JP2012142480A - Magnetic tunnel junction element, method of manufacturing the same and mram - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic tunnel junction element that is hardly affected by deposit generated at the side of a laminate structure.SOLUTION: A buffer layer formed of conductive material having a different etching characteristic from a lower electrode is disposed on a partial area of the lower electrode formed on a substrate. A first magnetization free layer formed of amorphous ferromagnetic material is disposed on the buffer layer. A second magnetization free layer formed of crystallized ferromagnetic material is disposed on the first magnetization free layer, and a tunnel barrier layer is disposed on the second magnetization free layer. A magnetization fixing layer whose magnetization direction is fixed is disposed on the tunnel barrier layer, and an upper electrode layer is disposed on the magnetization fixing layer.

Description

  The present invention relates to a magnetic tunnel junction element (MTJ element) used for an MRAM (magnetic random access memory), a magnetic head, and the like, and a method for manufacturing the same.

  The MTJ element has a structure in which a magnetization free layer in which the magnetization direction changes, a tunnel barrier layer, a magnetization fixed layer in which the magnetization direction is fixed, and an antiferromagnetic layer in which the magnetization direction is fixed by exchange coupling with the magnetization fixed layer are stacked. Have Anisotropic dry etching is applied when patterning this laminated structure. Further, isotropic dry etching is applied to remove conductive deposits attached to the side surfaces of the patterned laminated structure.

  A spin injection type MRAM in which the spin injection efficiency is increased and the current required for writing is reduced by using CoFeTa for the magnetization free layer is known.

JP 2006-165030 A JP 2007-48790 A

  If conductive deposits are generated on the side of the stacked structure after patterning the stacked structure of the MTJ element, the leakage current increases. It may be difficult to remove this deposit by etching. There is a need for a magnetic tunnel junction element that is less susceptible to the deposits produced on the side of the laminated structure and a method for manufacturing the same.

According to one aspect of the invention,
A lower electrode formed on the substrate;
A buffer layer disposed on a partial region of the lower electrode and formed of a conductive material having etching characteristics different from those of the lower electrode;
A first magnetization free layer disposed on the buffer layer and formed of an amorphous ferromagnetic material;
A second magnetization free layer disposed on the first magnetization free layer and formed of a crystallized ferromagnetic material;
A tunnel barrier layer disposed on the second magnetization free layer;
Provided is a magnetic tunnel junction element having a magnetization fixed layer disposed on the tunnel barrier layer and having a magnetization direction fixed, and an upper electrode layer formed on the magnetization fixed layer.

According to another aspect of the invention,
Forming a lower electrode layer on the substrate;
Forming a buffer layer formed of a conductive material having etching characteristics different from those of the lower electrode on the lower electrode layer;
Forming a first magnetization free layer made of an amorphous ferromagnetic material on the buffer layer;
Forming a second magnetization free layer made of an amorphous ferromagnetic material different from the first magnetization free layer on the first magnetization free layer;
Forming a tunnel barrier layer on the second magnetization free layer;
Forming a magnetization fixed layer having a magnetization direction fixed on the tunnel barrier layer;
Forming the magnetization fixed layer, and performing a heat treatment to crystallize the second magnetization free layer, the first magnetization free layer maintaining an amorphous state;
Forming a mask pattern on the magnetization fixed layer;
There is provided a method for manufacturing a magnetic tunnel junction element, comprising: using the mask pattern as an etching mask, etching using a reactive ion etching method from the magnetization fixed layer to the buffer layer, and exposing an upper surface of the lower electrode layer Is done.

  By disposing the buffer layer, the height from the lower electrode to the tunnel barrier layer can be increased. This makes the characteristics of the magnetic tunnel junction element less susceptible to the deposits deposited on the lower electrode during the process. By making the magnetization free layer into a two-layer structure of the first magnetization free layer and the second magnetization free layer, it is possible to suppress deterioration in quality at the interface between the magnetization free layer and the tunnel barrier layer.

1 is a cross-sectional view of a magnetic tunnel junction element according to Example 1. FIG. FIG. 3 is a cross-sectional view of the magnetic tunnel junction device according to Example 1 in the middle of manufacturing. FIG. 3 is a cross-sectional view of the magnetic tunnel junction device according to Example 1 in the middle of manufacturing. FIG. 3 is a cross-sectional view of the magnetic tunnel junction device according to Example 1 in the middle of manufacturing. It is a graph which shows the relationship between the thickness of the magnetization free layer of various magnetic tunnel junction elements from which the material of a lower electrode differs, and MR ratio. (4A) and (4B) are schematic cross-sectional views of the MTJ laminated structure when Ta and Ru are used for the lower electrode, respectively. (5A) is a graph showing the relationship between the thickness of the second magnetization free layer and the MR ratio, and (5B) is a graph showing the relationship between the thickness of the first magnetization free layer and the MR ratio. 6 is an equivalent circuit diagram of an MRAM according to Embodiment 2. FIG. 6 is a cross-sectional view of an MRAM according to Example 2 in the middle of manufacturing. FIG. 6 is a cross-sectional view of an MRAM according to Embodiment 3. FIG. FIGS. 9A and 9B are a front view and a cross-sectional view, respectively, of the air bearing surface of the magnetic head according to the fourth embodiment.

[Example 1]
FIG. 1 shows a cross-sectional view of an MTJ element according to the first embodiment. A lower electrode 11 is formed on a partial region of the substrate 10. For example, Ta is used for the lower electrode 11, and the thickness thereof is in the range of 5 nm to 50 nm, for example. The lower electrode 11 is not crystallized and is in an amorphous state. A buffer layer 12 is formed on a partial region of the lower electrode 11. For example, Ru, Pt, Cr, or the like is used for the buffer layer 12, and the thickness thereof is, for example, in the range of 3 nm to 15 nm. The buffer layer 12 is crystallized.

  An MTJ multilayer structure 15 is disposed on the buffer layer 12. The MTJ multilayer structure 15 has a multilayer structure in which a magnetization free layer 16, a tunnel barrier layer 20, a magnetization fixed layer 21, and an antiferromagnetic layer 27 are deposited in this order.

  The magnetization free layer 16 includes a first magnetization free layer 17 and a second magnetization free layer 18 formed thereon. The first magnetization free layer 17 is made of, for example, amorphous CoFeBTa, and has a thickness in the range of 0.8 nm to 2.5 nm. The second magnetization free layer 18 is formed of, for example, crystallized CoFeB, and the thickness thereof is in the range of 0.4 nm to 1.0 nm. The second magnetization free layer 18 is amorphous immediately after film formation, but crystallizes in a subsequent heat treatment process. The magnetization direction of the magnetization free layer 16 is easily changed by injection of spin-polarized electrons or application of an external magnetic field.

  The tunnel barrier layer 20 is made of, for example, MgO and has a thickness in the range of 0.8 nm to 1.1 nm. MgO has a body-centered cubic lattice structure, and MgO forming the tunnel barrier layer 20 is (001) -oriented.

  The magnetization fixed layer 21 has a stacked structure in which a first magnetization fixed layer 22, a second magnetization fixed layer 23, a spacer layer 24, and a third magnetization fixed layer 25 are deposited in this order. The first magnetization fixed layer 22 is made of, for example, CoFeB and has a thickness in the range of 1.5 nm to 2.5 nm. The second magnetization fixed layer 23 is made of, for example, CoFe and has a thickness of 0.5 nm or less. The spacer layer 24 is made of, for example, Ru and has a thickness in the range of 0.75 nm to 1.5 nm. The third magnetization fixed layer 25 is made of, for example, CoFe and has a thickness in the range of 1.5 nm to 2.5 nm. The magnetization fixed layer 21 has a laminated ferrimagnetic structure in which the upper and lower magnetization fixed layers are antiferromagnetic exchange coupled via the spacer layer 24.

  The antiferromagnetic layer 27 is made of, for example, IrMn and has a thickness in the range of 7 nm to 10 nm. The antiferromagnetic layer 27 may be formed of an antiferromagnetic material other than IrMn, such as PtMn. When PtMn is used for the antiferromagnetic layer 27, the thickness thereof is set in the range of 10 nm to 20 nm. The antiferromagnetic layer 27 is exchange-coupled with the third magnetization fixed layer 25 and fixes the magnetization direction of the third magnetization fixed layer 25.

  On the MTJ laminated structure 15, the upper electrode 31 and the connection layer 32 are laminated in this order. The upper electrode 31 is made of, for example, Ru and has a thickness of 3 nm to 10 nm. The connection layer 32 is made of, for example, Ta and has a thickness of 30 nm to 80 nm.

  An oxide layer 40 is formed on the lower electrode 11 on the side of the buffer layer 12. The oxide layer 40 is formed of the oxide of the lower electrode 11. The top surface of the oxide layer 40 is lower than the bottom surface of the tunnel barrier layer 20.

  With reference to FIGS. 2A to 2F, a method for manufacturing an MTJ element according to Example 1 will be described.

As shown in FIG. 2A, each layer from the lower electrode 11 to the connection layer 32 is formed on the substrate 10 by sputtering. At the time of film formation, the first magnetization free layer 17 and the second magnetization free layer 18 are in an amorphous state. After the connection layer 32 is formed, the second magnetization free layer 18 is crystallized by performing heat treatment in a magnetic field. This heat treatment is performed, for example, under the following conditions.
・ Pressure 1 × 10 ⁻⁵ Pa or less ・ Magnetic field 1T
・ Direction of magnetic field In-plane direction ・ Temperature 350 ℃
-Heat treatment time 2 hours-Temperature rise and temperature drop time 1 hour As shown in FIG. 2B, this heat treatment causes the inside of the second magnetization free layer 18 from the interface between the tunnel barrier layer 20 made of MgO and the second magnetization free layer 18. Crystallization proceeds toward. Thereby, the second magnetization free layer 18 is crystallized. In this specification, this heat treatment is referred to as “crystallization heat treatment”.

  CoFeBTa of the first magnetization free layer 17 is less likely to crystallize than CoFeB. Although the buffer layer 12 made of Ru under the first magnetization free layer 17 is crystallized, crystallization from the interface between the buffer layer 12 and the first magnetization free layer 17 into the first magnetization free layer 17 occurs. Absent. For this reason, the first magnetization free layer 17 remains in an amorphous state even after the crystallization heat treatment. Since the amorphous first magnetization free layer 17 is inserted between the second magnetization free layer 18 and the buffer layer 12, the crystal of the buffer layer 12 is crystallized when the second magnetization free layer 18 is crystallized. It is not affected by the structure.

As shown in FIG. 2C, the connection layer 32 is patterned so that the connection layer 32 is left in the region where the MTJ stacked structure is to be disposed. For patterning the connection layer 32, for example, a silicon oxide film is used as an etching mask, and a Cl ₂ gas is used as an etching gas. After the connection layer 32 is patterned, the silicon oxide film used as an etching mask is removed. The planar shape of the patterned connection layer 32 is, for example, a rectangle or an ellipse.

As shown in FIG. 2D, each layer from the upper electrode 31 to the buffer layer 12 is etched using the connection layer 32 as an etching mask. For this etching, reactive ion etching using a mixed gas of CO and NH ₃ is applied. The flow rate ratio between CO and NH ₃ is, for example, 1:10, and the pressure in the etching chamber is, for example, 10 Pa. Note that methanol gas can also be used as an etching gas.

  When the lower electrode 11 made of Ta is exposed, the surface of the lower electrode 11 is oxidized, and the tantalum oxide layer 40 is formed. Since the tantalum oxide layer 40 acts as an etching stopper, the lower electrode 11 is not etched. When the lower electrode 11 is oxidized, its volume expands, so that the upper surface of the tantalum oxide layer 40 becomes higher than the upper surface of the initial lower electrode 11. However, the thickness of the buffer layer 12 is set so that the top surface of the tantalum oxide layer 40 does not reach the bottom surface of the tunnel barrier layer 20.

  The stacked structure 41 including the buffer layer 12, the magnetization free layer 16, the tunnel barrier layer 20, the magnetization fixed layer 21, the antiferromagnetic layer 27, the upper electrode 31, and the connection layer 32 is formed through the steps so far.

  As shown in FIG. 2E, a photoresist film is formed on the laminated structure 41 and the oxide layer 40, and the photoresist film is patterned to form a resist pattern 43. The resist pattern 43 covers the stacked structure 41 and the surrounding oxide layer 40.

As shown in FIG. 2F, the oxide layer 40 and the lower electrode 11 are etched using the resist pattern 43 as an etching mask. For this etching, for example, reactive ion etching using Cl ₂ as an etching gas is applied. After the etching, the resist pattern 43 is removed. Through the steps so far, the MTJ element shown in FIG. 1 is formed.

  Hereinafter, the effect of disposing the buffer layer 12 will be described. When the magnetization free layer 16 is thickened, the volume of the ferromagnetic material whose magnetization is to be reversed increases, so that the current threshold necessary for writing increases. From the threshold value of the desired write current, the upper limit value of the thickness of the magnetization free layer 16 is determined. When the buffer layer 12 is not disposed, the height from the upper surface of the lower electrode 11 to the bottom surface of the tunnel barrier layer 20 is limited by the upper limit value of the thickness of the magnetization free layer 16.

  The oxide layer 40 grows downward and upward with reference to the upper surface of the lower electrode 11 before oxidation. The thickness of the oxide layer 40 depends on the reactive ion etching conditions shown in FIG. 2D. Under normal etching conditions, it is difficult to make the thickness of the portion of the oxide layer 40 grown upward less than the upper limit of the thickness of the magnetization free layer 16. For this reason, the oxide layer 40 reaches the tunnel barrier layer 20. Since the oxide of Ta has conductivity, when the oxide layer 40 comes into contact with the tunnel barrier layer 20, the leakage current increases.

  In Example 1, since the buffer layer 12 is inserted between the lower electrode 11 and the magnetization free layer 16, the bottom surface of the tunnel barrier layer 20 can be made higher than the upper surface of the oxide layer 40. Further, since the buffer layer 12 is not a ferromagnetic material, even if the buffer layer 12 is thickened, the threshold value of the current required for writing does not increase.

  Next, the effect of making the magnetization free layer 16 a two-layer structure will be described.

  FIG. 3 shows the measurement results of the MR ratio of various MTJ elements. The manufactured MTJ element includes a lower electrode 11, a magnetization free layer 16, a tunnel barrier layer 20, and a magnetization fixed layer 21. The magnetization free layer 16 is only one CoFeB layer. MgO was used for the tunnel barrier layer 20. The structure of the magnetization fixed layer 21 is the same as the structure of the magnetization fixed layer 21 of the first embodiment shown in FIG. Five types of samples using Ta, Ru, Cr, Pt, and Ti as the material of the lower electrode 11 were produced.

  The horizontal axis in FIG. 3 represents the thickness of the magnetization free layer 16 in the unit “nm”, and the vertical axis represents the MR ratio in the unit “%”. The element symbol attached to the curve in FIG. 4 indicates the material of the lower electrode 11. When Ta is used for the lower electrode 11, a larger MR ratio is obtained compared to the case where other materials are used.

  The reason why the MR ratio varies depending on the material of the lower electrode 11 will be described with reference to FIGS. 4A and 4B. 4A and 4B are schematic cross-sectional views of the MTJ stacked structure, and the direction of crystal growth during the crystallization heat treatment is indicated by arrows. As shown in FIG. 4A, when Ta is used for the lower electrode 11, crystallization proceeds downward from the tunnel barrier layer 20 in the magnetization free layer 18. Since Ta forming the lower electrode 11 is in an amorphous state, Ta in the lower electrode 11 does not become a crystal growth nucleus. For this reason, crystallization hardly proceeds from the lower electrode 11.

  As shown in FIG. 4B, when Ru is used for the lower electrode 11, the lower electrode 11 is crystallized. During the crystallization heat treatment, crystallization proceeding upward from the lower electrode 11 becomes dominant in the magnetization free layer 18. When crystal growth proceeds using crystallized Ru as a growth nucleus, CoFeB is (110) oriented. Further, the quality of the interface between the tunnel barrier layer 20 and the magnetization free layer 18 is degraded. For this reason, it is thought that MR ratio will become small. Similarly, when Cr, Pt, Ti, or the like is used for the lower electrode 11, it is considered that the quality of the interface between the tunnel barrier layer 20 and the magnetization free layer 18 is lowered.

  In the case of Example 1, since Ru is used as the buffer layer 12, if the second magnetization free layer 18 of CoFeB is directly deposited on the buffer layer 12, the second magnetization is the same as in the case of FIG. 4B. The quality of the interface between the free layer 18 and the tunnel barrier layer 20 is degraded. In the first embodiment, a first magnetization free layer 17 of CoFeBTa is inserted between the buffer layer 12 and the second magnetization free layer 18. CoFeBTa is harder to crystallize than CoFeB. Therefore, crystallization does not proceed upward from the buffer layer 12, and the first magnetization free layer 17 remains in an amorphous state even after the crystallization heat treatment.

  Since the first magnetization free layer 17 is not crystallized, crystallization does not proceed from the first magnetization free layer 17 in the second magnetization free layer 18. Accordingly, crystallization proceeds from the tunnel barrier layer 20 into the second magnetization free layer 18. Thereby, it is possible to prevent the quality of the interface between the second magnetization free layer 18 and the tunnel barrier layer 20 from being deteriorated.

  Furthermore, while CoFeBTa has magnetism, its saturation magnetization Ms is smaller than the saturation magnetization of CoFeB. The threshold value of the write current depends on the product of the saturation magnetization amount and the volume of the magnetization free layer 16. If the magnetization free layer 16 has a two-layer structure, the magnetization free layer 16 can be made thicker while maintaining the write current threshold constant as compared with the case where the magnetization free layer 16 has a single layer structure of CoFeB. . Conversely, when the thickness of the magnetization free layer is equal, the threshold value of the write current can be reduced by making the magnetization free layer 16 have a two-layer structure. By making the first magnetization free layer 18 thicker than the second magnetization free layer 17, the above-described effect appears remarkably.

5A and 5B show the measurement results of the MR ratio of the sample having the multilayer structure of the MTJ element according to Example 1. FIG. As the ferromagnetic material of the first magnetization free layer 17, (Co _0.42 Fe _0.42 B _0.16 ) _0.66 Ta _0.34 is used, and as the ferromagnetic material of the second magnetization free layer 18, Co _{0 .42} Fe _0.42 B _0.16 was used. For the measurement of MR ratio, CIPT (Current-In-Plane-Tunneling) method was used.

  The horizontal axis of FIG. 5A represents the thickness of the second magnetization free layer 18 in the unit “nm”, and the vertical axis represents the MR ratio in the unit “%”. A circle symbol and a triangle symbol indicate measurement results of a sample in which the thickness of the first magnetization free layer 17 is 1.0 nm and 1.2 nm, respectively.

  An MR ratio of 110% or more is obtained in a sample with a thickness of the second magnetization free layer 18 of 0.5 nm or more, whereas an MR ratio of a sample with a thickness of 0.4 nm is 100% or less. is there. In order to obtain an MR ratio larger than 100%, the thickness of the second magnetization free layer 18 is preferably 0.5 nm or more.

  The horizontal axis of FIG. 5B represents the thickness of the first magnetization free layer 17 in the unit “nm”, and the vertical axis represents the MR ratio in the unit “%”. A circle symbol, a triangle symbol, and a square symbol indicate measurement results of a sample in which the thickness of the second magnetization free layer 18 is 0.7 nm, 0.6 nm, and 0.5 nm, respectively.

  It can be seen that the change in the MR ratio is gradual with respect to the change in the thickness of the first magnetization free layer 17, and the preferred range of the thickness of the first magnetization free layer 17 is wide. As an example, it was confirmed that an MR ratio of 100% or more was obtained when the thickness of the first magnetization free layer 17 was in the range of 0.6 nm to 1.4 nm. A sample having a Ta content of 44 atomic% in the first magnetization free layer 17 was prepared and the MR ratio was measured, and it was confirmed that an MR ratio of 100% or more was obtained.

  In Example 1 described above, the first magnetization free layer 17 was formed of CoFeBTa, and the second magnetization free layer 18 was formed of CoFeB. In addition, a ferromagnetic material containing Fe, B, and Ta can be used for the first magnetization free layer 17. Further, the second magnetization free layer 18 may be made of a ferromagnetic material containing Fe and B and not containing Ta. The first magnetization free layer 17 and the second magnetization free layer 18 may further contain Co or Ni. The Fe content is preferably 10 atomic% or more. The B content is preferably in the range of 15 atomic% to 22 atomic%.

In order to prevent crystallization of the first magnetization free layer 17 and maintain an amorphous state, the Ta content is preferably set to 30 atomic% or more. When the Ta content is too high, the second magnetization free layer 18 and the first magnetization free layer 17 are continuously etched using the same etching gas, that is, a mixed gas of CO and NH _3. Becomes difficult. In order to etch the first magnetization free layer 17 with the same etching gas as the etching gas for the second magnetization free layer 18, the content of Ta is preferably set to 50 atomic% or less.

  Note that W may be used in place of Ta contained in the first magnetization free layer 17. Even if W is contained, the first magnetization free layer 17 is less likely to be crystallized than the second magnetization free layer 18.

  In Example 1, MgO having (001) orientation with a body-centered cubic lattice (bcc) structure was used for the tunnel barrier layer 20. In addition to MgO, other insulating materials that can become crystallization growth nuclei in the second magnetization free layer 18 may be used as the tunnel barrier layer 20 during the crystallization heat treatment. For example, MgZnO having a bcc structure and (001) orientation may be used, or MgAlO having a spinel crystal structure may be used.

  In the first embodiment, Ta is used for the lower electrode 11. However, other conductive materials such as Hf and TiN may be used. The lower electrode 11 functions as an etching stopper under the etching conditions of the buffer layer 12.

[Example 2]
FIG. 6 shows an equivalent circuit diagram of a spin torque injection type MRAM (STT-MRAM) according to the second embodiment. A plurality of word lines 53 extend in the vertical direction of FIG. 6, and a plurality of bit lines 65 extend in the horizontal direction of FIG. Corresponding to the intersection of the word line 53 and the bit line 65, a memory cell is arranged. The memory cell includes a MOS transistor 52 and an MTJ element 60. The gate electrode of MOS transistor 52 is connected to corresponding word line 53. One current terminal of the MOS transistor 52 is grounded, and the other current terminal is connected to the corresponding bit line 65 via the MTJ element 60.

  A method for manufacturing a spin torque injection type MRAM (STT-MRAM) according to the second embodiment will be described with reference to FIGS. 7A to 7C. 7A to 7C are cross-sectional views of a portion corresponding to one memory cell.

  As shown in FIG. 7A, an element isolation insulating film 51 is formed on a surface layer portion of a semiconductor substrate 50 such as silicon to define an active region. In this active region, a MOS transistor 52 is formed. The gate electrode of the MOS transistor 52 also serves as the word line 53 (FIG. 6). An interlayer insulating film 55 made of silicon oxide or the like is deposited on the semiconductor substrate 50 and the MOS transistor 52 by, for example, chemical vapor deposition (CVD). After deposition, the surface of the interlayer insulating film 55 is planarized by chemical mechanical polishing (CMP).

  A via hole is formed in the interlayer insulating film 55, and the via hole is filled with a conductive plug 56 such as tungsten. For example, TiN is used as the barrier metal. The conductive plug 56 is connected to one impurity diffusion region of the MOS transistor 52.

  A ground wiring 57 connected to the conductive plug 56 is formed on the interlayer insulating film 55. An interlayer insulating film 58 made of silicon oxide or the like is deposited on the interlayer insulating film 55 and the ground wiring 57 by, for example, CVD. After deposition, the surface of the interlayer insulating film 58 is planarized by CMP.

  As shown in FIG. 7B, via holes are formed in the interlayer insulating films 55 and 58, and the via holes are filled with a conductive plug 59 such as tungsten. For example, TiN is used as the barrier metal. Conductive plug 59 is connected to the other impurity diffusion region of MOS transistor 52.

  An MTJ element 60 is formed on the interlayer insulating film 58. The MTJ element 60 has the same stacked structure as the stacked structure from the lower electrode 11 to the connection layer 32 of the first embodiment shown in FIG. The MTJ element 60 is manufactured by the same method as in the first embodiment. The lower electrode 11 is connected to the conductive plug 59.

  As shown in FIG. 7C, an interlayer insulating film 63 made of silicon oxide or the like is deposited on the MTJ element 60 and the interlayer insulating film 59 by, for example, CVD. Thereafter, the surface of the interlayer insulating film 63 is planarized by CMP. A via hole is formed at a position overlapping with the MTJ element 60, and the via hole is filled with a conductive plug 64. For example, aluminum (Al) is used for the conductive plug 64.

  A bit line 65 is formed on the interlayer insulating film 63. For example, the bit line 65 has a three-layer structure in which a Ti layer having a thickness of 10 nm, a NiFe layer having a thickness of 30 nm, and an Al layer having a thickness of 600 nm are sequentially deposited. The bit line 65 is connected to the conductive plug 64.

  An upper wiring layer and electrode pads are formed on the bit line 65 and the interlayer insulating film 63 as necessary.

  An STT-MRAM in which the planar shape from the buffer layer 12 constituting the MTJ element 60 to the connection layer 32 (FIG. 1) was a rectangle of 80 nm × 170 nm was manufactured, and the write current threshold and the retention were measured. The thicknesses of the first magnetization free layer 17 and the second magnetization free layer 18 were 1 nm and 0.6 nm, respectively. For comparison, a sample according to a comparative example in which the magnetization free layer 16 is formed of a single layer made of CoFeB and having a thickness of 1.3 nm was manufactured.

  The threshold value of the write current was measured using a current pulse having a pulse width of 100 ns. Retention was estimated by a method in which measurement was repeated 300 times with a 100 ms current pulse and fitting was performed based on variations in write current.

  The threshold value of the write current of the sample having the structure of Example 1 was 0.5 mA. On the other hand, the write current threshold of the sample of the comparative example was 0.9 mA. By adopting the structure of the first embodiment, the threshold value of the write current can be reduced. Further, the retention of the sample having the structure of Example 1 was 44. In contrast, the retention of the sample according to the comparative example was 36. It can be seen that the retention is improved by adopting the structure of the first embodiment.

[Example 3]
FIG. 8 shows a cross-sectional view of the MRAM according to the third embodiment. In the following description, differences from the MRAM according to the second embodiment illustrated in FIG. 7C will be described, and description of the same configuration will be omitted. The MRAM of the second embodiment uses an STT-MTJ element. On the other hand, the MRAM of the third embodiment uses a wiring write type MTJ element.

  A write word line 67 is disposed below the MTJ element 60. The write word line 67 extends in a direction crossing the bit line 65. The magnetization direction of the magnetization free layer 16 is controlled by the combined magnetic field generated by the current flowing through the write word line 67 and the current flowing through the bit line 65.

  As the MTJ element 60, one having the same stacked structure as that of the MTJ element according to the first embodiment is used. For this reason, a high MR ratio can be obtained.

[Example 4]
FIG. 9A is a front view of the air bearing surface (surface facing the medium) of the magnetic head according to the fourth embodiment. An xyz orthogonal coordinate system is defined in which the air bearing surface is the xy plane, the trailing direction is the x axis, the track width direction is the y axis, and the direction perpendicular to the air bearing surface is the z axis. FIG. 9B shows a cross-sectional view parallel to the zx plane of the magnetic head.

  On the substrate 100 made of aluminum titanium carbide or the like, the reading element unit 105 and the recording element unit 115 are laminated in this order. The read element unit 105 includes a lower magnetic shield layer 101, a read element 102, and an upper magnetic shield layer 103. The recording element unit 115 includes a main magnetic pole 110, a main magnetic pole auxiliary layer 111, an auxiliary magnetic pole 112, and a connection part 114. The main magnetic pole 110, the main magnetic pole auxiliary layer 111, the auxiliary magnetic pole 112, and the connection portion 114 constitute a part of a magnetic path of a magnetic field generated during magnetic recording. A recording coil 113 is arranged so as to link with the magnetic path.

  For the reading element 102, the MTJ element according to the first embodiment is used. Thereby, a high MR ratio can be obtained.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

DESCRIPTION OF SYMBOLS 10 Substrate 11 Lower electrode 12 Buffer layer 15 MTJ laminated structure 16 Magnetization free layer 17 First magnetization free layer 18 Second magnetization free layer 20 Tunnel barrier layer 21 Magnetization fixed layer 22 First magnetization fixed layer 23 Second magnetization fixed layer 24 Spacer Layer 25 Third magnetization fixed layer 27 Antiferromagnetic layer 31 Upper electrode 32 Connection layer 40 Oxide layer 41 Laminated structure 43 Resist pattern 50 Semiconductor substrate 51 Element isolation insulating film 52 MOS transistor 53 Word line 55 Interlayer insulating film 56 Conductive plug 57 Ground wiring 58 Interlayer insulating film 59 Conductive plug 60 MTJ element 63 Interlayer insulating film 64 Conductive plug 65 Bit line 67 Write word line 100 Substrate 101 Lower shield film 102 Read element 103 Upper shield film 105 Read element section 110 Main magnetic pole 111 Main magnetic pole Auxiliary layer 112 Auxiliary magnetic pole 113 Recording coil 14 connection portion 115 recording element unit

Claims (7)

A lower electrode formed on the substrate;
A buffer layer disposed on a partial region of the lower electrode and formed of a conductive material having etching characteristics different from those of the lower electrode;
A first magnetization free layer disposed on the buffer layer and formed of an amorphous ferromagnetic material;
A second magnetization free layer disposed on the first magnetization free layer and formed of a crystallized ferromagnetic material;
A tunnel barrier layer disposed on the second magnetization free layer;
A magnetic tunnel junction element having a magnetization fixed layer disposed on the tunnel barrier layer and having a magnetization direction fixed, and an upper electrode formed on the magnetization fixed layer. The first magnetization free layer includes Fe, B, and Ta,
2. The magnetic tunnel junction device according to claim 1, wherein the second magnetization free layer contains Fe and B and does not contain Ta.   The magnetic tunnel junction element according to claim 2, wherein the Ta content of the first magnetization free layer is more than 30 atomic% and less than 50 atomic%.   4. The magnetic tunnel junction device according to claim 1, wherein the buffer layer includes at least one element selected from the group consisting of Ru, Pt, and Cr. 5.   The buffer layer has a thickness in a range of 3 nm to 15 nm, the first magnetization free layer has a thickness in a range of 0.8 nm to 2.5 nm, and the second magnetization free layer has a thickness. 5 is in the range of 0.4 nm to 1 nm. 5. The magnetic tunnel junction element according to claim 1. Forming a lower electrode layer on the substrate;
Forming a buffer layer formed of a conductive material having etching characteristics different from those of the lower electrode on the lower electrode layer;
Forming a first magnetization free layer made of an amorphous ferromagnetic material on the buffer layer;
Forming a second magnetization free layer made of an amorphous ferromagnetic material different from the first magnetization free layer on the first magnetization free layer;
Forming a tunnel barrier layer on the second magnetization free layer;
Forming a magnetization fixed layer having a magnetization direction fixed on the tunnel barrier layer;
Forming the magnetization fixed layer, and performing a heat treatment to crystallize the second magnetization free layer, the first magnetization free layer maintaining an amorphous state;
Forming a mask pattern on the magnetization fixed layer;
Etching from the magnetization fixed layer to the buffer layer using a reactive ion etching method using the mask pattern as an etching mask to expose the upper surface of the lower electrode layer. A plurality of word lines extending in a first direction;
A plurality of bit lines extending in a direction intersecting the word lines;
Memory cells arranged corresponding to the intersections of the word lines and the bit lines;
The memory cell is
A MOS transistor having one current terminal grounded and a gate electrode connected to a corresponding word line;
One electrode is connected to the other current terminal of the MOS transistor, and the other electrode includes an MTJ element connected to a corresponding bit line,
The MTJ element is
A lower electrode formed on the substrate;
A buffer layer disposed on a partial region of the lower electrode and formed of a conductive material having etching characteristics different from those of the lower electrode;
A first magnetization free layer disposed on the buffer layer and formed of an amorphous ferromagnetic material;
A second magnetization free layer disposed on the first magnetization free layer and formed of a crystallized ferromagnetic material;
A tunnel barrier layer disposed on the second magnetization free layer;
An MRAM comprising a magnetization fixed layer disposed on the tunnel barrier layer and having a magnetization direction fixed, and an upper electrode layer formed on the magnetization fixed layer.

Images