GB2167257A

GB2167257A - Surface acoustic wave device

Info

Publication number: GB2167257A
Application number: GB08525365A
Authority: GB
Inventors: Syuichi Mitsutsuka; Shoichi Minagawa
Original assignee: Clarion Co Ltd
Current assignee: Faurecia Clarion Electronics Co Ltd
Priority date: 1984-10-15
Filing date: 1985-10-15
Publication date: 1986-05-21
Also published as: NL8502807A; GB2167257B; DE3536704C2; FR2571907B1; GB8525365D0; FR2571907A1; DE3536704A1

Abstract

A surface acoustic wave device comprising a semi-conductor substrate 7, a piezoelectric film 8 disposed on the semi-conductor substrate, an input transducer 2, and an output transducer 3, includes a plurality of metal electrodes 10A, 10B, 10C, 10D disposed between said input and output electrodes, means for applying bias voltages to each of said metal electrodes, and means for regulating said bias voltages. The device may constitute a plurality of filters having different band widths. Alternatively, the device may constitute a variable delay line. <IMAGE>

Description

SPECIFICATION Surface acoustic wave device This invention relates to a surface acoustic wave (hereinbelow abbreviated to SAW) device comprising a semi-conductor substrate, a piezoelectric film disposed on the substrate and at least a pair of in/output transducers disposed adjacent opposed extremities of the substrate.

SAW filters are widely utilized as IF filters in television receivers, and as filters in various kinds of communication apparatus, but usually as fixed band width filters.

Fig. 7 of the drawings is a perspective view of a known SAW filter, in which reference number 1 indicates a piezoelectric substrate; 2' indicates comb-shaped electrodes constituting an input transducer 2; and 3' indicates comb-shaped electrodes constituting an output transducer 3.

Since the band width of such a filter is determined by the form and the number of pairs of the comb-shaped electrodes, it is a constant for the SAW filter and it is not possible to vary it. However, variable band width filters are strongly desired for communication apparatus, in which frequency band width varies with time, and various sorts of multi-channel communication apparatus.

Fig. 8 (A) is a top view of a known variable band width SAW filter, in which corresponding items are indicated by the same reference numerals as those in Fig. 7; 4 represents a switching circuit; 5 represents an input terminal; and 6 represents an output terminal. In the SAW filter indicated in Fig. 8(A) a plurality of SAW filters are mounted on a substrate so that the band widths of the filters are adjacent to each other, and one of the bands is selected by switching an external circuit 4. Fig.

8(B) illustrates the relationship between the frequency and the output, when the switch in the switching circuit 4 is at positions A, B, C, D and E in turn.

For such a prior art filter an external circuit 4 for switching is essential, this being disadvantageous with respect to the manufacturing cost and space saving. In addition, it has the disadvantage that the freedom for the shape of the pass band is small, because it is controlled only by switching on/off.

SAW delay lines are used for signal processing in radar devices, and as delay lines in SAW oscillators. In particular, development of a device the delay time of which can be varied, is desired, because such a device can be used in a variable frequency oscillator and in a ghost canceler for a television receiver.

Figs. 20 and 21 illustrate known devices used as SAW variable delay lines, in which the reference numeral 1 represents a piezoelectric substrate; 2' indicates comb-shaped electrodes constituting the input transducer 2; 3A, 3B, ... are comb-shaped electrodes constituting output transducers 3; 4 is a switching circuit; 5 is the input terminal; and 6 is the output terminal.

Fig. 20 illustrates a so-called delay line with taps, for which the area of the substrate may be small, but which has a disadvantage that interferences between different taps are produced due to reflection of waves at each of the taps (output comb-shaped electrodes 3A, 3B ). On the other hand, for the device illustrated in Fig. 21, although interferences between the output comb-shaped electrodes 3A, 3B, . . are small, it has a disadvantage that a large piezoelectric substrate 1 is necessary. Further, for both the devices an external circuit 4 for switching is necessary, and thus they are disadvantageous with respect to manufacturing cost and space saving.

According to this invention there is provided a surface acoustic wave device comprising a semi-conductor substrate; a piezoelectric film disposed on said semi-conductor substrate; an input transducer and an output transducer disposed adjacent opposed extremities on said piezoelectric film; a plurality of metal electrodes disposed between said input transducer and said output transducer; a bias voltage applying means for applying bias voltage to each of said metal electrodes; and a bias voltage regulating means for regulating said bias voltages.

The part where the metal electrodes are disposed has the structure of a so-called monolithic MIS (Metal/lnsulator/Semi-conductor). A surface acoustic wave propagating in such a structure varies considerably in its propagation loss, depending on the bias voltages applied between the metal electrodes and the semiconductor.

Fig. 9 of the drawings illustrates the relationship between the propagation loss and the bias voltage at various temperatures. As indicated, the propagation loss increases rapidly in a certain voltage domain. The domain where the vaive is attenuated rapidly corresponds to a voltage domain where the surface of the semi-conductor (interface piezoelectric substance/semi-conductor body) is strongly inverted.

Fig. 10 illustrates a comparison of a C-V characteristic (capacity-voltage characteristic) curve (curve b) with the propagation loss (curve a). As can be seen, the propagation loss increases rapidly when the semi-conductor surface is strongly inverted (domain at the left side of the broken line).

Therefore, by applying a great bias voltage producing a strongly inverted domain to the metal electrodes, it is possible to cut off the wave at this domain. Further, since it acts as a variable attenuator depending on the bias voltage, when the applied bias voltage is not so great, it is possible to obtain band characteristics having a large freedom, and to vary continuously the propagation loss of the wave by regulating the bias voltages applied to each of the metal electrodes.

This invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a perspective view of a variable band width SAW filter according to this invention; Figure 2 is a diagram showing the relationship between output and frequency in the filter of Fig. 1; Figure 3 is a top view of an SAW filter according to this invention; Figures 4, 5 and 6 are partial top views of three further SAW filters according to this invention; Figure 7 is a perspective view of a fixed band width prior art SAW filter; Figure 8A is a top view of a variable band width prior art SAW filter, and Fig. 8B a diagram illustrating the relationship between the output and frequency in the filter of Fig. 8A;; Figure 9 shows graphs illustrating the relationship between the propagation loss and the bias voltage at various temperatures in a device according to this invention; Figure 10 shows graphs illustrating the relationship between the bias volage and the propagation loss and the high frequency capacitance in a device according to this invention; Figure 11 is a perspective view illustrating a variable SAW delay line according to this invention; Figure 12 is a top view illustrating a variable SAW delay line according to this invention; Figures 13 to 16 are partial top views of four variable SAW delay lines according to this invention; Figures 17 to 19 are cross-sectional views of three variable SAW delay lines according to this invention; and Figures 20 and 21 are top views illustrating two prior art variable SAW delay lines.

Fig. 1 is a perspective view illustrating an embodiment of a variable band width SAW filter according to this invention. A semi-conductor substrate 7 and a piezoelectric film 8 disposed thereon constitute a piezoelectric substrate 1. On the other surface, which is opposite to the piezoelectric film 8, of the semi-conductor substrate 7 is disposed a back side electrode 9, which is earthed. At one end of the surface of the piezoelectric substrate 1 are disposed comb-shaped electrodes 2' constituting an input transducer 2 and at the other end comb-shaped electrodes 3A, 3B, 3C, 3D constituting output transducers 3, which are so formed that each of them responds only to SAW within an individual band width, which differ from each other. All the output comb-shaped electrodes 3A, 3B, 3C, 3D are connected to the output terminal 6.

Between the input comb-shaped electrodes 2' and the output comb-shaped electrodes 3A, 3B, 3C, 3D are disposed metal electrodes 10A, 10B, 10C, 10D. For each of the filters, one of bias voltages VA, VB, Vc, VD is applied between the metal electrode and the back side electrode 9. The band width of the whole filter is varied by controlling each bias voltage.

When the bias voltages VA, VB, Vc, VD applied to the metal electrodes 10A, 10B, 10C, 10D are varied, the propagation loss due to the insertion of each of the filters varies, and therefore the whole pass band width can be set differently by approximately combining the bias voltages.

Fig. 2 shows variations of the propagation loss due to the insertion of the filters in bands A, B, C, D, as a function of frequency for different bias voltages. The bias voltages can be set arbitrarily so that an output can be obtained e.g. as indicated by the full line.

In the device shown in Fig. 1, the output transducer 3 is divided into a plurality cf pairs of comb-shaped electrodes so that the whole output power is taken out as the sum of the outputs in parallel.

Otherwise, a structure as shown in Fig. 3 is possible.

On the back side surface of the device shown in Fig. 3 is disposed a back side electrode which is earthed, just as that described in the preceding embodiment, and the bias voltages VA, VB, Vc, VD and VE are applied to the metal electrodes 10A, 10B, 10C and 10D and 10E respectively in the same way as they are for the preceding embodiment. The difference between the device shown in Fig. 3 and that shown in Fig. 1 consists only in that the output transducer 3 consists of only one pair of comb-shaped electrodes 3' and that the pitch of the comb-shaped electrodes 3' varied discontinuously in the direction perpendicular to the propagation direction of SAW. Each of the pitches of the electrodes corresponds to one of the bands A, B, C. D, E and thus the device indicated in Fig. 3 works in the same way as that indicated in Fig. 1. The advantage of this form is that the bands can be divided more finely and that the bonding steps necessary for taking out the output to the exterior can be reduced.

Furthermore, if the metal electrodes 10 are formed to be finer and disposed to be closely adjacent to each other, and if the pitch of the comb-shaped electrodes 3' varies continuously in the direction perpendicular to the propagation direction of SAW, it is possible to adjust arbitrarily the pass band width by selecting the distribution of the bias voltages. The semi-conductor substrate in the devices indicated in Figs. 1, 3 and 4 can include an insulating film such as an oxide film, nitride film etc. obtained by oxidizing or nitribying its surface.

Further, the material of the metal electrodes can be same as that of the comb-shaped electrodes and they can be produced by the same process (photolithographic process) and at the same time as the comb-shaped electrodes.

Moreover, although the input transducer indicated in Figs. 1, 3 and 4 consists of only one pair of comb-shaped electrodes which are common to all the filters, it can consist of a plurality of pairs of comb-shaped electrodes 2A, 2B, 2C, 2D having different input characteristics. However, in this case, each pair of comb-shaped electrodes must be matched by using separate matching circuits 11 A, 11 B, 11C, 11D.

In addition, it is desirable to form the end surface of the metal electrodes 10A, 10B, .. so that it is inclined with respect to the propagation direction of SAW, because influences of the reflection of SAW at the end surface are reduced in this way.

Fig. 11 is a perspective view of a variable SAW delay line according to this invention.

A semi-conductor 7 and a piezoelectric film 8 disposed thereon constitute a piezoelectric substrate 1. On the other surface, which is opposite to the piezoelectric film 8, of the semi-conductor substrate 7, is disposed a back side electrode 9, which is earthed. At one end of the surface of the piezoelectric substrate 1 are disposed comb-shaped electrodes 2' constituting the input transducer 2 and at the other end comb-shaped electrodes 3A, 3B, 3C, 3D constituting the output transducers 3. The distances between the electrodes 3A, 3B, 3C, 3D and the input combshaped electrodes 2 are different from each other so that the time necessary for SAW emitted by the transducer 2 to reach the output transducers 3A, 3B, 3C, 3D are different from each other. All the electrodes 3A, 3B, 3C, 3D are connected to the output terminal 6.Metal electrodes 10A, 10B, 10C and 10D are mounted on the propagation paths of SAW from the input transducer 2 to the electrodes 3A, 3B, 3C and 3D respectively, and bias voltages VA, VB, Vc and VD are applied to these metal electrodes respectively. According to the working principle described above, when the bias voltages VA, VB, Vc, VD are so set that the surface of the semi-conductor substrate is strongly inverted, SAWs propagating in the portions under the metal electrodes 10A, 10B, 10C, 10D are rapidly attenuated.On the contrary, by setting the bias voltages so that the surface of the semi-conductor substrate is at the depletion state or weakly inverted or at a sufficiently charged state, it is also possible to make the attenuation of SAW sufficiently small. Consequently, by controlling the bias voltages VA, VB, Vc, VD applied to the metal electrodes 10A, 10B, 10C, 10D it is possible to switch on/off SAW reaching the output comb-shaped electrodes 3A, 3B, 3C, 3D. Further, since it is also possible to vary continuously the propagation loss of SAW by regulating the bias voltages VA, VB, Vc, VD the output level of each of the delay lines can be suitably controlled.

In this case, since it is possible to control the propagation of SAW simply by commuting the bias voltages, the device has the advantage that the circuit therefore can be simplified with respect to that of prior art devices, by which switching off SAW is effected by switching off RF signals. Further, since the semi-conductor substrate is cheaper and large size substrates are available more easily with respect to the piezoelectric body, the substrate has the advantage that it can be fabricated with a lower cost than the substrate used by the prior art techniques.

Although in the device Ilustrated in Fig. 11, the output transducer 3 is divided into a plurality of pairs of comb-shaped electrodes 3A, 3B, 3C, 3D and the final output is taken out in the form of a resultant output of plural circuits connected in parallel, a structure as illustrated in Fig. 12 is otherwise possible.

In the device illustrated in Fig. 12, the output transducer 3 consists of one pair of comb-shaped electrodes 3' which are constructed in the form of stairs so that the distance between the input transducer and the output comb-shaped electrodes in the propagation direction of SAW varies discontinuously in the direction perpendicular to the propagation direction of SAW. It is clear that the device illustrated in Fig. 12 works in the same manner as that illustrated in Fig. 11.

Furthermore, the output comb-shaped electrodes 3' can be formed as illustrated in Fig.

13.

In the devices illustrated in Figs. 12 and 13 the comb-shaped electrodes 3' of the output transducer 3 are so disposed that the distance between the comb-shaped electrodes 2' of the input transducer 2 and the comb-shaped electrodes 3' of the output transducer 3 varies discontinuously in the direction perpendicular to the propagation direction of SAW. In this way, it is possible to obtain advantages in that the delay time can be regulated more finely and that bonding steps necessary for taking out the output to the exterior can be reduced.

Similarly, the input transducer 2 may be divided into a plurality of electrodes 2A, 2B, 2C, 2D as indicated in Fig. 14, or only one pair of electrodes 2' may be constructed in the form of stairs, as indicated in Fig. 15, or they may be disposed inclined with respect to the propagation path of SAW, as indicated in Fig.

16.

Furthermore, it is desirable that the end surfaces of the metal electrodes 10A, 10B, are inclined with respect to the propagation direction of SAW, because in this way influences of the reflection of SAW at the end surfaces are reduced.

The semi-conductor substrate 7 used for this invention can be covered by an insulating film 11, such as an oxide film, a nitride film, etc. by oxidizing or nitrbying its surface, as indicated in Fig. 17.

In addition, for the piezoelectric substrate 1, as illustrated in Fig. 18, the propagation path of SAW can be divided into two regions, in one of which metal electrodes 10A, lOB ....

are disposed for controlling the propagation of SAW and in the other of which a metal film 12 is disposed at the interface between the piezoelectric film 8 and the semi-conductor substrate or the insulating film 11. For such a structue, in the portion where a metal film 12 exists, the potential of SAW is shielded and thus the semi-conductor substrate and SAW do not interfere with each other. This structure is advantageous when a long delay time is required, because attenuation of SAW is extremely small.

Furthermore, as indicated in Fig. 19, it is also possible to use a substrate structure in which the piezoelectric film 8 is disposed on the portion where the input comb-shaped electrodes 2' and the metal electrodes 10A, 10B, .... exist and not on the propagation paths of SAW therebetween. This structure is advantageous, similar to that indicated in Fig. 19 when a long delay time is required, because SAW propagates through the crystal semiconductor substrate 7, where the piezoelectric film 8 does not exist, and thus the attenuation of SAW is extremely small there.

As explained above, space saving and cost reduction can be achieved in relation to prior art variable band width SAW filters, because it is not necessary to dispose switching elements separately for each of the filters for selecting an output. In the device according to this invention, a function equivalent to such switching can be realised simply by switching on/off the bias voltages, and thus the circuit itself is simplified. Further, since the propagation loss for each of the bands can be varied in an analog manner by controlling the bias voltages in an analog manner, band width characteristics of large freedom can be obtained.

SAW filters according to this invention can be applied to all sorts of apparatus where SAW filters are used. However, they are particularly useful for communication apparatus, by which the frequency band is varied in time, such as CATV, communication apparatus.

Claims

1. A surface acoustic wave device comprising a semi-conductor sustrate; a piezoelectric film disposed on said semi-conductor substrate; an input transducer and an output transducer disposed adjacent opposed extremities on said piezoelectric film; a plurality of metal electrodes disposed between said input transducrer and said output transducer; a bias voltage applying means for applying bias voltages to each of said metal electrodes; and a bias voltage regulating means for regulating said bias voltages.

2. A surface acoustic wave device accordng to Claim 1, wherein said input transducer comprises at least one pair of input comb-shaped electrodes and said output transducer comprises a plurality of pairs of output comb-shaped electrodes, respectively associated with said metal electrodes.

3. A surface acoustic wave device according to Claim 1, wherein said input transducer and said output transducer each consist of a pair of comb-shaped electrodes, said combshaped electrodes of the output transducer being so formed that the pitch of the electrodes varies discontinuously in the direction perpendicular to the propagation direction of the surface wave.

4. A surface acoustic wave device according to Claim 1, wherein said input transducer and said output transducer each consist of a pair of comb-shaped electrodes, said combshaped electrodes of the output transducer being so formed that the pitch of the electrodes varies continuously in the direction perpendicular to the propoagation direction of the surface acoustic wave.

5. A surface acoustic wave device according to Claim 1 or Claim 2, wherein said input transducer consists of a plurality of pairs of input comb-shaped electrodes having identical input characteristics, respectively associated with said metal electrodes.

6. A surface acoustic wave device according to Claim 1 or Claim 2, wherein said input transducer consists of a plurality of pairs of input comb-shaped electrodes having different input characteristics, respectively associated with said metal electrodes, each pair of input comb-shaped electrodes being fed with input signals through a matching circuit.

7. A surface acoustic wave device according to Claim 1, wherein said input transducer and said output transducer each comprise at least one pair of input and output combshaped electrodes respectively, and said output comb-shaped electrodes are so disposed that the distance between said input and said output electrodes varies in the direction of the surface acoustic wave.

8. A surface acoustic wave device according to any preceding claim, wherein the end surface of each of said metal electrodes is inclined with respect to the propagation direction of the surface acoustic wave.

9. A surface acoustic wave device according to any preceding claim, wherein an insulating film is disposed between said semi-conductor substrate and said piezoelectric film.

10. A surface acoustic wave device according to Claim 9, wherein a metal film is disposed between said metal electrodes and said output transducer on said insulating film.

11. A surface acoustic wave device according to Claim 9, wherein said piezoelectric film is removed in a region between said metal electrodes and said output transducer.

12. A surface acoustic wave device substantially as hereinbefore described with reference to Figs. 1 and 2, Fig. 3, 4, 5, 6, 11, 12, 13, 14, 15, 16, 17, 18 or 19 of the drawings.