GB2206256A

GB2206256A - Surface acoustic wave device

Info

Publication number: GB2206256A
Application number: GB08811346A
Authority: GB
Inventors: Kazuyoshi Sugai
Original assignee: Clarion Co Ltd
Current assignee: Faurecia Clarion Electronics Co Ltd
Priority date: 1987-05-26
Filing date: 1988-05-13
Publication date: 1988-12-29
Also published as: JPS63294009A; FR2616021A1; GB8811346D0; DE3817728A1

Description

2'/"-'06250' 1 SURFACE ACOUSTIC WAVE DEVICE

FIELD OF THE INVENTION

This invention relates to an improvement of the surface acoustic wave device having a monolithic structure, in which a piezoelectric thin film is formed on a semiconductor substrate.

BACKGROUND OF THE INVENTION

Heretofore, as surface acoustic wave elements, there are known those having a structure, in which a piezoelectric thin film is formed on a semiconductor substrate. The surface acoustic wave elements having this structure are very important in many fields of use owing to their monolithic structure, in which a surface acoustic wave element and a peripheral active circuit therefor can be integrated on a common semiconductor substrate.

Similarly to the surface acoustic wave elements constituted by a piezoelectric monocrystal or a piezoelectric ceramic. it is required also for the surface acoustic wave elements, such as a filter, a resonator, an oscillator, etc., constituted by means of a structure of piezoelectric thin film/semiconductor substrate described above that the velocity and the central frequency of the surface acoustic wave as well as the delay time# etc. do not vary and are stable with respect to temperature variations. For the surface acoustic wave elements constituted by a piezoelectric monocrystal d' 1 2 or a piezoelectric ceramic, it is possible to realize a surface acoustic wave element having excellent temperature characteristics, for example, by selecting a substrate material such as ST cut crystallized quartz, showing good temperature characteristics, a crystallographical surface and a propagation direction.

Contrarily to the surface acoustic elements constituted by a piezoelectric monocrystal or a piezoelectric ceramic, for the monolithic structure constituted by a semiconductor substrate, since the substrate material is limited to semiconductor materials such as silicon (Si), gallium arsenide (GaAs), SOS (Silicon on Sapphire), it is generally impossible to realize satisfactory temperature characteristics only by selecting the uraterial, the crystallographical surface and the direction.

For this reason, heretoforey the temperature characteristics of an element have been compensated by forming it with composite materials including a glass layer such as a silicon oxide film relatively thick and the piezoelectric thin film. This is a technique for compensating temperature variations to reduce.the resultant temperature coefficient of the element due to a cancellation effect on the temperature coefficients obtained by using a thin film having a temperature coefficient, whose sign is opposite to that of the substrate material therein.

By the technique for compensating temperature variations described above attention is paid only to the effect obtained principally by the temperature coefficient of the thin film and as the technique for compensating temperature variations for the semiconductor substrate.

k 3 it is only effected there to select the materialf the crystallographical surface and the propagation direction so as to obtain temperature characteristics as good as possible, although this selection unsatisfactory and is done in narrow region. Furthermore, in effect, the selection of the substrate material is generally based on the field of use, the function, etc. in preference to the temperature characteristics.

OBJECT OF THE INVENTION The object of this invention is to provide a surface acoustic wave device capable of exhibiting excellent temperature characteristics by reducing the temperature coefficient of the material of the semicon- ductor substrate is made itself.

SUMMARY OF THE INVENTION

In order to achieve the above object, according to this invention, it is intended to resolve the problem stated above by introducing dopant at a high density in the semiconductor substrate.

Due to introduction of dopant at.high density in the semiconductor substrate, as described above, the temperature coefficient thereof can be reduced and then the temperature characteristics as a surface acoustic wave device is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a longitudinal cross-sectional view of a surface acoustic wave device, which is an embodiment 4 of this invention; Figure 2 is a longitudinal cross-sectional view of a surface acoustic wave device, which is another embodiment of this invention; and Figures 3 to 5 are graphs showing temperature characteristics of surface acoustic wave devices.

DETAILED DESCRIPTION

Figures 1 and 2 show two different embodiments of this invention, each of which is constituted by a silicon substrate doped at a high density.

In Figure 1 reference numeral 1 is a monocrystal silicon substrate; 2 is a low resistivity silicon layer formed thereon; 3 is a piezoelectric thin layer, such as ZnO, A1N, etc., formed on the low resistivity silicon layer; and 4 and 5 are interdigital electrodes (hereinbelow abbreviated to IDTY-formed on the piezoelectric thin layer for generating and detecting surface acoustic waves. The low resistivity silicon layer 2 stated above is the high density dopant layer. This low resistivity silicon layer 2, which is the high density dopant layer, can be formed by subjecting diffusion or ion-implantation method to the surface of the monocrystal silicon substrate 1 or by forming a silicon film doped at a high density on the monocrystal silicon substrate 1 due to use of the WD method.

In Figure 2 reference numeral 6 is a low resistivity monocrystal silicon substrater on which a piezoelectric thin layer 3 is formed and interdigital electrodes 4 and 5 are disposed further thereon. The low resistivity 1 1 monocrystal silicon substrate is the high density dopant layer.

The dopants described above are mainly phosphor (P)P arsen (As) and antimony (SW, which are n-conductivity type dopants and boron (B), which is a p-conductivity type dopant.

- In the structure of the surface acoustic wave devices indicated in Figures 1 and 2r the low resistivity silicon layer 2 doped at high densityr and the low resistivity monocrystal silicon substrate 6 have the effect to reduce the temperature coefficient of the surface acoustic wave elements.

As other embodimentst expecting both the passivation effect and the temperature coefficient reducing effect, there may be disposed glass layers such as silicon dioxide films at the interface between the low resistivity silicon layer 2 and the piezoelectric thin layer 3 and on the piezoelectric thin layer 3 in the structure indicated in Figure 1, and at the interface between the low resistivity monocrystal silicon substrate 6 and the piezoelectric thin layer 3 and on the piezoelectric thin layer 3 in the structure indicated in Figure 2.

Figures- 3 and 4 are graphs showing temperature characteristics of surface acoustic wave devices having the structure indicated in Figure 1, in which the abscissa represents the temperature T and the ordinate indicates frequency variations F.

In the graphst the broken lines indicate temperature characteristics of prior art surface acoustic wave

6 devices, in which piezoelectric thin films are formed on n-conductivity type monocrystal silicon substrates (dopant: Sb) of low relistivity of 1811000 2cm, which the full lines indicate temperature characteristics of surface acoustic wave devices according to this invention, in which low resistivity layers are disposed between piezoelectric thin layers and monocrystal silicon substrates by diffusing phosphor (P) as the dopant through the surface of the monocrystal silicon substrates described above at a high density.

The piezoelectric thin film in the structure described above is made of ZnO in the surface acoustic wave device for Figure 3, while it is made of A1N in the surface acoustic wave device for Figure 4, both of them being as thin as 500 nm.

The IDTs for generating and detecting the surface acoustic wave are formed on the piezoelectric thin film and the wavelength of the surface acoustic wave is 14 pm. The central frequency of the surface acoustic wave is about 350 MHz in both the cases.

The dopant density in the low resistivity silicon layer is 1.0 x 1020CM-3 at the surface and the depth of the diffusion layer is about 10 pm.

In the graphs indicated in Figures 3 and 4, the difference in the structure between the surface acoustic wave elements, whose characteristics are indicated by the full lines and the broken lines consists only in the presence or absence of the low resistivity layer doped with phosphor (P) at a high density. In either case of the surface acoustic wave elements having the character- 1 7 istics indicated in Figures 3 and 40 it can be understood that the temperature characteristics of an element having a low.resistivity layer are improved by about 57ppm/C and thus that the low resistivity layer improves the temperature characteristics of the element.

Figure 5 is a graph showing the temperature characteristics of a surface acoustic element having the structure indicated in Figure 2. The substrate is a p-conductivity type monocrystal silicon substrate (dopant:B) having a resistivity of 18/1000 2cm. The piezoelectric thin layer is made of A1N and 500 nm thick. The wavelength of the surface acoustic wave is 14 pm and the central frequency thereof is about 350 MHz. The dopant density of the low resistivity silicon monocrystal is 5 x 10 18 cm- 3 and it is uniform in the whole bulk.

As shown by the graph indicated in Figure 5, it can be understood that the presence of the low resistivity monocrystal silicon substrate has an effect to improve remarkably the temperature. characteristics of the surface acoustic wave element. Further measurement results show that. as the dopant, boron (B) has an extremely significant effect to improve the temperature characteristics.

As described above. it is possible to improve the temperature characteristics of the element by building a low resistivity silicon layer, in which the dopant is introduced at a high density in the surface acoustic wave element. The compensation effect of the temperature coefficient increases with increasing dopant density and is specifically remarkable when the density exceeds 18 -3 10 cm 8 Furthert the temperature variation compensation technique according to this invention can be applied also to electro-acoustic elements consisting of surface acoustic wave elements and which utilize interaction between carriers in semiconductor and the surface wave.

When a high resistivity silicon epitaxial layer having an dopant density suitable for a specified object is disposed on the low resistivity silicon layer or the low resistivity monocnystal silicon substrate described previously, it is possible to improve the temperature characteristics by means of the low resistivity layer and at the same time to realize necessary electric functions-by means of the high resistivity layer.

As explained above, according to this invention, it is possible to realize a surface acoustic wave device having excellent temperature characteristics by introducing dopants in the semiconductor substrate at a high density so that the temperature coefficient of the material itself of-the semiconductor substrate is reduced and or that the sign thereof is inversed at each other.

1 -Q l 7 9

Claims

Claims

A surface acoustic wave device comprising: a semiconductor substrate doped with dopants at a high density; a piezoelectric thin layer disposed thereon; and electrodes for generating and detecting a surface acoustic wave, which are disposed on said piezoelectric thin layer.
2. A surface acoustic wave device according to Claim 1, wherein said semiconductor substrate is constituted by a monocrystal silicon substrate and a low resistivity silicon layer disposed thereon, in which dopants are introduced at a high density.
3. A surface acoustic wave device according to Claim 1, wherein said semiconductor substrate is constituted by a low resistivity monocrystal silicon substrater in which dopants are introduced at a high density.
4. A surface acoustic wave device according to either one of Claims 1 to 3, wherein the dopant density is -3 of said semiconductor substrate is greater than 10 cm Published 1988 at The Patent 0Mce, State House, 66171 High Holborn. London WC1R 4TP. Further copies may be obtained from The Patent Mce, SAles Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Gray, Kent. Con. 1187.