Abstract:
The present invention provides a low resistance high magnetoresistance (MR) device comprised of a junction of two magnetic elements separated by a magnesium oxide (MgO) layer doped with such metals as Al and Li. Such device can be used as a sensor of magnetic field in magnetic recording or as a storage element in magnetic random access memory (MRAM). The invention provides a high-MR device possessing a diode function, comprised of a double junction of two outer magnetic elements separated by two MgO insulating layer and a center MgO layer doped with such metals as Al and Li. Such device provides design advantages when used as a storage element in MRAM. The invention with MR wherein a gate electrode is placed in electrical or physical contact to the center layer of the double tunnel junction.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to magnetic field sensors, memory elements, and three-terminal logic devices for recording head, external field sensing, MRAM and logic applications. In particular the invention provides improved magnetic junction devices based on doped MgO layers. 
       BACKGROUND OF THE INVENTION 
       [0002]    A device comprised of two ferromagnetic layers separated by an insulating tunnel barrier is known as a magnetic tunnel junction (MTJ). The cross-section of a conventional MTJ is illustrated in  FIG. 1 . It consists of bottom terminal  18  placed on a substrate or an underlayer, followed by the first magnetic layer  14 , the oxide tunnel barrier  10 , the second magnetic layer  12 , and the top terminal  16 . The resistance of such MTJ is a function of the relative orientation of the magnetizations of the two magnetic layers, being lowest for parallel (P) alignment and highest for anti-parallel (AP) alignment. One of the magnetic layers is made magnetically hard and the other magnetic layer is made magnetically soft such that the two layers switch in different magnetic fields. This can be achieved either by choosing magnetic materials of different magnetic anisotropy or by exchange biasing one of the layers using an antiferromagnetic layer. The P and AP states are realized by applying an external magnetic field sufficient to switch the soft layer but not the hard layer. The associated change in resistance can be used to sense magnetic field in such device as a read head of a hard drive in magnetic recording as described in [1]. Such high-MR MTJ&#39;s can also be used as storage elements in MRAM as described in [6]. Here the different resistance values for P and AP states of the junction correspond to stored values of the magnetic bit, “0” and “1”. 
         [0003]    It is essential for sensor applications that the signal to noise ratio (SNR) of the device is large. The signal for a magnetic junction sensor is the voltage given by the change in resistance caused by an applied magnetic field (ΔR−deltaR) times the biasing current flowing through the junction (I B ), V S =I B ΔR. The predominant noise mechanism of the magnetic junction is its thermal noise, which is proportional to the square root of the device resistance, R 0.5 . In an MTJ the device resistance is dominated by the resistance of the tunnel barrier, such as Al—O, with the leads and the ferromagnetic layers making a vanishingly small contribution. It is therefore highly desirable to decrease the resistance of the barrier (R) without reducing ΔR. R increases as the junction is made smaller in inverse proportion to the junction area (A), the product RA remaining essentially constant. Therefore, for smaller junctions of the future generations of field sensors the demand for reducing the resistance of the barrier is even higher. The desired values for the resistance-area product are RA&lt;1000 Ohm-um 2 , preferably RA&lt;10 Ohm-um 2  as described in [2]. Similar noise and scaling considerations apply to MTJ&#39;s used in MRAM [6]. Additionally, low resistance barriers are required for MRAM cells utilizing the effect of current induced magnetization switching, described in [3,4] and experimentally observed in [5]. Relatively high currents (typically a few mA) flowing through an MTJ are needed for producing switching of the soft magnetic layer. In order not to exceed the breakdown voltage of the MTJ (approximately 1 V) the resistance of the junction should preferably be smaller than 1 kohm. This corresponds to preferably RA&lt;10 Ohm-um 2  for a junction of ˜0.01 um 2  in area. 
         [0004]    The resistance of a tunnel barrier is an exponential function of the barrier thickness, with thinner barriers having lower resistance. For very thin barriers of thickness less then 1 nm, however, the useful signal (ΔR) decreases due to microscopic pinholes and other defects in the barrier as described in [2]. It is therefore desirable for sensor and MRAM applications to have an independent means of reducing the resistance of the barrier while preserving the barrier thickness, necessary for achieving high MR. 
         [0005]    Another desirable characteristic of a magnetic junction is current rectification. This property is conventionally realized in semiconductor diodes and is manifest in different currents through the device for a given bias voltage of positive and negative polarity. The ratio of these two current values is known as the rectification ratio (RR). Incorporating a diode in series with an MTJ in an MRAM cell can provide a significantly improved memory density as described in [6]. The diode blocks current for one bias polarity through MTJ&#39;s placed at cross-points of a 2D array of word and bit lines and allows current for the other bias polarity, thus providing a cell select mechanism built into the MTJ stack. However, fabricating efficient semiconductor diodes within metal-oxide MTJ stacks is a highly non-trivial task. It is highly desirable that the diode function is implemented within the same metal-oxide material system. Double tunnel junctions of the general structure magnet1/oxide/metal/oxide/magnet2 with atomically thin metal center electrodes and asymmetric oxide tunnel barriers can exhibit current rectification as described in [7]. Such diode structure is a variation of a conventional semiconductor Resonant Tunneling Diode described in detail in e.g. [8]. The spin sensitivity of the outer electrodes (magnet1 and magnet2) combined with transport through discrete energy states in the center electrode can provide high-MR and diode functionality—an ideal combination for use in MRAM applications. The thickness of the center metal layer must be comparable to its Fermi wavelength, which is smaller than 1 nm in most metals. For such thin metal layers dimensional quantization in the direction of current (perpendicular to the layers) results in transport through discrete electron states in the layer. Roughness of even one monolayer can significantly affect the performance of the device. The need for atomically thin and atomically smooth metal layers imposes practical limitations on the use of such a double barrier device where the center electrode is a metal. 
         [0006]    The electronic energy states of the center electrode in the double-junction described above can be affected electrically by an additional electrode placed in physical or electrical contact to the center electrode—a gate. Such a spin-dependent three-terminal device is a variation of a conventional Resonant Tunneling Transistor described in detail in e.g. [9]. The combination of MR and gate control can provides the basis for novel logic applications, in particular in reprogrammable logic [10]. 
         [0007]    Use of MgO yields MR (ΔR/R) of 100%-1000% in magnet1/MgO/magnet2 type junctions as described in [11]. The commonly used materials for the magnet layers are Fe, Co, Fe—Co, Fe—Ni, Fe—Co—B alloys. Magnetic junctions based on MgO are preferred in applications based on MR compared to junctions based on, for example, Al and Ti oxides where the typical MR value is ˜30%. Therefore, it is highly desirable for sensor, memory, and logic applications described above to improve the MgO barrier in such a way as to lower its resistance for a given thickness. 
         [0008]    Thus, it is in the art demonstrated the envisaged potentials of devices utilizing tunneling effects, comprising dual magnetic layers. However, the performance of such devices is presently often impaired by defects relating to the problems of providing thin enough layers separating the magnetic layers. 
       SUMMARY OF THE INVENTION 
       [0009]    Obviously an improved method and device, that makes it possible to fully take advantage of the possibilities of magnetic tunneling devices, are needed. 
         [0010]    The object of the present invention is to provide a device and method of producing such that overcome the drawbacks of the prior art techniques. This is achieved by the device as defined in claim  1 , and the method as defined in claim  18 . 
         [0011]    The present invention provides a low-resistance high-MR junction of two magnetic layers separated by a doped MgO layer. The magnets can be such known ferromagnetic metals as Fe, Co, Ni, Fe—Co, Ni—Fe, Fe—Co—B, other ferromagnetic or ferrimagnetic metal alloys or semiconductors. The MgO layer is doped with such elements as Al or Li to lower the resistance of the material. Such Mg x M y O layers (M=Al, Li, . . . ) are formed by depositing a film of Mg—Al or Mg—Li with a subsequent exposure to an oxygen atmosphere. Alternatively, the Mg x M y O layer can be formed by reactively sputtering the Mg—Al or Mg—Li alloy in a mixture of Ar—O 2 . The MTJ is formed by sequential deposition of magnetic layer  1 , Mg x M y O layer, and magnetic layer  2 . 
         [0012]    The present invention provides a magnetic double junction device having an asymmetric current-voltage characteristic (diode function). Such a device is formed by two magnetic layers such as Fe, Co, Ni, alloys Fe—Co, Ni—Fe, Fe—Co—B, or other ferromagnetic alloys or semiconductors. The two magnetic layers are separated by generally a tri-layer of MgO/Mg x M y O/MgO, where the thicknesses of the individual layers are adjusted to optimize the MR and the diode characteristic of the device. MgO/Mg x M y O/MgO tri-layer can be formed by a sequential sputtering of the Mg, Mg x M y  (M=Al, Li, . . . ), and Mg layers in a mixture of Ar—O 2 . The double-MTJ is formed by sequential deposition of magnetic layer  1 , MgO layer  1 , Mg x M y O layer, MgO layer  2 , and magnetic layer  2 . 
         [0013]    The present invention provides a three terminal magnetic junction. Such a device is formed by two magnetic layers such as Fe, Co, Ni, alloys Fe—Co, Ni—Fe, Fe—Co—B, or other magnetic alloys or semiconductors. The two magnetic layers are separated by generally a tri-layer of Mg/Mg x M y O/MgO, where the thicknesses of the individual layers are adjusted to optimize the MR and the trans-resistance of the device. An additional gate electrode is placed in physical or electrostatic contacts with the center Mg x M y O layer to affect its electronic configuration. In addition to MR the conduction through the device is sensitive to the potential on the gate electrode, making it a spin dependent trans-resistor. MgO/Mg x M y O/MgO tri-layer can be formed by sequential sputtering the Mg, Mg x M y  (M=Al, Li, . . . ), and Mg layers in a mixture of Ar—O 2 . The double-MTJ is formed by sequential deposition of magnetic layer  1 , MgO layer  1 , Mg x M y O layer, MgO layer  2 , and magnetic layer  2 . One implementation is to suitably pattern magnetic layer  2  to allow a contact to the top MgO layer of a metallic gate electrode. 
         [0014]    It will be understood by those skilled in the art that the said method of formation of the doped MgO layer by sputtering off an Mg target in Ar—O 2  mixture is one example. Other methods include, but are not limited to, sputtering of a composite Mg x M y O oxide target in Ar, sputtering off an MgO target in Ar—N 2  mixture, or sputtering off an Mg target in Ar—O 2 —N 2  mixture. In these cases M=Li, Al, N, . . . act as dopand atoms in MgO, with metal atoms preferentially substituting Mg and N atoms preferentially substituting O. 
         [0015]    Embodiments of the invention are defined in the dependent claims. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0016]      FIG. 1  illustrates a cross-section of a conventional magnetic junction having an MgO oxide layer separating two magnetic layers. 
           [0017]      FIG. 2  illustrates a cross-section of a magnetic junction having a doped MgO layer separating two magnetic layers, according to the present invention. 
           [0018]      FIG. 3  illustrates a cross-section of a magnetic junction having a doped MgO layer and two MgO oxide layers separating two magnetic layers, according to the present invention. 
           [0019]      FIGS. 4   a,b  illustrate a cross-section of a vertical magnetic junction having a doped MgO layer and two MgO oxide layers separating two magnetic layers and a third terminal in electrical (a) or physical (b) contact with the center Mg x M y O electrode, according to the present invention. 
           [0020]      FIG. 5  illustrates a cross-section of a lateral magnetic junction having a doped MgO layer and a MgO oxide layer separating two magnetic layers and a third terminal in the vicinity of the center Mg x M y O electrode, according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    As described in the background multilayered magnetic devices utilizing tunneling effects, hereinafter refereed to as magnetic tunneling devices, are difficult to manufacture, or alternatively do not give the desired effects. Of special interest is the tunnel barrier  10 , typically of MgO, in between the magnetic layers  14 ,  16  as illustrated in  FIG. 1 , and the center layer in magnetic double junctions. According to the present invention a tunnel barrier and/or a center layer is provided comprising a doped magnesium oxide (MgO). The dopand is preferably Al or Li. Also doping with B, Na, Si, P, S, K, Sc, Ti, Cu, or Rb is possible. The result of the doping, in the case of a MTJ, is that a material with reduced resistance compared to MgO is provided. Thereby the spacer layer can be made thicker without decreasing the performance of the device, or alternatively the performance may be increased maintaining the same thickness. 
         [0022]    Tardio et al. [12] describe an experiment of doping chemically or by ion implantation MgO crystals with Li. They observe very large increases of 7 to 14 orders of magnitude in the electrical conductivity of the doped material compared to the nominally insulating pure MgO. Mg is a valence  2  metal, so doping with Li of valence  1  leads to formation of holes and a p-type semiconducting behavior. Other dopands in MgO discussed in the literature include Al, Cu, Co, Ni [13, 14, 15, 16]. As in the case of Li-doping, larger concentrations of other dopands are expected to substantially increase the conductivity of nominally insulating MgO. Thus doping provides a means for increasing the conductivity of MgO, which is desirable for the sensor, memory, and logic devices discussed. 
         [0023]    In the case of devices utilizing a magnetic double junction, for example magnetic tunneling diodes and transistors, the doped MgO as a central layer results in a conductive center layer with significantly longer Fermi wavelength (lower carrier density) compared to the previously reported metal layer. The center layer according to the invention can be made thicker and is process compatible with the metal oxide stack. 
         [0024]    One embodiment of the present invention is a magnetic tunnel junction, MTJ, as illustrated in  FIG. 2 , fabricated on a substrate or an underlayer generally as a part of an electronic circuit. Terminals  26  and  28  are connected to magnetic layers  22  and  24 . The magnetic layers are preferably Fe, Co, Ni, alloys Fe—Co, Ni—Fe, Fe—Co—B, or other magnetic materials. Separating the magnetic layers is a layer of doped MgO, denoted Mg x M y O 20, produced for example by post-deposition oxidation or reactive sputtering in Ar—O 2  atmosphere of Mg, which is alloyed with such elements as Li or Al (M). The concentration of Mg is preferably x&gt;0.9. The concentration of the dopant is y&lt;&lt;1, preferably y&lt;0.1. The magnetic junction is formed by sequential deposition of layers  24 ,  20 ,  22 . The object of the doping of the Mg x M y O layer  20  is to provide a layer that has a smaller resistance than a corresponding layer of undoped MgO for layer thickness &gt;1 nm. The junction comprising a doped Mg x M y O layer  20  should preferably have a resistance of &lt;10% of that of a junction with an undoped MgO layer. The layer of doped Mg x M y O can be made in the order of 1 to 10 nm and still maintaining the performance of the MTJ, compared to around 1 nm in a MTJ using undoped MgO. As apparent to the skilled in the art this is a great advantage from a production point of view. 
         [0025]    A second embodiment of the present invention is a magnetic double junction as illustrated in  FIG. 3 , fabricated on a substrate or an underlayer generally as a part of an electronic circuit. Terminals  36  and  38  are connected to magnetic layers  32  and  34 . The magnetic layers are preferably Fe, Co, Ni, alloys Fe—Co, Ni—Fe, Fe—Co—B, or other magnetic materials. The magnetic layers are insulated with MgO layers  31  and  33  from a layer of doped Mg x M y O 30. Layers  30 ,  31 ,  33  are produced by post-deposition oxidation or reactive sputtering in Ar—O 2  atmosphere of doped and pure Mg in the case of layer  30  and layers  31  and  33 , respectively. The concentration of Mg in layer  30  is preferably x&gt;0.9. The concentration of the metal dopant is y&lt;&lt;1, preferably y&lt;0.1. The magnetic double junction is formed by sequential deposition of layers  34 ,  33 ,  30 ,  31 ,  32 . In addition to MR the device has diode functionality. The thicknesses of the two MgO barriers are individually varied from &lt;1 nm to approximately 3 nm to optimize the MR (preferably &gt;20%) and the diode effect (preferably RR&gt;10). Compared to the center metal layer in previous magnetic double junction devices the center layer in the device according to this embodiment of the invention can be made significantly thicker, 1-100 nm, and the device still maintaining its functionality as regards to MR and RR. 
         [0026]    A third embodiment of the present invention is a three terminal device as illustrated in  FIG. 4 , fabricated on a substrate or an underlayer generally as a part of an electronic circuit. Terminals  46  and  48  are connected to magnetic layers  42  and  44 . The magnetic layers are preferably Fe, Co, Ni, alloys Fe—Co, Ni—Fe, Fe—Co—B, or other magnetic materials. The magnetic layers are insulated with MgO layers  41  and  43  from a layer of doped Mg x M y O 40. Layers  40 ,  41 ,  43  are produced by post-deposition oxidation or reactive sputtering in Ar—O 2  atmosphere of doped and pure Mg in the case of layer  40  and layers  41  and  43 , respectively. The concentration of Mg in layer  40  is preferably x≧0.9. The concentration of the dopand is y&lt;&lt;1, preferably y&lt;0.1. The magnetic double junction is formed by sequential deposition of layers  44 ,  43 ,  40 ,  41 ,  42 . Layers  42  and  46  are patterned in such a way as to allow a third terminal ( 45 ) be placed in contact with MgO layer  41 . Changing the potential on this terminal affects through electrostatic fields the energy levels in layer  40  and thereby the resistance between terminal  1  and terminal  2 . Thus, in addition to MR the device has transistor functionality, with the third terminal ( 45 ) acting as a gate. Alternatively, a physical contact is made between the gate ( 45 ) and the center electrode ( 40 ) as indicated by the dashed area  47 . This can be achieved by a suitable pattering of layers  42 ,  46 ,  41 . 
         [0027]    A variation on the above three terminal vertical device is a lateral device illustrated in  FIG. 5 , fabricated on a substrate or an underlayer generally as a part of an electronic circuit. Terminals  50  and  56  are connected to magnetic layers  51  and  53 . The magnetic layers are preferably Fe, Co, Ni, alloys Fe—Co, Ni—Fe, Fe—Co—B, or other magnetic materials. The magnetic layers are insulated with MgO layer  54  from a layer of doped Mg x M y O 58. Layers  58  and  54  are produced by post-deposition oxidation or reactive sputtering in Ar—O 2  atmosphere of doped and pure Mg, respectively. The concentration of Mg in layer  58  is preferably x≧0.9. The concentration of the metal dopand is y&lt;&lt;1, preferably y&lt;0.1. Layers  50 ,  56 ,  51 , and  53  are patterned in such a way as to allow a third terminal ( 52 ) be placed in contact with MgO layer  54 . Changing the potential on this terminal affects through electrostatic fields the electronic states of layer  58 . Thus, in addition to MR the device has transistor functionality, with the third terminal ( 52 ) acting as a gate. 
         [0028]    It will be understood by those skilled in the art that the subdivision of the MgO/Mg x M y O/MgO spacer into three layers in the description of the above embodiments can as well be referred to as an inhomogeneous doping of a single MgO layer. The said method of formation of the doped MgO layer by sputtering off an Mg target in Ar—O 2  mixture is one example. Other methods include, but are not limited to, sputtering of a composite Mg x M y O oxide target in Ar, sputtering off an MgO target in Ar—N 2  mixture, or sputtering off an Mg target in Ar—O 2 —N 2  mixture. In these cases M=Li, Al, N, . . . act as dopand atoms in MgO, with metal atoms preferentially substituting Mg and N atoms preferentially substituting O. 
         [0029]    It will be further understood that various changes in form and detail can be made to the above illustrative embodiments without departing from the spirit and scope of the present invention. 
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