Deposition behavior and dielectric properties of the Ti-based oxide films for memory devices

Author
Seong Keun Kim
Year
2007
Abstract & Cover

High dielectric constant (high-k) - TiO2 thin films were deposited by Atomic Layer Deposition (ALD) with Ti(Oi-C3H7)4 (TTIP) and O3 as metal and oxygen source. TiO2 is well known as material having several polymorphs : rutile and anatase. Dielectric constant of anatase is 45 along a-axis and 25 along c-axis, respectively. On the other hand, rutile structured TiO2 have large dielectric constants at room temperature (170 along c-axis and 86 along a-axis). Although TiO2 thin films grown by ALD have been extensively studied, most of the TiO2 films grown by ALD have anatase structure, on the other hand, there are few reports on the formation of rutile-structured TiO2 films. However, in this study, the growth of rutile TiO2 thin films on Ru electrodes, having a k-value of 83-100, by using ALD technique at a growth temperature of 250 ℃ is reported. The structural compatibility of the RuO2 that was in-situ formed by the strong oxidation power of O3 with rutile TiO2 may constitute the main reason for obtaining rutile structured TiO2 with the high εr value from the O3-TiO2/Ru samples. Also, TiO2 thin films were grown on O3 pre-treated Ru electrodes by atomic layer deposition method with TTIP and H2O as reactants. TiO2 film grown on O3 pre-treated Ru electrode was crystallized in to rutile structure whereas crystalline structure of TiO2 film on Ru electrode without O3 pre-treatment was anatase and the dielectric constant of that TiO2 film was approximately doubled in the value without O3 pre-treatment.
It verifies that the growth of rutile structured TiO2 films on Ru electrodes is caused by structural compatibility with RuO2 formed in the interface. The crystallization behavior of TiO2 films was also promoted due to the local epitaxy growth.
For the application of the material and ALD process to DRAM devices with three-dimensional structures, contact hole type 3D MIM capacitors composed of Ru/TiO2/Ru layers were fabricated with different hole diameters and distances between the holes. The grown films exhibited a very high bulk k value (~100) due to the formation of randomly oriented rutile phase material. The conformity in film thickness and dielectric properties over the entire structured surface area was confirmed by capacitance variation vs. hole surface area experiments.
The equivalent oxide thickness of TiO2 thin films could be reduced up to 8 Å and the leakage current density of the films was less than 10-7 A/cm2 at the applied voltage of 0.8 V. Large leakage current of TiO2 thin films may be induced by low schottky barrier height of TiO2. In order to enhance the schottky barrier height, variation of the properties of the TiO2 films according to O3 concentration and feeding time were investigated. However, n-type of nature of TiO2 films did not change by the variation of the O3 concentration and feeding time even though the atomic O/Ti ratio of the films analyzed by XPS was about 2.2. Bulk properties such as density, crystalline structure, and composition were not also varied from the variation of the O3 concentration and feeding time. However, the higher O3 concentration and the longer O3 feeding time, leakage current of the films increased by increase of the roughness, which was induced by oxidation of Ru due to strong oxidation potential of O3.
Doping of Al ions in the TiO2 films were performed in order to improve the electrical properties eventually. Al and Ti precursor feeding ratio could control Al concentration in the films and Al ions were uniformly distributed in the films. The growth rate of Al-doped TiO2 films decreased with increasing Al precursor feeding ratio. It indicated that chemisorption site density of Ti precursor onto the Al-O layer might be lower than that of Ti precursor onto the Ti-O layer.
Although the dielectric constant of the Al-doped TiO2 films decreased with increasing the Al precursor feeding ratio, the dielectric constant of films which is in the range from 47 to 81, is higher than that of un-doped anatase structured TiO2, It means that the crystalline structure of Al-doped TiO2 films is not anatase but rutile structure after even Al doping. The rather higher leakage current of TiO2 films, which were induced by low Schottky barrier height, remarkably decreased by increasing the Schottky barrier height with the Al doping. In addition, leakage current density of the films was further improved by smoothing the film surface using a longer O3 purge time. Consequently, a minimum toxeq. of 4.8 Å with leakage current density < 1 x 10-7A/cm2 at applied voltage of 0.8 V. were achieved.
However, it is important to control proper Al concentration in the films through the variation of Al/[Al+Ti] precursor feeding ratio, because excess Al ions in the film generate defects like the dipole complexes.


 

Source of Information
Minsung Kim
University
Seoul National University
(Seoul, Korea)
Other notes
kindly provided by Seong Keun Kim
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