Effect of Ti interlayer on the residual stress and texture development of TiN thin films deposited by unbalanced magnetron sputtering

Effect of Ti interlayer on the residual stress and texture development of TiN thin films deposited by unbalanced magnetron sputtering

Surface & Coatings Technology 201 (2006) 3199 – 3204 www.elsevier.com/locate/surfcoat Effect of Ti interlayer on the residual stress and texture deve...

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Surface & Coatings Technology 201 (2006) 3199 – 3204 www.elsevier.com/locate/surfcoat

Effect of Ti interlayer on the residual stress and texture development of TiN thin films deposited by unbalanced magnetron sputtering Jia-Hong Huang a,⁎, Cheng-Hsin Ma b , Haydn Chen b,c,d a

b

Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA c Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong d Department of Physics, Tunghai University, Taichung, Taiwan Received 5 November 2005; accepted in revised form 27 June 2006 Available online 26 July 2006

Abstract The purpose of this study is to investigate the effects of Ti interlayer on the texture and residual stress of overlaying titanium nitride (TiN) thin films at a high residual stress state. Films were deposited by unbalanced magnetron (UBM) sputtering methods on substrates with or without a Ti interlayer. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to reveal the microstructure of the thin films. Residual stresses and pole figures of the grown films including Ti interlayer and TiN were measured by X-ray diffraction (XRD) techniques. Thin films with weak fiber texture were produced. The compressive residual stresses in the TiN films of all specimens were higher than −4.5 GPa; in contrast, the stresses in the Ti-interlayers were much lower at less than 0.72 GPa and in the tensile state. Results showed that the Ti interlayer had a relatively weaker effect on the texture of the TiN films prepared by UBM sputtering compared to those deposited by ion beam assisted deposition (IBAD) in the previous study. It was found that about 50% of the residual stress in the TiN film was relieved by the introduction of a Ti interlayer with a thickness of 200 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: Ti interlayer; Texture; Residual stress; TiN

1. Introduction Titanium nitride (TiN) thin films and coatings have been widely applied ranging from hard and protective coatings on mechanical tools, decorative coatings, to the diffusion barrier in microelectronic industry, owing to their high hardness, high thermal and chemical stability, and attractive color. Thin films deposited by vapor deposition, PVD or CVD, normally exhibit various degree of preferred orientations or textures. Most of the vapor deposition methods usually produce fiber textures. The film texture often affects its properties for example, the hardness of TiN film was reported to increase with increasing (111) texture [1–3], and TiN film with (111) texture was the most wear-resistant [4]. To improve the adhesion strength, it is common to incorporate a Ti interlayer between the TiN film and the substrate material. The Ti interlayer can also increase the corrosion re-

sistance of the TiN-coated metal substrate by isolating the corrosive medium through the pin-holes in the TiN coatings from reaching the substrate metal [5–7]. Moreover, the addition of Ti interlayer has been acknowledged to be an effective way to change the texture of the TiN film. The function of Ti interlayer is basically to provide a template for the growth of TiN film on top of the interlayer [8–10]. However, the addition of an interlayer will also relieve the stress in the TiN film. In our recent study [11], it is found that the change of texture is related to the Table 1 The deposition conditions for UBM sputtering processing Sample no.

Layer

N2 flow rate (sccm)

M1 M2

1 1 2 1 2

0.75 0 0.75 0 0.75

M3 ⁎ Corresponding author. Tel.: +886 35723916; fax: +886 35720724. E-mail address: [email protected] (J.-H. Huang). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.06.038

Thickness (nm) TiN on Si Ti on Si TiN on Si Ti on Si TiN on Si

150 100 150 200 200

Ar flow rate 15 sccm, negative bias 50 V, and substrate temperature 400 °C.

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Fig. 1. Pole figures of the TiN specimens with fiber textures deposited by UBM sputtering. (a) (111) pole figure of single-layer TiN specimen M1 showing weak (111) texture. (b) (0002) pole figure of Ti interlayer in specimen M2 showing (0002) fiber texture. (c) (111) pole figure of TiN/Ti specimen M2 showing weak (111) texture. (d) (200) pole figure of TiN/Ti specimen M2 showing no (200) texture. (e) (111) pole figure of TiN/Ti specimen M3 showing weak (111) texture.

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plasma-related physical vapor deposition (PVD) processes, such as magnetron sputtering and ion plating, the residual stress in the films may easily reach − 8 GPa and above [13,14]. It is unclear whether the Ti-interlayer can play a similar role on the residual stress and the texture development of the TiN thin film at a much higher residual stress level, for example higher than − 6 GPa. Our recent study [13] found that TiN thin films with thicknesses ranging from 140 to 244 nm deposited by unbalanced magnetron (UBM) sputtering may have very high compressive residual stresses, ranging from − 8.1 to − 10.2 GPa. Therefore, UBM sputtering was chosen for the preparation of TiN thin films with high residual stress, and the effect of Ti-interlayer on the texture and residual stress was investigated on TiN and TiN/Ti specimens. Texture analysis was conducted by pole figure measurement. Residual stress of the TiN and TiN/Ti specimens was measured by the modified sin2ψ method. The purpose of this study was to compare the effects of Ti interlayer on the texture and residual stress of the TiN films at high stress level, produced by UBM sputtering, and those at relatively lower stress level by IBAD method in the previous study [12]. 2. Experimental procedures

Fig. 2. Cross-sectional SEM images for (a) single layer TiN specimen M1, and (b) TiN/Ti specimen M2 deposited by UBM sputtering.

strain energy in the film. Consequently, the effect of interlayer on the texture of TiN film may not only relate to the local epitaxy, the change of residual stress may also play a role. In a previous study [12], using ion beam assisted deposition, we prepared TiN and TiN/Ti specimens with in-plane or fiber textures, and the effect of adding Ti-interlayer was investigated. The Ti-interlayer has an effect to change the in-plane textured TiN film from one type to another type by rotating the grain orientation 45° along the b100N axis with respect to the former texture; moreover, a strongly (0002) textured Ti-interlayer may further align the (111) plane in TiN with the orientation of Ti interlayer. For the TiN film with fiber texture, the insertion of Ti interlayer may change the texture of the TiN film from (002) to (111), and a strongly (0002) textured Ti interlayer is necessary to align the texture of the TiN film. The effect of Ti interlayer on the change of TiN texture occurs most evidently in the case of loose packing or low residual stress films. However, due to the intrinsic characteristics of the IBAD method, the residual stresses in the TiN films deposited by IBAD method are at a relatively lower range, usually less than − 2.2 GPa. On the other hand, the TiN or ZrN thin films deposited by other commonly used

TiN films were deposited using UBM sputtering method. Si (100) wafers with dimensions of 25.4 mm × 25.4 mm × 0.6 mm were used as the substrate materials, which were cleaned by successive rinses in ultrasonic baths of acetone and methanol and blown dried with N2 gas. For the UBM sputtering processing, TiN and TiN/Ti specimens were deposited using an unbalanced magnetron sputtering (STS-400) system. A rectangular titanium target (99.9% in purity) with dimensions of 248 mm × 130 mm × 10 mm was used. To reduce contamination, the chamber was heated to 300 °C during pumping. The base pressure of the chamber was 1.20 × 10− 3 Pa. Prior to deposition, the substrate was presputtered by argon discharge at a bias of − 1000 V for 10 min to remove the surface oxide layer. The argon pressure was fixed at 0.8 Pa and the current density was 0.15 mA/cm2 during presputtering. The target-to-substrate distance was 10 cm. During deposition, the direct-current (DC) target power supply was operated at 0.9 A using the constant-current mode; in other words, the target current density was 2.8 mA/cm2. For the specimens with Ti interlayer, the sputtering process was first conducted without nitrogen gas until the desired Ti layer thickness was reached and then N2 was introduced. High purity working gas, Ar (99.9995% in purity), and reactive gas, N2 (99.9995% in purity), were used; their flow rates were regulated by mass flow controllers. Ar and N2 gas flow rates were fixed at Table 2 Residual stress of TiN and TiN/Ti specimens Sample no. M1 M2 M3

TiN Ti TiN Ti TiN

Residual stress (GPa)

Correlation coefficient R (%)

− 8.79 0.26 − 7.05 0.72 − 4.53

97.1 25.9 97.4 76.0 97.0

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Fig. 3. Cross-sectional TEM images and the corresponding diffraction patterns for specimen M3 deposited by UBM sputtering. (a) Cross-sectional TEM image, (b) high-resolution TEM image, (c) diffraction pattern of TiN in (a), (d) diffraction pattern of Ti in (a), and (e) diffraction pattern including TiN/Ti in (a).

15 sccm and 0.75 sccm, respectively. Total gas pressure was controlled at 0.133 Pa during the deposition process. The negative substrate bias voltage was kept at 50 V. The coating temperature was 400 °C monitored using a thermocouple near the substrate. The deposition conditions are listed in Table 1. Both pole figures and residual stresses of the TiN and TiN/Ti films were measured by X-ray diffraction (XRD) using a 4-circle diffractometer with psi-goniometer geometry. The Cu Kα line at 0.15418 nm was used as the source for diffraction. The TiN (111), TiN(200) and Ti (0002) pole figures were determined by psi scan from + 85° to − 85° and phi scan from 0° to 360°, both with increment step of 5°. The residual stresses of TiN films and Ti interlayer were obtained using the grazing incident XRD sin2ψ method [15]. X-ray was incident at a grazing angle of 2° to increase the diffraction volume of the thin film specimen. The depth of penetration for Cu Kα radiation in TiN has been calculated to be 400 nm [16]. For the TiN films, (220) peak was chosen for the residual stress measurement because it provided sufficient intensity for precise determination of the peak position. The Ti (101¯1) peak was selected for measuring the residual stress of the Ti interlayer. The elastic constant of Ti(101¯1) was calculated according to the following equation [17]: 1 ¼ ð1−l32 Þs11 þ l34 s33 þ l22 ð1−l32 Þð2s13 þ s44 Þ; E where E is the Young's modulus at certain direction, li is the direction cosine, and sij is the compliance. Inserting the pub-

lished compliance data for α-Ti [18], the Young's modulus E101¯1 = 163.4 GPa can be obtained. This value together with the Poisson's ratio ν = 0.32 [19] were used for converting the measured lattice strain to residual stress of the Ti interlayer. The microstructure of the specimens was observed by using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The actual film thickness was measured from the cross-sectional view of SEM image. 3. Results and discussion The effects of the Ti interlayer on the texture of the TiN films deposited by the unbalanced magnetron (UBM) sputtering are quite different from those in the IBAD specimens. Fig. 1(a) shows the (111) pole figure of a TiN film without Ti interlayer (specimen M1), where the (111) poles are distributed in a dumbbell shape. The pole figures of the Ti interlayers with thicknesses of 100 nm (specimen M2) and 200 nm (specimen M3) are similar; a typical one is shown in Fig. 1 (b). It can be seen that the Ti interlayer with a strong (0002) fiber texture, with the (0002) pole slightly off, ∼ 5°, the substrate normal. The pole figures of the TiN films are similar for both specimens with 100 nm and 200 nm Ti interlayers, showing weak (111) fiber texture, as depicted in Fig. 1(c) and (e). The TiN (200) pole figure for the same specimen M2 is shown in Fig. 1(d), which indicates that there is no (200) texture in the specimen. Compared with Fig. 1(a), the distribution of (111) poles of the TiN

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specimens with Ti interlayer are not substantially different from the single-layer specimen. This suggests that unlike the cases in the IBAD specimens with fiber texture, in which the texture of the TiN film may switch from (200) to (111) due to the addition of Ti interlayer [12], the Ti interlayer has a relatively weaker effect on the texture of the TiN films grown by the UBM sputtering method. Moreover, the thickness of the Ti interlayer does not significantly affect the texture of the TiN film in any appreciable way. Cross-sectional SEM images reveal microstructures of the TiN and TiN/Ti films as shown in Fig. 2. Figs. 2 (a) and (b) show the typical microstructures of the TiN specimens deposited by UBM sputtering without and with a Ti interlayer of 200 nm, respectively. The specimens exhibit a very dense columnar structure with a column width ranging from 50 to 100 nm. Compared with the microstructure of the specimens with fiber texture deposited by IBAD, which displays looser and coarser columnar structure, the UBM sputtering specimen shows a much smoother surface and more compact structure. From Fig. 2, it can be found that the microstructure of the TiN films is similar for the specimen with and without Ti interlayer. Therefore, the addition of a Ti interlayer does not appear to substantially change the microstructure of the TiN film on top of the Ti layer for these UBM sputter-grown films. From the SEM pictures the thickness of the TiN and Ti interlayer is measured and the results are listed in Table 1. Fig. 3 shows the cross-sectional TEM images for the specimen M3 with 200 nm Ti interlayer. As shown in Figs. 3 (a) and (b), it can be seen that the film possesses columnar structure with a column width less than 50 nm, which is consistent with the results from SEM images, and both TiN and Ti interlayer are very dense without nanopipes between columns. The corresponding diffraction patterns for TiN, Ti and TiN/Ti are shown in Figs. 3 (c), (d) and (e), respectively. Both diffraction patterns of TiN and Ti display ring segments, indicating that the grain sizes in those two layers are very small. From Fig. 3 (b), the high resolution TEM image at TiN/Ti interface indicates that there is no substantial orientation relationship between TiN and Ti interlayer. The diffraction pattern depicted in Fig. 3(e) also reveals the same evidence. Therefore, TEM microstructure study confirms the results of pole figure measurements. The residual stresses in TiN film and Ti interlayer were measured using the modified sin2ψ method [15] with a grazing incident XRD geometry. The results and the correlation coefficients are listed in Table 2. It is found that the compressive residual stresses in the TiN films are quite high in all specimens, ranging from 4.53 to 8.79 GPa, compared with the IBAD specimens, less than 2.2 GPa, in the previous study. The residual stresses in the Ti-interlayer are much less than those in TiN films and in tensile state. The extent of residual stress is found to be consistent with the film microstructure. In the previous study [12], we have reported that for the specimens with residual stress lower than − 1 GPa, the TiN films possess a looser columnar structure, and therefore the growth of the TiN grains is easier to be aligned with the orientation of the Ti interlayer. In contrast, for specimens with compact microstructure they have higher residual stress and grains are more difficult to be aligned with the orientation of Ti interlayer. This argument is further confirmed in

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the present study. The specimens prepared by UBM sputtering, with a highly compact structure, are in a much higher stress range than those deposited by IBAD method, and the effect of interlayer texture becomes insignificant in the texture development of the TiN film. The effect of the Ti interlayer on the relief of residual stress is quite significant as the thickness of the interlayer increases by two fold. Comparing specimens M2 with M3, it is found that the residual stress was relieved by almost 50% when the thickness of the Ti layer was doubled. However, the stress relief was not obvious for a Ti interlayer of 100 nm thick, as compared with a single-layer TiN specimen. Although increasing the Ti interlayer thickness can lower the residual stress, its effect on texture change is not obvious. Specimens M2 and M3 do not exhibit a significant texture change when the thickness of the Ti interlayer is doubled. This also suggests that the relief of the residual stress is not necessarily accompanied by a texture change for the deposition conditions employed in this study. 4. Conclusions (1) The Ti interlayer has a relatively weak effect on the texture of the TiN films by UBM sputtering, and increasing the thickness of the Ti interlayer does not affect the texture of the TiN film. (2) About 50% of the residual stress in the TiN film is relieved by the Ti interlayer with a thickness of 200 nm. Acknowledgements This work was supported in part by the US Department of Energy, Office of Basic Energy Science, under the grant No. DEFG02-91ER45439 via the Frederick Seitz Materials Research Laboratory of the University of Illinois at UrbanaChampaign, and in part by the City University of Hong Kong via a SRG grant (Project No. 7001335). J.-H. Huang would like to acknowledge the support of National Science Council of the Republic of China under the grant No. NSC 93-2216-E-007-028 and the National Tsing Hua University in Taiwan during his sabbatical leave at the University of Illinois at Urbana-Champaign where this work started. The assistance of TEM analysis by Prof. F.R. Chen and TEM center at the Department of Engineering and System Science, National Tsing Hua University is appreciated. We also acknowledge Dr. Wen-Jun Chou of National Tsing Hua University for preparing the UBM specimens. References [1] W.J. Chou, G.P. Yu, J.H. Huang, Surf. Coat. Technol. 140 (2001) 206. [2] W.J. Chou, G.P. Yu, J.H. Huang, Surf. Coat. Technol. 149 (2002) 7. [3] H. Ljungcrantz, M. Oden, L. Hultman, J.E. Greene, J.E. Sundgren, J. Appl. Phys. 80 (12) (1996) 6725. [4] J.-E. Sundgren, Thin Solid Films 128 (1985) 21. [5] W.L. Pan, G.-P. Yu, J.-H. Huang, Surf. Coat. Technol. 110 (1998) 111. [6] B.F. Chen, W.L. Pan, J. Hwang, G.P. Yu, J.-H. Huang, Surf. Coat. Technol. 111 (1999) 16. [7] H.W. Wang, M.M. Stack, S.B. Lyon, P. Hovsepian, W.-D. Munz, Surf. Coat. Technol. 126 (2000) 279.

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