Fabrication of nanostructured titanium thin films via N ion implantation and postannealing treatment

Fabrication of nanostructured titanium thin films via N ion implantation and postannealing treatment

Surface & Coatings Technology 188 – 189 (2004) 260 – 264 www.elsevier.com/locate/surfcoat Fabrication of nanostructured titanium thin films via N ion...

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Surface & Coatings Technology 188 – 189 (2004) 260 – 264 www.elsevier.com/locate/surfcoat

Fabrication of nanostructured titanium thin films via N ion implantation and postannealing treatment S. Muraishia,*, T. Aizawaa, H. Kuwaharab a

Research Center for Advanced Science and Technology, The University of Tokyo, Komaba 4-6-1, Meguro-Ku, Tokyo 153-8904, Japan b Research Institute for Applied Science, Tanaka Ohichou 49, Sakyo-Ku, Kyoto 606-8202, Japan Available online 13 September 2004

Abstract Supersaturated Ti(N) thin films have been prepared by the combination of low temperature nonequilibrium processes of ion beam sputtering (IBS) and ion implantation method. Ti thin films of 150 nm in thickness have been deposited on (001) Si substrate by IBS. N+ ions have been penetrated into the films with the dose of 11017~21018 ion/cm2. The structural changes due to the N-implantation and successive heat treatments have been evaluated by X-ray photoelectron spectroscopy (XPS) chemical analysis and cross-sectional transmission electron microscopy (TEM) observation. The distribution of N atoms in N-implanted Ti films has been measured by XPS. The maximum concentration of N was achieved at the Ti/Si interface with beam energy of 100 keV. The linear increase of the binding energy shift in proportion to the N concentration suggests that N-implanted Ti films consist of Ti(N) supersaturated solid solution. From TEM observation, as-deposited Ti films show the conventional hcp structure with columnar grains 10 nm in diameter. These grains with growth direction of [0001]a develop perpendicular to the (001) Si substrate. N-implantation results in the film having the complex structure of aTi(N) solid solution with small amounts of q-Ti2N and y-TiN. Formation of the q-Ti2N is recognized from electron diffraction for the Nimplanted specimen with 11017 ion/cm2 and the y-TiN phase for the specimen with 51017 ion/cm2. The lattice constant has been measured from electron diffraction and the c/a ratio of hcp-Ti(N) increases in proportion to the N concentration toward the theoretical value for cubic structure. The XPS compositional measurement suggests that N-implantation induces the anomaly saturated N in hcp-Ti(N) lattice. The lattice constant of hcp-Ti(N) decreases by postannealing treatment at 773 K. Postannealing promotes the phase decomposition of supersaturated solid solution of hcp-Ti(N) into meta-stable nitrides. D 2004 Elsevier B.V. All rights reserved. Keywords: TEM; XPS; Ion Implantation; Nitride; Ion beam sputter

1. Introduction For the development of nanostructured coating, the nonequilibrium processes of ion beam sputtering (IBS) and N ion implantation have been conducted for fabrication of nanostructure within Ti–N films. The combination of IBS and ion implantation processes would make possible various material designs for the surface irrespective of thermal diffusion phenomena. These * Corresponding author. Department of Metallurgy and Ceramic Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Maguro-ku, Tokyo 152-8552, Japan. Tel.: +81 3 5734 3145; fax: +81 3 5734 3145. E-mail address: [email protected] (S. Muraishi). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.040

advanced processes are expected to produce a nanodistribution structure with functional particles on the surface. Usually, industrial transition metal (TM) nitrides have been used for protective coating against various severe environments because of their superior characteristics such as extreme hardness, high melting point and good thermal and electrical conductivity [1]. The nanocomposite structure of metal nitrides (TiN, AlN, ZrN) and nonmetal nitrides (SiN) are potential candidates for severe wear and corrosion environments; therefore, much attention has been given to the development of fabrication processes aimed at the evolution of nanocomposite nitrides coatings [2,3]. However, because the mixed nitrides sometimes exhibit highly complex

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structure with nonequilibrium phases (e.g., order–disorder, nanophase separation and amorphous), these precipitation processes and structures are not well understood. From this aspect, precise evaluation of the degree of the nonequilibrium state for a typical alloy system is required. Normally, a TM (Ti, Zr and Hf) is known to make large solubility regions with N, which occupies octahedral interstices (O-site) of the TM sublattice and increase the lattice constants [4]. These exhibits large solubility limits both in hcp-TM(N) and nonstoichiometric fcc-TM(N) nitride. However, excess N ratio decreases the lattice constants of TM(N) nitrides [5]. This is not unexpected because oxygen also plays the same role in Ti lattice and alternatively occupies the vacancy sites to decrease the lattice constants of TiOx [6]. Hence, the potential solubility limit in Ti–N system is still uncertain especially for thin films produced by nonequilibrium process, such N-implantation. In the present study, nonequilibrium Ti(N) films have been produced by a combination process of IBS and N-implantation methods. Structural changes due to N-implantation (0.1~2.61018 ion/cm2) have been investigated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) measurements. Postannealing effects are examined for the phase decomposition into nitride. Anomalous saturated structure of hcpTi(N) are mainly discussed from TEM diffraction and XPS chemical-state analysis results.

2. Experimental procedure Ti films were prepared by ion beam sputtering (IBS) method. A Ti target of 99.9% purity was sputtered by Ar+ with beam energy of 1 keV and deposited on the (001) Si substrate. The target and substrate were well cooled by water so that deposition was carried out under low temperature. The total pressure in the working chamber was 510 2 Pa during deposition. All specimens were made by 1800 s of sputter deposition and deposition rate was approximately 0.1 nm/s. N-implantation was conducted on as-deposited samples with beam energy of 100 keV and N dose of 0.1~2.61018 ion/cm2. Some of the specimens were heat-treated for 3600 s at 573 and 773 K in vacuum with the pressure of 210 4 Pa. The sample preparation for TEM was performed by ion milling equipment (IV4/HL, Technoorg Linda) with 4 keV Ar+ at the incident angle of 58. The final polishing treatment was made with 250 eV to remove irradiation damage on the surface. The structural observation of Ti films was done by TEM (JEOL 2010). The XPS measurements was carried out through Ti 2P3/2, N 1S, O 1S and Si 2P3/2 spectra by JPS 9200 T (JEOL). The depth profiling of N atoms was done after an iteration of 3 keV Ar+ sputtered etching. Chemical state analysis was performed for Ti 2P3/2 to evaluate the Ti–N bonding state.

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3. Results and discussion 3.1. TEM microstructure of as-deposited Ti film Titanium film of 150 nm thickness was deposited on (001) Si single crystal by IBS and the effect of cold sputter deposition on the structural evolution was investigated. The microstructure was observed by TEM. The crosssectional view of as-deposited Ti film is shown in Fig. 1(a). Columnar grains of diameter approximately 10 nm grew perpendicularly to the Si substrate. The faint contrast of the grain boundary indicated that formation and growth of Ti crystalline was suppressed by low-temperature sputtered deposition. The electron diffraction pattern in Fig. 1(b) showed the conventional hcp structure. Grains with strong texture were recognized in the figure with the orientation relationship of 0002Ti parallel to 002Si. The measurement of lattice constant was also conducted by correcting the camera constant with Au standard. Lattice constants of as-deposited Ti film were determined to be 0.304 nm in the a axis and 0.485 nm in the c axis. These lattice constants were somewhat larger than that of the bulk Ti specimen (a=0.2951 nm, c=0.4683 nm) and also the solid solution of hcp-Ti(N) (a=0.297 nm, c=0.478 nm) [7]. In Ref. [8], sputtered thin films exhibited different lattice constants from the bulk specimen, which resulted from the lattice mismatch at the interface. Hence, even if a small reactive gas component (approximately less than 5 at.% N) was mixed into the film during the deposition, it is suggested that the observed lattice constants of the asdeposited Ti film can be attributed to the nature of the sputtered thin film. 3.2. Structural changes induced by N-implantation with various N-dosed amounts N+ with beam energy of 100 keV was penetrated into asdeposited Ti film of 150 nm thickness. The structural change due to the N-implantation was investigated by TEM

Fig. 1. TEM cross-sectional view of as-deposited Ti film. Columnar growth structure with the direction of [0001]Ti//[001]Si was confirmed in the figure.

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formation of nitrides. In addition, because HREM observation for as-deposited sample found that columnar grains consisted of fine grains with low-angle grain boundaries, some grain refinement might occur in proportion to the development of graded N distribution within the film. Hence, the formation of equiaxed grains in N-implanted Ti films may be attributed to the structural accommodation against the compositional distribution of N atoms, not due to the phase separation into nitrides. 3.3. XPS measurements for N-implanted Ti films

Fig. 2. The structural changes due to N-implantation with the dose of 0.11018 ion/cm2 (a) and 2.61018 ion/cm2 (b). The equi-axed grains were introduced within columnar Ti grains. Electron diffraction patterns show hcp-Ti(N) phase was dominant even for the high dosed sample.

observation. N-implantation was made with doses of 0.1, 0.5 and 2.61018 ion/cm2. These N-implantation doses were estimated to be in the composition range of 10~74 at.% N, taking into account that 100-nm-thick Ti contains 61017 atoms/cm2. The composition range was quite large, which enabled the N concentration to reach over TiN1.0. Fig. 2 showed N-implanted Ti films with the dose of 1.01017 ion/cm2 (a) and 2.61018 ion/cm2 (b). The maximum penetration depth of N was calculated to be 210 nm, with average 150 nm, by SRIM-2003. This indicated that most of the N atoms would be stopped around the Ti/Si interface. As shown in Fig. 2(a) and (b), N-implantation induced the amorphous phase in the Si substrate. The formation of the amorphous phase increased in proportion to N-dosed amounts. In the Ti region, equiaxed grains were observed within the columnar Ti structure. These equi-axed grains increased in number and size especially for the large dosed sample. Diffraction patterns from N-implanted Ti films shows the formation of nitrides in Fig. 2. However, the hcp structure of Ti(N) was the dominant phase even after the N penetration of 2.61018 atoms/cm2. This indicated that the introduction of equiaxed grains was responsible for the dissolution of N atoms into hcp Ti lattice, not due to the

XPS measurements were performed for samples with Ndosed amounts of 0.1, 0.5 and 2.61018 ion/cm2. The distribution of N in Ti-film was measured by XPS depth profiling, and is shown in Fig. 3. The survey spectrum was obtained through Ti 2P, N 1S, O 1S and Si 2P spectra after etching with 3 keV Ar+. The change in N concentration was plotted against the normalized film depth which was determined from the survey counts of Si spectra. Previously [9], we reported that oxygen was only detected on the surface and it existed as oxides or adsorptions. As shown in Fig. 3, the N concentration gradually increased in proportion to the film depth. The maximum N concentration was achieved in the vicinity of Ti/Si interface irrespective of Ndosed amounts. The compositional distribution of N atoms was in good agreement with the TEM observation in Fig. 3 and the SRIM prediction of penetration depth. The average N concentration was calculated to be Ti–6 at.% N (N/ Ti=0.06) for the dose of 0.11018 ion/cm2, Ti–20 at.% N (N/Ti=0.25) for 0.51018 ion/cm2, Ti–38 at.% N (N/ Ti=0.61) for 2.61018 ion/cm2. The N compositional measurement revealed that observed hcp structures in Fig. 2 was the super saturated N solid solution in hcp-Ti(N).

Fig. 3. The XPS depth profiling for N-implanted Ti films with various doses. The maximum N concentration was achieved at Ti/Si interface for all the samples.

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Furthermore, since the maximum solubility limit of hcpTi(N) was on Ti–20 at.% N in the phase diagram, Nimplantation induced the compositional anomaly of hcpTi(N). In literature [10], N-implantation induced the formation of Ti2N and TiN phase on bulk Ti in wide range of N doses (0.1–1.01018 ion/cm2). In the present study, hcp structure was the dominant phase even for the Ti–38 at.% N film with N dose of 2.61018 ion/cm2. Hence, the formation of the compositional anomaly of hcp-Ti(N) solid solution was attributed to the highly nonequilibrium nature of IBS and N-implantation. In order to examine the binding energy between Ti and N, the chemical shift of Ti 2P spectra was measured for respective N-implanted Ti films. The change in chemical shift were plotted against the N concentration for various dosed sample and shown in Fig. 4. The binding energy of Ti 2P increased in proportion to the N concentration from pure Ti towards the reference value reported for TiN (455.0 eV) [11]. All the plots were located on a single line regardless of the N dose amounts. The linear increase of the chemical shift well described the tendency of the solid solution state of hcp-Ti(N). 3.4. Lattice constants change in proportion to N concentration and postannealing effect The lattice parameter change of hcp-Ti(N) was measured and plotted against the mean N concentration in Fig. 5. The linear increase of hcp-Ti lattice was confirmed for both c and a axes in proportion to the N concentration. In Ref. [7], N occupied octahedral interstices of hcp-Ti lattice in a random way and expanded the hcp-lattice up to the maximum solubility limit of Ti–20 at.% N. In the present study, lattice constants linearly increased to the Ti–38 at.% N (TiN0.6) over the maximum solubility limit of hcp-Ti(N); therefore, the dissolution of N into the Ti sublattice certainly occurred on the hcp-Ti(N) phase. In addition, the observed

Fig. 4. The chemical shift of Ti 2P against the N concentration for various dosed samples. The linear increase of chemical shift corresponded to the solid solution of hcp-Ti(N).

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Fig. 5. The change in lattice constants of hcp-Ti(N) against the average N concentrations. The linear increase of lattice constants was recognized for the a- and c-axes. The existence of anomaly saturated N in hcp-Ti(N) solid solution was suggested at higher N concentration.

lattice constant was quite large as comparison with the bulk hcp-Ti(N) (a=0.297 nm, c=0.478 nm) and the fcc-TiNx (hcp converted; a=0.298 nm, c=0.488 nm) [7]. Hence, the anomalous saturated structure of hcp-Ti(N) solid solution existed as the meta-stable phase in a wide composition range of the Ti–N system. The change in c/a ratio was plotted against the mean N concentration for N-implanted film and the postannealed samples in Fig. 6. The postannealing at 773 K for 3600 s was carried out for N-implanted Ti films in a vacuum of 2.010 4 Pa. The reaction between Ti and Si substrate was not confirmed in the present work. As shown in Fig. 6, the c/a ratio of hcp-Ti(N) increased linearly toward the theoretical value of cubic structure (c/a=1.633). This indicated that hcp-Ti(N)0.65 exhibited almost cubic sym-

Fig. 6. The c/a ratio of N-implanted Ti and postannealed samples. Nimplantation increased the c/a ratio of hcp-Ti(N) toward the theoretical value of cubic structure (c/a=1.633). The c/a ratio decreased by postannealing at 773 K, which indicated the occurrence of phase separation into nitrides.

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4. Conclusion

Fig. 7. The postannealing treatment was conducted to N-implanted Ti film. Formation of the meta-stable nitride was recognized in the figure. The dark field image was taken from the ordered spot, marked in the electron diffraction.

metry because of its high N/Ti ratio. Additional heat treatment decreased the c/a ratio of hcp-Ti(N), especially for the large N concentration. The c/a ratio of annealed samples seemed to remain constant irrespective to the N concentration. Because the solubility limit of hcp-Ti(N) was 5 at.% N at 773 K in phase diagram, the postannealing treatment might induce phase separation from metastable hcp-Ti(N) into stable hcp-Ti(N) and nitrides. TEM observation was conducted on N-implanted Ti film, and is shown in Fig. 7. Heat treatment at 573 K for 3600 s was added to the sample with the dose of 2.61018 ion/cm2. The dark field image was taken from the diffraction spot marked with the circle. This diffraction spot indicated an ordered structure with a lattice twice as large as the conventional lattice. The bright contrast was clearly observed to show the dispersed structure within the film. Because tetragonal Ti2N also exhibited the ordered spots around this site, ordered structure observed in Fig. 7 might be the tetragonal type nitride. However, because the lattice constant was not exactly corresponding to Ti2N phase [7,12], the ordered structure might still be the meta-stable phase thermodynamically.

For the development of advanced fabrication processes aimed at nanostructured material, a combination process of IBS and N-implantation was conducted on the Ti–N system. As-deposited Ti films showed conventional hcp structure with [0001] textured grains. N-implantation induced equiaxed grains within the columnar Ti film. The electron diffraction revealed that hcp-Ti(N) was the dominant phase even for the sample with 2.61018 ion/cm2. XPS measurements revealed that N-implantation induced the compositional anomaly of hcp-Ti(N) solid solution over the maximum solubility limit in phase equilibrium. The change in lattice constant in proportion to the N concentration indicated that N atoms dissolved into hcp-Ti(N) up to the N/ Ti ratio of 0.65. Postannealing treatment changed the lattice constant of hcp-Ti(N) according to the phase equilibrium and promoted the nanophase separation into the meta-stable nitride and hcp-Ti(N).

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