Patent Publication Number: US-11387782-B2

Title: Stacked-die bulk acoustic wave oscillator package

Description:
This application is a Continuation of application Ser. No. 16/747,679 filed Jan. 21, 2020, which is a Continuation of application Ser. No. 15/968,435 filed May 1, 2018, now U.S. Pat. No. 10,574,184. 
    
    
     FIELD 
     This Disclosure relates to packaged bulk acoustic wave oscillator devices. 
     BACKGROUND 
     Bulk acoustic wave (BAW) devices use the piezoelectric effect to convert electrical energy into mechanical energy resulting from an applied radio frequency (RF) voltage. BAW devices generally operate at their mechanical resonant frequency which is defined as that frequency for which the half wavelength of sound waves propagating in the device is equal to the total piezoelectric layer thickness for a given velocity of sound in the piezoelectric material. BAW resonators operating in the GHz range (e.g., at about 2 GHz) generally have physical dimensions of tens of microns in diameter with thicknesses of a few microns. 
     For functionality the piezoelectric layer of the BAW device is acoustically isolated from the substrate. There are two conventional device structures for acoustic isolation. The first is referred to as a Thin Film Bulk Acoustic Resonator (FBAR) device. In a FBAR device the acoustic isolation of the piezoelectric layer is achieved by removing the substrate or an appropriate sacrificial layer from beneath the electroded piezoelectric resonating component to provide an air gap cavity. 
     The second known device structure for providing acoustic isolation is referred to as a Solidly Mounted Resonator (SMR) device. In a SMR device the acoustic isolation is achieved by having the piezoelectric resonator on top of a highly efficient acoustic Bragg reflector that is on the substrate. The acoustic Bragg reflector includes a plurality of layers with alternating high and low acoustic impedance layers. The thickness of each of these layers is fixed to be one quarter wavelength of the resonant frequency. A variant of the SMR device adds a second Bragg mirror on the top of the piezoelectric resonator of BAW resonator. A conventional BAW oscillator leadframe package comprises a BAW die side-by-side with an oscillator circuit die that have bond pads which are coupled die-to-die by bond wires. Gold (Au) bond wires can be used for this die-to-die coupling. 
     SUMMARY 
     This Summary is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the Detailed Description including the drawings provided. This Summary is not intended to limit the claimed subject matter&#39;s scope. 
     Disclosed aspects recognize for a conventional BAW oscillator package with a BAW resonator die and oscillator circuit die side-by-side that are coupled together by bond wires, the bond wires generally add significant parasitic capacitance, and moreover a variation in this capacitance can degrade the performance of the BAW oscillator package. Reducing this parasitic capacitance by eliminating the bond wires while providing good stress isolation for the BAW resonator die can improve the overall BAW oscillator package performance by improving the performance of the BAW resonator die. Assembly manufacturing tolerances are also generally improved by eliminating bond wires for disclosed BAW die-to-oscillator circuit die coupling. 
     This Disclosure includes a stacked-die BAW oscillator package with bump coupling between a top BAW resonator die that is flip chip attached to a larger area bottom oscillator circuit die which replaces the conventional bond wires coupling the BAW resonator die to the oscillator circuit die. Disclosed aspects include a stacked-die oscillator package including an oscillator circuit die having inner bond pads and outer bond pads, and a BAW resonator die having a piezoelectric transducer thereon having a first and a second BAW bond pad on a same side of the BAW resonator die coupled to a top and bottom electrode layer, that are across a piezoelectric layer. A first metal bump is on the first BAW bond pad, and a second metal bump is on the second BAW bond pad, which are flip chip bonded to the inner bond pads of the oscillator circuit die. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, wherein: 
         FIG. 1A  is a cross sectional view of an example stacked-die BAW oscillator leadframe package with bumps showing bump coupling between bond pads on the BAW die and inner bond pads on the oscillator circuit die. 
         FIG. 1B  shows the bumps on the same side of the BAW resonator die. 
         FIG. 1C  shows the inner and outer bond pads on the oscillator circuit die. 
         FIG. 2A  is a cross sectional depiction that shows a layer stack for a BAW resonator die comprising a SMR device. 
         FIG. 2B  is a cross sectional depiction that shows a layer stack of a BAW resonator die comprising a dual-Bragg mirror. 
         FIG. 3A  is a cross sectional view of a printed circuit board (PCB) assembly comprising an example stacked-die BAW oscillator as a Wafer Chip Scale Package (WCSP) with bump coupling between bond pads on the BAW die and inner bond pads on the oscillator circuit die, where the WCSP is assembled onto land pads on the surface a PCB. 
         FIG. 3B  is a view from underneath the WCSP shown in  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments are described with reference to the drawings, wherein like reference numerals are used to designate similar or equivalent elements. Illustrated ordering of acts or events should not be considered as limiting, as some acts or events may occur in different order and/or concurrently with other acts or events. Furthermore, some illustrated acts or events may not be required to implement a methodology in accordance with this Disclosure. 
       FIG. 1A  is a cross sectional view of an example stacked-die BAW oscillator leadframe package  100  that includes an oscillator circuit die  130  on a leadframe and a BAW resonator die  120  flip chip bonded to the oscillator circuit die  130 , shown with a BAW bond pad  120   a  bonded by bump  111   a  to an inner bond pad  130   a  of the oscillator circuit die  130 . As compared to a conventional wire bonded die-to-die arrangement, the flip chip connection utilized by the stacked-die oscillator leadframe package  100  provides shorter electrical routing between the BAW resonator die  120  and oscillator circuit die  130  which reduces the parasitic capacitance. Disclosed bump connections eliminate the generally difficult conventional die-to-die wire bonding process while providing low capacitance interconnections that are more consistent from stacked-die oscillator package to package. 
     The leadframe includes a die pad  105  and a plurality of lead fingers shown as  106 ,  107 .  FIG. 1B  shows the bumps  111   a ,  111   b  on BAW bond pads  120   a ,  120   b  that are both on the same side of the BAW resonator die  120 , while  FIG. 1C  shows the inner bond pads  130   a ,  130   b  and outer bond pads  130   c ,  130   d , as well other outer bond pads (un-numbered) on the oscillator circuit die  130 . 
     Besides the first metal bump  111   a  shown in  FIG. 1A  on the first BAW bond pad  120   a  there is the second metal bump  111   b  shown in  FIG. 1B  on the second BAW bond pad  120   b , that are both flip chip bonded to the inner bond pads  130   a ,  130   b  of the oscillator circuit die  130 . Disclosed stacked-die oscillator package  100  maintains good stress isolation by having the BAW bond pads  120   a ,  120   b  and thus all the bumps  111   a ,  111   b  on one side of the BAW resonator die  120  to form a diving board type (linear or cantilever-like) bump configuration. 
     The bumps  111   a  and  111   b  can comprise a copper (Cu) post with a different metal cap thereon. A typical solder bump material is a Cu post with a Ni—Pd cap or a AgSn solder material cap. The bumps  111   a  and  111   b  can also comprises Au bumps. The outer edges of the bumps  111   a  and  111   b  are generally spaced apart by a minimum gap of 30 μm to provide a sufficient minimum clearance to help prevent shorts between the bumps. 
     The BAW resonator die  120  has a piezoelectric transducer  220  with a first and second BAW bond pad  120   a ,  120   b  on a same side of the die coupled to a top and bottom electrode layer across a piezoelectric layer.  FIG. 2A  described in more detail below shows a piezoelectric transducer  220  including a top electrode layer  224  and bottom electrode layer  221  across a piezoelectric layer  222 . Having all the BAW die connections to the oscillator circuit die  130  on one side of the BAW resonator die  120  forms a pivot point with a pivot on the bumps  111   a  and  111   b.    
     A low elastic modulus material  133  that generally comprises a polymer is shown in  FIG. 1A  that helps prevent package stress transferring into the BAW resonator die  120 . Being on one side a cantilever-like structure formed, with only two semi-flexible points of attachment being the bumps, surrounded with a low elastic modulus material  133  such as silicone or an epoxy, helps protect the BAW die  120  from external stresses. For example, stress from the mold compound  135  itself or from an external force can transfer stress to the BAW die  120  which can degrade its performance. Having the bump connections all on one side also helps to prevent coupling stresses from the oscillator circuit die  130  into the BAW die  120 . Although not shown, the BAW die  120  can have more than 2 bond pads each with bumps thereon, such as to add a ground connection to the substrate of the BAW die  120 , or to add bond pads and bumps for implementing an on-chip temperature sensor. 
     If the oscillator circuit die  130  on the bottom of the BAW oscillator package  100  bends for instance, this stress can end up moving the BAW resonator die  120  slightly in the low elastic modulus material  133  versus conventional bending the BAW die and thus inducing stress on the films within the BAW resonator die  120 . The outer bond pads  130   c ,  130   d  are shown wire bound by bond wire  143  and  144  to lead fingers  106  and  107  of the leadframe, respectively. The oscillator circuit die  130  is attached to the die pad  105  by a die attach material  114 , such as a conventional epoxy. 
     A polymer material  132  or a low elastic modulus material  133  that as described above is generally also a polymer is in a portion of a gap between the BAW resonator die  120  and oscillator circuit die  130  that functions as a stand-off on the side of the BAW die  120  opposite the bumps. Such a stand-off structure helps with planarity during BAW resonator die  120  flip chip attach. The polymer material  132  or a low elastic modulus material  133  when functioning as a standoff as opposed to a single feature can optionally be in the form of a plurality of stripes. The polymer material  132  or a low elastic modulus material  133  can be formed on the BAW die, formed on the oscillator circuit die  130 , or be formed on both of these die, generally to a thickness of about 10 μm. The bump is generally about 25 micron high and 30 μm in diameter, so that the bumps are generally taller as compared to a thickness of the polymer material  132 , which is shown in  FIG. 1A . 
     The polymer material  132  can comprise a polyimide. The polymer material  132  is also shown in other regions in  FIG. 1A  shown as regions  132   a  and  132   b  forming a surrounding dam for controlling possible bleed-out during the dispensing of a glob of the low elastic modulus material  133  described below which can help with die planarity during bumping. Polymers for the polymer material  132  can comprise polymers other than PI such as SU8 which comprises an epoxy-based material (conventionally used as a negative photoresist) comprising a Bisphenol A Novolac epoxy that is dissolved in an organic solvent (gamma-butyrolactone (GBL) or cyclopentanone, depending on the formulation) with up to 10 wt. % of mixed Triarylsulfonium/hexafluoroantimonate salt as the photoacid generator. 
     A mold compound  135  encapsulates the stacked-die oscillator package  100 , and a low elastic modulus material  133  (which can be the same material describe above that is in a portion of a gap between the BAW resonator die  120  and oscillator circuit die  130  that functions as a stand-off) is over the BAW resonator die  120  for encapsulating the BAW resonator die  120 , which can also filling any gaps under the BAW resonator die  120 . The low elastic modulus material  133 , such as silicone rubber, over the BAW die  120  helps isolate stress from the BAW resonator die  120 . For example, stress from the mold compound  135  or external forces can transfer stress to the BAW resonator die  120 . As described above, having the all bump connections on one side of the BAW resonator die  120  helps to prevent coupling stresses from the oscillator circuit die  130  with the oscillator into the BAW resonator die  120 . If the oscillator circuit die  130  bends for instance, the stress would end up moving the BAW resonator die  120  slightly in the low elastic modulus material  133  versus bending the BAW resonator die  120  that induces stress on the films within the BAW resonator die  120 . 
     As known in physics, an elastic modulus (or Young&#39;s Modulus) is defined as the ratio of longitudinal stress to longitudinal strain. Rubber-like behavior corresponds to a low elastic modulus value of about 10 6  N/m 2  (1 MPa) to 10 7  N/m 2  (10 MPa). A low elastic modulus material  133  as defined herein is a material that has a 25° C. elastic modulus of &lt;50 MPa. Silicone rubber has siloxane bonds (—Si—O—Si), and has a Young&#39;s modulus at 25° C. of about 10 to 20 MPa. The low elastic modulus material  133  can be selected to have an elastic modulus of &lt;10 MPa, such as 2 MPa to 10 MPa. 
       FIG. 2A  is a cross sectional depiction that shows a layer stack for a BAW resonator  200  comprising a SMR device. BAW resonator  200  includes a substrate  205  (e.g., silicon) having a top side surface  205   a  and a bottom side surface  205   b . A Bragg mirror  210  is on the top side surface  205   a  of the substrate. Bragg mirror  210  comprises a plurality of layers with alternating high and low acoustic impedance layers, with the relatively high acoustic impedance layers shown as  212 ,  214  and  216 , alternating with the relatively low acoustic impedance layers  211 ,  213 ,  215  and  217 . The thickness of each of these layers  211 - 217  is fixed to be about one quarter wavelength of the desired resonant frequency. 
     The piezoelectric transducer  220  shown includes a bottom electrode layer  221  that is on layer  217  of the Bragg mirror  210 , a piezoelectric layer  222  on the bottom electrode layer  221 , a dielectric layer  223  on the piezoelectric layer  222 , and a top electrode layer  224  on the dielectric layer. The dielectric layer  223  above the piezoelectric transducer  220  helps to reduce the temperature coefficient of frequency for BAW resonator  200 . Although not shown, BAW resonator  200  is generally in a hermetically sealed cavity to protect its top surface. 
       FIG. 2B  is a cross sectional depiction that shows a layer stack of a BAW resonator die  250  comprising a dual-Bragg mirror including both a bottom Bragg mirror  210 , and also top Bragg mirror  240 . The top Bragg mirror  240  being on top of the BAW resonator  200  shown in  FIG. 2A  results in a BAW resonator  250  becoming essentially resistant to frequency shifts caused by the deposition on contaminants on top of the piezoelectric transducer  220 . Analogous to bottom Bragg mirror  210 , the top Bragg mirror  240  comprises a plurality of layers with alternating high and low acoustic impedance layers, with the relatively high acoustic impedance layers shown as  242 ,  244  and  246 , alternating with the relatively low acoustic impedance layers  241 ,  243 ,  245  and  247 . The thickness of each of these layers  241 - 247  is fixed to be about one quarter wavelength of the desired resonant frequency. 
     As described above bumps (bumps  111   a ,  111   b  in  FIG. 1B ) connect the bond pads on the BAW resonator die  120  to inner bond pads  130   a ,  130   b  on the oscillator circuit die  130 . The signals that are sent through from the oscillator circuit die  130  travel through metal interconnect on the oscillator circuit die  130  through the inner bond pads  130   a ,  130   b  on the oscillator circuit die  130  through the bumps, then through the bond pads on the BAW resonator die  120  and to its resonator through the electrode metal (in layer  221 ,  224  in  FIG. 2A ) on the BAW resonator die  120 . 
       FIG. 3A  is a cross sectional view of a PCB assembly  300  comprising an example stacked-die BAW oscillator Wafer Chip Scale Package (WCSP)  310  assembled onto a PCB. The WSCP  310  has bump coupling with a bump  111   a  shown between a bond pad  120   a  on the BAW die  120  and an inner bond pad  130   a  on the oscillator circuit die  130 . The stacked die  120 / 130  is assembled by bumps  315  (typically solder balls) that couple the outer bond pads  130   c ,  130   d  of the oscillator die  130  onto land pads  322  (e.g., Solder Mask Defined (SMD) pads) on the surface of a printed circuit board (PCB)  320 . No bond wires or interposer connections are needed by WCSP  310 . A low elastic modulus material  133  is shown in a gap between the BAW resonator die  120  and the oscillator circuit die  130  on the side opposite the bumps shown as bump  111   a . Although shown flush to the BAW die  120  edges, the low elastic modulus material  133  will generally extend beyond the edges of the BAW die  120 , but can also be recessed relative to these edges. 
       FIG. 3B  is a view from underneath the example stacked-die BAW oscillator WCSP  310  shown in  FIG. 3A . In one specific example, the oscillator circuit die  130  is 1250 μm by 1500 μm in area, and the BAW die  120  is 550 μm by 878 μm in area, and has a thickness of 150 μm. 
     Disclosed stacked-die oscillator packages with bump coupling between the BAW resonator die  120  and oscillator circuit die  130  solve the problem for BAW technology needing good stress isolation and lower parasitic capacitance to provide improved stacked-die oscillator package performance. The disclosed bump die attach connections shorten the connection length between the BAW die  120  and the oscillator circuit die  130 , and also reduce the parasitic resistance, as well as the parasitic resistance. 
     Performance parameters for stacked-die oscillator packages include a series resistance resonance frequency (Fs), an anti-resonance or parallel resistance resonance frequency (Fp), and K 2 eff % value which is defined by the difference between Fs and Fp. The performance improvement provided by disclosed stacked-die oscillator packages comprises a reduced frequency shift due to less parasitic capacitance resulting from disclosed bump connections between the BAW die  120  and the oscillator circuit die  130  as compared to conventional wire bond connections for known side-by-side oscillator packages that are known to shift the frequency. A higher relative K 2 eff % value is also provided by reduced parasitic capacitance that is known to pull the Fs in closer to Fp. There is also less capacitive loading which results in a wider frequency oscillation window because of the reduced parasitic capacitance. Other advantages of disclosed stacked-die oscillator packages include a lower resistive path for connection of BAW die  120  to oscillator circuit die  130 . Shortened connections should also provide a lower risk for capacitive coupling of external noise which can couple in high frequency signals. 
     Those skilled in the art to which this Disclosure relates will appreciate that many other embodiments and variations of embodiments are possible within the scope of the claimed invention, and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of this Disclosure.