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Ellipsometric studies of N+ implanted Ti thin films
Thin Solid Films 518 (2010) 3754–3758
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Ellipsometric studies of N+ implanted Ti thin films S. Tripura Sundari ⁎, R. Krishnan, S. Dash Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
a r t i c l e
i n f o
Article history: Received 22 August 2008 Received in revised form 6 October 2009 Accepted 15 October 2009 Available online 24 October 2009 Keywords: Ellipsometry Ion irradiation Plasma energy
a b s t r a c t Spectroscopic ellipsometry was employed to study the optical response of N+ irradiated titanium thin films synthesized by pulsed laser deposition technique. The ellipsometric parameters were measured in the energy range of 1.5 to 5.5 eV. A combined Drude Lorentz (DL) model was employed to quantitatively describe the behavior of the dielectric response as a function of N+ irradiation fluence. A modeling based on effective medium approximation (EMA) was carried out to compute the volume fractions of Ti and TiN from the dielectric response functions. The plasma energy as computed from the DL model, decreased with increasing fluence. The results are explained on the basis of formation of TiN phase which was revealed from grazing incidence X-ray diffraction studies. This was further corroborated from the alterations on volume fraction of titanium as inferred from EMA based computation. © 2009 Elsevier B.V. All rights reserved.
1. Introduction TiN is a unique transition metal nitride that possesses a blend of ceramic and metallic properties. It is known to be hard, wear resistant and inert while being a good electrical and thermal conductor. The simultaneous occurrence of both covalent and metallic bonds permits its widespread usage in several technological applications. Specifically, this material has gained technological importance owing to high hardness, elasticity, electronic conductivity and chemical stability. These properties also allow applications in the area of microelectronic devices, solar cells and protective coatings etc. It has also been exploited in semiconductor device technology in ultra large scale integrated circuits. The covalent nature of Ti–N bonds and contribution of Ti d electrons to optical transitions has stimulated a number of fundamental studies on this transition metal nitride. Recently, TiN x (x = 1.1) [1] has been widely investigated owing to its better metallic and optical properties that are much sought after in electronic interconnects. The covalent bonding is responsible for the stability and exceptionally good mechanical properties such as hardness. The metallic behavior of TiN is due to the intersection of the Ti 3d electrons in valence band with the Fermi level. A number of research papers [2–5] have been published on the optical and electronic properties of TiN. However, in most of the studies, reactive RF magnetron sputtering has been used as the synthesis technique. The physical properties of thin film systems, especially,
⁎ Corresponding author. E-mail address: [email protected] (S. Tripura Sundari). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.10.122
the optical properties, are strongly dictated by the synthesis route. Although TiN has been synthesized by ion irradiation route [6], its optical properties have not yet been investigated. The present paper deliberates on N+ ion implantation induced evolution of TiN phase in pulsed laser deposition (PLD) grown Ti films and the optical characterization by spectroscopic ellipsometry. For the investigation of optical properties, ellipsometry is a unique non-destructive tool from which the complex dielectric functions can be obtained directly on a wavelength to wavelength basis without resorting to Kramers Kronig analysis. We address the study of optical properties of N+ implanted Ti thin films synthesized by PLD technique in terms of the variations in the dielectric functions. We model the system following a Drude Lorentz (DL) combined formalism which takes into account both the free electron and bound electron contributions respectively [5]. 2. Experimental details Thin films of titanium were deposited by ablating a high pure (99.99%) titanium metal target by a Q-switched Nd:YAG laser with a wavelength of 1064 nm, repetition rate of 10 Hz, pulse energy of 300 mJ and pulse width of 7 ns. An all-metal ultra high vacuum compatible PLD facility was used for this purpose. Exact details of this facility are published elsewhere [7]. Prior to deposition, the single crystal Si substrates were degreased and cleaned chemically in acidic and basic solutions and vacuum dried. The laser beam impacted a rotating titanium target, which was rotated at a constant velocity of 3 rpm to prevent crater formation and expose fresh surface of target for ablation at an oblique incidence of 45°. A target-substrate distance of 40 mm was maintained during the deposition. The nominal thicknesses of the films were 250 nm as measured by a surface profilometer.
S. Tripura Sundari et al. / Thin Solid Films 518 (2010) 3754–3758
The Ti films grown by PLD technique were irradiated at room temperature at normal incidence with 100 keV N+ ions for three different fluences namely, 1 × 1016, 5 × 1016 and 1 × 1017 ions/cm2 respectively, using a 150 keV ion accelerator. The beam current was kept below 800 nA/cm2 in order to avoid heating effect due to ion irradiation. The projected range of 100 keV nitrogen ions in titanium as calculated from a Monte Carlo based technique Stopping and Range of Ion in Matter code was found to be ~160 nm with an associated straggling of ~ 50nm[8]. The samples for transmission electron microscopy (TEM) were prepared by using freshly cleaved NaCl crystals as substrates for deposition of titanium thin films. After ion implantation, these films were fished out on copper grids by dissolving NaCl substrates in demineralized water. The TEM analysis was carried out using a TECNAI G2 F30 machine with an accelerating voltage of 300 keV. The as-deposited and implanted thin films were characterized by STOE grazing incidence X-ray diffraction (GIXRD) for their structural characterization. A SOPRA ESVG rotating polarizer spectroscopic ellipsometer was used to study the optical properties of the asdeposited and ion irradiated films. The optical measurements were performed at room temperature in the energy range of 1.5 to 5.5 eV in steps of 10 meV for three angles of incidence namely, 65°, 70° and 75°. Only results pertaining to spectroscopic ellipsometry are presented in this paper. 3. Theoretical background Spectroscopic ellipsometry is a non-destructive surface sensitive tool, which is capable of addressing the issue of change in optical properties caused by associated changes in microstructure in thin films, multilayer structure and bulk systems. This technique is inherently powerful as it is based on detecting the change in polarization state of incident light upon reflection from a surface. The measured ellipsometric parameter ρ which is the ratio of the parallel to perpendicular components of the reflection coefficients is decided by the dielectric function ε(E) [9] of the material under investigation. The complex psuedodielectric function ε(E) is related to the ellipsometric parameters by the relation 1−ρ 2 2 2 2 2 εðEÞ = No sin θ + sin θ tan θ 1+ρ
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part of the dielectric function is zero. It depends on the number density of conduction electrons N by the relation
ωpu
sffiffiffiffiffiffiffiffiffiffiffi Ne2 = m*εo
ð3Þ
where e is the electron charge, m* is the effective mass and εo is the permittivity of free space. The conduction electron density is a useful quantity to determine the metallic character. In most metals, the free electron contribution leads to a plasma frequency in the uv region of the electromagnetic spectrum. However, in noble metals like Ag, Au and Cu, the bound electron contribution to the interband transition drags down the plasma frequency to the visible region and is known as the screened plasma frequency (ωps). Because of the sensitivity of the screened plasma frequency to the electronic structure, it has been used to monitor the composition and stoichiometry of TiNx [1]. The optical properties of TiN arise from the electronic band structure in which the Fermi energy lies within the d band. The longitudinal excitation mode of the screened plasma energy is reported to be ~ 2.45–2.65 eV[11] depending on the stoichiometry. 4. Results and discussion Fig. 1a and b show the experimental real and imaginary parts of the psuedodielectric functions of as-deposited as well as irradiated films computed from the experimentally measured ellipsometric parameters using Eq. (1). As seen from the figure, the ε1 of as-
ð1Þ
where No is the refractive index of the ambient and θ is the angle of incidence. The data analysis in ellipsometry hinges on two broad based approaches. The first approach is the effective medium approximation (EMA) employed for the extraction of microstructure parameters like volume fraction of constituents, void fractions, thickness and roughness of layers, surfaces and interface layers etc. The second is the optical dispersion models such as DL that are used to infer about the underlying electronic structure [10]. The Drude model relation is given the relation ε = ε∞ −
ω2pu ω −iΓD ω 2
fj ωoj2 2 2 j = 1 ωoj −ω + 2
+ ∑
iγj ω
ð2Þ
In Eq. (2), ε∞ is the background constant, which is larger than unity due to the contribution from higher energy transitions that are not accounted for in the Lorentz relation. The Drude term is characterized by the damping factor ΓD and unscreened plasma frequency ωpu. The damping factor arises from scattering of electrons and is the inverse of its relaxation time. The bound electron contribution is given by the Lorentz term where fj, ωoj and γj are the strength, resonance energy and damping of the jth oscillator respectively. The plasma frequency (ωpu : unscreened plasma frequency) is defined as the frequency at which the real
Fig. 1. Real and imaginary parts of the experimental psuedodielectric function of asdeposited and irradiated specimens.
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deposited sample shows a metallic character but does not intercept the energy axis even though it is expected to do so as per the data reported in ref. [13]. In ref. [13], the ε1 goes to zero with a positive slope near 4.88 eV. As known from previous ellipsometry analysis, a prerequisite for the EMA analysis is that the dielectric function of Ti be known accurately. Several dielectric functions have been reported in literature, but the agreement among them is very poor owing to changes in microstructure. Ti is a transition metal and the measured optical constants depend on whether it is bulk Ti[14], polycrystalline or in the form of thin films[15] and they differ substantially from each other. There is a strong bound electron interband transition that arises from filled parts of the d bands to the empty parts of the d bands even though the bands are hybridized with s and p character. The difference in nature of the dielectric functions arise because of their inherent sensitivity to microstructure and stoichiometry and therefore to Ti/N ratio. For the as-deposited specimen, the EMA yielded a poor fit, while using the previously determined dielectric functions [14]. Hence, we used the dielectric functions from ref. [15] and incorporated voids and surface roughness into the modeling. Even then, the fitting was only reasonably good. The GIXRD pattern for this specimen showed the existence of Ti in the hcp phase as shown in Fig. 2 which is normally formed during thin film deposition. However, it is to be noted that metastable phases like fcc Ti are also formed by PLD technique. The difference in microstructure could be the reason for the not so good fitting that was obtained. In the present experiment, we are interested in the irradiation induced change in Ti thin films and subsequent nitride phase evolution on N+ irradiation. Therefore, we have chosen the as-deposited films as reference for the EMA analysis for the irradiated films. Our spectrum for the as-deposited films compared reasonably well with the spectrum reported in ref. [16] indicating sufficient volume fraction of voids in the film. From the figure, it is evident that there is no contribution from the underlying silicon substrate in any of the specimens. As the fluence is increased, the real and imaginary parts show a systematic variation. The most prominent ones being a decrease in ε2, accompanied with increase in ε1. A red shift in the plasma energy is also observed. The microstructure considered for the EMA analysis was constituted by a mixture of Ti, TiN and voids. The volume fractions of TiN in the irradiated specimens deduced from EMA were 25, 34 and 94% for the 1 × 1016, 5 × 1016 and 1 × 1017 ions/cm2 respectively. The GIXRD patterns of all irradiated films with fluences of 1 × 1016 and 5 × 1016 ions/cm2 showed hcp Ti and cubic TiN. Cubic
TiN was present for the specimen irradiated with 1 × 1017 ions/cm2. A typical effective medium approximation modeling for the specimen irradiated to a fluence of 1 × 1017 ions/cm2 is shown in Fig. 3a and b. In fact, it is known from EMA analysis in literature that for TiN thin films thicker than 60 nm, the ellipsometric technique directly yields the dielectric function of the films without any contribution from the Si substrate. A combined DL dispersion law was employed to model the contribution of the free and bound electrons to the dielectric dispersion. It is to be pointed out that the fitting was performed on the data measured at all three angles of incidence viz. 65°, 70° and 75°. A representative fit for the real and imaginary parts of the dielectric function using DL model is shown in Fig. 4a and b for specimen irradiated with a fluence of 1 × 1016 ions/cm2. The Drude part of the fit describes the optical response due to the Ti 3d electrons. The variation of the free electron contribution for as-deposited and the irradiated specimens are shown in Fig. 5. From the figure, the plasma energy shows a progressive red shift with increase in fluence indicating the increase in concentration of TiN [17]. This type of behavior is also observed in the case of TiNx films with a change in the nitrogen concentration. The decrease in plasma frequency is interpreted as a decrease in the conduction electron density. The contribution for conduction electrons comes from the Ti atoms. Hence, a decrease in plasma energy is construed as an increase in covalent bonds formed due to nitridation of Ti with increasing ion fluence. In the present experiment, the GIXRD pattern supports the coexistence of both Ti in the hcp phase and TiN in the cubic phase. Fig. 6 shows the volume fraction of TiN with increasing fluence and the values of plasma energy as deduced from the ε1(energy = 0) of Fig. 5. The plasma
Fig. 2. GIXRD pattern of as-deposited Ti thin films and specimen irradiated with a fluence of 5 × 1016 ions/cm2.
Fig. 3. Representative EMA modeling for the real and imaginary parts of the dielectric function for specimen irradiated to a fluence of 1 × 1017 ions/cm2.
S. Tripura Sundari et al. / Thin Solid Films 518 (2010) 3754–3758
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Fig. 6. The plasma frequency shift and volume fraction of TiN as a function of fluence. (Note: the lines are a guide to the eye).
Fig. 4. Experimental and fitted real and imaginary parts of the dielectric function using DL model for a representative irradiated specimen (1 × 1016 ions/cm2). The individual contributions of Drude and the two Lorentz oscillators are also shown.
energy for the specimen irradiated with 1 × 1017 ions/cm2 is ~2.7 eV which is marginally greater than values reported in literature so far. The broadening parameter computed from the Drude model for specimens irradiated with 1 × 1016, 5 × 1016 and 1 × 1017 ions/cm2 were 1.02 eV, 1.373 eV and 1.65 eV respectively which is considerably larger than that found in refs. [1] and [12]. In the case of specimen irradiated with 1 × 1017 ions/cm2, there is also an additional contribution arising from the nanocrystalline nature of TiN which was
Fig. 5. Drude contribution for the as-deposited and irradiated specimens.
confirmed from the indexed selected area electron diffraction (Fig. 7) analysis [18] which could lead to an increase in the plasma energy. However, the results obtained in the present experiment are substantially different from the nanocrystalline TiN thin films reported in ref. [1]. The studies reported in ref. [1] pertains to nanocrystalline TiNx films and the broadening decreased with increasing substrate temperature due to increasing grain size. The difference in these results is attributed to the progressive increase in defect production as fluence increases. Two reasons are attributed to the decreased relaxation time of electrons. One is the generation of defects during irradiation and the other is due to decreased grain size resulting from prevailing nanocrystalline nature. The best fit for the interband optical transition energy deduced from the Lorentz model were obtained near 2.4 eV and 5.5 eV for all three irradiated specimens. The positions of the oscillators did not change significantly with variation in fluence. The values of oscillator energies compare well with previous reports and confirm the formation of TiN. Adachi et al.[12] have observed transitions near 2.5 eV, 3.1 eV and 5.2 eV corresponding to E1, E2 and E3 transitions, respectively. The E1 transition corresponds to those between Γ15
Fig. 7. Selected area electron diffraction pattern of specimen irradiated with 1 × 1017 ions/cm2.
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valence band to the Γ12 conduction band [1] in TiN and its theoretical value is estimated to be ~ 2.3 eV. The E2 transition is interpreted as transitions between hybridized N p and Ti d bands at Γ15 → Γ12 bands while the transition near 5 eV is assigned to the transitions near the L, K and W points in the Brillouin zone. Patsalas et al. [1] have also observed transitions near 3 and 5.2 eV for TiN x nanocrystalline films. However, the lowest band gap transition near 1.0 eV occurring at the Γ point (Γv25Õ → Γc12) that is expected from theoretical calculations could not be observed by either of them. This is not easily identifiable experimentally because of the strong intraband transition occurring in this energy range. However, the oscillator at 5.2 eV was found to be very weak in the case of specimen irradiated to a fluence of 1 × 1017 ions/cm2. The peak near 5.5 eV is due to this. 5. Conclusions The optical response of thin films of Ti deposited on Si by PLD technique and irradiated with N+ were studied by spectroscopic ellipsometry in the energy range 1.5 to 5.5 eV. A combined Drude Lorentz model revealed distinct features and a red shift in the plasma energy indicative of formation and increasing volume fraction of TiN. The unscreened plasma frequency extracted from the Drude model decreased with increasing ion fluence signifying enhanced conversion of Ti to TiN. In order to understand more details and capture the onset of nitride formation, the experiment needs to be conducted in lower fluence ranges.
Acknowledgement The authors thank Mr. Amit Mondal of Institute Nano Initiative (INI), Indian Institute of Science, Bangalore for the TEM analysis.
References [1] P. Patsalas, S. Logothetidis, J. Appl. Phys. 90 (2001) 4725. [2] P. Patsalas, C. Charitidis, S. Logothetidis, Appl. Surf. Sci. 154–155 (2000) 256. [3] V.G. Kechagias, M. Gioti, S. Logothetidis, R. Benferhat, D. Teer, Thin Solid Films 364 (2000) 213. [4] J. Humlick, A. Nebojsa, J. Hora, M. Strasky, J. Spousta, T. Sikola, Thin Solid Films 332 (1998) 25. [5] P. Patsalas, S. Logothetidis, J. Appl. Phys. 93 (2003) 989. [6] P.A. Chen, T.T. Yang, Thin Solid Films 81 (1981) L91. [7] R. Krishnan, R. Ramaseshan, Mathews Tom, R. Nithya, S. Dash, A.K. Tyagi, Baldev Raj, Surf. Eng. 25 (2009) 218. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Matter, Pergamon, New York, 1985. [9] R.A.M. Azzam, N.M. Bashara, Ellipsometry and Polarized Light, North-Holland, Amsterdam, 1977. [10] S. Tripura Sundari, J. Appl. Phys. 92 (2002) 4367. [11] S. Logothetidis, I. Alexandrou, A. Papdopoulos, J. Appl. Phys. 77 (1995) 1043. [12] S. Adachi, M. Takahashi, J. Appl. Phys. 87 (2000) 1264. [13] D.W. Lynch, W.R. Hunter, in: E.D. Palik (Ed.), Optical Constants of Solids, Vol. II, Academic Press, San Diego CA, 1991, p. 240. [14] D.W. Lynch, C.G. Olson, Phys, Rev. B: Condens. Matter Mater. Phys. 11 (1975) 3617. [15] P.B. Johnson, R.W. Christy, Phys, Rev. B: Condens. Matter Mater. Phys. 9 (1974) 5056. [16] J.M.M. De Nijs, A.V. Silfhout, Thin Solid Films 173 (1989) 1. [17] J.H. Kang, K.J. Kim, J. Appl. Phys. 86 (1999) 346. [18] R. Krishnan, S. Amirthapandian, G. Mangamma, R. Ramaseshan, S. Dash, A.K. Tyagi, V. Jayaram, Baldev Raj, J. Nanosci. Nanotechnol. 9 (2009) 5461–5466.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Ellipsometric studies of N+ implanted Ti thin films S. Tripura Sundari ⁎, R. Krishnan, S. Dash Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
a r t i c l e
i n f o
Article history: Received 22 August 2008 Received in revised form 6 October 2009 Accepted 15 October 2009 Available online 24 October 2009 Keywords: Ellipsometry Ion irradiation Plasma energy
a b s t r a c t Spectroscopic ellipsometry was employed to study the optical response of N+ irradiated titanium thin films synthesized by pulsed laser deposition technique. The ellipsometric parameters were measured in the energy range of 1.5 to 5.5 eV. A combined Drude Lorentz (DL) model was employed to quantitatively describe the behavior of the dielectric response as a function of N+ irradiation fluence. A modeling based on effective medium approximation (EMA) was carried out to compute the volume fractions of Ti and TiN from the dielectric response functions. The plasma energy as computed from the DL model, decreased with increasing fluence. The results are explained on the basis of formation of TiN phase which was revealed from grazing incidence X-ray diffraction studies. This was further corroborated from the alterations on volume fraction of titanium as inferred from EMA based computation. © 2009 Elsevier B.V. All rights reserved.
1. Introduction TiN is a unique transition metal nitride that possesses a blend of ceramic and metallic properties. It is known to be hard, wear resistant and inert while being a good electrical and thermal conductor. The simultaneous occurrence of both covalent and metallic bonds permits its widespread usage in several technological applications. Specifically, this material has gained technological importance owing to high hardness, elasticity, electronic conductivity and chemical stability. These properties also allow applications in the area of microelectronic devices, solar cells and protective coatings etc. It has also been exploited in semiconductor device technology in ultra large scale integrated circuits. The covalent nature of Ti–N bonds and contribution of Ti d electrons to optical transitions has stimulated a number of fundamental studies on this transition metal nitride. Recently, TiN x (x = 1.1) [1] has been widely investigated owing to its better metallic and optical properties that are much sought after in electronic interconnects. The covalent bonding is responsible for the stability and exceptionally good mechanical properties such as hardness. The metallic behavior of TiN is due to the intersection of the Ti 3d electrons in valence band with the Fermi level. A number of research papers [2–5] have been published on the optical and electronic properties of TiN. However, in most of the studies, reactive RF magnetron sputtering has been used as the synthesis technique. The physical properties of thin film systems, especially,
⁎ Corresponding author. E-mail address: [email protected] (S. Tripura Sundari). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.10.122
the optical properties, are strongly dictated by the synthesis route. Although TiN has been synthesized by ion irradiation route [6], its optical properties have not yet been investigated. The present paper deliberates on N+ ion implantation induced evolution of TiN phase in pulsed laser deposition (PLD) grown Ti films and the optical characterization by spectroscopic ellipsometry. For the investigation of optical properties, ellipsometry is a unique non-destructive tool from which the complex dielectric functions can be obtained directly on a wavelength to wavelength basis without resorting to Kramers Kronig analysis. We address the study of optical properties of N+ implanted Ti thin films synthesized by PLD technique in terms of the variations in the dielectric functions. We model the system following a Drude Lorentz (DL) combined formalism which takes into account both the free electron and bound electron contributions respectively [5]. 2. Experimental details Thin films of titanium were deposited by ablating a high pure (99.99%) titanium metal target by a Q-switched Nd:YAG laser with a wavelength of 1064 nm, repetition rate of 10 Hz, pulse energy of 300 mJ and pulse width of 7 ns. An all-metal ultra high vacuum compatible PLD facility was used for this purpose. Exact details of this facility are published elsewhere [7]. Prior to deposition, the single crystal Si substrates were degreased and cleaned chemically in acidic and basic solutions and vacuum dried. The laser beam impacted a rotating titanium target, which was rotated at a constant velocity of 3 rpm to prevent crater formation and expose fresh surface of target for ablation at an oblique incidence of 45°. A target-substrate distance of 40 mm was maintained during the deposition. The nominal thicknesses of the films were 250 nm as measured by a surface profilometer.
S. Tripura Sundari et al. / Thin Solid Films 518 (2010) 3754–3758
The Ti films grown by PLD technique were irradiated at room temperature at normal incidence with 100 keV N+ ions for three different fluences namely, 1 × 1016, 5 × 1016 and 1 × 1017 ions/cm2 respectively, using a 150 keV ion accelerator. The beam current was kept below 800 nA/cm2 in order to avoid heating effect due to ion irradiation. The projected range of 100 keV nitrogen ions in titanium as calculated from a Monte Carlo based technique Stopping and Range of Ion in Matter code was found to be ~160 nm with an associated straggling of ~ 50nm[8]. The samples for transmission electron microscopy (TEM) were prepared by using freshly cleaved NaCl crystals as substrates for deposition of titanium thin films. After ion implantation, these films were fished out on copper grids by dissolving NaCl substrates in demineralized water. The TEM analysis was carried out using a TECNAI G2 F30 machine with an accelerating voltage of 300 keV. The as-deposited and implanted thin films were characterized by STOE grazing incidence X-ray diffraction (GIXRD) for their structural characterization. A SOPRA ESVG rotating polarizer spectroscopic ellipsometer was used to study the optical properties of the asdeposited and ion irradiated films. The optical measurements were performed at room temperature in the energy range of 1.5 to 5.5 eV in steps of 10 meV for three angles of incidence namely, 65°, 70° and 75°. Only results pertaining to spectroscopic ellipsometry are presented in this paper. 3. Theoretical background Spectroscopic ellipsometry is a non-destructive surface sensitive tool, which is capable of addressing the issue of change in optical properties caused by associated changes in microstructure in thin films, multilayer structure and bulk systems. This technique is inherently powerful as it is based on detecting the change in polarization state of incident light upon reflection from a surface. The measured ellipsometric parameter ρ which is the ratio of the parallel to perpendicular components of the reflection coefficients is decided by the dielectric function ε(E) [9] of the material under investigation. The complex psuedodielectric function ε(E) is related to the ellipsometric parameters by the relation 1−ρ 2 2 2 2 2 εðEÞ = No sin θ + sin θ tan θ 1+ρ
3755
part of the dielectric function is zero. It depends on the number density of conduction electrons N by the relation
ωpu
sffiffiffiffiffiffiffiffiffiffiffi Ne2 = m*εo
ð3Þ
where e is the electron charge, m* is the effective mass and εo is the permittivity of free space. The conduction electron density is a useful quantity to determine the metallic character. In most metals, the free electron contribution leads to a plasma frequency in the uv region of the electromagnetic spectrum. However, in noble metals like Ag, Au and Cu, the bound electron contribution to the interband transition drags down the plasma frequency to the visible region and is known as the screened plasma frequency (ωps). Because of the sensitivity of the screened plasma frequency to the electronic structure, it has been used to monitor the composition and stoichiometry of TiNx [1]. The optical properties of TiN arise from the electronic band structure in which the Fermi energy lies within the d band. The longitudinal excitation mode of the screened plasma energy is reported to be ~ 2.45–2.65 eV[11] depending on the stoichiometry. 4. Results and discussion Fig. 1a and b show the experimental real and imaginary parts of the psuedodielectric functions of as-deposited as well as irradiated films computed from the experimentally measured ellipsometric parameters using Eq. (1). As seen from the figure, the ε1 of as-
ð1Þ
where No is the refractive index of the ambient and θ is the angle of incidence. The data analysis in ellipsometry hinges on two broad based approaches. The first approach is the effective medium approximation (EMA) employed for the extraction of microstructure parameters like volume fraction of constituents, void fractions, thickness and roughness of layers, surfaces and interface layers etc. The second is the optical dispersion models such as DL that are used to infer about the underlying electronic structure [10]. The Drude model relation is given the relation ε = ε∞ −
ω2pu ω −iΓD ω 2
fj ωoj2 2 2 j = 1 ωoj −ω + 2
+ ∑
iγj ω
ð2Þ
In Eq. (2), ε∞ is the background constant, which is larger than unity due to the contribution from higher energy transitions that are not accounted for in the Lorentz relation. The Drude term is characterized by the damping factor ΓD and unscreened plasma frequency ωpu. The damping factor arises from scattering of electrons and is the inverse of its relaxation time. The bound electron contribution is given by the Lorentz term where fj, ωoj and γj are the strength, resonance energy and damping of the jth oscillator respectively. The plasma frequency (ωpu : unscreened plasma frequency) is defined as the frequency at which the real
Fig. 1. Real and imaginary parts of the experimental psuedodielectric function of asdeposited and irradiated specimens.
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deposited sample shows a metallic character but does not intercept the energy axis even though it is expected to do so as per the data reported in ref. [13]. In ref. [13], the ε1 goes to zero with a positive slope near 4.88 eV. As known from previous ellipsometry analysis, a prerequisite for the EMA analysis is that the dielectric function of Ti be known accurately. Several dielectric functions have been reported in literature, but the agreement among them is very poor owing to changes in microstructure. Ti is a transition metal and the measured optical constants depend on whether it is bulk Ti[14], polycrystalline or in the form of thin films[15] and they differ substantially from each other. There is a strong bound electron interband transition that arises from filled parts of the d bands to the empty parts of the d bands even though the bands are hybridized with s and p character. The difference in nature of the dielectric functions arise because of their inherent sensitivity to microstructure and stoichiometry and therefore to Ti/N ratio. For the as-deposited specimen, the EMA yielded a poor fit, while using the previously determined dielectric functions [14]. Hence, we used the dielectric functions from ref. [15] and incorporated voids and surface roughness into the modeling. Even then, the fitting was only reasonably good. The GIXRD pattern for this specimen showed the existence of Ti in the hcp phase as shown in Fig. 2 which is normally formed during thin film deposition. However, it is to be noted that metastable phases like fcc Ti are also formed by PLD technique. The difference in microstructure could be the reason for the not so good fitting that was obtained. In the present experiment, we are interested in the irradiation induced change in Ti thin films and subsequent nitride phase evolution on N+ irradiation. Therefore, we have chosen the as-deposited films as reference for the EMA analysis for the irradiated films. Our spectrum for the as-deposited films compared reasonably well with the spectrum reported in ref. [16] indicating sufficient volume fraction of voids in the film. From the figure, it is evident that there is no contribution from the underlying silicon substrate in any of the specimens. As the fluence is increased, the real and imaginary parts show a systematic variation. The most prominent ones being a decrease in ε2, accompanied with increase in ε1. A red shift in the plasma energy is also observed. The microstructure considered for the EMA analysis was constituted by a mixture of Ti, TiN and voids. The volume fractions of TiN in the irradiated specimens deduced from EMA were 25, 34 and 94% for the 1 × 1016, 5 × 1016 and 1 × 1017 ions/cm2 respectively. The GIXRD patterns of all irradiated films with fluences of 1 × 1016 and 5 × 1016 ions/cm2 showed hcp Ti and cubic TiN. Cubic
TiN was present for the specimen irradiated with 1 × 1017 ions/cm2. A typical effective medium approximation modeling for the specimen irradiated to a fluence of 1 × 1017 ions/cm2 is shown in Fig. 3a and b. In fact, it is known from EMA analysis in literature that for TiN thin films thicker than 60 nm, the ellipsometric technique directly yields the dielectric function of the films without any contribution from the Si substrate. A combined DL dispersion law was employed to model the contribution of the free and bound electrons to the dielectric dispersion. It is to be pointed out that the fitting was performed on the data measured at all three angles of incidence viz. 65°, 70° and 75°. A representative fit for the real and imaginary parts of the dielectric function using DL model is shown in Fig. 4a and b for specimen irradiated with a fluence of 1 × 1016 ions/cm2. The Drude part of the fit describes the optical response due to the Ti 3d electrons. The variation of the free electron contribution for as-deposited and the irradiated specimens are shown in Fig. 5. From the figure, the plasma energy shows a progressive red shift with increase in fluence indicating the increase in concentration of TiN [17]. This type of behavior is also observed in the case of TiNx films with a change in the nitrogen concentration. The decrease in plasma frequency is interpreted as a decrease in the conduction electron density. The contribution for conduction electrons comes from the Ti atoms. Hence, a decrease in plasma energy is construed as an increase in covalent bonds formed due to nitridation of Ti with increasing ion fluence. In the present experiment, the GIXRD pattern supports the coexistence of both Ti in the hcp phase and TiN in the cubic phase. Fig. 6 shows the volume fraction of TiN with increasing fluence and the values of plasma energy as deduced from the ε1(energy = 0) of Fig. 5. The plasma
Fig. 2. GIXRD pattern of as-deposited Ti thin films and specimen irradiated with a fluence of 5 × 1016 ions/cm2.
Fig. 3. Representative EMA modeling for the real and imaginary parts of the dielectric function for specimen irradiated to a fluence of 1 × 1017 ions/cm2.
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Fig. 6. The plasma frequency shift and volume fraction of TiN as a function of fluence. (Note: the lines are a guide to the eye).
Fig. 4. Experimental and fitted real and imaginary parts of the dielectric function using DL model for a representative irradiated specimen (1 × 1016 ions/cm2). The individual contributions of Drude and the two Lorentz oscillators are also shown.
energy for the specimen irradiated with 1 × 1017 ions/cm2 is ~2.7 eV which is marginally greater than values reported in literature so far. The broadening parameter computed from the Drude model for specimens irradiated with 1 × 1016, 5 × 1016 and 1 × 1017 ions/cm2 were 1.02 eV, 1.373 eV and 1.65 eV respectively which is considerably larger than that found in refs. [1] and [12]. In the case of specimen irradiated with 1 × 1017 ions/cm2, there is also an additional contribution arising from the nanocrystalline nature of TiN which was
Fig. 5. Drude contribution for the as-deposited and irradiated specimens.
confirmed from the indexed selected area electron diffraction (Fig. 7) analysis [18] which could lead to an increase in the plasma energy. However, the results obtained in the present experiment are substantially different from the nanocrystalline TiN thin films reported in ref. [1]. The studies reported in ref. [1] pertains to nanocrystalline TiNx films and the broadening decreased with increasing substrate temperature due to increasing grain size. The difference in these results is attributed to the progressive increase in defect production as fluence increases. Two reasons are attributed to the decreased relaxation time of electrons. One is the generation of defects during irradiation and the other is due to decreased grain size resulting from prevailing nanocrystalline nature. The best fit for the interband optical transition energy deduced from the Lorentz model were obtained near 2.4 eV and 5.5 eV for all three irradiated specimens. The positions of the oscillators did not change significantly with variation in fluence. The values of oscillator energies compare well with previous reports and confirm the formation of TiN. Adachi et al.[12] have observed transitions near 2.5 eV, 3.1 eV and 5.2 eV corresponding to E1, E2 and E3 transitions, respectively. The E1 transition corresponds to those between Γ15
Fig. 7. Selected area electron diffraction pattern of specimen irradiated with 1 × 1017 ions/cm2.
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valence band to the Γ12 conduction band [1] in TiN and its theoretical value is estimated to be ~ 2.3 eV. The E2 transition is interpreted as transitions between hybridized N p and Ti d bands at Γ15 → Γ12 bands while the transition near 5 eV is assigned to the transitions near the L, K and W points in the Brillouin zone. Patsalas et al. [1] have also observed transitions near 3 and 5.2 eV for TiN x nanocrystalline films. However, the lowest band gap transition near 1.0 eV occurring at the Γ point (Γv25Õ → Γc12) that is expected from theoretical calculations could not be observed by either of them. This is not easily identifiable experimentally because of the strong intraband transition occurring in this energy range. However, the oscillator at 5.2 eV was found to be very weak in the case of specimen irradiated to a fluence of 1 × 1017 ions/cm2. The peak near 5.5 eV is due to this. 5. Conclusions The optical response of thin films of Ti deposited on Si by PLD technique and irradiated with N+ were studied by spectroscopic ellipsometry in the energy range 1.5 to 5.5 eV. A combined Drude Lorentz model revealed distinct features and a red shift in the plasma energy indicative of formation and increasing volume fraction of TiN. The unscreened plasma frequency extracted from the Drude model decreased with increasing ion fluence signifying enhanced conversion of Ti to TiN. In order to understand more details and capture the onset of nitride formation, the experiment needs to be conducted in lower fluence ranges.
Acknowledgement The authors thank Mr. Amit Mondal of Institute Nano Initiative (INI), Indian Institute of Science, Bangalore for the TEM analysis.
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