Patent Publication Number: US-2022212231-A1

Title: Saw module, flavor inhaler, and method for manufacturing saw module

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Application No. PCT/JP2020/032308, filed on Aug. 27, 2020. 
    
    
     TECHNICAL FIELD 
     The invention relates to saw modules, flavor inhalers, and methods for manufacturing saw modules. 
     BACKGROUND ART 
     A saw module has been known, in which a piezoelectric element substrate with an IDT (Interdigital Transducer) comprising a comb-like electrode pair is used to generate a surface acoustic wave (SAW) and thus atomize liquid (see Patent Literatures 1 and 2, for example). 
     A technology of using the saw module in a flavor inhaler has been suggested (see Patent Literature 3. for example). A technology of forming an insulating coating layer on the surface of an IDT has also been suggested (see Patent Literature  4 , for example). 
     CITATION LIST 
     Patent Literature 
     PTI. 1: Japanese Unexamined Patent Application Publication (Kokai) No. 2012-24646 
     PTL 2: Japanese Unexamined Patent Application Publication (Kohyo) No. 2016-513992 
     PTL 3: U.S. Pat. No.2017/0280771 
     PTL 4: Japanese Unexamined Patent Application Publication (Kokai) No. 11-114467 
     SUMMARY OF INVENTION 
     Technical Problem 
     A first feature is a SAW module comprising a piezoelectric element substrate including a piezoelectric body, an IDT disposed on a front surface of the piezoelectric element substrate and comprising a comb-like electrode pair, and a coating layer covering the piezoelectric element substrate and the IDT. The coating layer is disposed on the front surface of the piezoelectric element substrate, an end surface of the piezoelectric element substrate, and a corner portion between the front surface of the piezoelectric element substrate and the end surface of the piezoelectric element substrate. 
     A second feature according to the first feature is that the coating layer has a thickness ranging from 600 nm to 1400 nm on the front surface of the piezoelectric element substrate. 
     A third feature according to the first or second feature is that, on the end surface of the piezoelectric element substrate, the coating layer has a thickness of 600 nm or more and the thickness of 50% or more of the thickness of the coating layer on the front surface of the piezoelectric element substrate. 
     A fourth feature according to any one of the first to third features is that, on the corner portion between the front surface of the piezoelectric element substrate and the end. surface of the piezoelectric element substrate, the coating layer has a thickness of 600 nm or more and the thickness of 50% or more of the thickness of the coating layer on the front surface of the piezoelectric element substrate. 
     A fifth feature according to any one of the first to fourth features is that the coating layer includes either one of SiO2 and SiN. 
     A sixth feature is a flavor inhaler comprising the SAW module according to any one of the first to fifth features. 
     A seventh feature is a method for manufacturing a SAW module, comprising the steps of forming a plurality of IDTs comprising a comb-like electrode pair on a front surface of a wafer of a piezoelectric element substrate including a piezoelectric body; cutting the wafer into module units; and forming a coating layer covering the piezoelectric element substrate that is cut and the IDTs. The step of forming the coating layer forms the coating layer on a front surface of the piezoelectric element substrate, an end surface of the piezoelectric element substrate, and a corner portion between the front surface of the piezoelectric element substrate and the end surface of the piezoelectric element substrate. 
     An eighth feature according to the seventh feature is that the step of forming the coating layer forms the coating layer by any one of a CVD method, an ALD method, and spray coating, 
     A ninth feature according to the seventh or eighth feature is that the method for manufacturing a SAW module further comprises the step of applying processing for enhancing a hydrophilic property to the coating layer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a SAW module according to an embodiment. 
         FIG. 2  is a planar view of the SAW module as viewed from a front surface side of a piezoelectric element substrate. 
         FIG. 3  is a sectional view of the SAW module. 
         FIG. 4  shows a coating layer that is exfoliated due to SAW vibration. 
         FIG. 5A  shows a section of the SAW module in which a coating layer with a thickness of 1000 nm is formed on the front surface of the piezoelectric element substrate, and which was driven for 250 times. 
         FIG. 5B  shows a part of the section on a front surface side of the SAW module, in which the coating layer with a thickness of 1000 nm is formed on the front surface of the piezoelectric element substrate, and which was driven for 250 times. 
         FIG. 5C  shows a part of the section on an end surface of the SAW module, in which the coating layer with a thickness of 1000 nm is formed on the front surface of the piezoelectric element substrate, and which was driven for 250 times. 
         FIG. 5D  shows a part of the section on a back surface side of the SAW module, in which the coating layer with a thickness of 1000 nm is formed on the front surface of the piezoelectric element substrate, and which was driven for 250 times. 
         FIG. 6  shows results of exfoliation of the coating layer and generation of a crack in the piezoelectric element substrate which are tested with respect to various thicknesses for the coating layer. 
         FIG. 7  shows the coating layer that is exfoliated in the end surface of the piezoelectric element substrate and the corner portion. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments will be discussed below. In the following explanation of the drawings, identical or similar elements are provided with identical or similar reference signs. It should be noted, however, that the drawings are schematic and that dimensional ratios and the like on the drawings may be different from actual ones. 
     Specific dimensions and the like should be determined in view of the following explanations. Needless to say, the drawings may differ from one another in terms of dimensional relationship and ratios. 
     SUMMARY OF DISCLOSURE 
     As mentioned under BACKGROUND ART, a technology of forming an insulating coating layer on a surface of an IDT has been suggested. However, when liquid is atomized on an end surface of a piezoelectric element substrate, a coating layer formed on a surface of IDT is sometimes exfoliated due to SAW vibration. If there is a microcrack in a front surface of the piezoelectric element substrate, a crack might be generated in the piezoelectric element substrate due to SAW vibration. A further improvement has therefore been desired in durability of SAW modules. 
     A SAW module according to the summary of disclosure comprises a piezoelectric element substrate including a piezoelectric body, an IDT disposed on the front surface of the piezoelectric element substrate and comprises a comb-like electrode pair, and a coating layer covering the piezoelectric element substrate and the IDT. The coating layer is disposed on the front surface of the piezoelectric element substrate, an end surface of the piezoelectric element substrate, and a corner portion between the front surface of the piezoelectric element substrate and the end surface of the piezoelectric element substrate. 
     According to the summary of disclosure, the coating layer is disposed on the front surface of the piezoelectric element substrate, the end surface of the piezoelectric element substrate, and a corner portion between the front surface of the piezoelectric element substrate and the end surface of the piezoelectric element substrate. This suppresses exfoliation of the coating layer and generation of a crack in the piezoelectric element substrate and thus improves the SAW module in durability. 
     Embodiments 
     (SAW Module) 
     A SAW module according to an embodiment will be explained below.  FIG. 1  shows a SAW module  30  according to the embodiment.  FIG. 2  is a planar view of the SAW module as viewed from a front surface side of a. piezoelectric element substrate  31 .  FIG. 3  shows a section of the SAW module  30 . The SAW module  30  is used, for example, in a flavor inhaler, not shown. The SAW module  30  atomizes liquid to be atomized which is supplied from a liquid storage portion, not shown. 
     As shown in  FIGS. 1 to 3 , the SAW module  30  includes the piezoelectric element substrate  31 , an electrode (IDT comprising a body portion  32  and a comb-like electrode pair  33 ), a coating layer  34 , and a heat dissipation mechanism  35 , The piezoelectric element substrate  31  is configured to atomize liquid using SAW vibration that is generated by applying voltage to the comb-like electrode pair  33  at high frequencies (resonant frequencies). 
     The piezoelectric element substrate  31 includes a front surface  31 F, on which the body portion  32  and the comb-like electrode pair  33  are arranged, a back surface  31 B disposed on an opposite side to the front surface  31 F, an end surface  31 E connecting the front surface  31 F and the back surface  31 B, and a corner portion  31 C between the front surface  31 F and the end surface  31 E. The piezoelectric element substrate  31  includes a piezoelectric body that is expanded/contracted by voltage application. The piezoelectric element substrate  31  includes a portion, on which the comb-like electrode pair  33  is arranged, and the portion may be referred to as an arrangement portion  30 A. The piezoelectric body may form at least the front surface  31 F. The piezoelectric body may comprise a known piezoelectric body made of ceramic, such as quartz, barium titanate, and lithium niobate, or another like material. 
     The body portion  32  is electrically connected to a power source of a flavor inhaler, not shown. The body portion  32  includes a first body portion  32 A that is formed integrally with a first comb-like electrode  33 A that is one of the comb-like electrode pair  33 , and a second body portion  32 B that is formed integrally with a second comb-like electrode  33 B that is the other of the comb-like electrode pair  33 . The first body portion  32 A and the second body portion  32 B are arranged with the arrangement portion  30 A located therebetween in an orthogonal direction B to a moving direction A of the SAW. Electric power that is outputted from the power source is supplied through the body portion  32  to the comb-like electrode pair  33 . 
     The comb-like electrode pair  33  includes the first comb-like electrode  33 A and the second comb-like electrode  33 B. The first comb-like electrode  33 A and the second comb-like electrode  33 B are alternately arranged in the moving direction A of the SAW. me first comb-like electrode  33 A has a shape extending from the first body portion  32 A along the orthogonal direction B. The second comb-like electrode  33 B has a shape extending from the second body portion  32 B along the orthogonal direction B. For example, the comb-like electrode pair  33  is made of metal formed by a sputtering method or evaporation method, or another like material. 
     In  FIG. 3 , the coating layer  34  is disposed to cover the front surface  31 F, the end surface  31 E, and the corner portion  31 C of the piezoelectric element substrate  31 . The coating layer  34  is an insulating layer that covers the piezoelectric element substrate  31 , the body portion  32 , and the comb-like electrode pair  33 . The coating layer  34  may be partially omitted in a region of the end surface  31 E of the piezoelectric substrate  31  or the like which is parallel to the moving direction A of the SAW and does not contribute to liquid atomization. 
     The coating layer  34  may be made of material that suppresses denaturalization of the piezoelectric element substrate  31  which is caused by liquid adhesion. The material of the coating layer  34  has a high electric resistance and a low water permeability and is equal to material of the piezoelectric element substrate  31  in Young&#39;s modulus, heat expansion coefficient, and density. For example, the coating layer  34  may be made of material such as Teflon, parylene, silicon dioxide, silicon nitride, and alumina. The coating layer  34  needs to evenly adhere to the front surface  31 F, the end surface  31 E, and the corner portion  31 C and is therefore formed by any one of a CVD method, an AID method, and spray coating. 
     As already mentioned, if the coating layer  34  is formed only on the surface of the IDT, the coating layer  34  might be exfoliated due to SAW vibration when the liquid is atomized on the end surface  31 E of the piezoelectric element substrate  31 .  FIG. 4  shows the coating layer  34  exfoliated due to SAW vibration. in  FIG. 4 , an area  36  enclosed by broken lines is a region in the end surface  31 E of the piezoelectric element substrate  31 , to which the liquid is supplied, and from which the coating layer  34  is exfoliated due to the SAW vibration. The exfoliation is considered to occur when the liquid enters through a gap between the front surface  31 F of the piezoelectric element substrate  31  and the coating layer  34  to be atomized by the SAW vibration, pushing the coating layer  34  off from the front surface  31 F of the piezoelectric element substrate  31 . 
     To solve the problem, the coating layer  34  is so disposed as to cover the front surface  31 F, the end surface  31 E, and the corner portion  31 C of the piezoelectric element substrate  31 , to thereby suppress contact of the liquid to the piezoelectric element substrate  31 . The coating layer  34  may also be disposed on the back surface  31 B of the piezoelectric element substrate  31  as well as the front surface  31 F, the end surface  31 E, and the corner portion  31 C of the piezoelectric element substrate  31 , Such a configuration further suppresses the contact of the liquid to the piezoelectric element substrate  31 . 
     The coating layer  34  also covers a microcrack, if any, in the front surface  31 F of the piezoelectric element substrate  31  and suppresses generation of a crack in the piezoelectric element substrate  31  due to SAW vibration. This suppression is considered to occur because the coating layer  34  suppresses the expansion/contraction of the piezoelectric element substrate  31  due to a temperature change caused by some of vibration energy. 
     The inventors and others involved in the invention conducted an experiment for optimizing thickness of the coating layer  34 . Specifically, the inventors and the others involved in the invention provided the coating layer  34  made of silicon dioxide onto the piezoelectric element substrate  31  including lithium niohate as the piezoelectric body by a plasma CVD method, to thereby produce a SAW module, and checked if the exfoliation of the coating layer  34  and the generation of a crack in the piezoelectric element substrate  31  occurred. The coating layer  34  was provided under coating conditions that pressure was 60 Pa; temperature was 300° C.; plasma application electric power was 1.2 W/cm2; precursor (Bis (ethylmethylamino) silane) flow rate was 5 to 10 scorn; Ar flow rate was 100 sccm; O2 flow rate was 150 sccm; and ES (electrode-substrate) distance was 30 mm. 
     The experiment for optimizing the thickness of the coating layer  34  was conducted from the following two perspectives. One of the perspectives is that, if the coating layer  34  is small in thickness, a crack is generated, for example, in the corner portion  31 C of the piezoelectric element substrate  31  due to SAW vibration, allowing the liquid to enter through the crack and cause the exfoliation of the coating layer  34 , or the coating layer  34  cannot suppress the expansion/contraction of the piezoelectric element substrate  31  and allows a crack to be generated in the piezoelectric element substrate  31 . 
     The other perspective is that, if the coating layer  34  is large in thickness, an effect of difference in heat expansion coefficient between the piezoelectric element substrate  31  and the coating layer  34  becomes more significant, which causes distortion between the piezoelectric element substrate  31  and the coating layer  34  and generates a crack in tae piezoelectric element substrate  31 . 
       FIGS. 5A to 5D  show a section of the SAW module  30 , in which the coating layer  34  with a thickness of 1000 nm is formed on the front surface  31 F, the end surface  31 E, and the corner portion  31 C of the piezoelectric element substrate  31 , and which was driven for  250  times (1750 sec).  FIG. 5A  shows the section of the SAW module  30 .  FIG. 5B  shows a part of the section on the front surface  31 F side of the SAW module  30 .  FIG. 5C  shows a part of the section of the end surface  31 E of the SAW module  30 .  FIG. 5D  shows a part of the section on the back surface  31 B side of the SAW module  30 . 
     The SAW module  30  was driven under conditions that a liquid supply rate is 3 μl/sec and that a driving electric power is 9 W.  FIGS. 5A to 5D  show that, when the coating layer  34  with a thickness of 1000 nm is formed on the front surface  31 F, the end surface  31 E, and the corner portion  31 C of the piezoelectric element substrate  31  as shown in the figures, neither the exfoliation of the coating layer  34  nor a crack in the piezoelectric element substrate  31  occurs in the front surface  31 F, the end surface  31 E, and the corner portion  31 C of the piezoelectric element substrate  31 . 
       FIG. 6  shows results of exfoliation of the coating layer  34  and generation of a crack in the piezoelectric element substrate  31  which are tested with respect to various thicknesses for the coating layer  34 . As shown in  FIG. 6 , it is confirmed that neither the exfoliation of the coating layer  34  nor a crack in the piezoelectric element substrate  31  occurs in the range of 600 nm to 1400 nm thickness of the front surface  31 F of the coating layer  34 . 
     The symbol ⊚ in  FIG. 6  means that neither the exfoliation of the coating layer  34  nor a crack in the piezoelectric element substrate  31  occurs even after the SAW module  30  is driven for 250 times (1750 sec). The symbol ◯means that the exfoliation of the coating layer  34  or a crack in the piezoelectric element substrate  31  occurs when the SAW module  30  is driven for about 150 times (1251) sec). The symbol X means that the exfoliation of the coating layer  34  or a crack in the piezoelectric element substrate  31  occurs when the SAW module  30  is driven for less than 10 times (50 sec). With regards to the case indicated by  0 , if the driving electric power of the SAW module  30  is reduced, the coating layer  34  is less likely to be exfoliated. 
       FIG. 7  shows the coating layer  34  exfoliated in the end surface  31 E and the corner portion  31 C of the piezoelectric element substrate  31 . In  FIG. 7 , the coating layer  34  with a thickness of  1000  urn is formed on the front surface  31 F of the piezoelectric element substrate  31 , and the coating layer  34  with a thickness of 400 nm is formed in the end surface  31 E and the corner portion  31 C of the piezoelectric element substrate  31 . 
       FIG. 7  shows that, when the coating layer  34  is thinner in the end surface  31 E and the corner portion  31 C of the piezoelectric element substrate  31  than in the front surface  31 F of the piezoelectric element substrate  31  as shown in  FIG. 7 , the coating layer  34  is exfoliated in the end surface  31 E and the corner portion  31 C of the piezoelectric element substrate  31 . 
     Based on the foregoing results and the fact that the coating layer  34  is more difficult to be formed in the end surface  31 E and the corner portion  31 C of the piezoelectric element substrate  31  than in the front surface  31 F of the piezoelectric element substrate  31 , the inventors and the others involved in the invention found desired thicknesses for the coating layer  34  in the front surface  31 F, the end surface  31 E, and the corner portion  31 C of the piezoelectric element substrate  31  as below. 
     The optimum thickness for the coating layer  34  in the front surface  31 F of the piezoelectric element substrate  31  ranges from 600 nm to 1400 nm, and preferably from 1000 nm to 1400 nm. The optimum thickness for the coating layer  34  on the end surface  31 E of the piezoelectric element substrate  31  is 600 nm or more and the thickness of 50% or more of the thickness of the coating layer  34  on the front surface  31 F of the piezoelectric element substrate  31 . The optimum thickness of the coating layer  34  on the corner portion  31 C of the piezoelectric element substrate  31  is 600 nm or more and the thickness of 50% or more of the thickness of the coating layer  34  on the front surface  31 F of the piezoelectric element substrate  31 . 
     If the thicknesses of the coating layer  34  in the front surface  31 F, the end surface  31 E, and the corner portion  31 C of the piezoelectric element substrate  31  are set as mentioned above, the exfoliation of the coating layer  34  and the generation of a crack in the piezoelectric element substrate  31  are suppressed, which improves the SAW module  30  in durability. 
     Referring to  FIG. 3  again, the heat dissipation mechanism  35  is a mechanism configured to conduct away heat that is generated by reflection of the surface acoustic wave in the corner portion  31 C of the piezoelectric element substrate  31 . The heat dissipation mechanism  35  includes at least either one of a heat dissipation layer and a Peltier element which are made of material having a higher thermal conductivity than the piezoelectric element substrate  31 . 
     According to an example shown in  FIG. 3 , the heat dissipation mechanism  35  is a heat dissipation layer that is arranged on the back surface  31 B of the piezoelectric element substrate  31 . However, the embodiment is not limited to this configuration For example, the heat dissipation mechanism  35  may be arranged on the front surface  31 F of the piezoelectric element substrate  31  as long as the heat dissipation mechanism  35  is in contact with the piezoelectric element substrate  31 . The heat dissipation mechanism  35  may he a Peltier element. The heat dissipation mechanism  35  may include both the heat dissipation layer and the Peltier element. 
     The heat dissipation layer may be made of, for example, metal, such as aluminum, copper, and iron, or made of carbon, aluminum nitride or ceramic. For example, the Peltier element may be bonded to the piezoelectric element substrate  31  with an adhesive agent (grease, epoxy resin or metal paste). The adhesive agent preferably has a heat conductivity that is higher than 0.1 W/m/K. More preferably, the heat conductivity of the adhesive agent is higher than 0.5 W/m/K. An adhesion layer is desirably as thinner as possible. A thin adhesion layer can be materialized by screen printing. 
     As shown in  FIG. 3 , a liquid supply unit  60  is disposed at the side of the back surface  31 B of the piezoelectric element substrate  31 . The liquid supply unit  60  is configured to supply liquid to the piezoelectric element substrate  31 . The liquid supply unit  60  supplies liquid to the front surface  31 F of the piezoelectric element substrate  31  through the end. surface  31  E of the piezoelectric element substrate  31 . For example, the liquid supply unit  60  is a syringe pump. The syringe pump may be of a manual or electric type. The liquid supply unit  60  may he a member that supplies liquid by capillary action. 
     In a portion between the comb-like electrode pair  33  and the end surface  31 E of the piezoelectric element substrate  31  which is supplied with liquid, at least a part of the coating layer  34  formed on the front surface  31 F of the piezoelectric element substrate  31  may be made hydrophilic by later-mentioned processing for enhancing a hydrophilic property. This makes it easy for the liquid supplied from the liquid supply unit  60  to move on the front surface  31 F of the piezoelectric element substrate  31  and thus facilitates the atomization of the liquid. 
     If the liquid storage portion is a cartridge, the liquid supply unit  60  may automatically supply the liquid to the SAW module  30  when the cartridge is installed. If a power switch for driving the flavor inhaler I is provided, the liquid supply unit  60  may automatically supply the liquid to the SAW module  30  when the power switch is turned on. 
     (Method for Manufacturing a SAW Module) 
     A method for manufacturing a SAW module according to the embodiment will be discussed below. First, a plurality of IDTs comprising the comb-like electrode pair  33  are formed on a front surface of a wafer of the piezoelectric element substrate  31  including the piezoelectric body. The wafer, on which the IDTs are formed, are then cut into module units for the SAW module  30 . 
     The coating layer  34  is formed, which covers the piezoelectric element substrate  31  that is cut and the IDTs. In the step of forming the coating layer  34 , the coating layer  34  is formed on the front surface  31 F of the piezoelectric element substrate  31 , the end surface  31 E of the piezoelectric element substrate  31 , and the corner portion  31 C between the front surface  31 F and the end surface  31 E of the piezoelectric element substrate  31 . At this point of time, the coating layer  34  may be formed by any one of the CVD method, the ALD method, and the spray coating. There may be the step of applying processing for enhancing a hydrophilic property to the coating layer  34 . The processing for enhancing the hydrophilic property, for example, further coats the surface of the coating layer  34  with a hydrophilic material or provides fine asperities to the surface of the coating layer  34  by physical fabrication. 
     If the coating layer  34  covering the piezoelectric element substrate  31  and the IDTs is formed after the wafer is cut into module units for the SAW module  30  as described above, the coatimz layer  34  can be formed on the front surface  31 F, the end surface  31 E, and the corner portion  31 C of the piezoelectric element substrate  31 . This suppresses the exfoliation of the coating layer  34  and the generation of a crack in the piezoelectric element substrate  31  and improves the SAW module  30  in durability. 
     According to the flavor inhaler I thus configured, the SAW module  30  comprises the piezoelectric element substrate  31  including the piezoelectric body, the IDTs disposed on the front surface  31 F of the piezoelectric element substrate  31  and comprising the comb-like electrode pair  33 . and the coating layer  34  covering the piezoelectric element substrate  31  and the IDTs. The coating layer  34  is disposed on the front surface  31 F of the piezoelectric element substrate  31 , the end surface  31 E of the piezoelectric element substrate  31 , and the corner portion  31 C between the front surface  31 F of the piezoelectric element substrate  31  and the end surface  31 E of the piezoelectric element substrate  31 . Consequently, the exfoliation of the coating layer  34  and the generation of a crack in the piezoelectric element substrate  31  are suppressed, and the durability of the SAW module  30  is improved. 
     The one embodiment of the invention has been discussed. The embodiment of the invention is intended to facilitate the understanding of the invention and not to limit the invention. The invention may be modified or improved without deviating from the gist thereof. The invention includes equivalents thereof The constituent elements mentioned in claims and description may be combined or omitted as long as the aforementioned problem is at least partially solved or as long as the advantageous effect is at least partially provided. 
     REFERENCE SIGN LIST 
       1  Flavor inhaler 
       30  SAW module 
       31  Piezoelectric element substrate 
       32  Body portion 
       33  Comb-like electrode pair 
       34  Coating layer 
       35  Heat dissipation mechanism 
       31 B Back surface 
       31 C Corner portion 
       31 E End suiface 
       31 F Front surface