Use of multi-layer thin films as stress sensors

Abstract

The present invention provides a pressure sensing device that includes at least one GMR sensor, and preferably an array of GMR sensors, with each GMR sensor having a conducting spacer layer interposed between two ferromagnetic layers. In an unbiased state, the magnetization vector of each of the ferromagnetic layers is preferably parallel to each other. Upon application of a current, however, the magnetization vector of each ferromagnetic layer is changed, preferably to an antiparallel position, in which state the sensor is used to then sense stress applied thereto. Upon application of stress, the magnetization vectors of both free magnetic layers will rotate, thus causing a corresponding and proportionally related change in the resistance of the sensor. This change in resistance can be sensed and used to calculate the stress applied thereto.

I claim:  
     
         1 . An apparatus for sensing pressure comprising: 
 a sensor including:    first and second free ferromagnetic layers; and    a non magnetic conducting layer disposed between the first and second free ferromagnetic layers, the non magnetic conducting layer providing ferromagnetic coupling of the first and second free ferromagnetic layers in an initial state such that magnetization vectors of the first and second free ferromagnetic layers are substantially parallel to each other, wherein applied pressure causes a change in the magnetization vectors of the first and second ferromagnetic layers through the property of non-zero magnetostriction and a corresponding change in resistance of the first and second ferromagnetic layers.    
     
     
         2 . An apparatus according to  claim 1  further including a plurality of sensors, each that are formed in a two dimensional array and operate as the sensor such that each sensor detects the pressure of an area associated with that sensor.  
     
     
         3 . An apparatus according to  claim 1  wherein the first and second ferromagnetic layers are comprised substantially of NiFe or CoFe, either alone or in combination.  
     
     
         4 . An apparatus according to  claim 1  wherein the thickness of each ferromagnetic layer is within the range of 0.5-15 nm.  
     
     
         5 . An apparatus according to  claim 4  wherein the thickness of the non-magnetic conducting layer is within the range of 0.1 to 10 nm.  
     
     
         6 . An apparatus according to  claim 1  wherein the sensor further includes a buffer layer disposed below the first ferromagnetic layer to assist in uniform epitaxial growth of the first ferromagnetic layer, and increased GMR response  
     
     
         7 . An apparatus according to  claim 6  wherein the buffer layer is NiFeCr.  
     
     
         8 . An apparatus according to  claim 6  further including a capping layer disposed above the second ferromagnetic layer.  
     
     
         9 . An apparatus according to  claim 1  further including a capping layer disposed above the second ferromagnetic layer.  
     
     
         10 . An apparatus according to  claim 1  further including an interlayer disposed between at least one of the first and second ferromagnetic layers and the nonmagnetic conducting spacer, the interlayer provided to increase GMR response and reduce interdiffusion.  
     
     
         11 . An apparatus according to  claim 10  wherein the interlayer is comprised of one of Co and CoFe.  
     
     
         12 . An apparatus according to  claim 1  further including an interlayer disposed between each of the first and second ferromagnetic layers and the nonmagnetic conducting spacer, each interlayer provided to increase GMR response and reduce interdiffusion.  
     
     
         13 . An apparatus according to  claim 12  wherein each interlayer is comprised of one of Co and CoFe.  
     
     
         14 . An apparatus according to  claim 1  further including a specular reflection underlayer disposed below the first ferromagnetic layer to assist in increased GMR response.  
     
     
         15 . An apparatus according to  claim 14  wherein the underlayer is one of NiO and gold.  
     
     
         16 . An apparatus according to  claim 1  further including a specular reflection layer disposed above the second ferromagnetic layer to assist in increased GMR response.  
     
     
         17 . An apparatus according to  claim 16  wherein the specular reflection layer is one of NiO and gold.  
     
     
         18 . An apparatus according to  claim 1  further including a specular reflection layer disposed above the second ferromagnetic layer to assist in increased GMR response.  
     
     
         19 . A apparatus according to  claim 18  wherein the specular reflection layer is one of NiO and gold.  
     
     
         20 . An apparatus according to  claim 1  wherein either one or both of the first and second ferromagnetic layers is a laminate of a plurality of layers that are antiferromagnetically coupled to each other.  
     
     
         21 . An apparatus according to  claim 1  further including a protective coating layer disposed above the second ferromagnetic layer, said protective coating layer having a surface energy that reduces deposits from adhering thereto.  
     
     
         22 . An apparatus according to  claim 21  wherein the protective coating layer is a carbon based material.  
     
     
         23 . An apparatus according to  claim 22  wherein the carbon based material is silicon carbide.  
     
     
         24 . An apparatus according to  claim 1  further including an insulating layer disposed over the second ferromagnetic layer and a conductive layer disposed over the insulating layer such that the conductive layer provides for protection from electrostatic discharge.