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Dielectric properties of ambient temperature grown nanocrystalline ZrTiO4 thin films using DC magnetron sputtering
Materials Science and Engineering B 168 (2010) 208–213
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Dielectric properties of ambient temperature grown nanocrystalline ZrTiO4 thin films using DC magnetron sputtering D. Pamu a,∗∗ , K. Sudheendran b , M. Ghanashyam Krishna b , K.C. James Raju b,∗ a b
Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781 039, India School of Physics, University of Hyderabad, Hyderabad 5000 546, India
a r t i c l e
i n f o
Article history: Received 26 July 2009 Received in revised form 8 November 2009 Accepted 6 December 2009 Keywords: Sputtering ZrTiO4 Optical Electrical and dielectric properties
a b s t r a c t Nanocrystalline zirconium titanate (ZrTiO4 ) thin films have been deposited at ambient temperature by DC reactive magnetron sputtering from individual titanium and zirconium metal targets on to platinized silicon and glass substrates. The present study demonstrates the possibility of growing zirconium titanium oxide films in 100% pure DC oxygen plasma. The processing conditions have been optimized to get the required stoichiometry of the films. The films got crystallized at temperatures below 100 ◦ C. Interestingly, the as-deposited films crystallized in orthorhombic phase. The crystallite size in the films varies between 13.2 and 28.6 nm as calculated from the X-ray diffraction patterns and is dependent on oxygen mixing percentage (OMP) in the sputtering gas. The refractive index is strongly dependent on the packing density of the films. The dielectric constant of these films did not show much dependence on frequency whereas the loss is higher at lower frequency region. The dielectric constant and loss of the films measured at frequencies in the range of 100 Hz–15 MHz ranged between 37–46.5 and 0.007–0.03, respectively. The magnitude of leakage current density is 9.03 × 10−7 A/cm2 at 10 MV/m for the films deposited at 40% OMP. It is found that all these properties of ZrTiO4 films are strongly dependent on processing parameters. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recent years are witnessing an increasing need for miniaturization and it has a dramatic impact in the field of microelectronics. The field of microwave ceramics has been continuously progressing in recent years, and there is an increasing interest to use dielectric properties of integrated thin films of these materials, in order to improve the performance of devices such as dynamic randomaccess memory (DRAM). Fabrication of thin film capacitors using high dielectric constant materials can provide higher charge storage densities provided they exhibit lower values of dielectric loss. The miniaturization of microwave circuits requires high dielectric constant and low loss materials that exhibit good temperature stability and that get processed at lower temperatures. ZrTiO4 (ZT) is one of the important materials, which satisfies the requirements for various microwave applications due to its high dielectric constant (38–40), high quality factor (tan ı ∼ 10−2 to 10−3 ) and high temperature stability ( f value of ±20 ppm/◦ C) [1]. Along with the good
∗ Corresponding author. Tel.: +91 40 2313 4305; fax: +91 40 2301 0227. ∗∗ Corresponding author. Tel.: +91 361 258 2721; fax: +91 361 258 2749. E-mail addresses: [email protected] (D. Pamu), [email protected] (K.C. James Raju). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.12.028
dielectric properties, ZT thin films also exhibit good optical and electrical properties to find applications in antireflection coatings, wave guides for light, insulator in electronic devices requiring high permittivity and as protective coatings [1]. Due to their excellent optical, electrical and dielectric properties, it is important to study the dependence of film properties on the deposition conditions and hence the present study. ZrTiO4 is a-PbO type, with orthorhombic symmetry and Pbcn space group with the Zr4+ and Ti4+ cations distributed randomly within the lattice. ZT has an incommensurate phase (IC) and is known to decrease the quality factor in comparison to normal and phases. It undergoes a second-order phase transition from normal phase to the structurally IC modulated phase at 1125 ◦ C [1]. ZrTiO4 films have been deposited by different techniques such as pulsed laser deposition [2], metal organic chemical vapor deposition [3], RF and DC reactive magnetron sputtering techniques [4,5]. Conventionally the deposition of multi-component materials is done by co-sputtering. In the current work it shown that multicomponent oxide such as ZrTiO4 can be deposited by sputtering from a single cathode with half the area covered by Zr and other half by Ti. A further innovation is DC sputtering in a 100% pure oxygen atmosphere. In general DC sputtering needs higher voltages in order for sputtering to take place and the films cannot be deposited at lower pressures. Reduced deposition rates for oxide films and resputtering effects are the other disadvantages.
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For industrial applications the deposition of the film processing should be simple and low cost. In the present study, ZT films were prepared using DC magnetron sputtering which is a low cost preparation method. The dielectric properties of the ZT thin films at low frequencies using DC magnetron sputtering technique from the individual Zr and Ti targets have not been reported earlier. In the current work we report the growth of nanocrystalline zirconium titanate thin films at ambient temperatures using DC reactive magnetron sputtering from Zr and Ti metal targets placed in a single cathode. The significant features of this study are the demonstration of the growth of zirconium titanate thin films from individual metal targets and the demonstration of ZrTiO4 thin films in 100% oxygen atmosphere during sputtering. The systematic study of structural, microstructural optical, electrical and the dielectric properties of the films are reported. 2. Experimental details The experiments were carried out in a DC magnetron-sputtering system equipped with a diffusion pump-rotary pump combination that gave an ultimate pressure of 5 × 10−6 Torr. Pure Zr (99.99%) and Ti (99.99%) metal foils placed in a single magnetron cathode were used as sputtering targets. To get the required composition, the ratios of Zr and Ti target area are varied and optimized. Pure oxygen (99.999%) and argon (99.999%) gases were used for the sputtering. The films were deposited on to platinized silicon at a target to substrate distance of (Ds-t ) 3 cm and different oxygen mixing percentages (OMPs) of 20, 40, 60, 80 and 100%. In each experiment the target was sputtered in an argon atmosphere until the surface oxide layers are removed from the target. The substrates were not heated during deposition and the temperature rise measured during the processing did not exceed 100 ◦ C independent of deposition parameters. Film thickness as derived from the measured spectral transmittance curves, varied between 200 and 450 nm. The rate of deposition varied between 6 and 10 nm/min at typical power densities of 1–1.5 W/cm2 . The crystal structure was determined by X-ray diffraction with Cu K␣ ( = 1.54056 Å) powder X-ray diffractometer (Philips PW 1830). Calibration using a Si standard was done to account for the instrumental broadening. For the electrical characterizations, metal insulator metal (MIM) type test structures were used. Spectral transmission characteristics in the wavelength range 200–1500 nm were measured using a JASCO V570 UV-VIS-NIR Spectrophotometer. The optical constants have been calculated using techniques described earlier [6]. Surface morphology of the films was examined by Atomic Force Microscopy (SPA-400, Seiko Instruments Inc., Japan with SPI-3800 probe station). The optical packing densities (p) of the films were calculated using the relation is given by [7] P=
n2f − 1 n2f
+2
×
n2b + 2 n2b
−1
(1)
where nb is the bulk refractive index (2.31) of ZrTiO4 and nf is the observed film refractive index (n600 and = 600 nm). ZT thin films were deposited on Pt-coated Si-substrates (2 cm2 × 2 cm2 ). 200 nm thick silver top electrodes were deposited by DC magnetron sputtering on to the ZrTiO4 through a shadow mask so as to define a capacitor structure (the diameter of top electrode is 50 m). The dielectric constant of the films is calculated using Eq. (2) C = εr ε0
A d
(2)
where C is the capacitance in F, A is the area of overlap of the two plates measured in m2 , εr is the relative static permittivity of the
Fig. 1. (a) XRD patterns of the films deposited on Pt–Si at different OMP, (I) 20%, (II) 40%, (III) 60% and (VI) 100%; (b) variation in crystallite size as a function of OMP for ZT films.
material between the plates, ε0 is the permittivity of free space and d is the separation between the plates, measured in m. The I–V characteristics of these test structures were measured using precision material analyzer (Radiant, USA). The low frequency dielectric spectroscopy at zero DC bias from 100 Hz to 15 MHz was performed by using an impedance analyzer (Agilent 4294 A) connected through a DC probe station (J micro technology, USA model LMS 2709). 3. Results and discussions 3.1. Crystal structure of ZrTiO4 thin films X-ray diffraction patterns of the films deposited on platinized silicon with different oxygen mixing percentages (OMPs) of 20, 40, 60, 80 and 100% are shown in Fig. 1(a). It is evident that the films crystallized in the orthorhombic phase with a strong glassy background. It is significant to note that the films crystallized although no external heating of the substrate was resorted to during the deposition. The variation in crystallite size as a function of OMP for films deposited on platinized silicon is shown in inset of Fig. 1(b). It is observed that the crystallite size decreases as the OMP increases. The crystallite sizes are estimated from the full width at half maximum (FWHM) of the 1 1 1 peaks using Scherrer’s method. The crystallite sizes are in the range of 13–28.6 nm for all the films. It is found that crystallite size decreases with increasing sputtering pressure but it is independent of input power density. Titania and zirconia thin films deposited by DC magnetron sputtering without substrate heating been reported to be amorphous earlier [8,9].
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3.2. Optical properties of ZrTiO4 thin films
Fig. 2. Transmittance spectra of ZrTiO4 films deposited on glass at different OMPs.
Oxide thin films naturally grow in the amorphous state unless activation is provided to the process either in the form of temperature or by ion bombardment. The nucleation of crystalline phase is probably due to the energetic particles impinging on growing film, which determines the phase to be crystalline or amorphous. The bombardment of the growing surface with low energy ions can create new nucleation sites. The films are crystalline in the pure orthorhombic phase. More significantly the films are nanocrystalline in the as-deposited state indicating that the intrinsic energies of the process are sufficient to cause crystallization, since the measured temperature rise during the deposition was only of the order of 100 ◦ C. As stated earlier, to get the required composition, the ratios of Zr and Ti target area are varied and optimized, after studying the growth of TiO2 and ZrO2 thin films individually [10,11]. The ability of sputtering ZrTiO4 in a 100% oxygen atmosphere even with the accompanying problem of target poisoning is important. These observations correlate well with recent reports on the growth of TiO2 films starting from TiO2−x target by DC magnetron sputtering [12]. These studies indicate that as long as the target oxidation remains incomplete, sputtering will continue to occur even in a DC magnetron sputtering geometry.
The spectral transmittance of the films deposited on glass at different OMP is shown in Fig. 2. The transmittance of all the films is around 85% in the region above the band gap. The films deposited at higher OMPs showed higher transmittance. With the further increase in OMP, the film has a porous structure and, as a porous structure has many voids, the light can pass through these voids without any loss and this result in the increase in the transmittance and is confirmed from DFM images that the films deposited in 100% O2 (Fig. 5(c)) is porous and also confirmed from packing density calculations (Fig. 3(b)). The reduction in transmittance for the films deposited at lower OMPs can be due to higher amount of grain boundaries resulting in higher scattering of light on grain boundaries. The refractive index (n) and extinction coefficient (k) of the films deposited on glass at different OMPs are shown in Fig. 3(a). It is observed that refractive index decreased with increase in OMP. The values of n ranged between 2 and 2.19 (at 600 nm) in the dispersion free region. It is observed that higher the refractive index, higher is the extinction coefficient and as the OMP increases the extinction coefficient decreases. These values are comparable with the one reported for RF sputtered films deposited at room temperature, and the increase in n is observed with increase in deposition temperature [5]. The variation in packing density for films deposited on glass as a function of OMP is shown in Fig. 3(b). The packing density of the films supported the refractive index values and followed the similar trend as function of OMP. It is seen that the film structure changes from a dense structure into a porous structure as the OMP increases. This means that the packing density of the films decreases and results in the decrease in the refractive index. Packing density or packing fraction is defined as the volume of the film that is occupied by the solid material. Thus, it is a measure of the porosity. A film with a pore-free microstructure will have a packing density of 1, the value of which decreases with increasing porosity. Therefore, the measured refractive index for film with packing density <1 is an effective refractive index of the solid (film material) and the material of the voids (either air or water vapor). As a consequence, there is an observed decrease in refractive index with increase in porosity. In addition stoichiometry and crystal structure can also affect the refractive index. The optical packing density values were ranged between 0.87 and 0.92. The low values of n observed in the present study may also be partly due to partial crystallinity of the films and low adatom mobility in the films at ambient temperatures. From this study it was observed that
Fig. 3. (a) Variation in refractive index and extinction coefficient of ZrTiO4 films deposited on glass as a function of OMP and (b) variation in packing density of ZrTiO4 films deposited on glass as a function of OMP.
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at low oxygen pressures will have large number of oxygen vacancies and which will result in high dielectric loss. As we increase the OMP, the dielectric properties are improved due to the reduction of oxygen vacancies. Further increase of OMP will result in the production of interstitial oxygen ions which in turn can give rise to dielectric loss. The changeover must be happening around 40% of OMP. Viticoli et al. [2] reported a dielectric constant of 41 and tan ı of 3.1 × 10−4 at 1 MHz for the films deposited by PLD at 550 ◦ C and Padeletti et al. [3] reported a dielectric constant of 40 at 1 MHz for the films deposited by MOCVD at 600 ◦ C. Kim et al. [4] reported a resonance in the dielectric response of ZrTiO4 film at about 1 MHz and in the present study apparently such a response was found above 18 MHz. It could be the effect of test fixture as it is known that a test fixture could give a resonance depending on the stray capacitances and inductances associated with it. Therefore dielectric constant and loss are measured till the onset of this resonance. It is relevant to note that the resonance type relaxation appears in a paraelectric material usually in the far infrared and optical range of frequencies [13].
3.4. Microstructure of ZrTiO4 thin films The DFM pictures of the ZT films deposited on glass are shown in Fig. 5(a–c), where a, b and c corresponds to the films deposited with 20, 40 and 100% of OMP. It is observed that the films deposited on glass showed the triangular grains and as the OMP in the sputtering atmosphere increases the grain size is decreased. 3.5. Electrical properties of ZrTiO4 thin films
Fig. 4. (a) Variation in dielectric constant of ZrTiO4 films as a function of frequency and deposited at different OMP; (a1) variation in dielectric constant and dielectric loss as a function of OMP measured at 5 MHz; (b) variation in dielectric loss of ZrTiO4 films as a function of frequency and deposited at different OMP.
the refractive index values were strongly dependent on packing densities. 3.3. Dielectric properties of ZrTiO4 thin films The variation in dielectric constant and loss of ZrTiO4 films deposited at different OMP as a function of frequency is shown in Fig. 4(a) and (b), respectively. It was observed that as the OMP in the sputtering atmosphere increases from 20 to 40%, the dielectric constants of the films increased and thereafter it started decreasing with increase in OMP. It was found that the films deposited with 40% of OMP exhibited higher dielectric constant and lower losses. The dielectric constant of these films did not show much dependence on frequency whereas the loss is higher at lower frequency region and can be due to the metal electrode and its interface. At higher frequency region, the loss tangent remained constant. The loss of the films deposited above and below 40% of OMP was higher. The films deposited in pure oxygen atmosphere showed lowest dielectric constant and highest losses. The dielectric constant and loss of the ZrTiO4 films plotted as a function of OMP measured at 5 MHz are shown in inset of Fig. 4(a1). The variation of the dielectric properties with OMP can be attributed to the presence of oxygen vacancies. It is well known that the films deposited
Fig. 6 shows the I–V curves of the ZT thin films deposited on Pt–Si at different OMP. It is observed that the films deposited at 40% OMP showed lowest leakage current density while films deposited in 100% OMP exhibited higher values. The leakage current present in the oxide thin films are due to the presence of oxygen vacancies. The films produced at 40% of OMP have got the lowest leakage current due to presence of less number of oxygen vacancies. The leakage current in oxide thin films is directly proportional to the dielectric loss tangent. The films with large dielectric loss tangent will show large leakage current. The magnitude of current density is 9.03 × 10−7 A/cm2 at 10 MV/m and the breakdown field strength is found to be 8 MV/m for the films deposited at 40% OMP. It is found that at other OMP values, the breakdown starts from lower field strengths. Microstructure, crystallographic orientation and process related defects and the presence of oxygen vacancies are the factors which affects the leakage characteristic of oxide thin films. Out of these, the presence of oxygen vacancies is the most prominent factor which affects the leakage characteristics. The films prepared at 20% of OMP are having larger leakage characteristic even though they have got smaller grains is because of the presence oxygen vacancies. In general, the polycrystalline films exhibit a lower leakage current than highly oriented films [14]. The leakage current is influenced by the grain size, crystallinity and surface morphology because of the presence of electrostatic barriers at the grain–grain interfaces which arise from the donor acceptor type traps localized at the grain boundaries [14]. Surface and grain boundaries introduce additional electronic states in the band gap because of the unsaturated bonds, which arise from the disruption of the periodical crystal lattice. Hence these grain boundaries act as charge carrier trap sites. Due to the presence of grain boundary space charge layer which is formed eventually, the flow of charges gets limited. As the microstructure contains many grains and grain boundaries, the leakage current will be lesser in these polycrystalline films. From
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Fig. 6. I–V curves of the films deposited at different OMP.
this study, it is clear that by optimizing the processing parameters we can reduce the leakage current density and increase the breakdown field strength. The dielectric properties of these films shows that they are promising candidates for applications in integrated circuits and the films deposited at 40% OMP gave the best results among all the cases studied. The values obtained in this study for all the films shows that it is an interesting material for application in integrated circuits. 4. Conclusions ZrTiO4 thin films were deposited using DC reactive magnetron sputtering from the individual metal targets (Zr and Ti) at different OMP on to glass and Pt–Si substrates. It is found that the asdeposited films both on glass and Pt–Si substrates are crystallized in orthorhombic phase at ambient temperatures. The refractive index is mainly dependent on the packing density of the deposited films and in addition stoichiometry and crystal structure can also affect the refractive index. Significantly, the ZT films were also obtained in the pure oxygen plasma. The dielectric constants and loss measured at 5 MHz ranged between 37–46.5 and 0.03–0.007, respectively. Leakage current density of 9 × 10−7 A/cm2 is obtained for the deposited films. It is fund that the electric, optical and dielectric properties of the films are strongly dependent on the OMP. Acknowledgements DP acknowledges Dr. Kothari’s postdoctoral fellowship from UGC and KS acknowledges a SRF from CSIR. Facilities provided by the UGC, UGC-CAS, UGC-UPE, DST-FIST and DST Centre for Nanotechnology are gratefully acknowledged. References
Fig. 5. DFM images of the ZT films coated on glass with different OMP. (a) 20%, (b) 40% and (c) 100%.
[1] D. Pamu, K. Sudheendran, M. Ghanashyam Krishna, K.C. James Raju, Eur. Phys. J. Appl. Phys. 44 (2008) 71–75. [2] M. Viticoli, G. Padeletti, S. Kaciulis, G.M. Ingo, L. Pandolfi, C. Zaldo, Mater. Sci. Eng. B 118 (2005) 87–91. [3] G. Padeletti, A. Cusma, M. Viticoli, G.M. Ingo, A. Mezzi, B. Watts, Mater. Sci. Eng. B 109 (2004) 104–112. [4] T. Kim, J. Oh, B. Park, S. Hong, Appl. Phys. Lett. 76 (2000) 3043–3045. [5] D.-A. Chang, P. Lin, T.-Y. Tseng, J. Appl. Phys. 77 (1995) 4445–4451. [6] R. Swanpoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214–1222. [7] K. Venkata Saravanan, K. Sudheendran, M. Ghanashyam Krishna, K.C. James Raju, A.K. Bhatnagar, Vacuum 81 (2006) 307–316. [8] M.H. Suhail, G. Mohan Rao, S. Mohan, J. Appl. Phys. 71 (1992) 1421–1427. [9] M.H. Suhail, G. Mohan Rao, S. Mohan, J. Vac. Sci. Technol. A 9 (1991) 2675–2677.
D. Pamu et al. / Materials Science and Engineering B 168 (2010) 208–213 [10] D. Pamu, M. Ghanashyam Krishna, K.C. James Raju, A.K. Bhatnagar, Solid State Commun. 135 (2005) 7–10. [11] D. Pamu, K. Sudheendran, M. Ghanashyam Krishna, K.C. James Raju, J. Vac. Sci. Technol. A 26 (2008) 185–192.
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[12] Y. Tachibana, H. Ohsaki, A. Hayashi, A. Mitsui, Y. Hayashi, Vacuum 59 (2000) 836–843. [13] B. Tareev, Physics of Dielectric Materials, Mir Publishers, Moscow, 1975. [14] L.-J. Wu, J.-M. Wu, J. Phys. D: Appl. Phys. 40 (2007) 4948–4952.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Dielectric properties of ambient temperature grown nanocrystalline ZrTiO4 thin films using DC magnetron sputtering D. Pamu a,∗∗ , K. Sudheendran b , M. Ghanashyam Krishna b , K.C. James Raju b,∗ a b
Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781 039, India School of Physics, University of Hyderabad, Hyderabad 5000 546, India
a r t i c l e
i n f o
Article history: Received 26 July 2009 Received in revised form 8 November 2009 Accepted 6 December 2009 Keywords: Sputtering ZrTiO4 Optical Electrical and dielectric properties
a b s t r a c t Nanocrystalline zirconium titanate (ZrTiO4 ) thin films have been deposited at ambient temperature by DC reactive magnetron sputtering from individual titanium and zirconium metal targets on to platinized silicon and glass substrates. The present study demonstrates the possibility of growing zirconium titanium oxide films in 100% pure DC oxygen plasma. The processing conditions have been optimized to get the required stoichiometry of the films. The films got crystallized at temperatures below 100 ◦ C. Interestingly, the as-deposited films crystallized in orthorhombic phase. The crystallite size in the films varies between 13.2 and 28.6 nm as calculated from the X-ray diffraction patterns and is dependent on oxygen mixing percentage (OMP) in the sputtering gas. The refractive index is strongly dependent on the packing density of the films. The dielectric constant of these films did not show much dependence on frequency whereas the loss is higher at lower frequency region. The dielectric constant and loss of the films measured at frequencies in the range of 100 Hz–15 MHz ranged between 37–46.5 and 0.007–0.03, respectively. The magnitude of leakage current density is 9.03 × 10−7 A/cm2 at 10 MV/m for the films deposited at 40% OMP. It is found that all these properties of ZrTiO4 films are strongly dependent on processing parameters. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recent years are witnessing an increasing need for miniaturization and it has a dramatic impact in the field of microelectronics. The field of microwave ceramics has been continuously progressing in recent years, and there is an increasing interest to use dielectric properties of integrated thin films of these materials, in order to improve the performance of devices such as dynamic randomaccess memory (DRAM). Fabrication of thin film capacitors using high dielectric constant materials can provide higher charge storage densities provided they exhibit lower values of dielectric loss. The miniaturization of microwave circuits requires high dielectric constant and low loss materials that exhibit good temperature stability and that get processed at lower temperatures. ZrTiO4 (ZT) is one of the important materials, which satisfies the requirements for various microwave applications due to its high dielectric constant (38–40), high quality factor (tan ı ∼ 10−2 to 10−3 ) and high temperature stability ( f value of ±20 ppm/◦ C) [1]. Along with the good
∗ Corresponding author. Tel.: +91 40 2313 4305; fax: +91 40 2301 0227. ∗∗ Corresponding author. Tel.: +91 361 258 2721; fax: +91 361 258 2749. E-mail addresses: [email protected] (D. Pamu), [email protected] (K.C. James Raju). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.12.028
dielectric properties, ZT thin films also exhibit good optical and electrical properties to find applications in antireflection coatings, wave guides for light, insulator in electronic devices requiring high permittivity and as protective coatings [1]. Due to their excellent optical, electrical and dielectric properties, it is important to study the dependence of film properties on the deposition conditions and hence the present study. ZrTiO4 is a-PbO type, with orthorhombic symmetry and Pbcn space group with the Zr4+ and Ti4+ cations distributed randomly within the lattice. ZT has an incommensurate phase (IC) and is known to decrease the quality factor in comparison to normal and phases. It undergoes a second-order phase transition from normal phase to the structurally IC modulated phase at 1125 ◦ C [1]. ZrTiO4 films have been deposited by different techniques such as pulsed laser deposition [2], metal organic chemical vapor deposition [3], RF and DC reactive magnetron sputtering techniques [4,5]. Conventionally the deposition of multi-component materials is done by co-sputtering. In the current work it shown that multicomponent oxide such as ZrTiO4 can be deposited by sputtering from a single cathode with half the area covered by Zr and other half by Ti. A further innovation is DC sputtering in a 100% pure oxygen atmosphere. In general DC sputtering needs higher voltages in order for sputtering to take place and the films cannot be deposited at lower pressures. Reduced deposition rates for oxide films and resputtering effects are the other disadvantages.
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For industrial applications the deposition of the film processing should be simple and low cost. In the present study, ZT films were prepared using DC magnetron sputtering which is a low cost preparation method. The dielectric properties of the ZT thin films at low frequencies using DC magnetron sputtering technique from the individual Zr and Ti targets have not been reported earlier. In the current work we report the growth of nanocrystalline zirconium titanate thin films at ambient temperatures using DC reactive magnetron sputtering from Zr and Ti metal targets placed in a single cathode. The significant features of this study are the demonstration of the growth of zirconium titanate thin films from individual metal targets and the demonstration of ZrTiO4 thin films in 100% oxygen atmosphere during sputtering. The systematic study of structural, microstructural optical, electrical and the dielectric properties of the films are reported. 2. Experimental details The experiments were carried out in a DC magnetron-sputtering system equipped with a diffusion pump-rotary pump combination that gave an ultimate pressure of 5 × 10−6 Torr. Pure Zr (99.99%) and Ti (99.99%) metal foils placed in a single magnetron cathode were used as sputtering targets. To get the required composition, the ratios of Zr and Ti target area are varied and optimized. Pure oxygen (99.999%) and argon (99.999%) gases were used for the sputtering. The films were deposited on to platinized silicon at a target to substrate distance of (Ds-t ) 3 cm and different oxygen mixing percentages (OMPs) of 20, 40, 60, 80 and 100%. In each experiment the target was sputtered in an argon atmosphere until the surface oxide layers are removed from the target. The substrates were not heated during deposition and the temperature rise measured during the processing did not exceed 100 ◦ C independent of deposition parameters. Film thickness as derived from the measured spectral transmittance curves, varied between 200 and 450 nm. The rate of deposition varied between 6 and 10 nm/min at typical power densities of 1–1.5 W/cm2 . The crystal structure was determined by X-ray diffraction with Cu K␣ ( = 1.54056 Å) powder X-ray diffractometer (Philips PW 1830). Calibration using a Si standard was done to account for the instrumental broadening. For the electrical characterizations, metal insulator metal (MIM) type test structures were used. Spectral transmission characteristics in the wavelength range 200–1500 nm were measured using a JASCO V570 UV-VIS-NIR Spectrophotometer. The optical constants have been calculated using techniques described earlier [6]. Surface morphology of the films was examined by Atomic Force Microscopy (SPA-400, Seiko Instruments Inc., Japan with SPI-3800 probe station). The optical packing densities (p) of the films were calculated using the relation is given by [7] P=
n2f − 1 n2f
+2
×
n2b + 2 n2b
−1
(1)
where nb is the bulk refractive index (2.31) of ZrTiO4 and nf is the observed film refractive index (n600 and = 600 nm). ZT thin films were deposited on Pt-coated Si-substrates (2 cm2 × 2 cm2 ). 200 nm thick silver top electrodes were deposited by DC magnetron sputtering on to the ZrTiO4 through a shadow mask so as to define a capacitor structure (the diameter of top electrode is 50 m). The dielectric constant of the films is calculated using Eq. (2) C = εr ε0
A d
(2)
where C is the capacitance in F, A is the area of overlap of the two plates measured in m2 , εr is the relative static permittivity of the
Fig. 1. (a) XRD patterns of the films deposited on Pt–Si at different OMP, (I) 20%, (II) 40%, (III) 60% and (VI) 100%; (b) variation in crystallite size as a function of OMP for ZT films.
material between the plates, ε0 is the permittivity of free space and d is the separation between the plates, measured in m. The I–V characteristics of these test structures were measured using precision material analyzer (Radiant, USA). The low frequency dielectric spectroscopy at zero DC bias from 100 Hz to 15 MHz was performed by using an impedance analyzer (Agilent 4294 A) connected through a DC probe station (J micro technology, USA model LMS 2709). 3. Results and discussions 3.1. Crystal structure of ZrTiO4 thin films X-ray diffraction patterns of the films deposited on platinized silicon with different oxygen mixing percentages (OMPs) of 20, 40, 60, 80 and 100% are shown in Fig. 1(a). It is evident that the films crystallized in the orthorhombic phase with a strong glassy background. It is significant to note that the films crystallized although no external heating of the substrate was resorted to during the deposition. The variation in crystallite size as a function of OMP for films deposited on platinized silicon is shown in inset of Fig. 1(b). It is observed that the crystallite size decreases as the OMP increases. The crystallite sizes are estimated from the full width at half maximum (FWHM) of the 1 1 1 peaks using Scherrer’s method. The crystallite sizes are in the range of 13–28.6 nm for all the films. It is found that crystallite size decreases with increasing sputtering pressure but it is independent of input power density. Titania and zirconia thin films deposited by DC magnetron sputtering without substrate heating been reported to be amorphous earlier [8,9].
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3.2. Optical properties of ZrTiO4 thin films
Fig. 2. Transmittance spectra of ZrTiO4 films deposited on glass at different OMPs.
Oxide thin films naturally grow in the amorphous state unless activation is provided to the process either in the form of temperature or by ion bombardment. The nucleation of crystalline phase is probably due to the energetic particles impinging on growing film, which determines the phase to be crystalline or amorphous. The bombardment of the growing surface with low energy ions can create new nucleation sites. The films are crystalline in the pure orthorhombic phase. More significantly the films are nanocrystalline in the as-deposited state indicating that the intrinsic energies of the process are sufficient to cause crystallization, since the measured temperature rise during the deposition was only of the order of 100 ◦ C. As stated earlier, to get the required composition, the ratios of Zr and Ti target area are varied and optimized, after studying the growth of TiO2 and ZrO2 thin films individually [10,11]. The ability of sputtering ZrTiO4 in a 100% oxygen atmosphere even with the accompanying problem of target poisoning is important. These observations correlate well with recent reports on the growth of TiO2 films starting from TiO2−x target by DC magnetron sputtering [12]. These studies indicate that as long as the target oxidation remains incomplete, sputtering will continue to occur even in a DC magnetron sputtering geometry.
The spectral transmittance of the films deposited on glass at different OMP is shown in Fig. 2. The transmittance of all the films is around 85% in the region above the band gap. The films deposited at higher OMPs showed higher transmittance. With the further increase in OMP, the film has a porous structure and, as a porous structure has many voids, the light can pass through these voids without any loss and this result in the increase in the transmittance and is confirmed from DFM images that the films deposited in 100% O2 (Fig. 5(c)) is porous and also confirmed from packing density calculations (Fig. 3(b)). The reduction in transmittance for the films deposited at lower OMPs can be due to higher amount of grain boundaries resulting in higher scattering of light on grain boundaries. The refractive index (n) and extinction coefficient (k) of the films deposited on glass at different OMPs are shown in Fig. 3(a). It is observed that refractive index decreased with increase in OMP. The values of n ranged between 2 and 2.19 (at 600 nm) in the dispersion free region. It is observed that higher the refractive index, higher is the extinction coefficient and as the OMP increases the extinction coefficient decreases. These values are comparable with the one reported for RF sputtered films deposited at room temperature, and the increase in n is observed with increase in deposition temperature [5]. The variation in packing density for films deposited on glass as a function of OMP is shown in Fig. 3(b). The packing density of the films supported the refractive index values and followed the similar trend as function of OMP. It is seen that the film structure changes from a dense structure into a porous structure as the OMP increases. This means that the packing density of the films decreases and results in the decrease in the refractive index. Packing density or packing fraction is defined as the volume of the film that is occupied by the solid material. Thus, it is a measure of the porosity. A film with a pore-free microstructure will have a packing density of 1, the value of which decreases with increasing porosity. Therefore, the measured refractive index for film with packing density <1 is an effective refractive index of the solid (film material) and the material of the voids (either air or water vapor). As a consequence, there is an observed decrease in refractive index with increase in porosity. In addition stoichiometry and crystal structure can also affect the refractive index. The optical packing density values were ranged between 0.87 and 0.92. The low values of n observed in the present study may also be partly due to partial crystallinity of the films and low adatom mobility in the films at ambient temperatures. From this study it was observed that
Fig. 3. (a) Variation in refractive index and extinction coefficient of ZrTiO4 films deposited on glass as a function of OMP and (b) variation in packing density of ZrTiO4 films deposited on glass as a function of OMP.
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at low oxygen pressures will have large number of oxygen vacancies and which will result in high dielectric loss. As we increase the OMP, the dielectric properties are improved due to the reduction of oxygen vacancies. Further increase of OMP will result in the production of interstitial oxygen ions which in turn can give rise to dielectric loss. The changeover must be happening around 40% of OMP. Viticoli et al. [2] reported a dielectric constant of 41 and tan ı of 3.1 × 10−4 at 1 MHz for the films deposited by PLD at 550 ◦ C and Padeletti et al. [3] reported a dielectric constant of 40 at 1 MHz for the films deposited by MOCVD at 600 ◦ C. Kim et al. [4] reported a resonance in the dielectric response of ZrTiO4 film at about 1 MHz and in the present study apparently such a response was found above 18 MHz. It could be the effect of test fixture as it is known that a test fixture could give a resonance depending on the stray capacitances and inductances associated with it. Therefore dielectric constant and loss are measured till the onset of this resonance. It is relevant to note that the resonance type relaxation appears in a paraelectric material usually in the far infrared and optical range of frequencies [13].
3.4. Microstructure of ZrTiO4 thin films The DFM pictures of the ZT films deposited on glass are shown in Fig. 5(a–c), where a, b and c corresponds to the films deposited with 20, 40 and 100% of OMP. It is observed that the films deposited on glass showed the triangular grains and as the OMP in the sputtering atmosphere increases the grain size is decreased. 3.5. Electrical properties of ZrTiO4 thin films
Fig. 4. (a) Variation in dielectric constant of ZrTiO4 films as a function of frequency and deposited at different OMP; (a1) variation in dielectric constant and dielectric loss as a function of OMP measured at 5 MHz; (b) variation in dielectric loss of ZrTiO4 films as a function of frequency and deposited at different OMP.
the refractive index values were strongly dependent on packing densities. 3.3. Dielectric properties of ZrTiO4 thin films The variation in dielectric constant and loss of ZrTiO4 films deposited at different OMP as a function of frequency is shown in Fig. 4(a) and (b), respectively. It was observed that as the OMP in the sputtering atmosphere increases from 20 to 40%, the dielectric constants of the films increased and thereafter it started decreasing with increase in OMP. It was found that the films deposited with 40% of OMP exhibited higher dielectric constant and lower losses. The dielectric constant of these films did not show much dependence on frequency whereas the loss is higher at lower frequency region and can be due to the metal electrode and its interface. At higher frequency region, the loss tangent remained constant. The loss of the films deposited above and below 40% of OMP was higher. The films deposited in pure oxygen atmosphere showed lowest dielectric constant and highest losses. The dielectric constant and loss of the ZrTiO4 films plotted as a function of OMP measured at 5 MHz are shown in inset of Fig. 4(a1). The variation of the dielectric properties with OMP can be attributed to the presence of oxygen vacancies. It is well known that the films deposited
Fig. 6 shows the I–V curves of the ZT thin films deposited on Pt–Si at different OMP. It is observed that the films deposited at 40% OMP showed lowest leakage current density while films deposited in 100% OMP exhibited higher values. The leakage current present in the oxide thin films are due to the presence of oxygen vacancies. The films produced at 40% of OMP have got the lowest leakage current due to presence of less number of oxygen vacancies. The leakage current in oxide thin films is directly proportional to the dielectric loss tangent. The films with large dielectric loss tangent will show large leakage current. The magnitude of current density is 9.03 × 10−7 A/cm2 at 10 MV/m and the breakdown field strength is found to be 8 MV/m for the films deposited at 40% OMP. It is found that at other OMP values, the breakdown starts from lower field strengths. Microstructure, crystallographic orientation and process related defects and the presence of oxygen vacancies are the factors which affects the leakage characteristic of oxide thin films. Out of these, the presence of oxygen vacancies is the most prominent factor which affects the leakage characteristics. The films prepared at 20% of OMP are having larger leakage characteristic even though they have got smaller grains is because of the presence oxygen vacancies. In general, the polycrystalline films exhibit a lower leakage current than highly oriented films [14]. The leakage current is influenced by the grain size, crystallinity and surface morphology because of the presence of electrostatic barriers at the grain–grain interfaces which arise from the donor acceptor type traps localized at the grain boundaries [14]. Surface and grain boundaries introduce additional electronic states in the band gap because of the unsaturated bonds, which arise from the disruption of the periodical crystal lattice. Hence these grain boundaries act as charge carrier trap sites. Due to the presence of grain boundary space charge layer which is formed eventually, the flow of charges gets limited. As the microstructure contains many grains and grain boundaries, the leakage current will be lesser in these polycrystalline films. From
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Fig. 6. I–V curves of the films deposited at different OMP.
this study, it is clear that by optimizing the processing parameters we can reduce the leakage current density and increase the breakdown field strength. The dielectric properties of these films shows that they are promising candidates for applications in integrated circuits and the films deposited at 40% OMP gave the best results among all the cases studied. The values obtained in this study for all the films shows that it is an interesting material for application in integrated circuits. 4. Conclusions ZrTiO4 thin films were deposited using DC reactive magnetron sputtering from the individual metal targets (Zr and Ti) at different OMP on to glass and Pt–Si substrates. It is found that the asdeposited films both on glass and Pt–Si substrates are crystallized in orthorhombic phase at ambient temperatures. The refractive index is mainly dependent on the packing density of the deposited films and in addition stoichiometry and crystal structure can also affect the refractive index. Significantly, the ZT films were also obtained in the pure oxygen plasma. The dielectric constants and loss measured at 5 MHz ranged between 37–46.5 and 0.03–0.007, respectively. Leakage current density of 9 × 10−7 A/cm2 is obtained for the deposited films. It is fund that the electric, optical and dielectric properties of the films are strongly dependent on the OMP. Acknowledgements DP acknowledges Dr. Kothari’s postdoctoral fellowship from UGC and KS acknowledges a SRF from CSIR. Facilities provided by the UGC, UGC-CAS, UGC-UPE, DST-FIST and DST Centre for Nanotechnology are gratefully acknowledged. References
Fig. 5. DFM images of the ZT films coated on glass with different OMP. (a) 20%, (b) 40% and (c) 100%.
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