Study On Thermal, Mechanical And Electrical Properties Of Meso-Porous Silicon Materials In MEMS

Due to its very large surface to volume ratio, intense visible photoluminescence, good chemical stability and easy compatibility with modern standard IC technology, porous silicon (PS) has received considerable attention in various fields such as silicon-on-insulator (SOI), micro-electro-mechanical-system (MEMS), micro-sensors technologies and so on. In recent years, with the rapid development of MEMS technologies, PS has been utilized as new kinds of sacrificial and thermal isolation materials. This very promising material has been an object of numerous MEMS applications in fabrication of chemical microsensors, thermal microsensors, optoelectronics and solar cell because of its excellent mechanical and thermal isolation properties. As a part of PS technologies, meso-PS has been effectively investigated in the above-mentioned MEMS applications for its moderate pore-size and porosity, excellent thermal isolation and mechanical properties.In this paper, meso-PS was prepared in a double-tank cell by using the electrochemical corrosion method. The thermal, mechanical and electrical properties of meso-PS in MEMS were investigated in detail. Besides, the thermal isolation properties of meso-PS as functional isolation layer used in metal film and semiconductor thermoresistors were also thoroughly analyzed.Thermal conductivity (TC) of meso-PS was measured using a direct non-contact and non-destuctive technique based on micro-Raman scattering spectroscopy. The affection of experimental conditions and post-oxidation process on its TC was studied. The comparison of measured data was also analyzed. TC values of meso-PS with respect to porosity and post-oxidation process were given. Theoretical models describing mechanisms of heat transfer in as-prepared and oxidized meso-PS based on the effective medium theory were brought forward. The factors affecting effective thermal conductivity (ETC) of as-prepared and oxidized meso-PS were analyzed theoretically, and the calculating formulas of ETC of as-prepared and oxidized meso-PS were given. The great difference between TC of meso-PS and that of silicon wafer was also revealed. It is shown that theoretical values are quite in good agreement with experimental data and this research has contributed to the construction of a systematic theoretical base for the usage of meso-PS as thermal isolation material in future. The characteristic dimensions of meso-PS films are so small that their mechanical parameters can not be measured by traditional mechanical testing methods. Due to its easy operation, high-resolution, characterization of mechanical behavior of materials in a very small region, nanoindentation technique has been widely adopted and used in the characterization of mechanical behavior of materials at small scales. For this reason, the method has become a primary technique for determining the mechanical properties of thin films and small structural features. The hardness and Young's elastic modulus of as-prepared and oxidized meso-PS with respect to nanoindentation depth were thoroughly investigated using nanoindentation. The mechanical properties of oxidized meso-PS under various oxidized temperatures were also compared. The experimental results reveal that the hardness and Young's elastic modulus of as-prepared meso-PS decrease with increasing porosities. SiO2 cladding layers are formed after post-oxidation process at different temperatures, and the mechanical properties of the films are distinctly improved.Based on the investigation of the thermal and mechanical properties of meso-PS, the electrical properties of as-prepared and oxidized meso-PS were deeply discussed. According to the theories of metal-semiconductor contacts, the longitudinal and transverse contact properties of Pt/as prepared or oxidized meso-PS/monocrystalline silicon microstructures were analyzed. Then their I-V characteristics with respect to preparation conditions and post-oxidation process were known. These results indicate that meso-PS layers have excellent electrical insulation properties and microdevices with meso-PS can obtain stable electrical contacts. I-V characteristics of meso-PS based microstructures exhibit nonrectifying contact properties and are mainly determined by the electrical properties of meso-PS layers.Owing to the favorable thermal isolation, mechanical stability and electrical insulation of meso-PS, its usage as thermal isolation layer in thermal microsensors was thoroughly studied. Taken Cu thin films exhibiting a positive temperature coefficient (PTC) characteristic and vanadium oxide thin films exhibiting a negative temperature coefficient (NTC) characteristic as thermal-sensitive devices, the analysis of thermal isolation properties of meso-PS functional structural layers were undergone. The resistance-temperature properties of corresponding thermal-sensitive devices were also investigated. It is found that these thermal sensitive films show fine resistance-temperature properties on account of the excellent thermal isolation of meso-PS. A wide range of materials can be utilized as thermal-sensitive materials in the fabrication of thermal microsensors with meso-PS functional structural layers, and consequently, the applications of meso-PS functional structure layers can be broadened.