image intensifier to study the temporal and spatial light intensity of the discharge. Additionally, a single Langmuir probe has been used to study the temporal evolution of the plasma potential and the charged particle density. The influence of varying parameters like geometry, gas composition, voltage, pressure, duration of the pulses, etc., on the spreading of the plasma has been investigated. Our experiments reveal that in the presence of electronegative species like TiCl4, which is a source gas for the production of titanium nitride, the spreading of the discharge along the substrates is slow, reaching some parts of the reactor with substantial delay. The result is a nonuniform plasma power density in front of the cathode as well a...

Creation of surface nanostructures by irradiation with slow, highly charged ions

It has recently been demonstrated that slow (v < < 1 a.u.) highly charged ions (HCIs) are able to generate nano-sized hillocks on cleaved CaF2(1 1 1) surfaces. The aim of the present study was to explore whether surface nanostructures can also be formed on other target materials by the impact of slow HCIs. To this purpose, we have irradiated LiF(0 0 1), diamond-like carbon (DLC) and Au(1 1 1) with slow Xe HCIs (up to charge state 44+) from the Heidelberg electron beam ion trap. After irradiation, the crystals were investigated by scanning force microscopy. Nanometric hillocks protruding from the surface were found in the topographic images for the case of Xe q+ on LiF(0 0 1) for charge states q ≥ 28, but not for DLC and Au(1 1 1).

DETERMINATION OF ATOMIC HYDROGEN DENSITIES IN GASES BY A DEUTERIUM BASED LYMAN-ALPHA ABSORPTION METHOD

We discuss the variant of a method for the determination of absolute hydrogen atom densities based on the absorption of the hydrogen Lyman‐alpha line. Previously, we demonstrated that by using a simple vacuum ultraviolet spectrometer with low resolution an accuracy of better than 50% for the determined hydrogen atom densities can be obtained for transmissions ranging from 10% to 90%. For transmissions outside of this range excessive errors occur, thus, limiting the usefulness of the method to a certain range of hydrogen atom densities, depending on absorber length and temperature. This range of atomic hydrogen densities accessible to the measurement can be extended by the new method that consists of using well‐defined mixtures of hydrogen and deuterium in the absorber. Using a source of either hydrogen or deuterium Lyman‐alpha radiation, only one sort of atoms contributes to the absorption. Thus, by selecting an appropriate mixture of hydrogen and deuterium, the range of atom densities accessible to the measurement can be extended to higher densities. Using well‐defined mixtures ranging from 1% hydrogen in deuterium to 2% deuterium in hydrogen as absorbers, we were able to determine hydrogen atom densities up to a factor 100 higher than those measured previously in pure hydrogen. Using mixtures down to the natural abundance of deuterium in hydrogen, the measurement of even higher atom densities seems possible. The effect of the difference of the energies of dissociation of hydrogen and deuterium has been investigated using a computer model of relevant chemical processes. In most situations, this effect is smaller than the experimental error. The main limitation of the method is its sensitivity to absorption by impurities, as many molecular gases absorb Lyman‐alpha radiation.

On the plasma chemistry of the C/H system relevant to diamond deposition processes

The chemistry of hydrogen-rich hydrocarbon-hydrogen mixtures is of primary interest for the understanding of the low-pressure synthesis of diamond. We per formed experiments under well-defined conditions like temperature, pressure, initial gas composition, etc. The gas composition at the end of a flow reactor was analyzed by a calibrated mass spectrometer and compared to results obtained from the Chemkin computer code. Residence thne in the reactor as well as other process parameters were similar to those of diamond-growing PA CVD processes performed earlier with the same experimental set-rip. Modeling and experiment under isothermal conditions show quantitative agreement. We realized time-resolved mass .spectrometry by means of a helium-flushed gas sampling probe. There is evidence that the commonly used reaction kinetic data for the dissociation C2H6 (+ M) ⇔ 2CH,(+M) gives 2 too small C2H4 concentrations for hydrogen-rich conditions. This could be attributed to the poorly known third-body efficiencies of the H2 molecules compared to Ar or C2H6 from which kinetic data are commonly derived.

Sputtered thermionic hexaboride coatings

Abstract Thermionic coatings based on the hexaborides of rare-earth elements (ReB 6 ) were deposited onto molybdenum and tungsten substrates using DC magnetron sputtering from LaB 6 , CeB 6 , SmB 6 and YB 6 targets. Films were investigated by means of scanning electron microscopy, electron probe microanalysis and X-ray diffraction. The work function and electron emission characteristics of the coatings have been studied by the thermionic emission method using coated tungsten filaments. After optimization of the sputtering parameters, extremely fine-columnar coatings consisting of predominantly (100)-orientated LaB 6 , CeB 6 and SmB 6 crystals were obtained. The Y-B films showed nearly amorphous film growth. All coatings were overstochiometric with B:Re atomic ratios ranging from 6.3 to 7.5. The work function was measured to be in the range 2.6–3.3 eV. Coated cathodes worked with comparable electron emission current at temperatures approximately 1000°C below the operating temperature of uncoated tungsten filaments. LaB 6 and CeB 6 were found to be the most promising electron emissive coating materials useful as low temperature emitters. However, satisfactory lifetimes may be only yielded at low operating temperatures and vacuum pressures.

Quasi-equilibria of gaseous species in the CH system

Abstract The influence of a constant concentration of atomic hydrogen on the steady state gas-phase composition (quasi-equilibrium) and its development in hydrocarbon-hydrogen mixtures was studied by means of computational modeling. In the thermodynamic equilibrium, methane is virtually the only stable gas species at temperatures below 1000 °C, whereas acetylene prevails at higher temperatures. The steady state of the gas phase in the presence of atomic hydrogen is also divided between these two stability regimes. The transition temperature between them, however, is considerably lowered even by small H concentrations. Concentrations of other species, especially CH 3 , are increased by several orders of magnitude over their equilibrium concentrations, having peak values in the vicinity of this transition temperature. Cyclic reaction sequences, consisting of H-consuming reactions, result in a catalytic effect on atomic hydrogen recombination. Generally, atomic hydrogen accelerates the development of a steady state, but not necessarily with a time constant decreasing with increasing temperature.

Plasma dynamics as a key to successful upscaling of pulsed plasma processes

In order to produce homogeneous hard coatings, e.g. titanium nitride, using the pulsed direct current plasma-assisted chemical vapour deposition technique, it is necessary to achieve a temporally and a spatially uniform plasma. In unipolar operation mode of the power supply, the spreading of the discharge is too slow under certain loading conditions. While the presence of this phenomenon is revealed by an oscillographic observation of the discharge current, a high-speed video camera is applied to investigate it in more detail. The problem can be remedied in a variety of way, e.g. by inserting additional anodes or by superimposing short high voltage pulses onto the normal discharge pulse. An attractive option supported by most commercially available power supplies is the use of additional pulses of reversed (positive) polarity, i.e. bipolar mode. Therefore, we investigated the effect of additional positive pulses on the dynamics of the discharge (bipolar operation mode). Our experiments show a clear improvement of the ignition and spreading behaviour of the discharge by the use of bipolar voltage pulses.

Developments in the deposition of hard coatings by plasma-based techniques

Abstract Titanium nitride (TiN), the most widely used hard material for coatings, offers an excellent example of developments in plasma-based techniques for the deposition of hard coatings. Chemical vapour deposition (CVD), which is widely used today, requires a deposition temperature of about 1000°C. The deposition of TiN at moderate temperature (⩽ 500°C) can be performed by plasma-based techniques. Reactive ion plating (RIP) was the first plasma-based technique for the deposition of high quality TiN and is now well established. To achieve good film uniformity it is necessary to rotate the work-pieces during the deposition procedure. Reactive high rate sputtering, which was developed later, faces the same problems of film uniformity if only one target is used. But this technique offers the possibility of using a double target arrangement, where the targets are situated on both sides of the work-pieces. With the superposed material flows from the targets achieved by this setup, a fairly good film uniformity can be obtained. Recent developments in plasma CVD show that high quality TiN coatings with good uniformity can also be obtained at moderate temperature by this technique, but special attention must be given to the design of the gas inlet and gas distribution system.

Diamond growth in a direct-current low-pressure supersonic plasmajet

Abstract A direct-current, water-cooled hydrogen plasmajet was designed and built. The arc burning between a thoriated tungsten cathode and a Laval nozzle acting as anode was operated at a power level of 4 kW. The plasmajet expands supersonically into a vacuum vessel in which a pressure of a few millibars is sustained. Methane as carbon-containing species is mixed into the decaying plasma close to the exit of the Laval nozzle. Molybdenum plates were used as substrates for diamond deposition. The substrates were placed on a water-cooled movable substrate holder, which allowed depositions at distances ranging from 2 to 30 cm. The bulk temperature of the substrate was monitored by a thermocouple. The deposited coatings were analysed by Raman spectroscopy for diamond phase purity and by scanning electron microscopy for morphology. The film thickness was determined by a Talley Surf profilometer and by optical thickness analysis. At the conditions investigated, a maximum growth rate of about 6 μm h−1 was observed. The growth rate showed a maximum at a distance of 6–10 cm from the nozzle. With increasing distance from the nozzle the growth rate as well as the crystallite size decreases and the diamond phase purity as reflected in the Raman spectra deteriorates. An alteration of arcjet power had only little effect on growth rate and no effect on film quality.