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Influence of ion bombardment on structure and properties of TiZrN thin film
Applied Surface Science 354 (2015) 155–160
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of ion bombardment on structure and properties of TiZrN thin film Yu-Wei Lin a,∗ , Jia-Hong Huang b , Ge-Ping Yu b , Chien-Nan Hsiao a , Fong-Zhi Chen a a b
Instrument Technology Research Center, National Applied Research Laboratories Department of Engineering and System Science National Tsing Hua University
a r t i c l e
i n f o
Article history: Received 3 October 2014 Received in revised form 26 February 2015 Accepted 27 February 2015 Available online 30 March 2015 Keywords: Ion bombardment TiZrN Hardness
a b s t r a c t The study is focused on the characterization of TiZrN thin film by controlling the behavior of ion bombardment. Thin films are grown using radio frequency magnetron sputtering process on Si wafer. The negative bias voltage ranging from −20 V to −130 V was applied to the substrate. The ion current density increases rapidly as substrate bias is lower than −60 V, then slightly increases as the critical value about −60 V is exceeded. At the substrate bias of −60 V, the ion current density is close to 0.56 mA/cm2 . The resistivity measured by four-point probe decreases from conditions −20 V to −60 V and then increases for substrate bias increases from −60 V to −130 V. The resistivity of TiZrN films is contributed from the packing factor. The N/TiZr ratios about 1 were measured by Rutherford backscattering spectrometer, and the packing factors of TiZrN films can also be obtained by the results of RBS. Field Emission scanning electron microscope (FEG-SEM) is used to characterize the thickness and structure of the deposited TiZrN film. X-ray diffraction (XRD) is used to determine the preferred orientation and lattice parameter. The precursor results of XRD show that all the coating samples exhibited (1 1 1) preferred orientation, and the hardness values of TiZrN films were ranging from 20 to 40 GPa. To sum up the precursor studies, the TiZrN films which can improve the properties from TiN and ZrN is a new ceramic material with higher potential. Following the advance process and analysis research, the structure and properties can be correlated and as a reference for industry application. © 2015 Elsevier B.V. All rights reserved.
1. Introduction TiN was used as the base for many coatings alloyed with other elements. These coatings exhibit a variety of mechanical and tribological properties. For example, most of them attain hardness approaching the superhard level (i.e. 35-40 GPa). Among the mechanical properties, hardness is a convenient index for evaluating the performance of the thin film. Some conflict trends between processing parameters and the hardness of thin film were reported from time to time [1–3]. A golden colored film, which is a characteristic of IV a group nitrides, was also obtained in the TiZrN thin films. Both TiN and ZrN crystallize in the cubic densest sphere packing, this is also the structure of the TiZrN. According to Vegard’s rule, the lattice parameter of the other mixed phase is, in proportion to the amounts of the alloying components, between the lattice parameters of the single phases. Studies of TiZrN coatings were deposited using various methods and show good adhesion and hardness [4].
This increased hardness shown by TiZrN coatings is probably due to a solid solution strengthening mechanism which provides an energy barrier to the movement of dislocations throughout the crystals by distortion of the periodic lattice. Preliminary studies on the effects of zirconium implantation have shown improved wear resistance of TiN coatings [5,6]. In the study, the TiZrN film deposition is carried out using a magnetron sputter system. There are several process parameters in the system such as substrate temperature, power of radio frequency, the position of the substrate, and the nitrogen flow rate affecting the properties and characterization of the deposited thin film. Ion bombardment is the main parameter to affect the characterization of TiZrN thin film. However, the different substrate bias on the TiZrN system has never been studied so far. This purpose of this paper is to study the Influence of ion bombardment on characterization of TiZrN deposited by sputtering system. 2. Experimental details
∗ Corresponding author. Tel.: +886 03 5779911 642. E-mail address: [email protected] (Y.-W. Lin). http://dx.doi.org/10.1016/j.apsusc.2015.02.190 0169-4332/© 2015 Elsevier B.V. All rights reserved.
In this research, TiZrN thin films were deposited on P-type (1 0 0) silicon wafers and AISI 304 stainless steel substrate by magnetron
156
Y.-W. Lin et al. / Applied Surface Science 354 (2015) 155–160
Table 1 Summary of the experimental results various negative bias.
20 40 60 80 100 120
Thickness (nm) 1827 1628 1461 1147 805 588
Resistivity (micro-Ohm cm)
Roughness (nm)
Hardness (GPa)
Grain size (nm)
Packing Factor
152.1 77.9 66.6 89.4 138.7 154.9
2.4 2.1 1.9 2.0 2.1 2.5
30.2 35.7 37.8 36.1 32.2 31.1
17.8 13.6 10.6 11.2 10.9 10.1
0.83 0.95 1 0.92 0.83 0.81
sputtering system. The sputtering system consists of two 4-inch sputtering guns, the deposition of TiZrN thin films was carried out by co-sputtering of Ti and Zr targets. The coating temperature was maintained at 400 ◦ C, which was monitored by placing a thermocouple on the back side of the heater during deposition, and the substrate temperature was calibrated by another thermocouple attaching on the specimen. The negative bias was applied onto the substrate which was rotated at a speed of 20 rpm during deposition process. TiZrN films were prepared in Ar + N2 plasma operated at a pressure of 6.7 × 10−1 Pa. In the first place, the TiZrN films were prepared from dual gun magnetron sputtering system of Ti target and Zr target, the ratios of Zr/(Zr + Ti) were dominating by changing Ti and Zr target power, and found out the optimum composition with desired properties. Next, the specimen S1∼S6 were based on the specimen that the ratio of Zr/(Zr + Ti) is close to 0.5 and deposited by radio frequency magnetron sputtering on various negative bias. The negative bias voltage ranging from −20 V to −120 V was applied to the substrate. The deposition time was 2 h. The coating conditions are listed in Table 1
3. Results Thin films of (Ti,Zr)N were deposited by radio frequency magnetron sputtering based on the previous optimum coating conditions (B10) with various negative bias. The negative bias voltage ranging from −20 V to −130 V was applied to the substrate. The increase of negative bias voltage applied during film deposition will concomitantly increase the energy and momentum of the incoming, bombarding ions. As such, the structure and the property of the growing film are altered, accordingly. Ar ions, Zr, Ti and N atoms are the main particle species present in the sputtering process. The ionization ratios of Zr, Ti and N are believed to be rather low in the magnetron plasma in discussion. As such, Zr, Ti and N are mostly present in atomic forms; however, argon exists virtually in ionic form. The negative substrate bias exerts no effect upon atomic species, Zr, Ti and while having a great influence on Ar ions. Both energy and momentum of Ar ions increase with the increasing negative substrate bias. Energy of the impinging Ar ions will increase the energy of surface adatoms and thus would facilitate their diffusion to more energy stable sites on the substrate surface. However, the highly energized plasma particles may also induce adverse effects, such as irradiation damage, re-sputtering effect, and lattice defects. The relationship between substrate bias voltage and ion current density is shown in Fig. 1. The ion current density increases rapidly as substrate bias is lower than −60 V, then slightly increases as the critical value about −60 V is exceeded. At the substrate bias of −60 V, the ion current density will be saturation and close to 0.56 mA/cm2 on the substrate surface. As substrate bias over −60 V, the ion current density does not increase. The ion current density is a major parameter in the study. Table 1 summarizes the experimental parameters employed for the film deposition in the present investigation along with the results of characterization for the films obtained.
2 ion current density (mA/cm )
Negative Bias (−V)
S1 S2 S3 S4 S5 S6
0.6 0.5 0.4 0.3 0.2 0.1 20
40
60 80 100 negative bias (-V)
120
140
Fig. 1. Substrate ion current density vs. bias voltage.
3.1. Microstructure of (Ti,Zr)N films with negative substrate bias The film thickness measured by SEM ranges from 588 to 1827 nm. Fig. 2 shows that the film thickness decreases with increasing substrate bias at a constant deposition time. The atomic packing factor and composition of the film were determined by RBS analysis. The RBS result of sample S1 with bias −20 V is shown in Fig. 3. The vertical axis is the normalized yield, which represents the number of scattering particle. The horizontal axis is the channel or energy. The atomic packing factor was calculated with a simulation code. The limitations of RBS have to be considered. The resolution of RBS can be lowered to several hundred Å of film. However, this method has a maximum detecting limit of 1 m, because the curve of Zr will overlap with that of Si substrate. Furthermore, there is no physical significance to simulate under 0.5 MeV as shown in Fig. 3. The reason is that the scattering cross section is inverse proportional to the square of energy, and 2000 1800 1600
Thickness (nm)
Specimen Nos.
1400 1200 1000 800 600 400 20
40
60
80
100
120
Negative Bias (-V) Fig. 2. The variations of the thickness with different substrate bias.
Y.-W. Lin et al. / Applied Surface Science 354 (2015) 155–160
157
Fig. 3. RBS result of sample S1 with bias −20 V.
the scattering cross section is proportional to the number of scattering particle. Thus, it can be seen that the result of normalized yield is steeply high at low channel or scatter energy. Fig. 4 shows that the packing factor of (Ti,Zr)N films relate with the substrate bias, there is a increase of the packing factor from 0.83 to 1 with low substrate bias ranging from −20 V to −60 V. The packing factors gradually decrease as substrate bias over −60 V. This indicates that the variation of substrate bias effectively influence atomic packing in growing films. The gradual increase of packing factor with increasing bias may be attributed to that the increase of ion bombardment may facilitate the adatoms to migrate to the equilibrium lattice sites and increases film crystallinity with fewer defects. However, a high bias ion beam may induce radiation damage to the growing film and produce lattice defects. With increasing number of defects, the packing factor consequently decreases. The preferred orientation may affect the properties of (Ti,Zr)N film; therefore, it is important to study the variation of preferred orientation with different deposition conditions. A few theories and hypotheses have been proposed to explain the formation of preferred orientation of thin films coated by PVD techniques. However, due to the variation of processing parameters with different PVD techniques, it is difficult to verify the significance of each specific parameter. Fig. 5 is the X-ray diffraction patterns of sample S1∼S6 with (Ti,Zr)N coated on silicon substrate, scanning from 2Â = 30◦ to 65◦ . Beside S1 with bias −20 V, (1 1 1) peak appears for all specimens,
Fig. 5. The X-ray diffraction patterns of sample S1–S6.
which indicate that the specimens have strongly (1 1 1) preferred orientation. Theoretically, (2 2 0) is the surface with lower energy in (Ti,Zr)N and all NaCl-type metal nitrides. For the film exhibiting a (1 1 1) preferred orientation, kinetic consideration must be taken into account. Compared with our previous work on TiN, ZrN has a stronger inclination to (1 1 1) preferred orientation than that of TiN. This is consistent with the results reported by Ensinger. Using IBAD to produce nitrides of the 4th group of transition metals, they pointed out that the tendency to (1 1 1) orientation is dependent on the atomic mass of the metal in the NaCl-type metal nitride. Therefore, it is more difficult to induce (2 0 0) growth by ion bombardment for heavy than for the lighter elements. Nanostructure and multiphase compositions lead to a greater flexibility in tailoring properties, enhanced mechanical performance [7,8], and the possibility to develop self-adaptive coatings for tribological application [9]. The grain size as shown as Fig. 6 of the (Ti,Zr)N thin films can be obtained from the FWHM of (1 1 1) or (2 0 0) peaks using Scherrer equation. The results are also listed in Table 1. The grain sizes of the deposited films are 10–18 nm. Fig. 7 is a plot of FWHM of (1 1 1) peak vs. bias, which shows that the FWHM of (1 1 1) peak decreases with the increasing bias. It indicates that the increase of bias can improve the crystalline quality of the (Ti,Zr)N films. This is one of the major advantages of ion
18 1.00
17 16
Grain size (nm)
Packing factor
0.95
0.90
0.85
15 14 13 12 11 10
0.80
9 20
40
60
80
100
120
Negative Bias (-V) Fig. 4. The packing factor of (Ti,Zr)N films relate with the substrate bias.
20
40
60
80
Negative Bias (-V) Fig. 6. Grain size vs. bias.
100
120
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Y.-W. Lin et al. / Applied Surface Science 354 (2015) 155–160
0.68 160
0.66 140
Resistivity (0-cm)
0.64
FWHM
0.62 0.60 0.58
120
100
80
0.56 60
0.54 20
20
40
60
80
100
40
120
60
80
100
120
Negative Bias (-V)
Negative Bias (-V) Fig. 9. The variation of resistivity of (Ti,Zr)N films with different substrate bias. Fig. 7. A plot of FWHM of (1 1 1) peak vs. bias.
bombardment. The quality of deposited films is frequently enhanced by the bombardment of high-energy ions or neutral atoms and molecules. The ion bombardment is correlated to the momentum transfer or the energy deposited in a certain volume on the surface. This momentum transfer or energy deposition can facilitate the adatoms movement on the surface to the equilibrium lattice site and therefore enhance the crystallinity of the film. The surface roughness and grain size of TiZrN films coated were investigated by AFM. The surface roughness of TiZrN film was evaluated by the software equipped on AFM. Table 1 lists the results of roughness for samples S1 to S6. It is found that each sample has different roughness. The mechanism to affect the surface roughness can be related to the impingement of bias on the surface. As shown in Fig. 8, the roughness decreases from 1.92 nm to 0.68 nm with low substrate bias ranging from −20 V to −60 V. The roughness gradually increases as substrate bias over −60 V. 3.2. Properties of TiZrN films with negative substrate bias The resistivity is related to the packing density and lattice defects in the thin film [10–12]. The population of lattice defects may be associated with the direction of ion incidence [13], deposition temperature [14], bias voltage [15], and deposition rate [16].
Table 1 shows the results of the resistivity and the packing factor of samples from S1 to S6. Fig. 9 displays the variation of electric resistivity of (Ti,Zr)N films with different substrate bias. From this figure, it can be seen that the resistivity decreases (from 152.1 to 66.6 cm) with increasing negative substrate bias from −20 V to −60 V. At high substrate bias of −60 V to −120 V, the value of resistivity rapidly increases to 154.9 cm. The results clearly indicate that the resistivity of (Ti,Zr)N thin film is sensitive to the substrate bias. The resistivity versus packing factor is shown in Fig. 10. It is found that the highest packing factor corresponds to the lowest resistivity. Consequently, the resistivity is related to the lattice defects in the (Ti,Zr)N film. Therefore, the resistivity can be used as an index to evaluate the quality of the (Ti,Zr)N film. Proper ion bombardment may enhance the mobility of the adatoms to stable site on the substrate surface. It will decrease the resistivity and increase the crystalline quality. However, a high bias ion beam may induce radiation damage to the growing film and produce lattice defects. The resistivity consequently increases with increasing number of defects. Hardness of the (Ti,Zr)N films was measured with a nanoindenter. Each reported hardness value was the average of 10 indentations. Fig. 11 shows the variation of the film hardness with substrate bias. The results show that the hardness is affected by
2.5
160
2.4
2.3
Resistivily
Roughness (nm)
140
2.2
2.1
120
100
80
2.0
60
1.9 20
40
60
80
100
120
Negative Bias (-V) Fig. 8. The roughness vs. substrate bias ranging from −20 V to −120 V.
0.80
0.85
0.90
0.95
1.00
Packing Factor Fig. 10. The lowest resistivity corresponds to the highest packing factor.
Y.-W. Lin et al. / Applied Surface Science 354 (2015) 155–160
40 38 36
Hardness (nm)
34 32 30 28 26 20
40
60
80
100
120
Negative Bias (-V) Fig. 11. The variation of the film hardness with substrate bias.
the substrate bias. Fig. 12 shows the film hardness with respect to packing factor. It can be seen that the hardness of (Ti,Zr)N films is sensitive to packing factor. 4. Discussion Nanotechnology has entered the thin film technology sector for quite a while, resulting in numerous research projects dealing with nanostructured coatings. There are two fundamental processes for controlling the grain size [17,18]: (1) mixing process and (2) lowenergy ion bombardment. The mixing process is based on the addition of one or several elements to the material of matrix. As at least two elements are present in the thin film. The mixing process increasing nucleation sites and restraining grain growth is an efficient method convenient for production of nanograin size films in the (Ti,Zr)N. In this study, the deposition of (Ti,Zr)N thin films was carried out by co-sputtering of Ti and Zr targets with variation power, the mixing process succeeded to prepare the sample of nano-grain size to 17 nm. Moreover, the specimen S1∼S6 were based on the specimen that the ratio of Zr/(Zr+ Ti) is close to 0.5. The negative bias voltage ranging from −20 V to −130 V was applied to the substrate, the ion bombardment during sputtering process succeeded to prepare the sample of nano-grain size of 10 nm.
Hard coatings prepared by various deposition techniques and conditions exhibit the widest variety of microstructures among materials in terms of phase composition, grain size, crystallographic orientation, lattice defects, and surface morphology. Depending on the size of the alloying element, a substitutional solid solution or an interstitial solid solution can form. In our cases, (Ti,Zr)N which accords to the Hume-Rothery rulesthe is proposed substitutional solid solution. In the part two of this study on substrate bias experiment, the research showed that preferred orientation of texture is an important factor in film hardness. Beside S1 with bias −20 V, (1 1 1) peak appears for S2∼S6 specimens, which indicate that the specimens have strongly (1 1 1) preferred orientation, and all specimens have high hardness greater than 30 GPa. For the transition metal nitrides with NaCl structure such as TiN, ZrN and CrN, the primary slip system is {1 1 0}<1 1 0> which has the smallest amount of displacement to restore the structure. Owing to the geometrical strengthening, the resolved stress on the slip system ({1 1 0}<1 1 0 > ) is a function of the direction of the applied force. An indentation at (1 1 1) plane has no resolved stress on the primary slip system, and hence (1 1 1) plane should be harder than other planes that have resolved stress on the slip systems. The Ti0.5Zr0.5N films of solid solution show excellent hardness of 37.8 GPa with exhibiting (1 1 1) preferred orientation. However, low energy ion bombardment is an effective method to promote the properties of (Ti,Zr)N thin film. The fact that the increase of packing factor of the films deposited with the increasing bias voltage maybe attributable to the increasing impact energy associated with bombarding ion flux, which may facilitate migration of adatoms to their equilibrium lattice sites, hence resulting in increased film’s denseness. Meantime, the amount of lattice defects generated by bombarding ions under negative bias voltages over −60 V can become significant, and thus is believed to be responsible for the abrupt increase of (Ti,Zr)N films’ resistivity. This offers the rationale for understanding the various film’s properties observed, i.e., surface roughness, resistivity and hardness. A suitable increase of ion bombardment may facilitate the adatoms to migrate to the equilibrium lattice sites and increases film crystallinity and packing factor. However, the ion-induced defects are believed to be the major factor contributing to the abrupt increase of resistivity over the negative substrate bias of −60 V. The hardness of nanocrystalline ZrN films is from 30 GPa to 38 GPa. Hardness may contribute from grain boundary strength, and increases with the increasing packing factor.
40
5. Conclusions
38
Nano-crystalline (Ti,Zr)N thin films were successfully produced using dual guns with Ti and Zr targets in magnetron sputtering. The negative bias voltage ranging from −20 V to −120 V was applied to the substrate. At the substrate bias of −60 V, the ion current density will be saturation and close to 0.56 mA/cm2 on the substrate surface. A suitable increase of ion bombardment may facilitate the adatoms to migrate to the equilibrium lattice sites and increases film crystallinity and packing factor. However, the highly energized plasma particles may also induce adverse effects, such as irradiation damage, re-sputtering effect, and lattice defects. The main hardening mechanisms of (Ti,Zr)N in the study are solid solution strengthening, nano-grain size effect, partial dislocation effect and crystallographic orientation. An indentation at (1 1 1) plane has no resolved stress on the primary slip system, the Ti0.5 Zr0.5 N films of solid solution show excellent hardness of 37.8 GPa with exhibiting (1 1 1) preferred orientation. The resistivity of thin film is inverse proportional to the packing density, the hardness of thin film is proportional to the packing density.
36 34
Hardness
159
32 30 28 26 60
80
100
120
140
Resistivity Fig. 12. The film hardness with respect to packing factor.
160
160
Y.-W. Lin et al. / Applied Surface Science 354 (2015) 155–160
References [1] [2] [3] [4] [5] [6] [7] [8]
P. Patsalas, C. Charitidis, S. Logothetidis, Surf. Coat. Technol. 125 (2000) 335. L. Combadiere, J. Machet, Surf. Coat. Technol. 88 (1996) 28. L. Karlsson, L. Hultman, J.E. Sundgren, Thin Solid Films 371 (2000) 167. L.A. Donohue, J. Cawley, J.S. Brooks, W.D. Munz, Surf. Coat. Technol. 74–75 (1995) 123. K.P. Purushotham, L.P. Ward, B. Narelle, P.J. Pigram, P. Evans, H. Noorman, R.R. Manory, Wear 254 (2003) 589. E. Lugscheider, O. Knotek, C. Barimani, T. Leyendecker, O. Lemmer, R. Wenke, Surf. Coat. Technol. 112 (1999) 146. M.A. Baker, P.J. Kench, C. Tsotsos, P.N. Gibson, A. Leyland, A. Matthews, J. Vac. Sci. Technol. A 23 (2005) 423. Y.T. Pei, D. Galvan, J.T.M. De Hosson, A. Cavaleiro, Surf. Coat. Technol. 198 (2005) 44.
[9] A.A. Voevodin, T.A. Fitz, J.J. Hu, J.S. Zabinski, J. Vac. Sci. Technol. A 20 (2002) 1434. [10] M. Kawamura, Y. Abe, H. Yanagisawa, K. Sasaki, Thin Solid Films 287 (1996) 115. [11] J. Stimmel, J. Vac. Sci. Technol. B 4 (6) (1986) 1377. [12] J.E. Sundgren, B.O. Johansson, S.E. Karlsson, H.T.G. Hentzell, Thin Solid Films 105 (1983) 367. [13] S. Horita, H. Akahori, M. Kobayashi, J. Vac. Sci. Technol. A 14 (1) (1996) 203. [14] S. Horita, M. Kobayashi, H. Akahori, T. Hata, Surf. Coat. Technol. 66 (1994) 318. [15] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [16] M. Yoshitake, T. Yotsuya, S. Ogawa, Jpn. J. Appl. Phys. 31 (1992) 4002. [17] J. Musil, J. Vlˇcek, Czech. J. Phys. 48 (10) (1998) 1209. [18] J. Musil, J. Vlˇcek, Thin Solid Films 343–344 (1999) 47.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of ion bombardment on structure and properties of TiZrN thin film Yu-Wei Lin a,∗ , Jia-Hong Huang b , Ge-Ping Yu b , Chien-Nan Hsiao a , Fong-Zhi Chen a a b
Instrument Technology Research Center, National Applied Research Laboratories Department of Engineering and System Science National Tsing Hua University
a r t i c l e
i n f o
Article history: Received 3 October 2014 Received in revised form 26 February 2015 Accepted 27 February 2015 Available online 30 March 2015 Keywords: Ion bombardment TiZrN Hardness
a b s t r a c t The study is focused on the characterization of TiZrN thin film by controlling the behavior of ion bombardment. Thin films are grown using radio frequency magnetron sputtering process on Si wafer. The negative bias voltage ranging from −20 V to −130 V was applied to the substrate. The ion current density increases rapidly as substrate bias is lower than −60 V, then slightly increases as the critical value about −60 V is exceeded. At the substrate bias of −60 V, the ion current density is close to 0.56 mA/cm2 . The resistivity measured by four-point probe decreases from conditions −20 V to −60 V and then increases for substrate bias increases from −60 V to −130 V. The resistivity of TiZrN films is contributed from the packing factor. The N/TiZr ratios about 1 were measured by Rutherford backscattering spectrometer, and the packing factors of TiZrN films can also be obtained by the results of RBS. Field Emission scanning electron microscope (FEG-SEM) is used to characterize the thickness and structure of the deposited TiZrN film. X-ray diffraction (XRD) is used to determine the preferred orientation and lattice parameter. The precursor results of XRD show that all the coating samples exhibited (1 1 1) preferred orientation, and the hardness values of TiZrN films were ranging from 20 to 40 GPa. To sum up the precursor studies, the TiZrN films which can improve the properties from TiN and ZrN is a new ceramic material with higher potential. Following the advance process and analysis research, the structure and properties can be correlated and as a reference for industry application. © 2015 Elsevier B.V. All rights reserved.
1. Introduction TiN was used as the base for many coatings alloyed with other elements. These coatings exhibit a variety of mechanical and tribological properties. For example, most of them attain hardness approaching the superhard level (i.e. 35-40 GPa). Among the mechanical properties, hardness is a convenient index for evaluating the performance of the thin film. Some conflict trends between processing parameters and the hardness of thin film were reported from time to time [1–3]. A golden colored film, which is a characteristic of IV a group nitrides, was also obtained in the TiZrN thin films. Both TiN and ZrN crystallize in the cubic densest sphere packing, this is also the structure of the TiZrN. According to Vegard’s rule, the lattice parameter of the other mixed phase is, in proportion to the amounts of the alloying components, between the lattice parameters of the single phases. Studies of TiZrN coatings were deposited using various methods and show good adhesion and hardness [4].
This increased hardness shown by TiZrN coatings is probably due to a solid solution strengthening mechanism which provides an energy barrier to the movement of dislocations throughout the crystals by distortion of the periodic lattice. Preliminary studies on the effects of zirconium implantation have shown improved wear resistance of TiN coatings [5,6]. In the study, the TiZrN film deposition is carried out using a magnetron sputter system. There are several process parameters in the system such as substrate temperature, power of radio frequency, the position of the substrate, and the nitrogen flow rate affecting the properties and characterization of the deposited thin film. Ion bombardment is the main parameter to affect the characterization of TiZrN thin film. However, the different substrate bias on the TiZrN system has never been studied so far. This purpose of this paper is to study the Influence of ion bombardment on characterization of TiZrN deposited by sputtering system. 2. Experimental details
∗ Corresponding author. Tel.: +886 03 5779911 642. E-mail address: [email protected] (Y.-W. Lin). http://dx.doi.org/10.1016/j.apsusc.2015.02.190 0169-4332/© 2015 Elsevier B.V. All rights reserved.
In this research, TiZrN thin films were deposited on P-type (1 0 0) silicon wafers and AISI 304 stainless steel substrate by magnetron
156
Y.-W. Lin et al. / Applied Surface Science 354 (2015) 155–160
Table 1 Summary of the experimental results various negative bias.
20 40 60 80 100 120
Thickness (nm) 1827 1628 1461 1147 805 588
Resistivity (micro-Ohm cm)
Roughness (nm)
Hardness (GPa)
Grain size (nm)
Packing Factor
152.1 77.9 66.6 89.4 138.7 154.9
2.4 2.1 1.9 2.0 2.1 2.5
30.2 35.7 37.8 36.1 32.2 31.1
17.8 13.6 10.6 11.2 10.9 10.1
0.83 0.95 1 0.92 0.83 0.81
sputtering system. The sputtering system consists of two 4-inch sputtering guns, the deposition of TiZrN thin films was carried out by co-sputtering of Ti and Zr targets. The coating temperature was maintained at 400 ◦ C, which was monitored by placing a thermocouple on the back side of the heater during deposition, and the substrate temperature was calibrated by another thermocouple attaching on the specimen. The negative bias was applied onto the substrate which was rotated at a speed of 20 rpm during deposition process. TiZrN films were prepared in Ar + N2 plasma operated at a pressure of 6.7 × 10−1 Pa. In the first place, the TiZrN films were prepared from dual gun magnetron sputtering system of Ti target and Zr target, the ratios of Zr/(Zr + Ti) were dominating by changing Ti and Zr target power, and found out the optimum composition with desired properties. Next, the specimen S1∼S6 were based on the specimen that the ratio of Zr/(Zr + Ti) is close to 0.5 and deposited by radio frequency magnetron sputtering on various negative bias. The negative bias voltage ranging from −20 V to −120 V was applied to the substrate. The deposition time was 2 h. The coating conditions are listed in Table 1
3. Results Thin films of (Ti,Zr)N were deposited by radio frequency magnetron sputtering based on the previous optimum coating conditions (B10) with various negative bias. The negative bias voltage ranging from −20 V to −130 V was applied to the substrate. The increase of negative bias voltage applied during film deposition will concomitantly increase the energy and momentum of the incoming, bombarding ions. As such, the structure and the property of the growing film are altered, accordingly. Ar ions, Zr, Ti and N atoms are the main particle species present in the sputtering process. The ionization ratios of Zr, Ti and N are believed to be rather low in the magnetron plasma in discussion. As such, Zr, Ti and N are mostly present in atomic forms; however, argon exists virtually in ionic form. The negative substrate bias exerts no effect upon atomic species, Zr, Ti and while having a great influence on Ar ions. Both energy and momentum of Ar ions increase with the increasing negative substrate bias. Energy of the impinging Ar ions will increase the energy of surface adatoms and thus would facilitate their diffusion to more energy stable sites on the substrate surface. However, the highly energized plasma particles may also induce adverse effects, such as irradiation damage, re-sputtering effect, and lattice defects. The relationship between substrate bias voltage and ion current density is shown in Fig. 1. The ion current density increases rapidly as substrate bias is lower than −60 V, then slightly increases as the critical value about −60 V is exceeded. At the substrate bias of −60 V, the ion current density will be saturation and close to 0.56 mA/cm2 on the substrate surface. As substrate bias over −60 V, the ion current density does not increase. The ion current density is a major parameter in the study. Table 1 summarizes the experimental parameters employed for the film deposition in the present investigation along with the results of characterization for the films obtained.
2 ion current density (mA/cm )
Negative Bias (−V)
S1 S2 S3 S4 S5 S6
0.6 0.5 0.4 0.3 0.2 0.1 20
40
60 80 100 negative bias (-V)
120
140
Fig. 1. Substrate ion current density vs. bias voltage.
3.1. Microstructure of (Ti,Zr)N films with negative substrate bias The film thickness measured by SEM ranges from 588 to 1827 nm. Fig. 2 shows that the film thickness decreases with increasing substrate bias at a constant deposition time. The atomic packing factor and composition of the film were determined by RBS analysis. The RBS result of sample S1 with bias −20 V is shown in Fig. 3. The vertical axis is the normalized yield, which represents the number of scattering particle. The horizontal axis is the channel or energy. The atomic packing factor was calculated with a simulation code. The limitations of RBS have to be considered. The resolution of RBS can be lowered to several hundred Å of film. However, this method has a maximum detecting limit of 1 m, because the curve of Zr will overlap with that of Si substrate. Furthermore, there is no physical significance to simulate under 0.5 MeV as shown in Fig. 3. The reason is that the scattering cross section is inverse proportional to the square of energy, and 2000 1800 1600
Thickness (nm)
Specimen Nos.
1400 1200 1000 800 600 400 20
40
60
80
100
120
Negative Bias (-V) Fig. 2. The variations of the thickness with different substrate bias.
Y.-W. Lin et al. / Applied Surface Science 354 (2015) 155–160
157
Fig. 3. RBS result of sample S1 with bias −20 V.
the scattering cross section is proportional to the number of scattering particle. Thus, it can be seen that the result of normalized yield is steeply high at low channel or scatter energy. Fig. 4 shows that the packing factor of (Ti,Zr)N films relate with the substrate bias, there is a increase of the packing factor from 0.83 to 1 with low substrate bias ranging from −20 V to −60 V. The packing factors gradually decrease as substrate bias over −60 V. This indicates that the variation of substrate bias effectively influence atomic packing in growing films. The gradual increase of packing factor with increasing bias may be attributed to that the increase of ion bombardment may facilitate the adatoms to migrate to the equilibrium lattice sites and increases film crystallinity with fewer defects. However, a high bias ion beam may induce radiation damage to the growing film and produce lattice defects. With increasing number of defects, the packing factor consequently decreases. The preferred orientation may affect the properties of (Ti,Zr)N film; therefore, it is important to study the variation of preferred orientation with different deposition conditions. A few theories and hypotheses have been proposed to explain the formation of preferred orientation of thin films coated by PVD techniques. However, due to the variation of processing parameters with different PVD techniques, it is difficult to verify the significance of each specific parameter. Fig. 5 is the X-ray diffraction patterns of sample S1∼S6 with (Ti,Zr)N coated on silicon substrate, scanning from 2Â = 30◦ to 65◦ . Beside S1 with bias −20 V, (1 1 1) peak appears for all specimens,
Fig. 5. The X-ray diffraction patterns of sample S1–S6.
which indicate that the specimens have strongly (1 1 1) preferred orientation. Theoretically, (2 2 0) is the surface with lower energy in (Ti,Zr)N and all NaCl-type metal nitrides. For the film exhibiting a (1 1 1) preferred orientation, kinetic consideration must be taken into account. Compared with our previous work on TiN, ZrN has a stronger inclination to (1 1 1) preferred orientation than that of TiN. This is consistent with the results reported by Ensinger. Using IBAD to produce nitrides of the 4th group of transition metals, they pointed out that the tendency to (1 1 1) orientation is dependent on the atomic mass of the metal in the NaCl-type metal nitride. Therefore, it is more difficult to induce (2 0 0) growth by ion bombardment for heavy than for the lighter elements. Nanostructure and multiphase compositions lead to a greater flexibility in tailoring properties, enhanced mechanical performance [7,8], and the possibility to develop self-adaptive coatings for tribological application [9]. The grain size as shown as Fig. 6 of the (Ti,Zr)N thin films can be obtained from the FWHM of (1 1 1) or (2 0 0) peaks using Scherrer equation. The results are also listed in Table 1. The grain sizes of the deposited films are 10–18 nm. Fig. 7 is a plot of FWHM of (1 1 1) peak vs. bias, which shows that the FWHM of (1 1 1) peak decreases with the increasing bias. It indicates that the increase of bias can improve the crystalline quality of the (Ti,Zr)N films. This is one of the major advantages of ion
18 1.00
17 16
Grain size (nm)
Packing factor
0.95
0.90
0.85
15 14 13 12 11 10
0.80
9 20
40
60
80
100
120
Negative Bias (-V) Fig. 4. The packing factor of (Ti,Zr)N films relate with the substrate bias.
20
40
60
80
Negative Bias (-V) Fig. 6. Grain size vs. bias.
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Y.-W. Lin et al. / Applied Surface Science 354 (2015) 155–160
0.68 160
0.66 140
Resistivity (0-cm)
0.64
FWHM
0.62 0.60 0.58
120
100
80
0.56 60
0.54 20
20
40
60
80
100
40
120
60
80
100
120
Negative Bias (-V)
Negative Bias (-V) Fig. 9. The variation of resistivity of (Ti,Zr)N films with different substrate bias. Fig. 7. A plot of FWHM of (1 1 1) peak vs. bias.
bombardment. The quality of deposited films is frequently enhanced by the bombardment of high-energy ions or neutral atoms and molecules. The ion bombardment is correlated to the momentum transfer or the energy deposited in a certain volume on the surface. This momentum transfer or energy deposition can facilitate the adatoms movement on the surface to the equilibrium lattice site and therefore enhance the crystallinity of the film. The surface roughness and grain size of TiZrN films coated were investigated by AFM. The surface roughness of TiZrN film was evaluated by the software equipped on AFM. Table 1 lists the results of roughness for samples S1 to S6. It is found that each sample has different roughness. The mechanism to affect the surface roughness can be related to the impingement of bias on the surface. As shown in Fig. 8, the roughness decreases from 1.92 nm to 0.68 nm with low substrate bias ranging from −20 V to −60 V. The roughness gradually increases as substrate bias over −60 V. 3.2. Properties of TiZrN films with negative substrate bias The resistivity is related to the packing density and lattice defects in the thin film [10–12]. The population of lattice defects may be associated with the direction of ion incidence [13], deposition temperature [14], bias voltage [15], and deposition rate [16].
Table 1 shows the results of the resistivity and the packing factor of samples from S1 to S6. Fig. 9 displays the variation of electric resistivity of (Ti,Zr)N films with different substrate bias. From this figure, it can be seen that the resistivity decreases (from 152.1 to 66.6 cm) with increasing negative substrate bias from −20 V to −60 V. At high substrate bias of −60 V to −120 V, the value of resistivity rapidly increases to 154.9 cm. The results clearly indicate that the resistivity of (Ti,Zr)N thin film is sensitive to the substrate bias. The resistivity versus packing factor is shown in Fig. 10. It is found that the highest packing factor corresponds to the lowest resistivity. Consequently, the resistivity is related to the lattice defects in the (Ti,Zr)N film. Therefore, the resistivity can be used as an index to evaluate the quality of the (Ti,Zr)N film. Proper ion bombardment may enhance the mobility of the adatoms to stable site on the substrate surface. It will decrease the resistivity and increase the crystalline quality. However, a high bias ion beam may induce radiation damage to the growing film and produce lattice defects. The resistivity consequently increases with increasing number of defects. Hardness of the (Ti,Zr)N films was measured with a nanoindenter. Each reported hardness value was the average of 10 indentations. Fig. 11 shows the variation of the film hardness with substrate bias. The results show that the hardness is affected by
2.5
160
2.4
2.3
Resistivily
Roughness (nm)
140
2.2
2.1
120
100
80
2.0
60
1.9 20
40
60
80
100
120
Negative Bias (-V) Fig. 8. The roughness vs. substrate bias ranging from −20 V to −120 V.
0.80
0.85
0.90
0.95
1.00
Packing Factor Fig. 10. The lowest resistivity corresponds to the highest packing factor.
Y.-W. Lin et al. / Applied Surface Science 354 (2015) 155–160
40 38 36
Hardness (nm)
34 32 30 28 26 20
40
60
80
100
120
Negative Bias (-V) Fig. 11. The variation of the film hardness with substrate bias.
the substrate bias. Fig. 12 shows the film hardness with respect to packing factor. It can be seen that the hardness of (Ti,Zr)N films is sensitive to packing factor. 4. Discussion Nanotechnology has entered the thin film technology sector for quite a while, resulting in numerous research projects dealing with nanostructured coatings. There are two fundamental processes for controlling the grain size [17,18]: (1) mixing process and (2) lowenergy ion bombardment. The mixing process is based on the addition of one or several elements to the material of matrix. As at least two elements are present in the thin film. The mixing process increasing nucleation sites and restraining grain growth is an efficient method convenient for production of nanograin size films in the (Ti,Zr)N. In this study, the deposition of (Ti,Zr)N thin films was carried out by co-sputtering of Ti and Zr targets with variation power, the mixing process succeeded to prepare the sample of nano-grain size to 17 nm. Moreover, the specimen S1∼S6 were based on the specimen that the ratio of Zr/(Zr+ Ti) is close to 0.5. The negative bias voltage ranging from −20 V to −130 V was applied to the substrate, the ion bombardment during sputtering process succeeded to prepare the sample of nano-grain size of 10 nm.
Hard coatings prepared by various deposition techniques and conditions exhibit the widest variety of microstructures among materials in terms of phase composition, grain size, crystallographic orientation, lattice defects, and surface morphology. Depending on the size of the alloying element, a substitutional solid solution or an interstitial solid solution can form. In our cases, (Ti,Zr)N which accords to the Hume-Rothery rulesthe is proposed substitutional solid solution. In the part two of this study on substrate bias experiment, the research showed that preferred orientation of texture is an important factor in film hardness. Beside S1 with bias −20 V, (1 1 1) peak appears for S2∼S6 specimens, which indicate that the specimens have strongly (1 1 1) preferred orientation, and all specimens have high hardness greater than 30 GPa. For the transition metal nitrides with NaCl structure such as TiN, ZrN and CrN, the primary slip system is {1 1 0}<1 1 0> which has the smallest amount of displacement to restore the structure. Owing to the geometrical strengthening, the resolved stress on the slip system ({1 1 0}<1 1 0 > ) is a function of the direction of the applied force. An indentation at (1 1 1) plane has no resolved stress on the primary slip system, and hence (1 1 1) plane should be harder than other planes that have resolved stress on the slip systems. The Ti0.5Zr0.5N films of solid solution show excellent hardness of 37.8 GPa with exhibiting (1 1 1) preferred orientation. However, low energy ion bombardment is an effective method to promote the properties of (Ti,Zr)N thin film. The fact that the increase of packing factor of the films deposited with the increasing bias voltage maybe attributable to the increasing impact energy associated with bombarding ion flux, which may facilitate migration of adatoms to their equilibrium lattice sites, hence resulting in increased film’s denseness. Meantime, the amount of lattice defects generated by bombarding ions under negative bias voltages over −60 V can become significant, and thus is believed to be responsible for the abrupt increase of (Ti,Zr)N films’ resistivity. This offers the rationale for understanding the various film’s properties observed, i.e., surface roughness, resistivity and hardness. A suitable increase of ion bombardment may facilitate the adatoms to migrate to the equilibrium lattice sites and increases film crystallinity and packing factor. However, the ion-induced defects are believed to be the major factor contributing to the abrupt increase of resistivity over the negative substrate bias of −60 V. The hardness of nanocrystalline ZrN films is from 30 GPa to 38 GPa. Hardness may contribute from grain boundary strength, and increases with the increasing packing factor.
40
5. Conclusions
38
Nano-crystalline (Ti,Zr)N thin films were successfully produced using dual guns with Ti and Zr targets in magnetron sputtering. The negative bias voltage ranging from −20 V to −120 V was applied to the substrate. At the substrate bias of −60 V, the ion current density will be saturation and close to 0.56 mA/cm2 on the substrate surface. A suitable increase of ion bombardment may facilitate the adatoms to migrate to the equilibrium lattice sites and increases film crystallinity and packing factor. However, the highly energized plasma particles may also induce adverse effects, such as irradiation damage, re-sputtering effect, and lattice defects. The main hardening mechanisms of (Ti,Zr)N in the study are solid solution strengthening, nano-grain size effect, partial dislocation effect and crystallographic orientation. An indentation at (1 1 1) plane has no resolved stress on the primary slip system, the Ti0.5 Zr0.5 N films of solid solution show excellent hardness of 37.8 GPa with exhibiting (1 1 1) preferred orientation. The resistivity of thin film is inverse proportional to the packing density, the hardness of thin film is proportional to the packing density.
36 34
Hardness
159
32 30 28 26 60
80
100
120
140
Resistivity Fig. 12. The film hardness with respect to packing factor.
160
160
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