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Preparation and characterization of (Ba,Sr)TiO3 thin films by liquid source chemical vapor deposition

Cheol-Hoon Yang, Young-Ki Han, Dong-Hyun Kim, Geun-Jo Han, Doo-Young Yang, KiYoung Oh, Juho Song, Jaehoo Park1, and Cheol-Seong Hwang1 JuSung Engineering Co., #49, Neungpyeong-Ri, Opo-Myeun, Kwangju-Si, Kyunggi-Do, 464-890, South Korea 1 School of Material Science and Engineering, Seoul National University, San #56-1, Shillim-dong, Kwanak-ku, Seoul, 151-742, South Korea ABSTRACT The cocktail source of BST was prepared by mixing of Ba, Sr, and Ti precursor solution with specific mole ratio. This cocktail source was vaporized and delivered into the warm wall reactor by liquid delivery system(LDS) and gaseous source was distributed by simple structure of gas injector instead of showerhead system. The thickness uniformity of BST on 8 inch wafers were less than 3%. The Ti composition uniformity of our films were less than the 1 at%(1 ) at stoichiometric and near stoichiometric. Their dielectric constant was about 230 and leakage current density was lower than 10-8 A/cm2 under ¢1V bias. Excellent step coverage and smooth (haze-free) surface morphology of BST films were obtained by a deposition using a noble dome type reactor. The merit of our warm wall type reactor also will be explained by excellent step coverage and the uniform composition with 3 dimensional structure. Our achievement should be applicable to the capacitor of next generation DRAM. INTORDUCTION The first trial to introduce the new dielectric of (Ba,Sr)TiO3 into a planarized DRAM(Dynamic Random Access Memory) cell design dates back to 1992.[1] Even after the long continuous efforts to integrate BST into a cost effective DRAM product, there are still many unresolved issues, among which is the unavailability of the reliable and mass-production compatible BST deposition system. However, there is a consensus that liquid delivery MOCVD would be the predominant production approach to deposit BST because of the reproducibility[2]. Numerous groups have described the effects of thickness[3], Ba/Sr ratio[4], and excess Ti[5,6] on the electrical properties of BST. Since the optimum composition of (BaxSr1-x)TiyO3 and the required thickness for actual application are well known, special emphasis was placed on the reliability and the longer uptime of the LDS(Liquid Delivery System) of the MOCVD reactor in this study. It is also known that precursor itself can play a significant role in the MOCVD of BST[7]. Due to the highly temperature dependent deposition properties, the composition can be different according to the different states of the wafer surface, i.e. SiO2 or film on the Si wafer. This is due to a temperature discrepancy between the wafers having SiO2 and Pt over-layers even at the same heater temperature. It can be clearly shown that the Pt over-layer increases the wafer temperature by more than 10 in the relevant temperature range[8]. In the case of BST film growth on a blanket wafer this may not be a serious problem as the wafer temperature.

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