Overcoming the Non-ideal Behavior of Atomic Layer Deposition of SrTiO3 Thin Films for DRAM Capacitor

Abstract

The current generation DRAM capacitors in mass production employ zirconium oxide as dielectric materials with an interposed alumina layer for leakage current density reducing. However, in order to further scale down DRAM devices to sub 20nm design rules, application of materials with higher dielectric constants is necessary. Although there are many candidate materials such as tantalum oxide, titanium oxide, and so on, strontium titanate (SrTiO3, STO) is one of the most promising next generation dielectric materials. In spite of its relatively small energy band gap, the superior dielectric constant of perovskite structured STO gives DRAM technology a real chance to overcome the adversities of extreme scaling. To adopt STO as the dielectric of next generation DRAM capacitors, three prerequisites must be fulfilled

a stable STO atomic layer deposition (ALD) process for good step coverage, easy access to noble metal electrodes such as ruthenium for low leakage current, and high temperature growth modes for in-situ crystallization. In this study, STO films were grown by ALD with various kinds of Sr and Ti precursors

Sr(iPr3Cp)2, [Sr(demamp)(tmhd)]2, Ti(O-iPr)2(tmhd)2, and Ti(Me5Cp)(OMe)3. The growth behaviors of STO deposition processes with various combinations of these precursors were investigated and film properties including the electrical properties of fabricated metal-insulator-metal (MIM) capacitors were evaluated. First, Sr(iPr3Cp)2 and Ti(O-iPr)2(tmhd)2 were employed for the Sr and Ti precursors, respectively, for the deposition of ALD STO. Through previous studies with the same precursors, Sr excess incorporation was observed during the initial stage of STO deposition on Ru substrates. Initial Sr overgrowth caused strontium carbonate formation which was undesirable since this not only made precise control of thickness difficult, but also aggravated the electrical properties of resultant MIM capacitors. Since the violent reaction between the gas phase Sr(iPr3Cp)2 and the oxygen ion containing Ru substrate was pointed out as the main cause of this unwanted abnormal growth, in previous studies, TiO2 layers with thicknesses larger than 3 nm was deposited before STO deposition to prevent this phenomenon. Interposing the TiO2 layer was successful in preventing the initial excess growth, but sparing a physical thickness of 3 nm to a buffer TiO2 film is not a negligible loss in total dielectric property when it comes to DRAM capacitor technology, considering that the whole dielectric film is only 10 nm. Therefore, in this study, Al2O3 which has lower oxygen diffusivity than TiO2, was adopted instead of TiO2 for barrier layer to resolve this issue. Only a 0.4-nm-thick Al2O3 layer could effectively suppress the initial excessive incorporation of Sr, and a 1-nm-thick Al2O3 layer had a blocking effect that was equivalent to that of a 3-nm-thick TiO2 layer. The STO films were crystallized in-situ at 370 oC without any additional annealing process by the assistance of a crystallized 3-nm-thick STO seed layer that was pre-deposited and annealed. The bulk dielectric constant of STO calculated by the slope of equivalent oxide thickness vs. physical thickness was 173 on the 1-nm-thick Al2O3/Ru substrate. However, the adverse contribution of the Al2O3 layers to capacitance was found to be more severe than the TiO2 layers. Also, the characteristics of ALD STO films were examined for metal-insulator-metal capacitors, with Cp-based precursors, Sr(iPr3Cp)2 and Ti(Me5Cp)(OMe)3, employed as the Sr and Ti precursors, respectively. The enhanced reactivity of this Ti precursor compared to Ti(O-iPr)2(tmhd)2 was found to suppress the unwanted excessive incorporation of Sr into the film in situ. Formation of strontium carbonate was also repressed. A possible mechanism for these phenomena is suggested in detail. By controlling the sub-cycle ratio of the SrO and TiO2 layers, stoichiometric STO could be obtained, even without employing a deleterious reaction barrier layer. This improved the attainable minimum equivalent oxide thickness of the RuO2/STO/Ru capacitor to 0.39 nm, with acceptable leakage current density (~9 x 10-8A/cm2). This indicates an improvement of ~46% in the capacitance density compared with previous work. However, the bulk dielectric constant was only about 100 which indicates the possibility to further improve the electrical property of capacitor. Careful analyses carried out on the films revealed the presence of microdefects in the STO films. The stress generated from the difference between the thermal expansion coefficients of the STO films and the substrate was found to be the main reason for the formation of these anomalies. By adjusting the cooling rate after crystallization annealing of seed layer, the STO films showed an improved bulk dielectric constant of 135, but only with worse interfacial characteristics. Finally, STO deposition using [Sr(demamp)(tmhd)]2 and Ti(Me5Cp)(OMe)3 for the Sr and Ti precursors, respectively, was investigated. This Sr precursor portrayed a similar saturated growth rate of SrO films with Sr(iPr3Cp)2. However, there was no portent of abnormal growth behavior in the initial stage of deposition. Although the deposition rate of the STO films was reduced to a certain extent, carbon contamination in STO film was decreased and no evidence of strontium carbonate formation was observed. Moreover, the decreased deposition rate seemed to produce rather denser films and avoid microdefect generation after crystallization annealing. With ideal ALD growth behavior, conformal STO films could be obtained on three dimensional hole structures with aspect ratios of 10:1. Since the observed bulk dielectric constant was ~143, this is still quite promising considering the fact that there is still room available to improve the electrical characteristics of the capacitor by controlling interface properties. In conclusion, by suppressing the non-ideal initial overgrowth of Sr with an interposed Al2O3 layer, in-situ crystallized STO was grown without SrCO3 formation by adopting a pre-crystallized STO seed layer. Moreover, with a more reactive Ti-precursor, stoichiometric STO was deposited without any barrier layer and the electrical property showed a tox of 0.39 nm with a leakage current density of 9 x 10-8 A/cm2. The microstructure was improved as well by adjusting the annealing condition. Finally, by employing a novel Sr-precursor with appropriate reactivity, ideal ALD growth behavior were acquired with satisfactory dielectric properties and conformal STO films were deposited in hole structures with the aspect ratio of ~10:1.