| Porous SiO2 films have been identified to possess many outstanding properties dependent on both chemical composition and structure, such as low dielectric constant, refractive index, and thermal conductivity, large specific surface area, high mechanical strength and thermal stability, and good adhesion to substrates. They have potential applications as low dielectric layers in semiconductor devices, thermal insulation layer in infrared detectors, and anti-reflection coatings in optical devices. The sol-gel method is an effective route to porous SiO2 films. However, the crucial problems in sol-gel process are the cracking and densification of gel-derived films during heating and isothermal processes. This dissertation focuses on the preparation of mesoporous SiO2 films with micron thickness and high porosity through a modified sol-gel method, and the properties of porous SiO2 films.Boiling-point-gradient solvent, consisted of ethanol, isopropanoland, and 2-methoxyethanol, were used to prepare SiO2 sol with tetraethoxysilane (TEOS) as silicon source, nitric acid or ammonia as catalyst. The property of SiO2 sol was studied and characterized by viscosity. The effect of factors (radios of raw materials and temperature) on viscosity of sol system was discussed in details. In addition, a kinetic equation was established for the nitric acid-catalyzed TEOS-ethanol system.Side-chain polyether modified polydimethylsiloxane terminated with Si-CH3 (CH3-PDMS) was introduced to the TEOS-sol system as modifier, resulting in a homogeneous hybrid SiO2 sol with low surface tension, high viscosity and stability, which was very suitable for the sol-gel process. The FT-IR, Solid 1H NMR, and DSC/TG analysis showed that the decomposition of CH3-PDMS varied in a wide temperature range, and the SiO2 system had a gentle decomposition process in the presence of CH3-PDMS.Porous silica films were fabricated via spin-coating process and annealed at 650oC. Crack-free silica films with single-coating thickness about 700nm could be obtained from CH3-PDMS modified sols. The acid-catalyzed sol was favorable coating sol since it could form better films than base-catalyzed sol. The CH3-PDMS, regarded as a viscoelastic material, played an important role in restraining stress during the annealing process. However, the cracking would occur with CH3-PDMS content beyond a certain range, and the relative reasons were discussed. The stress evolution on gel-to-ceramic film conversion was also investigated in situ on thin film stress measurement machine. The results revealed a slow decrease in stress below 250oC and a slow increase above 250oC, which was closely related to the alteration of chemical composition during the heat-treatment process. In addition, as another polyether-modified polydimethylsiloxane terminated with reactive Si-OC2H5 groups was employed to above hybrid silica sol to synthesize silica films, the stress of gel films during thermal treatment was further restrained, which was suited for large-area and crack-free porous silica film synthetization on silicon wafer. However, the polymer terminated with Si-OC2H5 groups was easily hydrolyzed, leading to inhomogenous system. Hence, the polymer with terminated with Si-CH3 (CH3-PDMS) was preferred.The CH3-PDMS is not only suggested as stress release agent but also as porogen, leading silica film to forming mesoporous structure with average pore size distribution in several nanometers. The porosity of porous silica films varied from 30% to 50% depending on the CH3-PDMS content. The role of porogen of CH3-PDMS was mainly achieved by the decomposition of polymer chains and the combustion of -CH3 groups. Polyethylene glycol (PEG) was introduced to the CH3-PDMS/SiO2 system to produce more pores in silica films, and the porosity could be increased to above 60%. Microcosmic structure characterization indicated that the annealed silica films presented better surface morphology, larger pore size but narrower pore size distribution. On this basis, oxygen instead of air was served as annealing atmosphere, highly porous silica films with more porous structure were obtained, and the porosity could be about 80%. Thick silica films were prepared by repeating spin-coating sol-gel process. In order to make sure the structural homogeneity of multilayer film with no cracking, CH3-PDMS modified silica sol derived from acid-base two steps were used as coating precursor, each layer of wet films was pretreated at 120oC, and then the obtained multilayer xerogel films were treated by a special annealing process: multi-step heat treatment under different atmospheres, respectively. As a result, crack-free mesoporous SiO2 films with a thickness of 5?m and uniform structure were obtained. Repeated experiments exhibited that the microstructure of silica films could be well controlled by varying some key process parameters.The dielectric properties of porous silica films were characterized by LCR meter test system. The dielectric properties of silica films annealed at different temperatures were investigated in the frequency range in 1200kHz. It was found that the silica films exhibited low dielectric constant (1.93), and the sample annealed at higher temperature exhibited lower dielectric constants but tended to be vulnerable to moisture, which was associated with the structure and chemical composition.The heat-resistant property of the silica films was studied by measuring thermal conductivity with 3?method. The results indicated that the thermal conductivity depended on film thickness, the porosity and annealing temperature. Among of them, the porosity had the greatest influence on the thermal conductivity: as porosity increased from?47 to 80%, corresponding thermal conductivity decreased from 0.262 W/(m·K) to 0.0513W/(m·K). With the increase of film thickness, the thermal conductivity presented an increased trend, which was confirmed to result from interfacial resistance. In addition, the thermal conductivity decreased with the increase in annealing temperature, which was mainly related to the structure and porosity of silica films. |
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