JP2007129141A - Semiconductor memory device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To acquire a structure that enables a multi-value memory of a memory element of an MTJ element, without impairing the degree of integration, in a semiconductor memory device. <P>SOLUTION: The MTJ element 81 of a first memory element is formed on a local interconnection 7 above a digit line 5. A contact plug 17 is formed on this MTJ element 81, electrically connected to the contact plug 17, and a first bit wire 101 is formed. A contact plug 19 is formed on the first bit wire 101, an MTJ element 82 of a second memory element is formed on the contact plug 19, a contact plug 23 is formed on the MTJ element 82, and a second bit line 102 is formed on the contact plug 23. The second bit line 102 has a portion formed in the same direction as the forming direction of the digit line 5, namely, a portion formed in the formation direction of the first bit wire 101, and the direction which perpendicularly intersects the MTJ elements 81 and 82. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a semiconductor memory device having a memory element such as an MTJ (Magnetic Tunneling Junction) element.

  An MRAM (Magnetic Random Access Memory) is an MTJ element including two magnetic layers (free layer and pinned layer) and a tunnel insulating layer sandwiched between these two magnetic layers as described in Non-Patent Document 1. Refers to a semiconductor memory device having a memory element as a storage element. Hereinafter, the structure and operation of the MRAM will be described. FIG. 32 is a cross-sectional view showing a memory cell structure of an MRAM including an MTJ element, cut along the bit line (BL) formation direction.

  As shown in the figure, an element isolation region 2 made of an insulator such as an oxide film is selectively formed in the upper layer portion of the semiconductor substrate 1. A gate insulating film 11 is selectively formed on the surface of the semiconductor substrate 1. A word line 3 made of a conductive film is formed on the gate insulating film 11. In the surface of the semiconductor substrate 1, source / drain regions 26 and 26 are formed with the surface (channel region) of the semiconductor substrate 1 below the word line 3 interposed therebetween.

  The word line 3 is also referred to as a “read word line” because a voltage is applied only during reading. Interlayer insulating film 12 is formed on the entire surface of semiconductor substrate 1 including word line 3. A contact hole 4 is selectively formed through the interlayer insulating film 12, and a contact plug 15 is formed by embedding a conductive film in the contact hole 4.

  A digit line 5 made of a conductive film is selectively formed on the interlayer insulating film 12. The digit line 5 is also referred to as a “write word line” because a voltage is applied only during writing. Interlayer insulating film 13 is formed on the entire surface of semiconductor substrate 1 (interlayer insulating film 12) including digit line 5. A contact hole 6 is selectively formed through the interlayer insulating film 13, and a contact plug 16 is formed by embedding a conductive film in the contact hole 6. The contact plug 16 is electrically connected to the contact plug 15 by being formed on the contact plug 15.

  A local wiring 7 made of a conductive film is selectively formed on the interlayer insulating film 13 so as to be electrically connected to the contact plug 16. An MTJ element 8 is selectively formed on the local wiring 7, and an interlayer insulating film 14 is formed on the entire surface of the semiconductor substrate 1 (interlayer insulating films 12 and 13) including the local wiring 7 and the MTJ element 8. A contact hole 9 is formed on the MTJ element 8 through the interlayer insulating film 14, and a contact plug 17 is formed by embedding a conductive film in the contact hole 9.

  A bit line 10 is provided on the interlayer insulating film 14 so as to be electrically connected to the contact plug 17, and an interlayer insulating film 25 is formed on the entire surface including the bit line 10.

  FIG. 33 is a perspective view showing a schematic configuration of the MTJ element. As shown in the figure, an ultrathin film insulator 8b is inserted between the lowermost ferromagnetic thin film 8a and the uppermost ferromagnetic thin film 8c.

  The ferromagnetic material has magnetization, and this magnetization exhibits a hysteresis curve as shown in FIG. 34 in response to an external magnetic field. When an external magnetic field is applied to the ferromagnetic thin film in the positive direction, the magnetic moment in the ferromagnetic body begins to align with the direction of the external magnetic field, and magnetization defined as the magnetic moment per unit volume occurs in the direction of the external magnetic field. start. When all the magnetic moments are aligned in the direction of the external magnetic field, the magnetization does not increase any more even if the external magnetic field is increased, and takes a saturation value. This value is called saturation magnetization.

  Conversely, when the external magnetic field is brought closer to “0”, the magnetization does not become “0” but remains at a finite value. This is called residual magnetization. Further, when the external magnetic field is increased in the negative direction, the magnetization directed in the positive direction discontinuously transitions in the negative direction at a certain external magnetic field value Hsw (−). The external magnetic field at this time is called a switching magnetic field or an inverted magnetic field. When the external magnetic field is increased further in the negative direction than the switching magnetic field, the magnetization eventually reaches saturation magnetization. The same thing happens with the reverse path starting from this state. That is, when the external magnetic field is brought closer to “0” from the negative direction, the magnetization does not become “0” but remains in the residual magnetization. Further, when the external magnetic field is increased in the positive direction, the magnetization directed in the negative direction is discontinuously transferred in the positive direction by the switching magnetic field (Hsw (+)). When the external magnetic field is increased further in the positive direction than the switching magnetic field, the magnetization eventually reaches saturation magnetization. In this way, the ferromagnetic thin film exhibits a hysteresis curve (hysteresis loop).

  The MRAM is a nonvolatile RAM that stores, as information, magnetization states that are opposite to each other in two directions appearing in a hysteresis curve. Returning to FIG. 33, the principle of information storage will be described. The ferromagnetic thin film material and structure are selected so that the switching magnetic field (Hsw, pin) of the ferromagnetic thin film 8a is considerably larger than the switching magnetic field (Hsw, free) of the ferromagnetic thin film 8c. At this time, the external magnetic field is set to a value Hext between Hsw, free and Hsw, pin. Then, since the external magnetic field is smaller than the switching magnetic field, the direction of the magnetization of the ferromagnetic thin film 8a is not reversed even when the external magnetic field is applied. For this reason, the ferromagnetic thin film 8a is called a pin layer. The magnetization of the ferromagnetic thin film 8c is reversed when an external magnetic field is applied because the external magnetic field is larger than the switching magnetic field. For this reason, the ferromagnetic thin film 8c is referred to as a free layer. Thus, by setting the switching magnetic field and the external magnetic field of the ferromagnetic thin films 8a and 8c, the MTJ element can take two states when there is no external magnetic field. That is, two different states of whether the (residual) magnetizations of the ferromagnetic thin films 8a and 8c are in a parallel state or in an antiparallel state can be stored information of the MTJ element.

  Next, the principle of reading stored information of the MTJ element will be described with reference to FIG. The stored information is read out by sensing a tunnel current flowing between the ferromagnetic thin films 8a and 8c via the ultrathin film insulator 8b. When the magnetizations of the ferromagnetic thin films 8a and 8c are in the parallel state, the electric resistance is lower than that in the antiparallel state. Therefore, when the same voltage is applied between the ferromagnetic thin films 8a and 8c, a larger current flows. . The stored information can be sensed by monitoring these two different current values.

  FIG. 35 to FIG. 38 are schematic diagrams of current differences in the case where the magnetizations of the ferromagnetic thin films 8a and 8c are in a parallel state and an antiparallel state. 35 and 37 show the MTJ element in the magnetization parallel state and its state, and FIGS. 36 and 38 show the MTJ element in the magnetization antiparallel state and its state.

  Inside the ferromagnetic body, electrons having spin are distributed in a state as shown in FIGS. The basic idea is that when the spin is oriented in the direction of magnetization, the overall energy becomes lower and the energy is more stable. If the magnetization direction of the pinned layer is spin up (↑) and the opposite direction is spin down (↓), in the parallel magnetization state, both the pinned layer and the free layer have the number of electrons of spin up (↑) (Npu, Nfu) Is larger than the number of electrons (Npd, Nfd) of spin down (↓).

  In the anti-magnetization state, the number of electrons (Npu) of spin up (↑) is larger than the number of electrons (Npd) of spin down (↓) in the pinned layer. The number of electrons (Nfu) is smaller than the number of electrons (Nfd) of spin down (↓). Electrons maintain the spin direction from the pinned layer to the free layer and tunnel the ultrathin film insulator 8b, so that the current is the number of spins up (↑) in the pinned layer (↑) and the spin in the free layer. product of up (↑) with the number of electrons (Nfu) and product of the number of electrons (Npd) of the spin down (↓) in the pinned layer and the number of electrons (Nfd) of the spin down (↓) in the free layer Is proportional to the sum of Therefore, the current flowing through the MTJ element 8 is larger in the spin parallel state than in the antiparallel state.

  FIG. 39 is a circuit diagram showing an equivalent circuit of a conventional MRAM. As shown in the figure, one end of the MTJ element MTJ0 (MTJ element 8) is connected to the bit line BL (bit line 10), and the other end is composed of a MOS transistor Q1 (word line 3, source / drain regions 26, 26). The source voltage Vs is applied to the source of the MOS transistor Q1. A digit line DL (digit line 5) is arranged in the vicinity of the MTJ element MTJ0.

  In such a configuration, a predetermined potential is applied to the bit line BL during reading, the digit line DL is brought into a floating state, the word line WL is activated, the MOS transistor Q1 is turned on, and the bit line is connected via the MTJ element MTJ0. By sensing the current flowing between BL and the source voltage Vs, the resistance value of the MTJ element MTJ0 can be recognized.

  The above is the basic property of the conventional MRAM. Further, as a storage device having a memory element including a ferromagnetic tunnel junction element corresponding to the MTJ element 8 described above and capable of storing multi-value information, for example, a storage device disclosed in Patent Document 1 is cited. It is done.

I.G Baek, et.al., IEDM Tech. Dig., Pp831-834 (2003), “MRAM with Lamellar Structure as Free Layer” JP 2001-217398 A

  Non-volatile memories are becoming multi-valued (for example, quaternary) due to difficulty in miniaturization and demand for large capacity of stored information. However, as described above, in the MRAM, the MTJ that is a memory storage node can take only two states of parallel and anti-parallel magnetization due to the characteristics of the ferromagnetic material, and therefore, a structure that stores multi-value information without impairing the degree of integration. There was a problem that it was difficult to obtain. The problem of deterioration in the degree of integration has not been solved even in the storage device disclosed in Patent Document 1.

  The present invention has been made to solve the above problems, and an object of the present invention is to enable multi-value storage in a semiconductor storage device having a storage element equivalent to an MTJ element without losing the degree of integration.

  According to a first aspect of the present invention, there is provided the semiconductor memory device according to the first aspect, wherein the first bit line is formed at a predetermined formation height and extends in the first direction, and the formation height is different from the first bit line. A digit line formed extending in a second direction different from the first direction, and arranged at a formation height opposite to the digit line with respect to the first bit line, A second bit line formed extending in a direction, and disposed at a formation height between the first bit line and the digit line, one end of which is electrically connected to the first bit line; A first memory element provided electrically independently; a bit line selection unit electrically connected to the other end of the first memory element; and between the first bit line and the second bit line Arranged at the formation height, and one end electrically connected to the second bit line. A second memory element having the other end electrically connected to the first bit line, and by supplying current to each of the first bit line, the second bit line, and the digit line, By setting the parallel / antiparallel state of magnetization between the pair of ferromagnets of each of the first and second memory elements, the combined resistance value of the first and second memory elements can be set.

  The semiconductor memory device according to claim 1 of the present invention differs depending on the parallel / antiparallel combination of magnetizations of the first and second memory elements, depending on the current supply to the first bit line, the second bit line, and the digit line. By setting multi-value (three or more types) combined resistance values, multi-value storage can be realized with the first and second memory elements as one unit of memory cells.

  At this time, by realizing the configuration in which one digit line is provided for the first and second memory elements, a multilevel memory semiconductor memory device can be obtained from the conventional binary memory configuration without losing the degree of integration. Can do.

<Embodiment 1>
(Construction)
FIG. 1 is a sectional view showing a memory cell configuration of an MRAM having an MTJ element according to the first embodiment of the present invention. FIG. 1 shows a cross-sectional structure cut along the formation direction of the first bit line.

  As shown in the figure, an element isolation region 2 made of an insulator such as an oxide film is selectively formed in the upper layer portion of the semiconductor substrate 1. A gate insulating film 11 is selectively formed on the surface of the semiconductor substrate 1. A word line 3 made of a conductive film is formed on the gate insulating film 11. In the surface of the semiconductor substrate 1, source / drain regions 26 and 26 are formed with the surface (channel region) of the semiconductor substrate 1 below the word line 3 interposed therebetween.

  Interlayer insulating film 12 is formed on the entire surface of semiconductor substrate 1 including word line 3. A contact hole 4 is selectively formed through the interlayer insulating film 12, and a contact plug 15 is formed by embedding a conductive film in the contact hole 4.

  A digit line 5 made of a conductive film is selectively formed on the interlayer insulating film 12. Interlayer insulating film 13 is formed on the entire surface of semiconductor substrate 1 (interlayer insulating film 12) including digit line 5. A contact hole 6 is selectively formed through the interlayer insulating film 13, and a contact plug 16 is formed by embedding a conductive film in the contact hole 6. The contact plug 16 is electrically connected to the contact plug 15 by being formed on the contact plug 15.

  A local wiring 7 made of a conductive film is selectively formed on the interlayer insulating film 13 so as to be electrically connected to the contact plug 16. Therefore, these contact plugs 15 and 16 function as a conductive layer (fourth conductive layer) that electrically connects one electrode region 26d, which is one of the source / drain regions 26 and 26, and the local wiring 7. . The interlayer insulating films 12 and 13 function as an interlayer insulating region (first interlayer insulating region) formed on the entire surface including the one electrode region 26d.

  An MTJ element 81 as a first memory element is selectively formed on the local wiring 7, and an interlayer insulating film is formed on the entire surface of the semiconductor substrate 1 (interlayer insulating films 12 and 13) including the local wiring 7 and the MTJ element 81. 14 is formed. A contact hole 9 is formed on the MTJ element 81 so as to penetrate the interlayer insulating film 14, and a contact plug 17 (first conductive layer) is formed by embedding a conductive film in the contact hole 9.

  A first bit line 101 is provided on the interlayer insulating film 14 and electrically connected to the contact plug 17, and an interlayer insulating film 18 is formed on the entire surface including the first bit line 101. Therefore, the MTJ element 81 is disposed at the formation height between the first bit line 101 and the digit line 5.

  A contact hole 19 is formed through the interlayer insulating film 18, and a conductive film is embedded in the contact hole 20, thereby forming a contact plug 19 (second conductive layer). The contact plug 19 is electrically connected to the first bit line 101 by being formed on the first bit line 101.

  An MTJ element 82 as a second memory element is formed on the contact plug 19. The MTJ elements 81 and 82 are composed of a pair of ferromagnetic thin films and an ultrathin film insulator formed between them, like the MTJ element 8 shown in FIG.

  Interlayer insulating film 22 is formed on the entire surface of semiconductor substrate 1 (interlayer insulating films 12 to 14, 18) including MTJ element 82. A contact hole 24 is formed through the interlayer insulating film 22 on the MTJ element 82, and a contact plug 23 (third conductive layer) is formed by embedding a conductive film in the contact hole 24.

  A second bit line 102 is formed on the contact plug 23. Accordingly, the second bit line 102 is disposed above the first bit line 101 in the direction opposite to the digit line 5. Further, the MTJ element 82 is formed at a formation height between the first bit line 101 and the second bit line 102.

  An interlayer insulating film 25 is formed on the entire surface of the semiconductor substrate 1 (interlayer insulating films 12 to 14, 18, 22) including the second bit line 102.

  The second bit line 102 is formed in the same direction as the digit line 5 formation direction and in a direction orthogonal to the formation direction of the first bit line 101.

  Table 1 shows the wiring layer comparison contents between the conventional MRAM shown in FIG. 32 and the multi-value (4-value) MRAM of the present embodiment. In Table 1, WL is a word line (corresponding to word line 3 in FIGS. 1 and 32), DL is a digit line (corresponding to digit line 5 in FIGS. 1 and 32), and MTJ1 is a first MTJ element (FIG. 1). MTJ element 81, corresponding to MTJ element 8 in FIG. 32), BL1 is a first bit line (corresponding to first bit line 101 in FIG. 1, bit line 10 in FIG. 32), and MTJ2 is a second MTJ element (in FIG. 1). The MTJ element 82) and BL2 mean the second bit line (corresponding to the second bit line 102 in FIG. 1).

  As shown in Table 1, in the memory cell of the first embodiment, the MTJ element 82 and the second bit line 102 which are the second MTJ element and the second bit line are newly added to the memory cell of the conventional configuration. It becomes the made component. However, the second bit line 102 can be formed in a layer used as a wiring also in the peripheral circuit portion. That is, by patterning the second bit line 102 using the same mask as the wiring of the peripheral circuit portion, if only the MTJ element 82 is added to the entire MRAM chip, basically, the MRAM of the conventional configuration is more than the MRAM. Value (4-value) MRAM can be designed.

  As a result, from the viewpoint of process steps, it is possible to add one mask step if the best method is taken. As will be described in detail later, the easy axis directions of the MTJ element 81 and the MTJ element 82 can also be the same as the bit line formation direction of the conventional MRAM. While applying a magnetic field, deposit thin-film ferromagnets, or at the final stage of the process, align the magnetization of the thin-film ferromagnets and initialize the information stored in the MTJ element, and then anneal in a strong magnetic field. In this case, it is essential that the easy axis directions of the MTJ element 81 and the MTJ element 82 are the same as the first bit line 101 formation direction. However, the request is satisfied.

  Table 2 shows quaternary accumulation information by the MRAM of the present embodiment. As shown in Table 2, in the MRAM according to the present embodiment, the stored information is defined by the combined resistance values R1 to R4 by connecting the MTJ element 81 and the MTJ element 82 in series. In Table 2, the resistance value of the MTJ element 81 is shown as (r1, ap) in the magnetization antiparallel state and (r1, p) in the magnetization parallel state. Similarly, the resistance value of the MTJ element 82 is the magnetization value. It is shown as (r2, ap) in the antiparallel state and (r2, p) in the magnetization parallel state.

  In Table 2, among the combined resistance values R1 to R4, R1, R2, R3, and R4 are set in order from the largest. That is, the combined resistance values R1 to R4 have a relationship satisfying the following expression (1).

  In the MTJ elements 81 and 82, the resistance value in the magnetization antiparallel state is expressed by the following equation (2), and the resistance value in the magnetization parallel state is expressed by the following equation (3).

In equations (2) and (3), “RA (Resistance Area Products)” means a standardized low resistance coefficient that is an index of MTJ resistance value, and “MR” is a high resistance value Rhigh. And the low resistance value Rlow means a resistance ratio determined by the following equation (4), S _mtr means a flat area of the MTJ element, and when the subscript k = 1, it means the MTJ element 81, k = 2 means the MTJ element 82.

  From the definitions shown in Table 2, “(r1, ap)> (r1, p)” and “(r2, ap)> (r2, p)” are established. The combined resistance of the entire MTJ is effective as the sum of the resistance value of the MTJ element 81 and the resistance value of the MTJ element 82, so that “(r1, p) + (r2, ap)> (r2, p) + (r1 , ap) ”,“ R1> R2> R3> R4 ”.

  That is, if the condition “(r2, ap) − (r2, p)> (r1, ap) − (r1, p)” is satisfied, the expression (1) is satisfied and the four-value accumulation information can be stored. Become. From the expressions (2) and (3), the left and right terms of the above conditional expression are expressed by the following expression (5).

  Then, when equation (5) is applied to the above condition, the following equation (6) is derived. Therefore, if the MTJ elements 81 and 82 have a relationship satisfying the expression (6), the expression (1) can be satisfied, and quaternary accumulation information can be stored.

For example, when MTJ elements 81 and 82 are formed under the same process conditions (RA) ₁ = (RA) ₂ and (MR) ₁ = (MR) ₂ in order to satisfy Expression (6), MTJ elements By making the plane area S _{mtr, 2} of 82 smaller than S _{mtr, 1} of the MTJ element 81, quaternary storage is possible with the MTJ element 81 and the MTJ element 82 as one unit of memory cells.

(Write operation)
2 to 5 are explanatory diagrams showing the contents of the write operation by the MRAM according to the first embodiment. 2 to 5 schematically show the planar structures of the MTJ elements 81 and 82 when viewed from above the structure of FIG.

  Table 3 shows the contents written in the MTJ resistance state (the combined resistance values R1 to R4) by the MTJ element 81 and the MTJ element 82.

  2 to 5, the positive direction ((+) direction) of the current flowing through the digit line DL (digit line 5) and the bit line BL2 (second bit line 102) is the upward direction (in FIG. 1, perpendicular to the paper surface). The positive direction of the current flowing through the bit line BL1 (first bit line 101) is defined as the right direction (the right direction in FIG. 1).

  As shown in FIGS. 2 to 5, the magnetization direction of the pin (Pin) layer of the MTJ elements 81 and 82 is fixed in the left direction in the figure. As described above, four types of writing with the combined resistance values R1 to R4 can be performed on the MTJ elements 81 and 82 fixed in the magnetization direction of the pinned layer.

  As shown in FIG. 1, since the first bit line 101 is provided between the MTJ elements 81 and 82, the MTJ elements 81 and 82 received by the write magnetic field generated by the current of the first bit line 101 is free ( The magnetization torque of the free layer is completely opposite between the MTJ element 81 and the MTJ element 82, and the magnetizations of both are directed to exactly the opposite.

  As shown in Table 3 and FIG. 2, the combined resistance value R1 is written by setting the word line WL to the off state (OFF), setting the current direction flowing through the digit line DL and the bit line BL1 to the positive direction, and setting the bit line BL2 to the positive direction. This is done by setting the direction of the flowing current to the negative direction ((−) direction).

  As shown in FIG. 2, with the write setting of the combined resistance value R1, in the MTJ element 81, the write magnetic field Hdl caused by the current flowing through the digit line DL (DL current) is directed to the right in the figure, and the current flowing through the bit line BL1 ( The write magnetic field Hbl1 due to (BL1 current) is upward in the figure. As a result, the magnetization of the free layer of the MTJ element 81 is between the right direction and the upward direction. When no current is applied, the direction in which the bit line BL1 (first bit line 101) is formed (BL1 direction; positive and negative direction) Because there is an easy axis, the magnetization settles in the right direction. The easy axis will be described in detail later.

  On the other hand, in the MTJ element 82, the write magnetic field Hbl1 due to the BL1 current is downward in the figure, and the write magnetic field Hbl2 due to the current flowing through the bit line BL2 (BL2 current) is rightward in the figure. As a result, the magnetization of the free layer of the MTJ element 82 is between the downward direction and the right direction, and when the current is not applied, since there is an easy axis in the BL1 direction (regardless of the positive and negative directions), the magnetization is right Settle in the direction.

  Accordingly, the free layers of the MTJ element 81 and the MTJ element 82 both have a magnetization direction opposite to the magnetization of the pinned layer, and both the MTJ element 81 and the MTJ element 82 have a high resistance state ((r1, ap) and (r2, ap) Thus, the accumulated information of the combined resistance value R1 having the highest resistance value as a whole can be written.

  As shown in Table 3 and FIG. 3, the combined resistance value R2 is written by setting the word line WL to the OFF state, setting the current direction flowing through the digit line DL and the bit line BL2 to the negative direction, and the current direction flowing through the bit line BL1. Is set in the positive direction.

  As shown in FIG. 3, in the MTJ element 81, the write magnetic field Hdl due to the DL current is directed to the left in the drawing and the write magnetic field Hbl1 due to the BL1 current is directed upward in the drawing due to the write setting of the combined resistance value R2. As a result, the magnetization of the free layer of the MTJ element 81 is between the left direction and the upward direction, and when the current is not applied, the magnetization settles in the left direction because there is an easy axis in the BL1 direction.

  On the other hand, in the MTJ element 82, the write magnetic field Hbl1 due to the BL1 current is downward in the figure, and the write magnetic field Hbl2 due to the BL2 current is rightward in the figure. As a result, the magnetization of the free layer of the MTJ element 82 is between the downward direction and the right direction, and when the current is not applied, since there is an easy axis in the BL1 direction, the magnetization settles in the right direction.

  Therefore, the free layer of the MTJ element 81 has the same magnetization direction as the magnetization of the pinned layer, the free layer of the MTJ element 82 has the magnetization direction opposite to the magnetization of the pinned layer, and the MTJ element 81 has a low resistance state ((r1, p)), and the MTJ element 82 is in the anti-parallel magnetization high resistance state ((r2, ap)), and the accumulated information of the combined resistance value R2 having the second highest resistance value as a whole can be written.

  As shown in Table 3 and FIG. 4, the combined resistance value R3 is written by setting the word line WL to the OFF state and setting the current direction flowing through the digit line DL, the bit line BL1, and the bit line BL2 to all positive directions. Is called.

  As shown in FIG. 4, in the MTJ element 81, the write magnetic field Hdl due to the DL current is directed rightward in the drawing and the write magnetic field Hbl1 due to the BL1 current is directed upward in the drawing due to the writing setting of the combined resistance value R3. As a result, the magnetization of the free layer of the MTJ element 81 is between the right direction and the upward direction, and when the current is not applied, the magnetization settles in the right direction because there is an easy axis in the BL1 direction.

  On the other hand, in the MTJ element 82, the write magnetic field Hbl1 due to the BL1 current is directed downward in the figure, and the write magnetic field Hbl2 due to the BL2 current is directed to the left in the figure. As a result, the magnetization of the free layer of the MTJ element 82 is between the downward direction and the left direction, and the magnetization settles in the left direction because there is an easy axis in the BL1 direction when no current is applied.

  Therefore, the free layer of the MTJ element 81 has the magnetization direction and the anti-body magnetization direction of the pinned layer, the free layer of the MTJ element 82 has the same magnetization direction as the magnetization of the pinned layer, and the MTJ element 81 has a high resistance state (( r1, ap)), and the MTJ element 82 is in a low-parallel resistance state ((r2, p)), and the accumulated information of the combined resistance value R3 having the third highest resistance value can be written as a whole.

  As shown in Table 3 and FIG. 5, the combined resistance value R4 is written by setting the word line WL to the OFF state, setting the current direction flowing through the digit line DL to the negative direction, and the current direction flowing through the bit line BL1 and the bit line BL2. Is set in the positive direction.

  As shown in FIG. 5, in the MTJ element 81, the write magnetic field Hdl due to the DL current is directed to the left in the figure and the write magnetic field Hbl1 due to the BL1 current is directed upward in the figure due to the write setting of the combined resistance value R4. As a result, the magnetization of the free layer of the MTJ element 81 is between the left direction and the upward direction, and when the current is not applied, the magnetization settles in the left direction because there is an easy axis in the BL1 direction.

  On the other hand, in the MTJ element 82, the write magnetic field Hbl1 due to the BL1 current is directed downward in the figure, and the write magnetic field Hbl2 due to the BL2 current is directed to the left in the figure. As a result, the magnetization of the free layer of the MTJ element 82 is between the downward direction and the left direction, and the magnetization settles in the left direction because there is an easy axis in the BL1 direction when no current is applied.

  Therefore, the free layers of the MTJ element 81 and the MTJ element 82 both have the same magnetization direction as the magnetization of the pinned layer, and both the MTJ element 81 and the MTJ element 82 are in the low resistance state ((r1, p) and (r2, p) Thus, the accumulated information of the combined resistance value R4 having the lowest resistance value as a whole can be written.

  Thus, as shown in Table 3, the MRAM according to the first embodiment performs a single write operation of setting the directions of the DL current, the BL1 current, and the BL2 current, thereby connecting the MTJ elements 81 and 82 in series. Any one of the combined resistance values R1 to R4 of the connection can be set.

  In the above description, the MTJ elements 81 and 82 have been described on the assumption that the easy axis direction or the pinned layer direction is the lower bit line (BL1) direction. Even if the axial direction or the direction of the pinned layer is the digit line (DL) direction, the reference direction in the explanation is only changed from the lower bit line direction to the digit line direction, and the conclusion with respect to the reference direction is maintained. .

  Further, by making the formation direction (second direction) of the second bit line 102 on the digit line 5 and the MTJ elements 81 and 82 orthogonal to the formation direction (first direction) of the first bit line 101, the MTJ It is possible to efficiently set the parallel / antiparallel state of magnetization between a pair of ferromagnetic thin films for each of the elements 81 and 82.

(Easy axis)
Hereinafter, the relationship between the easy axis of the magnetic material, the shape and the material will be described. As a simple example, a case where a rectangular thin film ferromagnetic material having uniaxial anisotropy undergoes magnetization reversal under an external uniform magnetic field applied in the uniaxial anisotropy direction will be described.

FIG. 6 is an explanatory diagram showing a schematic configuration of a rectangular thin film ferromagnetic body. The energy of the ferromagnetic material is composed of the following three components (E _{1 to} E ₃ ).

E ₁ : Uniaxial anisotropy energy of ferromagnetic material E ₂ : Interaction energy between external uniform magnetic field and magnetization in ferromagnetic material E ₃ : Strong due to ferromagnetic surface magnetic charge generated by magnetization in ferromagnetic material Electromagnetic energy inside the magnetic material From the energy principle, the total energy E of these three components is described as in equation (7), and the stable state of magnetization is determined by the minimum point of this total energy.

In accordance with this principle, the reversal of magnetization in a rectangular thin film ferromagnet having uniaxial anisotropy under an external uniform magnetic field is explained using a specific expression. Here, as shown in FIG. 6, the center of the rectangular thin film ferromagnet 80 is the origin, and the uniaxial anisotropy direction of the ferromagnet (or the direction of the applied external uniform magnetic field) is the z-axis (z− thin film of the rectangular thin film ferromagnet 80 so that the y-axis is perpendicular to the z-axis and is in the rectangular thin film ferromagnet 80, and the Cartesian coordinate system is the right-handed system. Take the x-axis perpendicular to the direction. The three components (E _{1 to} E ₃ ) of energy in the equation (7) are expressed by the following equations (8) to (10).

In the equations (8) to (10), V means the volume of the ferromagnetic material, K ₁ means the uniaxial magnetic anisotropy coefficient, and θ is the uniaxial direction of the ferromagnetic material and the uniform magnetization. This means the polar angle formed (see FIG. 6), and H means the magnitude of the external uniform magnetic field. In the integral display of the internal electromagnetic energy E ₃ , H _d is a demagnetizing field vector due to the ferromagnetic surface magnetic charge generated by the magnetization in the ferromagnetic body, and the volume integral is performed in the ferromagnetic region. M is a saturation magnetization vector of the ferromagnet, and its magnitude is M _s .

In general, H _d is not uniform inside the ferromagnetic material, but since the target ferromagnetic material is a thin film, it can be approximated by its center value H _dc . Since M is in the thin film plane of the rectangular thin film ferromagnet 80 and the saturation magnetization M _s has a constant value, the integrand function of the internal electromagnetic energy E ₃ becomes an integral variable in the volume integration region. It is a constant value that does not depend, and as a result, it can be output as a constant outside the volume. Further, the energy origin E ₀ is expressed by the following equation (11). In equation (11), l is the length in the uniaxial direction of the rectangular thin film ferromagnet (see FIG. 6), and w is the length in the direction perpendicular to the uniaxial axis of the rectangular thin film ferromagnet (see FIG. 6). , T means the thickness of the rectangular thin film ferromagnetic material (see FIG. 6).

When the magnitude of the demagnetizing field when the magnetization is oriented in the uniaxial direction is set as shown in the equation (12), the internal electromagnetic energy E ₃ is expressed by the following equation (13), and the energy sum E is It is represented by the formula (14) of the pair.

Here, when K _eff is _defined as in Expression (15), the energy sum E becomes Expression (16), and the derivative of the energy sum E with respect to θ is expressed by Expression (17). Note that θ is a polar angle formed by the uniaxial direction of the ferromagnetic material and uniform magnetization, and can take a value from “0” to π.

Therefore, the differential of the energy sum E becomes “0” at θ = 0, θ ₀ , and π, and these values are E (0), E (θ ₀ ), and E (π), respectively. (0), E (θ ₀ ), E (π), and cos θ ₀ are expressed by the following equations (18) to (21).

When K _eff > 0, in the holding state where no external uniform magnetic field is applied, θ = 0 and θ = π, the energy sum E takes the same minimum value, and θ = θo takes the maximum value. . Therefore, in the retained state, the magnetization is in a stable state when it is in one of the uniaxial anisotropy directions (either in the positive or negative z-axis direction). It corresponds to one memory retention state.

7 to 10 are explanatory diagrams showing states of the energy sum E when K _eff > 0 and H> 0.

As shown in FIG. 7, it is assumed that the actual holding state is θ = π (the negative direction of the z axis). A certain critical external uniform magnetic field H _c above the reversal magnetic field is _expressed by equation (22).

Then, when an external uniform magnetic field is applied in the positive direction in the range of 0 <H <H _c , as shown in FIG. 8, the energy of the stable point at θ = 0 decreases, and θ = π The energy of the stable point becomes larger, and the energy maximum point at θ = θ ₀ approaches θ = π. As shown in FIG. 9, when H = H _c , the maximum point θ = θ ₀ coincides with the stable point θ = π, and becomes a maximum point (more precisely, an inflection point). Then, as shown in FIG. 10, the stable point θ = π becomes an unstable point, and the magnetization transitions to the stable point θ = 0. This is the reversal of the magnetization from the negative direction to the positive direction. Therefore, the critical magnetic field H _c is a magnetization switching magnetic field. In this state, even if the external magnetic field is turned off, the magnetization remains in the state of θ = 0, the magnetization becomes stable, and the memory is held.

FIGS. 11-14 is explanatory drawing which shows the state of the energy sum total E in the case of _Keff > 0 and H <0.

As shown in FIG. 11, it is assumed that the actual holding state is θ = 0 (the positive direction of the z axis), contrary to the case of FIG. Then, when an external uniform magnetic field is applied in the negative direction in the range of 0>H> −H _c , as shown in FIG. 12, the energy of the stable point at θ = 0 increases, and θ = π The energy of the stable point at becomes smaller, and the energy maximum point at θ = θ ₀ approaches θ = 0. As shown in FIG. 13, the maximum point θ = θ ₀ coincides with the stable point θ = 0 at H = −H _c , and becomes a maximum point (more precisely, an inflection point). Then, as shown in FIG. 14, the stable point θ = 0 becomes an unstable point, and the magnetization transitions to the stable point θ = π. This is the reversal of the magnetization from the positive direction to the negative direction. Therefore, even if the external magnetic field is turned off in this state, the magnetization remains in the state of θ = π, the magnetization is in a stable state, and the memory is held.

In the case of K _eff <0, in a holding state where no external uniform magnetic field is applied, the energy sum E takes the same maximum value at θ = 0 and θ = π, and takes the minimum value at θ = θ ₀ . Accordingly, in the retained state, the magnetization is stable when it is oriented in the direction perpendicular to the uniaxial anisotropy direction (either the positive or negative direction of the y-axis). It corresponds to one memory retention state.

15-18 is explanatory drawing which shows the state of the energy sum total E in the case of _Keff <0, H> 0.

As shown in FIG. 15, the holding state is θ = π / 2 (in either the positive or negative direction of the y-axis), but the external uniform magnetic field is in the range of 0 <H <H _{c in} the positive direction. As shown in FIG. 16, the energy at the maximum point at θ = 0 decreases, the energy at the maximum point at θ = π increases, and the energy stable point at θ = θ ₀ becomes as shown in FIG. , Approaches θ = 0. Then, as shown in FIG. 17, when H = _Hc , the stable point θ = θ ₀ coincides with the maximum point θ = 0 and becomes the minimum point (more precisely, the inflection point). Then, as shown in FIG. 18, the maximum point θ = 0 becomes a stable point, and the magnetization transitions to the stable point θ = 0. However, when the external magnetic field is turned off in this state, the magnetization returns from θ = 0 to θ = π / 2, and the magnetization is not stable at the point θ = 0. The stable state of magnetization is a state of θ = π / 2, and when the magnetization is oriented in either the positive or negative direction of the y-axis, the memory retention state is established.

19-22 is explanatory drawing which shows the state of the energy sum total E in the case of _Keff <0, H <0.

As shown in FIG. 19, contrary to the case of FIG. 15, when an external uniform magnetic field is applied in the negative direction in the range of 0>H> −H _c , θ = The energy at the local maximum point at 0 increases, the energy at the local maximum point at θ = π decreases, and the energy stable point at θ = θ ₀ approaches θ = π. As shown in FIG. 21, when H = −H _c , the stable point θ = θ ₀ coincides with the maximum point θ = π, and becomes a minimum point (more precisely, an inflection point). Then, the maximum point θ = π becomes a stable point, and the magnetization transitions to the stable point θ = π. However, when the external magnetic field is turned off in this state, the magnetization returns from θ = π to θ = π / 2 as shown in FIG. 22, and the magnetization is not stable at the point θ = π. . The stable magnetization state is a state of θ = π / 2, and the memory holding state is when the magnetization is oriented in either the positive or negative direction of the y-axis.

From the above, regardless of the direction of the external uniform magnetic field, in the retained state, the magnetization is uniaxially anisotropic in the ferromagnetic material when K _eff > 0, and strong when K _eff <0. It can be seen that the magnetic material faces in the direction perpendicular to the uniaxial anisotropy direction. Therefore, the easy axis direction of the ferromagnetic material is the uniaxial anisotropic direction of the ferromagnetic material when K _eff > 0, and the uniaxial anisotropic direction of the ferromagnetic material when K _eff <0. It can be said that the direction is vertical.

_Let us look at the factor by which the polarity of K _eff determines the easy axis direction of the ferromagnetic material. The equation representing K _eff can be divided into a first term K _eff1 and a second term K _eff2 as in the following equations (23) to (25).

The first term (first axis determining first factor K _eff1 ) of the equation (23) shown in the equation (24) is a term caused by the material constant representing the crystallinity of the ferromagnetic material. The second term (the easy axis direction determination second factor K _eff2 ) of the equation (23) shown is a term caused by so-called shape magnetic anisotropy reflecting the shape effect (demagnetizing field) of the ferromagnetic material. The shape magnetic anisotropy based on the demagnetizing field is generally a great effect, but consider the case where it becomes clear. That is, when the aspect ratio (l / w) of the rectangular thin film ferromagnet is appropriately larger or smaller than “1”, K _eff is almost determined by the second term K _eff2 . Therefore, K _eff > 0 for l> w and K _eff <0 for l <w. That is, when the shape magnetic anisotropy effect is dominant, the long side direction of the rectangular thin film ferromagnetic material becomes the easy axis of the ferromagnetic material.

Next, consider the case where the aspect ratio (l / w) of the rectangular thin film ferromagnet becomes substantially equal to “1” and the shape magnetic anisotropy effect is disappearing. In this case, K _eff is almost determined by the first term K _eff1 . Therefore, in this case, K _eff > 0. That is, when the material constant of the ferromagnetic material is dominant, the easy axis is a uniaxial anisotropy direction.

  In summary, the effect caused by the material constant of the ferromagnet acts to make the easy axis in the uniaxial anisotropy direction, and the effect caused by the shape magnetic anisotropy becomes the length of the rectangular thin film ferromagnet. Acts in the side direction. In an actual device, the MTJ (MTJ elements 81 and 82) is manufactured so that one of these two effects is dominant in the easy axis.

  In the above example, the rectangular thin film ferromagnetic body 80 shown in FIG. 6 has been described as an example in order to avoid complicated expressions and place importance on the physical image. However, generally, when using the effect of shape magnetic anisotropy, an elliptical MTJ is applied, and when using the effect of the material constant of a ferromagnetic material, a circular MTJ is applied.

  In terms of process, a desired magnetization reversal magnetic field can be obtained by adjusting the ratio between the major axis and the minor axis of the thin film ferromagnet. That is, since the MTJ can be produced by a semiconductor process processing technique, it is easy to produce an MTJ using the effect of shape magnetic anisotropy. Therefore, the process is generally first developed in this direction. However, since the cell size increases in thin film ferromagnets with different major and minor axes, it is necessary to make an MTJ with a thin film ferromagnet with the major and minor axes aligned as the device is scaled. Comes out. In this case, the long axis and short axis of the thin film ferromagnet are adjusted (using shape magnetic anisotropy), and the desired magnetization reversal field cannot be obtained, and the material constant of the ferromagnet is adjusted. There is a need to. For this reason, it is necessary to develop materials in order to adjust the material constant as the device is scaled.

Finally, let's try a more intuitive explanation of shape magnetic anisotropy. Previously, the expression of the demagnetizing field H _dl when the magnetization is oriented in the uniaxial anisotropy direction is given, but in general, the demagnetizing field when the magnetization is oriented in an arbitrary direction can be obtained. Assuming that it is H _d (θ), it is expressed by the following equations (26) to (28) using the demagnetizing field H _dw when the magnetization is oriented in the direction perpendicular to the uniaxial anisotropic direction. .

  The demagnetizing field is an induced magnetic field generated by a magnetic charge formed on the surface of the ferromagnetic material by magnetization and having a direction opposite to the magnetization direction. Therefore, the action is to try to reverse the direction of magnetization.

When comparing H _dl and H _dw parallel and perpendicular to the uniaxial anisotropy direction, if the ratio (l / w) of the long side to the short side of the thin film ferromagnet is larger than “1”, H _dw Is larger than H _dl , the magnetization tends to go in the direction of uniaxial anisotropy where the demagnetizing field is small. On the contrary, when the ratio (l / w) of the long side to the short side of the thin film ferromagnet is smaller than “1”, since H _dl is larger than H _dw , the magnetization has a uniaxial anisotropy direction with a small demagnetizing field. Try to go in the vertical direction. That is, the effect caused by the shape magnetic anisotropy acts so that the easy axis is in the long side direction of the rectangular thin film ferromagnet.

  As described above, simplifying the situation when the easy axis is determined by the uniaxial anisotropy described by the material constants representing the spin exchange interaction phenomenologically and when it is determined by the shape magnetic anisotropy. explained. However, as described above, since the shape magnetic anisotropy effect is large, in reality, not all the magnetizations inside the ferromagnetic material are oriented in the easy axis direction due to the effect.

  FIG. 23 is an explanatory diagram showing the magnetization direction of the free layer of the MTJ element 81 (82) of an elliptical thin film. FIG. 24 is an explanatory view showing the magnetization direction of the free layer of the MTJ element 81 of a circular thin film.

  As shown in FIG. 23, in the elliptical thin film ferromagnet, an easy axis is formed in the major axis direction due to the shape magnetic anisotropy effect, and the magnetization tends to be aligned in that direction. However, at the edge of the ferromagnetic thin film in the short axis direction, the magnetization tends to be aligned in an elliptical shape due to the effect of the shape magnetic anisotropy. It does not align in the direction.

  As shown in FIG. 24, this edge effect appears more remarkably in the circular thin film ferromagnet. In the case of a circular thin-film ferromagnet, when a shape magnetic anisotropy disappears, a simple example of a rectangular magnetic body is shown. However, this description is an average interpretation. Actually, in the circular thin film ferromagnet, the above edge effect is larger than that of the elliptical thin film ferromagnet, and the magnetization is, on average, oriented in the uniaxial anisotropic direction, but perpendicular to the uniaxial anisotropic direction. Due to the edge effect in any direction, it is possible to obtain only a smaller magnetization than that originally obtained by the uniaxial anisotropy effect.

  An SAF (Synthetic Anti Ferromagnet) structure is a structure in which the edge effect due to the shape magnetic anisotropy is suppressed and more magnetization is directed in the easy axis direction.

  FIG. 25 is an explanatory diagram showing an MTJ element having a composite structure (SAF structure) that performs anti-exchange coupling (anti-parallel coupling). FIG. 26 is an explanatory diagram showing a composite MTJ element that performs exchange coupling (parallel coupling).

  As shown in FIG. 25, in the structure composed of the ferromagnetic thin film 8a, the ultrathin film insulator 8b, and the ferromagnetic thin film 8c, the free layer of the ferromagnetic thin film 8c is separated by two partial ferromagnetic thin films 8ca and 8cb. The SAF structure is realized by a three-layer structure of anti-exchange coupling formed with a metal thin film 8 cc interposed therebetween.

  As shown in FIG. 25, the spin repulsion occurs between the atomic layer electrons in contact with the metal thin film 8cc in the partial ferromagnetic thin film 8ca and the atomic layer electrons in contact with the metal thin film 8cc in the partial ferromagnetic thin film 8cb. The film thickness of the metal thin film 8cc is adjusted so that the exchange interaction acts, and the magnetizations of the partial ferromagnetic thin films 8ca and 8cb in contact with the metal thin film 8cc are made antiparallel. If the spin anti-exchange coupling is made strong enough to eliminate the shape magnetic effect, the magnetization tends to be aligned in the uniaxial direction due to the spin exchange coupling in the partial ferromagnetic thin films 8ca and 8cb, so that the edge effect can be suppressed. The magnetization can be aligned in the uniaxial anisotropy direction or the major axis direction. However, material development is necessary to make the spin anti-exchange coupling strong enough to eliminate the shape magnetic effect.

  However, the magnetizations in the partial ferromagnetic thin films 8ca and 8cb are opposite to each other. On the other hand, as shown in FIG. 26, the edge effect can be suppressed even if the thickness of the metal thin film 8cc is adjusted and the magnetizations of the partial ferromagnetic thin films 8ca and 8cb in contact with the metal thin film 8cc are parallel. However, since the reversal magnetic field of this composite thin film depends on the total magnetization of the composite thin film in consideration of the orientation of each ferromagnetic material, the magnetizations of the partial ferromagnetic thin films 8ca and 8cb at the portion in contact with the metal thin film 8cc are parallel. Then, the reversal field becomes too large, and the magnetization does not reverse with the desired external magnetic field, but if it is antiparallel, the reversal field is determined as the difference in magnetization of each ferromagnet, so the geometry of each ferromagnet Since the reversal magnetic field can be adjusted to a desired value depending on the physical structure and the material constant, the magnetizations of the partial ferromagnetic thin films 8ca and 8cb in contact with the metal thin film 8cc are made antiparallel.

  As a simple example, a case will be described in which a rectangular thin film composite ferromagnet having uniaxial anisotropy undergoes magnetization reversal under an external uniform magnetic field applied in the uniaxial anisotropy direction.

In this case, the energy of the ferromagnetic material is composed of the following six components.
1. 1. Uniaxial anisotropy energy of the partial ferromagnetic thin film 8ca 2. Uniaxial anisotropy energy of the partial ferromagnetic thin film 8cb 3. Interaction energy between uniform magnetic field (sum of external uniform magnetic field and external magnetic field caused by surface magnetic charge of partial ferromagnetic thin film 8cb) and magnetization in partial ferromagnetic thin film 8ca 4. Interaction energy between uniform magnetic field (sum of external uniform magnetic field and external magnetic field due to surface magnetic charge of partial ferromagnetic thin film 8ca) and magnetization in partial ferromagnetic thin film 8cb 5. Electromagnetic energy in the partial ferromagnetic thin film 8ca due to the surface magnetic charge of the partial ferromagnetic thin film 8ca generated by the magnetization in the partial ferromagnetic thin film 8ca Electromagnetic energy in the partial ferromagnetic thin film 8cb caused by the surface magnetic charge of the partial ferromagnetic thin film 8cb generated by the magnetization in the partial ferromagnetic thin film 8cb Therefore, from the energy principle, the behavior of the partial ferromagnetic thin film 8c is It is described by the total energy of these six components, and the stable state of magnetization is determined by the minimum point of this total energy.

  The expression in the case of the composite structure (SAF structure) of the anti-exchange bond shown in FIG. 25 can be obtained by replacing the expression in the case of not the composite structure with the following expressions (29) to (31). Can do.

  In addition, the expression in the case of the composite structure of exchange coupling shown in FIG. 26 can be obtained by replacing the expression in the case of not having the composite structure with the following expressions (32) to (34).

  The reversal magnetic field in the case of the SAF structure is determined by the total magnetization depending on the difference between the individual magnetizations, but the reversal magnetic field in the case of parallel coupling is determined by the total magnetization depending on the sum of the individual magnetizations. This is the reason for the antiparallel coupling. For this reason, the reversal magnetic field can be adjusted to a desired value depending on the geometric structure of each partial ferromagnetic thin film and the material constant.

(Read operation)
Table 4 shows application conditions of each signal line at the time of reading. As shown in Table 4, the word line WL (word line 3) is turned on to enable reading, the digit line DL (digit line 5) and the bit line BL1 (first bit line 101) are floated, and the bit A predetermined potential ((+) potential) is applied to the line BL2 (second bit line 102), and flows through the bit line BL2 through a selection transistor (a transistor constituted by the word line 3 and the source / drain regions 26 and 26). A read operation is performed by sensing the read current and recognizing accumulated information (any one of the combined resistance values R1 to R4).

  FIG. 27 is a circuit diagram showing an equivalent circuit of the MRAM according to the first embodiment. As shown in the drawing, one end of the second MTJ element MTJ2 (MTJ element 82) is connected to the bit line BL2, and the other end is connected to the bit line BL1. One end of the first MTJ element MTJ1 (MTJ element 81) is connected to the bit line BL1, and the other end of the MOS transistor Q1 (consisting of the word line 3 (gate electrode) and the source / drain region 26) functioning as a bit line selection unit. Connected to the drain (26d), the source voltage Vs is applied to the source of the MOS transistor Q1. A digit line DL (digit line 5) is arranged in the vicinity of the first MTJ element MTJ1 and electrically independent of the first MTJ element MTJ1.

  In such a configuration, a predetermined potential is applied to the bit line BL2 at the time of reading, the bit line BL1 and the digit line DL are brought into a floating state, the word line WL is activated, the MOS transistor Q1 is turned on, and the second MTJ element MTJ2 By sensing the current flowing between the bit line BL2 and the source voltage Vs via the bit line BL1 and the first MTJ element MTJ1, the combined resistance values R1 to R4 due to the serial connection of the second MTJ element MTJ2 and the first MTJ element MTJ1 are recognized. can do.

  As described above, the MRAM according to the first embodiment performs a single read operation of setting the potentials of the word line WL, the digit line DL, the bit line BL1, and the bit line BL2, as shown in Table 4. Based on the sense current obtained through the MOS transistor Q1, the resistance value of any one of the combined resistance values R1 to R4 of the MTJ elements 81 and 82 connected in series can be recognized.

(Layout structure)
FIG. 28 is a plan view showing a planar structure of the memory cell of the conventional MRAM shown in FIG. The AA cross section in FIG. 28 corresponds to the structure of FIG. 28 and 32, the plan view shown in FIG. 28 shows the dimensional accuracy more accurately.

  As shown in the figure, a plurality of (three in FIG. 28) bit lines 10 are formed in the horizontal direction in the figure, and a plurality of (two in FIG. 28) digit lines 5 are arranged in the vertical direction in the figure, that is, It is formed in a direction orthogonal to the bit line 10.

  In the plurality of bit lines 10, the positional relationship between the contact holes 4 and the MTJ elements 8 is formed so as to be reversed for each bit line 10. Then, in a plan view, the MTJ element 8 and the contact holes 4, 6, 9 are disposed so as to be substantially within the region where the local wiring 7 is formed.

  FIG. 29 is a plan view showing a planar structure (layout structure) of the memory cell of the MRAM according to the first embodiment shown in FIG. 29 corresponds to the structure shown in FIG. 1 and 29, the plan view shown in FIG. 29 shows the dimensional accuracy more accurately.

  As shown in the figure, a plurality (three in FIG. 29) of first bit lines 101 are formed in the horizontal direction in the figure, and a plurality (two in FIG. 29) of digit lines 5 are arranged in the vertical direction in the figure. A plurality of (three in FIG. 29) second bit lines 102 are formed while meandering along the first bit line 101 in the horizontal direction in the figure. That is, the second bit line 102 is formed to meander so as to extend in the vertical direction in the drawing only on the MTJ element 81 (82) formation region and the vicinity thereof. Therefore, the digit line 5 and the second bit line 102 are formed in a direction orthogonal to the first bit line 101 on the MTJ element 81 (82).

  In the plurality of first bit lines 101, the positional relationship between the contact hole 4 and the MTJ elements 81 and 82 is formed so as to be opposite for each first bit line 101. Then, in a plan view, the MTJ elements 81 and 82, the contact holes 4, 6, 9 and the like are arranged so as to be substantially contained in the region where the local wiring 7 is formed.

  Furthermore, on the MTJ element 81 (82), the portion extending in the vertical direction of the second bit line 102 and the digit line 5 are formed overlapping in plan view, and the MTJ element 81 and MTJ element 82 are first viewed in plan view. It is formed in the formation region of the 2-bit line 102 and the digit line 5.

  As is clear from the comparison between FIG. 28 and FIG. 29, the first embodiment in which the MTJ element 82, the second bit line 102, etc. are added from the conventional structure is realized in the planar structure without any loss of integration. is doing.

  That is, in the MRAM according to the first embodiment, as shown in FIG. 29, the portion extending in the vertical direction of the second bit line 102 and the digit line 5 are formed on the MTJ element 81 (82) so as to overlap in plan view. Then, by forming the MTJ elements 81 and 82 in a plan view in the portion of the second bit line 102 extending in the vertical direction in the figure and the digit line 5 formation region, the selection MOS transistor Q1 (word line 3,. Since the circuit area is not increased by providing the second bit line 102 and the MTJ element 82 added from the conventional configuration including the source / drain regions 26 and 26), the degree of integration is deteriorated. Absent.

  29 shows an example in which the easy axis direction of the MTJ element 81 (82) or the direction of the pinned layer is the direction of the digit line 5, the examples shown in FIGS. As shown, the easy axis direction of the MTJ element 81 can of course be the direction of the first bit line 101.

<Embodiment 2>
(Construction)
FIG. 30 is a sectional view showing a memory cell configuration of an MRAM having an MTJ element according to the second embodiment of the present invention. FIG. 30 shows a cross-sectional structure taken along the formation direction of the first bit line.

  As shown in the figure, the cross-sectional structure from the semiconductor substrate 1 to the first bit line 101 is the same as that of the first embodiment shown in FIG. Hereinafter, a cross-sectional structure from the first bit line 101 to the interlayer insulating film 25 will be described.

  An interlayer insulating film 18 is formed on the entire surface including on the first bit line 101, a contact hole 20 is formed through the interlayer insulating film 18, and a contact plug 19 is formed by embedding the conductive film in the contact hole 20. The The contact plug 19 is electrically connected to the first bit line 101 by being formed on the first bit line 101.

  A local wiring 28 is formed on the interlayer insulating film 18 in electrical connection with the contact plug 19. Therefore, the contact plug 19 functions as a conductive layer (second conductive layer) that electrically connects the first bit line 101 and the local wiring 28. The interlayer insulating film 18 functions as an interlayer insulating region (second interlayer insulating region) formed on the entire surface including on the first bit line 101.

  Then, the MTJ element 83 is selectively formed on the local wiring 28. The MTJ element 83 is composed of a pair of ferromagnetic thin films and an ultrathin film insulator formed between them, like the MTJ element 8 shown in FIG.

  An interlayer insulating film 22 is formed on the entire surface of the semiconductor substrate 1 (interlayer insulating films 12 to 14, 18) including the MTJ element 83 and the local wiring 28. A contact hole 24 is formed through the interlayer insulating film 22 on the MTJ element 83, and a contact plug 23 is formed by embedding a conductive film in the contact hole 24.

  A second bit line 102 is formed on the contact plug 23. An interlayer insulating film 25 is formed on the entire surface of the semiconductor substrate 1 (interlayer insulating films 12 to 14, 18, 22) including the second bit line 102. Second bit line 102 has a portion formed on MTJ element 83 in the same direction as digit line 5 is formed. Therefore, the second bit line 102 has a portion formed on the MTJ element 83 so as to be orthogonal to the formation direction of the first bit line 101.

(Layout structure)
FIG. 31 is a plan view showing a planar structure of the memory cell of the MRAM according to the second embodiment shown in FIG. The CC cross section in FIG. 31 corresponds to the structure of FIG. 30 and 31, the plan view shown in FIG. 31 shows the dimensional accuracy more accurately.

  As shown in the figure, a plurality (three in FIG. 31) of first bit lines 101 are formed in the horizontal direction in the figure, and a plurality (two in FIG. 31) of digit lines 5 are arranged in the vertical direction in the figure. A plurality of (three in FIG. 31) second bit lines 102 are formed in the horizontal direction in the figure while meandering on the first bit line 101. That is, the second bit line 102 is formed to meander so as to extend in the vertical direction in the drawing only in the formation region of the MTJ element 83. Therefore, the second bit line 102 is formed in the direction orthogonal to the first bit line 101 on the MTJ element 83, and the digit line 5 and the first bit line 101 are formed in the direction orthogonal to the MTJ element 81. The

  The plurality of first bit lines 101 are formed such that the positional relationship between the MTJ element 81 and the MTJ element 83 and the like is reversed for each first bit line 101. Then, the MTJ elements 81 and 83, the contact holes 4, 6, 9 and the like are arranged so as to be substantially contained in the formation region of the local wirings 7 and 28 in plan view.

  Further, as shown in FIGS. 30 and 31, the local wiring 7 is formed to extend between the MTJ element 81 and the contact hole 4 in a plan view, and the local wiring 28 is formed between the MTJ elements 81 and 83 in a plan view. It is formed to extend.

  In addition, the MTJ elements 81 and 83 are formed at a distance d in plan view, the MTJ element 81 is formed in the formation region of the digit line 5 in plan view, and the MTJ element 83 and the contact holes 4 and 6 are The second bit line 102 is formed in a formation region of a portion extending in the vertical direction in the drawing in plan view.

  As is clear from the comparison between FIG. 28 and FIG. 31, the second embodiment in which the MTJ element 83, the second bit line 102, etc. are added from the conventional structure is similar to the first embodiment in the conventional planar structure. This is achieved without compromising the degree of integration.

  That is, the MTJ element 81 is formed in the digit line 5 formation region in plan view, and the MTJ element 83 and the contact holes 4 and 6 are formed in a portion extending in the vertical direction in the drawing in plan view. By forming in the region, the circuit area by providing the second bit line 102 and the MTJ element 83 from the conventional configuration including the MOS transistor Q1 for selection (configured from the word line 3 and the source / drain regions 26 and 26) can be reduced. Since the increase is not caused, the integration degree is not deteriorated.

  Further, in the second embodiment, unlike the first embodiment, the MTJ element 83 is formed on the local wiring 28 and the planar position of the MTJ element 83 is arranged not on the MTJ element 81 but on the contact hole 4. Yes.

  As described above, in the MRAM according to the second embodiment, the MTJ element 83 corresponding to the MTJ element 82 according to the first embodiment is formed on the local wiring 28. Since the local wiring 28 is formed on the interlayer insulating film 18 that is not affected by the contact hole 20, the surface flatness is superior to the contact plug 19 that is affected by the contact hole 20. Therefore, the MTJ element 83 formed on the local wiring 28 can form the MTJ element 83 with good flatness as compared with the case where the MTJ element 82 is directly formed on the contact plug 19, and as a result, the MTJ element 83. Improvement in device characteristics can be expected.

  That is, since the MTJ elements 81 and 83 are formed on the local wiring 7 and the local wiring 28 formed on the interlayer insulating film 13 and the interlayer insulating film 18, they are formed directly on the contact plug 16 and the contact plug 19. Compared with the case, it can be formed with good flatness. As a result, since the MRAM according to the second embodiment can form the MTJ element 83 with good flatness in addition to the MTJ element 81, the MTJ elements 81 and 83 having good element characteristics can be obtained. .

  Even in the first embodiment, when priority is given to flatness, the contact plug 19 is formed at a position shifted from the lower portion of the MTJ element 82, and the MTJ element 82 is formed on the local wiring using the local wiring. Thus, the device characteristics can be improved.

  Furthermore, in the first embodiment, the MTJ elements 81 and 82 are arranged at the same plane position, but as shown in FIG. 31, in the second embodiment, the MTJ elements 81 and 83 are separated from each other by a distance d in plan view. By disposing, the distance between the MTJ elements 81 and 83 can be set longer than that between the MTJ elements 81 and 82 of the first embodiment. As a result, the influence of interference between the MTJ element 81 and the second bit line 102 and between the MTJ element 83 and the digit line 5 is caused between the MTJ element 81 and the second bit line 102 of the first embodiment, Since the influence of interference between the MTJ element 82 and the digit line 5 is smaller, the effect of solving the interference problem between the MTJ element and the farther wiring can be exhibited more than in the first embodiment.

  In the layout example shown in FIG. 31, an example in which the easy axis direction of the MTJ element 81 (83) or the direction of the pinned layer is the direction of the digit line 5 is shown. Thus, it is of course possible to make the easy axis direction of the MTJ element 81 the direction of the first bit line 101.

  In the second embodiment, the local wiring 28 is used. However, the lower plug of the MTJ element 83 may be directly connected to the first bit line 101 as in the first embodiment, with a slight sacrifice in flatness. good. Thereby, it is possible to reduce the cost by reducing the mask for forming the local wiring 28.

It is sectional drawing which shows the memory cell structure of MRAM which has the MTJ element which is Embodiment 1 of this invention. FIG. 6 is an explanatory diagram illustrating a write operation content (part 1) by the MRAM according to the first embodiment; FIG. 11 is an explanatory diagram illustrating a write operation content (part 2) by the MRAM according to the first embodiment; FIG. 6 is an explanatory diagram illustrating a write operation content (part 3) by the MRAM according to the first embodiment; FIG. 6 is an explanatory diagram showing a write operation content (part 4) by the MRAM according to the first embodiment; It is explanatory drawing which shows schematic structure of a rectangular thin film ferromagnetic. It is explanatory drawing which shows the state (the 1) of the energy sum total E in the case of _Keff > 0 and H> 0. It is explanatory drawing which shows the state (the 2) of the energy sum total E in the case of _Keff > 0 and H> 0. It is explanatory drawing which shows the state (the 3) of the energy sum total E in the case of _Keff > 0 and H> 0. It is explanatory drawing which shows the state (the 4) of the energy sum total E in the case of _Keff > 0 and H> 0. It is explanatory drawing which shows the state (the 1) of the energy sum total E in the case of _Keff > 0 and H <0. It is explanatory drawing which shows the state (the 2) of the energy sum total E in the case of _Keff > 0 and H <0. It is explanatory drawing which shows the state (the 3) of the energy sum total E in the case of _Keff > 0 and H <0. It is explanatory drawing which shows the state (the 4) of the energy sum total E in the case of _Keff > 0 and H <0. It is explanatory drawing which shows the state (the 1) of the energy sum total E in the case of _Keff <0, H> 0. It is explanatory drawing which shows the state (the 2) of the energy sum total E in the case of _Keff <0, H> 0. It is explanatory drawing which shows the state (the 3) of the energy sum total E in the case of _Keff <0, H> 0. It is explanatory drawing which shows the state (the 4) of the energy sum total E in the case of _Keff <0, H> 0. It is explanatory drawing which shows the state (the 1) of the energy sum total E in the case of _Keff <0, H <0. It is explanatory drawing which shows the state (the 2) of the energy sum total E in the case of _Keff <0, H <0. It is explanatory drawing which shows the state (the 3) of the energy sum total E in the case of _Keff <0, H <0. It is explanatory drawing which shows the state (the 4) of the energy sum total E in the case of _Keff <0, H <0. It is explanatory drawing which showed the magnetization direction of the free layer of the MTJ element of an elliptical thin film. It is explanatory drawing which showed the magnetization direction of the free layer of the MTJ element of a circular thin film. It is explanatory drawing which shows the MTJ element of the composite structure (SAF structure) which performs anti-exchange coupling. It is explanatory drawing which shows the MTJ element of the composite structure which performs exchange coupling. FIG. 3 is a circuit diagram illustrating an equivalent circuit of the MRAM according to the first embodiment. It is a top view which shows the planar structure of the memory cell of MRAM of a conventional structure. 3 is a plan view showing a planar structure of a memory cell of the MRAM according to the first embodiment. FIG. It is sectional drawing which shows the memory cell structure of MRAM which has the MTJ element which is Embodiment 2 of this invention. 6 is a plan view showing a planar structure of a memory cell of an MRAM according to a second embodiment. FIG. It is sectional drawing which shows the memory cell structure of MRAM containing the conventional MTJ element. It is a perspective view which shows schematic structure of an MTJ element. It is a graph which shows the influence by the external magnetic field of magnetization of a ferromagnetic material. It is explanatory drawing which shows typically the magnetization of a ferromagnetic thin film in a parallel state. It is explanatory drawing which shows typically the magnetization of a ferromagnetic material thin film in an antiparallel state. It is explanatory drawing which shows typically the state of a spin electron in case the magnetization of a ferromagnetic thin film is a parallel state. It is explanatory drawing which shows typically the state of a spin electron in case the magnetization of a ferromagnetic thin film is an antiparallel state. It is a circuit diagram which shows the equivalent circuit of MRAM of the conventional structure.

Explanation of symbols

1 semiconductor substrate, 2 element isolation region, 3 word line, 4, 6, 9, 20, 24 contact hole, 5 digit line, 7, 28 local wiring, 11 gate insulating film, 12-14, 18, 22, 25 interlayer Insulating film, 15-17, 19, 23 contact plug, 26 source / drain region, 81-83 MTJ element, 101, first bit line, 102 second bit line.

Claims (9)

A first bit line formed at a predetermined formation height and extending in a first direction;
A digit line disposed at a different formation height from the first bit line and extending in a second direction different from the first direction;
A second bit line having a portion extending in the second direction and disposed at a formation height opposite to the digit line with respect to the first bit line;
A first memory element disposed at a formation height between the first bit line and the digit line, one end of which is electrically connected to the first bit line and is electrically independent from the digit line When,
A bit line selector electrically connected to the other end of the first memory element;
The first bit line is disposed at a formation height between the second bit line, one end is electrically connected to the second bit line, and the other end is electrically connected to the first bit line. Two storage elements,
By supplying current to each of the first bit line, the second bit line, and the digit line, a parallel / antiparallel state of magnetization between a pair of ferromagnetic materials of each of the first and second memory elements is set. Thus, a combined resistance value of the first and second memory elements can be set.
Semiconductor memory device. The semiconductor memory device according to claim 1,
The first and second directions are substantially orthogonal to each other;
Semiconductor memory device. A semiconductor memory device according to claim 1 or 2,
The first bit line is disposed above the digit line, the second bit line is disposed above the first bit line, and the first and second storage elements are viewed in plan view. A bit line, a portion extending in the second direction of the second bit line, and a digit line forming region;
Semiconductor memory device. A semiconductor memory device according to claim 1 or 2,
The first bit line is disposed above the digit line; the second bit line is disposed above the first bit line;
The second bit line and the digit line are formed without overlapping in a plan view, and the first and second storage elements are formed at a predetermined distance in a plan view,
The first memory element is formed in a formation region of the first bit line and the digit line in plan view,
The second memory element is formed in a formation region of a portion extending in the second direction of the first bit line and the second bit line in plan view.
Semiconductor memory device. A semiconductor memory device according to claim 3 or 4, wherein
The first memory element is electrically connected to the first bit line through a first conductive layer provided on the top,
The second memory element is electrically connected to the first bit line via a second conductive layer provided below, and the second bit line via a third conductive layer provided above. Electrically connected with,
Semiconductor memory device. 6. The semiconductor memory device according to claim 5, wherein
The bit line selection unit includes a selection transistor having one electrode, the other electrode, and a control electrode,
One electrode region of the selection transistor is connected to the fourth conductive layer provided above and the first local wiring provided above the fourth conductive layer and below the first memory element. Electrically connected to the other end of the first memory element,
The first local wiring is formed to extend between the first memory element and the fourth conductive layer in plan view.
Semiconductor memory device. A semiconductor memory device according to claim 3 or 4, wherein
The first memory element is electrically connected to the first bit line through a first conductive layer provided on the top,
The second memory element is provided on a second local wiring, and is connected to the first bit line via the second conductive layer and the second local wiring provided below the second local wiring. Electrically connected and electrically connected to the second bit line via a third conductive layer provided on the upper part,
Semiconductor memory device. The semiconductor memory device according to claim 7,
The bit line selection unit includes a selection transistor having one electrode, the other electrode, and a control electrode,
One electrode region of the selection transistor is connected to the fourth conductive layer provided above and the first local wiring provided above the fourth conductive layer and below the first memory element. Electrically connected to the other end of the first memory element,
The first local wiring is formed to extend between the first memory element and the fourth conductive layer in plan view,
The second local wiring is formed to extend between the first and second memory elements in a plan view.
Semiconductor memory device. 9. The semiconductor memory device according to claim 8, wherein
A first interlayer insulating region formed on the entire surface including one electrode region of the selection transistor;
A second interlayer insulating region formed on the entire surface including on the first bit line,
The second conductive layer is formed on the first bit line through the second interlayer insulating region,
The fourth conductive layer is formed on one electrode region of the selection transistor through the first interlayer insulating region;
The first local wiring is formed on the first interlayer insulating region so as to be electrically connected to the fourth conductive layer,
The second local wiring is formed on the second interlayer insulating region so as to be electrically connected to the second conductive layer.
Semiconductor memory device.

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