JP2651784B2 - Ferroelectric thin film having superlattice structure and infrared sensor / pressure sensor provided with the thin film - 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 having a superlattice structure and an infrared sensor and a pressure sensor provided with the ferroelectric thin film.

[0002]

2. Description of the Related Art It is known that ferroelectric materials exhibit larger dielectric properties (such as permittivity and spontaneous polarizability) as the amount of displacement of each constituent atom from a normal position increases. FIGS. 6 and 7 show schematic diagrams of the crystal structures of PbTiO ₃ and BaTiO ₃ , which are typical ferroelectrics, respectively. The spontaneous polarizability is calculated as follows from these schematic diagrams and the displacement of ions. You. (1) In the case of PbTiO ₃ Sum of dipole moments per unit cell P = 8 × (2e / 8) × 0.47 × 10 ⁻¹⁰ + 4e × 0.3 × 10 ⁻¹⁰ = 3.47 × 10 ⁻²⁸ [C · m] Volume of unit cell V = 63.0 × 10 ⁻³⁰ [m ³ ] Spontaneous polarization Ps = 55 [μC / cm ² ] (2) In the case of BaTiO ₃ Sum of dipole moment per unit cell P = 8 × (2e / 8) × 0.061 × 10 ⁻¹⁰ + 4e × 0.12 × 10 ⁻¹⁰ + 2 × [(− 2e) / 2] × (−0.036) × 10 ⁻¹⁰ = 1.08 × 10 ⁻²⁹ [C · m] Volume of unit cell V = 64.3 × 10 ⁻³⁰ [m ³ ] Therefore, spontaneous polarization (sum of dipole moments per unit volume) is Ps = P / V = 17 [ μC / cm ² ]

The calculated value of the above-mentioned spontaneous polarizability shows a value close to a value actually observed. Therefore, in order to obtain a large dielectric property, the crystal may be artificially displaced. For this reason, experiments on hydrostatic pressure and the like have been actively attempted, and the pressure effect has been reported. It is also known that, apart from the above-mentioned pressure effect, by combining specific elements, extremely large dielectric properties are exhibited as compared with a single compound (for example, BaTiO ₃ and SrTiO 3).
₃ or PbTiO ₃ and PbZrO ₃ ).

[0004] JP-A-5-70103 discloses that 0.0
A method of manufacturing an inorganic dielectric thin film such as PbTiO ₃ , BaTiO _3, or LiTaO ₃ on a substrate by an evaporation method using a laser beam in an oxygen atmosphere of 6 Torr or more is described. Japanese Patent Application Laid-Open No. 2-258700 describes a method of forming a BaTiO ₃ thin film on a substrate by a vacuum evaporation method using oxygen plasma.

[0005]

However, in the above-mentioned experiment of hydrostatic pressure, it was only possible to press the material isotropically, and it was impossible to apply pressure only in one axial direction. Also, in the above method of combining specific elements,
These elements are randomly mixed in the solid solution, and it was impossible to artificially control the mixing mode of the elements.

[0006] Table 1 shows the change in dielectric properties of (Ba _1-x Sr _x ) TiO ₃ ceramics depending on the composition. The above can be seen from Table 1. Note that SrTi
O ₃ : BaTiO ₃ = 50: 50 (atomic (at)%) corresponds to SrTiO ₃ : BaTiO ₃ = 60: 40 (wt%), and the relative dielectric constant (25 ° C.) at this time is about 600.

[0007]

[Table 1]

Further, the methods described in JP-A-5-70103 and JP-A-2-258700 disclose a method in which one
There is no disclosure of alternately stacking at least two different types of ferroelectrics for forming different types of inorganic dielectric thin films.

The present invention has been made in view of the above points, and a first object of the present invention is to fabricate a ferroelectric superlattice in which two or more ferroelectrics having different lattice constants are alternately laminated. Due to the lattice constant difference, it is possible to generate a pressure due to lattice strain in the in-plane direction of the thin film surface, thereby making it possible to artificially increase the displacement of the stationary position of the constituent ions artificially. It is to provide a ferroelectric thin film having the following. A second object of the present invention is to provide an infrared sensor and a pressure sensor having excellent characteristics, each of which is composed of the ferroelectric thin film having the superlattice structure.

[0010]

In order to achieve the above object, a ferroelectric thin film having a superlattice structure according to the present invention is characterized in that at least two types of different ferroelectrics are laminated. It is desirable that one of the ferroelectrics be BaTiO ₃ . In addition, PbTiO ₃
Or a compound in which some elements are substituted with Zr or La,
That is, (Pb _x La _{1 -x} ) (Ti _y Zr _{1 -y} ) O ₃
(However, it is preferable that 0 ≦ x ≦ 1, 0 ≦ y ≦ 1). Further, it is desirable to use LiTaO ₃ or a compound in which Ta is substituted with Nb, that is, Li (Ta _x Nb _{1 -x} ) O ₃ (where 0 ≦ x ≦ 1).

In addition, elements that exhibit large dielectric properties by forming a solid solution are combined. As such elements, Ba, Sr, Pb, Li, Nb, Zr, La,
Ca and the like can be mentioned. Then, the lattice constants of at least two types of ferroelectrics are made different. In addition, one or more of the two or more compounds having different lattice constants can be made of a ferroelectric substance. FIG. 1 shows a substrate 10 (for example, SrTiO ₃ −
A schematic diagram of a ferroelectric thin film having a superlattice structure formed by laminating a plurality of SrTiO ₃ thin films 12 and BaTiO ₃ thin films 14 on Nb) is shown.

In FIG. 1, it is possible to generate a pressure due to lattice distortion in the in-plane direction of the thin film surface due to a difference between two or more lattice constants. Thereby, the displacement of the configuration position can be artificially increased. Further, it is possible to artificially modulate the concentration of each element in a direction perpendicular to the surface of the thin film by the laminated structure of the thin film superlattice.

Further, the pyroelectric characteristics and the piezoelectric characteristics are improved by these effects. When an infrared sensor, a pressure sensor, and the like are formed using the ferroelectric thin film of the present invention, the characteristics of these sensors are improved. be able to.

[0014]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the present invention may be practiced by appropriately changing the gist of the invention. Is possible. FIG. 2 shows a schematic configuration of the apparatus used in the example. In FIG. 2, reference numeral 1 denotes a film forming container (vacuum chamber), 2 denotes a laser oscillator, 3 denotes a target (raw material compound), 4 denotes a substrate, and 5 denotes a gas introduction path for introducing oxygen gas or the like.
In this case, an excimer laser, which is ultraviolet light, is oscillated from the laser oscillator 2, and the laser beam 6 passes through the lens 7 and the laser inlet 8 provided in the film forming container 1, and passes through the target 7. 3 is irradiated. Excited species and the like generated from the target 3 are vapor-deposited on the substrate 4 disposed so as to face the target 3, whereby a film is formed. This film forming apparatus is characterized in that an energy source (laser) 2 and a film forming container (vacuum chamber) 1 are separated from each other.
It is possible to form a film under a pressure of 0 ^-1 Torr to normal pressure. In the above-described apparatus, when two or more different types of ferroelectrics, for example, two types of different ferroelectrics are laminated on the substrate 4, it is performed as follows. By setting two types of targets A and B in the vacuum chamber 1 and irradiating them alternately with a laser beam, a laminated structure of A / B / A / B... Can be obtained.

Example 1 Using the apparatus shown in FIG. 2, a ferroelectric thin film having a superlattice structure was produced under the following conditions. Target composition: SrTiO ₃ and BaTiO ₃ Film forming method: Ablation method using excimer laser (wavelength: 193 nm) Laser intensity: about 1 J / cm ² Frequency: 10 Hz Film forming speed: 0.1 μm / hr Film forming temperature : 600 ° C. Gas pressure: 1 × 10 ⁻³ Torr Substrate: Nb-doped single crystal SrTiO ₃

Under the above conditions, SrTiO ₃ thin films and BaTiO ₃ thin films were alternately laminated by excimer laser ablation to produce a ferroelectric superlattice (multilayer film). In particular, the thickness of each layer to be laminated should be 1
000A to 20A. However, the number of laminations was adjusted so that the thickness of the entire thin film was 4000 A.

FIGS. 3 and 4 show the relationship between the thickness of each layer and the lattice constant. From 1000 A to 100 A, as the thickness of each layer becomes thinner, the lattice constant becomes S
It can be seen that both rTiO ₃ and BaTiO ₃ change, and the effect of distortion due to lattice mismatch is clearly observed. Correspondingly, as shown in FIG.
The dielectric properties also change, and the relative dielectric constant increases as the thickness of each layer decreases. In addition, the substrate is Pt
When a single crystal MgO coated with (platinum) was used and the experiment was conducted under the same conditions as above, almost the same results as above were obtained.

As a comparison, SrTiO ₃ (dielectric constant: 30)
The value of the relative dielectric constant calculated assuming that 0) and BaTiO ₃ (1000) are connected in series as a capacitor is 460. Furthermore, as shown in Table 1, the solid solution S
The relative dielectric constant of r _0.5 Ba _0.5 TiO ₃ is about 600 as described above.
This was first observed in a superlattice artificially subjected to concentration modulation of Sr and Ba. The method of forming a multilayer film described in the embodiment is not limited to the laser ablation method, but may be another film formation method, for example, a high frequency (rf) sputtering method,
Direct current (dc) sputtering, chemical vapor deposition (CV)
D), molecular beam epitaxy, ion beam sputtering, reactive chemical vapor deposition, and the like are also applicable.

[0019]

As described above, the present invention has the following effects. (1) Due to the difference between the lattice constants of two or more ferroelectrics,
It is possible to generate pressure due to lattice strain in the in-plane direction of the thin film surface. For this reason, the displacement of the stationary position of the constituent ions can be artificially increased. (2) Due to the laminated structure of the thin film superlattice, it is possible to artificially modulate the concentration of each element in a direction perpendicular to the surface of the thin film. (3) When an infrared sensor, a pressure sensor, or the like is formed using the ferroelectric thin film of the present invention, the pyroelectric characteristics and piezoelectric characteristics of these sensors can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic view of a ferroelectric thin film having a superlattice structure according to the present invention.

FIG. 2 is a schematic configuration diagram showing an apparatus used in an example.

FIG. 3 is a diagram showing the relationship between the lamination thickness of each layer and the lattice constant a-axis (front-back axis) length.

FIG. 4 is a diagram showing the relationship between the lamination thickness of each layer and the lattice constant c-axis (vertical axis) length.

FIG. 5 is a diagram showing the relationship between the lamination thickness of each layer and the relative permittivity.

FIG. 6 is a schematic view of a crystal structure of PbTiO ₃ .

FIG. 7 is a schematic diagram of the crystal structure of BaTiO ₃ .

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Film-forming container (vacuum chamber) 2 Laser oscillator 3 Target (raw material compound) 4 Substrate 5 Gas introduction path 6 Laser beam 7 Lens 8 Laser introduction port 10 Substrate 12 SrTiO ₃ thin film 14 BaTiO ₃ thin film

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical indication location G01J 1/02 G01J 1/02 C H01B 3/00 H01B 3/00 F 3/12 3/12 H01L 37/02 H01L 37/02 41/08 41/08 Z

Claims (8)

(57) [Claims] 1. A ferroelectric thin film having a superlattice structure, wherein at least two types of different ferroelectrics are laminated. 2. One of the ferroelectrics is BaTiO _3.
The ferroelectric thin film having a superlattice structure according to claim 1, wherein 3. A ferroelectric material comprising PbTiO _3.
Or (Pb _x La _{1- x} ) (Ti _y Zr _1-y ) O ₃ (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1). Ferroelectric thin film. 4. One of the ferroelectrics is LiTaO _3.
Or Li (Ta _x Nb _1-x ) O ₃ (where 0 ≦ x ≦
The ferroelectric thin film having a superlattice structure according to claim 1, wherein 1) is satisfied. 5. The ferroelectric thin film having a superlattice structure according to claim 1, wherein elements exhibiting large dielectric properties by forming a solid solution are combined. 6. The ferroelectric thin film having a superlattice structure according to claim 1, wherein at least two kinds of ferroelectrics have different lattice constants. 7. An infrared sensor comprising the ferroelectric thin film according to claim 1. 8. A pressure sensor comprising the ferroelectric thin film according to claim 1.