Analysis of TiO₂ for microelectronic applications: effect of deposition methods on their electrical properties

Abstract

Metal oxide semiconductor (MOS) device down-scaling is a powerful driving force for the evolution of microelectronics. The downsizing rate of metal oxide semiconductor field effect transistors (MOSFETs) is really marvelous. Silicon dioxide (SiO₂) has served as a perfect gate dielectric for the last four decades. Due to physical limitations, leakage current, high interface trap charge it now needs to be replaced with higher permittivity dielectric material. Keeping the motivation for the search of high-k materials, extensive studies have been carried out on several metal oxides, such as ZrO₂, Ta₂O₅, TiO₂, Al₂O₃ and HfO₂ for the replacement of SiO₂. The high dielectric constant (k) of titanium dioxide (TiO₂) will open multifaceted prospects for the use of this material in microelectronic devices. In this paper, a comparative study of various deposition methods for fabrication of thin TiO₂ films has been presented. This work uses a combination of simulation results, experimental data and critical analysis of published data. Further, an experiment using sol-gel method has been carried out to deposit thin films of TiO₂. It has been characterized and compared with the earlier reported fabrication methods. The X-ray diffraction analyses and Raman spectra indicate the presence of anatase TiO₂ phase in the film. The dielectric constant as calculated using capacitance-voltage (C-V) analysis was found to be 23. The refractive index of the film was 2.43. The TiO₂ films studied for microelectronic applications and present acceptable properties such as low leakage current density of 1.0×10^-5 A/cm at 1 V and band gap of 3.6 eV. The leakage current has been found to be dominant by the Schottky emission at lower electric field, while Flower-Nordheim (F-N) tunneling occurs at higher biasing voltages.

Key words： titanium dioxide (TiO₂); high-k material; sol-gel; thin films; atomic force microscopy (AFM); capacitance-voltage (C-V) analysis

Introduction

During the last few years, it has been observed that a transparent oxide semiconductor having optical band gap wider than 3 eV can be applied in microelectronic applications [1], i.e., UV photodiodes, transparent transistor, transparent integrated circuits, transparent electrodes, optical radiation photo detectors, wave length selective devices and solar cells, etc. The titanium dioxide (TiO₂) is the one which meets not only this requirement but also the outstanding properties, non toxicity and chemical stability in hostile environment which makes its study interesting [2]. It exists in amorphous form and crystallizes in three distinct crystallographic structures: two tetragonal phases, anatase (a = b = 3.785 Å, c = 9.514 Å) and rutile (a = b = 4.587 Å, c = 2.953 Å) shown in Fig. 1, and a third orthorhombic phase, brookite (a = 5.456 Å, b = 9.182 Å, c = 5.143 Å).

Fig.1 Unit lattice crystalline structure

Full size|PPT slide

Band gap of TiO₂ is determined by the crystal modification and in the case of thin films the reported values are 3.5 eV for amorphous films, 3.2 eV for crystalline films in the anatase phase and 3.0 eV for crystalline films in the rutile phase. As TiO₂ thin films can be applied in gate oxide in metal oxide semiconductor field effect transistors (MOSFETs) [3], it is the most promising material in photo catalytic application due to its strong oxidizing powder and high photo stability [4]. It has a high refractive index [5], high dielectric constant [6] and transparent to visible light [7]. TiO₂ films have successfully been used for photodecomposition of water and for environmental purification [8]. TiO₂ has also successfully been used as gas sensor and antireflection coating [9], as UV light emitting devices, sensors, laser diodes and other high speed electronic devices. It can be used for photo electrochemical solar cells and is also a promising material for quantum dot sensitized solar cells optical brightener in wall colors, ingredient in sun cream and bone implants photo catalysis, electro chromic devices and photovoltaic cells because of its biocompatibility [10], thermal stability, strong oxidized stability, non-toxicity and long term photo-stability. TiO₂ based gate insulators are seriously being considered for the applications of the next generation metal oxide semiconductor (MOS) field effect transistors.

In every logic circuit, the key element is the MOS transistor, which basically constitutes an electrical switch [11]. The ever present development of the transistor manifests itself in two ways. Firstly, it has become smaller, and secondly, the number of transistors interconnected on state-of-the-art chips will be in excess of 10 billion [12]. The MOS transistor scaling beyond the present 32 nm design makes it difficult to grow high quality ultra thin oxides. Even though it is necessary to scale down the gate oxide thickness, there are two major limiting factors to overcome. First is the reliability, as the gate oxide thickness decreases, the breakdown voltage also decreases because of the increased electric field at the same gate bias; second, is leakage current especially, when the thickness of SiO₂ reaches its direct tunneling limit below 7 nm. Therefore, reducing the electric field can be reduced by increasing the physical thickness of the gate oxide or decreasing the power supply voltage. Several conduction mechanisms [5,13] of leakage current occur across SiO₂ MOS devices, i.e., Fowler-Nordheim (F-N) tunneling, space charge limited current (SCLC) mechanism, Schottky emission (SE) and Poole-Frenkel (PF) conduction, etc. But, whatever the mechanism, the electric filed was a common and basic parameter, which determines the amount of leakage current. Therefore, the challenge is to increase the physical thickness of gate dielectric, in order to reduce the tunneling current while maintaining an equivalent oxide thickness (EOT). So to keep the trend of scaling down of complementary metal oxide semiconductors (CMOS) transistors, require the replacement of conventional SiO₂ layer with higher dielectric constant (k) material for gate dielectric [14].

Many sophisticated fabrication techniques, namely, vacuum evaporation [15], molecular beam epitaxial [16], laser-assisted vacuum evaporation [17], chemical vapor deposition [18], various sputtering methods, reactive direct current (DC) or radio frequency (RF) magnetron sputtering [19], ion beam techniques [20], are used for fabrication of thin films. Although these techniques have merits of their own of having a control over the growth and for obtaining pure material but all these methods require complexity in process and of course costly equipments [21,22]. The present study aims to investigate the sol-gel process to deposit nano crystalline TiO₂ thin films on silicon substrate with very simple experimental setup. This may also be suitable for mass production. In this paper, various electrical characteristics of samples deposited by various methods have been compared with author’s deposited samples by sol-gel methods.

Fabrication of ultra thin high-k films

Various thin film deposition techniques such as thermal/electron beam evaporation, pulsed laser deposition, DC/RF sputtering chemical vapor deposition and ion beam deposition have been widely used for the deposition of TiO₂ thin films. But every method has its merits and demerits in terms of quality of the deposited films as well as applications.

Chowdhury et al. [23] deposited thin film of titanium of thickness 15–20 nm using dc magnetron sputtering system on n-type Si substrate at room temperature. The growth of uniform TiO₂ films was reported using thermal oxidation of e-beam evaporated Ti films in O₂ ambient [23]. It has been shown that it was possible to achieve the leakage current density of 1×10^-8 A/cm² for the films annealed at 550°C for 30 min. The flat band was estimated to be -0.6 V. The oxide charge density Q_i, is estimated to be of the order of 2.35×10¹²cm^-2. The metal and semiconductor work function potential difference was found to be 0.35 V. At low temperature, F-N tunneling of electron was observed to be dominating while SCLC current mechanism was found to dominant at higher voltages. The TiO₂ films used in this study were deposited by Hitchman et al. [24] low pressure chemical vapor deposition (CVD) at various temperatures from 257°C to 400°C. Anatase peaks appear at 144, 197, 397, 515 and 637 cm^-1 and rutile peaks were found at 448 and 612 cm^-1. Rutile has a larger value of peaks than anatase, while refractive indices were found 2.57 for anatase and 2.74 for rutile. In case of solar cell devices, it was reported that increasing film thickness may increase the resistance of the film because of the polycrystalline structure, and this will lead to a slight decrease in the photocurrent. These parameters are best suitable for the potential application in Solar cell and photodiodes. Kaliwoh et al. [25] reported the deposition of TiO₂ films on crystalline silicon by plasma impulse chemical vapor deposition (PICVD). The thickness of grown films was 20 to 510 nm with refractive indices of 2.20 to 2.54 at temperature between 50°C and 350°C. The deposition rate reported by this method was 50 nm/min at 350°C. Device fabricated may have potential application in solar cell, insulating memory devices, and fiber optical sensors. Babelon et al. [26] discussed the effect of growth parameters on different characteristics of TiO₂ thin films grown on 100 Si had been studied. The grain size increases from 30 nm 450°C to 50 nm at 550°C. Binding energy at peak was reported approximately 500 eV and band gap around 5.7 eV. However, the channel mobility was found to be very low, probably due to the presence of large interface states. Electrical properties of ultrathin TiO₂ films deposited at 150°C on strained-Si heterolayers by microwave plasma enhanced chemical vapor deposition (PECVD) had been investigated by Chakraborty et al. [26]. The extracted values of interface charge (D_it) were 1.19×10¹² and 3.36×10¹¹ eV^-1·cm^-2 for as-deposited and annealed samples, and the field oxide charge densities are 5.07×10¹² and 4.01×10¹² cm^-2, respectively. Two main conduction mechanisms were invoked to explain the current transport in TiO₂ thin films, i.e., SE and F-N tunneling [23,27,28]. The SE is a process occurring across the interface between a semiconductor (or metal) and an insulating film as a result of barrier lowering due to applied field. The leakage current is found to be dominated by the SE at a low electric field (<1 MV/cm) for both as-deposited and annealed samples, whereas PF effect appears only for the deposited samples at moderate electric field (>1 MV/cm) [29,27,28].

Dalapati et al. [30] provides another possible synthesis route for forming Si_1-y C_y layer. Samples of rapid thermal were annealed in QUPLAS reactor under flowing nitrogen at 1046°C for 30 s. In order to study the electrical characteristics of the TiO₂ films deposited on strained Si_1-y C_y layers, MIS structures were fabricated with Al gate (area: 1.96×10^-3cm^-2). A separation of 5.6 eV between two peaks of Ti 2P confirms the formation of TiO₂. Inversion capacitance of the MIS capacitors is found to increase with the increase in carbon concentration due to an increase of donor like centers in the Si_1-y C_y layers. The value of D_it was found to be 1.5×10¹² cm²/eV for the continuum model. The current-voltage characteristics of the MIS capacitors were measured and observed that the current density (J) at 1 V is 10^-5 A/cm² increases sharply with bias and then almost saturates. The leakage current has been found to be dominated by the SE at a low electric field, whereas PF emission takes over at higher electric field. So may be used for the next generation metal oxide semiconductor field effect transistors [30]. Zhang and Han [31] discussed the TiO₂ anatase thin films of 1000 nm were deposited by dielectric barrier discharge enhanced chemical vapor deposition (DBD-CVD) method at 400°C for glass substrate under working pressures from 200 Pa to atmospheric pressure. The film surface is closely covered by small Particles with size about 20–50 nm but the TiO₂ film deposited at 2000 Pa with size about 100–200 nm. Depending on discharge conditions, different kinds of discharges can be generated, like glow-like discharge, corona- like discharge, or filamentary streamers. The glow-like discharge could produce more homogeneous transient plasma and it normally appears at lower pressure. Obviously it cannot be useful for microelectronics application but suitable for display devices and photo detectors. Ivan et al. [32] presented nanocrystalline Tatiana thin films deposited at ambient temperature by DC magnetron sputtering, the triangular columnar grains of the order of 70–100 nm were obtained at 400°C with pressure of 32 m Torr. The anatase and rutile films of thickness of the order of 500 nm were reported. The refractive index of the films deposited at different O₂ partial pressure was between 1.8 and 2.25 in the dispersion free region. The optical band gap values to lie between 3.3 and 3.5 eV. The band gap values for anatase TiO₂ is 3.20 eV and up to 3.70 eV have been reported for the amorphous phase. The results presented that the refractive index and crystallite size decrease with an increase in the percentage of oxygen in the sputtering environment. The band gap increase with decrease in crystallite size, which is frequently deemed to indicate the onset of quantum confinement effects, light-emitting diode (LED), etc. These may be quite expensive when large-scale production is needed. It is one of the most promising gas-sensing materials due to its high temperature stability, harsh environment tolerance and catalytic properties. Bendavid et al. [33] deposited thin films of titanium dioxide on conducting (100) silicon wafers by filtered arc deposition. The refractive index values of the amorphous, anatase and rutile films were found to be 2.56, 2.62 and 2.72 at wavelength of 550 nm, respectively. The morphology of TiO₂ on silicon substrates changes from anatase to amorphous and then to rutile phase without auxiliary heating and, by using an appropriate substrate bias. The author reported that the densities of TiO₂ films had a significant effect on the optical and mechanical properties. The effect of surface morphology of the electrodeposited TiO₂ was studied by changing the precursor concentrations of the electrochemical bath. It can be a promising as a preparation method for industrial applications. Ohsaka et al. [34] obtained the TiO₂ layers of anatase phase, well-adhered, homogenous, with good secularity and colored by interference of reflected light. Their thickness was in the range of 100±500 nm. Microwave heating required short times, low temperatures and is relatively inexpensive. TiO₂ films on conducting glass were used in new types of solar cells. However, all above methods have high costs, and the preparation of films in a large area is technically difficult. Recently, wet processes, such as sol-gel and electrochemical deposition, have emerged as an alternative route for the preparation of the crystalline TiO₂ thin films.

Sol-gel method

In our work, we have opted for sol-gel spin coating method and have tried to make a comparison of various fabrication methods and their characteristics required for the application of microelectronics.

Experimental detail

TiO₂ thin films were deposited using sol-gel spin coating technique. Titanium isoproxide was used as the Titania precursor. The sol was prepared with absolute ethanol and acetic acid in the molar ratio of 1∶0.9∶0.1. The role of acetic acid acted as catalyst. The sol was prepared as shown in Fig. 2 and stirred for 30 min using magnetic-stirrer at 90°C. Then substrate was placed on spinner and drops of the above mentioned solution were placed on surface of RCA cleaned P(100) substrate. The substrate was then allowed to spin for 2 min with spinning rate of 3000 r/min. Then wafer was baked for 20 min at 90°C. The film was then sintered in a temperature controlled resistance furnace under dynamic air at 550°C and 850°C in nitrogen ambient.

The TiO₂ film thickness and optical characteristics were determined by ellipsometry. Scanning electron micrograph (SEM) were obtained using JEOL-1200 EX model. The accelerating voltage was kept at 5 kV. X-Ray diffraction (XRD) Philips model (PW 1729) was used for XRD analyses. The target was consists of copper metal, where as nickel metal is used as β-filter. The accelerating voltage was kept at 30 kV. The tube current was kept 15 mA. For electrical measurements of the structure Al/TiO₂/Si MOS capacitor, current voltage and capacitance voltage were taken by Keithley 2400 source meter and Agilent model (4284 A) LCR analyzer respectively in a probe station. Lab view program was used to interface LCR meter with computer for plotting data.

Fig.2 Experimental setup for chemical process

Full size|PPT slide

Experimental results

Measurements of samples were carried out at room temperature by SEM (Fig. 3(a)) and atomic force microscopy (AFM) (Fig. 3(b)). The surface morphology was obtained using AFM and field emission scanning electron microscopy (FESEM) as shown in Fig. 3, the porous nature with better crystallinity is clearly visible. The AFM topography of the as deposited TiO₂ films and annealed at 550°C revealed that surface is smooth and compact over whole substrate. The AFM images were subjected to flattering process. Then according to quantitative analysis measured value of average roughness (R_a), root mean square (RMS) value, and coefficients of kurtosis (R_ku) were 0.33, 0.42, and 0.72 nm respectively at 550°C. These results were in well agreement with the literature.

Fig.3 SEM and AFM images of thin TiO₂ films annealed at 550ºC. (a) FESEM image; (b) AFM image

Full size|PPT slide

Figure 4 shows the XRD pattern of the thin TiO₂ films annealed at 550°C in Fig. 4(a) and 850°C in Fig. 4(b) respectively. The amorphous nature of thin TiO₂ films was confirmed by XRD analysis. The horizontal axis represents the angle 2θ (in degrees) and the vertical axis represents the intensity of the diffracted X-Ray beam from the sample in arbitrary units. The grain size was calculated by the Scherer’s formula D = 0.89λ/(β_1/2cosθ), where λ is X-ray wavelength, β_1/2 is full width at half maximum (FWHM) of diffractions line and θ is diffraction angle, and confirmed by SEM. The XRD exhibit different crystalline phase of TiO₂ thin film and the calculated grain size of TiO₂(004), TiO₂(200) and TiO₂(211) were 23, 37 and 54 nm respectively. Figure 4(b) shows the XRD pattern of TiO₂ film deposited at Si wafer and annealed 850°C in nitrogen ambient. The calculated grain size of TiO₂(004), TiO₂(200), TiO₂(211) was 71, 69, and 63 nm respectively after annealing at 850ºC.

Fig.4 XRD spectra of (a) TiO₂ film deposited on silicon wafer annealed at 550°C; (b) titanium oxide film annealed at 850°C

Full size|PPT slide

The Raman spectrum of TiO₂ nanocrystalline film anealed at 550°C is shown in Fig. 5. The spectrum is typical of anatase TiO₂ phase and also support the results of XRD analysis. The three Raman peaks at 144, 192 and 634 cm^-1 are aggined to the E_g mode of anantase phase, which supports the result of literature [34]. The peak at 390 cm^-1 was obtained correponding to B_1g mode and 519 cm^-1 for the A_1g and B_1g modes. So TiO₂ have six Raman active modes A_1g+2B_1g+3E_g. The Raman spectra shows well defined peak and the absence of overlapping peaks confirms the well crystallinity of thin films with low number of imprefection sites.

Fig.5 Raman spectra of TiO₂ thin film annealed at 550°C

Full size|PPT slide

The variation of the capacitance (C) with gate voltage (V_G) ranging from -4.0 V to+4.0 V with frequency 100 kHz was obtained using Keithley 590 capacitance-voltage (C-V) analyzer for TiO₂ layer as shown in Fig. 6. The oxide capacitance (C_ox) is the high frequency capacitance when the device is biased for strong accumulation and found to be 54 pF. The dielectric constant of TiO₂ (high-k) 29 was observed by calculation from the knowledge of the capacitance (C_ox), film thickness (d), the free space charge permittivity (Є₀) and the area of the capacitor (A) using the relation K=Cd/(Є₀ A). Thickness was measured by using stylus profiler and found to be 52 nm for TiO₂ thin film.

Fig.6 C-V characteristics of Si/TiO₂/Al structure

Full size|PPT slide

By comparing the characteristics of the capacitor with ideal stimulated C-V curves, the flat band voltage of the capacitor was calculated 1.02 V using the relation K=C_oxd/(Є₀A) where C_ox (54 pF) is taken from C-V graph from Fig. 6. Extrinsic Debye length calculated using the formula λ = [Є_SKT/(q²N_X)]^1/2 and is found to be 184.92 Å. The gate area was calculated 1×10^-5 cm^-2 using the expression πr² with r = 0.72 mm. Flat band capacitance (C_FB) can be calculated using the relation

(1)C_{\mathrm{F}\mathrm{B}}={\displaystyle \frac{C_{\mathrm{O}\mathrm{X}}{ϵ}_{\mathrm{S}}A/(\mathrm{1}\times {\mathrm{10}}^{-\mathrm{4}})(\lambda )}{(\mathrm{1}\times {\mathrm{10}}^{-\mathrm{12}})(C_{\mathrm{O}\mathrm{X}})+({ϵ}_{\mathrm{S}}A)(\mathrm{1}\times {\mathrm{10}}^{-\mathrm{4}})(\lambda )}},

was found to be 51 pF. Where C_ox is the oxide capacitance, Є_S is the permittivity of substrate material, λ is the extrinsic Debye length, A is the gate area. W_MS (metal semiconductor work function difference) it is found to be (-1.025 V) using the relation W_MS=W_M-(W_S- Ф_B+0.5E_G). D_it was found to be 1.02×10¹¹ cm^-2·eV^-1, using the relation 1×10¹² C_it/(Aq) where here C_it is the interface state capacitance and A is electrode area. The effective oxide charge (Q_EFF) represents the sum of the oxide fixed charge (Q_F), the mobile charge (Q_M), and the oxide trapped charge (Q_OT). The calculation of Q_EEF is based on the assumption that the charge is located in a sheet at the silicon to TiO₂ interface. Its value is found to be 12.9×10^-8 C·cm^-2 using the relation Q_EEF = C_ox(V_FB- W_MS). And the effective oxide charge concentration (N_EFF) is computed using the equation N_EFF = Q_EFFq^-1, and is found to be 8.06×10¹¹ unit/cm³. I-V characteristics of TiO₂ thin films have been studied with Al/TiO₂/Si MOS capacitor structure. The input voltage is swept from 0 to±6 V and the gate leakage current is determined. The current-voltage characteristics were shown in Fig. 7. Gate voltage is considered positive when top electrode (Al) is more positively biased then ohmic contact of MOS structure. The electrical properties of TiO₂ films as the gate bias were also investigated. It can be noted that the as deposited film showed a relatively low leakage current (J_L) of about 10^-6 A/cm² at zero bias and 5.32×10^-5 A/cm² at a gate bias of+1 V.

Fig.7 Current voltage characteristics of MOS capacitor with TiO₂ film with Al top electrode

Full size|PPT slide

For higher gate voltages two prominent leakage current mechanisms were invoked namely SE and FN tunneling which were similar to reported values elsewhere [27-29]. The I-V relationship for the SE emission can be expressed as

(2)J=A_{\mathrm{t}\mathrm{h}}{T}^{\mathrm{2}}\left[\exp \left(-{\displaystyle \frac{q{W}_{\mathrm{M}\mathrm{S}}}{k_{\mathrm{B}}T}}\right)\right]\exp \left[{\displaystyle \frac{q}{k_{\mathrm{B}}T}}{\left({\displaystyle \frac{q{V}_{\mathit{B}}}{\mathrm{4}\mathrm{\pi }{ϵ}_{\mathrm{r}}{ϵ}_{\mathrm{o}}{d}_{\mathrm{o}\mathrm{x}}}}\right)}^{\frac{\mathrm{1}}{\mathrm{2}}}\right],

where A_{\mathrm{t}\mathrm{h}}, W_MS, {ϵ}_{\mathrm{r}}, {ϵ}_0 and d_{\mathrm{o}\mathrm{x}} were the Richardson-Dushmann constant, metal work function, relative dielectric constant, vacuum permittivity and oxide thickness respectively. Figure 8 shows the current (I) versus V^{\frac{\mathrm{1}}{\mathrm{2}}} plot in semi-log scale of the observed data in the voltage range of 0.2 to 0.7 V. It is clear that the current was linear with applied electric field confirms the SE process at lower biases<1 MV/cm. The conduction mechanism at lower biases region is dominant by thermally excited carriers known as hopping mechanism which can confirmed by Fig. 8. As the bias is increased, the current increases exponentially confirm the SE behavior. It can be observed that hopping conduction mechanism is dominant below gate voltage<1 V while further increasing the gate bias, the conduction was invoked by F-N relationship as shown in Fig. 8. F-N tunneling can be expressed as

(3)J_{\mathrm{F}\mathrm{N}}=A_{\mathrm{F}\mathrm{N}}{V}^{\mathrm{2}}\exp \left(-{\displaystyle \frac{B_{\mathrm{F}\mathrm{N}}}{V_{\mathrm{B}}}}\right),

(4)B_{FN}={\displaystyle \frac{8\pi \sqrt{2m-m_o}\left(q{W}_{MS}^{3/2}\right)d_{ox}}{3qh}},

where A_{\mathrm{F}\mathrm{N}} and B_{\mathrm{F}\mathrm{N}} are constants.

Fig.8 SE plot of measured I-V data

Full size|PPT slide

The F-N plot of MOS capacitor is shown in Fig. 9 with conduction band offset value of 0.87 eV while theoretically it should be 1.02 eV. In case of Al top electrode, Ti ions in the TiO₂ film is reduced due to oxidation of Al. Thus aluminum oxide film continuous to grow a heterogeneous mixture of the aluminum created on top of the TiO₂ and results to change in conduction offset value. The device reliability was confirmed by the time dependent dielectric breakdown analysis (TDDB). The analysis was performed for 20 min on the MOS capacitor and constant leakage current was observed for whole operation. The increase in temperature affects the crystal size and leakage current significantly. The increase in grain size of as deposited thin TiO₂ film with respect to temperature reduces the boundary area, and so reduces the scattering electron on the surface and increase in carrier concentration, which take participate in current flow. Hence increase the conductivity with temperature. This effect on lattice structure can be confirmed with XRD analysis in Fig. 3 and also by DSC-TGA graph as shown in Fig. 10.

Fig.9 F-N tunneling for TiO₂ gives conduction band offset 0.83 eV. Upper inset shows the current-voltage plot in the range of 0 to±6 V

Full size|PPT slide

The crystalline temperature (T_c) was 310°C. This transition from amorphous solid to crystalline solid is an exothermic process, and results in a peak in the DSC signal as shown in Fig. 10. Thermal characterization was performed on DSC/TGA instrument with heating rate of 5°C/min in N₂ atmosphere. From the TGA curve, in the temperature range from 100°C to 400°C, a considerable weight loss of 17% is observed which is imputing the loss of water and transformation of Ti-peroxide to TiO₂. DSC shows endothermic at 250°C and exothermic peaks at around 450°C and 800°C. The endothermic peak around 10°C due to loss of water and other solvents, peak at 450°C is due to partially change of anatase to rutile phase, while another peak at 800°C completely transformation of rutile phase.

Fig.10 DSC-TGA analysis of TiO₂ thin films

Full size|PPT slide

Electrical characteristics of various deposition methods

Table 1 shows measured and calculated data about TiO₂, electrical characteristics by considering T = 300 K, standard physical constant (q, K, Є₀), and electrode area 1×10^-5 cm². Table 2 summarizes the major electrical and technological characteristics for various deposition methods.

Tab.1 Various characteristics of nanocrystalline TiO₂ ultra thin film grown by sol-gel

dielectric constant	band gap /eV	formation temperature/°C	silicide formation	thermal stability/°C	interface trap density/(eV-1·cm-2)	oxide trap density /cm2	breakdown field/(MV·cm-1)	capacitance (Cox)/(F·cm-2)	flat band voltage (Vfb)/V	threshold voltage (Vt)/V	bulk potential(Φf)	low leakage current density wrt SiO2/(A·cm-2)
22-40	3.6	400	NA	550	1.1 × 1012	8.9 × 1012	<4	5.85×10-8	0.37	1.2	0.35	101-102

Tab.2 Comparison of main features of existing deposition methods versus our sample

electric field /(MV·cm-1)	thermal (12 nm)	e-beam (15 nm)	sputtering (13.6 nm)	PECVD (15 nm)	sol-gel spin (52 nm)
leakage current density	0	0		3.16 × 10-6
	0.5	3.16 × 10-8	1.00 × 10-6	1.25 × 10-5	3.98 × 10-7	1.00 × 10-4
	1	3.16 × 10-7	1.00 × 10-6	1.58 × 10-5	6.30 × 10-5	1.00 × 10-3
	1.5	3.98 × 10-6	3.16 × 10-5	1.99 × 10-4	7.94 × 10-5	1.26 × 10-3
	2	6.30 × 10-6	3.98 × 10-4	5.01 × 10-3	1.00 × 10-4	1.58 × 10-3
	2.5	1.58 × 10-5	6.30 × 10-4	6.30 × 10-3	1.25 × 10-4	1.58 × 10-3
	3	5.01 × 10-5	1.00	7.94 × 10-3	1.58 × 10-4	1.58 × 10-3
	3.5			1.00	1.99 × 10-4	1.99 × 10-3
refractive index	2.2-2.5	2.1	2.53-2.72	2.12-2.56	2.33
optical dielectric constant	4.84-6.25	4.41	6.4009	4.4944	5.428
band gap/eV	3.2-3.5	2.69	3.0	3.25	3.4
heat treatment/°C	550-750	600	700-1000	300-450	400-550
dielectric constant	15-40	23	15-25	20-35	26-80
merits/application	can be used as gate dielectric material	best photocataystics material, junction based devices	microelectronic materials, enhance mobility MOSFET	solar cell, insulating memory devices, fiber optical sensors	next generation high-k dielectric material for MOS, chip
drawbacks	higher leakage current density	5 nm roughness in surface, leakage current	mass fabrication is expensive	mass fabrication is expensive	higher silicon/ dielectric interface density

Notes;Data from Refs. [18, 20, 24, 27, 35-38] and some other sources, slightly different values of those parameters were report time to time.

We now have much better understanding on leakage current and method option for high-k gate insulator. The effect of the parameters for film production to the mechanisms of film growth is understood to some extent. There has much more research to be performed in order to clarify the relationship between microscopic aspects (interfaces, structure, and bonding) and macroscopic features and properties (crystalline, texture, etc.). The understanding of such connections is essential for establishing defined and reproducible processes. The leakage current depends upon the deposition method as shown in Table 2.

Discussions

In the present study, it was found that TiO₂ is a possible candidate among the gate insulators because its anatase type has a high dielectric constant of approximately 27 and good thermal stability on Si. Conduction band offset of 1.2 eV with silicon can act as better gate dielectric with high gate capacitance with less leakage current in sub-micron regimes of CMOS ultra large scale integration. The band gap of material was reported 3.6 eV. The acceptance for most modern CMOS fabrication facilities, TiO₂ has extensively been investigated as a possible SiO₂ replacement [39] for the fabrication of new generation of dynamic random access memories (DRAMS) and micro-electromechanical systems. A smaller grain size increases the surface area to volume ratio that leads to an enhancement of surface processes, as photo catalytic activity and humidity adsorption capacity [38]. TiO₂ have too low conduction band offset and can react with the silicon substrate. In addition, TiO₂ is found to have a low crystallization temperature of about 400°C which is below most of the processing temperatures in the present CMOS technology. The grains of the titanium dioxide film were of the order of nanometers. It was also observed that grain size improved with increased annealing temperature. The further improvements in deposition process are required to make thin films suitable for MOS devices.

Conclusion

Main aspects for film deposition methods are economy and ecology, to make the technique an increasing demand of future. In the past ten years, various methods have been employed to coat TiO₂ thin films on supporting substrates for substantial applications, like water and air purification, self-cleaning, fabrication of various semiconductor devices in microelectronics industries etc. among which, the sol-gel process has been long utilized to prepare Anatase TiO₂ films. However, it still remains a problem for a large scale production in the long run. The commercially more established process of CVD seems to be mostly suitable for large-scale industrial production of TiO₂ thin films. Solid phase epitaxial method also has potential but cathode electro deposition seems promising as a preparation method for mass production and industrial applications. The sol-gel method is simple, inexpensive, non-vacuum, and low temperature technique for deposition of films. TiO₂ by thermal oxidation is considered as one of the potential methods because the current density was observed three orders of less magnitude than other methods. Generally, high crystalline nature of TiO₂ is desired for its applications in CMOS devices and as an electrode of a photovoltaic device. When oxide thickness is in nanometer scale, even small non-uniformity either in chemical composition or even at surface fluctuation leads to increase leakage current by 10 fold. It has been reported that EOT of 3.6-13 nm is achieved with TiO₂ as gate dielectric and also reduced leakage current by two orders of magnitude is achieved.

In this study, we successfully deposited thin TiO₂ films by sol-gel method and compared with other traditional methods. It has been observed that the dielectric constant is 26 and grain size which is found to be tens of nm in case of silicon wafer in air at 550ºC and grain sizes increases with 850ºC in nitrogen ambient. The X-Ray diffraction studies show that the nature of deposited film was amorphous. The high value of leakage current and interface trap density requires more research. Further improvements can be done in deposition process and annealing process in order to bring the interface trap charge density and leakage current value at minimum level. So these parameters enhance the acceptability of TiO₂ for the fabrication of quantum dot solar cell and for ultra thin gate electrodes. This may make the thin TiO₂ films suitable for futuristic CMOS devices.

Acknowledgements

The author would like to thanks Assistant Professor, Dr. Savita Rathee from Department of Mathematics, Maharshi Dayanand University Rohtak, India, for her cooperation and regular guidance. The author also wants to thank Mr Naveen Goel, H.O.D M. Tech, Department of Electronics & Communications, Vaish College College of Engineering, Rohtak, India for his many insightful discussions.

References

1 Borkowska A, Domaradzki J, Kaczmarek D. Characterization of TiO₂ and TiO₂-HfO₂ transparent thin films for microelectronics applications. In: 2006 International students and Young Scientist Workshop, Photonic and Microsystems. 2006: 5–8

2 Masuda Y, Jinbo Y, Yonzawa T, Koumoto K. Templeted site selective deposition of Titanium dioxide and self assembled monolayer. Chemistry of Materials, 2002, 14(3): 1236–1241

3 Fuyuki T, Matsunami H. Electronic properties of the interface between Si and TiO₂ deposited at very low temperatures. Japanese Journal of Applied Physics, 1986, 25(9): 1288–1291

4 Su C, Hong B Y, Tseng C M. Sol-gel preparation and photocatalysis of titanium dioxide. Catalysis Today, 2004, 96(3): 119–126

5 Wong H, Iwai H. On the scaling issues and high-κ replacement of ultrathin gate dielectrics for nanoscale MOS transistors. Microelectronic Engineering, 2006, 83(10): 1867–1904

6 Gan J Y, Chang Y C, Wu T B. Dielectric property of (TiO₂)_x-(Ta₂O₅)_1-x thin films. Applied Physics Letters, 1998, 72(3): 332

7 Westlinder J. Investigation of novel metal gate and high-k dielectric materials for CMOS technologies. PhD Thesis Uppsala: Acta Universitatis Upsaliensis, 2004: 8–72 www.uu.diva-portal.org/smash/get/diva2:165233/FULLTEXT01

8 Zhang L, Mu J M. Nanomaterial and Nanostructure. Bejing: Science Press, 2001

9 Kostlin H, Frank G, Hebbinghaus G, Auding H, Denissen K. Optical filters on linear halogen-lamps prepared by dip-coating. Journal of Non-Crystalline Solids, 1997, 218: 347–353

10 Corma A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chemical Reviews, 1997, 97: 2373–2420

11 Pomoni K, Vomvas A, Trapalis C. Transient photoconductivity of nanocrystalline TiO₂ sol-gel thin films. Thin Solid Films, 2005, 479(1–2): 160–165

12 ITRS 2003, Edition, Semiconductor Industry Association (SIA), Austin, SEMATECH USA, 2706 from: www.itrs.net/links/2003

13 Kurakula S R. Studies on the electrical properties of titanium dioxide thin film dielectrics for microelectronic applications. Dissertation for the Master’s Degree. Indian Institute of Science, 2007: 1–45

14 Gusev E P, Cartier E, Buchanan D A, Gribelyuk M, Copel M, Okorn-Schmidt H, D’Emic C. Ultrathin high-K metal oxides on silicon: processing, characterization and integration issues. Microelectronic Engineering, 2001, 59(1–4): 341–349

15 Löbl P, Huppertz M, Mergel D. Nucleation and growth in TiO₂ films prepared by sputtering and evaporation. Thin Solid Films, 1994, 251(1): 72–79

16 Georgia J, Armynov S, Volva E, Oulios I P, Sotiropoulos S. Preparation and photoelectrochemical characterisation of electrosynthesised titanium dioxide deposits on stainless steel substrates. Electrochimica Acta, 2006, 51(10): 2076–2087

17 Battiston G A, Gerbai R, Porchia M, Margio A. Influence of substrate on structural properties of TiO₂ thin films obtained via MOCVD. Thin Solid Films, 1994, 239(2): 186–191

18 Löbl H P, Huppertz M, Mergel D. ITO films for antireflective and antistatic tube coatings prepared by direct current magnetron sputtering. Surface and Coatings Technology, 1996, 82(1–2): 90–98

19 Meng L J, dos Santos M P. Investigations of titanium oxide films deposited by direct current reactive magnetron sputtering in different sputtering pressures. Thin Solid Films, 1993, 226(1): 22–29

20 Martin N, Rousselt C, Savll C, Palmino F. Characterizations of titanium oxide films prepared by radio frequency magnetron sputtering. Thin Solid Films, 1996, 287 (1–2): 154–163

21 Fernandez L A, Espinos J P, Belderrain T R, Gonzalez-Elipe A R.Ion beam induced chemical vapor deposition procedure for the preparation of oxide thin films. II. Preparation and characterization of Al_xTi_yO_z thin films. Journal of Vacuum Science and Technology A: Vacuum, Surfaces, and Films, 1996, 14 (5): 2842–2848

22 Liu H M, Yang W S, Ma Y, Cao Y A, Yao J N, Zhang J, Hu T D. Synthesis and characterization of titania prepared by using a photoassisted Sol–Gel method. Langmuir, 2003, 19(7): 3001–3005

23 Chowdhury P, Barshilia Harish C, Selvakumar N, Deepthi B, Rajam K S, Chaudhuri A R, Krupanidhi S B. The structural and electrical properties of TiO₂ thin films prepared by thermal oxidation. Physica B, Condensed Matter, 2008, 403(19–20): 3718–3723

24 Hitchman M L, Tian F. Studies of TiO₂ thin films prepared by chemical vapour deposition for photocatalytic and photoelectrocatalytic degradation of 4-chlorophenol. Journal of Electroanalytical Chemistry, 2002, 538-539: 165–172

25 Kaliwoh N, Zhang J Y, Boyd I W. Characterisation of TiO₂ deposited by photo-induced chemical vapour deposition. Applied Surface Science, 2002, 186(1–4): 241–245

29 Babelon P, Dequiedt A S, Mostéfa-Sba H, Bourgeois S, Sibillot P, Sacilotti M. SEM and XPS studies of titanium dioxide thin films grown by MOCVD. Thin Solid Films, 1998, 322(1–2): 63–67

26 Chakraborty S, Bera M.K., Bhattachary S, Maiti C K. Current conduction mechanism in TiO₂ gate dielectrics. Microelectronic Engineering, 2005, 81: 188–193

27 Chong L H, Malik K, de Groot C H, Kersting R. The structural and electrical properties of thermally grown TiO₂ thin films. Journal of Physics Condensed Matter, 2006, 18(2): 645

28 Sze S M. Physics of Semiconductor Devices. New York: Wiley-Interscience, 1969, 496

30 Dalapati G K, Chatteraje S, Shrama S K, Nandi S K, Bose P K, Varma S, Patil S, Maiti C K. Electrical properties of ultrathin TiO₂ films on Si_1-yC_y heterolayers. Solid-State Electronics, 2003, 47(10): 1793–1798

31 Zhang X W, Han G R. Microporous textured titanium dioxide films deposited at atmospheric pressure using dielectric barrier discharge assisted chemical vapor deposition. Thin Solid Films, 2008, 516(18): 6140–6144

32 Ivan H, Pullmannov A, Martin P, Juraj H, Kups T, Spiess L. Communications structural and morphological investigations of TiO₂ sputtered thin films. Communications, 2009, 60(6): 354–357

33 Bendavid A, Martin P J, Takikawa H. Deposition and modification of titanium dioxide thin films by filtered arc deposition. Thin Solid Films, 2000, 360(1–2): 241–249

34 Ohsaka T, Izumi F, Fujiki Y. Raman spectrum of anatase, TiO₂. Journal of Raman Spectroscopy, 1978, 7(6): 321–324

35 Vigil E, Saadoun L, Ayllón J A, Domènechc X, Zumetaa I, Rodrı́guez-Clemente R. TiO₂ thin film deposition from solution using microwave heating. Thin Solid Films, 2000, 365(1): 12–18

36 Rathee D, Kumar M, Arya S K. CMOS Development optimization, scaling issue and replacement with high-k material for future microelectronics. International Journal of Computer Application, 2010, 8(5): 10–17

37 Zhang H Z, Banfield J F. Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: insights from TiO₂. Journal of Physical Chemistry B, 2000, 104(15): 3481–3487

38 Rathee D S, Sharma R, Pandey M. The Roadmap for CMOS scaling and optoelectronics devices. In: Proceedings of National Conference ITM. 2007, 82–87

39 Jang H D, Kim S K, Kim S J. Effect of particle size and phase composition of titanium dioxide nanoparticles on the photocatalytic properties. Journal of Nanoparticle Research, 2001, 3(2–3): 141–147