Research On Magnetoresistance Film Stacks And Dual-axis Giant Magnetoresistance Sensor

With the development of information technology,magnetic sensors have been widely used in many fields such as the Internet of Things(IoTs),aerospace,automobiles,consumer electronics and biomedicine.Recently,the output value of magnetic sensor chips has exceeded 2 billion US dollars and is still growing at an annual growth rate of 8%.In recent years,the urgent demands of high detectivity,small size and low power consumption make Hall sensor difficult to meet the request.However,emerging spintronic sensors have the advantages of low power consumption and high integration,this provides an opportunity to develop the spintronic sensors.Due to their high sensitivity,small size,low power consumption and high integration,the spintronics sensors based on giant magnetoresistance(GMR)and tunneling magnetoresistance(TMR)are expected to become the mainstream products of the next generation of sensors and have attracted great attention of many research institutions in the world as well as the giant sensor companies such as Infineon and Honeywell,etc.Generally,the design of film stacks of high sensitivity sensors and sensors based on full Wheatstone bridge structure are a valuable research topic and highly required.In this dissertation,we have done a series work on the optimization of film stacks,fabrication of a full Wheatstone bridge sensors on one wafer and construction monolithic dual-axis full Wheatstone bridge sensors.The main contents of this thesis are as follows:1.Optimization of IrMn-based magnetic tunnel junctions(MTJs).By optimizing the synthetic antiferromagnetic,the strength of anti ferromagnetic coupling can up to 7005 Oe,and the stray field is zero.We have investigated the structure,magnetic properties,and TMR ratio for the IrMn-based MTJ with seed layers of Ta,Ta/Ru,NiFeCr and Ta/NiFe,annealed at different temperatures.The experimental results show that the Ta/Ru seed layer is beneficial to the formation of IrMn(111)texture.More importantly,the TMR ratio for the film stack with seed layer of Ta/Ru is much higher than that of the film stacks with other seed layers,which is probably due to the weak interdiffusion at the interfaces of CoFeB/MgO/CoFeB.Besides,we give some suggestions on the design of TMR sensors after detailly on each layer’s functions.2.Manipulation on pinning direction in MTJs under magnetic field annealing with zero magnetic field cooling.During the magnetic field annealing with zero magnetic field cooling process of MTJ film stack with a wedge antiferromagnetic(AFM)layer,we find the pinning direction reversed with increasing thickness of AFM.Based on Stoner Wohlfarth model analyses,the pinning direction is mainly determined by the competition between exchange bias coupling and interlayer coupling.Then we deign a set of experiments to verify the simulation results which shows good conformity.These unique properties provide a new design thought for developing full Wheatstone-bridge-based MR sensors.3.Construction of the full Wheatstone-bridge GMR sensors through post annealing.We calculate the output curve of magnetoresistance(MR)ratio versus magnetic field under the±45° GMR strips and analyses a full Wheatstone bridge output.Then,the experimental verification is carried out based on simple and mature crafts of GMR.We fabricate them to±45° devices by standard micro-nano processing.Finally,the full Wheatstone-bridge GMR sensors is realized under post annealing.The sensitivity of full Wheatstone-bridge is up to 0.2 mV/V/Oe.4.Construction of monolithic dual-axis full Wheatstone-bridge GMR sensors.We propose a way of spin-orbit-torque(SOT)configured pinning direction,which is utilized to construct dual-axis full Wheatstone-bridge GMR sensors.We deposit a series of IrMn/CoFe multilayers to demonstrate that the field-like field of spin current from IrMn layer dominates the switching of pinning direction.Furthermore,through this electrical controllable technology,we can set the pinning direction of adjacent GMR bridge arms opposite with each other,monolithic dual-axis full Wheatstone-bridge GMR sensor is constructed.Towards practical applications,we also perform X-axis and Y-axis output measurement and total noise power spectral density(PSD)characterization to reveal the performance of the constructed dual-axis GMR sensors.As a result,high sensitivity and low noise level are achieved,which contributes to excellent detectivities of 9.45 nT Hz-1/2 and 12.3 nT Hz-1/2 at 10 Hz for X-axis and Y-axis sensing axis,respectively.We also design a simple peripheral circuit for the GMR sensor to implement fully automatic detection of a magnetic field angle.