JP2003282835A - Lower electrode for ferroelectric memory, ferroelectric capacitor using it, and method of manufacturing lower electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lower electrode for ferroelectric thin film capacitor that can reveal a high ferroelectric property, and to provide a capacitor using the electrode and a method of manufacturing the electrode. <P>SOLUTION: The uppermost part of this lower electrode which comes into contact with a ferroelectric substance is constituted of a polycrystalline film of a noble metal and crystal grains constituting the polycrystalline film are oriented. The (111)-orientation axis of the crystal grains is inclined with respect to the (111)-orientation surface of the polycrystalline film by 5-20°C. At the time of producing the lower electrode for ferromagnetic memory, a substrate, an SiO<SB>2</SB>layer, a Ti layer, and the polycrystalline film of the noble metal are laminated upon another in this order and, after the Ti layer is formed, heat treatment is performed for 20-60 minutes under an oxygen-coexisting condition or an oxygen atmosphere of ≥0.01 MPa in pressure at a temperature of 800-1,200°C. Thereafter, a ferroelectric thin film having the composition of SrxBiyTa<SB>2</SB>O<SB>9</SB>(wherein, x denotes ≥0.6 and <0.8 and y denotes ≥2.05 and <2.4) is provided on the electrode film composed of the noble metal. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric thin film capacitor excellent in remanent polarization for use in various devices such as a non-volatile memory utilizing ferroelectric characteristics.

[0002]

2. Description of the Related Art In recent years, much research has been conducted on applying a ferroelectric thin film of a Pb-based perovskite oxide such as PZT or a Bi-based layered perovskite oxide such as SBT to a nonvolatile memory. Especially SBT is PZT
Compared with the above, it has attracted attention because it has the characteristics of excellent polarization reversal fatigue characteristics and is capable of reversing polarization at low voltage, but on the other hand, it has a problem of low residual polarization value. . Since the higher remanent polarization value becomes indispensable as the integration density of the memory advances, the improvement of the remanent polarization value becomes an increasingly important issue.

Ferroelectricity is exhibited in these ferroelectrics by displacing the atomic position in a specific crystal direction or rotating the crystal. Therefore, it is known that the ferroelectric characteristics of a thin film change greatly depending on the orientation of the film.

Generally, in a ferroelectric memory device, since the ferroelectric substance is an oxide, a noble metal such as Pt, Ir or Ru is used as an electrode in order to prevent oxidation of the electrode when forming the ferroelectric film. It is known that the crystal orientation of these films is strongly oriented to (111). This ferroelectric thin film exhibits ferroelectricity in the a-axis and b-axis directions in, for example, SBT, but the ferroelectric layered on this oriented electrode film has random crystal directions, so the characteristics of the ferroelectric are A value averaged by the orientation axis ratio as compared with the single crystal, specifically, 2/3 is expected as the theoretical maximum value.

[0005]

According to the present invention, the alignment axis of the noble metal layer of the lower electrode is shifted from (111),
An object of the present invention is to provide a lower electrode for a ferroelectric thin film capacitor which substantially changes the orientation of the ferroelectric film and has a large ferroelectricity, a capacitor using the same, and a method for manufacturing the lower electrode.

[0006]

The inventors have found that the orientation of the noble metal electrode slightly changes when the manufacturing process of the conventional ferroelectric thin film capacitor is changed. Further, there is a constant value between the orientation of the noble metal electrode and the orientation of the crystals forming the ferroelectric thin film when the ferroelectric thin film, particularly the metal oxide thin film composed of the constituent elements of SBT is crystallized into the ferroelectric thin film. The present invention has been accomplished by finding out that there is a relationship.

The first invention for solving the above-mentioned problems is a lower electrode for a ferroelectric memory, wherein the uppermost part of the lower electrode in contact with the ferroelectric is composed of a noble metal polycrystalline film. The crystal grains that make up the film show an orientation,
11) The orientation axis is originally shown by the precious metal polycrystalline film (11
1) A lower electrode for a ferroelectric memory, which is tilted 5 to 20 degrees with respect to the orientation plane. In the present invention, it is more preferable to use Pt, Ir, Ru as a noble metal as a constituent material from the viewpoint of preventing an increase in electric resistance value due to oxidation during the formation of the ferroelectric layer.

A second aspect of the present invention is a method of manufacturing a lower electrode as described above, wherein a substrate, Si and
O ₂ , Ti, and Pt are stacked in this order, and after forming the Ti layer, the temperature is 800 to 1200 ° C., in the coexistence of oxygen, or 0.01 M.
The heat treatment is performed for 20 to 60 minutes in an oxygen atmosphere of Pa or more. In the above method, more preferable results can be obtained by stacking SiO ₂ , Ti, Pt and the like using any one of dry film forming methods such as DC sputtering method, high frequency sputtering method, magnetron sputtering method and ECR plasma sputtering method.

The third invention is a ferroelectric capacitor, and in the ferroelectric capacitor having the lower noble metal electrode structure of the first invention, a ferroelectric is provided on the noble metal electrode film, The composition of the ferroelectric is SrxBi
yTa ₂ O ₉ , x is 0.6 or more and less than 0.8, y is 2.0
It is 5 or more and less than 2.4.

In the present invention, the orientation of the noble metal film is difficult to understand by ordinary .theta.-2.theta. X-ray diffraction, and therefore the reciprocal lattice mapping method is used for the measurement.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below. The first invention is a lower electrode for a ferroelectric memory, in which the uppermost portion of the lower electrode in contact with the ferroelectric is
A noble metal polycrystal film is formed, and the crystal grains constituting the noble metal polycrystal film exhibit orientation, and the (111) orientation axis thereof is
The lower electrode for a ferroelectric memory is characterized in that it is originally inclined by 5 to 20 degrees with respect to the (111) orientation plane of the noble metal polycrystalline film. The inclination of 5 to 20 degrees means that the perpendicular of the crystal plane is originally inclined by 5 to 20 degrees from the perpendicular of the noble metal polycrystal film. When the (111) crystal axis is within the range of this inclination with respect to the film surface, it is possible to obtain a ferroelectric thin film capacitor exhibiting an extremely high remanent polarization value of 25 μC / cm ² or more. This value is close to the expected theoretical value of remanent polarization possessed by the single crystal having the composition.

In the above, Pt and I are used as the noble metal.
Although r, Ru and the like are used, it is more preferable to use Pt from the viewpoint of preventing oxidation.

A second aspect of the present invention is a method of manufacturing the above lower electrode. Conventionally, the manufacturing process of a ferroelectric thin film capacitor includes (1) a process of forming an adhesion layer made of a Ti thin film on SiO ₂ , and (2) a noble metal thin film of Pt, Ir, Ru or the like on the adhesion layer. A step of forming a lower electrode layer, (3) a step of forming a metal oxide thin film layer made of a constituent metal element of a ferroelectric material on the lower electrode layer, and (4) a heat treatment to crystallize the metal. Step of forming oxide thin film into ferroelectric thin film layer (hereinafter abbreviated as ferroelectric thin film layer forming step), (5) Upper electrode layer consisting of metal thin film or conductive oxide thin film on ferroelectric thin film layer Forming process, (6) etching process to form a capacitor structure, (7)
It is constituted by a step of re-heat treatment if necessary.

According to the present invention, a predetermined condition is provided between the step of forming a contact layer made of a Ti thin film shown in (1) and the step of forming a lower electrode layer made of a metal thin film shown in (2). A step of heat treatment is added. As a result, the crystal grains of the metal thin film formed afterward show the orientation, and the (111) orientation axis originally shows the noble metal polycrystalline film (1
11) It is possible to shift the orientation plane by 5 to 20 degrees.

As a predetermined condition, the temperature is 800 to 12
Oxygen coexistence at 00 ° C or oxygen partial pressure 0.01MP
The processing time is 20 to 60 minutes.

Here, the reason why the heat treatment temperature is set to 800 ° C. or higher is that the metal thin film produced after 800 ° C. has a deviation of the (111) orientation axis within the above range, as will be shown in later examples. Because there is no. If the temperature exceeds 1200 ° C, the Si substrate is deformed. Further, considering that the hydrogen treatment temperature of Si is 1000 ° C. or lower, 1000
C. or less is more preferable.

The reason why the present invention is used in the coexistence of oxygen or the oxygen partial pressure is set to 0.01 MPa or more is also the same, because if these conditions are not removed, a sufficient effect cannot be obtained. It is preferable that the oxygen partial pressure applied during the heat treatment for crystallization of SBT be equal to or higher than the oxygen partial pressure, because the (111) orientation axis of the metal thin film will not be further displaced in the subsequent operation.

There is no particular limitation on the furnace that can be used for the heat treatment of Ti, but it is necessary that the atmosphere can be adjusted. When the amount of processed material is small, it is possible to use, for example, a tubular furnace maintained at a constant temperature as shown in this example. If the heat treatment time is less than 20 minutes, Ti
The annealing effect is insufficient, and considering the treatment in a short time, 60 minutes or less is preferable.

The effect of the addition of the heat treatment step of the present invention is remarkable when the SBT ferroelectric thin film is formed by various methods such as a DC sputtering method, a high frequency sputtering method, a magnetron sputtering method and an ECR plasma sputtering method.

The ferroelectric capacitor of the third aspect of the present invention is the ferroelectric capacitor having the lower noble metal electrode structure of the first aspect of the present invention, in which the ferroelectric substance is provided on the noble metal electrode film, There SrxBiyTa ₂ O _9, x is 0.
It is 6 or more and less than 0.8, and y is 2.05 or more and less than 2.4. Pt, Ir, Ru or the like is used as the noble metal, but it is more preferable to use Pt from the viewpoint of preventing oxidation as described above.

It is not known exactly as to why the present invention is effective. However, we have
In the ferroelectric thin film capacitor having the lower electrode structure of the present invention, the (111) orientation axis of the noble metal layer is 5 to 20 from the film surface.
The inclination causes the fine structure of the surface of the noble metal layer to change, and the ferroelectric polycrystal itself provided with random orientation on the surface of the noble metal is forcibly tilted, and the ferroelectric composed relatively of the polycrystal. The effect of the c-axis component of the body membrane is diluted,
It is assumed that the effects of the a-axis and the b-axis are strengthened, and as a result, the ferroelectric characteristics are improved.

[0022]

EXAMPLES Next, the present invention will be further described with reference to examples. (Examples 1 to 5) Ferroelectric thin film capacitors were manufactured by the following method. (1) First, a Si group was treated by a thermal oxidation method to form a SiO ₂ film having a film thickness of 500 nm on the surface thereof. (2) A Ti thin film having a film thickness of 50 nm was formed on the SiO ₂ / Si wafer by DC magnetron sputtering using a Ti target having a diameter of 6 inches under the conditions of an output of 200 W and an Ar pressure of 0.3 Pa. (3) After that, at 800 (Example 1), 900 (Example 2), 1000 (Example 3), 1100 (Example 4), 1200 (Example 5) ° C., in an oxygen atmosphere, respectively. A heat treatment (hereinafter referred to as Pre-anneal) was performed at (0.1 MPa) for 30 minutes. (4) Then, using a Pt target with a diameter of 6 inches,
Output 100 by RF magnetron sputtering
Pt with a film thickness of 100 nm under the conditions of W and Ar pressure of 0.3 Pa
A thin film was formed on the Ti thin film. (5) Further, on each Pt thin film, a Sr _0.60 Bi _2.23 Ta _2.00 O _y target having a composition of φ6 inch was used, and a film thickness of 200 was obtained under the conditions of an output of 200 W and an Ar pressure of 3.0 Pa by an RF magnetron sputtering method.
A metal oxide thin film composed of the SBT constituent metal element having a thickness of nm was formed. (6) Next, these were heat-treated at 800 ° C. in oxygen for 30 minutes to crystallize the metal oxide thin film to obtain an SBT thin film. (7) On top of this, under the same conditions as the Pt thin film formation in (4) above,
A Pt thin film having a diameter of 0.5 mm and a film thickness of 100 nm was formed. (8) Finally, reheat treatment was performed at 600 ° C. in oxygen for 30 minutes.

When the composition of the SBT ferroelectric thin film thus produced was analyzed by ICP emission spectroscopy, it was found that Sr / Bi / Ta = 0.70 / 2.
It was 19 / 2.00.

Next, the ferroelectric characteristics of each of the obtained ferroelectric thin film capacitors were evaluated under the condition of an applied voltage of 5V. The equipment used was TF ANALY made by aixACCT in Germany.
It is ZER2000FE. The results obtained are shown in Table 1. In addition, the reciprocal lattice map of each Pt film obtained in the above step (4) is used as X'Pert-ProMP manufactured by Philips.
The measurement was performed using D, and the inclination of the (111) oriented surface from the film surface was obtained. The obtained results are shown in Table 1. In addition, as an example of the result, the reciprocal lattice mapping of the heat-treated Ti film at 900 ° C. is shown in FIG. Comparing FIG. 1 and FIG. 2 in the case where the Ti film is not heat-treated, there is no change in the peak position of Pt in the measurement at Ψ = 0, θ−2θ, but the contour line of the X-ray diffraction intensity in the Ψ direction. It can be seen from FIG. 1 that the peak of Pt deviates from Ψ = 0. θ = 40
°, Ψ of Pt peak position in Pt (111) diffraction line
The amount of deviation in the direction was defined as the inclination of the (111) orientation plane.

Table 1 Heat treatment temperature Remanent polarization value (2Pr) (111) Inclination of orientation plane ° C μC / cm ² degrees 800 28.7 7 900 29.3 9 1000 29.5 11 1100 29.8 12 1200 30.1 14

(Examples 6 to 8) The oxygen partial pressure at the time of Pre-anneal is 0.01 MPa (Example 6), 0.02M.
A ferroelectric thin film capacitor was prepared in the same manner as in Example 2 except that Pa (Example 7) and 0.05 MPa (Example 8) were used. Obtained remanent polarization values (2Pr) and (111)
The tilt of the orientation plane is shown in Table 2. Table 2 Oxygen partial pressure Remanent polarization value (2Pr) (111) Inclination of orientation plane MPa μC / cm ² degree 0.01 27.8 5 0.02 28.2 6 0.05 28.5 7

(Examples 9 to 11) Pre-anneal
Processing time of 20 minutes (Example 9), 40 minutes (Example 1)
A ferroelectric thin film capacitor was prepared in the same manner as in Example 2 except that the time was 0) and 60 minutes (Example 11). The obtained remanent polarization value (2Pr) and the inclination of the (111) oriented surface are shown in Table 3.
It was shown to. Table 3 Processing time Remanent polarization value (2Pr) (111) Inclination of orientation plane μC / cm ² degrees 20 26.3 5 40 28.9 7 60 60 29.3 9

(Example 12) A ferroelectric thin film capacitor was prepared in the same manner as in Example 2 except that Ir was used instead of Pt. The obtained residual polarization value (2Pr) was 28.3.
It was μC / cm ² .

(Example 13) A ferroelectric thin film capacitor was prepared in the same manner as in Example 2 except that Ru was used instead of Pt. The obtained residual polarization value (2Pr) is 27.9.
It was μC / cm ² .

(Comparative Examples 1 to 3) The same as Example 1 except that the temperatures during the Pre-anneal were 400 ° C. (Comparative Example 1), 500 ° C. (Comparative Example 2) and 700 ° C. (Comparative Example 3). As a result, a ferroelectric thin film capacitor was manufactured. The composition of the SBT ferroelectric thin film thus prepared is
When analyzed by P issue spectroscopy
It was r / Bi / Ta = 0.70 / 2.19 / 2.00.

Next, the ferroelectric characteristics of each of the obtained ferroelectric thin film capacitors were evaluated under the condition of an applied voltage of 5V. The equipment used was TF ANALY made by aixACCT in Germany.
It is ZER2000FE. The results obtained are shown in Table 3. In addition, the reciprocal lattice map of each Pt film obtained in the above step (4) is a X'Pert-Pro MP manufactured by Philip Company.
The measurement was performed using D, and the inclination of the (111) oriented surface from the film surface was obtained. The obtained results are shown in Table 4. In addition, as an example of the result, the reciprocal lattice mapping of the heat-treated Ti film at 500 ° C. is shown in FIG.

Table 4 Heat treatment temperature Remanent polarization value (2Pr) (111) Inclination of orientation plane ° C μC / cm ² degree 400 18.1 0 500 25.1 0 700 700 25.7 2

(Comparative Examples 4 to 5) Ferroelectric thin film capacitors were prepared in the same manner as in Comparative Example 1 except that the oxygen partial pressure during Pre-anneal was 0.001 MPa (Comparative Example 4). Table 5 shows the obtained remanent polarization value (2Pr) and the inclination of the (111) oriented surface. Table 5 Oxygen partial pressure Remanent polarization value (2Pr) (111) Inclination of orientation plane MPa μC / cm ² degrees 0.001 25.1 0

(Comparative Examples 5 to 6) The processing time at the time of Pre-anneal was 10 minutes (Comparative Example 9) and 120 minutes (Comparative Example 1).
A ferroelectric thin film capacitor was prepared in the same manner as in Comparative Example 1 except that it was set to 0). Table 6 shows the obtained remanent polarization value (2Pr) and the inclination of the (111) oriented surface. Table 6 Treatment time Remanent polarization value (2Pr) (111) Gradient of orientation plane μC / cm ² degrees 10 25.3 3 120 29.6 13

From the above results, it was found that it is possible to obtain a ferroelectric capacitor having an extremely high remanent polarization value in the examples of the present invention. In Comparative Example 6, although a good remanent polarization value was obtained, a long treatment of 120 minutes as an annealing time was not realistic from the economical point of view and was out of the range of the present invention. From this, it is clear that the heat treatment after forming the Ti thin film is very effective in improving the remanent polarization value of the SBT ferroelectric thin film. The composition of the ferroelectric thin film of the present invention is Sr / Bi / Ta = 0.70 / 2.19 /
It is 2.00, and a slight amount of Sr and oxygen is deficient. Therefore, the remanent polarization value 2Pr assumed when a ferroelectric material having the composition is a single crystal is a physical review B 2001 Vol.
According to 4102 40 .mu.C / cm ² or so, thus the theoretical residual polarization value 2Pr when formed into a random polycrystalline film will be 26.7μC / cm ² about two thirds.

[0036]

As described in detail above, according to the method for producing a ferroelectric thin film capacitor of the present invention, the (111) orientation axis of the lower noble metal layer constituting the ferroelectric thin film capacitor is 5 to 20 from the film surface. The inclination causes the fine structure of the surface of the noble metal layer to change, and the ferroelectric polycrystal itself provided with random orientation on the surface of the noble metal is forcibly tilted, and the ferroelectric composed relatively of the polycrystal. The effect of the c-axis component of the body film is weakened, and the effects of the a-axis and the b-axis are strengthened, and a ferroelectric thin film capacitor having a ferroelectric thin film exhibiting an extremely high remanent polarization value can be formed with good reproducibility. It is possible.

[Brief description of drawings]

FIG. 1 is a diagram showing a result of reciprocal lattice mapping according to a second embodiment.

FIG. 2 is a diagram showing a result of reciprocal lattice mapping in Comparative Example 2.

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Claims (6)

[Claims] 1. In a lower electrode for a ferroelectric memory, the uppermost part of the lower electrode in contact with the ferroelectric is composed of a noble metal polycrystal film, and the crystal grains forming the noble metal polycrystal film exhibit orientation. A lower electrode for a ferroelectric memory, wherein a (111) orientation axis is originally inclined by 5 to 20 degrees with respect to a (111) orientation plane indicated by the noble metal polycrystalline film. 2. The lower electrode for a ferroelectric memory according to claim 1, wherein the noble metal is composed of Pt, Ir, Ru. 3. A ferroelectric capacitor having a lower noble metal electrode structure according to claim 1, wherein a ferroelectric substance is provided on the noble metal electrode film, and the ferroelectric substance is SrxByTa.
_A ferroelectric capacitor having a composition of ₂ O ₉ , x of 0.6 or more and less than 0.8, and y of 2.05 or more and less than 2.4. 4. A substrate, Si, for forming a lower electrode.
A method for manufacturing a lower electrode for a ferroelectric memory, which comprises laminating O ₂ , Ti, and Pt in this order, and annealing after forming a Ti layer at a temperature of 800 to 1200 ° C. in the coexistence of oxygen. 5. The method of manufacturing a lower electrode according to claim 4, wherein the annealing is performed at an oxygen partial pressure of 0.01 MPa or more. 6. The manufacturing method according to claim 4, wherein the laminating method is at least one of a DC sputtering method, a high frequency sputtering method, a magnetron sputtering method, an ECR plasma sputtering method and the like.