Thin films of ferroelectric materials are of much interest for fabricating novel functional devices. Ferroelectric thin films can not only miniaturize devices, but they can also increase the efficiency of operation of devices owing to their scale-reduction and configuration. Many attempts have been made to apply ferroelectric thin films to a variety of devices including high-value capacitor for DRAM, nonvolatile memory, infrared sensors, ultrasonic sensors, and optical switches. There is a critical need for chip integrated dielectric materials with low leakage current, high dielectric strength, and high dielectric constant for a number of applications including charge-storage capacitors for ULSI DRAM and low-inductance decoupling capacitors for the control of simultaneous switching noise(SSN) in high-speed switching ULSI chips. An analysis of the requirements of 1Gb DRAM with planar capacitors concludes that charge-storage densities, leakage-current densities, and dielectric thickness in the ranges of 25~40 μC/㎠, 0.1~0.2μA/㎠, and 0.05~0.2㎛, respectively, are required.
La-modified lead titanate(PLT) thin films are a very interesting optoelectronic, pyroelectric, and ferroelectric materials. The properties of PLT, such as shapes of hysteresis, surface microstructure, transparency, and electric conductance, are changed with addition of lanthanum.
In this study, I describe the deposition process of PLT thin film at 500℃ under the pressure of 1000 mTorr by hot-wall type LP-MOCVD method, using $Pb(dpm)_2$, $La(dpm)_3$, and titanium tetra-isopropoxide${Ti[OCH(CH_3)_2}_4}$ as the source materials. PLT films deposited at 500℃ were annealed at 600℃ for 10 minutes with oxygen ambient. I have investigated crystallinity, surface morphology, and electrical properties, such as polarization-electric field(P-E) and current-voltage(I-V), capacitance-voltage(C-V), capacitance-frequency(C-F) and current-time(I-t), with various lanthanum molar fraction. Also we have discussed conduction mechanism of 180 nm-PLT(La 12 mole%) film. With increasing La mole%, surface morphology and densification were improved, and c/a ratio was decreased. PLT films wtih La 34 mole% had a cubic perovskite structure at room temperature. Ferroelectric properties, such as saturated polarization, remanent polarization and coercive field, and leakage current density decreased with increasing La mole%. In case of 180 nm-PLT(La 12 mole%) films showed the relative dielectric constant of 1000 1200 under the zero bias and 100 kHz, and the leakage current density of 1.5×10^{-7}A/㎠ under the 3 volts(167 kV/cm). The effective storage charge density($P_s-P_r$) of 180 nm-PLT(La 12 mole%) was 26.7 ㎛/㎠ at an appied voltage of 3 volts(167 kV/cm). This value is equal to the capacitance of $90fF/㎛^2$ and a $SiO_2$ equivalent thickness of 3.64Å. Pt/PLT(12)/Pt capacitors exhibited the Schottky conduction mechanism within the electric field range of 0.16~0.25 MV/cm. Schottky barrier height$(θ_o$) was 1.63 eV. PLT films with La 34 mole% revealed the relative dielectric constant of 700 under the measuring conditions of zero bias and 100 kHz, leakage current density of $2.0×10^{-8}A/㎠$, and effective storage charge density of 7.0 $μC/㎠$ at 167 kV/cm, respectively. From the above results, I would conclude that PLT films are one of the very attractive materials for dielectric of DRAM capacitor and power supply decoupling of high speed ULSI package.
