JP2615020B2 - Ultrasonic transducer and surface wave device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a surface acoustic wave device and an ultrasonic transducer in which a partially domain-inverted region is formed in a surface layer of a single crystal piezoelectric substrate.

(Prior art) Conventionally, an interdigital transducer (hereinafter referred to as IDT) electrode is formed on a surface of a LiTaO ₃ substrate with a suitable conductive thin film at a predetermined pitch, and a portion opposite to the spontaneous polarization of the substrate is formed on the surface layer of the substrate. Transducers and surface acoustic wave devices formed with a magnetic domain inversion region are known.

However, conventional ultrasonic transducers and surface acoustic wave devices have a drawback that energy conversion efficiency is low because LiTaO ₃ having a relatively small electromechanical coupling coefficient is used as a substrate material.

In addition, the conventional surface acoustic wave device requires the pitch of the IDT electrode to be reduced in order to cope with the recent increase in the frequency of electronic devices, which is extremely difficult not only in manufacturing but also as the electrode finger gap becomes narrower. There is a drawback that a short circuit occurs even by the presence of a large metal piece.

(Object of the Invention) It is an object of the present invention to provide a high-efficiency ultrasonic transducer and a surface acoustic wave device which replace the ultrasonic transducer and the surface acoustic wave device having a low energy conversion efficiency using the conventional LiTaO ₃ substrate as described above. I do.

(Summary of the Invention) In order to achieve the above object, an ultrasonic transducer according to the present invention comprises a TiN thin film partially adhered to a + C surface of a LiNbO ₃ substrate, and then the substrate is placed in a gas atmosphere of air or an inert gas. Heat treatment at a temperature lower than the Curie point of 1030 ° C.
The Ti is diffused to form a domain-inverted region having a polarity opposite to the spontaneous polarization of the substrate, and an IDT electrode is arranged on the domain-inverted region and an alternating electrode is applied thereto to excite ultrasonic waves in the substrate thickness direction. It is something to do. Next, the surface acoustic wave device according to the present invention is a LiNbO ₃ substrate + C
After the Ti thin film is partially adhered to the surface, the substrate is subjected to a heat treatment at a temperature lower than the Curie point of the substrate and at a temperature of 1030 ° C. or higher in a gas atmosphere of air or an inert gas, thereby
Ti is diffused immediately below the Ti thin film attachment region to form a domain-inverted region having a polarity opposite to the spontaneous polarization of the substrate, and the domain-inverted region is disposed on both sides of the IDT electrode or between two IDT electrodes. .

In another surface acoustic wave device according to the present invention, a partially domain-inverted region is formed in a + C plane side surface layer portion of a LiNbO ₃ substrate, and a uniform electrode is attached to the front and back surfaces of the substrate in part or all of the region. By applying an alternating electric field between the uniform electrodes on the front and back of the substrate, surface waves can be excited without using an IDT electrode.

Further, still another surface acoustic wave device according to the present invention includes Li
A partially domain-inverted region is formed on the + C face side surface portion of the NbO ₃ substrate, uniform electrodes are provided on the front and back of the substrate in a part of the region, and a part of the partial domain-inverted region is used as a reflector. A surface wave is excited without using an IDT electrode by applying an alternating electric field between the uniform electrodes on the front and back of the substrate.

(Example) Hereinafter, an example of the present invention will be described in detail. Here, a method of manufacturing a LiNbO ₃ substrate having a partially domain-inverted region which is a main component of the ultrasonic transducer and the surface acoustic wave device according to the present invention, and an optical modulation device which is an application example of the LiNbO ₃ substrate Will also be described.

(I) Manufacturing method of LiNbO ₃ substrate having partially domain-inverted regions Ti is deposited in a grating shape to a thickness of about 1,000 mm at a predetermined interval on the + C surface of a LiNbO ₃ z plate having a C axis perpendicular to the plate surface. This is subjected to a heat treatment for 5 to 10 hours in an Ar gas containing air or water vapor (for the purpose of preventing external diffusion of Li). The heat treatment temperature at this time is maintained at about 1,030 ° C., which is slightly lower than the Curie point temperature of LiNbO ₃ (about 1,150 ° C.).

As a result, as shown in FIG. 1A, a domain-inverted region having a polarity opposite to that of spontaneous polarization is formed immediately below the Ti vapor-deposited film in a grating shape, and its thickness does not generally exceed 12 to 15 μm.

When the same operation was performed on a 36-degree rotated Y plate whose C axis was inclined 54 ° from the plate surface normal, no domain-inverted region occurred at a heat treatment temperature of 1,030 ° C. Was formed.

It should be noted that the appearance of the domain-inverted regions as described above occurs when the domain lines are separated from each other as shown in FIG. 1 (a) and when the domain-inverted regions adjacent to each other are connected as shown in FIG. 1 (b). In some cases, the area boundary becomes sinusoidal. The cause is unknown at present, but it seems to be due to the state of Ti diffusion.

(II) partially polarizing applications of LiNbO ₃ substrate having an inversion region, the LiNbO ₃ substrate having the partial domain inversion regions by methods such as described in (I) is can be applied to such following devices It is.

(A) Surface-excited ultrasonic transducer FIGS. 2A and 2B are cross-sectional views showing an embodiment in which a LiNbO ₃ substrate having a partially domain-inverted region is used as the ultrasonic transducer according to the present invention. .

That is, LiNb having a grating-like partially domain-inverted region formed at a predetermined interval as shown in FIG.
If an IDT electrode is made of Al or the like over the domain boundary on the surface of the O ₃ Z plate and an alternating electric field is applied to the IDT electrode similarly to the SAW resonator, a transverse bulk wave is generated.

If IDT electrodes are alternately provided in the domain-inverted region and the non-inverted region of the LiNbO ₃ rotating Y plate and excited as shown in FIG. 3B, longitudinal waves are generated, and both of them are radiated as ultrasonic waves. The fact that LiTaO ₃ is more efficient than that is apparent from the difference in the electromechanical coupling coefficient between the two.

(B) the mutual light deflection or conventional light deflection or light modulating device application to an optical modulation device and the ultrasonic waves and light propagating example LiNbO ₃ crystal adhered ultrasonic transducer in the LiNbO ₃ crystal Some devices use black diffraction based on the action, but such devices have defects such as difficulty in bonding the ultrasonic transducer to the LiNbO ₃ crystal and difficulty in increasing the frequency.

If an optical device of this type uses a LiNbO ₃ substrate having a partially domain-inverted region according to the present invention, an IDT electrode is attached to the surface where the partially domain-inverted region is formed as shown in FIG. It will not be necessary to say that a light deflection or light modulation device can be easily obtained simply by doing.

(C) Application to reflectors for SAW resonators
Since it is clear that W) is reflected, it can be used as a reflector such as a SAW resonator. In particular, if a substrate as shown in FIG. 1 (b), in which the domain lines are continuous in the vicinity of the LiNbO ₃ crystal surface, is used, the reflection of the SAW changes continuously, so that a reflector using a conventional conductor grating can be used. As the reflected wave is less likely to be converted into another bulk wave and become a loss as described above, a high Q SA
It is possible to get a W device.

FIG. 4 shows a cross-sectional view thereof. An IDT electrode can be provided on the surface of the partial domain-inverted region forming portion. For example, an internal reflection type unidirectional SAW transducer can be obtained as shown in FIG. 5 (a) or (b). Can be

In this case, although the reflection of the SAW is mainly controlled by the domain-inverted region, the influence of the IDT electrode is exerted, so that the mutual positional relationship between the domain-inverted region and the IDT electrode is determined by the step of the conventional IDT electrode. It is not as simple as the structure and must be determined by experiment.

(D) The surface acoustic wave device of the present invention which does not require an IDT excitation electrode As described above, in the conventional SAW device, the frequency is determined by the pitch of the IDT excitation electrode. It is necessary to reduce the pitch of the IDT electrodes, which is extremely difficult in manufacturing. In addition, as the electrode finger gap becomes narrower, short-circuiting often occurs due to the presence of fine metal pieces.

Such a problem is caused by LiNbO ₃ having a partially domain-inverted region.
Using a substrate, for example, as shown in FIG. 6 (a), a partially uniform electrode is provided on the front and back of the center of the substrate, an alternating electric field is applied thereto, and the domain-inverted rows on both sides of the uniform electrode are used as reflectors. Completely solved. That is, as shown in FIG. 1 (b), the domain-inverted region may be incomplete even if the separation is incomplete. This is because there is no difference in characteristics.

Further, it is sufficient that the pitch of the vapor-deposited Ti strip is twice as large as that of a normal IDT electrode finger.

The one-port SAW device (resonator) has been described above. The one-port SAW device (resonator) has a completely planar electrode as shown in FIG. 3B, or a two-port device as shown in FIG. 3C or FIG. It is also possible to use

(Effects of the Invention) According to the present invention, the following excellent effects can be obtained.

(1) claims ultrasonic transducer and claims the surface wave device in the range the second term to the fourth claim of the range preceding claim in becomes large electromechanical coupling coefficient than LiTaO ₃ LiNbO ₃
Is used as a substrate material, so that the energy conversion efficiency is high.

(2) The surface acoustic wave device according to claim 2 is:
By arranging the domain-inverted region on both sides of the IDT electrode or between the two IDT electrodes, the energy conversion efficiency can be further improved by using the domain-inverted region as a reflector.

(3) The surface acoustic wave device according to claims 3 and 4, wherein the surface acoustic wave is applied without using the IDT electrode by applying an alternating electric field between the uniform electrodes on the front and back of the LiNbO ₃ substrate. Since it can be excited, manufacturing for high frequency is easy.

(4) The surface acoustic wave device according to claim 4 is:
Since a part of the partially domain-inverted region is used as a reflector, the energy conversion efficiency can be further improved.

[Brief description of the drawings]

FIGS. 1 (a) and 1 (b) show different structures of a LiNbO ₃ substrate having a partially domain-inverted region which is a main component of the ultrasonic transducer and the surface acoustic wave device of the present invention, respectively. Figure (a) and (b) illustrate an example of application shows the structure of the ultrasonic transducer has different Tsu using LiNbO ₃ substrates each having a partial domain inversion regions, FIG. 3 is the same to the optical deflector FIG. 4 is a block diagram showing an example in which this is applied to a reflector of a SAW resonator, and FIGS. 5 (a) and 5 (b) show different internal reflection type unidirectional SAW transducers. FIG. 6 (a) is a block diagram showing an example of application to the present invention.
(D) is a configuration diagram showing an embodiment of the surface acoustic wave device according to the present invention.

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-58-42968 (JP, A) JP-A-58-21211 (JP, A) JP-B-54-18385 (JP, B2) JP-B-58-212 32527 (JP, B2)

Claims (4)

(57) [Claims] The present invention relates to a method for manufacturing a semiconductor device, comprising: depositing a Ti thin film partially on the + C surface of a LiNbO ₃ substrate; and performing heat treatment at a temperature lower than the Curie point of the substrate and at least 1030 ° C. in a gas atmosphere of air or an inert gas. Just below the Ti thin film deposition area
Is diffused to form a domain-inverted region having a polarity opposite to the spontaneous polarization of the substrate, and an IDT electrode is arranged on the domain-inverted region, and an alternating electrode is applied thereto to excite ultrasonic waves in the substrate thickness direction. An ultrasonic transducer characterized by the above. 2. The method according to claim 1, further comprising: depositing a Ti thin film partially on the + C surface of the LiNbO ₃ substrate; and performing heat treatment at a temperature lower than the Curie point of the substrate and at least 1030 ° C. in a gas atmosphere of air or an inert gas. Just below the Ti thin film deposition area
A surface-inverted region having a domain-inverted region having a polarity opposite to the spontaneous polarization of the substrate, and the domain-inverted region is disposed on both sides of the IDT electrode or between two IDT electrodes. 3. A partially domain-inverted region is formed in a surface layer portion on the + C face side of a LiNbO ₃ substrate, and a uniform electrode is provided on the front and back surfaces of the substrate in part or all of the region. A surface wave device characterized in that a surface wave is excited without using an IDT electrode by applying an alternating electric field between the electrodes. 4. A partially domain-inverted region is formed in a surface layer portion on the + C plane side of a LiNbO ₃ substrate, a uniform electrode is provided on the front and back of the substrate in a part of the region, and a partial domain-inverted region is formed. A surface wave device characterized in that a surface wave is excited without using an IDT electrode by using a portion as a reflector and applying an alternating electric field between uniform electrodes on the front and back of the substrate.