Energy and mass spectroscopy studies during triode ion plating of TiN films in two different layouts

Energy and mass spectroscopy studies during triode ion plating of TiN films in two different layouts

Surface and Coatings Technology 113 (1999) 149–156 Energy and mass spectroscopy studies during triode ion plating of TiN films in two different layou...

327KB Sizes 0 Downloads 23 Views

Surface and Coatings Technology 113 (1999) 149–156

Energy and mass spectroscopy studies during triode ion plating of TiN films in two different layouts M. Macˇek a,*, B. Navinsˇek b, P. Panjan b, S. Kadlec c, S. Wouters d, C. Quaeyhaegens e, L.M. Stals e a University of Ljubljana, Faculty of Electrical Engineering, Trzˇasˇka 25, 1000 Ljubljana, Slovenia b Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia c Institute of Physics, Academy of Sciences, Na Slovance 2, 18040 Prague 8, Czech Republic d Katholieke Hogeschool voor Limburg (IWT), Universitaire Campus, B-3590 Diepenbeek, Belgium e Limburgs Universitair Centrum, Institute for Materials Research, Universitaire Campus, Wetenschapspark 1, B-3590 Diepenbeek, Belgium Received 15 June 1998; accepted 5 December 1998

Abstract Plasma properties are compared in two triode ion-plating systems using a low-voltage arc discharge. The systems differ in the configuration of the chamber and magnetic field. Mass and energy distributions of positive ions during deposition of TiN films were measured by an energy-resolved mass spectrometer. Plasma parameters were determined also by Langmuir probe diagnostics. In both systems, single and multiple charged ions of Ar and N gas and the evaporated Ti were detected. The system with a 2 strong magnetic field confinement exhibits a higher plasma potential (U =55 V ) under standard deposition conditions, the peak p ion energy is higher and the energy distribution is narrower than in the system without magnetic confinement (U =12 to 35 V ). p The probability for the multiple charged ions in this system is much higher than in the system without the magnetic confinement, so the population of Ti++ in this system is even higher than that of Ti+. The presence of high-energy neutral Ti is also observed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Plasma diagnostics; TiN coatings; Triode ion plating; Energy-resolved mass spectroscopy

1. Introduction Low-pressure plasma is a basic medium, used in triode ion plating systems for preparation of hard coatings like TiN, CrN and Ti(C,N ). The triode ion plating systems use a filament-based ionization source forming a lowvoltage (LV ) arc, which expands into the vessel. Such plasma is an efficient source of low-energy electrons, used for enhancing ionization during deposition as well as for the substrate heating and plasma etching processes. These heating and etching processes by the LV arc plasma are used prior to the deposition itself to prepare the surfaces of the substrates [1–3]. The processes on the surfaces of the substrates during heating, etching and deposition strongly depend on the mass and energy distribution of the principal species in plasma, especially of the heavy particles. The mass and energy distributions may also be influenced by the

* Corresponding author. E-mail: [email protected]

overall chamber geometry and plasma confinement in the chamber by magnetic and electrical fields. This paper compares the mass and energy distributions of ions during deposition of TiN films in two triode ion plating systems with different configurations of the chamber and the magnetic field. An energyresolved mass spectrometer was used for characterization of positive ions from the plasma. Plasma parameters were also determined by Langmuir probe diagnostics.

2. Experimental Energy distributions of positive ions have been measured in two triode ion plating systems, referred to as System A and B, with different configurations of the chamber and the magnetic fields. Both systems are shown schematically in Fig. 1. System A with a magnetic confinement is a Balzers BAI 730 M, and system B is a system with only local magnetic fields (a Balzers BAI

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 83 9 - 1

150

M. Macˇek et al. / Surface and Coatings Technology 113 (1999) 149–156

parison of properties of plasmas in these two systems, based on the same physical background, may help in the understanding of the common processes and the differences in the system layouts. The following paragraphs describe the common features of Systems A and B and the specific features of each system. 2.1. Common features of systems A and B

(a)

Both ion plating systems use a vacuum vessel containing a crucible, an auxiliary anode and substrate holders with substrates. A filament-based ionization source forms the LV arc. The power supply for this arc is electrically floating with respect to the chamber walls. Its negative pole is connected to the arc cathode, and the positive one can be connected to different anodes, depending on the operation mode (heating, etching and evaporation). During the heating mode, the substrates serve as the anode for the LV arc. In the etching mode, the electron flux is directed to the auxiliary anode, and ions produced in the plasma are extracted by a negative potential to the substrates. In the deposition mode, the LV arc is directed towards the crucible containing the metal ( Ti) to be melted. For TiN deposition, titanium is evaporated in a mixture of argon and nitrogen gases. The Ar inlet is in the LV ionization source chamber, and N comes directly 2 into the main vessel through the chamber wall. 2.2. System A

(b) Fig. 1. Schemes of the triode ion plating systems: (a) System A (Balzers BAI 730 M ) and (b) System B (Balzers BAI 640).

640). The systems have some features in common, but they differ in a number of aspects. Although, from the pure scientific point of view, it might sound interesting to compare deposition systems differing just in one aspect, we have chosen two commercially available triode ion plating systems that have proven the ability to deposit various nitride, carbide and carbonitride hard coatings. We hope that the com-

The ion plating system A has an axial symmetry of the overall vessel with a vertical axis. The inner diameter of the vessel is 70 cm, and the height equals 60 cm. Two Helmholtz coils producing an axial magnetic field [Fig. 1(a)] are placed at the top and bottom of the vessel and are separated by 52 cm. The crucible is located centrally on the bottom, and it can be moved vertically up. All the measurements presented in this paper were done with the crucible at the bottom position. The LV arc source is mounted centrally on the top of the chamber. The LV beam is focused by the axial magnetic field. Thus, the Ti metal is melted directly by the axial LV arc. Substrates are placed on 12 rotating holders distributed symmetrically along the chamber axis at a radius of 26 cm. The crucible is surrounded by the auxiliary anode in the form of a concentric ring with an outer diameter of 26 cm. This anode is used mainly for etching and is electrically floating during deposition. The standard parameters during the evaporationmode are: arc current I =200 A, substrate voltage arc U =−130 V, argon pressure p =0.15 Pa, total pressub Ar sure p =0.20 Pa and current in both coils I =15 A. tot c The typical magnetic field in the center of the vessel is 7 mT at this coil current. Under these conditions, the

M. Macˇek et al. / Surface and Coatings Technology 113 (1999) 149–156

typical arc voltage is 56 V, whereas the voltages measured on the crucible and the auxiliary anode are 53 and 52 V, respectively. The titanium evaporation rate under typical conditions is 0.3 g min−1, which results in a TiN deposition rate of about 50–70 nm min−1 on the rotating substrates. The spectrometer is located on a side in the middle of the chamber, and the analyzer axis is horizontal, crossing the chamber axis. 2.3. System B The ion plating system B has a different configuration of the deposition chamber and no major magnetic field [Fig. 1(b)]. The apparatus consists of an upright rectangular vessel with the dimensions 100×80×80 cm. The substrates are placed on top of the vessel on a substrate plate. A rotating crucible is located at the bottom of the vessel away from the center of the vessel. The LV arc source is mounted on the sidewall of the chamber. This LV arc is focused only locally by the magnetic field by a coil surrounding the LV ionization source. The auxiliary anode is a vertical plate behind the crucible. In contrast to System A, there is a high-voltage (HV ) electron gun in System B, which helps to sustain the evaporation of titanium from the crucible. This E-gun uses another local magnetic field to deflect the electron beam for 270° toward the crucible. The standard process parameters in System B are I =100 A, U =−110 V, p =0.15 Pa, p =0.20 Pa. arc sub Ar tot The HV gun operates with a current I =0.4 A and E voltage U =7 kV. Under these conditions, the total arc E voltage is 40 V, and the crucible voltage is +24 V with respect to the ground. The floating anode potential is close to +2 V. The titanium evaporation rate under typical conditions is 0.5 g min−1, which results in a deposition rate of about 70 nm min−1 for TiN on the substrates. The mass analyzer is located at the top of the chamber, and the analyzer axis is directed towards the crucible.

151

The measured energy spectrum is actually a spectrum of stopping potential; it means that for multiple charged ions, the real energy is the product of the measured stopping potential and the ion charge. Of course, the energy of an ion striking the surface of the biased specimen equals the sum of the stopping potential and the bias voltage multiplied by its charge. Under some conditions, the energy distributions of the positive ions depend on the bias voltage at the substrate (or at the extraction electrode). This is especially true when the mean free paths for charge-exchange and/or other collisions are low compared to the thickness of the plasma sheath [7,8]. In our case, the gas pressure is low, and the sheath thickness is much lower than the mean free path [4]. Thus, the measured energy distributions are valid not only for grounded surfaces but also for the case of a negative d.c. bias up to several hundreds of volts. Again, there is a shift of the real ion energy distributions at the biased surface corresponding to the bias potential and to the ion charge. 2.5. Langmuir probe measurements Langmuir probe measurements in System A were performed by a semispherical probe with a diameter of 1 mm, which is almost directionally insensitive in the spherical angle of 2p. The probe is located similarly as the spectrometer, in the middle, on a side of the chamber. It can be moved horizontally from the chamber wall close to the chamber axis. Data were collected using a data acquisition system controlled by a personal computer. In system B, cylindrical and two planar probes (normal and perpendicular to the crucible) have been used. During the TiN deposition, the probe was positioned at different vertical positions from the crucible between 4 and 55 cm. More details on measurements are given in Refs. [3,4]. From the measured I–V characteristics, the electron current was calculated as a sum of the measured total probe current and the extrapolated ion current. The plasma potential, U , was defined from the maximum p of the first derivative of the electron current.

2.4. Energy and mass spectrometry The energy distributions of positive ions have been measured using the energy and mass analyzer Balzers PPM 421. The orifice of the energy and mass analyzer is floating. The ions are focused by the ion optics, filtered by the cylindrical mirror energy analyzer and then passed through the quadrupole mass analyzer. The secondary electron multiplier counts the ions. It can give readings from 0.1 to 107 counts per second. More details are given in Refs. [3–6 ]. The analyzer can perform a mass scan at a selected ion energy or an energy scan at selected mass numbers.

3. Results 3.1. Plasma potential 3.1.1. System A The plasma potential measured by the plasma probe in System A is very uniform across the chamber in the radial direction, as shown in Fig. 2. At the standard arc current for the deposition of TiN in this system (I =200 A), the plasma potential, measured from a 12.5 cm from the center axis up to the 30 cm (5 cm from

152

M. Macˇek et al. / Surface and Coatings Technology 113 (1999) 149–156

Fig. 2. Radial profile of plasma potential U in System A measured by p plasma probe. The insert shows a typical measured I–V plot with calculated electron current, I , and first derivative, dI /dV. e e

the sidewall ), is almost constant. Only a slight tendency to be higher near the center of the vessel is observed. The average plasma potential is U =52±1.4 V. This is p very close to the potential measured on the centrally located crucible (53 V relative to the grounded vessel walls) and surrounding auxiliary anode (52 V ). This uniformity is also preserved for the very low arc currents, as shown on the same figure. 3.1.2. System B Measurements of the plasma potential performed in Refs. [3,4] show that the plasma potential in System B is much less uniform. Typical measured values vary from 16 to 19 V at 30 cm from the crucible, and are as low as 10–12 V at 55 cm. Unfortunately, the measurements close to the crucible had to be performed at a lower arc current (I =60A) a due to overheating of the probe. An experiment was performed 4 cm from the crucible. Under these conditions, the plasma potential was at 23 eV, i.e. a value close to the crucible potential. Another difficulty was that the I–V characteristic was rather complicated, probably reflecting plasma oscillations in the vicinity of the crucible [4,9]. Nevertheless, it is reasonable to expect that the plasma potential at I =100 A will be even a slightly higher than 23 V and that the gradient of the plasma potential in the chamber is as high as 10–15 V [3,4]. 3.2. Energy spectra Energy spectra of 40Ar+ (m/q=40), 48Ti+ (m/q=48), 48Ti++ (m/q=24) and 14N+ (m/q=14) ions during deposition in System A are represented in Fig. 3(a) and for System B in Fig. 3(b).

Fig. 3. Typical energy spectra of selected ions during deposition of TiN in the ion plating systems: (a) System A and (b) System B. The selected ions are 14N+, (mass/charge ratio m/q=14), 40Ar++ (m/q=48), 48Ti+ (m/q=48) and 48Ti++ (m/q=24).

3.2.1. System A As can be seen from Fig. 3(a), the signal is low for energies below 45 eV and rises sharply for all masses at energies above 45 eV. The peak is centered on 55 eV. This value is close to the measured plasma potential shown in Fig. 2. The plasma potential in this system is close to the value of the highest electrode potential measured in the vessel (relative to the grounded vessel walls). The uniformity of the radial profile of the U p explains why the signal at low energies is almost five orders of magnitude lower than the peak value. If all the ions in the plasma are at the same plasma potential, then they are accelerated to at least the minimum energy in the sheath adjacent to the spectrometer (provided that collisions in the sheath could be neglected [7]). This minimum energy is defined by the difference between the plasma potential and ground. The high-energy tail of the distribution may reflect the distribution of the kinetic energy of ions in the plasma. The other effects that may influence the measured energy distribution are the radial distribution of the plasma potential near the LV arc column where the probe characteristics could not be measured, or timedependent fluctuations of the plasma potential [9,10]. Similar shapes of the measured energy distribution are observed for single and multiple charged ions (not only for Ti+ and Ti++ but also for other ions, namely 48Ti3+ (m/q=16), 40Ar++ (m/q=20), and 14N++ (m/q=7) [11]). The measurement thus indicates that the ion energy for single and multiple charged ions is delivered by acceleration in the same potential profile in the plasma. Of course, the real energy of the multiple charged ions is correspondingly higher than for single charged ions. The intensity of the double charged Ti ion is at least 20 times higher than the intensity of the single charged

M. Macˇek et al. / Surface and Coatings Technology 113 (1999) 149–156

Ti ion, which is an indication of a very high ionization probability in this type of deposition system. In fact, we have measured relatively strong intensities also for triple and quadruple charged Ti ions [see also Fig. 4(a)]. 3.2.2. System B The energy spectra measured in System B [Fig. 3(b)] are different, with regard to the shape and peak energies

153

compared to System A. The 40Ar+ (m/q=40) peak position is different from the 48Ti+ (m/q=48) peak. The latter is a broad and centered on 30 eV and the former is sharp, peaking slightly below 10 eV. The 48Ti+ energy is close to the measured plasma potential in the densest plasma near the crucible [3], whereas the 40Ar+ energy corresponds to the plasma potential far from the crucible. The Ti+ distribution in System B differs also from

(a)

(b) Fig. 4. Typical mass spectra during (a) the deposition of TiN in the ion plating System A at the peak energy for the Ti+ ion at 55 eV and during (b) the deposition of Ti in the ion plating System B at the peak energy for the Ti+ ion at 28 eV. The insert in (a) shows the fine structure of the Ti3+ and Ti4+ peaks for Ti isotopes of masses 46, 47,48, 49 and 50.

154

M. Macˇek et al. / Surface and Coatings Technology 113 (1999) 149–156

the energy distribution of the nitrogen 14N+ and other ions originating from the gas phase (e.g. 14N+ , 1H+). 2 2 These distributions are more similar to the Ar+ energy distribution [5]. The energy distribution of the single charged Ti is also quite different from that measured in System A. Whereas, in System A, the distribution is sharp and located at around 55 eV, the distribution in the case of System B is broad, expanding from the energies corresponding to the measured plasma potential (10–15 V ) up to about 60 eV. A broad peak is observed at about 30 eV. This energy is related to the higher local plasma potential in the area adjacent to the crucible [3]. The difference in the shape of the distribution of the Ar and Ti ions corresponds to the different origin of the species. Whereas Ar comes into the vessel through the LV arc source, the Ti evaporates from the crucible and passes through the crucible plasma area. The width of the distribution for both Ar and Ti ions reflects the spatial non-uniformity of the plasma potential in the chamber of System B [5]. Opposite to the spectra in System A, the intensity of the double charged Ti ions (m/q=24) is lower than the intensity of the single charged Ti ion (m/q=48). This can be attributed to the lower ionization probability in this system under the selected ‘‘standard’’ conditions. It means that the axial magnetic field in System A has a very beneficial influence on the plasma confinement and on the ionization probability, at least for the Ti ion. 3.3. Mass spectra Fig. 4 shows typical mass spectra during the deposition of TiN in System A [Fig. 4(a)] and System B [Fig. 4(b)] measured at the peak energy for the Ti+ ion in each system. 3.3.1. System A The mass spectrum plotted in Fig. 4(a) was measured at the energy of 55 eV. Titanium shows a typical spectrum with five peaks corresponding to the Ti isotopes with masses 46, 47, 48, 49 and 50. This is clearly seen not only for the single charged Ti peaks centered at mass 48 (with relative abundance 73%), but also for multiple charged ions centered at m/q=24 ( Ti++), m/q=16 ( Ti3+) and even for m/q=12 ( Ti4+) as shown in the inserts of Fig. 4(a). The small shift of the mass scale (0.1–0.2 a.m.u.) is related to the calibration of the mass spectrometer. The fine structure of the peaks around 12 and 16 a.m.u. indicates that these peaks are mostly related to multiple charged Ti ions and not to 12C+ or 16O+ ions, as may be expected. A high ionization efficiency of System A is evident also from the relatively strong peaks of Ar++, N+ and N++ ions compared to the intensity of Ar+ and N+. 2

3.3.2. System B The mass spectrum plotted in Fig. 4(b) was measured at the energy of 28 eV, which is the peak energy of the Ti+ ion, but not of the Ar+ [as seen in Fig. 3(b)]. Nevertheless, this figure shows again the lower probability for ionization of Ti to the double charged state than to the single charge. Other measurements revealed that the probability for the ionization of the Ar and N to the double charged state is also lower in System B than in System A for standard deposition conditions.

4. Discussion The form of the mass spectrum in System A with a Ti++ signal 20 times higher than Ti+ and even with high Ti3+ and Ti4+ peaks indicates that this triode ion plating system has an extremely high ionization efficiency. System B does not have that high ionization efficiency to multiple charged states, but it is efficient enough to ionize the majority of Ti particles [2]. Moreover, the measurements of the energy distribution of Ti neutrals in System B showed a majority of neutral Ti atoms having energies of 2–30 eV at the substrate position [10,12]. Thus, both ion-plating systems investigated have very different features than just a thermal evaporation. The properties of the evaporated metal particles are more similar to those often reported just for cathodic arc evaporation [13]. The difference between System A and B apparently affects the measured ion energy distribution. There are obvious differences in the relative positions of the crucible, the LV arc, the auxiliary anode and the substrates. Also, the LV arc current is different during deposition in both systems. Moreover, the magnetic field is present in the whole vessel of System B. We believe that the plasma confinement by the combined action of the magnetic and electric field from the arrangement of electrodes is the most important factor for the observed differences in energy and mass distributions for both systems. The magnetic field in System A forms a weak mirror magnetic vessel in which at least some plasma electrons are confined on spiral paths oscillating between the two top and bottom magnetic mirrors. For the typical measured electron energy (1 eV ) and magnetic field intensity (7 mT ), the Larmor radius is less than 1 mm. Even the low energy ions can have a smaller Larmor radius than the dimensions of the vessel; Ar ions with an energy of 1 eV have a radius of 10 cm at a magnetic field intensity of about 10 mT. The ions are, however, not magnetized because the collision frequency for them is higher than their cyclotron frequency [14]. This axial magnetic field with the high potential on the crucible forms a high plasma potential along the

M. Macˇek et al. / Surface and Coatings Technology 113 (1999) 149–156

central axis. This potential expands to the wider area with the effect of the floating auxiliary anode. In this way, a rather uniform plasma potential is formed in a large volume surrounded by the substrates. This is true not only for high arc currents used in System A (200 A) but also for lower currents comparable with the typical arc current in System B (100 A). For example, at I =50 A, the plasma potential equals 33 V almost over arc the whole area ( Fig. 2). This plasma confinement increases the plasma density in a large volume, and in this way, it increases the ionization probability compared with System B. Plasma potential in System B is not as uniform as in A. This is confirmed by the probe measurement of the spatial distribution of the plasma potential [12]. The reason for this is the absence of the major magnetic field and focusing of the LV arc plasma to the crucible. Therefore, a gradient of the plasma potential is formed between the crucible area and central plasma, so that the whole LV arc current is collected by the crucible [3,10]. This plasma potential gradient explains the broad energy distribution as well as the different peak energies of Ar and Ti ions. The differences in energy and mass spectra, as well as in the plasma potential, are reflected in the properties of the deposited TiN films. It was shown in Ref. [15] that the preferential 111 growth in System A (with higher plasma potential and with predominately double charged Ti ions) starts already at 25 V of negative bias applied to the substrates. The TiN layer with the same texture grows in System B at negative bias voltages over 100 V. This can easily be understood, taking into account that the average energy of the particles bombarding the surface equals n E= ∑ [eEj+ +je(U +U )]nj+, 0 b pl j=0 with charge je, initial energy E and fluxes of species 0 nj+. An effective bias, U , therefore depends linearly eff on the bias, U , applied to the substrates, b U =E9 +q: U +q: U . The proportionality factor is the 0 pl b eff average charge of species in the flux q: =

1 n ∑ jnj+. n j=0

Calculations in Ref. [15] show that the same effective bias (U =80 V ) is achieved at U =−40 V in System eff b A, whereas about 60 V higher bias is needed in System B for the same U (U =−100 V ). eff b 5. Conclusions Both triode ion-plating systems studied showed that the LV arc is a very efficient tool for ionizing both the

155

gas and the evaporated species. In both systems, there are single and multiple charged ions that are accelerated towards the biased substrates with a wide range of energies. The average energy of the species in System A with a strong axial magnetic field is much higher than in System B with only a weak magnetic field. This is partially due to the higher plasma potential, and partially due to the higher probability of multiple ionization in a strong magnetic field. With these differences in mind, it is obvious that different deposition parameters, such as substrate bias, must be carefully adjusted for an optimized deposition process in different systems. The presence of high-energy neutral Ti and a high ionization probability enhance the ion-assisted growth of TiN films on the substrates. This is apparently the reason why these systems have been successful in the deposition of high-quality TiN and other hard coatings.

Acknowledgements This work was supported by the Ministry of Science and Technology of the Republic of Slovenia and was included in the program of the COST 515 action. A part of this research was executed in the framework of the Objective-2-region program 1996–1998 for Limburg (Belgium) and financed by the EU ( EFRD-action) and the Flemish Government (Limburgfonds). This paper presents research results of the Belgian program P4/33 on Inter-University Attraction Poles initiated by the Belgian state Prime Minister’s office (Brussels), Science Policy Programming. We acknowledge also the support of the Grant Agency of the Czech Republic, Grant no 106/96/K245. This work was supported also by a NATO-Collaborative Research Grant, Project No. HTECH.CRG 970529 and by the project ME110, The Czech Ministry of Education.

References [1] E. Mol, E. Bergmann, Surf. Coat. Technol. 37 (1989) 483. [2] E. Bergmann, Surf. Coat. Technol. 57 (1993) 133. [3] M. Nesla´dek, C. Quaeyhaegens, S. Wouters, L.M. Stals, E. Bergmann, G. Rettinghaus, Surf. Coat. Technol. 68/69 (1994) 339. [4] S. Wouters, Ph.D. thesis, Limburgs Universitair Centrum, 1998. [5] S. Wouters, S. Kadlec, M. Nesla´dek, C. Quaeyhaegens, L.M. Stals, Surf. Coat. Technol. 76 (1995) 135. [6 ] S. Wouters, S. Kadlec, C. Quaeyhaegens, L.M. Stals, Surf. Coat. Technol. 97 (1997) 114. [7] W.D. Davis, T.A. Vanderslice, Phys. Rev. 131 (1963) 219. [8] M. Zeuner, H. Neumann, J. Meichsner, Jpn. J. Appl. Phys. 36 (1987) 4711. [9] M. Macˇek, S. Kadlec, Characterization of Plasma in BAI 730M Deposition System, Res. Report, Jozˇef Sˇtefan Institut, Ljubljana, Slovenia, 1997, 58 pp.

156

M. Macˇek et al. / Surface and Coatings Technology 113 (1999) 149–156

[10] S. Wouters, S. Kadlec, C. Quaeyhaegens, L.M. Stals, Surf. Coat. Technol. (in press). [11] M. Macˇek, B. Navinsˇek, P. Panjan, S. Kadlec, Proc. 33rd Int. Conf. Microelectronics, Devices and Materials, Gozd Martuljk, SI, 24–26 September, 1997. [12] S. Wouters, S. Kadlec, C. Quaeyhaegens, L.M. Stals, J. Vac. Sci. Technol. A 16 (5) (1998) 2816.

[13] J. Vyskocˇil, J. Musil, J. Vac. Sci. Technol. A 10 (1992) 1740. [14] F.F. Chen, Introduction to Plasma Physics, Plenum, New York, 1974. [15] S. Kadlec, M. Macˇek, S. Wouters, B. Meert, B. Navinsˇek, P. Panjan, C. Quaeyhaegens, L.M. Stals, Surf. Coat. Technol., submitted.