Eighty years have passed since Langmuir's first use of the word 'plasma' for describing ionized media. On this occasion we would like to present selected recent results on the application of Langmuir probes to diagnose plasma jet systems. To demonstrate the versatility of the Langmuir probe method we give examples of measurements of the spatial distribution of the plasma parameters as well as their temporal dependence in cases when the plasma jet system operates in a pulsed regime. A part of this paper is devoted to introducing the Langmuir probe technique, especially with regard to its application under conditions when collisions between the charged and neutral particles in the probe sheath cannot be neglected and when the electron energy distribution function in plasma cannot be approximated by a Maxwellian one. In the experimental part we present Langmuir probe measurement of low-pressure and atmospheric-pressure plasma jet systems that are currently used for experiments with deposition of materials with special properties.Most of the experimental results presented in this paper are original data. However, in order to fill in the picture we have used in the case of a barrier-torch discharge three figures that have already been published.

We applied the single and the double hollow cathode plasma jet systems for deposition of BaxSr1-xTiO3 (BSTO) thin films on Si and on multi-layer Si/Sio(2)/TiO2/Pt substrates. Two ceramic inserts in a single nozzle or two separate nozzles made of BaTiO3 (BTO) and SrTiO3 (STO) ceramics were reactively sputtered in the RF modulated plasma jet. The substrate was simultaneously heated on approximately 500 degrees C. Plasma parameters during the deposition process were determined by time-resolved measurements over the working (active pulse) and the idle part of the period. Electron density and electron effective temperature at the substrate position were determined by Langmuir probe technique, temperature of neutral particles and ratio of sputtered atoms (especially Ba and Sr) were estimated by optical emission spectroscopy. Elemental composition of the deposited films was estimated by electron microprobe analysis. Measured electron concentration in the substrate position reached value 2x10(16) m(-3) during the active part of the duty cycle and resulting effective electron temperature was approximately 5 eV. Temperature of neutral particles measured by means of rotational temperature of OH radicals was lower than 500 K and high correlation between ratio of spectral intensity of Ba and Sr lines and ratio of Ba and Sr atoms in BSTO thin film was observed. Knowledge of this correlation was used for deposition of compositional gradient BSTO thin films. XRD diffraction confirmed presence of BSTO and STO perovskite phase in the films.

Pulse modulated double hollow cathode plasma jet system was used for deposition of Ba x Sr 1− x TiO 3 (BSTO) thin films on Si and on multi-layer Si/SiO 2 /TiO 2 /Pt substrates. Both separate nozzles made of BaTiO 3 and SrTiO 3 ceramics were reactively sputtered in the RF modulated plasma jet. Measurements of plasma parameters during deposition process were done. Electron density, electron temperature and electron energy probability function were determined by Langmuir probe, temperature of neutral particles and ratio of sputtered atoms (especially Ba and Sr) were estimated by optical emission spectroscopy. Measured electron concentration in the substrate position reached value 2 × 10 16 m −3 during active part of the duty cycle and resulting effective electron temperature was approximately 5 eV. Temperature of neutral particles measured by means of rotational temperature of OH radicals was lower than 500 K and high correlation between ratio of spectral intensity of Ba and Sr lines and ratio of Ba and Sr atoms in BSTO thin film was observed. Knowledge of this correlation was used for deposition of compositional gradient BSTO thin films.

Low temperature low pressure hollow cathode plasma jet system working in static magnetic field was investigated by means of time resolved Langmuir probe technique. The hollow cathode discharge was excited in a cylindrical nozzle fabricated from Ti by continuous DC, pulsed DC, continuous wave radio‐frequency (RF) and pulse‐modulated RF power. Pure Ar was used as working gas. Electron distribution, electron density, and effective electron temperature were measured and compared between the different plasma jet excitation modes. Time evolution of plasma parameters was studied and compared with time evolution of power absorbed in the discharge in pulsed DC and pulse‐modulated RF discharge excitation modes. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The RF modulated plasma jet system was used for the deposition of BaxSr1-xTiO3(BST), and SrTiO3 (STO) thin films on Si and multi-layer system Si/SiO2/TiO2/Pt substrates. BSTO and STO thin films were analyzed using X-ray diffraction. During deposition, the substrate was heated at 500 degrees C. Emission spectroscopy of the plasma was used to control the concentration of sputtered particles from the hollow cathode, mainly Sr and Ba atoms. XRD diffraction confirmed the presence of BSTO and STO perovskite phase in the films. The optical band gap obtained, from spectroscopic ellipsometry was correlated with composition.

Pulse modulated hollow cathode RF plasma jet system was used for deposition of BaxSr1-xTiO3 (BSTO) and SrTiO3 (STO) thin films on Si and on multi-layer Si/SiO2/TiO2/Pt substrates. Optical emission spectroscopy was used for control of con- centration of particles sputtered from the hollow cathode. Relation between ratio of spec- tral intensity of Ba, Ba + , Sr and Sr + lines and ratio of Ba and Sr concentration in the de- posited film was found. Deposited thin films were analyzed by X-ray diffraction, which confirmed presence of BSTO and STO perovskite phase in the films. All films with high enough Ba concentration (0.5 ≤ x ≤ 0.9) prepared by low pressure RF plasma jet sputter- ing system exhibited good shaped dielectric hysteresis loops typical for ferroelectric phase.

Two types of plasma jet systems were used for deposition of oxide layers on polymer substrates. The target was to deposit such kind of thin films with crystalline structure at low temperature in order to avoid polymer substrate damages. Both systems were excited by RF source working in pulse modulated mode. This modulation allowed exciting high density plasma in the active part of the duty cycle and simultaneously keeping the neutral gas in the plasma jets at the substrate cold thus protecting polymer substrate from thermal damages. The first system was the low pressure plasma jet system excited by RF hollow cathode. The hollow cathode works simultaneously as a nozzle. This system works in vacuum system which was continuously pumped. The low pressure plasma jet system was used for deposition of PbZr_xTi_(1-x)O_3 (PZT) perovskite thin films on kapton (polymer) foil with Pt electrode layer. The RF hollow cathode nozzle was fabricated from PZT ceramics and was reactively sputtered in Ar and O_2 high-density hollow cathode pulse modulated plasma. The substrate bias and ion flux current were monitored during the deposition process. Langmuir probe system was used for measurement of plasma parameters in different part of the modulation cycle in the location of the substrate. This measurement was combined with monitoring of RF voltage, current and their relative phase in the RF hollow cathode. The second system was the RF barrier torch atmospheric discharge plasma jet. This system works at open air without any vacuum system. This system was used for a low temperature deposition of thin conductive oxide thin films at atmospheric pressure on polymer substrates. Under certain condition in the atmospheric plasma jet, these films have crystalline structure. ZnO films deposited directly on polymer contained hexagonal crystalline phase, they were optically transparent and have electrical conductivity sigma~10^-1-100 S/cm. As growth precursors for ZnO films, Zn-acetylacetonate vapors were used.

RF and DC plasma jet sputtering systems were investigated as sources for deposition of ZnO thin films. Deposited zone films have a strong orientation of hexagonal crystallites with the ‘c’ axis perpendicular to the substrate surface. Temperature of the substrate did not exceed 150 °C during the deposition. Langmuir probe measurement performed at the substrate revealed two groups of electrons with different temperatures in the DC plasma jet. A single group of electrons was found in the RF plasma jet system. Electron concentration in the RF jet was found to be about neRF≈5×109–1010 cm−3 with temperature TeRF≈2.5–3.5 eV. In the DC plasma jet, concentration of cold electrons was usually nec≈109 cm−3 with temperature Tec≈0.5 eV and concentration of hot electrons was neh≈108 cm−3 with temperature Teh≈2–3.5 eV. Photographs also confirmed that the plasma density is higher in the position of the substrate in the RF jet than in the DC jet. Other obtained macroscopic parameters as plasma potential and floating potential relative to the grounded reactor are presented.

Accurate control of plasma microparameters at the position of the substrate is crucial factor in applications of the barrier-torch plasma source for technological purposes. We present measurements of the electron temperature Te in the RF barrier-torch discharge by means of the planar RF-compensated Langmuir probe. The probe was mounted at the substrate position. The error caused by collisions of charged particles with neutrals in the space-charge sheath around the probe (collision probe working regime) at atmospheric pressure is discussed. In order to minimize this error the single probe technique was used to acquire the probe data, which were then recalculated to get the double probe characteristic. From this the electron temperature Te has been obtained in usual manner. The Te was measured at the position of the substrate in the single- and multi-torch barrier atmospheric plasma-jet systems. Using He as a working gas Te was found to be in the interval Te=2.7–6 eV depending on the applied RF power and system configuration. The neutral gas temperature has been measured by optical diagnostics and found to be 400–800 K. The plasma of the RF barrier-torch discharge is therefore strongly non-isothermal even at such high operation pressure.

The description and investigation of a new atmospheric plasma source for the treatment and coatings of surfaces are presented in this paper. This new system is a modification of a well-known atmospheric torch discharge stabilized by a flowing channel of the working gas through an RF powered nozzle. The new version of this source prevents the transition to the regime with hot electrodes. This modification is suitable for surface and coatings applications of such substrates sensitive to overheating causing undesirable phase transition or melting. The new source called in our paper as an RF barrier-torch atmospheric discharge employs dielectrically coated nozzles instead of bare metallic ones. In that case, the plasma jet has quite different properties, as it is clear from presented experiments. The new version allowed excitation of the atmospheric plasma channel interacting with the substrate independently on the conductivity of the substrate. Simultaneously it is possible to hold the substrate temperature under atmospheric jet interaction below the limit point of aluminium melting or below an even lower limit of 80°C in a pulse-modulated mode. Extension to the multi-nozzle barrier-torch system was attained with the application of nine quartz nozzles. This multi-plasma jet excitation and its interaction was demonstrated with the substrate made of either quartz or aluminium plate, both possibly with non-flat shape. Emission spectroscopy and RF voltage and current amplitude measurements were employed in order to characterize the RF barrier-torch discharge.

In the recent decade an RF driven, low-pressure plasma reactor with supersonic plasma jet was developed (RPJ). This reactor was successfully used for deposition of thin films of various materials. The deposition of thin films indicates that the properties of the deposited films are dependent on the sputtering or reactive sputtering processes appearing inside the nozzle (hollow athode). The nozzle (hollow athode) fabricated of different kinds of materials and alloys works both as a cathode of the radio frequency (RF) hollow cathode discharge and as a nozzle for plasma jet channel generation as well. The RF hollow cathode discharge is a secondary discharge, which is induced by the primary RF plasma generated in the reactor chamber. The present paper deals with the experimental study of this RF hollow cathode discharge. The stress is laid on the investigation of the axial distribution of discharge parameters and sputtering processes inside the nozzle. On the base of experiments, the simple model of the axial distribution of the investigated RF hollow cathode discharge has been developed.

The structure, magnetic and electric properties of Fe-Ta-O films prepared by the RF plasma jet technique were investigated. With increasing content of O the conductance changes from metallic to tunnelling one. In some films with the Fe/Ta ratio close to 0.25 the superparamagnetic properties and spin-dependent tunnelling magneto resistance (with Delta rho/rho up to 1.5% at 77.3 K) were observed. The behavior can be explained by the nanogranular structure consisting of ferromagnetic metallic grains embedded in an insulating amorphous matrix. (C) 2001 Elsevier Science B.V. All rights reserved.

A low pressure pulsed RF supersonic plasma jet system (RPJ) has been used for deposition of CNx thin films. The aim of the CNx thin films deposition was an application for tribological coatings. Chemical composition, mechanical and optical properties of deposited CNx films have been measured. The obtained parameters were found to be similar to those of CNx films prepared by DC magnetron sputtering. The deposition rate for the CNx films prepared in RPJ reactor was of approximately 2 μm/h. During the deposition process, the substrate temperature did not exceed 250°C, as required for certain kinds of machine tribological coatings. A strong correlation between DC bias magnitude, nitrogen concentration and mechanical properties was found. The value of the substrate bias VDC=−100 V was found to be optimal for deposition of hard CNx films with maximum microhardness H=22 GPa. The films with the highest microhardness had the lowest atomic concentration of nitrogen. Analogous correlation has been found in ‘Diamond Relat. Mater. 7 (1998) 417’, although the deposition method and conditions were quite different. The chemically active plasma has been investigated in the RF supersonic plasma jet channel during the deposition process by means of ‘in situ’ emission spectroscopy. The mechanism of CNx formation has been studied as well.

A plasma-chemical reactor with a multi-plasma-jet RF hollow-cathode system has been developed for the deposition of alloy and composite thin films. Two primary plasma-jet channels and one secondary plasma channel were created in the volume of the reactor. High-density plasma flowing in these plasma-jet channels is generated inside the nozzles. These work simultaneously as RF hollow cathodes. An RF hollow cathode discharge is generated in these nozzles and is subsequently blown out of the reactor, creating flowing plasma jets. These jets were used for the deposition of SiGe and ZrCN thin films as an example of the deposition of alloy and composite thin films with this system. Co-sputtering or reactive co-sputtering of the nozzle material using high-density RF hollow-cathode plasma was applied for the deposition process. Control of the composition of the thin films was accomplished by setting the relative distance between the nozzle outlets without changing the plasma density inside the RF hollow cathodes. The composition of films was investigated with an electron microprobe system equipped with an X-ray microanalyser. This apparatus allowed quantitative analysis of the films. The stoichiometric homogeneity of the films was studied with this technique. Optical emission spectroscopy was used to investigate the secondary plasma-jet channel.

The influence of deposition conditions on the structure, magnetic properties, and electrical resistivity of Fe-Hf-O films, prepared by the supersonic plasma jet deposition technique, was investigated. Composition of the films was controlled by the nozzle composition and the working gas. It varied in the limits: 15-68 at.% of Fe, 0.5-8 at.% of Hf and 29-80 at.% of O. The films were mainly X-ray amorphous. Some of them showed a weak and broad peak near the [110] reflection of alpha-Fe indicating the presence of Fe-rich clusters in an amorphous matrix. Depending on the deposition parameters the magnetic properties vary from paramagnetic to ferromagnetic ones. The electrical resistivity changes from the metallic to the hopping type. In some samples a large negative magnetoresistance is observed.

In the present paper the Langmuir probe technique was applied for plasma monitoring in the Ar and n-hexane mixture plasma during the polymerisation process. The experiments were performed in a stainless steel DC magnetron reactor with a planar magnetron cathode. Within the investigated range of the gas pressure, no time-stable condition of the discharge was found. The regular oscillations, of a frequency of approximately 32 kHz, were observed in the Ar and 20% n-hexane mixture at the total pressure of 10 Pa and the discharge current of 8 mA. The experimentally obtained probe characteristics were used to determine the density and the average energy of electrons. In the DC magnetron stainless steel reactor the electron average energy in the Ar and n-hexane mixture plasma was about one order of magnitude smaller than in the case in which the working gas inside the reactor was the pure Ar.

The RF plasma chemical reactor with low pressure supersonic plasma jet system (RPJ) has been used for deposition of Cu3N thin films. From comparison of experimental values of composition weight per cent with theoretically predicted ones, and from XRD analysis, it follows that if the RF power absorbed in the reactor does not exceed 75 W, stoichiometric Cu3N films are achieved. The typical value of deposition growth rate was found to be on the order of 16 nm/min for RF power Pw≈40 W. The optical energy gap Eg and microhardness H of the deposited Cu3N thin films increased with decreasing RF power. They are Eg=1.24 eV and H=8.8 GPa for the sample deposited at RF power 40 W. For RF power higher than approximately 75 W, it appears that a small amount of Cu microparticles, up to ∼1 μm, is diluted in the Cu3N film. The deposition of the microparticles can be explained by local overheating of the probe surface by RF power absorbed in RF hollow cathode discharge.

A low-pressure r.f. supersonic plasma jet reactor (RPJ) has been used for deposition of Cu3N thin films. From a comparison of experimental values of composition weight per cent with the theoretically predicted values, it follows that if the r.f. power absorbed in the reactor does not exceed 75W, stoichiometric Cu3N films can be obtained. The typical value of the deposition growth rate was found to be in the order of 16nmmin−1 for r.f. power Pw≈40W. The optical energy gap, Eg, microhardness, H, and Young's modulus, E, of the deposited Cu3N thin films increase with decreasing r.f. power. They are Eg=1.24eV, H=8.8GPa and E=146GPa for the sample deposited with a r.f. power of 40W. Deposition of Cu3N thin films by means of the modificated RPJ reactor (with a multi-jet system) on to internal walls of cavities, holes and on the surface of complex shapes of hollow substrates can be useful for surface-treatment technology.

We present a plasma source which works on the principle of the arc torch discharge. The powered electrode of the arc torch discharge was made from a thin pipe that simultaneously acts as the nozzle through which the working gas flows to the discharge region. The flow of the working gas stabilizes the arc torch discharge and a well defined plasma channel is created. The advantage of this system is that it is able to work at high pressure of working gas up to atmospheric pressure inside the plasma-chemical reactor and also in free space.