薄膜光学参数的P偏振测量法及其在光气敏传感器中的应用

光化学传感器是近年来传感器研究领域一个热门课题。气体传感器与人类生活密切相关，因而倍受人们的关注。溶胶一凝胶（Sol-gel）技术是当前正在发的、最具竞争力的制备性能优越的光学气敏薄膜的工艺，借助于先进的检测手段，可望获得高灵敏的光气敏薄膜传感器。论文围绕这一研究主题，就薄膜光学参数的P－偏振光双面反射法测量原理及应用、溶胶－凝胶二氧化锡气敏薄膜的制备及特性、基于P－偏振光双面反射的新型薄膜传感器、以及表面等离子体激元共振（SPR）溶胶－凝胶薄膜传感器等四个方面展开了一系列研究工作。论文第一章概述了光学薄膜的成膜技术及其发展方向，重点阐述了溶胶－凝胶薄膜在光学上的应用，并对薄膜光学参数的测量方法作了较为全面的综述。同时，简要介绍了气体传感器的研究现状及基动态，在此基础上，全面概括了光化学传感器的机理，重点讨论了溶胶－凝胶工艺与等离子体激元共振效应在光化学传感器中的应用，并指出了目前存在的问题及今后的发展方向。论文第二、三、四章是论文工作的第一部分，对P－偏振光双面反射法的测量原理及其在膜层参数分析中的应用，作了系统的讨论。第二章首次将P－偏振光双面反射法的原有的数学模型加以推广与改善，用于单面涂膜薄膜系统光学参数的测量。利用平板侧膜系的不对称性，分别测量F－P和P－F两种方式下样品的光强反射比调制曲线，不但可减小系统误差，而且只需单波长的光进行测量。实验上准确测量了偶氮染料掺杂聚甲基丙烯酸甲脂（PMMA）薄膜的光学参数，薄膜折射率、消光系数及厚度的测量精度达10~（－4）、10~（－4）、10~（－1）nm。在此基础上，将4400信号检测分析系统引入薄膜参数的P－偏振光双面反射测量中，理论和实验上考察分析了高斯光束发散角对测量的影响，进一步提高了测量精度，同时，采用等高线分析法对薄膜光学参数测量值的唯一性进行了讨论。第三章中，作者就薄膜的特殊结构－玻璃表面层进行了全面的研究。首先，实验测量了不同化学清洗条件下K9玻璃表面层的光学参分布，并与观测的表面粗糙度及激光损伤阈值相比较，发现经酸、碱洗的玻璃表面较未化学清洗的光洁度高、表面吸收小、且抗激光损伤阈值高，从而为制备高性能的玻璃基底及其薄膜提供了有效的依据。其次，理论上分析了玻璃表面层对不同薄膜系统膜层光学参数测量的影响，结果表明，对于双面镀膜的薄膜系统，折射率与基底相近的弱吸收膜参数的测定，以及单面镀膜的薄膜系统中，折射率与基底相近有较强吸收薄膜参数的测定，必须考虑表面层的影响。实验上测定了PMTES以及SnO_2等薄膜的光学参数，当计及表面层时，理论拟合曲线与实验结果吻合很好。作为P－偏振光双面反射法的一个典型应用，第四章中作者利用该方法研究了光存储相变薄膜光学参数的温度特性。实验上制备并测定了酞菁LB膜及旋涂膜在不同温度退火后的折射率n和消光系数k，发现这两种薄膜不仅表现出了相似的光学常数的数值，而且具有相似的随温度变化的趋势，实验结果与观察到的吸收光谱一致，即薄膜在150 ℃退火时发生由单体向聚集体的转变，而在300 ℃退火后发生聚集体向单体的转变。另外，首次将P－偏振双面反射法与椭偏光谱测量法相结合，用于研究不同温度下制备的Ge-Te半导体薄膜样品的光学参数，准确地获得了被测薄膜材料的光学参数，及其在250nm～830nm波段范围的复折射率曲线，获得的结果对Ge-Sb-Te三元相变光盘的设计制作具有一定的参考价值。本论文的第二部分即第五章中，实验制备了Sb掺杂的溶胶－凝胶SnO_2薄膜，以及Sb:SnO_2/SiO_2复合膜。利用X射线衍射仪、傅立叶红外谱仪、原子力显微镜、UV／VIS／NIR光谱仪等分析手段，研究了两种薄膜的结构、表面形貌及光学性能。实验结果表明Sb掺杂的二氧化锡复合膜具有很高的透过率（可见光波段约95％）、及较强的抗机械损坏能力；薄膜孔隙率高、颗粒尺寸小、比表面积大，因而在气敏材料和光电子学领域中具有潜在的应用前景。论文第六章即论文的第三部分，将P－偏振光双面反射法用于光化学气体传感器的信号检测，建立了一种新型传感器结构。理论上分析了此类传感器的灵敏度随膜层折射率、消光系数及膜厚等光学参数变化的关系。数值模拟表明，通过选择合适的参数，该传感器对膜层折射率、消光系数及膜厚的灵敏度高达10~4、10~2及10~3以上，相应的侧量分辨率为10~（－7）、10~（－5）及10~（－3）nm。实验上考察了提拉速率与热处理温度对溶胶－凝胶SnO_2膜光学参数的影响，薄膜的气敏实验则充分证明了光学参数设计的必要性。气体选择性实验表明，Sb掺杂的SnO_2膜及复合膜对C＿2H＿5OH具有更高的灵敏度。因此，P－偏振光双面反射法可作为光化学传感器的高灵敏度检测方法。论文第七章即论文的第四部分，首次采用溶胶－凝胶金属氧化物半导体薄膜，作为表面等离子体激元共振（SPR）效应的光化学传感器的传感介质。理论分析了该传感器的灵敏度随膜层光学参数变化的关系，拟合结果表明，选择合适的参数该传感器对膜层折射率的灵敏度高达10~5以上。通过在金属膜与传感层之间增设中间层，可进一步改善和提高传感器的灵敏度，经优化的灵敏度较没有中间层结构的传感器高出约40％。实验上选择SnO_2薄膜及SiO_2薄膜作为传感层与中间层，气敏实验充分证明，结构优化的传感器具有更高的灵敏度。因此，SPR溶胶－凝胶薄膜传感器，可望在光化学传感器及生物传感器中得到广泛作用。The study on the optical chemical sensor is an attractive research project in recent years. In view of being closely related to human life, the gas sensor is taken very seriously. Sol-gel technique is developing and challenging process technology, which can prepare optical and gas sensitive materials with good performance. With the aid of advanced ways and means of signal detection, the optical gas sensor with high sensitivity is probably acquired. In this dissertation, some new results on the application and principle of measurement of the film optical parameter by p-polarized reflectance, preparation and characteristic of sol-gel-derived tin oxide films, novel film sensor based on p-polarized reflectance, and surface plasmon resonance based sol-gel film sensor are presented. In the first part (i.e. chapter one), the optical film preparation technology are reviewed, as well as their developing tendency. A survey of sol-gel film applications in optics is elucidated in detail, and a comprehensive overview on the measurement of film optical parameters is given. In addition, the state-of-the-art of gas sensor is briefly discussed. On the basis of this, the author epitomizes the mechanism of optical chemical sensor, and especially introduces the applications of surface plasmon resonance (SPR) and sol-gel process in optical chemical sensor. Further, the main problems to be solved are pointed out, and an overview is presented of the further development. The measurement principle of p-polarized reflectance and its application in film parameter analysis is studied systematically in the second part (i.e. chapter two, three, and four). In chapter two, the method of p-polarized reflectance is first applied in measuring the optical parameters of films coated on one side of the transparent plane plate by improving the original mathematical model. Based on the asymmetry of this film system, the angular modulation curves of the reflectance ratio of p-polarized reflectance are measured under two incident modes (i.e. F-P and P-F), which is helpful to reduce systematic error and measure with single wavelength light source. Experimentally we have measured precisely the optical parameters of the azo-dye doped PMMA films. The resolution of measuring the extinction coefficient, refractive index and thickness of single films are 10~(-5), 10~(-4) and 0.1nm, respectively. In addition, 4400 signal processor is drawn into p-polarized reflectance measurement, and an influence of divergent angle of Gaussian beam on measurement is analyzed, which improves the measuring precision. Furthermore, the uniqueness of solution of optical parameters is discussed by means of isohypse analysis. In chapter three, the surface layers of the glass-the particular structure of the thin films are studied elaborately. First, the optical profiles of surface layers on K9 glass are measured under different chemical cleaning. By observing their morphology and laser induced damage, it can be seen that the glass surface through acid and alkali cleaning possess high smoothness, laser induced damage threshold, and minor absorption, compared with the glass cleaned by water. This established a solid foundation for preparing high quality glass-substrate film. Second, an influence of glass layers on the measurement of the optical parameters of the films is analyzed. It is clear that glass surface must be taken into account under two circumstance: (1) the weakly absorbing films coated on both sides of the glass substrate whose refractive index is near to that of the films; (2) the higher absorbing films coated on one side of the glass substrate whose refractive index is near to that of the films. Experimentally the author has measured the optical parameters of PMTES and SnO_2 films. The results indicate that the experimental results coincide well with the theoretical simulation only if the glass surface layers are considered. As a typical application, the method of p-polarized reflectance is applied to study the temperature characteristic of optical storage phase-change films in chapter four. The optical parameters of spin-coated and LB films of tetra-neopentoxy phthalocyanine Zinc (TNPPcZn) annealing at different temperatures have been analyzed. The similar value of n and k, as well as the similar changing trend with varied annealing temperatures, is found between LB films and spin-coated films. The results correspond with their absorbance curve in visible range. When the annealing temperature increases to 150 ℃, the monomer of TNPPcZn in both kinds of films transforms to aggregate, n and k of them increases. Furthermore, n and k of them decrease as aggregate changes back to monomer again at the annealing temperature of 300 ℃. In addition, the optical parameters of GeTe semiconductor films at different temperatures have been measured carefully by combining the method of p-polarized reflectance with ellipsometry. The optical parameters of the samples are precisely obtained, as well as the complex refractive index curves of them in the spectrum range from 250nm to 830nm. The results are of some value to design and fabrication of Ge-Sb-Te phase-change disk. In the second part (i.e. chapter five), Sb doped sol-gel SnO_2 films and Sb:SnO_2/SiO_2 compound films are prepared and studied. Their structure, morphology, and optical property are analyzed by use of X-ray diffractometer, Fourier transform infrared spectrometer, ATM microscopy, and UV/VIS/NIR spectrometer. The experimental results show that Sb:SiO_2 compound films have higher transmissivity (visible region: ～95%), and good mechanical performance. So this kind of SnO_2 films probably has a broad prospect for application in gas sensitive material and optoelectronics. In the third part (i.e. chapter six), a novel film sensor based on p-polarized reflectance has been built up. The dependence of sensitivity of this sensor to refractive index, extinction coefficient and thickness of the films on the optical parameters is obtained theoretically. Data simulation shows that the sensitivities of this scheme to refractive index, extinction coefficient and thickness of the films are predicted to be more than 10~4, 10~2 and 10~3, respectively, provided the film optical parameters are suitable. Accordingly the resolutions of refractive index, extinction coefficient and thickness of the films are predicted to be less than 10~(-7), 10~(-5) and 10~(-2)nm, respectively. Experimentally, the influences of dip rate and annealing temperature on optical parameters of sol-gel SnO_2 films are researched. Gas sensitive test shows it is indispensable to design optical parameter elaborately. Further gas selectivity experiment indicates that Sb:SnO_2 and Sb:SnO_2/SiO_2 films have higher sensitivity to C_2H_5OH. Therefore, the method of the p-polarized reflectance can be used as high sensitivity detection strategy in optical chemical sensors. In the last part (i.e. chapter six), the metal oxide semiconductor films have been used as sensing medium in the optical chemical sensor based on SPR for the first time. The influences on the sensor sensitivity of the optical parameters of Ag and sensing films are analyzed. Data simulation shows that the sensitivity of this scheme to refractive index of the films is predicted to be more than 10~5. By adding an intermediate layer between the metal films and the sensing films, the sensitivity can be raised further. Further calculation shows the sensitivity of this sensor with optimal optical parameters can increase by 40% over that of only sensing films. Experimentally, the author has selected SnO_2 and SiO_2 films as sensing and intermediate layers, and made the gas sensing tests to NH_3, C_2H_5OH and C_3H_8. The results indicate the sensor with optimized structure has a higher sensitivity. Consequently, SPR sol-gel film sensor will be widely used in high sensitivity optical chemical and biochemical sensors