【作者】 吴剑；

【导师】 王继扬； 张怀金；

【作者基本信息】 山东大学 ， 材料学， 2008， 硕士

【摘要】 随着通讯、信息产业的迅速发展,各种光电子元件得到快速的发展并趋于高性能化。为此,对光电子元件的关键零件—基片的材料和加工精度提出了新的要求。要求基片晶体材料具有优良的压电、光电和热电性能;对元件基片加工精度的要求甚至达到纳米级;要求基片晶格具有无畸变的超光滑无损伤表面等。基片表面存在任何微小缺陷都会破坏晶体材料表面性能,甚至导致其特性的变化,影响元件的工作精度和可靠性。此外,还要求高的加工成品率。光电子元件基片为典型的超薄零件,如手机声表面波滤波器基片厚度350 um,刚性差,难以有效满足加工精度和加工质量要求。因此,要满足现代光电子元件苛刻的精度和表面质量要求,如何获得具有超光滑表面的基片,便成为从事该领域研究的人们多年来苦苦探寻的工艺问题。钽酸钾（KTaO₃）晶体是一种典型的钙钛矿型晶体,从室温到熔点间无相变,具有不潮解,热机械性能优良等优点,可以采用提拉法获得大尺寸高质量的单晶。此晶体与各种钙钛矿型氧化物超导薄膜具有良好的品格匹配和结构匹配,在高温下具有较好的化学稳定性、热传导性和绝缘性,在高额时具有很小的介电常数和损耗,是一种很有实际应用的高温超导（High Temperature Superconductor）薄膜的衬底材料。由于晶体加工表面严重地影响在其表面所生长的高温超导薄膜的性能,所以如何获得超光滑无损伤的基片表面是一个亟待解决的问题。目前,实际生产所采用的机械抛光工艺存在抛光效率低、加工质量不稳定、碎片率高等问题。目前对单晶KTaO₃超精密加工技术的研究较少,没有一套完整的专门适合于单晶KTaO₃的超精密加工工艺。因此,本文对KTaO₃晶体的生长、结构、缺陷、基本性质及其加工进行了系统的探索,其主要包括以下工作:一、KTaO₃晶体生长采用纯度为99.99%的K₂CO₃和Ta₂O₅通过固相反应合成出的KTaO₃多晶原料,利用熔体提拉法成地生长出了一系列大尺寸、无开裂、高质量的KTaO₃单晶。由于晶体生长是一个复杂的物理-化学过程,我们结合晶体生长热力学和动力学规律,系统讨论了影响晶体生长和晶体质量的主要因素。包括优质多晶原料的制备和建立合理温场的设计;控制合适的生长工艺参数,选用优质籽晶,消除外界因素的影响等。同时针对不同因素的产生原因和影响特点,提出了相应的解决办法。二、KTaO₃晶体组分、结构与缺陷利用JXA—8800型电子探针技术分别测量了K元素、Ta元素和O元素在生长的KTaO₃单晶不同位置的浓度。测量结果表明在富K体系所生长的KTaO₃单晶是非化学计量比的,组分均匀性良好不过仍存在氧空位缺陷。利用X射线粉末衍射和锥光干涉图对生长的KTaO₃晶体进行了结构研究。根据X射线粉末衍射结果,确定生长的KTaO₃晶体属于立方晶系,利用Dicvol91计算立方晶胞参数为:a=3.9884（?）。采用化学腐蚀法和高分辨X射线衍射观测了KTaO₃晶体缺陷的完整性,并结合晶体生长探讨了缺陷的形成机制,为生长优质晶体提供了依据。在KTaO₃晶体中存在的主要缺陷为位错和小角晶界。根据这些缺陷的形成机制,采取了相应措施,不断改进生长工艺条件,有效地减少或消除了晶体中的这些缺陷。三、KTaO₃晶体的基本性质测量了KTaO₃晶体的密度、硬度、线性光学性质和热学性质,并讨论了这些基本性质对晶体生长和应用的影响。采用浮力法测得KTaO₃晶体的密度为7.0146 g/cm³。利用莫氏硬度计测得KTaO₃晶体的莫氏硬度为6左右。采用V型棱镜法测量了KTaO₃晶体在入射光波波长633 nm和1539 nm的折射率分别为2.2256和2.1520。利用Hitachi U—3500型IR—UV分光光度计,测量了室温下KTaO₃晶体在340～3500 nm波段范围内的透过光谱,晶体在此波段的透过率为70～80%,仅在波长2875 nm附近有一OH^-吸收峰。系统地研究了KTaO₃晶体的热学性质。通过高温差示扫描量热仪,在303.15K～843.15K的温度范围内测得KTaO₃晶体的比热为KTaO₃:0.371～0.385jg^-1K^-1(99.428～103.185 Jmol^-1K^-1)。采用热膨胀仪在298.15～773.15K温度范围内测量获得a_c=6.44×10^-6/℃,a_a=a_b=6.63×10^<sub>-6/℃。我们还测量了KTaO₃晶体的热扩散系数并计算了热导率。在室温附近,KTaO₃晶体的热扩散系数为3.259 mm²s^-1;而该晶体在室温下相应的热传导系数为8.551 Wm^-1K^-1。讨论了热学性质对KTaO₃晶体的生长和应用所产生的影响。四、KTaO₃基片的超光滑表面加工用化学机械抛光（Chemical Mechanical Polishing,CMP）方法加工单晶基片表面,通过化学和机械的交互作用去除表面损伤层,高效地获得超光滑无损伤的单晶KTaO₃基片表面。并在不同工艺参数下分别对单晶KTaO₃基片进行CMP实验,对比分析了不同工艺参数的作用机理及抛光效果,选择出一种适合单晶KTaO₃基片CMP的参数。通过正交实验,对CMP工艺参数进行优化,获得了较好的基片表面质量,表面粗糙度Ram值达到7.52（?）（AFM测量结果）,提高了加工效率,降低了成本。在大量的CMP实验基础上,探讨了影响CMP过程的主要因素（如抛光垫、抛光液、抛光压力、抛光盘转速等）对单晶KTaO₃基片抛光效果的影响规律。更多还原

【Abstract】 With the rapid development of communication and information,diversified photoelectronic devices develop at a tremendous speed and come to the trend of advanced performance.Therefore,the quality for the key part of photo-electronic devices-the materials of substrates and the degree of processing nicety-has to face newer and stricter standard.That is to say,substrates single crystal material must have excellent qualities of piezo-electricity,photo-electricity,thermo-electricity;the degree of processing nicety of device substrates must amount to the level of nanometer;the crystal lattices must have super-smooth surfaces without distortion or damage.Because any tiny flaw in surface of substrates will destroy the surface performance of crystal materials and even arouse to the change of the structure of crystal lattices,witch will affect the processing nicety and reliability of devices.Furthermore,high rate of finished products also be required.However,the substrate of photo-electronic devices,as typical ultra-thin part,such as the thickness of the substrate of the surface wave filter of cell-phone just amount to 350 um,has bad stiffness and cannot meet the requirement of processing nicety and quality.As a result,how to meet the requirement of the high nicety of modern photo-electronic devices and surface quality and how to gain the substrate with super-smooth surface are the very processing methods that people in this field have explored for many years.Potassium Tantalate single crystal（KTaO₃）which has no phase transition from room temperature to melting is a topically isostructural perovskite.This crystal is good thermal and mechanical properties,can be grown for high quality by using the Czochralski method.There is small crystal lattice mismatching and structure matching for extending kinds of perovskite oxide film.KTaO₃ crystal has such performances as good chemical stability,heat-conduction and insulation under the high temperature, very low dielectric constant and dissipation in high frequency.The substrate surface quality,such as surface roughness,micro-structure directly influences the quality of High Temperature Superconductor thin film grown on it.So,it is an important problem how to get KTaO₃ substrate with high surface quality.Now,there are some disadvantages in mechanical polishing of single crystal KTaO₃ substrates,such as low material removal rate（MRR）,poor surface quality and high percent off ragment waste. Moreover,to the author’s best knowledge,systematic study on ultra-precision polishing of KTaO₃ substrates has not been reported.Thereby,in this dissertation,the growth, structure,defects,properties and ultra-precision polishing of KTaO₃ crystal are systematiclly studied and discussed.The outline of this dissertation is as follows:1.Crystal GrowthKTaO₃ polycrystalline material was synthesized by means of a solid-state reaction with 4N K₂CO₃ and Ta₂O₅.Free-cracked KTaO₃ single crystals with high optical quality and large dimensions were successfully grown by using Czochralski method.Single crystal growth is a complex process,the factors that affect quality of crystals during were analyzed,according to the theories of thermodynamics and dynamics.High quality polycrystalline material and a suitable filed of temperature are the precondition to grow an excellent crystal;the optimal technics is the key for crystal growth;using a good crystal seed and eliminating outside influence can ensure the quality of crystals.We also presented the resolvents according to the characteristics of these factors.2.Crystal Components,Structure and DefectsThe concentrations of K,Ta and O elements in the different parts of as-grown KTaO₃ single crystal were measured by the JXA-8800 electron probe analysis method. The measured results indicate that as-grown KTaO₃ in rich K melt is non stoichimetric KTaO₃ and has high homogeneousness in components,yet exist obvious O vacancy in crystals.The structure of as-grown KTaO₃ single crystal was studied by X-ray powder diffraction combined with conoscopic diagram method.The X-ray diffraction confirmed that as-grown KTaO₃ single crystal belongs to the cubic crystal system,and calculated unit cell dimensions a=3.9884（?）by Dicvol 91 software.Defects in the as-grown KTaO₃ single crystal have been studied by using the chemical etching method and high-resolution X-ray diffraction.The results of observations and discussion of growth defects and formation mechanisms in as-grown KTaO₃ single crystal were presented in this dissertation.It is important for us,since the investigation can guide the growth of single crystal with high perfection.The main defects in the as-grown KTaO₃ single crystal are dislocations and sub-grain boundaries. These defects seriously affected the perfection of crystals.In order to avoid or decrease these defects,we take some corresponding steps according to the above results.3.Basic PropertiesThe density,hardness,linear optical properties and thermal properties of the as-grown KTaO₃ single crystal were measured,and influences of these properties on crystal growth and applications were also discussed.The density of KTaO₃ crystal was measured by using the buoyancy method at room temperature and its value is 7.0146 g/cm³.The Mohs hardness of KTaO₃ was about 6 by using Mohs sclerometer.The refractive indices of KTaO₃ single crystal was measured by using V-prism method at Hg lamp e line（λ=633nm andλ=1539nm）and its value is 2.2256 and 2.1520.The transmission spectra were measured with Hitachi U-3500 FIIR spectrometer at room temperature and the results show that KTaO₃ crystal has a broad transparent wavelength band.The cut-off wavelength of the crystals is about 340 nm.From 340 nm to 3 500 nm,only an absorption band between 2 847 nm and 2 910 nm appears,and the peak is located at about 2 875 nm.The thermal properties of KTaO₃ single crystal were systemically studied.The specific heat were measured by the method of differential scanning calorimetry,which values is 0.371～0.385 jg^-1K^-1in the temperature range of 303.15 K～843.15 K.The thermal expansion of KTaO₃ single crystal was measured by using a thermal dialatometer and the average linear thermal expansion coefficient were calculated in the temperature from 298.15 to 773.15K,which values are ac=6.44×10^-6/℃,a_a=a_b= 6.63×10^-6/℃.The thermal diffusion coefficient of KTaO₃ single crystal was measured by the laser flash method in the temperature range from 298.15 K to 773.15 K and its value decreases with increasing temperature.At room temperature,the thermal diffusion coefficients are 3.259 mm²s^-1,the calculated thermal conductivities of KTaO₃ single crystal are 8.551 Wm^-1K^-1.The influences of these thermal properties on crystal growth and applications were also analyzed.4.Crystal Ultra-precision PolishingThe chemical mechanical polishing（CMP）technology of KTaO₃ substrate is studied in order to efficiently obtain the ultra smooth and free damage surface of KTaO₃ substrates by use of the chemical and mechanical interaction.A series of CMP experimental studies on different technics parameter on the surface quality of KTaO₃ substrate were carried out.According to analysis result of different parameter,we have compared to different polish mechanism and polish effect,summed up optimal CMP technics parameter for KTaO₃ crystal.Finally,the suitable CMP parameters for single crystal KTaO₃ are obtained by using the orthogonal experiment method.It is proved that the surface roughness of KTaO₃ substrate can reach Ram=0.752 nm and the MRR of KTaO₃ substrate is improved.A series of CMP experimental studies on the influences of the main factors（polishing pad,polishing slurry,polishing load,table rotational speed） on the surface quality of substrate were carried out.更多还原