Study On The Low-power Technology Of Micro Fluxgate Based On Soft Magnetic Porous Thin Film

Fluxgate is a magnetic field sensor with very good comprehensive performance. It has been widely used in the fields, such as geomagnetic study, oil well logging, space magnetic field detection, aerospace, micro satellites, bio medical, current measurements, and so on. However, the structure and working principle of fluxgate result in larger volume and high power consumption. The micro fluxgate fabricated by MEMS technology cut the device size successfully, but failed to reduce power consumption significantly. Because of the heat generated in the work focused on small area, it brings cooling problems which threaten the thermal stability of the sensor. It is more urgent to reduce the power consumption of micro fluxgate. In addition, due to the introduction of micro processing technology, the performance of fluxgate declined obviously, including lower sensitivity and higher noise. Thus, how to solve the problems in the miniaturization of fluxgate becomes a key technical issue. Based on the analysis of micro fluxgates, in this dissertation, a low-power micro fluxgate based micro-porous core was proposed and a fabricated method of nano-porous soft magnetic thin film were studied. From structural optimization and core performance improvement, the dissertation investigated and discussed the relevant questions.The main innovative achievements of this dissertation include:(1) A low consumption method based micro-porous core of micro fluxgate is proposed. Contracted core structure is beneficial for reducing the excitation current and power consumption, but the flux leakage of core also increased. The effect of reducing power consumption is not obvious. This dissertation proposed a low-power micro fluxgate with micro-porous core. The porous core structure can reduce the flux leakage and increase the equivalent cross-sectional area of core in excitation coil. The topology structure of hole can affect porous magnetic core. This dissertation studied the impact of hole topology factors on the properties of porous core, analyzed their contribution to the reduction of fluxgate power consumption, and optimized the core topology structure. The porous core adopted hexagonal holes, 5:1 shrink ratio and array distribution. After tested the micro fluxgate obtained by controlling topological parameters of porous core, the results show that the optimized topology conforms to the requirements of low-power micro fluxgate. The preparing process balanced the sensor sensitivity and the yield of MEMS technology. The overall performance of porous core micro fluxgate is superior to the contracted core micro fluxgate obtained in the same condition. Compared with the contracted core micro fluxgate, the sensitivity and power consumption of porous micro fluxgate were respectively improved by 13% and reduced by 40.4%.(2) An optimum excitation current estimation method of micro-porous core micro fluxgate is proposed, in which the structural parameters were considered as calculation parameters. Optimum excitation current is an important index of the power consumption of fluxgate. In low-power optimization of micro fluxgate, the optimum excitation current can be reduced by changing the structure parameters. The finite element analysis of porous core micro fluxgates with different structure parameters occupy lots of resources and it takes a long computing time. This dissertation proposed an optimum excitation current estimation method based on structural parameters through small amount of finite element analysis and data fitting calculation. This method can be applied to various porous core micro fluxgates to save a lot of resources and shorten simulation process.(3) The preparation technology of anodic alumina template on silicon substrate which was suitable for nano-porous soft magnetic thin film is proposed. Conventional alumina templates have been restricted for application in the fabrication of nano-porous soft magnetic thin film due to limitations such as flatness and brittleness. This dissertation proposed a novel fabrication process of alumina porous template on silicon wafer based on two-step anodization. Through micro-structure characterization and thickness measurement, the pore diameters and porosities of nano-porous film are fully applicable to the function as a template. Thickness of template can be precisely controlled. In fabrication process, the pore diameters and porosities can be adjusted by changing preparation conditions for various micro-structures. The alumina template can be used to fabricate porous soft magnetic thin films. The preparation process of template is compatible with MEMS process.(4) The preparation technology of nano-porous soft magnetic thin film which was suitable for low-power micro fluxgate core is proposed. How to improve the performance of thin film core prepared with MEMS process is the urgent task in the development of micro fluxgate.This dissertation proposed a novel fabrication process of nano-porous thin film core based on two-step replication from alumina porous template. The process is compatible with MEMS process and can change the micro-structure of thin film. The nano-porous structure of thin film can make the distribution of grain more uniform, decrease magnetocrystalline anisotropy, reduce material abnormal loss, and release the additional stress in thin film. After tested the porous thin film core, the results show that coercive force and saturation magnetic field intensity decreased effectively while magnetization performance improved obviously. The nano-porous thin film core can reduce optimum excitation current, decrease power consumption and improve the performance of micro fluxgate.This dissertation was supported by the National Natural Science Foundation of China called “Study on the low-power technology of micro fluxgate” and Specialized Research Fund for the Doctoral Program of Higher Education of China called “Study on the low-power technology of micro fluxgate based on soft magnetic porous thin film”. This dissertation will be of great beneficial in improving the performance and expanding the application fields for micro fluxgate.