JP2005332838A - Tunneling magneto-resistive element and its manufacturing method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tunneling magneto-resistive element containing an insulation layer which is formed of aluminum nitride which allows the formation of the insulation layer with good controllability, and which has a TMR ratio nearly the same as that of aluminum oxide, and also to provide its manufacturing method and apparatus. <P>SOLUTION: The tunneling magneto-resistive element is such that the insulation layer formed of aluminum nitride (Al-N) is sandwiched between two ferromagnetic layers (fixed magnetization layer and free magnetization layer). The insulation layer has a barrier height of 2 eV or above. The manufacturing method of the insulation layer comprises a nitriding process for nitriding a metal Al film by plasmas, and an annealing process of heat-treating the nitrided Al-N film. The plasma has a low electron temperature of 2 eV or lower, and as nitriding species, and serves to excite nitrogen molecule radicals or nitrogen atom radicals. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a tunnel type magnetoresistive element used for a magnetic device such as a GMR type read head or a magnetic memory (MRAM), and a method and apparatus for manufacturing the same, and in particular, aluminum nitride (Al-N) is used for the tunnel insulating layer. The present invention relates to a tunnel type magnetoresistive element and a method and apparatus for manufacturing the same.

  In recent years, tunneling magnetoresistive (TMR) elements have been proposed, and the development of various magnetic devices to which the tunneling magnetoresistive elements are applied is also progressing rapidly. This tunnel-type magnetoresistive element (TMR element) has a structure in which a thin insulating layer is sandwiched between two ferromagnetic layers, and generally includes a ferromagnetic layer (magnetization fixed layer) / insulation layer / ferromagnetic layer (magnetization free layer). It has a laminated structure. Here, the magnetization fixed layer is a layer in which the magnetization direction is fixed in one direction by exchange coupling with the antiferromagnetic layer, and the magnetization free layer is a magnetization direction that fluctuates freely by an external magnetic field, for example. Is a layer. As a result, when the magnetization direction of the magnetization free layer varies with respect to the magnetization fixed layer whose magnetization direction is fixed in one direction, the tunnel magnetoresistive effect (TMR effect) appears, and the change of the external magnetic field is changed to the resistance value. It is possible to perceive it as a change.

  Examples of the magnetic device to which this tunnel type magnetoresistive element is applied include a GMR type reproducing head in a magnetic recording apparatus, a non-volatile magnetic memory (MRAM: magnetic random access memory) in which data is stored even when the power is turned off, etc. There is.

  The above magnetic memory uses a tunnel type magnetoresistive element having a laminated structure of a first ferromagnetic layer (magnetization fixed layer) / insulating layer / second ferromagnetic layer (magnetization free layer) as a memory element. An MRAM memory cell is configured by combining a tunnel type magnetoresistive element and a MOSFET.

  In such a tunnel type magnetoresistive element, a CoFe-based alloy is often used as a material constituting the first and second ferromagnetic layers because of its high polarizability. As a material constituting the insulating layer, an aluminum (Al) oxide film is often used because a high TMR ratio (tunnel magnetoresistance change rate) can be obtained. As a method for forming this Al oxide film, various methods such as natural oxidation by oxygen, oxidation by oxygen radicals, oxidation by oxygen plasma, etc. have been proposed, but oxidation by oxygen plasma (plasma oxidation method) is the mainstream. It is.

  Patent Document 1 has been filed by the same inventors as the present application regarding a method and apparatus for manufacturing a tunnel magnetoresistive element using a plasma oxidation method and a magnetic device using this element. To cite the description of the publication of Patent Document 1, Patent Document 1 states that “a heat treatment at a low temperature is possible and the barrier barrier height of the obtained insulating layer is sufficiently high, and as a result, a high TMR ratio is realized. For the purpose of providing a tunneling magnetoresistive element, a magnetic device using the same, and a manufacturing method and manufacturing apparatus therefor, the tunneling magnetoresistive element includes a first ferromagnetic layer, an insulating layer, The second ferromagnetic layer is sequentially laminated, and the insulating layer is formed into a metal oxide by oxidizing the deposited metal layer or alloy layer by a plasma processing apparatus using a radial line slot antenna (RLSA). The technique is disclosed in which the resistance value is arbitrarily controlled within a predetermined resistance value range by heat treatment after film formation.

  Patent Document 1 also widely discloses the configuration of a tunnel-type magnetoresistive element and a magnetic device using this element. With respect to these outlines, the description of Patent Publication 1 is partially cited below. In the following.

  FIG. 10 (corresponding to FIG. 1 of Patent Document 1) is a schematic cross-sectional view showing a tunnel type magnetoresistive element. In FIG. 10, reference numeral 1 denotes a tunnel type magnetoresistive element, and the first ferromagnetic layer (conductor). 2, an insulating layer (insulator) 3, and a second ferromagnetic layer (conductor) 4 are sequentially stacked.

  The first ferromagnetic layer 2 is made of an alloy exhibiting ferromagnetism at least partially, and the magnetization direction is fixed in one direction as shown by an arrow in FIG. The first ferromagnetic layer 2 is formed by, for example, a sputtering method, a vapor deposition method, or other thin film forming means in an ultrahigh purity argon (Ar) gas atmosphere having an impurity gas concentration of less than 1 ppb. A magnetron sputtering method is preferred.

  The alloy exhibiting ferromagnetism is not particularly limited as long as it exhibits ferromagnetism, but the barrier barrier height is sufficiently secured, and a high TMR ratio (tunnel magnetoresistance change rate) is obtained. In consideration of obtaining, Co, Fe and other ferromagnetic metals, Co alloys, Fe alloys, Ni alloys, NiFe alloys, CoFe alloys, CoNi alloys, CoFeNi alloys, MnAs, MnSb, MnBi Alloy compounds of Mn and VA group of periodic table (N, P, As, Sb, Bi), Ni such as NiAs, NiSb, NiBi and VA group of periodic table (N, P, As, Sb, An alloy compound with Bi) is preferably used.

Among these alloys, a CoFe-based alloy represented by the composition formula Co _x Fe _100-x is preferable. In the above composition formula, a preferable range of x (at%) is 50 ≦ x ≦ 85. Note that all of the first ferromagnetic layer 2 may be a metal or alloy having the above composition, and the first ferromagnetic layer 2 is a laminate composed of a plurality of ferromagnetic layers having different compositions, and at least one layer is formed. A metal or alloy having the above composition may be used. Further, although not described in Patent Document 1, suitable ferromagnetic materials that have recently attracted attention include CoFeB having an amorphous structure and Fe ₃ O ₄ · CoMnSi · CoCrFeAl having a half metal structure.

  The insulating layer 3 is made of an insulating material such as a metal oxide (for example, aluminum oxide). The formed metal layer or alloy layer is oxidized to form a metal oxide, and the resistance value is arbitrarily controlled within a predetermined resistance value range by heat treatment after film formation. The thickness of the insulating layer 3 is preferably extremely thin so that the tunnel effect can be exhibited, and is, for example, 0.5 to 2.0 nm. As the insulating layer, it is known that aluminum nitride, aluminum oxynitride, magnesium oxide, strontium titanate, or the like can be used in addition to the aluminum oxide (for example, see Patent Document 2).

  The second ferromagnetic layer 4 is made of an alloy showing at least part or all of ferromagnetism, and the direction of magnetization is changed by changing the direction of the external magnetic field 5 as shown by the arrows in FIG. Changes to the right or left.

  The second ferromagnetic layer 4 is formed in the same manner as the first ferromagnetic layer 2, for example, in an ultra-high purity argon (Ar) gas atmosphere having an impurity gas concentration of less than 1 ppb, by sputtering, vapor deposition, or other methods. It is formed by a thin film forming means, and a magnetron sputtering method is particularly suitable. The second ferromagnetic layer 4 is preferably made of a metal, alloy, alloy compound or the like having the same ferromagnetism as the first ferromagnetic layer 2.

  FIG. 11 (corresponding to FIG. 4 of Patent Document 1) is a schematic cross-sectional view showing a tunnel type magnetoresistive element having a form different from that of FIG. 10, and this tunnel type magnetoresistive element 51 is placed on a Si substrate 52 below. The ground layer 53, the antiferromagnetic layer 54, the first ferromagnetic layer (magnetization fixed layer) 55, the insulating layer 56, the second ferromagnetic layer (magnetization free layer) 57, and the electrode layer 58 are sequentially laminated ( For details, see Patent Document 1).

  Next, based on FIGS. 12 to 14, the outline of various magnetic devices to which the tunnel type magnetoresistive element is applied will be described with reference to the description of Patent Document 1. FIG.

  FIG. 12 is a partially broken perspective view showing a GMR reproducing head and a recording / reproducing separated type magnetic head in which this reproducing head and an induction recording head are combined. FIG. 13 is a cross-sectional view showing the main part of the GMR reproducing head. It is. 12 and 13, reference numeral 100 denotes a tunnel type magnetoresistive element, 108 denotes an MR electrode, 109 denotes a hard film, 111 denotes a GMR type reproducing head, and 112 denotes a GMR type reproducing head 111 that also serves as a lower magnetic pole (124) of the recording head. 113, 114 are nonmagnetic insulating films, 115 is a lower shield of the GMR reproducing head 111, 121 is a recording head, 122 is an upper pole of the recording head 121, 123 is a coil made of a conductor, and 124 is a GMR. This is the lower magnetic pole of the recording head that also serves as the upper shield (112) of the mold reproducing head 111. Since the tunnel type magnetoresistive element 100 is substantially the same as the tunnel type magnetoresistive element 51 of the form shown in FIG. 11, the same components as the tunnel type magnetoresistive element 51 are denoted by the same reference numerals. The description is omitted.

  Next, FIG. 14 will be described. FIG. 14 is a cross-sectional view showing a memory cell which is a main part of a nonvolatile magnetic memory (MRAM). The memory cell 200 includes a MOSFET 201 which is a cell selection element, a word line 202, a bit line 203, and a tunnel magnetoresistive element 204. The word line 202 is connected to the MOSFET 201, the bit line 203 is connected to the tunnel type magnetoresistive element 204, and the tunnel type magnetoresistive element 204 is connected to the MOSFET 201 via the connection wiring 205. The tunnel type magnetoresistive element 204 is substantially the same as the tunnel type magnetoresistive element 100 of the form shown in FIG.

  By the way, also in the magnetic random access memory (MRAM), the increase in storage capacity expected in the near future will not only reduce the size of the ferromagnetic tunnel junction (MTJ) element but also increase the reading speed. Due to the demand for reduction in device resistance, the resistance area product (R × A) of the junction must be reduced. In order to reduce the junction resistance without impairing the tunnel magnetoresistance change rate (TMR ratio), the tunnel insulating film, which is currently about 0.8 nm thick, can be used without causing deterioration of the barrier performance due to pinhole contamination. It is essential to reduce its thickness. In this case, the tunnel barrier film forming process in which the plasma oxidation method is mainly used has the following problems.

  In order to maintain a high TMR ratio in the MTJ film, it is essential to maintain a high spin polarizability of the two ferromagnetic layers. The plasma oxidation method, in which a metal Al film is formed and then oxidized with oxygen plasma to form an insulating layer, is generally considered suitable as a method for forming a tunnel barrier film because a high TMR ratio is easily obtained. . In order to reduce the junction resistance by reducing the thickness of the tunnel barrier film without impairing the high TMR ratio, the surface of the underlying lower ferromagnetic layer is used in the oxidation process of the ultrathin metal Al film. Without oxidizing the Al film, it is necessary to oxidize the Al film precisely to the interface with the lower ferromagnetic layer.

  However, when the metal Al film thickness is reduced, the strong oxidation activity of atomic oxygen radicals in particular makes it difficult to precisely control the oxidation process of the ultrathin metal film (for details, This will be described later in comparison with the embodiment of the present invention).

  By the way, focusing on Al-N with respect to Al-O, which is generally used for tunnel barrier layers, the difference in affinity of metal elements for nitrogen gas and oxygen gas, as well as in oxides or nitrides, respectively. From the viewpoint of the difference in the diffusion coefficient of the gas elements, the nitridation process of the metal Al thin film is expected to proceed more slowly than the oxidation process, and is considered suitable for the formation of an ultrathin tunnel barrier film. So far, the tunnel magnetoresistance effect of an MTJ film having a Co—Fe / Al—N / Co—Fe structure has been studied (for example, see Non-Patent Document 1).

However, the maximum value of TMR ratio in aluminum nitride (Al—N) reported in Non-Patent Document 1 is about 33%, and 40 to 50% or more when aluminum oxide (Al—O) is used. Compared to the TMR ratio, there is an amber color. Such a relatively low TMR ratio is thought to hinder application studies of Al-N tunnel barrier layers.
JP 2003-101098 A JP 2002-176221 A Jianguo Wang et al., “Tunnel junctions with AlN barriers and FeTaN electrodes”, Journal of. Applied. Physics., Vol. 89, No. 11, p6868 ~ 6870, (1 June 2001)

  As described above, when the insulating layer of the tunnel magnetoresistive element is extremely thinned, when the insulating layer of aluminum oxide is formed by plasma oxidation of metal Al, the characteristics of the TMR ratio are good, but the oxidation rate Therefore, it is difficult to optimize the oxidation conditions. For example, there is a problem that the TMR ratio is drastically reduced due to insufficient oxidation or excessive oxidation. That is, there is a problem that the controllability of forming the insulating layer is poor and it is difficult to stably manufacture a high quality product.

  On the other hand, when forming an insulating layer of aluminum nitride by plasma nitriding, the controllability is good because the nitriding rate is about 10 to 100 times slower than the oxidation rate. Therefore, it is difficult to obtain a TMR ratio (about 40%) or more required in the MRAM.

  The present invention has been made in view of the above points, and an object of the present invention is an insulating layer made of aluminum nitride having good controllability of forming an insulating layer, and having an TMR ratio comparable to that of aluminum oxide. It is an object of the present invention to provide a tunnel type magnetoresistive element having a layer and a method and apparatus for manufacturing the same.

  The above-mentioned subject is achieved by the following. That is, in a tunnel magnetoresistive element in which an insulating layer made of aluminum nitride (Al—N) is sandwiched between two ferromagnetic layers (a magnetization fixed layer and a magnetization free layer), the barrier height of the insulating layer is 2 eV or more. (Claim 1). By setting the barrier height of the insulating layer to 2 eV or more, the tunnel magnetoresistance change rate can be set to 40% or more as desired in the MRAM. Details will be described later.

  Further, as a manufacturing method for obtaining the tunnel type magnetoresistive element, the inventions of the following claims 2 to 8 are preferable. That is, in the method for manufacturing a tunnel magnetoresistive element in which an insulating layer made of aluminum nitride (Al—N) is sandwiched between two ferromagnetic layers (a magnetization fixed layer and a magnetization free layer), the insulating layer forming step Includes a nitriding process for nitriding a metal Al film with plasma and an annealing process for heat-treating the nitrided Al—N film, and the plasma has a low electron temperature of 2 eV or less. The nitriding species excites a nitrogen molecular radical or a nitrogen atom radical (claim 2).

  As an embodiment of the invention of claim 2, the inventions of claims 3 to 8 below are preferable. That is, in the manufacturing method according to claim 2, the nitriding step is performed using a microwave-excited plasma processing apparatus including a radial line slot antenna (claim 3).

  Furthermore, in the manufacturing method according to claim 2 or 3, the nitriding treatment step is performed in a nitrogen atmosphere or an inert gas atmosphere containing at least nitrogen (claim 4).

  Furthermore, in the manufacturing method according to claim 4, the inert gas is at least one of helium gas, argon gas, krypton gas, and xenon (claim 5).

  Further, in the manufacturing method according to any one of claims 2 to 5, the resistance value of the Al-N film and the magnetoresistance change rate of the magnetoresistive element are controlled by a heat treatment temperature in the annealing process. (Claim 6).

  Furthermore, in the manufacturing method of the said Claim 6, the said heat processing temperature shall be 240-340 degreeC (Claim 7).

  The manufacturing method according to any one of claims 2 to 7, wherein the tunneling magnetoresistive element is an element for a magnetic device such as a GMR reproducing head or a magnetic memory. (Claim 8).

  According to the method of manufacturing a tunnel magnetoresistive element of the present invention, a high-quality tunnel insulating layer having a barrier height of 2 eV or more can be stably formed, and as a result, the required specifications of the MRAM are satisfied. High TMR ratio (tunnel magnetoresistance change rate). In particular, in the case of nitriding with plasma, when nitrogen ions are present, the nitrogen ions are accelerated by the electric field gradient in the plasma, causing damage to the ultrathin aluminum film, resulting in characteristic deterioration. Since the plasma in the plasma has a low electron temperature of 2 eV or less and excites nitrogen molecular radicals or nitrogen atom radicals as nitriding species, it can suppress the occurrence of the damage and controllability. Therefore, a stable high TMR ratio can be realized. Details will be described later.

  Next, as a method for manufacturing a tunnel type magnetoresistive element of the present invention, the invention of claim 9 is preferable. That is, in a tunnel magnetoresistive element manufacturing apparatus in which an insulating layer made of aluminum nitride (Al—N) is sandwiched between two ferromagnetic layers (a magnetization fixed layer and a magnetization free layer), the ferromagnetic layer is formed. A sputtering apparatus for forming a film; a sputtering apparatus for forming a metal Al film; a microwave-excited plasma processing apparatus for nitriding the metal Al film; and an annealing apparatus for annealing the nitrided Al—N film. (Claim 9).

  According to the present invention, a tunneling magnetoresistive element comprising an insulating layer made of aluminum nitride having good controllability of forming an insulating layer and having an TMR ratio comparable to that of aluminum oxide, its manufacturing method, and its manufacturing A device can be provided.

  Next, based on FIG. 1 and FIG. 2, an example of an embodiment relating to a method and apparatus for manufacturing a tunnel magnetoresistive element of the present invention will be described. Further, in the section of Examples described later, the experimental example of aluminum nitride according to the present invention and the experimental example of conventional aluminum oxide will be compared and described in detail. In the following description, the pressure is expressed in units of Torr. However, when this is converted into Pa (Pascal) which is an SI unit, it may be converted by 1 Torr = 133 Pa. In some cases, the film thickness is expressed in units of Å, but when this is converted into nm (nanometer), which is an SI unit, conversion may be made by 1 Å = 0.1 nm. Further, the magnetic field may be expressed in units of Oe (Oersted), but when this is converted to A / m (ampere per meter), which is an SI unit, it may be converted by 1 Oe = 79.58 A / m. .

  FIG. 1 is a schematic cross-sectional view showing a multi-sputter apparatus 11 used in manufacturing the tunnel type magnetoresistive element 1 of the present invention. The multi-sputter apparatus 11 is a chamber serving as a processing space isolated from the outside. 12, a sputtering apparatus 13 for forming a ferromagnetic material, a sputtering apparatus 14 for forming an aluminum metal that is nitrided by nitriding to be described later to be an insulator, and a substrate 15 that is an object to be processed is a sputtering apparatus 13. And a substrate holder 16 that moves between the sputtering devices 14.

  The sputtering devices 13 and 14 include a sputtering target 17 and a cathode 18. The substrate holder 16 is configured to move the substrate 15 between the sputtering apparatus 13 and the sputtering apparatus 14 by rotating the holder portion 21 that holds the substrate 15 around the shaft 22. Further, an exhaust port 23 connected to a vacuum device (not shown) is formed on the side wall of the chamber 12, and the inside of the chamber 12 is maintained at a predetermined pressure by the vacuum device. The inside of the chamber 12 is formed in a state in which, for example, the impurity gas concentration is 1 ppb or less and the pressure is filled with high-purity argon (Ar) gas of, for example, 0.1 to 10 mTorr.

  The sputtering apparatuses 13 and 14 are not particularly limited as long as they can form a ferromagnetic layer and an aluminum metal layer by a sputtering method. However, the film thickness can be excellent and the film can be formed in a large area. Considering these points, a magnetron sputtering apparatus is suitable.

  FIG. 2 is a schematic cross-sectional view showing a microwave-excited plasma processing apparatus 31 including a radial line slot antenna (RLSA) used when manufacturing the tunnel type magnetoresistive element 1 of the present invention. This is for nitriding the metal aluminum layer formed by the above method.

  The microwave-excited plasma processing apparatus 31 includes a chamber 32 serving as a processing space S, the inside of which is isolated from the outside, and a quartz plate 34 that is an insulating plate that is airtightly provided at the bottom of the chamber 32 via a sealing material 33. A radial line slot antenna 35 arranged parallel to the quartz plate 34 at a predetermined interval, a disk-shaped dielectric plate 36 in which the radial line slot antenna 35 is embedded, and a microwave generator (not shown). The bottomed cylindrical radial waveguide portion 38 having the waveguide 37 is provided, and a support base 42 that is disposed on the ceiling portion of the chamber 32 and supports the substrate 41.

  A gas introduction port 43 connected to a gas supply means for introducing an inert gas such as Ar or Kr and an exhaust port 44 connected to a vacuum device (not shown) are formed in the side wall of the chamber 32. The chamber 32 is maintained in a vacuum state at a predetermined pressure by this vacuum device. On the other hand, an electrostatic chuck or a clamping mechanism for supporting the substrate 41 on which the ferromagnetic layer and the aluminum metal layer are laminated is provided on the lower surface of the support base 42. The support base 42 is biased via a wiring (not shown). It is connected to a high frequency power supply.

  Next, a method for nitriding the aluminum metal layer formed by the sputtering apparatus 14 using the microwave-excited plasma processing apparatus 31 will be described. First, the first ferromagnetic layer 2 and the aluminum metal layer are formed on the upper surface of the substrate 15 by using the multi-sputter apparatus 11 to form the first ferromagnetic layer 2 and the aluminum metal layer that is nitrided to become an insulator. Is prepared. Next, the substrate 41 is accommodated in the chamber 32 using a transfer device, and the substrate 41 is fixed on the support table 42 using an electrostatic chuck or a clamp mechanism.

Next, the inside of the chamber 32 is maintained in a predetermined vacuum state, for example, 1 × 10 ⁻⁶ to 1 × 10 ⁻¹¹ Torr, and an inert gas such as Ar containing nitrogen is introduced from the gas introduction port 43. The pressure of the inert gas is, for example, 200 to 2000 mTorr. As the inert gas, Ar gas, Kr gas, or Xe gas containing 1 to 30 v / v% of nitrogen (N ₂ ) is preferably used.

  At the same time, the microwave generated from the microwave generator is introduced into the chamber 32 through the waveguide 37, the radial line slot antenna 35 and the quartz plate 34, and the plasma P is generated in the chamber 32. Nitriding treatment is applied to the aluminum metal layer on the substrate 41. By this nitriding treatment, the aluminum metal layer is nitrided to become an insulating layer made of high resistance aluminum nitride.

  The substrate 41 is again carried into the film forming apparatus 11, and the second ferromagnetic layer 4 is formed on the insulating layer using the sputtering apparatus 13. As a result, a tunneling magnetoresistive element having an insulating layer sandwiched between the two ferromagnetic layers 2 and 4 can be obtained. Next, the tunnel type magnetoresistive element is taken out from the film forming apparatus, and annealed in a vacuum heat treatment furnace (not shown) provided separately from the film forming apparatus. By setting the heat treatment temperature in the annealing of the insulating layer in the range of 240 to 340 ° C., the resistance value changes corresponding to the heat treatment temperature within a predetermined resistance value range. By this annealing treatment, the insulating layer becomes the insulating layer 3 having a predetermined resistance value. In addition, a magnetoresistive element having a desired magnetoresistance change rate can be obtained. This makes it possible to secure a sufficient barrier barrier height of the tunneling magnetoresistive element, and as a result, a high TMR ratio can be realized.

  The apparatus for performing the nitriding treatment is not limited to the microwave-excited plasma processing apparatus as described above as long as the following basic matters are satisfied. In the nitriding treatment step, basically important matters are to use a plasma having a low electron temperature and to use a nitrogen molecular radical or a nitrogen atom radical as a nitriding species. If ions are present in the plasma, the ions are accelerated by the space potential of the plasma, damaging the metal Al film that forms the ultrathin insulating layer and degrading the properties, so that neutral nitrogen radicals (molecular or atomic) It is necessary to excite the state.

The electron temperature will be described in detail below with reference to FIG. FIG. 3 shows the distance from the window of the plasma processing apparatus toward the processing chamber (distance from the quartz plate 34 to the plasma in FIG. 2) z (mm), the electron temperature Te (eV), and the electron density (cm ⁻³ ). This relationship is shown for the examples of the inert gases He, Ar, and Kr. In FIG. 3, the microwave wavelength is 2.45 GHz, the power is 1.42 W / cm 2, and the processing pressure is 1 Torr.

  As shown in FIG. 3, the electron temperature and the electron density vary depending on the type of inert gas and the distance from the window. In the case of plasma nitriding, nitriding is usually performed by mixing a rare gas with nitrogen gas. Some rare gases ionize with lower energy than nitrogen. Therefore, when the electron temperature is high, these ions are accelerated and damage the aluminum nitride thin film. If the nitriding species is radical (neutral), there is no problem even if the electron temperature is high, but the electron temperature is preferably 2 eV or less from the viewpoint of suppressing damage due to ionization of the rare gas. According to FIG. 3, for example, when z = 50 mm, the value of the electron temperature Te is about 1.1 eV for Kr gas, about 1.3 eV for Ar gas, and about 2 eV for He gas. From the viewpoint of suppressing the damage by lowering the electron temperature, Kr gas or Ar gas is more preferable than He gas.

  Hereinafter, according to an embodiment of an aluminum nitride ferromagnetic tunnel junction (MTJ) device according to the present invention, as compared with a conventional aluminum oxide (comparative example), a film formation process, a film formation speed, an appropriate plasma exposure amount, The MTJ TMR effect, insulating layer parameter evaluation, and the like will be described in more detail.

(Process for forming MTJ film in Examples and Comparative Examples)
MTJ film consists of substrate / underlying lead electrode layer / Mn-Ir 100 A / Co ₇₀ Fe ₃₀ 40 A / Al-N or Al-O / Co ₇₀ Fe ₃₀ 40 A / Ni-Fe 200 A / upper lead electrode MTJ having a junction area of 25 mm ² to 2500 mm ² was prepared by photolithography and ion milling. The film thickness (d _Al ) of the oxidized / nitrided metal Al film was 8, 10, 15 A. Various concentrations of the oxygen gas and nitrogen gas added to the rare gas and the plasma treatment time were changed. Wavelength of the microwaves to be introduced for plasma excitation was 2.45 GHz, power constant at 1.1 W / cm ^2. The treatment pressure was set to 1 Torr except for experiments in which the pressure was changed. The fabricated MTJ was subjected to sequential heat treatment on the same sample in an applied magnetic field of 1 kOe.

(Al-N and Al-O insulating film formation rate and appropriate plasma exposure)
Next, based on FIG. 4 and FIG. 5, the experimental results relating to the formation rate of the insulating film and the appropriate plasma exposure amount will be described.

FIG. 4 shows the plasma exposure dose dependency of the MTJ film junction resistance (R × A) immediately after fabrication. Plasma exposure is defined as the product of the partial pressure (P) of oxygen or nitrogen gas in the rare gas and the plasma treatment time (t), expressed in Langmuir units (L = 1 x 10 ^-6 Torr sec) Yes. In FIG. 4, the film thickness (d _Al ) 8 A is ○ ●, 10A is Δ ▲, 15A is ▽ ▼, and black marks indicate the rare gas Ar and white indicates Kr. Also, in the curve of FIG. 4, the left side shows oxidation and the right side shows nitriding.

In each of the oxidized sample and the nitride sample shown in FIG. 4, it can be seen that the increase in the junction resistance with respect to the plasma exposure time has a substantially single tendency regardless of the film thickness (d _Al ). Further, comparing the oxidized sample and the nitrided sample, it can be seen that the amount of plasma exposure necessary to obtain the same junction resistance is one to two orders of magnitude greater for the nitrided sample than for the oxidized sample. This suggests that the nitriding process of the metal Al film proceeds more slowly than the oxidizing process, as described above.

As for the effect of the rare gas to be mixed, the oxidized sample in the case of d _Al = 15 A (marked with ▽ ▼ in the figure) compared to the case where Ar was used when Kr was used as the rare gas. Even with the same plasma exposure, it shows a large junction resistance, suggesting that the oxidation rate of the metal Al film is changed by the mixed rare gas. On the other hand, in the nitrided sample, the use of Kr as the rare gas always shows a slightly higher junction resistance, but it is not as large as the difference seen in the oxidized sample, and the influence of the mixed rare gas in the plasma nitridation process It turns out that it is not so big. This corresponds to the fact that in a separately performed emission spectroscopic analysis of nitrogen plasma, the nitriding species excited in the plasma does not change much when the mixed rare gas is changed between Ar and Kr. This is a significant difference from the case of oxidation plasma.

Next, FIG. 5 re-plots the plasma exposure conditions shown in FIG. 4 with respect to d _Al , and the range for appropriate oxidation / nitridation conditions determined from the results of FIGS. 6 and 6 described later. Is shaded. The maximum TMR ratio obtained in the heat treatment process of a typical MTJ and the junction resistance at that time are also shown in the figure.

Looking at the oxidized MTJ (bottom of Fig. 5), when d _Al = 15 A, the appropriate oxygen plasma exposure is 1 to 2 x 10 ⁵ L, whereas the metal Al film thickness decreases. Over time, the appropriate plasma exposure will be drastically reduced, and when d _Al = 8 A, it is only 2-5 × 10 ³ L, which is nearly two orders of magnitude lower than when d _Al = 15 A. I understand. In the region of the exposure amount (0.5 to 1 × 10 ⁵ L) when the appropriate plasma exposure amount for d _Al = 15 A is simply about half of the 8 A metal Al film thickness, see figure It can be seen that MTJ is in a peroxidized state. This means that in the process of plasma oxidation of the metal Al film, the formation rate of the oxide film (Al-O) is not linear with respect to the film thickness direction, and the oxide film is quickly formed on the extreme surface layer of the Al film. Means.

On the other hand, looking at the nitrided MTJ (upper side in FIG. 5), it can be seen that the change of the appropriate plasma exposure amount relative to d _Al is more gradual than in the case of oxidation. From this, it is presumed that in the process of plasma nitriding of the metal Al film, rapid nitridation of the extreme surface layer is unlikely to occur, and nitridation proceeds gradually and gradually in the thickness direction of the metal Al film. From the above, it can be said that nitriding is advantageous from the viewpoint of controllability when a tunnel barrier film is formed by plasma exposure of an extremely thin metal Al film.

(TMR effect of MTJ of comparative example using Al-O)
FIG. 6 shows the TMR ratio and the dependence of the junction resistance (R × A) on the heat treatment temperature (T _a ) ° C. in the annealed MTJ samples prepared under various conditions. A sample under oxidation conditions in which a relatively large TMR ratio was obtained at each d _Al in the case where Kr was used as the mixed rare gas to the oxidation plasma is shown. In FIG. 6, ◯ indicates 8 A, and ▽ indicates 15 A. The rare gas is Kr, P × t, ○ is 4.0 × 10 ³ L, ● is 4.8 × 10 ⁴ L, and ▽ is 1.6 × 10 ⁵ L.

According to FIG. 6, in the case of d _Al = 15 A, the TMR ratio, which showed about 15% in the state immediately after fabrication (T _a = 100 to 120 ° C.), increases as the heat treatment temperature rises. _a = Maximum at 300 ° C. At this time, the junction resistance tends to gradually decrease as the heat treatment temperature increases. The same tendency can be observed in the sample (d) in the case of d _Al = 8 A and plasma exposure 4.0 × 10 ³ L. Although the maximum value of the TMR ratio obtained at T _a = 300 ° C. is different, the tendency of the change with respect to the heat treatment temperature is similar. From this, it is considered that the 8 A-thick metal Al film is appropriately oxidized in this sample.

On the other hand, the sample with d _Al = 8 A and plasma exposure of 4.8 × 10 ⁴ L (marked with ● in the figure) shows a completely different tendency from the above two samples in both TMR ratio and junction resistance. It turns out that it shows. The TMR ratio, which was about 5% immediately after fabrication, decreased with increasing heat treatment temperature up to 300 ° C, then suddenly increased, and reached _a maximum value of 44% at T _a = 340 ° C. It can be seen that the junction resistance at this time increases abruptly by an order of magnitude at T _a = 300 ° C. or higher.

In the case of the d _Al = 8 A sample, when the plasma exposure is 4.0 × 10 ³ L, only the metal Al film is appropriately oxidized, whereas the plasma exposure is increased to 4.8 × 10 ⁴ L. This means that the surface of the lower ferromagnetic layer is oxidized, so-called overoxidation.

(TMR effect of MTJ of Example using Al-N)
FIG. 7 shows the heat treatment temperature (T _a ) ° C. dependence of annealing of the TMR ratio and junction resistance (R × A) of nitrided MTJ samples prepared under various conditions. For both the case of using the Kr and Ar to a mixed rare gas into the plasma nitride, a relatively large TMR ratio is shown for samples obtained nitrided condition in each d _Al. In FIG. 7, the film thickness (d _Al ) 8 A is ○ ●, 10A is Δ ▲, 15A is ▽ ▼, and black marks indicate the rare gas Ar and white indicates Kr.

When Ar is used as the rare gas (black symbols in the figure), when d _Al = 15 A, the TMR ratio of about 20% increases as the heat treatment temperature increases immediately after fabrication. , T _a = 250 ° C., which is 35% of the maximum value reported in the non-patent document. In the case of d _Al = 10 A, the tendency of the TMR ratio to increase with the increase of the heat treatment temperature is the same, but the maximum value of the TMR ratio (49%) and the increase of the heat treatment temperature indicating it (280 °) C) is permitted. The TMR ratio of 49% at room temperature is larger than that of any MTJ using Al-N reported so far, and is similar to the MTJ using Al-O. Further, when the thickness of the metal Al film is reduced to 8A, the maximum value of the TMR ratio decreases, but the heat treatment temperature showing the maximum increases to 300 ° C.

In the above process, the junction resistance tends to be almost constant or slightly decreased with the increase of the heat treatment temperature, and there is no rapid increase in junction resistance as seen in the peroxide sample in the case of oxidized MTJ. It was. Even when the rare gas is changed to Kr, the tendency of change of the TMR ratio is almost the same. From the above, it is considered that in the nitrided MTJ film shown in FIG. 7, appropriate nitridation is realized for each thickness of the metal Al film. Incidentally, preferable range of heat treatment temperature T _a in the annealing varies depending dAl and other conditions, including the results of experiments except as disclosed in FIG. 7, Taken together, TMR ratio of required specifications MRAM 40 In order to satisfy%, T _a = 240 to 340 ° C. is preferable.

Note that a high TMR ratio and a low R × A are indispensable for applying a ferromagnetic tunnel junction film (MTJ) to a magnetic random access memory (MRAM). According to FIG. 7, a low R × A in the range of 10 ³ to 10 ⁴ Ωμm ² is obtained for R × A, and the nitrided MTJ film of the example is suitable for MRAM.

(Insulation layer parameter evaluation)
Next, the parameter evaluation of the insulating layer will be described with reference to FIGS. FIG. 8 is a graph plotting the barrier height and the barrier width with respect to the embodiment of FIG. 7 using Al—N described above. In FIG. 8, the film thickness (d _Al ) 8 A is ○ ●, 10A is Δ ▲, 15A is ▽ ▼, and black marks indicate the rare gas Ar and white indicates Kr. FIG. 9 is a diagram showing the relationship between the barrier height and the TMR ratio.

  The barrier height is a parameter related to the quality of the insulating film, and the barrier width is a parameter related to the substantial thickness of the insulating layer. It is obtained by fitting to "Simmons' formula". In general, the metal aluminum film thickness does not necessarily become the insulating film thickness. In the case of nitriding, the film volume is increased by bonding with nitrogen atoms, and if the nitriding conditions are not good, there is a portion that cannot be completely nitrided and remains as a metal. Therefore, the substantial thickness of the insulating layer formed by a plasma nitriding process or the like is referred to as “barrier width”.

  The barrier height and the barrier width are evaluation parameters of the insulating layer. From the viewpoint of forming a high-quality insulating layer, the barrier height is important. The higher the barrier height, the higher the quality. In the case of conventional Al—N, the maximum value of the barrier height is about 2.0 eV, but according to the embodiment of FIG. 8, an Al—N insulating film of 2 to 3 eV can be obtained. Further, according to FIG. 9, in order to obtain a high TMR ratio, a high barrier height is required, and in order to satisfy the TMR ratio of 40% of the required specification of MRAM, the barrier height is at least 2 eV or more. is necessary.

The typical sectional view showing the multi sputter device for tunnel type magnetoresistive element manufacture of the present invention. The typical sectional view showing the microwave excitation plasma processing device provided with the radial line slot antenna for tunnel type magnetoresistive element manufacture of the present invention. The figure which shows the relationship between the distance from the window of a plasma processing apparatus to the inside of a process chamber, and electron temperature and electron density about the example of inert gas He, Ar, and Kr. The figure which shows the plasma exposure amount dependence of the junction resistance (RxA) of MTJ film | membrane. FIG re plotted conditions of the plasma exposed to the film thickness d _Al. The figure which shows the heat processing temperature dependence in annealing of TMR ratio and junction resistance (RxA) about the comparative example using Al-O. The figure which shows the heat processing temperature dependence in annealing of a TMR ratio and junction resistance (RxA) about the Example using Al-N. The figure which plotted the barrier height and the barrier width regarding the Example of FIG. The figure which showed the relationship between barrier height and TMR ratio. A typical sectional view of a tunnel type magnetoresistive element. FIG. 11 is a schematic cross-sectional view of a tunnel magnetoresistive element having a different form from that of FIG. 10. FIG. 3 is a partially broken perspective view showing a GMR reproducing head and a recording / reproducing separated magnetic head in which the reproducing head and an induction recording head are combined. Sectional drawing which shows the principal part of a GMR type | mold reproducing head. Sectional drawing which shows the memory cell which is the principal part of a non-volatile magnetic memory (MRAM).

Explanation of symbols

1, 51, 100, 204 Tunnel-type magnetoresistive element 2 First ferromagnetic layer 3 Insulating layer 4 Second ferromagnetic layer 11 Multi-sputtering device 13, 14 Sputtering device 15, 41 Substrate 31 Microwave excitation plasma processing device 32 Chamber 34 Quartz plate 35 Radial line slot antenna 37 Waveguide 43 Gas inlet 111 GMR type reproducing head 200 Memory cell

Claims (9)

  In a tunnel magnetoresistive element in which an insulating layer made of aluminum nitride (Al—N) is sandwiched between two ferromagnetic layers (a magnetization fixed layer and a magnetization free layer), the barrier height of the insulating layer is set to 2 eV or more. A tunnel type magnetoresistive element.   In the method of manufacturing a tunnel magnetoresistive element in which an insulating layer made of aluminum nitride (Al-N) is sandwiched between two ferromagnetic layers (a magnetization fixed layer and a magnetization free layer), the step of forming the insulating layer includes: A nitriding treatment step of nitriding a metal Al film with plasma; and an annealing treatment step of heat-treating the nitrided Al—N film, and the plasma has a low electron temperature of 2 eV or less and is nitrided A method for producing a tunneling magnetoresistive element, characterized by exciting a nitrogen molecular radical or a nitrogen atom radical as a seed.   3. The method of manufacturing a tunnel magnetoresistive element according to claim 2, wherein the nitriding process is performed using a microwave-excited plasma processing apparatus having a radial line slot antenna.   4. The method of manufacturing a tunnel magnetoresistive element according to claim 2, wherein the nitriding step is performed in a nitrogen atmosphere or an inert gas atmosphere containing at least nitrogen.   5. The method of manufacturing a tunnel magnetoresistive element according to claim 4, wherein the inert gas is at least one of helium gas, argon gas, krypton gas, and xenon.   6. The tunnel-type magnetism according to claim 2, wherein a resistance value of the Al—N film and a magnetoresistance change rate of the magnetoresistive element are controlled by a heat treatment temperature in the annealing process. A method of manufacturing a resistance element.   The method of manufacturing a tunneling magnetoresistive element according to claim 6, wherein the heat treatment temperature is 240 to 340 ° C.   8. The method of manufacturing a tunnel magnetoresistive element according to claim 2, wherein the tunnel magnetoresistive element is an element for a magnetic device such as a GMR read head or a magnetic memory. . Forming a ferromagnetic layer in a tunnel magnetoresistive element manufacturing apparatus in which an insulating layer made of aluminum nitride (Al-N) is sandwiched between two ferromagnetic layers (a magnetization fixed layer and a magnetization free layer) A sputtering apparatus, a sputtering apparatus for forming a metal Al film, a microwave-excited plasma processing apparatus for nitriding the metal Al film, and an annealing apparatus for annealing the nitrided Al-N film. An apparatus for manufacturing a tunnel type magnetoresistive element.

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