JP2012041580A - Method for manufacturing magnesium oxide thin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an MgO thin film excellent in crystallinity.SOLUTION: The method for manufacturing an MgO thin film, includes a heating step and a deposition step to manufacture the MgO thin film on a ferromagnetic body. The heating step is performed at the temperature of 200°C or more, while the deposition step is performed by sputtering a target composed of MgO and MgF. In the target, the approximate charge amount of the MgFto the MgO is controlled between 2.3-7.0 atom%. Thereby, (100) the MgO thin film excellent in orientation and crystallinity can be manufactured.

Description

  The present invention relates to a magnesium oxide thin film excellent in crystallinity used as a barrier film of a tunnel magnetoresistive element.

  As a high-sensitivity sensor of a magnetic random access memory or a magnetic head, a tunnel magneto-resistance (hereinafter, referred to as a TMR) element has attracted attention. The TMR element has a structure in which an extremely thin tunnel barrier layer is sandwiched between two ferromagnetic layers, and the resistance value of the element varies greatly depending on the magnetization directions of the two ferromagnetic layers. At this time, if the resistance value when the magnetization directions of the two ferromagnetic layers are in a semi-parallel state is defined as Ra and the resistance value when in the parallel state is defined as Rp, the rate of change (Ra−Rp) / Rp is This is called a magnetoresistance ratio (hereinafter referred to as MR ratio). The performance of the TMR element is determined by this MR ratio, and improvement of the MR ratio is required for various device applications.

  The MR ratio of the TMR element greatly depends on the material of the tunnel barrier layer. Currently, a (100) -oriented magnesium oxide (hereinafter referred to as MgO) thin film is mainly used as a tunnel barrier layer, and is formed on a ferromagnetic thin film such as CoFeB by a sputtering method.

  A TMR element using (100) -oriented single crystal MgO is predicted to obtain a large MR ratio of 1000% or more in theoretical calculation. However, since the MgO thin film formed by the sputtering method is a polycrystal and has a large influence of crystal grain boundaries and the like, a large MR ratio as described above cannot be obtained in a TMR element using this. In order to realize a TMR element having a larger MR ratio, it is required to realize a MgO thin film having a large domain size to reduce the number of crystal grain boundaries and excellent crystallinity with few impurities in the crystal. It was.

  As a conventional method for producing a MgO thin film having excellent crystallinity, a metal having a higher gettering effect on oxidizing gas than MgO is attached to a component in the film forming chamber before forming the MgO film, and then MgO is added. There existed what formed a film (for example, refer patent documents 1 and nonpatent literature 1). In Patent Document 1 and Non-Patent Document 1, an oxide gas such as oxygen or water is removed with a metal having a high gettering effect, and the MgO thin film is formed in a state where there is little residual gas. It can be improved.

  As another method for producing an MgO thin film having excellent crystallinity, there has been a method in which a film forming chamber is baked at a high temperature before film formation, and then MgO is formed (see Non-Patent Document 2). When the film forming chamber is baked for a long time at a temperature of 100 ° C. or higher, the moisture adhering to the wall surface in the film forming chamber can be reduced. It can be improved.

  Further, when forming an MgO thin film by oxidizing magnesium (hereinafter referred to as Mg), there has been an attempt to improve the characteristics of the MgO thin film by oxidizing in an atmosphere containing fluorine (see Non-Patent Document 3). ). In Non-Patent Document 3, when an Mg thin film is oxidized using oxygen plasma, fluorine is generated by inserting a Teflon (registered trademark) ring into the plasma, and an attempt is made to improve the barrier properties of the MgO thin film with the fluorine. ing.

JP 2007-266584 A

Yoshinori Nagamin et al., “Ultra resistance-area product of 0.4 Ωcm2 and high magnetism tolerance 50% inCoFeB / MgO / CoFeBungent p. 89, 2006, p. 1625071-1625073 Se Young O et al., "X-ray Diffraction study of the Optimization of MgO growth conditions for magnetic tunnel junctions," Journal of Hopy Applications. 103, 2008, pp. 07A920-07A923 Jun Hyung Kwon et al., “Effect of F-induction in nm-thick MgO barrier”, Current Applied Physics, Vol. 9, 2009, pp. 788-791

  The manufacturing method of the MgO thin film shown in Patent Literature 1, Non-Patent Literature 1 and Non-Patent Literature 2 has an effect of reducing impurities taken in during the MgO film formation, but does not promote MgO crystal growth itself. . Therefore, even if the manufacturing method of the MgO thin film described in these conventional examples is used, there is a limit to the crystallinity of MgO that can be manufactured. Actually, when the MgO thin film having a thickness of 50 nm was manufactured by using the above method, the obtained MgO thin film had a peak height of the MgO (200) plane in X-ray diffraction (hereinafter referred to as XRD). It was 128 counts per second (hereinafter referred to as cps) and the half width was about 2,44 °, which was not sufficient as crystallinity.

  Moreover, in the manufacturing method of the MgO thin film shown by the nonpatent literature 3, although the oxidation process has changed by adding fluorine, the effect of improving the characteristic of the obtained MgO crystal cannot be acquired. Actually, in Non-Patent Document 3, even when the oxidation is performed in an atmosphere containing fluorine, the barrier height is hardly changed as compared with the case where fluorine is not contained. Although the crystallinity of the MgO thin film is not described, it is presumed that there is almost no change from the above results.

  An object of the present invention is to solve the conventional problems described above, and to provide a method for producing an MgO thin film having a large domain size and excellent crystallinity.

In order to solve the above-mentioned conventional problems, the method for producing an MgO thin film according to the present invention is a method for producing an MgO thin film on a ferromagnetic material comprising a heating step and a deposition step, and the heating step is performed at 200 ° C. or higher. The deposition step is performed by sputtering a target made of MgO and MgF _{2. In the} target, the approximate charge amount of the MgF ₂ with respect to the MgO is 2.3 atomic percent or more, 7 0.0 atomic percent or less. With this configuration, an MgO thin film having (100) orientation and excellent crystallinity can be produced.

  Further, the MgO thin film manufacturing method may be characterized in that the ferromagnetic material is a ferromagnetic material having an amorphous structure mainly composed of Co, Fe, and B. With this configuration, an MgO thin film having excellent crystallinity that can be used as a barrier film of a TMR element can be realized.

  In the method for producing an MgO thin film of the present invention, a trace amount of fluorine can be added to the MgO thin film, and the effect of the fluorine can realize an MgO thin film having a large domain size and excellent crystallinity.

The flowchart of the manufacturing process of the MgO thin film in Embodiment 1 of this invention The figure which shows an example of the deposition process of the MgO thin film in Embodiment 1 of this invention It shows the amount of fluorine contained in MgO thin film manufactured in the first embodiment of the present invention, the approximate MgF ₂ charge amount dependent The XRD peak height and half-width of the MgO thin film in Example 1 of the present invention, showing the approximate MgF ₂ charge amount dependent

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 is a flowchart of a method for manufacturing an MgO thin film according to Embodiment 1 of the present invention. As shown in FIG. 1, the manufacturing method of the MgO thin film according to the first embodiment includes a “ferromagnetic film forming process”, a “substrate heating process”, and an “MgO thin film deposition process” as bases. The entire process is carried out continuously without exposure to the atmosphere. By the above process, an MgO thin film having better crystallinity than conventional can be produced on the ferromagnetic film. Hereinafter, details of each of the above-described steps will be described.

First, the film forming process of the ferromagnetic film will be described. A sputtering method can be used as a method for forming the ferromagnetic film. At this time, the composition of the ferromagnetic film is not particularly limited, but CoFeB having an amorphous structure is desirable for realizing the TMR element. In addition, as a substrate for forming the ferromagnetic film, a general substrate such as Si, SiO ₂ , or SiN can be used.

  Next, the substrate heating process will be described. The substrate heating step is a step of raising the substrate on which the base is formed to the deposition temperature of MgO using a direct or indirect method. As the heater for heating the substrate in the first embodiment, any heater can be used as long as it can maintain a constant temperature by external control, such as a lamp heater or a resistance wire. Good.

  At this time, the heating temperature needs to be 200 degrees or more. In the case of 200 degrees or less, the effect of improving the crystallinity of MgO cannot be obtained even if fluorine is added. The heating temperature may be 200 ° C. or higher. However, in order to maintain CoFeB in an amorphous state without being crystallized, it is desirable not to raise the temperature excessively, specifically 350 ° C. or lower.

Next, the deposition process of the MgO thin film will be described. FIG. 2 is a diagram showing a method for depositing an MgO thin film according to the first embodiment. As shown in FIG. 2, a second target containing magnesium fluoride (hereinafter referred to as MgF ₂ ) in MgO and a first target 13 mainly composed of MgO with respect to a substrate 11 heated using a heater 12. By sputtering the target 14 simultaneously, a MgO film containing fluorine is deposited on the substrate.

At this time, a single crystal MgO target, a sintered MgO target, or the like can be used as the first target 13. As the second target, it is possible to use a target obtained by mixing and sintering MgO and MgF ₂ powder, a target obtained by attaching a MgF ₂ chip on MgO, or the like.

At this time, the amount of fluorine contained in the MgO thin film can be controlled by controlling the amount of MgF ₂ charged in the second target. Moreover, it is possible to control the approximate amount of charged MgF ₂ by controlling the film formation rate ratio between the first target and the second target.

Here, the approximate charge amount is an approximate value of the charge amount of the target component when the two targets sputtered simultaneously are assumed to be one target. This approximate charge amount can be estimated from the composition of each target and the deposition rate ratio. In the first embodiment, a charge of MgF ₂ in the second target by multiplying the ratio of the deposition rate of the second target single relative deposition rates during co-sputtering, the approximate MgF ₂ charged amount I'm calculating.

At this time, the approximate amount of MgF ₂ charged and the amount of fluorine contained in the actual MgO thin film have a linear relationship as shown in FIG. 3, and by controlling the approximate amount of MgF ₂ charged, It is possible to control the fluorine content.

  Note that the fluorine content in the MgO thin film obtained in Embodiment 1 can be quantified by using secondary ion mass spectrometry (hereinafter, SIMS). Quantitative addition of fluorine can be done by comparing the reference sample and the manufactured MgO thin film with SIMS measurement results using a fluorine ion beam injected into MgO thin film containing no fluorine as a reference sample. Is possible.

Usually, when foreign substances such as fluorine are mixed during the formation of a thin film such as MgO, the crystallinity is lowered by the amount of mixing. However, as a result of diligent investigation, after heating the substrate, the approximate MgF ₂ charge amount is 2.3 atomic percent (hereinafter, at.%) Or more, 6.9 at. It has been found that when sputtering is performed using a target having a range of not more than%, an extremely small amount of fluorine is contained in the MgO thin film, and the crystallinity is improved contrary to the usual case. Derivation of the optimum range of the approximate amount of MgF ₂ charge will be described in the examples described later.

  The reason why the crystallinity of the MgO thin film is improved by adding fluorine in the method of manufacturing the MgO thin film shown in the first embodiment is not clear in detail. Changes in nucleation probability and growth mode of MgO crystals ”and“ substances that hindered crystal growth, such as magnesium hydroxide, were decomposed and removed by fluorine ”. It is speculated that this has led to improvement.

In addition, since the MgO thin film has high crystallinity even when fluorine is added, fluorine is not contained in the MgO crystal but remains as MgF _{2 at the} crystal grain boundary. Inferred. Since MgF ₂ has a larger band gap than MgO, it is considered that it does not cause a leak of the TMR element and does not cause a decrease in the MR ratio of the TMR element.

  Note that in the sputtering method of the first embodiment, the gas species used in the discharge can be used without any problem as long as they do not form a compound with magnesium such as argon, helium, and xenon.

Example 1
Hereinafter, the first embodiment will be described in detail using examples.

First, as Example 1-1, the approximate MgF ₂ charge amount was 2.3 at. The production of the MgO thin film when% is described.

  In Example 1-1, a substrate in which an oxide film was formed on the surface of a (100) -oriented Si wafer was used. The size of the substrate was 30 mm square and the thickness was 0.5 mm. The oxide film on the surface of the substrate was formed by a thermal oxidation method, and the thickness was about 100 nm.

  A CoFeB film was formed on the substrate using a DC / RF sputtering apparatus (manufactured by ULVAC). Deposition conditions are listed in Table 1 as condition (1). The composition of CoFeB used as the target was Co: 40 at. %, Fe: 40 at. %, B: 20 at. %Met. The film obtained on the substrate using CoFeB having the above composition had an amorphous structure. Also, before the film formation of CoFeB, as shown in Non-Patent Document 2, baking is performed for 50 hours at a high temperature (120 ° C.) sufficient to remove moisture, and water components in the film formation chamber are removed. It has been removed.

  Film formation was performed for about 30 seconds under the film formation conditions shown in Table 1 (1), and an amorphous CoFeB film of 3.0 nm was formed on the substrate.

Next, the substrate was heated. The substrate was heated using a lamp heater in a vacuum of 5 × 10 ⁻⁶ Pa or less, and the temperature of the substrate surface was increased to 200 ° C.

  After heating the substrate, an MgO thin film was continuously formed. Prior to the deposition of the MgO thin film, as shown in Patent Document 1, a substance containing Al or B was attached to a member in the deposition chamber, and the oxidizing gas was removed by gettering. At this time, when a component of residual gas in the gas phase before and after gettering was observed using a quadrupole mass spectrometer, it was confirmed that the components derived from water and oxygen were reduced.

The MgO thin film was formed by the method shown in FIG. At this time, the first target is a single crystal MgO target (manufactured by Tateho Chemical Industry Co., Ltd.), and the second target is a sintered MgO target (manufactured by Kojundo Chemical Laboratory Co., Ltd.). A fixed MgF ₂ sintered body (manufactured by Kojundo Chemical Laboratory Co., Ltd.) was used. At this time, in the second target, MgF ₂ was fixed so as to cover 15% of the surface area of the sintered MgO target.

Details of the film forming conditions are shown as (2) in Table 1. When film formation was performed under the conditions of (2) in Table 1, the film formation rate of the first target was 0.151 Å / second, and the film formation rate of the second target was 0.028 Å / second. From this rate ratio, the approximate charge amount of MgF ₂ in all targets is calculated to be 2.3 at. %Met.

  Sputtering was performed for about 2790 seconds under the conditions of Table 1 (2), thereby depositing an MgO thin film of about 50 nm on CoFeB. When XRD was performed on the deposited MgO thin film, a peak could be confirmed in the vicinity of 2θ = 42.9 °. This coincides with the (200) plane peak of the MgO crystal. At this time, other orientation peaks such as 2θ = 36.9 ° and 62.3 ° were not confirmed, and it was confirmed that a (100) -oriented MgO thin film was deposited on the substrate.

  From the XRD measurement results, the peak height and half width were derived by fitting the peak (2θ = 42.9 °) on the MgO (200) plane using the Voigt function. The peak height was 502.9 cps and the half width was 1.07 °.

  Further, the amount of fluorine contained in the manufactured MgO film was measured. When the amount of fluorine in the MgO thin film of Example 1 was identified by SIMS, 0.04 at. % Content was confirmed.

(Effect of substrate heating and fluorine addition)
Next, studies were conducted to clarify the effects of substrate heating and fluorine addition. In Comparative Example 1-1, “substrate heating” and “fluorine addition” were removed from the conditions of Example 1-1, and the MgO thin film was manufactured. In Comparative Example 1-2, only “Substrate Heating” was removed from Example 1-1, and in Comparative Example 1-3, only “Addition of Fluorine” was removed from Example 1-1. Are manufactured respectively. Table 2 shows the conditions of the substrate temperature and the input power to the target in Comparative Examples 1-1 to 1-3. In addition, except the conditions shown in Table 2, film-forming was implemented on the completely same conditions as Example 1-1. The production procedure was also the same as that in Example 1-1.

  The MgO thin film was manufactured under the conditions shown in Table 2, and the XRD of the obtained thin film was measured. Table 3 shows the results of extracting the peak height and half width from the measurement results.

  As shown in Table 3, only Example 1-1 had a large peak height and a narrow half-value width. It is known that the half width is inversely proportional to the domain size. Under the conditions of Example 1-1, it was confirmed that the domain size increased and the crystallinity was improved. At this time, the method for improving the crystallinity shown in the conventional example is carried out under any of the conditions of Comparative Example 1-1, Comparative Example 1-2, and Comparative Example 1-3. It was confirmed that the production method 1 was more effective than the conventional method.

(Change due to the amount of fluorine added)
Next, the optimum range of the approximate MgF ₂ charge amount was examined. As shown in Table 4, as Example 1-2, Example 1-3, and Comparative Example 1-4, the MgO thin film was deposited while increasing the approximate MgF ₂ charge. In addition, except the conditions shown in Table 4, film-forming was implemented on the completely same conditions as Example 1-1.

XRD was measured with respect to the manufactured MgO thin film, and the peak height and half value width of (200) plane were extracted from the measurement result. The results are shown in Table 5. FIG. 4 is a diagram showing the dependence of the XRD peak height and half-value width of the manufactured MgO thin film on the approximate MgF ₂ preparation amount.

As shown in Table 5 and FIG. 4, the approximate MgF ₂ charge amount is 2.3 to 7.0 at. %, The XRD peak height is remarkably increased and the full width at half maximum is greatly decreased, from 2.3 to 7.0 at. It was confirmed that the domain size in the MgO thin film was increased in the range of%.

Moreover, the estimated MgF ₂ charge shown as Example 1-4 is 15.0 at. In the case of%, the full width at half maximum is narrower than that of Comparative Example 1-3, but the peak height is reduced. This is because the approximate MgF ₂ charge is 15.0 at. In the case of more than%, the domain size is increased, but the amount of fluorine is too much, so there are many parts that have an amorphous structure or a crystal structure other than (100), and the number of (100) MgO crystals per unit volume is remarkably reduced. It is speculated that. Such an MgO thin film cannot be expected to have a large MR ratio. Therefore, the optimum range of the approximate MgF ₂ preparation amount in the first embodiment is 2.3 at. % Or more, 7.0 at. % Or less.

  In Example 1, Co: 40 at. %, Fe: 40 at. %, B: 20 at. % CoFeB was used as the ferromagnetic material, but Co: 20 at. %, Fe: 60 at. %, B: 20 at. It was confirmed that the same effect can be obtained even when a ferromagnetic material having a composition of% is used. Therefore, the manufacturing method of the MgO thin film according to the first embodiment can be used on ferromagnetic materials having various compositions.

  From the above, it was confirmed that an MgO thin film having excellent crystallinity can be formed on a ferromagnetic material by performing the MgO thin film forming step shown in the first embodiment.

  The method for producing an MgO thin film according to the present invention is useful as a method for forming a tunnel barrier film such as a TMR element because an MgO thin film having excellent crystallinity can be produced.

11 Substrate 12 Heater 13 First Target 14 Second Target

Claims (2)

A method for producing a magnesium oxide thin film on a ferromagnetic material comprising a heating step and a deposition step,
The heating step is performed at a temperature of 200 ° C. or higher,
The deposition step is performed by sputtering a target made of magnesium oxide and magnesium fluoride,
The method for producing a magnesium oxide thin film characterized in that, in the target, an approximate charge amount of the magnesium fluoride with respect to the magnesium oxide is 2.3 atomic percent or more and 7.0 atomic percent or less. 2. The method for producing a magnesium oxide thin film according to claim 1, wherein the ferromagnetic material is a ferromagnetic material having an amorphous structure mainly composed of Co, Fe, and B.

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