JP3858312B2 - Surface acoustic wave device and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface acoustic wave element (SAW device) and a method for manufacturing the same, and more particularly to an electrode protective film and a method for manufacturing the same.
[0002]
[Prior art]
A surface mount type using a ceramic package has become the mainstream for miniaturization of SAW devices. In such a package structure, the interdigital transducer electrode portion of the SAW device is short-circuited due to a minute metal scrap due to plating of the lid portion or a solder jump when the lid and the package are soldered, and cannot be used. It was happening.
[0003]
As a method for preventing such defects due to metal scrap, a method described in Japanese Patent Application Laid-Open No. 6-132775 has been known. FIG. 9 shows a conventional structure, where 1 is a first package, 2 is a second package, 3 is a surface acoustic wave element pattern, 4 is a conductive adhesive, 5 is a lead wire, and 6 is elastic. It is a small piece of a substrate on which a surface wave element pattern is formed.
[0004]
In this conventional method, a first package 1 and a second package 2 are prepared, respectively, and a part or all of the surface acoustic wave element pattern 3 is joined by anodic bonding in the first package 1, and metal The second package 2 made in step 2 has a two-stage structure that seals the entire surface acoustic wave element.
[0005]
[Problems to be solved by the invention]
In order to prevent the occurrence of defects due to shorting of metal scrap, it is required that the SAW characteristics are not affected as much as possible, and can be manufactured by a simple process at a low cost. An object of the present invention is to provide a SAW device and a method for manufacturing the SAW device that prevent a short circuit by forming an insulating film only on the surface of an electrode in a simple process and without causing large fluctuations in characteristics. .
[0006]
[Means for Solving the Problems]
In order to solve this problem, the present invention provides an electrode by applying a photosensitive resin after forming an electrode pattern of an interdigital transducer electrode having a comb shape and exposing the electrode pattern from the back surface of the piezoelectric substrate by a self-alignment method. In this structure, selective film formation is possible by exposing only to the photosensitive resin, and an insulating film is formed only on the electrode surface.
[0007]
As a result, since the insulating film is not formed on the surface of the piezoelectric substrate, it is possible to prevent a short circuit without significantly changing the surface acoustic wave element characteristics, and the frequency characteristics can be adjusted by etching the insulating film formed on the surface. Is also possible.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The invention described in claim 1 of the present invention includes a piezoelectric substrate, a comb-shaped interdigital transducer electrode provided on the piezoelectric substrate, and an insulating film formed on the interdigital transducer electrode , The insulating film is a surface acoustic wave element that is formed on the interdigital transducer electrode and part of both side surfaces, and is not formed on the piezoelectric substrate surface between the electrodes of the interdigital transducer electrode. It has the effect of preventing defects due to short-circuiting of the interdigital transducer electrodes caused by metal scraps without increasing the element characteristics, particularly insertion loss.
[0009]
According to a second aspect of the present invention, there is provided a step of forming a comb-shaped interdigital transducer electrode on the surface of the piezoelectric substrate, and a negative type on the surface of the piezoelectric substrate on which the interdigital transducer electrode is formed. Forming on the piezoelectric substrate by applying and drying photosensitive resin and irradiating light for exposing the photosensitive resin from the surface of the piezoelectric substrate where the interdigital transducer electrode is not formed Exposing the exposed photosensitive resin, removing the photosensitive resin formed on the interdigital transducer electrode by development, and on the surface of the piezoelectric substrate on which the interdigital transducer electrode is formed. Forming an insulating film on the photosensitive resin, and simultaneously dissolving and removing the photosensitive resin. A method of manufacturing a surface acoustic wave device including a step of removing the insulating film, and a non-photosensitive material can be used as the insulating film, so that the range of materials that can be used is widened, depending on the material of the piezoelectric substrate. It has the effect | action that an optimal material can be used.
[0010]
According to a third aspect of the present invention, the thickness after drying of the negative photosensitive resin formed on the piezoelectric substrate on which the interdigital transducer electrode is formed is determined from the thickness of the interdigital transducer electrode. 3. The method of manufacturing a surface acoustic wave device according to claim 2 , wherein the insulating film is formed from the surface of the interdigital transducer electrode to a part of both ends by forming the thickness as described above. In addition, the effect of preventing the short circuit of the electrode due to the metal scrap can be more reliably performed.
[0011]
Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 and 2.
[0012]
(Embodiment 1)
FIG. 1 shows a cross-sectional structure of an interdigital transducer electrode portion of a surface acoustic wave device according to a first embodiment of the present invention, wherein 1 is a piezoelectric substrate, 21 is an interdigital transducer electrode, and 31 is an insulating film according to the present invention.
[0013]
In FIG. 1, an insulating film 31 functions to prevent a metal scrap (not shown) on the interdigital transducer electrode 21 from dropping from a lid and short-circuiting during or after soldering. Made of a conductive polyimide film.
[0014]
In this description, the example in which the insulating film is made of positive photosensitive polyimide has been described. However, the present invention can be similarly implemented by using other organic resins, silicon oxide, silicon nitride oxide, organosilicate, and the like.
[0015]
(Embodiment 2)
FIG. 2 shows a cross-sectional structure of the interdigital transducer electrode portion of the surface acoustic wave device according to the second embodiment of the present invention, wherein 1 is a piezoelectric substrate, 21 is an interdigital transducer electrode, and 31 is an insulating film according to the present invention.
[0016]
In FIG. 2, an insulating film 31 serves to prevent a metal scrap (not shown) on the interdigital transducer electrode 21 from dropping from the lid and short-circuiting during or after soldering. It is made up of.
[0017]
In the present embodiment, the insulating film 31 is formed not only on the surface of the interdigital transducer electrode 21 but also on a part of both side surfaces, and has an effect of more reliably preventing a short circuit due to metal scrap.
[0018]
In this description, the example in which the insulating film is formed of a silicon oxide film has been described. However, the present invention can be similarly implemented by using other organic resin, silicon nitride oxide, organosilicate, or the like.
[0019]
【Example】
Next, specific examples of the present invention will be described.
[0020]
Example 1
The surface acoustic wave device according to the first embodiment shown in FIG. 1 was produced as follows. First, an 872 MHz ladder type surface acoustic wave filter having an electrode pattern configuration (a) and an equivalent circuit configuration (b) as shown in FIG. 3 was used. Reference numeral 1 denotes a piezoelectric substrate, and a 36 ° YX lithium tantalate crystal is used in this embodiment. 2 is an interdigital transducer electrode and 3 is a reflector electrode, which are made of an aluminum film.
[0021]
Although it produced using the board | substrate with which such a filter element was formed, the cross-sectional structure of the element explaining the creation process is shown in FIG. FIG. 4A shows a cross section of the interdigital transducer electrode portion of the filter shown in FIG. 1 is a piezoelectric substrate, and 21 is an interdigital transducer electrode. A positive photosensitive polyimide (Nissan Chemical Co., Ltd.) was applied to the entire surface of the piezoelectric substrate including the interdigital transducer electrode 21 by spin coating on the surface on which the electrode was formed, prebaked and dried. This film formation state is shown in FIG.
[0022]
Next, as shown in FIG. 4C, the positive photosensitive resin between the electrodes was exposed by irradiating ultraviolet light from the back surface of the piezoelectric substrate 1. Thereafter, the exposed portion was removed with a developing solution and cured by heating at about 300 ° C., thereby producing a surface acoustic wave device having the configuration shown in FIG. At this time, the thickness of the polyimide film as the insulating film 31 was set to 70 nm, but the frequency change was smaller than the initial value by about 2.5 MHz, but the insertion loss only increased by 0.1 dB or less, and the surface acoustic wave There were no practical problems with the characteristics.
[0023]
This element was examined for an effect of preventing a short circuit when a metal scrap falls. In the evaluation method, after several Ni particles having a shape of about 20 μm were dropped on the interdigital transducer electrode, a voltage at which the insulation between the electrodes was broken by the CV charge method was obtained.
[0024]
As a result, the insulation property was destroyed at a voltage of 1 V or less when there was no polyimide film as an insulation film, but it was not destroyed even at a voltage of 100 V or more when formed to 70 nm. In this embodiment, the film thickness is set to 70 nm. However, the film thickness for obtaining the short-circuit preventing effect may be 40 nm or more, and if the insertion loss is allowed to 0.5 dB or less, it is preferably 150 nm or less. When an insulating film was formed between the electrodes by spin coating under the same conditions, drying, and immediately heating and curing, the insertion loss was 0.5 dB.
[0025]
(Example 2)
Using a quartz substrate with an oscillator having a center frequency of 224 MHz, positive photosensitive polyimide is applied over the entire surface, dried, and then irradiated with ultraviolet light from the back of the substrate to expose the resin between the electrodes and develop. Then, the resin was removed and heat-cured at 300 ° C. to form an element having the structure shown in the first embodiment.
[0026]
For the polyimide film thus formed and the element that was applied and dried over the entire surface, immediately heated and cured, and the insulating film was left between the electrodes, the relationship between the frequency variation and the insertion loss was determined by changing the film thickness. . The result is shown in FIG.
[0027]
In the figure, (A) shows the characteristics of the device according to the present invention, and (B) shows the device characteristics when polyimide is left on the entire surface. Although the element (A) according to the present invention was prepared under the same conditions, the insertion loss did not change even when the frequency was changed, whereas the element (B) in which the polyimide film was formed on the entire surface was obtained. The insertion loss increases simultaneously with the frequency change, and it has been found that not forming a polyimide film between the electrodes has a great effect on reducing the influence on the device characteristics. This element was also evaluated for short-circuit prevention effect, but the film thickness necessary for short-circuit prevention was about 40 nm.
[0028]
Example 3
The surface acoustic wave device according to the first embodiment shown in FIG. 1 was produced as follows. First, the insulating film 31 was formed on the surface acoustic wave device having the electrode pattern configuration as shown in FIG. The piezoelectric substrate 1 uses lithium niobate (LiNbO ₃ ), the center frequency is 902 MHz, and the electrode design is a longitudinal mode system. FIG. 7A shows a cross-sectional shape of the interdigital transducer electrode portion of the element.
[0029]
1 is a piezoelectric substrate, and 21 is an interdigital transducer electrode. On this board | substrate, it apply | coated by the spin coat using the negative photosensitive photoresist, and was dried. The cross-sectional shape at this time is shown in FIG. In the figure, 41 is a negative resist.
[0030]
Then, as shown in FIG.7 (c), the ultraviolet light was irradiated from the back surface of the board | substrate, and the resist currently formed between electrodes was exposed. Thereafter, when development is performed, as shown in FIG. 7D, the resist on the electrode is removed and the surface of the electrode is exposed, and a silicon oxide film of 40 nm is formed on this surface by sputtering (FIG. 7E). ).
[0031]
Finally, the negative resist was removed with acetone, and at the same time, the silicon oxide film on the resist was removed, thereby producing the surface acoustic wave device of the second embodiment shown in FIG. As for the characteristics of the device thus fabricated, the frequency change was 1.5 MHz and the insertion loss was only 0.02 dB or less. Further, the short-circuit preventing effect was measured by the CV charge method, but it had a withstand voltage of 70 V or more.
[0032]
In this embodiment, the silicon oxide film is formed by sputtering. However, the present invention is not limited to sputtering, and may be vacuum vapor deposition or chemical vapor reaction (CVD), spin coating, printing, or A method such as dipping may be used, and the film forming method is not particularly limited. In addition, the insulating film is not limited to silicon oxide, and is not particularly limited as long as it is a material that is not eroded by the negative resist stripping removal liquid and has a heat resistance that is smaller than the density of the electrode film material and can withstand solder reflow. There is nothing.
[0033]
Example 4
The surface acoustic wave device according to the second embodiment shown in FIG. 2 was produced as follows. First, an insulating film was formed on a surface acoustic wave device having an electrode pattern configuration as shown in FIG. The substrate is lithium niobate (LiNbO ₃ ), the center frequency is 902 MHz, and the electrode design is a longitudinal mode system. FIG. 8A shows the cross-sectional shape of the interdigital transducer electrode portion of the element.
[0034]
1 is a piezoelectric substrate, and 21 is an interdigital transducer electrode. On this substrate, a negative photosensitive photoresist was applied by spin coating and dried. The resist film thickness at this time is formed slightly thinner than the electrode film thickness as shown in FIG. In the figure, 41 is a negative resist.
[0035]
Thereafter, ultraviolet light is irradiated from the back surface of the substrate, the resist formed between the electrodes is exposed, and development is performed. As shown in FIG. 8C, the resist on the electrodes is removed and the electrode surface is exposed. A silicon oxide film having a thickness of about 40 nm was formed on this surface by sputtering. At this time, the silicon oxide is partially formed not only on the electrode surface but also on both sides as shown in FIG.
[0036]
Finally, the negative resist was removed with acetone, and at the same time, the silicon oxide film on the resist was removed, thereby producing the surface acoustic wave device of the second embodiment shown in FIG. In the surface acoustic wave device thus prepared, the insulating film 31 is not formed on the piezoelectric substrate. However, not only the surface but also a part of both side surfaces are formed on the electrode surface, so that a short circuit can be prevented. There was a greater effect, and even with a film thickness of 40 nm, a withstand voltage of 100 V was obtained by the CV charge method. The frequency change and insertion loss were almost the same as in Example 3.
[0037]
(Example 5)
About 150 nm of positive photosensitive polyimide was formed on an oscillator having a center frequency of 224 MHz using the quartz substrate shown in Example 2. The frequency at this time was 224.1 MHz, which was out of specification. However, as a result of performing dry etching on this substrate in a mixed gas atmosphere of CF ₄ and O ₂ gas for 30 s, the frequency should be 224.0 MHz, which is within the standard value. Was made. The film thickness of the polyimide after etching remained about 50 nm, and the short-circuit preventing effect was sufficient.
[0038]
In this example, etching was performed in the substrate state, but after being divided into each element and die-bonded to the package and connected to the wire, dry etching was performed with CF ₄ and O ₂ gas while measuring the frequency in vacuum. More accurate frequency adjustment was possible by finishing etching at the frequency.
[0039]
In this embodiment, the polyimide film is dry-etched, but it is not particularly limited to polyimide as long as it has etching selectivity with respect to the substrate and the electrode material. Further, although quartz is used as the piezoelectric substrate, there is no particular problem in using a substrate such as lithium tantalate, lithium niobate, or lithium tetraborate used for the surface acoustic wave device.
[0040]
【The invention's effect】
As described above, according to the present invention, the insulating film can be formed only on the electrode surface by the self-alignment exposure method, and the characteristics of the surface acoustic wave device can prevent the short circuit without increasing the particularly important insertion loss. An advantageous effect and the frequency can be adjusted to a desired value by etching the insulating film, and a great effect can be obtained in improving the yield.
[Brief description of the drawings]
1 is a cross-sectional structure diagram of an interdigital transducer section of a surface acoustic wave device according to a first embodiment of the present invention. FIG. 2 is a cross-sectional structure diagram of an interdigital transducer section of a surface acoustic wave element according to a second embodiment of the present invention. 3A is an electrode pattern configuration diagram of a surface acoustic wave device used in Embodiment 1 of the present invention. FIG. 3B is an equivalent circuit configuration diagram of the surface acoustic wave device. FIG. FIG. 5 is a process chart for fabricating the surface acoustic wave device of FIG. 5. FIG. 5 shows the characteristics of the surface acoustic wave device according to the first embodiment of the present invention compared with the case where an insulating film is formed on the entire surface. FIG. 6 is a configuration diagram of an electrode pattern used in the surface acoustic wave device according to the first embodiment of the present invention. FIG. 7 is another process diagram for creating the surface acoustic wave device according to the first embodiment of the present invention. FIG. 8 shows surface elasticity according to the second embodiment of the present invention. Process diagram for creating device 9 (a) a plan view showing a planar shape of a conventional surface acoustic wave device (b) cross-sectional view showing a sectional shape of a conventional surface acoustic wave device [Description of symbols]
DESCRIPTION OF SYMBOLS 1 Piezoelectric substrate 2 Interdigital transducer electrode 3 Reflector electrode 21 Interdigital transducer electrode 31 Insulating film 41 Negative photosensitive resin

Claims (3)

A piezoelectric substrate; a comb-shaped interdigital transducer electrode provided on the piezoelectric substrate ; and an insulating film formed on the interdigital transducer electrode , the insulating film on the interdigital transducer electrode and on both sides A surface acoustic wave element that is formed up to a part of the surface and is not formed on the surface of the piezoelectric substrate between the electrodes of the interdigital transducer electrode.   Forming a comb-shaped interdigital transducer electrode on the surface of the piezoelectric substrate; applying and drying a negative photosensitive resin on the surface of the piezoelectric substrate on which the interdigital transducer electrode is formed; and Exposing the photosensitive resin formed on the piezoelectric substrate by irradiating light for exposing the photosensitive resin from a surface side of the piezoelectric substrate where the interdigital transducer electrode is not formed; Removing the photosensitive resin formed on the interdigital transducer electrode by development; forming an insulating film on the surface of the piezoelectric substrate on which the interdigital transducer electrode is formed; The step of removing the insulating film formed on the photosensitive resin simultaneously with dissolving and removing the resin The method of manufacturing a surface acoustic wave element having. 3. The surface acoustic wave device according to claim 2 , wherein the thickness of the negative photosensitive resin formed on the piezoelectric substrate on which the interdigital transducer electrode is formed is smaller than the thickness of the interdigital transducer electrode. Production method.

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