JP2547203B2 - Method for forming bismuth titanate thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention refers to bismuth titanate (hereinafter referred to as Bi ₄ Ti ₃ O ₁₂ ) used for applications such as field effect transistors (FETs), optical switches, and memories. ) It relates to a method for forming a thin film.

(Prior Art) The thin film of Bi ₄ Ti ₃ O ₁₂ has a possibility of being used for FETs, optical switches, memories and the like. The single crystal of this material has a space group of B2cb (a = 5.448, b = 32.83, e = 5.411, z
= 4), the b-axis is perpendicular to the layer as shown in FIG. This substance has a Curie point at 675 ° C., exhibits ferroelectricity below this temperature, and a dipole vector exists on the ab plane. This dipole is used in the above-mentioned optical switch or the like by utilizing the characteristic that the direction of the dipole changes by the application of an electric field. Since the switching operation at this time has a threshold voltage of 3 to 4 kV / cm, it is suitable as a memory device.

However, as described above, since the threshold voltage is large, there is a problem that the operating voltage for switching is too high with a single crystal having a certain thickness. For this reason Bi ₄ Ti ₃ O ₁₂
It has been proposed to form a thin film and reduce the operating voltage (for example, Japanese Patent Laid-Open Nos. 61-6123 and 61-6124). This proposal is intended to form a Bi ₄ Ti ₃ O ₁₂ film by a sputtering method, 60～80Omegati% a Bi ₄ Ti ₃ O ₁₂ as a target in planar magnetron sputtering, Bi ₁₂ TiO
₂₀ ) is used in a proportion of 40 to ₂₀ wt%.

(Problems to be Solved by the Invention) However, since the vapor pressure of Bi and the vapor pressure of Ti are different, it is quite difficult to obtain a homogeneous substance having a constant composition ratio.

Also, when the substrate temperature is less than about 600 ° C, Bi ₂ O ₃ , TiO ₂ , Ti
There has been a problem that the desired thin film of Bi ₄ Ti ₃ O ₁₂ cannot be obtained because it becomes a mixture thin film of ₂ O ₃ or the like.

Therefore, the conventional vapor deposition method requires a substrate temperature of 600 to 750 ° C.
Very high or vapor deposition first at room temperature, then 600 ~ 7
It was heat treated at 00 ° C. to form a Bi ₄ Ti ₃ O ₁₂ thin film.

Also, it is generally difficult to obtain a thin film having a good crystal axis orientation (for example, a b-axis orientation film), and the orientation of crystals deposited on the surface is influenced by the substrate material, and thus the substrate material As a result, only a very limited substrate material can be used.

The present invention solves the above problems, that is, the control of the composition ratio of the Bi component and the Ti component is unstable, the substrate temperature is high, and only a limited substrate material can be used. It is what

(Means for Solving Problems) The above-mentioned problems are caused by a substrate on which vaporized substances evaporated from the raw material containing at least titanium atoms and the raw material containing at least bismuth atoms are arranged at desired positions. In the method of forming a bismuth titanate thin film to be vapor-deposited on, both of the vaporized substances are vapor-deposited by an ion beam vapor deposition method to form a thin film of Bi ₄ Ti ₃ O ₁₂ on the substrate. This is solved by the method of forming a bismuth thin film.

This allows any substrate, such as various glass plates,
On the surface of silicon plate, various ceramic plate, oxide single crystal plate, metal plate or alloy plate, Bi ₄ Ti ₃ O ₁₂ polycrystal film, amorphous film, or uniaxially oriented film can be freely formed to obtain the required crystallinity. It is possible to obtain a Bi ₄ Ti ₃ O ₁₂ thin film having

 Hereinafter, the present invention will be described in detail with reference to the drawings.

 FIG. 2 shows a schematic diagram of a vapor deposition apparatus used in the present invention.

In FIG. 2, 1 is the substrate and 2 is the formed Bi ₄ T.
i ₃ O ₁₂ thin films 3, 4 are crucibles containing raw materials, 5, 6
Is an ionization unit for ionizing the evaporation material emitted from the crucible, 7 and 8 are acceleration electrodes for accelerating the ionized evaporation material, and 9 and 10 are ion beams of the evaporation material evaporated from the crucibles 3 and 4. . Further, 11 and 12 are accelerating power supplies for accelerating the vaporized substances, and in this device, the substrate potential is set to the ground potential and acceleration up to about 10 kV is possible. 13
Is a vacuum container. The temperature of the substrate 1, the crucibles 3, 4
The temperature, the ionization current in the ionization units 7 and 8, and the vacuum degree of the vacuum container 13 can be independently controlled by a device (not shown).

Using such a device, a Bi ₄ Ti ₃ O ₁₂ thin film can be formed on the substrate 1 as follows.

First, the vacuum degree of the vacuum container 13 is set to a vacuum higher than about 10 ⁻⁶ Torr, and the temperature of the substrate 1 is set to a predetermined temperature of 300 ° C. or lower. Into the crucible 3, either Bi or bismuth oxide such as Bi ₂ O ₃ is put as a raw material containing Bi atoms, and the crucible is heated so that the vapor pressure becomes about 10 ⁻⁴ Torr or more. The crucible 4 contains Ti and TiO ₂ as raw materials containing Ti atoms.
Alternatively, add one of titanium oxides such as Ti ₂ O ₃ and add B
It is heated in the same manner as the source material containing i atoms. At this time, the composition ratio of the Bi component and the Ti component is measured by a thin film monitor (for example, a crystal oscillator, not shown) installed around the substrate 1, and the temperature of the crucible is adjusted so that the desired ratio is obtained. . The ionization current and the acceleration voltage of the ionization units 5 and 6 and the acceleration electrodes 7 and 8 are set according to the crystal state of the target Bi ₄ Ti ₃ O ₁₂ thin film. Generally, in order to obtain an oriented film with good crystallinity, it is desirable to ionize and accelerate the vaporized substances from the respective raw materials together.
In order to remove unreacted components such as i, Bi ₂ O ₃ , Ti, TiO ₂ , and Ti ₂ O ₃ from the obtained thin film 2, the evaporation material from the raw material containing Bi atoms is accelerated more strongly, It is better to ionize more.

Table 1 shows an example of the combination of the ionization current Ii and the acceleration voltage Va. Bi ₂ O ₃ for crucible ₃ and T for crucible 4
The results of the Bi ₄ Ti ₃ O ₁₂ thin film when i was put in and quartz glass was used as the substrate and the substrate temperature was set to 250 ° C., in the case of the combination of No. 1 in Table 1, the X-ray of the obtained thin film The diffraction pattern is as shown in FIG. 3, and the (171) diffraction line is slightly recognized at the portion A in the figure, but only the halo pattern of the substrate is obtained. Also, the (171) peak was slightly strongly observed under the condition of No. 3 in Table 1, but the results were similar to those in FIG. However, under the conditions of No. 2 in Table 1, the X-ray diffraction pattern is as shown in Fig. 4, which is the X-ray of the Bi ₄ Ti ₃ O ₁₂ powder.
It corresponds well to the line diffraction pattern. In this state, the intensity of the diffraction line observed for the halo pattern from the substrate is high. In addition, the ionization current and acceleration voltage
Under the conditions of Nos. 4 and 5 in Table 1, the X-ray diffraction patterns shown in FIGS.
_{A 4} Ti ₃ O ₁₂ thin film is obtained. As described above, the crystal state of Bi ₄ Ti ₃ O ₁₂ obtained can be freely changed by changing the vapor deposition conditions in the ion beam vapor deposition method.

Further, it is possible to lower the substrate temperature by changing the shape of the crucibles 3 and 4 in FIG. 2, for example, by installing a nozzle at the outlet of the evaporation material and changing the shape of the nozzle. is there.

(Example) Hereinafter, the Example of this invention is described.

Example 1 Using the apparatus shown in FIG. 2, Bi ₄ Ti ₃ O was prepared as follows.
₁₂ thin films were prepared.

First, Bi ₂ O ₃ as a raw material containing Bi atoms and Ti as a raw material containing Ti atoms are placed in the crucibles 3 and 4, respectively (purity is 99.9% or more in each case). Corning 7059 glass was used as the substrate 2, and its temperature was set to 250 ° C. Adjust the vacuum degree of the vacuum vessel 13 to be 2 × 10 ^-6 Torr,
Oxygen gas was introduced at a rate of 9 ml / sec from an inlet not shown in FIG. Table 1 shows the ionization current and accelerating voltage of each component
It was set to No.1 condition. Adjust the temperature of crucibles 3 and 4,
Deposition was performed at a rate of 2-3 Å / sec near the substrate. Bi obtained
_{The 4} Ti ₃ O ₁₂ thin film had a thickness of 2000 Å, and an amorphous thin film was obtained with the same X-ray diffraction pattern as in FIG.

Example 2 Bi ₂ O ₃ was selected as the source material containing Bi atoms, and Ti was selected as the source material containing Ti atoms. Corning 7059 glass and quartz glass were used as the substrate, and the temperature was set to 200 ° C.
It vapor-deposited like Example 1 on condition of No. 2 in Table 1. Film thickness
The X-ray diffraction pattern of the 1500 Å Bi ₄ Ti ₃ O ₁₂ thin film was equivalent to that in FIG.

Example 3 Bi ₂ O ₃ was selected as the raw material containing Bi atoms, Ti was selected as the raw material containing Ti atoms, and the substrate was Corning 7059 glass.
Quartz glass, slide glass (S111) and (100) surface Si wafer were used and the temperature was set to 200 ° C. It vapor-deposited like Example 1 on condition of No. 3 in Table 1. Film thickness 20
The X-ray diffraction pattern of the 00Å Bi ₄ Ti ₃ O ₁₂ thin film was the same as in FIG.

[Example 4] Raw material containing Bi atoms and either Bi or Bi ₂ O ₃ , and Ti, TiO ₂ , or Ti ₂ O as raw material containing Ti atoms
Select any one of ₃ and use Corning 705 as the substrate.
9 glass, quartz glass, slide glass (S111), gold, platinum, copper, palladium electrodes attached on the glass, MgO single crystal, PLZT ceramics and (100) plane
A Si wafer was used. The temperature of these substrates was set to 250 ° C., and vapor deposition was carried out in the same manner as in Example 1 under the conditions of No. 4 in Table 1. The Bi ₄ Ti ₃ O ₁₂ thin film thus obtained had an X-ray diffraction pattern equivalent to that shown in FIG. Bi on metal electrode and low resistance Si wafer
_When an electrode such as Al was mounted on the ₄ Ti ₃ O ₁₂ thin film and the dielectric constant was measured, a value of about 135 was obtained, which was in good agreement with the value of the single crystal.

[Embodiment 5] An inner diameter of 2 mmφm at the outlet of the crucibles 3 and 4 in FIG.
A nozzle having a height of 2 mm was attached, the same substrate as in Example 4 was used, and the substrate temperature was set to 150 ° C. When vapor deposition was performed under these conditions in the same manner as in Example 4, a Bi ₄ Ti ₃ O ₁₂ thin film showing an X-ray diffraction pattern equivalent to that shown in FIG. 5 was obtained, and the electrical characteristics were also good.

Example 6 Using the same raw material and substrate containing Bi and Ti atoms as in Example 4, the substrate temperature was adjusted to 200 ° C. It vapor-deposited like Example 1 on condition of No. 5 in Table 1. The obtained Bi ₄ Ti ₃ O ₁₂ thin film was equivalent to the X-ray diffraction pattern in FIG. 6 and had good electrical characteristics.

[Embodiment 7] At the outlets of the crucibles 3 and 4 in FIG.
A nozzle having a height of 0.5 mm was attached, the same substrate as in Example 4 was used, and the substrate temperature was set to 120 ° C. Deposition was performed under the same conditions as in Example 6. The obtained Bi ₄ Ti ₃ O ₁₂ thin film had the same X-ray diffraction pattern as shown in FIG. 6 and good electrical characteristics as in Example 4.

In the above embodiment, the ionization current and the acceleration voltage are limited to those shown in Table 1. In the case of Bi ₄ Ti ₃ O ₁₂ thin film, when the ionization current is 300mA and the acceleration voltage is 7kV or more, Bi on the substrate
_It is difficult to control the film thickness because the ₄ Ti ₃ O ₁₂ thin film is sputtered. However, if the value is less than the above value, the orientation of the obtained thin film can be controlled by selecting the vapor deposition conditions.

Further, in the above-mentioned embodiment, the introduced oxygen is neutral oxygen, but by ionizing this, the substrate temperature can be lowered and a thin film having a higher withstand voltage can be obtained.

Furthermore, by changing the shape of the nozzle attached to the crucible, Bi ₄ Ti ₃ O ₁₂ with a more precise and flat film surface can be obtained.
A thin film can be produced.

(Effects of the Invention) As described above, according to the present invention, the Ti component and
Since the Bi components are deposited by the ion beam deposition method respectively, Bi ₄ Ti ₃ O ₁₂ such as amorphous, single crystal, b-axis oriented, etc.
There is an effect that a thin film can be formed on a desired substrate at a low temperature of 300 ° C. or lower.

The composition ratio of Bi and Ti can be controlled within 5% with respect to the theoretical value Bi: Ti = 4: 3 (atomic ratio).

[Brief description of drawings]

FIG. 1 is a crystal structure diagram of Bi ₄ Ti ₃ O ₁₂ , FIG. 2 is a schematic diagram of an apparatus used for carrying out the present invention, and FIGS. 3 to 6 are Bi ₄ Ti ₃ O 12.
It is a figure of the X-ray-diffraction pattern of ₁₂ thin films. 1 ... Substrate, 2 ... Bi ₄ Ti ₃ O ₁₂ thin film 3, 4 ... Crucible, 5, 6 ... Ionization unit 7, 8 ... Accelerating electrode, 9, 10 ... Ion beam 11, 12 ... Acceleration Power supply, 13 ... vacuum container

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masato Nibe 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Yasuko Motoi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Takeo Tsukamoto 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) Reference JP 61-6123 (JP, A) JP 61-6124 (JP, A) JP-B-55-11245 (JP, B2)

Claims (5)

(57) [Claims] 1. A raw material containing at least titanium atoms,
In a method of forming a bismuth titanate thin film, wherein vaporized substances vaporized from respective source substances containing at least bismuth atoms are vapor-deposited on a substrate placed at a desired position, both of the vaporized substances are ion beam evaporated. A method for forming a bismuth titanate thin film, which comprises depositing a thin film of Bi ₄ Ti ₃ O ₁₂ on the substrate. 2. The method for forming a bismuth titanate thin film according to claim 1, wherein the raw material containing titanium atoms is Ti, TiO ₂ or Ti ₂ O ₃ . 3. The raw material containing a bismuth atom is Bi or Bi.
_The method for forming a bismuth titanate thin film according to claim 1, which is ₂ O ₃ . 4. The bismuth titanate according to claim 1, wherein the glass substrate, silicon plate, ceramic plate, oxide single crystal plate, oxide single crystal plate, metal plate or alloy plate is used as the substrate. Method of forming thin film. 5. The method for forming a bismuth titanate thin film according to claim 1, wherein the substrate is kept at 300 ° C. or lower.