Design Of Magnetic Sensor Based On AMR Effect And The Interface Circuit

With the development of integrated circuit and sensor industry,magnetic sensor has a broad application prospect because of its characteristics of ultra-high sensitivity and noncontact.Among them,the sensor based on anisotropic magnetoresistance(AMR)effect is widely concerned and has a large market due to its advantages of high sensitivity,strong stability,convenience for integration and low cost,etc.Based on the principle of the AMR effect,the studies on the AMR sensor are conducted in the thesis,including the modeling,the simulation,the fabrication process preparation and the interface circuit design.Firstly,the structural parameters of the sensor are optimized by the modeling and the simulation.Then,the optimized parameters are applied to the designed sensor and the corresponding fabrication process is proposed.After that,the sensor chip with superior performance is fabricated.Finally,according to the characteristics of the output signal of the fabricated sensor chip,the interface circuit is designed for the amplification of the output signal.The main contents of this thesis include:(1)The design principle of the AMR sensor is introduced,including the principle of the AMR effect,the magnetic properties of Permalloy,the bias effect principle of Barber electrode and the principle of Wheatstone bridge,etc.Based on the formula of the principle,the relationship between the resistivity of the sensor and the angle between the current and the magnetization direction is obtained.Meanwhile,the ideal output curve of the AMR sensor is drawn.(2)Aiming at the optimization problems of the important structural parameters in the AMR sensor,a sensor model is established,and a new simulation scheme is proposed.During simulation,aiming at the parameter optimization of Permalloy film’s thickness and aspect ratio,the hysteresis loops with different parameters are simulated by the simulation software.Its magnetic properties are obtained with the different coercivities and hysteresis losses;Aiming at the parameter optimization of the Barber electrode’s width and spacing,the simulation software is used to simulate the direction and magnitude of the current density under different parameters.The deflection angles of the current densities at the electrode are calculated.The bias effect of the Barber electrode is compared in terms of the angle size and the uniformity of distribution.The most optimized situation can be listed as the following: the thickness,length and width of Permalloy is 20 nm,200 μm and 10 μm,respectively,as well as the width of Barber electrode is 10 μm and the spacing is 8 μm.(3)Aiming at the related process problems of the practical fabrication of the AMR sensor,the whole process and the test scheme are designed,including magnetron sputtering,photography,electron beam evaporation,ion beam etching and many other processes.In the process of preparation,aiming at the anisotropy of Permalloy shape,the magnetic resistance strip pattern of Permalloy is successfully prepared by the combination of lithography and ion beam etching.To meet the requirements of Barber electrode bias effect of 45°,lithography process combined with lift-off process is used to successfully prepare the pattern of parallelogram with 45°.In view of the influence of different annealing temperature and time on the output curve of the sensor,the probe bench test system is used to analyze and compare the output curves of the sensor.350 ℃ annealing temperature and 3 h annealing time are determined as the best process parameters.Based on the actual fabrication result,the sensitivity of the sensor is 0.162 m V/V/Oe,and the linear operating range is ±5 Oe.(4)Aiming at the issue that the AMR sensor output signal needs to be amplified,a novel interface circuit for the AMR sensor is designed.The main scheme of the amplifier is similar to the improved three-op instrument amplifier circuit.Aiming at solving the problem of high power consumption and large noise of the traditional instrument amplifier circuit,the first stage of the designed circuit is the combination of high gain folding common-source common-gate amplifier and bias transistors to reduce the value of the resistance.Aiming at the output distortion caused by the excessive voltage swing after the first stage,the second stage is designed as trans-resistance-amplifying circuit,which amplifies the small current signal into voltage signal.Simulation results show that the performance of the circuit is good and stable for the signal amplification of the designed AMR sensor.