JPS6367363B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave device, and more particularly to a surface acoustic wave device in which an excitation transducer and a reception transducer are formed on a piezoelectric crystal plate having pyroelectric properties.

A surface acoustic wave element has an excitation transducer, a reception transducer, etc. formed on a piezoelectric plate.The excitation transducer converts an electrical signal into a surface acoustic wave signal, and the reception transducer converts a surface acoustic wave signal into an electrical one. Convert to signal. Such a surface acoustic wave element is a filter,
Used in delay lines, oscillators, etc.

Fig. 1 shows an example of the configuration of a surface acoustic wave filter in which such a surface acoustic wave element is applied to the filter, and Fig. 1a is a perspective view of the surface acoustic wave element, and Fig. 1b is a perspective view of the surface acoustic wave element.
1 is a diagram schematically showing a general filter circuit including a surface acoustic wave filter. A surface acoustic wave filter has a filter function by using surface acoustic waves of a specific frequency that propagate through a piezoelectric material due to a piezoelectric phenomenon. In the figure, 11 is a piezoelectric substrate, and the surface of this piezoelectric substrate A conductive thin film of aluminum or the like is formed in the shape of a blind by photoetching or the like. One of the interdigital thin films is the excitation converter 1.
2S and the other interdigital thin film function as a receiving transducer 12R, and the piezoelectric substrate 11 and each interdigital thin film form a surface acoustic wave element. Now, the excitation converter 12S of the surface acoustic wave filter 10
The high frequency input signal fin is connected to the input terminal 13in.
Suppose that is input. At this time, the sound speed of the surface acoustic wave W propagating on the surface of the surface acoustic wave filter 10 is u
Then, the wavelength λ of the surface wave is expressed as λ=u/. However, is the high frequency input signal
This is the frequency of fin that should be waved.

By the way, on the excitation converter 12S side, the interdigital electrode 14 extending from the input electrode S and the ground electrode E
When the high frequency input signal is applied between the transducer and the interdigital electrode 14' extending from the transducer, the piezoelectric material existing between these electrodes expands and contracts due to the piezoelectric phenomenon, inducing surface acoustic waves along the surface layer. Therefore, the frequency to be waved is set as shown in the previous equation, and the arrangement pitch of the interdigital electrodes 14 extending from the input electrode S and the interdigital electrode 14' extending from the ground electrode E are determined.
If the array pitches of both are set equal to λ, only the surface acoustic waves of the wavelength λ, that is, the frequency, will propagate on the surface of the surface acoustic wave filter 10, and an output signal of the frequency will be obtained from the output terminal 13out.
is sent to load R.

The surface acoustic wave filter that operates as described above can be used particularly effectively as a bandpass filter in a high frequency band, and its external dimensions are small, at most a few mm square, making it extremely effective. Although quartz crystal can be used as the piezoelectric substrate 11, since the electromechanical coupling coefficient of quartz crystal is small, it is difficult to create a surface acoustic wave filter with low loss. Therefore, LiNbO ₃ , which has a large electromechanical coupling coefficient, is generally used.
LiTaO ₃ etc. are used for piezoelectric substrates. However, since LiNbO ₃ and LiTaO ₃ have the property of pyroelectricity, which will be described later, if there is a sudden temperature change, for example, a temperature change of about 40°C over a period of about 3 minutes to 1 hour, the surface acoustic wave filter will fail. A plurality of unnecessary pulses due to discharge appear at the input terminal or output terminal of the filter, leading to malfunction of equipment using the filter. As explained above about surface acoustic wave filters, the input and output terminals of not only surface acoustic wave filters but also general surface acoustic wave elements using piezoelectric crystal plates with pyroelectricity are pyroelectrically generated when there is a sudden temperature change. Unwanted pulses appear due to gender-based discharges. Pyroelectricity here means that when the temperature of a crystal is changed, a change in the crystal's spontaneous polarization 1P (1P is a function of temperature) appears on the crystal surface, and a polarization occurs between two points on the crystal. A phenomenon that generates a potential difference.

The phenomenon in which unnecessary pulses are generated at the input terminal of a surface acoustic wave element using a pyroelectric piezoelectric crystal plate when a sudden temperature change occurs can be explained as follows. In other words, the charge generated by pyroelectricity is not uniform due to the structural non-uniformity of the crystal, creating a strong electric field on the crystal surface, which exceeds the dielectric breakdown voltage of that area and causes the fluorescence to change. An accompanying discharge occurs, and a pulsed signal is detected at the input and output electrodes due to the discharge.

In fact, when a surface acoustic wave device substrate is rapidly cooled or heated between -30°C and 50°C, flashing due to discharge is observed, and the gradient of temperature change (temporal temperature change rate) is increased from 10°C/min to 1°C. When the temperature was changed in degrees Celsius/min, it was found that the steeper the slope, the more pulses were generated.

Figure 2 explains the generation of such a flash of light. Figures a and b are both diagrams of the flash light generation state recorded on a photographic plate, where a is the temperature during one temperature cycle and b is the temperature. FIG. 3 is a diagram showing a flashlight generation state during three cycles. Note that the same parts as in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted.

In the figure, reference numeral 21 indicates a discharge area that appears white on the photographic plate. During one temperature cycle, flashing due to discharge occurs at the edge of the surface acoustic wave element, and after three cycles, the surface acoustic wave element Flashing light due to discharge is occurring over the entire surface of the area.

If discharge occurs due to pyroelectricity in this way, pulsed unnecessary signals are generated at the input/output electrodes, causing equipment malfunction.

For this reason, the present inventor has already proposed a surface acoustic wave element that can prevent such discharge.
This already proposed surface acoustic wave device will be explained below.

FIG. 3 is an explanatory diagram for explaining discharge generation, and FIG. 4 is an equivalent circuit diagram of FIG. 34 and 35 are lead wires, and 36 is dirt such as moisture and dust on the substrate.

Now, assuming that the magnitude of spontaneous polarization generated per unit area of the piezoelectric crystal plate 31 is Ps, the pyroelectric current ip flowing from the unit area is expressed by equation (1).

ip=dPs/dt=dPs/dT・dT/dt (1) If the surroundings are surrounded by an ideal dielectric, this component is stored as a change in electrical displacement D, but in reality it is stored on the crystal plane. It is thought that the discharge is gradually caused by dirt and moisture on the battery. Fig. 4 is an equivalent circuit diagram of Fig. 3, where _rL is the resistance due to moisture, dirt, etc., Cx is the capacitance of the creeping discharge path, Cd is the electrode 32,
33 is the capacitance of the crystal.

In the formula (1), dPs/dT is constant between 0℃ and 400℃ in the case of LiNbO ₃ , and -4×10 ^-5 (q/
m ² )/°C. Therefore, according to equation (1), the more rapid the temperature change is, in other words, the larger dT/dt is, the larger the pyroelectric current ip becomes, and the larger the potential difference V _L between the electrodes 32 and 33 becomes. When this potential difference V _L exceeds the creeping discharge potential Vc (corresponding to the breakdown voltage of Cx), Cx
causes dielectric breakdown and rapid discharge occurs. This situation will be analyzed below using the equivalent circuit shown in FIG.

As is clear from the equivalent circuit, the transient voltage V _L
(t) is obtained as a solution to the following differential equation.

(Cd+Cx)・dV _L /dt+V _L /r _L = dPs/dt=ip (2) Here, dPs/dt can be expressed by the following formula.

dPs/dt=dPs/dT・dT/dt=Pc・dT/dt (3) Note that Pc is a pyroelectric constant and becomes a constant value at temperatures far from the Kiuri point.

If we solve equation (2) in the general equation, the solution will be complicated, so we will consider the case where the temperature change is linear, that is, dT/dt = α (constant) (4). Therefore, To is the reference temperature, α
If (°C/sec) is a time distribution, the temperature T after t seconds can be expressed as T=αt+To (5).

Substituting equations (3) and (4) into equation (2) yields (Cd+Cx)・dV _L /dt+V _L /r _L =Pc・α (6).

If we solve equation (6) considering the initial condition V _L (o)=o, we get V _L (t)=Pc・α・r _L {1−exp(−t/(Cd+Cx)r _L )} (7 ) becomes.

As a special case, in the case of a clean and perfect crystal, that is, in the ideal case of r _L →∞, V _L (t) is expressed as the following differential equation (Cd + Cx)・dV _L /dt=Pc・α ( 8), and under the initial condition of V _L (o) = o, V _L = Pc・α・t/(Cd+Cx) (9). This means that in an ideal system, the pyroelectric component is stored in Cd and Cx as an electrostatic displacement component, which is nothing but what electrostatics teaches.

However, since there is generally a slight stain on the crystal surface and r _L cannot be ignored, the explanation will be given below using equation (7).

When the change in V _L is illustrated using (t/(Cd+Cx)r _L ) as an independent variable in equation (7), it becomes as shown in FIG. 5. Now r _L = r _L0 V _Lnax = Pc・α・r _L0 , and creeping discharge potential Vc is Vc=0.393・V _Lnax =0.393・Pc・α・r _L0 (10) then t/(Cd+Cx) r _L0 Creeping discharge occurs when = 0.5. In addition, in Fig. 5, the solid line is V _L when lineside discharge does not occur, and Vc =
If creeping discharge occurs at 0.393V _Lnax (Cd
+Cx) is repeatedly charged and discharged, and V _L changes as shown by the dashed line in the figure.

From the above, if we assume that the creeping discharge path remains unchanged and therefore the creeping discharge potential Vc is constant at 0.393Pc・α・r _L0 , then by actively reducing the resistance r _L , we can always reduce the creeping _discharge It can be said that the potential can be lowered to below Vc, and the occurrence of creeping discharge can be prevented. That is, the value of the resistance r _L can be changed to r _L1 by some method.
(<r _L0 ), and |Pc・α・r _L1 |<|Vc|
If the following is satisfied, V _L will change as shown by the two-dot chain line in Fig. 5, and will not exceed Vc, and creeping discharge will not occur. However, | Pc・α・r _L1 |
Creating a state that satisfies <|Vc| is an ideal, but cannot be easily achieved. For example, if the entire surface of a piezoelectric crystal substrate is covered with a conductor, the purpose is achieved, but it cannot be used as a surface acoustic wave device.

However, in reality, if it can withstand temporal changes within a limited temperature range, |Sc・α・r _L1 |>|Vc
| In most cases, it is acceptable.
This is because of the following reasons.

Now, if 10 minutes after the temperature change starts (to=10), 60
Assuming that there is a change in temperature (α=0.1℃/
sec), the voltage V _L (to) after time to (this V _L (to) is α
= 0.1℃/sec, r _L = r _L1 , which can be obtained from equation (7)) is smaller than the creeping discharge potential Vc, |
This is because creeping discharge does not occur even if the resistance r _L1 is such that Pc・α・r _L1 |>|Vc|.

Therefore, in the previously proposed surface acoustic wave device,
The region including the excitation transducer and the reception transducer is covered with a resistive film having a resistance value high enough not to cause creeping discharge, and the surface of the other region is covered with a resistive film having a resistance value lower than the resistance value of the resistive film. However, by covering it with an electrically conductive layer made of metal or a resistor, r _L is reduced and discharge generated on the pyroelectric voltage crystal substrate due to temperature changes is suppressed.

FIG. 6 is a perspective view of the previously proposed surface acoustic wave device, and the same parts as those in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted.

41 is a pyroelectric piezoelectric crystal plate, and an electric conductor layer 42 is uniformly formed on the crystal surface by vapor deposition or the like, except for the excitation transducer 12S and the reception transducer 12R. Moreover, the excitation converter 12s and the reception converter 1
The entire surface of 2R is covered with chromium Cr and nickel chromium NiCr.
Alternatively, a resistive film 43 made of tungsten W or the like is similarly formed uniformly by vapor deposition or the like.

FIG. 7 is a perspective view of another embodiment of the previously proposed surface acoustic wave device, and the same parts as in FIG. 6 are given the same reference numerals. The difference from FIG. 6 is that a resistive film 43 is also present between the excitation converter 12S and the reception converter 12R. Note that a low-resistance resistive film may be attached instead of the electrical conductor layer 42, and the piezoelectric crystal plate 4
A resistive film 43 may be attached to the entire surface of the crystal 1.

FIG. 8 is a cross-sectional view of another embodiment of a previously proposed surface acoustic wave device, in which a resistive film 43 is attached to the entire crystal surface, and then an electrical conductor layer 42 is attached to the bottom and side surfaces.

The analysis of the discharge phenomenon was carried out between two specific points on the crystal plate (Figure 3), but in reality, due to the imperfections of the crystal, discharge occurs throughout the crystal plate. Conceivable. Therefore, since it is difficult to quantitatively elucidate the discharge phenomenon, the effect of reducing the resistance r _L was experimentally investigated.

According to experimental results, at a temperature gradient of about 60°C/hour, the resistance film 43 with a resistance of σ/h = 10 ¹² Ω or less
(However, σ and h are the specific resistance and film thickness of the resistive film 43, respectively.) Also, at a temperature gradient of about 12°C/min, a resistive film with σ/h = 10″Ω or less is attached, and rapid cooling is not possible. In a rapidly heated state, if a resistive film of σ/h=10 ¹⁰ Ω or less was applied, discharge was significantly suppressed.

However, in this previously proposed surface acoustic wave element, no matter how small the σ/h of the resistive film 43 is, the discharge does not completely disappear and still remains, causing equipment malfunction.

Therefore, an object of the present invention is to provide a surface acoustic wave element that can completely eliminate discharge, and this object is achieved by forming an excitation transducer and a reception transducer on a piezoelectric crystal plate having pyroelectric properties. A surface acoustic wave element comprising: a piezoelectric crystal plate whose surface is chemically polished, and a surface other than the surface and an inner wall surface of a crack formed on the surface are etched with an etching solution; a receiving converter provided on the surface of the piezoelectric crystal plate in correspondence with the exciting converter; a resistive film having a resistance value high enough to prevent generation of electricity; and an electrically conductive layer covering an area other than the areas of the excitation transducer and the receiving transducer and having a resistance value lower than that of the resistive film. This is achieved by providing a surface acoustic wave element characterized by the following.

Embodiments of the present invention will be described in detail below with reference to the drawings.

First, we will examine the reason why the discharge does not completely disappear in the surface acoustic wave elements shown in FIGS. 6 to 8.

Regarding the surface acoustic wave element shown in FIG. 6, when examining the location where discharge occurs, it was observed that discharge occurred on the back surface of the piezoelectric crystal substrate 41. This is due to the following reason.

LiNbO ₃ crystals, LiTaO ₃ crystals, etc. can be obtained by a pulling method called the Czyochoralski method, but due to the uneven temperature distribution at the solid-liquid interface during pulling, dislocation layers and impurity layers may be formed in the crystals. There are some imperfect crystal parts. Even if polarization processing is performed to align the direction of spontaneous polarization in such a state, a perfect single-domain structure will not result, and it is thought that there are microscopically incomplete domain parts. It will be done. Since it is thought that this incomplete crystal part serves as a power source for discharge, there is a possibility of discharge occurring on all crystal surfaces, regardless of size.

The piezoelectric crystal substrate 41 is obtained by cutting and shaping these piezoelectric crystals at a cutting angle suitable for surface wave excitation, but those using only mechanical polishing have countless layers called processed layers on the surface. It is known to have cracks. usually,
The surface on which the excitation transducer 12S and the reception transducer 12R are formed (hereinafter referred to as the surface) is finally polished smooth by chemical polishing, so no cracks remain, and microscopically it is smooth with no processed layer. It has become a face. However, each converter 12S, 12R
The surfaces (back surface and side surfaces) that are not formed are rough to diffusely reflect bulk waves.
There are countless cracks of all sizes. For this reason, even if an electrically conductive layer such as chromium is deposited, the electrically conductive layer cannot penetrate into the very thin cracks on the back side and cover the cracks, and if there is a sudden temperature change, Pyroelectricity causes localized electrical discharge within the crack.

FIG. 9 is an enlarged cross-sectional view of the main parts of the surface acoustic wave element after the electrically conductive layer has been deposited, and the same parts as in FIG. 6 are given the same reference numerals. In the figure, 51a~
Numeral 51c is a crack, and the inner wall of the crack is not coated with the electrically conductive layer 42. Therefore, as described above, local discharge occurs within the cracks 51a to 51c.

Therefore, it is presumed that if the hole diameter of the crack is made large and the surface is smooth, the inner wall surface of the crack can be covered with the electrically conductive layer, and the discharge will be completely extinguished for the above-mentioned reason.

Therefore, in the present invention, a piezoelectric crystal plate, for example,
After chemically polishing the surface of the LiNbO ₃ or LiTaO ₃ crystal plate to make it smooth and roughening the back surface, at least the back surface of the crystal plate was coated with an aqueous solution of ammonium fluoride (NH ₄ F) and hydrogen fluoride ( HF)
or a mixture of nitric acid (HNO ₃ ) and HF.
The processed layer is removed by immersion in an etching solution such as a mixed solution for 10 to 30 minutes, and then each transducer 12S, 12R is formed on the surface, and then the electrical conductor layer 42 and the resistor are formed by vapor deposition. A film 43 was deposited to obtain the surface acoustic wave device shown in FIGS. 6 to 8. In addition, during the etching process, a resist such as a vapor-deposited thin film of Au-Cr is deposited on the surface in advance.
This may protect the surface, and then the entire crystal plate may be immersed in an etching solution to remove the processed layer.

As a result of the above-described etching, the pore diameter of the crack was enlarged, and the inner wall surface of the crack could be covered with the electrically conductive layer 42 or the resistive film 43. According to experiments, σ/h = 10 ¹² Ω or less for a temperature gradient of 60°C/hour, and σ/h = 10 ¹¹ Ω or less for a temperature gradient of 12°C/min. In the rapidly heated state, discharge could be completely removed by depositing a resistive film 43 having a resistance of σ/h=10 ¹⁰ Ω or less.

FIG. 10 is an enlarged view of the main parts of a surface acoustic wave device according to the present invention in which an electric conductor layer is deposited by vapor deposition after etching, and the same parts as in FIG. 9 are given the same reference numerals. As is clear from FIG. 10, the hole diameters of the cracks 51a to 51c are larger than those of FIG. 9, and the electrically conductive layer 42 is adhered to the inner walls thereof.

As explained in detail above, in the present invention, since the surfaces other than the surface of the piezoelectric crystal plate and the inner wall surface of the cracks formed on these surfaces are etched with an etching solution, the cracks become larger, and as a result, the piezoelectric crystal plate An electrically conductive layer applied to the crack penetrates into the crack and is applied to the inner wall surface. and,,
The resistive film covering the areas of the excitation transducer and reception transducer has a resistance value high enough not to generate creeping discharge on the piezoelectric crystal plate, and excludes the area of the excitation transducer and reception transducer. Since the electrically conductive layer covering the area has a resistance value lower than that of the resistive film, no discharge due to pyroelectricity occurs on any surface of the surface acoustic wave element due to a sudden change in temperature. Therefore, no noise is generated from the surface acoustic wave element. Therefore, equipment using this device will not malfunction as in the past.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing an example of a surface acoustic wave filter, in which Fig. 1a is a schematic perspective view, Fig. 1b is a diagram schematically showing a general filter circuit including a surface acoustic wave filter, and Fig. 2 These are explanatory diagrams explaining the generation of flashing light. Figures a and b are diagrams of the occurrence of flashing light recorded on a photographic plate, Figure 3 is an explanatory diagram explaining the generation of electric discharge, and Figure 4 is a diagram of the occurrence of flashing light recorded on a photographic plate. Equivalent circuit diagram, Figure 5 is a potential difference V _L -time characteristic diagram between two points on a pyroelectric piezoelectric substrate, and Figures 6, 7, and 8 are perspective views of previously proposed surface acoustic wave elements. 9 is an enlarged cross-sectional view of a main part of a previously proposed surface acoustic wave element, and FIG. 10 is an enlarged cross-sectional view of a main part of a surface acoustic wave element according to the present invention. 11...Piezoelectric substrate, 12s...Excitation converter, 12R
...Receiving transducer, 14, 14'...Bib-shaped electrode, 2
1... Discharge portion on photographic plate, 31... Piezoelectric crystal substrate having pyroelectricity, 32, 33... Electrode, 34, 35
...Lead wire, 36...Dirty, 41...Piezoelectric crystal plate, 4
2... Electric conductor layer, 43... Resistive film, 51a to 51
c... Kratsk.

Claims (1)

[Claims] 1. In a surface acoustic wave device in which an excitation transducer and a reception transducer are formed on a piezoelectric crystal plate having pyroelectric properties, the surface is chemically polished, and the surface excluding the surface and the surface a piezoelectric crystal plate whose inner wall surface of the crack formed in the crack is etched with an etching solution; an excitation transducer formed on the surface of the piezoelectric crystal plate; and a piezoelectric crystal plate provided on the surface of the piezoelectric crystal plate corresponding to the excitation transducer. a receiving transducer; covering areas of the excitation transducer and the receiving transducer;
and a resistive film having a high resistance value that does not cause creeping discharge on the piezoelectric crystal plate, and a resistive film that covers an area excluding the areas of the excitation converter and the receiving converter, and has a resistance value lower than the resistance value of the resistive film. A surface acoustic wave device comprising: an electrically conductive layer having a resistance value; 2. The surface acoustic wave device according to claim 1, wherein a surface portion of the piezoelectric crystal plate sandwiched between the excitation transducer and the reception transducer is covered with a resistive film.