JP2007336417A - Surface acoustic wave element and method for manufacturing the same - Google Patents

Abstract

An electrode having a side surface perpendicular to a substrate is formed to increase reflection of surface acoustic waves.
A film forming process for forming an electrode metal film on a surface of an LBO wafer, a resist film forming process for forming a photoresist film on the metal film, and exposing and developing the resist film. Then, a mask forming step of patterning into a predetermined shape, and using the patterned resist film 40 as a mask, the exposed portion of the metal film 38 is irradiated with plasma of a predetermined gas to remove the metal film 38, and the LBO wafer 36. And an etching process for removing the exposed surface layer portion of the LBO wafer, and a cleaning step for washing the exposed portion of the LBO wafer with water and removing the damage layer 46 caused by the plasma generated on the LBO wafer 36.
[Selection] Figure 3

Description

  The present invention relates to a surface acoustic wave element piece and a manufacturing method thereof, and more particularly to a surface acoustic wave element piece using a lithium tetraborate single crystal substrate as a piezoelectric substrate and a manufacturing method thereof.

2. Description of the Related Art In recent years, in the field of wireless communication devices such as mobile phones, the progress of high frequency and downsizing has been remarkable, and surface acoustic wave filters that are small and excellent in mass productivity are widely used for wireless communication devices. A surface acoustic wave filter is an IDT (Interdigital) comprising interdigital electrodes on a piezoelectric substrate.
A surface acoustic wave element provided with a transducer is a main component. Although there are various piezoelectric materials, quartz, lithium tantalate (LiTaO ₃ : LT), and lithium niobate (LiNbO ₃ : LN) have been conventionally used as piezoelectric substrates for surface acoustic wave element pieces. A surface acoustic wave element piece made of a quartz substrate has a small amount of frequency fluctuation (frequency temperature characteristic) with respect to a temperature change. For this reason, it is used for a resonator, a narrow band filter, and the like. On the other hand, although the LT substrate and the LN substrate have poor frequency temperature characteristics, the electromechanical coupling coefficient is much larger than that of quartz, and a broadband filter can be realized.

By the way, in recent years, a lithium tetraborate (Li ₂ B ₄ O ₇ : LBO) single crystal having relatively good frequency-temperature characteristics and a much larger electromechanical coupling coefficient than quartz has attracted attention as a piezoelectric material. Use on devices is ongoing. In particular, a lithium tetraborate single crystal (hereinafter sometimes simply referred to as LBO) is difficult to realize with quartz, and has an excellent advantage in a medium band filter in which frequency temperature characteristics are problematic in LT and LN.

  Surface acoustic wave filters (SAW filters) include a resonator type in which a lattice-like reflector is provided with an IDT interposed therebetween, and a transversal type having no reflector. A transversal SAW filter generally has a drawback of large insertion loss. However, this insertion loss has been reduced by adopting a unidirectional electrode. The unidirectional electrode utilizes reflection of surface acoustic waves at the end surfaces (side surfaces) of electrode fingers constituting the IDT. The intensity of reflection of the surface acoustic wave at the electrode end face depends on the material of the electrode, but varies depending on the shape of the electrode. In the LBO substrate, when the electrode (IDT) is formed of aluminum (aluminum), the reflection of the surface acoustic wave increases as the thickness of the electrode increases. Also, the surface of the electrode is more strongly reflected when it stands vertically than when the end surface (side surface) is inclined with respect to the substrate.

  However, LBO has a deliquescence property that absorbs moisture in the atmosphere and dissolves when exposed to the atmosphere. LBO does not dissolve in an alkaline organic solvent, but is slightly etched in pure water, and has a property of reacting with acid in particular. For this reason, when an IDT is formed by forming an aluminum film on an LBO substrate and etching the aluminum film, it is not possible to use an acid etching method that is used in many piezoelectric materials such as quartz, LT, and LN. That is, when the surface acoustic wave element piece is formed using the LBO substrate, if the aluminum film is etched with an acid, the exposed portion of the LBO is dissolved by the acid, and the LBO under the electrode is also affected. As a result, when an ordinary acid etching is performed using an LBO substrate, the electrical characteristics of the surface acoustic wave element piece are deteriorated, which may lead to problems such as electrode peeling. Therefore, a method has been developed in which a positive resist film is formed on an aluminum film, and the resist film and the aluminum film are simultaneously patterned with an organic alkaline developer (Patent Document 1).

On the other hand, when the aluminum film is anisotropically etched using plasma of a chlorine-based gas, an electrode whose side surface (end surface) is perpendicular to the substrate can be formed. When performing etching using plasma, so-called over-etching, in which etching is performed up to the surface layer portion of the substrate, is generally performed in order to completely eliminate the residue of the aluminum film. However, if overetching is performed, the substrate is damaged by plasma. Therefore, an etching method has been developed in which a base film that can be etched with a gas that is not etched with a chlorine-based gas and that does not damage the substrate is provided under the aluminum film (Patent Document 2).
Japanese Patent Publication No. 5-24684 Japanese Patent No. 3266109

  The etching method described in Patent Document 1 has an advantage that the LBO substrate is not damaged by the etching solution because the etching solution is not acidic. However, when an aluminum film is etched with an organic alkaline developer, the side surface of the aluminum electrode is etched due to isotropic etching, and the side surface of the electrode is inclined with respect to the substrate. For this reason, the etching method described in Patent Document 1 has a drawback that the reflection of the surface acoustic wave on the side surface of the electrode becomes weak and it is difficult to reduce the insertion loss.

  The method of etching by providing a base film under the aluminum film described in Patent Document 2 can avoid damage due to plasma due to overetching. However, etching must be performed using two kinds of gases when forming the IDT electrode. For this reason, in order to form an electrode, the number of processes increases and it becomes difficult to reduce cost.

  The present invention has been made to solve the conventional drawbacks, and has an object to increase the reflection of surface acoustic waves by forming an electrode having a side surface perpendicular to a substrate.

Another object of the present invention is to make it possible to remove substrate damage caused by etching.
An object of the present invention is to reduce the insertion loss of a piezoelectric device.

  When the inventors performed plasma etching using a chlorine-based gas as a reactive gas, the Cl-ion of the chlorine-based gas diffuses into the crystal of the LBO substrate and disturbs the crystallinity. It was judged that plasma damage would occur.

  By the way, "LBO is pure water (1-2 μm / It is slightly etched in “day)”. 1 to 2 μm / day corresponds to etching of 6 to 14 angstroms (0.6 to 1.4 nm) per minute with pure water. For example, in the case of a filter having a center frequency around 300 MHz, the width of the electrode finger of the IDT is 1.4 to 2.8 μm (1400 to 2800 nm), depending on the design method. For example, even if the LBO substrate under the aluminum electrode finger is etched by 6 to 14 Å by cleaning the LBO substrate with pure water, the etching amount is 0 for the entire electrode finger width when the center frequency is 300 MHz. . Stays below 2%. For this reason, even if the inventor washed the LBO substrate with pure water for a short time, the inventor determined that the influence on the peeling of the electrode and the characteristics of the surface acoustic wave element piece was small. Further, the inventor speculated that by cleaning and etching the LBO substrate with pure water, the surface layer portion of the LBO substrate whose crystallinity was disturbed by Cl ions, that is, a damaged layer caused by plasma, could be removed.

The present invention has been made based on the above consideration. In order to achieve the above object, the surface acoustic wave element according to the present invention is provided on a lithium tetraborate substrate and the lithium tetraborate substrate. A surface acoustic wave element provided with interdigital electrodes, wherein the lithium tetraborate substrate is characterized in that a damaged layer due to etching generated during the formation of the interdigital electrodes is removed.

  In the present invention as described above, the damage layer due to the plasma of the lithium tetraborate substrate generated when the interdigital electrode is formed by plasma etching is removed. Therefore, when the interdigital electrode is formed, the influence of the damaged layer of the substrate caused by etching on the propagation of the surface acoustic wave can be avoided, and the deterioration of the characteristics of the surface acoustic wave element can be prevented. Further, anisotropic etching by plasma can be used, and the side surface (end surface) of the electrode can be formed perpendicular to the substrate. Therefore, the reflection of the surface acoustic wave at the electrode end face can be increased, and the insertion loss of the surface acoustic wave device can be reduced.

  The method for producing a surface acoustic wave element according to the present invention includes a film forming step of forming a metal film for an electrode on the surface of a lithium tetraborate substrate, and a resist film that forms a photoresist film on the metal film. A forming step; a mask forming step of exposing and developing the resist film to pattern the resist film into a predetermined shape; and exposing the exposed portion of the metal film to a predetermined gas plasma using the patterned resist film as a mask. Removing the metal film, exposing the lithium tetraborate substrate, and removing the exposed surface layer portion of the lithium tetraborate substrate; and the remaining in the lithium tetraborate substrate And a resist removal step for removing the resist film.

  In the present invention as described above, the side surface (end surface) of the electrode forming the interdigital electrode by anisotropic etching using plasma can be made perpendicular to the substrate. For this reason, the reflection of the surface acoustic wave at the electrode end face can be increased, and the insertion loss of the surface acoustic wave device can be reduced.

  The etching step includes washing the exposed portion of the lithium tetraborate substrate with water after plasma etching, removing the damaged portion caused by the plasma generated on the lithium tetraborate substrate, and drying the washed lithium tetraborate substrate. And a drying step.

  The lithium tetraborate (LBO) substrate is soluble in water. Therefore, by washing LBO with water, the lithium tetraborate substrate is etched with water. Therefore, by etching the LBO substrate with water, the damaged layer caused by the plasma can be removed, the influence of the damaged layer on the propagation of the surface acoustic wave is eliminated, and the deterioration of the characteristics of the surface acoustic wave element piece is prevented. Can do. Further, since the cleaned substrate is dried, the influence of the cleaning liquid remaining on the substrate can be eliminated.

The method for manufacturing a surface acoustic wave element according to the present invention includes a film forming step of forming a metal film for an electrode on a surface of a lithium tetraborate substrate, and a resist film that forms a photoresist film on the metal film. A forming step; a mask forming step of exposing and developing the resist film to pattern the resist film into a predetermined shape; and exposing the exposed portion of the metal film to a predetermined gas plasma using the patterned resist film as a mask. Removing the metal film, exposing the lithium tetraborate substrate, and removing the exposed surface layer portion of the lithium tetraborate substrate; and the remaining in the lithium tetraborate substrate A resist removal step for removing the resist film, and washing the exposed portion of the lithium tetraborate substrate with water to prevent damage caused by the plasma generated on the lithium tetraborate substrate. A cleaning step of removing the parts, is characterized by having a step of drying the washed the lithium tetraborate substrate. The cleaning process of the tetraborate substrate may be performed after removing the resist film.

The electrode metal film may be aluminum or an aluminum alloy. The gas used for plasma etching may be a chlorine-based gas. An aluminum or aluminum alloy film can be easily etched by plasma of a chlorine-based gas such as chlorine (Cl ₂ ), carbon tetrachloride (CCl ₄ ), or boron trichloride (BCl ₃ ).

  The washing step can include an alkali washing step of washing the lithium tetraborate substrate with an alkaline aqueous solution and a water washing step of washing the lithium tetraborate substrate with water adjusted to a predetermined pH. When the interdigital electrode is formed of aluminum or an aluminum alloy, the etching gas residue can be easily removed by washing the LBO substrate with an alkaline aqueous solution.

  The water whose pH is adjusted may be pure water. Pure water has a constant pH, that is, pH = 7, and does not require any pH adjustment. Moreover, since pure water does not contain impurities, the substrate can be kept clean. The water washing step is preferably performed immediately after the alkali washing step. According to the inventors' research, the damage layer can be easily and reliably removed by washing the lithium tetraborate substrate with water as soon as possible after washing with an alkaline aqueous solution.

  The drying process desirably includes a liquid draining process for forcibly removing the cleaning liquid adhering to the substrate. In order to avoid the influence of the cleaning liquid of the lithium tetraborate substrate, particularly moisture, it is desirable to remove the cleaning liquid adhering to the substrate. Thereby, etching of the LBO substrate under the electrode can be prevented, and variations in etching of the LBO substrate due to water can be suppressed. The liquid draining step is preferably performed immediately after the washing step is finished. The liquid draining process is preferably performed as soon as possible after the cleaning process is completed in order to prevent variation in etching amount and deterioration of characteristics. The liquid draining step can be performed, for example, by rotating the substrate at a high speed with a spin dryer or blowing hot air onto the substrate.

  The surface acoustic wave element according to the present invention is manufactured by any one of the manufacturing methods described above. Thereby, said effect can be obtained.

Preferred embodiments of a surface acoustic wave element piece and a method for producing the same according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a plan view of a surface acoustic wave element piece according to the present invention, which is a plan view of a surface acoustic wave element piece for a transversal surface acoustic wave filter, and FIG. 2 is a line AA in FIG. FIG. In these drawings, the surface acoustic wave element piece 10 is provided with a pair of IDTs 14 and 16 formed of interdigital electrodes on a piezoelectric substrate 12 having a rectangular shape in plan view. The piezoelectric substrate 12 is made of lithium tetraborate (Li ₂ B ₄ O ₇ : LBO). For example, the cut angle, that is, the cut-out angle and the propagation direction of the surface acoustic wave are expressed in Euler angles (45 °, 90 °). , 90 °).

The IDTs 14 and 16 are provided along the propagation direction of the surface acoustic wave excited by the piezoelectric substrate 12. Each of the IDTs 14 and 16 includes a pair of comb-shaped electrodes 18 (18a and 18b) and 20 (20a and 20b). The comb-shaped electrodes 18a and 18b constituting the IDT 14 each include a plurality of electrode fingers 22 (22a and 22b) and a bus bar 24, and one end of the electrode finger 22 is connected to the bus bar 24. Each electrode finger 22 is formed orthogonal to the propagation direction of the surface acoustic wave. The IDTs 14 are arranged at equal intervals so that the electrode fingers 22a and 22b of the pair of comb-shaped electrodes 18a and 18b are engaged with each other, and form an interdigital shape. ID
T16 is arranged with an appropriate distance from IDT14. The pair of comb electrodes 20 constituting the IDT 16 includes electrode fingers 26 (26 a and 26 b) and a bus bar 28, and is formed in the same manner as the comb electrode 18. The IDTs 16 are interdigitally arranged at equal intervals so that the electrode fingers 26 of the pair of comb-shaped electrodes 20 mesh with each other as in the IDT 14.

  One of the IDTs 14 and 16, for example, the IDT 14 is an input electrode, and the other IDT (in this case, IDT 16) is an output electrode. One of the pair of comb-shaped electrodes 18a and 18b of the IDT 14 serving as an input electrode (in this embodiment, the comb-shaped electrode 18b) is grounded, and the other comb-shaped electrode is connected to the input circuit. In the IDT 16 serving as the output electrode, one of the pair of comb electrodes 20a and 20b (in the embodiment, the comb electrode 20a) is grounded, and the other comb electrode is connected to the output circuit. The pair of comb-shaped electrodes 18 of the IDT 14 generates a surface acoustic wave having a predetermined frequency by exciting the piezoelectric substrate 12 with an input electric signal (voltage) having a predetermined frequency. The IDT 16 receives the surface acoustic wave excited by the IDT 14, converts it into an electrical signal (voltage), and outputs it.

  The surface acoustic wave element piece 10 according to the embodiment is provided with a shield electrode 30 between the two in order to weaken the electromagnetic coupling between the IDTs 14 and 16. The shield electrode 30 is connected to a ground circuit of a circuit board (not shown). Further, the surface acoustic wave element piece 10 is coated with a sound absorbing material 32 made of silicon rubber or the like at both ends in the longitudinal direction of the piezoelectric substrate 12, that is, in the propagation direction of the surface acoustic wave, to suppress unnecessary reflected waves. ing. The sound absorbing material 32 may not be provided.

  In the embodiment, the IDTs 14 and 16 and the shield electrode 30 are formed of an aluminum (aluminum) thin film. The IDTs 14 and 16 and the shield electrode 30 are formed by anisotropic etching using plasma using a chlorine-based gas, as will be described in detail later. In the embodiment, the substrate 12 made of LBO is overetched with the electrode aluminum film, and the surface layer portion of the substrate 12 between the electrode fingers (electrodes) is removed as shown in FIG. Is formed. Further, in the surface acoustic wave element piece 10, after the electrodes are formed, the substrate 12 is washed with water, and the damage layer due to the plasma generated in the bottom portion of the groove 34 is removed.

  In the surface acoustic wave element piece 10 according to the embodiment as described above, the damage layer due to the plasma generated on the substrate 12 is removed, and therefore, there is no influence on the propagation of the surface acoustic wave by the damage layer. Therefore, the surface acoustic wave element does not deteriorate in characteristics due to the plasma damage layer. In addition, since the electrode fingers are formed by anisotropic etching using plasma, the side surfaces (end surfaces) of the electrode fingers can be formed perpendicular to the surface of the piezoelectric substrate 12. For this reason, the reflection of the surface acoustic wave at the electrode end face is increased, and the insertion loss can be reduced.

  FIG. 3 is a process diagram of the method for manufacturing the surface acoustic wave element according to the first embodiment of the present invention. First, an LBO wafer (substrate) 36 is prepared and a film forming process is performed. As shown in FIG. 3A, an electrode metal film 38 is formed on the surface of the polished LBO wafer 36 to a predetermined thickness. Film. In the case of the embodiment, the metal film 38 is made of aluminum and formed by sputtering. The electrode metal film 38 may be an aluminum alloy, and the film may be formed by other methods such as vacuum deposition. Further, although not shown, the LBO wafer 36 has a rough surface with a predetermined roughness by sandblasting or the like on the back surface (lower surface) in order to reduce the reflection of bulk waves.

Next, as shown in FIG. 3B, a resist film forming step is performed in which a photoresist is applied to the surface of the metal film 38 and solidified to form a resist film 40. The photoresist may be positive or negative. Thereafter, a mask forming process for patterning the resist film 40 is performed. In the mask formation step, first, a photomask (not shown) having a predetermined pattern is disposed above the resist film 40, and the resist film 40 is irradiated with light such as ultraviolet rays through the photomask. To expose. Next, the exposed resist film 40 is immersed in a predetermined developer, and development is performed to remove unnecessary portions of the resist film 40. Thereby, as shown in FIG. 3C, the resist film 40 is patterned into a predetermined shape and becomes a mask for plasma etching.

Next, the electrode metal film 38 is etched using the patterned resist film 40 as a mask. That is, anisotropic etching is performed by irradiating the entire surface of the LBO wafer 36 with a chlorine gas plasma such as chlorine gas (Cl ₂ ) or carbon tetrachloride (CCl ₄ ) through the patterned resist film 40. As a result, the metal film 38 exposed from the resist film 40 is etched into the electrode 42 having a predetermined shape. Further, the portion of the LBO wafer 36 corresponding to the etched portion of the metal film 38 is exposed (FIG. 3 (4)). In this plasma etching process, so-called over-etching is performed to remove the surface layer portion of the LBO wafer 36 so that the exposed metal film 38 can be completely removed. The etching amount (etching depth) of the LBO wafer 36 by this over-etching may be 100 angstroms or less as long as the exposed metal film 38 can be completely removed. Thus, by performing anisotropic etching using plasma, the side surface (end surface) of the electrode 42 made of the metal film 38 can be made perpendicular to the surface of the LBO wafer 36. Therefore, the reflection of the surface acoustic wave at the end face of the electrode 42 can be increased, and the insertion loss can be reduced when a surface acoustic wave device such as a surface acoustic wave filter is used. In addition, because of anisotropic etching, the LBO wafer 36 under the electrode 42 is hardly etched, and the electrode 42 is not peeled off from the LBO wafer 36.

  When the etching of the metal film 38 is completed, the chlorine-based gas residue 44 may adhere to the exposed portion of the LBO wafer 36 as shown in FIG. Further, since the metal film 38 is over-etched, a damaged layer 46 due to plasma is generated in the etched portion of the LBO wafer 36. Therefore, a cleaning process is performed in order to remove the chlorine-based gas residue 44 and the damaged layer 46. In the case of the embodiment, the cleaning process includes a process of removing the chlorine-based gas residue 44 and a process of removing the damaged layer 46.

  The chlorine gas residue 44 is easily dissolved in an alkaline solution. Therefore, in the embodiment, first, an alkali cleaning process is performed in which the LBO wafer 36 on which the electrode 42 is formed is cleaned with an alkaline aqueous solution. As the alkaline aqueous solution for washing, for example, a sulfur compound aqueous solution in which 1% by weight of a sulfur compound is dissolved in pure water can be used. The cleaning time may be about 30 seconds. As a result, as shown in FIG. 3 (5), the chlorine-based gas residue 44 attached to the LBO wafer 36 is removed, and the time-dependent change due to the chlorine-based gas residue 44 of the surface acoustic wave element piece 10 can be eliminated. , Can increase the reliability. It is desirable to perform the alkali cleaning process as soon as possible after plasma etching.

  Next, a water washing step of washing the LBO wafer 36 that has completed the alkali washing step with pure water was performed to remove the damaged layer 46. That is, immediately after finishing the alkali cleaning step, the LBO wafer 36 was cleaned with pure water for 1 minute to remove the damaged layer 46 (see FIG. 3 (6)). This cleaning with pure water is preferably performed as soon as possible after cleaning with an alkaline aqueous solution. According to the inventor's research, the longer the time from plasma etching to the start of the washing process, the smaller the washing effect. In addition, this water washing process with pure water also serves as washing of the alkaline aqueous solution for washing away the alkaline aqueous solution. Further, since the back surface (lower surface) of the LBO wafer 36 is adjusted to a rough surface having a predetermined roughness, it is desirable that pure water does not rotate on the back surface.

The LBO wafer 36 is subjected to a liquid draining process, which is a part of the drying process, immediately after finishing the water washing process. In the liquid draining step, for example, the LBO wafer 36 is rotated at a high speed by a spin dryer, and pure water adhering to the LBO wafer 36 is forcibly removed. Hot air may be blown onto the LBO wafer 36. Thereby, the pure water adhering to the LBO wafer 36 can be removed in a short time, and the LBO wafer 36 under the electrode 42 can be prevented from being etched. In addition, variations in etching due to pure water can be suppressed. Thereafter, the LBO wafer 36 is heated to around 120 ° C., and the LBO wafer 36 is completely dried.

  When the drying process is completed, a resist removing process is then performed, and the resist film 40 remaining on the LBO wafer 36, that is, remaining on the electrode 42, as shown in FIG. Remove. In the embodiment, in the resist removing process, the resist film 40 is removed by ashing using, for example, oxygen plasma which does not affect the electrode 42 and the LBO wafer 36. In the embodiment, the ashing time is 60 seconds or less. When the resist film 40 is ashed, a gas resist residue 48 adheres to the LBO wafer 36 as shown in FIG. Therefore, the LBO wafer 36 that has undergone the resist removal process is cleaned to remove the gas residue 48 as shown in FIG. In the case of the embodiment, the LBO wafer 36 was washed by shaking the LBO wafer 36 in tap water for 30 seconds, and immediately after that, rinsed with pure water for 2 to 3 seconds. Thereafter, the LBO wafer 36 is dried in the same manner as described above, and then cut into the surface acoustic wave element pieces 10. Thereby, the surface acoustic wave element piece 10 is obtained.

  In order to confirm the effect of the manufacturing method according to the first embodiment, after washing with an alkaline aqueous solution, the manufacturing method of the present invention in which a washing process with pure water is performed and the conventional manufacturing method without performing the washing process are elastic. Surface wave element pieces were manufactured and compared. The manufactured surface acoustic wave element piece is for a transversal SAW filter having a center frequency of 300 MHz.

  When the insertion loss was calculated | required about the 24 surface acoustic wave element pieces obtained with the conventional manufacturing method without the water washing process, the average value was a little over 10.3 dB. On the other hand, when the insertion loss was determined for the same number of surface acoustic wave element pieces obtained by the manufacturing method of the embodiment in which the water washing step with pure water for 1 minute was performed after the alkali washing step, the average value was 10.1 dB. Therefore, the insertion loss was improved by 0.2 dB. Moreover, the variation in insertion loss was also improved when washed with pure water for 1 minute. Further, when the insertion loss was obtained for 24 surface acoustic wave element pieces obtained by washing the LBO wafer with pure water for 5 minutes in the water washing step, the average value was 10.5 dB. From this, it was found that the long-time water washing of the LBO wafer greatly deteriorates the characteristics of the surface acoustic wave element.

  Japanese Patent Application Laid-Open No. 7-154179 discloses a technique for adjusting the frequency by immersing and etching an LBO substrate on which an electrode pattern is formed in pure water. That is, the technique described in JP-A-7-154179 measures the representative value or average value of the surface acoustic wave element frequency for each wafer, and adjusts the frequency of each wafer to a desired frequency for each wafer. Then, the etching time is calculated, and each wafer is etched for the calculated time to make the frequencies of all the wafers substantially coincide with each other.

  On the other hand, the present invention is the same in that the surface of the wafer is etched with pure water, but the object is to remove plasma damage on the surface layer portion. That is, an object of the present invention is to remove the surface layer portion of each wafer by a certain thickness. Therefore, in the present invention, the etching time with pure water is the same for all wafers.

Note that even the surface acoustic wave element piece using the LBO substrate has a frequency ranging from several tens of MHz to a high frequency around 500 MHz. In the surface acoustic wave element piece, the width dimension of the electrode fingers constituting the IDT varies depending on the frequency. That is, in the case of a low frequency of several tens of MHz, the width of the electrode finger exceeds 10 μm. On the other hand, when the frequency is around 500 MHz, the width of the electrode finger becomes narrow to around 1 μm. Therefore, the cleaning time with pure water that improves insertion loss without adversely affecting reliability such as electrode peeling needs to be varied depending on the frequency of the surface acoustic wave element piece and the width of the electrode finger (electrode width). . In the case of the above 300 MHz filter, the washing time with pure water was good for 1 minute. However, when the frequency is several tens of MHz to around 500 MHz, it is necessary to set the cleaning time with pure water to an optimal cleaning time between 10 seconds and 2 minutes. This cleaning time can be obtained by experiment or simulation. Here, the reason why the lower limit of the cleaning time is 10 seconds is that the LBO etching reaction does not start unless the minimum time is given.

  FIG. 4 is a process diagram of a method for manufacturing a surface acoustic wave element according to a second embodiment of the present invention. The manufacturing method of the second embodiment is different from the manufacturing method of the first embodiment in that the cleaning process is performed before the resist removing process in the first embodiment but after the resist removing process. That is.

  In the method of manufacturing the surface acoustic wave element according to the second embodiment, first, as shown in FIG. 4A, an electrode metal film 38 made of aluminum or an aluminum alloy is formed on the surface of the LBO wafer 36. A film forming process is performed. Next, a resist film forming step is performed to form a resist film 40 on the surface of the metal film 38 (FIG. 4B). Thereafter, as shown in FIG. 4C, a mask forming process for patterning the resist film 40 is performed. Next, an etching process is performed in which the electrode metal film 38 is etched with chlorine-based plasma using the patterned resist film 40 as a mask. In this etching step, as shown in FIG. 4D, overetching is performed so that the surface layer portion of the LBO wafer 36 is etched as in the first embodiment. The etching amount (etching depth) of the LBO wafer 36 by over-etching may be 100 angstroms or less.

  In the manufacturing method of the second embodiment, after the etching process is completed, a resist removal process is performed, and the resist film 40 remaining on the electrode 42 is removed by ashing (FIG. 4 (5)). In general, when a substrate is exposed by etching a metal film with a plasma of chlorine-based gas, a residue due to chlorine ions of chlorine-based gas is generated on the surface of the substrate, and the gas is removed as shown in FIG. Impurities 50 are attached. Therefore, an alkali cleaning process is performed in which the LBO wafer 36 from which the resist film 40 has been removed is washed with an alkaline aqueous solution, and the impurities 50 adhering to the LBO wafer 36 are removed as shown in FIG. As the alkaline aqueous solution for washing, the above-described aqueous sulfur compound solution can be used. As described above, if the alkali cleaning process is performed after the removal of the resist film 40, it is not necessary to clean the LBO wafer 36 with a chemical each time an unnecessary material adheres to the LBO wafer 36 by etching. Therefore, man-hours can be reduced, the amount of chemicals used can be reduced, and costs can be reduced.

  When the alkali cleaning process is completed, a water cleaning process for immediately cleaning the LBO wafer 36 with pure water is performed, and the surface layer portion of the LBO wafer 36 is etched with pure water. As a result, as shown in FIG. 4 (7), the damage layer 46 caused by the plasma generated on the LBO wafer 36 by over-etching the electrode metal film 38 can be removed. This cleaning with pure water also serves as cleaning of the alkaline aqueous solution. The alkali cleaning step is desirably performed as soon as possible after the resist film removing step, and the water washing step is desirably performed as soon as possible after the alkali cleaning step.

  Thereafter, the LBO wafer 36 is set on a spin dryer immediately after the water washing step is completed, and the attached pure water is forcibly removed by rotating at high speed. Next, the LBO wafer 36 is heated to about 120 ° C., dried, and cut into individual surface acoustic wave element pieces.

A surface acoustic wave element piece for a transversal SAW filter having a center frequency of 300 MHz was produced by the production method of the second embodiment, and compared with a surface acoustic wave element piece produced by a conventional production method having no water washing step. The number of surface acoustic wave element pieces produced is 24 each. In the manufacturing method of the second embodiment, the time from the end of the etching process to the start of the resist film stripping process, the time from the end of the resist stripping process to the start of the alkali cleaning process, and the end of the alkali cleaning process to the start of the water washing process Each time is 30 seconds.

  When the insertion loss of the surface acoustic wave element piece manufactured by the conventional method without water washing was determined, the average value was a little over 10.3 dB. On the other hand, the average value of the insertion loss of the surface acoustic wave element pieces washed with pure water for 1 minute in the washing step of the manufacturing method of the second embodiment was 10.1 dB. That is, by performing the water washing for 1 minute, the insertion loss was improved by 0.2 dB compared with that manufactured by the conventional manufacturing method. In addition, the one that was washed with pure water for 1 minute also showed less variation in insertion loss. In the manufacturing method of the second embodiment, a surface acoustic wave element piece having a center frequency of 300 MHz was manufactured with a cleaning time of pure water in the water washing step being 5 minutes. The average value of these 24 insertion losses was 10.6 dB. Therefore, it has been found that the cleaning with pure water for a long time also deteriorates the characteristics of the surface acoustic wave in the manufacturing method of the second embodiment.

  As described above, the manufacturing method according to the embodiment can use (1) a dry etching process common to the manufacture of other surface acoustic wave element pieces, thereby reducing costs. (2) By using the over-etching method of the metal film 38, metal residues can be eliminated, and deterioration of the characteristics of the surface acoustic wave element can be prevented. (3) By cleaning the LBO wafer with pure water for a short time, the surface layer portion of the LBO wafer can be removed, the insertion loss of the surface acoustic wave element piece is reduced, and the individual difference can be reduced. (4) By setting the time from the completion of etching by plasma to washing with water to a short time and minimizing the entry of Cl ions into the LBO wafer, it is possible to suppress the deterioration of the insertion loss of the surface acoustic wave element piece. it can. (5) By rotating the wafer at high speed after water washing and drying it in a short time, it is possible to prevent a decrease in reliability such as peeling of the electrode due to unnecessary etching of the LBO wafer under the electrode. Effects such as can be obtained.

  In addition, the said embodiment is 1 aspect of this invention, Comprising: It is not limited to the said embodiment. For example, in the above-described embodiment, the case where washing is performed in the washing step using pure water having a pH of 7.0 is described. However, water used in the washing step is not limited to pure water, and is neutral water, For example, tap water, ground water (well water), etc. may be sufficient. Moreover, although the case where the neutral water whose pH is 7.0 was used was demonstrated in the said embodiment, it may not be neutral water. In this case, since the LBO etching rate differs depending on the pH of water, the pH of water should be adjusted to a predetermined value. In the above embodiment, the surface acoustic wave element piece is used for a transversal SAW filter. However, the surface acoustic wave element piece may be used for a resonator type filter or a resonator.

It is a top view of the surface acoustic wave element piece concerning an embodiment. FIG. 2 is a partial cross-sectional view taken along line AA in FIG. 1. It is process drawing of the manufacturing method of the surface acoustic wave element piece concerning 1st Embodiment. It is process drawing of the manufacturing method of the surface acoustic wave element piece concerning 2nd Embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ......... Surface acoustic wave element piece, 12 ...... Lithium tetraborate (piezoelectric substrate), 14, 16 ... Interdigital electrode (IDT), 18a, 18b, 20a, 20b ...... Comb-shaped electrode, 22a, 22b, 26a, 26b... Electrode fingers, 36... LBO wafer, 38... Electrode metal film, 40.

Claims (11)

A surface acoustic wave element comprising a lithium tetraborate substrate and a comb electrode provided on the lithium tetraborate substrate,
A surface acoustic wave element piece in which the lithium tetraborate substrate has a damaged layer due to etching generated when the interdigital electrode is formed. A film forming step of forming a metal film for an electrode on the surface of the lithium tetraborate substrate;
A resist film forming step of forming a photoresist film on the metal film;
A mask forming step of exposing and developing the resist film and patterning the resist film into a predetermined shape;
Using the patterned resist film as a mask, the exposed portion of the metal film is irradiated with plasma of a predetermined gas to remove the metal film, thereby exposing the lithium tetraborate substrate and exposing the exposed four An etching process for removing the surface layer portion of the lithium borate substrate;
A resist removing step of removing the resist film remaining on the lithium tetraborate substrate;
A method for producing a surface acoustic wave element, comprising: In the manufacturing method of the surface acoustic wave according to claim 2,
The etching step includes washing the exposed portion of the lithium tetraborate substrate with water after plasma etching, and removing the damaged portion caused by the plasma generated on the lithium tetraborate substrate;
A drying step of drying the washed lithium tetraborate substrate;
A method of manufacturing a surface acoustic wave element, comprising: A film forming step of forming a metal film for an electrode on the surface of the lithium tetraborate substrate;
A resist film forming step of forming a photoresist film on the metal film;
A mask forming step of exposing and developing the resist film and patterning the resist film into a predetermined shape;
Using the patterned resist film as a mask, the exposed portion of the metal film is irradiated with plasma of a predetermined gas to remove the metal film, thereby exposing the lithium tetraborate substrate and exposing the exposed four An etching process for removing the surface layer portion of the lithium borate substrate;
A resist removing step of removing the resist film remaining on the lithium tetraborate substrate;
Washing the exposed portion of the lithium tetraborate substrate with water, and removing a damaged portion caused by the plasma generated on the lithium tetraborate substrate;
A drying step of drying the washed lithium tetraborate substrate;
A method for producing a surface acoustic wave element, comprising: In the manufacturing method of the surface acoustic wave element piece according to any one of claims 2 to 4,
The electrode metal film is made of aluminum or an aluminum alloy,
The gas is chlorinated,
A method for manufacturing a surface acoustic wave element. In the manufacturing method of the surface acoustic wave element piece according to claim 5,
The washing step includes
An alkaline washing step of washing the lithium tetraborate substrate with an alkaline aqueous solution;
A water washing step of washing the lithium tetraborate substrate with water adjusted to a predetermined pH;
A method for producing a surface acoustic wave element, comprising: In the manufacturing method of the surface acoustic wave element piece according to claim 6,
The method for producing a surface acoustic wave element according to claim 1, wherein the pH-adjusted water is pure water. In the manufacturing method of the surface acoustic wave element piece according to claim 6 or 7,
The method for producing a surface acoustic wave element piece, wherein the water washing step is performed immediately after the alkali washing step. In the manufacturing method of the surface acoustic wave element piece according to any one of claims 3 to 8,
The method for manufacturing a surface acoustic wave element piece, wherein the drying step includes a liquid draining step for forcibly removing the cleaning liquid adhering to the substrate. In the manufacturing method of the surface acoustic wave element piece according to claim 9,
The method for producing a surface acoustic wave element, wherein the liquid draining step is performed immediately after the cleaning step.   A surface acoustic wave element piece manufactured by the manufacturing method according to claim 2.

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