Research On Noise Optimization Of AMR Linear Magnetic Sensor

With a wide range of applications,magnetic field sensor plays a very important role in the fields such as national defense construction,science and technology,medical care and health service,which is a main branch of the sensor industry.AMR linear magnetic field sensor is one of the most widely used magnetic field sensors because of its high sensitivity,simple process,easy integration,low cost,and low noise.The performance of the AMR linear magnetic field sensor is not only affected by the fabrication process,but also affected by the design of geometric parameters.Recently,most researchers focus on films that constitute the sensor and their fabrication processes,but there are few researches on sensor size design.Therefore,this thesis mainly studied the influence of different design parameters on the performance of AMR linear magnetic field sensor in order to find the most suitable design parameters and optimize the sensor’s performance.In view of the above research topics,the main study contents of this dissertation include:First,the design parameters of the sensor are determined by analyzing the relevant parameters that affect the performance of the sensor.The design of AMR linear magnetic field sensor were mainly divided into two parts:magnetoresistive strip and Barber electrode.The main design parameters of the magnetoresistive strip were its thickness,width and length.The thicknesses of the magnetoresistive strips were designed as 28nm,32nm,35nm,and the widths were designed as 16μm,20μm,24μm,28μm,32μm and 36μm.The length of all strips was 400μm.The parameters of the distance between Barber electrodes were designed as 10μm,12μm,14μm,16μm,and the electrode angles were designed as 40°,45°,50°.Besides,the electrode width was designed as 10μm.There were a total of 216 devices based on combinations with different design parameters.Second,the fabrication process of the sensor was explored.In this thesis,Ta/Fe Ni/Ta three-layer structure was adopted to make the magnetoresistive film.Permalloy（Fe Ni）was selected as a magnetoresistive sensitive material,Tantalum film as the buffer layer.And the magnetoresistive film was made by magnetron sputtering.Moreover,the Barber electrode structure was adopted to make the sensor output linearly near the zero magnetic field by changing the direction of the current flowing through the magnetoresistive strip.The Barber electrode was made of aluminum with a thickness of 500nm and was made by electron beam evaporation.A layer of titanium was evaporated between the aluminum film and the magnetoresistive film as an adhesion layer with a thickness of 15nm.The metal aluminum was stripped by using the lift-off process.Then,the prepared devices were tested.It was found that the voltage sensitivity of the magnetoresistive strip per unit length was inversely proportional to the ratio of the thickness to the width of the magnetoresistive strip.The voltage sensitivity of sensor increased with decreasing the ratio of thickness to width.If the thickness of the magnetoresistive film was constant,the sensitivity increased when the width of the magnetoresistive strip broadened.If the width of the magnetoresistive film was constant,the sensitivity of the sensor could be improved by appropriately reducing the thickness of the magnetoresistive film.When the ratio of thickness to width were down to less than 10^-3,there was no obvious improvement effect on the sensitivity of the sensor.Therefore,it was recommended to design the ratio to approximately 10^-3 which was appropriate.As the distance between Barber electrodes increased,the sensitivity of the sensor decreased.When the distance between electrodes was 10μm,the sensitivity was the best.The smaller the electrode angle,the greater the sensitivity.The devices with an electrode angle of 40° in this article has the best sensitivity.After the noise of designed devices were tested,it was found that the thicker the magnetoresistive film was,the lower the noise of the sensor was.The narrower the magnetoresistive film was,the greater the noise would be.The noise of a sensor with a smaller Barber electrode angle was lower than other sensors.The sensor with the thickness of 35nm,width of 36μm,and the electrode angle of 40° made the lowest noise.When the frequency was 1Hz,the noise density of its output voltage was 34.3n V/√Hz.Finally,the self-made AMR linear magnetic field sensor was compared with Honeywell’s HMC1021 sensor.It was found that the sensitivity temperature coefficient and bridge resistance temperature coefficient of the device,with the width of 36μm,thickness of 35nm,electrode spacing of 10μm,and the electrode angle of 40°,were close to HMC1021 sensor.Furthermore,the sensitivity,bridge bias voltage and 1/f Noise of the device were better than the HMC1021 sensor.