Spectroscopic study on amorphous tantalum oxynitride thin films prepared by reactive gas-timing RF magnetron sputtering

Applied Surface Science 492 (2019) 99–107

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Spectroscopic study on amorphous tantalum oxynitride thin films prepared by reactive gas-timing RF magnetron sputtering

T

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T. Lertvanithphola, , W. Rakreungdetb, C. Chananonnawathorna, P. Eiamchaia, S. Limwicheana, N. Nuntawonga, V. Patthanasettakula, A. Klamchuenc, N. Khemasirid, J. Nukeawd, K. Seawsakule, ⁎ C. Songsiriritthigulf, N. Chanlekf, H. Nakajimaf, P. Songsiriritthigule, M. Horprathuma, a

National Electronics and Computer Technology Center (NECTEC), National Science and Technology Development Agency (NSTDA), PathumThani 12120, Thailand The Institute for the Promotion of Teaching Science and Technology (IPST), KlongToey, Bangkok 10110, Thailand c National Nanotechnology Center, National Science and Technology Development Agency, Pathumthani 12120, Thailand d College of Nanotechnology, King Mongkut's Institute of Technology Ladkrabang, Ladkrabang, Bangkok 10520, Thailand e Research Network NANOTECH-SUT on Advanced Nanomaterials and Characterization, School of Physics, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand f Synchrotron Light Research Institute, Maung, Nakhon Ratchasima 30000, Thailand b

A R T I C LE I N FO

A B S T R A C T

Keywords: TaOxNy thin films Sputtering Reactive gas-timing (RGT) Ellipsometer X-ray photoelectron spectroscopy (XPS) X-ray absorption spectroscopy (XAS)

The amorphous tantalum oxynitride (TaOxNy) thin films were prepared on silicon (100) substrates by magnetron sputtering system with different techniques of conventional reactive sputtering and reactive gas-timing (RGT). The films were studied via spectroscopic ellipsometry (SE) measured in the range of 0.75–5.0 eV with 0.025 eV interval at 70° incident angle, and the optical model based on Tauc-Lorentz function was constructed to extract the properties of the films. The SE results indicated that all prepared films were grown homogeneously and show different optical properties upon their deposition conditions and techniques. The optical properties of film prepared by conventional reactive sputtering were close to the tantalum oxide film (TaO). The refractive index and optical band gap (Eg) of RGT samples changed with the oxygen timing and correlated with the change of oxygen and nitrogen concentration of the films. In addition, the morphologies, crystallinities, atomic concentrations and distributions of nitrogen atoms in the films analyzed by field-emission scanning electron microscopy, glazing-incident X-ray diffraction, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy are also discussed.

1. Introduction Tantalum oxynitride (TaOxNy) has been a well-known material for various fields of applications e.g. optical coatings [1–4], photocatalysis [5–8], memories devices [9–11], hard coatings [12,13] and bio-compatible coatings [14] due to its wide band gap, high dielectric constant, excellent hardness and bio-compatibility [1–14]. The TaOxNy thin films are conventionally fabricated via physical vapor deposition technique, especially reactive magnetron sputtering. The metallic Ta target was used as sputtered target while Ar, O2 and N2 were used as sputtered and reactive gases, respectively [1–7,10,12–14]. The properties of TaOxNy films were normally related with their atomic composition. It could be tuned through the variation of O2:N2 gases ratio during deposition [1–6,14]. However, it is difficult to incorporate nitrogen into the films due to its reactivity lower than that of oxygen [5]. Therefore, the

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reactive gas-timing (RGT) technique has been proposed as a suitable technique for oxynitride thin film deposition with RF-reactive sputtering system [15–20]. The energy per atom and reactivity of reactive gases including O2 and N2 can be enhanced by pausing in the supply of the gases for a while during the deposition. The composition of the films could be tuned in a wide range by varying the pausing time [15–20]. Spectroscopic ellipsometry is an optical technique that determines the change of polarization of reflected light from the thin film. It could be used to extract the properties of interest including film's morphologies, index dispersions and band gap with high accuracy through a suitable optical model [21]. Several reports of the TaOxNy films study via ellipsometry technique are available [1–3,5,7,10]. However, the properties of the TaOxNy films related with their atomic compositions and optical band gaps (Eg) remain elusive due to the uncertainty of incorporation of the nitrogen atoms into the films [1,3,5,7].

Corresponding authors. E-mail addresses: [email protected] (T. Lertvanithphol), [email protected] (M. Horprathum).

https://doi.org/10.1016/j.apsusc.2019.06.199 Received 18 February 2019; Received in revised form 12 June 2019; Accepted 19 June 2019 Available online 20 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 492 (2019) 99–107

T. Lertvanithphol, et al.

In this report, we studied the properties of TaOxNy thin films grown by the RGT technique and conventional reactive sputtering through the proposed optical model especially for the morphologies, crystallinities and optical properties. All the ellipsometry results were found to be consistent with those from the field-emission scanning electron microscopy (FE-SEM), glazing-incident X-ray diffraction (GI-XRD), X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS).

in the photon energy range of 0.75–5.0 eV at 0.025 eV interval. The number of revolutions per measure was set at 29. The Ψ and Δ values were measured and defined by the ellipsometric ratio, ρ, as follows:

ρ=

2.1. Sample preparation The samples of thin film were deposited by a radio-frequency (13.56 MHz) reactive magnetron sputtering system (AJA international, Inc.; ATC 2000-F) on the silicon wafer (100) substrate. High purity argon (99.999%), oxygen (99.999%) and nitrogen (99.999%) were flowed into the chamber as sputtered and reactive gasses, respectively. The 2-in. diameter Ta (99.995%, KJ Lesker) was used as a sputtered target 90-mm away from substrate. Prior to the deposition, the base pressure of the deposition chamber was obtained at the ultra-high vacuum (5 × 10−4 mTorr) using mechanical pump (ALCATEL) and turbomolecular pump (Shimazu, TMP-803-LM). Moreover, the silicon substrate was cleaned by argon plasma for 10 min in order to remove surface contamination and the Ta target was pre-sputtered under the argon plasma in order to remove excessive oxide surface layer. During the deposition, the applied power was fixed at 200 W and the nominal thickness was set at 200 ± 30 nm. In the mass flow controller, the Ar, O2 and N2 flow rates were set at 10, 11 and 8 sccm, respectively. In addition, the partial pressures of all gas were closely to 1.25 mTorr. The TaOxNy thin films deposited with the reactive gas-timing (RGT) and conventional reactive sputtering were prepared in the following ways. The RGT technique, the O2 and N2 were alternatively flowed into the chamber during the deposition while the Ar flow rate was kept constant. The timing of oxygen flow was varied either at 5 or 15 s, while the timing of nitrogen flow was always fixed at 60 s, as shown in S1. The operated pressure was approximately constant in range between 2.2 and 2.5 mTorr during the deposition. In the conventional technique, the TaOxNy and TaOx thin films were prepared with a constant flow of Ar, O2 and N2 and the operated pressure was 3 mTorr for TaOxNy and 2.7 mTorr for TaOx. In addition, all the deposition conditions summarized in Table 1.

ε1 (E ) = ε1 ( ∞ ) +

TaOxNy RGT 5:60 TaOxNy RGT 15:60 TaOxNy TaOx

N2timing

MSE =

PN2

60

2.2–2.5

1.25

1.25

1.25

15

60

2.2–2.5

1.25

1.25

1.25

3.2

1.25

1.25

1.25

2.7

1.25

–

1.25

Conventional technique Conventional technique

∞

ξε (ξ )

∫ ξ 2 −2 E 2 dξ .

(3)

Eg

1 2N − M

N

2

2

exp mod − Δiexp ⎞ ⎤ ⎡ Ψimod − Ψ i ⎞ ⎛ Δi ⎟ + ⎜ ⎟ ⎥, exp exp σΨ, i σΔ, i ⎠ ⎝ ⎠ ⎦ ⎣⎝

∑ ⎢ ⎛⎜ i=1

(4)

3. Results and discussion As shown in Fig. 1(a), the physical model was constructed as multilayer of silicon substrate/native oxide layer/homogeneous film layer/ surface roughness (Srough) layer. The native oxide layer generated from oxygen and moisture in air was fixed at the thickness of 2 nm [23]. The optical properties of the film and Srough layer were defined using the TL function and Bruggemann effective medium approximation (BEMA) theory with 50%:50% of film:void, respectively [21]. In this study, the values of the model including thicknesses and TL parameters were fitted in comparison with the ‘exp’ ellipsometric spectra. Then, the ‘mod’ spectra were generated from the model with those fitted parameters. In comparison, the proposed model with TL function is the well fitted with all ellipsometry data as shown in Fig. 1(b)–(e). This indicated that the TaOxNy films from both RGT and conventional techniques were grown homogeneously. In order to confirm the film morphologies, the prepared samples were characterized by FE-SEM in cross-sectional view as illustrated in

PAr

5

2 p π

where N represents the number of measured Ψ and Δ pairs, M the total number of variable parameters and σ the standard deviations [21]. To clarify the type of ellipsometric data, the ‘mod’ and ‘exp’ were used to indicate the spectra from optical model and experimental measurement, respectively. The morphologies of the prepared films were characterized by field emission scanning electron microscopy (FE-SEM; Hitachi High Tech. SU8030). The X-ray technique including glazing-incident X-ray diffraction (GI-XRD; BL.7.2W at SLRI), X-ray photoelectron spectroscopy (XPS; PHI 5000 VersaProbe II, BL.5.3 SUT-NANOTEC-SLRI XPS) and Xray absorption spectroscopy (XAS; BL.3.2Ua at SLRI) were also used to investigate their crystallinity, atomic concentrations and distribution of oxygen and nitrogen atoms in the films, respectively.

Partial pressure (mTorr) PO2

(2)

The parameters of the model were varied and analyzed by the commercial software WVASE32 (J.A. Woollam Co., Inc.). The fitting minimizes the differences of values between experiment and generated data from the model. In addition, the mean square error (MSE) was used to determine the goodness of fit in the following equation:

Table 1 Deposition condition of sputtered TaOxNy and TaO thin film growth by RGT technique and conventional reactive sputtering.

O2timing

(1)

where A represents the amplitude, E0 the peak transition energy, C the broadening constant, and Eg the optical band gap. After applying ε2 with the Kramers-Kronig relation,ε1 can be derived as [21]:

The properties of the prepared samples were characterized via variable-angle spectroscopic ellipsometry (VASE2000; J.A. Woollam). The ellipsometric measurements were performed at 70° incident angle,

Operated pressure (mTorr)

= tan(Ψ) exp(iΔ)

AE0 C (E − Eg )2 1 ⎤ ε2 (E ) = ⎡ 2 , E > Eg , ⋅ ⎢ (E − E02 )2 + C 2E 2 E ⎥ ⎣ ⎦ ε2 (E ) = 0, E ≤ Eg ,

2.2. Sample characterization

GST technique

Rs

where Rp/Rs are the complex reflection coefficients of the parallel and perpendicular components to the incident polarized light, respectively [21]. The optical model based on the film structure and the dielectric function (ε2) of the thin film layer using Tauc-Lorentz (TL) function was constructed. In addition, the TL dispersion has been proposed by combining the Tauc band edge with the classical Lorentz broadening function as follows [22]:

2. Experimental details

Sample

Rp

100

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Fig. 1. (a) The physical models proposed in this study in order to represent the TaOxNy thin films based on the spectroscopic ellipsometry. Measured and calculated ellipsometric Ψ and Δ spectra for(b) and (c)the TaOxNy thin films deposited by RGT technique at the O2:N2 timing ratio of 5 s:60 s and 15 s:60 s, respectively, (d) and (e) the TaOxNy and TaO thin films deposited by conventional reactive sputteringon the silicon substrate at 70° angle of incident.

Fig. 2(a)–(d). The images clearly showed that the films were grown homogeneously and the same structure as the proposed model. In addition, the film thicknesses obtained from both techniques were in good agreement as shown in Fig. 2(e). The crystallinities in samples were also analyzed by GI-XRD which determined the images of 2D diffraction pattern. From the results as represented in Fig. 3(a)–(d), no diffraction peak was observed at all 2θ angles which indicated that the prepared films were completely amorphous. Indeed, the as-deposited TaOx thin films without substrate

heating or substrate bias were mostly amorphous due to high binding energy of tantalum atom [24]. This also validates the proposed model based on the TL function that is suitable for amorphous metal-oxide and metal-oxynitride thin films [22,25,26]. From the model, the dispersion of refractive index (n) and extinction coefficient (k) of the films were extracted and plotted as shown in Fig. 4(a)–(b). These results indicated that the refractive indices in the visible region of TaOxNy thin films prepared by RGT technique, both with timing ratio of 5:60 and 15:60, were higher than those of TaOxNy 101

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Fig. 2. (a)–(d) cross-section FE-SEM images of all prepared TaOxNy and TaO thin film. (e) Film thickness of the TaOxNy and TaO thin film growth by RGT technique and conventional reactive sputtering as analyzed by the SE and FE-SEM.

and TaOx films from conventional technique. In the k-spectra, the RGT films showed higher k in the overall range used in the measurement than the conventional films. In particular, the absorption coefficients (α) were calculated from a relationship of α = 4πk/λ where λ is the wavelength of photon. The Eg of the films was later determined in the equation as follows:

αhν = A(hν − Eg ) m ,

and TaOx samples, respectively. As shown in Fig. 5, the indices and Eg values of prepared films in this work were compared with the results from the other research groups [3,5,7]. From the publication of S. Venkataraj et al. and J. Rezek et al., the optical properties of the TaOxNy films were varied with the O:N partial pressure which affected to oxygen and nitrogen concentrations of the films [3,5]. However, the variation of concentrations was in wide range due to the long range of reactive gases flows [3] and the large ion fluxes of sputtered particle by high-power impulse magnetron sputtering (HiPIMS) system [5], respectively. Moreover, from the report of J. H. Hsieh et al., the properties of the TaOxNy could be varied by changing (O + N): Ar at a constant O:N partial pressure. However, the rapid thermal annealing process was required [7]. In our work, the TaOxNy and TaOx films prepared by conventional reactive sputtering technique are close to the bulk tantalum oxide [3] while the samples from RGT show a characteristic of tantalum oxynitride. The

(5)

where A represents a constant, hν the photon energy and m a factor of the transition nature of the film material (m = 2 for direct transition and m = ½ for an indirect transition) [27]. Here, m was set to ½ in this analysis because TaOxNy and TaOx were known as indirect band gap semiconductor materials [1,7,28]. Then the Eg was obtained from the linear extrapolating of Tauc plot between (αhν)1/2 and hν as illustrated in Fig. 4(c). The values of Eg were determined as 2.17, 2.88, 4.21 and 4.21 eV for TaOxNy RGT 5:60, TaOxNy RGT 15:60, conventional TaOxNy 102

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Fig. 3. 2D-GIXRD pattern of the sputtered TaOxNy and TaO thin film growth by RGT technique and conventional reactive sputtering.

results also indicated that the films optical characteristics from RGT could be tuned by varying the oxygen timing. The shorter the oxygen timing was, the lower the Eg and the higher the refractive index would be. In addition, the change of optical properties was related with the nitrogen concentration in the films. Normally, the oxynitride films with high nitrogen incorporation show high refractive index and low band gap [3,5,7]. From this point, it should be inferred that the RGT effectively supplied nitrogen into the films better than the conventional reactive method due to the energy enhancement in short time during the oxygen supply was closed [1,15–20]. When the oxygen supply closed and the nitrogen flow was immediately switched on, the operated pressure would drift to a lower pressure state in a short amount of time. During that time, the sputtered atoms, which kept long mean free paths, now experienced fewer collisions. They therefore maintained relatively high particle energies to reach the substrates and allowed a different nitrogen content with a wide range of band gaps from the prepared films [1]. In order to clarify the difference of nitrogen concentration in the films related with the deposition technique, the prepared samples were further investigated by XPS. The monochromatic AlKα X-ray (1486.6 eV) was used as an excitation source. The XPS spectra in the

binding energy range of 0–600 eV were utilized to identify the elemental composition. Ta, O, N and C were observed in the films from RGT and conventional techniques as illustrated in Fig. 6(a). In addition, the carbon was presented as the contamination on the film surface during air exposure via sample transfer to XPS instrument and C1s peak was used as a reference of binding energy at 284.6 eV. The high resolution XPS spectra of N1s, Ta4f and O1s were determined for the atomic compositions through the Gaussian fitting on N1s and O1s and pseudo-Voigt fitting (Gaussian 80% and Lorentzian 20%) on Ta4f fitting with the full-width half maximum of 1.2–1.5 eV as shown in Figs. 6(b)–(d). In the N1s spectra, only the TaOxNy prepared by the RGT technique showed strength signal at 396 eV while no peak appeared on the surface of TaOxNy prepared by the conventional technique as represented in Fig. 6(b). This confirms that the nitrogen was successfully added into the oxynitride thin films using the RGT technique. Moreover, the area of N1 s Gaussian peak related to the atomic percentage of nitrogen was varied with the oxygen timing. The longer the oxygen timing, the lower the nitrogen percentage should be. As shown in Fig. 6(c), the peaks corresponding to Ta4f7/2 (~25.7 eV) and Ta4f5/2 (~27.6 eV) were observed from the all prepared thin films. The binding energy of Ta4f7/2 was higher than that of Ta metal (Ta4f7/2: 21.7, 103

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Fig. 4. The results of the optical characteristics of the TaOxNy thin films deposited by RGT technique, the TaOxNy and TaO thin films deposited by conventional reactive sputtering, as investigated by the spectroscopic ellipsometry.

Ta4f5/2: 21.9 eV [4]) and closed to that of Ta2O5 (Ta4f7/2: 26.5, Ta4f5/2: 26.7 eV [4,14]). Additional doublet peaks at Ta4f7/2 (24.9 eV) and Ta4f5/2 (26.8 eV) on the spectra of TaOxNy from RGT technique appear close to those reported in Ta3N5 (Ta4f7/2: 24.8 eV, Ta4f5/2: 26.7 eV) corresponding to the tantalum oxynitride on the film surfaces. The trend of Ta4f7/2 binding energy suggested that the tantalum atom in the films was positively charged and mostly forms the covalent bonds with oxygen and nitrogen. The O1s spectra were fitted to double Gaussian peaks as illustrated in Fig. 6(d). The peaks at 530.5 and 531 eV were respectively assigned to oxygen bonding with tantalum and carbon species, i.e. CeOH and C]O group or oxygen deficiencies in TaO [29]. After fitting, the relative percentages of film composition were determined by evaluating the integrated area of Ta4f, O1s and N1s and plotted as shown in Fig. 6(e). In summary, as the oxygen timing increased from 5 to 15 s, the oxygen content in the TaOxNy thin films increased while the nitrogen content decreased from 11.5 to 7.6%. The N K-edge XAS spectra of prepared samples were measured in

Fig. 5. Optical band gaps of all prepared TaOxNy and TaO thin film as a reflective index at 550 nm. Lines were only intended as guides for the eye.

104

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Fig. 6. (a) Survey spectra of the all prepared TaOxNy and TaO thin film. High resolution XPS spectra with deconvolution of the sputtered TaOxNy and TaO thin film; (b) N, (c) Ta and (d) O. (e)composition percentage of the sputtered TaOxNy and TaO thin film growth by RGT technique and conventional reactive sputtering. Lines were only intended as guides for the eye.

the total fluorescence yield (TFY) mode to analyze the nitrogen chemical states inside the films because the penetration depth of soft X-ray fluorescence (about several 100 nm) is much longer than that of electron analyzed in XPS (several nm) [30]. N K-edge spectra shown in Fig. 7 represent the transition from 1s core level electron to the unoccupied 2p π*/σ* molecular orbitals or conduction bands resulting in the convolution between occupied and unoccupied states of nitrogen atoms in the films. All the TFY spectra were calibrated in the carbon K edge (C1s – π* peak at 285.4 eV) of graphite. The TFY spectra of RGT samples showed a single peak at 400.6–400.9 eV with a broad density of states (DOS) emerged from about 396.2 eV beyond 410 eV. The onset of DOS is almost proportional to the square root of energy in the intensity corresponding to the bottom of conduction band in TaOxNy due to the hybridized bonds among Ta5d, N2p, and O2p in the films

[31–33]. Furthermore, the onset of DOS shifts from 396.2 eV (5:60TaON) to 396.6 eV (15:60TaON) corresponding to the shift of the conduction band partly related to the increase in the energy band gap. In contrast, the spectra of TaOxNy from conventional technique represented two peaks at 401.0 eV and 405.4 eV without an increase of baseline at 395 eV observed in those in RGT samples. It should be noted that Ta4p3/2 is located at 404 eV, which is closed to the peak at 405.4 eV. However, the Ta4p3/2 peak is distributed in the range of 400–410 eV as a bump, which might be due to the self-absorption effect, and does not change the discussion of fundamental N K edge profile in TFY spectrum as seen in TaO of Fig. 7. The peaks in 400.6–401.0 eV seen in TaOxNy from both techniques are in good agreement with the 1s → π* transition in nitrogen gas molecules trapped within the films [34]. The peak of 405.4 eV observed in TaOxNy 105

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Fig. 7. The N K-edge XAS spectra of sputtered TaOxNy and TaO thin film growth by RGT technique and conventional reactive sputtering.

from conventional technique corresponds to the 1s - σ* transition in either molecular nitrogen including nitro group (-NO2) or sp3 configuration suggested by the reported experiment and model calculation [34–36]. As a result, most of nitrogen atoms in TaOxNy from conventional technique are weakly trapped in TaO film resulting in the lack of nitrogen atoms at the film surface as observed in XPS after the exposure to air. The RGT samples still contain the molecular nitrogen in the films, while their effectively involve TaON bonds. 4. Conclusion The TaOxNy thin films were deposited by magnetron sputtering system with conventional reactive sputtering and reactive gas-timing techniques. The thicknesses and optical properties of the films were investigated via SE. The results showed that the films were grown homogeneously with varied optical properties depending upon the deposition conditions. The values of the refractive index and Eg of conventional film were close to those of tantalum oxide while those of RGT films were varied with the oxygen timing. The variation was observed because the timing affected directly on the energy. In addition, the shorter the oxygen timing was, the lower the Eg and the greater the nitrogen concentration would be. Based only various investigation techniques, this work therefore proved that the conventional TaON coatings only showed weakly trapped nitrogen atoms within the TaO matrics, while the proposed RGT deposition showed obvious TaON bonds. Our claims have been validated from the morphologies, crystallinities, atomic concentrations and nitrogen distributions of the obtained films. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.06.199. Acknowledgments The authors gratefully acknowledge the National Electronics and Computer Technology Center (Grant No. P1551655) and Synchrotron Light Research Institute (SLRI), Thailand for financial and instrument supports. N.C. and P.S. would like to thank for the partial financial support from the Research Network NANOTEC program of the National Nanotechnology Center (NANOTEC) of Thailand. References [1] N. Khemasiri, S. Jessadaluk, C. Chananonnawathorn, S. Vuttivong, T. Lertvanithphol, M. Horprathum, P. Eiamchai, V. Patthanasettakul, A. Klamchuen, A. Pankiew, S. Sorntheeraphat, J. Nukeaw, Optical band engineering of metaloxynitride based on tantalum oxide thin film fabricated via reactive gas-timing RF

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functionalities and formation mechanisms, Carbon 114 (2017) 566–578. [35] N. Jiang, D.G. Georgiev, A.H. Jayatissa, R.W. Collins, J. Chen, E. McCullen, Zinc nitride films prepared by reactive RF magnetron sputtering of zinc in nitrogen containing atmosphere, J. Phys. D. Appl. Phys. 45 (2012) 135101. [36] I.I. Vlasov, S. Turner, G. Van Tendeloo, A.A. Shiryaev, Chapter 9 – Recent results on characterization of detonation nanodiamonds, Ultrananocrystalline Diamond, 2nd edition, William Andrew Publishing, Oxford, 2012, pp. 291–326.

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