JPH0817678A - Ferroelectric material thin film - Google Patents

Abstract

PURPOSE:To obtain a ferroelectric material thin film, which can bring out fully the intrinsic characteristics of a ferroelectric material film without restricing the film thickness of the ferroelectric material film and the condition of formation of the ferroelectric material film even in a film having a crack, a pinhole or the like and a film of a low specific resistance. CONSTITUTION:A ferroelectric material thin film layer 4 consisting of a thin film, which shows a ferroelectric property, is held between insulating thin films 3 and 5 consisting of a meterial having electrical insulation properties higher than those of this layer 4 to constitiute a ferroelectric material thin film and electrons from an electrode are prevented from being injected in the ferroelectric material thin film to keep an electric field in the layer 4 constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a ferroelectric thin film, and particularly to a ferroelectric thin film effective in forming a capacitor having a sandwich structure (MIM structure) constituted by sandwiching a dielectric between electrodes. Regarding

[0002]

2. Description of the Related Art Ferroelectric materials have a high dielectric constant, a polarization history phenomenon,
Since it exhibits peculiar properties such as a piezoelectric effect, a pyroelectric effect, and an electro-optical effect, it is highly expected as a highly functional material. If a ferroelectric thin film obtained by thinning this ferroelectric can be obtained, application to an ultrahigh-performance electronic device is considered. For example, if a capacitor is formed of a ferroelectric thin film having a high dielectric constant in an LSI, a large capacity and highly integrated DRA
M can be obtained, and a non-volatile memory can be realized by forming the gate oxide film with a ferroelectric thin film. Furthermore, piezoelectric filters and vibrators that utilize the piezoelectric effect,
It can be considered to be applied to various elements such as an infrared sensor utilizing the pyroelectric effect and an optical switch utilizing the electro-optical effect.

However, when a ferroelectric material is formed into a thin film, cracks and pinholes are generated in the film due to defective film formation.
Due to the low specific resistance of the film or the like and the film itself, there is a problem that the leak current increases and causes insulation failure. This is greatly affected particularly when forming a capacitor having an MIM structure, and the original characteristics of the ferroelectric could not be fully exhibited.

Therefore, as a method of improving the insulating property of the ferroelectric thin film, when the ferroelectric material is formed, the thickness of the ferroelectric film is limited to 30 to 150 nm, and the annealing is performed in the subsequent heat treatment. -500 temperature and time
The temperature is set to 1 hour, and this is repeated twice or more to stack thin films to obtain a ferroelectric thin film having a desired film thickness (see Japanese Patent Laid-Open No. 1-93089). The thickness of the ferroelectric film is set to 30 to 150 nm and the annealing temperature is set to 500.
The temperature is set to ° C because when the temperature is 150 nm or more or when the annealing temperature is 500 ° C or more, fine cracks are generated in the film or the film is peeled off to cause insulation failure of the ferroelectric film. Is.

[0005]

However, according to the above method, when obtaining a ferroelectric thin film having a film thickness of 300 nm, for example, it is necessary to repeat the steps of film formation and heat treatment a plurality of times, and the steps are complicated. However, there is a problem in that the yield decreases as it becomes.

The present invention has been made in view of the above circumstances, and does not limit the film thickness or manufacturing conditions of the ferroelectric film even in a film having cracks, pinholes, etc. or a film having a low specific resistance. An object of the present invention is to provide a ferroelectric thin film that can sufficiently bring out the original characteristics of the ferroelectric.

[0007]

[Means for Solving the Problems] The virgin sample of the ferroelectric substance is
It is composed of a large number of domains as shown in FIG. 5, and in the initial state, the mutual relation of the polarization directions of the domains is completely disordered, so that it does not have spontaneous polarization as a whole. When an electric field E is applied to this sample, when the electric field E is equal to or higher than the coercive electric field, the respective polarization directions are aligned with the direction of the electric field. For example, when barium titanate is used as the ferroelectric sample, the polarization direction is likely to be 90 ° which is orthogonal to 180 ° which is antiparallel. That is, as shown in FIG. 6, the domain showing the polarization direction that is the same as or closest to the direction of the electric field E is present as it is, and the domain having the opposite polarization is rotated by 180 ° in the spontaneous dipole moment. In the rest of the domains, the rotation of the spontaneous dipole moment by 90 ° increases the volume of the domains with spontaneous polarization in the direction of the electric field.
As a whole, it shows a large spontaneous polarization. As described above, in order to efficiently generate spontaneous polarization in the ferroelectric substance,
It is important that an electric field higher than the coercive electric field is applied in a constant direction throughout the film.

However, cracks and pinholes are formed in the ferroelectric film.
If a short occurs between the upper and lower electrodes, the potential difference between the electrodes disappears and the predetermined electric field is not applied to the ferroelectric substance. Further, lattice defects are likely to occur at the interface of crystal grains in the ferroelectric film, and the electrons injected from the upper and lower electrodes freely move around the interface due to hopping conduction and are detected as a leak current. The equivalent circuit in such a case is considered to be a resistor network as shown in FIG. 7, and its direction and size are random. Therefore, the direction and magnitude of the electric field applied to the large number of domains are not uniform, and it is not possible to form polarization in one direction in the film as a whole.

Therefore, the ferroelectric thin film according to the present invention is shown in FIG.
As shown in (1), it is characterized in that a ferroelectric thin film layer made of a thin film exhibiting ferroelectricity is sandwiched between thin films made of a material having higher electric insulation than the ferroelectric thin film layer.

[0010]

According to the present invention, the thin ferroelectric layer is sandwiched by the thin film having excellent electrical insulation properties to prevent the injection of electrons from the electrode arranged outside the thin film, thereby preventing the ferroelectric thin layer from being injected. The electric field in the layer is constant, and the electric field applied to each domain can be uniform in magnitude and direction.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. Fig. 1 shows the M
The example which comprised the capacitor of IM structure is shown. The ferroelectric thin film is formed by sandwiching a ferroelectric thin film layer 4 made of a thin film exhibiting ferroelectricity between insulating thin films 3 and 5 made of a material having higher electric insulation than the ferroelectric thin film layer 4. Furthermore,
A capacitor having an MIM structure is formed by sandwiching the lower electrode 2 and the upper electrode 6 formed of a metal thin film.

Next, a procedure for manufacturing the above capacitor will be described. First, a lower electrode 2 is formed by depositing an unoxidized metal film such as Au or Pt on a substrate 1 such as glass or Si wafer. The reason why the metal film is not oxidized is that an oxidizing atmosphere process such as a firing process is performed in a later process. The thickness of the metal film is 100 to 400n
m. Next, a Ta ₂ O ₅ film having excellent electrical insulation and a large relative dielectric constant is deposited on the lower electrode 2 by sputtering, sol-gel, MOD or the like to form an insulating thin film 3. The film thickness of the insulating thin film is set to about 300 nm in consideration of the thickness required to withstand the voltage and the thickness at which peeling does not occur. The relative permittivity was about ε = 28.

I of the Ta ₂ O ₅ film deposited by sputtering
The -V characteristic is shown in FIG. The insulating thin film had a thickness of about 300 nm, and a Ta ₂ O ₅ sintered body was used as a target.
From FIG. 2, when the applied voltage is 10 V, the leak current is
It can be suppressed to as small as 10 ⁻⁸ A / cm.

Next, Li is deposited on the insulating thin film 3 by the MOD method.
A NbO ₃ film is deposited to form the ferroelectric thin film layer 4. LiNbO ₃ used as the ferroelectric thin film layer 4 in this example
Since the film has a small relative permittivity of about ε = 30 in the ferroelectric substance, it has advantages that the effective applied voltage can be made high, and the film is transparent and has excellent optical characteristics. The LiNbO ₃ film can be formed by sputtering, sol-gel, MOD, MOCVD or the like. When using sputtering or MOCVD,
Since a vacuum device is used, a dense film can be formed and a film having excellent pressure resistance can be obtained. In this embodiment, it is formed by the MOD method as described above. This is Li, Nb
Weigh out the organic acid solution of to obtain stoichiometry,
A film is formed on the substrate 1 by a spinner or printing and dried,
This is a method of obtaining a uniform film through a firing step, and has the advantages of simple steps and low cost.

[0015] showing an I-V characteristic of LiNbO ₃ film when the LiNbO ₃ film formed directly on the formed at Au electrodes in FIG. The thickness of the LiNbO ₃ film was about 600 nm. The leak characteristics are 1 compared with the Ta ₂ O ₅ film shown in FIG.
^{0-4 is} bad. This is because the LiNbO ₃ film is a two-component system, so that composition control is difficult, a stoichiometric film cannot be formed after firing, and oxygen defects occur.
In the case of such a film having many oxygen defects, electrons move to a position where defects are generated by hopping conduction, and the leak current tends to increase.

Ferroelectric thin film layer 4 composed of LiNbO ₃ film
A Ta ₂ O ₅ film is again deposited as an insulating thin film 5 under the same conditions as above. Then, Au or Pt is deposited on the insulating thin film 5 to form the upper electrode 6.

As shown in the above embodiment, the insulating thin film 3,
According to the ferroelectric thin film 10 having the laminated structure including the ferroelectric thin film layer 4 and the insulating thin film 5, the ferroelectric thin layer 4 (LiNb
(O ₃ film) is sandwiched between insulating thin films 5 and 6 (Ta ₂ O ₅ film), the IV characteristics thereof are the same as those of LiNb shown in FIG.
The characteristics of the O ₃ film are closer to the characteristics of the Ta ₂ O ₅ film shown in FIG. 2, and the leak current can be reduced even if a LiNbO ₃ film having poor IV characteristics is used. This is because by sandwiching the ferroelectric thin layer 4 (LiNbO ₃ film) with an insulating thin film (Ta ₂ O ₅ film) having excellent electrical insulation,
This is because the injection of electrons from the electrode side is prevented.

When the hysteresis characteristic of the ferroelectric thin film 10 was measured, the loop shown in FIG. 4 was obtained.
In FIG. 4, no Lissajous waveform is observed, a clean hysteresis loop is drawn, and the residual polarization Pr is 0.5 μC.
/ Cm ² , and the coercive electric field Ec was 6 kV / cm. From this, according to the structure of the ferroelectric thin film 10, even if a LiNbO ₃ film having a poor leakage current characteristic is used, a Lissajous waveform is not generated, and the residual dielectric polarization inherent in the ferroelectric is derived. You can This is because the ferroelectric thin layer 4 (LiN) is formed by the insulating thin films (Ta ₂ O ₅ films) 3 and ₅ having excellent electric insulation.
(bO ₃ film) to prevent the injection of electrons from the electrode and make the electric field in the ferroelectric thin layer 4 constant,
This is because the electric field applied to each domain in the ferroelectric thin layer 4 can be made uniform in magnitude and direction.

In this embodiment, the ferroelectric thin layer 4 is made of Li.
Although the NbO ₃ film is used, a LiTaO ₃ film may be used, or a barium titanate film may be used. Further, since the insulating thin films (Ta ₂ O ₅ films) 3 and 5 have a structure to prevent injection of electrons from the electrode side, a film having a low specific resistance can be used as the ferroelectric thin layer 4.

[0020]

According to the present invention, since the ferroelectric thin layer is sandwiched by the insulating thin film to form the ferroelectric thin film, the injection of electrons from the electrode arranged outside the insulating thin film is prevented. Even with a film that can be prevented by an insulating thin film and has cracks, pinholes, etc., or a film with a low specific resistance, the original characteristics of the ferroelectric substance can be maintained without limiting the film thickness and manufacturing conditions of the ferroelectric thin film. It can be pulled out enough.

[Brief description of drawings]

FIG. 1 is a cross-sectional explanatory view showing a capacitor having an MIM structure using a ferroelectric thin film of the present invention.

FIG. 2 is a graph showing a leak current characteristic of a Ta ₂ O ₅ film.

FIG. 3 is a graph showing a leak current characteristic of a LiNbO ₃ film.

FIG. 4 is a graph showing the hysteresis characteristic of the ferroelectric thin film of the present invention.

FIG. 5 is a schematic diagram illustrating an internal structure of a ferroelectric thin film in an initial state.

FIG. 6 is a schematic diagram illustrating an internal structure of a ferroelectric thin film in a state where an electric field is applied.

FIG. 7 is a schematic diagram illustrating a current path in a ferroelectric thin film.

FIG. 8 is a schematic diagram illustrating an internal structure of the ferroelectric thin film of the present invention in a state where an electric field is applied.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate, 2 ... Lower electrode, 3 ... Insulating thin film, 4 ... Ferroelectric thin layer, 5 ... Insulating thin film, 6 ... Upper electrode, 10
… Ferroelectric thin film

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H01L 21/8242 27/108 49/02 H01L 27/10 325 J

Claims (1)

[Claims] 1. A ferroelectric material comprising a ferroelectric thin film layer made of a thin film exhibiting ferroelectricity, sandwiched between thin films made of a material having a higher electric insulation than the ferroelectric thin film layer. Thin film.