JP2001196895A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve problems of a conventional ZnO thin film on a crystal being a surface acoustic wave device aiming at temperature stability that cannot have thickened the film thickness of the ZnO resulting in decreasing the k2 of the entire element because the integration with other electronic device is difficult, the isolation of the ZnO is prone to decrease, and the absolute value of a negative TCF value of the crystal is small. SOLUTION: Laminating a ZnO piezoelectric layer 13 oriented in the (001) with at least any of Li, Cu, Ag attached thereto on a Si substrate 11 oriented in the (100) can enhance the piezoelectricity and increase the k2 and depositing a SiO2 protection layer 16 containing at least any of Li2O, MgO, Al2O3 and having a negative center frequency temperature coefficient (TCF) value on the layer 13 can nullify the TCF value of the entire element without decreasing the k2 value so as to realize a communication device such as a high frequency filter and an oscillator with a small insertion loss and excellent temperature stability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave device used in the field of information communication, and more particularly to a surface acoustic wave device using a piezoelectric thin film.

[0002]

2. Description of the Related Art The demand for surface acoustic wave devices has been rapidly expanding with the remarkable development of the communication field centering on mobile communication such as mobile phones. The direction of development of surface acoustic wave devices is in the direction of miniaturization, high efficiency, and high frequency, and for that purpose, a larger electromechanical coupling coefficient (hereinafter k ² ),
More stable temperature characteristics, larger surface acoustic wave propagation velocity,
Is required. The surface acoustic wave device is used for a filter, a resonator, or the like. For example, when the surface acoustic wave device is used as an oscillator, it is desirable that the temperature characteristic is such that its center frequency temperature coefficient (TCF) is as close to zero as possible. Conventionally, a surface acoustic wave device is mainly composed of an inter-digital electrode (Inter-Digital Transducc) on a piezoelectric single crystal.
er (hereinafter, IDT) has been used. However, in the case of an IF filter or an oscillator that requires stable temperature characteristics, quartz having a TCF close to zero has been used as a piezoelectric substrate.

On the other hand, in the case of a piezoelectric material having inferior temperature characteristics such as zinc oxide (hereinafter referred to as ZnO), for example, “Surface Wave Devices and Their Applications”, edited by Electronic Materials Industry Association, published in 1978, p.
As described on pages 106 to 108, materials having different signs of TCF, for example, silicon dioxide (hereinafter, SiO ₂ )
Attempts have been made to improve the temperature characteristics by laminating them. Jpn. J. Appl. Phys. Vol. 36
(1997) p. 3076-3080 and JP-A-9-19-1
As described in US Pat. No. 30192, a negative TCF on a quartz substrate with a cut angle and propagation direction having a positive TCF value.
Attempts have been made to adjust the TCF to near zero by forming a ZnO piezoelectric film having a certain value and optimizing the film thickness. In this case, by optimizing the film thickness of each layer, it is possible to improve not only the temperature characteristics but also the k ² and the propagation speed of the surface wave.

[0004]

However, the conventional surface acoustic wave device using a ZnO thin film on quartz has the following problems.

First, since a quartz substrate must be used, it has been difficult to integrate it with other electronic devices. Therefore, if a structure in which the film is directly deposited on a silicon (hereinafter, Si) substrate can be realized, it is very useful including reduction in manufacturing cost. Si has a surface acoustic wave propagation velocity of 50
A large value of about 00 m / s is also useful for increasing the frequency.

Next, since ZnO is a material generally exhibiting the properties of an n-type semiconductor, when oxygen defects are generated by a film forming process in a low oxygen atmosphere, conductivity appears to cause a decrease in leak current and piezoelectric constant. Problems such as were easy to occur. Therefore, in order to secure the insulating property of ZnO, it is desirable to add a metal having a lower valence than Zn, that is, a metal that easily becomes a monovalent cation.

On the other hand, the SiO ₂ having a positive TCF is to offset the negative TCF of ZnO it is necessary to increase the film thickness to some extent, in which case the influence of small k ² of SiO _2, There is a drawback that k ^{2 of the} entire device is reduced. Therefore, TCF is better than SiO ₂
It is desirable to select a material having a large thermal expansion coefficient, and a material having a negative coefficient of thermal expansion is promising.

The present invention solves the above-mentioned problems, and provides a surface acoustic wave device using a thin film which is excellent in temperature characteristics and has a high k ² and can cope with a higher frequency.

[0009]

According to a first aspect of the present invention, there is provided a surface acoustic wave device comprising: a (100) Si substrate;
It is characterized by comprising a buffer layer made of iO ₂ and a piezoelectric layer made of ZnO on the SiO ₂ buffer layer, wherein an IDT is formed immediately below or directly above the ZnO piezoelectric layer.

According to the above configuration, Zn is directly formed on the Si substrate.
A structure in which an O piezoelectric material is deposited can be realized, which is advantageous in reducing manufacturing costs and integrating other electronic devices.

According to a second aspect of the present invention, there is provided a surface acoustic wave device comprising ZnO
The piezoelectric layer has a composition in which Zn is partially replaced with at least one of Li, Cu, and Ag.

According to the above configuration, there is an effect that problems such as a leak current of the ZnO piezoelectric layer and a decrease in the piezoelectric constant are improved.

According to a third aspect of the present invention, there is provided a surface acoustic wave device comprising ZnO
In the piezoelectric layer, the c-axis is oriented perpendicular to the silicon substrate, and the a-axis is an epitaxial film oriented in a plane parallel to the silicon substrate.

According to the above configuration, since a flat and dense epitaxial film can be obtained, there is an effect that a loss caused by leaky wave propagation is reduced.

According to a fourth aspect of the present invention, there is provided a surface acoustic wave device comprising ZnO
It is characterized by having a protective layer made of SiO ₂ on the piezoelectric layer.

According to the above configuration, S having a positive TCF
The iO ₂ layer can cancel the negative TCF of the ZnO layer to make the TCF of the entire device zero, thereby improving temperature stability.

According to a fifth aspect of the present invention, there is provided a surface acoustic wave device comprising:
_{2 In the} protective layer, besides SiO ₂ , Li ₂ O, MgO, A
It is characterized by containing at least one of l ₂ O ₃ .

According to the above configuration, the positive TC of the SiO ₂ layer
Since the F value can be increased, the thickness of the SiO ₂ protective layer can be reduced, and k ^{2 of the} entire device can be improved.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments.

(Embodiment 1) FIG. 1 is a view showing a first embodiment of a surface acoustic wave device according to the present invention.

The substrate temperature was adjusted to 60 by laser ablation using a ZnO target to which 3 mol% of Li was added.
At 0 ° C. and an oxygen partial pressure of 3 × 10 ⁻³ Torr, a 2 μm ZnO piezoelectric layer 3 was deposited on the Si substrate 1. At this time,
The SiO ₂ oxide film layer 2 is formed between the Si substrate 1 and the ZnO piezoelectric layer 3 by thermal oxidation. However, substrate temperature,
The oxygen partial pressure is not limited to this, and the amount of Li added is not limited to this.

Next, after depositing metal aluminum, a continuous process of resist coating, exposure, dry etching, and patterning by removing the resist is performed to form a ZnO piezoelectric layer 3.
IDT electrodes 4 and 5 were formed thereon.

When the obtained surface acoustic wave device is expressed in pseudo cubic index, the (001) ZnO /
(100) Si, [100] ZnO // [10
0] It was an oriented film of Si. With respect to the obtained surface acoustic wave device, the sound velocity determined from the delay time V _open of the surface acoustic wave between the IDT electrodes 4 and 5 was 4000 m / s. I
When calculated from the difference from the delay time V _short of the surface acoustic wave when the space between the DT electrodes 4 and 5 is covered with a metal thin film, k ² is 0.0
It became 1. When using a ZnO target without added Li, since the sound velocity is 4000 m / s, k ² is 0.001, it is clear that improved piezoelectric properties by addition of Li.

Similar effects were obtained when ZnO to which Cu or Ag was added instead of Li was used for the piezoelectric layer. The same effect was obtained when the IDT electrode was formed under the piezoelectric layer. As described above, Li, C
By depositing a (001) oriented ZnO piezoelectric thin film on a (100) Si substrate using a ZnO target to which either u or Ag is added, k ² can be improved.

(Embodiment 2) FIG. 2 is a view showing a surface acoustic wave device according to a second embodiment of the present invention.

The substrate temperature was adjusted to 60 by laser ablation using a ZnO target to which 3 mol% of Li was added.
Under a condition of 0 ° C. and an oxygen partial pressure of 3 × 10 ⁻³ Torr, a 2 μm ZnO piezoelectric layer 13 was deposited on the Si substrate 11. At this time, the SiO ₂ oxide film layer 12 is formed between the Si substrate 11 and the ZnO piezoelectric layer 13 by thermal oxidation. However,
The substrate temperature and the oxygen partial pressure are not limited to those described above.
The amount of addition is not limited to this.

Next, after vapor deposition of metal aluminum, a continuous process of resist coating, exposure, dry etching, and patterning by resist removal is performed to obtain a ZnO piezoelectric layer 13.
IDT electrodes 14 and 15 were formed thereon.

Finally, a ₂ μm SiO ₂ protective layer 16 was deposited by laser ablation using a Li ₂ O.Al ₂ O ₃ .4SiO ₂ target under the conditions of a substrate temperature of 25 ° C. and an oxygen partial pressure of 3 mTorr. However, the substrate temperature and the oxygen partial pressure are not limited to these.

When the obtained surface acoustic wave device is represented by a pseudo cubic index, the (001) ZnO /
(100) Si, [100] ZnO // [10
0] It was an oriented film of Si. Further, the SiO ₂ protective layer was amorphous.

For the obtained surface acoustic wave device, IDT
The speed of sound obtained from the delay time V _open of the surface acoustic wave between the electrodes 14 and 15 was 4000 m / s. IDT electrode 1
When calculated from the difference from the delay time V _short of the surface acoustic wave when the area between 4 and 15 was covered with a metal thin film, k ² was 0.01. TCF was -1 ppm / ° C at 25 ° C. This is 4000 m / s in sound velocity and k ² = as compared with the case of using a SiO ₂ target to which Li ₂ O or Al ₂ O ₃ is not added.
0.01 is the same, but the TCF is −10 pp at 25 ° C.
It was much smaller than the value of m / ° C.

Similar effects were obtained when ZnO to which Cu or Ag was added instead of Li was used for the piezoelectric layer. Further, instead of Li ₂ O of _{Li 2 O · Al 2 O 3} · 4SiO 2, MgO, containing one of Al ₂ O ₃ S
Similar effects were obtained when iO ₂ was used for the protective layer. The same effect was obtained when the IDT electrode was formed under the piezoelectric layer.

As described above, on a (100) Si substrate,
Temperature characteristics can be improved by depositing a ZnO piezoelectric thin film to which any of Li, Cu, and Ag are added, and further depositing a SiO ₂ protective layer containing Li ₂ O, MgO, and Al ₂ O _3. Becomes

[0033]

As described above, according to the surface acoustic wave device of the present invention, by depositing the Li-doped ZnO piezoelectric layer on the Si substrate, the piezoelectricity is improved and the value of k ² is increased. And Li ₂ O, MgO,
S containing at least one of Al ₂ O ₃ and having a positive TCF
By depositing an iO ₂ protective layer, the negative T
By canceling CF, the TCF of the entire element can be reduced to zero, and a communication device such as a high-frequency filter or an oscillator having low insertion loss and excellent temperature stability can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a surface acoustic wave device having a Li-doped ZnO piezoelectric layer according to an embodiment of the present invention.

FIG. 2 is a sectional view of a surface acoustic wave device having a Li-doped ZnO piezoelectric layer and a SiO ₂ protective layer containing Li ₂ O and Al ₂ O _{3 according} to an embodiment of the present invention.

[Explanation of symbols]

1. (100) Si substrate 2. SiO ₂ oxide film layer (001) ZnO piezoelectric layer IDT electrode 5. IDT electrode 11. (100) Si substrate 12. SiO ₂ oxide film layer 13. 13. (001) ZnO piezoelectric layer IDT electrode 15. IDT electrode 16. SiO ₂ protective layer

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hiroshi Miyazawa 3-3-5 Yamato, Suwa City, Nagano Prefecture Seiko Epson Corporation F-term (reference) 5J097 AA06 AA21 AA23 DD29 EE08 FF02 FF05 HA02 HA03 KK09

Claims (5)

[Claims] 1. A (100) silicon substrate, comprising: a buffer layer made of silicon dioxide on the silicon substrate; and a piezoelectric layer made of zinc oxide on the silicon dioxide buffer layer. A surface acoustic wave device comprising an interdigital electrode formed immediately below or directly above. 2. A method according to claim 1, wherein said zinc oxide piezoelectric layer has a part of Zn
2. The surface acoustic wave device according to claim 1, having a composition substituted by at least one of i, Cu, and Ag. 3. The zinc oxide piezoelectric layer is an epitaxial film having a c-axis oriented perpendicular to the silicon substrate and an a-axis oriented in a plane parallel to the silicon substrate. Item 3. A surface acoustic wave device according to item 1. 4. The surface acoustic wave device according to claim 1, further comprising a protective layer made of silicon dioxide on said zinc oxide piezoelectric layer. 5. The silicon dioxide protective layer according to claim 1, further comprising: SiO ₂ ;
Li ₂ O, MgO, surface acoustic wave device according to claim 4, characterized in that it comprises at least one of Al ₂ O _3.