Patent Publication Number: US-6985381-B2

Title: System and method for reading magnetization orientation of MRAM cells

Description:
THE FIELD OF THE INVENTION 
     The present invention generally relates to magnetic memory cells. More particularly, the present invention relates to a method for reading the magnetization orientation of such devices. 
     BACKGROUND OF THE INVENTION 
     Magnetic random access memory (MRAM) is a non-volatile thin-film memory that is used for data storage. A typical MRAM device includes an array of memory cells. Conductive traces (commonly referred to as word lines and bit lines) are routed across the array of memory cells. Word lines extend along rows of the memory cells, and bit lines extend along columns of the memory cells. Each memory cell is located at a cross point of a word line and a bit line, and stores a single bit of information. 
     The memory cells may be magnetic memory cells, such as spin dependent tunneling junctions. A typical magnetic memory cell includes a layer of ferromagnetic film in which the magnetization orientation is alterable (referred to as a sense layer or a data storage layer), and a layer of ferromagnetic film in which the magnetization orientation is fixed in a particular direction (referred to as a reference layer or a pinned layer). An insulating tunnel barrier is sandwiched between the ferromagnetic layers. 
     A logic value may be written to a magnetic memory cell by applying a magnetic field that sets the relative orientations of the memory cell&#39;s sense layer and reference layer to either parallel (logic “0”) or anti-parallel (logic “1”). The magnetization orientation in the sense layer aligns along an axis of the sense layer that is commonly referred to as its easy axis. External magnetic fields are applied to flip the magnetization orientation in the sense layer along its easy axis to either a parallel or anti-parallel orientation with respect to the magnetization orientation of the reference layer, depending on the desired logic state. The magnetization orientation of each memory cell will thus assume one of two stable orientations at any given time (i.e., parallel or anti-parallel). The parallel or anti-parallel orientation of the memory cell&#39;s ferromagnetic layers determines the resistance state of the memory cell, with a parallel orientation corresponding to a low resistance state, and an anti-parallel orientation corresponding to a high resistance state. 
     The external magnetic fields used to flip the magnetization orientation of the sense layer in a selected memory cell are created by supplying current to the word line and the bit line crossing the selected memory cell. The currents in the word line and bit line create magnetic fields that, when combined, can switch the magnetization orientation of the selected memory cell from parallel to anti-parallel or vice versa. Other unselected memory cells receive only a single magnetic field from either the word line or the bit line crossing the unselected memory cells. The magnitudes of the magnetic fields are chosen to be low enough so that the unselected memory cells do not switch their magnetization orientations when subjected to a single magnetic field from either the word line or the bit lines. An undesired switching of a memory cell that is subject only to the word line magnetic field or the write line magnetic field is commonly referred to as half-select switching. 
     As noted above, the logic value stored in a magnetic memory cell is determined by the parallel or anti-parallel orientation of the memory cell. Also, the parallel or anti-parallel orientation of the memory cell determines the resistance state of the memory cell. Thus, the logic value stored in the memory cell may be read by sensing the resistance state of the memory cell. However, the absolute difference between the resistance of a memory cell having a parallel orientation and the resistance of a memory cell having an anti-parallel orientation may be very small. Therefore, the act of measuring the resistance (i.e., reading the data in the memory cell) can itself introduce some uncertainty into the accuracy of the measurement. The act of measuring the resistance may also alter the magnetization orientation of the memory cell. If the magnetization orientation of the memory cell is altered (i.e., the reading operation is destructive), the data must also be written back into the memory cell after the data is read. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for reading the magnetization orientation of a memory cell. In one embodiment according to the invention, the method comprises applying a magnetic field to the memory cell, observing any change in resistance of the memory cell as the magnetic field is applied, and determining the magnetization orientation based upon the observed change in resistance of the memory cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. 
         FIGS. 1   a  and  1   b  are top and profile views of a prior art MRAM array. 
         FIGS. 2   a  through  2   c  are profile and side views of a prior art MRAM memory cell illustrating an orientation of magnetization of active and reference magnetic films. 
         FIG. 3  is a profile view of a prior art memory cell, its write lines, and magnetic fields generated by currents flowing through the write lines. 
         FIG. 4  is an example of a hysteresis loop showing resistance versus applied magnetic field in a memory cell whose magnetization orientation can be read according to embodiments of the invention. 
         FIG. 5  is an illustration of magnetic fields in a memory cell whose magnetization orientation can be read according to embodiments of the invention, the memory cell displaying a parallel magnetization orientation. 
         FIGS. 6   a – 6   c  are illustrations of magnetic fields in the sense layer of a memory cell whose magnetization orientation can be read according to embodiments of the invention, the illustrated magnetization orientation moving progressively from a parallel orientation toward an anti-parallel orientation as the magnetic field strength increases toward a critical value. 
         FIG. 7  is an illustration of magnetic fields in a memory cell whose magnetization orientation can be read according to embodiments of the invention, the memory cell displaying an anti-parallel magnetization orientation. 
         FIGS. 8   a – 8   c  are illustrations of magnetic fields in the sense layer of a memory cell whose magnetization orientation can be read according to embodiments of the invention, the illustrated magnetization orientation remaining unmoved from an anti-parallel orientation toward a parallel orientation at magnetic strengths less than a critical value. 
         FIG. 9   a  illustrates δR/δH of the hysteresis loop of  FIG. 4 , as the magnetization orientation of the memory cell changes from anti-parallel to parallel, which is used by embodiments of the invention for reading the magnetization orientation of the memory cell. 
         FIG. 9   b  illustrates δR/δH of the hysteresis loop of  FIG. 4 , as the magnetization orientation of the memory cell changes from parallel to anti-parallel, which is used by embodiments of the invention for reading the magnetization orientation of the memory cell. 
         FIGS. 10   a – 10   d  are exemplary shapes of a memory cell sense layer which enhance the edge domain effect in a memory cell whose magnetization orientation can be read according to embodiments of the invention. 
         FIG. 11  is a block diagram illustrating one embodiment of a system for implementing the various embodiments for reading the magnetization orientation of a memory cell according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a top plan view of a simplified prior art MRAM array  100 . The array  100  includes memory cells  120 , word lines  130 , and bit lines  132 . The memory cells  120  are positioned at each intersection of a word line  130  with a bit line  132 . Most commonly, the word lines  130  and bit lines  132  are arranged in orthogonal relation to one another. The memory cells  120  are positioned between the word and bit lines  130 , 132 , as illustrated in  FIG. 1   b . For example, the bit lines  132  can be positioned above the memory cells  120  and the word lines  130  can be positioned below. Because the word lines and the bit lines operate in combination to switch the magnetization orientation of the selected memory cell (i.e., to write the memory cell), the word lines and bit lines can be collectively referred to as write lines. 
       FIGS. 2   a  through  2   c  illustrate the storage of a bit of data in a single memory cell  120 . In  FIG. 2   a , the memory cell  120  includes a sense layer  122  and a reference layer  124  which are separated by a dielectric region  126 . The magnetization orientation in the sense layer  122  is not fixed and can assume either of two stable orientations as shown by arrow M 1 . On the other hand, the reference layer  124  has a fixed magnetization orientation shown by arrow M 2 . The sense layer  122  rotates its magnetization orientation in response to electrical currents applied to the write lines ( 130 , 132 , not shown) during a write operation to the memory cell  120 . The first logic state of the data bit stored in memory cell  120  is indicated when M 1  and M 2  are parallel to each other as illustrated in  FIG. 2   b . For instance, when M 1  and M 2  are parallel a logic “0” state is stored in the memory cell  120 . Conversely, a second logic state is indicated when M 1  and M 2  are anti-parallel to each other as illustrated in  FIG. 2   c . Similarly, when M 1  and M 2  are anti-parallel a logic “1” state is stored in the memory cell  120 . In  FIGS. 2   b  and  2   c  the dielectric region  126  has been omitted. Although  FIGS. 2   a  through  2   c  illustrate the sense layer  122  positioned above the reference layer  124 , the reference layer  124  can be positioned above the sense layer  122 . 
     The resistance of the memory cell  120  differs according to the relative orientations of M 1  and M 2 . When M 1  and M 2  are anti-parallel, i.e., the logic “1” state, the resistance of the memory cell  120  is at its highest. On the other hand, the resistance of the memory cell  120  is at its lowest when the orientations of M 1  and M 2  are parallel, i.e., the logic “0” state. As a consequence, the logic state of the data bit stored in the memory cell  120  can be determined by measuring its resistance. The resistance of the memory cell  120  is reflected by a magnitude of a sense current  123  (referring to  FIG. 2   a ) that flows in response to read voltages applied to the word and bit lines  130 , 132 . 
     In  FIG. 3 , the memory cell  120  is positioned between the write lines  130 , 132 . The separate sense and reference layers  122 ,  124  are not shown in  FIG. 3 . The magnetization orientation of the memory cell  120  is altered in response to a current I x  that generates a magnetic field H y  and a current I y  that generates a magnetic field H x . The magnetic fields H x  and H y  act in combination to rotate the magnetization orientation of the memory cell  120 . 
     As can be seen from  FIG. 4 , as the magnetization orientation of the memory cell  120  changes from one magnetization orientation to another (i.e., parallel to anti-parallel or anti-parallel to parallel), a plot of the memory cell resistance R versus the applied magnetic field H follows a hysteresis loop  200 . The shape of the resistance R versus magnetic field Hx plot is different, depending on whether the memory cell  120  is going from low resistance (parallel state) to high resistance (anti-parallel state), or from high resistance to low resistance. Specifically, the change from a high resistance to a low resistance is more abrupt than the change from a low resistance to a high resistance. This shape difference of the hysteresis loop  200  may be exploited to read the data in the memory cell  120  with a high degree of accuracy, without altering the resistance state (and thus the data) stored in the memory cell  120 . 
     The reason for the shape of the hysteresis loop  200  can be better understood by examining the magnetization orientations of the sense layer  122  and reference layer  124  in memory cell  120  as the memory cell orientation switches from parallel (low resistance) to anti-parallel (high resistance) and from anti-parallel to parallel. In  FIG. 5 , a parallel magnetization orientation of M 1  in sense layer  122  and M 2  in reference layer  124  is illustrated. For purposes of explanation and clarity, in  FIG. 5  top views of sense layer  122  and reference layer  124  are illustrated side-by-side and separately, although in actuality they would be stacked on top of each other. As illustrated in  FIG. 5 , in a parallel state, sense layer  122  and reference layer  124  both have similar net charges on the same sides of the layers (e.g., a net negative charge on the left hand side and a net positive charge on the right hand side). 
     In a parallel magnetization orientation state of memory cell  120 , as shown in  FIG. 5 , the net magnetic charges of sense layer  122  and reference layer  124  force the formation of edge domains  210  in sense layer  122 . As a negative magnetic field Hx (i.e. a field forcing the magnetic field M 1  of sense layer  122  to an anti-parallel orientation) is applied to sense layer  122 , the edge domains  210  slowly grow until negative magnetic field Hx reaches a critical strength and the magnetization orientation of M 1  in sense layer  122  abruptly switches direction to an anti-parallel state. This effect is illustrated in  FIGS. 6   a – 6   c , in which only sense layer  122  is shown. In  FIG. 6   a , the applied magnetic field Hx is 0. In  FIG. 6   b , the applied magnetic field Hx is −0.25Hc, where Hc is the critical magnetic field that will cause the magnetization orientation of M 1  in sense layer  122  to switch direction to an anti-parallel state. In  FIG. 6   c , the applied magnetic field Hx is −0.50Hc. As the magnitude of the applied magnetic field Hx grows toward the critical magnetic field Hc, edge domains  210  continue to grow toward the center of sense layer  122 , until total reversal of the magnetization orientation of M 1  in sense layer  122  occurs. As illustrated by portion  202  of hysteresis loop  200  in  FIG. 4 , the parallel to anti-parallel reversal of the magnetization orientation of M 1  in sense layer  122  may be characterized by a gradual and then increasingly rapid change from low resistance to high resistance as the magnitude of the negative magnetic field is increased. After the magnitude of the applied magnetic field Hx surpasses the critical magnetic field Hc, any further increase in the magnitude of the magnetic field has no effect on the resistance R. 
     For comparison, in  FIG. 7  an anti-parallel magnetization orientation of M 1  in sense layer  122  and M 2  in reference layer  124  is illustrated. For purposes of explanation and clarity, in  FIG. 7  top views of sense layer  122  and reference layer  124  are illustrated side-by-side and separately, although in actuality they would be stacked on top of each other. As illustrated in  FIG. 7 , in an anti-parallel state, sense layer  122  and reference layer  124  have different net charges on the sides of the layers (e.g., reference layer  124  has a net negative charge on its left hand side and a net positive charge on its right hand side, while sense layer  122  has a net positive charge on its left hand side and a net negative charge on its right hand side). 
     In an anti-parallel magnetization orientation state of memory cell  120 , as shown in  FIG. 7 , the net magnetic charges of sense layer  122  and reference layer  124  effectively cancel each other and no edge domains are formed in sense layer  122 . As a positive magnetic field Hx (i.e. a field forcing the magnetic field of sense layer to a parallel orientation) is applied to sense layer  122 , no effect is seen in the magnetic orientation of M 1  in sense layer  122 . Only when magnetic field Hx reaches a critical strength Hc will the magnetization orientation of M 1  in sense layer  122  abruptly switches direction to a parallel state. This effect is illustrated in  FIGS. 8   a – 8   c , in which only sense layer  122  is shown. In  FIG. 8   a , the applied magnetic field Hx is 0. In  FIG. 8   b , the applied magnetic field Hx is 0.25Hc, where Hc is the critical magnetic field that will cause the magnetization orientation of M 1  in sense layer  122  to switch direction to a parallel state. In  FIG. 8   c , the applied magnetic field Hx is 0.50Hc. In contrast to the gradual parallel to anti-parallel switch illustrated in  FIGS. 6   a – 6   c , in the anti-parallel to parallel switch illustrated in  FIGS. 8   a – 8   c  as the applied magnetic field Hx grows toward the critical magnetic field Hc, no effect is seen in the magnetization orientation of M 1  in sense layer  122  (or in the resistance of memory cell  120 ) until magnetic field Hx reaches a critical strength Hc. When the magnetic field reaches the critical strength Hc, the magnetization orientation of M 1  in sense layer  122  abruptly switches direction to a parallel state. As illustrated by portion  204  of hysteresis loop  200  in  FIG. 4 , the anti-parallel to parallel reversal of the magnetization orientation of M 1  in sense layer  122  may be characterized by a sudden change from high resistance to low resistance (i.e., a step transition) as the magnitude of the positive magnetic field is increased. After the magnitude of the applied magnetic field Hx surpasses the critical magnetic field Hc, any further increase in the magnitude of the magnetic field has no effect on the resistance R. 
     In one embodiment according to the invention, an electric current is supplied to one or both of write lines  130 ,  132  to create a magnetic field Hx. The resistance R of memory cell  120  is measured at a plurality of values of magnetic field Hx, and curves depicting the rate of change of the resistance R with changing magnetic field strength Hx (hereinafter “δR/δH curves”) for the memory cell  120  are determined. The measured δR/δH curves are compared to model curves as illustrated in  FIGS. 9   a  and  9   b . The model δR/δH curves of  FIGS. 9   a  and  9   b  are derivatives of the hysteresis loop  200  in  FIG. 4 , and reflect the non-symmetrical shape of hysteresis loop  200 .  FIG. 9   a  shows δR/δH as the memory cell  120  transitions from an anti-parallel state to a parallel state (portion  204  of hysteresis curve  200 ), while  FIG. 9   b  shows δR/δH as a memory cell  120  transitions from a parallel state to an anti-parallel state (portion  202  of hysteresis curve  200 ). 
     As can be seen, the model δR/δH curves of  FIGS. 9   a  and  9   b  are significantly different from each other, and can convey more conclusive information about the magnetization orientation of a memory cell than simple resistance measurements. If the measured δR/δH curve is similar to the model curve shown in  FIG. 9   a , the memory cell  120  is at a high resistance (i.e., anti-parallel) state, and the data stored in the memory cell is a logic value of “1”. If the measured δR/δH curve is similar to the model curve shown in  FIG. 9   b , the memory cell  120  is at a low resistance (i.e., parallel) state, and the data stored in the memory cell is a logic value of “0”. 
     To obtain complete measured δR/δH curves, it is necessary to measure resistance R at magnetic field Hx values around the expected critical magnetic field Hc. As discussed above, if the magnitude of Hx exceeds Hc, the measurements will alter the magnetization orientation of memory cell  120  (i.e., destructively read memory cell  120 ), and thus require that memory cell  120  be rewritten after its magnetization orientation is determined. Such rewriting of the memory cell  120  is not necessarily problematic, except in so far as it requires an additional step in the reading of data from the memory cell. In some instances, however, it is desirable if memory cell  120  is not destructively read, and thus rewriting of memory cell  120  is not required. 
     In another embodiment according to the invention, only partial δR/δH curves are obtained. In particular, resistance R is measured at magnetic field Hx values which are selected to avoid exceeding Hc, such that the magnetization orientation of the memory cell  120  is not altered. As illustrated in  FIGS. 9   a  and  9   b , even for values of Hx that will not alter the magnetization orientation of the memory cell  120 , the shape of the δR/δH curves are different enough from each other to allow comparison of the partial measured δR/δH curves against the model curves of  FIGS. 9   a  and  9   b.    
     Measuring the absolute resistance values of a memory cell to create either complete or partial δR/δH curves can be difficult, particularly when the change from parallel orientation to anti-parallel orientation or vice versa is not complete (such as along curved portion  202  of hysteresis loop  200  in  FIG. 4 ). The ability to determine the magnetization orientation of a memory cell without having to measure absolute resistance values or absolute changes in resistance (as is necessary to determine δR/δH curves) is desirable. 
     In another embodiment according to the invention, the magnetization orientation of memory cell  120  is determined without the need for measuring the absolute resistance values or absolute change in resistance values of the memory cell to create either complete or partial δR/δH curves, and without risk of altering the magnetization orientation of the memory cell (i.e., a non-destructive read operation). In this embodiment according to the invention, only a relative change in the memory cell resistance R need be observed. Referring to the hysteresis loop  200  of  FIG. 4 , it can be seen that if there is any change in resistance of the memory cell under the application of a magnetic field Hx (either positive or negative) having a magnitude less than the critical magnetic field Hc, then memory cell  120  is in the parallel state. If there is no change in the resistance of memory cell  120 , then memory cell  120  is in the anti-parallel state. For example, if memory cell  120  is in the parallel state and a negative magnetic field Hx is applied, a small increase in resistance R will be observed; if a positive magnetic field Hx is applied, no change in resistance R will be observed. If, on the other hand, memory cell  120  is in the anti-parallel state and a negative magnetic field Hx is applied, no change in resistance R will be observed; similarly, if a positive magnetic field Hx is applied, no change in resistance R will be observed. Thus, the application of only a negative magnetic field Hx allows determination of the magnetization orientation of memory cell  120  based upon the observed change, if any, in resistance R of memory cell  120 . 
     In one embodiment of the invention, a single conductor (e.g., only one of bit line  132  or word line  130 ) is energized with an electrical current to create a negative magnetic field Hx around the energized line. The magnitude of the negative magnetic field Hx is kept smaller than the critical magnetic field Hc required to flip the magnetization orientation, so that the magnetization orientation of memory cell  120  is not altered. In one embodiment, the magnitude of Hx is as large as approximately 0.6Hc. As the negative magnetic field Hx is applied, the resistance of memory cell  120  is observed to detect any change in resistance R when the negative magnetic field is applied. As described above, if there is any change in resistance R under the application of a negative magnetic field, then memory cell  120  is in the parallel state. If there is no change in the resistance of memory cell  120 , then memory cell  120  is in the anti-parallel state. 
     In another embodiment according to the invention, the magnetization orientation of memory cell  120  may be determined when both word line  130  and bit line  132  are energized. This method may be used, for example, when one of the lines  130 ,  132  is being used to write data to a memory cell  120  different from the memory cell  120  being read. Assuming one of word or bit lines  130 ,  132  is “on” (i.e., supplied with an electric current), the other line is supplied with an electric current to create a negative magnetic field Hx. As described above, the magnitude of magnetic field Hx is kept smaller than the critical magnetic field Hc necessary to flip the magnetization orientation, so that the magnetization orientation of memory cell  120  is not altered. The resistance of memory cell  120  is observed to detect any resistance change when the negative magnetic field is applied. As described above, if there is any resistance change under the application of a negative magnetic field, then memory cell  120  is in the parallel state. If there is no resistance change under the application of a negative magnetic field Hx, then memory cell  120  is in the anti-parallel state. 
     Whether one or both of write lines  130 ,  132  are supplied with an electric current to create magnetic fields about the write lines, it is not necessary to know the polarity (positive or negative) of the magnetic field Hx to determine the magnetization orientation of memory cell  120 . In one embodiment according to the invention, data may be read (that is, the magnetization orientation may be determined) from a selected memory cell  120  in an array of memory cells by supplying a first current to either one of write lines  130 ,  132  and creating either a positive or negative magnetic field in the selected write line  130 ,  132 . Memory cell  120  is observed to detect any change in resistance R as the first current is supplied to the selected write line. The first current is then reversed in the selected write line, and memory cell  120  is again observed to detect any change in resistance R as the reversed current is supplied to the selected write line. The magnetization orientation of memory cell  120  may then be determined based on the detected change (if any) in resistance R as the first current and the reversed current are supplied to the selected write line. If no change in resistance R is observed with either the first current or the reversed current, then memory cell  120  is in the anti-parallel state. If a change in resistance R is observed with either the first current or the reversed current, then memory cell  120  is in the parallel state. This same method may also be used if one of write lines  130 ,  132  is constantly energized, while the other of write lines  130 ,  132  has its current reversed. 
     In each of the embodiments according to the invention, sense layer  122  may be designed or shaped to enhance the edge domain effect described above with respect to  FIGS. 6   a – 6   c  and  8   a – 8   c . In particular, sense layer  122  may be created with shapes similar to those described in  FIGS. 10   a – 10   d . The shapes illustrated in  FIGS. 10   a – 10   d  effectively change the slope of hysteresis loop as the magnetization orientation changes from parallel to anti-parallel, such that the gradual change in resistance occurs over a greater range of the magnetic field. 
     One exemplary system  300  for implementing the various embodiments for reading the magnetization orientation of a memory cell is illustrated in  FIG. 11 . System  300  includes a magnetic memory device  302  having one or more memory cells  120  as described above. Within the magnetic memory device  302 , each memory cell  120  is operatively positioned between a conductive word line  130  and a conductive bit line  132 . A variable current source  304  is connected to the magnetic memory device  302 , such that a current of variable strength and polarity may be independently applied to one or both of the word line  130  and bit line  132  of a selected memory cell  120 . The applied current causes a corresponding magnetic field Hx of variable strength and polarity or be applied to selected memory cell  120 . 
     A resistance measurement module  306  is also connected to the magnetic memory device  302 , such that the resistance of the selected memory cell  120  can be observed or measured as the strength and/or polarity of the applied current (and thus the applied magnetic field Hx) is varied. Resistance measurement module  306  may be either software, hardware, or a combination of both, and may be capable of determining relative and/or absolute change in the resistance of selected memory cell  120 , depending upon which of the various embodiments for reading the magnetization orientation is being implemented. 
     The observed or measured change in resistance of the selected memory cell  120  as a function of the applied magnetic field Hx strength and/or polarity is supplied to a comparator module  308 . Comparator module  308  compares the measured or observed behavior of the resistance of memory cell  120  (obtained from resistance measurement module  306 ) to a model behavior of a memory cell (represented by box  310 ). Comparator module  308  may be either software, hardware, or a combination of both. The model behavior of memory cell resistance with change as a function of applied magnetic field can be δR/δH curves as discussed above with reference to  FIGS. 9   a  and  9   b . Alternately, the model behavior of memory cell resistance as a function of applied magnetic field can be a hysteresis loop as discussed above with reference to  FIG. 4 . Based upon comparison of the measured or observed behavior of memory cell  120  to the model behavior, comparator module  308  determines the status  312  of the magnetic orientation (parallel or anti-parallel) of the selected memory cell  120 .