JP6069904B2 - Magnetoresistive memory - Google Patents

Description

  The present invention relates to a magnetoresistive memory.

  In recent electronic devices, the importance of large-capacity nonvolatile memories that can be embedded at low cost with respect to silicon (Si) CMOS logic is increasing.

  Magnetoresistive random access memory (MRAM) is capable of unlimited rewriting of information in addition to non-volatility of information. For this reason, attention is paid to a novel nonvolatile memory that can replace not only a ROM memory such as a flash memory but also a RAM memory such as an SRAM or a DRAM.

  MRAM uses a magnetic tunnel junction (MTJ) in which a ferromagnetic metal electrode is arranged above and below a tunnel insulating film and the tunnel resistance changes depending on the relative magnetization direction of the ferromagnetic metal electrode. Is realized. A ferromagnetic metal electrode whose magnetization direction is fixed is called a magnetization fixed layer, and a ferromagnetic metal electrode whose magnetization direction can be reversed is called a magnetization free layer. Until now, the direction of the magnetization free layer has been reversed using a magnetic field induced by passing a current through the wiring. However, in recent years, it has been found that magnetization reversal of the magnetization free layer is possible by spin-transfer torque (STT) (spin injection magnetization reversal). Thereby, the current required for rewriting can be greatly reduced, and the possibility of practical use of MRAM is further increased.

  In the following description, MgO used as a material for realizing the magnetic tunnel junction is magnesium oxide, Co is cobalt, Fe is iron, B is boron (boron), Ta is tantalum, Ru is ruthenium, Si represents silicon, O represents oxygen, and N represents nitrogen. Further, Pt represents platinum, Ar represents argon, Cu represents copper, Pd represents palladium, and Mg represents magnesium.

  An MTJ of CoFeB / MgO / CoFeB using MgO for the tunnel insulating film and CoFeB for the ferromagnetic metal electrodes above and below the tunnel insulating film is known. In this MTJ, by applying a template crystallization technique in which CoFeB is also crystallized while being affected by the (001) orientation of MgO, electrons with a high Δ1 band spin polarization are preferential. Pass MgO through the tunnel. Thereby, it is known that a high MR ratio can be obtained. The MR ratio is defined as MR ratio (%) = 100 × (Rap−Rp) / Rp when the resistance Rp is in the low resistance state and the resistance Rap is in the high resistance state.

  Furthermore, by utilizing the interface perpendicular magnetization induced at the interface between MgO and CoFeB, it became clear that the MTJ structure of CoFeB / MgO / CoFeB, which had been in-plane magnetization, can be used as the perpendicular magnetization type. It was. The perpendicular magnetization type MTJ has higher efficiency of spin injection magnetization reversal than the in-plane magnetization type MTJ, and when the MRAM is manufactured, the information can be rewritten with a lower rewriting current with the same thermal stability. It is expected.

  Based on these research results, research and development of an interface perpendicular magnetization type STT-MRAM using an interface perpendicular MTJ having a CoFeB / MgO / CoFeB structure has been actively conducted.

Japanese Patent No. 4082711 JP 2006-080116 A JP 2007-184063 A

S. Ikeda et al., Nature materials, Vo. 9, pp.721-724, September 2010 W. Kim et al., IEDM11-531 Tech. Dig. 24.1.1-24.1.4 2011 D.C.Worledge et al., Appl. Phys. Lett. 98, 022501 (2011) Z. Diao et al., Appl. Phys. Lett. 90, 132508 (2007)

  The thermal stability Δ of STT-MRAM (magnetoresistance memory element) is expressed by E / kBT. Here, E is the energy of the element, kB is the Boltzmann constant, and T is the temperature. In the STT-MRAM, the rewrite current Ic and the thermal stability Δ are both proportional to the element volume, and both are in a trade-off relationship. Therefore, when the element volume is reduced by miniaturization, it is preferable because the rewrite current Ic is reduced. On the other hand, there is a problem that the thermal stability Δ is also reduced and the non-volatility of the memory cannot be guaranteed. The interface perpendicular magnetization type STT-MRAM is also continuously miniaturized, but it is desirable to improve the trade-off relationship in miniaturization.

According to the embodiment, the perpendicular magnetization type magnetoresistive memory has a plurality of perpendicular magnetization type magnetoresistive memory cells. Each memory cell includes a tunnel insulating film, a magnetization free layer and a magnetization fixed layer that are two layers of ferromagnetic metal electrodes formed so as to face each other with the tunnel insulation film interposed therebetween, and a tunnel insulation film of the magnetization free layer. And an additional layer formed at the opposite interface. The additional layer has at least one set including an interface cap layer and a ferromagnetic metal electrode auxiliary layer. The perpendicular magnetization type magnetoresistive memory stores data by utilizing the interface perpendicular magnetization between the tunnel insulating film and the two layers of ferromagnetic metal electrodes.
Each memory cell includes a tunnel insulating film, a magnetization free layer and a magnetization fixed layer that are two layers of ferromagnetic metal electrodes formed so as to face each other with the tunnel insulating film interposed therebetween, and a tunnel insulating film of the magnetization free layer And an interface cap layer of an insulating film formed at the opposite interface to store data using the interface perpendicular magnetization of the tunnel insulating film and the two layers of ferromagnetic metal electrodes.

  According to the embodiment, a perpendicular magnetization type magnetoresistive memory is realized in which the thermal stability is improved without increasing the rewrite current Ic and the trade-off relationship is improved.

FIG. 1 is a diagram illustrating a memory cell of an interface perpendicular magnetization type STT-MRAM according to an embodiment. FIG. 2 shows an example of a cross-sectional structure of two adjacent memory cells connected to the same bit line and source line and a transistor in a peripheral circuit portion in the layout of the memory cell shown in FIG. FIG. FIG. 3 is a diagram showing an example of magnetization characteristics of a sample assuming that the tunnel insulating film is made of MgO, the magnetization free layer is made of CoFeB, and the cap layer is made of MgO. FIG. 4 is a diagram illustrating an example of magnetization characteristics of a sample assuming that the tunnel insulating film is made of MgO, the magnetization free layer is a multilayer of CoFeB / CoFeBTa / CoFeB, and the cap layer is made of MgO. FIG. 5 is a diagram showing the layer structure of a sample prepared for measuring the bias dependence of the coercive force of the ferromagnetic layer. FIG. 6 is a diagram showing measurement results of bias dependency of holding force of samples A and B in FIG. 7A and 7B are diagrams showing a basic structure of the memory cell of the perpendicular magnetization type magnetoresistive memory according to the embodiment. FIG. 7A shows a bottom pin type, and FIG. 7B shows a top pin type. FIG. 8 is a view showing a layer structure of a sample of an embodiment in which the tunnel insulating film and the interface cap layer are formed of MgO and the magnetization free layer is a multilayer structure in the bottom pin type layer structure of FIG. is there. FIG. 9 is a diagram illustrating an RV characteristic by DC measurement of the MTJ sample of the embodiment of FIG. FIG. 10 shows the rewrite current Ic and the thermal stability Δ of the memory cell having the general MTJ layer structure of FIG. 5A and the memory cell having the MTJ layer structure of the embodiment of FIG. It is a figure which shows a relationship. FIG. 11 is a diagram illustrating a manufacturing process of the MTJ portion in the embodiment. FIG. 12 is a diagram illustrating the manufacturing process of the MTJ portion in the embodiment. FIG. 13 is a block diagram of a semiconductor device in which the interface perpendicular magnetization type STT-MRAM of the embodiment having the MTJ described so far is embedded in a CMOS circuit. FIG. 14 is a block diagram of the MRAM.

  FIG. 1 is a diagram showing a memory cell of an interface perpendicular magnetization type STT-MRAM according to an embodiment, in which (A) shows an electrical equivalent circuit of one memory cell, and (B) shows a plurality of memory cells. A memory cell array is shown.

  As shown in FIG. 1A, the memory cell 10 includes a transistor (nMOSFET) 11 and a variable resistance element 12 in which a resistance value is set. The variable resistance element 12 includes the interface perpendicular MTJ, and the magnetization direction of the magnetization free layer is set according to the stored data. One end of the variable resistance element 12 is connected to the bit line 14, and the other end of the variable resistance element 12 is connected to one controlled terminal (drain) of the transistor 11. The control terminal (gate) of the transistor 11 is connected to the word line 13, and the other controlled terminal (source) of the transistor 11 is connected to the source line 15.

  When data is written to the memory cell 10, the selection voltage (H) is applied to the word line 13 to turn on the transistor 11, and between the bit line 14 and the source line 15 according to the data (H or L) to be written. A voltage is applied so that currents having different polarities flow. Thereby, the magnetization direction of the magnetization free layer of MTJ is set according to the data to be written, and the variable resistance element 12 exhibits different resistance values. When reading data from the memory cell 10, a selection voltage (H) is applied to the word line 13 to turn on the transistor 11, and a voltage smaller than that at the time of writing is applied between the bit line 14 and the source line 15. As a result, a current flows between the bit line 14 and the source line 15 via the transistor 11 and the variable resistance element 12, but the current flowing according to the resistance value of the variable resistance element 12 varies, so that the difference in the amount of current is caused. Correspondingly stored data is detected.

  In FIG. 1A, the bit line 14 and the source line 15 are orthogonal to each other, but it is desirable that the bit line 14 and the source line 15 are adjacently arranged in parallel from the relationship between the write and read operations. FIG. 1B shows a layout of the memory cell when the bit line 14 and the source line 15 are arranged in parallel. As shown in FIG. 1B, the word line 13 is arranged in a direction orthogonal to the set of the bit line 14 and the source line 15. In FIG. 1B, the sources of the transistors 11 of two adjacent memory cells connected to the same bit line 14 and source line 15 are connected, and the connection node is connected to the source line 15.

FIG. 2 shows an example of a cross-sectional structure of two adjacent memory cells connected to the same bit line 14 and source line 15 and transistors in the peripheral circuit portion in the memory cell layout shown in FIG. FIG.
As shown in FIG. 2A, functional elements such as transistors are formed in the layer 22 on the substrate 21 in the memory cell portion and the peripheral circuit portion. In the contact layer CT, gate electrodes 23A and 23B, drain electrodes 24A and 24B, and a source electrode 25 are formed. M1 to M5 represent metal layers, respectively, and V1 to VM4 represent via layers. Although not shown, the gate electrodes 23A and 23B are connected to a word line provided in one of the metal layers and extending in a direction perpendicular to the paper surface. The source electrode 25 is connected to a source line provided in any metal layer and extending in the horizontal direction on the paper surface. The drain electrodes 24A and 24B are guided to the upper layer through the metal layers M1 to M4 and the via layers V1 to V4, and are connected to the lower electrode 26. The above structure is the same in the memory cell portion and the peripheral circuit portion. In the memory cell portion, the MTJ 30 is formed between the lower electrode 26 and the upper electrode 28. The upper electrode 28 is disposed on the metal layer M5 and connected to a bit line extending in the horizontal direction on the paper surface.

  Since parts other than the MTJ 30 are realized by applying a wiring layout and manufacturing method that have been widely used so far, description thereof will be omitted, and only the MTJ will be described.

  In order to improve the trade-off relationship between the rewrite current Ic and the thermal stability Δ, factors related to the rewrite current Ic and the thermal stability Δ were examined.

  As a factor related to the rewrite current Ic, Gilbert's damping constant α is known. The damping constant α is a value that expresses the friction component of the spin precession. If this value is small, the rewriting current decreases. On the other hand, the damping constant α is basically irrelevant to the thermal stability, and it has been noted that the trade-off between the rewriting current and the thermal stability is improved if α can be reduced.

  Until now, in the STT-MRAM, the tunnel insulating film is generally formed of MgO, the magnetization fixed layer and the magnetization free layer are formed of CoFeB, and the cap layer of the magnetization free layer is generally formed of Ta or CoFeBTa. On the other hand, it was examined whether the damping constant α could be reduced by forming the cap layer with MgO, which is the same material as the tunnel insulating film.

  FIG. 3 is a diagram showing an example of magnetization characteristics of a sample assuming that the tunnel insulating film is made of MgO, the magnetization free layer is made of CoFeB, and the cap layer is made of MgO. 3A shows the layer structure of the solid film sample, and FIG. 3B shows the magnetization characteristics (M−H) measured by the vibration sample type magnetization measuring device (VSM).

  As shown in FIG. 3A, the sample has a layer structure in which a 1.1 nm CoFeB layer 32 is formed on an MgO film 31 and an MgO film 33 is formed thereon.

  In FIG. 3B, the horizontal axis indicates the applied magnetic field strength (M), and the direction of the applied magnetic field is reversed with respect to zero. In FIG. 3B, the vertical axis shows the normalized amount of magnetization of the magnetized sample. If the direction of the magnetic flux measured by the vibration sample type magnetization measuring device is a direction parallel to the surface, the in-plane direction H is measured, and if the direction is perpendicular, the vertical direction H is measured.

  In general, in the evaluation before processing using a VSM of a solid film of MgO / CoFeB / Ta or Ta / CoFeB / MgO, it was known that when CoFeB is thin, the easy axis of magnetization shows a perpendicular direction due to interface perpendicular magnetization. . However, it is already known as a double-junction MTJ that MgO / CoFeB / MgO exhibits in-plane magnetization type MTJ characteristics when the thickness of CoFeB is relatively large.

  However, as shown in FIG. 3B, in this sample, the easy axis of magnetization was the in-plane direction. This is unexpected considering that the interface between MgO and CoFeB that induces perpendicular magnetization increases from one to two. In any case, it has been found that the layer structure of FIG. 3A is not suitable for use as a perpendicular magnetization type even when CoFeB is thin (about 1 nm).

  Therefore, further experiments on the material of the magnetization free layer were further conducted, and the magnetization characteristics (M−H) were obtained when the magnetization free layer was separated into a three-layer structure of a lower ferromagnetic layer, an intermediate (ferromagnetic) layer, and an upper ferromagnetic layer. It was measured.

  FIG. 4 is a diagram illustrating an example of magnetization characteristics of a sample assuming that the tunnel insulating film is made of MgO, the magnetization free layer is a multilayer of CoFeB / CoFeBTa / CoFeB, and the cap layer is made of MgO. 4A shows the layer structure of the solid film, and FIG. 4B shows the magnetization characteristics (M−H) measured by VSM.

  As shown in FIG. 4A, in the sample, a 0.6 nm CoFeB layer 321 is formed on the MgO film 31, a 0.6 nm CoFeBTa layer 322 is formed thereon, and a 0.6 nm CoFeB layer is formed thereon. 321 is formed, and a MgO film 33 is further formed thereon. In other words, the layer structure of FIG. 4A is the layer structure of FIG. 3A, and the magnetization free layer is composed of three layers of the lower ferromagnetic layer 321, the intermediate layer 322, and the lower ferromagnetic layer 323. This is a structure of MgO / CoFeB / CoFeBTa / CoFeB / MgO.

  FIG. 4B is a diagram corresponding to FIG. 3B, in which the horizontal axis indicates the magnetic field strength (M) and the vertical axis indicates the normalized magnetization amount of the sample. As shown in FIG. 4B, in the structure of FIG. 4A, it can be seen that the easy axis of magnetization is in the vertical direction. From this, it can be seen that the magnetization free layer of CoFeB / CoFeBTa / CoFeB behaves as a single layer of ferromagnetic material having an axis of easy magnetization in the vertical direction.

  As described above, if a multilayer structure of CoFeB / CoFeBTa / CoFeB is used as the magnetization free layer, even if the cap layer is made of the same material as the tunnel insulating film and the damping constant α is made of small MgO, the perpendicular magnetization type I found out that

  On the other hand, an auxiliary magnetization free layer (auxiliary layer) (referred to as a dummy magnetization free layer) that is magnetically coupled to the magnetization free layer via the interface cap layer and always faces the same direction as the magnetization free layer due to the leakage magnetic field of the magnetization free layer. In some cases). As a material for forming the auxiliary layer, CoFeB capable of utilizing interface perpendicular magnetization with MgO of the cap layer was used. The auxiliary layer is always oriented in the same direction as the magnetization free layer by magnetic interaction with the magnetization free layer. If the magnetization of the auxiliary layer is reduced to reduce the coercive force, the influence of the leakage magnetic field on the auxiliary layer due to the orientation of the magnetization free layer becomes relatively large. The magnetization free layer leakage magnetic field causes the auxiliary layer to become a magnetization free layer. Magnetizing in the same direction is relatively easy. On the other hand, in order to increase the leakage magnetic field received by the magnetization free layer by the auxiliary layer, it is desirable to increase the magnetization of the auxiliary layer.

  FIG. 5 is a diagram showing the layer structure of a sample prepared for measuring the bias dependence of the coercive force of the ferromagnetic layer. FIG. 5A shows the layer structure of Sample A for measuring the bias dependence of the coercive force of a ferromagnetic layer that operates only as a magnetization free layer, and FIG. 5B operates as an auxiliary layer. The layer structure of Sample B for measuring the bias dependence of the coercive force of the ferromagnetic layer is shown. Sample B has the same structure as the layer structure of the embodiment described later.

  As shown in FIG. 5A, the sample A has a CoFeB film 42 as a magnetization fixed layer, a MgO film 41 as a tunnel insulating film, and a 1.2 nm layer as a magnetization free layer on the lower metal layer 44. A CoFeB film 43 and an upper metal layer 45 were formed.

  As shown in FIG. 5B, the sample B has a CoFeB film 42, an MgO film 41, a CoFeB film 431, a CoFeBTa film 432, a CoFeB film 433, an MgO film 46, and a CoFeB film 47 on the lower metal layer 44. And the upper metal layer 45 was formed. The thickness of the MgO film 41 is 0.9 nm, and the thickness of the MgO film 46 is 0.7 nm. The CoFeB film 431 has a thickness of 0.6 nm, the CoFeBTa film 432 has a thickness of 0.6 nm, the CoFeB film 433 has a thickness of 0.5 nm, and the CoFeB film 47 has a thickness of 1.2 nm.

  Samples A and B were processed into microelements with a diameter of 50 nm, and the bias dependency of the holding force was measured.

  FIG. 6 is a diagram showing the measurement results of the bias dependence of the holding force of samples A and B in FIG. In FIG. 6, the horizontal axis indicates the bias voltage to be applied, the vertical axis indicates the normalized value of the coercive force exhibited by the element when the bias voltage is applied, the white square F indicates the characteristics of sample B, and the black circle S indicates sample A. The characteristics are shown respectively.

  As shown in FIG. 6, the coercive force of the sample B, that is, the magnetization free layer, shows the highest coercive force at 0V with respect to the change of the bias voltage, and gradually decreases as the bias voltage goes away from 0V. On the other hand, the holding force of sample A, that is, the auxiliary layer shows the highest holding force at 0V with respect to the change of the bias voltage, similar to sample B, but the bias voltage is 0V compared to sample B. Decreases rapidly as you move away from. In other words, it has been found that the holding force of the auxiliary layer varies greatly depending on the bias applied to the element. For example, in a state where a low voltage (about −0.2 V) indicated by the arrow Z is applied, the decrease in the coercive force of the magnetization free layer is small, but the coercive force of the auxiliary layer is almost zero. Therefore, even if the magnetization of the auxiliary layer is increased to some extent, when the orientation of the magnetization free layer is rewritten by spin injection, the coercive force of the auxiliary layer itself is reduced, and the inversion of the magnetization free layer and the auxiliary layer can be synchronized. It turned out to be.

  7A and 7B are diagrams showing a basic structure of the memory cell of the perpendicular magnetization type magnetoresistive memory according to the embodiment. FIG. 7A shows a bottom pin type, and FIG. 7B shows a top pin type.

  In the bottom pin type of FIG. 7A, the magnetization fixed layer 52, the tunnel insulating film 51, the magnetization free layer 53, the interface cap layer 56, the auxiliary (dummy magnetization free) layer 57, and the upper metal are formed on the lower metal layer 54. The layer 55 is formed in order.

  The top pin type of FIG. 7B has an auxiliary (dummy magnetization free) layer 67, an interface cap layer 66, a magnetization free layer 63, a tunnel insulating film 61, a magnetization fixed layer 62, an upper part on a lower metal layer 65. It has a structure in which a metal layer 64 is formed in order.

  The description so far and the following is an example of the bottom pin type, but the description is equally valid for the top pin type.

  In the rewritten state in which a bias is applied in the structure of FIG. 7A, the auxiliary (dummy magnetization free) layer 57 has almost no coercive force as shown in FIG. When the magnetization direction of the magnetization free layer 53 is reversed in this state, the direction of the leakage magnetic field from the magnetization free layer 53 is reversed, so that the magnetization direction of the auxiliary layer 57 is easily reversed. Even in a data holding state in which no bias is applied, it is advantageous in terms of energy that the magnetization direction of the auxiliary layer 57 faces the same direction as the magnetization free layer 53 due to the influence of the leakage magnetic field from the magnetization free layer 53. Therefore, the auxiliary layer 57 is always magnetized in the same direction as the magnetization free layer 53. Conversely, in the data retention state, the thermal stability of the magnetization free layer 53 is increased by the leakage magnetic field from the auxiliary (dummy magnetization free) layer 57 facing the same direction as the magnetization free layer 53.

  As described above, by using a material having a small damping constant α as the interface cap layer 56, only the rewriting current Ic can be reduced without sacrificing thermal stability. When MgO is used as a material having a small damping constant α and the tunnel insulating film 51 and the interface cap layer 56 are formed of MgO, it is desirable that the magnetization free layer 53 has a multilayer structure as described above. It is desirable to form CoFeBTa.

  As described above, the perpendicular magnetization type magnetoresistive memory cell having the basic structure shown in FIGS. 7A and 7B reduces the rewrite current Ic with respect to the normal MTJ, and has the thermal stability Δ. Can be improved.

  FIG. 8 is a view showing a layer structure of a sample of an embodiment in which the tunnel insulating film and the interface cap layer are formed of MgO and the magnetization free layer is a multilayer structure in the bottom pin type layer structure of FIG. is there.

  As shown in FIG. 8, the sample of the embodiment includes a CoFeB film 72, a MgO film 71, a CoFeB film 731, a CoFeBTa film 732, a CoFeB film 733, a MgO film 76, a CoFeB film 77 and an upper part on the lower metal layer 74. A metal layer 75 was formed. The lower metal layer 74 and the upper metal layer 75 correspond to the lower metal layer 54 and the upper metal layer 55 in FIG. The thickness of the MgO film 71 corresponding to the tunnel insulating film 51 is 0.9 nm, and the thickness of the MgO film 76 corresponding to the interface cap layer 56 is 0.7 nm. The thickness of the CoFeB film 731 is 0.6 nm, the thickness of the CoFeBTa film 732 is 0.6 nm, the thickness of the CoFeB film 733 is 0.5 nm, and the thickness of the CoFeB film 77 is 1.2 nm. A stack of the CoFeB film 731, the CoFeBTa film 732, and the CoFeB film 733 functions as a single layered ferromagnetic free magnetization layer 53 having an easy axis in the vertical direction.

  FIG. 9 is a diagram illustrating an RV characteristic by DC measurement of the MTJ sample of the embodiment of FIG. If the magnetization direction of the auxiliary layer (CoFeB film) 77 is opposite to the magnetization direction of the magnetization free layer (lamination of the CoFeB film 731, the CoFeBTa film 732, and the CoFeB film 733), the resistance increases at that point. Should show -V characteristics. However, the RV characteristic in FIG. 9 is the same as the normal MTJ characteristic. Therefore, it was confirmed that the auxiliary layer 77 was always oriented in the same direction as the magnetization free layer (lamination of the CoFeB film 731, the CoFeBTa film 732, and the CoFeB film 733).

  FIG. 10 shows the rewrite current Ic and the thermal stability Δ of the memory cell having the general MTJ layer structure of FIG. 5A and the memory cell having the MTJ layer structure of the embodiment of FIG. It is a figure which shows a relationship. Ic and Δ were obtained by a standard method for measuring the pulse width dependency of the rewrite current Ic (for example, the method described in Non-Patent Document 1). As is apparent from FIG. 10, in the memory cell of the embodiment, the thermal stability Δ is greatly improved in spite of the decrease in the rewrite current Ic, and the rewrite current Ic and the thermal stability Δ It was found that the trade-off relationship can be improved.

  Next, with reference to FIG. 11 and FIG. 12, the manufacturing process of the MTJ portion in the embodiment of FIG. 8 will be described. As shown in FIG. 2, the manufacturing process of the MTJ portion is performed after the via layer VM4 is formed, so that the description will be made from the state in which the via connecting the transistor and the lower electrode (BEL) 74 is exposed. .

In a normal CMOS process, the via material is Cu, and a via (Cu) 81 is formed in the SiO ₂ layer 82 as shown in FIG. As shown in FIG. 11A, a Ta layer (3 nm) 83, a Ru layer (25 nm) 84, and a Ta layer (15 nm) 85 to be the lower electrode (BEL) 74 are sequentially formed by sputtering. The intermediate Ru layer 84 of the BEL 74 has an effect of lowering the sheet resistance, and the surface flatness is improved as compared with the case where the same sheet resistance is obtained with a Ta single film. The Ta layer 85 on the Ru layer 84 functions as an etching stopper layer when the MTJ is dry-etched (RIE).

Next, as shown in FIG. 11B, a total of 14 nm of Co (0.4 nm) / Pt (0.6 nm), which is a part of the magnetization fixed layer 72, is formed by sputtering. Subsequently, Ru is formed to a thickness of 0.9 nm, and subsequently Co (0.4 nm) / Pt (0.6 nm) is formed to a total thickness of 4 nm. Thereby, the (Co / Pt) ₁₄ / Ru / (Co / Pt) ₄ layer 86 is formed. Subsequently, a Ta layer (0.2 nm) 87 is formed, and a CoFeB film (1.0 nm) 88 is further formed. The composition ratio of CoFeB is adjusted so as to be Co: Fe: B = 20: 60: 20 (atomic%) by adjusting the target composition including the subsequent layers. The [Co / Pt] ₁₄ layer and the [Co / Pt] ₄ / Ta / CoFeB layer are antiferromagnetically coupled via the Ru layer to form a magnetization fixed layer with a small leakage magnetic field.

  Next, as shown in FIG. 11C, a Mg film having a thickness of 0.8 nm is formed by sputtering, followed by oxidation of Mg in an oxygen atmosphere for 30 seconds, and then Mg to prevent interface overoxidation. By depositing 0.35 nm, an MgO layer 89 that is a tunnel insulating film is formed. Next, a CoFeB film 90 that is a part 731 of the magnetization free layer is formed by sputtering to a thickness of 0.6 nm. Next, a CoFeBTa layer (0.6 nm) 91 which is a part 732 of the magnetization free layer is formed by sputtering. The composition ratio of the CoFeBTa layer 91 is adjusted to be CoFeB: Ta = 75: 25 (atomic%) by adjusting the deposition power. Next, a CoFeB layer (0.5 nm) 92 which is a part 733 of the magnetization free layer is formed by sputtering.

  Next, as shown in FIG. 11D, Mg is deposited to a thickness of 0.8 nm by sputtering, and then Mg is oxidized for 15 seconds in an oxygen atmosphere, and then Mg is added to prevent over-oxidation at the interface. The MgO layer 94 that is the interface cap layer 76 is formed by forming the film to 0.35 nm. Next, a CoFeB layer (1.2 nm) 94 as the auxiliary layer 77 is formed by sputtering.

Next, as shown in FIG. 12A, a Ta layer (1 nm) 95 which is an interface cap layer 76 is formed by sputtering. Next, a Ru layer (7 nm) 96 as an etching stopper for the upper electrode and a Ta layer (80 nm) 97 as an upper electrode (TEL) are formed by sputtering, followed by a hard mask SiO ₂ layer (100 nm by CVD). ) 98 is deposited.

Next, as shown in FIG. 12B, the resist pattern <TEL> of MTJ is exposed to the SiO ₂ layer 98 in a circle having a diameter of 50 nm by immersion ArF lithography and a three-layer resist process. Further, the etching is performed up to the three-layer resist, hard mask, TEL, and MTJ by dry etching, and is stopped by Ta of BEL. Note that the etching of the three-layer resist, the SiO ₂ hard mask, and the TEL is not limited to the MRAM element, and details thereof are omitted. For the etching from the upper etching stopper to the magnetization fixed layer containing Co / Pt, etching is performed using a gas obtained by diluting methanol (CH ₃ OH) with argon (Ar) as an etching gas, and the Ta layer as an etching stopper At 85, the etching is stopped. At the stage where the etching is completed, the hard mask SiO ₂ has disappeared due to the etching, and the TEL Ta is exposed.

Next, as shown in FIG. 12C, a SiN layer (30 nm) 99 which is an interlayer insulating film is formed, and then a SiO ₂ layer (100 nm) 100 which is a thick interlayer insulating film is formed. Perform flattening. Further, in order to electrically separate the MTJ, the BEL resist pattern <BEL> is exposed so as to cover the MTJ and the Cu plug by immersion ArF lithography and a three-layer resist process. Further, the dry etching is performed up to the three-layer resist, SiO ₂ , SiN, and BEL 83 and is stopped at the SiO ₂ below the BEL 83.

After that, it is almost the same as the normal Cu dual damascene process. As shown in FIG. 12D, a SiO ₂ layer (300 nm) which is an interlayer insulating film is formed and planarized by a CMP method. Furthermore, a via is opened in a portion where a via is necessary, the TEL layer 97 is exposed by etching the wiring portion above the MTJ, and the Cu layer 101 is buried and planarized by CMP to form an upper wiring.

  Subsequent processes are not particularly limited to the elements of the embodiment, and if an upper Cu wiring and Al pad are formed, an MRAM CMOS-embedded LSI is completed.

  Note that the material, film thickness, conditions, and the like used in the present embodiment are an optimal example, and the embodiment is not limited thereto. For example, the film thickness of MgO which is a tunnel insulating film or a cap interface layer may be 0.6 nm or more, and the MgO film formation method may be a method of oxidizing metal Mg after sputtering film formation as in this embodiment, or MgO. Direct sputtering using a target may also be used. However, in order to prevent an increase in device resistance and a decrease in MR ratio, it is desirable that the resistance value of the cap interface layer is lower than that of the tunnel insulating film.

  It is known that MgO, which is an interface cap layer, is better as it is thinner, but in order for the MgO / CoFeB interface to induce interface perpendicular magnetization, the thickness of MgO is required to be 0.6 nm or more. Of course, the material of the cap layer may be other than MgO. Furthermore, the cap layer does not have to be a single layer film, and a multi-layered structure can be used as long as the material can couple the magnetization free layer and the auxiliary (dummy magnetization free) layer ferromagnetically (the magnetization direction is the same). good.

  In this embodiment, the upper CoFeB is the auxiliary (dummy magnetization free) layer, the intermediate CoFeB / CoFeBTa / CoFeB is the magnetization free layer, and the lower (Co / Pt) / Ru / (Co / Pt) / Ta / CoFeB is The bottom pin structure which is a magnetization fixed layer is assumed. However, a top pin structure in which this vertical relationship is reversed may be used. Further, the film thickness of CoFeB / CoFeBTa / CoFeB only needs to be in the range of 0.3 to 1.0 nm for each layer, and the composition ratio of CoFeB or CoFeBTa is not limited to the example. However, CoFeBTa is preferably 10% or more and 50% or less as the composition of Ta. The material of the magnetization free layer and the auxiliary layer is not limited to CoFeB, CoFeBTa, and combinations thereof. For example, ferromagnetic materials having perpendicular magnetization such as Co / Pt, Co / Pd, and Co / Ni, and CoFeB and CoFeBTa. A film in combination with may be used.

  As described above, in the MTJ structure of the embodiment, the rewriting current is reduced and the thermal stability is improved as compared with the normal MTJ. Therefore, it was found that the trade-off relationship between the two can be greatly improved by the MTJ of the embodiment.

FIG. 13 is a block diagram of a semiconductor device in which the interface perpendicular magnetization type STT-MRAM of the embodiment having the MTJ described so far is embedded in a CMOS circuit.
As illustrated in FIG. 13, the semiconductor device (chip) 200 includes an MRAM 220 and a CMOS circuit unit 210 other than the MRAM 220. The CMOS circuit unit 210 includes, for example, a logic circuit unit 211 such as a processor, an analog circuit unit 212, a power supply circuit, and the like.

FIG. 14 is a block diagram of the MRAM 220.
As illustrated in FIG. 14, the MRAM 220 includes a memory cell array 301, a row decoder 302, a column decoder 303, a selection switch row 304, a write amplifier 305, a sense amplifier 306, a data I / O unit 307, and a control unit 308. The row decoder 302, the column decoder 303, the data I / O unit 307, and the control unit 308 receive the address signal, input / output data, and control signal from the CMOS circuit unit 210 and access the memory cell array 301. Since the configuration and operation of the MRAM 220 are widely known, a description thereof will be omitted.

  The embodiment has been described above, but all examples and conditions described herein are described for the purpose of helping understanding of the concept of the invention applied to the invention and technology. In particular, the examples and conditions described are not intended to limit the scope of the invention, and the construction of such examples in the specification does not indicate the advantages and disadvantages of the invention. Although embodiments of the invention have been described in detail, it should be understood that various changes, substitutions and modifications can be made without departing from the spirit and scope of the invention.

12 Variable resistance element 30 MTJ (Magnetic Tunnel Junction)
51, 61 Tunnel insulating film 52, 62 Magnetization fixed layer 53, 63 Magnetization free layer 54, 65 Lower metal layer 55, 64 Upper metal layer 56, 66 Interface cap layer 57, 67 Auxiliary (dummy magnetization free) layer

Claims (6)

Comprising a plurality of perpendicular magnetization type magnetoresistive memory cells,
Each memory cell
A tunnel insulating film;
A magnetization free layer and a magnetization fixed layer which are two layers of ferromagnetic metal electrodes formed so as to face each other with the tunnel insulating film interposed therebetween;
An additional layer formed at an interface opposite to the tunnel insulating film of the magnetization free layer,
The additional layer includes at least one set including an interface cap layer and a ferromagnetic metal electrode auxiliary layer,
The magnetization free layer is a multilayer film of CoFeB / CoFeBTa / CoFeB,
The auxiliary layer has a coercive force smaller than that of the magnetization free layer,
A perpendicular magnetization type magnetoresistive memory, wherein data is stored by utilizing perpendicular magnetization of the interface between the tunnel insulating film and the two layers of ferromagnetic metal electrodes. The perpendicular magnetization type magnetoresistive memory according to claim 1, wherein the auxiliary layer is always magnetized in the same direction as the magnetization free layer. The perpendicular magnetization type magnetoresistive memory according to claim 1, wherein the interface cap layer is an insulating film. The perpendicular magnetization type magnetoresistive memory according to claim 3 , wherein a resistance value of the interface cap layer is smaller than a resistance value of the tunnel insulating film. 5. The perpendicular magnetization type magnetoresistive memory according to claim 4, wherein the tunnel insulating film and the interface cap layer are MgO films, and the auxiliary layer is a CoFeB film. A tunnel insulating film;
A magnetization free layer and a magnetization fixed layer which are two layers of ferromagnetic metal electrodes formed so as to face each other with the tunnel insulating film interposed therebetween;
An additional layer formed at an interface opposite to the tunnel insulating film of the magnetization free layer,
The additional layer includes at least one set including an interface cap layer and a ferromagnetic metal electrode auxiliary layer,
The magnetization free layer is a multilayer film of CoFeB / CoFeBTa / CoFeB,
The auxiliary layer has a coercive force smaller than that of the magnetization free layer,
A perpendicular magnetization type magnetoresistive memory, wherein data is stored by utilizing perpendicular magnetization of the interface between the tunnel insulating film and the two layers of ferromagnetic metal electrodes.

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