JPH0656592A - Piezoelectric litao3 thin film - Google Patents

Abstract

PURPOSE:To obtain an LiTaO3 thin film having piezoelectricity and faster in surface acoustic wave velocity than that of a bulk single crystal of LiTaO3 by forming the film having piezoelectricity on a sapphire substrate and specifying the surface acoustic wave velocity. CONSTITUTION:The LiTaO3 thin film having piezoelectricity and the surface acoustic wave velocity faster than 3500m/s is formed on the sapphire substrate. In the synthetic method of the piezoelectric body thin film, a laser abrasion method by which a ratio of the elements constituting a target is 0.5<=Li/(formula II)<=3.5 when a thin film of formula I (0<=x<=1) is synthesized is preferred, and the objective thin film can be obtained especially by using mixed gas of oxygen and ozone as oxidizing gas within the limited pressure range. The LiTaO3 thin film can be used as a surface acoustic wave device and a surface acousto-optical element, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to surface acoustic waves (Surfa).
ce acoustic wave (hereinafter abbreviated as SAW) and a novel piezoelectric thin film used for devices and optical element materials.

[0002]

2. Description of the Related Art LiTaO ₃ single crystals have excellent properties in terms of electromechanical coupling constants, electro-optical effects, non-linear optical effects, etc., and are regarded as promising optical elements while being put to practical use as materials for SAW devices. It is a material. With the recent progress in semiconductor technology and the increasing integration and miniaturization of electronic components, ferroelectric elements and piezoelectric elements have also been miniaturized.
Thinning is progressing, and the demand for thinning the LiTaO ₃ single crystal is strong, and many studies have been conducted.

Conventionally, a LiTaO ₃ thin film has been produced by a liquid phase epitaxial method, a CVD method, a vacuum deposition method, a sol-gel method, a sputtering method, etc., and various academic reports have been made and a manufacturing method patent has been issued. . The obtained film is crystallographically analyzed by X-ray diffractometry or electron beam diffractometry, chemical composition is analyzed by secondary ion mass spectrometry or Auger electron spectroscopy, and optical properties such as refractive index and light propagation loss are analyzed. Analysis has been done. However, there are very few reports on the piezoelectric characteristics of the LiTaO ₃ thin film, which should be a piezoelectric body, and there are almost no reports of probable values.
There are no reports of W filters being formed.
This is because the thin film obtained so far, which is considered to be LiTaO ₃ , is crystallographically, optically and chemically compositionally LiTa.
It is similar to O ₃ , but is incomplete, and it shows that a single domainized LiTaO ₃ thin film has not yet been obtained with an area large enough to form a device such as a SAW filter.

[0004]

SUMMARY OF THE INVENTION The present invention is to provide a LiTaO ₃ thin film having piezoelectricity which can be used as a SAW device or a surface acousto-optic device.

[0005]

The present invention is characterized by being formed on a sapphire substrate, having piezoelectricity, and having a surface acoustic wave velocity of more than 3500 m / s.
₃ thin film. As a result of intensive studies by the inventors, when a LiTaO ₃ thin film was synthesized by a laser ablation method, a LiTaO ₃ thin film having piezoelectricity was obtained.
Although the reason is not clear, the laser ablation method has the advantage that the amount of impurities is extremely small, so it seems that there is no decrease in piezoelectricity due to the inclusion of Fe, etc. It is unlikely that the surface will be damaged and that the film formation rate can be adjusted freely, so that it is possible to synthesize a thin film with extremely good crystal orientation, and other advantages such as the piezoelectricity are exerted. It is considered that the LiTaO ₃ thin film shown was obtained. In other words, it seems that a LiTaO ₃ thin film that completely prevents the mixing of impurities, has a good crystal orientation, and has a small surface damage due to plasma or the like will exhibit piezoelectricity regardless of the synthesis method.

In the present invention, the crystallinity is evaluated by X-ray rocking curve [X-ray analyzer R manufactured by Rigaku Denki Co., Ltd.
AD-C] is performed with a full width at half maximum. At that time, the divergence slit and the scattering slit are 1/6 °, and the light receiving slit is 0.15 mm. The crystallinity is preferably 0.90 ° or less, more preferably 0.65 ° or less. Crystallinity is 0.90
If it exceeds 0 °, the piezoelectricity deteriorates, sufficient characteristics cannot be obtained, and in some cases, the measurable piezoelectricity is not exhibited.

Impurities in the thin film were analyzed for all elements having an atomic number of 5 or more by fluorescent X-ray analysis [Shimadzu Corporation SXF-1100]. The impurity concentration detected by this analysis method is 0.1 wt% or more. For impurities below this concentration, Table 1 and Table 2
It is shown as none in. The effect of impurities on piezoelectricity is not clear, but at least 0.1 wt%
Piezoelectricity is impaired by the above-mentioned mixture of Fe, and 2 wt%
From the above, it has been confirmed that the piezoelectric property does not show a measurable piezoelectric property, and it is not preferable to mix it. Further, it is not preferable to mix the constituent elements and elements other than Fe.
The reason why the SAW velocity on the LiTaO ₃ thin film according to the present invention is higher than that of the bulk single crystal is not clear, but
It is considered that the substrate is affected by the SAW speed of sapphire. The synthetic methods that easily reach the present invention are described below.

A suitable synthesis method is the laser ablation method disclosed in Japanese Patent Application No. 3-243209, that is, a Li (Nb _x Ta _1-x ) O ₃ thin film (0≤x≤1) is used. During synthesis, the ratio of elements composing the target is 1.5 ≦ Li / (Nb _x Ta _1-x ) ≦ 3.5.
The method for synthesizing a piezoelectric thin film by the laser ablation method is suitable, and the present invention can be achieved by limiting the pressure range and using a mixed gas of oxygen and ozone as an oxidizing gas. The laser is an excimer laser (ArF
193 nm), the laser frequency is 20 Hz, and the laser output is 150 mJ. The target is Li ₂ CO
Sintered body made of ₃ and Ta ₂ O ₅ (size 15m
mφ × 3 mm ^t ) is used, and the pressure is a mixed gas of oxygen and ozone, and the pressure is 0.0005 to 0.005 torr. The ozone concentration is preferably 1 vol% or more.

[0009]

EXAMPLES Next, the present invention will be described in more detail by way of examples. The synthesis of the LiTaO ₃ thin film is performed by the laser ablation method. FIG. 1 shows an outline of the device. The sintered target is irradiated with laser light to form a film on the substrate in a mixed gas atmosphere of oxygen and ozone. The conditions used in the synthesis are listed below.

Synthesis conditions Single crystal substrate Sapphire C, A, R, M surface Substrate temperature 800 ° C. Introduced gas Oxygen and ozone (8 vol%)
The mixed gas of the target and the substrate 3 cm Reaction pressure 0.001 torr Target Li / Ta 2.2 Laser wavelength 193 nm (ArF excimer laser) Laser output 150 mJ Laser frequency 20 Hz Reaction time 120 minutes Using the above conditions, LiTaO ₃ thin film The synthesis was carried out.
The results of X-ray diffraction of the obtained thin film are shown in FIG. 2 (on sapphire C surface), FIG. 3 (on sapphire A surface), and FIG. 4 (sapphire R).
Surface) and FIG. 5 (on the sapphire M surface). Sapphire C
(006) on the surface and (11 on the sapphire A surface.
0), (012) on sapphire R surface, sapphire M
A (300) oriented LiTaO ₃ thin film is obtained on the upper surface. The fact that these thin films are epitaxially grown is confirmed by high-speed electron beam diffraction (RHEED), four-axis X-ray diffraction method and the like. A comb-shaped electrode (Inter Digit) was formed on the synthesized film by a photolithography process.
tal Transducer, hereinafter abbreviated as IDT). The IDT is a normal type electrode, the number of pairs of electrode fingers is 64 pairs for both input and output, and the wavelength (λ) is 10 μm. The SAW filter characteristics of each sample were evaluated by this IDT to obtain Vs. The results are shown in Table 1 together with the analysis results of crystallinity and impurities. SAW filter characteristics on LiTaO ₃ thin film deposited on sapphire R surface 1
An example is shown in FIG. It is clear that the LiTaO ₃ thin film thus obtained has piezoelectricity.

[0011]

Comparative Example As a comparative example, L
The iTaO ₃ thin film was synthesized. The synthesis conditions are shown below. Synthesis conditions Single crystal substrate Sapphire C, A, R, M surface Substrate temperature 800 ° C. Introduced gas Oxygen + Ar (1: 1) Distance between target and substrate 4 cm Reaction pressure (P) 0.005-0.001 to
Li / Nb of rr target 2.2 High frequency 13.56 MHz rf-power (RF) 80 to 150 W Reaction time 120 minutes Results of X-ray diffraction of the obtained thin film are shown in FIG. 7 (on sapphire C surface) and FIG. 8 (sapphire C surface). 9) (Sapphire R)
Surface) and FIG. 10 (on sapphire M surface). crystalline,
Table 2 shows the results of analysis of impurities and the evaluation of SAW characteristics. X
Judging from the line diffraction, it seems that a LiTaO ₃ thin film was obtained, but impurities were detected in any of the films, the crystal orientation was lower than that of the example, and a measurable piezoelectric property was exhibited. There wasn't.

[0012]

[Table 1]

[0013]

[Table 2]

[0014]

The present invention has piezoelectricity and SAW.
LiTa with a larger rate than the bulk single crystal of LiTaO _3.
An O ₃ thin film can be provided.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an example of a synthesizing apparatus used in the present invention.

FIG. 2 shows L obtained on a sapphire C plane in the example.
ITaO ₃ is a spectrum diagram showing the X-ray diffraction pattern of the thin film.

FIG. 3 shows L obtained on a sapphire A surface in an example.
ITaO ₃ is a spectrum diagram showing the X-ray diffraction pattern of the thin film.

FIG. 4 shows L obtained on a sapphire R surface in an example.
ITaO ₃ is a spectrum diagram showing the X-ray diffraction pattern of the thin film.

FIG. 5 shows L obtained on the M-plane of sapphire in the example.
ITaO ₃ is a spectrum diagram showing the X-ray diffraction pattern of the thin film.

FIG. 6 is a spectrum diagram showing an example of SAW filter characteristics obtained in Examples.

FIG. 7 shows L obtained on a sapphire C surface in a comparative example.
ITaO ₃ is a spectrum diagram showing the X-ray diffraction pattern of the thin film.

FIG. 8 shows L obtained on a sapphire A surface in a comparative example.
ITaO ₃ is a spectrum diagram showing the X-ray diffraction pattern of the thin film.

FIG. 9 shows L obtained on a sapphire R surface in a comparative example.
ITaO ₃ is a spectrum diagram showing the X-ray diffraction pattern of the thin film.

FIG. 10 is a spectrum diagram showing an X-ray diffraction result of a LiTaO ₃ thin film obtained on a sapphire M surface in a comparative example.

[Explanation of symbols]

 1 ArF excimer laser 2 lens 3 window 4 target 5 substrate 6 film thickness measuring device 7 gas inlet

Claims (1)

[Claims] 1. A LiTaO ₃ thin film formed on a sapphire substrate, having piezoelectricity, and having a surface acoustic wave velocity higher than 3500 m / s.