| All the time, magnetism plays a very important role in the progress of human history. Especially, as magnetoelectronics springing up, the weak magnetic field detection related to geomagnetic field has obtained a new developing opportunity. Weak magnetic applications such as magnetic anti-submarine with unmanned aviation vehicles, geomagnetic navigation and intelligent magnetic fuses urgently require the tri-axial magnetic sensors with high performance and miniaturization. For the advantages of small size, low power dissipation and middle high sensitivity, giant magnetoresistance(GMR) sensors have become the hot candidate for high performance magnetic sensors. However, the progress of GMR sensors has been severely hampered by the common problems such as 1/f noise, magnetic hysteresis and tri-axial nonorthogonality. This thesis presents the fruitful investigations on theory exploration, simulation design, fabrication process, and experimental tests for tri-axial MEMS/GMR integrated magnetic sensors, which is financially supported by NSFC. 1. A magnetic sensing theory with micro mechatro-electromagnetic structures has been proposed, which presents a solution method for the common problems such as 1/f noise, magnetic hysteresis and triaxial nonorthogonality.Utilizing electromagnetic theory and finite element stimulation, the basic subjects, including magnetic concentration, flux turning, and vertical motion modulation, magnetic field tracking and compensation, united regulation and control to 3D magnetic field, and coupling of mechatro-electromagnetic noise and disturbance, have been investigated. Based on that, the magnetic sensing theory with micro mechatro-electromagnetic structures has been proposed. Vertical motion modulation, which originated from the idea of magnetic flux dynamically shunting, can directly modulate the detected magnetic field with mechatro-electromagnetic structures vibrating at high frequency, and will overcome the 1/f noise limit of GMR sensors in low-frequency magnetic detection ability. Due to high modulation efficiency and simplicity in structures and processes, vertical motion modulation is better than other modulation methods. Flux turning, as a new method for magnetic regulation and control, can redirect the vertical Z component to X-Y plane with special soft magnetic structures and acheive the planar measurement for 3D magnetic field in GMR sensors, which results in the promotion of triaxial orthogonality. Based on magnetic domain motion the principle of hysteresis in GMR elements has been revealed. The magnetization state of GMR elements can be roughly constant with tracking and compensating the detected magnetic field, for which magnetic hysteresis and nonlinearity will be suppressed. According to the magnetic regulation and control methods, a united regulation and sensing theory for 3D magnetic field has been established, which can effectively promote the comprehensive performance of triaxial GMR magnetic sensors. By means of equivalent circuit analysis, the coupling principle models of electrostatic disturbance and mechanical vibration noise have been built, which reveals the main rules of multiphysic field coupling in micro mechatro-electromagnetic structures and largely enhances the forementioned magnetic sensing theory. 2. A noval scheme with its design and optimization methods has been proposed for the triaxial MEMS/GMR integrated magnetic sensors.According to the magnetic sensing theory with micro mechatro-electromagnetic structures, the structures including GMR elements, vertical motion modulation films, soft magnetic bulk for flux turning, micro silicon/PZT gyro-beam, and micro compensation coils were integrated, and a noval design scheme for the triaxial MEMS/GMR integrated magnetic sensors has been proposed. Using electromagnetic and structural finite element simulation, the design and optimization methods containing the core indexes such as modulation efficiency, turning efficiency, modal frequency and inductance constant were established with the design idea of symmetry, multilayers and from whole to part. The design and optimization of vertical modulation film and turning structure was achieved with ANSYS magnetic field finite element analysis, and the stimulation values of modulation efficiency and turning efficiency are 30% and 0.65, respectively. After the analysis of structure modal and anti-disturbance ability, the size optimization of the micro silicon/PZT gyro beam was conducted with COMSOL structure finite element method by the indexes of static displacement and first-order modal frequency. With the soft magnetic micro structures, the magnetic excitation rules of coil-turns were investigated by MAXWELL electromagnetic finite element method, and the micro compensation coils were completely designed in terms of electromagnetic inductance constan, resistance and power dissipation. 3. Some critical processes such as preparation of micro silicon structures with soft magnetic material, highly precise fabrication of micro silicon structures, preparation of micro compensation coils, and micro multi-layered structures bonding at low temperature, have been broke through and a fabrication process system has also been established for the triaxial MEMS/GMR integrated magnetic sensing heads.Utilizing sputtering seed layer and model electroplating method, the micro soft magnetic structures were prepared with high quality and large thickness. The thickness and relative magnetic permeability of as-prepared Ni Fe films are above 10μm and 1000, respectively, which are completely sufficient with the requirement of soft magnetic micro structures. Aiming at the issue of large errors in an inheritance mark system, we have established a reference mark system for the micro structure fabrication of the triaxial MEMS/GMR integrated magnetic sensors. As the critical conditions including temperature, concentration and cleanness, a control method for wet etching of single crystal silicon has been established, and can effectively improve the surface and shape precision of micro silicon structures. With thickness reduction with timing, the thickness of height adjustment layer can be under precise control. The pyramid hills on thinning surfaces were suppressed with strengthening cleanness and reducing etchant temperature and their roughness can reach 30 nm. By oxygen plasma process, the combination between glass substrate and Cr/Cu/Cr metal composite film was enhanced. Then the micro compensation coils with high aspect ratio were fabricated by undercutting compensation and etching condition optimization. To solve the issue of multilayered micro structures bonding at low temperature, we have proposed the high-precision alignment method based on water film attachment for separated micro structures and the bonding process based on spinning-dipping glue, which can effectively improve the fabrication precision for the prototype sensing heads. 4. The technologies of high-resolution signal detection and highly precise measurement and control for GMR sensors have been broke through, and a prototype has also been built for the triaxial MEMS/GMR integrated magnetic sensors.The input module has been paid more attentions, and the equivalent input noise voltage of the pre-amplification circuit is reduced to a high-grade level of 3.75 n V/√Hz @7.0k Hz by source resistance matching, offset voltage adjustment and static operation point design. Based on numerical simulation and experimental tests, the signal detection ability of digital lock-in amplification and spectrum calibration(Rife and M-Rife) was comparatively investigated, and a new high-resolution algorithm called moving window based M-Rife has been suggested and the RMS errors of frequency and amplitude can be reduced by half. The driving voltage amplitude of the micro silicon/PZT gyro-beams is precisely measured with moving window based M-Rife, which results in the gyro-beam can oscillate steadily with digital PI control method. Its oscillation amplitude and frequency are approximately 10 μm and 7.2 k Hz respectively and the fluctuations of amplitude and frequency are less than 0.005 μm and 0.5 Hz in 20 minutes, respectively. By the methods of multi measurement range, D/A gain adjustment, and digital PID control, the compensation current was produced and controlled in high precision which can be better than 0.05%. 5. A serial of test and estimate methods for weak magnetic sensors have been established, and the magnetic sensing theory with micro mechatro-electromagnetic structures has been roughly verified.After the test system for weak magnetic sensors was set up, the characteristics of the the prototype sensor were tested and assessed in response, noise and mechatroelectromagnetic coupling. And then the performance indexes including minimum detectable magnetic field, magnetic hysteresis, and nonlinearity were scientifically assessed. At last, a serial of test and estimate methods were established for weak magnetic sensors. The minimum detectable magnetic field of the prototype sensor reaches 77 p T/√Hz, which is better than the highest level of the reported GMR sensors(133 p T/√Hz,@1Hz). Magnetic flux vertical motion modulation can reduce 1/f noise limit by more than two orders(larger than 220 times), and its modulation efficiency is 29.5% which is well in accordance with the design value 30% and surpasses the highest level reported so far. Magnetic field tracking and compensation method can obviously reduce the hysteresis and nonlinearity of GMR elements, and the corresponding indexes are approximately 10 times improved. 3D magnetic field can be measured in a plane with flux turning and the turning efficiency reaches 0.68 which is well in agreement with the design value 0.65. With highly precise impedance measurements, the theory models for electrostatic disturbance and mechanical vibration noise are proved in rationality. The magnetic sensing theory with micro mechatro-electromagnetic structures is verified well in effectiveness and correction by the aforementioned experimental results. |
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