- Email: [email protected]
VN nanoscale multilayer coatings by energy-filtered TEM
Surface and Coatings Technology 151 – 152 (2002) 209–213
Investigation of intermixing in TiAlNyVN nanoscale multilayer coatings by energy-filtered TEM b ¨ ¨ b, J.M. Rodenburgb,* H. Meidiaa, A.G. Cullisa, C. Schonjahn , W.D. Munz a
Electronic and Electrical Engineering Department, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK b Materials Research Institute, Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB, UK
Abstract We employ high-resolution energy-filtered cross-sectional transmission electron microscopy (EFTEM) to estimate the degree of layer intermixing in a TiAlNyVN low friction, wear-resistant nanoscale multilayers with a periodicity of approximately 4 nm grown by industrial-scale unbalanced magnetron ion-assisted PVD. Coatings are examined by high-resolution TEM imaging. A series of high-resolution elemental distribution maps are obtained by tuning the electron energy-loss imaging filter to the core loss energies of the elements present. We compare the experimental composition profiles with calculations, which model intermixing at the interface of the layers by a convolution. Although no physical shutter is employed in the industrial process (the substrate is passed in front of alternating targets in a confined plasma), the resulting layers are well differentiated. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: PVD; TiAlNyVN; Superlattice; Intermixing; EELS
1. Introduction We examine the microstructure and atomic-scale composition of a nanoscale TiAlNyVN superlattice coating grown by PVD in an industrial-scale coating unit. Such coatings have been reported to exhibit enhanced hardness values of up to 5000 HV w1,2x. The nature of these high hardness values has been discussed in terms of the layer thickness relationships, the related diffusibility of crystalline defects, and the shear modulus of the materials involved w3x. As the layer periodicity is varied, physical properties can significantly (and beneficially) depart from the bulk attributes of the constituent components. In order to understand these effects we need to have a detailed knowledge of the degree of intermixing of separate components at the layer boundaries. Sharp changes of stoichiometry from one layer to the next is not possible when the coating is prepared on an industrial scale. The system we employ cannot physically shutter each component during PVD deposition, but it can rotate substrates periodically in front of different sputtering targets. Although it is well established that * Corresponding author. Tel.: q44-114-225-4037; fax: q44-114225-3501. E-mail address: [email protected] (J.M. Rodenburg).
this method creates multilayers, the sharpness of the interfaces is difficult to measure because it is on the scale of less than approximately ten unit cells. In this paper, we study a coating at high resolution and measure this intermixing via filtered electron energy loss imaging. In principle, the technique has a much higher spatial resolution (less than 1 nm) than conventional X-ray analysis, and has the further advantage of being sensitive to local bonding configurations. 2. Experimental The TiAlNyVN coating investigated here is dedicated to tribological applications and has commercial applications because of its low friction coefficient and extremely low sliding wear coefficient w4x. It has been deposited in a industrial sized four target PVD coating unit (Hauzer Techno Coating HTC 1000-4) using the arc bond sputter (ABS) process, which is a combined steered arcyunbalanced magnetron sputtering system w5,6x. The sputtering occurs in the ArqN2 atmosphere without any shutter or shielding, although the magnetic confinement of the unbalanced magnetron does effectively introduce periodicity into the coating as the substrate is passed from in front of one target to the
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 6 2 1 - 8
H. Meidia et al. / Surface and Coatings Technology 151 – 152 (2002) 209–213
210
Fig. 2. HREM image showing the crystal misorientation.
Fig. 1. The structure of superlattice TiAlNyVN. (a) BF image, (b) DF image.
next. The deposition rate of the coating was 15 nmy min, with a substrate temperature of 450 8C and a negative bias of 95 V. Cross-sectional samples were prepared by ion beam thinning and were examined in a field emission gun (FEG) JEOL 2010F transmission electron microscope with a point-to-point resolution of 0.19 nm. Bright field (BF), dark field (DF) and high-resolution electron microscopy (HREM) images were taken to study the microstructures at various length scales. The microscope is equipped with a Gatan imaging filter (GIF) which is attached under the camera chamber and gives us the
capability to carry out energy-selected imaging (ESI) and electron energy loss spectroscopy (EELS). We obtained energy filtered TEM (EFTEM) images by selecting only the inelastically scattered electrons corresponding to the inner shell losses of the element edge of interest. In this way we can obtain a high resolution map of the element distribution within the sample. The resolution (0.5–1 nm) of the energy loss signal is not as high as the atomic-resolution BF image, but it is sufficient to measure the 4-nm layer periodicity we describe below. We used three-window background subtraction to image the distribution of Ti–L2,3 edge (455.5 eV) and the V–L2,3 edge (513 eV). Two windows were placed in front of the edge (pre-edge 1 and pre-edge 2) and one behind the edge (post edge) as illustrated in Fig. 4. Table 1 gives the position of these windows. The images and spectra were recorded in the GIF on a 1k=1k pixel, cooled CCD camera. 3. Results and discussion The TiAlNyVN coating consists of multilayers with 4-nm periods grown on a stainless steel substrate. Fig. 1a shows the cross-section BF image of the TiAlNyVN coating appearing with columnar grains. It should be noted that in this region the competitive growth which
Table 1 The position of pre-edges and post edge windows
Ti – L2,3 V – L2,3
Energy (eV)
Pre-edge 1 (eV)
Pre-edge 2 (eV)
Post edge (eV)
Slit width (eV)
455.5 513
431 480
446 502
470 526
20 20
H. Meidia et al. / Surface and Coatings Technology 151 – 152 (2002) 209–213
Fig. 3. The EEL spectrum with the location of the three windows background subtraction for Ti and V maps.
initially occurs at the substrate has settled into clearlydefined microcrystallites. The micrograph has been taken at large defocus in order to enhance the multi-layer contrast. The dark region on the right of the field of view is an abutting crystallite which has been orientated to give strong diffraction contrast. In the DF image in Fig. 1b we see that this same region gives rise to speckle-like intensity, which is discussed further below. The banding of the superlattice indicates the evolution of the growth surface; as commonly observed, these corrugations tend to be flatter as the bias voltage is increased. We note that bright-field atomic fringes are visible in all the grains (e.g. Fig. 2, which shows the boundary between two grains at high resolution), and that the orientation of these fringes is consistent in any one grain
211
over its whole extent, despite the presence of the 4-nm wavy bands in Fig. 1a. This implies that the grains grow epitaxially despite curvature of the actual growth surface, and means that the multi-layers in any one grain do not grow normal to any one particular crystalline orientation. All the grain boundaries we have observed (e.g. Fig. 2) do not exhibit voids, although, as expected, they meet with no obvious preferred orientations. Fig. 2 has been imaged through a slightly tilted low-angle grain boundary. Detailed examination of the BF image show areas of well-ordered fringes, but these do not persist over more than 2–8 nm, although the orientation of all such fringes is constant in any one grain. This observation, coupled with the speckle-like contrast in Fig. 1b which on the same scale as these well-ordered areas, imply, as expected, that although crystalline, the specimen was heavily faulted with a complex sub-grain structure. Energy loss spectroscopy (EELS) was applied to sample the chemical composition across the superlattice structures. Fig. 3 shows the N, Ti and V spectrum that has a pronounced near edge fine structures. The N Kedge has sawtooth-shape with distinct fine structures. In Ti L2,3-edge, the magnitude of L2 binding energy is slightly higher than L3 due to the spin–orbit splitting. We also observed the separation of two threshold peaks that increases with increasing atomic number as seen in V L2,3-edge. The elemental maps across the multilayer were taken to examine the superlattice interfaces. To obtain these maps, we acquired a thickness map (tyl where t is thickness of sample and l is the mean free path for inelastic scattering) as shown in Fig. 4. Its profile shows the tyl variation across the layers due to the elemental difference between the layers (VN and TiAlN). This ty
Fig. 4. The thickness mapping taken on the interest region and obtained by dividing an unfiltered TEM image by the zero loss-filtered image and logarithm of this image, tylsln(ItotyI0). (a) Thickness (tyl) mapping, (b) its profile.
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H. Meidia et al. / Surface and Coatings Technology 151 – 152 (2002) 209–213
Fig. 5. The elemental mapping taken on the region of interest (a) BF image, (b) Ti map, (c) V map.
l map was taken of a region of interest and gives values -0.5 for tyl, thus the plural scattering can be ignored w7x, and the specimen is thin enough for quantitative analysis. A three-windows background subtraction technique was used to remove the background contribution. Two windows were placed in front of the edge and one below the edge as seen in Fig. 3. The windows positions are not in scale on Fig. 3. The exact position of the windows can be found in Table 1. The pre-edge windows are used to evaluate the background parameters, A and r (assuming the background can be fitted to a function of the form IB(E)sAEyr) and then the background contribution to the post-edge can be calculated. All images were acquired by a CCD camera and all processing was performed using Gatan Digital Micrograph software. Ti and V maps (Fig. 5) were obtained to study the interfaces and the elemental variation across the layers. The intensity profiles of these two maps are shown in Fig. 6. The width of the TiAlN and VN can be estimated from these profiles, which are approximately 4 nm. This
Fig. 6. The profile of the elemental maps across the square. (a) Ti profile, (b) V profile.
is in good agreement with the periodicity measured by low angle X-ray diffraction where an average value of 4.01 nm was measured w4x. Ti and V profiles are complementary to each other, as expected. The map profiles indicate there was some excess V in TiAlN layers and vice versa, and hence we infer the shuttering provided by the magnetic confinement is imperfect. The interpretation of the elemental distribution in this sample is not straight forward because there are at least three sources of blurring that will cause the actual measured stoichiometry to depart from the ideal squarewave function: blurring caused by the deposition process
Fig. 7. (a) Comparison between the experimental result, the calculation with blurring effect and the ideal flux profiles for titanium. ((b) Comparison between the experimental result, the calculation with blurring effect and the ideal flux profiles for vanadium.
H. Meidia et al. / Surface and Coatings Technology 151 – 152 (2002) 209–213
itself; interdiffusion between VN and TiAlN layers; and the limitation from microscope resolution. The resolution of the microscope in the energy-filtered mode has been measured independently using a SiGe multilayer standard and found to be better than 1 nm w8x. In order to estimate the total blurring from all these effects, we have convolved an ideal square-wave profile, f (x) with a total Gaussian response function g(x) (Fig. 7) to reproduce the experimental profile h(x). By varying the width of g(x) until it matches the experimental response function h(x), we can thus estimate the total blurring. Fig. 7a shows a function where the ideal profile was convoluted with a blurring function assuming a resolution of 1.3 nm (series 2) and the experiment result of titanium (series 3). We can thus conclude that the measured atomic intermixing (whether due to the deposition process or later diffusion) is on a larger scale than the impulse response of the microscope. The same procedure was applied for to the V map (Fig. 7b). In order to fit these experimental data only a blurring of 1 nm (0.3 nm less than in case of Ti) was required. The 0.3-nm difference may have arisen from differences in the interdiffusion rates of these elements, or could be caused by the deposition process itself. 4. Conclusions The complexity of TiAlNyVN superlattice structure was studied by using HREM technique. Coherent growth
213
at the multilayer interfaces was observed. The elemental distribution across the layers was investigated by EFTEM. The elemental maps show the formation of the superlattice coating although there is no physical shutter employed in the process. The intermixing of TiAlNy VN layer was analysed by comparing the experimental data with calculations assuming the blurring (convolution) effects. We found that the intermixing of titanium was more pronounced than that of vanadium, although more work will be required to establish why this is the case. References w1x U. Helemersson, S. Todorova, S.A. Barnett, J.-E. Sundgren, L.C. Markert, J.E. Greene, J. Appl. Phys. 62 (1987) 481. w2x W. Chu, M.S. Wong, W.D. Sproul, S.L. Rohde, S.A. Barnett, J. Vacuum Sci. Technol. A10 (1992) 1604. w3x S.A. Barnett, in: M.H. Francombe, J.L. Vossen ŽEds.., Physics of Thin Films, 17, Academic Press, Boston, 1993. w4x W.-D. Munz, D.B. Lewis, P.E.h. Hovsepian, C. Schonjahn, A. Ehisarian, I.J. Smith, Surf. Eng. 17y1 (2001) 15. w5x W.-D. Munz, D. Schulze, F.J.M. Hauzer, Surf. Coat. Technol. 50 (1992) 169. w6x P.E.h Hovsepian, D.B. Lewis, W.D. Munz, Surf. Coat. Technol. 133 – 134 (2000) 166. w7x P.A. Crozier, Ultramicroscopy 58 (1995) 157. w8x D.J. Norris, A.G. Cullis, T.J. Grasby, E.H.C. Parker, J. Appl. Phys. 86 (12) (1999) 7183.
Investigation of intermixing in TiAlNyVN nanoscale multilayer coatings by energy-filtered TEM b ¨ ¨ b, J.M. Rodenburgb,* H. Meidiaa, A.G. Cullisa, C. Schonjahn , W.D. Munz a
Electronic and Electrical Engineering Department, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK b Materials Research Institute, Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB, UK
Abstract We employ high-resolution energy-filtered cross-sectional transmission electron microscopy (EFTEM) to estimate the degree of layer intermixing in a TiAlNyVN low friction, wear-resistant nanoscale multilayers with a periodicity of approximately 4 nm grown by industrial-scale unbalanced magnetron ion-assisted PVD. Coatings are examined by high-resolution TEM imaging. A series of high-resolution elemental distribution maps are obtained by tuning the electron energy-loss imaging filter to the core loss energies of the elements present. We compare the experimental composition profiles with calculations, which model intermixing at the interface of the layers by a convolution. Although no physical shutter is employed in the industrial process (the substrate is passed in front of alternating targets in a confined plasma), the resulting layers are well differentiated. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: PVD; TiAlNyVN; Superlattice; Intermixing; EELS
1. Introduction We examine the microstructure and atomic-scale composition of a nanoscale TiAlNyVN superlattice coating grown by PVD in an industrial-scale coating unit. Such coatings have been reported to exhibit enhanced hardness values of up to 5000 HV w1,2x. The nature of these high hardness values has been discussed in terms of the layer thickness relationships, the related diffusibility of crystalline defects, and the shear modulus of the materials involved w3x. As the layer periodicity is varied, physical properties can significantly (and beneficially) depart from the bulk attributes of the constituent components. In order to understand these effects we need to have a detailed knowledge of the degree of intermixing of separate components at the layer boundaries. Sharp changes of stoichiometry from one layer to the next is not possible when the coating is prepared on an industrial scale. The system we employ cannot physically shutter each component during PVD deposition, but it can rotate substrates periodically in front of different sputtering targets. Although it is well established that * Corresponding author. Tel.: q44-114-225-4037; fax: q44-114225-3501. E-mail address: [email protected] (J.M. Rodenburg).
this method creates multilayers, the sharpness of the interfaces is difficult to measure because it is on the scale of less than approximately ten unit cells. In this paper, we study a coating at high resolution and measure this intermixing via filtered electron energy loss imaging. In principle, the technique has a much higher spatial resolution (less than 1 nm) than conventional X-ray analysis, and has the further advantage of being sensitive to local bonding configurations. 2. Experimental The TiAlNyVN coating investigated here is dedicated to tribological applications and has commercial applications because of its low friction coefficient and extremely low sliding wear coefficient w4x. It has been deposited in a industrial sized four target PVD coating unit (Hauzer Techno Coating HTC 1000-4) using the arc bond sputter (ABS) process, which is a combined steered arcyunbalanced magnetron sputtering system w5,6x. The sputtering occurs in the ArqN2 atmosphere without any shutter or shielding, although the magnetic confinement of the unbalanced magnetron does effectively introduce periodicity into the coating as the substrate is passed from in front of one target to the
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 6 2 1 - 8
H. Meidia et al. / Surface and Coatings Technology 151 – 152 (2002) 209–213
210
Fig. 2. HREM image showing the crystal misorientation.
Fig. 1. The structure of superlattice TiAlNyVN. (a) BF image, (b) DF image.
next. The deposition rate of the coating was 15 nmy min, with a substrate temperature of 450 8C and a negative bias of 95 V. Cross-sectional samples were prepared by ion beam thinning and were examined in a field emission gun (FEG) JEOL 2010F transmission electron microscope with a point-to-point resolution of 0.19 nm. Bright field (BF), dark field (DF) and high-resolution electron microscopy (HREM) images were taken to study the microstructures at various length scales. The microscope is equipped with a Gatan imaging filter (GIF) which is attached under the camera chamber and gives us the
capability to carry out energy-selected imaging (ESI) and electron energy loss spectroscopy (EELS). We obtained energy filtered TEM (EFTEM) images by selecting only the inelastically scattered electrons corresponding to the inner shell losses of the element edge of interest. In this way we can obtain a high resolution map of the element distribution within the sample. The resolution (0.5–1 nm) of the energy loss signal is not as high as the atomic-resolution BF image, but it is sufficient to measure the 4-nm layer periodicity we describe below. We used three-window background subtraction to image the distribution of Ti–L2,3 edge (455.5 eV) and the V–L2,3 edge (513 eV). Two windows were placed in front of the edge (pre-edge 1 and pre-edge 2) and one behind the edge (post edge) as illustrated in Fig. 4. Table 1 gives the position of these windows. The images and spectra were recorded in the GIF on a 1k=1k pixel, cooled CCD camera. 3. Results and discussion The TiAlNyVN coating consists of multilayers with 4-nm periods grown on a stainless steel substrate. Fig. 1a shows the cross-section BF image of the TiAlNyVN coating appearing with columnar grains. It should be noted that in this region the competitive growth which
Table 1 The position of pre-edges and post edge windows
Ti – L2,3 V – L2,3
Energy (eV)
Pre-edge 1 (eV)
Pre-edge 2 (eV)
Post edge (eV)
Slit width (eV)
455.5 513
431 480
446 502
470 526
20 20
H. Meidia et al. / Surface and Coatings Technology 151 – 152 (2002) 209–213
Fig. 3. The EEL spectrum with the location of the three windows background subtraction for Ti and V maps.
initially occurs at the substrate has settled into clearlydefined microcrystallites. The micrograph has been taken at large defocus in order to enhance the multi-layer contrast. The dark region on the right of the field of view is an abutting crystallite which has been orientated to give strong diffraction contrast. In the DF image in Fig. 1b we see that this same region gives rise to speckle-like intensity, which is discussed further below. The banding of the superlattice indicates the evolution of the growth surface; as commonly observed, these corrugations tend to be flatter as the bias voltage is increased. We note that bright-field atomic fringes are visible in all the grains (e.g. Fig. 2, which shows the boundary between two grains at high resolution), and that the orientation of these fringes is consistent in any one grain
211
over its whole extent, despite the presence of the 4-nm wavy bands in Fig. 1a. This implies that the grains grow epitaxially despite curvature of the actual growth surface, and means that the multi-layers in any one grain do not grow normal to any one particular crystalline orientation. All the grain boundaries we have observed (e.g. Fig. 2) do not exhibit voids, although, as expected, they meet with no obvious preferred orientations. Fig. 2 has been imaged through a slightly tilted low-angle grain boundary. Detailed examination of the BF image show areas of well-ordered fringes, but these do not persist over more than 2–8 nm, although the orientation of all such fringes is constant in any one grain. This observation, coupled with the speckle-like contrast in Fig. 1b which on the same scale as these well-ordered areas, imply, as expected, that although crystalline, the specimen was heavily faulted with a complex sub-grain structure. Energy loss spectroscopy (EELS) was applied to sample the chemical composition across the superlattice structures. Fig. 3 shows the N, Ti and V spectrum that has a pronounced near edge fine structures. The N Kedge has sawtooth-shape with distinct fine structures. In Ti L2,3-edge, the magnitude of L2 binding energy is slightly higher than L3 due to the spin–orbit splitting. We also observed the separation of two threshold peaks that increases with increasing atomic number as seen in V L2,3-edge. The elemental maps across the multilayer were taken to examine the superlattice interfaces. To obtain these maps, we acquired a thickness map (tyl where t is thickness of sample and l is the mean free path for inelastic scattering) as shown in Fig. 4. Its profile shows the tyl variation across the layers due to the elemental difference between the layers (VN and TiAlN). This ty
Fig. 4. The thickness mapping taken on the interest region and obtained by dividing an unfiltered TEM image by the zero loss-filtered image and logarithm of this image, tylsln(ItotyI0). (a) Thickness (tyl) mapping, (b) its profile.
212
H. Meidia et al. / Surface and Coatings Technology 151 – 152 (2002) 209–213
Fig. 5. The elemental mapping taken on the region of interest (a) BF image, (b) Ti map, (c) V map.
l map was taken of a region of interest and gives values -0.5 for tyl, thus the plural scattering can be ignored w7x, and the specimen is thin enough for quantitative analysis. A three-windows background subtraction technique was used to remove the background contribution. Two windows were placed in front of the edge and one below the edge as seen in Fig. 3. The windows positions are not in scale on Fig. 3. The exact position of the windows can be found in Table 1. The pre-edge windows are used to evaluate the background parameters, A and r (assuming the background can be fitted to a function of the form IB(E)sAEyr) and then the background contribution to the post-edge can be calculated. All images were acquired by a CCD camera and all processing was performed using Gatan Digital Micrograph software. Ti and V maps (Fig. 5) were obtained to study the interfaces and the elemental variation across the layers. The intensity profiles of these two maps are shown in Fig. 6. The width of the TiAlN and VN can be estimated from these profiles, which are approximately 4 nm. This
Fig. 6. The profile of the elemental maps across the square. (a) Ti profile, (b) V profile.
is in good agreement with the periodicity measured by low angle X-ray diffraction where an average value of 4.01 nm was measured w4x. Ti and V profiles are complementary to each other, as expected. The map profiles indicate there was some excess V in TiAlN layers and vice versa, and hence we infer the shuttering provided by the magnetic confinement is imperfect. The interpretation of the elemental distribution in this sample is not straight forward because there are at least three sources of blurring that will cause the actual measured stoichiometry to depart from the ideal squarewave function: blurring caused by the deposition process
Fig. 7. (a) Comparison between the experimental result, the calculation with blurring effect and the ideal flux profiles for titanium. ((b) Comparison between the experimental result, the calculation with blurring effect and the ideal flux profiles for vanadium.
H. Meidia et al. / Surface and Coatings Technology 151 – 152 (2002) 209–213
itself; interdiffusion between VN and TiAlN layers; and the limitation from microscope resolution. The resolution of the microscope in the energy-filtered mode has been measured independently using a SiGe multilayer standard and found to be better than 1 nm w8x. In order to estimate the total blurring from all these effects, we have convolved an ideal square-wave profile, f (x) with a total Gaussian response function g(x) (Fig. 7) to reproduce the experimental profile h(x). By varying the width of g(x) until it matches the experimental response function h(x), we can thus estimate the total blurring. Fig. 7a shows a function where the ideal profile was convoluted with a blurring function assuming a resolution of 1.3 nm (series 2) and the experiment result of titanium (series 3). We can thus conclude that the measured atomic intermixing (whether due to the deposition process or later diffusion) is on a larger scale than the impulse response of the microscope. The same procedure was applied for to the V map (Fig. 7b). In order to fit these experimental data only a blurring of 1 nm (0.3 nm less than in case of Ti) was required. The 0.3-nm difference may have arisen from differences in the interdiffusion rates of these elements, or could be caused by the deposition process itself. 4. Conclusions The complexity of TiAlNyVN superlattice structure was studied by using HREM technique. Coherent growth
213
at the multilayer interfaces was observed. The elemental distribution across the layers was investigated by EFTEM. The elemental maps show the formation of the superlattice coating although there is no physical shutter employed in the process. The intermixing of TiAlNy VN layer was analysed by comparing the experimental data with calculations assuming the blurring (convolution) effects. We found that the intermixing of titanium was more pronounced than that of vanadium, although more work will be required to establish why this is the case. References w1x U. Helemersson, S. Todorova, S.A. Barnett, J.-E. Sundgren, L.C. Markert, J.E. Greene, J. Appl. Phys. 62 (1987) 481. w2x W. Chu, M.S. Wong, W.D. Sproul, S.L. Rohde, S.A. Barnett, J. Vacuum Sci. Technol. A10 (1992) 1604. w3x S.A. Barnett, in: M.H. Francombe, J.L. Vossen ŽEds.., Physics of Thin Films, 17, Academic Press, Boston, 1993. w4x W.-D. Munz, D.B. Lewis, P.E.h. Hovsepian, C. Schonjahn, A. Ehisarian, I.J. Smith, Surf. Eng. 17y1 (2001) 15. w5x W.-D. Munz, D. Schulze, F.J.M. Hauzer, Surf. Coat. Technol. 50 (1992) 169. w6x P.E.h Hovsepian, D.B. Lewis, W.D. Munz, Surf. Coat. Technol. 133 – 134 (2000) 166. w7x P.A. Crozier, Ultramicroscopy 58 (1995) 157. w8x D.J. Norris, A.G. Cullis, T.J. Grasby, E.H.C. Parker, J. Appl. Phys. 86 (12) (1999) 7183.