Abstract:
A method of manufacturing a piezoelectric thin film resonator which can reduce variations in resonant frequency and resonant resistance by uniformly planarizing a structural film. The method of manufacturing the piezoelectric thin film resonator includes the steps of forming sacrifice layer patterns on an upper surface of a mother substrate; forming a dielectric film on the sacrifice layer patterns; processing a surface of the dielectric film by a plasma treatment; forming vibration portions on the dielectric film, the vibration portions each being composed of two excitation electrodes and a piezoelectric thin film provided therebetween; etching the sacrifice layer patterns; and cutting the mother substrate into separate piezoelectric thin film resonators.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of application Ser. No. 11/714,870, filed Mar. 6, 2007, which is a continuation of International Application No. PCT/JP2005/022300, filed Dec. 5, 2005, claiming priority to Japanese Patent Application No. JP2004-373761, filed Dec. 24, 2004, the entire contents of each of these applications being incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a piezoelectric thin film resonator and a manufacturing method thereof. 
     BACKGROUND OF THE INVENTION 
     In a so-called air-bridge type piezoelectric thin film resonator, in order to acoustically separate a vibration portion, which is formed of a pair of excitation electrodes facing each other and a piezoelectric thin film provided therebetween, from a substrate, the structure is formed in which a thin film member (membrane) partly floats over the substrate with an airspace layer provided there between.
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 61-218214   

     Heretofore, planarization has not been studied for an air-bridge type piezoelectric thin film resonator. 
     The orientation property of a piezoelectric thin film influences resonant properties. In order to improve the resonant properties, the orientation property of a piezoelectric thin film must be improved, and in order to improve the orientation property thereof, the flatness of a structural film, which is provided under an excitation electrode, must be improved. 
     As a method for planarizing the structural film provided under the excitation electrode, for example, polishing by CMP (chemical mechanical polishing) may be mentioned. 
     In an air-bridge type piezoelectric thin film resonator, a structural film is formed on a sacrifice layer which is used for forming an airspace layer. Hence, irregularities are formed on the structural film which is to be polished. By polishing, convex portions (projecting surfaces) can only be planarized, and in addition, since slurry is trapped in concave portions, peripheries of the convex portions are likely to be polished as compared to central portions thereof. Accordingly, even when the structural film on the sacrifice layer is polished, film-thickness distribution is generated, and as a result, superior resonant properties cannot be obtained. In order to avoid this problem, a method may be used in which after a burying material is buried in concave portions, planarization is performed by CMP, and the burying material is then removed; however, in this case, the manufacturing process becomes complicated, and hence the manufacturing cost is increased. 
     In addition, when a plurality of piezoelectric thin film resonators is simultaneously formed using a wafer, it is difficult to uniformly polish structural films of individual resonators in the wafer on the order of nanometers in thickness. The resonant frequency and the resonant properties of a resonator largely depend on the thickness of the structural film. When the thicknesses of the structural films vary within a wafer, the yield may be decreased, or the number of steps, such as frequency adjustment, may be increased, and as a result, the manufacturing cost is increased. 
     SUMMARY OF THE INVENTION 
     The present invention has been conceived in consideration of the above circumstances, and an object of the present invention is to provide a piezoelectric thin film resonator having superior resonant properties by uniformly planarizing a structural film and a manufacturing method of the above piezoelectric thin film resonator. 
     In order to achieve the above object, the present invention provides a piezoelectric thin film resonator having the following structure. 
     The piezoelectric thin film resonator includes a substrate having at least one flat major surface; a dielectric film having two support portions supported by the major surface of the substrate and a floating portion which is connected to the support portions and which is disposed over the major surface of the substrate with an airspace layer provided therebetween; and a vibration portion which is formed of a pair of electrodes and a piezoelectric thin film provided therebetween and which is provided on the floating portion of the dielectric film at a side opposite to the airspace layer. In this piezoelectric thin film resonator, a surface of the dielectric film which is located at a side opposite to the substrate is planarized by a plasma treatment using an inert gas or a gas containing an element forming the dielectric film. 
     According to the structure described above, since the dielectric film is planarized by a plasma treatment, the orientation property of the excitation electrode formed on the dielectric film is improved, and hence the orientation property of the piezoelectric thin film formed on the excitation electrode is also improved. Since the orientation property of the piezoelectric thin film is improved, variations in resonant properties and resonant resistance can be reduced. In addition, since the orientation property of the excitation electrode is improved, a piezoelectric thin film resonator having superior resistance against electric power can be formed. 
     In addition, when a plurality of piezoelectric thin film resonators is simultaneously formed using a wafer, the thickness of the structural film of each resonator can be controlled on the order of nanometers. Accordingly, since variation in resonant frequency in the wafer surface and the resonant properties can be controlled with high accuracy, the yield can be increased, and hence the manufacturing cost can be reduced. 
     Furthermore, since a plasma treatment is performed using an inert gas or a gas containing an element forming the dielectric film, the dielectric film can be planarized without forming a compound layer thereon which is different from a dielectric material. 
     The dielectric film is preferably a film formed of a material selected from the group consisting of Si 3 N 4 , SiO 2 , and Al 2 O 3 . 
     According to the above structure, since the dielectric film is an amorphous film, planarization can be easily performed by a plasma treatment. 
     In addition, the present invention provides the following method for manufacturing piezoelectric thin film resonators. 
     The method for manufacturing piezoelectric thin film resonators, includes: i) a first step of forming sacrifice layer patterns on an upper surface of a mother substrate; ii) a second step of forming a dielectric film on the sacrifice layer patterns; iii) a third step of processing a surface of the dielectric film by a plasma treatment; iv) a fourth step of forming vibration portions on the dielectric film, the vibration portions each being composed of two excitation electrodes and a piezoelectric thin film provided therebetween; v) a fifth step of etching the sacrifice layers; and vi) a sixth step of cutting the mother substrate to separate the piezoelectric thin film resonators. 
     Since the dielectric film is planarized in the above third step, the orientation property of the excitation electrode formed on the dielectric film is improved, and the orientation property of the piezoelectric thin film formed on the excitation electrode is also improved. Since the orientation property of the piezoelectric thin film is improved, a piezoelectric thin film resonator having superior resonant properties can be manufactured. 
     The third step described above preferably includes the substeps of: a) fitting the mother substrate provided with the dielectric film formed thereon to a substrate plate and then placing the mother substrate in a sputtering chamber; b) supplying a gas in the sputtering chamber; and c) performing sputtering etching of the surface of the dielectric film, which is formed on the mother substrate fitted to the substrate plate, with plasma of the gas generated by supplying an RF voltage to the substrate plate while the substrate plate is being electrically floated from the sputtering chamber. 
     In this case, in the third step, the dielectric film can be planarized without forming a compound layer thereon which is different from a dielectric material. In addition, a dielectric film patterning step can be stably performed when etching holes are formed. 
     In particular, the following embodiments may be mentioned. 
     As one embodiment, the gas may be an inert gas selected from the group consisting of Ar and He. 
     As another embodiment, the gas may be a gas containing an element forming the dielectric film. 
     In said another embodiment, the dielectric film is preferably formed of SiO 2  or Al 2 O 3 , and oxygen is preferably used as the gas. In this case, since the insulating properties are excellent, the resonant properties are not degraded. 
     In said another embodiment, the dielectric film is preferably formed of Si 3 N 4 , and nitrogen is preferably used as the gas. In this case, since the insulating properties are excellent, the resonant properties are not degraded. 
     According to the piezoelectric thin film resonator and the manufacturing method thereof, by uniformly planarizing the structural film, a resonator having superior resonant properties can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1(   a ) is a cross-sectional view of a piezoelectric thin film resonator, and  FIG. 1(   b ) is a plan view thereof (example). 
         FIG. 2  is a plan view of a piezoelectric thin film resonator, which corresponds to  FIG. 1(   b ) (modified example). 
       REFERENCE NUMERALS 
       
           
           
             
                 10 ,  10   a  piezoelectric thin film resonator 
                 11  substrate 
                 13  airspace layer 
                 14  lower electrode (excitation electrode) 
                 16  piezoelectric thin film 
                 17  sacrifice layer 
                 17   x  end portion 
                 18  upper electrode (excitation electrode) 
                 19  vibration portion 
             
           
         
      
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, examples will be described as embodiments according to the present invention with reference to  FIG. 1 .  FIG. 1(   a ) is a cross-sectional view taken along the line A-A in  FIG. 1(   b ), and  FIG. 1(   b ) is a plan view. 
     As schematically shown in  FIG. 1 , a piezoelectric thin film resonator  10  is formed of a substrate  11  and a thin film member (membrane) provided thereon, the thin film member including a dielectric film  12 , a lower electrode  14 , a piezoelectric thin film  16 , and an upper electrode  18 . An airspace layer  13  (see  FIG. 1(   a )) is formed between the substrate  11  and the dielectric film  12 . The dielectric film  12  includes support portions supported by the substrate  11  and a floating portion floating over the substrate  11 . On the floating portion of the dielectric film  12  at a position opposite to the airspace layer  13 , a vibration portion  19  floating over the substrate  11  is formed in a region in which the electrodes  14  and  18  are overlapped with each other. The vibration portion  19  is composed of parts of the lower electrode  14 , the piezoelectric thin film  16 , and the upper electrode  18 , which are present in the above region. The airspace layer  13  is formed by removing a sacrifice layer  17  (see  FIG. 1(   b )) which is formed between the substrate  11  and the dielectric film  12 . 
     Next, a manufacturing method of the piezoelectric thin film resonator  10  will be described. 
     &lt;Sacrifice Layer&gt; 
     First, the sacrifice layer  17  is formed on the substrate  11 . As the substrate  11 , a substrate which is inexpensive and which has superior machinability is used. A Si or a glass substrate having a flat surface is more preferable. On this substrate  11 , by methods such as a sputtering method and a photo-etching method, the sacrifice layer  17  for forming an airspace layer is formed using, for example, zinc oxide which is likely to be chemically dissolved. 
     As a material for the sacrifice layer  17 , it is preferably used the material can withstand a high temperature generated when the piezoelectric thin film  16  is formed, and which can be easily removed. For example, a metal such as Ge, Sb, Ti, Al, or Cu, a malate silicate glass (PSG), or a polymer may be used. As the polymer, for example, polytetrafluoroethylene or its derivative, poly(phenylene sulfide), poly(ether ether ketone), polyimide, poly(imide siloxane), vinyl ether, polyphenyl, parylene-n, parylene-f, or benzocyclobutene is preferable. 
     The sacrifice layer  17  should have a sufficient thickness so that the vibration portion  19  is not brought into contact with the substrate  11  even when the membrane is warped. For easy formation, the thickness of the sacrifice layer  17  is preferably in the range of 50 nm to several micrometers. In addition, the minimum distance between the vibration portion  19  and an end portion  17   x  of the sacrifice layer  17  is set to be not more than 50 times the thickness of the vibration portion  19 . 
     &lt;Formation of Dielectric Film&gt; 
     Next, the dielectric film  12  is formed. The dielectric film  12  is formed so as to cover the entire surface of the substrate  11  by a method such as sputtering, CVD, or electron beam deposition. This dielectric film  12  has an effect of protecting the vibration portion  19 , which includes the electrodes  14  and  18  and the piezoelectric thin film  16 , and may be formed using a nitride such as silicon nitride having superior passivation properties or an oxide such as silicon oxide. 
     In addition, for this dielectric film  12 , when a material having a temperature coefficient of frequency (TCF) opposite to that of a material used for the piezoelectric thin film  16  is used, the change in frequency with respect to the change in temperature of a resonator or a filter is decreased, and hence the properties are improved. For example, when zinc oxide or aluminum nitride is used for the piezoelectric thin film, silicon oxide having a TCF opposite to that thereof may be used. 
     In addition, aluminum nitride which is an insulating material and which has high thermal conductivity may also be used. 
     This dielectric film  12  is planarized in a subsequent step. As a material suitable for the planarization, an insulating film may be used, and as a preferable material, for example, an amorphous material such as silicon nitride, silicon oxide, or aluminum oxide may be used. 
     &lt;Planarization of Dielectric Film&gt; 
     Next, the dielectric film  12  is planarized. That is, the dielectric film  12  is planarized by dry etching. 
     As the dry etching, either ion etching or plasma etching may be performed. In the case of ion etching, an inert gas such as Ar or He is discharged by RF power, and sputtering etching is performed by self-bias, so that planarization may be performed. That is, by plasma (electrons and positive ions) generated by supplying an RF voltage to a substrate plate (electrically floating from a sputtering chamber) in the sputtering chamber, the substrate plate is biased to a negative potential with respect to a standard potential, so that sputtering etching is performed. 
     In the case in which the dielectric material is an oxide such as silicon oxide, an oxygen gas may be used. Positive ions are Ar +  ions when the gas is argon and are O + , O 2   +  (two-atom ion) and the like when the gas is oxygen. In addition, planarization may be performed by reactive ion etching using a chemically active gas such as a halogen compound. Since a plasma treatment is performed using an inert gas or a gas containing an element forming a dielectric film, the dielectric film  12  can be planarized without forming a compound layer thereon which is different from a dielectric material. 
     A preferable surface roughness (Ra) of the dielectric film  12  is 2.0 nm or less. This surface roughness (Ra) is called an arithmetic average roughness and is an average value obtained such that a standard length L is extracted from a roughness measurement curve in a direction of its average line, and the absolute values of deviations from the average line of this extracted portion to a measurement curve are summed and averaged. 
     As one example of plasma treatment conditions, when an SiO 2  film is processed at an RF power of 6 mW/mm 2  for a treatment time of 10 minutes using an O 2  gas, an initial surface roughness Ra of 2 to 10 nm is improved to a surface roughness Ra of 2 nm or less by the treatment. 
     SiO 2  is obtained as an amorphous material by common sputtering. Even when the same treatment as described above is performed for a ZnO film which is likely to be uniaxially oriented, the surface roughness is not so much improved. 
     &lt;Formation of Lower Electrode&gt; 
     Next, the lower electrode  14  is formed on the dielectric film  12  processed by the planarization treatment. The lower electrode  14  is formed by film formation using sputtering, plating, CVD, electron beam deposition or the like, followed by patterning using a photolithographic technique. The lower electrode  14  is primarily formed from a metal material, such as Mo, Pt, Al, Au, Cu, or Ti, to have a belt shape extending from the sacrifice layer  17  to one side of the substrate  11  (right side in the figure). Since the dielectric film  12 , which is a layer provided below the lower electrode  14 , is planarized, the lower electrode  14  can be formed to have a flat surface. A preferable surface roughness (Ra) of the lower electrode  14  is 2.0 nm or less. 
     &lt;Formation of Piezoelectric Thin Film&gt; 
     Next, the piezoelectric thin film  16  is formed on the lower electrode  14 . By film formation using sputtering or the like and by lift-off using patterning by a photolithographic technique, the piezoelectric thin film  16  is formed using zinc oxide, aluminum nitride, or the like. When aluminum nitride is used for forming the piezoelectric thin film  16 , by lift-off using zinc oxide, aluminum nitride is patterned. Since the dielectric film  12  of silicon oxide or the like is formed over the entire surface of the sacrifice layer  17  formed of zinc oxide, although zinc oxide used for lift-off is wet-etched when it is patterned or when aluminum nitride is processed by lift-off, zinc oxide used for forming the sacrifice layer  17  is not etched. 
     &lt;Formation of Upper Electrode&gt; 
     Next, the upper electrode  18  is formed. The upper electrode  18  is formed on the piezoelectric thin film  16  in a manner similar to that for the lower electrode  14 . The upper electrode  18  is formed to have a belt shape extending from the piezoelectric thin film  16  to the other side of the substrate  11  (left side in the figure). 
     &lt;Formation of Etching Hole&gt; 
     Next, etching holes, which are penetrating portions for exposing the sacrifice layer  17 , are formed. After a photoresist or the like is patterned by photolithography, by reactive ion etching (RIE), wet etching, or the like, parts of the dielectric film  12  provided on the sacrifice layer  17 , which are not covered with a photoresist pattern, are removed. A photoresist pattern shown in  FIG. 1(   b ) has a rectangular shape covering the lower electrode  14 , the piezoelectric thin film  16 , and the upper electrode  18 . The end portions  17   x  of the sacrifice layer  17  under the insulating film  12  are extended outside from the resist pattern. The photoresist pattern may have a cross-shape as shown in  FIG. 2 . For example, when silicon oxide is used for the dielectric film  12 , reactive ion etching is performed using a fluorinated gas such as CF 4 . Alternatively, wet etching may be performed using a hydrofluoric acid solution. After the etching, an etching mask such as a photoresist is removed using an organic solvent such as acetone. Dry etching using oxygen plasma may also be performed. 
     &lt;Formation of Airspace Layer&gt; 
     Next, the sacrifice layer  17  is etched, so that the airspace layer  13  is formed. After a photoresist or the like is patterned by photolithography, by reactive ion etching, wet etching, or the like, the sacrifice layer  17  is removed. For example, when zinc oxide is used for forming the sacrifice layer  17 , it is removed using an acidic solution containing hydrochloric acid, phosphoric acid, or the like. After the etching, an etching mask such as a photoresist is removed using an organic solvent such as acetone. When the sacrifice layer  17  is etched using a solution which does not etch the piezoelectric thin film  16 , the dielectric film  12 , and the electrodes  14  and  18 , a process including patterning by photolithography and removal of an etching mask can be omitted. For example, when aluminum nitride is used for the piezoelectric thin film  16 , silicon oxide is used for the dielectric film  12 , and Pt, Au, Ti, or the like is used for the electrodes  14  and  18 , zinc oxide forming the sacrifice layer  17  can be removed by a mixed aqueous solution composed, for example, of acetic acid and phosphoric acid without performing patterning. After the etching, replacement using a volatile solution such as pure water or IPA is sufficiently performed, followed by drying, so that the airspace layer  13  is formed. 
     When mass production of the piezoelectric thin film resonator  10  is performed, the piezoelectric thin film resonators  10  are simultaneously formed by the above manufacturing method using a wafer (mother substrate) as the substrate  11  and are then separated by dicing or the like, so that individual piezoelectric thin film resonators  10  are obtained. Alternatively, after a packaging substrate having lands is prepared, and the upper and the lower electrodes of the mother substrate are bonded to the lands by bump-bonding before individual piezoelectric thin film resonators are separated by cutting, the peripheries of the piezoelectric thin film resonators may be encapsulated for packaging. 
     The piezoelectric thin film resonator  10  thus described has the following operations and advantages. 
     (1) Since the dielectric film  12  used as an underlayer provided below the lower electrode  14  is planarized, the lower electrode  14  can be formed to be flat and to have a superior orientation property, and the piezoelectric thin film  16  having superior quality can be formed thereon; hence, the piezoelectric thin film resonator  10  having superior properties can be obtained. In addition, since the orientation property of the lower electrode  14  is improved, the piezoelectric thin film resonator  10  can be formed to have superior resistance against electric power. 
     (2) Since a dry process is used for planarization instead of CMP, the thickness of the dielectric film  12 , which is used as an underlayer provided below the lower electrode  14 , can be uniformly controlled on the order of nanometers in the substrate surface. Accordingly, since the variation in resonant frequency in the wafer surface and the resonant properties can be controlled with high accuracy, the yield can be increased, and hence the manufacturing cost can be reduced. 
     That is, the resonant frequency and the resonant properties of a resonator considerably depend on the thickness of its constituent film. When the thickness of the constituent film varies in a wafer surface, the yield may be decreased, and/or the number of steps such as frequency adjustment may be increased; hence, as a result, the manufacturing cost is increased. When a plurality of resonators is simultaneously formed using a wafer, the thickness of the constituent film of each resonator can be controlled on the order of nanometers by a plasma treatment, and hence the variation in resonant frequency in the wafer surface and the resonant properties can be controlled with high accuracy. As a result, the yield can be increased, and the manufacturing cost can be reduced. 
     (3) In the case in which zinc oxide is used both for the sacrifice layer  17  for forming the airspace layer  13  and a lift-off mask used when the piezoelectric thin film  16  made of aluminum nitride is formed, when the dielectric film  12  is formed over the entire surface of the sacrifice layer  17  when aluminum nitride is patterned, the shape of the sacrifice layer  17  is not damaged when the aluminum nitride is processed by lift-off. 
     (4) When structural films other than the sacrifice layer  17  have resistance against an etching solution for the sacrifice layer  17 , a patterning step for sacrifice-layer etching can be omitted, and as a result, because of process stabilization and decrease in number of steps, the cost can be reduced. 
     (5) After the sacrifice layer  17  is wet-etched, the etchant is sufficiently replaced with a volatile solution such as pure water or IPA. When the replacement is performed using a volatile solution, time required for a drying step following the removal of the sacrifice layer can be reduced, and hence the cost can be reduced. 
     The present invention is not limited to the above examples, and various modifications may be performed without departing from the scope of the present invention. The present invention may also be applied to a ladder type and a lattice type piezoelectric filter using a plurality of piezoelectric thin films.