Investigation On The Fabrication And Thermoelectric Properties Of Silicon Microchannel Plates

Silicon as the common semiconductor material, has been developed for fifty years, and is widely applied in many fields, such as electronics, physics, chemistry, biology and materials. In recent years, silicon based three-dimensional materials has attracted tremendous attention following the revolution of micro electro mechanical systems (MEMS). As the silicon based material, they are not only compatible with the process of the integrated circuit, but also feature many novel physical and chemical properties, thus are used to the fabrication of sensor and power generation device with high efficiency. The thesis focuses on the fabrication and thermoelectric properties of silicon micro-channel plates (Si MCPs) prepared by electrochemical etching.First, the fabrication process of Si MCPs was developed based on the forming theory of porous silicon. After the amelioration of the photo-assisted electrochemical etching (PAECE) devices and the optimization of the etching conditions, including the etchant concentration, current density, illumination and reaction temperature, the samples with desirable morphology and high aspect ratio could be fabricated. The thickness of the silicon micro-channel layer can even reach 400μm. Meanwhile, when proper etching conditions was addressed, the silicon micro-channel layer could break away from the substrate automatically, which was much valuable to the subsequent applications of the Si MCPs.It is known to all that the bulk silicon has very poor thermoelectric properties. However, the Si MCPs with three-dimensional structure have improved thermoelectric properties. The measurement indicated that the pore size and the thickness of the Si MCPs had influence on the thermoelectric properties while the decline angle of the channel direction from the perpendicular (7°) did not have any impact. And then, the samples with pore dimensions of 5μm x 5μm and 3μm x 3 μm were used to temperature sensing, and showed temperature sensitivities of 1.88 mV/℃and 0.93 mV/℃, while the linearity was 0.998 and 0.997, respectively. In addition, both samples had anisotropic thermoelectric properties along the surface. The Si MCPs with high reliability, simple structure and compatibility with IC process have a promising future in precision temperature measurement and monitoring of industry, environment and other fields.In order to improve the thermoelectric properties of the Si MCPs, the silicon layer of the pore wall was thinned by oxidation and etching. After oxidation from 30 minutes to 70 minutes and removing the silicon dioxide layer by buffered hydrofluoric acid, the samples show an improved coefficient as high as 1219μV/K after repeating oxidation and etching 5 times, and the thermoelectric properties are enhanced. Moreover, to decrease the very high resistivity of the Si MCPs, boron doping was introduced in different doping time. The electrical resistivity of the doped samples was determined by a four point probe. Boron doping changed the electrical resistivity of the samples from 1.5 x 105Ω·1cm to 5.8 x 10-3Ω·cm, and the absolute Seebeck coefficient deteriorated relatively slightly from 674μV/K to 159μV/K. According to the Seebeck coefficient and electrical conductivity, the power factor was calculated and a peak value of 4.7 x 10-1 mW m-1K-2 was obtained. The results reveal that Si MCPs doped with boron are promising silicon-based thermoelectric materials.After undergoing boron and phorophorous doping, the p-type and n-type Si MCPs were obtained. Based on the boron and phosphorus doped samples, a simple thermoelectric junction was fabricated to connect the two type Si MCPs with copper sheet. When the current flowing from the n-type to p-type side was below 100 mA, a temperature below 25℃could be observed, and the maximum temperature difference in this range was about 1℃. By comparing the theoretically calculated and experimental results, the intrinsic figure-of-merit ZT of the unicouple device and thermal conductivity of the p-type and n-type MCP legs are estimated to be 0.007 and 50 Wm-1K-1, which suggests that the holey structure could decrease the thermal conductivity effectively. The research is benefit to the fabrication of silicon-based cooling device.Some silicon alloys are thermoelectric materials with excellent properties. The iron film was deposited on the surface and pore wall of the Si MCPs. Then the silicon and iron can be synthesized toβ-FeSi2/Si MCPs by annealing at the temperature of 700℃. The structure can maintain the brilliant high temperature thermoelectric properties ofβ-FeSi2 while reducing the thermal conductivity.In conclusion, the Si MCPs fabricated by PAECE are suitable in thermoelectric applications and can be optimized further. It is of great significance for the development of novel sensors, green energy and cooling device due to the practicability and innovation.