Abstract:
Simultaneous setting of exchange pinning field magnetization in more than one direction for several thin film structures on a single substrate has been achieved by first orienting the structures as needed. A layer of hard magnetic material is then deposited, suitably patterned to control the direction of its flux, and then magnetized through a single exposure to a strong magnetic field. The assemblage is then thermally annealed (in the absence of any applied field) at a temperature higher than the AFM material blocking temperature, following which the thin film structures are magnetically pinned in the intended directions.

Description:
This application is related to HMG07-007, filed on Jul. 26, 2007 as application Ser. No. 11/881,349, now U.S. Pat. No. 7,394,247, and herein incorporated, by reference, in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to the general field of magnetized thin films with particular reference to achieving more than one direction of exchange pinned magnetization on a given substrate simultaneously. An important application of the invention is to a magnetic field angle sensor using highly sensitive MTJ or GMR islands. 
     BACKGROUND OF THE INVENTION 
     An example of a prior art structure is a conventional angle sensor in which the sensing islands are four long anisotropic magneto-resistance (AMR) stripes  12  oriented in a diamond shape with the ends connected together by metallization to form a Wheatstone bridge, as shown in  FIG. 1 . Typically, these anisotropic magneto-resistance (AMR) stripes are made of Permalloy material. The top and bottom connections of the four identical islands are given a direct current stimulus in the form of a supply voltage (Vs), with the remaining side connections to be measured. In the absence of a magnetic field, the side contacts should be at the same voltage. To have the island magnetization direction align with an externally applied magnetic field, the latter must be large enough to saturate the Permalloy material. 
     With the AMR islands connected in this fashion to form the Wheatstone bridge, these side contacts will produce a different voltage (ΔV) which is a function of the supply voltage, the MR ratio, and the angle between the island current flow (I) and island magnetization (M). If there is only one such bridge, angle measurement is limited to a range of from −45 degree to +45 degree. When combined with a second bridge, which is rotated 45-degrees relative to the first bridge, a wider range of angle, from −90 to +90 degrees, can be measured. 
     In the prior art, due to the characteristics of the AMR effect in which the resistance change is a function of cos 2 (θ), where θ is the angle between the magnetization and current flowing direction, one AMR Wheatstone only detects 90-degree angle while two AMR Wheatstone bridges with 45-degrees orientation difference only allow a measurement of 180-degree angle. In order to measure a full 360-degree angle, an additional Hall sensor must be used in combination with the two Wheatstone bridges. 
     Due to the characteristics of GMR or MTJ devices, in which the resistance change is a function of cos(θ), where θ is the angle between the free layer magnetization and the pinned reference layer magnetization, they have the ability to detect the full 360-degree magnetic field. However, in an angle sensor using GMR or MTJ devices, it is required that the reference layers be pinned in various directions, thereby introducing a major challenge to GMR or MTJ based sensor development. In order to achieve maximum sensitivity and accuracy, the pinned magnetizations of the reference layers need to be in both anti-parallel and orthogonal directions as in, for example, an ideal arrangement of eight sensing islands  21  that have identical geometry, differing only in their pinned directions, as shown schematically in  FIG. 2 . 
     With continuing advances in micro-magnetic technology, both in regard to the structures formed and the processes needed to form them, the need arises for the ability to apply exchange pinning fields on two or more reference magnetic layers that share the same substrate, in different directions. Typically, an anti-ferromagnetic material (AFM) layer deposited directly underneath or on top of a soft ferromagnetic material layer is utilize to generate an exchange pinning field on the soft ferromagnetic layer through a thermal annealing process. The exchange pinning field direction is in the same direction of the thermal annealing field, i.e., the external field direction during the thermal annealing process. The problem with prior art approaches that uses GMR or MTJ devices has been that if, after applying an exchange pinning field on a first reference ferromagnetic layer in a first direction, it is attempted to apply another exchange pinning field on a second reference ferromagnetic layer in a different direction, application of the second thermal annealing field causes the direction of the first exchange pinning field on the first reference ferromagnetic layer to change. 
     The prior art approach to dealing with this problem has been to cut out individual sensing islands, all of which have been magnetized in the same direction on a single wafer, and to then rotate them through different angles, following which they are cemented in place and then connected together to form the completed angle sensor. Fabricating the latter in this fashion is expensive, limits the accuracy of the angle sensor, increases the size of the full structure, and is susceptible to the introduction of assembly errors. 
     A routine search of the prior art was performed with the following references of interest being found: 
     M. Ruhrig, et al., proposed a single-chip solution using pulsed electric currents to local conductor stripes to set magnetization directions of reference layers [3]. However, in this solution, one has to locally apply current pulse one by one to each GMR or MTJ island, which is costly and is not practical for a mass production.
 
A. Jander, et al., U.S. Pat. No. 7,054,114 proposed a different solution without the need to apply local current pulses to set each individual GMR or MTJ island&#39;s pinning direction. In this prior art, four soft magnetic shields are arranged to have four orthogonal gaps regions where four active sensing islands are located. During thermal annealing process, as an external magnetic field is applied, the fields inside the shield gaps are magnified and their field directions altered to be essentially perpendicular to the gaps. As a result, the pinning directions of these four sensing islands are set perpendicular to gaps that they are located between them. However, using this approach, one still is not able to set pinning magnetizations with anti-parallel directions as required, for example, by an angle sensor based on a simple Wheatstone bridge.
 
     In addition to the above references, the following publications were also found to be of interest:
     1. Honeywell application note “Applications of Magnetic Position Sensors”   2. Taras Pokhil, et., “Exchange Anisotropy and Micromagnetic Properties of PtMn/NiFe bilayers,” J. Appi. Phys. 89, 6588 (2001)   3. M. Ruhrig, et al., “Angular Sensor Using Tunneling Magnetoresistive Junctions With An artificial Antiferromagnet Reference Electrode and Improved Thermal Stability,” IEEE Trans. Magn. V. 40, p. 101, January 2004   

     SUMMARY OF THE INVENTION 
     It has been an object of at least one embodiment of the present invention to provide a method for simultaneously generating AFM exchange pinning fields on two or more thin film objects in different directions through the same thermal annealing process, said thin film objects being on the same substrate. 
     Another object of at least one embodiment of the present invention has been that the thin film objects to which said method is applicable include both GMR and MTJ devices. 
     Still another object of at least one embodiment of the present invention has been that said method require only a single exposure to a magnetizing field. 
     A further object of at least one embodiment of the present invention has been that said magnetization exchange pinning directions on the same substrate may have values that span a full 360 degree range. 
     These objects have been achieved by using a novel method wherein a layer of hard magnetic material is first deposited on and/or close to the objects that are to be simultaneously magnetized in multiple directions. Typically, the objects in question would be GMR or MTJ devices but this is not a requirement, the invention being more general than this. Said hard magnetic layer is then magnetized through exposure to a strong magnetic field. 
     By shaping the hard layer so that it includes restricted areas and/or suitably oriented non-magnetic gaps, flux from the magnetized hard layer can be directed to run locally in any desired direction. With the magnetized hard layer in place, the assemblage is heated (in the absence of any applied field) to a temperature that lies between the Curie point of the hard layer and the blocking temperature of AFM layer of the objects being magnetized. Once they cool below the AFM blocking temperature, objects will have been magnetized or pinned in the intended directions. 
     While the method taught by the present invention is of a general nature, being suitable for application to any magnetizable thin film structure, we have applied it mainly to controlling the directions of magnetization of the pinned (reference) layers of assemblages of GMR and MTJ devices. Such assemblages, wherein the pinned layers are not all magnetized in the same direction, are useful for the construction of GMR or MTJ-based magnetic angle sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . is a schematic view of an angle sensor of the prior art. An electric current flows across a single Wheatstone bridge made up of four identical AMR stripes. 
         FIG. 2  is a schematic view of an angle sensor formed from two Wheatstone bridges. 
         FIG. 3  is a schematic cross-section of a GMR or MTJ device. 
         FIG. 4  illustrates the basic structure that is used to implement the method taught by the present invention. 
         FIG. 5  is a plan view of  FIG. 5  following shaping of the hard magnetic layer relative to the GMR or MTJ structures (A, B, C and D) and then magnetizing said hard layer. Thick arrows represent magnetizations in hard magnetic layer and thin arrows represent local bias fields generated by magnetizations in hard magnetic layer. 
         FIG. 6  shows cross-sectional views of two the two shapes given to the hard layer relative to the GMR or MTJ devices. 
         FIG. 7  illustrates the starting point for manufacturing one of the embodiments of the invention. 
         FIG. 8  shows the structure of  FIG. 7  after it has been given sloping sidewalls through use of ion beam milling, 
         FIG. 9  shows  FIG. 8  after the deposition of the hard magnetic layer and following the removal of the liftoff mask. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention discloses a solution to the problem of forming, on a single substrate, multiple structures with exchange pinned ferromagnetic layers in different directions. This technique is then applied to the manufacture of a MTJ or GMR 360-degree magnetic field angle sensor that comprises both anti-parallel and orthogonal predetermined AFM pinning directions. In each MTJ or GMR sensor, there are multiple MTJ islands or GMR stripes that have identical geometries except for their different AFM pinning orientations. 
     Formation of the GMR/MTJ devices is effected through deposition of a multi-layer structure of the general form (bottom layer listed first)): 
     seed-layer/AFM/AP2/Ru/AP1/Cu or tunneling barrier/free layer/capping layer 
     In  FIG. 3  seed layer  37  is deposited on common substrate  35 . Ferromagnetic sub-layer  32  (AP2), non-magnetic antiferromagnetic coupling layer  33  (generally of Ru), and ferromagnetic reference sub-layer  34  (AP1) together form a synthetic antiferromagnetic structure which is pinned by antiferromagnetic (AFM) layer  36 . The pinning field, or exchange anisotropy, is related to the exchange coupling at the interface between an AFM layer and a ferromagnetic sub-layer (AP2). One class of antiferromagnetic materials suitable for layer  36  includes the ordered tetragonal (fct) alloys such as PtMn, PtPdMn, NiMn, etc. In their as-deposited state these materials have a non-magnetic fcc structure so the ferromagnetic sub-layer has no exchange bias (pinning) with similar coercivity to the purely ferromagnetic layer. In order to be re-ordered them into their AFM tetragonal (fct) state, a thermal anneal at a temperature higher than the AFM material blocking temperature must be performed. The remaining layers shown in  FIG. 3  include Cu spacer (or tunneling barrier) layer  31 , free layer  33 , and capping layer  39 . 
     Once the layers listed above have been deposited, it is necessary to set the pinning directions of the synthetic AFM layers along their required predetermined directions. How this is achieved is a critical feature of the invention (for both GMR and MTJ devices). As a requirement of the invention, the magnetic moment of sub-layer AP2 is designed to be greater than that of the reference sub-layer AP1, giving a non-zero net magnetic moment to the synthetic layer. Before thermal annealing, local magnetic fields would rotate the net moments of the synthetic layers toward the field directions. 
     After deposition, the GMR or MTJ film stack is patterned into rectangular stripes  42  having large aspect ratios and different orientations of their long axes. This is achieved through use of ion beam etching (IBE) or reactive ion etching (RIE) in conjunction with suitable etch masks. 
     Now follows a key feature of the invention. Referring next to  FIG. 4 , protective insulating layer  41  is deposited over stripes  42  (viewed end-on in the figure) and hard magnetic layer  43  (typically a material such as CoPt, or CoCrPt is deposited on layer  41 . 
     Then, using appropriate masking and etching techniques, hard magnet layer  43  is patterned into two sets of shapes. As seen in  FIG. 5 , in shape I-II, on the right, hard magnetic layer  43  abuts previously patterned GMR/MTJ stripes  42  along both their edges but does not overlap them to any extent, while for shape III, layer  43  directly overlies previously patterned GMR/MTJ stripes  42  with no overlap beyond either edge. 
       FIG. 6  shows a cross-sectional view in the vicinity of two of the GMR/MTJ devices seen in  FIG. 5 . Device  42   a  on the left is an example of shape III. It is magnetized in direction  61  by the return flux that completes the local magnetic flux loop associated with the magnetization  62  of hard magnetic layer  643   a . The device on the right is an example of shape I-II. It is magnetized by hard magnets  643   b  in direction  63  by flux that follows the path of lowest reluctance between I and II. 
     Once formation of the various shapes into which the hard magnetic material is patterned has been completed, the newly formed hard magnet shapes are simultaneously magnetized by exposure to a large, externally generated, magnetic field (typically a field of at least 500 Oe). Following removal of the latter, the magnetization of the hard magnets remains unchanged resulting in the generation of biasing magnetic fields which are applied to each individual GMR or MTJ stripe in a directions that is perpendicular to local long edges of hard magnets and determined locally by the relative positions of the hard magnets and the GMR or MTJ stripes, as shown by thin arrows in  FIG. 5 . 
     It is important to note that, while shape I-II and shape III have been drawn with the same orientation in  FIG. 5 , this is not necessary for the invention to operate as described below, the invention being, in fact, independent of what the relative orientations of the different GMR or MTJ stripes might be for any given application of the invention. 
     Once the required magnetized hard magnet structure is in place, a thermal annealing process is conducted for between about 5 and 500 minutes by heating the assemblage to a temperature of between about 200 and 400 deg. C. in the absence of any external magnetic field. This anneal temperature is set to be below the Curie point of the hard magnetic material but above the blocking temperature of the GMR or MTJ devices&#39; AFM layers. In this way the magnetization of each device&#39;s reference layer gets to be pinned (by its re-oriented AFM structure) along the direction set by the hard magnetic layer, as discussed above. Since these patterned GMR or MTJ islands (A, B, C and D as shown in  FIG. 5 ) experience bias fields in different directions determined locally by their adjacent hard magnets, magnetizations in their AP2 sub-layers are set along their local bias field directions during the full thermal annealing process. After the thermal annealing process when the temperature falls below the AFM blocking temperature, exchange pinning fields are induced on the AP2 sub-layers, and pinning directions are set along their original local bias field directions. Due to the strong anti-parallel coupling between the AP1 and AP2 sub-layers, the whole synthetic layer is pinned by AFM exchange field. 
     At the conclusion of the thermal annealing process, all hard magnetic material used to magnetize the devices is removed by use of suitable selective etching processes. 
     Note that the various GMR or MTJ shapes described above are merely examples of shaped magnetic layers, disposed to have different relative orientations, that were simultaneously magnetized or exchange pinned in multiple directions on a single substrate through application of the present invention. The method we disclose is not limited to either those shapes or those relative orientations. Rather it is applicable to any set of shaped magnetic layers. 
     In some cases it is possible to omit protective insulating layer  41  from the magnetization process that we have just described. Omission of insulating layer  41  from the process (where feasible) offers the advantages of better hard magnet-to-GMR (or MTJ) alignment and stronger local hard bias field. We will illustrate this alternate embodiment of the invention using a GMR or MTJ device as the vehicle but it will be understood by those skilled in the art that this alternate embodiment of the invention is applicable to any plurality of thin film structures that are to be permanently magnetized on the same substrate in directions that vary from one structure to the next. 
     Referring now to  FIG. 7 , capping layer  39  is deposited on GMR or MTJ stack  71  (see layers  31 - 37  in  FIG. 3 ) which lies on substrate  35 . Liftoff mask  72 , comprising an upper etch resistant portion  73  that overhangs lower, readily etched portion  74 , is formed on the surface of capping layer  39 . Then, as illustrated in  FIG. 8 , Ion Beam Etching (IBE), schematically shown as arrows  81  which represent energetic ions, usually, but not necessarily, of argon, is used to pattern the formless layered structure of  FIG. 7  into individual GMR or MTJ devices, each having sloping sidewalls  82 , and shaped as large rectangular stripes having a large aspect ratio. The various devices that are so formed will, in general, have different relative orientations of their long axes. 
     Then, as illustrated in  FIG. 9 , layer of hard magnetic material  93  is deposited over the entire surface followed by liftoff of mask  72  through application of a solvent known to readily attack lower mask layer  74 . Next, a large external magnetic field (typically a field of at least 500 Oe) is applied to set all hard magnets and is then removed. This is followed by thermal annealing at a temperature between about 200 and 400 deg. C. for between about 5 and 500 minutes, in the absence of an external magnetic field, to set the AFM and their associated pinned reference layers into their intended directions. 
     Following selective removal of the hard magnetic material, the GMR/MTJ structures are isolated, as appropriate, so that they can operate independently and the structure of which they are a part is completed through the addition of leads, formation of interconnections between devices, formation of I/O connections, etc.