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Synthesis and characterization of low pressure chemically vapor deposited titanium nitride films using TiCl4 and NH3
December 2002
Materials Letters 57 (2002) 261 – 269 www.elsevier.com/locate/matlet
Synthesis and characterization of low pressure chemically vapor deposited titanium nitride films using TiCl4 and NH3 N. Ramanuja a, R.A. Levy a,*, S.N. Dharmadhikari a, E. Ramos a, C.W. Pearce b, S.C. Menasian b, P.C. Schamberger c, C.C. Collins c a
Physics Department, New Jersey Institute of Technology, 323 King Boulevard, University Heights, Newark, NJ 07102, USA b Agere Systems, Allentown, PA 18103, USA c Diamonex Incorporated, Allentown, PA 18104, USA Received 22 February 2002; accepted 23 February 2002
Abstract This study investigates the inter-relationships governing the growth kinetics, composition, and properties of titanium nitride (TiN) films synthesized by low pressure chemical vapor deposition (LPCVD) using titanium tetrachloride (TiCl4) and ammonia (NH3) as reactants. In the deposition temperature regime of 450 to 600 jC, an Arrhenius dependence was observed from which an activation energy of 42 kJ/mol was calculated. The growth rate dependencies on the partial pressures of NH3 (50 to 100 mTorr) and TiCl4 (1 to 12 mTorr) yielded reaction rate orders of 1.37 and 0.42, respectively. RBS spectrometry was used for establishing the Ti/N ratio and the chlorine content of the films as a function of the processing variables. Films with compositions trending towards stoichiometry were produced as the deposition temperature was decreased and the NH3 partial pressure was increased. The chlorine concentration in the films was observed to decrease from 7.2% (a/o) at the deposition temperature of 450 jC down to 0.15% at 850 jC. The film density values increased from 3.53 to 5.02 g/cm3 as the deposition temperature was increased from 550 to 850 jC. The resistivity of the films was dependent on changes in deposition temperature and flow rate ratios. The lowest resistivity value of 86 AV cm was measured for a deposition temperature of 600 jC and an NH3/TiCl4 flow ratio of 10/1. The film stress was found to be tensile for all deposits and to decrease with higher deposition temperatures. Nano-indentation measurements yielded values for the hardness and Young’s modulus of the films to be around 15 and 250 GPa, respectively. X-ray diffraction measurements revealed in all cases the presence of cubic TiN phase with a preferred (200) orientation. For the investigated aspect ratios of up to 4:1, the deposits were observed to exhibit conformal step coverage over the investigated range of processing conditions. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium nitride films; TiCl4; NH3
1. Introduction Titanium nitride has been extensively investigated because of its numerous applications which include its *
Corresponding author. Tel.: +1-973-596-3561; fax: +1-973596-8369. E-mail address: [email protected] (R.A. Levy).
use as a wear resistant coating on tools [1], a gold substitute in decorative items, a solar energy absorber [2], an IR reflector, and a thin film resistor [3]. Due to its thermal stability, good diffusion barrier properties, and low electrical resistivity [4,5], TiN is being considered as a contact/barrier layer to silicon and as a gate electrode in MOS circuits [6,7]. The properties of TiN generally depend on composition (Ti/N
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 7 7 6 - 0
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ratio), impurity content (i.e. O, Cl), and structure, which are established by the choice in deposition technique and processing parameters. Although TiN films have been conventionally deposited by reactive sputtering [8], LPCVD offers the superior step coverage required of high aspect ratio structures used in integrated circuits (ICs). This study investigates the inter-relationships governing the growth kinetics, resulting compositions, and properties of titanium nitride (TiN) films synthesized by low pressure chemical vapor deposition (LPCVD) using titanium tetrachloride (TiCl4) and ammonia (NH3) as reactants. The growth kinetics are examined as a function of deposition temperature and partial pressure of reactants in order to determine the activation energy and rate constants of the reaction. The electrical, mechanical, and structural properties of the films are determined and correlated to the processing parameters and film compositions. In view of the reliability threat created by the presence of chlorine in films used in microelectronic fabrication, the processing conditions leading to minimal chlorine incorporation in the films are also examined.
2. Experimental procedures Synthesis of the TiN films was carried out in an Advanced Semiconductor Materials (ASM) micropressure CVD system (Fig. 1). The horizontal reaction chamber consists of a 13.5-cm diameter and 135-cm long fused silica tube mounted within a 10-kW Thermco three-zone heating furnace. The furnace temperature was kept constant across all three zones and confirmed to be uniform within 1% over a 30-cm region using a calibrated type K thermocouple. The back end of the reaction chamber was connected to a vacuum station consisting of a Leybold – Heraeus TRIVAC dual stage rotary vane pump and a Leybold – Heraeus RUVAC Roots blower. The TiCl4, which is a light yellow corrosive liquid with a boiling point of 136.4 jC and freezing point of 25 jC at atmospheric pressure, was held at 50 jC and the vapor was introduced to a MKS vapor source mass flow controller leading to the reactor. Two UNIT mass flow controllers (UFC-1100) were used to control the flow of ammonia and the argon gas into the reaction chamber. The pressure in the reactor was measured at the reactor inlet using an MKS baratron pressure
Fig. 1. Schematic representation of the LPCVD reactor.
N. Ramanuja et al. / Materials Letters 57 (2002) 261–269
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gauge. The partial pressure of the reactants was calculated from the reactant flows fed into the reactor, using the following expression:
Preactant ¼
reactant flow rate ðsccmÞ Ptotal : total flow rate ðsccmÞ
P-type <111> single side polished silicon wafers with a diameter of 10 cm and thickness of around 525 Am were used as substrates. RBS measurements using a 2-MeV He+ beam were taken to determine the stoichiometry and chlorine concentration of the film. The stress in the films was determined by measuring the changes in radius of curvature of a wafer resulting from deposition on a single side using a dual laser beam apparatus, before and after film deposition [9]. Film thickness and sheet resistance were determined using a Sloan II-A Dektak profilometer and a Veeco four point probe, respectively, with the product of these two parameters yielding a value for resistivity. For all deposits, the thickness uniformity across wafers was found to be within F5%. The hardness and Young’s modulus of the TiN films were measured using a Nano Instruments NANO-II nanoscope indenter. A Berkovich style indenter was employed to form five indents at three different indent depths (50, 100, and 150 nm) with an indenter load of 0 to 20 mN. Silicon was analyzed as a hardness standard. These values are reported as an average of the five indents referenced to silicon as 10.8 GPa. X-ray diffraction data were taken on a Phillips X-ray diffractometer using Cu ka radiation at 40 kV and 45 mA. SEM cross-sections were used to reveal the morphology and step coverage of deposits made in trenches of differing aspect ratios.
3. Results and discussion 3.1. Growth kinetics and film composition At a fixed total pressure of 70 mTorr and NH3 flow rate of 50 sccm, the growth rate dependence on deposition temperature is shown in Fig. 2 for NH3/ TiCl4 flow rate ratios of 5:1 and 10:1. For both these flow rate ratios, an apparent Arrhenius behavior was observed yielding an apparent activation energy of 42
Fig. 2. Variation of growth rate as a function of reciprocal temperature.
kJ/mol. For the NH3/TiCl4 flow rate ratio of 10:1, the growth rate is seen to saturate above 550 jC reflecting the onset of the mass transfer limited regime. No such saturation was noted for the flow rate ratio of 5:1 up to the investigated deposition temperature of 850 jC. It is worth noting that NH3 and TiCl4 react to form complexes (TiCl4.nNH3) [10 – 13] in the gaseous phase. Among them, the yellow powder of TiCl4.2NH3 can be formed between room temperature and 250 jC [13]. Formation of that powder was indeed evident in the front and exhaust ends of the reactor tube, which was situated outside the heating zone of the furnace. In order to minimize this undesirable by-product formation, the TiCl4 gas was introduced through a stainless steel tube into the hot reaction zone and allowed to mix with the NH3 in proximity to the wafers. Fig. 3 illustrates the dependence of elemental film composition on deposition temperature in the two regimes where the NH3/TiCl4 flow rate ratio was 5:1 and 10:1. For both these flow rate ratios, it appears that the films are Ti rich and become progressively richer as the deposition temperature is increased. At the lowest investigated deposition temperature of 450 jC and flow rate ratio of 10:1, the Ti/N ratio was closest to the stoichiometric value. The increase in Ti concentration with higher deposition temperatures was compensated by a decrease in the Cl content of the films. Over the temperature range of 450– 850 jC, the Cl content in the films decreased from 7.2% (a/o) down to 0.15%. The reaction orders for the TiCl4 and NH3 reaction were determined for a deposition temperature of 600 jC and a total pressure of 110 mTorr. These con-
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Fig. 5. Growth rate dependence on partial pressure of NH3. Fig. 3. Dependence of elemental film composition on deposition temperature.
ditions were selected to allow a sufficient partial pressure span, while maintaining a reasonable deposition rate and a Cl content deemed acceptable. Keeping the NH3 partial pressure constant at 50 mTorr, the partial pressure of TiCl4 was varied from 1 to 12 mTorr while also varying the argon carrier gas partial pressure in order to maintain a constant total pressure within the reaction chamber. Similarly, while keeping the TiCl4 partial pressure constant at 8 mTorr, the partial pressure of NH3 was varied from 50 to 100 mTorr with corresponding variations in argon partial pressure to maintain the aforementioned total pressure. The growth rates, plotted on a double logarithmic scale as a function of the partial pressures of TiCl4 and NH3, are shown in Figs. 4 and 5, respectively. These plots indicate an increase in growth rate with the decrease in TiCl4 partial pressure and the increase in NH3 partial pressure in agreement with the results of Buiting et. al. [10]. From the slopes of these
Fig. 4. Growth rate dependence on partial pressure of TiCl4.
plots, the reaction orders with respect to TiCl4 and NH3 were calculated as 0.42 and 1.37, respectively. Knowing the values of the activation energy and the rate orders with respect to NH3 and TiCl4 from the experimental deposition rate results, the following rate equation is established: r ¼ 4:35 105 expð5150=T ÞðPNH3 Þ1:37 ðPTiCl4 Þ0:42 where: r=deposition rate, mol cm2 min1; T=deposition temperature, K; Pa=partial pressure of reactant a, Pa. The occurrence of the negative order of the reaction with respect to TiCl4 suggests that the reaction mechanism may involve: (1) competitive adsorption of two or more gases on the surface and a reaction rate which is dependent on the surface fractions covered by these two gases [Langmuir – Hinshelwood-like mechanism] and/or (2) competitive reaction paths consisting of the intended deposition reaction and parallel reactions. In the later case, increasing partial pressure of TiCl4 could drive the reaction towards the complex formation, which in turn results in the decrease in TiN formation, as less NH3 is available. The rate equation obtained is consistent with the model put forth by Imhoff et al. [11] using Langmuir– Hinshelwood kinetics for the case of competitive adsorption. At low partial pressures of the reactant, the growth rate increases with the partial pressure of the reactant resulting in a positive reaction order. At high partial pressures of the reactant, the growth rate decreases with increasing reactant partial pressure resulting in a negative reaction order. Pintchovski et al. [14] reported positive reaction orders with
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Fig. 6. Variation of elemental film composition with TiCl4 partial pressure.
respect to both NH3 and TiCl4. This result does not conflict with our observations because their experiments were performed by using both NH3 and TiCl4 injections far from the substrate. Neither NH3 nor TiCl4 can act as the poisonous species, so growth rate always increases with increasing NH3 or TiCl4 partial pressures. Imhoff et al. [11] have injected NH3 near the substrate and reported a reaction order of 0.63. In the present study, TiCl4 having being introduced close to the wafers may also explain the observed negative rate order. Our results are consistent with those of Buiting et al. [10] who similarly introduced TiCl4 close to the wafers and reported rate orders of 0.5 and 1.37 for TiCl4 and NH3, respectively. Moreover, Buiting et al. [10] operated in a deposition temperature regime of 400 to 700 jC and at a pressure of 150 mTorr which resulted in an apparent activation
Fig. 7. Variation of elemental film composition with NH3 partial pressure.
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Fig. 8. Variation of film resistivity and chlorine content with deposition temperature.
energy of 61 KJ/mol, while Imhoff et al. [11] operated from 700 to 850 jC at pressures varying from 150 to 550 mTorr and reported an apparent activation energy of 78.9 kJ/mol. The differences among Buiting et al. [10], Imhoff et al. [11], and our value for the apparent activation energy as well as the rate orders with respect to NH3 and TiCl4 are consistent if the relative strength of adsorption of TiCl4 is greater than NH3 above 700 jC while that of NH3 is greater than TiCl4 below 700 jC. Evidence of this was observed at 700 jC where the growth rate is seen to be independent of TiCl4 flow indicating a transition in the strength of adsorption from NH3 to TiCl4. The availability of sufficient TiCl4 for the reaction may explain the absence of a mass transfer controlled regime for NH3/TiCl4 flow ratio of 5/1 up to the investigated deposition temperature of 850 jC.
Fig. 9. Film stress dependence on deposition temperature.
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partial pressures of both reactants appear to have no effect on the incorporated Cl content of the films. 3.2. Characterization study
Fig. 10. Variation of Hardness and Young’s modulus with deposition temperature.
The dependence of film composition on partial pressure of the reactants at 600 jC is shown in Figs. 6 and 7. The Ti/N ratio appears not to vary significantly with changes in the TiCl4 partial pressure. This may be explained by the preferential adsorption of that specie as a result of the close proximity of the TiCl4 delivery tube to the wafers. On the other hand, the increase in NH3 partial pressure has the effect of reducing the Ti/N ratios to values that progressively approach stoichiometry. This results from the higher number of NH3 molecules available to compete with TiCl4 for available adsorption sites. Changes in the
The color of the TiN deposits was found to be dependent on deposition temperature. In the temperature range of 450 – 600 jC, the films appear to be reddish while from 600 to 800 jC, the color turned to brownish. At 800 jC and above, the deposits exhibited the golden color characteristic of TiN. This color change is believed to be due to higher concentrations of Ti in the films as the temperature is increased which causes a shift in the absorbed spectra towards higher energies [15]. The density of the films was determined by dividing weight gain by film volume as calculated from the product of film thickness and known area of the wafer. For the two investigated flow rate ratios, the density was observed to increase from 3.53 to 5.02 g/cm3 as the temperature was increased from 550 to 850 jC. This variation is indicative of the deviation from stoichiometry and the Ti enrichment that occurs with higher deposition temperatures. The highest observed density value is near the bulk 5.22 g/cm3 reported in the CRC handbook for cubic TiN [16]. At a constant deposition temperature of 600 jC and total pressure of 110 mTorr, the density of the films showed no significant variation with changes in the partial pres-
Fig. 11. Typical X-ray diffraction spectrum of a deposited TiN films.
N. Ramanuja et al. / Materials Letters 57 (2002) 261–269
sures of either NH3 or TiCl4. This is indicative of the minor compositional changes occurring with these two parameters. The variation in film resistivity and chlorine composition as a function of deposition temperature for the two investigated flow rate ratios is shown in Fig. 8. It appears that both resistivity and chlorine content decrease monotonically with higher deposition temperature. At the NH3/TiCl4 flow rate ratio of 10:1 and the total pressure of 70 mTorr, an increase in temperature from 450 to 600 jC causes the resistivity to drop from 140 to 86 AV cm and the chlorine content to drop from 7.2% to 0.15% (a/o). This trend holds for the lower flow rate ratio of 5:1 where the resistivity drops from f230 to f135 AV cm while the Cl content drops from 2% to 0.15% (a/o) with higher deposition temperatures. The parallel drop between resistivity and the chlorine content leads to the conclusion that chlorine incorporation directly affects the film resistivity. The increase in NH3/TiCl4 flow rate ratio at a fixed temperature causes the films to approach the stoichiometric composition, which explains the trend towards lower resistivity values. Our lowest observed value is comparable to that of 80 AV cm reported by Yokoyama et al. [17] for a LPCVD TiN deposited at 700 jC. Those values are still approximately a factor of two higher than the 39 AV cm reported for r.f. sputtered TiN films [18] with the difference being possibly attributed to the absence of Cl in the sputtered films. It is worth noting that both PVD and CVD techniques yield resistivity values that are significantly higher than the 22 AV cm reported for bulk TiN [3]. All films in this study showed excellent adherence to the substrate and no signs of cracking. The temperature-dependent behavior of film stress for the two investigated flow rate ratios is shown in Fig. 9. The stress was found to be tensile in all cases and to decrease with higher deposition temperatures. For the NH3/TiCl4 flow rate ratio of 10/1, this decrease may be attributed to the drop in the Cl content from 7.2% down to 0.15% (a/o). For the NH3/TiCl4 flow rate ratio of 5/1, the decrease in stress may be explained by considering the fact that as the deposition temperature is increased from 600 to 800 jC, the intrinsic stress decreases due to the promotion of defect annealing and grain growth. At the constant deposition temperature of 600 jC, the film stress data tracked closely
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the compositional changes resulting from the variation in the reactant partial pressures. For the film composition that was closest to stoichiometry (NH3 partial pressure of 100 mTorr), the stress had a value of 197 MPa. Fig. 10 illustrates the variation in the values of hardness and Young’s modulus as a function of deposition temperature for both investigated flow rate ratios. The hardness is seen to decrease from 17 to 13 GPa while the Young’s modulus increases from 80 to 350 GPa over the temperature range of 575– 850 jC. This behavior reflects the increase in Ti concentration
Fig. 12. SEM micrographs of TiN filled vias at deposition temperatures of 450 (a and c) and 600 jC (b).
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occurring with higher deposition temperatures. Titanium being a relatively soft element with high strength, its incremental presence in the film would explain this observed dependency. A typical X-ray diffraction pattern of LPCVD TiN ˚ with scan film having a thickness of around 3000 A duration of over 6 h is shown in Fig. 11. All the major peaks, e.g. (111), (200) and (220) were identified, confirming the presence of cubic TiN phase. From the ˚ for the (200) plane, the lattice d-value of 2.13 A ˚ . An examination constant was calculated as 4.26 A of the relative intensity indicates that the films exhibit a preferred (200) orientation, consistent with the findings of Oh and Je [19], who reported similar ˚. results for films thinner than f6000 A Fig. 12 illustrates the step coverage of TiN films deposited at 475 jC for aspect ratios of 2:1 and 4:1 and at 600 jC for an aspect ratio of f2:1. It’s apparent that the step coverage is quite conformal at both deposition temperatures and for both aspect ratios. A plug exhibiting no voids is readily observed for the aspect ratio of 4:1. Columnar growth is evident in all three micrographs, which impacts the surface topography and is typical for metallic films deposited at this temperature regime.
of temperature was found to follow an Arrhenius behavior in the temperature range of 450 –550 jC, yielding an apparent activation energy of 42 kJ/mol. The RBS results show that the films produced were nearly stoichiometric with chlorine atomic concentration ranging from 0.15% to 7.2% (a/o). The electrical, mechanical, and structural properties of the TiN films were also determined. The film density values were seen to increase from 3.53 to 5.02 g/cm3 as the deposition temperature was increased from 550 to 850 jC. The resistivity of the films was found to be dependent on changes in deposition temperature and flow rate ratios. The lowest resistivity value of 86 AV cm was measured for a deposition temperature of 600 jC and an NH3/TiCl4 flow ratio of 10/1. The film stress was tensile for all deposits and was seen to decrease with higher deposition temperatures. Nanoindentation measurements yielded values for the hardness and Young’s modulus of the films to be around 15 and 250 GPa, respectively. X-ray diffraction studies indicated that the cubic phase was present in the TiN films while the SEM revealed that the films were highly conformal.
Acknowledgements 4. Conclusions In this study, we have investigated the inter-relationships governing the growth kinetics, resulting compositions, and properties of titanium nitride (TiN) films synthesized by low pressure chemical vapor deposition (LPCVD) using titanium tetrachloride (TiCl4) and ammonia (NH3) as reactants. The kinetics of the reaction between TiCl4 and NH3 was first established resulting to formulation of a rate equation showing the dependence of the reaction on the partial pressures of both reactants, which is shown as follows: r ¼ 4:35 105 expð5150=T ÞðPNH3 Þ1:37 ðPTiCl4 Þ0:42 : For the reactor geometry used, the reaction orders with respect to NH3 and TiCl4 were found to be 1.37 and 0.42, respectively. Deposition rate as a function
The authors would like to thank V. Sigal for his invaluable help in maintaining the experimental apparatus as well as J.M. Grow and N.M. Ravindra for the many fruitful discussions.
References [1] W. Schintlmeister, O. Pacher, K. Pfaffinger, J. Electrochem. Soc. 123 (1976) 924. [2] L. Roux, J. Hanus, J.C. Francois, M. Sigirist, Sol. Energy Mater. 7 (1982) 299. [3] S.R. Kurtz, R.G. Gordon, Thin Solid Films 140 (1986) 277. [4] M.A. Nicolet, Thin Solid Films 52 (1978) 415. [5] M. Ostling, S. Nygren, C.S. Petersson, H. Norstrom, R. Butcha, H.O. Blom, S. Berg, Thin Solid Films 145 (1986) 81. [6] M. Wittmer, J. Noser, H. Melchoir, J. Appl. Phys. 52 (1981) 6659. [7] C.Y. Ting, J. Vac. Sci. Technol. 21 (1982) 14. [8] M. Wittmer, B. Studer, H. Melchoir, J. Appl. Phys. 52 (1981) 5722. [9] R.A. Levy, J.M. Grow, G.S. Chakravarthy, Chem. Mater. 5 (1993) 1710.
N. Ramanuja et al. / Materials Letters 57 (2002) 261–269 [10] M.J. Buiting, A.F. Otterloo, A.H. Montree, J. Electrochem. Soc. 138 (1991) 500. [11] L. Imhoff, A. Bouteville, J.C. Remy, J. Electrochem. Soc. 145 (1998) 1672. [12] J. Joho, A. Kato, J. Jpn. Ceram. Soc. 89 (1981) 277. [13] G.W.A. Fowles, F.H. Pollard, J. Chem. Soc., (1953) 2588. [14] F. Pintchovski, T. White, E. Travis, P.J. Tobin, J.B. Price, Tungsten and Other Refractory Metals for ULSI Applications IV, MRS, Pittsburgh, PA, 1989, p. 323.
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[15] Ph. Roquiny, F. Bodart, G. Terwagne, Surface and Coatings Technology 116 – 119 (1999) 278 – 283. [16] CRC Handbook of Chemistry and Physics, 60th edn., CRC Press, Boca Raton, 1979 – 1980. [17] N. Yokoyama, K. Hinode, Y. Homma, J. Electrochem. Soc. 138 (1991) 190. [18] F. Marchetti, M. Dapor, S. Girardi, F. Giacomozzi, A. Cavalleri, Mater. Sci. Eng., A 115 (1989) 217. [19] U.C. Oh, J.H. Je, J. Mater. Res. 13 (1998) 1225.
Materials Letters 57 (2002) 261 – 269 www.elsevier.com/locate/matlet
Synthesis and characterization of low pressure chemically vapor deposited titanium nitride films using TiCl4 and NH3 N. Ramanuja a, R.A. Levy a,*, S.N. Dharmadhikari a, E. Ramos a, C.W. Pearce b, S.C. Menasian b, P.C. Schamberger c, C.C. Collins c a
Physics Department, New Jersey Institute of Technology, 323 King Boulevard, University Heights, Newark, NJ 07102, USA b Agere Systems, Allentown, PA 18103, USA c Diamonex Incorporated, Allentown, PA 18104, USA Received 22 February 2002; accepted 23 February 2002
Abstract This study investigates the inter-relationships governing the growth kinetics, composition, and properties of titanium nitride (TiN) films synthesized by low pressure chemical vapor deposition (LPCVD) using titanium tetrachloride (TiCl4) and ammonia (NH3) as reactants. In the deposition temperature regime of 450 to 600 jC, an Arrhenius dependence was observed from which an activation energy of 42 kJ/mol was calculated. The growth rate dependencies on the partial pressures of NH3 (50 to 100 mTorr) and TiCl4 (1 to 12 mTorr) yielded reaction rate orders of 1.37 and 0.42, respectively. RBS spectrometry was used for establishing the Ti/N ratio and the chlorine content of the films as a function of the processing variables. Films with compositions trending towards stoichiometry were produced as the deposition temperature was decreased and the NH3 partial pressure was increased. The chlorine concentration in the films was observed to decrease from 7.2% (a/o) at the deposition temperature of 450 jC down to 0.15% at 850 jC. The film density values increased from 3.53 to 5.02 g/cm3 as the deposition temperature was increased from 550 to 850 jC. The resistivity of the films was dependent on changes in deposition temperature and flow rate ratios. The lowest resistivity value of 86 AV cm was measured for a deposition temperature of 600 jC and an NH3/TiCl4 flow ratio of 10/1. The film stress was found to be tensile for all deposits and to decrease with higher deposition temperatures. Nano-indentation measurements yielded values for the hardness and Young’s modulus of the films to be around 15 and 250 GPa, respectively. X-ray diffraction measurements revealed in all cases the presence of cubic TiN phase with a preferred (200) orientation. For the investigated aspect ratios of up to 4:1, the deposits were observed to exhibit conformal step coverage over the investigated range of processing conditions. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium nitride films; TiCl4; NH3
1. Introduction Titanium nitride has been extensively investigated because of its numerous applications which include its *
Corresponding author. Tel.: +1-973-596-3561; fax: +1-973596-8369. E-mail address: [email protected] (R.A. Levy).
use as a wear resistant coating on tools [1], a gold substitute in decorative items, a solar energy absorber [2], an IR reflector, and a thin film resistor [3]. Due to its thermal stability, good diffusion barrier properties, and low electrical resistivity [4,5], TiN is being considered as a contact/barrier layer to silicon and as a gate electrode in MOS circuits [6,7]. The properties of TiN generally depend on composition (Ti/N
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 7 7 6 - 0
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ratio), impurity content (i.e. O, Cl), and structure, which are established by the choice in deposition technique and processing parameters. Although TiN films have been conventionally deposited by reactive sputtering [8], LPCVD offers the superior step coverage required of high aspect ratio structures used in integrated circuits (ICs). This study investigates the inter-relationships governing the growth kinetics, resulting compositions, and properties of titanium nitride (TiN) films synthesized by low pressure chemical vapor deposition (LPCVD) using titanium tetrachloride (TiCl4) and ammonia (NH3) as reactants. The growth kinetics are examined as a function of deposition temperature and partial pressure of reactants in order to determine the activation energy and rate constants of the reaction. The electrical, mechanical, and structural properties of the films are determined and correlated to the processing parameters and film compositions. In view of the reliability threat created by the presence of chlorine in films used in microelectronic fabrication, the processing conditions leading to minimal chlorine incorporation in the films are also examined.
2. Experimental procedures Synthesis of the TiN films was carried out in an Advanced Semiconductor Materials (ASM) micropressure CVD system (Fig. 1). The horizontal reaction chamber consists of a 13.5-cm diameter and 135-cm long fused silica tube mounted within a 10-kW Thermco three-zone heating furnace. The furnace temperature was kept constant across all three zones and confirmed to be uniform within 1% over a 30-cm region using a calibrated type K thermocouple. The back end of the reaction chamber was connected to a vacuum station consisting of a Leybold – Heraeus TRIVAC dual stage rotary vane pump and a Leybold – Heraeus RUVAC Roots blower. The TiCl4, which is a light yellow corrosive liquid with a boiling point of 136.4 jC and freezing point of 25 jC at atmospheric pressure, was held at 50 jC and the vapor was introduced to a MKS vapor source mass flow controller leading to the reactor. Two UNIT mass flow controllers (UFC-1100) were used to control the flow of ammonia and the argon gas into the reaction chamber. The pressure in the reactor was measured at the reactor inlet using an MKS baratron pressure
Fig. 1. Schematic representation of the LPCVD reactor.
N. Ramanuja et al. / Materials Letters 57 (2002) 261–269
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gauge. The partial pressure of the reactants was calculated from the reactant flows fed into the reactor, using the following expression:
Preactant ¼
reactant flow rate ðsccmÞ Ptotal : total flow rate ðsccmÞ
P-type <111> single side polished silicon wafers with a diameter of 10 cm and thickness of around 525 Am were used as substrates. RBS measurements using a 2-MeV He+ beam were taken to determine the stoichiometry and chlorine concentration of the film. The stress in the films was determined by measuring the changes in radius of curvature of a wafer resulting from deposition on a single side using a dual laser beam apparatus, before and after film deposition [9]. Film thickness and sheet resistance were determined using a Sloan II-A Dektak profilometer and a Veeco four point probe, respectively, with the product of these two parameters yielding a value for resistivity. For all deposits, the thickness uniformity across wafers was found to be within F5%. The hardness and Young’s modulus of the TiN films were measured using a Nano Instruments NANO-II nanoscope indenter. A Berkovich style indenter was employed to form five indents at three different indent depths (50, 100, and 150 nm) with an indenter load of 0 to 20 mN. Silicon was analyzed as a hardness standard. These values are reported as an average of the five indents referenced to silicon as 10.8 GPa. X-ray diffraction data were taken on a Phillips X-ray diffractometer using Cu ka radiation at 40 kV and 45 mA. SEM cross-sections were used to reveal the morphology and step coverage of deposits made in trenches of differing aspect ratios.
3. Results and discussion 3.1. Growth kinetics and film composition At a fixed total pressure of 70 mTorr and NH3 flow rate of 50 sccm, the growth rate dependence on deposition temperature is shown in Fig. 2 for NH3/ TiCl4 flow rate ratios of 5:1 and 10:1. For both these flow rate ratios, an apparent Arrhenius behavior was observed yielding an apparent activation energy of 42
Fig. 2. Variation of growth rate as a function of reciprocal temperature.
kJ/mol. For the NH3/TiCl4 flow rate ratio of 10:1, the growth rate is seen to saturate above 550 jC reflecting the onset of the mass transfer limited regime. No such saturation was noted for the flow rate ratio of 5:1 up to the investigated deposition temperature of 850 jC. It is worth noting that NH3 and TiCl4 react to form complexes (TiCl4.nNH3) [10 – 13] in the gaseous phase. Among them, the yellow powder of TiCl4.2NH3 can be formed between room temperature and 250 jC [13]. Formation of that powder was indeed evident in the front and exhaust ends of the reactor tube, which was situated outside the heating zone of the furnace. In order to minimize this undesirable by-product formation, the TiCl4 gas was introduced through a stainless steel tube into the hot reaction zone and allowed to mix with the NH3 in proximity to the wafers. Fig. 3 illustrates the dependence of elemental film composition on deposition temperature in the two regimes where the NH3/TiCl4 flow rate ratio was 5:1 and 10:1. For both these flow rate ratios, it appears that the films are Ti rich and become progressively richer as the deposition temperature is increased. At the lowest investigated deposition temperature of 450 jC and flow rate ratio of 10:1, the Ti/N ratio was closest to the stoichiometric value. The increase in Ti concentration with higher deposition temperatures was compensated by a decrease in the Cl content of the films. Over the temperature range of 450– 850 jC, the Cl content in the films decreased from 7.2% (a/o) down to 0.15%. The reaction orders for the TiCl4 and NH3 reaction were determined for a deposition temperature of 600 jC and a total pressure of 110 mTorr. These con-
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Fig. 5. Growth rate dependence on partial pressure of NH3. Fig. 3. Dependence of elemental film composition on deposition temperature.
ditions were selected to allow a sufficient partial pressure span, while maintaining a reasonable deposition rate and a Cl content deemed acceptable. Keeping the NH3 partial pressure constant at 50 mTorr, the partial pressure of TiCl4 was varied from 1 to 12 mTorr while also varying the argon carrier gas partial pressure in order to maintain a constant total pressure within the reaction chamber. Similarly, while keeping the TiCl4 partial pressure constant at 8 mTorr, the partial pressure of NH3 was varied from 50 to 100 mTorr with corresponding variations in argon partial pressure to maintain the aforementioned total pressure. The growth rates, plotted on a double logarithmic scale as a function of the partial pressures of TiCl4 and NH3, are shown in Figs. 4 and 5, respectively. These plots indicate an increase in growth rate with the decrease in TiCl4 partial pressure and the increase in NH3 partial pressure in agreement with the results of Buiting et. al. [10]. From the slopes of these
Fig. 4. Growth rate dependence on partial pressure of TiCl4.
plots, the reaction orders with respect to TiCl4 and NH3 were calculated as 0.42 and 1.37, respectively. Knowing the values of the activation energy and the rate orders with respect to NH3 and TiCl4 from the experimental deposition rate results, the following rate equation is established: r ¼ 4:35 105 expð5150=T ÞðPNH3 Þ1:37 ðPTiCl4 Þ0:42 where: r=deposition rate, mol cm2 min1; T=deposition temperature, K; Pa=partial pressure of reactant a, Pa. The occurrence of the negative order of the reaction with respect to TiCl4 suggests that the reaction mechanism may involve: (1) competitive adsorption of two or more gases on the surface and a reaction rate which is dependent on the surface fractions covered by these two gases [Langmuir – Hinshelwood-like mechanism] and/or (2) competitive reaction paths consisting of the intended deposition reaction and parallel reactions. In the later case, increasing partial pressure of TiCl4 could drive the reaction towards the complex formation, which in turn results in the decrease in TiN formation, as less NH3 is available. The rate equation obtained is consistent with the model put forth by Imhoff et al. [11] using Langmuir– Hinshelwood kinetics for the case of competitive adsorption. At low partial pressures of the reactant, the growth rate increases with the partial pressure of the reactant resulting in a positive reaction order. At high partial pressures of the reactant, the growth rate decreases with increasing reactant partial pressure resulting in a negative reaction order. Pintchovski et al. [14] reported positive reaction orders with
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Fig. 6. Variation of elemental film composition with TiCl4 partial pressure.
respect to both NH3 and TiCl4. This result does not conflict with our observations because their experiments were performed by using both NH3 and TiCl4 injections far from the substrate. Neither NH3 nor TiCl4 can act as the poisonous species, so growth rate always increases with increasing NH3 or TiCl4 partial pressures. Imhoff et al. [11] have injected NH3 near the substrate and reported a reaction order of 0.63. In the present study, TiCl4 having being introduced close to the wafers may also explain the observed negative rate order. Our results are consistent with those of Buiting et al. [10] who similarly introduced TiCl4 close to the wafers and reported rate orders of 0.5 and 1.37 for TiCl4 and NH3, respectively. Moreover, Buiting et al. [10] operated in a deposition temperature regime of 400 to 700 jC and at a pressure of 150 mTorr which resulted in an apparent activation
Fig. 7. Variation of elemental film composition with NH3 partial pressure.
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Fig. 8. Variation of film resistivity and chlorine content with deposition temperature.
energy of 61 KJ/mol, while Imhoff et al. [11] operated from 700 to 850 jC at pressures varying from 150 to 550 mTorr and reported an apparent activation energy of 78.9 kJ/mol. The differences among Buiting et al. [10], Imhoff et al. [11], and our value for the apparent activation energy as well as the rate orders with respect to NH3 and TiCl4 are consistent if the relative strength of adsorption of TiCl4 is greater than NH3 above 700 jC while that of NH3 is greater than TiCl4 below 700 jC. Evidence of this was observed at 700 jC where the growth rate is seen to be independent of TiCl4 flow indicating a transition in the strength of adsorption from NH3 to TiCl4. The availability of sufficient TiCl4 for the reaction may explain the absence of a mass transfer controlled regime for NH3/TiCl4 flow ratio of 5/1 up to the investigated deposition temperature of 850 jC.
Fig. 9. Film stress dependence on deposition temperature.
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partial pressures of both reactants appear to have no effect on the incorporated Cl content of the films. 3.2. Characterization study
Fig. 10. Variation of Hardness and Young’s modulus with deposition temperature.
The dependence of film composition on partial pressure of the reactants at 600 jC is shown in Figs. 6 and 7. The Ti/N ratio appears not to vary significantly with changes in the TiCl4 partial pressure. This may be explained by the preferential adsorption of that specie as a result of the close proximity of the TiCl4 delivery tube to the wafers. On the other hand, the increase in NH3 partial pressure has the effect of reducing the Ti/N ratios to values that progressively approach stoichiometry. This results from the higher number of NH3 molecules available to compete with TiCl4 for available adsorption sites. Changes in the
The color of the TiN deposits was found to be dependent on deposition temperature. In the temperature range of 450 – 600 jC, the films appear to be reddish while from 600 to 800 jC, the color turned to brownish. At 800 jC and above, the deposits exhibited the golden color characteristic of TiN. This color change is believed to be due to higher concentrations of Ti in the films as the temperature is increased which causes a shift in the absorbed spectra towards higher energies [15]. The density of the films was determined by dividing weight gain by film volume as calculated from the product of film thickness and known area of the wafer. For the two investigated flow rate ratios, the density was observed to increase from 3.53 to 5.02 g/cm3 as the temperature was increased from 550 to 850 jC. This variation is indicative of the deviation from stoichiometry and the Ti enrichment that occurs with higher deposition temperatures. The highest observed density value is near the bulk 5.22 g/cm3 reported in the CRC handbook for cubic TiN [16]. At a constant deposition temperature of 600 jC and total pressure of 110 mTorr, the density of the films showed no significant variation with changes in the partial pres-
Fig. 11. Typical X-ray diffraction spectrum of a deposited TiN films.
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sures of either NH3 or TiCl4. This is indicative of the minor compositional changes occurring with these two parameters. The variation in film resistivity and chlorine composition as a function of deposition temperature for the two investigated flow rate ratios is shown in Fig. 8. It appears that both resistivity and chlorine content decrease monotonically with higher deposition temperature. At the NH3/TiCl4 flow rate ratio of 10:1 and the total pressure of 70 mTorr, an increase in temperature from 450 to 600 jC causes the resistivity to drop from 140 to 86 AV cm and the chlorine content to drop from 7.2% to 0.15% (a/o). This trend holds for the lower flow rate ratio of 5:1 where the resistivity drops from f230 to f135 AV cm while the Cl content drops from 2% to 0.15% (a/o) with higher deposition temperatures. The parallel drop between resistivity and the chlorine content leads to the conclusion that chlorine incorporation directly affects the film resistivity. The increase in NH3/TiCl4 flow rate ratio at a fixed temperature causes the films to approach the stoichiometric composition, which explains the trend towards lower resistivity values. Our lowest observed value is comparable to that of 80 AV cm reported by Yokoyama et al. [17] for a LPCVD TiN deposited at 700 jC. Those values are still approximately a factor of two higher than the 39 AV cm reported for r.f. sputtered TiN films [18] with the difference being possibly attributed to the absence of Cl in the sputtered films. It is worth noting that both PVD and CVD techniques yield resistivity values that are significantly higher than the 22 AV cm reported for bulk TiN [3]. All films in this study showed excellent adherence to the substrate and no signs of cracking. The temperature-dependent behavior of film stress for the two investigated flow rate ratios is shown in Fig. 9. The stress was found to be tensile in all cases and to decrease with higher deposition temperatures. For the NH3/TiCl4 flow rate ratio of 10/1, this decrease may be attributed to the drop in the Cl content from 7.2% down to 0.15% (a/o). For the NH3/TiCl4 flow rate ratio of 5/1, the decrease in stress may be explained by considering the fact that as the deposition temperature is increased from 600 to 800 jC, the intrinsic stress decreases due to the promotion of defect annealing and grain growth. At the constant deposition temperature of 600 jC, the film stress data tracked closely
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the compositional changes resulting from the variation in the reactant partial pressures. For the film composition that was closest to stoichiometry (NH3 partial pressure of 100 mTorr), the stress had a value of 197 MPa. Fig. 10 illustrates the variation in the values of hardness and Young’s modulus as a function of deposition temperature for both investigated flow rate ratios. The hardness is seen to decrease from 17 to 13 GPa while the Young’s modulus increases from 80 to 350 GPa over the temperature range of 575– 850 jC. This behavior reflects the increase in Ti concentration
Fig. 12. SEM micrographs of TiN filled vias at deposition temperatures of 450 (a and c) and 600 jC (b).
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occurring with higher deposition temperatures. Titanium being a relatively soft element with high strength, its incremental presence in the film would explain this observed dependency. A typical X-ray diffraction pattern of LPCVD TiN ˚ with scan film having a thickness of around 3000 A duration of over 6 h is shown in Fig. 11. All the major peaks, e.g. (111), (200) and (220) were identified, confirming the presence of cubic TiN phase. From the ˚ for the (200) plane, the lattice d-value of 2.13 A ˚ . An examination constant was calculated as 4.26 A of the relative intensity indicates that the films exhibit a preferred (200) orientation, consistent with the findings of Oh and Je [19], who reported similar ˚. results for films thinner than f6000 A Fig. 12 illustrates the step coverage of TiN films deposited at 475 jC for aspect ratios of 2:1 and 4:1 and at 600 jC for an aspect ratio of f2:1. It’s apparent that the step coverage is quite conformal at both deposition temperatures and for both aspect ratios. A plug exhibiting no voids is readily observed for the aspect ratio of 4:1. Columnar growth is evident in all three micrographs, which impacts the surface topography and is typical for metallic films deposited at this temperature regime.
of temperature was found to follow an Arrhenius behavior in the temperature range of 450 –550 jC, yielding an apparent activation energy of 42 kJ/mol. The RBS results show that the films produced were nearly stoichiometric with chlorine atomic concentration ranging from 0.15% to 7.2% (a/o). The electrical, mechanical, and structural properties of the TiN films were also determined. The film density values were seen to increase from 3.53 to 5.02 g/cm3 as the deposition temperature was increased from 550 to 850 jC. The resistivity of the films was found to be dependent on changes in deposition temperature and flow rate ratios. The lowest resistivity value of 86 AV cm was measured for a deposition temperature of 600 jC and an NH3/TiCl4 flow ratio of 10/1. The film stress was tensile for all deposits and was seen to decrease with higher deposition temperatures. Nanoindentation measurements yielded values for the hardness and Young’s modulus of the films to be around 15 and 250 GPa, respectively. X-ray diffraction studies indicated that the cubic phase was present in the TiN films while the SEM revealed that the films were highly conformal.
Acknowledgements 4. Conclusions In this study, we have investigated the inter-relationships governing the growth kinetics, resulting compositions, and properties of titanium nitride (TiN) films synthesized by low pressure chemical vapor deposition (LPCVD) using titanium tetrachloride (TiCl4) and ammonia (NH3) as reactants. The kinetics of the reaction between TiCl4 and NH3 was first established resulting to formulation of a rate equation showing the dependence of the reaction on the partial pressures of both reactants, which is shown as follows: r ¼ 4:35 105 expð5150=T ÞðPNH3 Þ1:37 ðPTiCl4 Þ0:42 : For the reactor geometry used, the reaction orders with respect to NH3 and TiCl4 were found to be 1.37 and 0.42, respectively. Deposition rate as a function
The authors would like to thank V. Sigal for his invaluable help in maintaining the experimental apparatus as well as J.M. Grow and N.M. Ravindra for the many fruitful discussions.
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