Preparation and Characterization of TiO 2 and SiO 2 Thin Films

Although scaling will continue for couple of decades but device geometries reaches to atomic size and limitation of quantum mechanical physical boundaries. To address these problems there is need of innovation in material science & engineering, device structure, and new nano devices based on different principle of physics. So TiO2 thin films have been grown on well clean N-type silicon substrates via a sol–gel spin coating method. MOS capacitor were fabricated and characterized with SiO2 and TiO2 as dielectric material on N-type silicon wafer. The thickness was measured by stylus profiler and found to be 510 Å and 528 Å for SiO2 and TiO2 respectively. Some of the material parameters were found from the measured Capacitance -Voltage (C-V) curve obtained by SUPREM-III (Stanford University Process Engineering Model Version 0-83) for SiO2 and C-V Keithly 590 analyzer for TiO2 thin films. The result shows that obtained TiO2 film present a dielectric constant of approximately 80. The refractive index was found to be 2.4 and optical constant was 5.43 obtained from Ellipsometry. Band gap 3.6 eV of TiO2 was calculated by spectrophotometer and Surface morphology was obtained using Scanning Electron Microscope (SEM-JEOL) micrograph. The aluminum (Al) metal was deposited by the thermal evaporation system on the back side of the sample for the ohmic contact. Analysis shows that TiO2 may be acceptable as a viable substitute for high k dielectric in order to prevent the tunneling current problems.


Introduction
The silicon industry has been scaling SiO 2 aggressively for low power, high performance CMOS logic applications.The tunneling current increases exponentially as the thickness of the dielectric decreases.Now a day's TiO 2 oxide layers are being studied intensively as one of the promising high K dielectric for high density DRAM applications [1].According to International Technology Roadmap for Semiconductor (ITRS) a dielectric constant higher than 25 is needed to meet the scaling goal and at the same time to keep the gate leakage current within tolerable limit (10 A/cm 2 ) [2].There are various challenges with new high K dielectric i.e must have large band gap, nature must be amorphous, thermodynamically stability, and good interface quality with Si [3,4].Several high K materials are considered to replace SiO 2 gate di-electric among them metal oxides having value high than 25 are of interest [4].Here we present the study of Titanium dioxide (TiO 2 ) as a gate dielectric layer.

Experimental
The films were deposited on N-type <100> silicon substrate in the 2-10 Ω cm resistivity range.The silicon wafer was chemically cleaned by RCA standard cleaning procedure to remove insoluble metallic and organic contaminants.

Nanocrystalline Dielectric Film Deposition
TiO 2 thin films were deposited on Silicon <100> substrate using Sol-Gel method.Titanium isoproxide (TIP) was used as the Titania precursor.The matrix sol was prepared by mixing TIP with absolute ethanol and acetic acid in the molar ratio of 0.1:8:0.1.The substrate was placed on spinner and drops of above mentioned solution were placed on substrate.The substrate was then allowed to spin for 2 minutes with spinning rate 1500 rpm.Then sample was baked for 20 minutes at 95˚C.The film was than annealed in dynamic air at 550˚C for 30 minutes to treat the adsorbed film.While SiO 2 film was grown on silicon substrate using dry oxidation process at 650˚C for 60 minutes with a pre ramp of 5˚C per minute so that we reach reached to 950˚C and than flat temp of 950˚C for the 110 minutes at last post-ramp of 5˚C per minutes for 60 minutes with N 2 (12) and O 2 (32).

MOS Capacitor Fabrication
MOS capacitors were fabricated on a 4 inch diameter & 800 micron thick N-type silicon wafer.The dielectric layer of 510 Å was grown on silicon wafer by dry oxidation at 950˚C for 2 hours.About 3000 Å of aluminum was then deposited over oxide layers using the sputtering technique.The 10 -2 cm 2 capacitor contact was defined by physical mask.The contacts were annealed in gas atmosphere H 2 , & N 2 at 350˚C for 25 minutes.The fabricated capacitors electrically tested to characterize the material and to inspect the device performance.The variation of capacitance (C) with voltage V G ranging -5 Volts to +5 Volts is shown in Figure 2.
In this work MOS capacitor were fabricated utilizing TiO 2 film deposited by Sol Gel spin coating method and SiO 2 films were thermally grown on <100> silicon substrate.The C-V measurement for TiO 2 was obtained using C-V analyzer [5,6], while SUPREM-III tool was used for SiO 2 layer.SUPREM-III is a computer program that allows the user to simulate the various processing steps used in manufacturing of the Silicon integrated or discrete devices [7].The characterization of the samples was carried out at room temperature.Two dimensional SEM images of the TiO 2 film of thickness 52 nm (average film thickness for three sol-gel immersion cycles) were obtained (using JEOL-JM-6510 model shown in Figure 1), the accelerating voltage was kept at 5 kV.The optical properties were studied using UV-Visible spectrophotometer.

Film Characetrization
The porous nature of the film is clearly visible.The refractive index (n) of the TiO 2 film measured by Ellipsometer and was found to be 2.33.The optical dielectric constant can be determined from the refractive index of

MOS Capacitor Results
The variation of the capacitance (C) with gate voltage (VG) ranging from -5.0 V to +5.0 V with frequency 10 KHz was obtained using SUPREM -III for SiO 2 di electric and by Keithley 590 CV analyzer for TiO 2 layer as shown in Figure 2. The oxide capacitance ( ) is the high frequency capacitance when the device is biased for strong accumulation as shown in While in the inversion region, where the total capacitance per unit area ( ,min a C ) is the series combination of the oxide capacitance and the steady minimum depletion capacitance.
The inversion capacitance per unit area was calculated 46.17 pF for TiO 2 by equation where ox oxide capacitance, electronic charge, Si C  is the permittivity of the substrate = 11.7 × 8.85 × 10 -14 , d is density of carrier concentration in the doped substrate, ni is carrier concentration in intrinsic semiconductor.

N
While in the inversion region, where the total capacitance per unit area ( ,min a ) is the series combination of the oxide capacitance and the steady minimum depletion capacitance.

C
The inversion capacitance per unit area was calculated 46.17 pF for TiO 2 by equation where λ is the extrinsic Debye length, as calculated 5 2 1.279 10 where kT is thermal energy at room temperature.Debye length indicates that how far an Electrical event can be sensed with in the semiconductor.The flat band capacitance FB = 39.7 pF, ox is the oxide capacitance = 41.4 pF and A is gate area = 3.13 × 10 -6 m 2 .This .
By the C-V characteristics of the capacitor the flat band voltage V FB of the capacitor can be estimated.Now the threshold voltage V TH for MOS-C from a C-V curve as follows.
ss than the 6.5 pF By comparing the C-V characteristics of the capacitor with the ideal simulated C-V curves, the flat band voltage of the capacitors was calculated 2V.The oxide charge density of capacitor was calculated by where C ox , W MS , V FB and A are oxide capacitance, metal semiconductor work function difference, flat band voltage, and electrode area A. The value of oxide charge density for structure Si/TiO 2 /Al (Q it ) 7.48 × 10 12 and interface trap density (D it ) 4.32 × 10 13 eV -1 •cm -2 were slightly higher than the values reported in literature [5].
The deposited TiO 2 thin presented a dielectric constant value of approximately 80, an order of magnitude higher than the obtained for SiO 2 (3.9).MOS capacitors were fabricated and it presented a leakage current density of 10 mA/cm 2 , acceptable for high performance logic circuits (maximum of 100 A/cm 2 ) and low power circuits (10 mA/cm 2 ) device [8].The leakage current density value is higher than thermally grown SiO 2 film with approximately same thickness as shown in Figure 4.These results indicate that TiO 2 deposited on silicon with right recipe may be strong candidate to substitute the current dielectric in CMOS fabrication.

Conclusions
MOS capacitors were fabricated successfully by using TiO 2 and SiO 2 as gate dielectric.The capacitor uses Si substrate with Al as another terminal.The C-V and I-V results shows that the as deposited TiO 2 Nanocrystalline films present a dielectric constant of approximately 73, with good interface quality with silicon and with leakage current density, for 1V of 10 mA/cm 2 , which may be acceptable for fabricating high performance and low power logic circuits.Also the measured band gap of TiO 2 thin film was 3.6 eV may be applied in microelec tronics applications.The higher value of leakage current density and interface trap density of MOS capacitors requires further improvement in the deposition process.The measured results shows that thermally grown oxide has desirable properties in terms of leakage current and interface trap density but another obtained parameter like higher dielectric constant and small band gap are also considerable.More research is needed in case of TiO 2 to solve leakage current problem without decreasing in effective dielectric constant.
TiO 2 opt  = n 2 = 2.33 2 = 5.4389.The band gap was calculated 3.6 eV by using equation  = c/λ,  is photon energy, c speed of light and λ cut of wavelength by the help of Spectrophotometer as shown in Figure 1(b).

Figure 2 .
In case of SiO 2 dielectric it was found to be 5.202 pF, and 41.4 pF for TiO 2 as shown in Figures 2(a) and 2(b) respectively.ox C The dielectric constant of TiO 2 (high-k) 80 was observed by calculation from the knowledge of the capacitance ( ox ), film thickness (d), the free space charge permittivity ( o C  ) and the area of the capacitor (A) using the relation K = Cd/ o  A. Thickness was measured by using stylus profiler and found to be 528 A for TiO 2 and 510 A in case of SiO 2 Figure 3(a).

Figure 1 .
Figure 1.(a) SEM images of TiO 2 deposited on silicon wafer annealed at 550 C; (b) The absorption spectra of TiO 2 thin film.

Figure 2 .
Figure 2. (a) C-V Characteristics of MOS capacitors at room temperature (Si/TiO 2 /Al) (b) Si/SiO 2 /Al C-V Characteristics of MOS capacitors at room temperature.