JP2000156620A - Center frequency adjustment method for surface acoustic wave device and production of the device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily adjust the center frequency of an SAW(surface acoustic wave) filter by measuring the input/output characteristic including the frequency characteristic of an SAW device and repeating both etching and oxide forming steps until the desired input/output characteristic is obtained. SOLUTION: A piezoelectric substrate 50 is placed in a chamber 52 having a terminal 70 which can measure the input/output characteristic including the frequency characteristic of an SAW device. In an etching step, the surface of an interdigital electrode constructing the SAW device is shaped by a method such as the dry etching. In an oxide forming step, an oxide is formed on the surface of the interdigital electrode. In a repeating step, the input/output characteristic including the frequency characteristic of the SAW device is measured via the terminal 70. Then both etching and oxide forming steps are repeated until the desired input/output characteristic is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a SAW (surface acoustic wave) device. In particular, it relates to a method of adjusting the center frequency of the SAW device during a manufacturing process.

[0002]

2. Description of the Related Art SAW devices are widely used as frequency filters. The present inventors have developed a filter using a quartz substrate whose resonance frequency is narrower.

The high-frequency narrow-band filter being developed is a STW (Surface Transverse Wave) on a quartz substrate.
The acoustic velocity of the surface acoustic wave is about 5100 m / s, which is suitable for higher speed and higher frequency than conventional devices. The load Q value of the device under development is about 1500.
Further, since this STW device uses a quartz substrate, the frequency temperature dependence of the conventional SAW device is -18.
It exhibits a frequency temperature characteristic of about 1 to 4 ppm / ° C. with respect to ppm / ° C. As a result, the developed STW device has a frequency variation of about 10 for a temperature range of 100 ° C.
It is about 0 ppm (0.08 MHz).

[0004] Because of the high Q value, the developed STW device has a pass band width of about 0.5 MHz, for example, in an 800 MHz band filter, which was about 2.4 MHz in the past. . That is, ultra-narrow band has been achieved for the conventional SAW device.
The specific bandwidth of the developed STW device is 0.07%
It is. Also, the out-of-band attenuation is 35 dB at the center frequency of ± 2 MHz, whereas the conventional device has 20 dB at the center frequency of ± 2 MHz, which is a significant improvement. Further, the developed STW device can be applied to a VCO or the like as a high Q resonator.

Since the speed of sound of surface acoustic waves is high, the developed S
The TW device handles a signal of 1 GHz and has an electrode line width of about 1.3 μm. Therefore, under the current electrode processing technology, it is possible to manufacture a filter that can handle a signal frequency up to about 1 to 2 GHz.

However, in the STW device developed by the inventors of the present invention, the resulting Q size and the narrow passband width cause problems in finished dimensional variations such as electrode finger line width and electrode film thickness. In particular, a change in the center frequency of the filter due to a variation in the electrode film thickness resulted in a significant reduction in the manufacturing yield.

The sensitivity of the center frequency depending on the electrode film thickness of this STW device is about 0.2 MHz / 10 angstroms. The manufacturing accuracy of the Al (aluminum) vapor deposition apparatus is about 30 Å to 60 Å. As a result, the center frequency may fluctuate up to about 1.2 MHz. In the conventional SAW filter, such a change in the center frequency slightly affects the yield. However, in the newly developed quartz STW filter, the pass band may be completely different as a result of the change in the center frequency. is there. Therefore, it is sufficiently assumed that there is no filter in one wafer (or in one batch) that can obtain a desired frequency characteristic.

Therefore, the inventors of the present invention have proposed a new STW device, which has not been regarded as a problem until now, due to the critical processing accuracy of a vapor deposition apparatus and the variation in film formation state (electrode film thickness, electrode finger line width). It became necessary to investigate the frequency fluctuation and establish a technique for adjusting the center frequency.

FDB Manufacturing Method FIG. 5 is an explanatory diagram showing a conventional FDB method of manufacturing a SAW filter. In the FDB system, a SAW chip input / output pad and a pattern formed inside a package 12 accommodating a SAW chip 10 are connected to each other via gold bumps 14 so that electrical connection and mechanical holding are simultaneously performed. Is the way.

Since this manufacturing method does not use an adhesive or the like, the internal state of the package is stable.
Suitable for the manufacture of various narrow band SAW filters, including W filters. Also, since the SAW device can be downsized,
It is widely used for mass-produced varieties.

Current Manufacturing Method and Adjustment Method of Center Frequency The manufacturing process of the SAW filter can be largely divided into the first half wafer process and the second half assembly process. FIG. 6 is a flowchart showing this manufacturing process.

In the wafer process 20, electrode patterns and pad patterns are formed on the piezoelectric substrate, and other substrate surface treatments are performed. The resist coating is performed in step S6-1, and the exposure and development are performed in step S6-2 and step S6-3. Further, Al deposition is performed in step S6-4. Further, a lift-off operation is performed in step S6-5. Each of these processes is performed for each pattern.

Since the deposited film thickness of the electrode pattern directly affects the frequency characteristics of the filter, controlling the film thickness is important. There is known a technology for forming a high accuracy of several thousand angstroms ± 40 angstroms. In a conventional SAW device, the pass bandwidth is about 2.4 MHz, and the production yield is poor, but the need for frequency adjustment is small. However, the crystal ST developed by the present inventors has
Since a very narrow pass band is realized in the W filter, a process for adjusting the frequency must be included in the manufacturing process.

In step S6-6, the center frequency f0 is checked. If it is determined that the center frequency f0 needs to be adjusted, the center frequency f0 is adjusted in step S6-7. Then, step S6
At -8, bump formation is performed.

Next, in an assembly process 22, a SAW chip is assembled. Dicing is performed in step S6-9. Further, chip cleaning is performed in step S6-10. In addition, an FDB process such as packing is performed in step S6-12. Finally, a sealing process is performed in step S6-13.

The final product inspection is performed in step S6-14.
And those that pass the inspection are shipped.

As described above, FIG. 6 is a flowchart showing a process for manufacturing a quartz STW filter. An RF prober measuring process and an f0 adjusting process are added to the conventional SAW manufacturing process. The above-mentioned f0 adjustment is performed by an RIE device (dry etching) or a developing device (wet etching). Manufacturing costs increase due to additional steps.

The frequency adjustment is performed for each wafer. At present, it is difficult to perform this for each chip. Adjustment of the frequency is performed by minutely shaving the electrodes or the surface of the piezoelectric substrate of all the chips in one wafer. As the adjusting device, a reactive ion etching (RIE) device has the highest accuracy and has little manufacturing variation. This apparatus adjusts the frequency by a method of removing an electrode material (Al-Cu) using a chlorine-based gas. In the wafer process 20, there is a method in which an apparatus for developing a resist is used and wet etching is performed to cut the electrode surface. Although this method has large variations in manufacturing accuracy, it is simple, and therefore, in many cases, frequency adjustment is performed using this apparatus in actual sites.

The above RIE apparatus is suitable for precise frequency adjustment. The problem with this device is how far the etch rate can be reduced. On the other hand, wet etching is inferior in terms of uniformity and reproducibility of adjustment.
Electrodes of about 0 Å to 100 Å are shaved to adjust the frequency up to about 2 MHz.

As described above, there has been a wet etching or dry etching technique as a center frequency adjusting technique for a SAW device formed on a piezoelectric substrate.

For example, wet (Al)
For the etching, a method using an etchant diluted with hydrofluoric acid, nitric acid or the like is generally used. An organic alkali-based developer can be used to perform a small amount of etching. The electrode thickness is reduced by etching the electrode material, and the center frequency of the element is increased.

As dry etching, reactive ion etching (RIE) technology is widely used. The electrode material reacts with the chlorine gas system in a high frequency electric field. The center frequency can be increased by etching the electrode material.

[0023]

When the surface acoustic wave device is operated at a frequency higher than the 800 MHz band, the IDT
The electrode width is reduced to 1 μm and the electrode thickness is reduced to 100 nm or less. Further, a film thickness control technique of about 10 Å or less is required. In this case, the processing accuracy of the electrode affects the center frequency of the device. In addition, it is necessary to pay attention to the oxidation phenomenon on the electrode surface. The reason is that the thickness of the oxide film affects the center frequency of the device. Further, a technique for adjusting the frequency more accurately is also desired.

Since the oxidation phenomenon progresses gradually, the center frequency measurement on the wafer must be performed in a state where the oxide film is stable. If the measurement is performed in an unstable state, the center frequency may change between the time of measurement and the time of product shipment due to an oxidation phenomenon after the assembly process.

The above-described conventional manufacturing method and frequency adjustment method are a single process of wet (dry) etching → center frequency measurement, and are not repeated due to the nature of the operation. Accuracy in controlling the amount of etching is required.

In particular, wet etching has a poor controllability because the etching solution erodes the substrate. If you reduce,
The frequency change is large, and it is difficult to adjust the center frequency to a desired frequency. Further, since no consideration is given to the formation of the oxide film, there is a possibility that the center frequency may change over time. Specifically, the following can be said.

The electrode fingers of the SAW filter are formed by vacuum evaporation of Al-Cu. Al chemically reacts with oxygen in the air to become an oxidized compound. The thickness of the oxidized compound gradually increases with time and does not progress further when it reaches a certain thickness determined by the ambient temperature of the device. This thickness is believed to be several Angstroms to 100 Angstroms.

As described above, after the manufacture of the SAW filter, the oxidation reaction proceeds gradually, and the oxide film thickness on the electrode surface changes. When the oxide thickness changes, the center frequency of the filter also shifts. In the prior art, there are few products that need to consider the effect on the characteristics of the oxide film. However, as the band width of the SAW filter progresses, the center frequency is adjusted in consideration of the effect on the characteristics. A method is desired.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method of adjusting a center frequency of a SAW filter, in which the center frequency can be easily adjusted and the center frequency of the filter does not change with time. That is.

[0030]

According to the present invention, in order to solve the above-mentioned problems, f0 adjustment is performed by etching an Al-Cu electrode using a chlorine-based gas using an RIE apparatus. Then, the present invention proposes a new frequency adjustment method in consideration of the influence of the oxide film.

As described above, the fluctuation of the center frequency with respect to the film thickness has a sensitivity of about 0.2 MHz / 10 angstroms. Even if the thickness of the oxide film changes by about several + angstroms, the center frequency is slightly shifted due to the influence. In addition, if the oxide film is naturally formed without controlling and managing the oxide film thickness, the center frequency changes over time, which greatly affects the reliability of the device. Further, when etching is performed over the oxide film layer and the aluminum layer at the time of electrode etching, the etching rate changes due to the different properties of the materials, making accurate etching difficult.

In order to prevent aging, the simplest solution is to intentionally form an oxide film on the electrode. In order to form an oxide film, it is preferable to raise the substrate temperature in ozone gas. Since the RIE is a method of reacting with a gas in a chamber, an oxide film can be easily formed by replacing the gas with ozone. Moreover, it can be operated in the same batch, which is preferable in the production process.

FIG. 1 is an enlarged view of a section of an electrode finger of a SAW filter arranged inside an RIE etching manufacturing chamber.

FIG. 1A is an explanatory view showing formation of an oxide film. First, ozone gas is introduced into the RIE apparatus chamber. Chamber internal temperature (1
By controlling the temperature (about 50 ° C. to 200 ° C.), an oxide film d having a thickness equal to or greater than the natural oxide film thickness determined by the ambient temperature of the device is formed in advance. After the formation, the center frequency is measured by RF measurement, and the etching amount (cut film thickness) at which the target frequency is obtained is determined.

Next, in FIG. 1B, the gas in the chamber is replaced with a chlorine-based gas, and the gas concentration, RF power, etching time and the like of the RIE device are controlled to perform etching to a target film thickness. Fig. 1 Etching thickness
The thickness is set within the oxide film thickness formed by the formation of the oxide film described in (1). If more cutting is required, see Figure 1
The steps from (1) and FIG. 1 (2) to the following FIG. 1 (3) are repeated.

Next, in FIG. 1C, an ozone gas is again introduced into the chamber to form an oxide film. This oxide film is formed for the purpose of supplementing the oxide film thickness corresponding to the electrode film thickness cut by RIE. The oxide film obtained by forming this oxide film has the same thickness as the oxide film formed in the process described with reference to FIG. As a result, even if the device is taken out of the chamber and left, the fluctuation of the center frequency of the device after the electrode etching can be prevented, and the adjusted center frequency is maintained.

With the above-described method, electrode etching and center frequency adjustment can be performed in consideration of the influence of the electrode oxide film.

In the case where the desired center frequency cannot be adjusted, the case where the adjustment is performed more accurately, the case where the etching is performed to the thickness of the oxide film or more, etc., the above-mentioned FIGS.
It is preferable to repeat the operation (3) again.

In addition to raising the center frequency, the center frequency can be lowered by replacing the reactive gas with fluorine and etching the quartz substrate surface. When the quartz substrate surface is etched with a fluorine-based gas, it is not necessary to form an oxide film again because the electrode oxide film thickness does not change. That is, the reaction gas is changed from a chlorine-based gas to a fluorine-based gas in the processing step described in FIG. 1 (2) without executing the processing step described in FIG. 1 (1) and FIG. The surface of the quartz substrate is cut by a thickness to obtain the frequency.

In any of the methods described above, the frequency of the SAW device in the substrate can be adjusted in one batch without opening and closing the chamber.

Specifically, the present invention employs the following means.

According to a first aspect of the present invention, there is provided a method for adjusting a center frequency of a surface acoustic wave device when a surface acoustic wave device formed on a piezoelectric substrate is manufactured. An arrangement step of disposing the piezoelectric substrate in a chamber having a terminal capable of measuring output characteristics, and an etching step of shaving the surface of the interdigital electrode constituting the surface acoustic wave device using a method such as dry etching, An oxide forming step of forming an oxide on the surface of the interdigital electrode, and measurement of input / output characteristics including frequency characteristics of the elastic surface device via the terminals is performed until a desired characteristic is obtained. A repetitive step of repeatedly performing an etching step and the oxide forming step. It is the center frequency adjustment method.

The second invention is a method for manufacturing a surface acoustic wave device using the method of the first invention.

A third aspect of the present invention is a surface acoustic wave device manufactured using the method of the first aspect of the present invention.

Further, a fourth invention is a communication apparatus using a surface acoustic wave device manufactured by using the method of the first invention.

[0046]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1 FIG. 2 is an explanatory diagram showing how the center frequency of a SAW device is adjusted.

As shown in this figure, a piezoelectric substrate 50 on which an electrode pattern of a SAW device shown in FIG. Then, a gas is introduced into the chamber 52 to generate plasma.
A support 62 for the piezoelectric substrate 50 is provided in the chamber 52. The support 62 is formed using a quartz plate. A metallic positive electrode 51 is attached below the support 62, and the positive electrode 51 is
The output of the F power supply is connected via a capacitor. On the other hand, a grounded ground electrode 53 is mounted above the piezoelectric substrate 50 so as to be parallel to the plus electrode 51. By applying a high-frequency voltage to the plus electrode 51 and the ground electrode 53 by the RF power source, ions generated in the plasma can be accelerated by an electric field and anisotropically etched on the piezoelectric substrate 50. ing.

At the lower part of the chamber 52, there is provided a structure through which a coaxial line for measuring electric characteristics of the SAW device penetrates. As shown in FIG. 4, the SAW device is provided in advance with a pad pattern 60 for contacting a terminal at the end of the coaxial line. Contact to the tip is piezoelectric substrate 5
The position of the measurement terminal 70 at the distal end portion fixed to the support table 62 is variably adjusted. For this purpose, a position adjusting knob 64 of the measuring terminal 70 is provided on the support 62. Adjustment of the position of the measurement terminal 70 at the tip is performed in advance with the chamber 52 opened. That is, the measuring terminal 7 at the tip end
0 is a SAW electrode measurement pad pattern 60 (see FIG. 4).
In this way, electrical characteristics can be measured by contacting the surface.

After mounting the piezoelectric substrate 50 on the support 62,
The lid 66 of the housing of the chamber 52 is closed, and the chamber 5 is closed.
Exhaust the inside of 2. When the inside of the chamber 52 reaches a certain degree of vacuum, a gas for oxidizing the electrode surface is first introduced. As the gas, a gas that causes an oxidation reaction with aluminum, which is an electrode material, such as ozone is selected. When such a gas is introduced into the chamber 52, the support 62
The temperature controller (not shown) mounted on the substrate is driven to heat the piezoelectric substrate 50. By heating the piezoelectric substrate 50, an oxidation reaction on the electrode surface is promoted, and an oxide film having a desired thickness can be formed in a short time. The temperature controller can be formed by using a conventional technique.
A pipe may be embedded in the second or positive electrode 51 and a heat exchange medium may be passed through the pipe.

The thickness of the oxide film to be formed is not less than the oxide film thickness determined by the operating environment temperature of the SAW device, and is set so that oxidation does not proceed further at the operating environment temperature. After the oxide film formation is completed, the center frequency is measured. The center frequency measured at this time is called the f0 reference value of the device. As described above, by measuring the center frequency after forming the oxide film, stable f
A measurement value of 0 (center frequency) can be obtained.

After the reference f0 measurement is completed in this manner, the inside of the chamber 52 is evacuated, and then a chlorine-based gas is introduced into the chamber 52. After the introduction of the gas, RF power is applied to perform a dry etching process. Optimum conditions regarding RF power and gas concentration during the dry etching process are determined in advance by inspection. Dry etching is performed under the optimal conditions. The control of the etching amount is performed by the RF power application time or the like. The center frequency is increased by etching. The etching time is set short so as not to exceed the target center frequency.

After the dry etching process is completed, the air is exhausted and an oxide film is formed again. When the target oxide film thickness is obtained, the center frequency measurement is performed again. If the target frequency is not reached, the air is further exhausted, and the above-described steps of dry etching and oxide film formation are repeated. When the target center frequency is obtained, the air is exhausted and the substrate is taken out.

As described above, the chamber 5 having the structure shown in FIG.
The frequency is measured while always forming a constant oxide film by using No. 2.

As described above, in this embodiment, f0 adjustment, frequency stabilization (oxide film formation), and frequency measurement are repeatedly performed within one patch. Therefore, permanent frequency measurement can be performed. Further, since the repetitive processing can be performed, the dry etching processing is initially performed roughly (that is, the etching amount is set large), and when the center frequency approaches the target frequency, the processing conditions are changed and fine adjustment is performed (that is, the etching amount is changed). To a small value). Therefore, the processing can be performed accurately in a short time.

According to this embodiment, since the center frequency of the SAW device is directly measured, erroneous etching is performed even if the gas or the situation inside the chamber 52 changes, and the target frequency is significantly different from the target frequency. There is no risk of adjusting to the frequency.

Further, according to the present embodiment, since the oxide film is formed, it is possible to prevent a frequency shift after a lapse of time. Therefore, it is not necessary to consider the frequency fluctuation due to aging.

Embodiment 2 FIG. 3 is an explanatory diagram showing how the center frequency is adjusted in another embodiment.

As shown in FIG. 3, the SAW device is not diced, but is diced and flip-chip mounted. That is, it is supported in the form shown in FIG.

In flip chip mounting, always use SA
A space corresponding to the height of the gold bump 14 is provided between the W chip 10 and the package 12. In the case of the flip chip type surface acoustic wave device using the gold bump 14, approximately 3
There is a space of 0 μm. A space of about 50 μm is also provided between the edge of the SAW chip 10 and the package 12. As described above, the excited active species reach the chip electrode surface through the space between the package 12 and the SAW chip 10. As a result, the material on the elastic wave propagation region on the surface of the SAW chip 10 is etched. Measurement terminal 70
Is in contact with the pattern of the package 12 from the back of the support 62, and electrical measurement is possible. That is, the supporting table 62 is provided with two small holes for inserting a high-frequency coaxial line, and the measuring terminals 70 are respectively brought into contact with the pattern of the package 12 through the small holes. And the measuring terminal 7
Good contact is achieved by fine-tuning the zero position.

As described above, even when the package 12 on which the chip is mounted is mounted in the chamber 52, and the exhaust, dry etching, exhaust, electrode oxidation, and measurement of the electric characteristics are performed, the same effects as those of the first embodiment can be obtained. Can be

[0062]

As described above, according to the present invention, it is not necessary to pay attention to the influence of a natural oxide film which progresses with time, and accurate center frequency adjustment of a SAW device is possible. In the present invention, since the oxide film is formed in advance in the adjustment stage, it is possible to provide a manufacturing method capable of obtaining a device having stable performance over a long period of time.

Further, according to the present invention, RF measurement, f0
Since adjustment (electrode etching) and f0 stabilization (oxide film formation) are simultaneously performed by a single apparatus, the number of steps and cost can be reduced.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an operation of adjusting a center frequency of a SAW device according to a first embodiment.

FIG. 2 is an explanatory diagram illustrating an operation of adjusting the center frequency of the SAW device according to the first embodiment.

FIG. 3 is an explanatory diagram illustrating an operation of adjusting a center frequency of a SAW device according to the second embodiment.

FIG. 4 is an explanatory diagram showing a chip pattern including a measurement pad pattern in the first embodiment.

FIG. 5 is an explanatory view showing a conventional FDB type SAW filter manufacturing method.

FIG. 6 is a flowchart illustrating a manufacturing process of a conventional SAW filter.

[Explanation of symbols]

10 SAW chip, 12 package, 14 gold bump, 20 wafer process, 22 assembly process, 50 piezoelectric substrate, 51 plus electrode, 52
Chamber, 53 ground electrode, 60 measurement pad pattern, 62 support base (quartz plate), 64 position adjustment knob, 66 lid, 70 measurement terminal.

Claims (4)

[Claims] 1. A method of adjusting a center frequency of a surface acoustic wave device when manufacturing a surface acoustic wave device formed on a piezoelectric substrate, wherein input / output characteristics including frequency characteristics of the surface acoustic wave device can be measured. An arranging step of arranging the piezoelectric substrate in a chamber having various terminals; an etching step of shaving the surface of an interdigital electrode constituting the surface acoustic wave device by using a method such as dry etching; and the interdigital electrode. An oxide forming step of forming an oxide on the surface of the surface, measuring the input / output characteristics including the frequency characteristics of the elastic surface device via the terminals, until the desired characteristics are obtained, the etching step and the oxidation A center frequency of the surface acoustic wave device, comprising: Adjustment method. 2. A method for manufacturing a surface acoustic wave device using the method according to claim 1. 3. A surface acoustic wave device manufactured by using the method of claim 1. 4. A communication apparatus using a surface acoustic wave device manufactured by using the method according to claim 1.