Abstract:
A process for manufacturing a TMR sensor is disclosed wherein the blocking temperature of the AFM layer in the TMR sensor has been raised by inserting a magnetic seed layer between the AFM layer and the bottom shield. This gives the device improved thermal stability, including improved SNR and BER.

Description:
TECHNICAL FIELD 
     This application relates to the general field of magnetic sensors with particular reference to tunneling magneto-resistive (TMR) sensors and how to reduce high temperature noise. 
     BACKGROUND 
     A conventional TMR sensor includes a seed layer, an anti-ferromagnetic (AFM) layer having a blocking temperature T b , synthetic anti-parallel (SyAP) layers, one of which serves as the reference layer, a barrier layer, a free layer, and one or more capping layers. As TMR sensors become smaller, sensor noise improvement, especially at high temperatures, is critical for maintaining or improving the signal to noise ratio (SNR) and the bit error ratio (BER) (the number of bit errors divided by the total number of transferred bits). 
     A Ta based seed layer, such as Ta/Ru, Ta/Cr, Ta/Cu or Ta/NiCr, is usually used as the seed layer for a TMR sensor but it is very difficult to improve the thermal stability of such seeds without increasing AFM and/or seed thickness. This leads to an increase in shield-to-shield spacing, resulting in a resolution penalty. 
       FIG. 1  shows a TMR sensor of the prior art. Depicted there are bottom and top shields,  11  and  21  respectively, conventional seed layer  13  (ranging in thickness from 5 Å to 30 Å), AFM layer  14  (ranging in thickness from 50 Å to 100 Å), antiparallel (AP)2 layer  15 , AP coupling layer  16  (usually of ruthenium), and AP1 (ranging from 10 Å to 40 Å), the latter also serving as the reference layer. AP1 is followed by insulated tunnel barrier layer  18  on which lies free layer  19  followed by capping layer(s)  20 . 
     SUMMARY 
     It has been an object of at least one embodiment of the present disclosure to raise the blocking temperature of a TMR sensor&#39;s AFM layer, thereby improving its thermal stability. 
     Another object of at least one embodiment of the present disclosure has been to reduce shield to shield spacing thereby achieving better spatial resolution. 
     Still another object of at least one embodiment of the present disclosure has been to provide a process for manufacturing the disclosed TMR sensor. 
     A still further object of at least one embodiment of the present disclosure has been for the above process to be fully compatible with currently used processes. 
     These objects have been achieved by inserting a magnetic seed layer, between the AFM layer and the bottom shield layer. The first seed layer can be magnetic or non-magnetic, This raises the blocking temperature of the AFM layer, improving high temperature noise, including improved SNR and BER. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . Schematic drawing of conventional TMR sensor 
         FIG. 2   a . Schematic drawing of a first embodiment of the disclosed TMR sensor with a magnetic seed layer inserted to be part of the stack 
         FIG. 2   b . Schematic drawing of a second embodiment of the disclosed TMR sensor with a magnetic seed layer inserted as part of the shield structure 
     
    
    
     DETAILED DESCRIPTION 
     Better thermal stability with much higher blocking temperature has been achieved by inserting a NiFeX based layer between the bottom shield and the AFM layer. Examples of NiFeX materials include, but are not limited to, NiFe, NiFeCr, NiFeW, NiFeRh, NiFeTa, NiFeMo, NiFeHf, NiFeNb, NiFeZr, NiFeTi and NiFeCo. This improvement in the thermal properties is achieved without degrading the sensor properties. 
     We disclose below how the blocking temperature of a TMR device is improved by replacing the conventional (non-magnetic) seed layer with a NiFeX based magnetic seed. The NiFeX based magnetic seed provides a better substrate on which to grow the AFM layer by enhancing the AFM crystal orientation. 
     The magnetic seed can also serve as a shield for the TMR sensor so, by removing the non-magnetic seed layer, the shield-to-shield spacing can be decreased. 
       FIGS. 2   a  and  2   b  disclose two configurations that we have used to improve the Tb of a TMR sensor when NiFeX based magnetic seed layers replace conventional non-magnetic seed layers. 
     First seed layer  13  (which can be magnetic or non-magnetic) of an amorphous or fine-grained material is deposited first (on lower shield  11 ) to reduce/eliminate crystallographic influence of an underlying NiFe shield layer. The first seed layer ranges in thickness from 5 to 30 Å and includes, but is not limited to, Ta, CoFeB, CoFeZr, CoNiFeZr, CoFeHf, CoNiFeHf, CoFeTa, CoFeNiTa, CoFeNbZr and FeTa. 
     NiFeX type magnetic seed layers  23  are then deposited to provide the template for AFM growth (to a thickness in the of 20 to 200 Å range). Non-magnetic layer  22 , such as Ru, Cr, Cu, Al, NiFeCr or NiCr, is then deposited on magnetic seed layer  23  (to a thickness in a range of 5 to 20 Å) followed by the deposition (on non-magnetic layer  22 ) of AFM layer  14 . Note that non-magnetic layer  22  serves to eliminate exchange coupling between the AFM layer and the magnetic seed layer 
     Also shown, though not part of the invention, are isolation layers  12  and the locations of the longitudinal bias layers. 
     Together with the device&#39;s magnetic first seed layer, magnetic seed layer  23  will be magnetically coupled to bottom shield  11 , thereby acting as a shield itself, which reduces shield to shield spacing of the TMR sensor. For a thin non-magnetic first seed layer less than 10 Å thick, magnetostatic coupling between magnetic seed and bottom shield will reduce the effective shield to shield spacing of a TMR sensor. 
     The NiFeX based seed layer can be a single layer or a multi-layer, provided at least one of its NiFeX layers contains at least 40 atomic percent of Fe. Different compositions of NiFeX layers (including combination with CoFeX) can be used in the multilayer configurations in order to optimize the magnetic properties (including magnetostriction) of the magnetic seed layer. CoFeX examples include, but are not limited to, CoFe, CoFeB, CoFeNb and CoFeNi. Also, the smoothness of the magnetic seed surface is usually improved with a multilayer configuration, which provides a better template to grow the TMR sensors. 
       FIGS. 2(   a ) and  2 ( b ) show examples of two different milling depths—(a) leaving only the bottom shield unmilled and (b) terminating milling just before the NiFeX magnetic seed layer is reached. The milling depth that is selected is a matter of designer choice, usually dictated by the needs of the bias layers. For example, with the magnetic seed layer the blocking temperature of the sensor can be improved to be as high as 40 degrees, which provides advantages for thermal stability. 
     X-ray diffraction analysis showed that the NiFeX magnetic seed enhanced AFM crystallographic orientation, which is important for improved thermal stability.