Abstract:
Packaged film bulk acoustic resonators may be tuned after packaging by exposing them to irradiation. In one embodiment, transparent covers may be provided so that the film bulk acoustic resonator filter may be exposed to laser irradiation to either add or remove material from the filter and to thereby adjust its frequency.

Description:
BACKGROUND 
     This invention relates generally to radio frequency filters, including film bulk acoustic resonators (FBARs). 
     Film bulk acoustic resonators have many advantages over other techniques, such as surface acoustic wave (SAW) devices and ceramic filters, particularly at high frequencies. For example, SAW filters begin to experience excessive insertion losses above 2.4 gigahertz and ceramic filters are much larger in size and become increasingly difficult to fabricate at increased frequencies. 
     A conventional FEAR filter may include two sets of FBARs to achieve the desired filter response. The series FBARs have one resonant frequency and the shunt FBARs have another resonant frequency. The frequency of an FBAR is mainly determined by the thickness of its piezoelectric film, which approximately equals a half wavelength of the acoustic wave. The frequencies of FBARs need to be precisely set to achieve a desired filter response. 
     For example, for a 2 gigahertz FBAR, the thickness of the piezoelectric film may be approximately 2 microns. A one percent error or variation in piezoelectric film thickness may change the frequency of the filter by approximately 20 megahertz, which is not acceptable if a frequency precision or accuracy of 16 megahertz is required. 
     A variety of techniques are known for adjusting the frequency of an FBAR filter. However, the frequency of an FBAR filter may shift during or after packaging. 
     Therefore, there may be a need to tune the frequency of FBAR filters that were tuned prior to packaging, again after packaging. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged cross-sectional view of one embodiment of the present invention; 
     FIG. 2 is an enlarged cross-sectional view of another embodiment of the present invention; 
     FIG. 3 is an enlarged cross-sectional view of another embodiment of the present invention; and 
     FIG. 4 is an enlarged cross-sectional view of another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a film bulk acoustic resonator  10  may include a silicon substrate  26  having a cavity  33  formed therein. A piezoelectric film  32  may be formed over a lower electrode  24  and under an upper electrode  22 . That structure may then be enclosed beneath a transparent cover  12  and above a transparent cover  30 . Contacts  28  may be provided for electrically connecting to the upper and lower electrodes  22  and  24 . A sealing material  14  seals the cover  12  to the rest of the film bulk acoustic resonator  10 . 
     A tuning material  18  may be deposited on the inner surface of the upper transparent cover  12 . In one embodiment, the material  18  may be patterned as an array of dots. After the filter  10  has been fabricated with a frequency higher than the design or intended value, the frequency of the filter  10  can be tuned to the desired value by decomposing the tuning material  18 . In one embodiment, a laser (laser  1 ) may be utilized to expose all or part of the tuning material  14 , to decompose that material, and to cause it to be deposited on the upper electrode  22  through the cavity  20 . 
     The filter  10  frequency is then altered due to the mass loading effect or the change of the affected filter thickness. The tuning material may be a non-conductive material, such as A1 2 O 3 , to avoid any electrical shorting. 
     The size of the spot created on the material  18  by the laser may be smaller than the size of the tuning material  18  so that scanning is needed to cover the full device. Alternatively, the size of the spot may be larger, so only one shot is needed to finish the tuning of each filter  10 . 
     Referring next to FIG. 2, in accordance with another embodiment of the present invention, material may be removed from the upper electrode  22  to tune the filter after packaging. In this case, the filter  10   a  may be formed with a target frequency lower than the design value. Laser light is shined through the upper cover  12  and focused on the top surface of the filter electrode  22 . The filter  10   a  can be tuned to the desired value by removing part of, or thinning, the top electrode  22  of the filter  10   a . Alternatively, an additional layer  23  may be positioned on top of the electrode  22 , which then may be selectively removed. 
     The size of the laser spot on the electrode  22  may be smaller than the size of the electrode  22  so that scanning is needed to cover the entire electrode  22 . Alternatively, the size of the laser spot may be larger, so that only one shot is needed to finish tuning each filter  10   a.    
     Referring next to FIG. 3, an approach using both addition and removal of material for tuning is illustrated. In this case, two lasers, laser  1  and laser  2 , may be used so that both addition and removal of material can be applied. In this example, laser  1  is used for addition of material from the material  18  to the electrode  22 . The laser  2  is used for removal of material from the electrode  22 . 
     Two different frequencies of laser may be used for this purpose. Advantageously, in one embodiment, the tuning material  18  may be transparent to the light generated by the laser  2 , but is absorbed by the light generated by the laser  1 . Alternatively, the material  18  may be patterned or positioned to permit the laser  2  to pass without affecting the material  18  while the laser  1  is aimed to hit the patterned or positioned tuning material  18 . 
     Referring to FIG. 4, in accordance with another embodiment of the present invention, addition and removal may be accomplished from opposite sides of the package  10   c  using the transparent covers  12  and  30 . For example, material may be added, when a frequency reduction is needed, from the top side using the laser  1  to cause material to be added to electrode  22 . Material may be removed from the electrode  24 , when a frequency increase is needed, from the bottom side using laser  2 . In this way, gravity may be used to assist the tuning process. 
     While an embodiment is illustrated using laser irradiation, other forms of exposure may be used including energy exposure such as heat exposure and other types of irradiation, such as infrared irradiation, may be applied through the package to tune a resonator. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.