Abstract:
An acoustic reflector ( 48 ) is applied over a thin-film piezoelectric resonator ( 41, 61 ) which is supported on a semiconductor or semiconductor-compatible substrate ( 42, 62 ) of a microelectronic device ( 40, 60 ), enabling an encapsulant ( 49 ) to be applied over the reflector-covered resonator without acoustically damping the resonator. In one embodiment, alternating high and low acoustic impedance layers ( 51, 53 . . . 55 ) of one-quarter wavelength thicknesses constructively reflect the resonating wavelength to make an encapsulant in the form of an inexpensive plastic molding compound appear as a “clamping” surface to a resonator ( 41 ) peripherally supported over an opening ( 43 ) on a silicon substrate ( 42 ). In another embodiment, an encapsulant- and reflector-covered resonator ( 61 ) is mechanically supported above a second reflector ( 68 ) which eliminates the need for peripheral support, making substrate ( 68 ) also appear as a clamping surface. The invention enables low cost plastic packaging of resonators and associated circuitry on a single monolithic structure. A radio frequency transceiver front-end application is given as an exemplary implementation.

Description:
This application is a division of Ser. No. 09/022,905, filed Feb. 12, 1998, now U.S. Pat. No. 6,087,198 which claims priority under 35 U.S.C. §119(e)(1) of provisional U.S. Ser. No. 60/039,577 filed Feb. 28, 1997, entitled “Low Cost Packaging for Thin-Film Resonators and Thin-Film Resonator-Based Filters,” the entirety of which is incorporated herein by reference. 
     This invention relates generally to microelectronic devices and, in particular, to microelectronic devices including a frequency selective component of the thin-film acoustic resonator type, and to methods of fabricating and packaging the same. 
    
    
     BACKGROUND OF THE INVENTION 
     Microelectronic devices including frequency selective components are important for many electronic products requiring stable frequency signals or ability to discriminate between signals based on frequency diversity. These functions are difficult to reliably and inexpensively realize together with other circuitry in monolithic form on a silicon substrate. 
     One approach to providing microelectronic devices with frequency selective functions employs a mass allowed to vibrate in one or more dimensions (e.g., a pendulum). Such a mass is conveniently realized as a thin film supported at critical points, for example, peripherally or alternatively along one edge or end, forming a thin-film resonator structure. The term “thin film” in this context refers to a material deposited using, for example, chemical vapor deposition, evaporation, sputtering or other integrated circuit chip fabrication techniques, and having a thickness in the range of from about a few (2-3) atomic layers to about a few (1-5) microns, with typical thicknesses being about 2-4 microns. Such thin-film structures provide clearly defined mechanical resonances having significant utility, for example as filters in cellular phones and other communications devices or as frequency stabilizing feedback elements in oscillator circuits. 
     A significant drawback to such suspended mass resonators has been the need to fabricate the free-standing thin-film membrane. Typically, this is done by depositing the thin-film membrane over a sacrificial layer, then selectively removing the sacrificial layer to leave the thin film self-supported. Alternatively, the substrate is etched from the back to provide an opening extending up to the bottom of the membrane. Another approach is to form a cantilevered beam, capacitively coupled to adjacent structures (e.g., by means of a conductor placed beneath the beam). The beam is free to vibrate and has one or more resonance frequencies. Disadvantages of these approaches include the need to form free-standing structures and also a tendency of the beam to “stick” to adjacent structures if or when the beam comes into contact therewith. A need to remove any sacrificial layer and/or underlying substrate material limits fabrication ease and results in structures which are extremely fragile with respect to externally applied forces. These factors contribute to reduced fabrication yields and reduced robustness of the finished resonator component. 
     Dworsky et al., U.S. Pat. No. 5,373,268, the totality of which is incorporated herein by reference, describes a thin-film resonator having solid mechanical support. In the &#39;268 patent, a piezoelectric element, sandwiched between two electrodes, is supported on an acoustic reflector which presents a high acoustic impedance, analogous to a clamped surface, when situated atop a low acoustic impedance substrate. The reflector has an odd number of alternating layers of high and low acoustic impedance materials of one-quarter wavelength thickness that act to exhibit the transmission line Ferranti effect, whereby the low impedance of the substrate at one end of the reflector transmission line is transformed to a high impedance at the other end closest to the bottom electrode. This enables a one-quarter wavelength thickness resonator to be supported atop the reflector with its bottom electrode effectively “clamped” at the high acoustic impedance end of the reflector. The &#39;268 patent mentions that the same impedance transformation effect can be realized using tapered or other tailored impedance profiles. 
     Thin-film resonators (and filters based on the use of thin-film resonators) require packaging similar to that used for crystal resonators. In a crystal resonator element, a plate of piezoelectric crystal is suspended inside a hermetic package. Even though both surfaces are metallized, the crystal quality factor and frequency are very sensitive to particulates and changes in humidity. Likewise, thin-film resonators, including those described in the &#39;268 patent which have a reflector structure for mounting on a substrate, require environmental isolation from particulates and humidity. To achieve this isolation, present thin-film resonators must be packaged in high-cost, hermetically sealed metal packages, evacuated or back-filled with inert gas. Inexpensive plastic molding compounds used for packaging integrated circuit dies (viz. compounds with low acoustic impedance and thicknesses greater than one wavelength) are not usable for encapsulating thin-film resonators because they dampen vibration and interfere with acoustic operation. Moreover, to avoid interference with acoustic operation, present-day resonators are not even coated with chemically vapor deposited silicon oxide or silicon nitride, spin-on glass, spin-on polyimide, or similar passivation insulator films (low acoustic impedance and typical thicknesses of 500 to 4,000 Angstroms) deposited as a last line of defense against moisture and other contaminants for integrated circuits. 
     There is a need for inexpensive microelectronic devices including frequency selective components of the thin-film acoustic resonator type. Thus, there is also a need for low cost environmental isolation and packaging for thin-film resonators and thin-film resonator-based filters. 
     SUMMARY OF THE INVENTION 
     The invention provides lower cost environmental isolation and packaging for microelectronic devices including a frequency selective component of the thin-film acoustic resonator type, and lower cost microelectronic devices and electronic systems incorporating the same. 
     In accordance with one aspect of the invention, an acoustic transformer is applied over a thin-film acoustic resonator which is supported on a semiconductor or semiconductor-compatible substrate of a microelectronic device and an encapsulant is applied over the transformer-resonator structure. The acoustic transformer transforms the low acoustic impedance of the encapsulant into high acoustic impedance at the resonator/transformer interface. 
     In one embodiment, described in greater detail below, the resonator is a piezoelectric element, sandwiched between upper and lower electrodes, and the acoustic transformer takes the form of an acoustic reflector having alternating layers of high and low acoustic impedance materials. A flowable protective encapsulant material is flowed over the reflector-coupled resonator and cured in place to provide isolation from particulates and humidity. The reflector acts to make the protective encapsulant look like a high impedance to the resonator, with the upper electrode effectively “clamped.” 
     In another embodiment, a similar acoustic transformer and encapsulant are applied to a resonator which is mechanically supported on a substrate by means of a second acoustic reflector below the lower electrode which transforms low acoustic impedance at the substrate into high acoustic impedance at the resonator, effectively clamping the lower electrode, as well. 
     The invention enables low cost integrated circuit passivation layer and plastic packaging techniques to be applied to thin-film resonators. It also enables thin-film resonators to be usefully fabricated, environmentally isolated and packaged along with semiconductor diodes, transistors and other integrated circuit components in a single monolithic structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention have been chosen for purposes of illustration and description, and are shown with reference to the accompanying drawings, wherein: 
     FIGS. 1 and 2 are side sectional views of microelectronic devices including acoustic resonator components in accordance with the prior art; 
     FIGS. 3 and 4 are similar views of microelectronic devices including acoustic resonator components in accordance with the invention; and 
     FIG. 5 is a schematic diagram of a radio frequency apparatus including acoustic resonator components in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a microelectronic device  10  including a thin-film acoustic resonator  11  peripherally supported in conventional manner above an opening  18  on a substrate  12 . Resonator  11  comprises piezoelectric layer  14  sandwiched between electrode layers  15 ,  16 . Thickness  17  of resonator  11  is typically chosen to be one-half of an acoustic wavelength or an odd multiple thereof. Substrate  12  provides mechanical support for piezoelectric layer  14  and electrodes  15 ,  16  and for any ancillary components such as transistors, diodes, capacitors, resistors and the like included as part of a larger microelectronic device  10 . Substrate  12  conveniently comprises semiconductor (e.g., silicon or germanium) or semiconductor compatible (e.g., silicon on sapphire, cadmium sulfide on glass, etc.) material. 
     Acoustic impedance Z a  of resonator  10  varies with mass density ρ (rho) and stiffness c as 
     
       
           Z   a =(ρ* c ) 0.5 ,  (1) 
       
     
     while acoustic velocity v a  varies as 
     
       
           v   a =( c /ρ) 0.5 .  (2) 
       
     
     One component of acoustic reflection coefficient Γ z  arises from acoustic impedance mismatch (at an interface between different media, for example) and is found analogously to the familiar impedance mismatch reflection coefficient formula 
     
       
         Γ z =( Z   t   −Z   0 )/( Z   0   +Z   t ),  (3) 
       
     
     where Z t  represents a terminating impedance and Z o  represents a characteristic impedance of a transmitting medium. A free surface thus corresponds to an extremely low impedance and provides a reflection coefficient of    31   1 while a surface having infinite density and/or stiffness provides a reflection coefficient of +1. A free surface allows particle motion and a high impedance surface “clamps” particle motion. Examples of high impedance terminations thus comprise stiff, dense materials (e.g., tungsten), while low impedance terminations are materials having low stiffness and low mass density (e.g., silicone rubber, air). 
     FIG. 2 shows a microelectronic device  20  including an acoustic resonator  21  mounted on a substrate  22  in accordance with U.S. Pat. No. 5,373,268. Resonator  21  comprises electrode layers  25 ,  26  deposited on either side of piezoelectric layer  24  and has a thickness  27  of one-quarter wavelength. Resonator  21  is disposed atop acoustic reflector  28 , which, in turn, is disposed atop substrate  22 . Acoustic reflector  28  comprises a plurality of layers of material, represented here as layer  31  having thickness  32 , layer  33  having thickness  34  and layer  35  having thickness  36 . Further layers disposed between layers  33 ,  35  are denoted by dot-dashed lines. Layer  35  is disposed on substrate  22 , analogous to substrate  12  of device  10  of FIG.  1 . 
     Layers  31 ,  32  . . .  35  of acoustic reflector  28  are chosen to each have thicknesses  32 ,  34  . . .  36  equal to one-quarter of a characteristic fundamental acoustic wavelength of the associated resonator and are also chosen to have different acoustic impedances. In a typical embodiment, acoustic reflector  28  comprises alternating layers of high and low acoustic impedance materials (e.g., layer  35  having higher acoustic impedance than substrate  22 ). Acoustic reflector  28  thus manifests the Ferranti effect, whereby a low impedance (that of substrate  22 , for example) at a first end of a transmission line such as acoustic reflector  28  is transformed to a high impedance at a second end (adjacent electrode  25 , for example) when acoustic reflector  28  consists of an odd number of layers  31 ,  33  . . .  35 . As discussed in the &#39;268 patent, acoustic reflector  28  may thus be employed to present a high impedance to the piezoelectric resonator, analogous to a clamped surface, when acoustic reflector  28  is situated atop a material (e.g., substrate  22 ) having a low acoustic impedance and when acoustic reflector  28  comprises an odd number of layers each one-quarter of an acoustic wavelength in thickness. 
     The effective impedance which acoustic reflector  28  presents increases when the first, third, fifth, etc. layers (e.g., layers  31 ,  35 ) have high acoustic impedance and the second, fourth, sixth, etc. layers (e.g., layer  33 ) have low acoustic impedance. As mentioned in the &#39;268 patent, impedance transformation may also be realized through different techniques, such as tapered impedance media (exponentially/hyperbolically/otherwise tailored impedance profiles may be usefully employed, for example) optimized to provide high impedance or reduced length (thickness) to effect a given impedance transformation. Such variation of composition can be realized, e.g., by sequentially sputtering from a plurality of targets and varying dwell time on each target, or by depositing thin layers of material and varying the number of sequential layers of each material. 
     In accordance with the &#39;268 patent, when acoustic reflector  28  provides a high acoustic impedance at one end thereof, an acoustic resonator  21  disposed at that one end and comprising electrodes  25 ,  26  and piezoelectric material  24  may usefully be one-quarter of the characteristic resonating acoustic wavelength in thickness. This is possible because a lower surface of the acoustic resonator is clamped. 
     Both thin-film resonators (and resonator-based) filters, whether fabricated in accordance with the structures shown in FIG. 1 or FIG. 2, have an upper electrode ( 16  or  26 ) that is uncoated (unprotected by a passivation layer) and interfaces with air, so utilize high cost hermetic packaging to provide sealed cavity isolation from adverse effects of humidity and particulates. The high cost of hermetic packaging is a major factor in the high cost of the overall microelectronic device. 
     FIG. 3 shows a microelectronic device  40  including a thin-film acoustic resonator  41  mounted on a substrate  42 . Resonator  41  is analogous to resonator  11  (FIG. 1) and comprises electrodes  45 ,  46  on either side of piezoelectric layer  44 . Resonator  44  has a thickness  47  which is one-quarter of an acoustic wavelength in a preferred embodiment. Electrode  45  corresponds to electrode  15  (FIG. 1) and is disposed atop substrate  42 , with its edges peripherally supported above an opening formed in the substrate. Electrode  45 , piezoelectric element  44  and electrode  46  may, for example, be layers of material successively deposited and patterned using known integrated circuit deposition and patterning techniques. Piezoelectric layer  44  may, for example, comprise a thin layer of piezoelectric (or electrostrictive) material(s) having acceptable electromechanical coupling coefficients, such as AlN, ZnO, CdS and the like. Electrode layers  45 ,  46  may, for example, be thin layers of metallic materials having low acoustic losses and favorable electrical conductivity properties (e.g., aluminum or aluminum alloys). The substrate opening over which the resonator is clamped may, for example, be formed by etching substrate  42  from the back to give the opening  43  shown in FIG.  3 . Alternatively, the opening may be formed by depositing resonator  41  over a sacrificial layer, and then etching away the sacrificial layer. For a 1 GHz frequency resonator fabricated with ZnO, for example, electrode layers  45 ,  46  may have 0.3 micron thicknesses and layer  44  may be 1 micron thick (one wavelength=6.4 microns). 
     In accordance with the invention, a laminate acoustic reflector  48  is deposited on top of the upper electrode  46  and a protective molding compound  49  is formed in place over the reflector covered resonator. Reflector  48  comprises a plurality of superposed layers of material, shown here as layer  51  having thickness  52 , layer  53  having thickness  54  and layer  55  having thickness  56 . Further layers disposed between layers  53 ,  55  are denoted by dot-dashed lines. Layer  51  is disposed on upper electrode  46 . Compound  49  is disposed over layer  55 . Layers  51 ,  52  . . .  55  are chosen to have thicknesses  52 ,  54  . . .  56  equal to one-quarter of an acoustic wavelength and comprise alternating layers of high and low impedance materials Layers  51 ,  52  . . .  55  desirably comprise layers of material(s) having low acoustic propagation losses. In a typical embodiment, reflector  48  comprises alternating layers of high and low impedance materials (e.g., layer  55  having higher acoustic impedance than the cured molding compound  49 ). Acoustic reflector  48  thus acts to transform low impedance at the compound  49  to high impedance at electrode  46 , when reflector  48  consists of an odd number of layers, or given other tailored impedance profile to exhibit the similar Ferranti effect. 
     Reflector  48  is conveniently realized as layers of metal because such an arrangement facilitates contacting electrode  46  and because metal films are conveniently fabricated and patterned in modern microfabrication facilities. Longitudinal acoustic properties of:a few representative materials are summarized in U.S. Pat. No. 5,373,268. The resonator is constructed to provide acoustic impedance mismatches at one-quarter wavelength spacings, with reflections occurring in-phase with the incident wave at the mismatches to provide constructive interference at the resonant frequency. There, thus, appears an acoustic mirror above the resonator which enables the encapsulant (viz. passivation layer and/or glob top/overmold material) to be applied directly in contact with the reflector, without damping. The layers are preferably chosen to have matching coefficients of thermal expansion with each other and with the resonator components. In a typical embodiment, the layers will have lateral dimensions on the order of 400 microns across, and thicknesses of about 1-2 microns. The layers can be deposited using standard integrated circuit metallization deposition and patterning techniques. 
     Electrodes  45 ,  46  provide electrical connections to the piezoelectric element  44 . Metal layer vias or similar known methods are used to provide electrical connections of electrodes  45 ,  46  to other components formed elsewhere on the same substrate  42 . Electrode  45  may, for example, comprise an input electrical port having an associated motional capacitance C m1 , while electrode  46  may comprise an output electrical port having an associated motional capacitance C m2 . Motional capacitance C m  is related to static capacitance C 0  through the electromechanical coupling coefficient K 2 , i.e., C m /C 0 =8K 2 /π 2 . 
     Compound  49  is preferably an inexpensive plastic molding compound deposited over a spin-on glass passivation layer. The molding compound may be of the type used for encapsulating integrated circuit dies and which is brought into a fluid state, deposited from a reservoir onto the reflector-covered resonator, then cured in place. It may, for example, be an epoxy novalac-based resin or other epoxy, polyimide or silicone resin deposited using a reactive polymer processing technique. Reactive polymer processing is the combined polymerization and processing of reactive polymers or prepolymers in a single operation, and encompasses numerous processing methods such as transfer molding (viz. compressing heated preform in a mold cavity), conformal spread coating (viz. spinning, spraying, vapor deposition), radial-spread (or “glob top”) coating (viz. dispensing glob of material from hollow needle), and reaction-injection molding (combining two-part reactive polymers into a mold cavity). Further details of reactive polymer processing methods are found in Chapter 8 of R. Tummala &amp; E. Rymaszewski, Microelectronics Packaging Handbook (1989 Von Nostrand Reinhold), which is incorporated herein by reference. 
     FIG. 4 shows a microelectronic device  60  including a thin-film acoustic resonator  61  mounted on a substrate  62  according to another embodiment of the invention. Resonator  61  is analogous to resonator  21  (FIG. 2) and comprises electrodes  65 ,  66  on either side of piezoelectric layer  64 . Like resonator  11 , resonator  61  has a thickness  67  which is one-half wavelength in a preferred embodiment. Electrode  65  corresponds to electrode  25  (FIG. 2) and is disposed atop an acoustic reflector  68  which is analogous to, and may have the same structure as, reflector  28  of FIG.  2 . Reflector  68  is, in turn, disposed atop substrate  62 . Reflector  68  comprises an odd number of layers  71 ,  73  . . .  75  of quarter-wavelength thicknesses  72 ,  74  . . .  76  of alternating high and low acoustic material, dimensioned and configured to transform a low impedance at substrate  62  into a high impedance at electrode  65 . An acoustic reflector  48 , like that described previously in reference to FIG. 3, is deposited on top of the upper electrode  46  and a protective molding compound  49  is formed in place over the reflector covered resonator. As before, reflector  48  may comprise an odd-numbered plurality of superposed layers  51 ,  53  . . .  55  of quarter-wavelength thicknesses  52 ,  54  . . .  56  of alternating high and low impedance materials, dimensioned and configured to transform a low impedance at the cured molding material  49  into a high impedance at the upper electrode  66 . As with reflector  48  of FIG. 3, reflector  48  of FIG. 4 provides a mechanism for protecting the upper surface of the resonator from particulates and moisture, while simulating clamping of the upper electrode. This enables the reflector-covered resonator to be encapsulated with passivation layers and epoxy resin or other inexpensive overmold packaging method utilized for packaging integrated circuit semiconductor components. Thus, thin-film resonators and thin-film resonator filters can be incorporated in post-processing on the same substrate with other components, and be adapted to the same packaging procedures that would be used for those components in the absence of incorporation of the resonators or filters. 
     FIG. 5 illustrates an example of a system application of devices  40 ,  60  or similar devices. Radio frequency receiver front-end circuit  80  comprises an antenna  81  which sends or receives signals from a power divider  82  which is coupled to receive a transmitter signal from a transmit filter  83 . Power divider  82  sends an incoming signal to a first receive filter  84  which passes the resulting band limited signal to an amplifier  85 . From amplifier  85 , the amplified signal passes through a second, band-limiting receive filter  86  to a mixer  87 . Mixer  87  also receives a signal from a local oscillator (LO)  88  through a band-limiting oscillator filter  89 . The resulting signal from mixer  87  passes to a receiver intermediate frequency (IF) output  90 . 
     Filters  83 ,  84 ,  86  and/or  89  are advantageously thin-film resonator filters of the type described here and made according to the structure and method of the present invention, but of varying frequency or other properties according to the particular desired function. For example, filter  84  removes input RF frequencies outside the band in which the receiver is intended to operate. This is particularly useful for narrow band receivers such as are frequently required in cellular phone and paging applications and the like. Additionally, local oscillator  88  may employ a thin-film resonator in accordance with the present invention for a frequency stabilizing element. Filter  86  may have the same or a different pass band than filter  84  and removes any undesired harmonics generated by amplifier  85  or other out-of-band signals not removed by filter  84 . Filter  89  desirably passes the local oscillator frequency and stops undesired harmonics thereof. 
     Thus, electronic apparatus, and especially radio frequency circuitry can benefit from the packaging cost savings which result from the use of microelectronic devices including thin-film resonator filters in accordance with the present invention. The invention enables the practical formation of packaged microelectronic devices including deposited resonator filters for applications such as cellular phones, direct broadcast satellite, television set-top boxes, laptop computer transceivers, etc. 
     Those skilled in the art to which the invention relates will appreciate that additions, substitutions and modifications can be made to the described embodiments, without departing from the spirit and scope of the invention as described by the claims below.