JP2000091668A - Manufacture of ferromagnetic tunnel junction element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferromagnetic tunnel junction element of low resistance value and high current density which is required for a magnetic head and magnetic memory by forming a tunnel barrier layer of high quality under good control. SOLUTION: A first ferromagnetic layer 11 and a conductive layer 12 are continuously formed in vacuum, pure oxygen is introduced without breaking the vacuum so that the surface of the conductive layer 12 is naturally oxidized to form a tunnel barrier layer 13, and after the oxygen is evacuated, a second ferromagnetic layer 14 is formed to complete a basic structure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetoresistive element suitable for a reproducing magnetic head and a high density magnetic memory (MRAM) in a high density magnetic disk drive.

[0002]

2. Description of the Related Art A ferromagnetic tunnel junction device has a structure in which a tunnel barrier layer made of a thin insulator having a thickness of several nm is sandwiched between two ferromagnetic layers. In this device, when an external magnetic field is applied in the plane of the ferromagnetic layer with a constant current flowing between the ferromagnetic layers, a magnetoresistance effect phenomenon in which the resistance value changes according to the relative angle of the magnetization of the two magnetic layers appears. When the magnetization directions are parallel, the resistance value is minimum, and when the magnetization directions are antiparallel, the resistance value is maximum. Therefore, by providing a coercive force difference between the two magnetic layers, a parallel and anti-parallel state of magnetization can be realized according to the strength of the magnetic field, so that a magnetic field can be detected by a change in resistance.

In recent years, the use of an Al surface oxide film as a tunnel barrier layer has made it possible to obtain a ferromagnetic tunnel junction element having a magnetoresistance change rate of nearly 20%. The potential for application has increased. As a representative example of such a large rate of change in magnetoresistance, "Journal of
Applied Physics, 79, 4724-4729 (Jour
nal of Applied Physics, vol. 79, 4724-4729, 199
6) ".

[0004] This technique will be described with reference to the drawings. As shown in FIG. 8, C is formed on a glass substrate 81 using an evaporation mask.
A first ferromagnetic layer 82 of oFe is vacuum-deposited [FIG.
(A)] Then, the mask is replaced, and 1.2 to 2.0 n
An m-thick Al layer 83 is deposited [FIG. 8 (b)]. This Al
By exposing the surface of the layer 83 to oxygen glow discharge, Al ₂ O ₃
Forming a tunnel barrier layer 84 of FIG.
(C)]. Finally, a second ferromagnetic layer 85 made of Co is formed so as to overlap with the first ferromagnetic layer 82 via the tunnel barrier layer 84 to complete a cross-electrode ferromagnetic tunnel junction device [ FIG. 8 (d)]. In this method, a large value of a maximum magnetoresistance change of 18% is obtained.

As another example, see Japanese Patent Application Laid-Open No. 5-63254.
JP-A-6-244777, JP-A-8-70148
JP-A-8-70149, JP-A-8-316548
And `` 1997, Journal of the Japan Society of Applied Magnetics, 21, 493
~ 496 pages ". Here, as a method of forming a tunnel barrier layer, a method is proposed in which an Al layer is formed and then exposed to the atmosphere to grow Al ₂ O ₃ .

[0006]

In order to apply a ferromagnetic tunnel junction device to devices such as a magnetic head and a magnetic memory, it is necessary to have a somewhat low resistance value in practical device dimensions in order to reduce the influence of thermal noise. However, it has been difficult to realize the conventional tunnel barrier forming method. Further, in application to a magnetic head corresponding to high density, the magnitude of a signal output voltage is key, but there is a problem that a sufficient current density cannot be obtained without deteriorating element characteristics in the related art. Furthermore, in the prior art, there is a large variation in device characteristics within a wafer or between lots, and it has been difficult to obtain a sufficient production yield for practical use.

It is considered that these problems are mainly caused by the conventional method of forming a tunnel barrier layer. In other words, in the method using the oxygen glow discharge, active oxygen in the form of ions or radicals is used for oxidizing the conductive layer, so that it is difficult to control a thin oxide film thickness, that is, to control the element resistance. There is a problem that the tunnel barrier layer is contaminated by the impurity gas and the bonding quality is deteriorated. On the other hand, in the method based on natural oxidation in the atmosphere, many dusts in the atmosphere cause pinholes in the tunnel barrier layer and are contaminated by moisture, carbon oxides, nitrogen oxides, etc. I have a problem.

An object of the present invention is to provide a method of manufacturing a ferromagnetic tunnel junction device which solves the problems of the prior art described above, has a resistance value and a signal output voltage characteristic necessary for practical use, and has an improved manufacturing yield. It is in.

[0009]

According to the above object, a method of manufacturing a ferromagnetic tunnel junction device according to the present invention has a structure in which a tunnel barrier layer is sandwiched between a first ferromagnetic layer and a second ferromagnetic layer. In a method of manufacturing a ferromagnetic tunnel junction device having
After forming a conductive layer made of metal or semiconductor in vacuum,
A step of introducing pure oxygen and naturally oxidizing the surface of the conductive layer to form a tunnel barrier layer.

Also, particularly preferably, the first ferromagnetic layer,
The tunnel barrier layer and the second ferromagnetic layer are continuously formed without being exposed to an impurity gas. After the first ferromagnetic layer is formed, pure oxygen is introduced into vacuum to form the first ferromagnetic layer. A step of oxidizing the surface of the ferromagnetic layer, wherein the first and second ferromagnetic layers are Fe, Co, Ni, or an alloy containing these elements, and the conductive layer is Characterized by being composed of any of Al, Mg, and metals belonging to lanthanoids, wherein the oxygen partial pressure of pure oxygen is 2
It is characterized by a range of 0 mTorr to 200 Torr.

In the present invention, since pure oxygen is introduced into a vacuum and the surface of the conductive layer is naturally oxidized to form a tunnel barrier layer, a thermal equilibrium state is maintained in a clean atmosphere unaffected by impurity gas. An oxide layer can be grown, a high quality tunnel barrier layer can be formed with good controllability, and the above object can be achieved.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a method for manufacturing a ferromagnetic tunnel junction device according to the present invention will be described with reference to the drawings.

As shown in FIG. 1, a first ferromagnetic layer 11,
The conductive layer 12 is continuously formed in a vacuum [FIG. 1 (a)], and then pure oxygen is introduced without breaking the vacuum, and the surface of the conductive layer 12 is naturally oxidized to form a tunnel barrier layer 13 [FIG. 1 (b)]. FIG. 1B shows a case where an unoxidized portion of the conductive layer 12 remains at the interface with the first ferromagnetic layer 11 even after the oxidation of the conductive film 12, but depending on the oxidation conditions. It is also possible to completely oxidize. Next, after oxygen is evacuated, a second ferromagnetic layer 14 is formed to complete the basic structure of the ferromagnetic tunnel junction device [FIG. 1 (c)].

The ferromagnetic layers 11 and 14 are made of Fe, Co, Ni.
Alternatively, when an alloy containing these elements is used, the conductive layer 1
By selecting Al having a surface free energy value smaller than that of the ferromagnetic layer as 2, in particular, the first substrate serving as an underlayer is selected.
Good ferromagnetic layer 11 is provided. As a result, in the completed device, good characteristics without an electrical short between the ferromagnetic layers due to pinholes can be obtained. Also, A
l of free energy of formation per atom of oxygen is Fe,
Since it is larger than Co and Ni, Al ₂ O _{3 serving} as a tunnel barrier layer is thermally stable at the junction interface.

When Mg or a metal belonging to the lanthanoid is selected for the conductive layer 12, for the same reason, the first ferromagnetic layer 11 serving as an underlayer has good coverage and a thermally stable tunnel barrier. A layer is obtained.

Next, a second embodiment of the present invention will be described.
This will be described with reference to the drawings. As shown in FIG. 2, a first ferromagnetic layer 11 is formed (FIG. 2A), and oxygen is introduced into a vacuum to form an oxide layer 21 on the surface of the first ferromagnetic layer 11.
Is formed [FIG. 2 (b)]. When this step is added, oxygen diffusion occurs from the first ferromagnetic layer 11 to the conductive layer 12 when the conductive layer 12 is formed in the next step, and the first ferromagnetic layer 11
In addition, a reduction region 22 is formed, and an oxide layer 23 is formed on the lower surface of the conductive layer 12 (FIG. 2C).

According to this step, the conductive layer 1 is formed on at least both interfaces (all regions in FIG. 2D) in contact with the ferromagnetic layer.
Since the oxide film of No. 2 is formed, an element having more excellent thermal stability is realized. In order to form a stable oxide layer on the conductive layer 12 side, the free energy of formation per atom of oxygen in the conductive layer 12 may be set to be larger than that of the element forming the first ferromagnetic layer 11. When Fe, Co, Ni or an alloy containing them is used for the ferromagnetic layer, Al,
It is effective to use Mg or a metal belonging to the lanthanoid.

Subsequently, as in the first embodiment,
By forming the tunnel barrier layer 24 and the second ferromagnetic layer 14, a basic structure of a ferromagnetic tunnel junction device can be obtained [FIG. 2 (e)].

[0019]

<Embodiment 1> A first embodiment of the present invention will be described in detail with reference to the drawings.

First, as shown in FIG. 3A, a first wiring layer 32 made of an Al film having a thickness of 50 nm is formed on a Si substrate 31 whose surface is thermally oxidized. A magnetic layer 33 and a conductive layer 34 of an Al layer having a thickness of 2 nm were continuously deposited by sputtering. For this film formation, a high-frequency magnetron sputtering apparatus provided with four targets each having a diameter of 4 inches was used. The sputtering conditions were all such that the background pressure was 1 × 10 ⁻⁷ Torr or less, and the Ar pressure was 10 mTorr.
r, high-frequency power 200 W. Next, pure oxygen is introduced into the sputtering apparatus, and the oxygen pressure is increased from 20 mTorr to 200 mTorr.
The surface of the Al conductive layer was oxidized to hold the tunnel conductive layer at Torr for 10 minutes to form a tunnel barrier layer. After oxygen was exhausted to reach the background pressure, a second ferromagnetic layer 36 of a 20 nm-thick CoFe film was sputter-deposited to complete the coupling constituent layer.

Next, using a normal photolithography technique and an ion milling technique, all of the bonding constituent layers were processed into a lower wiring shape [FIG. 3 (b)]. Second ferromagnetic layer 36
A resist pattern 37 for defining a junction size was formed thereon, and ion milling was performed up to the first ferromagnetic layer 33 (FIG. 3C). 300 nm with this resist left
After electron beam evaporation of the insulating layer 38 made of a thick Al ₂ O ₃ film, the resist was lifted off [FIG. 3 (d)].
Next, a resist pattern 39 for forming an upper wiring is formed.
[FIG. 3 (e)], and then the exposed sample surface was subjected to reverse sputter cleaning to obtain electrical contact between the second ferromagnetic layer 36 and the second wiring layer 40. Subsequently, a second wiring layer 40 made of an Al film having a thickness of 200 nm was deposited, and finally the resist was lifted off to form an upper wiring, thereby completing a ferromagnetic tunnel junction device [FIG. 3 (f)].

FIG. 4 shows a typical magnetoresistance curve of the ferromagnetic tunnel junction device manufactured as described above. The junction area of this element was 40 × 40 μm ² , and the oxygen pressure of Al when forming the tunnel barrier layer was 20 Torr. By changing the applied magnetic field from -300 0e to 300 0e and, conversely, from 300 0e to -300 0e, the resistance value changes along the curve of A → B → C → D → E → F → G → H → A. Moving. The rising edges of B and F and the falling edges of D and H correspond to the coercive forces of Fe and CoFe, respectively. A and E indicate that the magnetization directions of Fe and CoFe are realized in a parallel state, and C and G indicate that an antiparallel state is realized. The rate of change in magnetoresistance read from this magnetoresistance curve is about 5%, and the junction area (2 × 2 μm ² to 40 × 4)
Approximately the same value was obtained irrespective of 0 μm ² ). on the other hand,
As shown in FIG. 5, the resistance value showed a clean inverse proportional relationship with the junction area. The slope determined by the least squares method is -1.004, which indicates that the element is manufactured with very good controllability. The resistance value standardized by the junction area was 1.5 × 10 ⁻⁵ Ωcm ² , and by lowering the oxygen pressure at the time of forming the tunnel barrier layer to 20 mTorr, a resistance value smaller by one digit or more was obtained. These resistance values are two to three orders of magnitude lower than the lowest resistance values obtained by conventional methods.

FIG. 6 shows the current density dependency of the resistance value and the magnetoresistance ratio at 10 × 10 μm ² . Even if the current density is increased for both the resistance value and the magnetoresistance change rate, it is 10 ³ A / c.
No change was observed at all up to m ² . 5 × 10 ³ A /
Even at cm ² , there was almost no change in the resistance value, and the magnetoresistance change rate was only reduced by about 10%. In addition, this value returned to the original state when the current density was reduced. These results, when obtaining the signal output voltage of the ferromagnetic tunnel junction element as the magnetoresistive effect element, in 10 ³ about a current density of ^{A / cm 2 1mV, 5 ×} 10 3 A / cm 2 to about 3mV met Was. When this element is used for a reproducing magnetic head, it is considered that the latter can correspond to a recording density of 10 Gb / in ² or more.

<Embodiment 2> Next, a second embodiment of the present invention will be described.

The conductive layer 34 in the step of FIG.
In place of the 2 nm Al film, an Mg film having the same thickness was used. Other steps were exactly the same as in Example 1. The obtained magnetoresistance ratio is about 8%, and the bonding area (2 × 2
μm ² to 40 × 40 μm ² ), and almost the same value was obtained. The resistance value normalized by the junction area is 1.6 × 10 ⁻⁵ at an oxygen pressure of 20 Torr at the time of forming the tunnel barrier layer.
Ωcm ² , and by lowering the resistance to 20 mTorr, a value smaller by one digit or more was obtained. These resistance values are two to three orders of magnitude lower than the lowest resistance values obtained by conventional methods. Both the resistance value and the magnetoresistance change rate did not change at all up to the current density of 10 ³ A / cm ² . The signal output voltage under this condition is about 1.3
mV.

<Embodiment 3> Next, a third embodiment of the present invention will be described.

The conductive layer 34 in the step of FIG.
In place of the 2 nm Al film, an La film having the same thickness was used. Other steps were exactly the same as in Example 1. The obtained magnetoresistance ratio is about 6%, and the bonding area (2 × 2
μm ² to 40 × 40 μm ² ), and almost the same value was obtained. The resistance value normalized by the junction area is 2.7 × 10 ⁻⁵ at an oxygen pressure of 20 Torr at the time of forming the tunnel barrier layer.
Ωcm ² , and by lowering the resistance to 20 mTorr, a value smaller by one digit or more was obtained. These resistance values are two to three orders of magnitude lower than the lowest resistance values obtained by conventional methods. Both the resistance value and the magnetoresistance change rate did not change at all up to the current density of 10 ³ A / cm ² . The signal output voltage under this condition is about 1.
6 mV.

In this embodiment, La is used as the metal belonging to the lanthanoid used for the conductive layer 34.
Almost the same results can be obtained by using a metal of the same group such as Nd, Sm, and Lu.

<Embodiment 4> A fourth embodiment of the present invention will be described.

First, as shown in FIG. 7A, a first wiring layer 32 made of an Al film having a thickness of 50 nm is formed on a Si substrate 31 whose surface is thermally oxidized. The magnetic layer 33 was continuously sputter deposited. For this film formation, a high-frequency magnetron sputtering apparatus provided with four targets each having a diameter of 4 inches was used. The sputtering conditions were all such that the background pressure was 1 × 10 ⁻⁷ Torr or less, the Ar pressure was 10 mTorr, and the high frequency power was 200 W. next,
Pure oxygen is introduced into the sputtering device, and the oxygen pressure is 200To.
rr for 10 minutes, and the FeOx layer 71 on the Fe film surface.
Was formed. After the oxygen was exhausted to reach the background pressure, a conductive layer 34 made of an Al film having a thickness of 2 nm was sputter-deposited [FIG. 7 (b)]. At that time, Fe on the Fe film surface
Ox oxygen diffuses into the Al film, and an Al ₂ O ₃ layer 7
3 was formed. Once again, pure oxygen was introduced into the sputtering apparatus, the oxygen pressure was maintained at a range of 20 mTorr to 200 Torr for 10 minutes, and the surface of the Al conductive layer 34 was oxidized to form a tunnel barrier layer 74. After exhausting oxygen, the second ferromagnetic layer 3 made of a CoFe film having a thickness of 20 nm is formed.
6 was sputter-deposited to complete the bonding constituent layer [FIG.
(C)]. In this embodiment, the Al film serving as the tunnel barrier layer 74 was completely oxidized.

Next, using a normal photolithography technique and an ion milling technique, all of the bonding constituent layers were processed into a lower wiring shape [FIG. 7 (d)]. Second ferromagnetic layer 36
A resist pattern 37 for defining a junction size was formed thereon, and ion milling was performed up to the first ferromagnetic layer 33 (FIG. 7E). 300 nm with this resist left
After an insulating layer 38 made of a thick Al ₂ O ₃ film was subjected to electron beam evaporation, the resist was lifted off [FIG. 7 (f)].
Next, a resist pattern 39 for forming an upper wiring is formed.
[FIG. 7 (g)], and then the exposed sample surface was subjected to reverse sputter cleaning in order to obtain electrical contact between the second ferromagnetic layer 36 and the second wiring layer 40. Subsequently, a second wiring layer 40 made of an Al film having a thickness of 200 nm was deposited, and finally the upper wiring was formed by lifting off the resist, thereby completing a ferromagnetic tunnel junction device [FIG. 7 (h)].

The obtained magnetoresistance ratio was about 10%, and almost the same value was obtained irrespective of the junction area (2 × 2 μm ² to 40 × 40 μm ² ). The resistance value normalized by the bonding area is the oxygen pressure at the time of forming the tunnel barrier layer of 20 Torr.
r is 5 × 10 ⁻⁵ Ωcm ² , and by lowering the value to 20 mTorr, a value smaller by at least one order of magnitude was obtained. These resistance values are two to three orders of magnitude lower than the lowest resistance values obtained by conventional methods. Both the resistance value and the magnetoresistance change rate are a current density of 1.5 × 10 ³ A / c.
No change was observed at all up to m ² . The signal output voltage under this condition was about 7.5 mV. When this element is used for a reproducing magnetic head, it is considered that the element can cope with a recording density of ²⁰ Gb / in ² or more.

<Embodiment 5> Next, a fifth embodiment of the present invention will be described.

The conductive layer 34 in the step of FIG.
In place of the 2 nm Al film, an Mg film having the same thickness was used. Other steps were exactly the same as in Example 4. The obtained magnetoresistance ratio is about 9%, and the bonding area (2 × 2
μm ² to 40 × 40 μm ² ), and almost the same value was obtained. The resistance value normalized by the junction area is 6 × 10 ⁻⁵ Ωc at an oxygen pressure of 20 Torr at the time of forming the tunnel barrier layer.
m ² , and by lowering the value to 20 mTorr, a value smaller by one digit or more was obtained. These resistance values are two to three orders of magnitude lower than the lowest resistance values obtained by conventional methods. In both the resistance value and the magnetoresistance change rate, no change was observed up to a current density of 1.5 × 10 ³ A / cm ² . The signal output voltage under this condition is about 8.
It was 1 mV.

<Embodiment 6> Next, a sixth embodiment of the present invention will be described.

The conductive layer 34 in the step of FIG.
In place of the 2 nm Al film, an La film having the same thickness was used. Other steps were exactly the same as in Example 4. The obtained magnetoresistance change rate is about 12%, and the bonding area (2 ×
Almost the same value was obtained irrespective of ² μm ² to 40 × 40 μm ² ). The resistance value normalized by the junction area is 4 × 10 ⁻⁵ Ω at an oxygen pressure of 20 Torr at the time of forming the tunnel barrier layer.
cm ² , and by lowering the value to 20 mTorr, a value smaller by one digit or more was obtained. These resistance values are two to three orders of magnitude lower than the lowest resistance values obtained by conventional methods. Both the resistance value and the rate of change in magnetoresistance showed no change up to a current density of 1.5 × 10 ³ A / cm ² . The signal output voltage under this condition is about 7.
It was 2 mV.

In this embodiment, La is used as the metal belonging to the lanthanoid used for the conductive layer 34.
Almost the same results can be obtained by using a metal of the same group such as Nd, Sm, and Lu.

[0038]

According to the present invention, an oxide layer can be grown in a clean atmosphere which is not affected by an impurity gas while maintaining a thermal equilibrium state. Therefore, a high quality tunnel barrier layer can be formed with good controllability. it can. Further, by controlling the oxygen pressure and the substrate temperature, it is possible to easily obtain an element having a low resistance value and a high current density necessary for application of a device such as a magnetic head or a magnetic memory. Further, it is possible to manufacture a device having excellent uniformity of device characteristics in a wafer and reproducibility between lots.

[Brief description of the drawings]

FIGS. 1A to 1C are process diagrams for explaining a first embodiment of the present invention.

FIGS. 2A to 2E are process diagrams for explaining a second embodiment of the present invention.

FIGS. 3A to 3F are process diagrams for explaining the first embodiment.

FIG. 4 is a magnetoresistance curve diagram of the ferromagnetic tunnel junction device manufactured in Example 1.

FIG. 5 is a diagram showing the relationship between the resistance and the junction area of the ferromagnetic tunnel junction device manufactured in Example 1.

FIG. 6 is a diagram showing the current dependence of the resistance and the magnetoresistance change rate in a 10 μm square junction of the ferromagnetic tunnel junction device manufactured in Example 1.

FIGS. 7A to 7H are process diagrams for explaining Example 4. FIGS.

FIGS. 8A to 8D are process diagrams for explaining a conventional method of manufacturing a ferromagnetic tunnel junction device.

[Description of Signs] 11 First ferromagnetic layer 12 Conductive layer 13 Tunnel barrier layer formed by natural oxidation of pure oxygen 14 Second ferromagnetic layer 21 Surface oxide film of first ferromagnetic layer 22 First strength Reduction region of magnetic layer 23 Oxide layer on lower surface of conductive layer 24 Tunnel barrier layer formed by natural oxidation of pure oxygen

Claims (6)

[Claims] 1. A method for manufacturing a ferromagnetic tunnel junction device having a structure in which a tunnel barrier layer is sandwiched between a first ferromagnetic layer and a second ferromagnetic layer, wherein the conductive layer is made of a metal or a semiconductor in a vacuum. And forming a tunnel barrier layer by introducing pure oxygen and naturally oxidizing the surface of the conductive layer after forming the film. 2. The method for manufacturing a ferromagnetic tunnel junction device according to claim 1, wherein the first ferromagnetic layer, the tunnel barrier layer, and the second ferromagnetic layer are continuously formed without being exposed to an impurity gas. . 3. The method according to claim 1, further comprising the step of introducing pure oxygen into a vacuum to oxidize the surface of the first ferromagnetic layer after forming the first ferromagnetic layer. A method for manufacturing the ferromagnetic tunnel junction device according to the above. 4. The method according to claim 1, wherein the first and second ferromagnetic layers include Fe,
The method for manufacturing a ferromagnetic tunnel junction device according to any one of claims 1 to 3, wherein the method is Co, Ni, or an alloy containing these elements. 5. The method for manufacturing a ferromagnetic tunnel junction device according to claim 1, wherein the conductive layer is made of any one of Al, Mg, and a metal belonging to a lanthanoid. . 6. The oxygen partial pressure of pure oxygen is 20 mTorr to 2
The method of manufacturing a ferromagnetic tunnel junction device according to claim 1, wherein the pressure is 00 Torr.