Spin-Coated Mesoporous Silica Films from a Surfactant-Templated Method: Preparation and Applications as Low-k Materials and Antireflection Films

• Main Authors: Chih-Yuan Ting, 丁致遠
• Other Authors: Ben-Zu Wan
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/97926441556379886789

博士 === 國立臺灣大學 === 化學工程學研究所 === 92 === The preparation of spin-on mesoporous silica films and their applications as low dielectric constant (low-k) films and anti-reflection (AR) films were studied in this dissertation. The pores were introduced via the surfactant-templated method, and the film was prepared via the spin-coating process. The film thickness control, the effects of the spin-coating process, the effects of the templates, the correlations between the film architecture and the film properties, and the development of the ultra low-k materials along with the AR materials were investigated. The thickness of the spin-on films is related to the solution composition or the spin recipe. In order to control the film thickness, it was expected to find out the significant factors influencing the thickness. To understand more about the spin-coating, the theoretical models developed in the literatures were reviewed. Moreover, it was hoped to evaluate the possibility for developing a theoretical model that was capable of describing the relationship between the film thickness and the spin-coating conditions for the system in this dissertation. However, it was concluded from this research that to obtain an empirical correlation between the thickness and the operating conditions (including the coating solution composition and the process parameters) was more practical and easier than to derive a theoretical model. Therefore, the effects of the solution composition, the spin recipe, and the calcination on the thickness were examined and compared experimentally. From the results of this research, it was found that the most significant factor in the thickness control was the concentration of tetraethylorthosilicate (TEOS) in the colloid solution. The refractive index, the optical microscope, the X-ray diffraction and the nitrogen adsorption/desorption were used for the characterization of the effects of spin-coating and calcination on the mesoporous films made with different templates. The spin-on films were spin-coated at 2600 rpm for 0.5 min. The uniformity of the spin-on films was demonstrated by the uniform refractive indices at different positions. Moreover, the XRD results showed that the spin-coating only caused slightly poorer structural periodicity and sometimes minor variation of the d-spacing. However, the more notable effect on the films was the phase separation of some templates from the coating solution during the spin-coating. For the films templated with the block copolymer Pluronic P123, the phase separation of P123 was observed when the more concentrated P123 coating solution (P123/TEOS=41wt%) was used. The phase separation of P123 resulted in a crack film, whose porosity was less than that prepared with the less concentrated P123 solution (P123/TEOS=20wt%). For the effect of the calcination, on the other hand, it was observed that the calcination only caused the structure of the film templated with cetyltrimethylamonium bromide (C16TMABr) to collapse and shrink. For the films templated with non-ionic surfactants or copolymers, significant shrinkage of the structure was not observed. The thicker walls around the pores in the films templated with non-ionic templates may strengthen the structure and resist the shrinkage. The correlations between the film properties and the film architecture were also studied. It was concluded that the k value and the refractive index can be easily controlled by tuning the porosity of the film; however, both the larger leakage current density and the weaker mechanical strength for the more porous films were critical challenges for practical applications. For the film templated with C16TMABr, an ionic surfactant, the leakage current was too large to be measured. The flat band voltage of the film was positive. It indicated that there were a lot of negative charges (possibly residual Br－ ions from C16TMABr) within the film. These residual ions may result in the ultra high leakage current. Due to the ultrahigh leakage current exhibited by the film templated with C16TMABr, the ionic surfactants did not seem to be appropriate templates for the application as the low-k films. For the films prepared with different types of nonionic templates, the flat band voltages were negative. It indicated that there were positive charges within the film. Moreover, the flat band voltage of the more porous film shifted to a more negative value. It indicated that there were more positive charges within the more porous film. These positive charges may result from the protons of the silanol groups in the film structure. It suggested that there were more silanol groups within the more porous film. The increase of the silanol groups should cause the elevation of the leakage current density, which was consistent with the higher leakage current density of the more porous film observed in this research. For the development of the ultra low-k films, an ultra low dielectric constant (1.47) at 1 MHz was achievable by tuning the parameters such as the colloid composition and the mixing time of the coating solution. Polyoxyethylene(20) sorbitan monooleatel, also known as Tween80, was used as the template. By controlling the concentration of the reactant (H2O), the concentration of the catalysts (HCl), and the reaction time, the hydrolysis and the polycondensation reactions of the sol-gel process can be facilitated. The dielectric constants of the films were found to be decreased with the extent of the hydrolysis and the polycondensation reactions within the sol-gel solution. There may be two reasons for the reduction of the k values. First, the film structure was stronger and thereby retained more pore volume if the hydrolysis and the polycondensation reactions were more complete. Second, the more polarizable silanol groups should be reduced when the polycondensation reactions were more complete. The leakage current densities can be of 10-7 A/cm2 order or lower under an electric field of 1 MV/cm. Moreover, the dielectric constants kept almost the same after the films were exposed in the atmosphere for one month. For the development of the AR films, a high transmittance (99%) at 590nm (wavelength) was achievable by tuning the refractive index and the film thickness of the AR films on the glass (the transmittance was originally about 92%). The advantage, that both the refractive index and the film thickness can be easily controlled, makes the surfactant-templated film quite attractive for being an optical filter material.