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Development of texture in TiN films deposited by filtered cathodic vacuum arc
Journal of Crystal Growth 252 (2003) 257–264
Development of texture in TiN films deposited by filtered cathodic vacuum arc Y.H. Chenga,b,*, B.K. Tayc a
State Key Laboratory of Plastic Forming Simulation and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China b State Key Laboratory of Laser Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China c School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Received 8 June 2002; accepted 9 January 2003 Communicated by R. S. Feigelson
Abstract X-ray diffraction was used to characterize the texture in TiN films deposited by an off-plane double bend filtered cathodic vacuum arc technique. The influence of deposition pressure, substrate bias, and deposition temperature on the texture of the films was studied systematically. The texture develops from a random orientation to a mixed (1 1 1) and (2 2 0) preferred orientation with increasing deposition pressure. As substrate bias is increased, the texture evolves from a (2 0 0) preferred orientation to a mixed (1 1 1) and (2 2 0) preferred orientation at a substrate bias of 100 V. However, further increase of substrate bias changes the texture back to the (2 0 0) preferred orientation again. The deposition temperature has no significant effect on the development of the film texture. Comparing the texture with the internal stress and surface roughness, it is observed directly that the texture in the TiN films is controlled by the internal stress and surface roughness. r 2003 Published by Elsevier Science B.V. PACS: 85.40.Ls; 81.15.Ef; 61.66. Fn; 61.10.Eq Keywords: A1. Texture; A1. X-ray diffraction; A3. Filtered cathodic vacuum arc; B1. TiN films
1. Introduction TiN films have been widely used in the tooling industry as a wear resistant coating due to the high *Corresponding author. Institut fur . Physik, Technische Universitat Chemnitz, D-09107 Chemnitz, Germany. Tel.: +49-371-531-3570; fax: +49-371-531-3042. E-mail address: yh [email protected] (Y.H. Cheng).
hardness and high wear resistance for many years [1,2]. Recently, TiN films were also used as an important diffusion barrier material for the design of very large-scale integrated (VLSI) and ultralarge-scale integrated (ULSI) devices due to its excellent diffusion barrier characteristics, very good electrical conductivity, and excellent adhesion/glue layer performance [3,4]. TiN films deposited by physical vapor deposition and
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chemical vapor deposition are often reported to exhibit a preferred orientation. The properties of TiN films are strongly dependent on the preferred orientation. It has been reported that the wear resistance of tool materials depends on the orientations of the TiN films, and the films with a (1 1 1) preferred orientation have the highest value of wear resistance [5]. Recently, it was reported that the texture of metal lines in interconnect thin film stacks in integrated circuits is greatly affected by the texture of the underlayer TiN films. Knorr [6] found that Al appears to inherit its texture from the TiN as TiNo1 1 1>/ Alo1 1 1> and TiNo3 1 1>/Alo3 1 1>. Tracy [7] observed that a strongly textured underlayer such as Tio0 0 2> or Tio0 0 2>/TiNo1 1 1> induces a similarly strong o1 1 1> texture in AlCu, and a nearly random texture in TiN significantly weakens the texture in subsequent metal films. Abe [8,9] also reported that the Cu (1 1 1) crystallographic orientation is enhanced on a TiN film having a strong (1 1 1) orientation as in the case of Al interconnects. Therefore, it is useful to study the development of the preferred orientation in the TiN films with deposition processes. The preferred orientation of TiN films is greatly affected by the deposition techniques and deposition processes. The dependence of preferred orientation of TiN films deposited by sputtering on the deposition processes has been widely studied. Kobayashi [10] observed a change of the preferred orientation from (2 0 0) to (1 1 1) and then to (2 2 0) with increasing bias voltage in a sputtering process. Jeong [11] reported a change of the preferred orientation from (1 1 1) to (2 0 0) by increasing N/Ti atomic ratio from 0.45 to 1.0. Jones [12] found that the TiN films deposited by RF reactive magnetron sputtering developed a strong (1 1 1) orientation, but the intensity of the (1 1 1) orientation diminished with increasing deposition pressure, as well as with increasing substrate temperature. Oh [13] found that the preferred orientation of the TiN films changed from the (2 0 0) plane through the (1 1 0) plane, and then finally to the (1 1 1) plane with the film thickness. Greene [14] found that the preferred orientation of the TiN films was controllably varied from (1 1 1) to completely (0 0 2) by varying
the incident ion/metal flux ratio with the N+ 2 ion energy maintained constant at 20 eV. Filtered cathodic vacuum arc (FCVA) technique, which employs electro-magnetic and mechanical filtering techniques to remove unwanted macro-particles and neutral atoms, can be used to deposit high quality films. The nature of the fully ionized plasma makes it possible to deposit conformal TiN films into deep trenches and vias with high aspect ratio [15,16]. However, only a few reports on the texture of the TiN films deposited by FCVA can be seen in the literature [17–20]. For the application of the TiN films as barrier layer in deep vias or contact holes in ULSI by using filtered cathodic vacuum arc technique, it is important to study the correlation between the deposition processes and the preferred orientation. In this study, TiN films were deposited by an off-plane double bend filtered cathodic vacuum arc technique. The texture of the films is identified by glazing angle X-ray diffraction (XRD). The influence of deposition pressure, substrate bias, and deposition temperature on the texture of the TiN films was studied systematically.
2. Experimental details TiN films were deposited by an off-plane double bend FCVA technique. The detailed deposition apparatus used in this work is published elsewhere [21]. A 99.999% pure Ti target with a diameter of 80 mm and thickness of 70 mm was used as cathode. High purity nitrogen gas, which was directly introduced into the deposition chamber, was used as working gas. The base pressure was 2.0 106 Torr. A curvilinear axial magnetic field (40 mT) was introduced in the system via an offplane, double-bend curved toroidal duct. The arc current was set at 145 A. The substrates used were (1 0 0) p-type silicon wafers with a thickness of 400725 mm. The substrates were heated using a quartz halogen lamp whose power was controlled by varying the input voltage. The substrate temperature was monitored by a thermocouple attached to the back of the substrate. During the deposition, the deposition pressure, substrate bias, and deposition temperature can be varied in the
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range 1 105–6 104 Torr, 0–500 V, and 50– 500 C, respectively. The film thickness was kept at 100–150 nm. The phase and crystal structure of the deposited films were identified by using an X-ray diffractometry (Rigaku, Japan) with a thin film attachment. ( was A Cu Ka X-ray source (wavelength of 1.54 A) used at 50 kV and 20 mA. The degree of preferred orientation for the coatings was determined, following subtraction of background radiation, by calculating the texture coefficient of the (h k l) plane, Thkl, defined as [12,22]. Thkl ¼
Im ðh k l Þ=I0 ðh k l Þ ; 1 Pn Im ðh k l Þ=I0 ðh k l Þ n 1
where Im (h k l ) is the measured relative intensity of the reflection from the (h k l ) plane, I0 (h k l ) is the relative intensity from the same plane in a standard reference sample (JCPDS 38-1420), and n is the total number of the reflection peaks from the coating. The value of the texture coefficient for the peaks under investigation ranges from unity for a randomly oriented sample, to n for a sample having a complete preferential orientation. The root mean square (RMS) surface roughness over an area of 1 1 mm2 of the TiN films was obtained using an atomic force microscopy (AFM) in the tapping mode (Dimension 3000 Scanning Probe Microscope from Digital Instrument). The internal stress in TiN films was determined by a substrate curvature method. The radius of curvature and the film thickness were measured by a surface profilometer (Tencor P10).
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deposition pressure are shown in Fig. 1. In this series of experiments, substrate bias and temperature remain constant at –100 V and 450 C, respectively. It can be seen that the texture coefficients of the three planes are greatly affected by the deposition pressure. For the films deposited at 1 104 Torr, the texture coefficients of the three peaks are all close to 1.0, indicating that the films have a random orientation. The increase of deposition pressure leads to a great decrease of T200 and T111, and an increase of T220. However at a deposition pressure above 2 104 Torr, the increase of deposition pressure results in a decrease of T220 and an slight increase of T111, but T200 remains almost unchanged. Due to the change in the texture coefficients of the three planes, the films deposited at a pressure above 2 104 Torr show a mixed (1 1 1) and (2 2 0) preferred orientation. This is different from Jone’s results [12], which showed that increasing deposition pressure result in a decrease in the texture coefficient of the (1 1 1) plane, and an increase in the texture coefficient of the (2 0 0) and (2 2 0) plane of the TiN films deposited by RF reactive magnetron sputtering. Fig. 2 shows the influence of deposition pressure on the internal stress and RMS surface roughness of the TiN films. The internal stress decreases slightly as the deposition pressure is increased, reaching a minimum at a pressure of 2 104 Torr, then increases gradually with further increases in deposition pressure. The surface roughness decreases greatly with increasing deposition pressure to 2 104 Torr, then remains
3. Results XRD results show that all films deposited at a pressure above 1 104 Torr are single-phase face centered cubic (FCC) type TiN films. Three peaks at 2y of 36.9 , 42.3 and 62.0 , which corresponds to the (1 1 1), (2 0 0), and (2 2 0) plane of cubic TiN, respectively, can be observed in all the XRD patterns. The calculated texture coefficients, T111, T200, and T220, of the (1 1 1), (2 0 0), and (2 2 0) plane, respectively, of the films as a function of the
Texture coefficient
2.0
1.5
T111
1.0
T200 T220
0.5
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Deposition pressure ( mTorr) Fig. 1. Dependence of the T111, T200, and T220 of the TiN films deposited at a substrate bias and temperature of 100 V and 450 C, respectively, on the deposition pressure.
5
0.9 0.8
4
0.7 3 0.6 2 1
Internal stress
0.5
RMS roughness
0.4
0
0.3 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Deposition pressure ( mTorr) Fig. 2. Dependence of the internal stress and RMS surface roughness of the TiN films deposited at a substrate bias and temperature of 100 V and 450 C, respectively, on the deposition pressure.
Texture coefficient
1.6
T111
1.2
T200 T220 0.8
0.4
0
100
200
300
400
500
600
Substrate bias (-V) Fig. 3. Influence of the substrate bias on the T111, T200, and T220 of the TiN films deposited at a deposition pressure and temperature of 2 104 Torr and 450 C, respectively.
almost constant at higher deposition pressure. Comparing Figs. 2 and 1, it is interesting to note that the variation trend of T111 and T200 with deposition pressure is similar to that of the internal stress and RMS surface roughness of the films, respectively. The influence of substrate bias on the T111, T200, and T220 of the TiN films is shown in Fig. 3. In this series of experiments, deposition pressure and substrate temperature remain constant at 1 104 Torr and 450 C, respectively. The preferred orientation of the films is strongly dependent on the substrate bias. As the substrate bias is increased, T200 decreases deeply, reaching a minimum at a bias of 100 V, then increases rapidly. The trends in T111 and T220 with substrate bias are opposite to that of T200, which show a rapid
increase with increasing substrate bias up to – 100 V, then decreases greatly. The dependence of the film texture on the substrate bias can be divided into three ranges. At a substrate bias below 50 V, TiN films exhibit a (2 0 0) preferred orientation. In the substrate bias range –50– 200 V, TiN films show a mixed (1 1 1) and (2 2 0) preferred orientation. At a substrate bias above –200 V, the preferred orientation changes to the (2 0 0) plane again. Our results are consistent with Hoang’s reports [23], which show a evolution of the preferred orientation in TiN films deposited by a helicon activated reactive evaporation system from the (2 0 0) to (1 1 1), then to (2 0 0) with increasing substrate bias, but is different from the reports of Zhao [18], who observed a change of the preferred orientation from the (1 1 1) to (2 2 0) plane with increasing substrate bias. The dependence of the internal stress and RMS surface roughness on the substrate bias is shown in Fig. 4. As the substrate bias is increased to 100 V, the internal stress increases to a maximum and the surface roughness decreases rapidly to a minimum. Further increase of substrate bias leads to a great decrease of the internal stress and a great increase of the surface roughness. It can also be noted by comparing Figs. 3 and 4 that the trends of the internal stress and surface roughness with substrate bias fit well with that of the T111 and T200 of the TiN films, respectively.
5.0
1.0
4.5
0.9
4.0 0.8 3.5 0.7 3.0 0.6
Internal stress
2.5
RMS roughness (nm)
1.0
Internal stress (GPa)
Internal stress (GPa)
6
RMS roughness (nm)
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RMS roughness 2.0 0
100
200
300
400
500
0.5 600
Substrate bias (-V) Fig. 4. Influence of the substrate bias on the internal stress and RMS surface roughness of the TiN films deposited at a deposition pressure and temperature of 2 104 Torr and 450 C, respectively.
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Texture coefficient
1.5
T 111
1.0
T 200 T 220 0.5
0.0 0
100
200
300
400
500
600
Deposition temperature (˚C) Fig. 5. Influence of the deposition temperature on the T111, T200, and T220 of the TiN films deposited at a deposition pressure and substrate bias of 2 104 Torr and 100 V, respectively.
.7 Internal stress RMS roughness
10
.6
9 8
.5 7 6
.4
5 4 0
100
200
300
400
RMS roughness (nm)
Internal stress (GPa)
11
.3 600
500
Deposition temperature (˚C) Fig. 6. Influence of the deposition temperature on the internal stress and RMS roughness of the TiN films deposited at a deposition pressure and substrate bias of 2 104 Torr and 100 V, respectively.
1.5
Texture coefficient
Fig. 5 shows the dependence of the T111, T200, and T220 of the TiN films on the deposition temperature. In this series of experiments, deposition pressure and substrate bias remain constant at 1 104 Torr and 100 V, respectively. All the TiN films show a mixed (1 1 1) and (2 2 0) preferred orientation. The influence of deposition temperature on the texture coefficients of the three planes is not so significant as that of the deposition pressure and substrate bias. At a deposition temperature below 100 C, the increase of deposition temperature results in a slight decrease of T111 and a slight increase of T200. At a higher deposition temperature, T200 remains almost constant, but T111 decreases further with increasing deposition temperature. There is no significant change for T220 in the whole temperature range. Fig. 6 shows the influence of deposition temperature on the internal stress and surface roughness of TiN films. The increase of deposition temperature results in a linear decrease of internal stress and a linear increase of surface roughness. Comparing Figs. 2, 4 and 6, it can be seen that the RMS surface roughness of all films in this series of experiments is very small and the surface roughness varies slightly with deposition temperature, while the variation of the internal stress in the TiN films is much greater than that in the TiN films studied in the other two series of experiments. It is interesting to note that the trends of T111 and T200 are similar to that of the internal stress and RMS surface roughness, respectively, in all
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1.0
T 111 T 200 T 220
0.5
0.0 2
4
6
8
10
12
Internal stress (GPa) Fig. 7. Dependence of the T111, T200, and T220 on the internal stress of the TiN films deposited at different deposition processes.
three series of experiments, which may indicate that the texture in the TiN films are correlated to the internal stress and surface roughness. In order to investigate the influence of the internal stress and surface roughness on the texture in the TiN films, the texture coefficients of the three planes were plotted as a function of the internal stress and RMS surface roughness, respectively. Fig. 7 shows the dependence of T111, T200, and T220 on the internal stress. At an internal stress below 4.5 GPa, the texture coefficients of the three planes are strongly dependent on the internal stress: T111 and T220 increase drastically, while T200 drastically decreases. Further increase of internal stress results in a slight increase of T111 and T220, and a slight decrease of T200. It is worth noting that the trend of T220 with internal stress is the same as that
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of T111. The development of the texture coefficient of the three planes with increasing internal stress results in the evolution of the texture from a (2 0 0) preferred orientation, to a random orientation at an internal stress of 3.5 GPa, then to a mixed (1 1 1) and (2 2 0) preferred orientation. Fig. 8 shows the dependence of the texture coefficient of the three planes on the RMS surface roughness. As the surface roughness is increased, a continuous decrease of T111 and T220, and increase of T200 can be observed, which leads to the change of the texture from a mixed (1 1 1) and (2 2 0) preferred orientation to a (2 0 0) preferred orientation. It could also be found that the trend of T220 with surface roughness is the same as that of T111.
4. Discussions Preferred orientation has been widely observed in the TiN films deposited by vapor deposition. Most explanations for the preferred orientation in the films were given in terms of strain energy, surface energy, and kinetic factors [14,24]. Pelleg et al. [24] suggested that the preferred orientation of the TiN thin films is caused by the driving force to lower the overall energy of the film which is composed of the surface energy and strain energy. According to the theoretical calculation, the degree of the surface energy of TiN crystal, Shkl, is S200oS110oS111, and the degree of the strain
Texture coefficient
1.5
1.0
0.5
T 111 T 200 T 220
0.0 0.2
0.4
0.6
0.8
1.0
1.2
RMS roughness (nm) Fig. 8. Dependence of the T111, T200, and T220 on the RMS surface roughness of TiN films deposited at different deposition processes.
energy per unit volume, uhkl, is u111ou110ou200. TiN films would grow toward the orientation of the (2 0 0) plane with the lowest surface energy when the surface energy is dominant, and toward the orientation of the (1 1 1) plane with the lowest strain energy when the strain energy is dominant. At intermediate stress levels, the (1 1 0) direction might be preferred in order to minimize the total energy (strain and surface energy combined). However, the kinetic factor theory suggested that the preferred orientation is determined by the growth velocity differences between crystal faces. Zhao [18] has used this theory to explain the development of the (2 2 0) texture in the TiN films deposited by cathodic arc at the high substrate bias. As shown in Figs. 7 and 8, the texture coefficients of the three planes are strongly dependent on the internal stress and RMS surface roughness, and the dependence of the texture coefficient of the (2 2 0) plane on the internal stress and RMS roughness is the same as that of the texture coefficient of the (1 1 1) plane. As we know, the internal compressive stresses in the films have been proposed to originate from the ‘‘atomic peening’’ mechanism [25–27]. The bombarding ions or atoms knock surface atoms deeper into the subsurface of the growing films and get themselves trapped in the layer. The associated additional volume in the constrained layer leads to an expansion of the film outwards from the substrate, but the film is not free to expand and the entrapped atoms cause macroscopic compressive stress. Therefore, the internal stress correlates to the strain energy in the deposited films. At the same time, the RMS surface roughness of the deposited films determines the surface area, and therefore the surface energy. As shown in Fig. 2, for the films deposited at a pressure below 2 104 Torr, the increase of deposition pressure results in a slight decrease of internal stress and a great decrease of RMS roughness, leading to a slight decrease of the strain energy and a great increase of the surface energy in the deposited films, respectively. According to the energy minimization model, in order to minimize the total energy of the system, the TiN films will develop to a crystallographic texture
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with slightly higher T111 and much lower T200. For the films deposited at a pressure of 2 104 Torr, both internal stress and surface roughness are the lowest, a texture with highest T220 may be preferred in order to minimize the total energy. Further increase of deposition pressure leads to a slight increase in the internal stress but no obvious change in the RMS roughness, which may contribute to the slight increase of T111 and the constant T200 with increasing deposition pressure. For the films deposited at a substrate bias of 0 V, the internal stress is lowest and the RMS roughness is highest. This indicates that the surface energy is dominant, which may result in a (2 0 0) preferred orientation growth. The increase of substrate bias leads to a great increase of the internal stress and decrease of the RMS roughness, indicating the increase of the strain energy and the decrease of the surface energy, respectively, which may result in a increase of T111 and decrease of T200, respectively. At a substrate bias above 100 V, the increase in bias results in a decrease of internal stress and an increase of RMS roughness, leading to a decrease of strain energy and an increase of surface energy, and therefore a decrease of T111 and an increase of T200. For the films deposited in the bias range 50–100 V, the low surface roughness and moderate internal stress may lead to the development of the texture to the highest T220. For the films deposited at room temperature, the internal stress is fairy high while the surface roughness is very low. In this case, the strain energy is dominant over the surface energy in the films. As a result, the films exhibit a mixed (1 1 1) and (2 2 0) preferred orientation with high T111, T220 and low T200. Fig. 6 shows that the increase of deposition temperature results in a linear decrease of internal stress and a linear increase of surface roughness, indicating a linear decrease of strain energy and a linear increase of surface energy, which leads to the decrease of T111 and the increase of T200 as shown in Fig. 5. However, it should be noted that the internal stress is much higher and the surface roughness is much lower in this series of films as compared with the films studied in the other two series. The strain energy is dominant in all this series of films. Therefore, all
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the films exhibit a mixed (1 1 1) and (2 2 0) preferred orientation, and the variation of the texture coefficient for all three planes is insignificant. In addition, it should be pointed out that the increase of deposition temperature might decrease the defect density in the deposited films, which might reduce the area of the inner surface of the defect, and therefore the surface energy. This may compensate the increase of the surface energy due to the increase of the surface roughness, which corresponds to the development of the texture with almost constant T200 for the films deposited at a higher temperature.
5. Conclusions The texture in the TiN films deposited by an offplane double bend FCVA is greatly affected by deposition pressure, substrate bias, and deposition temperature. As the deposition pressure is increased, T111 decreases slightly and T200 decreases greatly. Further increase of deposition pressure results in a slight increase of T111 but no significant change of T200, and the film texture develops from a random orientation to a mixed (1 1 1) and (2 2 0) preferred orientation. As the substrate bias is increased, T200 decreases greatly to a minimum and T111 increases greatly to a maximum at a substrate bias of 100 V, which leads to the evolvement of the film texture from a (2 0 0) preferred orientation to a mixed (1 1 1) and (2 2 0) preferred orientation. A further increase in substrate bias causes a rapid increase of T200 and decrease of T111, resulting in a change of the film texture to a (2 0 0) preferred orientation again. The films deposited at different temperature exhibit a mixed (1 1 1) and (2 2 0) preferred orientation growth, and the increase of deposition temperature results in a slight increase of T200 and decrease of T111. The variation trend of T111 and T200 with deposition pressure, substrate bias and deposition temperature is consistent with that of the internal stress and surface roughness, respectively, which may suggest that the internal stress and surface roughness are the main factors that affect the development of the texture in the TiN films.
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Acknowledgements The first author would like to express gratitude for the support of the State Key Laboratory of Laser Technology of China and the State Key Laboratory of Plastic Forming Simulation and Die & Mould Technology of China.
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Development of texture in TiN films deposited by filtered cathodic vacuum arc Y.H. Chenga,b,*, B.K. Tayc a
State Key Laboratory of Plastic Forming Simulation and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China b State Key Laboratory of Laser Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China c School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Received 8 June 2002; accepted 9 January 2003 Communicated by R. S. Feigelson
Abstract X-ray diffraction was used to characterize the texture in TiN films deposited by an off-plane double bend filtered cathodic vacuum arc technique. The influence of deposition pressure, substrate bias, and deposition temperature on the texture of the films was studied systematically. The texture develops from a random orientation to a mixed (1 1 1) and (2 2 0) preferred orientation with increasing deposition pressure. As substrate bias is increased, the texture evolves from a (2 0 0) preferred orientation to a mixed (1 1 1) and (2 2 0) preferred orientation at a substrate bias of 100 V. However, further increase of substrate bias changes the texture back to the (2 0 0) preferred orientation again. The deposition temperature has no significant effect on the development of the film texture. Comparing the texture with the internal stress and surface roughness, it is observed directly that the texture in the TiN films is controlled by the internal stress and surface roughness. r 2003 Published by Elsevier Science B.V. PACS: 85.40.Ls; 81.15.Ef; 61.66. Fn; 61.10.Eq Keywords: A1. Texture; A1. X-ray diffraction; A3. Filtered cathodic vacuum arc; B1. TiN films
1. Introduction TiN films have been widely used in the tooling industry as a wear resistant coating due to the high *Corresponding author. Institut fur . Physik, Technische Universitat Chemnitz, D-09107 Chemnitz, Germany. Tel.: +49-371-531-3570; fax: +49-371-531-3042. E-mail address: yh [email protected] (Y.H. Cheng).
hardness and high wear resistance for many years [1,2]. Recently, TiN films were also used as an important diffusion barrier material for the design of very large-scale integrated (VLSI) and ultralarge-scale integrated (ULSI) devices due to its excellent diffusion barrier characteristics, very good electrical conductivity, and excellent adhesion/glue layer performance [3,4]. TiN films deposited by physical vapor deposition and
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chemical vapor deposition are often reported to exhibit a preferred orientation. The properties of TiN films are strongly dependent on the preferred orientation. It has been reported that the wear resistance of tool materials depends on the orientations of the TiN films, and the films with a (1 1 1) preferred orientation have the highest value of wear resistance [5]. Recently, it was reported that the texture of metal lines in interconnect thin film stacks in integrated circuits is greatly affected by the texture of the underlayer TiN films. Knorr [6] found that Al appears to inherit its texture from the TiN as TiNo1 1 1>/ Alo1 1 1> and TiNo3 1 1>/Alo3 1 1>. Tracy [7] observed that a strongly textured underlayer such as Tio0 0 2> or Tio0 0 2>/TiNo1 1 1> induces a similarly strong o1 1 1> texture in AlCu, and a nearly random texture in TiN significantly weakens the texture in subsequent metal films. Abe [8,9] also reported that the Cu (1 1 1) crystallographic orientation is enhanced on a TiN film having a strong (1 1 1) orientation as in the case of Al interconnects. Therefore, it is useful to study the development of the preferred orientation in the TiN films with deposition processes. The preferred orientation of TiN films is greatly affected by the deposition techniques and deposition processes. The dependence of preferred orientation of TiN films deposited by sputtering on the deposition processes has been widely studied. Kobayashi [10] observed a change of the preferred orientation from (2 0 0) to (1 1 1) and then to (2 2 0) with increasing bias voltage in a sputtering process. Jeong [11] reported a change of the preferred orientation from (1 1 1) to (2 0 0) by increasing N/Ti atomic ratio from 0.45 to 1.0. Jones [12] found that the TiN films deposited by RF reactive magnetron sputtering developed a strong (1 1 1) orientation, but the intensity of the (1 1 1) orientation diminished with increasing deposition pressure, as well as with increasing substrate temperature. Oh [13] found that the preferred orientation of the TiN films changed from the (2 0 0) plane through the (1 1 0) plane, and then finally to the (1 1 1) plane with the film thickness. Greene [14] found that the preferred orientation of the TiN films was controllably varied from (1 1 1) to completely (0 0 2) by varying
the incident ion/metal flux ratio with the N+ 2 ion energy maintained constant at 20 eV. Filtered cathodic vacuum arc (FCVA) technique, which employs electro-magnetic and mechanical filtering techniques to remove unwanted macro-particles and neutral atoms, can be used to deposit high quality films. The nature of the fully ionized plasma makes it possible to deposit conformal TiN films into deep trenches and vias with high aspect ratio [15,16]. However, only a few reports on the texture of the TiN films deposited by FCVA can be seen in the literature [17–20]. For the application of the TiN films as barrier layer in deep vias or contact holes in ULSI by using filtered cathodic vacuum arc technique, it is important to study the correlation between the deposition processes and the preferred orientation. In this study, TiN films were deposited by an off-plane double bend filtered cathodic vacuum arc technique. The texture of the films is identified by glazing angle X-ray diffraction (XRD). The influence of deposition pressure, substrate bias, and deposition temperature on the texture of the TiN films was studied systematically.
2. Experimental details TiN films were deposited by an off-plane double bend FCVA technique. The detailed deposition apparatus used in this work is published elsewhere [21]. A 99.999% pure Ti target with a diameter of 80 mm and thickness of 70 mm was used as cathode. High purity nitrogen gas, which was directly introduced into the deposition chamber, was used as working gas. The base pressure was 2.0 106 Torr. A curvilinear axial magnetic field (40 mT) was introduced in the system via an offplane, double-bend curved toroidal duct. The arc current was set at 145 A. The substrates used were (1 0 0) p-type silicon wafers with a thickness of 400725 mm. The substrates were heated using a quartz halogen lamp whose power was controlled by varying the input voltage. The substrate temperature was monitored by a thermocouple attached to the back of the substrate. During the deposition, the deposition pressure, substrate bias, and deposition temperature can be varied in the
Y.H. Cheng, B.K. Tay / Journal of Crystal Growth 252 (2003) 257–264
range 1 105–6 104 Torr, 0–500 V, and 50– 500 C, respectively. The film thickness was kept at 100–150 nm. The phase and crystal structure of the deposited films were identified by using an X-ray diffractometry (Rigaku, Japan) with a thin film attachment. ( was A Cu Ka X-ray source (wavelength of 1.54 A) used at 50 kV and 20 mA. The degree of preferred orientation for the coatings was determined, following subtraction of background radiation, by calculating the texture coefficient of the (h k l) plane, Thkl, defined as [12,22]. Thkl ¼
Im ðh k l Þ=I0 ðh k l Þ ; 1 Pn Im ðh k l Þ=I0 ðh k l Þ n 1
where Im (h k l ) is the measured relative intensity of the reflection from the (h k l ) plane, I0 (h k l ) is the relative intensity from the same plane in a standard reference sample (JCPDS 38-1420), and n is the total number of the reflection peaks from the coating. The value of the texture coefficient for the peaks under investigation ranges from unity for a randomly oriented sample, to n for a sample having a complete preferential orientation. The root mean square (RMS) surface roughness over an area of 1 1 mm2 of the TiN films was obtained using an atomic force microscopy (AFM) in the tapping mode (Dimension 3000 Scanning Probe Microscope from Digital Instrument). The internal stress in TiN films was determined by a substrate curvature method. The radius of curvature and the film thickness were measured by a surface profilometer (Tencor P10).
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deposition pressure are shown in Fig. 1. In this series of experiments, substrate bias and temperature remain constant at –100 V and 450 C, respectively. It can be seen that the texture coefficients of the three planes are greatly affected by the deposition pressure. For the films deposited at 1 104 Torr, the texture coefficients of the three peaks are all close to 1.0, indicating that the films have a random orientation. The increase of deposition pressure leads to a great decrease of T200 and T111, and an increase of T220. However at a deposition pressure above 2 104 Torr, the increase of deposition pressure results in a decrease of T220 and an slight increase of T111, but T200 remains almost unchanged. Due to the change in the texture coefficients of the three planes, the films deposited at a pressure above 2 104 Torr show a mixed (1 1 1) and (2 2 0) preferred orientation. This is different from Jone’s results [12], which showed that increasing deposition pressure result in a decrease in the texture coefficient of the (1 1 1) plane, and an increase in the texture coefficient of the (2 0 0) and (2 2 0) plane of the TiN films deposited by RF reactive magnetron sputtering. Fig. 2 shows the influence of deposition pressure on the internal stress and RMS surface roughness of the TiN films. The internal stress decreases slightly as the deposition pressure is increased, reaching a minimum at a pressure of 2 104 Torr, then increases gradually with further increases in deposition pressure. The surface roughness decreases greatly with increasing deposition pressure to 2 104 Torr, then remains
3. Results XRD results show that all films deposited at a pressure above 1 104 Torr are single-phase face centered cubic (FCC) type TiN films. Three peaks at 2y of 36.9 , 42.3 and 62.0 , which corresponds to the (1 1 1), (2 0 0), and (2 2 0) plane of cubic TiN, respectively, can be observed in all the XRD patterns. The calculated texture coefficients, T111, T200, and T220, of the (1 1 1), (2 0 0), and (2 2 0) plane, respectively, of the films as a function of the
Texture coefficient
2.0
1.5
T111
1.0
T200 T220
0.5
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Deposition pressure ( mTorr) Fig. 1. Dependence of the T111, T200, and T220 of the TiN films deposited at a substrate bias and temperature of 100 V and 450 C, respectively, on the deposition pressure.
5
0.9 0.8
4
0.7 3 0.6 2 1
Internal stress
0.5
RMS roughness
0.4
0
0.3 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Deposition pressure ( mTorr) Fig. 2. Dependence of the internal stress and RMS surface roughness of the TiN films deposited at a substrate bias and temperature of 100 V and 450 C, respectively, on the deposition pressure.
Texture coefficient
1.6
T111
1.2
T200 T220 0.8
0.4
0
100
200
300
400
500
600
Substrate bias (-V) Fig. 3. Influence of the substrate bias on the T111, T200, and T220 of the TiN films deposited at a deposition pressure and temperature of 2 104 Torr and 450 C, respectively.
almost constant at higher deposition pressure. Comparing Figs. 2 and 1, it is interesting to note that the variation trend of T111 and T200 with deposition pressure is similar to that of the internal stress and RMS surface roughness of the films, respectively. The influence of substrate bias on the T111, T200, and T220 of the TiN films is shown in Fig. 3. In this series of experiments, deposition pressure and substrate temperature remain constant at 1 104 Torr and 450 C, respectively. The preferred orientation of the films is strongly dependent on the substrate bias. As the substrate bias is increased, T200 decreases deeply, reaching a minimum at a bias of 100 V, then increases rapidly. The trends in T111 and T220 with substrate bias are opposite to that of T200, which show a rapid
increase with increasing substrate bias up to – 100 V, then decreases greatly. The dependence of the film texture on the substrate bias can be divided into three ranges. At a substrate bias below 50 V, TiN films exhibit a (2 0 0) preferred orientation. In the substrate bias range –50– 200 V, TiN films show a mixed (1 1 1) and (2 2 0) preferred orientation. At a substrate bias above –200 V, the preferred orientation changes to the (2 0 0) plane again. Our results are consistent with Hoang’s reports [23], which show a evolution of the preferred orientation in TiN films deposited by a helicon activated reactive evaporation system from the (2 0 0) to (1 1 1), then to (2 0 0) with increasing substrate bias, but is different from the reports of Zhao [18], who observed a change of the preferred orientation from the (1 1 1) to (2 2 0) plane with increasing substrate bias. The dependence of the internal stress and RMS surface roughness on the substrate bias is shown in Fig. 4. As the substrate bias is increased to 100 V, the internal stress increases to a maximum and the surface roughness decreases rapidly to a minimum. Further increase of substrate bias leads to a great decrease of the internal stress and a great increase of the surface roughness. It can also be noted by comparing Figs. 3 and 4 that the trends of the internal stress and surface roughness with substrate bias fit well with that of the T111 and T200 of the TiN films, respectively.
5.0
1.0
4.5
0.9
4.0 0.8 3.5 0.7 3.0 0.6
Internal stress
2.5
RMS roughness (nm)
1.0
Internal stress (GPa)
Internal stress (GPa)
6
RMS roughness (nm)
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RMS roughness 2.0 0
100
200
300
400
500
0.5 600
Substrate bias (-V) Fig. 4. Influence of the substrate bias on the internal stress and RMS surface roughness of the TiN films deposited at a deposition pressure and temperature of 2 104 Torr and 450 C, respectively.
Y.H. Cheng, B.K. Tay / Journal of Crystal Growth 252 (2003) 257–264
Texture coefficient
1.5
T 111
1.0
T 200 T 220 0.5
0.0 0
100
200
300
400
500
600
Deposition temperature (˚C) Fig. 5. Influence of the deposition temperature on the T111, T200, and T220 of the TiN films deposited at a deposition pressure and substrate bias of 2 104 Torr and 100 V, respectively.
.7 Internal stress RMS roughness
10
.6
9 8
.5 7 6
.4
5 4 0
100
200
300
400
RMS roughness (nm)
Internal stress (GPa)
11
.3 600
500
Deposition temperature (˚C) Fig. 6. Influence of the deposition temperature on the internal stress and RMS roughness of the TiN films deposited at a deposition pressure and substrate bias of 2 104 Torr and 100 V, respectively.
1.5
Texture coefficient
Fig. 5 shows the dependence of the T111, T200, and T220 of the TiN films on the deposition temperature. In this series of experiments, deposition pressure and substrate bias remain constant at 1 104 Torr and 100 V, respectively. All the TiN films show a mixed (1 1 1) and (2 2 0) preferred orientation. The influence of deposition temperature on the texture coefficients of the three planes is not so significant as that of the deposition pressure and substrate bias. At a deposition temperature below 100 C, the increase of deposition temperature results in a slight decrease of T111 and a slight increase of T200. At a higher deposition temperature, T200 remains almost constant, but T111 decreases further with increasing deposition temperature. There is no significant change for T220 in the whole temperature range. Fig. 6 shows the influence of deposition temperature on the internal stress and surface roughness of TiN films. The increase of deposition temperature results in a linear decrease of internal stress and a linear increase of surface roughness. Comparing Figs. 2, 4 and 6, it can be seen that the RMS surface roughness of all films in this series of experiments is very small and the surface roughness varies slightly with deposition temperature, while the variation of the internal stress in the TiN films is much greater than that in the TiN films studied in the other two series of experiments. It is interesting to note that the trends of T111 and T200 are similar to that of the internal stress and RMS surface roughness, respectively, in all
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1.0
T 111 T 200 T 220
0.5
0.0 2
4
6
8
10
12
Internal stress (GPa) Fig. 7. Dependence of the T111, T200, and T220 on the internal stress of the TiN films deposited at different deposition processes.
three series of experiments, which may indicate that the texture in the TiN films are correlated to the internal stress and surface roughness. In order to investigate the influence of the internal stress and surface roughness on the texture in the TiN films, the texture coefficients of the three planes were plotted as a function of the internal stress and RMS surface roughness, respectively. Fig. 7 shows the dependence of T111, T200, and T220 on the internal stress. At an internal stress below 4.5 GPa, the texture coefficients of the three planes are strongly dependent on the internal stress: T111 and T220 increase drastically, while T200 drastically decreases. Further increase of internal stress results in a slight increase of T111 and T220, and a slight decrease of T200. It is worth noting that the trend of T220 with internal stress is the same as that
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of T111. The development of the texture coefficient of the three planes with increasing internal stress results in the evolution of the texture from a (2 0 0) preferred orientation, to a random orientation at an internal stress of 3.5 GPa, then to a mixed (1 1 1) and (2 2 0) preferred orientation. Fig. 8 shows the dependence of the texture coefficient of the three planes on the RMS surface roughness. As the surface roughness is increased, a continuous decrease of T111 and T220, and increase of T200 can be observed, which leads to the change of the texture from a mixed (1 1 1) and (2 2 0) preferred orientation to a (2 0 0) preferred orientation. It could also be found that the trend of T220 with surface roughness is the same as that of T111.
4. Discussions Preferred orientation has been widely observed in the TiN films deposited by vapor deposition. Most explanations for the preferred orientation in the films were given in terms of strain energy, surface energy, and kinetic factors [14,24]. Pelleg et al. [24] suggested that the preferred orientation of the TiN thin films is caused by the driving force to lower the overall energy of the film which is composed of the surface energy and strain energy. According to the theoretical calculation, the degree of the surface energy of TiN crystal, Shkl, is S200oS110oS111, and the degree of the strain
Texture coefficient
1.5
1.0
0.5
T 111 T 200 T 220
0.0 0.2
0.4
0.6
0.8
1.0
1.2
RMS roughness (nm) Fig. 8. Dependence of the T111, T200, and T220 on the RMS surface roughness of TiN films deposited at different deposition processes.
energy per unit volume, uhkl, is u111ou110ou200. TiN films would grow toward the orientation of the (2 0 0) plane with the lowest surface energy when the surface energy is dominant, and toward the orientation of the (1 1 1) plane with the lowest strain energy when the strain energy is dominant. At intermediate stress levels, the (1 1 0) direction might be preferred in order to minimize the total energy (strain and surface energy combined). However, the kinetic factor theory suggested that the preferred orientation is determined by the growth velocity differences between crystal faces. Zhao [18] has used this theory to explain the development of the (2 2 0) texture in the TiN films deposited by cathodic arc at the high substrate bias. As shown in Figs. 7 and 8, the texture coefficients of the three planes are strongly dependent on the internal stress and RMS surface roughness, and the dependence of the texture coefficient of the (2 2 0) plane on the internal stress and RMS roughness is the same as that of the texture coefficient of the (1 1 1) plane. As we know, the internal compressive stresses in the films have been proposed to originate from the ‘‘atomic peening’’ mechanism [25–27]. The bombarding ions or atoms knock surface atoms deeper into the subsurface of the growing films and get themselves trapped in the layer. The associated additional volume in the constrained layer leads to an expansion of the film outwards from the substrate, but the film is not free to expand and the entrapped atoms cause macroscopic compressive stress. Therefore, the internal stress correlates to the strain energy in the deposited films. At the same time, the RMS surface roughness of the deposited films determines the surface area, and therefore the surface energy. As shown in Fig. 2, for the films deposited at a pressure below 2 104 Torr, the increase of deposition pressure results in a slight decrease of internal stress and a great decrease of RMS roughness, leading to a slight decrease of the strain energy and a great increase of the surface energy in the deposited films, respectively. According to the energy minimization model, in order to minimize the total energy of the system, the TiN films will develop to a crystallographic texture
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with slightly higher T111 and much lower T200. For the films deposited at a pressure of 2 104 Torr, both internal stress and surface roughness are the lowest, a texture with highest T220 may be preferred in order to minimize the total energy. Further increase of deposition pressure leads to a slight increase in the internal stress but no obvious change in the RMS roughness, which may contribute to the slight increase of T111 and the constant T200 with increasing deposition pressure. For the films deposited at a substrate bias of 0 V, the internal stress is lowest and the RMS roughness is highest. This indicates that the surface energy is dominant, which may result in a (2 0 0) preferred orientation growth. The increase of substrate bias leads to a great increase of the internal stress and decrease of the RMS roughness, indicating the increase of the strain energy and the decrease of the surface energy, respectively, which may result in a increase of T111 and decrease of T200, respectively. At a substrate bias above 100 V, the increase in bias results in a decrease of internal stress and an increase of RMS roughness, leading to a decrease of strain energy and an increase of surface energy, and therefore a decrease of T111 and an increase of T200. For the films deposited in the bias range 50–100 V, the low surface roughness and moderate internal stress may lead to the development of the texture to the highest T220. For the films deposited at room temperature, the internal stress is fairy high while the surface roughness is very low. In this case, the strain energy is dominant over the surface energy in the films. As a result, the films exhibit a mixed (1 1 1) and (2 2 0) preferred orientation with high T111, T220 and low T200. Fig. 6 shows that the increase of deposition temperature results in a linear decrease of internal stress and a linear increase of surface roughness, indicating a linear decrease of strain energy and a linear increase of surface energy, which leads to the decrease of T111 and the increase of T200 as shown in Fig. 5. However, it should be noted that the internal stress is much higher and the surface roughness is much lower in this series of films as compared with the films studied in the other two series. The strain energy is dominant in all this series of films. Therefore, all
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the films exhibit a mixed (1 1 1) and (2 2 0) preferred orientation, and the variation of the texture coefficient for all three planes is insignificant. In addition, it should be pointed out that the increase of deposition temperature might decrease the defect density in the deposited films, which might reduce the area of the inner surface of the defect, and therefore the surface energy. This may compensate the increase of the surface energy due to the increase of the surface roughness, which corresponds to the development of the texture with almost constant T200 for the films deposited at a higher temperature.
5. Conclusions The texture in the TiN films deposited by an offplane double bend FCVA is greatly affected by deposition pressure, substrate bias, and deposition temperature. As the deposition pressure is increased, T111 decreases slightly and T200 decreases greatly. Further increase of deposition pressure results in a slight increase of T111 but no significant change of T200, and the film texture develops from a random orientation to a mixed (1 1 1) and (2 2 0) preferred orientation. As the substrate bias is increased, T200 decreases greatly to a minimum and T111 increases greatly to a maximum at a substrate bias of 100 V, which leads to the evolvement of the film texture from a (2 0 0) preferred orientation to a mixed (1 1 1) and (2 2 0) preferred orientation. A further increase in substrate bias causes a rapid increase of T200 and decrease of T111, resulting in a change of the film texture to a (2 0 0) preferred orientation again. The films deposited at different temperature exhibit a mixed (1 1 1) and (2 2 0) preferred orientation growth, and the increase of deposition temperature results in a slight increase of T200 and decrease of T111. The variation trend of T111 and T200 with deposition pressure, substrate bias and deposition temperature is consistent with that of the internal stress and surface roughness, respectively, which may suggest that the internal stress and surface roughness are the main factors that affect the development of the texture in the TiN films.
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Acknowledgements The first author would like to express gratitude for the support of the State Key Laboratory of Laser Technology of China and the State Key Laboratory of Plastic Forming Simulation and Die & Mould Technology of China.
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