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Tailoring the structural and optical properties of TiN thin films by Ag ion implantation
Nuclear Instruments and Methods in Physics Research B 389–390 (2016) 33–39
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Tailoring the structural and optical properties of TiN thin films by Ag ion implantation M. Popovic´ ⇑, M. Novakovic´, Z. Rakocˇevic´, N. Bibic´ University of Belgrade, VINCˇA Institute of Nuclear Sciences, 11001 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 18 July 2016 Received in revised form 4 October 2016 Accepted 10 November 2016 Available online 22 November 2016 Keywords: TiN Implantation Spectroscopic ellipsometry Surface plasmon resonance Resistivity
a b s t r a c t Titanium nitride (TiN) thin films thickness of 260 nm prepared by dc reactive sputtering were irradiated with 200 keV silver (Ag) ions to the fluences ranging from 5 1015 ions/cm2 to 20 1015 ions/cm2. After implantation TiN layers were annealed 2 h at 700 °C in a vacuum. Ion irradiation-induced microstructural changes were examined by using Rutherford backscattering spectrometry, X-ray diffraction and transmission electron microscopy, while the surface topography was observed using atomic force microscopy. Spectroscopic ellipsometry was employed to get insights on the optical and electronic properties of TiN films with respect to their microstructure. The results showed that the irradiations lead to deformation of the lattice, increasing disorder and formation of new Ag phase. The optical results demonstrate the contribution of surface plasmon resonace (SPR) of Ag particles. SPR position shifted in the range of 354.3– 476.9 nm when Ag ion fluence varied from 5 1015 ions/cm2 to 20 1015 ions/cm2. Shift in peak wavelength shows dependence on Ag particles concentration, suggesting that interaction between Ag particles dominate the surface plasmon resonance effect. Presence of Ag as second metal in the layer leads to overall decrease of optical resistivity of TiN. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Transition metal nitrides, such as TiN, have attractive combination of physical and chemical properties. TiN coatings, depending on their microstructure and Ti/N stoichiometry, exhibit high hardness, high temperature stability, chemical inertness and low resistivity [1–5]. Due to its good biocompatibility, it can be also used as a biomedical material. Besides, addition of Ag in TiN system has been observed to result in substational improvement of corrosion and antibacterial properties of TiN coatings [6]. Namely, silver is known as an inherently antibacterial material. On the other side, TiN as a refractory material also exhibits a plasmonic resonance in the visible-NIR range, enabling plasmonic absorption in this range [7,8]. Surface plasmons are collective oscillation of the free electrons localized at the surfaces of metals structures [9,10]. This effect results in resonant optical absorbance, transmittance and/or reflectance spectra of the plasmonic materials, such as gold, silver and copper nanoparticles. Silver has some advantages over Au and Cu, because of the stronger plasmon resonance effect. Numerous ⇑ Corresponding author at: Institute of Nuclear Sciences VINCˇA, Mike Petrovic´a Alasa 12-14, 11 000 Belgrade, Serbia. E-mail address: [email protected] (M. Popovic´). http://dx.doi.org/10.1016/j.nimb.2016.11.013 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
works have been dedicated to researching nanocomposite materials, which consist of noble metal particles (Au, Ag) and dielectric matrices (SiO2, SiNx, ZnO, TiO2) [11–14]. In contrast to this, only a few studies have been performed related to the formation of embbending metallic nanoparticles in non-dielectric matrices [6]. But, little is known about optical properties of TiN coatings with addition of Ag. Led by these considerations, we have carried out a series of experiments, in which TiN layers were irradiated with 200 keV Ag ions. Ion implantation of silver was used as a versatail method for the formation of silver rich layer in the TiN structure. The spectral response of silver as a function of ion fluence, and the increase in silver concentration and particles agglomeration was investigated. The Ag particles agglomeration results in inter-particle plasmon interaction followed by a shift in surface plasmon resonance (SPR) band in the visible region. Following the combined Drude Tauc-Lorenc model, we investigated the optical properties of silver implanted TiN thin films in terms of their optical parameters measured by spectroscopic ellipsometry. We have analyzed the refractive index and extinction coefficient of implanted TiN thin films as a function of silver ion fluence. It was interesting to find that Ag nanoparticles interaction plays significant role for the shift of the surface plasmon resonance peak when increasing ion fluence. Additional analysis by Rutherford backscattering spectrometry, X-ray diffraction technique,
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transmission electron validated the results insights in thin films the correlation with implanted TiN films.
microscopy and atomic force microscopy obtained by ellipsometry and provided microstructure and morphology enabling the optical and electronic properties of
2. Experiment TiN films were deposited using dc reactive sputtering onto 550 lm thick Si(1 0 0) substrate in a Balzer Sputtron II System. The silicon substrates were chemically etched in a dilute HF solution and washed in deionized water prior to insertion in the vacuum chamber and were sputter cleaned by a flux of argon ions just before deposition. The chamber was evacuated to 1 104 Pa and during deposition the nitrogen gas was introduced into the chamber to the partial pressure of 3 102 Pa. Experimental parameters were chosen so that the deposition rate at the substrate was 6.5 nm/min. The layers were deposited to the thickness of 260 nm, as determined by using the Talystep profilometer. Then, TiN samples dimensions of 2 2 cm2 were subjected to implantaˇ A 500 kV ion implanter. The tion of 200 keV Ag+ ions using VINC irradiations of the samples were performed at room temperature under high vacuum (104 Pa) in the fluence range between 5 1015 and 20 1015 ions/cm2. The beam current density was kept at 1 lA/cm2 in order to avoid thermal degradation during ion implantation. Irradiation energy was chosen in such a way that the projected range of ions and energy deposition density were in TiN, according to simulation done with the SRIM 2008 code [15]. After implantation the samples were annealed for 2 h at 700 °C in a tube furnace under a vacuum of some 106 Pa. The structural characterization of TiN thin films was done by Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), transmission electron microscopy (TEM) together with energy dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM). For studying the optical properties of the films spectroscopic ellipsometry was used. Depth profiling was performed by 900 keV He++ ion beam provided by the Gottingen IONAS implanter [16]. RBS spectra were taken with two silicon surface barrier detectors mounted at ±165° with respect to the beam, having an energy resolution of 13 keV FWHM. The software used in simulation of RBS spectra was WiNDF code [17]. Philips PW1050 Xray diffractometer with monochromatized CuKa radiation (k = 0.15418 nm) in a 2h range of 35–45° was used to analyze the structure of the TiN films. TEM and EDS analyses of the TiN samples were performed by using PHILIPS CM30 microscope operated at 300 kV. In order to investigate the crystalline structure of the samples we also used the micro-diffraction (MD) technique. The surface topography of the samples was analyzed by means of atomic force microscopy using Quadrex Multimode IIIe (Veeco Instruments Inc.). Data were taken in ambient air and in tapping mode. Ellipsometric spectra have been measured by HORIBAJobin Yvon UVIS ellipsometer in an energy range of 0.4–4.8 eV. The measurements were performed under an incident angle of 70 degrees with respect to the surface normal, at room temperature. 3. Results and discussion 3.1. Silver depth profiles Irradiation energy of 200 keV was chosen to ensure the maximum Ag concentration to be situated well within TiN layer. Fig. 1a shows the depth profiles of Ag in TiN layer, extracted with WiNDF code from experimental RBS spectra taken from the sam-
ple irradiated to the fluence of 20 1015 ions/cm2 and the sample annealed at 700 °C after irradiation. This analysis reveals that Ag has a maximum concentration of 3.5 at.% inside of TiN layer at a depth of around 33 nm. Due to the diffusion process after annealing the depth profile was slightly wider with the maximum at around 38 nm in depth. The process of Ag ions irradiation was simulated using SRIM code, as also shown in Fig. 1a (solid line). SRIM calculation gave a mean projected range Rp = 48 nm and straggling of DRp = 16 nm for the above experimental parameters. It can be seen that the experimentally obtained Ag concentration maximum was shifted towards the surface as compared to the SRIM Rp calculation. The obvious difference can be due to the changes of the density of the layer as well as to the surface erosion that occurs during irradiation. The sputtering effect was calculated from experimentally obtained RBS spectra and it was found to be 10 nm. Fig. 1b shows the EDS elemental mapping of Ag in the cross-section of the sample annealed after irradiation to the fluence of 20 1015 ions/cm2. The presence of Ag inside of TiN layer with the maximum concentration at the depth of around 40 nm was confirmed, demonstrating good agreement with RBS measurements. 3.2. XRD analysis of silver implanted TiN thin films Fig. 2(a–d) shows the X-ray diffraction (XRD) patterns obtained from as deposited, implanted and after irradiation annealed samples in h–2h geometry. Both as deposited and irradiated samples exhibit a typical polycrystalline TiN structure, showing only (1 1 1) and (2 0 0) diffraction peaks. Two observations are apparent indicating a decrease in crystallinity of the films and compressive stress/strain development during irradiation. It can be clearly seen that intensity of diffraction peaks decreased and its full width of half maximum (FWHM) increased after irradiation with Ag+ ions. On the other side, diffraction peaks of implanted samples shift towards higher angles in comparison with of as deposited TiN film, demonstrating that the lattice undergoes a shrinkage of 1% from 4.293 Å to 4.261 Å after irradiation. This lattice compression appears to be consistent with compressive stress/strain developed during irradiation. In addition, for the sample implanted to the fluence of 20 1015 ions/cm2 a small peak with the lattice spacing of 0.2383 nm corresponding to (1 1 1) Ag reflection appears, suggesting the Ag metallic clusters agglomeration for the highest ion fluence. Note that the intensity of the peaks increased after annealing at 700 °C, indicating that the crystallinity of the TiN films was improved. The average size of Ag crystalline grains for the sample implanted to the fluence of 20 1015 ions/cm2 was found to be 7 nm. Post-implantation annealing leads to increase of Ag clusters to the value of 16 nm. 3.3. TEM analysis of silver implanted TiN thin films Fig. 3a shows the cross-sectional bright field image taken from the sample irradiated to 20 1015 Ag/cm2 and then annealed at 700 °C. It can be seen that the TiN layer has a dense columnar structure partially damaged after 200 keV Ag+ ions irradiation. The damage region is situated within 100 nm at surface of the TiN layer and is stabilized after annealing. The presence of nanocrystalline grains is confirmed by the micro-diffraction (MD) patterns taken from cross-sectional TEM as presented in Fig. 3b. Besides of (1 1 1) and (2 0 0) patterns of TiN phase the presence of Ag nanograins arranged in (1 1 1) and (2 0 0) orientations are observed. Fig. 3c is a high-resolution image of (1 1 1) lattice fringes from a typical Ag grain in the sample implanted to the fluence of 20 1015 Ag/cm2. It can be seen that the silver nanoparticles have a spherical shape with diameter of 7 nm. The lattice spacing is
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Fig. 1. (a) Ag concentration depth profiles of implanted TiN sample to the fluence of 20 1015 ions/cm2 before and after annealing at 700 °C, compared to the SRIM simulation and (b) EDS elemental map for silver extracted from the cross-section TiN sample irradiated to the fluence of 20 1015 ions/cm2 and then annealed 2 h at 700 °C.
3.4. AFM surface topography and surface roughness of silver implanted TiN thin films Changes in surface morphology of the TiN layers induced by Ag ion irradiation were also analyzed by AFM. Fig. 4 shows twodimensional AFM morphology images taken from the surface of TiN sample before and after implantation of 20 1015 Ag/cm2 in the as-implanted state and also after annealing at 700 °C. The difference in roughness for as deposited and then irradiated sample can be observed. The as deposited TiN layer exhibit smooth surface morphology with surface roughness value of 0.4 nm. After Ag ion implantation to the fluence of 20 1015 Ag/cm2 the surface becomes smoother with the roughness of 0.2 nm. This comes as a consequence of the sputtering of atoms from the surface layer and of the interaction of incident energetic Ag ions with the atoms of TiN during ion bombardment. Further, annealing of implanted sample changed the value of roughness to 0.3 nm. 3.5. Spectroscopic ellipsometry of silver implanted TiN thin films
Fig. 2. XRD spectra of TiN samples: (a) as deposited; (b) Ag ion irradiated to the fluence of 5 1015 ions/cm2; (c) Ag ion irradiated to the fluence of 20 1015 ions/ cm2; (d) Ag ion irradiated to the fluence of 20 1015 ions/cm2 and then annealed 2 h at 700 °C.
0.2362 nm which is in agreement with the value of 0.2383 nm obtained from the XRD analysis. The theoretical value for the cubic Ag is 0.2359 nm [18].
TiN samples were subjected to spectroscopic ellipsometry measurements in order to get information about the optical properties of the films. The two experimental quantities, which are directly obtained trough this technique, are W and D. These parameters are indication of relative changes in the amplitude and the phase of a linearly polarized monochromatic incident light upon an oblique reflection from a sample surface. In order to obtain optical parameters from these measured values it is necessary to construct appropriate optical model of the samples. Therefore we analyzed the experimental data of as deposited and annealed samples using a three layer model, consisting of a few nm thick oxygen layer on the top, the homogenous TiN layer thickness of 260 nm and 550 lm silicon substrate. On the other hand, each ion beam implanted sample was treated as a four layer system comprising an oxygen layer, modified and non-modified TiN layer and finally Si substrate. The optical models used in simulations of TiN samples are: Tauc Lorenc oscillator for the interband transitions due to valence electrons and the Drude term which considers the contribution of the intraband transitions (free electron transitions in the conduction band) [19–21]. The optical constants n (refractive index) and k (extinction coefficient) can be obtained by fitting the experimental W and D values. This was done by DeltaPsi2 software (HORIBA) [22]. The spectral variation of n and k for as deposited and Ag ions irradiated
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Fig. 3. TEM analysis of TiN samples: (a) low magnification bright-field image and (b) corresponding MD diffraction pattern of the TiN layer irradiated with Ag ions to the fluence of 20 1015 ions/cm2 and then annealed 2 h at 700 °C; (c) HRTEM image of isolated Ag particle taken from sample implanted to the fluence of 20 1015 ions/cm2.
Fig. 4. Surface topography AFM images 1 lm 1 lm of TiN sample: (a) as deposited; (b) irradiated to the fluence of 20 1015 ions/cm2 and (c) irradiated to the fluence of 20 1015 ions/cm2 and subsequent annealed 2 h at 700 °C.
samples are shown in Fig. 5(a and b). The n and k spectra for as deposited TiN layer show good agreement with data previously reported [23]. It can be observed that Ag ions irradiation changes optical response of the TiN thin films. In particular, a broad band in the spectra of n and k in visible region was observed. There is also a shift towards larger wavelengths with increasing ion fluence. The similar behavior was observed for the samples annealed at 700 °C after irradiation, as shown in Fig. 6(a and b). These results demonstrate the contribution of the surface plasmon resonance of Ag particles. The existence of a SPR was predicted by the Mie theory [24]. This is effect of absorption and scattering of light by small spherical particles. Namely, Mie theory shows that light absorption exhibit a single SPR, yielding a dipolar charge distribution on the surface. Our results show that single SPR position shifted in the range of 354.3–476.9 nm when Ag ion fluence varied from 5 1015 ions/cm2 to 20 1015 ions/cm2. These observed surface plasmon resonance bands are indication of Ag nanoparticles
formation in the TiN matrix. The SPR generated in Ag irradiated TiN samples arises from the collective oscillations of the conduction band electrons induced by the incident light. A classical Mie theory describes SPR for small isolated spherical particles, ie. for low particle volume concentration V (V 1). With increasing of ion fluence, the Ag concentration increases and interaction between particles cannot be neglected. In this case the MaxwelGarnett‘s effective medium theory [25–27] can be applied to interpret the results. According to Maxwel-Garnett‘s theory, the SPR wavelength, kSPR = 2pc/xSPR, is given by:
kSPR ¼ kP
1=2 2þq eext þ 1 1q
ð1Þ
where kP = 2pc/xP is the Ag’s bulk plasmon wavelength, xP = 9.2 eV for Ag, eext is the interparticle dielectric constant and q is the particle filling factor. As Ag ion fluence increases one can
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Fig. 5. Refractive index (a) and extinction coefficient (b) spectra as a function of wavelength for as deposited and silver ion implanted TiN thin films.
Fig. 6. Refractive index (a) and extinction coefficient (b) spectra as a function of wavelength for TiN thin films annealed 2 h at 700 °C after irradiation.
expect the increase of the particle filling factor q. As a result, this will favors the kSPR shift towards longer wavelengths, as it was observed. To further elucidate the effects of Ag ions irradiation on optical properties of TiN layers, we have plotted the n and k values (taken at 434 nm) as a function of ion fluence. The results are shown in Fig. 7. It can be seen that as the fluence of Ag ions increases from 5 1015 ions/cm2 to 20 1015 ions/cm2, the refractive index also increases. At the same time the k values decrease. We can assume that this can be due to the structural changes induced by Ag ions irradiation. The complex processes of ion-solid interaction lead to deformation (compaction) of lattice, increasing disorder (damage) and formation of new Ag phase (agglomeration of Ag atoms). For the samples annealed after irradiation the refractive index exhibits small difference and higher values. The difference is a consequence of the increase in Ag grain size as well as the grain size of the host TiN layer. The HRTEM and XRD measurements showed that the Ag grain size increased from 7 nm to 16 nm after annealing. The mean crystalline size of the host TiN was found to be 10 nm and increase to 12 nm after annealing. The analysis clearly demonstrate that postimplantation annealing resulted in recovery of polycrystalline structure implying the grain size effect on the optical properties of the Ag irradiated and annealed TiN samples. Spectroscopic ellipsometry was also used for measuring the optical resistivity of TiN thin films. The values of optical resis-
tivites were obtained using formula q ¼
Cd
e0
1
x2p
, where Cd and
xp are parameters known from fitting of (W, D) spectra. The calculated optical resistivites for as deposited sample, Ag treated and/or annealed TiN layers are shown in Fig. 8. The optical resistivity of as deposited TiN thin film found to be 587 lX cm, in agreement with the value of 527 lX cm already reported by Patsalas at al. [28]. We found that the optical resistivity, q, was always lower in implanted layers than in as deposited TiN film. However, this optical resistivity does not decrease monotonically, as expected. The largest reduction of resistivity relative to the unimplanted layer was observed for the sample implanted to the fluence of 5 1015 ions/cm2, with the values around the 20 lX cm. Then, optical resistivity stays almost constant in the ion fluence range of 10 1015–15 1015 ions/cm2. For the highest ion fluence of 20 1015 ions/cm2, the optical resistivity achieves the lowest value of 4 lX cm. Obviously, presence of Ag as a second metal in the layer has a predominant role on metallic behavior of TiN over the damage and leads to overall decrease of optical resistivity of TiN. Having this in mind the resistivity of TiN can be easily changed in the wide range by varying Ag concentration in the layers. However, we observed no effect of post-thermal annealing on resistivity of the layers, indicating that the structure of TiN is preserved at high temperatures. Indeed, these assumptions were confirmed by the observations obtained by TEM analysis.
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Fig. 7. The optical constants n and k (at 434 nm) of Ag implanted TiN samples as a function of ion fluence.
ence, the concentration of Ag nanoparticles increases and interaction between particles cannot be neglected. Shift in kSPR induced by interaction among the particles becomes more significant with increasing Ag concentration. Another important point clarified by spectroscopic ellipsometry is related to the modifications of n and k values. As the Ag+ ions fluence increases, the n also increase and k values decrease. At the post-implantation annealing of 700 °C an increase of Ag clusters and TiN lattice recovery were observed. This provides conditions for the slight increase of the n values. The presence of Ag as a second metal in the layer has a great influence on metallic behavior of TiN films. As the concentration of Ag ions increases the film becomes more metallic.
Acknowledgement
Fig. 8. The calculated optical resistivity for as deposited, Ag treated and/or annealed TiN films.
This work was supported by the Ministry of Education and Science of the Republic of Serbia (Project No. III 45005). We would like to thank I. Peterka, D. Purschke, M. Mitric´ and P Wilbrandt for their support during the experiments.
References 4. Summary The structural and optical properties of the TiN layers irradiated with 200 keV Ag+ ions have been analyzed as a function of ion fluence and post-implantation thermal annealing at 700 °C. The formation of Ag metallic clusters inside of TiN layers at a depth of around 40 nm was observed. XRD patterns clearly demonstrate a decrease of crystallinity of the TiN layers and compressive stress/ strain development during irradiation. According to HRTEM analyses the Ag metallic phase has a nanometric nature of grains, with a spherical shape and grain diameter of 7 nm. The above observations are in agreement with the spectroscopic ellipsometry analyses. The surface plasmon resonance of Ag particles was confirmed by a broad band in the spectra in visible region. The SPR shifted in the range of 354.3 nm to 476.9 nm when Ag ion fluence varies from 5 1015 ions/cm2 to 20 1015 ions/cm2. With increasing ion flu-
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Tailoring the structural and optical properties of TiN thin films by Ag ion implantation M. Popovic´ ⇑, M. Novakovic´, Z. Rakocˇevic´, N. Bibic´ University of Belgrade, VINCˇA Institute of Nuclear Sciences, 11001 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 18 July 2016 Received in revised form 4 October 2016 Accepted 10 November 2016 Available online 22 November 2016 Keywords: TiN Implantation Spectroscopic ellipsometry Surface plasmon resonance Resistivity
a b s t r a c t Titanium nitride (TiN) thin films thickness of 260 nm prepared by dc reactive sputtering were irradiated with 200 keV silver (Ag) ions to the fluences ranging from 5 1015 ions/cm2 to 20 1015 ions/cm2. After implantation TiN layers were annealed 2 h at 700 °C in a vacuum. Ion irradiation-induced microstructural changes were examined by using Rutherford backscattering spectrometry, X-ray diffraction and transmission electron microscopy, while the surface topography was observed using atomic force microscopy. Spectroscopic ellipsometry was employed to get insights on the optical and electronic properties of TiN films with respect to their microstructure. The results showed that the irradiations lead to deformation of the lattice, increasing disorder and formation of new Ag phase. The optical results demonstrate the contribution of surface plasmon resonace (SPR) of Ag particles. SPR position shifted in the range of 354.3– 476.9 nm when Ag ion fluence varied from 5 1015 ions/cm2 to 20 1015 ions/cm2. Shift in peak wavelength shows dependence on Ag particles concentration, suggesting that interaction between Ag particles dominate the surface plasmon resonance effect. Presence of Ag as second metal in the layer leads to overall decrease of optical resistivity of TiN. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Transition metal nitrides, such as TiN, have attractive combination of physical and chemical properties. TiN coatings, depending on their microstructure and Ti/N stoichiometry, exhibit high hardness, high temperature stability, chemical inertness and low resistivity [1–5]. Due to its good biocompatibility, it can be also used as a biomedical material. Besides, addition of Ag in TiN system has been observed to result in substational improvement of corrosion and antibacterial properties of TiN coatings [6]. Namely, silver is known as an inherently antibacterial material. On the other side, TiN as a refractory material also exhibits a plasmonic resonance in the visible-NIR range, enabling plasmonic absorption in this range [7,8]. Surface plasmons are collective oscillation of the free electrons localized at the surfaces of metals structures [9,10]. This effect results in resonant optical absorbance, transmittance and/or reflectance spectra of the plasmonic materials, such as gold, silver and copper nanoparticles. Silver has some advantages over Au and Cu, because of the stronger plasmon resonance effect. Numerous ⇑ Corresponding author at: Institute of Nuclear Sciences VINCˇA, Mike Petrovic´a Alasa 12-14, 11 000 Belgrade, Serbia. E-mail address: [email protected] (M. Popovic´). http://dx.doi.org/10.1016/j.nimb.2016.11.013 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
works have been dedicated to researching nanocomposite materials, which consist of noble metal particles (Au, Ag) and dielectric matrices (SiO2, SiNx, ZnO, TiO2) [11–14]. In contrast to this, only a few studies have been performed related to the formation of embbending metallic nanoparticles in non-dielectric matrices [6]. But, little is known about optical properties of TiN coatings with addition of Ag. Led by these considerations, we have carried out a series of experiments, in which TiN layers were irradiated with 200 keV Ag ions. Ion implantation of silver was used as a versatail method for the formation of silver rich layer in the TiN structure. The spectral response of silver as a function of ion fluence, and the increase in silver concentration and particles agglomeration was investigated. The Ag particles agglomeration results in inter-particle plasmon interaction followed by a shift in surface plasmon resonance (SPR) band in the visible region. Following the combined Drude Tauc-Lorenc model, we investigated the optical properties of silver implanted TiN thin films in terms of their optical parameters measured by spectroscopic ellipsometry. We have analyzed the refractive index and extinction coefficient of implanted TiN thin films as a function of silver ion fluence. It was interesting to find that Ag nanoparticles interaction plays significant role for the shift of the surface plasmon resonance peak when increasing ion fluence. Additional analysis by Rutherford backscattering spectrometry, X-ray diffraction technique,
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transmission electron validated the results insights in thin films the correlation with implanted TiN films.
microscopy and atomic force microscopy obtained by ellipsometry and provided microstructure and morphology enabling the optical and electronic properties of
2. Experiment TiN films were deposited using dc reactive sputtering onto 550 lm thick Si(1 0 0) substrate in a Balzer Sputtron II System. The silicon substrates were chemically etched in a dilute HF solution and washed in deionized water prior to insertion in the vacuum chamber and were sputter cleaned by a flux of argon ions just before deposition. The chamber was evacuated to 1 104 Pa and during deposition the nitrogen gas was introduced into the chamber to the partial pressure of 3 102 Pa. Experimental parameters were chosen so that the deposition rate at the substrate was 6.5 nm/min. The layers were deposited to the thickness of 260 nm, as determined by using the Talystep profilometer. Then, TiN samples dimensions of 2 2 cm2 were subjected to implantaˇ A 500 kV ion implanter. The tion of 200 keV Ag+ ions using VINC irradiations of the samples were performed at room temperature under high vacuum (104 Pa) in the fluence range between 5 1015 and 20 1015 ions/cm2. The beam current density was kept at 1 lA/cm2 in order to avoid thermal degradation during ion implantation. Irradiation energy was chosen in such a way that the projected range of ions and energy deposition density were in TiN, according to simulation done with the SRIM 2008 code [15]. After implantation the samples were annealed for 2 h at 700 °C in a tube furnace under a vacuum of some 106 Pa. The structural characterization of TiN thin films was done by Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), transmission electron microscopy (TEM) together with energy dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM). For studying the optical properties of the films spectroscopic ellipsometry was used. Depth profiling was performed by 900 keV He++ ion beam provided by the Gottingen IONAS implanter [16]. RBS spectra were taken with two silicon surface barrier detectors mounted at ±165° with respect to the beam, having an energy resolution of 13 keV FWHM. The software used in simulation of RBS spectra was WiNDF code [17]. Philips PW1050 Xray diffractometer with monochromatized CuKa radiation (k = 0.15418 nm) in a 2h range of 35–45° was used to analyze the structure of the TiN films. TEM and EDS analyses of the TiN samples were performed by using PHILIPS CM30 microscope operated at 300 kV. In order to investigate the crystalline structure of the samples we also used the micro-diffraction (MD) technique. The surface topography of the samples was analyzed by means of atomic force microscopy using Quadrex Multimode IIIe (Veeco Instruments Inc.). Data were taken in ambient air and in tapping mode. Ellipsometric spectra have been measured by HORIBAJobin Yvon UVIS ellipsometer in an energy range of 0.4–4.8 eV. The measurements were performed under an incident angle of 70 degrees with respect to the surface normal, at room temperature. 3. Results and discussion 3.1. Silver depth profiles Irradiation energy of 200 keV was chosen to ensure the maximum Ag concentration to be situated well within TiN layer. Fig. 1a shows the depth profiles of Ag in TiN layer, extracted with WiNDF code from experimental RBS spectra taken from the sam-
ple irradiated to the fluence of 20 1015 ions/cm2 and the sample annealed at 700 °C after irradiation. This analysis reveals that Ag has a maximum concentration of 3.5 at.% inside of TiN layer at a depth of around 33 nm. Due to the diffusion process after annealing the depth profile was slightly wider with the maximum at around 38 nm in depth. The process of Ag ions irradiation was simulated using SRIM code, as also shown in Fig. 1a (solid line). SRIM calculation gave a mean projected range Rp = 48 nm and straggling of DRp = 16 nm for the above experimental parameters. It can be seen that the experimentally obtained Ag concentration maximum was shifted towards the surface as compared to the SRIM Rp calculation. The obvious difference can be due to the changes of the density of the layer as well as to the surface erosion that occurs during irradiation. The sputtering effect was calculated from experimentally obtained RBS spectra and it was found to be 10 nm. Fig. 1b shows the EDS elemental mapping of Ag in the cross-section of the sample annealed after irradiation to the fluence of 20 1015 ions/cm2. The presence of Ag inside of TiN layer with the maximum concentration at the depth of around 40 nm was confirmed, demonstrating good agreement with RBS measurements. 3.2. XRD analysis of silver implanted TiN thin films Fig. 2(a–d) shows the X-ray diffraction (XRD) patterns obtained from as deposited, implanted and after irradiation annealed samples in h–2h geometry. Both as deposited and irradiated samples exhibit a typical polycrystalline TiN structure, showing only (1 1 1) and (2 0 0) diffraction peaks. Two observations are apparent indicating a decrease in crystallinity of the films and compressive stress/strain development during irradiation. It can be clearly seen that intensity of diffraction peaks decreased and its full width of half maximum (FWHM) increased after irradiation with Ag+ ions. On the other side, diffraction peaks of implanted samples shift towards higher angles in comparison with of as deposited TiN film, demonstrating that the lattice undergoes a shrinkage of 1% from 4.293 Å to 4.261 Å after irradiation. This lattice compression appears to be consistent with compressive stress/strain developed during irradiation. In addition, for the sample implanted to the fluence of 20 1015 ions/cm2 a small peak with the lattice spacing of 0.2383 nm corresponding to (1 1 1) Ag reflection appears, suggesting the Ag metallic clusters agglomeration for the highest ion fluence. Note that the intensity of the peaks increased after annealing at 700 °C, indicating that the crystallinity of the TiN films was improved. The average size of Ag crystalline grains for the sample implanted to the fluence of 20 1015 ions/cm2 was found to be 7 nm. Post-implantation annealing leads to increase of Ag clusters to the value of 16 nm. 3.3. TEM analysis of silver implanted TiN thin films Fig. 3a shows the cross-sectional bright field image taken from the sample irradiated to 20 1015 Ag/cm2 and then annealed at 700 °C. It can be seen that the TiN layer has a dense columnar structure partially damaged after 200 keV Ag+ ions irradiation. The damage region is situated within 100 nm at surface of the TiN layer and is stabilized after annealing. The presence of nanocrystalline grains is confirmed by the micro-diffraction (MD) patterns taken from cross-sectional TEM as presented in Fig. 3b. Besides of (1 1 1) and (2 0 0) patterns of TiN phase the presence of Ag nanograins arranged in (1 1 1) and (2 0 0) orientations are observed. Fig. 3c is a high-resolution image of (1 1 1) lattice fringes from a typical Ag grain in the sample implanted to the fluence of 20 1015 Ag/cm2. It can be seen that the silver nanoparticles have a spherical shape with diameter of 7 nm. The lattice spacing is
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Fig. 1. (a) Ag concentration depth profiles of implanted TiN sample to the fluence of 20 1015 ions/cm2 before and after annealing at 700 °C, compared to the SRIM simulation and (b) EDS elemental map for silver extracted from the cross-section TiN sample irradiated to the fluence of 20 1015 ions/cm2 and then annealed 2 h at 700 °C.
3.4. AFM surface topography and surface roughness of silver implanted TiN thin films Changes in surface morphology of the TiN layers induced by Ag ion irradiation were also analyzed by AFM. Fig. 4 shows twodimensional AFM morphology images taken from the surface of TiN sample before and after implantation of 20 1015 Ag/cm2 in the as-implanted state and also after annealing at 700 °C. The difference in roughness for as deposited and then irradiated sample can be observed. The as deposited TiN layer exhibit smooth surface morphology with surface roughness value of 0.4 nm. After Ag ion implantation to the fluence of 20 1015 Ag/cm2 the surface becomes smoother with the roughness of 0.2 nm. This comes as a consequence of the sputtering of atoms from the surface layer and of the interaction of incident energetic Ag ions with the atoms of TiN during ion bombardment. Further, annealing of implanted sample changed the value of roughness to 0.3 nm. 3.5. Spectroscopic ellipsometry of silver implanted TiN thin films
Fig. 2. XRD spectra of TiN samples: (a) as deposited; (b) Ag ion irradiated to the fluence of 5 1015 ions/cm2; (c) Ag ion irradiated to the fluence of 20 1015 ions/ cm2; (d) Ag ion irradiated to the fluence of 20 1015 ions/cm2 and then annealed 2 h at 700 °C.
0.2362 nm which is in agreement with the value of 0.2383 nm obtained from the XRD analysis. The theoretical value for the cubic Ag is 0.2359 nm [18].
TiN samples were subjected to spectroscopic ellipsometry measurements in order to get information about the optical properties of the films. The two experimental quantities, which are directly obtained trough this technique, are W and D. These parameters are indication of relative changes in the amplitude and the phase of a linearly polarized monochromatic incident light upon an oblique reflection from a sample surface. In order to obtain optical parameters from these measured values it is necessary to construct appropriate optical model of the samples. Therefore we analyzed the experimental data of as deposited and annealed samples using a three layer model, consisting of a few nm thick oxygen layer on the top, the homogenous TiN layer thickness of 260 nm and 550 lm silicon substrate. On the other hand, each ion beam implanted sample was treated as a four layer system comprising an oxygen layer, modified and non-modified TiN layer and finally Si substrate. The optical models used in simulations of TiN samples are: Tauc Lorenc oscillator for the interband transitions due to valence electrons and the Drude term which considers the contribution of the intraband transitions (free electron transitions in the conduction band) [19–21]. The optical constants n (refractive index) and k (extinction coefficient) can be obtained by fitting the experimental W and D values. This was done by DeltaPsi2 software (HORIBA) [22]. The spectral variation of n and k for as deposited and Ag ions irradiated
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Fig. 3. TEM analysis of TiN samples: (a) low magnification bright-field image and (b) corresponding MD diffraction pattern of the TiN layer irradiated with Ag ions to the fluence of 20 1015 ions/cm2 and then annealed 2 h at 700 °C; (c) HRTEM image of isolated Ag particle taken from sample implanted to the fluence of 20 1015 ions/cm2.
Fig. 4. Surface topography AFM images 1 lm 1 lm of TiN sample: (a) as deposited; (b) irradiated to the fluence of 20 1015 ions/cm2 and (c) irradiated to the fluence of 20 1015 ions/cm2 and subsequent annealed 2 h at 700 °C.
samples are shown in Fig. 5(a and b). The n and k spectra for as deposited TiN layer show good agreement with data previously reported [23]. It can be observed that Ag ions irradiation changes optical response of the TiN thin films. In particular, a broad band in the spectra of n and k in visible region was observed. There is also a shift towards larger wavelengths with increasing ion fluence. The similar behavior was observed for the samples annealed at 700 °C after irradiation, as shown in Fig. 6(a and b). These results demonstrate the contribution of the surface plasmon resonance of Ag particles. The existence of a SPR was predicted by the Mie theory [24]. This is effect of absorption and scattering of light by small spherical particles. Namely, Mie theory shows that light absorption exhibit a single SPR, yielding a dipolar charge distribution on the surface. Our results show that single SPR position shifted in the range of 354.3–476.9 nm when Ag ion fluence varied from 5 1015 ions/cm2 to 20 1015 ions/cm2. These observed surface plasmon resonance bands are indication of Ag nanoparticles
formation in the TiN matrix. The SPR generated in Ag irradiated TiN samples arises from the collective oscillations of the conduction band electrons induced by the incident light. A classical Mie theory describes SPR for small isolated spherical particles, ie. for low particle volume concentration V (V 1). With increasing of ion fluence, the Ag concentration increases and interaction between particles cannot be neglected. In this case the MaxwelGarnett‘s effective medium theory [25–27] can be applied to interpret the results. According to Maxwel-Garnett‘s theory, the SPR wavelength, kSPR = 2pc/xSPR, is given by:
kSPR ¼ kP
1=2 2þq eext þ 1 1q
ð1Þ
where kP = 2pc/xP is the Ag’s bulk plasmon wavelength, xP = 9.2 eV for Ag, eext is the interparticle dielectric constant and q is the particle filling factor. As Ag ion fluence increases one can
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Fig. 5. Refractive index (a) and extinction coefficient (b) spectra as a function of wavelength for as deposited and silver ion implanted TiN thin films.
Fig. 6. Refractive index (a) and extinction coefficient (b) spectra as a function of wavelength for TiN thin films annealed 2 h at 700 °C after irradiation.
expect the increase of the particle filling factor q. As a result, this will favors the kSPR shift towards longer wavelengths, as it was observed. To further elucidate the effects of Ag ions irradiation on optical properties of TiN layers, we have plotted the n and k values (taken at 434 nm) as a function of ion fluence. The results are shown in Fig. 7. It can be seen that as the fluence of Ag ions increases from 5 1015 ions/cm2 to 20 1015 ions/cm2, the refractive index also increases. At the same time the k values decrease. We can assume that this can be due to the structural changes induced by Ag ions irradiation. The complex processes of ion-solid interaction lead to deformation (compaction) of lattice, increasing disorder (damage) and formation of new Ag phase (agglomeration of Ag atoms). For the samples annealed after irradiation the refractive index exhibits small difference and higher values. The difference is a consequence of the increase in Ag grain size as well as the grain size of the host TiN layer. The HRTEM and XRD measurements showed that the Ag grain size increased from 7 nm to 16 nm after annealing. The mean crystalline size of the host TiN was found to be 10 nm and increase to 12 nm after annealing. The analysis clearly demonstrate that postimplantation annealing resulted in recovery of polycrystalline structure implying the grain size effect on the optical properties of the Ag irradiated and annealed TiN samples. Spectroscopic ellipsometry was also used for measuring the optical resistivity of TiN thin films. The values of optical resis-
tivites were obtained using formula q ¼
Cd
e0
1
x2p
, where Cd and
xp are parameters known from fitting of (W, D) spectra. The calculated optical resistivites for as deposited sample, Ag treated and/or annealed TiN layers are shown in Fig. 8. The optical resistivity of as deposited TiN thin film found to be 587 lX cm, in agreement with the value of 527 lX cm already reported by Patsalas at al. [28]. We found that the optical resistivity, q, was always lower in implanted layers than in as deposited TiN film. However, this optical resistivity does not decrease monotonically, as expected. The largest reduction of resistivity relative to the unimplanted layer was observed for the sample implanted to the fluence of 5 1015 ions/cm2, with the values around the 20 lX cm. Then, optical resistivity stays almost constant in the ion fluence range of 10 1015–15 1015 ions/cm2. For the highest ion fluence of 20 1015 ions/cm2, the optical resistivity achieves the lowest value of 4 lX cm. Obviously, presence of Ag as a second metal in the layer has a predominant role on metallic behavior of TiN over the damage and leads to overall decrease of optical resistivity of TiN. Having this in mind the resistivity of TiN can be easily changed in the wide range by varying Ag concentration in the layers. However, we observed no effect of post-thermal annealing on resistivity of the layers, indicating that the structure of TiN is preserved at high temperatures. Indeed, these assumptions were confirmed by the observations obtained by TEM analysis.
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Fig. 7. The optical constants n and k (at 434 nm) of Ag implanted TiN samples as a function of ion fluence.
ence, the concentration of Ag nanoparticles increases and interaction between particles cannot be neglected. Shift in kSPR induced by interaction among the particles becomes more significant with increasing Ag concentration. Another important point clarified by spectroscopic ellipsometry is related to the modifications of n and k values. As the Ag+ ions fluence increases, the n also increase and k values decrease. At the post-implantation annealing of 700 °C an increase of Ag clusters and TiN lattice recovery were observed. This provides conditions for the slight increase of the n values. The presence of Ag as a second metal in the layer has a great influence on metallic behavior of TiN films. As the concentration of Ag ions increases the film becomes more metallic.
Acknowledgement
Fig. 8. The calculated optical resistivity for as deposited, Ag treated and/or annealed TiN films.
This work was supported by the Ministry of Education and Science of the Republic of Serbia (Project No. III 45005). We would like to thank I. Peterka, D. Purschke, M. Mitric´ and P Wilbrandt for their support during the experiments.
References 4. Summary The structural and optical properties of the TiN layers irradiated with 200 keV Ag+ ions have been analyzed as a function of ion fluence and post-implantation thermal annealing at 700 °C. The formation of Ag metallic clusters inside of TiN layers at a depth of around 40 nm was observed. XRD patterns clearly demonstrate a decrease of crystallinity of the TiN layers and compressive stress/ strain development during irradiation. According to HRTEM analyses the Ag metallic phase has a nanometric nature of grains, with a spherical shape and grain diameter of 7 nm. The above observations are in agreement with the spectroscopic ellipsometry analyses. The surface plasmon resonance of Ag particles was confirmed by a broad band in the spectra in visible region. The SPR shifted in the range of 354.3 nm to 476.9 nm when Ag ion fluence varies from 5 1015 ions/cm2 to 20 1015 ions/cm2. With increasing ion flu-
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