JP3157969B2 - Manufacturing method of surface acoustic wave device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a surface acoustic wave device using a lithium tetraborate single crystal substrate.

[0002]

2. Description of the Related Art A surface acoustic wave resonator is characterized in that a direct oscillation by a fundamental wave can be obtained, a high Q can be obtained, and its resonance characteristics are determined by an electrode design for forming a piezoelectric substrate surface.

[0003] Due to such properties, it is suitable for forming a small and lightweight resonator that can operate in the frequency range of the VHF to UHF bands, and uses this frequency band for RF modulators of TVs, various wireless telephones, and communication devices. Applied to equipment.

On the other hand, it is well known that a surface acoustic wave filter is also widely used in AV equipment and communication equipment as an intermediate frequency filter in the above-mentioned frequency band.

[0005] As a substrate material of the surface acoustic wave element,
Lead titanate (PT), lead zirconate titanate (PZ
Various piezoelectric materials such as T), lithium tantalate (LT), lithium niobate (LN), and quartz are used. Among these conventional materials, PT and PZT are sintered bodies, and are advantageous in cost. However, the heat resistance is low, LN has a large electromechanical coupling coefficient but a large temperature coefficient, and quartz has good temperature characteristics, but has advantages and disadvantages such as a small electromechanical coupling coefficient and a large insertion loss. Have been used properly.

For example, in a surface acoustic wave filter used in the field of communication equipment, quartz has been frequently used in order to conform to specifications with strict temperature characteristics.

However, in the field of mobile communication, there is a movement to shift from analog communication to digital communication. In this case, the surface acoustic wave filter used in this case needs to have a small temperature coefficient and a large relative bandwidth. Therefore, the electromechanical coupling coefficient must be large, and it has been difficult to cope with the conventional piezoelectric material.

Among them, lithium tetraborate (hereinafter simply referred to as LBO) has a large electromechanical coupling coefficient, and the temperature coefficient can be reduced by setting the crystal orientation to, for example, 45 ° X cut and selecting the propagation direction in the Z direction. Because of this feature, the frequency 250 for digital mobile communications
Applications to intermediate frequency bandpass filters of about MHz are being considered.

The LBO is characterized in that the reflection efficiency of the surface wave is large, and the number of the electrode stripes of the reflector can be reduced in the resonator or the resonator type filter, so that the substrate size is 10 to 20% smaller than the conventional one. Can be

As described above, LBO is a material having potential as a substrate material of a surface acoustic wave device, but has a problem that its handling is difficult. That is, LBO has the property of dissolving in acid and water. Therefore, various considerations are required in the manufacturing process. For example, in the electrode pattern forming step, the etching method cannot be adopted because the substrate surface is affected by the etchant, and the lift-off method is used.

[0011] In addition, the frequency adjustment step also poses a problem. Generally, when a surface acoustic wave device is manufactured, a frequency adjustment step is indispensable in order to increase the yield during mass production. This will be described taking a surface acoustic wave resonator or a resonator type filter as an example. One set of surface acoustic wave resonators is composed of an interdigital excitation electrode and a pair of reflection electrodes. In order to obtain frequency characteristics, the electrode pitch is determined substantially by dividing the sound speed of the piezoelectric substrate by the frequency. In an actual surface wave, since the electrode film has a mass, the propagation speed becomes smaller than a predetermined value as it is. Therefore, when designing the element, the optimum film thickness is selected in consideration of this fact, and the electrode pattern is designed.However, actually, the electrode pattern width, the electrode film thickness, the cut direction of the substrate, etc. Due to the variation, the resonance frequency has a distribution from a predetermined value. Therefore, frequency adjustment is performed after the electrode pattern is formed.

It is known that the frequency is adjusted by controlling the thickness of the electrode film after forming the electrode pattern. If the thickness of the electrode film is reduced by etching, the propagation speed increases and the frequency increases.
_{Even if two} films are formed, the frequency increases.

Here, in the case of a surface acoustic wave device using an LBO substrate, a usual electrode film etching method cannot be used because the substrate material is soluble in acid and water as described above. Therefore, it has been proposed to form an electrode on a substrate surface once formed of a SiO ₂ thin film as a protective layer, and then perform normal electrode etching or etching with a heated alkaline solution. Attention has been paid to whether frequency adjustment should be performed while preventing noise.

The heated alkaline solution etches the Al electrode, but hardly reacts with the LBO substrate.

[0015]

Among the methods proposed above, the method of interposing an SiO ₂ thin film is originally unnecessary for the structure of the device itself, and the number of steps for formation is increased. In addition, since the thin film forming apparatus becomes large-scale, it is not suitable for mass production. In the method using a heated alkaline solution, cleaning after etching is performed with hydrophilic acetone or isopropyl alcohol (IPA). However, a problem remains because the cleaning property of the alkaline solution is poor.

Further, in order to perform frequency adjustment by performing electrode etching by the above-described method, the etching conditions are determined in advance by a preliminary test. Even if the etching conditions are determined, the repeated use of the etching solution is required. The etching ability gradually decreases due to the fatigue caused by the gas. Therefore, there is a problem that strict control of the etchant is required.

In addition, it is not known during the etching process that the frequency has actually reached the proper frequency, but only after the process is completed.

The present invention has been devised in view of the above problems, and has as its object to adjust the resonance frequency of a surface acoustic wave element using an LBO substrate easily and reliably, and to provide a dry process. Accordingly, it is an object of the present invention to provide a method of manufacturing a surface acoustic wave device whose handling is extremely simple.

[0019]

Means for Solving the Problems The present invention relates to a method for manufacturing a surface acoustic wave device in which an electrode pattern of a predetermined shape is formed on a lithium tetraborate single-crystal substrate, wherein lithium tetraborate from which a plurality of device regions can be extracted is provided. Forming a resist layer having an opening corresponding to the electrode pattern on the surface of each element region of the large single crystal substrate, and forming a metal layer serving as the electrode pattern on the large single crystal substrate through the resist layer. Forming, removing the resist layer to form an electrode pattern set in advance to be higher than a predetermined resonance frequency value, separating the large single crystal substrate for each element, A step of performing layering for each frequency value, collectively exposing the plurality of layered elements to an activated fluoride gas, and selectively selecting only a substrate surface portion excluding the electrode pattern portion. And adjusting the resonance frequency to a predetermined resonance frequency value.

[0020]

[0021]

DETAILED DESCRIPTION OF THE INVENTION The present invention, a surface acoustic wave device provided with electrode patterns, such as Al as advance the resonance frequency on the LBO substrate becomes higher, after forming an electrode pattern, for example by plasma fluoride gas such as CF ₄ By activating and exposing for a predetermined time, the surface of the LBO substrate can be etched to lower the resonance frequency.

That is, after an electrode pattern designed in advance so that the resonance frequency is higher than a predetermined value is formed on the surface of the LBO substrate and then exposed to an activated fluoride gas, the electrode pattern formed of Al or the like is etched. Instead, only the portion of the LBO substrate exposed from between the electrode patterns is etched. This is presumably because fluorine radicals in the plasma selectively act on the LBO substrate surface to decompose and remove LBO.

Since only the substrate portion is etched as described above, a step between the electrode surface and the substrate surface becomes large, an action equivalent to adding a mass to the electrode occurs, and the propagation speed of the surface wave decreases. , The resonance frequency decreases.

As a result, the resonance frequency is adjusted to a predetermined value.

Since gas etching is a completely dry process, cleaning after etching is not necessary. In addition, since the supply and discharge of gas are always performed in a steady state in a steady state, there is no problem that the etching rate is reduced due to fatigue of the liquid when etching is performed repeatedly as in the case of liquid etching.

In addition, since the treatment is performed in a gas, the resonance frequency can be measured during the etching step, and the etching is stopped when the resonance frequency reaches a predetermined value, thereby increasing the adjustment accuracy.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for manufacturing a surface acoustic wave device according to the present invention will be described with reference to cross-sectional views in main steps of FIGS. 1 (a) to 1 (g). 1A to 1E show a large LBO substrate from which a plurality of surface acoustic wave elements, for example, a surface acoustic wave resonator and a surface acoustic wave filter, can be extracted. 1) to 1 (g) are cross-sectional views after each element is cut.

First, a 45 ° X-cut LBO single crystal substrate 1 having a predetermined thickness and a completely polished surface to remove a strain layer is prepared.

Next, as shown in FIG. 1A, a positive resist layer 2 is formed by spin coating on the entire surface of the LBO single crystal substrate 1.

Next, an exposure process is performed as shown in FIG.

Specifically, a mask 3 on which a negative electrode pattern corresponding to a plurality of elements is formed is brought into close contact with the substrate 1 and exposed to ultraviolet light 4. The portion 3a of the mask 3 is a light-transmitting portion corresponding to the portion where the electrode is formed, and the portion 3b is a light-shielding portion.

Next, as shown in FIG. 1C, a resist pattern is developed.

More specifically, the resist layer is dissolved and removed only in the exposed portion, that is, the electrode forming portion by immersing the substrate in an etching solution for a positive resist and developing, thereby forming a resist pattern 2b.

Next, as shown in FIG. 1D, a metal layer serving as an electrode pattern is formed.

Specifically, Al is vapor-deposited on the entire surface of the LBO substrate 1 on which the resist pattern 2b is formed, and a metal layer serving as an electrode layer is formed as a film. The metal layer includes an electrode pattern portion 5a formed directly on the surface of the substrate 1 and a resist pattern 2
5b formed on the resist pattern 2b. Here, since a positive type resist is used, the step portion of the resist pattern 2b has an inverted mesa structure.
Is not continuous. Thereby, in the subsequent process, the edge shape of the electrode pattern 5a after lift-off becomes sharp.

The metal layer is not limited to Al but may be gold or the like.

Next, as shown in FIG. 1E, an electrode pattern is formed.

Specifically, the electrode pattern 5a is formed by a lift-off method. That is, the LB on which the metal layer is formed
The O-substrate 1 is put into a liquid serving as an ultrasonic medium, and a predetermined ultrasonic wave is applied to remove the resist pattern 2b and the metal layer 5b formed thereon, thereby removing only the electrode pattern portion 5a. Let it remain. In this state, each element has an average resonance frequency of 0.1 to 0.2 from the target value of the center frequency, taking into account the variation in the process and the thickness of the substrate.
%.

Next, as shown in FIG. 1 (f), element isolation / layering processing is performed.

More specifically, FIG.
(E) is cut at the dotted line, separated into individual elements, the resonance characteristic of each element is measured, and the resonance frequency is stratified into groups of several stages according to the deviation from the target value.

Next, a frequency adjustment process is performed.

One group of the element groups separated into layers is put together in, for example, one batch in a plasma etching apparatus in a state where it is arranged in a tray, evacuated, then CF ₄ gas is introduced, and high frequency power is applied. Then, the gas is activated and predetermined etching is performed to lower the resonance frequency to a target value.

As a method for activating the gas in the etching process, a plasma method in which high-frequency power is applied to the gas is preferably used as described above, but a method in which another energy such as ultraviolet light is applied to excite the gas may be used.

As the fluoride gas, carbon tetrafluoride (CF ₄ ) is preferably used because it is chemically stable and has no toxicity. However, as long as the electrode material of the element is not corroded, excitation of fluorine by activation is performed. Other gases that produce seeds, for example, Si
F _4, etc. can be used.

Here, the LBO substrate 1 exposed from the space between the electrode patterns 5a is etched on the side where the electrode patterns 5a are formed by using these gases, whereby FIG.
A surface acoustic wave device as shown in FIG. Adjustment of the resonance frequency is achieved by the partial etching of the LBO substrate 1.

According to the experiment of the present inventor, it has been found that the amount of change in the frequency can be estimated depending on the etching rate, and the adjustment of the frequency is further facilitated.

That is, the CF ₄ gas and the Ar gas are supplied for 10 seconds each.
LB by exciting the plasma by applying a high frequency of 13.56 MHz and 200 W while maintaining the gas pressure at 0.005 torr while maintaining the gas pressure at 0.005 torr.
When the O substrate 1 was etched, the etching rate was 90 ° / min. The relationship between the change in the thickness of the substrate 1 and the change in the resonance frequency differs depending on the electrode pattern 5a. In the case of a resonator having a center frequency of about 250 MHz, the change in the frequency corresponding to the etching amount is 13%.
It was 5 KHz / min.

The element whose frequency has been adjusted as described above is mounted on a package, connected to a lead terminal by wire bonding or the like, and then hermetically sealed with a lid to complete a surface acoustic wave device.

The sealing structure of the package needs to completely prevent the penetration of moisture as described above.
This may be a conventional metal hermetic case with lead terminals that is used for resistance welding, but for the recently demanded surface mount type, a ceramic package with seam welding and sealing is suitable. ing.

The internal space of the sealed package is N ₂ ,
An inert gas such as Ar or He is filled to prevent the electrodes of the surface acoustic wave element from being corroded or oxidized, thereby preventing the resonance characteristics from changing over time.

[0051]

[0052]

According to the present invention, only the exposed surface of the LBO substrate is selectively etched by gas-etching the surface acoustic wave element after the electrode pattern is formed, and the resonance frequency is lowered to a predetermined value. Can be.

Further, since this gas etching is a dry process, conventionally troublesome steps such as cleaning after the etching can be omitted, and the etching rate can be easily controlled.

Further, according to the method of the present invention, the resonance characteristics of the elements can be adjusted while individually monitoring during the etching, so that an element with extremely high frequency accuracy can be obtained.

As a result, a surface acoustic wave device having a stable resonance characteristic can be manufactured with a simplified process and a high yield.

[Brief description of the drawings]

FIGS. 1A to 1G are cross-sectional views illustrating main steps of a method for manufacturing a surface acoustic wave device according to the present invention.

[Explanation of symbols]

 1 ... LBO substrate 5a ... electrode pattern

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-5-90865 (JP, A) JP-A-4-81110 (JP, A) JP-A-1-231412 (JP, A) JP-A-56- 103513 (JP, A) JP-A-2-189011 (JP, A) JP-A-63-160409 (JP, A) JP-A-61-251221 (JP, A) JP-A-4-365212 (JP, A) JP-B 61-41446 (JP, B2) JP-B 3-37329 (JP, B2) JP-B 1-15166 (JP, B2) (58) Fields studied (Int. Cl. ⁷ , DB name) H03H 3/08-3/10

Claims (2)

(57) [Claims] 1. A four to lithium borate single crystal substrate A method of manufacturing a surface acoustic wave device provided with electrode patterns of a predetermined shape, large single of lithium tetraborate which a plurality of element regions can be extracted
Corresponds to the electrode pattern on the surface of each element area of the crystal substrate
Forming a resist layer having a shaped opening , the electrode on a large single crystal substrate through the resist layer
Forming a metal layer serving as a pattern, removing the resist layer, and previously setting a predetermined resonance frequency value
Steps to form an electrode pattern set to be higher
The large single crystal substrate is separated for each element, and each element is resonated.
A step of performing layering for each frequency value ,
Exposure to substrate gas, excluding the electrode pattern
Only a predetermined frequency
Adjusting the value to a value . 2. The method for manufacturing a surface acoustic wave device according to claim 1, wherein a resonance frequency is simultaneously measured during the etching process, and the etching process is terminated when the resonance frequency reaches a predetermined resonance frequency.