Patent Publication Number: US-2005140247-A1

Title: Film bulk acoustic wave resonator device and manufacturing method thereof

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a film bulk acoustic wave resonator (hereinafter, referred to as an FBAR), and more particularly to an FBAR device and a manufacturing method thereof, which can achieve ease of frequency adjustment, and minimize a risk in generation of poor products during a packaging process.  
      2. Description of the Related Art  
      According to the recent trend wherein mobile communication terminals have tended to become much leaner and enhanced and diversified in their quality and functions, techniques related with constituent components of the mobile communication terminals, for example radio frequency (RF) components, are rapidly being developed. Among the RF components, especially, an FBAR (film bulk acoustic wave resonator) is in the spotlight as an essential passive filter component of the mobile communication terminals by virtue of its advantages in that it has a lower insertion loss than other filters, and it can achieve a desired level of integration and miniaturization.  
      In general, an FBAR device is a thin film type device wherein a piezoelectric thin film layer made of ZnO or AlN is formed on a semiconductor substrate made of silicon or GaAs, resulting in a resonant frequency from the combination of a mechanical stress and a load produced at a surface of the piezoelectric thin film layer. The resonant frequency of such an FBAR device is determined by the total thickness of its resonance unit comprising upper and lower electrodes as well as the piezoelectric thin film layer. With the present technical level, however, it is substantially impossible to make respective FBAR devices in a wafer to have the same thickness as each other within a tolerance range of approximately 1 percent of the thickness. Moreover, the upper electrode made of metal tends to cause an oxidation phenomenon thereof, resulting in disadvantageous variation in the resonant frequency of the FBAR device. Such a frequency variation problem of the FBAR device, especially, may be increased during a packaging process due to oxidation.  
      The FBAR device, therefore, sincerely requires a solution for adjusting a resonant frequency thereof to have a constant value, and stabilizing the adjusted resonant frequency.  
      Considering one example of conventional solutions for adjusting the resonant frequency of the FBAR device, it adjusts the overall thickness of the FBAR device through etching or vapor deposition implemented on an upper metal layer, namely, an upper metal electrode of the FBAR device. This solution, however, still exhibits an oxidation problem of the upper metal electrode during a subsequent process.  
      In order to solve this oxidation problem, U.S. patent publication No. 2003-0098631 (achieved by an applicant named in AGILENT TECHNOLOGIES, INC) discloses an FBAR device wherein a thermal oxide film having a predetermined thickness is additionally formed by performing an intentional thermal oxidation process of its upper electrode made of molybdenum (Mo) in an atmosphere of oxygen. The obtained structure of such a FBAR device with the thermal oxide film is schematically shown in  FIGS. 1   a  and  1   b.    
      Referring to  FIGS. 1   a  and  1   b , the FBAR device of the type as discussed above, designated as reference number  10 , comprises a silicon substrate  11  formed at an upper surface thereof with a resonance unit. The resonance unit comprises a lower electrode  14 , a piezoelectric layer  15 , and an upper electrode  16 , which are successively stacked on an air gap (A) defined in the silicon substrate  11 . Said U.S. patent publication No. 2003-0098631 proposes a frequency adjustment solution using a thermal oxide film  18 , which is formed at an upper surface of the upper electrode  16  at a relatively low temperature (for example, approximately 200 to 300° C.) by means of a hot plate process or RTA (rapid thermal annealing) equipment. As a result of forming the thermal oxide film  18  having an appropriate thickness, the FBAR device  10  enables a resonant frequency thereof to be accurately adjusted from a present value to a desired target value. The thermal oxide film  18  further functions to restrict excessive oxidation of the upper electrode  16 , resulting in stabilization in the adjusted frequency.  
      The above described conventional FBAR device  10 , however, has a problem in that there is a limitation of the thickness of the thermal oxide film  18  obtainable through the thermal oxidation process, resulting in a considerable restriction in adjustable frequency range. In view of frequency stabilization, further, the presence of the thermal oxide film  18  only affects to delay an oxidation speed of the upper electrode made of metal during a subsequent process, and thus it is difficult to completely prevent an actual oxidation progress itself.  
      Furthermore, since the thermal oxide film  18  formed on the upper electrode  16  tends to be damaged in a subsequent process, especially, in the manufacture of a package accompanying with a photoresist process or slicing process, there may be a risk of an unintentional sudden frequency variation.  
      As can be seen from the above description related to the prior art, there has been required a new solution in the art for adjusting a resonant frequency up to a desired sufficient level as well as stably maintaining the adjusted resonant frequency even during a subsequent packaging process.  
     SUMMARY OF THE INVENTION  
      Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an FBAR device, which can achieve appropriate adjustment in a resonant frequency thereof and stable protection of its resonance unit during a subsequent process. This objective is accomplished by virtue of a passivation layer formed substantially throughout the resonance unit, rather than being formed only on an upper electrode of the resonance unit.  
      It is another object of the present invention to provide an FBAR device, and a method of manufacturing an FBAR device package, which shows additional advantages in relation to the formation of a cap structure in a chip scale packaging or wafer level packaging process.  
      In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a film bulk acoustic wave resonator (FBAR) device comprising: a substrate; a resonance unit including a lower electrode, a piezoelectric film, and an upper electrode, which are successively stacked on the substrate; and a passivation layer formed substantially throughout an upper surface and a peripheral surface of the resonance unit in order to protect the resonance unit, wherein a partial region of the passivation layer located on at least the upper electrode has a thickness required to compensate for a difference between a resonant frequency of the resonance unit and a desired target resonant frequency.  
      Preferably, the passivation layer may be made of an oxide or nitride composed of elements selected from among the group consisting of Si, Zr, Ta, Ti, Hf, and Al, and more preferably, the passivation layer may be made of a material selected from among the group consisting of SiO 2 , Si 3 N 4 , HfO, Al 2 O 3 , AlN and AlNO x . The passivation layer may be formed by sputtering, evaporation, or chemical vapor deposition (CVD).  
      Preferably, the FBAR device in accordance with an embodiment of the present invention may further comprise connection pads formed on the substrate so that they are connected to the upper and lower electrodes, respectively, and the connection pads may be made of Au.  
      In general, the FBAR device is basically classified into an air gap manner device and a bragg reflection manner device according to an insulation structure between the substrate and a resonance unit, and the present invention can be effectively applied into both the devices. Therefore, the substrate may include an air gap formed at a region where the resonance unit is formed thereabove. Alternatively, the substrate may have a reflective film structure obtained through bragg reflection.  
      In accordance with another aspect of the present invention, the above and other objects can be accomplished by the provision of a method of manufacturing an FBAR device comprising the steps of: a) preparing a substrate; b) forming a resonance unit by successively stacking a lower electrode, a piezoelectric film, and an upper electrode on the substrate; c) calculating a thickness of the resonance unit required to compensate for a difference between a resonant frequency of the resonance unit and a desired target resonant frequency; and d) forming a passivation layer substantially throughout an upper surface and a peripheral surface of the resonance unit for protecting the resonance unit so that a partial region of the passivation layer located on at least the upper electrode has the calculated thickness.  
      Preferably, before the step d), the method of the present invention further comprises the step of: e) forming connection pads on the substrate so that they are connected to the upper and lower electrodes, respectively.  
      Preferably, the step d) may include the steps of: d-1) forming the passivation layer on the substrate above the resonance unit so that the partial region of the passivation layer located on at least the upper electrode has the calculated thickness; and d-2) selectively removing the passivation layer so that partial regions of the connection pads to be bonded to an exterior circuit are exposed to the outside.  
      Preferably, the step a) may include the steps of: a-1) forming a sacrificial material region at the substrate, the sacrificial material region being for use in the formation of an air gap; and a-2) forming an insulation layer on the sacrificial material region, and the method of the present invention may further comprise the steps of: f) selectively removing the insulation layer, so as to form a via hole communicating with the sacrificial material region; and g) removing the sacrificial material region through the via hole, so as to form the air gap.  
      Preferably, the step d-2) and the step f) may be simultaneously performed through a single process using a photoresist film, the sacrificial material region may be made of a polysilicon material, the step g) may be an etching step of the sacrificial material region using XeF 2 . Advantageously, in the step g), the passivation layer may protect the upper electrode.  
      In accordance with yet another aspect of the present invention, the above and other objects can be accomplished by the provision of a method of manufacturing an FBAR device package comprising the steps of: a) preparing a substrate; b) forming a resonance unit by successively stacking a lower electrode, a piezoelectric film, and an upper electrode on the substrate; c) calculating a thickness of the resonance unit required to compensate for a difference between a resonant frequency of the resonance unit and a desired target resonant frequency; d) forming a passivation layer substantially throughout an upper surface and peripheral surface of the resonance unit for protecting the resonance unit so that a partial region of the passivation layer located on at least the upper electrode has the calculated thickness; and e) forming a cap structure so as to seal the resonance unit formed with the passivation layer.  
      In the above package manufacturing method in accordance with the present invention, various kinds of cap structures can be employed. When the cap structure is formed by making use of dry films, the step e) may include the steps of: e-1) forming a side wall structure surrounding the resonance unit by applying a first dry film; and e-2) forming a roof structure on the side wall structure by applying a second dry film thereon. Preferably, after the step e-1) and before the step e-2), the method of the present invention may further comprise the step of: f) removing the sacrificial material region for the formation of the air gap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
       FIGS. 1   a  and  1   b  are a side sectional view and a plan view, respectively, illustrating a conventional FBAR device;  
       FIGS. 2   a  and  2   b  are a side sectional view and a plan view, respectively, illustrating an FBAR device in accordance with an embodiment of the present invention;  
       FIGS. 3   a  to  3   h  are side sectional views, respectively, illustrating the sequential steps of manufacturing an FBAR device in accordance with the present invention; and  
       FIGS. 4   a  to  4   e  are sectional views, respectively, illustrating the sequential steps of manufacturing an FBAR device package in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Now, the present invention will be described, with reference to the accompanying drawings.  
       FIGS. 2   a  and  2   b  are a side sectional view and a plan view, respectively, illustrating an FBAR device in accordance with an embodiment of the present invention.  
      Referring to  FIG. 2   b  along with  FIG. 2   a , the FBAR device of the present invention, designated as reference numeral  20 , comprises a substrate  21  formed at an upper surface thereof with a resonance unit. The resonance unit comprises a lower electrode  24 , a piezoelectric layer  25 , and an upper electrode  26 , which are successively stacked on the substrate  21  so that they are positioned above an air gap (A) defined in the substrate  21 . Mainly, the substrate  21  may be a silicon substrate, the upper and lower electrodes  26  and  24  may be made of molybdenum (Mo), and the piezoelectric layer  25  may be made of aluminum nitride (AlN), but are not limited thereto.  
      The FBAR device  20  in accordance with the present invention further comprises a passivation layer  29 . Preferably, as shown in  FIGS. 2   a  and  2   b , the passivation layer  29  is formed substantially throughout an upper surface and peripheral surface of the resonance unit, while remaining certain partial regions of the upper and lower electrodes  26  and  24  where connectors  26   a  and  24   a  will be formed.  
      The passivation layer  29  enables easy and effective adjustment in a resonant frequency of the FBAR device  20 . That is, as shown in  FIG. 2   a , the passivation layer  29  is formed at the upper surface of the resonance unit, namely, at an upper surface of the upper electrode  26  in such a manner that it increases a thickness of a certain region corresponding to an actual resonance region. Such a thickness increase in the actual resonance region enables easy adjustment in the resonant frequency. In the present invention, therefore, the resonant frequency of the FBAR device can be realized in such a manner that, after completion of the resonance unit, a resonant frequency produced by the resonance unit is measured, and then the passivation layer is formed into a thickness suitable for compensating for a difference between the measured resonant frequency and a desired target frequency.  
      The passivation layer  29  further enables stable maintenance a resulting adjusted resonant frequency in view of frequency stabilization. This is accomplished since the passivation layer  29  has substantially no risk of its thickness change or damage due to oxidation during a subsequent process. The passivation layer  29  furthermore functions to safely protect the resonance unit during the manufacture of an FBAR device package accompanying a photoresist process or slicing process.  
      The passivation layer  29  may be made of an oxide or nitride composed of elements selected from among the group consisting of Si, Zr, Ta, Ti, Hf, and Al. More preferably, the passivation layer  29  may be made of a material selected from among the group consisting of SiO 2 , Si 3 N 4 , HfO, Al 2 O 3 , AlN and AlNO x . Differently from a conventional thermal oxide film, the passivation layer  29  can be sufficiently grown up to a desired thickness, and has an easy formation process.  
      Several other advantages and effects of the present invention will be understood by reading a follow description related to an FBAR device manufacturing method and an FBAR device package manufacturing method in accordance with the present invention. According to the present invention, especially, a formation process of the passivation layer  29  provides several advantages as it is usefully combined with an air gap formation process using a sacrificial material region and a package manufacturing method.  
       FIGS. 3   a  to  3   h  are side sectional views, respectively, illustrating the sequential steps of manufacturing an FBAR device in accordance with the present invention.  
      As shown in  FIG. 3   a , first, a silicon substrate  31  of the FBAR device is prepared so that a cavity (C) is formed at an upper surface thereof. The cavity (C) is for the formation of an air gap serving as isolation means between the substrate  31  and a resonance unit to be formed in a subsequent process.  
      Then, as shown in  FIG. 3   b , the cavity (C) of the silicon substrate  31  is filled with a sacrificial material, thereby forming a sacrificial material region  33 . The sacrificial material may be a polysilicon material. Meanwhile, before the formation of the sacrificial material region  33 , a first insulation layer  32   a  is formed throughout an inner surface of the cavity (C) defined in the silicon substrate  31 , in order to protect the silicon substrate  31  from an etching process for the formation of an air gap. Similarly, after the formation of the sacrificial material region  33 , the silicon substrate  31  is formed at the upper surface thereof with a second insulation layer  32   b . The second insulation layer  32   b  serves to prevent the etching of a lower electrode, which is designated as reference numeral  34  in  FIG. 3   c.    
      Referring to  FIG. 3   c , on a portion of the insulation layer  32   b  located above the sacrificial material region  33  are successively stacked the lower electrode  34 , a piezoelectric film  35 , and an upper electrode  36 , resulting in a resonance unit. The upper and lower electrodes  36  and  34  and piezoelectric film  35  can be formed by repeating respective film growth processes and etching processes. As a result of the appropriate etching processes, a via hole h 1  is formed to vertically penetrate through the piezoelectric film  35  as shown in  FIG. 3   c . Such a via hole h 1  is for use in an etching process of the sacrificial material region  33 .  
      After completing the formation of the resonance unit, as shown in  FIG. 3   d , connection pads  37  and  38  are formed on the silicon substrate  31  so that they are connected to the upper and lower electrodes  36  and  34 . The connection pads  37  and  38  may be made of Au. In a subsequent process, the connection pads  37  and  38  serve as connectors to be connected with an exterior circuit. The connection pad  37  made of Au, especially, serves to connect the upper electrode  36  to a certain region of the silicon substrate  31  to be connected to an exterior circuit.  
      In succession, as shown in  FIG. 3   e , a passivation layer  39  is formed to cover all of the above enumerated components including the upper and lower electrodes  36  and  34 , piezoelectric film  35 , and connection pads  37  and  38 . The passivation layer  39  may be made of oxide or nitride composed of elements selected from among the group consisting of Si, Zr, Ta, Ti, Hf, and Al. More preferably, the passivation layer  39  may be made of a material selected from among the group consisting of SiO 2 , Si 3 N 4 , HfO, Al 2 O 3 , AlN and AlO x . The passivation layer  39  may be formed through a conventional film growth process such as sputtering, evaporation, or chemical vapor deposition (CVD). In this case, a portion of the passivation layer  39  provided on the upper electrode  36  is a portion for use in the adjustment of a resonant frequency, and has a thickness suitable for adjusting the previously measured resonant frequency of the resonance unit to a desired target frequency.  
      Next, as shown in  FIG. 3   f , a photoresist film  40  is applied onto the passivation layer  39 , and is patterned so that partial regions of the passivation layer  39  located on the connectors to be bonded to an exterior circuit and a portion of the second insulation layer  32   b  communicating with the via hole h 1  are exposed to the outside.  
      Then, as shown in  FIG. 3   g , an etching process of the passivation layer  39  using the patterned photoresist film  40  is performed. Through this etching process, partial regions of the connection pads  37  and  38  to be bonded to the exterior circuit are exposed to the outside, and the portion of the second insulation layer  32   b  is selectively removed, thereby producing a via hole h 2  communicating with the sacrificial material region  33 .  
      Finally, as shown in  FIG. 3   h , the photoresist film  40  is removed and then the sacrificial material region  33  is removed, resulting in an air gap (A). The sacrificial material region  33  may be made of a polysilicon material, and in this case, the sacrificial material region  33  can be removed by using XeF 2 . The use of XeF 2  does not affect the connection pads  37  and  38  made of Au, but may affect the upper and lower electrodes  36  and  34  made of Mo as an etchant. In order to eliminate such an unintentional influences, according to the present invention, the passivation layer  39  can act as a protecting layer for the upper electrode  36 , and the like.  
      Although  FIGS. 3   a  to  3   h  illustrate an embodiment wherein an air gap (A) is formed through the formation of the cavity C, those skilled in the art will appreciate that the present invention can be similarly applied to another embodiment wherein an air gap is formed by constructing a separate membrane structure on a substrate. Furthermore, the present invention is applicable to a certain manner wherein a substrate is structured by using a bragg reflection method for allowing the substrate to function as isolation means between the substrate and a resonance unit to be formed thereon.  
       FIGS. 4   a  to  4   e  are sectional views illustrating the sequential steps of manufacturing an FBAR device package in accordance with the present invention. The FBAR device package manufacturing method of the present invention basically comprises the above described FBAR device manufacturing method. That is,  FIGS. 4   a  to  4   e  are sectional views illustrating a method of manufacturing a cap structure for use in the formation of a package in a state wherein the FBAR device is previously manufactured. The cap structure employed in the present embodiment is a cap structure using a dry film, but the present invention is not limited thereto.  
      Referring to  FIG. 4   a , first, it shows the same state as  FIG. 3   g , except that the photoresist film  40  is removed from the FBAR device. That is, a silicon substrate  41  of the FBAR device is formed with a cavity, and inside the cavity are formed a first insulation layer  42   a  for the protection of the substrate  41 , and a sacrificial material region  43 . Then, after a second insulation layer  42   b  for the protection of a lower electrode  44  is formed at an upper surface of the silicon substrate  41 , the lower electrode  44 , a piezoelectric film  45 , and an upper electrode  46  are successively stacked on the silicon substrate  41  above the sacrificial material region  43 , thereby forming a resonance unit. In succession, a pair of connection pads  48  and  47  are formed so that they are connected to the upper and lower electrodes  46  and  44 , respectively, and a passivation layer  49  is formed so that it covers all of the constituent components as stated above. After that, through a photoresist process as shown in  FIGS. 3   f  and  3   g , certain partial regions of the passivation layer  49  is selectively etched so that partial portions of the connection pads  47  and  48  to be bonded to an exterior circuit and a portion of the sacrificial material region  43  are exposed to the outside. Finally, as the photoresist film  40  is removed, the FBAR device shown in  FIG. 4   a  can be completed.  
      In a state wherein the FBAR device is prepared as stated above, referring to  FIG. 4   b , a side wall structure  51  is formed by using a dry film so that it surrounds the resonance unit. This process is performed in such a manner that, after the dry film is applied throughout an upper surface of the FBAR device, it is selectively removed. In the present embodiment, during the formation of the side wall structure  51 , the sacrificial material region  43  is not removed. It ensures increased structural stability of the FBAR device compared to a case of previously forming an air gap, even when the dry film is formed throughout the upper surface of the FBAR device for the formation of the side wall structure  51 . Meanwhile, when the dry film is selectively removed to complete the side wall structure  51 , the passivation layer  49  serves to protect the resonance unit including the upper electrode  46 .  
      After forming the side wall structure  51 , as shown in  FIG. 4C , the sacrificial material layer  43  is removed, resulting in an air gap (A). During this etching process, although the photoresist film  40  (referring to  FIG. 3   g ) must be previously removed for the formation of the side wall structure  51 , the upper electrode  46  can be appropriately protected by the passivation layer  49 .  
      In succession, as shown in  FIG. 4   d , another dry film is applied onto the side wall structure  51 , so as to form a roof structure  52 . In this way, an FBAR device package  60  using dry films can be completed. The FBAR device package  60  is connectable to an exterior circuit in a wire bonding manner by making use of the connection pads  47  and  48 , which are exposed out of a resulting cap structure  50 , as shown in  FIG. 4   e . Although the present embodiment illustrates a wire bonding structure, the present invention is not limited thereto, and the wire bonding structure may be substituted with a flip chip bonding structure.  
      In the present embodiment, although an example of the formation of the cap structure using dry films is explained, the FBAR device package of the present invention may be embodied to a wafer level package using a cap wafer, which is made of a material similar to that of a device wafer.  
      As apparent from the above description, the present invention provides an FBAR device which is configured so that a passivation layer is formed to completely cover a resonance unit including upper and lower electrode and a piezoelectric layer, thereby enabling appropriate easy adjustment of a resonant frequency of the resonance unit and protecting the resonance unit from unintentional influences of subsequent processes.  
      Further, according to the present invention, through the formation of the passivation layer, during an air gap formation process and a cap structure formation process included in a chip scale packaging or wafer level packaging, the resonance unit of the FBAR device can be safely protected, resulting in stable maintenance of the appropriately adjusted resonant frequency thereof, and considerable enhancement in reliability of the FBAR device.  
      Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.