Study On Basic Characteristics And Thermal Isolation Properties Of Porous Silicon Used In MEMS

Since 1990, porous silicon (PS) attracted people's attention because of its excellent photoluminescence property. With the development of Micro-Electro- Mechanical System (MEMS) technology, porous silicon has become a new kind of sacrificial or thermal isolation materials used in MEMS due to its excellent mechanical and thermal isolation properties in recent years.The basic characters of PS and the application of PS as thermal isolation layer in MEMS were studied. In this paper, PS was prepared by double-cell electrochemical etching and galvanic element method. Some characters such as influences factors of porosity and etching velocity of PS, pore size, pore wall thickness, surface and cross-section analysis were studied in detail.Through analyzing the morphological characters of PS by AFM, it was found that the PS belonged to Mesoporous silicon, the distribution of pores were uniform and the pore diameter were in the range of 15~50nm. Thickness and porosity of PS, which are affected by the etching time and etching current density, were discussed briefly. The results showed that the etching speed of PS is constant at the early stage, but at the later stage, the speed became slow with the increased depth of the hole. It was also found that the porosity values of PS samples prepared by different current density had the same trend that the porosity increased at the beginning and then decreased with etching time.In order to meet the requisition of preparing PS on large size silicon chip, primary study was performed on preparing PS by galvanic element method. It was found that the uniformity of PS can be improved and the pole scale can be decreased by increasing thickness of Pt film electrode. The thickness of PS had a close relation with ration between Pt film electrode area and etching area.To study the crack phenomenon of PS, Micro-Raman spectroscopy (MRS) was used to measure the residual stress of PS. The results showed that the tensile stresses increased with porosity of PS. The thermal conductivity (TC) of PS was also measured successfully by MRS. It was found that the TC values of PS decreased rapidly with increasing porosity and thickness, and the lowest TC value of PS was 0.624W/(m·K).To contrast the thermal isolation efficiency between PS and silicon, VxOy thermalsensitive resistances and Cu films were deposited respectively on the PS layer and the silicon substrate. It was found that the sensitivity of resistance deposited on PS layer was better than the resistance deposited on silicon substrate and the resistance's sensitivity was improved with increasing porosity and thickness of PS.