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Ar gas mixtures
Materials Chemistry and Physics 68 (2001) 266–271
Characterization of tantalum nitride films deposited by reactive sputtering of Ta in N2 /Ar gas mixtures Wen-Horng Lee, Jing-Cheng Lin, Chiapyng Lee∗ Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan, ROC Received 14 February 2000; received in revised form 16 June 2000; accepted 28 June 2000
Abstract Tantalum nitride (TaN) films are deposited on silicon substrates by radio frequency (RF) reactive sputtering of Ta in N2 /Ar gas mixtures at a bias of 0 V. The deposition rate, chemical composition, and crystalline microstructure are investigated by cross-sectional transmission electron microscopy (XTEM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), X-ray diffraction (XRD), and atomic force microscopy (AFM), respectively. According to those results, the deposition rate, film composition, and microstructure correlate with the N2 /Ar flow ratio. In addition, the deposition mechanism which controls the film characteristics is presented as well. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Tantalum nitride; Reactive sputtering; Diffusion Barrier
1. Introduction
2. Experimental
Owing to their high stability and excellent conductivity, refractory metal nitrides are widely recognized as an attractive class of materials which can be used as diffusion barriers in metal–semiconductor contacts [1]. Among those refractory metal nitrides, tantalum nitride (TaN) has received extensive interest as a thin film diffusion barrier between silicon and metal overlayers of Ni [2], Al [3–6] and most recently Cu [7–11]. TaN has been prepared as thin films by sputtering tantalum in a mixture of nitrogen and argon [8,12–14]. Tantalum nitride has a defective structure [15], and deviations from stoichiometry are common. Consequently, the properties of TaN thin films are extremely sensitive to the film’s microstructure and growth morphology as well as to deviations from stoichiometry. According to previous literature [8,12–14], the chemical composition, microstructure, and hence, properties of TaN films heavily depend on the deposition parameters. In this work, the variation of deposition rate, film chemical composition, and crystalline microstructure as a function of N2 /Ar flow ratio is studied at a bias of 0 V.
Silicon wafers, h1 0 0i oriented, p-type with a resistivity of 2–5 cm, cut into 10×10 mm2 pieces, were used as substrates for the growth of TaN films. The wafers were cleaned with acetone in an ultrasonic bath for 30 min prior to deposition. TaN films were grown in a radio frequency (RF) magnetron sputtering system from a 7.5 cm planar Ta target in a N2 /Ar ambient. The substrate holder was placed about 5 cm below the target. After wafer loading, the system was pumped down to the 3.0×10−7 Torr base pressure for 120 min by a diffusion pump prior to deposition. During deposition, the total pressure was maintained constant at 3.0×10−2 Torr. The RF power was maintained at 250 W. The substrate bias was kept constant at 0 V. The N2 /Ar flow ratio was adjusted by varying the N2 flow rate while maintaining the Ar flow rate at 30 sccm. Notably, the substrate was not heated during deposition. Deposition was performed at a static mode for 4 min in all cases. The crystalline structures of TaN films were examined by cross-sectional transmission electron microscopy (XTEM). A JEOL 2000 FXII STEM was used to examine samples which were prepared in a cross-sectional view. Film thickness was directly measured from the XTEM micrographs. The XTEM is a destructive analysis technique commonly used to observe the deposited film with electron beams perpendicular to the wafer surface normal. As XTEM can be used to observe the deposited film and the film/substrate
∗ Corresponding author. E-mail address: [email protected] (C. Lee).
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 7 0 - 9
W.-H. Lee et al. / Materials Chemistry and Physics 68 (2001) 266–271
267
Fig. 3. Auger depth profile for a reactively sputtered TaN film with a N2 /Ar flow ratio of 0.25.
Fig. 1. Deposition rate of reactively sputtered TaN films vs. N2 /Ar flow ratio. Solid line indicates the trend of these data.
interface simultaneously, it becomes the most direct and precise means of determining the crystalline phase and structure of the deposited films. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses were performed in a VG Microtech MT-500 spectrometer. The spectrometer was equipped with a hemispherical analyzer. All XPS data presented herein were acquired using the Al K␣ X-rays (1486.6 eV). Peaks positions were then calibrated with respect to the C 1s peak at 284.6 eV from the adventitious hydrocarbon contamination. The AES analysis was performed using an electron beam having a kinetic energy of 5 keV. Next, a PHILIPS PW1710 X-ray diffractometer was used to examine the sample’s crystalline structure. Micrographs
Fig. 2. Ta 4f XPS spectra obtained from various compositional TaN films: (a) bulk Ta; (b) TaN film prepared at N2 /Ar flow ratio of 0.08; (c) 0.13; (d) 0.17; (e) 0.2; (f) 0.25; (g) 0.33.
Fig. 4. N/Ta ratio of TaN films as a function of N2 /Ar flow ratio. Solid line indicates the trend of these data.
Fig. 5. XRD patterns of TaN films deposited at various N2 /Ar flow ratios: (a) 0.08; (b) 0.17; (c) 0.2; (d) 0.25; (e) 0.27; (f) 0.33; (g) 0.42.
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Fig. 6. Bright-field XTEM micrographs and diffraction patterns of the TaN films deposited at a N2 /Ar flow ratio of (a) 0.08, (b) 0.25, and (c) 0.33.
of the samples in ambient conditions were obtained with an atomic force microscope, the Nanoscope III (Digital Instruments), according to conventional procedures.
3. Results and discussion Fig. 1 presents the deposition rate of films for various N2 /Ar flow ratios, in which the deposition rate was deduced from the thickness measurements. A step change in deposition rate was observed at a N2 /Ar flow ratio of 0.2 from about 47.0 nm min−1 at lower flow ratios to about 27.0 nm min−1
at higher flow ratios. This 0.2 N2 /Ar flow ratio corresponds to a nitrogen partial pressure of 5×10−3 Torr. By using RF reactive sputtering, this work investigates how N2 /Ar flow ratio influences the growth of TaN at 0 V bias. Fig. 1 clearly indicates an abrupt stepwise decrease in the deposition rate at a flow ratio of 0.2. Sun et al. [12] observed a similar phenomenon in deposition rate for RF-sputtered TaN films. This nitrogen pressure may be connected with the formation of TaN on the target. As generally assumed, the decrease in the deposition rate is correlated with the decrease in the sputtering rate of the target because numerous sites on it are occupied by TaN and N2 , thereby
W.-H. Lee et al. / Materials Chemistry and Physics 68 (2001) 266–271
diminishing the ion efficiency. For the RF reactive sputtering of zirconium in a mixture of nitrogen and argon, Shinoki and Itoh [16] proposed a kinetic model which considers the gettering action of the deposited material. Their model also shows an abrupt stepwise decrease in the sputtering rate at a definite partial pressure of nitrogen. Fig. 2 displays the high resolution XPS spectra of the films deposited at a bias of 0 V and different N2 /Ar flow ratios. Curve (a) in this figure illustrates the XPS spectrum for pure Ta and the observed binding energies correlate well with the values found in the handbook [17]. Increasing the N2 /Ar flow ratio from 0.08 to 0.33 causes the Ta 4f7/2 peak to shift from 21.8 eV (elemental Ta) to 23.2 eV. Fig. 3 presents the normalized atomic concentrations of sample (f) in Fig. 2 obtained by AES analysis. This typical depth profile reveals that the components (Ta and N) are uniformly distributed throughout the “bulk” of the film. Fig. 4 displays the film composition analyzed by XPS as a function of N2 /Ar flow ratio with a bias of 0 V during deposition. This figure indicates that, as the N2 /Ar flow ratio increases, the N/Ta ratio rises sharply at first and then becomes saturated around 1.2 at 0.25 N2 /Ar flow ratio and above. The argon and oxygen concentrations in the film were about <1 and 4 at.% as determined by XPS, respectively. Tantalum nitride has a defective structure [15] and deviations from stoichiometry are frequent. This finding corresponds to the XPS observations in Fig. 2 that the Ta 4f peaks shift upon the variation in N2 /Ar flow ratio, and in Fig. 4 that the amount of N in the deposited film increases with the flow ratio of N2 /Ar, since nitrogen doping should affect the chemical environment on the outermost electron orbitals of Ta. Nicolet and coworkers [12] showed a homogeneity range of TaN less than 53 at.% N which is consistent with the 1.2 N/Ta ratio observed in this study. Fig. 5 displays the X-ray diffraction (XRD) spectra of TaN films deposited at a bias of 0 V and with various N2 /Ar flow ratios. The major peaks are characteristic of the rock-salt structure of the TaN phase and correspond to the (1 1 1), (2 0 0), and (2 2 0) orientations, respectively [14,18,19]. At a N2 /Ar flow ratio of 0.08, the XRD spectrum exhibits a highly (1 1 1) preferred orientation (Fig. 5a). On increasing the N2 /Ar flow ratio to 0.25 (Fig. 5d), the intensities of (1 1 1), (2 0 0) and (2 2 0) peaks broaden corresponding to smaller grains. However, when the N2 /Ar flow ratio exceeds 0.33, only dispersive peaks appear in the XRD pattern of the film (Fig. 5f and g). In addition, the peak shifts to a lower diffraction angle 2θ with an increasing N2 /Ar flow ratio as shown in Fig. 5. Fig. 6 depicts XTEM bright-field micrographs with electron diffraction patterns of the films grown at a bias of 0 V and different N2 /Ar flow ratios. Fig. 6a indicates that at a N2 /Ar flow ratio of 0.08, the deposited films on Si are of polycrystalline TaN. According to Fig. 5a, the grains in the bright-field image (Fig. 6a) have a columnar structure, indicating the existence of a preferred orientation (1 1 1). Increasing the N2 /Ar flow ratio to 0.25 allows us to obtain TaN
269
Fig. 7. AFM micrographs of TaN films deposited at a N2 /Ar flow ratio of (a) 0.08, (b) 0.25, and (c) 0.33.
films with columnar grains again as shown in Fig. 6b. This finding corresponds to the observation in Fig. 5e that (1 1 1) is still the preferred orientation. In addition, microcrystalline TaN is observed at N2 /Ar flow ratios above 0.33. Fig. 6c presents the typical XTEM micrograph of microcrystalline TaN deposited at 0.33 flow ratio, which correlates with the dispersive peaks in Fig. 5f and g. Moreover, the diffraction patterns in Fig. 6 reveal that the film is a rock-salt structure which is identical to that reported by Gerstenberg and Calbick[19]. Fig. 7 depicts the atomic force microscopy (AFM) micrographs of the TaN films prepared at a bias of 0 V and N2 /Ar
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W.-H. Lee et al. / Materials Chemistry and Physics 68 (2001) 266–271
Fig. 8. Grain density of reactively sputtered TaN films vs. N2 /Ar flow ratio.
flow ratios of 0.08, 0.25 and 0.33, respectively. The micrographs contain spherical bumps of similar sizes. The spatial arrangement of the grains appears random, and the lines separating adjacent grains are assumed to be grain boundaries. In Fig. 7c, very fine grains of TaN are observed. In addition, Fig. 7 reveals that the grain size decreases with an increasing N2 /Ar flows ratio. Fig. 8 plots the grain density measured from the AFM micrographs versus N2 /Ar flow ratio. The grain density also takes off around the 0.2 N2 /Ar flow ratio. Reactively sputtered TaNx film has a NaCl structure with a series of disordered vacancies mainly at N sublattice site if x<1.0 and mainly at Ta sublattice sites if x>1.0 [20]. Fig. 9 illustrates lattice constant changes, obtained from X-ray anal-
ysis (Fig. 5), as a function of film composition. In sum, the TaN lattice is distorted due to the change of N concentration and formation of a nitrogen-defect lattice. The lattice parameters for superstoichiometric TaNx (x>1.0) are usually larger than the equilibrium bulk value(4.40 Å), indicating that the interstitial N atoms should also be present besides Ta vacancies. Based on the above discussion, we can conclude that this compound exhibits a considerable homogeneity range extending from the stoichiometric composition to significantly lower N concentrations. During the deposition, the growing TaN film is under the bombardment of positive ions as observed by Shinoki and Itoh [16]. Their work [16] used a mass spectrometer system capable of determining the mass and energy spectra of positive ionic species impinging on the substrate surface during the sputter deposition of thin films. The sputtering system was a planar diode RF sputtering configuration. According to their results, the N2 + and N+ ion currents increased with an increase of nitrogen partial pressure. At N2 /Ar flow ratios less than 0.33, the XRD spectrum showed a highly (1 1 1) preferred orientation (Fig. 5). This phenomenon may be attributed to that under ion irradiation, the polycrystalline film consists mainly of crystals with their (1 1 1) crystallographic axis parallel to the surface normal. This is the energetically most stable growth direction.
4. Conclusion This work deposits tantalum nitride (TaN) films on silicon substrates by RF sputtering of Ta in N2 /Ar gas mixtures at a bias of 0 V. Experimental results indicate that the deposition rate, chemical composition, and crystalline structure of the deposited film depend on the N2 /Ar flow ratio. A stepwise change in deposition rate is observed at a N2 /Ar flow ratio of 0.2. As the N2 /Ar flow ratio increases, the N/Ta ratio increases steeply at first and, then, becomes saturated around 1.2 at 0.25 N2 /Ar flow ratio and above. The TaN lattice is distorted due to the change of N concentration and a nitrogen-defect lattice is formed. Crystalline TaN can be deposited on Si substrates at a N2 /Ar flow ratio of 0.25 and lower, whereas microcrystalline TaN is obtained at N2 /Ar flow ratios higher than 0.25. References [1] [2] [3] [4] [5] [6]
Fig. 9. Lattice constant of FCC-TaN, deduced from XRD analysis (Fig. 5), as a function of film composition. Solid line indicates the trend of these data.
M.-A. Nicolet, Thin Solid Films 52 (1978) 415. S. Kanamori, Thin Solid Films 136 (1986) 195. B. Mehrotra, J. Stimmell, J. Vac. Sci. Technol. B 5 (6) (1987) 1736. M. Wittmer, J. Appl. Phys. 53 (1982) 1007. M. Wittmer, Appl. Phys. Lett. 37 (1980) 540. M.A. Farooq, S.P. Murarka, C.C. Chang, F.A. Baiocchi, J. Appl. Phys. 65 (8) (1989) 3017. [7] T. Oku, E. Kawakami, M. Uekubo, K. Takahiro, S. Yamaguchi, M. Murakami, Appl. Surf. Sci. 99 (1996) 265. [8] A. Noya, K. Sasaki, M. Takeyama, Jpn. J. Appl. Phys. 32 (1993) 911.
W.-H. Lee et al. / Materials Chemistry and Physics 68 (2001) 266–271 [9] S.C. Sun, M.H. Tsai, C.E. Tsai, H.T. Chiu, Proceedings of the International IEEE VLSI Multilevel Interconnection Conference, Vol. 12, 1995, p. 157. [10] J.O. Olowolafe, C.J. Mogab, R.B. Gregory, M. Kottke, J. Appl. Phys. 72 (9) (1992) 4099. [11] K. Holloway, P.M. Fryer, C. Cabral Jr., J.M.E. Harper, P.J. Bailey, K.H. Kelleher, J. Appl. Phys. 71 (11) (1992) 5433. [12] X. Sun, E. Kolawa, J.S. Chen, J.S. Reid, M.-A. Nicolet, Thin Solid Films 236 (1993) 347. [13] K. Sasaki, A. Noya, T. Umezawa, Jpn. J. Appl. Phys. 29 (1990) 1043. [14] B. Mehrotra, J. Stimmell, J. Vac. Sci. Technol. B 5 (6) (1987) 1736.
271
[15] L.E. Thod, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [16] F. Shinoki, A. Itoh, J. Appl. Phys. 46 (1975) 3381. [17] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Eden Praine, 1995. [18] Q.Y. Zhang, B. Chen, G.B. Li, S. Jin, F.X. Chen, D.Z. Yang, Surf. Coat. Technol. 66 (1994) 468. [19] D. Gerstenberg, C.J. Calbick, J. Appl. Phys. 35 (1964) 402. [20] S.Q. Wang, I. Raaijmakers, B.J. Burrow, S. Suthar, S. Redkar, K.B. Kim, J. Appl. Phys. 68 (1990) 5176.
Characterization of tantalum nitride films deposited by reactive sputtering of Ta in N2 /Ar gas mixtures Wen-Horng Lee, Jing-Cheng Lin, Chiapyng Lee∗ Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan, ROC Received 14 February 2000; received in revised form 16 June 2000; accepted 28 June 2000
Abstract Tantalum nitride (TaN) films are deposited on silicon substrates by radio frequency (RF) reactive sputtering of Ta in N2 /Ar gas mixtures at a bias of 0 V. The deposition rate, chemical composition, and crystalline microstructure are investigated by cross-sectional transmission electron microscopy (XTEM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), X-ray diffraction (XRD), and atomic force microscopy (AFM), respectively. According to those results, the deposition rate, film composition, and microstructure correlate with the N2 /Ar flow ratio. In addition, the deposition mechanism which controls the film characteristics is presented as well. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Tantalum nitride; Reactive sputtering; Diffusion Barrier
1. Introduction
2. Experimental
Owing to their high stability and excellent conductivity, refractory metal nitrides are widely recognized as an attractive class of materials which can be used as diffusion barriers in metal–semiconductor contacts [1]. Among those refractory metal nitrides, tantalum nitride (TaN) has received extensive interest as a thin film diffusion barrier between silicon and metal overlayers of Ni [2], Al [3–6] and most recently Cu [7–11]. TaN has been prepared as thin films by sputtering tantalum in a mixture of nitrogen and argon [8,12–14]. Tantalum nitride has a defective structure [15], and deviations from stoichiometry are common. Consequently, the properties of TaN thin films are extremely sensitive to the film’s microstructure and growth morphology as well as to deviations from stoichiometry. According to previous literature [8,12–14], the chemical composition, microstructure, and hence, properties of TaN films heavily depend on the deposition parameters. In this work, the variation of deposition rate, film chemical composition, and crystalline microstructure as a function of N2 /Ar flow ratio is studied at a bias of 0 V.
Silicon wafers, h1 0 0i oriented, p-type with a resistivity of 2–5 cm, cut into 10×10 mm2 pieces, were used as substrates for the growth of TaN films. The wafers were cleaned with acetone in an ultrasonic bath for 30 min prior to deposition. TaN films were grown in a radio frequency (RF) magnetron sputtering system from a 7.5 cm planar Ta target in a N2 /Ar ambient. The substrate holder was placed about 5 cm below the target. After wafer loading, the system was pumped down to the 3.0×10−7 Torr base pressure for 120 min by a diffusion pump prior to deposition. During deposition, the total pressure was maintained constant at 3.0×10−2 Torr. The RF power was maintained at 250 W. The substrate bias was kept constant at 0 V. The N2 /Ar flow ratio was adjusted by varying the N2 flow rate while maintaining the Ar flow rate at 30 sccm. Notably, the substrate was not heated during deposition. Deposition was performed at a static mode for 4 min in all cases. The crystalline structures of TaN films were examined by cross-sectional transmission electron microscopy (XTEM). A JEOL 2000 FXII STEM was used to examine samples which were prepared in a cross-sectional view. Film thickness was directly measured from the XTEM micrographs. The XTEM is a destructive analysis technique commonly used to observe the deposited film with electron beams perpendicular to the wafer surface normal. As XTEM can be used to observe the deposited film and the film/substrate
∗ Corresponding author. E-mail address: [email protected] (C. Lee).
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 7 0 - 9
W.-H. Lee et al. / Materials Chemistry and Physics 68 (2001) 266–271
267
Fig. 3. Auger depth profile for a reactively sputtered TaN film with a N2 /Ar flow ratio of 0.25.
Fig. 1. Deposition rate of reactively sputtered TaN films vs. N2 /Ar flow ratio. Solid line indicates the trend of these data.
interface simultaneously, it becomes the most direct and precise means of determining the crystalline phase and structure of the deposited films. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses were performed in a VG Microtech MT-500 spectrometer. The spectrometer was equipped with a hemispherical analyzer. All XPS data presented herein were acquired using the Al K␣ X-rays (1486.6 eV). Peaks positions were then calibrated with respect to the C 1s peak at 284.6 eV from the adventitious hydrocarbon contamination. The AES analysis was performed using an electron beam having a kinetic energy of 5 keV. Next, a PHILIPS PW1710 X-ray diffractometer was used to examine the sample’s crystalline structure. Micrographs
Fig. 2. Ta 4f XPS spectra obtained from various compositional TaN films: (a) bulk Ta; (b) TaN film prepared at N2 /Ar flow ratio of 0.08; (c) 0.13; (d) 0.17; (e) 0.2; (f) 0.25; (g) 0.33.
Fig. 4. N/Ta ratio of TaN films as a function of N2 /Ar flow ratio. Solid line indicates the trend of these data.
Fig. 5. XRD patterns of TaN films deposited at various N2 /Ar flow ratios: (a) 0.08; (b) 0.17; (c) 0.2; (d) 0.25; (e) 0.27; (f) 0.33; (g) 0.42.
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W.-H. Lee et al. / Materials Chemistry and Physics 68 (2001) 266–271
Fig. 6. Bright-field XTEM micrographs and diffraction patterns of the TaN films deposited at a N2 /Ar flow ratio of (a) 0.08, (b) 0.25, and (c) 0.33.
of the samples in ambient conditions were obtained with an atomic force microscope, the Nanoscope III (Digital Instruments), according to conventional procedures.
3. Results and discussion Fig. 1 presents the deposition rate of films for various N2 /Ar flow ratios, in which the deposition rate was deduced from the thickness measurements. A step change in deposition rate was observed at a N2 /Ar flow ratio of 0.2 from about 47.0 nm min−1 at lower flow ratios to about 27.0 nm min−1
at higher flow ratios. This 0.2 N2 /Ar flow ratio corresponds to a nitrogen partial pressure of 5×10−3 Torr. By using RF reactive sputtering, this work investigates how N2 /Ar flow ratio influences the growth of TaN at 0 V bias. Fig. 1 clearly indicates an abrupt stepwise decrease in the deposition rate at a flow ratio of 0.2. Sun et al. [12] observed a similar phenomenon in deposition rate for RF-sputtered TaN films. This nitrogen pressure may be connected with the formation of TaN on the target. As generally assumed, the decrease in the deposition rate is correlated with the decrease in the sputtering rate of the target because numerous sites on it are occupied by TaN and N2 , thereby
W.-H. Lee et al. / Materials Chemistry and Physics 68 (2001) 266–271
diminishing the ion efficiency. For the RF reactive sputtering of zirconium in a mixture of nitrogen and argon, Shinoki and Itoh [16] proposed a kinetic model which considers the gettering action of the deposited material. Their model also shows an abrupt stepwise decrease in the sputtering rate at a definite partial pressure of nitrogen. Fig. 2 displays the high resolution XPS spectra of the films deposited at a bias of 0 V and different N2 /Ar flow ratios. Curve (a) in this figure illustrates the XPS spectrum for pure Ta and the observed binding energies correlate well with the values found in the handbook [17]. Increasing the N2 /Ar flow ratio from 0.08 to 0.33 causes the Ta 4f7/2 peak to shift from 21.8 eV (elemental Ta) to 23.2 eV. Fig. 3 presents the normalized atomic concentrations of sample (f) in Fig. 2 obtained by AES analysis. This typical depth profile reveals that the components (Ta and N) are uniformly distributed throughout the “bulk” of the film. Fig. 4 displays the film composition analyzed by XPS as a function of N2 /Ar flow ratio with a bias of 0 V during deposition. This figure indicates that, as the N2 /Ar flow ratio increases, the N/Ta ratio rises sharply at first and then becomes saturated around 1.2 at 0.25 N2 /Ar flow ratio and above. The argon and oxygen concentrations in the film were about <1 and 4 at.% as determined by XPS, respectively. Tantalum nitride has a defective structure [15] and deviations from stoichiometry are frequent. This finding corresponds to the XPS observations in Fig. 2 that the Ta 4f peaks shift upon the variation in N2 /Ar flow ratio, and in Fig. 4 that the amount of N in the deposited film increases with the flow ratio of N2 /Ar, since nitrogen doping should affect the chemical environment on the outermost electron orbitals of Ta. Nicolet and coworkers [12] showed a homogeneity range of TaN less than 53 at.% N which is consistent with the 1.2 N/Ta ratio observed in this study. Fig. 5 displays the X-ray diffraction (XRD) spectra of TaN films deposited at a bias of 0 V and with various N2 /Ar flow ratios. The major peaks are characteristic of the rock-salt structure of the TaN phase and correspond to the (1 1 1), (2 0 0), and (2 2 0) orientations, respectively [14,18,19]. At a N2 /Ar flow ratio of 0.08, the XRD spectrum exhibits a highly (1 1 1) preferred orientation (Fig. 5a). On increasing the N2 /Ar flow ratio to 0.25 (Fig. 5d), the intensities of (1 1 1), (2 0 0) and (2 2 0) peaks broaden corresponding to smaller grains. However, when the N2 /Ar flow ratio exceeds 0.33, only dispersive peaks appear in the XRD pattern of the film (Fig. 5f and g). In addition, the peak shifts to a lower diffraction angle 2θ with an increasing N2 /Ar flow ratio as shown in Fig. 5. Fig. 6 depicts XTEM bright-field micrographs with electron diffraction patterns of the films grown at a bias of 0 V and different N2 /Ar flow ratios. Fig. 6a indicates that at a N2 /Ar flow ratio of 0.08, the deposited films on Si are of polycrystalline TaN. According to Fig. 5a, the grains in the bright-field image (Fig. 6a) have a columnar structure, indicating the existence of a preferred orientation (1 1 1). Increasing the N2 /Ar flow ratio to 0.25 allows us to obtain TaN
269
Fig. 7. AFM micrographs of TaN films deposited at a N2 /Ar flow ratio of (a) 0.08, (b) 0.25, and (c) 0.33.
films with columnar grains again as shown in Fig. 6b. This finding corresponds to the observation in Fig. 5e that (1 1 1) is still the preferred orientation. In addition, microcrystalline TaN is observed at N2 /Ar flow ratios above 0.33. Fig. 6c presents the typical XTEM micrograph of microcrystalline TaN deposited at 0.33 flow ratio, which correlates with the dispersive peaks in Fig. 5f and g. Moreover, the diffraction patterns in Fig. 6 reveal that the film is a rock-salt structure which is identical to that reported by Gerstenberg and Calbick[19]. Fig. 7 depicts the atomic force microscopy (AFM) micrographs of the TaN films prepared at a bias of 0 V and N2 /Ar
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W.-H. Lee et al. / Materials Chemistry and Physics 68 (2001) 266–271
Fig. 8. Grain density of reactively sputtered TaN films vs. N2 /Ar flow ratio.
flow ratios of 0.08, 0.25 and 0.33, respectively. The micrographs contain spherical bumps of similar sizes. The spatial arrangement of the grains appears random, and the lines separating adjacent grains are assumed to be grain boundaries. In Fig. 7c, very fine grains of TaN are observed. In addition, Fig. 7 reveals that the grain size decreases with an increasing N2 /Ar flows ratio. Fig. 8 plots the grain density measured from the AFM micrographs versus N2 /Ar flow ratio. The grain density also takes off around the 0.2 N2 /Ar flow ratio. Reactively sputtered TaNx film has a NaCl structure with a series of disordered vacancies mainly at N sublattice site if x<1.0 and mainly at Ta sublattice sites if x>1.0 [20]. Fig. 9 illustrates lattice constant changes, obtained from X-ray anal-
ysis (Fig. 5), as a function of film composition. In sum, the TaN lattice is distorted due to the change of N concentration and formation of a nitrogen-defect lattice. The lattice parameters for superstoichiometric TaNx (x>1.0) are usually larger than the equilibrium bulk value(4.40 Å), indicating that the interstitial N atoms should also be present besides Ta vacancies. Based on the above discussion, we can conclude that this compound exhibits a considerable homogeneity range extending from the stoichiometric composition to significantly lower N concentrations. During the deposition, the growing TaN film is under the bombardment of positive ions as observed by Shinoki and Itoh [16]. Their work [16] used a mass spectrometer system capable of determining the mass and energy spectra of positive ionic species impinging on the substrate surface during the sputter deposition of thin films. The sputtering system was a planar diode RF sputtering configuration. According to their results, the N2 + and N+ ion currents increased with an increase of nitrogen partial pressure. At N2 /Ar flow ratios less than 0.33, the XRD spectrum showed a highly (1 1 1) preferred orientation (Fig. 5). This phenomenon may be attributed to that under ion irradiation, the polycrystalline film consists mainly of crystals with their (1 1 1) crystallographic axis parallel to the surface normal. This is the energetically most stable growth direction.
4. Conclusion This work deposits tantalum nitride (TaN) films on silicon substrates by RF sputtering of Ta in N2 /Ar gas mixtures at a bias of 0 V. Experimental results indicate that the deposition rate, chemical composition, and crystalline structure of the deposited film depend on the N2 /Ar flow ratio. A stepwise change in deposition rate is observed at a N2 /Ar flow ratio of 0.2. As the N2 /Ar flow ratio increases, the N/Ta ratio increases steeply at first and, then, becomes saturated around 1.2 at 0.25 N2 /Ar flow ratio and above. The TaN lattice is distorted due to the change of N concentration and a nitrogen-defect lattice is formed. Crystalline TaN can be deposited on Si substrates at a N2 /Ar flow ratio of 0.25 and lower, whereas microcrystalline TaN is obtained at N2 /Ar flow ratios higher than 0.25. References [1] [2] [3] [4] [5] [6]
Fig. 9. Lattice constant of FCC-TaN, deduced from XRD analysis (Fig. 5), as a function of film composition. Solid line indicates the trend of these data.
M.-A. Nicolet, Thin Solid Films 52 (1978) 415. S. Kanamori, Thin Solid Films 136 (1986) 195. B. Mehrotra, J. Stimmell, J. Vac. Sci. Technol. B 5 (6) (1987) 1736. M. Wittmer, J. Appl. Phys. 53 (1982) 1007. M. Wittmer, Appl. Phys. Lett. 37 (1980) 540. M.A. Farooq, S.P. Murarka, C.C. Chang, F.A. Baiocchi, J. Appl. Phys. 65 (8) (1989) 3017. [7] T. Oku, E. Kawakami, M. Uekubo, K. Takahiro, S. Yamaguchi, M. Murakami, Appl. Surf. Sci. 99 (1996) 265. [8] A. Noya, K. Sasaki, M. Takeyama, Jpn. J. Appl. Phys. 32 (1993) 911.
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