JP2002232254A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drastically reduce the cost of a SAW filter by enhancing the judgment accuracy of acceptance very much at a wafer stage by enabling a high frequency probe to measure/select non-defective of the frequency characteristic of the SAW filter formed on a wafer while preventing comb-shaped electrode fingers from being subjected to discharge breakdown by pyroelectric effect due to rapid temperature fluctuation in a SAW filter producing process. SOLUTION: The relation between the total area of the formation region of a wire electrode and comb-shaped electrodes connected to the wire electrode and the inter-electrode finger distance of an IDT electrode meets the following expression. S <=1.49×105×√(Lidt-0.4) (however, S: total area (μm2) of wire electrode and comb-shaped electrode connected to wire electrode, and Lidt: inter-electrode finger distance (μm) of IDT electrode)).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter (hereinafter referred to as SA) for use in mobile communication equipment such as a portable telephone.
Surface acoustic wave device such as a W filter).
The present invention relates to an electrode structure of a surface acoustic wave device formed by providing a plurality of IDT electrodes formed of a pair of comb-shaped electrodes.

[0002]

2. Description of the Related Art In recent years, many surface acoustic wave devices have been used as components such as filters, delay lines, and oscillators of electronic devices utilizing radio waves. In particular, a SAW filter which is small and lightweight and has a high sharp cutoff performance as a filter has been widely used in the mobile communication field as a filter of an RF stage and an IF stage of a portable terminal device, and has a low loss and a low loss. As the cutoff characteristics outside the pass band, high attenuation characteristics and a wide bandwidth are required.

One of the filters used in the RF stage of a mobile phone or the like
A so-called ladder-type SAW in which a plurality of one-port surface acoustic wave resonators (hereinafter, referred to as SAW resonators) are arranged on the same piezoelectric substrate, and the SAW resonators are connected in series and in a ladder shape.
There is a filter. The ladder-type SAW filter is small in size and low in loss, and can realize a filter having a steep attenuation characteristic. Therefore, the ladder-type SAW filter is widely used as an RF filter for a mobile phone or the like.

As a piezoelectric substrate used in the ladder type SAW filter, lithium tantalate is widely used. One reason for this is that the electromechanical coupling coefficient of lithium tantalate is well adapted to achieve the required bandwidth for RF stage filters in mobile phones in different countries. Further, the piezoelectric substrate has a smaller frequency temperature coefficient and a smaller change in center frequency due to a change in ambient temperature than lithium niobate, which is known as a piezoelectric material often used in an RF stage.

However, since lithium tantalate is a material having a large pyroelectric property, charges are generated by a pyroelectric effect due to a rapid temperature change, and an electrode formed on the substrate surface due to a difference in distribution of the charges, that is, a potential difference. Discharge breakdown occurs between the patterns, and a problem often occurs in which a fine pattern such as a comb-like electrode finger of an IDT electrode is broken.

Many measures have been proposed to prevent the discharge breakdown due to the electric charge. One of the measures has been known as shown in FIG. FIG. 10 is an enlarged view of a ladder-type SAW filter element formed in a large number on the wafer 1 shown in FIG. In FIG. 2, 6 is a dicing line, 8 is a SAW filter element before dicing, and 9 is an Al solid portion. The ladder-type SAW filter element shown in FIG.
IDT electrodes and SAW resonators 13 composed of grating reflectors arranged on both sides of the IDT electrodes are arranged side by side on the piezoelectric substrate 14 at intervals that do not affect each other, and they are arranged in parallel and in series. , Parallel,... Are sequentially connected using the wiring electrode 5 so as to form a ladder type (ladder type) structure. Here, 2 is an input electrode, 3 is an output electrode, 4 is a ground electrode, 6 is a dicing wire, 7 is a thin wire provided to prevent discharge breakdown due to static electricity generated by the pyroelectric effect, , 4 are all connected to a dicing line 6.

The dicing lines 6 around each ladder type SAW filter element are also electrically connected to each other. With this configuration, even if the wafer 1 shown in FIG. 2 is exposed to a rapid temperature change and charges are generated on the surface of the wafer 1 due to the pyroelectric effect, the thin wire 7 for preventing discharge breakdown and the dicing wire 6 and the electric charge moves instantaneously, and there is no excessive potential difference between the electrodes.
It has become possible to greatly reduce damage to the comb-shaped electrode fingers due to electric discharge breakdown.

A dicing line 6 formed so as to surround the periphery of each SAW filter element is used as a mark when the wafer is cut into individual pieces by dicing.
Since the element is cut into individual pieces along the dicing line 6 and the dicing line 6 is cut off at the same time, a desired operation as an element is obtained after being cut into individual pieces.

[0009]

However, in the above-described SAW filter, since a large number of SAW filter elements formed on the wafer surface are electrically short-circuited, the SAW filter elements such as the frequency and insertion loss of each SAW filter element. Even if the high frequency probe is brought into contact with the input / output electrode pad of the SAW filter element to measure the frequency characteristics, a large number of surface acoustic wave device elements other than the desired SAW filter element are electrically connected to the high frequency probe. As a result, there is a problem in that it is not possible to measure desired characteristics unique to the SAW filter element.

As a conventional example, not shown,
When the thin wire 7 of FIG. 10 using a conductive thin film is formed in a meandering line shape, the thin wire has low resistance in DC and high impedance in AC. In addition, the pass band characteristics of the ladder-type SAW filter element on the wafer 1 could be made closer to the pass characteristics of the finished product.

However, if the DC resistance of the thin wire is unnecessarily high, it may hinder the measures against discharge breakdown. Therefore, the width, line length, and film thickness of the conductive thin wire 7 must be appropriately set. In addition, since the conductive thin wires 7 are arranged in a meandering line on the piezoelectric substrate, there is a problem that the chip size becomes unnecessarily large (Japanese Patent Laid-Open No.
000-40932).

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a SAW filter and a method of manufacturing the SAW filter which are capable of performing a discharge breakdown prevention measure and capable of checking the frequency characteristics of each SAW filter element in a wafer state. The purpose is to provide.

[0013]

In order to solve the above-mentioned problems, a surface acoustic wave device according to the present invention comprises: a wiring electrode;
When the relationship between the electrode and the electrode finger distance satisfies the following expression, it is possible to prevent the comb-shaped fine electrode of the IDT electrode from being destroyed by electric discharge due to the pyroelectric effect of the piezoelectric substrate without increasing the chip size. I do.

S ≦ 1.49 × 10 ⁵ × (Lidt−0.4) (However, Δ (Lidt−0.4) = (Lidt−0.
4) ^1/2 , S: total area (μm ² ) of the wiring electrode and the comb-like electrode connected to it, Lidt: distance between the electrode fingers of the IDT electrode (μm)) In order to control the total area of the connected region of the comb-shaped electrode, a cutout pattern is used in the formation region of the wiring electrode and the comb-shaped electrode connected to the wiring electrode. Note that a blank pattern refers to a pattern in which a film is not partially formed, for example, in a mesh shape or an island shape, in a formation region of a wiring electrode and a comb-like electrode connected thereto.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a surface acoustic wave device according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing the structure of a SAW filter according to the present invention. In the drawing, reference numeral 1 denotes a piezoelectric substrate (or wafer), on which a thin film 2-7, 12-13, Al
And a metal electrode such as Al-Cu. Reference numeral 2 denotes an input electrode, which is connected to a half of the IDT electrode. Reference numeral 3 denotes an output electrode, which is similarly connected to half of the comb-shaped electrode. Reference numeral 4 denotes a ground electrode, which is connected to a dicing wire 6 used for dicing around the outer periphery of the SAW filter element by a thin wire 7. Here, reference numeral 5 denotes an electrode group arranged opposite the grounded IDT electrode and the fine electrode, and is shown in black in the figure.

Next, FIG. 2 shows a SAW filter patterned on a lithium tantalate wafer. SA
The W filter element 8 is connected to the outer peripheral portion 9 of the wafer by the dicing line 6.
It is connected to the. Reference numeral 9 denotes a solid metal region, which is always grounded during the electrode manufacturing process of the SAW filter. here,
Most of the static electricity generated by the temperature rise and fall during the electrode manufacturing process escapes to the ground electrode by the dicing wire 6, but the static electricity generated in the floating electrodes 5 and 2 and 3 in FIG. 1 has no place to escape. For this reason, a potential difference occurs between the floating electrode and the comb-shaped electrode of the grounded IDT electrode, and discharge breakdown occurs.

The principle will be described in more detail. In the IDT electrode shown in FIG. 3, static electricity is uniformly generated on the left and right comb teeth due to a pyroelectric effect due to a rapid temperature change. Although no discharge breakdown occurs in this state, the ground electrode 4 in FIG. 3 has a potential of 0 because it is grounded.
A potential difference of several kV occurs between the DT electrode (counter electrode) 5. For this reason, the distance between the electrode fingers of the fine comb-shaped electrode becomes a discharge gap, and discharge breakdown occurs.

FIG. 4 shows the mechanism of discharge breakdown in different cases. (A) is a case where the distance between the ground electrode 4 and the counter electrode 5 is relatively large, and the potential difference is large, but the electric field intensity is not so large and does not lead to discharge breakdown because the distance between the electrodes is large. (B) shows a case where the distance between the ground electrode 4 and the counter electrode 5 is small, and a high electric field is generated between the electrode fingers and discharge breakdown occurs. (C) shows a case where the area of the counter electrode is large.
Even when the amount of electric charge 10 charged in 5 is large and the distance between ground electrode 4 and counter electrode 5 is large to some extent, a large electric field is generated between ground electrode 4 and discharge breakdown occurs.

As described above, in order to prevent the discharge breakdown in the comb-teeth-shaped electrode portion, the distance between the electrode fingers is large as shown in FIG. What is necessary is just to reduce the area of the comb-shaped electrodes to be formed and the wiring connecting these.

The following experiment was performed to verify this. FIG. 5 shows that a temperature gradient is applied to a lithium tantalate wafer from 32 ° C. to a rate of 360 ° C./Hr to 120 ° C., and then the temperature is reduced from 120 ° C. to 36 ° C. by natural cooling.
FIG. 6 is a plot of the voltage on the wafer surface when the temperature is lowered to ° C. FIG. The measurement was performed using a non-contact charging voltage measuring device. Although there are discontinuities around 50-60 ° C,
Even in a temperature cycle from room temperature to about 120 ° C, about -1 to 2k
As much as V voltage was generated.

FIG. 6 shows the distance between the IDT electrodes = 0.5 μm.
In the case of m, it is a graph plotting the relationship between the total area of the wiring electrode of the SAW filter and the formation area of the comb-shaped electrode connected to the wiring electrode and the discharge breakdown probability of the IDT electrode. In the experiment, six types of SAW filters were prepared, and the failure rate due to discharge breakdown when a steep temperature gradient was applied to each SAW filter was examined. As a result, the total area was 22 μm ² and 36 μm ² and 4
A defect rate of 0% was achieved with three filters of 5 μm ² .
All the other three filters having a large total area were defective due to discharge breakdown. (However, each sample number = 2
0) Further, the dependence of the total electrode area on the distance between the electrode fingers without causing discharge breakdown was examined. Conditions are from 0.3 μm to 1.0
When the process up to μm was performed in increments of 0.1 μm, it was found that the total electrode area where no discharge breakdown occurred was a region inside the curve as shown in FIG.

That is, the total electrode area of the wiring electrode and the comb-like electrode connected to the wiring electrode is S (μm ² ), the distance between the electrode fingers of the IDT electrode (the space between an arbitrary electrode finger and an adjacent electrode finger). Where S ≦ 1.49
It was found that discharge breakdown did not occur at all in the region of × 10 ⁵ × √ (Lidt-0.4).

In FIG. 1, as a method of reducing the total area of the wiring electrode and the formation area of the comb-shaped electrode connected to the wiring electrode, not only the method of reducing the thickness of the wiring 5 connecting the IDT electrode, As shown in FIG. 8, a pattern for removing the mesh-like or striped wiring 12 may be used. This means that the LT board and the Al
Since the area ratio of the solid part can be changed freely,
The change in the frequency characteristic can be kept smaller.

Here, FIG. 9 (A) shows a case where the white portion is square, (B) is a parallelogram, (C) is a right-angled isosceles triangle, (D) is a circle, (E) is a rhombus, and (F) shows a conventional Al solid. In order to avoid an increase in insertion loss due to partial concentration of current density, it is more preferable that the blanking pattern includes a shape (for example, a circle (D)) constituted by a curve as much as possible.

Next, a description will be given of an embodiment in which a ladder-type SAW filter according to the present invention is experimentally manufactured. 42 ° Y-cut LiT
Al (98wt%)-Cu (2wt%) on aO3 substrate
To form an electrode pattern. For pattern production, a reduction projection exposure machine (stepper) and RIE (Reac
Photolithography was performed using a five ion etching apparatus. First, a lithium tantalate wafer as a substrate material was ultrasonically cleaned with an organic solvent such as acetone and IPA to wash organic components. Next, after the substrate was sufficiently dried by a clean oven, an electrode was formed. For electrode formation, a sputtering apparatus was used to form an Al-Cu alloy material. The electrode film thickness was about 0.2 μm. Next, apply photoresist for about 0.5
Spin coating was performed to a thickness of μm, and desired patterning was performed using a reduction projection exposure apparatus (stepper). next,
Unnecessary portions of the resist were dissolved with an alkaline developing solution in a developing device to expose a desired pattern. In the steps up to this point, there are several temperature cycles from room temperature to about 100 ° C. twice during the spin coating and development of the photoresist.

Next, the exposed portion of the Al-Cu electrode was etched by the RIE apparatus, and the patterning of the electrode was completed. Thereafter, the process proceeds to a process of forming a protective film.
SiO2 is deposited by CVD (Chemical Vapor D)
The resist was patterned by photolithography again, and the pad portion for wire bonding was subjected to SiO2 etching with an RIE device or the like, thereby completing a protective film pattern. A temperature of about 100 ° C. is applied to the wafer in the RIE apparatus and about 300 ° C. in the CVD apparatus.

Then, the piezoelectric substrate was cut along the dicing line and divided for each SAW filter element. And
Each SAW filter element was picked up by a die bonding device and die-bonded in a SMD package with a resin containing Si resin as a main component. Thereafter, the Si resin was dried and cured at a temperature of 200 ° C. As the SMD package, a 3 mm square laminated type shown in FIG. 11 was used. Next, 30 in FIG.
After the μmφAu wire 15 was ball-bonded on the pad portion of the SMD package 16 and the Al pad on the SAW filter element, the lid was put on the package, and the package was welded and sealed with a sealing machine to complete the process.

The ground electrode patterns on the SAW filter element were separately wired and electrically connected to the ground pads on the package by Au ball bonding. In this wire bonding step, the temperature of about 150 ° C. is applied to the piezoelectric substrate. In addition, sealing is performed by seam welding, in which case a temperature of 200 ° C. was applied for annealing.

In the steps so far, particularly in the protective film forming step, the die bonding step, the wire bonding step, and the annealing step in which the piezoelectric substrate is exposed to a high temperature, the discharge breakdown of the comb-shaped electrode is liable to occur conventionally, and many defects are caused. Was occurring. However, when the area of the wiring electrode was changed as in the present invention, no failure occurred due to discharge breakdown.

The SAW resonator constituting the ladder-type SAW filter has 40 to 120 pairs of IDT electrodes (1/2 of the number of IDT electrodes) and a cross width of 10 to 30λ (λ is the wavelength of a surface acoustic wave). Although the wavelength λ of the surface acoustic wave differs between the series and the parallel, it is approximately 2 μm. Here, the number of reflective electrodes is 20 for both the series resonator and the parallel resonator. An example of the filter configuration is as shown in FIG. FIG. 1 shows an example of a 2.5-stage T-type having three series resonators and two parallel resonators.

[0032]

According to the surface acoustic wave device of the present invention, the relationship between the total area of the wiring electrodes and the comb-shaped electrodes connected to the wiring electrodes and the distance between the electrode fingers of the IDT electrode is optimized. The frequency characteristics of the surface acoustic wave device formed on the wafer are measured and selected using a high-frequency probe while preventing the comb-shaped electrode fingers of the IDT electrode from being destroyed by the pyroelectric effect due to the rapid temperature fluctuation in. be able to. As a result, the pass / fail judgment accuracy can be increased at the wafer stage, and an inexpensive surface acoustic wave device with excellent reliability can be provided.

Further, in order to reduce the total area of the formation area of the wiring electrode and the comb-shaped electrode connected to the wiring electrode, a cutout pattern is formed, and the shape of the cutout area is formed by a curve, so that electrostatic breakdown is minimized. It is possible to provide an excellent surface acoustic wave device that prevents the change in frequency characteristics and prevents the change.

[Brief description of the drawings]

FIG. 1 is a plan view schematically showing a structural example of a SAW filter according to the present invention.

FIG. 2 is a plan view schematically showing an arrangement of SAW filter chips patterned on a lithium tantalate single crystal wafer.

FIG. 3 is a schematic view of the structure of an IDT electrode.

FIGS. 4A to 4C are schematic diagrams illustrating discharge breakdown caused by a pyroelectric effect.

FIG. 5 is a graph illustrating an amount of a charging voltage generated by rapid temperature rise and fall.

FIG. 6 shows a wiring electrode of the present invention and I connected to the wiring electrode.
5 is a graph illustrating a relationship between a total area of a formation region of a DT electrode and a discharge breakdown probability of an IDT electrode.

FIG. 7 shows a total area of a wiring electrode and a formation area of an IDT electrode connected to the wiring electrode where no discharge breakdown occurs according to the present invention;
5 is a graph showing a relationship between an IDT electrode and a distance between electrode fingers.

FIG. 8 is a plan view schematically showing another embodiment of the SAW filter of the present invention.

FIG. 9 shows a wiring electrode according to the present invention and I connected to the wiring electrode.
It is a partial enlarged view which shows typically the example which reduces the total area of the formation area | region of a DT electrode, (A) is a case where a blank pattern is a square, (B) is a case where a blank pattern is a parallelogram,
(C) shows a case where the blank pattern is a right-angled isosceles triangle.
(D) shows the case where the cut pattern is circular, (E) shows the case where the cut pattern is rhombic, and (F) shows the case where the conventional solid pattern is used.

FIG. 10 is a plan view showing the structure of a conventional SAW filter that does not cause discharge breakdown.

FIG. 11 is a plan view schematically showing a structure of a SAW filter according to the present invention which does not cause discharge breakdown, which is mounted in a package.

[Explanation of symbols]

 1: Wafer (piezoelectric substrate) 2: Input electrode 3: Output electrode 4: Ground electrode 5: Wiring electrode 6: Dicing wire 7: Fine wire 8: SAW filter chip 9: Al solid portion 10: Generated by pyroelectric effect Charge 11: Discharge breakdown 12: Drain pattern 13: SAW resonator 14: Piezoelectric substrate 15: Wire bonding wire 16: Package

Claims (2)

[Claims] 1. A piezoelectric substrate comprising a lithium tantalate single crystal is provided with a plurality of IDT electrodes comprising a pair of comb-like electrodes and wiring electrodes for connecting the comb-like electrodes of the IDT electrodes to each other. A surface acoustic wave device, wherein a total area of the wiring electrode and a comb-like electrode connected to the wiring electrode;
A surface acoustic wave device characterized in that the relationship between the DT electrode and the electrode finger distance satisfies the following expression. S ≦ 1.49 × 10 ⁵ × √ (Lidt−0.4) (where, S: total area (μm ² ) of the wiring electrode and the comb-like electrode connected thereto, Litt: between the electrode fingers of the IDT electrode Distance (μm) 2. The surface acoustic wave device according to claim 1, wherein the wiring electrode and a comb-like electrode connected to the wiring electrode are formed in a cut pattern.