JPS5864815A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To decrease the propagation loss at high frequencies, by constituting in such a way that a sapphire substrate, a silicon single crystal layer, aluminum nitride single crystal expitaxial layer and electrodes are formed in the order. CONSTITUTION:A sapphire substrate 1 is normally cut at a plane (R plane) equivalent to the (0112) crystal plane, a silicon single crystal layer 1' consists of a plane (R plane) equivalent to the (001) crystal plane, T in the film thickness and is formed on the sapphire substrate 1. An aluminum nitride single crystal epitaxial layer 2 is formed on the silicon single crystal layer 1' and its piezoelectric axis is formed in parallel with the plane of the sapphire substrate 1. Comb type surface acoustic wave generating and detecting electrodes 3, 4 are formed of the surface of the aluminum nitride single crystal epitaxial layer 2. Thus, the propagation loss in high frequencies can be decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave element with a new structure and excellent characteristics.

Elastic surface tEL (S surface Acoust 1c
Conventionally, the structures (substrates) constituting elastic cloth surface wave elements for handling various electrical signals are: 1. Structures consisting only of piezoelectric substrates (piezoelectric single-crystal substrates, 2. Structure 3 in which a piezoelectric film is formed on a non-piezoelectric substrate, structure 3 in which a piezoelectric film is formed on a semiconductor substrate, etc. are known.

Among these structures, the monolithic structure (3), in which a surface acoustic wave element is formed together with an integrated circuit on the same semiconductor substrate, is advantageous in terms of applications and is expected to be further developed in the future.

By the way, 3 above. At present, the monolithic structure is silicon single crystal (hereinafter SO8) formed on a silicon (Si) single crystal substrate or a sapphire substrate.
= 5iliconOnSapphire) by sputtering method etc. l111: Zinc erioxide film (Zn
A structure in which ZnO) is formed is known, but this ZnO film has the following drawbacks, which poses problems for practical use.

1. Voltage application (electrical instability occurs).

2. Because it is difficult to form a high-quality film, it is difficult to obtain a film with sufficient reproducibility in terms of resistivity, piezoelectricity, etc.

3. A protective film (5iO2) is required on the silicon single crystal layer.

4. Surface acoustic wave propagation loss is large in the high frequency region.

5. Large distribution in surface acoustic wave propagation characteristics.

6. Not consistent with normal silicon IC process.

The present invention has been made to address these problems, and its fundamental feature is to use an elastic body structure (substrate) in which an aluminum nitride single crystal epitaxial layer is formed on an SO8 substrate. 001) It is an object of the present invention to provide a specific surface acoustic wave device using a silicon single crystal layer consisting of crystal planes. Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a cross-sectional view showing a surface acoustic wave device according to an embodiment of the present invention, in which 1 is a sapphire substrate cut in a plane (R plane) equivalent to the normal (0112) crystal plane, and 1' is a sapphire substrate. A silicon single-crystal layer 2 formed on the silicon single-crystal layer 1 and having a thickness T and consisting of a plane equivalent to the normal (100) plane;
) A single crystal evixial layer is formed so that its piezoelectric axis is parallel to the surface of the sapphire substrate. 3.4 is a comb-shaped surface acoustic wave generation electrode and detection electrode formed on the surface of the aluminum nitride single crystal epitaxial layer 2, and H is the film thickness of the aluminum nitride single crystal epitaxial layer 2.

FIG. 2 (al, (bl U) is a sectional view showing another embodiment of the present invention, in which (a) a surface acoustic wave generation electrode 3 and a detection electrode 4 are formed on the surface of a silicon single crystal substrate 1'. After that, an aluminum nitride single crystal epitaxial layer 2 is formed to cover these. This shows a structure in which a comb-shaped surface acoustic wave generation electrode 5 and a machining detection electrode 6 made of low resistance silicon are embedded in the layer, and then an aluminum nitride single crystal epitaxial layer 2 is formed to cover them. It is.

FIG. 3 (al) and (b) show other embodiments of the present invention, in which (al) a pair of flexible electrodes 8 are formed partially on the surface of the silicon single crystal substrate 1 as second electrodes. After that, an aluminum nitride single crystal epitaxial layer 2 is formed to cover these, and a surface acoustic wave detection electrode 3 and a roughness detection electrode 4 are formed on this surface as a first electrode. After forming a low-resistance silicon layer 9 partially on the surface of the silicon/single crystal substrate 1', a silicon nitride single crystal epitaxial layer 2 is formed to cover the silicon nitride single crystal epitaxial layer 2, and the generation electrode is formed on this surface as a first electrode. 3 and a structure in which a detection electrode 4 is formed.

Figure 4 (al, (bl) shows other embodiments of the present invention, (a) shows a surface acoustic wave generating electrode 3 used as a first electrode on the surface of a silicon single crystal substrate 1' for detecting machining. After forming the electrodes 4, an aluminum nitride single crystal epitaxial layer 2 is formed to cover them, and a pair of flexible electrodes 8 are partially formed on the surface thereof as second electrodes, (b) shows a structure. A high resistance layer 7 or a depletion layer is partially formed on the surface of the silicon single crystal substrate 1', and the generation electrode 5 as a hollow-shaped first electrode made of low resistance silicon is formed on these layers. After the detection electrodes 6 are embedded, an aluminum nitride single crystal epitaxial layer 2 is formed to cover them, and a pair of shielding electrodes 8 are formed as second electrodes on the surface of the aluminum nitride single crystal epitaxial layer 2.

1 to 4 (al, (bl), a surface acoustic wave is excited in a direction equivalent to the piezoelectric axis direction of the aluminum nitride single crystal epitaxial film 2. When propagation), the surface acoustic wave velocity dispersion characteristics as shown in Figure 5 were obtained.In the figure, the horizontal axis represents the standardized thickness H of the aluminum nitride single crystal epitaxial film 2. is expressed as 2πh/λ (here, λ is the wavelength of the surface acoustic wave), and the vertical axis indicates the phase velocity Vp of the surface acoustic wave.

Further, '1=-2πT/λ indicates the normalized thickness T of the silicon single crystal layer 1', and is expressed as a parameter.

As is clear from the figure, two modes, the first mode A and the second mode B, appear in the velocity dispersion characteristic, and when the above parameter KT is increased in the range of 0 to 1.2, the first mode
In mode A, the phase velocity shows a minimum value near 2.0 of the normalized film thickness 2πH/λ. In addition, when 2πH/λ is in the range of 0.5 to 5.0, KT is 0 compared to the structure where KT=Q (that is, the structure in which the aluminum nitride single crystal epitaxial layer 2 is directly formed on the sapphire substrate 1). As it increases from .5 to 0.8 to 1,0 to 1.2, the dispersion of the phase velocity Vp decreases, and 0.8 or more is particularly effective.

, Figure 6 shows the characteristic curve of the electromechanical coupling coefficient,
The horizontal axis shows 2πH/λ as in FIG. 5, and the vertical axis shows the square value of the electromechanical coupling coefficient 2 as a percentage.

In the same figure, A, Hl, C4I) are 1 respectively in Fig. 1,
Figure 2 (al, (b), Figure 3 (al, (b),! 4
It corresponds to the structures in figures (a) and (b), and K T =
The characteristics are shown at 1.2 vr-. 2πH/
Desirable K is obtained when λ is in the range of 0.5 to 5o,
This shows that it has excellent piezoelectricity. The characteristics shown in FIG. 6 are for KT=1.2, but no difference was observed in the range of KT from 0 to 1.2.

By forming an aluminum nitride single crystal epitaxial layer on the 808 substrate as in the structure of the above embodiment, it is possible to reduce the frequency dispersion of the surface acoustic wave velocity and to improve the piezoelectricity. In other words, in a surface acoustic wave (SAW) wideband signal processing device, when the frequency dispersion of the SAW velocity becomes large, waveform distortion accompanying signal propagation becomes impossible to ignore and device operation deteriorates.
By adopting the structure of the above embodiment, KH is 0.5~
5.0 and KT in the range of θ to 1.2, waveform distortion can be kept small and piezoelectricity can be improved. Examples of the above-mentioned wideband signal processing elements include slow ttJJ, filters, convolvers, correlators, Fourier transformers, parametric interaction elements, etc., but the application is not limited to these, but can also be applied to narrowband signal processing elements such as cooperators and oscillators. Similar effects can be obtained by using the same method, and since it becomes easier to manufacture elements that match the designed operating frequency, yields can be improved and costs can be reduced.

FIG. 7 shows the characteristic of the square f[K of the electromechanical coupling coefficient in the case of the second mode B. As is clear from the figure, the value is small and the piezoelectricity is small, but as is clear from Fig. 5, the SAW sound velocity is faster than in the first mode A, and the dispersion of the sound velocity is also small. It is sufficiently practical if applied to an element that does not require piezoelectricity, such as a resonator.

Furthermore, since the sound velocity is higher than in the first mode A, there is also the advantage that the electrode pattern can be easily processed.

Aluminum nitride films have inherently better insulating properties and electrical stability than conventional zinc oxide films, and
Since it is harmless to the process, it can be formed on the same substrate as the semiconductor integrated circuit. In this case MO-CVD
It can be easily formed using technology, and M
O-CVD technology is compatible with semiconductor IC processes. Furthermore, since the aluminum nitride film has uniform film quality, it is possible to suppress propagation loss at high frequencies.

According to the present invention explained above, the following effects can be obtained.

1. Since an epitaxial aluminum nitride film is used, the film quality is uniform and the propagation loss at high frequencies is small.

2. Since the surface acoustic wave has a high sound velocity, the wavelength at high frequencies becomes large, making it easier to manufacture square-shaped electrodes and the like.

3. Since the frequency dispersion of surface acoustic wave sound speed is kept small, waveform distortion accompanying signal propagation is reduced.

4. A monolithic structure is possible in which integrated circuits and surface acoustic wave devices are formed on a common semiconductor substrate.

5. Aluminum nitride film has a large band gap f) i of about 66.2 eV and a resistivity of 10 Ωm or more, so it is electrically stable and can be easily formed using MO-CVD technology. This makes it compatible with silicon IC processes.

As described above, the structure according to the present invention can be applied to a wide range of uses, especially since a surface acoustic wave element and a semiconductor integrated circuit can be formed on the same semiconductor substrate.

[Brief explanation of the drawing]

Figure 1, Figure 2 (a), (bl, Figure 3 (al, (t))
l-! Fig. 4 (al, b) are all cross-sectional views showing examples of the present invention, and Figs. 5 to 7 are characteristic diagrams showing the results obtained by the present invention. 1... Sapphire Substrate, 1'...Silicon single crystal layer, 2...Aluminum nitride single crystal epitaxial layer,
3.4.5.6... Comb-shaped electrode, 7... High resistance silicon layer, 8... Shiyahei electrode, 9... Low resistance silicon layer. Patent applicant Noriyoshi Mikoshiba Kazuo Otsubochi Agent Patent attorney Nagai 1) Takesanbu-〇2 [H/8-2 in H/Yuashi 7 Figure 0123456 2EH1 in → Procedural amendment (side 1, case Showa % Patent Application No. 163793 2 Name of the invention Surface acoustic wave device 3 Relationship with the person making the amendment Patent applicant 4, agent 105 Address 3-2-14 Shiba, Minato-ku, Tokyo Shiba 3-chome pill February 1981 Tabi (Shipping - Katsuichi)

Claims (1)

[Claims] 1. A sapphire substrate, a silicon single-crystal layer formed thereon, an aluminum nitride single-crystal epitaxial layer formed on the silicon single-crystal layer and having a piezoelectric axis oriented; What is claimed is: 1. A surface acoustic wave element comprising: electrodes formed at positions. 2. A patent characterized in that the silicon single crystal layer is formed of a (001) crystal plane or a plane equivalent thereto, and the piezoelectric axis of the aluminum nitride single crystal epitaxial layer is parallel to the silicon single crystal layer. A surface acoustic wave device according to claim 1. 3. The surface acoustic wave device according to claim 2, wherein a surface acoustic wave is propagated in a direction equivalent to the piezoelectric axis of the aluminum nitride single crystal epitaxial layer. 4. The film thickness H of the aluminum nitride single crystal epitaxial layer is 0.5 (2πH/λ(5,0 (however, λ
The surface acoustic wave element according to claim 2 or 3, wherein the surface acoustic wave element belongs to a range of 1. 5. The thickness T of the silicon single crystal layer is 2πT/λ(
The surface acoustic wave device according to any one of claims 2 to 4, wherein the surface acoustic wave device belongs to the range of 1 and 2 (where λ indicates the wavelength of the elastic waveform). 7. The surface acoustic wave device according to any one of claims 1 to 6, wherein the electrode is formed on the surface of an aluminum nitride single crystal epitaxial layer. 8. The surface acoustic wave device according to any one of claims 1 to 6, wherein the electrode is formed between a silicon single crystal layer and an aluminum nitride single crystal epitaxial layer. 9. The electrodes are formed as a pair of first ift electrodes on the surface of the aluminum nitride single crystal epitaxial layer, and a pair of thin electrodes are formed as second electrodes between the silicon single crystal layer and the aluminum nitride single crystal epitaxial layer. A surface acoustic wave element according to any one of claims 1 to 6, characterized in that the surface acoustic wave element is formed. 10. The electrodes are formed as a pair of first electrodes between the silicon single crystal layer and the aluminum nitride single crystal epitaxial layer, and a pair of thin electrodes are formed as second electrodes on the surface of the aluminum nitride single crystal epitaxial layer. Claims 1 to 6 characterized in that
3. The surface acoustic wave device according to any one of the above.