Abstract:
Techniques, apparatus and circuits based on magnetic or magnetoresistive tunnel junctions (MTJs). In one aspect, a programmable circuit device can include a magnetic tunnel junction (MTJ); a MTJ control circuit coupled to the MTJ to control the MTJ to cause a breakdown in the MTJ in programming the MTJ; and a sensing circuit coupled to the MTJ to sense a voltage under a breakdown condition of the MTJ.

Description:
BACKGROUND 
     This application relates to magnetic materials and structures, and devices based on such materials and structures. 
     Various magnetic materials use multilayer structures which have at least one ferromagnetic layer configured as a “free” layer whose magnetic direction can be changed by an external magnetic field or a control current. One example for such a multilayer structure is a magnetic or magnetoresistive tunnel junction (MTJ) which includes at least three layers: two ferromagnetic layers and a thin layer of a non-magnetic insulator as a barrier layer between the two ferromagnetic layers. MTJ can be used in various applications, including sensing devices, memory devices and circuits. 
     SUMMARY 
     This application describes, among other features, techniques, apparatus and circuits based on magnetic or magnetoresistive tunnel junctions (MTJs). In one aspect, a programmable circuit includes a magnetic tunnel junction (MTJ); a MTJ control circuit coupled to the MTJ to control the MTJ to cause a breakdown in the MTJ in programming the MTJ, thus forming a breakdown region with a low resistance; and a sensing circuit coupled to the MTJ to sense a voltage under a breakdown condition of the MTJ. 
     In another aspect, a programmable circuit includes a first magnetic tunnel junction (MTJ) and a second MTJ that are connected to each other in series; a sensing circuit coupled to a sensing location in a conductive path between the first MTJ and second MTJ to sense a voltage at the sensing node; a first MTJ control circuit coupled to the first MTJ to control the first MTJ; and a second MTJ control circuit coupled to the second MTJ to control the second MTJ. One of the first and second MTJ control circuits operates to supply a high voltage to break down a respective MTJ to increase a sensing margin at the sensing location. 
     In another aspect, a programmable circuit includes a magnetic tunnel junction (MTJ); a circuit element that has a structure different from the MTJ and connected to the MTJ in series; a sensing circuit coupled to a sensing location in a conductive path between the MTJ and the circuit element to sense a voltage at the sensing node; a MTJ control circuit coupled to the MTJ to supply a high voltage to break down the MTJ to increase a sensing margin at the sensing location; and a circuit control circuit coupled to the circuit element to control the circuit element. 
     In yet another aspect, a method for providing a programmable circuit includes connecting a magnetic tunnel junction (MTJ) in a programmable circuit to supply a high resistance or a low resistance to the programmable circuit; and controlling a current injected into the MTJ to cause a breakdown in the MTJ in programming the programmable circuit to form a breakdown region with a low resistance. In one implementation, this method may also include connecting a second MTJ to the MTJ in series in the programmable circuit; controlling a current injected into the second MTJ to avoid a breakdown in the second MTJ in programming the programmable circuit to make the second MTJ exhibit a high resistance; and sensing a voltage at a sensing location in a conductive path between the MTJ and the second MTJ. 
     These and other aspects and implementations, their variations and modifications are described in greater detail in the attached drawings, the detailed description, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates one example of a MTJ cell structure. 
         FIG. 2  illustrates a part of a circuit for operating a MTJ cell based on the spin-transfer torque effect. 
         FIG. 3  illustrates a programmable circuit with two cascaded MTJ cells. 
         FIGS. 4 and 5  illustrate programmable circuits with one MTJ cell and one non-MTJ circuit element connected in series. 
     
    
    
     DETAILED DESCRIPTION 
     A magnetic tunnel junction (MTJ) can be implemented to use an insulator for the middle barrier layer between the two ferromagnetic layers. When the thickness of the insulator is sufficiently thin, e.g., a few nanometers or less, electrons in the two ferromagnetic layers can “penetrate” through the thin layer of the insulator due to a tunneling effect under a bias voltage applied to the two ferromagnetic layers across the barrier layer. Notably, the resistance to the electrical current across the MTJ structure varies with the relative direction of the magnetizations in the two ferromagnetic layers. When the magnetizations of the two ferromagnetic layers are parallel to each other, the resistance across the MTJ structure is at a low value R L . When the magnetizations of the two ferromagnetic layers are anti-parallel with each other, the resistance across the MTJ is at a high value R H . The magnitude of this effect is commonly characterized by the tunneling magnetoresistance (TMR) defined as (R H −R L )/R L . The ratio between the two values, R H /R L , can be, for example, up to 2 or 3 in some MTJ devices. 
     The relationship between the resistance to the current flowing across the MTJ and the relative magnetic direction between the two ferromagnetic layers in the TMR effect may be used for nonvolatile magnetic memory devices to store information in the magnetic state of the MTJ. Magnetic random access memory (MRAM) devices based on the TMR effect, for example, may be an alternative to electronic RAM devices. In such MRAM devices, one ferromagnetic layer is configured to have a fixed magnetic direction and the other ferromagnetic layer is a “free” layer whose magnetic direction can be changed to be either parallel or opposite to the fixed direction. Information is stored based on the relative magnetic direction of the two ferromagnetic layers on two sides of the barrier of the MTJ. For example, binary bits “1” and “0” may be recorded as the parallel and anti-parallel orientations of the two ferromagnetic layers in the MTJ. Recording or writing a bit in the MTJ can be achieved by switching the magnetization direction of the free layer, e.g., by a writing magnetic field generated by supplying currents to write lines disposed in a cross stripe shape, by a current flowing across the MTJ based on the spin-transfer torque effect, or by other means. In the spin-transfer switching, the current required for changing the magnetization of the free layer can be small (e.g., 0.1 mA or lower) and can be significantly less than the current used for the field switching. Therefore, the spin-transfer switching in a MTJ can be used to significantly reduce the power consumption of the cell. 
       FIG. 1  illustrates an example of a MTJ  100  formed on a substrate  101  such as a Si substrate. The MTJ  100  is constructed on one or more seed layers  102  directly formed on the substrate  101 . On the seed layers  102 , an antiferromagnetic (AFM) layer  113  is first formed and then a first ferromagnetic layer  111  is formed on top of the AFM layer  113 . After the post annealing, the ferromagnetic layer  111  later is pinned with a fixed magnetization. In some implementations, this fixed magnetization may be parallel to the substrate  101  (i.e., the substrate surface). On top of the first ferromagnetic layer  111  is a thin insulator barrier layer  130  such as a metal oxide layer. In the MTJ  100 , a second ferromagnetic layer  112  is formed directly on top of the barrier layer  130 . In addition, at least one capping layer  114  is formed on top of the second ferromagnetic layer  112  to protect the MTJ. 
     The magnetization of the ferromagnetic layer  112  is not pinned and can be freely changed to be parallel to or anti-parallel to the fixed magnetization of the pinned layer  111  under a control of either an external control magnetic field or a driving current perpendicularly flowing through the MTJ. For this reason, the layer  112  is a free layer (FL). A magnetic field in the field operating range, or an applied current across the junction in the current operating range, can force the magnetization of the free layer  112  to be substantially parallel to or substantially antiparallel to the fixed magnetization of the pinned layer  111 . Many magnetic systems have competing energy contributions that prevent a perfect parallel or antiparallel alignment of the magnetic domains or nanomagnets in each ferromagnetic layer. In MTJs, the dominant contribution to the energy state of the nanomagnets within the free layer  112  tends to force the nanomagnets into the parallel or antiparallel alignment, thus producing a substantial parallel or antiparallel alignment. 
     A MTJ also exhibits a breakdown effect when the electrical voltage applied across the MTJ junction exceeds a breakdown voltage that breaks down the insulator material for the barrier layer  130 . After the occurrence of the breakdown, the MTJ exhibits a low resistance R bk  across the barrier layer  130  that is less than R L  and, like R H  and R L , can vary with material compositions and structures of the MTJ. The ratio between R bk  and R H , R H /R bk , is much larger than the ratio of R H /R L . As an example, some MTJs can have the following resistance values: R L =2 Kohm to 5 Kohm, R H =4 Kohm to 15K ohm, and R bk =50 ohm to 200 ohm, where the worst case ratio between R H  and R bk  is R H /R bk =20 (R H =4 Kohm and R bk =200 ohm). 
     The breakdown property of the MTJ allows the MTJ to be used as a programmable electrical fuse element for various circuits. Such MTJ programmable fuses can be designed to have large sensing margin between the two logic states “0” and “1” and to provide good data retention. A MTJ can be relatively easy to program by controlling the voltage applied across the MTJ junction to the breakdown voltage. 
     An MTJ can be structured to operate under a spin-transfer torque effect where a spin polarized current is injected into the MTJ to flow in a direction perpendicular to the free layer and to control the direction of the magnetization of the free layer. The direction of magnetization can be controlled and switched via spin exchange between the electrons in the driving current and the free layer without requiring an external magnetic field.  FIG. 2  depicts another implementation, a device  200  having a magnetic element  201  based on the spin-transfer torque effect having at least one low magnetization free layer. A conductor line  210  labeled as “bit line” is electrically coupled to the magnetic element  201  by connecting to one end of the magnetic element  201  to supply an electrical drive current  240  through the layers of the magnetic element  201  to effectuate the spin-transfer torque effect in the magnetic element  201 . An electronic isolation device  230 , such as an isolation transistor, is connected to one side of the magnetic element  201  to control the current  240  in response to a control signal applied to the gate of the transistor  230 . A second conductor line  220  labeled as “word line” is electrically connected to the gate of the transistor  230  to supply that control signal. 
     In operation, the drive current  240  flows across the layers in the magnetic element  201  to change magnetization direction of the free layer when the current  240  is greater than a switching threshold which is determined by materials and layer structures of the magnetic element  201 . The switching of the free layer in the magnetic element  201  is based on the spin-transfer torque caused by the drive current  240  alone without relying on a magnetic field produced by the lines  210  and  220  or other sources. In various applications, the magnetic cell shown in  FIG. 2  is implemented as a unit cell of a magnetic array. 
       FIG. 3  shows one example of a programmable circuit  300  based on two programmable MTJs based on spin transfer effect. MTJa  310  and MTJb  320  are connected in series between  301  Vcc and  302  Vss and the magnetization of each MTJ is controlled by the current injected into each MTJ. The MTJa  310  has one terminal connected to the Vcc and another terminal connected to one terminal of the MTJb  320  while the other terminal of the MTJb  320  is connected to the Vss. A sensing circuit  330  is connected to a sensing location  303  in the conductive path between the MTJa  310  and MTJb  320  to measure the voltage Vsense at the location  303  to produce an output Vout. 
     A write circuit  312  is coupled to the MTJa  310  to control the electrical bias applied to the MTJa  310  and a separate write circuit  322  is coupled to the MTJb  320  to control the electrical bias applied to the MTJb  320 . A circuit multiplexer MUXa  314  is coupled between the Vcc  301  and the MTJa  310  to program the MTJa  310 . The MUXa  314  is used to either: (1) connect the MTJa write circuit  312  to the MTJa  310  while isolating and disconnecting the MTJa  310  from the Vcc  301 ; or (2) connect the MTJa  310  to the Vcc  301  while isolating and disconnecting the MTJa  310  from the write circuit  312 . 
     Similarly, for the write circuit  322 , a circuit multiplexer MUXb  324  is coupled between the Vss  302  and the MTJb  320  to program the MTJb  320 . The MUXb  324  is used to either (1) connect the MTJb write circuit  322  to the MTJa  320  while isolating and disconnecting the MTJb  320  from the Vss  302 ; or (2) connect the MTJb  320  to the Vss  302  while isolating and disconnecting the MTJb  320  from the write circuit  322 . 
     In one operation of the above circuit  300 , for example, the MTJb  320  can be programmed into the breakdown condition (e.g., R bk =200 ohm) and MTJa  310  is set into the high junction resistance R H  to increase the logic “0” sense margin. In order to program MTJb  320  into the breakdown condition, the MUXb circuit  324  is set so that MTJb  320  is connected to MTJb write circuit  322  and is disconnected from Vss  302 . Under this connection configuration, a higher than normal write current is provided from the MTJb write circuit  322  to flow through MTJb  320  to cause the MTJb junction  320  to breakdown. After the programming, the MUXb circuit  324  is set so that the MTJb  320  is connected to Vss  302  and disconnected from the MTJb write circuit  322  for normal circuit operation. 
     In programming the MTJa  310  into the breakdown condition (e.g., Rbk=200 ohm), the MTJb  320  is set into the high junction resistance R H . The MUXa circuit  314  is set so that MTJa  310  is connected to MTJa write circuit  312  and is disconnected from Vcc  301 . Under this connection configuration, a higher than normal write current is provided from the MTJa write circuit  312  to flow through MTJa  310  to cause the MTJa junction  310  to breakdown. After the programming, the MUXa circuit  314  is set so that the MTJa  310  is connected to Vcc  301  and disconnected from the MTJa write circuit  312  for normal circuit operation. This condition of MTJa  310  at R bk  and the MTJb  320  at R H  to increase the logic “0” sense margin in the circuit  300 . 
     For example, Vsense can be set at 0.95V as the logic “1” state and 0.05V as logic “0” state in the circuit  300  in  FIG. 3  with Vcc being set at 1V above Vss. The voltage difference, Delta V=(0.95V−0.05V)=0.9V, which is 90% of Vcc. This larger DeltaV can provide a greater sense margin for the circuit  300 . 
     In comparison, the logic “1” at Vout can be defined as MTJa=RL and MTJb=RH and the logic “0” at Vout can be defined as MTJa=RH and MTJb=RL. With Rratio=R H /R L =3 at Vcc=1.0V, Vsense=0.75V as logic “1” and Vsense=0.25V as logic “0” as an example. The Delta V is (0.75V−0.25V)=0.5V, which is 50% of Vcc. Hence, using the MTJ breakdown characteristics for programming the circuit can improve the sensing margin. 
       FIG. 4  shows another example of a programmable circuit  400  based on two programmable devices  410  and  420 , one of which can be a programmable MTJ while the other can be either a MTJ circuit element or a non-MTJ circuit element such as a MOS transistor or a resistor load. The two devices  410  and  420  are connected in series between  401  Vcc and  402  Vss. The device  410  has one terminal connected to the Vcc and another terminal connected to one terminal of the device  420  while the other terminal of the device  420  is connected to the Vss. A sensing circuit  430  is connected to a sensing location  403  in the conductive path between the devices  410  and  420  to measure the voltage Vsense at the location  403  to produce an output Vout. Two device control circuits  412  and  422  are provided to control the two devices  410  and  420 , respectively. 
       FIG. 5  shows one implementation  500  based on the circuit design in  FIG. 4  where the device  410  is implemented as a programmable MTJ  310  as shown in  FIG. 3  and the device  420  is a non-MTJ circuit element, such as a MOS transistor or a resistor load element. 
     Alternatively, another implementation of the circuit design in  FIG. 4  can use a resistor load as the device  410  and a programmable MTJ  320  as shown in  FIG. 3  as the device  420 . In operation, the initial logic “1” is obtained by programming MTJb  420  at its high resistance state R H , the device  410  at its low resistance state R L ; for example, the device  410  is at RH=4 kohm, the device  420  is set at RL=2 k (MTJ or MOS transistor/resistor value of 2 kohm), the output at Vout will be “1”. If the MTJb  420  is programmed into breakdown state, e.g., MTJb=200 ohm, the ratio between the devices  410  and  420  is 2 kohm/200 ohm=10/1 and Vout will be “0” state because the MTJ  420  is in a breakdown state. 
     Another application of the device in  FIG. 4  is having initial logic “0”. i.e., Device_a=MTJa=RH and Device_b=(MTJb or MOS transistor or resistor); for example MTJa=RH=4 kohm, and Device_b=2 kohm, the Vout is “0” initially. And, if ONLY Device_a is programmed into Rbk=200 ohm, the resistor is 200 ohm/2 kohm=1/10 and Vout=“1” as results of the breakdown process. Again, ONLY Device_a is used for programming and Device_b is used for resistor load. 
     While the specification of this application contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     Only a few implementations are described. Variations, modifications and enhancements of the described implementations and other implementations can be made based on what is disclosed in this document.