Abstract:
A complex RF device is provided which is composed of two RF circuits stacked vertically. The complex RF device comprises a substrate, a second RF circuit provided on the substrate, and a first RF circuit which is provided on the second RF circuit and does not require a substrate. The first RF circuit is formed on another substrate before being transferred onto the second RF circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to discrete radio frequency circuit devices (hereinafter referred to as RF devices), such as a filter, a duplexer, a switch (SW), a low noise amplifier (LNA), a power amplifier (PA), and the like, which are used in mobile communication radio circuits, such as mobile telephones, wireless LAN, and the like, or a complex RF device composed thereof, and a method for manufacturing the complex RF device.  
         [0003]     2. Description of the Background Art  
         [0004]     Mobile apparatuses and the like require smaller-size and lower-profile radio circuits. To this end, regarding filters and radio ICs which are incorporated into electronic apparatuses (e.g., mobile apparatuses, etc.), there is an active trend toward a complex device in which different devices are integrated together so as to achieve a small size.  
         [0005]      FIG. 7  is a cross-sectional view of a structure of a complex RF device employing a conventional IC chip. See, for example, Japanese Patent Laid-Open Publication No. H05-13663.  
         [0006]     A first IC chip  901  is provided on a second IC chip  902  by face-up mounting. The second IC chip  902  is provided on a substrate  903  made of a ceramic or a resin by face-up mounting. An electrode  904  provided on the first IC chip  901  is connected to an electrode  906  provided on the substrate  903  by wire bonding, so that the first IC chip  901  and the substrate  903  are electrically connected together. An electrode  905  provided on the second IC chip  902  is connected to the electrode  906  provided on the substrate  903  by wire bonding, so that the second IC chip  902  and the substrate  903  are electrically connected together. With this structure, a complex RF device having each of the functions of the first IC chip  901  and the second IC chip  902  is achieved with a small area.  
         [0007]     However, in the structure of this conventional complex RF device, the first IC chip  901 , the second IC chip  902 , and the substrate  903  each have a thickness of several hundreds of micrometers, and therefore, when they are mounted in a stacked manner, the whole complex RF device has a large thickness. Therefore, a technique for reducing the thickness of the whole complex RF device has been proposed.  
         [0008]      FIG. 8  is a cross-sectional view of a structure of a conventional complex RF device which employs a piezoelectric filter and solves the above-described problem. See, for example, P. Ancey (ST Microelectronics), “BAW &amp; MEMS above silicon for RF applications”, IEEE MTT-S 2005 International Microwave Symposium Workshop.  
         [0009]     An electrode  1002  provided inside and on a surface of a substrate is used to form an IC substrate  1001  having functions of a switch, a low noise amplifier, a power amplifier or the like. On the IC substrate  1001 , an insulator element  1004 , a lower electrode  1005 , a piezoelectric element  1006 , and an upper electrode  1007  are stacked in this order via a cavity  1003  to form a piezoelectric resonator  1008 . A plurality of piezoelectric resonators  1008  are combined to operate as a piezoelectric filter. The IC substrate  1002  and the piezoelectric filter are connected together to form a complex RF device.  
         [0010]     With this structure, although the IC substrate  1001  still has a thickness of several hundreds of micrometers, the piezoelectric resonator  1008  has a thickness of about 10 micrometers or less (in a microwave region which is used for mobile telephones or the like, though also depending on the resonance frequency), so that a complex RF device in which a piezoelectric filter having a small thickness is stacked can be achieved.  
         [0011]     However, in the conventional structure of  FIG. 8 , the electrode  1002 , the insulator  1004 , and a sacrifice layer so as to form the cavity  1003  and the like need to be successively deposited on the IC substrate  1001 . Therefore, the evenness of a surface of the IC substrate  1001  is deteriorated before the lower electrode  1005 , the piezoelectric element  1006 , and the upper electrode  1007  are deposited, so that the crystallinity of the lower electrode  1005 , the piezoelectric element  1006 , and the upper electrode  1007 , which are formed as thin films, is impaired. This reduces a Q value indicating the performance of the piezoelectric resonator  1008 , leading to an increase in insertion loss of the piezoelectric filter.  
       SUMMARY OF THE INVENTION  
       [0012]     Therefore, an object of the present invention is to provide a small-size and low-profile complex RF device having a plurality of functions in a high-quality state without impairing the crystallinity of a piezoelectric layer thereof.  
         [0013]     The present invention provides a complex RF device composed of two RF circuits stacked vertically, comprising a substrate, a second RF circuit provided on the substrate, and a first RF circuit provided on the second RF circuit, the first RF circuit not requiring a substrate. The first RF circuit is formed on another substrate before being transferred onto the second RF circuit.  
         [0014]     The first RF circuit and the second RF circuit may be electrically connected to each other via first and second support members.  
         [0015]     Typically, the first RF circuit is one selected from the group consisting of a piezoelectric resonator, a piezoelectric switch, a piezoelectric filter, and a duplexer which do not require a substrate, and the second RF circuit is one selected from the group consisting of a power amplifier, a switch, an LNA, and an RF-IC which do require a substrate.  
         [0016]     Note that the complex RF device functions singly, and may be incorporated into a filter, a duplexer, and a communication apparatus.  
         [0017]     The complex RF device is manufactured by the steps of forming a first RF circuit on a first substrate, forming a first support member on the first substrate, forming a second RF circuit on a second substrate, forming a second support member on the second substrate, bonding the first support member and the second support member together, and after the bonding step, removing the first substrate, and transferring the first RF circuit onto the second RF circuit.  
         [0018]     Typically, after the transferring step, a predetermined electrode is formed on the first RF circuit.  
         [0019]     Preferably, the first and second support members are made of a metal material which can electrically connect the first RF circuit and the second RF circuit together.  
         [0020]     According to the present invention, it is possible to provide a small-size and low-profile complex RF device having a plurality of functions in a high-quality state without impairing the crystallinity of a piezoelectric layer thereof.  
         [0021]     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a perspective view illustrating a structure of a complex RF device according to an embodiment of the present invention;  
         [0023]      FIG. 2  is a cross-sectional view of the complex RF device, taken along line A-A of  FIG. 1 ;  
         [0024]      FIG. 3  is an equivalent circuit diagram of the complex RF device of  FIG. 1 ;  
         [0025]      FIGS. 4A  to  4 D are cross-sectional views illustrating exemplary structures of other complex RF devices which can be achieved by the present invention;  
         [0026]      FIGS. 5A and 5B  are diagram roughly illustrating a method for manufacturing a complex RF device of the embodiment of the present invention;  
         [0027]      FIG. 6  is a diagram illustrating an exemplary configuration of a communication apparatus employing a complex RF device of the embodiment of the present invention; and  
         [0028]      FIGS. 7 and 8  are cross-sectional views of a structure of a conventional complex RF device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.  
       Exemplary Structure of Complex RF Device  
       [0030]      FIG. 1  is a perspective view illustrating a structure of a complex RF device according to an embodiment of the present invention.  FIG. 2  is a cross-sectional view of the complex RF device, taken along line A-A of  FIG. 1 .  FIG. 3  is an equivalent circuit diagram of the complex RF device of  FIG. 1 . In FIGS.  1  to  3 , a duplexer employing a piezoelectric filter is illustrated as an example of the complex RF device.  
         [0031]     The complex RF device of this embodiment has a transmission terminal  101   a , a reception terminal  101   b , and an antenna terminal  101   c , and is composed of a transmission filter  110  connected to the transmission terminal  101   a , a reception filter  120  connected to the reception terminal  101   b , and a phase-shift circuit  102  provided between the transmission filter  110  and the reception filter  120 , and the antenna terminal  101   c . As illustrated in  FIG. 1 , the complex RF device has a structure in which the transmission filter  110  (first RF circuit) is provided at an upper portion thereof and the reception filter  120  (second RF circuit) is provided at a lower portion thereof.  
         [0032]     Referring to  FIG. 2 , the transmission filter  110  is composed of piezoelectric resonators  112   a  and  112   b  connected in series between the transmission terminal  101   a  and the antenna terminal  101   c , a piezoelectric resonator  113  connected in parallel therebetween, and an inductor  114  via which the piezoelectric resonator  113  is grounded. The reception filter  120  is composed of piezoelectric resonators  122   a  and  122   b  connected in series between the reception terminal  101   b  and the antenna terminal  101   c , a piezoelectric resonator  123  connected in parallel therebetween, and an inductor  124  via which the piezoelectric resonator  123  is grounded. In the example of  FIG. 2 , as the phase-shift circuit  102 , an inductor via which a connection point of the transmission filter  110  and the reception filter  120  is grounded, is employed.  
         [0033]     Note that the above-described circuit configurations of the transmission filter  110  and the reception filter  120  are only for illustrative purposes, and a similar effect can be obtained when other numbers of stages or other circuit configurations are employed. Also, the phase-shift circuit  102  may have other circuit configurations, depending on transmission/reception intervals or the impedances of the transmission filter  110  and the reception filter  120 .  
         [0034]     Referring to the cross-sectional view of  FIG. 3 , in the complex RF device of this embodiment, the piezoelectric resonator  123  which belongs to the second RF circuit and is composed of an upper electrode  125 , a lower electrode  126 , and a piezoelectric element  203 , is formed on a substrate  201  made of GaAs or the like. On the piezoelectric resonator  123 , the piezoelectric resonator  112   a  which belongs to the first RF circuit and is composed of an upper electrode  115 , a lower electrode  116 , and a piezoelectric element  202 , is formed. The first RF circuit is formed via a metal column  117  made of a gold-tin alloy or the like above the second RF circuit so that a manufacturing method described below can be used. Note that the shape of the metal column  117  is not limited to that of  FIG. 3 .  
         [0035]     Thus, in the present invention, parts requiring a substrate, such as a power amplifier, a switch, an LNA, or an RF-IC, or the like, are formed in the lower second RF circuit, and parts not requiring a substrate, such as a piezoelectric resonator, a MEMS switch, or a piezoelectric filter or a duplexer employing these, or the like, are formed on the upper first RF circuit.  
         [0036]      FIGS. 4A  to  4 D are cross-sectional views illustrating exemplary structures of other complex RF devices which can be achieved by the present invention.  FIG. 4A  illustrates an exemplary structure of a complex RF device in which a cantilever MEMS switch is provided in the first RF circuit and a piezoelectric resonator is provided in the second RF circuit.  FIG. 4B  illustrates an exemplary structure of a complex RF device in which a duplexer employing a piezoelectric filter is provided in the first RF circuit and a power amplifier is provided in the second RF circuit.  FIG. 4C  illustrates an exemplary structure of a complex RF device in which a duplexer employing a piezoelectric filter is provided in the first RF circuit and a piezoelectric filter is provided in the second RF circuit.  FIG. 4D  illustrates an exemplary structure of a complex RF device in which a piezoelectric switch is provided in the first RF circuit and a power amplifier is provided in the second RF circuit.  
       Exemplary Method for Manufacturing Complex RF Device  
       [0037]      FIGS. 5A and 5B  are diagram roughly illustrating a method for manufacturing a complex RF device of this embodiment. In this manufacturing method, the complex RF device of  FIG. 3  is manufactured by a wafer-to-wafer bonding method.  
         [0038]     Initially, a film-formation substrate  511  made of silicon, glass, sapphire or the like is prepared. An electrode film  513  made of molybdenum (Mo) or the like is formed on the film-formation substrate  511  (step a of  FIG. 5A ). Note that an even thermal oxide film (not shown) is previously formed as an insulating film on the film-formation substrate  511 . Next, a piezoelectric layer  202  made of aluminum nitride (AlN) or the like is formed on the electrode film  513  (step b of  FIG. 5A ). For example, when a piezoelectric resonator having a 2-GHz band is formed, the piezoelectric layer  202  is designed to have a thickness of about 1100 nm, and the electrode film  513  is designed to have a thickness of about 300 nm. In this example, the piezoelectric layer  202  is formed via the electrode film  513  on the even film-formation substrate  511 , there is not an influence of a discontinuity occurring in the electrode film  513 , a degradation in a surface of the electrode film  513  occurring when during patterning, or the like, thereby making it possible to obtain the piezoelectric layer  202  having a satisfactory level of crystallinity.  
         [0039]     Next, an electrode film  512  made of molybdenum or the like is formed on the piezoelectric layer  202  (step c of  FIG. 5A ). Thereafter, the electrode film  512  is patterned into a predetermined shape by typical photolithography to form a lower electrode  115  (step d of  FIG. 5A ). Next, a support member  117   a  which is to be a part of the support portion  117  is formed on the piezoelectric layer  202  by electron beam vapor deposition, sputtering, or the like (step e of  FIG. 5A ). In this example, the support member  117   a  is formed by electron beam vapor deposition of Ti/Au/AuSn in this order using a lift-off technique. Thereby, preparation of the film-formation substrate  511  is completed.  
         [0040]     Next, the substrate  201  is prepared, and the piezoelectric resonator  123  composed of the upper electrode  125 , the lower electrode  126  and the piezoelectric layer  203  is formed in a similar manner (step f of  FIG. 5A ). Note that an even thermal oxide film or the like (not shown) is previously formed as an insulating film on the substrate  201 . Next, a support member  117   b  which is to be a part of the support portion  117  is formed on the piezoelectric layer  203  by electron beam vapor deposition, sputtering, or the like (step g of  FIG. 5B ). In this example, the support member  117   b  is formed by electron beam vapor deposition of Ti/Au/AuSn in this order using a lift-off technique so that, when the substrate  201  is disposed, facing the film-formation substrate  511 , the AuSn alloy layer of the support member  117   b  contacts the AuSn alloy layer of the support member  117   a . Note that the pattern of the support member  117   b  formed on the substrate  201  does not need to completely match the pattern of the support member  117   a  formed on the film-formation substrate  511 , and a margin is preferably provided in view of the accuracy of positioning both the substrates.  
         [0041]     Next, the support member  117   a  of the film-formation substrate  511  and the support member  117   b  of the substrate  201  are caused to face each other, and are bonded together by eutectic crystallization of gold and tin (step h of  FIG. 5B ). In this case, a pressure is applied to both the substrates. In this example, a press pressure of three atmospheres is applied so as to bond the substrates. Also, the bonded substrates are heated, so that AuSn contacting each other are melted, and thereafter, by reducing the temperature, firm metal bond can be obtained. Thereby, a piezoelectric resonator having an excellent level of reliability of bonding can be obtained.  
         [0042]     Although a AuSn alloy is used in the support portion  117  in this example, the present invention is not limited to this. For example, when the two substrates are bonded together via a half-melted or melted state of the support portion  117 , the melting point (solidus temperature) may be higher than solder reflow temperature at which the piezoelectric resonator is mounted on a mother board, and may be lower than the melting points of an electrode material and the like of the piezoelectric resonator. Also, the support portion  117  may be bonded by diffusion bonding due to mutual diffusion of metals below the melting point, or alternatively, may be bonded at room temperature by surface activation of bonding surfaces using a plasma treatment or the like. By room-temperature bonding, residual thermal stress can be eliminated from the vibrating portion, thereby making it possible to obtain a piezoelectric resonator having a high manufacturing yield and a small change over time in frequency fluctuation or the like.  
         [0043]     Next, the film-formation substrate  511  is removed from the product obtained by bonding the two substrates together (step i of  FIG. 5B ). For example, the film-formation substrate  511  can be removed by dry etching. By steps g to i, the first RF circuit which is originally present on the film-formation substrate  511  is transferred to the substrate  201  on which the second RF circuit is formed. Finally, the electrode film  513  is patterned into a predetermined shape by typical photolithography to form an upper electrode  116  (step j of  FIG. 5B ). Thereby, the complex RF device of  FIG. 3  is completed.  
         [0044]     Although the film-formation substrate  511  is removed by, for example, etching in the above-described manufacturing method, a come-off layer may be provided between the electrode film  513  and the film-formation substrate  511  so that the film-formation substrate  511  can be detached along with the come-off layer. Alternatively, the electrode film  513  may not be formed, and a come-off layer and the piezoelectric layer  202  may be stacked on the film-formation substrate  511 . In this case, after the film-formation substrate  511  is detached, the upper electrode  116  needs to be formed by patterning. When gallium nitride (GaN), which has optical characteristics different from those of AlN, is used as the come-off layer, AlN can be transferred by decomposing only GaN by irradiation with laser. Alternatively, as the come-off layer, a metal film which has a small affinity with the electrode film  513 , a metal film or an oxide substance which is dissolved in a solvent or the like, glass, or the like may be used.  
         [0045]     As described above, according to the embodiment of the present invention, a small-size and low-profile complex RF device having a plurality of functions can be achieved in a high-quality state without impairing the crystallinity of the piezoelectric layer.  
       Exemplary Configuration Employing Complex RF Device  
       [0046]      FIG. 6  is a diagram illustrating an exemplary configuration of a communication apparatus employing a complex RF device of the present invention. In the communication apparatus of  FIG. 6 , two transmission/reception circuits  603  and  604  are connected and switched by a switch  602  so as to support a plurality of bands.  
         [0047]     A signal input through an antenna  601  is separated and input by the switch  602  into the first transmission/reception circuit  603  which is operated at a low frequency band (first band) and the second transmission/reception circuit  604  which is operated at a high frequency band (second band). In the first transmission/reception circuit  603 , a first-band transmission signal input through a transmission terminal  605   a  is passed through an RF-IC  606   a , a power amplifier  607   a , and a transmission filter  609   a  of a duplexer  608   a , and is transmitted via the switch  602  from the antenna  601 . Also, a first-band reception signal input through the antenna  601  is passed and transferred through the switch  602 , a reception filter  610   a  of the duplexer  608   a , an LNA  611   a , and the RF-IC  606   a , to a reception terminal  612   a.    
         [0048]     Similarly, in the second-band transmission/reception circuit  604 , a second-band transmission signal input through a transmission terminal  605   b  is passed through an RF-IC  606   b , a power amplifier  607   b , and a transmission filter  609   b  of a duplexer  608   b , and is transmitted via the switch  602  from the antenna  601 . Also, a second-band reception signal input through the antenna  601  is passed and transferred through the switch  602 , a reception filter  610   b  of the duplexer  608   b , an LNA  611   b , and the RF-IC  606   b , to a reception terminal  612   b . With this configuration, a communication apparatus which has low loss and low power consumption can be achieved.  
         [0049]     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.