JP3214031B2 - Ferroelectric thin film element and method of manufacturing the same - Google Patents

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 device and a method of manufacturing the same, and more particularly, to a ferroelectric thin film device formed using a ferroelectric single crystal thin film and a method of manufacturing the same.

[0002]

2. Description of the Related Art Ferroelectric thin film elements are made of PbTiO ₃ or PbTiO _3.
It is configured by directly sputtering a thin film made of a ferroelectric material such as b (Ti _1-x Zr _x ) O ₃ on a substrate or by sputtering on an electrode formed on the substrate. It is used for infrared sensors utilizing the pyroelectric phenomenon, ferroelectric capacitors for nonvolatile memories, and the like.

In order to sufficiently exhibit the ferroelectricity of a ferroelectric thin film formed on a substrate, the axis of easy polarization of the ferroelectric thin film is preferably aligned with the crystal direction of the thin film. It is preferable to use a ferroelectric single crystal thin film having a high anisotropy. Conventionally, when a ferroelectric single crystal thin film is formed on a substrate, selection of a substrate material has been a very important problem. That is, in order to obtain a single crystal thin film made of PbTiO ₃ , it is indispensable to use a (100) MgO single crystal substrate as a substrate in order to form a single crystal thin film of c-axis orientation. In order to form a single crystal thin film, it was essential to use a C-plane or R-plane sapphire single crystal substrate. This was thought to be because, when a ferroelectric single crystal thin film was formed on a substrate, the crystallinity of the substrate material played a significant role. It has also been reported that when a ferroelectric single-crystal thin film is formed on a substrate, the difference in thermal expansion coefficient between the ferroelectric single-crystal thin film and the substrate material has a large effect (Journal of the Japan Society of Applied Physics, No. Vol. 30, No. 9B, Nos. 2145-2148
Page (1991)).

[0004]

As described above, when a ferroelectric thin film element is formed using a ferroelectric single crystal thin film, a specific crystal orientation is determined according to the crystallinity of the ferroelectric single crystal thin film. Had to choose a substrate material. That is, when an attempt was made to manufacture a ferroelectric thin film element using a ferroelectric single crystal thin film, the restrictions on the substrate material were very severe.

SUMMARY OF THE INVENTION An object of the present invention is to reduce a restriction on crystallinity of a substrate material, and to form a ferroelectric single-crystal thin film using a substrate material which has not been conventionally used. An object of the present invention is to provide an element and a method for manufacturing the same.

[0006]

According to another aspect of the present invention, there is provided a ferroelectric thin film element comprising: a rectangular plate-like substrate having a width a and a length b and b / a ≧ 5; A ferroelectric thin-film element comprising a ferroelectric material having a coefficient different from the thermal expansion coefficient of a material constituting the substrate, and a ferroelectric single-crystal thin film formed directly or indirectly by sputtering on the substrate. is there.

The method for manufacturing a ferroelectric thin film element of the present invention comprises:
A step of preparing a rectangular plate-shaped substrate having a width a and a length b and b / a ≧ 5, wherein the coefficient of thermal expansion at the Curie point is different from the coefficient of thermal expansion of the material constituting the substrate at the same temperature Forming a ferroelectric single crystal thin film by sputtering a ferroelectric material on the substrate.

[0008]

As described in the prior art described above,
When forming a dielectric single crystal thin film, the thermal expansion of the substrate material
Coefficient and thermal expansion coefficient of single-crystal thin film greatly affect
You. That is, this prior art includes PbTiO._ThreeCerami
Using a quartz glass-based target
PbTiO on board_ThreeSingle crystal thin film by sputtering
Example of formation and PbTiO on MgO single crystal substrate_Three
An example in which a single crystal thin film is formed is disclosed. And P
bTiO_ThreePbTiO at Curie point_Three, MgO
The thermal expansion coefficient of single crystal and quartz glass is MgO single crystal ≧
PbTiO _ThreeFerroelectric single crystal thin film ≧ quartz glass
For this reason, on a quartz glass substrate, the ferromagnetic crystals oriented in the a-axis
An electric single crystal thin film is formed, and on the MgO single crystal substrate, c
Assuming that an axially oriented ferroelectric single crystal thin film is formed
I have.

That is, as shown in FIG. 1A, when a PbTiO ₃ single crystal thin film 2 is formed on a quartz glass substrate 1, the thermal expansion coefficient of the PbTiO ₃ single crystal thin film 2 is larger. Upon cooling after passing through the Curie point after the formation of the thin film, a tensile stress is applied to the ferroelectric thin film 2 side. As a result, it is shown that the ferroelectric thin film 2 becomes an a-axis oriented single crystal thin film. Further, as shown in FIG. 1B, when a PbTiO ₃ thin film 4 is formed on a MgO single crystal substrate 3, when the PbTiO ₃ thin film 4 is cooled to room temperature after passing through the Curie point, it is illustrated by the above-mentioned difference in thermal expansion coefficient. It is reported that a compressive stress is applied to the ferroelectric thin film 4 as shown by an arrow, thereby forming a c-axis oriented single crystal thin film.

The present inventors have paid attention to the fact that the crystal orientation of the ferroelectric single crystal thin film is controlled by the difference in thermal expansion coefficient between the substrate material and the ferroelectric single crystal. We believe that if the thermal expansion coefficient difference with the thin film is positively utilized, a ferroelectric single crystal thin film with a-axis or c-axis orientation can be formed even when combined with a substrate material that could not be used conventionally. , Studied diligently. As a result, they have found that a ferroelectric single crystal thin film having a desired crystal orientation can be obtained by devising the shape of the substrate material.

That is, in the present invention, since the substrate has a width a and a length b and b / a ≧ 5, when a ferroelectric single crystal thin film is formed on a substrate by sputtering, The tensile stress or the compressive stress resulting from the difference in the thermal expansion coefficient is effectively applied to the ferroelectric thin film in combination with the stress due to the dimensional anisotropy of the substrate. Therefore, even when a combination of substrate materials that cannot be used conventionally is used, an a-axis or c-axis oriented ferroelectric single crystal thin film can be manufactured. In addition, about the thickness t of a board | substrate,
Preferably, the width a is inside t ≦ a / 2. If the thickness t exceeds a / 2, it is difficult for the tensile stress and the compressive stress indicated by the arrows in FIG. 1 to become sufficiently large.

[0012]

EXPLANATION OF EXPERIMENT 1 First, as the substrate 5 shown in FIG. 2, a substrate made of quartz glass having five kinds of dimensions shown in Table 1 below was prepared. The thermal expansion coefficient of quartz glass ranges from room temperature to a wide temperature range including the Curie temperature of the PbTiO ₃ -based material described later, and is almost zero.
It is.

[0013]

[Table 1]

Next, Pb is formed on the substrate 5 under the following conditions.
TiO ₃ was sputtered to form a 1.5 μm thick ferroelectric thin film. Equipment using sputtering conditions : RF magnetron sputtering equipment Target: PbTiO ₃ ceramics (c / a
Ratio: 1.032, Curie point: 320 ° C. and thermal expansion coefficient at the Curie point: 100 × 10 ⁻⁷ / ° C.) Substrate temperature: 650 ° C. Sputtering gas pressure: 20 mTorr Sputtering gas: ArO ₂ in a volume ratio of 90: 1.
Mixed gas containing 0 ratio RF power: 200 W / (per 4 inch diameter target) Sputtering time: 2 hours

As shown in Table 1, when the substrates of Sample Nos. 4 and 5 satisfying b / a ≧ 5 were used by the above-mentioned sputtering, a PbTiO ₃ single crystal thin film oriented in the a-axis was formed. That is, as shown in the X-ray diffraction pattern of the thin film formed in the case of the sample No. 4 in FIG. 3, the a-axis oriented PbTiO ₃ is formed, the spot shape is shown by the reflection high-energy electron diffraction (RHEED), It was confirmed that it was a crystalline thin film.

On the other hand, when the substrates of sample numbers 1 to 3 where b / a <5 are used, the a-axis orientation PbTiO ₃
However, reflection high-energy electron diffraction (RHEED) showed a ring shape and was a polycrystalline thin film. This is considered for the following reason. Since the thermal expansion coefficient at the Curie point of PbTiO ₃ is 100 × 10 ⁻⁷ / ° C., which is larger than the thermal expansion coefficient of the substrate made of quartz glass at the same temperature, the ferroelectric thin film exerts a tensile stress upon cooling after sputtering. receive. In sample numbers 4 and 5, the above b / a ≧ 5
Therefore, it is considered that a large difference occurs in the tensile stress between the long side and the short side of the substrate 5, whereby two-dimensional two-directional tensile stress acts in the substrate 5, and single crystallization progresses. Can be That is, by setting the ratio b / a to the width of the length of the substrate to be used to be 5 or more, a large difference occurs in the amount of strain applied to the long side and the short side of the ferroelectric thin film formed on the substrate 5. It is considered that a single crystal thin film was obtained.

Experiment 2 Next, in place of the quartz glass substrate, crystallized glass containing α-quartz or α-cristobalite as the main component (having a coefficient of thermal expansion of 150 × 10 ^{−7) having the} dimensions shown in Table 2 below.
/ ° C.) using the substrate, PbTiO ₃ under the same conditions as in Experiment 1
A thin film was formed by sputtering. As a result, Table 2
As shown in (a), in samples 6 to 8, c-axis oriented polycrystalline films were formed, whereas in samples 9 and 10, c-axis oriented single crystal thin films were formed.

[0018]

[Table 2]

For example, in the thin film formed on the substrate in sample No. 9, c-axis oriented PbTiO ₃ is formed as shown in the X-ray diffraction pattern diagram in FIG. It showed a spot shape and was confirmed to be a single crystal thin film. This is because the crystallized glass has a coefficient of thermal expansion at the Curie point of 150 × 10 ⁻⁷ /
° C, which is larger than that of PbTiO ₃ , and it is considered that a large compressive stress was applied in the direction of the arrow shown in FIG. 1B during cooling after sputtering, and the ferroelectric thin film was distorted. Then, sample numbers 9 and 10
Since a substrate with b / a ≧ 5 is used, there is a large difference in the magnitude of strain between the long side and the short side of the ferroelectric thin film. Working,
While a c-axis oriented single crystal thin film was formed, sample number 6
In Nos. To 8, it is probable that single crystallization did not occur because the difference in strain amount between the long side and the short side was small.

In the above Experiments 1 and 2,
Used quartz glass and α-quartz glass.
In the ferroelectric thin film element of the invention and the method of manufacturing the same,
Substrate materials other than the above materials can be used. Also strong
As for the dielectric material, PbTiO_ThreeMaterials other than
If Pb (Ti _1-xZr_x) O_ThreeEtc. can also be used
You. In short, the key of the ferroelectric material should be
Coefficient of thermal expansion at Curie point> at the same temperature of substrate material
Material and substrate material in relation of thermal expansion coefficient
Uses to form ferroelectric single crystal thin film oriented in a-axis
The heat of the ferroelectric material at the Curie point
Expansion coefficient <thermal expansion coefficient of substrate material at the same temperature
In the case of using the relevant combination, it was oriented to the c-axis.
A ferroelectric single crystal thin film can be formed.

Therefore, according to the present invention, a ferroelectric single crystal thin film oriented in the a-axis or the c-axis can be formed by using a substrate material which has not been conventionally used. In Experiments 1 and 2, the ferroelectric single crystal thin film was formed directly on the substrate. However, an electrode may be formed on the substrate, and then the ferroelectric single crystal thin film may be formed.

The present invention relates to a ferroelectric thin-film element and a method of manufacturing the same. The ferroelectric thin-film element has a structure in which the ferroelectric single-crystal thin film is formed directly or indirectly on a substrate. The other structure is not particularly limited as long as it is provided. That is, as shown in FIG.
An electrode 12 is formed on a substrate 11, a ferroelectric single crystal thin film 13 is formed on the electrode 12, and a ferroelectric single crystal thin film 1 is further formed.
Any structure, such as a structure in which the electrode 14 is formed on 3, can be adopted according to the application. Similarly, in the method of manufacturing a ferroelectric thin film element of the present invention, except for the step of directly or indirectly forming a ferroelectric single crystal thin film on a substrate, a conventionally known method of manufacturing a ferroelectric thin film element is used. It can be done according to the method.

[0023]

As described above, according to the method of manufacturing a ferroelectric thin-film element of the present invention, the substrate has a shape satisfying the relationship of b / a ≧ 5, and is thermally expanded with the ferroelectric material. Since the ferroelectric thin film is formed using substrates made of materials having different coefficients, a ferroelectric single crystal thin film oriented in the a-axis or the c-axis can be formed without being restricted by the crystallinity of the substrate material. Therefore, it is possible to provide a ferroelectric single crystal thin film element having a substrate made of a material that could not be used conventionally.

Further, in the ferroelectric thin film element of the present invention, the substrate has the above specific shape, and has the above-mentioned difference in thermal expansion coefficient from the ferroelectric material. Since a ferroelectric thin-film element using a substrate made of such a material can be obtained, it is possible to widen the range of selection of a substrate material.

[Brief description of the drawings]

FIGS. 1A and 1B are cross-sectional views for explaining directions in which stress is applied based on a difference in thermal expansion coefficient between a substrate and a ferroelectric thin film.

FIG. 2 is a perspective view for explaining the shape of a substrate used in the embodiment.

FIG. 3 is an X-ray diffraction pattern diagram of a ferroelectric single crystal thin film formed in Experiment 1.

FIG. 4 is an X-ray diffraction pattern diagram of a ferroelectric single crystal thin film formed in Experiment 2.

FIG. 5 is a sectional view showing an example of a ferroelectric thin film element to which the present invention is applied.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Ferroelectric thin film 3 ... Substrate 4 ... Ferroelectric thin film

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification code FI H01L 41/24 H01L 41/18 101A

Claims (2)

(57) [Claims] 1. A rectangular plate-like substrate having a width a and a length b and b / a ≧ 5, and a ferroelectric material having a coefficient of thermal expansion at the Curie point different from the coefficient of thermal expansion of the substrate material at the same temperature A ferroelectric single-crystal thin film made of a material and formed directly or indirectly by sputtering on the substrate. 2. A width a and a length b, and b / a ≧ 5.
A step of preparing a rectangular plate-shaped substrate which is a ferroelectric material having a coefficient of thermal expansion at the Curie point different from the coefficient of thermal expansion of the material constituting the substrate at the same temperature, which is ferroelectrically sputtered on the substrate. Forming a ferroelectric thin film element.