JP2534041B2 - Surface acoustic wave device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a semiconductor substrate, a piezoelectric film formed thereon, a pair of surface acoustic wave transducers provided on the surface thereof, and the pair of surface acoustic waves. The present invention relates to a surface acoustic wave device having a gate made of a metal film provided between transducers.

B. Outline of the Invention In a surface acoustic wave device in which a piezoelectric film is laminated on a semiconductor substrate, and a pair of transducers is provided on the piezoelectric film, and a gate electrode is provided between them, the transducers of the gate electrode on the transducer side are provided. Strip lines are connected at both ends.

C. Conventional Technology Conventionally, as a convolver using surface acoustic waves, a separation medium structure, an elastic structure, and a layered structure in which a semiconductor and a piezoelectric body are combined are known.

In the separation medium structure, for example, silicon (S
It is necessary to combine i) and the lithium niobate (LiNbO ₃ ) which is a piezoelectric material with a slight gap between them. This structure has an advantage that the characteristics of the semiconductor and the piezoelectric body can be used independently, but since the void needs to be on the order of 1000Å, it has a drawback that productivity is poor due to assembly problems and reproducibility problems.

In the elastic structure, the electrode pattern may be formed on a piezoelectric substrate such as a lithium niobate (LiNbO ₃ ) substrate, and there is no problem in assembling. However, it is necessary to devise means for producing elastic nonlinearity of the piezoelectric body in the elastic structure. However, since elastic nonlinearity is not generally large, there is a drawback that the efficiency as a convolver is poor.

The layered structure is formed by forming a piezoelectric film such as zinc oxide (ZnO) on a semiconductor substrate such as silicon (Si) by means such as sputtering. Therefore,
Because of the monolithic type as well as the elastic structure,
It has the advantage of being easy to configure. In addition, the layered structure has an advantage that the efficiency of the convolver is high because the nonlinearity of the depletion layer capacitance of the semiconductor is used.

FIG. 9 is a sectional view of the layered structure convolver, and FIG. 10 is a plan view thereof. In the figure, 1 is a semiconductor substrate made of silicon or the like, 2
Is a piezoelectric film such as a zinc oxide film, 3 is a surface acoustic wave transducer, and 4 is an output gate. The surface acoustic wave transducer 3 and the output gate 4 are made of aluminum or the like. The output gate has a rectangular shape having a length of 1 in the propagation direction of the surface acoustic wave and a width of b in the orthogonal direction.

In order to increase the process gain, which is the figure of merit of the convolver, the product of the bandwidth B (Hz) and the delay time T (seconds) of the output gate 4 (T = l / v, v is the velocity of the surface acoustic wave). It is necessary to increase a certain B · T product.

A surface acoustic wave convolver is used, for example, for broadband communication (Spread S
When applied to pectrum communication, etc., the bandwidth B corresponds to the clock frequency of the PN code that produces the spread spectrum. Although it is possible to increase the bandwidth B by designing the transducer 3, the clock frequency for the PN code has a frequency limitation caused by the logic circuit. Therefore, in order to increase the B · T product, it is necessary to increase not only the bandwidth B but also the delay time T, that is, the gate length l.

As shown in Fig. 11, surface acoustic wave transducers 7, 8
At terminals 5 and 6 of g ₁ (t) e ^jωt and g ₂ (t) e, respectively.
^When the input signal of ^jωt is applied, the convolution signal h (t) represented by the following equation is obtained from the terminal 9 of the ^convolver output gate 10. It should be noted that for the coordinates, the left end of the gate in FIG. 1 is taken as the origin, and the gate length is l.

As can be seen from the equation (1), the convolution output signal has a form in which the convolution signal generated at an arbitrary position x of the gate 10 is integrated with respect to the gate length 1.
Therefore, it is necessary to make the amplitude and the phase of the output signal in each part on the gate 10 as uniform as possible, and this is called the uniformity of the distribution in the gate.

When the gate length 1 is increased, the uniformity of the distribution within the gate is disturbed. As can be seen from the structure of the gate 10 in FIG. 11, since the gate has an elongated shape (1 >> b), it can be considered as a stripline having a distributed constant. Therefore the first
The gate 10 in Fig. 11 can be regarded as a distributed constant line with many signal sources, and both ends (x = 0, 1) are open, so
At this position the output convolutional electrical signal is reflected. Therefore, the standing wave is generated on the gate 10, so that the distribution in the gate is greatly deteriorated. In order to improve this distribution in the gate, as shown in FIG. 12, both ends 11 and 1 of the gate 10 are
A structure in which 2 is terminated with an inductance L (13,14) has been proposed. Here, the value of the inductance L is the gate 10
Is the stripline characteristic impedance Z _g
And has the relationship of

Z _g = ωL (2) where ω is the output signal angular frequency.

In the elastic structure, the distribution in the gate can be improved by terminating the gate end with the inductance L as shown in FIG. On the other hand, in the case of the layered structure, as shown in FIG. 13, a bias voltage is generally required to optimize the operating point of the convolver. In FIG. 13, reference numeral 15 is a bias power source, and 16 is a bias resistor. In the configuration shown in FIG. 13, since the gate 10 is terminated by the inductance L (13, 14) at the end portions 11, 12, the bias voltage is short-circuited by the inductance, and the bias voltage cannot be applied to the gate. The operating point of the convolver cannot be optimized. To solve this, instead of terminating with an inductance,
You can end it with. However, in the resistance termination, since the energy of the convolution output signal is consumed in this portion, there is a drawback that the efficiency is deteriorated.

D. Problems to be Solved by the Invention The present invention has been made in order to overcome the above-mentioned drawbacks, and an object of the present invention is to maintain high efficiency in a layered structure surface acoustic wave convolver and improve distribution in the gate. It is to provide a device structure capable of

E. Means for Solving the Problems To achieve the above object, a surface acoustic wave device of the type mentioned at the outset according to the invention has a stripline connected to both ends of the gate and from the gate end. The gist is that the impedance seen from the strip line side is inductive, and the end of the strip line is open.

In an advantageous embodiment of the invention, where the stripline length is a and the wavelength for electrical signals on the stripline is λ, And the characteristic inductance of the gate is Z _g , the characteristic impedance of the strip line is Z _o , and the length of the strip line is a, Where β is the phase constant and λ is the wavelength for the electrical signal on the stripline. Is.

Further, a bias voltage is applied to the gate electrode.
The semiconductor substrate is advantageously silicon and the piezoelectric film is zinc oxide. A silicon oxide film may be provided between the semiconductor substrate and the piezoelectric film.
The strip line may be formed on the surface acoustic wave device or on another substrate, and when a silicon oxide film is provided between the semiconductor substrate and the piezoelectric film, It can also be formed on the silicon oxide film. The strip line may be parallel or perpendicular to the surface acoustic wave propagation direction. It is advantageous to use a Sezawa wave as the surface acoustic wave.

F. Action Fig. 3 shows the characteristic impedance Z _o connected to the gate end
5 schematically shows a strip line having a length of a. However, in FIG. 3, the stripline is terminated with a load impedance Z _L.

The impedance Z _in seen from the gate end is expressed by the following equation.

Here, β is the phase constant, λ is the wavelength of the output electric signal on the strip line, and the following equation holds.

Since the end of the strip line is open, equation (3)
At Z _L → ∞. Z _in at this time is In order for Z _in (open) _in equation (5) to be inductive, −tan βa> 0 (6) Must.

If the characteristic inductance of the gate is Z _g , In other words, with the strip line having the length a that satisfies the expressions (7) and (8), the same effect as the expression (2) can be obtained.

G. Examples Hereinafter, the present invention will be described in more detail with reference to the drawings with reference to the Examples, but these are merely examples, and various modifications and improvements can be made without departing from the scope of the present invention. Of course it is possible.

1 and 2 are a top view and a cross-sectional view, respectively, of a surface acoustic wave device according to the present invention, in which 7 and 8 are shown.
Is a surface acoustic wave transducer, 9 is an output terminal, 10 is an output gate, 11 and 12 are gate ends, 15 is a bias power supply, 16
Is a bias resistor, 17 and 18 are strip lines, 19 is a piezoelectric film, and 20 is a semiconductor substrate.

A film 19 made of piezoelectric material such as zinc oxide (ZnO) and aluminum nitride (AlN) is formed on a semiconductor substrate 20 such as silicon (Si). Surface acoustic wave transducers 7 and 8 made of a metal such as aluminum (Al), an output gate 10, and strip lines 17 and 18 drawn from the output gate ends 11 and 12 are formed thereon.

The output gate 10 is connected to an output terminal 9 drawn out from the center of the gate and a bias power supply 15 for optimizing the operating point of the convolver via a bias resistor 16.

When an input electric signal is applied to the surface acoustic wave transducers 7 and 8, the electric signal is converted into a surface acoustic wave and propagates toward the gate 10 in the center. These surface acoustic waves propagating in opposite directions generate the convolution output signal h (t) of the equation (1) via the non-linearity of the depletion layer capacitance of the semiconductor immediately below the gate electrode 10, It is taken out to the output terminal 9.

Here, in order to make the distribution within the gate on the gate 10 of the convolver uniform, the strip lines 1
Form 7,18. The end of this stripline 17,18 19,
20 is opened, and in this state, the length and width of the strip line are determined so that the impedance seen from the gate ends 11 and 12 on the strip line side is inductive so as to satisfy the conditions described in the section of action.

If a strip line satisfying the equation (7) or the equations (7) and (8) is formed at the gate ends 11 and 12 in FIG.
A bias voltage can be applied to the gate electrode 10 in order to open it in terms of direct current, and operation can be performed with an optimum bias. Further, since the gate end is terminated with an inductance at high frequencies, the distribution in the gate is improved.

Figure 4 shows the calculation results for a convolver with a gate length l of 20 mm. The solid line in FIG. 4 shows the intra-gate distribution of the amplitude according to the present invention in which a strip line satisfying the formulas (7) and (8) is formed at the gate end, and the dotted line shows the characteristic for the conventional structure without the strip line. The distribution of the phase in the gate is uniform in both cases. As described above, it is understood that the device structure of the present invention can greatly improve the distribution in the gate as compared with the conventional structure.

The above is effective for the layered structure in which the piezoelectric film 19 is formed on the semiconductor substrate 20 as shown in FIG. 2. Particularly, silicon (Si) is used as a semiconductor and zinc oxide (Zn is used as a piezoelectric body).
The ZnO / Si structure formed by O) has a large electro-mechanical coupling coefficient, so that the bandwidth can be widened and the efficiency of the convolver can be increased.

Regarding the electro-mechanical coupling coefficient, the cut surface of silicon and the strip lines 17 and 18 can be manufactured in the same process as the surface acoustic wave transducers 7 and 8 and the gate electrode 10 without increasing the number of processes, and No external additional circuit is required. Depending on the propagation direction of the surface acoustic wave,
Propagation in the [100] direction on the Si (100) plane (hereinafter abbreviated as Si (100) [100] in the present specification), Si (100) [11
0] and Si (110) [100] are effective. For the Sezawa wave, which is the higher mode of the Rayleigh wave,
This structure has a great advantage because of its large mechanical coupling coefficient. (Acoustic Society of Acoustics, Asai et al., October 1982, No. 1)
591 to 592).

In FIG. 1, the bias power supply 15 and the bias resistor 16 are provided for the purpose of optimizing the operating point. However, those elements may be omitted for an element having an optimum operating point at zero bias. Of course you can.

In FIG. 1, the strip lines 17 and 18 are directly connected to the gate electrode 10 on the surface acoustic wave element.
As shown in the figure, the striplines 25 and 26 may be formed on the stripline substrate 28 outside the surface acoustic wave device 27. In the figure, 21 is a bonding wire for connecting both ends 11 and 12 of the gate 10 and the external lead terminal 22, and 23 and 2
Reference numeral 4 is a connection point between the external lead terminal 22 and the strip lines 25 and 26. By doing so, the surface acoustic wave device 27 and the strip lines 25 and 26 that determine the load conditions at the gate end can be designed independently, which has the advantage of increasing the degree of freedom in selecting the strip line substrate 28. As shown in FIG. 6, a silicon oxide film SiO ₂ 2 which is an insulating film is formed between silicon (Si) 20 which is a semiconductor and zinc oxide (ZnO) 19 which is a piezoelectric substance.
9 may be used to improve the temperature characteristics of the protective film on the silicon surface or the ZnO / Si structure.

Further, in FIG. 1, the strip lines 17 and 18 are formed in a direction perpendicular to the propagation direction of the surface acoustic wave.
As shown in FIG. 7, by forming a strip line in parallel with the propagation direction, the overall element size can be reduced.

Further, in the structure shown in FIG. 6, the strip line is a surface acoustic wave transducer 7, 8 and a gate 1 on the piezoelectric film 19.
Although it is formed on the same plane as 0, in the device shown in FIG. 8, since the strip lines 17 and 18 are on the silicon oxide film 29, the distributed capacitance becomes large. Therefore, when the characteristic impedance Z ₀ of the strip line is defined, the strip width can be reduced, and the element size can be further reduced.

H. Effects of the Invention As described above, according to the present invention, a monolithic surface acoustic wave convolver having a layered structure has a high convolver efficiency and a large electromechanical coupling coefficient, so that a wide band can be achieved. Further, by terminating the gate end, the distribution in the gate can be improved even if the gate length is increased.
The product is large, and therefore, there is an advantage that a high-performance surface acoustic wave convolver can be obtained.

[Brief description of drawings]

1 and 2 are a top view and a sectional view, respectively, of a surface acoustic wave device according to the present invention, and FIG. 3 is a schematic view of a strip line of length a having a characteristic impedance Z ₀ connected to a gate end, FIG. 4 is a graph showing the calculation result of the in-gate distribution of amplitude, FIG. 5 is a top view of a surface acoustic wave convolver according to another embodiment of the present invention, and FIG. 6 is another view of the present invention. FIG. 7 is a top view of a surface acoustic wave convolver according to an embodiment, FIG. 7 is a top view of a surface acoustic wave convolver according to yet another embodiment of the present invention, and FIG. 8 is another embodiment of the present invention. FIG. 9 is a perspective view of a surface acoustic wave convolver according to FIG. 9, FIG. 9 is a sectional view of a general surface acoustic wave convolver, and FIG. 11 is a schematic view for explaining the operation of the surface acoustic wave convolver. Figures 12 and 13 are conventional It is a schematic diagram showing the configuration of a surface acoustic wave convolver. 7,8 ... Surface acoustic wave transducer, 10 ... Output gate, 9 ... Output terminal, 15 ... Bias power supply, 16 ... Bias resistor, 17,18 ... Strip line, 19 ... Piezoelectric film, 20 …… Semiconductor substrate.

Claims (11)

(57) [Claims] 1. A semiconductor substrate, a piezoelectric film formed on the semiconductor substrate, a pair of surface acoustic wave transducers provided on the surface of the semiconductor substrate, and a metal film provided between the pair of surface acoustic wave transducers. A surface acoustic wave element having a gate, and (b) a strip line connected to both ends of the gate, the impedance seen from the gate end to the strip line side is inductive, and the end of the strip line is open. A surface acoustic wave device characterized in that 2. When the length of the strip line is a and the wavelength for an electric signal on the strip line is λ, The surface acoustic wave device according to claim 1, wherein 3. When the characteristic impedance of the gate is Zg, the characteristic impedance of the stripline is Zo, and the length of the stripline is a, Where β is the phase constant and λ is the wavelength for the electrical signal on the stripline. The surface acoustic wave device according to claim 1 or 2, wherein 4. A surface acoustic wave device according to any one of claims 1 to 3, wherein a bias voltage is applied to the gate electrode. 5. The elastic surface according to any one of claims 1 to 4, wherein the semiconductor substrate is silicon and the piezoelectric film is zinc oxide. Wave device. 6. The surface acoustic wave device according to claim 5, further comprising a silicon oxide film between the semiconductor substrate and the piezoelectric film. 7. The surface acoustic wave device according to claim 1, wherein the strip line is formed on a surface acoustic wave element. 8. The elastic surface according to any one of claims 1 to 7, characterized in that the stripline is formed parallel to the propagation direction of the surface acoustic wave. Wave device. 9. The strip line is outside the surface acoustic wave device and is connected to a gate end portion, according to any one of claims 1 to 6. The surface acoustic wave device described. 10. A strip line is formed on a silicon oxide film and is connected to both ends of a gate, according to any one of claims 1 to 8. The surface acoustic wave device described. 11. The surface acoustic wave device according to claim 1, wherein a Sezawa wave is used as the surface acoustic wave.