JPS6320907A - Frequency adjusting method for piezoelectric vibrator - Google Patents

Abstract

PURPOSE:To contrive to suppress the change in a static capacitance Cd and a capecitance ratio gamma by applying laser radiaton onto an electrode face so as to apply mesh spot trimming thereby attaining the trimming with respect to a decreasing frequency as well as an increasing frequency. CONSTITUTION:The mesh spot trimming is applied to the electrode face by using a laser to adjust the frequency. The frequency is adjusted in both increasing and decreasing directions by varying a trimming laser output as shown in figure A. The resonance frequency tends to increase as the laser output is increased as the result of experiment, but, on the other hand, the frequency has a decreasing tendency with further increase of the resonance frequency. That is, the resonance frequency is decreased proportional to number of spots with the mesh trimming at a large laser spot diameter, while the resonance frequency is increased proportional to the spot number with the trimming at a smaller spot diamter. Moreover, the static capacitance Cd and the capacitance ratio gamma are almost unchanged independenly of the trimming as shown in figures B and C.

Description

[Detailed Description of the Invention] [Summary] A frequency adjustment method that performs spot trimming by irradiating a metal film electrode with a laser as a method of adjusting the resonance frequency of a piezoelectric vibrator in an upward or downward direction.

[Industrial application field]

The present invention relates to a method for adjusting the frequency of a piezoelectric vibrator using spot trimming.

A piezoelectric vibrator consists of both sides of a thin plate made of a dielectric material with a large electromechanical coupling coefficient, such as a piezoelectric crystal such as lithium niobate (LiNbOz), lithium tantalate (LiTaOz) + quartz, or a piezoelectric ceramic such as lead zirconate titanate (PZT). metal electrodes are provided by vacuum evaporation or sputtering, and when an alternating current electric field is applied between these electrodes, stress with a frequency equal to this is generated due to the piezoelectric effect, and the frequency of the electric field becomes equal to the natural frequency of the vibrator. If it loses, it will cause a strong resonance.

Such piezoelectric vibrators are used for clock signals for microcomputers and as vibrators for voltage-controlled oscillators.

[Conventional technology]

Gold (Au) and silver (
Ag) is used.

That is, LiNbO3 and LiTa are used as piezoelectric substrate materials.
For those using 0°, the base layer has a thickness of approximately 500°.
A nichrome (Ni-Cr) film with a thickness of 1,000 to 5,000 people is used as an electrode.
Vapor-deposited film is used.

Generally, a vibrator exhibits thickness shear vibration, and the resonant frequency is uniquely determined by the thickness of the diaphragm (hereinafter referred to as the substrate), and this relationship is expressed by the following equation.

fr = v / 2t Here, f7 is the resonant frequency, ■ is the propagation speed of thickness vibration, and t is the thickness of the board. Therefore, to adjust the value of the resonant frequency f, just change the value of the board thickness t. However, since the thickness t of the substrate cannot be changed after the electrodes are formed, the propagation velocity V is changed.

Here, it is known that the propagation velocity V changes depending on the following factors.

■ Elastic constant and density of the substrate, (can be changed by ion implantation, heat treatment, etc.) ■ Added mass of the electrode, (possible by changing the electrode material or film thickness) ■ Electromechanical coupling coefficient of the substrate, (determined by the material) ■ The dimensions of the electrode (can be changed by trimming) Therefore, in general, the resonant frequency f1 is adjusted using method (■).
i1 adjustment is being performed.

Here, conventional trimming methods include longitudinal trimming, which uses a laser to cut the electrode film in the longitudinal direction of the substrate, and widthwise trimming, which cuts the electrode film in the width direction of the substrate. The resonant frequency increases, and the terminal capacitance and capacitance ratio tend to change.

Figure 2 shows the equivalent circuit of a piezoelectric vibrator, where C4 is the capacitance between opposing electrodes, L is the equivalent inductance of the series resonant circuit, C is the equivalent capacitance of the series resonant circuit, and R is the series resonance. The equivalent resistance of the circuit is expressed as follows, and the capacitance ratio γ is expressed as r=c,/C, and is also used as a guideline to express the frequency interval between the resonant frequency f and the anti-resonant frequency f3.

r=K fr/(fm-rr) Here K is a proportionality constant.

Also, the Q value is Q=ωL/R Here ω is the angular frequency, There is a relationship between

Now, when adjusting the resonant frequency of a piezoelectric vibrator by trimming the electrode film, it is desirable that the resonant frequency can be changed in either an upward direction or a downward direction, and the capacitance C6 between the electrodes and the capacitance ratio γ are It is desirable that it does not change.

However, in reality, these characteristics change significantly when longitudinal trimming or widthwise trimming is performed.

The broken line 1 in FIGS. 3(A) to 3(C) shows the conventional trend; the resonant frequency shifts to an upward direction as the electrode area decreases as shown in FIG. ), the capacitance 1 ca tends to decrease, and the capacitance ratio γ tends to increase, as shown in (C) of the same figure.

[Problem that the invention seeks to solve]

As mentioned above, according to the conventional trimming method, the resonant frequency always tends to rise, and the capacitance C4 decreases.
Although the capacitance ratio γ is increasing, the challenge is to find a trimming method that can lower the resonance frequency and minimize changes in the capacitance C4 and the capacitance ratio γ.

[Means for solving problems]

The above problem can be solved by irradiating the electrode surface with a laser and performing spot trimming in a mesh pattern.

[Effect]

FIG. 1 is a perspective view of a piezoelectric vibrator embodying the present invention, in which frequency adjustment is performed by spot trimming in a mesh pattern using a laser.

Actual vA2 in Figures 3 (A) to (C) shows the tendency when laser trimming is performed by applying the present invention, and by changing the laser output for trimming, as shown in Figure 3 (A), It can be adjusted both in the upward and downward directions.

Experimental results show that as the laser output increases, the resonance frequency tends to increase, but as the laser output increases further, the resonance frequency tends to decrease.

That is, when trimming is performed in a mesh pattern with a large laser spot diameter, the resonant frequency decreases in proportion to the number of spots.

On the other hand, when trimming is performed with a small spot diameter, the resonance frequency increases in proportion to the number of spots.

Note that, as shown in FIGS. 3B and 3C, the values of the capacitance C6 and the capacitance ratio T hardly change even after trimming.

This effect was discovered as a result of experiments, and the reason is not clear, but the diameter of the laser spot is small, at most 50 μm, and the holes are not exactly opposite to the upper and lower electrode films. Therefore, even if holes are made in the electrode film on the substrate in a mesh pattern, the electric field is applied uniformly, and therefore, it is thought that the change in capacitance C6 is small.

Furthermore, the reason for the small change in the capacitance ratio γ is presumed to be that the decrease in the amount of energy trapped due to the decrease in the electrode area can be ignored.

〔Example〕

Using LiTa0= as the piezoelectric crystal, 3° around the Y axis
The rotated X cut is the board surface, and the longitudinal direction is Y.
The direction rotated 50' clockwise from the axis was defined as the in-plane direction.

The size of this board is 3.5 mm (length') x 48
The dimensions are 0 μm (width) x 167 μm (thickness), and the length of the electrode film is 1.4 mm.

Here, the electrode film was formed by forming an Au film with a thickness of about 2000 mm on a NiCr film with a thickness of about 500 mm.

Next, the laser light source to be trimmed has a rated output of 101
A YAG (yttrium aluminum garnet) laser was used.

FIG. 4 shows changes when the horizontal axis is the laser output (evaluated using a dial scale) and the vertical axis is the rate of change in the resonance frequency, and the table shows the measurement results.

By selecting the table or laser power, the resonant frequency can be adjusted in either direction.

Next, in Figure 5, the resonance frequency is set to 6000ppm (0.6%).
This graph shows the changes in the equivalent series resistance R3 and capacitance C4 when the Q value is adjusted to 9. The broken line 3 is the result of conventional trimming in the longitudinal direction, and the solid line 4 is the result of trimming by the method of the present invention. This is the result.

Here, trimming is performed with a spot diameter of 43.2 μm.
Adjusted to reduce the high resonance frequency.

As is clear from the figure, according to the conventional method, the capacitance C1 decreases in proportion to the amount of adjustment, but according to the present invention, it hardly changes.

Also, as the amount of adjustment increases, the equivalent series resistance Rs tends to increase and the Q tends to decrease, but the Q is still around 4000.
value can be maintained.

〔Effect of the invention〕

As described above, by implementing the present invention, it is possible to perform not only frequency increasing type trimming but also decreasing type trimming as in the past.
Therefore, the production yield is improved and the capacitance ffi Cd
Therefore, the change in the capacitance ratio T can be suppressed to a small extent.

[Brief explanation of drawings]

Figure 1 is a perspective view of a piezoelectric vibrator embodying the present invention, Figure 2 is an equivalent circuit of the piezoelectric vibrator, Figures 3 (A) to (C) are explanatory diagrams showing the effects of electrode trimming, and Figure 4. Figure 5 shows the relationship between laser output and resonant frequency, and Figure 5 shows the relationship between Q and R as a function of the rate of change in resonant frequency. and a characteristic diagram of C4.

Claims (1)

[Scope of Claims] A vibrator comprising metal film electrodes facing each other on both sides of a diaphragm made of a piezoelectric crystal or piezoelectric ceramic, and electrode extraction portions provided in mutually opposite directions from the electrodes, the electrode surface as described above. A method for adjusting the frequency of a piezoelectric vibrator, characterized by irradiating the laser with a laser and performing spot trimming in a mesh pattern.