Integrated GMR Switch Circuit Research

The integrated magnetic switch is a kind of magnetic switch device which uses magneticsensor as core components, and combined with IC process technology. It features small size, highaccuracy, low power, high reliability, and anti-vibration, widely used in automatic control, security,medical electronics etc.Currently, the common integrated magnetic switches are mostly based on three effects:Hall-effect, Anisotropic Magnetoresistive (AMR) and Giant Magnetoresistive (GMR). Among them,the Hall-effect switch features simple structure and process, low cost, widely applied in theconventional field. But it cannot be used in some special environment such as medical, military,aerospace etc., because of its large chip size and great temperature offset. AMR switch caneliminate such defects that Hall-effect switch has. But its application is limited because thesaturation field of AMR is very small. However, the GMR switch features higher accuracy, lowerpower, smaller size, better temperature characteristic, and higher saturation field. So it has becomethe focus of industry. So far, only NVE Corporation successfully achieves industrialization inworld-wide, and just budding in China. This means the integrated GMR switch has huge potentialboth in domestic and international market.This project is carried on the basis of the GMR multi-layer sensor, which is developed byCenter for Integrated Spintronic Devices (CISD) of Hangzhou Dianzi University with thecorporation of SPINIC INC. The configuration the sensor is Wheatstone bridge with differentialoutputs. It features good heat stability and linearity, as well as little hysteresis. The proposed circuitof the integrated GMR switch in this project includes two main parts: amplifier module andcomparator module. According to the signal feature of Wheatstone bridge, the 3-Op Ampinstrument amplifier is used in amplifier module for the high CMRR and input impedancedemanding. Meanwhile, the hysteresis comparator is used in comparator module to raise the noisemargin to avoid the noisy response for the variation near the threshold points. Besides, a Buffer isadded to the output port, which can raise the driving capacity, as well with a PNP transistor that cansupply current sink output.Before the designing work of the circuit, this project carries on the testing and analysis on thefeatures of GMR multi-layer sensor. It is found that the GMR sensor is sensitive to angular variationand can be characterized with a cosine function. Meanwhile, a modified model is established byanalysing the non-ideal effects to make it fit with the test data. The designed circuit is prototyped by CSMC Technologies Corporation with 0.5μm MixedSignal process. After the sample dies testing and verifying, the results are concluded as following:the instrument amplifier has an acceptable performance; the hysteresis comparator’s hysteresis zonehas some obvious shift, but the function is good; unfortunately, the layout of PNP transistor has amistake, where the emitter shorts to ground, which result in the failure of the current sink. Thedeficiencies mentioned above will be modified and improved in further work.To sum up, this project finished the research and development of the integrated GMR switchbased on the testing and analysis results of GMR multi-layer sensor. The fabrication process ofintegrated GMR switch including System-in-Package (SiP) process and System-on-Chip (SoC) willbe studied in further work. These results will accumulate experience and lay foundation for thefurther study of GMR isolator and MRAM circuits design in future.