Abstract:
A surface acoustic wave device is enable to prevent electrode erosion, without any specific environmental process. The surface acoustic wave device includes a piezoelectric substrate, an electrode for the formation of surface acoustic wave, being formed on the piezoelectric substrate, on the piezoelectric substrate, a frame-shaped layer surrounding the electrode for the formation of surface acoustic wave, and a lid body formed on the frame-shaped layer by bonding, so as to form a hollow portion between the lid body and the electrode for the formation of surface acoustic wave. The frame-shaped layer and the lid body include photosensitive resin, and the lid body includes a through hole, and the through hole is sealed with a halogen-free thermosetting resin.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-266214, filed on Oct. 12, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a surface acoustic wave device for use in mobile communication equipment etc., and a manufacturing method therefor. 
     2. Description of the Related Art 
     The surface acoustic wave device has widely been used conventionally, as a duplexer and a filter for use in the mobile communication equipment etc. 
     As the prior art,  FIG. 1  shows an exemplary configuration of the surface acoustic wave device described in Patent document 1. 
     In  FIG. 1 , there is shown a schematic configuration of the cross section of the surface acoustic wave device being individually cut out. A surface acoustic wave (SAW) device chip  1  is disposed on a circuit substrate  2 , with each electrode for the formation of surface acoustic wave being placed on the upper side. 
     The electrode for the formation of surface acoustic wave disposed on SAW device chip  1  includes an interdigital transducer (IDT)  10  for exciting a surface acoustic wave and a pair of reflective electrodes  11  each disposed on each side of the IDT  10 . The electrode is connected to an electrode  3  on the circuit substrate  2  side by means of a bonding wire  12  via a metal post. 
     Further, in the configuration shown in  FIG. 1 , SAW device chip  1  is covered with an insulating rim  4  and a lid  5 , so that a hollow  13  is formed on a surface acoustic wave propagation path. 
     Now, in Patent document 1, as an effect of covering SAW device chip  1  with insulating rim  4  and lid  5 , there is described that “because protection is made while a hollow ( 9 ) is maintained on the surface of a functional portion, stable manufacturing can be made without a risk of damaging a functional portion ( 1   a ) by mistake when the chip ( 1 ) is handled. Also, because the functional portion  1   a  is simply sealed with first and second insulating films  2   a ,  2   b , it is not always necessary to tightly seal as in the conventional surface acoustic wave equipment. Thus, low-cost surface acoustic wave equipment is obtainable” (in the paragraph 0017 of Patent document 1). 
     In contrast, because the material forming the IDT and the reflective electrode is an aluminum alloy, it is known that the electrode becomes deteriorated because of eroded aluminum when being exposed in the atmosphere having high humidity in a long term (for example, lines 17-19 of page 1 in Patent document 2). 
     Specifically,  FIG. 2  is a diagram illustrating the structure of a SAW device having the structure shown in Patent document 2. A connection electrode of SAW device chip  1  is connected to an electrode  3  on the circuit substrate  2  side through a metal ball  14 . Thus, by means of metal ball  14 , a hollow  13  is formed between SAW device chip  1  and circuit substrate  2 . 
     Further, a resin cover  6  is formed on the upper face and the side faces of the SAW device chip  1 . In order to prevent erosion of the electrode, with regard to the above resin cover  6 , it is described that the resin cover  6  has to be formed of a resin having a small chlorine ion content. 
     [Patent document 1] Japanese Unexamined Patent Publication No. 2000-114918. 
     [Patent document 2] WIPO international publication No. WO 02/061943. 
     In the above Patent document 1, it is shown that photosensitive films are used for the insulating rim  4  and lid  5  (refer to FIG. 3 of Patent document 1). Further, it is obvious that the photosensitive film includes a halogen compound from the characteristic thereof, and in the manufacturing process of the SAW device, at the time of heating such as reflow, a portion of the halogen gas is emitted from the photosensitive film to the hollow ( 9 ) 
     Therefore, similarly to the case described in the aforementioned Patent document 2, there is a risk of bad influence such as erosion produced on the IDT, which is an electrode for the formation of surface acoustic wave disposed in the hollow, and the reflective electrode. However, in Patent document 1, there is neither disclosure nor suggestion about the influence on the electrodes caused by the halogen gas, not to mention the avoidance of the influence on such electrodes. 
     Meanwhile, as described above, the problem of an eroded electrode is shown in Patent document 2. As a method to solve the problem, it is disclosed that, through predetermined environmental processes (heating and pressing processes), a resin material of reduced chlorine ion content is to be used as a coating material. 
     SUMMARY OF THE INVENTION 
     From the viewpoint described above, the objective of the present invention is to provide a surface acoustic wave device enabling prevention of electrode erosion, without requiring such predetermined environmental processes as described in Patent document 2 to obtain a resin material having a small chlorine ion content, and the manufacturing method therefor. 
     In order to achieve the above object, according to a first aspect of the present invention there is provided a surface acoustic wave device having a piezoelectric substrate, an electrode for the formation of surface acoustic wave, being formed on the piezoelectric substrate, on the piezoelectric substrate, a frame-shaped layer surrounding the electrode for the formation of surface acoustic wave, and a lid body formed on the frame-shaped layer by bonding, so as to form a hollow portion between the lid body and the electrode for the formation of surface acoustic wave, wherein the frame-shaped layer and the lid body include photosensitive resin, and the lid body includes a through hole, and the through hole is sealed with a halogen-free thermosetting resin. 
     As such, the frame-shaped layer and the lid body include photosensitive resin, and the lid body has the through hole. Further, the through hole is sealed with the halogen-free thermosetting resin. By this, a halogen compound included in the photosensitive resin and produced through a heating process, which is essentially needed in the manufacturing process of the SAW device, is emitted via the through hole as a halogen gas. Accordingly, no halogen gas remains in the hollow sealed by the halogen-free thermosetting resin, enabling prevention of the IDT electrode from being eroded caused by the halogen gas. 
     A manufacturing method of a surface acoustic wave device according to the present invention comprises the processes of, on a piezoelectric substrate, forming an electrode for the formation of surface acoustic wave, forming a frame-shaped layer surrounding the electrode for the formation of surface acoustic wave being formed on the piezoelectric substrate, on the frame-shaped layer, forming a lid body by bonding, having a through hole, so as to form a hollow between the lid body and the electrode for the formation of surface acoustic wave, removing a halogen gas generated from the frame-shaped layer and the lid body via the through hole, by heating in vacuum in a state of being bonded with the lid body, and after removing the halogen gas, sealing the through hole with a halogen-free thermosetting resin. 
     Thus, according to the manufacturing method of the present invention, because no halogen gas remains in the hollow sealed by the halogen-free thermosetting resin, it is possible to obtain a highly reliable SAW device in which the erosion of the IDT electrode caused by halogen gas is prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary configuration of the surface acoustic wave device described in Patent document 1; 
         FIG. 2  is a diagram illustrating the structure of a SAW device having the structure shown in Patent document 2; 
         FIG. 3  shows a diagram illustrating a schematic cross section of a first embodiment of the surface acoustic wave (SAW) device according to the present invention. 
         FIG. 4A  is a plan viewed from the solder ball  14  side; 
         FIG. 4B  is a cross section along the A-A line shown in  FIG. 4A ; 
         FIG. 5  is a process diagram explaining the manufacturing process of a first embodiment of the a surface acoustic wave device according to the present invention; 
         FIG. 6  is a diagram illustrating a schematic cross section of a second embodiment of the SAW device according to the present invention; 
         FIG. 7  is a process diagram explaining the manufacturing process according to the second embodiment; 
         FIG. 8  is an example showing the effects to remove the halogen gas generated after the vacuum baking was performed; and 
         FIG. 9  is an example showing the effects to remove the halogen gas generated after H 2 O ashing was performed. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 3  shows a diagram illustrating a schematic cross section of a first embodiment of the surface acoustic wave (SAW) device according to the present invention.  FIG. 5  is a process diagram explaining the manufacturing process thereof. 
     Here, the SAW device shown in the figures represents only one SAW device, on the assumption of a state such that each SAW device is formed by separating a plurality of SAW devices, simultaneously formed on a wafer, into individual pieces by dicing at the completion of manufacturing. 
     In  FIG. 3 , a piezoelectric substrate  1  formed of LiTaO 3  etc. is prepared (processing process P 1 ). As the electrode to form surface acoustic wave, on the piezoelectric substrate  1 , there are formed an IDT  10  constituted of aluminum alloy of Al—Cu, which is a drive electrode portion to excite surface acoustic wave, and a reflective electrode  11 . Also, a wiring electrode  15  for external connection is formed. Further, a protection film  18  constituted of a silicon compound (such as SiO 2  and SiN) is formed over piezoelectric substrate  1 , the drive electrode portion and wiring electrode  15  (processing process P 2 ). 
     At this time, to ensure connection to a metal post  17 , a barrier metal  16  constituted of Ti/Au is formed on wiring electrode  15  (processing process P 2 ). 
     Next, on piezoelectric substrate  1 , an epoxy photosensitive negative resist is spin coated with a thickness of 30 μm. Then, through patterning exposure and development, the resists in a drive electrode portion area A and a metal post formation portion B (refer to  FIG. 5 ) are removed (processing process P 3 ). 
     Through the above processing, there is formed a frame-shaped layer  20  in which only the drive electrode portion A and the metal post formation portion B are not covered with the resist. 
     Next, a photosensitive film negative resist  21  to form a lid body  21  with a thickness of 30 μm is bonded on frame-shaped layer  20  by means of a tenting method. Similar to the case of the processing process P 3 , through patterning exposure and development, the resists in the metal post formation portion B and a through hole portion  30 A are removed (processing process P 4 ). With this, the drive electrode portion A comes to have a hollow structure having through hole  30 A. 
     Thereafter, heating is performed in vacuum (approximately 1 Torr) to 200-250° C. By this, although a halogen gas is generated by heating from frame-shaped layer  20  and lid body  21 , which are photosensitive negative resists, the generated halogen gas can be removed via through hole  30 A. 
     Additionally, in order to efficiently remove the halogen gas generated from frame-shaped layer  20  and lid body  21 , it is preferable to heat in vacuum after being retained for a predetermined time under a high humidity environment (85-100% RH) and a high temperature environment (100-120° C.). Alternatively, it may also be possible to heat (100-250° C.) piezoelectric substrate  1  in H 2 O plasma. 
     Next, it is possible to seal the through hole  30 A by means of a print method using a liquid halogen-free thermosetting resin  30 . With this, it is possible to form hollow  13  of drive electrode portion A not including the halogen gas (processing process P 5 ). 
     Thereafter, there are formed nickel metal post  17  on the metal post formation portion B, and a solder ball  14  of SnAgCu alloy thereon (processing process P 6 ). 
     Here,  FIGS. 4A ,  4 B are diagrams explaining a ratio of the size of through hole  30 A, capable of effectively discharging the halogen gas, to the size of hollow  13 , as an embodiment. 
       FIG. 4A  is a plan viewed from the solder ball  14  side, and  FIG. 4B  is a cross section along the A-A line shown in  FIG. 4A . In  FIG. 4A , the area size of the plane of hollow  13  is shown with broken lines in a perspective manner. Understandably, there are formed two surface acoustic wave device areas. 
     As an embodiment, the area ratio of through hole  30 A to a plane area of hollow  13  is 0.1 in approximation. 
     Second Embodiment 
       FIG. 6  is a diagram illustrating a schematic cross section of a second embodiment of the SAW device according to the present invention. 
       FIG. 7  is a process diagram explaining the manufacturing process according to the second embodiment. In  FIG. 7 , the processing processes P 1  through P 4  are similar to the processes of the first embodiment shown in  FIG. 5 . 
     Next, in the processing process P 4 , in a state that a hollow structure is formed on the drive electrode portion A, heating is made in vacuum (appropriately 1 Torr) to 200-250° C. With this, the halogen gas, which is included in the photosensitive resist of frame-shaped layer  20  and lid body  21  and generated by heating, can be removed via through hole  30 A. 
     To remove the halogen gas more efficiently, it is preferable to heat in vacuum after being retained for a predetermined time under a high humidity environment (85-100% RH) and a high temperature environment (100-120° C.). Alternatively, it may also be possible to heat the substrate (100-250° C.) in H 2 O plasma. 
     Thereafter, a halogen-free thermosetting film resist  31  is bonded on lid body  21  by means of the tenting method (processing process P 7 ). With this, it is possible to form hollow  13  of the drive electrode portion A including no halogen gas. 
     In a portion corresponding to the metal post formation portion B, a thermosetting film resist  31  is holed using laser (processing process P 8 ). Alternatively, it is also possible to use the thermosetting film resist on which a hole is formed in advance corresponding to the metal post formation portion B. 
     Thereafter, there are formed a nickel metal post  17  on the metal post formation portion B, and a solder ball  14  of SnAgCu alloy thereon (processing process P 9 ). 
       FIGS. 8 and 9  are diagrams explaining effects by the present invention. In particular, there are shown diagrams (part  1  and part  2 ) explaining the effects of a vacuum heating process (vacuum baking) to efficiently remove the halogen gas generated from frame-shaped layer  20  and lid body  21 , in the above explanation of the embodiments. 
     In the example shown in  FIG. 8 , the vacuum baking was performed for one hour (1 Torr, 200° C.) after lid body  21  was formed in the first embodiment. Further, after through hole  30 A was sealed with a halogen-free resin, the sample was dipped in boiled deionized water (100° C.) so as to confirm the effect against erosion, and the erosion of the drive electrode (IDT) was confirmed. 
     For the sample on which the vacuum baking was not performed, the entire IDTs were eroded after being dipped for 60 minutes. In contrast, in regard to the sample on which the vacuum baking was performed, it was confirmed that only 7.5% was eroded even after being dipped for one hour. 
     Further, in the example shown in  FIG. 9 , after the lid body was formed, H 2 O ashing was performed (at 250° C.). Further, after through hole  30 A was sealed with a halogen-free resin, the sample was examined under a pressure cooker test (PCT) to confirm the effect against erosion. After the sample has been retained for 12 hours under the environments of 121° C., 2 atm and 95% RH, the characteristic of the sample was confirmed. 
     As a result, it has been understood from  FIG. 9  that, as the H 2 O ashing time is set longer, there occurs a smaller variation of insertion loss after the PCT, as well as a smaller erosion of the drive electrode.