JPH06104680A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To obtain the surface acoustic wave element in which a ZnO film with high quality is formed even on an IDT lower electrode and to obtain the surface acoustic wave element optimum to a surface acoustic wave convolver or the like with a high K<2> and high input efficiency by forming the element with a silicon substrate having a low resistance layer and an interdigital electrode(IDT) or the like placed to a position corresponding to the low resistance layer on the surface of a zinc oxide layer. CONSTITUTION:An oxide film 3 is formed on the surface of an n/n<+> epitaxial Si substrate 2 with thermal oxidation and a resist pattern 9 of an IDT lower electrode is formed on the oxide film 3. Then Phos<+> ions are injected by ion implantation. Then the resist pattern 9 is removed, after anneal processing, the oxide film 3 is removed. Thus, an n<+> layer of a low resistance is formed in the n-epitaxial Si layer of the substrate 2 to form the lower electrode. Then the thermal oxidation film is formed by applying thermal oxidation to the wafer. The very smooth surface is obtained in this stage. Then the oxide film on the rear side of the wafer is removed to form a substrate electrode 1. Then the ZnO layer 5 is depositted onto the substrate 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device,
In particular, surface acoustic waves (S
AW) Concerning an element suitable for a convolver or the like, the present invention relates to an improvement of a structure in which a lower electrode is provided on a portion of a semiconductor substrate surface facing an input comb-shaped electrode (hereinafter referred to as IDT).

[0002]

_2. Description of the Related Art A sectional view of a conventional SAW convolver having a ZnO / SiO ₂ / Si structure is shown in FIG. In the figure,
1 is a substrate electrode, 2 is an n / n ⁺ Si epitaxial substrate,
3 is a Si thermal oxide film, 5 is a ZnO film layer, 6 is an output electrode, 7
Reference numerals a and 7b are IDTs, and 4 is an IDT lower electrode composed of a metal thin film layer. The IDT lower electrode 4 is provided for the following two purposes. (I) A part of the input signal applied to the IDTs 7a and 7b is prevented from leaking to the n-Si layer of the substrate 2 by shielding with the IDT lower electrode 4. As a result, the noise of the output from the output electrode 6 can be reduced. (Ii) By introducing the IDT lower electrode 4, the electromechanical coupling constant k ² of the SAW convolver is theoretically increased, and the input efficiency of the convolver can be increased. This IDT
As the lower electrode 4, a metal film having a film thickness of several tens to several hundreds nm, for example, an Al thin film having good adhesion with SiO ₂ is often used.

[0003]

However, when an attempt is made to actually manufacture a convolver having such a structure, ZnO
When the film layer 5 is formed by, for example, a sputtering method, the surface of the IDT lower electrode 4 is not a smooth surface such as a SiO ₂ surface, so that the ZnO film layer 5 does not grow on the IDT lower electrode 4. As a result, many hillocks were generated, resulting in IDT7
ZnO film layer 5 under a and 7b has a poor crystallinity and a large k ²
There was a problem that could not be obtained.

It is an object of the present invention, even on an IDT bottom electrode,
By obtaining a high-quality ZnO film with few hillocks, k
^It is to obtain a surface acoustic wave device suitable for a SAW convolver or the like having a large ^{2 and} a high input efficiency.

[0005]

In order to achieve the above object, the surface acoustic wave of the present invention is formed on a silicon substrate having a low resistance layer formed at a predetermined position on the surface and on the silicon substrate. A silicon dioxide film layer, a zinc oxide film layer formed on the silicon dioxide film, and a comb-shaped electrode (IDT) provided at a position corresponding to the low resistance layer on the surface of the zinc oxide film layer. The point is to make it.

[0006]

Since the low resistance layer as the IDT lower electrode is provided in the silicon substrate, this portion has good smoothness, and a zinc oxide film of good film quality can be formed thereon.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a SAW convolver (d) as a surface acoustic wave device according to an embodiment of the present invention and manufacturing steps (a to d) thereof. In the figure, the same reference numerals as those in FIG. 2 indicate the same or similar portions, 8a and 8b are high-concentration impurity diffusion layers, and 9 is a photoresist pattern.

First, as shown in FIG. 1A, an oxide film 3 having a thickness of, for example, 50 nm is formed on the surface of an n / n ⁺ epitaxial Si substrate 2 by thermal oxidation, and a resist for an IDT lower electrode is formed by a photolithography process. Pattern 9 on this oxide film 3
Form on top. Next, by ion implantation, Phos ⁺ is implanted at 2 × 10 ¹⁵ cm ⁻² at an acceleration voltage of 180 keV, for example.

Next, as shown in FIG. 1B, the resist pattern 9 is removed, an annealing process is performed, and then the oxide film 3 is entirely removed. As a result, the n-epitaxial S of the substrate 2 is
A low resistance n ⁺ layer is formed in the i layer to form a lower electrode.

Next, as shown in FIG. 1C, the wafer shown in FIG. 1B is thermally oxidized to form a thermal oxide film 3'having a thickness of 100 nm on the surface. A very smooth surface is obtained at this stage. Then, the oxide film on the back surface of the wafer is removed to form the substrate electrode 1.

Next, as shown in FIG. 1D, Z is formed on this substrate.
The nO film layer 5 is deposited by sputtering, for example, to a thickness of ˜5 μm. After that, the input / output electrodes 6, 7a, 7b on the surface are formed.

Although the case where the IDT lower electrode is an n ⁺ -Si layer is described here as an example, for example,
It is also possible to diffuse B ⁺ or the like to form a p ⁺ -Si layer, and a low resistance layer IDT lower electrode composed of a high concentration impurity diffusion layer may be provided.

Thus, Si is used as the IDT lower electrode.
Since the high-concentration impurity diffusion layer is provided in the epitaxial layer n, the smoothness of the substrate surface before the formation of the ZnO film layer 5 is remarkably improved, and as a result, ZnO having a good film quality which is the same as other regions on the IDT lower electrode is also obtained. The film layer can be formed, the k ^{2 of} the IDT section is increased, and the input efficiency of the convolver is improved.

[0014]

As described above, according to the present invention, I
A good quality ZnO film can be formed on the DT lower electrode, and a surface acoustic wave device having a large k ^{2 and} a high input efficiency can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a SAW convolver according to the present invention and a manufacturing process thereof.

FIG. 2 is a schematic diagram showing a conventional SAW convolver.

[Explanation of symbols]

1 substrate electrode 2 n / n ⁺ Si epitaxial substrate 3 Si oxide film 4 IDT lower electrode (metal thin film layer) 5 ZnO film layer 6 output electrode 7 input electrode 8 IDT lower electrode (high concentration impurity diffusion layer) 9 photoresist pattern

Claims (4)

[Claims] 1. A silicon substrate having a low resistance layer formed at a predetermined position on the surface, a silicon dioxide film layer formed on the silicon substrate, and zinc oxide formed on the silicon dioxide film. A surface acoustic wave device comprising: a film layer; and a comb-shaped electrode provided at a position corresponding to the low resistance layer on the surface of the zinc oxide film layer. 2. The surface acoustic wave device according to claim 1, wherein the low resistance layer comprises a high concentration impurity diffusion layer. 3. The surface acoustic wave according to claim 2, wherein the silicon substrate is an n / n ⁺ Si epitaxial substrate, and the high-concentration impurity diffusion layer is formed in the Si epitaxial layer. element. 4. The surface acoustic wave device according to claim 1, wherein a pair of the resistance layer and the comb-shaped electrode are provided, and an output electrode is provided on the surface of the zinc oxide film layer between them.