Deposition Of Nanoscale Films And Ellipsomctry Study Of Their Optical Properties

Historically, almost every step up the ladder of science is implied by the advance of measurement technology. In some sense, advances in science can be attributed to various measurement techniques which are used for research. In the past fifty years, many new characterization techniques have been established, such as scanning tunneling microscope (STM) which has lead a revolution in surface science and has improved the rapid progress of surface science.Nevertheless, ellipsometry, which is an important technique for surface science, was established more than100years ago, but it has been seen as an’unproductive instrument" until recently. In1990s, with the rapid progress of computer, automatic measurement and data analysis of ellipsometry was realized. In the mid of1990s, the commercialization of spectroscopic ellipsometry started. By now, spectroscopic ellipsometry has become a widely used technique which is applied in semiconductor, biology and many other fields. In recent years, spectroscopic ellipsometry has realized the real-time characterization of film growth and has been applied in anisotropic optics. Spectroscopic ellipsometry has established its position in optical high-precision characterization. It has been used by more and more universities, departments and companies for fundamental research, energy industry, biology and medical fields. However, the principles of ellipsometry are usually thought to be difficult. It is partly because people have little knowledge of polarization light which is used as probe in ellipsometry and partly because the ellipsometry is an indirect measurement and needs a unique data analysis.With the rapid development of micro-electronics, integrated optics, solar cell industry and medical industry, thin film has played a more and more important role in our life. Due to its extremely high precision, convenient operation and non-destructive character, spectroscopic ellipsometry has irreplaceable importance in films research. Recently, with the improvement of industrial technology and processing level, people begin to realize the importance of ultrathin films which has significantly different properties from bulk material. Human thirst for ultrathin films which are tens of nanometers or even several nanometers thick has grown. In this situation, the research for ultrathin films has become essentially important. However, ellipsometry has trouble with ultrathin films measurement. When the thickness of ultrathin films is small enough, the ellipsometry measured data is not sensitive to the variety of film thickness or optical constants, which means traditional ellipsometry is not capable to obtain film thickness and optical constants of ultrathin films simultaneously.Based on spectroscopic ellipsometry, our group has studied some metal, nonmetal and semiconductor films with nanoscale thicknesses. We have investigated the technology of PECVD deposited silica films and its effects on films properties. Optical constants and bandgap of topological insulator materials films have been measured. We have studied the optical constants of ultrathin aluminum films and their thickness dependence. We also investigated the technology of magnetron sputtering of tantalum films and its relationship between the sputtering condition and the properties of deposited films. Based on previous research, we have improved the traditional ellipsometry and proposed an original and effective method to solve the problem of ellipsometry investigation on ultrathin films. The work of this dissertation will be described in the following aspects:Firstly, we explored the properties of PECVD deposited silica films. And we have studied the effect of substrate temperature on the optical properties of deposited silica films with PECVD by using spectroscopic ellipsometry and XPS. We found that with increase of the substrate temperature, the impurities of silica films deposited by PECVD decrease dramatically and the density of silica films have increased.We have studied the optical properties and bandgap of Bi2O3and Bi2Se3ultrathin films prepared by MBE technique by using spectroscopic ellipsometry. By updating the evaluation function, we obtain and firstly report the optical constants of Bi2O3and Bi2Se3ultrathin films in the visible range. We found that the thickness dependence of refractive index of these two films are similar but and the thickness dependence of extinction coefficient of these two films are different. The calculated bandgap of them are smaller than corresponding bulk materials.Secondly, we studied the thickness dependence of ultrathin aluminum films and firstly report the optical constants of ultrathin aluminum films with thickness of2-5nm. By the construction and comparison of different models, we found the best model to describe our samples and obtain the optical constants of ultrathin aluminum films with thickness of2-16nm. We found that the thickness dependence of refractive index and extinction coefficient of these ultrathin aluminum films are opposite. An absorption peak of ultrathin aluminum films with thickness of12nm and16nm is observed and it shifted to longer wavelength with the increase of film thicknesses.Thirdly, we have studied the effect of sputtering power and substrate temperature onto the morphology and optical constants of ultrathin tantalum films.1. A set of tantalum films were deposited by magnetron sputtering and measured by AFM to observe their morphology. The effect of sputtering power and substrate temperature has been explored. The formation of hillocks on the surface has been studied. We eventually find the optimized condition to obtain tantalum films with hillock-free surface.2. The tantalum films were measured by spectroscopic ellipsometry and a dispersion law of Drude function and Lorentz function was constructed to describe the optical constants of ultrathin tantalum films. Then the optical constants of ultrathin tantalum films deposited under different conditions were obtained by a fitting procedure. We analyze the effect of sputtering power and substrate temperature onto the optical constants of tantalum films. Abnormal low optical constants were observed in the tantalum film with extremely low film stress.Fourthly, a method of combing ellipsometry and transmission has been used to investigate the optical constants with different thicknesses.1. We originally perform an ex situ ellipsometry measurement of ultrathin iron films by covering a protective layer of tantalum material.2. We perform a combined ellipsometry and transmission measurement of double-layer ultrathin metal films by depositing the same films on transparent and opacity substrates. Study on an extra single-layer tantalum film reduced the unknowns in the double-layer ultrathin metal films and make it possible to investigate them by spectroscopic ellipsometry.3. A point-to-point fitting program based on Matlab was written by using the ant colony algorithm. We have optimized the program to make it effective when searching the solution space and improve its ability to search the global optimal solution.4. A uniqueness test proved that the combined ellipsometry and transmission measurement has improved the uniqueness of the obtained film thickness a lot and we obtain the film thickness of ultrathin iron films by it. Then the optical constants of ultrathin iron films with different thicknesses have been obtained. The results show that the optical constants strongly depend on film thicknesses. The ultrathin iron film with a thickness of7.1nm has significantly different optical constants from other samples. There is a broad absorption peak observed at the370nm and it shifted to longer wavelength with the increase of film thicknesses. We have explained the mechanism of it theoretically.Fifthly, we have originally performed an ellipsometry study on the ultrathin native oxide film of iron by using an effective-substrate method. We have obtained the film thickness and optical properties of the ultrathin native oxide film of iron.1. Spectroscopic ellipsometry measurements have been performed continuously on the iron substrate exposed to natural environment. We found that with the increase of exposure time, the shape of the curve of measured data does not change obviously but gradually decreases.2. A spectroscopic ellipsometry study of an unknown ultrathin film with unknown film thickness on an unknown substrate, i.e. an ultrathin native oxide layer of iron with unknown film thickness and optical constants on an iron substrate with unknown optical constants was originally performed by using an effective-substrate method with the help of calculation of pseudo dielectric function.3. We originally obtain the film thickness of ultrathin iron native oxide layer which is1.5nm by a two-dimensional uniqueness test. We also obtain the optical constants and bandgap of the oxide film which is smaller than the artificially deposited ferric oxide films and then the optical constants of the iron substrate are also obtained.Sixthly, a spectroscopic ellipsometry measurement system with high-precision temperature control is established. We study the optical constants of GaAs substrate by using the system and realize a spectroscopic ellipsometry with automatic high-precision temperature control function during a wide temperature range. We update the Duwar-flask by adding measurement windows, inject hole of liquid nitrogen, vacuum valve, electric circuit and cold-heading on it. By the balance of liquid nitrogen and heating-wire, we realize the spectroscopic ellipsometry in vacuum with high-precision temperature control function.In a conclusion, in this dissertation we explore the depositing technology of silica films and the relationship between the deposition and its optical properties. We also investigate the ultrathin film of topological materials and firstly report their optical constants in the visible wavelength range. We obtain the optical constants of ultrathin aluminum films with different film thicknesses by a spectroscopic ellipsometry measurement and analysis. The relationship between the depositing condition and the surface morphology and optical constants of ultrathin tantalum films has been investigated. Based on previous work, we proposed original methods to solve the difficulties of spectroscopic ellipsometry study on ultrathin films. We eventually solve problems of ex situ spectroscopic ellipsometry investigation of ultrathin iron films and unknown ultrathin films on an unknown substrate by using our proposed methods.