Abstract:
Methods are disclosed for manufacturing piezoelectric devices. An exemplary method comprises the step of bonding a lid wafer, a piezoelectric frame wafer (having a vibrating piece and a outer frame surrounding the vibrating piece), and a base wafer (having at least one wiring through-hole) together. A surface of a unit (typically ball-shaped) of eutectic metal is cleaned and then arranged on the through-hole. The unit of eutectic metal is then melted in a vacuum or inert gas environment to allow the eutectic metal to enter the through-hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Japan Patent Application No. 2008-259377, filed on Oct. 6, 2008, in the Japan Patent Office, the disclosure of which is incorporated herein by reference in its entirety. 
     1. Field 
     The present invention relates to a manufacturing method of forming piezoelectric devices using sealing through-holes. 
     2. Description of the Related Art 
     Nowadays, crystal vibrating devices used for mobile communication device or Office Automation (“OA”) equipment must be increasingly miniaturized, provide a thinner profile, or operate at higher frequencies so as to be correspondingly accommodated in electronic devices that likewise are miniaturized or operate at higher frequencies. 
     In the conventional art, a piezoelectric vibrator has a piezoelectric vibrating piece contained in a package, and the piezoelectric vibrating piece is connected to an electrode arranged in the package. Generally, the package is made of glass or ceramic, and the package comprises a designated space within the package. On the bottom surface of the package, through-holes which penetrate through the bottom surface are formed. The through-holes are sealed by, for example, eutectic metals. In a reflow step, which seals the through-holes formed on the package by eutectic metal, components of a eutectic metal spread to form an electrode film on the piezoelectric vibrating piece. This spread is also accompanied by migration of gold (Au) in the electrode film, which causes fluctuations in crystal impedance (CI) value and oscillation frequency. 
     For example, a sealing method for a ceramic package is described in Japan Unexamined Patent Application No. 2003-158439 (the “&#39;439 application”), which describes a method of using an adhesive, such as a brazing filler metal, applied on a top surface of a package. The adhesive is melted in a heatable chamber by loading from the top, and then the package is pressure bonded together. Then, ball-shaped units of gold-germanium (Au/Ge) alloy are arranged on the through-holes and irradiated by laser light (within a vacuum chamber or a chamber filled with inert gas) to seal the package. 
     In the method disclosed in the &#39;439 application the sealing material is described as being arranged on through-holes formed on the bottom surface of the package. The sealing materials are melted by laser-light irradiation to seal the through-holes. However, this step must be done one-by-one, so mass production is difficult. Also, the ball-shaped units of gold-germanium (Au/Ge) alloy are easily oxidized. Thus, an oxidized film can easily form on the balls. The presence of an oxidized film makes the alloy difficult to flow and makes the sealing process more difficult. Further, segregated regions of germanium (Ge) can be found on the surface of the gold-germanium alloy ball, in which the concentration of germanium (Ge) is high to allow the germanium to flow to the electrodes of the crystal vibrating piece when the balls are melted. The combination of a gold surface and germanium is good for electrical conduction, and allows a flow speed that is faster than that of gold-tin (Au/Sn). Therefore, if germanium flows to an electrode of the crystal vibrating piece, the resistance characteristic of the electrode may undesirably change. This can also cause undesirable fluctuations in vibration frequency. Also, if the germanium flows to an electrode pattern, the film itself may become so thin that it causes disconnection. This effect also causes frequency instability. 
     The purpose of present invention is to control the spreading of eutectic metal to the electrode film as well as prevent migration of gold (Au) in an electrode film via the through-holes. The invention also provides a method of manufacturing airtight piezoelectric devices. 
     Further, respective wafers on which package lids, package bases, and crystal vibrating pieces are formed can be handled at the wafer level. During manufacture of piezoelectric devices, the package pieces on the respective wafers are bonded together by siloxane (Si—O—Si) bonding. 
     SUMMARY 
     A method of manufacturing a piezoelectric device has a first aspect comprising a step of bonding and sandwiching a piezoelectric frame wafer between a lid wafer and a base wafer having through-hole wiring. The piezoelectric frame wafer includes a vibrating piece and an outer frame surrounding the vibrating piece. The method includes a step of cleaning a surface of a eutectic metal, a step of arranging the eutectic metal on an already-cleaned surface of the through-hole wiring, and a step of melting the eutectic metal in a vacuum or inert-gas atmosphere. 
     According to this configuration, the oxidized film formed on the surface of the eutectic metal is removed, allowing the eutectic metal to be melted properly when heated. Thus, it does not affect excitation electrodes of the vibrating piece so that the resonant frequency of the vibrating piece will be stable. 
     A method of manufacturing a piezoelectric device has a second aspect that comprises a bonding step using siloxane bonding to bond together the base wafer, the lid wafer, and the piezoelectric-frame wafer made of a crystal material. 
     When multiple piezoelectric devices are bonded together in large numbers, bonding thereof by siloxane bonding is preferred. 
     A method of manufacturing a piezoelectric device has a third aspect that comprises a surface-cleaning step that includes a light-etching step to reduce the concentration of hydrofluoric acid relative to the eutectic metal. 
     A method of manufacturing a piezoelectric device according to a fourth aspect comprises a surface-cleaning step that further includes a UV light-irradiation step to irradiate the eutectic metal using ultraviolet (UV) light. 
     By conducting light-etching or UV light irradiation of the eutectic metal, an oxidized film formed on the surface of a film (wherein the oxidized film is a highly concentrated component of the eutectic metal) is removed. 
     A method of manufacturing a piezoelectric device further comprises an annealing step for annealing the eutectic metal after cleaning the eutectic metal with, e.g., water between the light-etching step and the arranging step. 
     Gas components may be produced when the eutectic metal is melted, but the remaining gas components on the eutectic metal ball can be removed after performing the annealing step. 
     A method of manufacturing a piezoelectric device has a sixth aspect comprising a step of dicing the piezoelectric frame wafer, the base wafer, and the lid wafer that have been bonded together. 
     Because piezoelectric devices in certain embodiments of the disclosed technology have eutectic metal balls that are cleaned, melted eutectic metal spreads evenly in the through-hole wiring, thus enabling manufacture of air-tight piezoelectric devices. Also, spreading of eutectic metal components to the electrode film of a piezoelectric vibrating piece or migration of gold (Au) in the electrode film is prevented. Further, the piezoelectric devices of the disclosed technologies can be bonded at the wafer level so that mass production and cost reduction can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective exploded view in which a base  40  of a piezoelectric device  100  is separated. 
         FIG. 1B  is a cross-sectional view of a piezoelectric device  100  along the line A-A in  FIG. 1A . 
         FIG. 2  is a flow chart for performing light-etching of a eutectic metal ball  60  by wet-etching. 
         FIG. 3  is a flow chart for performing light-etching of a eutectic metal ball  60  by wet-etching. 
         FIG. 4  is a flow chart for performing light-etching of a eutectic metal ball  60  by UV irradiation. 
         FIG. 5A  is an enlarged view around a first through-hole  41  before a eutectic metal ball  60  is melted. 
         FIG. 5B  is an enlarged view around a first through-hole  41  after a eutectic metal ball  60  has been melted and the through-hole has been sealed. 
         FIG. 6  is a perspective view before a lid wafer LW, a frame wafer VW, and a base wafer BW are layered together. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  is a perspective exploded of a piezoelectric device  100 , in which the base is uppermost, and  FIG. 1B  is a cross-sectional view of the piezoelectric device  100  along the line A-A in  FIG. 1A . 
     As  FIG. 1A  shows, the piezoelectric device  100  comprises three layers: a lid  10  made of a crystal “board,” a piezoelectric frame  20  having a tuning-fork-type crystal vibrating piece  30 , and a base  40  made of a crystal “board.” 
     The piezoelectric frame  20  is comprised of a tuning-fork-type crystal vibrating piece  30  situated in the center. The vibrating piece has a pair of supporting arms  25  and an outer frame  21 . These components are formed uniformly with crystal boards having substantially uniform thickness. The tuning-fork-type crystal vibrating piece  30  of the piezoelectric frame  20  includes connection portions  26  by which the outer frame  21  and the supporting arms  15  are connected together. The piezoelectric frame  20  includes a first base electrode  33  and a second base electrode  34  on the outer frame  21  and on the base  23 . A void  22  exists between the tuning-fork-type crystal vibrating piece  30  and the outer frame  21 . The void  22  defines a profile outline of the tuning-fork-type crystal vibrating piece  30  as formed by crystal etching. The tuning-fork-type crystal vibrating piece  30  includes a base  23  and a pair of vibrating arms  31  that extend from the base  23 . The vibrating arms  31  have grooves  27 , excitation electrodes  35 , and weights  37  at their distal end. 
     As shown in  FIG. 1B , the lid  10  comprises a concavity  17  facing one surface of the piezoelectric frame  20 . The base  40  comprises a concavity  47  that faces the other surface of the piezoelectric frame  20 . The base  40  has a first through-hole  41 , a second through-hole  42 , and a step  49 . At the step  49 , the first connecting electrode  42  and a second connecting electrode  44  are connected to the first through-hole  41  and the second through-hole  43 . A first external electrode  45  and a second external electrode  46  are formed on the bottom surface of base  40 . 
     On the inner surfaces of the first and second through-holes  41 ,  42  of the base  40  is a metal film  15 . The metal film  15  is formed concurrently with the first and second connecting electrodes  42 ,  44  and the first and second external electrodes  45 ,  46  by a photolithography method. The metal film  15  comprises two layers: a gold (Au) layer formed on a nickel (Ni) layer. The nickel layer is 150 to 700 Ångstroms thick and the gold layer is 400 to 1000 Ångstroms thick. Instead of the nickel layer, a chrome (Cr) or titanium (Ti) layer can be used. 
     The first connecting electrode  42  is electrically connected to the first external electrode  45  formed at the bottom of the base  40  via the first through-hole  41 . The second connecting electrode  44  is electrically connected to the second external electrode  46  formed at the bottom of the base  40  via the second through-hole  43 . 
     The base  40  is bonded to a “top” surface of the piezoelectric frame  20  comprising a tuning-fork-type crystal vibrating piece  30 , and the lid is bonded to a “bottom” surface of the piezoelectric frame  20  to form the piezoelectric device  100  with the crystal vibrating piece located in the center of the sandwich. Thus, the first base electrode  33  is connected to the first connecting electrode  42 , and the second base electrode  34  is connected to the second connecting electrode  44 , respectively. The base  40  is bonded to the piezoelectric frame  20  and the lid  10  is bonded to the piezoelectric frame  20 , respectively, by siloxane bonding. 
     After bonding the package  80  together by siloxane bonding, eutectic metal balls  60  are washed by light-etching and treated by deaeration. The eutectic metal balls  60  are arranged on the first through-holes  41  and second through-holes  43  as the holes face upwards. Then, the package  80  on which the eutectic metal balls  60  are arranged is heated for a designated time within a reflow furnace, filled with an inert gas atmosphere or vacuum, in which the eutectic metal balls  60  are melted. As an eutectic metal ball  60  is melted, it retains its ball shape due to surface tension within the melt. The ball is then flattened using a tool (not shown). Once the eutectic metal balls  60  are flattened, the eutectic metal flows along the metal film  15  to seal the first and second through-holes  41 ,  43 . 
     For a given eutectic metal ball  60 , gold-germanium (Au 12 Ge) alloy to which a light-etching process explained in  FIGS. 2 to 4  is applied. The eutectic metal balls  60  are cleaned by light-etching and treated by deaeration to remove oxidized film from their surfaces and also to control the amount and behavior of germanium (Ge), which is highly concentrated on the surfaces of the balls. Thus, the phenomenon that otherwise would spread germanium to the first and second connecting electrodes  42 ,  44  can be controlled. Also, the phenomenon exhibited by eutectic metal, in which gold (Au) tends to migrate on the first and second connecting electrodes  42 ,  44 , is controlled. In this way, electrodes, such as the first and second connecting electrodes  42 ,  44 , are made stable so that a piezoelectric device having a more stable vibration frequency can be manufactured. 
     Surface Treatment of Eutectic Metal Ball 
       FIGS. 2 to 4  show flow charts for surface treatment of a eutectic metal ball  60 .  FIG. 2  is a flow chart for a light-etching process for a eutectic metal ball  60  by wet-etching. 
     In step S 102 , in order to remove contaminants, such as organic substances and moisture on the surface of eutectic metal ball  60 , UV light irradiation or heat treatment is performed. UV light irradiation is performed using UV light having a wavelength of 165 nm and 254 nm to irradiate the ball. A cover is used for preventing leakage of UV light. The heat treatment is conducted using a hot plate or infrared light lamp to heat the eutectic metal ball  60  to 150° C. to 200° C. to degrade organic substances and remove them from the ball. 
     In step S 104   a , DHF (Dilute Hydrogen Fluoride) cleaning or BHF (Buffered Hydrogen Fluoride: NH 4 F, HF, H 2 O) cleaning (both of which are light-etching treatments) is performed. During DHF cleaning after irradiation of UV light or heating treatment, the eutectic metal ball  60  is placed in a netting container. Then the ball is soaked in hydrofluoric acid solution, having a concentration of 0.5 Wt %, for thirty seconds. The eutectic metal ball  60  is etched by about 5 nm. With this treatment, the oxidized surface of the ball  60  and highly concentrated germanium on the surface of the ball can be controlled when the ball is melted. 
     During a BHF cleaning, the eutectic metal ball  60  after irradiation of UV light or heating treatment is soaked in 200:1 BHF for thirty seconds, and a similar effect as obtained with the DHF cleaning is achieved. 200:1 BHF is a liquid containing 0.25 Wt % hydrofluoric acid (HF) and 39.8 Wt % ammonium fluoride (NH 4 F). 
     In step S 104   b , DHF spraying of the balls is performed to achieve DHF cleaning (a light-etching treatment) of the balls. During DHF spraying, hydrofluoric acid at a concentration of 0.5 Wt % is sprayed onto the eutectic metal balls  60  after the balls have had UV light irradiation or heat treatment. A similar effect as obtained from DHF cleaning is achieved. Note that either step S 104   a  or step S 104   b , not necessarily both, can be conducted as a light-etching treatment. 
     In step S 106 , a eutectic metal ball  60  after being treated by light-etching is kept in the netting container and placed in a rotary drive device to remove diluting solution by rotation. Next, cleaning fluid is applied to remove the diluting solution, and then the balls are washed by purified water. The eutectic metal balls  60  are dried by infrared light irradiation or by heated air. With these steps, any organic substances on the surface of eutectic metal ball  60  are degraded and removed. Also removed are the oxidized surface and highly concentrated germanium. 
     In step S 108 , a eutectic metal ball  60  after completion of light-etching treatment is placed in a vacuum heating furnace heated to a designated temperature to conduct vacuum annealing by maintaining the ball at temperature and in a vacuum state for approximately ten minutes. Remaining gas components on the eutectic metal ball  60  are removed by the annealing treatment. Note that the designated temperature of the vacuum heating furnace should be set slightly lower or equal to the melting temperature of the eutectic metal ball  60 . 
       FIG. 3  is a flow chart for light-etching a eutectic metal ball  60  by wet-etching. 
     In step S 202 , to remove contaminants (particles) such as organic substances and moisture on the surface of eutectic metal ball  60 , UV light irradiation or heat treatment is performed. During UV light irradiation, for example, UV light having wavelengths of 185 nm and 254 nm is irradiated onto the eutectic metal ball  60 . The heat treatment is conducted using a hot plate or infrared-light lamp to heat the eutectic metal ball  60  up to 150° C. to 200° C. to degrade organic substances and remove them from the balls. 
     With this heating treatment, adherence of particles on the surface of the eutectic metal balls  60  becomes weak so that the particles can be removed properly and efficiently by, e.g., spraying hot air from an air nozzle onto the surfaces of the balls. 
     In step  5204 , light-etching treatment by gas-phase etching using hydrofluoric acid vapor is performed. A preferable temperature for the treatment of gas-phase etching is 40° C. to 80° C., allowing the eutectic metal balls  60  to be retained near the optimum temperature. The hydrofluoric acid solution used for gas-phase etching is controlled at a concentration (1 atmosphere, 39.6 Wt % concentration at room temperature). With such a composition, pseudo-hydrofluoric acid is formed at a rate equal to the evaporation rate of hydrogen and hydrogen fluoride. Nitrogen gas is infused as a carrier gas to a hydrofluoric acid vapor-generating container filled with a hydrofluoric acid solution providing the desired concentration of pseudo-hydrofluoric acid composition. Then, a light-etching treatment is performed on the balls by providing the vapor on the surface of a eutectic metal ball  60 . 
     In step  5206 , a eutectic metal ball  60  is washed by purified water and then dried by irradiation of infrared light or by application of heated air to the balls. 
     In step  5208 , a eutectic metal ball  60 , after completion of a light-etching treatment, is placed in a vacuum-heating furnace at 320° C. for 10 minutes in a vacuum state, thereby performing a vacuum-annealing treatment. With the vacuum-annealing treatment, remaining gas components on the eutectic metal ball  60  are removed. 
       FIG. 4  is a flow chart for light-etching a eutectic metal ball  60  by UV irradiation. 
     In step S 302 , the surface of a eutectic metal ball  60  is light-etched by irradiating with UV light having wavelengths of 185 nm and 254 nm in a vacuum state in a vacuum heating furnace. By this light-etching by irradiation of UV light, the organic substances on the surface of the eutectic metal ball  60  are degraded and removed. Also, the oxidized surface and highly concentrated germanium on the surface of the ball are removed. 
     Next, in step  5304 , after completion of light-etching, the eutectic metal ball  60  is placed in a vacuum-heating furnace at 320° C. for 10 minutes, while maintaining a vacuum state to conduct a vacuum-annealing treatment. With the vacuum-annealing treatment, the remaining gas components on the eutectic metal ball  60  are removed. 
       FIG. 5A  is an enlarged view of the vicinity of a first through-hole  41  before the eutectic metal ball  60  is melted, and  FIG. 5B  is an enlarged view of the vicinity of a first through-hole  41  after the eutectic metal ball  60  is melted and the through-hole is sealed. The second through-hole  43  is the same. 
     As shown in  FIG. 5A , the lid  10 , the first piezoelectric frame  20 , and the first base  40 , bonded by siloxane bonding, are arranged upside-down. Then, a eutectic metal ball  60  which has been light-etched, is placed on the first through-hole  41 . The ball starts to melt at about 350° C. in the vacuum-reflow furnace, so the ball can be flattened using a tool. The time to achieve melting of the eutectic metal ball  60  that has been treated by light-etching, is unchanged so this process can be done at the wafer level. 
     Then, as  FIG. 5B  shows, the melted eutectic metal ball  60  spreads in the first through-hole  41  and seals the hole. When the eutectic metal ball  60  is melted in this way, germanium (Ge) in the ball  60  does not spread rapidly to the first and second base electrodes  33 ,  34 . Because the ball  60  is etched to a depth of 5 nm by the light-etching step, the oxidized film on its surface and highly concentrated germanium (Ge) are removed. Thus, spread of metal to the electrodes and migration of gold are limited. Thus, the characteristic vibration frequency of the tuning-fork-type crystal vibrating piece  30  is stabilized before and after the reflow step. 
     Manufacturing Piezoelectric Devices 
       FIG. 6  is a perspective view before a lid wafer LW, a frame wafer VW, and a base wafer BW are layered. The lid wafer LW includes a plurality of lids  10 . The frame wafer VW includes a plurality of tuning-fork-type crystal vibrating pieces  30  and respective piezoelectric frames  20 . The base wafer BW includes a plurality of bases  40 . For purpose of explanation, the lids  10  are illustrated with virtual lines as are the piezoelectric frames  20  on the frame wafer VW and bases  40  on the base wafer BW. Note that, while 42 pieces of the lid  10 , the piezoelectric frame  20  and the base  40  are respectively illustrated on each crystal wafer depicted in  FIG. 6 , hundreds or thousands of piezoelectric devices  100  can be formed on the bonded crystal wafers. 
     Before the crystal wafers are layered, the concavity  17  of the lid  10  is formed by etching. Also formed are the concavity  47  of the base  40  and the first and second connecting electrodes  42 ,  44  (not shown). On the tuning-fork-type crystal vibrating piece  30  are formed the first and second base electrodes  33 ,  34  and excitation electrode  35 . For bonding, the contacting surfaces of the lid  10  and base  40  with the frame  20  are activated by rendering them as mirror surfaces and then treating them with plasma or an irradiating ion beam. 
     For example, the size of each activated crystal wafer (containing multiple devices) has a diameter of 4 inches. The wafers include respective orientation flats by which the wafers are aligned and layered correctly. The three sandwiched crystal wafers are bonded strongly together by siloxane bonding to form packages  80 . After forming a package  80 , sealing of the first and second through-holes  41 ,  43  ( FIG. 1 ) is performed in a vacuum state or inactive atmosphere. For sealing of through-holes, eutectic metal balls  60  treated with light-etching are used. 
     The lid wafer LW, the base wafer BW, and the frame wafer containing piezoelectric frames  20  are bonded together as one 3-wafer sandwich. Then the sandwich in which the first and second through-holes  41  and  43  have been sealed is cut by a dicing saw or laser saw to complete formation of the piezoelectric devices  100 . Packaging and sealing of through-holes are conducted on the sandwich before cutting so that mass manufacturing can be achieved. 
     Multiple embodiments are described above. But, it will be understood by persons of ordinary skill in the relevant art that any of said embodiments, as well as any other embodiments within the scope of the invention, can be modified or changed. For example, cleaning by water in the light-etching process or a drying step can be omitted. Further, a base or lid not having concavities can be used, which allows the thickness of tuning-fork-type piezoelectric vibrating piece  30  to be thinner than the outer frame. 
     In the described embodiments, the eutectic metal balls  60  are made of, for example, Au 12 Ge alloy. Alternatively, gold-tin (Au 20 Sn) alloy (melting temp. 280° C.) or gold-silicon (Au 3.15 Si) alloy (melting temp. 363° C.) can be used.