【作者】 杨海平；

【导师】 唐谟堂； 杨声海；

【作者基本信息】 中南大学 ， 材料冶金， 2011， 博士

【摘要】 随着超大规模集成电路的发展,器件的特征尺寸越来越小,栅介质层不断减薄。隧穿电流随栅介质层厚度减薄而呈指数级增加,导致功耗的急剧增加和器件的可靠性降低。解决这一问题的方法之一是使用高介电常数材料代替传统Si02栅介质层。Ta2O5因其具有较高的介电常数(约26),以及与目前集成电路加工相兼容等突出特点,已被认为是有希望作为动态随机存储器电容元件材料和甚大规模集成电路栅介质材料的替代品之一。钽醇盐是化学气相沉积及原子层沉积制备氧化物薄膜的常用前驱体。针对目前氨法制备钽醇盐存在流程长、环境差、成本高、回收率低等缺点,我们提出了电化学合成与纯化制备钽醇盐新技术。本论文系统研究了钽在含有支持电解质乙醇溶液中的电化学行为、乙醇钽的电合成及其精馏提纯的工艺和理论,得出了一些有意义的结果。运用循环伏安、线性极化、计时电流和交流阻抗等电化学测试方法,研究了钽在Et4NCl、Et4NBr和Bu4NHSO4为支持电解质的乙醇体系中的电化学行为。结果表明,当给钽施加正电位时,钽在Et4NCl或Et4NBr乙醇溶液中发生点蚀,在Bu4NHSO4乙醇溶液中发生均匀腐蚀。扫描速率增加时,钽在Cl-或Br-中的点蚀电位升高；而在HSO4中的电流密度和扫描速率的平方根成正比。溶液温度升高会加速钽在乙醇中的溶解,钽在Cl-\Br-1以及HSO4乙醇溶液中发生电化学反应的活化能分别为36.25、44.17、43.39kJ/mol。随着支持电解质浓度增加,点蚀电位降低；而钽在HSO4乙醇溶液中的钝化膜厚度会随HSO4浓度增加而减薄。在含有Cl-或Br-的乙醇中,电位大于2.0 V时发生点蚀,且水含量增加,点蚀电位和保护电位增加,电流密度急剧下降；在含HSO4的乙醇体系中,钽仍处于点蚀诱导期,但电流密度随水含量增加有所增加。电化学合成过程中,槽电压在电化学合成初期急剧下降,在中期趋于稳定,在后期逐步上升；电流密度和槽电压基本成线性关系。在相同条件下,以Bu4NHSO4为支持电解质的槽电压和电流效率最高,Et4NCl其次,Et4NBr最低。槽电压随极距、电流密度增加而增加,随Bu4NHSO4浓度、电化学合成温度增加而减小。电流效率均超过100%,可能和钽存在低价态有关。精馏提纯乙醇钽过程中,在气相温度为46.7℃、气压为200Pa时,有饱和碳氢化合物馏出。精馏时间对产品质量的影响较小,精馏中期产品稍微好于精馏前期和精馏后期。随金属钙添加量的增加,乙醇钽产率减小且精馏时间延长。回流比的增加使乙醇钽纯度提高,特别是铌和硫含量大幅降低,但产品直收率急剧下降,渣率明显上升。以精馏回收后的乙醇为溶剂电化学合成乙醇钽,回收率显著提高,升至90%以上。当采用纯度较高的钽板为原料并在密封性好的精馏装置中进行提纯时,乙醇钽纯度可以达到99.999%,主族元素、活泼金属、过渡金属、铝和铌等杂质元素的含量均优于SAFC Hitech(原Epichem)公司的5N标准。红外光谱和化学键一一对应,并和文献报道结果十分吻合。核磁共振相应的各峰面积之比与氢原子数量比例相吻合。以钽板为阳极,不锈钢板为阴极,以季铵盐溶于相应的醇为电解液,通过电化学合成并蒸馏提纯,成功制备出丙醇钽、异丙醇钽及丁醇钽。产物经红外光谱、拉曼光谱检测,吸收峰和各化学键很好对应。核磁共振对应的各峰面积之比与氢原子数量比例相吻合。ICP-Mass分析结果表明,丙醇钽、异丙醇钽和丁醇钽的纯度分别为99.998%、99.998%和99.997%。丙醇钽挥发性最好,丁醇钽其次,异丙醇钽最差,800℃热处理后的总失重率分别为95%、88%和75%。通过苯甲酸失重速率计算出用于后续计算钽醇盐蒸汽压的k值,通过朗缪尔方程并结合上述k值,计算出三种钽醇盐在蒸发阶段的蒸汽压,和文献报道的数据基本一致。通过克劳修斯-克拉佩龙方程计算出三种钽醇盐的蒸发焓。本论文研究了电化学合成乙醇钽过程中钽阳极的电化学行,可以为合成钽醇盐提供理论指导；探讨了各种因素对电化学合成乙醇钽及其精馏提纯的影响,可以为小规模生产高纯钽醇盐提供技术指导,为产业化的设备选择提供技术原型；研究了钽醇盐热分解特性,可以为MOCVD前驱体的选择提供理论依据。更多还原

【Abstract】 With the fast development of very large scale integration (VLSI), the device feature size scales down, resulting in the thinning of the gate dielectric. The tunneling current increases exponentially as the gate dielectric thickness decreases, which leads to unbearable power consumption and device performance problems. A solution is to use materials with large dielectric constants to replace conventional SiO2 dielectric layer. Tantalum pentoxide (Ta2O5) is considered one of the promising candidates as a capacitor insulator in dynamic random access memories (DRAM) and as an ultra large scale integrated circuits (ULSI) gate dielectric due to its high dielectric constant (about 26) and compatibility with ULSI processing. Tantalum alkoxides are widely used precursors in chemical vapor depositon (CVD) and atomic layer deposition (ALD) processes of oxides. Aimed at some shortages such as long flow path, bad work environment, high cost and low recovery rate, a new preparation technology of tantalum alkoxides using electrosynthesis and purification was proposed in this paper. The electrochemical behaviors of tantalum in ethanol containing supporting electrolytes, the electrosynthesis of tantalum ethoxide and its technology and theory of distillation purification were systematically investigated, and some meaningful results were obtained.The electrochemical behaviors of tantalum in ethanol solutions containing Et4NOl, Et4NBr and BU4NHSO4 were investigated using cyclic voltammetry, linear polarization, potentiostatic current-time transient and impedance techniques. The results revealed that, when an anodic potential was imposed on the tantalum, it suffered from pitting corrosion in Et4NCl or Et4NBr ethanol solution, while uniform corrosion occurred in BU4NHSO4 ethanol solution. The pitting potential of tantalum in ethanol solution containing Cl- and Br- increased with the increase of scan rate, while current density of tantalum in ethanol solution containing HSO4 was proportional to the square root of the scan rate. The increase of solution temperature could accelerate the dissolution of tantalum, and the apparent activation energy of tantalum in ethanol solution containing Cl-, Br- and HSO4 ions were 36.25,44.17 and 43.39 kJ/mol, respectively. The pitting potential of tantalum in ethanol solution decreased with the increase of supporting electrolyte concentrations, while the passive film thickness of tantalum in ethanol solution containing HSO4 began to thin with the increase of HSO4 concentrations. In the ethanol solution containing Cl- or Br-, the pitting corrosion occurred when the potential was higher than 2.0 V, the pitting potential and repassivation potential increased but the current density decreased with the increase of water content, while in the ethanol solution containing HSO4, tantalum was still in the incubation period and the current density increased with the increase of water content.The cell voltage sharply decreased at the early stage of electrolysis, and then began to stabilize in the medium term, finally increased gradually. The current density was basically linear with the cell voltage. Under the same conditions, the cell voltage and current density of tantalum in the solution containing BU4NHSO4 were the highest, followed by Et4NCl, Et4NBr was the lowest. The cell voltage increased with the increase of electrode distance and current density, but decreased with the increasing BU4NHSO4 concentration and solution temperature. All the current efficiency was higher than 100%, which was related to low valence of tantalum.Saturated hydrocarbon was distilled when gas phase temperature was 46.7℃at 200 Pa pressure. The distillation time had little effect on the product quality, and the quality of the middle stage was somewhat better than that of the early and late stage. The yield of tantalum ethoxide decreased and the distillation time was prolonged with the increase of calcium weight. When increasing the reflux ratio, the purity of tantalum ethoxide was enhanced, the direct recovery rate of the product sharply decreased, and the residue rate rose obviously. The recovery rate significantly increased and it could rise to 90%when using the ethanol recovered from distillation as solvent in the electrosyntheis of tantalum ethoxide. The purity of tantalum ethoxide could reach 99.999%by using high purity tantalum plates as raw materials and purifying in the distillation device with excellent sealing performance, and the impurity contents such as main group elements, reactive metals, transition metals, aluminium and niobium were superior to that of 5N standard of SAFC Hitech company. Infraed spectrum conformed to chemical bonds excellently, which was consistent with the results reported in literature. The peak area ratio of nuclear magnetic resonance coincided with number ratio of hydrogen atoms.Tantalum propoxide (Ta(OPrn)5), tantalum isopropoxide (Ta(OPr’)5) and tantalum butoxide (Ta(OBun)5) were successfully prepared by electrosynthesis and distillation purification, using tantalum plate as anode, stainless steel plate as cathode and solutions of quaternary ammonium salt in the corresponding alcohol as electrolytes. Measured by fourier transform infrared spectra (FT-IR) and raman spectra, the absorption peaks of the product were excellently consistent with chemical bonds. The peak area ratio of nuclear magnetic resonance coincided with number ratio of hydrogen atoms. The analysis results of ICP-Mass revealed that the purity of Ta(OPrn)5, Ta(OPrl)5 and Ta(OBun)5 was 99.998%,99.998% and 99.997%, respectively. The volatility of Ta(OPrn)5 was the best, followed by Ta(OBun)5, and Ta(OPr’)5 was the poorest, and the total weight loss rate(heat treatment at 800℃) was 95%,88% and 75%, respectively. The value of k was obtained by calculating the weight loss rate of benzoic acid, and the vapor pressure of tantalum alkoxides was calculated using Langmuir equation and k value aforementioned. The data of vapor pressure were basically consistent with the results reported in literature. Enthalpies of vaporization for tantalum alkoxides were calculated from the vapor press-temperature data using the Clausius-Clapeyron equation.In this paper, the electrochemical behaviors of tantalum in the electrosynthesis of tantalum ethoxide were investigated, which could provide theoretical guidance for the electrosynthesis of tantalum alkoxides; the influence of various factors on the electrosynthesis and distillation purification of tantalum ethoxide was studied, which could provide technical guidance for the small scale production of high purity tantalum alkoxides, and could provide technical prototype for the equipment selection of industrialization; the thermal decomposition characteristics of tantalum alkoxides were also studied, which could provide theoretical basis for the selection of MOCVD precursors.更多还原