Abstract:
A memory device includes a memory cell having a read margin that exceeds the MR ratio of the memory cell&#39;s MR element. The memory cell includes a MR element, a reference transistor, and an amplifying transistor. In some embodiments, the MR element can include a magnetic tunneling junction sandwiched between electrode layers. One of the electrode layers can be connected to an input node, which is also connected to the drain or source node of the reference transistor and the gate node of the amplifying transistor. The drain node of the amplifying transistor is connected to a sense amplifier via a conductive program line. The memory cell uses the current through the MR element to control the gate-source voltage of the amplifying transistor, and senses the state of the memory cell based on the voltage drop (or current loss) across the amplifying transistor.

Description:
TECHNICAL FIELD 
   This application relates to random access memory devices that include magnetoresistive memory elements. 
   CROSS REFERENCE TO RELATED APPLICATIONS 
   This application relates to a commonly-assigned patent application, entitled “Magnetic Random Access Memory Device”. 
   BACKGROUND 
   Magnetic random access memory (MRAM) is a type of non-volatile memory that uses magnetism rather than electrical power to store data.  FIG. 1  shows a schematic diagram of a portion  10  of an MRAM array, which includes a plurality of memory cells  12 – 19 . Each memory cell  12 – 19  includes a magnetoresistive (MR) element  20 – 27  and a transistor  30 – 37 . The transistors  30 – 33  are coupled to each other via a word line (WL 1 )  40 , and transistors  34 – 37  are coupled to each other via a word line (WL 2 )  41 , where the word lines  40 ,  41  form the gate electrode for the transistors  30 – 37 . The transistors  30 – 33  are also coupled to each other via a program line (PL 1 )  42 , and transistors  34 – 37  are coupled via a program line (PL 2 )  43 , where the program lines  42 ,  43  serve as virtual ground lines. Similarly, the MR elements  20  and  24  are coupled to each other by bit line (BL 1 )  45 , MR elements  21  and  25  are coupled to each other by bit line (BL 2 )  46 , MR elements  22  and  26  are coupled to each other by bit line (BL 3 )  47 , and MR elements  23  and  27  are coupled to each other by bit line (BL 4 )  48 . The bit lines  45 – 48  are typically somewhat perpendicular to the word lines  40 ,  41  and the program lines  42 ,  43 . 
   Each of the MR elements  20 – 27  is a multi-layer magnetoresistive element, such as a magnetic tunneling junction (MTJ) or a giant magnetoresistive (GMR) element.  FIG. 2  shows an example of a typical MTJ element  50 . The MTJ element  50  includes the following layers: a top electrode layer  52 , a free layer  53 , a spacer  54  which serves as a tunneling barrier, a pinned layer  55 , a pinning layer  56 , and a bottom electrode  57 . The free layer  53  and the pinned layer  55  are constructed of ferromagnetic material, for example cobalt-iron or nickel-cobalt-iron. The pinning layer  56  is constructed of antiferromagnetic material, for example platinum manganese. Magnetostatic coupling between the pinned layer  55  and the pinning layer  56  causes the pinned layer  55  to have a fixed magnetic moment. The free layer  53 , on the other hand, has a magnetic moment that, by application of a magnetic field, can be switched between a first orientation, which is parallel to the magnetic moment of the pinned layer  55 , and a second orientation, which is antiparallel to the magnetic moment of the pinned layer  55 . 
   The spacer  54  interposes the pinned layer  55  and the free layer  53 . The spacer  54  is composed of insulating material, for example aluminum oxide, magnesium oxide, or tantalum oxide. The spacer  54  is formed thin enough to allow the transfer (tunneling) of spin-aligned electrons when the magnetic moments of the free layer  53  and the pinned layer  55  are parallel. On the other hand, when the magnetic moments of the free layer  53  and the pinned layer  55  are antiparallel, the probability of electrons tunneling through the spacer  54  is reduced. This phenomenon is commonly referred to as spin-dependent tunneling (SDT). 
   As shown in  FIG. 3 , the electrical resistance through the MTJ  50  (e.g., through layers  52 – 57 ) increases as the moments of the pinned and free layers become more antiparallel and decreases as they become more parallel. In an MRAM memory cell, the electrical resistance of the MTJ  50  can therefore be switched between first and second resistance values representing first and second logic states. For example, a high resistance value can represent a logic state “1” and a low resistance value can represent a logic state “0”. The logic states thus stored in the memory cells can be read by passing a sense current through the MR element and sensing the resistance. For example, referring back to  FIG. 1 , the logic state of memory cell  12  can be read by passing a sense current through bit line (BL 1 )  45 , activating transistor  30  via word line (WL 1 )  40 , and sensing the current passing to program line (PL 1 )  42 . 
   During a write operation, electrical current flows through a program line  42 ,  43  and a bit line  45 – 48  that intersect at the target memory cell  12 – 19 . For example, in order to write to memory cell  13 , a current is passed through program line (PL 1 )  42  and a current is passed through bit line (BL 2 )  46 . The magnitude of these currents is selected such that, ideally, the resulting magnetic fields are not strong enough on their own to affect the memory state of the MR elements  20 – 23  and  25 , but the combination of the two magnetic fields (at MR element  21 ) is sufficient for switching the memory state (e.g., switching the magnetic moment of the free layer  53 ) of the MR element  21 . 
   SUMMARY 
   In a memory cell such as those shown in  FIG. 1 , the difference (read margin) between a current representative of logic state “1” and another current representative of logic state “0” depends directly on the magnetoresistive ratio (MR ratio) of the MR element. Thus, in such memory cells a high MR ratio is desired in order to be able to discern the difference between the to logic states. The MR ratio of an MR element varies according to applied voltage, for example as the applied voltage increases, the MR ratio decreases. Thus, since a high MR ratio is usually necessary for MRAM operation, it is necessary to keep the applied voltage relatively low so that the MR ratio does not drop to a point where the memory cell is unreadable. However, this limits access speed since a higher access speed requires a higher voltage. 
   Disclosed herein is an improved magnetoresistive memory device that includes a memory cell having a read margin that exceeds the MR ratio of the memory cell&#39;s MR element. The memory cell includes a MR element, a reference transistor, and an amplifying transistor. In some embodiments, the MR element can include a magnetic tunneling junction sandwiched between upper and lower electrode layers. The upper electrode layer can be connected to a conductive bit line. The lower electrode layer can be connected to an input node, which is also connected to the drain or source node of the reference transistor and the gate node of the amplifying transistor. The drain node of the amplifying transistor is connected to a sense amplifier via a conductive program line. Instead of passing a portion of a read current through an MR element and sensing the remaining read current as is done in prior memory cells, the present memory cell uses the current through the MR element to control the gate-source voltage of the amplifying transistor, and senses the state of the memory cell based on the voltage drop (or current loss) across the amplifying transistor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments are illustrated by way of example in the accompanying figures, in which like reference numbers indicate similar parts, and in which: 
       FIG. 1  shows a schematic diagram of a portion of an MRAM array; 
       FIG. 2  shows a schematic block diagram of a typical MTJ structure; 
       FIG. 3  shows a graph of the relationship between resistance and the relative magnetic orientations of the free and pinned layers in the MTJ shown in  FIG. 2 ; 
       FIG. 4  shows a schematic diagram of a memory cell having a magnetoresistive element and two transistors; and 
       FIG. 5  shows a simplified plan view of a memory array including memory cells such as the one shown in  FIG. 4 . 
   

   DETAILED DESCRIPTION 
     FIG. 4  shows a schematic diagram of a portion  100  of an MRAM array, which includes a memory cell  102 . The memory cell  102  includes a magnetoresistive (MR) element  104 , a reference transistor  106 , and an amplifying transistor  108 . The MR element  104  can include layers  52 – 57  shown in  FIG. 2  and described above. The reference transistor  106  has a gate node coupled to a word line (WL)  110 , a source node coupled to either a predetermined voltage V DD  or a signal ground V SS  (depending on which of the read schemes described below is used), and a drain node coupled to the bottom electrode ( 57  in  FIG. 2 ) of the MR element  104 . The amplifying transistor  108  has a gate node connected to the drain node of the reference transistor  106  and to the bottom electrode  57  of the MR element  104 . The amplifying transistor  108  also has a drain node connected to a program line (PL)  114  and a source node connected to V SS . The top electrode ( 52  in  FIG. 2 ) of the MR element  104  is coupled to a bit line (BL)  112 . The program line (PL)  114  extends in the vicinity of the MR element  104  for write operations. 
   Components of the MRAM array external to the memory cell  102  include a sense amplifier  116  connected to the PL  114 . During a read operation, the sense amplifier  116  can determine the logic state of the memory cell  102  based on whether the voltage (or current) on the PL  114  is higher or lower than a reference voltage (or current). In some embodiments, the reference voltage (or current) can come from an optional reference cell  117  connected to the sense amplifier  102 . The reference cell  117  can include an MR element fixed at a midpoint resistance level. In other embodiments, a fixed voltage (or current) can be supplied to the sense amplifier  116  for use as a reference voltage (or current). 
   The MRAM array can further include a column selector  120  and a row selector  122 . The column and row selectors  120 ,  122  are used for addressing cells of the MRAM array. For this purpose, the column selector  120  controls the voltage level of the WL  110  and the row selector controls the voltage level of the BL  112 . 
   As mentioned above, there are multiple options for read schemes for the portion  100  of the MRAM array shown in  FIG. 4 . 
   A first read scheme can be used when the reference transistor  106  has its source node connected to signal ground V SS  (e.g., where V SS  is signal ground). In order to read the data bit stored in the MR element  104 , the column selector  120  sets the WL  110  to a predetermined voltage, for example a voltage in a range of 0.3V to 1.8V. The row selector  122  sets the BL  112  to a predetermined voltage, for example a voltage in a range of 0.3V to 1.5V. The voltage VIN at input node  118  will depend on the resistance of the MR element  104  as follows: 
                   V   IN     =       V   BL     ⁢       R   REF         R   REF     +     R   MR                   (   1   )               
where R REF  is the resistance across the reference transistor  106  and R MR  is the resistance across the MR element  104 . The current or voltage level of the PL  114  can then be detected by the sense amplifier  116  in order to detect the logic state stored in the memory cell  102 . In embodiments that include a reference cell  117 , for example, the sense amplifier  116  can detect the logic state of the memory cell  102  based on a comparison of the voltage level of the PL  114  to a reference voltage level received from the reference cell  117 .
 
   Alternatively, the polarity across the reference transistor  106  and the MR element  104  can be reversed. Specifically, a second read scheme can have the BL  112  set to signal ground V SS  and the source node of the reference transistor  106  connected to a predetermined voltage V DD , for example a voltage in a range of 0.3V to 1.5V. The WL  110  is still set to a predetermined voltage, for example a voltage in a range of 0.3V to 1.8V, in order to read the data bit stored in the MR element  104 . A predetermined voltage level V DD , for example a voltage in a range of 0.3V to 1.8V, is applied to the PL  114 . As in the first read scheme, the voltage VIN at input node  118  will depend on the resistance of the MR element  104  according to Equation (1) above. The current or voltage level of the PL  114  can then be detected by the sense amplifier  116  in order to detect the logic state stored in the memory cell  102 . In embodiments that include a reference cell  117 , for example, the sense amplifier  116  can detect the logic state of the memory cell  102  based on a comparison of the voltage level of the PL  114  to a reference voltage level received from the reference cell  117   
   As a result of including the amplifying transistor  108  in the memory cell  102  and using a read operation such as those described above, a greater voltage margin for reading the memory cell can be obtained. For example, in the prior memory cell  12  shown in  FIG. 1 , the bit line current is sensed during a read operation, and varies based on the resistance of the MR element  20  according to Equation (2) below. 
                   I   BL     =       V   BL         R   MR     +     R   TR                 (   2   )               
In Equation (2), I BL  is the current of the bit line  45 , V BL  is the voltage of the bit line  45 , R MR  is the resistance of MR element  20 , and R TR  is the resistance across the transistor  30 . If the MR ratio of the MR element  20  is 30%, and R MR  &gt;&gt;R TR , then the difference between I BL  “High” (e.g., representative of a logic state “0”) and I BL  “Low” (e.g., representative of a logic state “1”) provides for a read margin of only about 30%.
 
   In contrast, for the memory cell  102  shown in  FIG. 4 , the logic state can be sensed by detecting current on the program line  114 , which varies according to the voltage at the input node  118 . In this case, if the MR ratio is 30% and the resistance R REF  across the reference transistor  106  that is close to the resistance R MR  across the MR element  104 , then the difference between I PL  “High” (e.g., representative of a logic state “0” and I PL  “Low” (e.g., representative of a logic state “1”) can provide for a read margin in a range of 50% to 200%. 
   The increased read margin is particularly advantageous for embodiments that include a reference cell  117 . In such embodiments, a read operation depends on the ability of the sense amplifier  116  to accurately determine a logic state based on whether the voltage from the memory cell  102  is higher or lower than the reference voltage received from the reference cell  117 . However, in a large array of memory cells  102 , slight differences between MR elements  104  can result in variations among the read voltages received from different memory cells  102 . If the read margin is too low, as in prior devices, such deviations in read voltages can result in false readings. On the other hand, by increasing the read margin according to the present application, the impact of differences among the MR elements  104  is greatly reduced if not eliminated. As a result, a more reliable memory device can be realized. 
   A write operation can be performed by passing sufficiently high currents through the PL  114  and the BL  112 . The magnitude of these currents is selected such that, ideally, the resulting magnetic fields are not strong enough on their own to affect the memory state of the MR element  104  (or other MR elements not shown), but the combination of the two magnetic fields (at MR element  104 ) is sufficient for switching the memory state (e.g., switching the magnetic moment of the free layer  53 ) of the MR element  104 . During a write operation, the WL  110  is set to signal ground V SS . 
     FIG. 5  shows a simplified plan view of an exemplary layout of an MRAM array composed of memory cells  102 . The memory cells  102  are arranged in rows and columns. Each memory cell  102  of a particular row is connected by a bit line  112 , while each memory cell  102  of a particular column is connected by a program line  114  and a word line  110 . 
   While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and are not limiting. Thus, the breadth and scope of the invention(s) should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and elements accomplishing any or all of the above advantages. 
   Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” such claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.