JPS59188221A - Preventing method of aging change in resonance frequency of piezoelectric device - Google Patents

Abstract

PURPOSE:To prevent the aging change in resonance frequency by vapor-depositing magnesium fluoride on the interdigital transducer electrode of a piezoelectric device or sticking surface between it and a reflector. CONSTITUTION:The interdigital transducer (IDT) electrode 2 is stuck to the surface of a piezoelectric substrate 1. Then, magnesium fluoride 9 is vapor- deposited on the IDT electrode 2 or the surface of the piezoelectric substrate where the reflector 3 is stuck to specific thickness. Therefore, the inclination of a decrease in resonance frequency considered as results from the condensation of an adhesive for fixing the piezoelectric substrate to a holder is canceled by the inclination of an increase in resonance frequency due to an aging change of vapor deposited magnesium fluoride.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention prevents the resonant frequency of a piezoelectric device using surface acoustic waves (hereinafter referred to as SAW), space waves (hereinafter referred to as BSW), etc. from deteriorating over time. Regarding the method.

Conventionally, as in a SAW resonator, an interdigital transducer electrode (hereinafter referred to as an IDT electrode) is attached to the surface of a pressure-sensitive substrate, and by applying a full alternating current electric field to this, the waves excited within the substrate surface or bulk are generated. The resonant frequency of the piezoelectric device used generally exhibits negative aging characteristics. The reason for this is not clear, but the solvent of the adhesive for fixing the entire piezoelectric substrate to the holder adheres to the surface of the piezoelectric substrate, and as the adhesive condenses, the piezoelectric substrate bends and reduces the IDT electrode finger spacing. It is thought that this is due to the combined effects of expansion.

The non-invention is the conventional SAW dollar vibrator, etc. ID as mentioned above.
This was done in view of the problems of piezoelectric devices that utilize waves excited by piezoelectric electrodes, and the IDT of piezoelectric devices
It is an object of the present invention to provide a method for preventing changes in resonance frequency over time of a piezoelectric device 7j, in which changes in resonance frequency over time are canceled by changes in the resonant frequency over time by completely depositing magnesium fluoride on an electrode or the attachment surface of the electrode and a reflector. purpose.

Hereinafter, the present invention will be explained in detail based on examples.

FIG. 1 is a diagram showing an example of a SAW resonator to which the present invention is fully applied.

That is, a T-electrode 2 of a predetermined logarithm and thickness is formed from aluminum on a quartz iso-piezoelectric substrate 1, and reflectors 3, 3 are formed on both sides thereof by gold capping and etching, and this is placed in a cage as shown in FIG. Store it. The holder is composed of a base 4 and a sealing lid 5, and the piezoelectric substrate 1 of the SAW resonator is fixed onto the base 4 with an adhesive 6, and its IDT
The electrode is coupled to a lead wire 8.8 which hermetically passes through the base 4 with a glass 7 and is electrically insulated from the base.

The resonant frequency of the SAW resonator configured in this way decreases over time, and for example, as shown in FIG.
In the second SAW resonator, the number of units per day is already 20.
ppmf and its change trend follows an exponential curve.

As mentioned above, the cause of such a change over time is probably due to the adhesion of the adhesive solvent to the surface of the piezoelectric substrate and the distortion of the piezoelectric substrate due to the condensation of the adhesive.

So this problem? To solve this problem, in the present invention, a magnesium fluoride film 9 is deposited on the surface of the piezoelectric substrate 1 to which the IDT electrode 2 and the reflector 3,3 are attached through a suitable mask.

Since it is difficult to control the amount of magnesium fluoride (b4gFz) evaporated while directly measuring the thickness of the evaporated film, it is easy and reliable to control it while observing the synchronized frequency f during evaporation.

Figure 5 shows a crystal SAW# having the aging characteristics shown in Figure 3.
Initial resonance frequency change amount △f/f'I3ooppm in the vibrator
FIG. 4 is a diagram showing the results of an experiment in which changes in resonance frequency over time were investigated when magnesium fluoride was deposited in amounts of 1,000 ppm and 1,000 ppm.

As is clear from this figure, when the amount of deposited magnesium fluoride exceeds the appropriate amount, the time-dependent characteristic of the resonance frequency becomes positive.

When considering the reason for this effect, it is generally found that ID
If a vapor-deposited film, whether metal or non-metal, is attached to the surface of the piezoelectric substrate on which all the T' electrodes are attached, the resonant frequency will be lowered due to the mass loading effect, but even if all the substances with extremely low bulk wave propagation speed are attached, the mass loading will be reduced. It seems that the speed of waves propagating near the surface of the piezoelectric substrate decreases due to reasons other than loading, and when this material changes over time to a material with a high bulk wave propagation speed, the entire wave propagates near the surface of the piezoelectric substrate. It can be seen that this increases the speed of the waves.

In other words, it is known that the vapor-deposited film of magnesium fluoride is initially in an amorphous state and its filling rate is low and that it recrystallizes over time and the filling rate increases, but magnesium fluoride has a low filling rate. If it is low, the sound speed is low and as the filling rate increases due to recrystallization, the sound speed increases, so IDT fi
It is thought that if a substance with a sonic velocity between the other poles is attached, the velocity of the waves propagating all over the substrate surface or just below it will also be the same, and the resonance frequency will increase.

Therefore, it goes without saying that the amount of magnesium fluoride deposited must be determined after experimentally understanding the aging characteristics of each resonator, but this is not the case for resonators that are mass-produced through the same processing steps. It is possible to perform batch processing.

However, as a result of vapor depositing magnesium fluoride, the Q of the device decreases somewhat, but the degree of decrease is at most a large number compared to the appropriate amount of vapor deposition, as shown in Figure 6, and no particular problem arises. do not have.

Since the present invention uses the method described above, it is extremely easy and reliable to achieve resonance in piezoelectric devices such as SA-included camera cameras, filters, etc. using the waves excited on the surface or inside of the piezoelectric substrate by one pole of the IDT1. Change in frequency over time?
Since it is possible to prevent this, it is extremely effective in ensuring the characteristics of the piezoelectric device over a long period of time.

Similarly, in the present invention, magnesium fluoride does not necessarily have to be entirely deposited on the entire surface of the (II) T electrode and reflector as shown in the embodiment, but may be partially deposited.

Furthermore, the method according to the present invention need not be limited to SAW devices only (it is applicable to any type of device in which bulk waves are excited in a piezoelectric substrate by an IDT electrode).

[Brief explanation of drawings]

Figure 1 shows the structure of a SAW resonator to which the present invention is applied, and Figure 2 shows the SAW resonator shown in Figure 1 housed in a holder. 3 is a diagram of experimental data showing the change in resonance frequency over time. FIG. 4 is a diagram showing the state of gold deposited on the SAW resonator shown in FIG.
6 and 6 are diagrams of experimental data showing the relationship between the amount of magnesium fluoride vapor deposited and the amount of change in the resonant frequency and Q of the resonator, respectively. 1...Piezoelectric substrate, 2...
・Interdigital transducer electrode, 3...
...Reflector. 9...Magnesium fluoride vapor deposited film patent applicant Toyo Tsushinki Co., Ltd. 1 Figure P 2 ω 3 fiJ Ariρ Kaya, / Ii

Claims (1)

[Claims] In a piezoelectric device, an interdigital transducer electrode is attached to the surface of a piezoelectric substrate, and an alternating current electric field is applied to the electrode. A method for preventing changes in the resonant frequency of a piezoelectric chair over time, which is characterized by depositing a predetermined thickness of magnesium fluoride on the surface of the piezoelectric substrate to which the interdigital transducer electrode or the reflector are attached.