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Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation
Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation Katarzyna Nowakowska-Langier a,⇑, Rafal Chodun b, Roman Minikayev c, Lukasz Kurpaska a, Lukasz Skowronski d, G.W. Strzelecki a, Sebastian Okrasa b, Krzysztof Zdunek b a
National Centre for Nuclear Research (NCBJ), A. Soltana 7, 05-400 Otwock, Poland Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, Warsaw, Poland Polish Academy of Sciences, Institute of Physics, Poland d Institute of Mathematics and Physics, UTP University of Science and Technology, Poland b c
a r t i c l e
i n f o
Article history: Received 10 December 2016 Accepted 19 April 2017 Available online xxxx Keywords: Pulsed magnetron sputtering Modulation of power frequency Copper nitride layers
a b s t r a c t This work concerns the synthesis of copper nitride Cu3N layers by means of the Pulsed Magnetron Sputtering method (PMS), operating with various conditions of frequency modulation. We have studied how this parameter affect the mechanism of layer growth. Reported studies focus on the structure of the Cu–N layers. The results showed that the synthesis processes strongly depend of sputtering parameters. The modulation of sputtering frequency had an effect on the structure, phase composition and consequently, the properties of synthesized layers. Our results revealed that the studied parameter should be considered as an another important factor for optimization of layers synthesis and opens new paradigm in future implementation of PMS-based methods. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Magnetron sputtering (MS) method is the most widely used plasma surface engineering method of the layers synthesis. One of the advantage of MS method is its potential for continued development but still basing on elementary assumptions of the method. Successive improvements provide new opportunities or intensify already existing ones. Examples of such solutions aiming toward perfection can be, for example: HiPIMS e.g. [1,2], GIMS [3–5] but also the PMS [e.g. [6–8]] methods. The scope of interest of the society is particularly optimization the process of synthesis by various factors like: kinetics of growth, ionization state of growth, structure of layers, chemical and phase composition etc. In our experiment the layers were synthetized by using the Pulsed Magnetron Sputtering method (PMS) that can work in different conditions of frequency modulation (fmod) [6,9]. It means that the pulsed manner of plasma generation with the main frequency of power source (100 kHz) was modulated by another frequency. Through the possibility of changing the electrical parameters directly affecting the generation of plasma conditions such as frequency and lifetime of plasma, we are able to influence the synthesis environment. Presented study concerns the structure ⇑ Corresponding author. E-mail address: [email protected] (K. Nowakowska-Langier).
and properties of the Cu–N layers inducted by the applied process parameters. In our experiments, we focused on the possibility of controlling the electrical parameters that directly affecting the conditions of plasma generation. Therefore we focused on elementary phenomena of vapour excitation (time of active state of plasma) which are responsible for the temporary sputtering of the target material. We believed that it may affect both the quantity and the activity of ions involved in the synthesis as well as their interaction with the substrate and the growing layers. Therefore it can be a very important and effective parameter of synthesis process in future implementation of PMS-based methods. 2. Experimental part The Cu–N layers were synthesized by the Pulsed Magnetron Sputtering (PMS) method in vacuum chamber equipped with circular magnetron. The magnetron was powered by a 10 kW pulse power supply, operating in DC mode with a frequency of 100 kHz modulated by frequency fmod 10 Hz and 1000 Hz [6,9]. Additionally, during the synthesis, we used various power values: P1 = 150 W, P2 = 300 and P3 = 500 W corresponding to the duration of an individual pulse of plasma [10]. Fig. 1 illustrates the how the modulation frequency and power drive the duration of plasma pulse. The magnetron target (50 mm in diameter and 6 mm thick) was made of pure copper. The layers were deposited on a
http://dx.doi.org/10.1016/j.nimb.2017.04.070 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Nowakowska-Langier et al., Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.070
2
K. Nowakowska-Langier et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Fig. 1. The principle of operation where is shown how the modulation frequency and power drive the duration of plasma pulse.
non-heated silicon substrate, located parallel to the targets at 100 mm and 160 mm distance. The processes were carried out in a nitrogen atmosphere under a constant pressure of pN2 = 1 Pa. In case of Cu–N coatings synthesis, the use of neutral sputtering gas is not recommended because of the relatively high sputtering rate of copper. The morphology and phase composition of the Cu–N layers was characterized by using a scanning electron microscopy (SEM) and X-ray diffraction measurements using Cu–Ka radiation. Optical properties of Cu–N coatings were studied by means of spectroscopic ellipsometry (V-VASE; J. A. Woollam Co., Inc.). To extract optical constants of the coatings the five-medium optical model was applied (ambient/rough layer/Cu–N/SiO2/Si). The optical constants of Cu–N coatings were described in terms of Lorentzian- and Gaussian-type oscillators. Additionally, for metallic-type Cu–N layers, the Drude term was taken into account. The band-gap energy was estimated using the Tauc-plot method. Nanomechanical properties were investigated by using NanoTest Vantage system provided by Micro Materials Laboratory. Measurements were performed with Berkovitch-shaped indenter using control depth option up to approx.. 80 nm depth. This corresponds to approx. 20% thickness of modified layer. According to our previous studies and literature data [11–14], in order to obtain valuable mechanical data, nanoindentation depth should be set in the range of 20–50% of the total thickness of the tested layer. These conditions assure collecting mechanical response of the system without influence of the bulk material [14]. Each measurement was repeated at least 8 times. Oliver and Pharr method [15] was applied for calculation of nanomechanical parameters such as nanohardness and Young modulus.
Fig. 2. Cross section view of the Cu–N layers obtained with different frequency (fmod) and power (P) of plasma generations. SEM.
3. Result and discussion Fig. 2 shows cross section view of the Cu–N layers deposited at variable frequency fmod and power P. The Cu–N layers obtained with high fmod, in all cases of power, exhibit a columnar structure. The higher used power was, the more defragmented structure appeared, but kept similar roughness. Observed columnar structure is very typical for magnetron sputtering method, which is well described by Messier and Thornton models of layers growth [16,17]. The XRD measurements (Fig. 3) show that such a conditions favor the polycrystalline structure of stoichiometric Cu3N phase [18] with preferential growth direction (1 0 0), and a lattice parameters ranging from 3.817 to 3.826 A, that corresponds well with the literature e.g. [19,20]. The Cu–N layers exhibit semiconducting (1000 Hz, P1) and metallic-like (10 Hz, P1 and P2) properties (see Table 1). One sample (1000 Hz, P2) exhibit both semiconducting and metallic-like features. Its optical band-gap energy (1.85 eV) is lower than energy found for the semiconducting sample (2.02 eV), while its optical resistivity (2600 mXcm) is 5–10 times larger, than corresponding values reported for metallic-like coatings (260–470 mXcm). The conducting behavior
Fig. 3. h/2h diffraction patterns of Cu–N layers obtained under frequency modulation equals 1000 Hz (top) and 10 Hz (bottom) in function of P.
of Cu–N coatings is also visible in the relaxation time of free electrons (see Table 1). It should be noted, that the optical resistivity and relaxation time, determined for the sample synthesized at 1000 Hz and using the effective power 300 W is poorly estimated, which confirms the transitional behavior on the coating. As one can see, the different morphology of structure was obtained from the synthesis process characterized by low fmod-the fine grained, compacted and uniform structure, in contrast to the high fmod case.
Please cite this article in press as: K. Nowakowska-Langier et al., Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.070
3
K. Nowakowska-Langier et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx Table 1 Band gap, relaxation time of free electrons and resistivity of the Cu–N layers.
10 Hz 1000 Hz
Power Peff
Eg (eV)
qopt (lXcm)
s(fs)
P1 P2 P1 P2
Metallic-like Metallic-like 2.02 ± 0.01 1.85 ± 0.02
472 ± 6 262 ± 11 Semiconductor 2600 ± 5600
0.721 ± 0.008 0.908 ± 0.018 0.062 ± 0.095
Fig. 5. Schematic diagram showing the essence of the mechanism of the layers growth occurring during Pulsed Magnetron Sputtering with variable frequency of plasma generations. Fig. 4. Nanohardness of the Cu–N layers synthetized by PMS method with different frequency and power of plasma generations.
Moreover, structure of the layers changes with the power increase. It means that these conditions of synthesis disturb the columnar growth of the layers. Also, the Cu3N phase is characterized by another preferred growth direction [1 1 1] (Fig. 3, bottom) It was concluded before that the copper adatoms during the layer growth are responsible for privileging this direction [21]. Additionally the lattice parameter increase with the increase of the power from 3.820 to 3.865 A, and we observe a peaks shifted to lower 2theta position. We suppose that relatively long duration of plasma pulse is responsible for intensifying the sputtering of copper target. The practical result of the generation plasma enriched by copper particles should be strong reflected in composition of deposited layers, and in fact, it is. Larger lattice parameter calculated from diffraction patterns and peaks’ shifts observed come from Cu3N phase which is supersaturated by copper vacancies. These results show that the synthesis under conditions of high fmod characterize by sufficient reactivity of plasma components which explains the nucleation of stoichiometric phase. Low fmod contributes to the fragmentation of the structure, disturbs the columnar growth of the layers and enriches the synthesis environment by copper particles, which leads to supersaturating the structure of synthesized layers. The effects of the fmod were revealed also in the nanoindentation measurements. As is shown in Fig. 4, the layer synthetized with the 10 Hz fmod, and fine grain structure, is characterized by nanohardness two fold higher than in the case of the layer obtained with 1000 Hz fmod. With the increase of the P the nanohardness decrease was observed. It is probably connected with the increase of copper content in the layer deposited under these conditions. The increase of nanohardness for layer deposited with 1000 Hz with the increase of P is a result of its structural features. Uniform and ultra-fine structures characterize by better mechanical properties. We believe that various conditions of plasma generation, like the discussed the duration of each pulse, affect the nature and intensity of interaction of plasma components with the surface region of the substrate and growing layer interface. In the case of low fmod, probably the more intense interaction of plasma components with the surface of the substrate occurs, causing interruption of continuous growth-, sputtering the material of growing layer
and its re-condensation on the surface. A confirmation can be a structure obtained under these conditions but for the greater substrate – target distance (dS-T) – Fig. 2. It is known that structure of films depends of plasma particles energy [17,22]. The energy of plasma components reduces with the distance from the plasma source, because of phenomena of energy dissipation on non elastic collisions with neutral gas molecules. So it is expected that layers at higher distance grow in the regime of relatively lower energy exchange with the plasma particles. In the case of presented results the structure shows tendency to columnar growth. Therefore we can conclude that the differences in morphology originate from the different growth mechanisms accompanying synthesis during various frequency of plasma generation can be expressed by the schematic structural diagram of the layer growth proposed in Fig. 5. It appears that fmod can be a tool to control the mechanism of growth of layers. 4. Conclusion The Cu–N layers were synthetized by use of the pulsed magnetron sputtering with various frequency of plasma generation. Changing conditions of plasma generation resulting from the possibility of controlling its frequency and life time of plasma, has an strong affect on quantity and activity of plasma particles and their interaction with the substrate. It allows to control the mechanism of the layers growth, their microstructure, and phase composition, as well as mechanical and optical properties. The low frequency modulation contribute to the fragmentation of the structures and perturb columnar growth that are characteristic for the structure of the Cu–N layers synthetized under high frequency modulation. Our investigation revealed that the use of variable frequency modulation is a very important parameter of synthesis process that can inaugurate new paradigm in future implementation of PMSbased methods. Acknowledgement This work was financially supported by the National Science Centre within the project 2014/15/B/ST8/01692.
Please cite this article in press as: K. Nowakowska-Langier et al., Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.070
4
K. Nowakowska-Langier et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Reference [1] K. Macak, V. Kouznetsov, J. Schneider, U. Helmersson, I. Petrov, J. Vac. Sci. Technol. A18 (2000) 1533. [2] U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson, Thin Solid Films 513 (2006) 1. [3] K. Zdunek, K. Nowakowska-Langier, J. Dora, R. Chodun, Surf. Coat. Technol. 228 (2013) S367. [4] K. Zdunek, K. Nowakowska-Langier, R. Chodun, J. Dora, S. Okrasa, E. Talik, Mater. Sci. Poland 32 (2) (2014) 171. [5] K. Zdunek, L. Skowrónski, R. Chodun, K. Nowakowska-Langier, A. Grabowski, W. Wachowiak, S. Okrasa, A. Wachowiak, O. Strauss, A. Wronkowski, P. Domanowski, Mater. Sci. Poland 34 (1) (2016) 137. [6] W.M. Posadowski, Thin Solid Films 343–344 (1999) 85. [7] L. Skowronski, K. Zdunek, K. Nowakowska-Langier, R. Chodun, M. Trzcinski, M. Kobierski, M.K. Kustra, A.A. Wachowiak, W. Wachowiak, T. Hiller, A. Grabowski, L. Kurpaska, M.K. Naparty, Surf. Coat. Technol. 282 (25) (2015) 16. [8] K. Nowakowska-Langier, R. Chodun, R. Nietubyc, R. Minikayevc, K. Zdunek, Appl. Surf. Sci. 275 (2013) 14.
[9] W.M. Posadowski, A. Wiatrowski, J. Dora, Z.J. Radzin´ski, Thin Solid Films 516 (14) (2008) 4478. [10] A. Wiatrowski, W.M. Posadowski, Mater. Sci. Poland 34 (2) (2016) 374. [11] C.K. Dolph et al., J. Nucl. Mater. 481 (2016) 33. [12] L. Kurpaska, J. Jagielski, K. Nowakowska-Langier, Nucl. Instrum. Methods Phys. B 379 (2016) 112. [13] L. Kurpaska, J. Jagielski, Nucl. Instrum. Methods Phys. B 379 (2016) 107. [14] L. Kurpaska, M. Gapinska, J. Jasinski, M. Lesniak, M. Sitarz, K. NowakowskaLangier, J. Jagielski, K. Wozniak, J. Mol. Struct. 1126 (2016) 226. [15] C. Olivier, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [16] J.A. Thornton, J. Vac. Sci. Technol. 11 (4) (1974) 666. [17] R. Messier, A.P. Giri, R.A. Roy, J. Vac. Sci. Technol A2 (2) (1984) 500. [18] JCPDS 47–1088. [19] T. Maruyama, T. Morishita, J. Appl. Phys. 78 (1995) 4104. [20] J.F. Pierson, Vacuum 66 (2002) 59. [21] G. Sahoo, S.R. Meher, M.K. Jain, Mater. Sci. Eng. B 191 (2015) 7. [22] A. Anders, Thin Solid Films 518 (2010) 4087.
Please cite this article in press as: K. Nowakowska-Langier et al., Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.070
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation Katarzyna Nowakowska-Langier a,⇑, Rafal Chodun b, Roman Minikayev c, Lukasz Kurpaska a, Lukasz Skowronski d, G.W. Strzelecki a, Sebastian Okrasa b, Krzysztof Zdunek b a
National Centre for Nuclear Research (NCBJ), A. Soltana 7, 05-400 Otwock, Poland Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, Warsaw, Poland Polish Academy of Sciences, Institute of Physics, Poland d Institute of Mathematics and Physics, UTP University of Science and Technology, Poland b c
a r t i c l e
i n f o
Article history: Received 10 December 2016 Accepted 19 April 2017 Available online xxxx Keywords: Pulsed magnetron sputtering Modulation of power frequency Copper nitride layers
a b s t r a c t This work concerns the synthesis of copper nitride Cu3N layers by means of the Pulsed Magnetron Sputtering method (PMS), operating with various conditions of frequency modulation. We have studied how this parameter affect the mechanism of layer growth. Reported studies focus on the structure of the Cu–N layers. The results showed that the synthesis processes strongly depend of sputtering parameters. The modulation of sputtering frequency had an effect on the structure, phase composition and consequently, the properties of synthesized layers. Our results revealed that the studied parameter should be considered as an another important factor for optimization of layers synthesis and opens new paradigm in future implementation of PMS-based methods. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Magnetron sputtering (MS) method is the most widely used plasma surface engineering method of the layers synthesis. One of the advantage of MS method is its potential for continued development but still basing on elementary assumptions of the method. Successive improvements provide new opportunities or intensify already existing ones. Examples of such solutions aiming toward perfection can be, for example: HiPIMS e.g. [1,2], GIMS [3–5] but also the PMS [e.g. [6–8]] methods. The scope of interest of the society is particularly optimization the process of synthesis by various factors like: kinetics of growth, ionization state of growth, structure of layers, chemical and phase composition etc. In our experiment the layers were synthetized by using the Pulsed Magnetron Sputtering method (PMS) that can work in different conditions of frequency modulation (fmod) [6,9]. It means that the pulsed manner of plasma generation with the main frequency of power source (100 kHz) was modulated by another frequency. Through the possibility of changing the electrical parameters directly affecting the generation of plasma conditions such as frequency and lifetime of plasma, we are able to influence the synthesis environment. Presented study concerns the structure ⇑ Corresponding author. E-mail address: [email protected] (K. Nowakowska-Langier).
and properties of the Cu–N layers inducted by the applied process parameters. In our experiments, we focused on the possibility of controlling the electrical parameters that directly affecting the conditions of plasma generation. Therefore we focused on elementary phenomena of vapour excitation (time of active state of plasma) which are responsible for the temporary sputtering of the target material. We believed that it may affect both the quantity and the activity of ions involved in the synthesis as well as their interaction with the substrate and the growing layers. Therefore it can be a very important and effective parameter of synthesis process in future implementation of PMS-based methods. 2. Experimental part The Cu–N layers were synthesized by the Pulsed Magnetron Sputtering (PMS) method in vacuum chamber equipped with circular magnetron. The magnetron was powered by a 10 kW pulse power supply, operating in DC mode with a frequency of 100 kHz modulated by frequency fmod 10 Hz and 1000 Hz [6,9]. Additionally, during the synthesis, we used various power values: P1 = 150 W, P2 = 300 and P3 = 500 W corresponding to the duration of an individual pulse of plasma [10]. Fig. 1 illustrates the how the modulation frequency and power drive the duration of plasma pulse. The magnetron target (50 mm in diameter and 6 mm thick) was made of pure copper. The layers were deposited on a
http://dx.doi.org/10.1016/j.nimb.2017.04.070 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Nowakowska-Langier et al., Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.070
2
K. Nowakowska-Langier et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Fig. 1. The principle of operation where is shown how the modulation frequency and power drive the duration of plasma pulse.
non-heated silicon substrate, located parallel to the targets at 100 mm and 160 mm distance. The processes were carried out in a nitrogen atmosphere under a constant pressure of pN2 = 1 Pa. In case of Cu–N coatings synthesis, the use of neutral sputtering gas is not recommended because of the relatively high sputtering rate of copper. The morphology and phase composition of the Cu–N layers was characterized by using a scanning electron microscopy (SEM) and X-ray diffraction measurements using Cu–Ka radiation. Optical properties of Cu–N coatings were studied by means of spectroscopic ellipsometry (V-VASE; J. A. Woollam Co., Inc.). To extract optical constants of the coatings the five-medium optical model was applied (ambient/rough layer/Cu–N/SiO2/Si). The optical constants of Cu–N coatings were described in terms of Lorentzian- and Gaussian-type oscillators. Additionally, for metallic-type Cu–N layers, the Drude term was taken into account. The band-gap energy was estimated using the Tauc-plot method. Nanomechanical properties were investigated by using NanoTest Vantage system provided by Micro Materials Laboratory. Measurements were performed with Berkovitch-shaped indenter using control depth option up to approx.. 80 nm depth. This corresponds to approx. 20% thickness of modified layer. According to our previous studies and literature data [11–14], in order to obtain valuable mechanical data, nanoindentation depth should be set in the range of 20–50% of the total thickness of the tested layer. These conditions assure collecting mechanical response of the system without influence of the bulk material [14]. Each measurement was repeated at least 8 times. Oliver and Pharr method [15] was applied for calculation of nanomechanical parameters such as nanohardness and Young modulus.
Fig. 2. Cross section view of the Cu–N layers obtained with different frequency (fmod) and power (P) of plasma generations. SEM.
3. Result and discussion Fig. 2 shows cross section view of the Cu–N layers deposited at variable frequency fmod and power P. The Cu–N layers obtained with high fmod, in all cases of power, exhibit a columnar structure. The higher used power was, the more defragmented structure appeared, but kept similar roughness. Observed columnar structure is very typical for magnetron sputtering method, which is well described by Messier and Thornton models of layers growth [16,17]. The XRD measurements (Fig. 3) show that such a conditions favor the polycrystalline structure of stoichiometric Cu3N phase [18] with preferential growth direction (1 0 0), and a lattice parameters ranging from 3.817 to 3.826 A, that corresponds well with the literature e.g. [19,20]. The Cu–N layers exhibit semiconducting (1000 Hz, P1) and metallic-like (10 Hz, P1 and P2) properties (see Table 1). One sample (1000 Hz, P2) exhibit both semiconducting and metallic-like features. Its optical band-gap energy (1.85 eV) is lower than energy found for the semiconducting sample (2.02 eV), while its optical resistivity (2600 mXcm) is 5–10 times larger, than corresponding values reported for metallic-like coatings (260–470 mXcm). The conducting behavior
Fig. 3. h/2h diffraction patterns of Cu–N layers obtained under frequency modulation equals 1000 Hz (top) and 10 Hz (bottom) in function of P.
of Cu–N coatings is also visible in the relaxation time of free electrons (see Table 1). It should be noted, that the optical resistivity and relaxation time, determined for the sample synthesized at 1000 Hz and using the effective power 300 W is poorly estimated, which confirms the transitional behavior on the coating. As one can see, the different morphology of structure was obtained from the synthesis process characterized by low fmod-the fine grained, compacted and uniform structure, in contrast to the high fmod case.
Please cite this article in press as: K. Nowakowska-Langier et al., Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.070
3
K. Nowakowska-Langier et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx Table 1 Band gap, relaxation time of free electrons and resistivity of the Cu–N layers.
10 Hz 1000 Hz
Power Peff
Eg (eV)
qopt (lXcm)
s(fs)
P1 P2 P1 P2
Metallic-like Metallic-like 2.02 ± 0.01 1.85 ± 0.02
472 ± 6 262 ± 11 Semiconductor 2600 ± 5600
0.721 ± 0.008 0.908 ± 0.018 0.062 ± 0.095
Fig. 5. Schematic diagram showing the essence of the mechanism of the layers growth occurring during Pulsed Magnetron Sputtering with variable frequency of plasma generations. Fig. 4. Nanohardness of the Cu–N layers synthetized by PMS method with different frequency and power of plasma generations.
Moreover, structure of the layers changes with the power increase. It means that these conditions of synthesis disturb the columnar growth of the layers. Also, the Cu3N phase is characterized by another preferred growth direction [1 1 1] (Fig. 3, bottom) It was concluded before that the copper adatoms during the layer growth are responsible for privileging this direction [21]. Additionally the lattice parameter increase with the increase of the power from 3.820 to 3.865 A, and we observe a peaks shifted to lower 2theta position. We suppose that relatively long duration of plasma pulse is responsible for intensifying the sputtering of copper target. The practical result of the generation plasma enriched by copper particles should be strong reflected in composition of deposited layers, and in fact, it is. Larger lattice parameter calculated from diffraction patterns and peaks’ shifts observed come from Cu3N phase which is supersaturated by copper vacancies. These results show that the synthesis under conditions of high fmod characterize by sufficient reactivity of plasma components which explains the nucleation of stoichiometric phase. Low fmod contributes to the fragmentation of the structure, disturbs the columnar growth of the layers and enriches the synthesis environment by copper particles, which leads to supersaturating the structure of synthesized layers. The effects of the fmod were revealed also in the nanoindentation measurements. As is shown in Fig. 4, the layer synthetized with the 10 Hz fmod, and fine grain structure, is characterized by nanohardness two fold higher than in the case of the layer obtained with 1000 Hz fmod. With the increase of the P the nanohardness decrease was observed. It is probably connected with the increase of copper content in the layer deposited under these conditions. The increase of nanohardness for layer deposited with 1000 Hz with the increase of P is a result of its structural features. Uniform and ultra-fine structures characterize by better mechanical properties. We believe that various conditions of plasma generation, like the discussed the duration of each pulse, affect the nature and intensity of interaction of plasma components with the surface region of the substrate and growing layer interface. In the case of low fmod, probably the more intense interaction of plasma components with the surface of the substrate occurs, causing interruption of continuous growth-, sputtering the material of growing layer
and its re-condensation on the surface. A confirmation can be a structure obtained under these conditions but for the greater substrate – target distance (dS-T) – Fig. 2. It is known that structure of films depends of plasma particles energy [17,22]. The energy of plasma components reduces with the distance from the plasma source, because of phenomena of energy dissipation on non elastic collisions with neutral gas molecules. So it is expected that layers at higher distance grow in the regime of relatively lower energy exchange with the plasma particles. In the case of presented results the structure shows tendency to columnar growth. Therefore we can conclude that the differences in morphology originate from the different growth mechanisms accompanying synthesis during various frequency of plasma generation can be expressed by the schematic structural diagram of the layer growth proposed in Fig. 5. It appears that fmod can be a tool to control the mechanism of growth of layers. 4. Conclusion The Cu–N layers were synthetized by use of the pulsed magnetron sputtering with various frequency of plasma generation. Changing conditions of plasma generation resulting from the possibility of controlling its frequency and life time of plasma, has an strong affect on quantity and activity of plasma particles and their interaction with the substrate. It allows to control the mechanism of the layers growth, their microstructure, and phase composition, as well as mechanical and optical properties. The low frequency modulation contribute to the fragmentation of the structures and perturb columnar growth that are characteristic for the structure of the Cu–N layers synthetized under high frequency modulation. Our investigation revealed that the use of variable frequency modulation is a very important parameter of synthesis process that can inaugurate new paradigm in future implementation of PMSbased methods. Acknowledgement This work was financially supported by the National Science Centre within the project 2014/15/B/ST8/01692.
Please cite this article in press as: K. Nowakowska-Langier et al., Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.070
4
K. Nowakowska-Langier et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Reference [1] K. Macak, V. Kouznetsov, J. Schneider, U. Helmersson, I. Petrov, J. Vac. Sci. Technol. A18 (2000) 1533. [2] U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson, Thin Solid Films 513 (2006) 1. [3] K. Zdunek, K. Nowakowska-Langier, J. Dora, R. Chodun, Surf. Coat. Technol. 228 (2013) S367. [4] K. Zdunek, K. Nowakowska-Langier, R. Chodun, J. Dora, S. Okrasa, E. Talik, Mater. Sci. Poland 32 (2) (2014) 171. [5] K. Zdunek, L. Skowrónski, R. Chodun, K. Nowakowska-Langier, A. Grabowski, W. Wachowiak, S. Okrasa, A. Wachowiak, O. Strauss, A. Wronkowski, P. Domanowski, Mater. Sci. Poland 34 (1) (2016) 137. [6] W.M. Posadowski, Thin Solid Films 343–344 (1999) 85. [7] L. Skowronski, K. Zdunek, K. Nowakowska-Langier, R. Chodun, M. Trzcinski, M. Kobierski, M.K. Kustra, A.A. Wachowiak, W. Wachowiak, T. Hiller, A. Grabowski, L. Kurpaska, M.K. Naparty, Surf. Coat. Technol. 282 (25) (2015) 16. [8] K. Nowakowska-Langier, R. Chodun, R. Nietubyc, R. Minikayevc, K. Zdunek, Appl. Surf. Sci. 275 (2013) 14.
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Please cite this article in press as: K. Nowakowska-Langier et al., Structure of Cu–N layers synthesized by pulsed magnetron sputtering with variable frequency of plasma generation, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.070