Method for manufacturing elastic wave element

The present invention relates to a method for manufacturing an elastic wave element such as a surface acoustic wave (SAW) element or an elastic boundary wave element.

The elastic wave element is lithium tantalate (LiTaO).₃: LT) substrate and lithium niobate (LiNbO)₃: LN) An element formed by forming a comb-shaped electrode (Inter-Digital Transducer: IDT) on a piezoelectric substrate such as a substrate. The coefficient of thermal expansion of LT and LN is about 6 times that of silicon (silicon about 2.6 x 10).^-6LT about 16 × 10 for / K^-6/ K, LN about 15 × 10^-6Since it is as large as / K), when an LT substrate or LN substrate is used for an elastic wave element, a change in filter characteristics due to a temperature change becomes a big problem. Therefore, temperature compensation is performed by various methods.

For example, Patent Document 1 discloses a technique for joining a thinned piezoelectric substrate and an amorphous piezoelectric substrate for temperature compensation directly or via an inorganic thin film layer. Further, Patent Document 2 discloses that a piezoelectric substrate thinned after forming an IDT and an insulating substrate for temperature compensation are joined by an adhesive member made of a vitreous material.<patcit num="1"><text>Japanese Unexamined Patent Publication No. 6-326553</text></patcit><patcit num="2"><text>Japanese Patent Application Laid-Open No. 2002-16468</text></patcit>

When a bonded substrate formed by bonding a piezoelectric substrate and a temperature compensation substrate is used in a wafer process, for example, when an IDT for an elastic wave element is formed on the bonded substrate, the piezoelectric substrate and temperature special compensation are performed by a heating process during the process. Stress due to the difference in linear expansion coefficient between the substrate and the substrate is generated and the piezoelectric substrate is warped, and the line width machining accuracy is lowered accordingly. Further, in the wafer process, processing may be performed at a temperature of 200 ° C. or higher, and when the bonded substrate is subjected to such a temperature, stress due to the difference in the coefficient of linear expansion is generated and the substrate is cracked or the bonded surface is cracked. May cause peeling.

The present invention has been made in view of this point, and is an elastic wave that has excellent temperature coefficient of frequency (TCF ), high processing accuracy of IDT pattern, and can withstand high temperature processing of 200 ° C or higher. It is an object of the present invention to provide a method for manufacturing an element.

The method for manufacturing an elastic wave element of the present invention includes a step of forming an IDT on one main surface of a piezoelectric substrate and a linear expansion coefficient of the piezoelectric substrate on the other main surface of the piezoelectric substrate after forming the IDT. It is characterized by comprising a step of forming a film of a material having a linear expansion coefficient smaller than that by thermal spraying.

According to this method, after forming the IDT on the piezoelectric substrate, a sprayed film exhibiting a temperature compensation effect is formed. First, since IDT is formed on the piezoelectric substrate, warping of the substrate due to stress generation due to the difference in linear expansion coefficient and substrate cracking in the high temperature process, which are problems in the bonded substrate, do not occur. Then, since the IDT can be formed in a state where the substrate is not warped, it is possible to obtain an elastic wave element having high processing accuracy of the IDT. Furthermore, since the thermal spray film that exerts the temperature compensation effect is formed after the IDT is formed, the elastic wave element obtained by this is further exerting the temperature compensation effect while the processing accuracy of the IDT is high. Can be done.

In the method for manufacturing an elastic wave element of the present invention, it is preferable to further include a step of roughening the other main surface of the piezoelectric substrate before forming a film by thermal spraying. According to this method, an elastic wave element that is not affected by bulk waves can be obtained. In this method, it is preferable that the other main surface has Ra = 0.01 μm to 3 μm.

In the method for manufacturing an elastic wave element of the present invention, it is preferable to further include a step of thinning the other main surface of the piezoelectric substrate before film formation by thermal spraying. According to this method, an elastic wave element having a higher temperature compensation effect can be obtained.

In the method for producing an elastic wave element of the present invention, the material is preferably at least one selected from the group consisting of mullite, alumina, silicon and yttria.

In the method for manufacturing an elastic wave element of the present invention, it is preferable that the piezoelectric substrate is a lithium tantalate substrate or a lithium niobate substrate.

In the method for manufacturing an elastic wave element of the present invention, it is preferable to further include a step of filling the pores formed in the film formed by the thermal spraying with a filler. According to this method, the rigidity of the sprayed film can be increased to improve the frequency temperature characteristics.

According to the method for manufacturing an elastic wave element of the present invention, an IDT is formed on one main surface of the piezoelectric substrate, and the coefficient of linear expansion of the piezoelectric substrate is formed on the other main surface of the piezoelectric substrate after the IDT is formed. Since a material with a coefficient of linear expansion smaller than that is formed by spraying, it has excellent temperature coefficient of frequency (TCF), and the substrate does not warp during the process, and it is treated at a high temperature of 200 ° C or higher. It is possible to obtain an elastic wave element that can withstand the above.

<figref num="1">It is a figure which shows the relationship between the thickness of a piezoelectric substrate and the temperature compensation effect.</figref><figref num="2">(a) to (c) are diagrams for explaining a method for manufacturing an elastic wave element according to an embodiment of the present invention.</figref><figref num="3">(a) to (d) are diagrams for explaining another example of the method for manufacturing an elastic wave element according to the embodiment of the present invention.</figref><figref num="4">It is a figure which shows the relationship between the temperature of PEB and the amount of warpage of a wafer at the time of forming IDT.</figref><figref num="5">It is a figure which shows the relationship between the pattern line width processing accuracy R and PEB temperature in a wafer surface.</figref><figref num="6">(a) and (b) are diagrams showing the influence of bulk waves of elastic wave elements.</figref>

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the method for manufacturing an elastic wave element of the present invention, an IDT is formed on one main surface of the piezoelectric substrate, and the linear expansion coefficient of the piezoelectric substrate is obtained on the other main surface of the piezoelectric substrate after the IDT is formed. A material having a small linear expansion coefficient is formed by thermal spraying. Here, the elastic wave element indicates an elastic surface wave element and an elastic boundary wave element.

Examples of the piezoelectric substrate include a lithium tantalate substrate (LT substrate) and a lithium niobate substrate (LN substrate).

Here, the relationship between the thickness of the piezoelectric substrate and the temperature compensation effect will be described. The temperature compensation effect of an elastic wave element can be obtained by examining the temperature coefficient of frequency (TCF). FIG. 1 is a characteristic diagram showing the relationship between the LT substrate thickness ratio (LT substrate thickness / total substrate thickness) and the temperature compensation effect. Here, an LT substrate was used as the piezoelectric substrate. As can be seen from FIG. 1, the smaller the ratio of the LT substrate thickness to the total substrate thickness, the higher the temperature compensation effect. As described above, from the viewpoint of the temperature compensation effect, it is desirable to make the ratio of the piezoelectric substrate thickness to the total substrate thickness as small as possible, that is, to make the thickness of the LT substrate as thin as possible in the fixed total substrate thickness.

However, in the conventional bonded substrate, the surface to be bonded is mirror-finished in order to bond the substrates to each other. When the surface is mirror-finished in order to perform good bonding in this way, it is necessary to polish the surface at the time of mirroring. In this polishing process, a relatively high processing stress is applied to the substrate. Therefore, if the thickness of the piezoelectric substrate is reduced in order to enhance the temperature compensation effect, the piezoelectric substrate may be cracked due to the stress in the polishing process. Therefore, in the bonded substrate, it is difficult to reduce the thickness of the piezoelectric substrate. On the other hand, in the method of the present invention, since it is not necessary to consider the viewpoint of bonding the substrates, polishing is not required, and the piezoelectric substrate can be thinned in order to enhance the temperature compensation effect.

In an elastic wave element, a bulk wave of an elastic wave generated on one main surface forming an IDT is reflected by the other main surface, and this reflected wave interferes with an elastic wave generated on one main surface. In order to reduce the influence of such bulk waves, it is preferable that the back surface of the piezoelectric substrate, that is, the surface on which the thermal spray film is formed is roughened. By roughening the back surface of the piezoelectric substrate in this way, the bulk wave of the elastic wave generated on one main surface forming the IDT is reflected on the other main surface, and this reflected bulk wave is reflected on one of the main surfaces. It is possible to suppress interference with elastic waves generated on the main surface. Further, in order to improve the adhesion (anchor bonding effect) between the piezoelectric substrate and the sprayed film, it is preferable that the back surface of the piezoelectric substrate is a roughened surface. For example, the surface roughness Ra of the back surface is preferably 0.01 μm to 3 μm.

Regarding the roughening, examples of the processing applied to the surface of the substrate include grinding, blasting, and lapping. Since such processing can reduce the stress on the substrate as compared with the polishing processing, it is advantageous in that the influence on the substrate is reduced. The roughness of the back surface of the piezoelectric substrate is preferably Ra = 0.01 μm to 3 μm in consideration of the effect of suppressing the influence of reflected bulk waves and the adhesion of the sprayed film to the piezoelectric substrate.

Examples of the material having a coefficient of linear expansion smaller than the coefficient of linear expansion of the piezoelectric substrate (material of the thermal spray film) to be used for thermal spraying include alumina, mullite, silicon, and yttria. In the thermal spray film formation method, electric energy (arc, plasma) or combustion energy is used as a heat source, and powder or rod-shaped material of the adherend material is put into the heat source and sprayed onto the surface of the substrate as fine particles in a molten or semi-molten state. This is a method of forming a film. By adopting the thermal spray film formation method, it is possible to suppress the heat effect on the piezoelectric substrate during film formation as much as possible. This makes it possible to suppress cracking and peeling due to a temperature rise during substrate processing.

Since the sprayed film is generally porous and its rigidity is relatively small, CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), and SOG (Spin On Glass) are used to fill the pores. It is preferable to fill the pores by such a method. As a result, the rigidity of the sprayed film can be increased and the frequency temperature characteristics can be improved.

Further, the thermal sprayed film may be composed of a plurality of layers. By forming the sprayed film with a plurality of layers in this way, various materials can be combined, so that the coefficient of linear expansion of the sprayed film can be easily adjusted.

In the method for manufacturing a piezoelectric substrate of the present invention, the piezoelectric substrate 1 is prepared as shown in FIG. 2 (a), and IDT2 is formed on one of the main surfaces 1a as shown in FIG. 2 (b). .. When forming IDT2, an IDT electrode material is adhered on the main surface 1a of the piezoelectric substrate 1, a resist layer (not shown) is formed on the IDT electrode material layer (not shown), and a resist layer (not shown) is formed through a photomask. The resist layer in the region where the IDT is formed is exposed and developed to pattern the resist layer. Then, the IDT electrode material layer is etched using the patterned resist layer as a mask, and then the remaining resist layer is removed. There are no particular restrictions on the shape of the IDT and the method of forming it.

Next, as shown in FIG. 2 (c), it is composed of a material having a coefficient of linear expansion smaller than the coefficient of linear expansion of the piezoelectric substrate 1 by the thermal spray film formation method on the other main surface (back surface) 1b of the piezoelectric substrate 1. The thermal spray film 3 is formed. Further, if necessary, the sprayed film is impregnated with SOG or the like and cured to improve the rigidity of the sprayed film.

Further, in the method for manufacturing a piezoelectric substrate of the present invention, the piezoelectric substrate 1 is prepared as shown in FIG. 3 (a), and IDT2 is placed on one of the main surfaces 1a as shown in FIG. 3 (b). After the formation, as shown in FIG. 3 (c), the piezoelectric substrate 1 is thinned from the back surface 1b side, and then, as shown in FIG. 3 (d), a thermal spraying method is performed on the back surface 1b of the piezoelectric substrate 1. The sprayed film 3 may be formed by the above method. For thinning the piezoelectric substrate 1, for example, blasting, lapping, grinding, or the like is used.

Here, FIG. 4 shows the relationship between the temperature of PEB (Post Exposure Bake) during the formation of IDT and the amount of warpage in the heated state of the wafer. Here, an LT substrate was used as the piezoelectric substrate. The amount of warpage in the heated state was measured by core9037a (manufactured by Cores, trade name). FIG. 4 shows the amount of warpage ( plot) of the 4-inch LT substrate alone and the amount of warpage ( plot) of the 4-inch bonded substrate (LT substrate + Si substrate). As can be seen from FIG. 4, at the temperature shown in the figure, at a temperature equal to or lower than the temperature at which the substrate breaks, the amount of warpage of the LT substrate alone is much smaller than that of the bonded substrate (LT substrate + Si substrate). Therefore, by forming the IDT in the state of the LT substrate alone, which has a small amount of warpage in the heated state, the pattern line width processing accuracy R in the wafer surface is increased. Further, at a temperature higher than the range shown in this figure, the bonded substrate cracks and peels off.

Figure 5 shows the relationship between the pattern line width processing accuracy R and the PEB temperature in the wafer surface. Here, an LT substrate was used as the piezoelectric substrate. Figure 5 shows the line width processing accuracy R (line width processing accuracy when directly patterning on the LT substrate: plot) and the 4-inch bonded substrate (LT substrate + Si) in the wafer surface of the 4-inch LT substrate alone. The line width processing accuracy R (line width processing accuracy when pattern processing on a bonded substrate: plot) in the wafer surface of the substrate) is shown. As can be seen from FIG. 5, according to the method of the present invention, the IDT is formed in the state of the LT substrate alone, which has a small amount of warpage in the heated state, so that the line width processing accuracy is high. On the other hand, in the case of a bonded substrate (LT substrate + Si substrate), since the amount of warpage is large in the heated state, the PEB temperature in the wafer surface varies, and therefore the resist line width varies greatly. As a result, the line width processing accuracy R deteriorates. In particular, when the PEB temperature is 130 ° C or higher, the line width machining accuracy varies by 10% or more.

In this way, the elastic wave element obtained by thermal spraying after forming the IDT on the piezoelectric substrate, that is, the elastic wave element obtained by first pattern-processing the piezoelectric substrate, forms the IDT in a state where the warpage of the substrate is small. As a result, the processing accuracy of IDT is high. Further, by roughening the sprayed surface of the piezoelectric substrate, the influence of bulk waves can be suppressed. Further, in the present invention, since there is no bonding between the substrates, it is not necessary to polish the surface of the substrate, and it is possible to prevent the piezoelectric substrate from cracking due to high processing stress at the time of thinning. Further, in the method of the present invention, since the thermal spray film formation method is adopted, an expensive bonding device is not required, and an elastic wave element having excellent frequency temperature characteristics can be manufactured at low cost.

Next, an example carried out for clarifying the effect of the present invention will be described. First, the high temperature treatment characteristics will be described with reference to Example 1. (Example 1) The coefficient of linear expansion is 16 × 10^-6The coefficient of linear expansion is 1 × 10 by the thermal spray film formation method on one main surface of a lithium tantalate substrate (LT substrate) with a thickness of / K and a thickness of 0.02 mm.^-6A 4-inch substrate was prepared by forming a film of mullite, which is / K, with a thickness of 0.33 mm. This 4-inch substrate was heat-treated. At this time, the 4-inch substrate did not crack at the heating temperatures of 180 ° C and 200 ° C, but the 4-inch substrate cracked at the heating temperatures of 250 ° C and 350 ° C.

Also, instead of the LT substrate, the coefficient of linear expansion is 15 × 10.^-6A 4-inch substrate was prepared in the same manner as above using a lithium niobate substrate (LN substrate) having a thickness of / K and a thickness of 0.02 mm. This 4-inch substrate was heat-treated. At this time, the 4-inch substrate did not crack at the heating temperatures of 180 ° C and 200 ° C, but the 4-inch substrate cracked at the heating temperatures of 250 ° C and 350 ° C.

In addition, the coefficient of linear expansion is 16 × 10.^-6On one main surface of a lithium tantalate substrate (LT substrate) with a diameter of 4 inches and a thickness of 0.25 mm, the coefficient of linear expansion is 3 × 10.^-6A silicon substrate with a diameter of 4 inches and a thickness of 0.33 mm for temperature compensation, which is / K, is directly bonded by the surface activation method, and then the silicon surface is ground and polished to form a thin layer to a thickness of 0.02 mm. A substrate was prepared. The surfaces to be bonded to each other of the LT substrate and the silicon substrate were mirror-finished in advance. Next, the bonded substrate was heat-treated. At this time, the LT substrate cracked even at heating temperatures of 180 ° C and 200 ° C.

When forming an IDT after applying a temperature compensation process to a piezoelectric substrate such as an LT substrate or an LN substrate, there is a high temperature process of about 200 ° C such as a resin layer curing process. Therefore, from the above results, if the IDT is formed after the piezoelectric substrate is subjected to the temperature compensation treatment, the piezoelectric substrate may be cracked in the high temperature process. However, according to the method according to the present invention, after forming the IDT on the piezoelectric substrate in advance, the thermal spray film formation is performed as the treatment for temperature compensation, so that the piezoelectric substrate subjected to the treatment for temperature compensation is subjected to the high temperature process. There is nothing. Therefore, it is possible to manufacture an elastic wave element having excellent frequency and temperature characteristics without cracking the substrate.

Next, the temperature characteristic evaluation and the influence of bulk waves will be described with reference to Example 2. (Example 2) The coefficient of linear expansion is 16 × 10^-6IDT was formed on one main surface of a lithium tantalate substrate (LT substrate) having a diameter of 4 inches and a thickness of 0.25 mm at / K. In the formation of this IDT, a chemically amplified resist was used as the resist, and the temperature of PEB (Post Exposure Bake) was 110 ° C. Next, the other main surface of this LT substrate was thinned to a thickness of 0.01 to 0.04 mm by grinding and roughened to Ra 0.1 μm. Next, powder was sprayed on the other roughened main surface in the order of silicon and alumina by a thermal spraying method to form a film so that the total thickness of the LT substrate and the thermal sprayed film was 0.25 mm. The thermal spraying treatment was carried out using a DC plasma spraying device, using Ar plasma gas, and having a power output of 40 kW.

Regarding the IDT-equipped wafer thus obtained, the in-plane line width processing accuracy R was investigated. This line width processing accuracy R was measured by a length measuring SEM (Scanning Electron Microscope). As a result, the line width processing accuracy R in the wafer surface was about 2%. It is considered that this is because, as described above, the warpage of the wafer in the high temperature treatment is suppressed and the variation in the resist line width is suppressed to be small.

Further, when the wafer thus obtained was diced to produce an elastic wave element (SAW device) and the temperature compensation effect was examined, an improvement effect was observed as shown in FIG. The waveform of the elastic wave element is shown in FIG. 6 (a). Here, for reference, the waveform of the elastic wave element obtained by using the bonding substrate produced in Example 1 is shown in FIG. 6 (b). Part X in Fig. 6 (b) is the ripple due to the influence of bulk waves. As can be seen from FIG. 6A, the elastic wave element obtained by the method according to the present invention did not show ripple due to bulk waves. This is because the bulk wave of the elastic wave generated on one main surface forming the IDT is reflected on the other main surface due to the roughening of the surface to be sprayed, and this reflected bulk wave is generated on one main surface. It is considered that this is because it was possible to suppress the interference with the elastic wave.

The present invention is not limited to the above-described embodiment, and can be implemented with various modifications. The shape, dimensions, material, and the like in the above-described embodiment are examples, and can be appropriately modified as long as the effects of the present invention are not impaired. In addition, the present invention can be implemented with various modifications without departing from the scope of the present invention.