JPS61117913A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To suppress the increase in a resonance resistance lower even at a high frequency and to prevent short-circuit between electrodes due to a conductive foreign material by coating an insulator on a reed screen electrode formed separately of Al or Al alloy film by means of the anodic oxidation. CONSTITUTION:The reed screen electrodes 2 are formed separately by applying anodic oxidation method to an Al or an Al alloy film and the insulator 5 is coated on them. For example, a crystal substrate subject to mirror grinding is used as the piezoelectric substrate 1 and an Al is vapor-deposited by sputtering on the substrate 1. Then the reed screen electrodes 2 are formed separately by oxidizing parts other than the reed screen electrodes 2 by means of the anodic oxidation method to form alumina parts 7. Further, reflectors 3 are formed by the wet photo etching method. Moreover, the insulator 5 is formed by applying sputter vapor deposition to a silicon dioxide onto the reed screen electrodes 2 and the alumina parts 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a surface acoustic wave element applied to a delay line 1 oscillator, a filter, etc.

"Prior art and its problems" Surface acoustic wave elements have traditionally been used for special military purposes, but in recent years they have begun to be used in consumer equipment such as FM tuners and TVs, and have suddenly come into the spotlight. Specifically, the 6 surface acoustic wave elements that have become available are commercialized as delay elements, oscillators, filters, and the like. The characteristics of these various surface acoustic wave devices are that they are small, lightweight, and highly reliable.
Another advantage is that the manufacturing process is similar to that of integrated circuits, making it highly suitable for mass production. Nowadays, it is mass-produced as an indispensable electronic component.

To explain an example of a conventional surface acoustic wave device using a surface acoustic wave device as an example, as shown in FIGS. 7 and 8, an interdigital electrode 2 made of a conductive material is formed on a piezoelectric substrate 1. There is. in this case.

The piezoelectric substrate 1 is, for example, a piezoelectric single crystal such as quartz, lithium niobate, or a piezoelectric ceramic.

Alternatively, a glass surface with a piezoelectric thin film formed thereon is used. Further, the interdigital electrode 2 is formed by depositing a metal such as aluminum or gold on the piezoelectric substrate 1.
It can be formed by photoetching. On both sides of this interdigital electrode 2, there are formed 1 pairs of lattice reflectors 3.3 each consisting of a ridge made of a dielectric, a conductor, a groove, etc.

When a voltage of a specific frequency is applied to the interdigital electrodes 2, an electric field is applied to the surface of the piezoelectric substrate 1 in the gap between the interdigital electrodes 2, and a strain proportional to the voltage is generated due to the piezoelectricity of the piezoelectric substrate l.
The strain propagates toward the image side as a surface wave at a sound speed determined by the material of the piezoelectric substrate 1. This surface wave is reflected by the grating reflectors 3.3 on both sides, returns to the interdigital electrode 2 again, and resonates.

By the way, these various surface acoustic wave elements are generally sealed with a metal container called a hermetic seal 4 as shown in FIG. The hermetic seal 4 is usually plated with nickel or the like in consideration of sealing properties, corrosion resistance, etc.

However, in such conventional surface acoustic wave elements, conductive foreign matter mixed in before the hermetic seal 4 is sealed, peeled off plating of the hermetic seal, etc. adhere to the interdigital electrodes, causing a short circuit phenomenon between the electrodes. was there. This causes problems such as changes in electrical impedance, lowers the reliability of the surface acoustic wave element, and hinders mass production.

Therefore, the present inventors have discovered that by coating the interdigital electrode portion with an insulating material, it is possible to prevent short-circuiting between the electrodes due to peeled off plating of the hermetic seal or other conductive foreign matter, and have already filed a patent application.

FIG. 10 shows an example of such a surface acoustic wave element. That is, after forming a film of a metal such as AI by, for example, sputter deposition on a piezoelectric substrate l such as a quartz substrate, the interdigital electrode 2 and the reflector 3 are formed by a normal wet etching method.
form. In this case, the metal film thickness is 1, for example, 130M.
For surface acoustic wave devices in the Hz band, it is about i p-m,
For surface acoustic wave devices in the 800 MHz band, it is approximately 0.15 mm. Further, as the etching solution, for example, a mixed solution of phosphoric acid and nitric acid is used. Then, an insulator 5 such as silicon dioxide is formed on the interdigital electrode 2 by, for example, sputter deposition. For film formation, 1, e.g., substrate temperature 200°C, film formation rate) 0.15es8r, Ar+02 mixed gas pressure 3
This is carried out while the substrate 1 is rotating around its axis at X 10-3 Torr.

However, in this surface acoustic wave element, in order to effectively prevent the phenomenon of short circuit between electrodes, the film thickness of the insulator 5 is increased to 2000 mm.
It is necessary that the number of people is at least the same level as that of a person.

If the insulator 5 is formed, a problem arises in that the resonance resistance of the surface acoustic wave element increases (for example, 90) IHz.
In a band surface acoustic wave device, when the film thickness of the insulator 5 is 2000 MHz, the resonance resistance increase rate is about 10% at maximum, which can be accommodated in the design, so it is not a big problem.However, for example, for an 800 MHz band elastic surface It was found that in the wave element, when the film thickness of the insulator 5 was 2000 mm, the resonance resistance increase rate reached a maximum of 25%, the variation increased, and the productivity decreased. Therefore, this surface acoustic wave element has, for example, VH
Although it is sufficiently practical in the F band, the increase in resonance resistance cannot be ignored in higher frequencies such as the UHF band.

"Objective of the Invention" The object of the present invention is to prevent short-circuiting between electrodes caused by peeled off plating of hermetic seals or other conductive foreign matter;
Another object of the present invention is to provide a surface acoustic wave element that can suppress an increase in resonance resistance even in the case of high frequencies.

"Structure of the Invention" In the surface acoustic wave element according to the present invention, the interdigital electrodes are formed by separating and forming an AI or Al alloy film by an anodizing method, and the interdigital electrode portions are coated with an insulator.

The present invention investigates the problems of the surface acoustic wave element shown in FIG. 1O, and further improves the problems. That is, in the surface acoustic wave element shown in FIG. 10, as shown in FIG. 11, the interdigital electrode 2 is etched so as to protrude from the piezoelectric substrate l, so when it is covered with the insulator 5, The insulator 5 actually has an uneven state as shown in the figure. For this reason, for example, in the portion indicated by A in the figure, the film thickness of the insulator 5 becomes extremely thin, and the v'Jk effect of the inter-electrode short circuit phenomenon is weakened.

The surface acoustic wave element according to the present invention has AI or
After forming the first alloy film, this is anodized to form the interdigital electrode 2. That is, the portions other than the interdigital electrode 2 were oxidized by anodic oxidation to form a non-conductive alumina portion 7, thereby forming the interdigital electrode 2 separately. As a result, the alumina portion 7 is connected to the interdigital electrode 2.
The upper surface is slightly more raised than the upper surface, but the upper surface is considerably flattened as a whole. Therefore, when the insulating film 5 is coated on this, the insulating film 5 becomes relatively uniform and covers the interdigital electrode 2 and the alumina part 7. Therefore, the thickness of the insulating WI5 is set to 100xs degree, for example. Also, sufficient insulation properties can be obtained, and by reducing the thickness of the aSS film, the increase in resonance resistance can also be suppressed. As described above, according to the surface acoustic wave element of the present invention, the resonance resistance does not increase significantly even in the case of high frequencies such as UHFHF.

According to a preferred embodiment of the present invention, the insulator 5 is
Oxides such as Sing, TaNx, Al0K, Sing
, τaNx, or an inorganic insulator made of a composite thereof.

According to a further preferred embodiment of the present invention, the insulator 5 has a thickness of 500 to 3000 layers. The film thickness of the insulator 5 is 50
If the number of participants is less than 0, it will be difficult to obtain a sufficient effect of preventing short-circuiting between electrodes, and if the number of participants exceeds 3,000, the resonance resistance will tend to increase.

The surface acoustic wave element of the present invention may also have a structure as shown in FIGS. 3 and 4, that is, in this example,
After the interdigital electrodes 2 are separated and formed by an anodizing method, the upper surface of the interdigital electrodes 2 is also oxidized by an anodizing method, so that the entire interdigital current collection fI2 is covered with an alumina portion 7. Therefore, in this example, the alumina portion 7' covering the upper surface of the interdigital electrode 2 is substantially the insulator 5.
It plays the role of

``Embodiments of the Invention'' Example A mirror-polished quartz substrate was used as a piezoelectric substrate 1, and Al was sputter-evaporated onto it to a film thickness of 2G00.
Next, by using a well-known anodic oxidation method, the portion other than the interdigital electrode 2 is oxidized to form the alumina portion 7.
The interdigital electrode 2 was formed separately. Further, a reflector 3 was formed by a conventional wet photoetching method. Furthermore, on the interdigital electrode 2 and the alumina part 7, silicon dioxide is formed into a film at a substrate heating temperature of 200°C (L/-)0.15#.
Lm8r, Ar10, total pressure 3 with mixed gas X1O-3
The insulator 5 was formed by sputter deposition while rotating the substrate 1 under Torr. In this way, a surface acoustic wave device having a structure as shown in FIGS. 1 and 2 was manufactured.

Next, in order to test the effect of preventing short circuit between electrodes of this surface acoustic wave element, as shown in FIG.

AI was sputter-deposited on the insulator 5 to a thickness of 2,000 yen by a mask evaporation method to form a pseudo-conductive foreign material 8.

The pseudo-conductive foreign matter 8 is sized to span the interdigital electrode 2 when viewed from above, and the insulating material covering the interdigital electrode 2 is
Several pieces were attached to arbitrary locations on the 15. After forming the pseudo-conductive foreign matter 8, the direct current resistance failure rate was measured to examine the short-circuit phenomenon between the interelectrode of the interdigital electrode 2. It has been found through experiments that this test method allows for more reliable and rigorous testing with a smaller number of products than the conventionally used vibration test.

And the 1st r! 4 and the surface acoustic wave device shown in FIG.
, IQooA, 2,000, and 3,000 people were manufactured, and the DC resistance failure rate and co-removal resistance increase rate were measured for each by the method described above. The result is the 8th rI! Note that this surface acoustic wave element shown in I is of the 800 MHz band. From 881 m onwards, this surface acoustic wave element has a film thickness of the insulator 5 of too much, and the DC resistance failure rate due to the pseudo-conductive foreign matter 8 becomes zero, and at that point, the surface acoustic wave element becomes #! The resistance increase rate is also 5% on average and 10% at maximum.
It can be seen that it is.

In addition, the material of the insulator 5 is TaNx, ^10! , SiNx
, Similar results were obtained even when TaNx was used instead. Further, similar results were obtained in the surface acoustic wave elements shown in FIGS. 3 and 4 in which the entire interdigital electrode 2 was covered with the alumina portion 7.

Comparative Example After sputter-depositing AI to a thickness of 2,000 yen on a piezoelectric substrate 1 made of a quartz substrate, interdigital electrodes 2 and reflectors 3 were formed by a conventional wet etching method. Then, on the interdigital electrode 2, silicon dioxide was deposited at a substrate temperature of 200°C at a film deposition rate of 0.15xm/hr, ^j+%.
The insulator 5 was formed by sputter deposition using a mixed gas at a total pressure of 3 Xl0-'' Tarr while rotating the substrate 1. In this way, the surface acoustic wave device shown in FIGS. 1O and 11 was manufactured. The surface acoustic wave element is manufactured by using a film thickness of the insulator 5 made of silicon dioxide of 1000, 2000, or 30.
00 people, and the direct current resistance failure rate and resonance resistance increase rate were measured for each of them by the method described above. The results are shown in FIGS. 12 and 13. FIG. 12 shows the case of a 90 MHz band surface acoustic wave element, and 113
The figure shows the case of an 800 MHz band surface acoustic wave element. 1
As shown in Figure 12, in the 90 MHz band, the DC resistance failure rate due to conductive foreign matter 8 becomes zero when the film thickness of the insulator 5 is 2000, and the resonant resistance increase rate at that point is at most 1O
%, it is sufficiently practical. however,
As shown in Figure 13, in the 800MHz band, the insulator 5
It can be seen that when the film thickness is 2000, the increase rate of both 4[Jl resistance increases to a maximum of about 25%, and the variation also increases, resulting in poor productivity.

[Effects of the Invention] As explained above, according to the present invention, since the interdigital electrodes are coated with an insulating material, the phenomenon of short circuit between the interelectrode of the interdigital electrodes can be almost completely prevented. Also, the interdigital electrode is A
Since it is made of I or Al alloy film separated by anodization, the upper surface of the interdigital electrode is smooth.
Even with a relatively thin insulating film, it is possible to sufficiently prevent the inter-electrode short circuit phenomenon of the interdigital electrodes, thereby making it possible to extremely reduce the rate of increase in resonance resistance even at high frequencies. Therefore, according to the present invention, it is possible to dramatically improve the reliability of the 91-frequency surface wave device, extremely reduce the occurrence of defective products even during mass production, and simplify work such as inspection during manufacturing. I can do it.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of a surface acoustic wave device according to the present invention, FIG. 2 is a partially enlarged sectional view of the same surface acoustic wave device,
FIG. 3 is a sectional view showing another embodiment of the surface acoustic wave device according to the present invention, FIG. 4 is a partially enlarged sectional view of the same surface acoustic wave device, and FIG. 5 is a test method for investigating the phenomenon of short circuit between electrodes. FIG. 8 is a diagram showing the direct current resistance failure rate and resonant resistance increase rate of the surface acoustic wave device according to the present invention. FIG. 7 is a plan view showing an example of a conventional surface acoustic wave device. is a sectional view of the same surface acoustic wave device, FIG. 9 is a perspective view showing a product form in which the surface acoustic wave device is sealed with a hermetic seal, and FIG. 1O is a sectional view showing an example of a surface acoustic wave device other than the present invention. , FIG. 11 is a partially enlarged cross-sectional view of the same surface acoustic wave element, the first
Figure 2 is a chart showing the direct current resistance failure rate and resonance resistance increase rate at 90 MHz of the same surface acoustic wave element, and Figure 13 is a chart showing the DC resistance failure rate and resonance resistance increase rate at 60 MHz of the same surface acoustic wave element 1. be. In the figure, 1 is a piezoelectric substrate, 2 is an interdigital electrode, 3 is a reflector,
5 is an insulator, and 7.7 is an alumina part. Fig. 1 Fig. 3 Fig. 5 Fig. 6 Between the two roosters and I. Fig. 7 Fig. 8 Fig. 10

Claims (3)

[Claims] (1) In a surface acoustic wave device in which interdigital electrodes are formed on a piezoelectric substrate, the interdigital electrodes are formed by separately forming an Al or Al alloy film by an anodizing method, and an insulator is provided in the interdigital electrode portions. A surface acoustic wave element characterized by being coated with. (2) In claim 1, the insulator is S
A surface acoustic wave element that is a composite of one or more selected from the group consisting of iOx, TaOx, AlOx, SiNx, and TaNx. (3) A surface acoustic wave device according to claim 1 or 2, wherein the insulator has a thickness of 500 to 3000 Å.