Abstract:
A piezoelectric resonator include a multi-layer top electrode. The multi-layer top electrode includes at least a top metal layer and a bottom metal layer. A top metal layer edge is recessed compared to a bottom metal layer edge allowing conformal deposition of a passivation layer. The passivation layer covers and protects the underlying layers from subsequent etching, thereby preventing etch undercut of the top electrode. In some embodiments, the multi-layer top electrode is configured as a bi-layer. In other embodiments, an extra layer is configured between the top metal layer and the bottom metal layer, for example a shunt load layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of piezoelectric resonators. More particularly, the present invention relates to the field of a bulk acoustic wave (BAW) resonator and a method of controlling a top electrode edge of a BAW resonator during patterning. 
     BACKGROUND OF THE INVENTION 
     Piezoelectric resonators are primarily used in RF filters and oscillators. RF filters are increasingly being used in mobile communications devices. These resonators are commonly referred to as bulk acoustic wave (BAW) resonators. Other acronyms for the same or similar devices include FBAR (thin-film bulk acoustic resonators), SMR (solidly mounted resonators), TFR (thin film resonators), or SCR (stacked crystal resonators). The resonators are interconnected, typically at the upper metal level, to build RF filters. 
     It is known that a bulk acoustic wave (BAW) resonator in general comprises a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. BAW resonators are typically fabricated on silicon (Si), gallium arsenide (GaAs), glass, or ceramic substrates. BAW resonators are typically manufactured using various thin film manufacturing techniques, such as for example sputtering, vacuum evaporation or chemical vapor deposition. BAW resonators utilize a piezoelectric thin film layer for generating the acoustic bulk waves. The resonance frequencies of typical BAW resonators range from 0.5 GHz to 5 GHz, depending on the size and materials of the device. BAW resonators exhibit the typical series and parallel resonances of crystal resonators. The resonance frequencies are determined mainly by the material of the resonator and the dimensions of the layers of the resonator. 
     A typical BAW resonator consists of an acoustically active piezoelectric layer, electrodes on opposite sides of the piezoelectric layer, and an acoustical isolation from the substrate. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is a predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased. 
     The material used to form the electrode layers is an electrically conductive material. The acoustical isolation is produced with a substrate via-hole, a micromechanical bridge structure, or with an acoustic mirror structure. In the via-hole and bridge structures, the acoustically reflecting surfaces are the air interfaces below and above the devices. The bridge structure is typically manufactured using a sacrificial layer, which is etched away to produce a free-standing structure. Use of a sacrificial layer makes it possible to use a wide variety of substrate materials, since the substrate does not need much modification, unlike with the via-hole structure. A bridge structure can also be produced using an etch pit structure, in which case a pit is etched in the substrate or the material layer below the BAW resonator to produce the free standing bridge structure. 
       FIG. 1  illustrates a cross-section of a conventional piezoelectric resonator. The piezoelectric resonator includes a substrate  10 , an acoustic mirror or acoustic reflector  20 , a bottom electrode  30 , a piezoelectric layer  40 , and a top electrode  50 . The top electrode can be constructed from several metallic and dielectric layers. The bottom electrode  30  can also be constructed from several metallic and dielectric layers; molybdenum is typically used. The top electrode  50  shown in  FIG. 1  includes a bottom metal layer  52  of a material with high acoustic impedance, and a top metal layer  54  of a material with low impedance. 
       FIGS. 2-5  illustrate conventional fabrication steps to form the top electrode  50 . As illustrated in  FIG. 2 , the bottom metal layer  52  is deposited on the piezoelectric layer  40 . The top metal layer  54  is then deposited upon the bottom layer  52 . The top metal layer  54  is then patterned, selectively to under-laying layer  52  ( FIG. 3 ) and resist removed. Bottom metal layer  52  is then etched using top metal layer  54  as a hard mask ( FIG. 4 ). As a result, a portion  56  of the bottom metal layer  52  is also etched forming a recess underneath the top metal layer  54  ( FIG. 5 ). Additionally, when an interconnect metal layer is fabricated to make contact with the top electrode  50 , further undercutting of the top layer  54  can occur during an etch step of the interconnect metal layer. In either case, undercutting of the top metal layer  54  negatively impacts the performance of the piezoelectric resonator. 
     During fabrication of conventional piezoelectric resonators, such as the process illustrated in  FIGS. 2-5 , problems arise during etching of the bi-layer top electrode and during etching of the metallization layer that contacts the top electrode bi-layer stack. The difficulty is finding etches that do not preferentially attack the bottom metal layer  52  causing undercut voiding at the device periphery. This is particularly true if wet etching is employed because of various galvanic and catalytic reactions that do occur. 
     SUMMARY OF THE INVENTION 
     Embodiments of a piezoelectric resonator include a multi-layer top electrode. The multi-layer top electrode includes at least a top metal layer and a bottom metal layer. A top metal layer edge is recessed compared to a bottom metal layer edge allowing conformal deposition of a passivation layer. The passivation layer covers and protects the underlying layers from subsequent etching, thereby preventing etch undercut of the top electrode. Preventing etch undercut provides a superior piezoelectric resonator architecture because of good control of the active area in-plane dimensions. In some embodiments, the multi-layer top electrode is configured as a bi-layer. In other embodiments, an extra layer is configured between the top metal layer and the bottom metal layer, for example a shunt load layer. 
     Standard IC fabrication methods are used for the basic manufacturing sequences, including depositions, photolithography, and etch processes. MEMS techniques can also be employed for packaging and resonator acoustic isolation from the substrate. 
     The multi-layer top electrode construction is desirable in BAW resonator design that require stringent acoustic, electrical, and process integration requirements. By using two distinct materials for the multi-layer configuration, it is possible to benefit from the unique properties of each material. 
     In one aspect of the present invention, a piezoelectric resonator is disclosed. The piezoelectric resonator includes a bottom electrode, a piezoelectric layer coupled to the bottom electrode, a top electrode coupled to the piezoelectric layer, and a passivation layer coupled to the top electrode. The top electrode comprises a bottom metal layer and a top metal layer configured such that a perimeter edge of the top metal layer is recessed relative to a perimeter edge of the bottom metal layer. The passivation layer is coupled to the top electrode such that the top metal layer and the bottom metal layer are isolated from a subsequent metal etch step. The bottom metal layer can be a refractory metal. The bottom metal layer can be a tungsten alloy. The bottom metal layer can be molybdenum, ruthenium, tungsten, platinum, osmium, rhenium or iridium. The top metal layer can be aluminum, gold, platinum, or an aluminum alloy composition. The top metal layer can be a metal alloy composition. The passivation layer can include an etched contact area, and the piezoelectric resonator can also include an interconnect metal layer coupled to the top electrode at the etched contact area. In some embodiments, the interconnect metal layer can include a first interconnect metal layer and a second interconnect metal layer. In this case, the first interconnect metal layer can be titanium tungsten, and the second interconnect metal layer can be aluminum copper or copper. In other embodiments, the interconnect metal layer can be titanium tungsten, tungsten, or molybdenum. The interconnect metal layer can be a metal material that is selectively etched relative to the passivation layer. The piezoelectric resonator can be a bulk acoustic wave resonator. The passivation layer is initially configured such that all exposed portions of the top metal layer and the bottom metal layer are covered. 
     In another aspect of the present invention, a method of fabricating a piezoelectric resonator is disclosed. The method includes providing a bottom electrode. A piezoelectric layer is coupled to the bottom electrode. A bottom metal layer of a top electrode is deposited on the piezoelectric layer. A top metal layer of the top electrode is deposited on the bottom metal layer. A photoresist layer is deposited on the top metal layer and then patterned and etched. The top metal layer is etched while the etched photoresist layer remains in place as an etch mask. The bottom metal layer is etched such that a perimeter edge of the top metal layer is recessed relative to a perimeter edge of the bottom metal layer. The etched photoresist layer is removed. A passivation layer is deposited on the top metal layer and the bottom metal layer such that the top metal layer and the bottom metal layer are isolated from a subsequent metal etch step. The passivation layer initially covers all exposed portions of the top metal layer and the bottom metal layer. The method can also include patterning and etching the passivation layer to form a contact area for the top electrode, depositing an interconnect metal layer on the passivation layer, and patterning and etching the interconnect metal layer to form a metal interconnect for the top electrode at the contact area. In some embodiments, the interconnect metal layer can be a first interconnect metal layer and a second interconnect metal layer. In this case, the first interconnect metal layer can be titanium tungsten, and the second interconnect metal layer can be aluminum copper or copper. In other embodiments, the interconnect metal layer can be titanium tungsten, tungsten, or molybdenum. The interconnect metal layer can be a metal material that is selectively etched relative to the passivation layer. The piezoelectric resonator can be a bulk acoustic wave resonator. The bottom metal layer can be a refractory metal. The bottom metal layer can be a tungsten alloy. The bottom metal layer can be molybdenum, ruthenium, tungsten, platinum, osmium, rhenium or iridium. The top metal layer can be aluminum, gold, platinum, or an aluminum alloy composition. The top metal layer can be a metal alloy composition. Etching the top metal layer can be performed by a wet etch process. Etching the bottom metal layer can be performed by a dry etch process. 
     In yet another aspect of the present invention, another piezoelectric resonator is disclosed. The piezoelectric resonator includes a bottom electrode, a piezoelectric layer coupled to the bottom electrode, a top electrode coupled to the piezoelectric layer, a passivation layer coupled to the top electrode, and an interconnect metal layer coupled to the top electrode. The top electrode comprises a bottom metal layer and a top metal layer configured such that a perimeter edge of the top metal layer is recessed relative to a perimeter edge of the bottom metal layer. The passivation layer includes an etched portion configured to provide a contact point with the top electrode, further wherein the passivation layer is further configured to isolate a remaining portion of the top electrode. The interconnect metal layer is coupled to the top electrode at the contact point through the etched portion of the passivation layer. The bottom metal layer can be a refractory metal. The bottom metal layer can be a tungsten alloy. The bottom metal layer can be molybdenum, ruthenium, tungsten, platinum, osmium, rhenium or iridium. The top metal layer can be aluminum, gold, platinum, or an aluminum alloy composition. The top metal layer can be a metal alloy composition. The interconnect metal layer is configured to be in physical contact with the top metal layer and the bottom metal layer at the contact point. In some embodiments, the interconnect metal layer can be a first interconnect metal layer and a second interconnect metal layer. In this case, the first interconnect metal layer can be titanium tungsten, and the second interconnect metal layer can be aluminum copper or copper. The interconnect metal layer can be titanium tungsten, tungsten, or molybdenum. The interconnect metal layer can be a metal material that is selectively etched relative to the passivation layer. The piezoelectric resonator can be a bulk acoustic wave resonator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-section side view of a conventional piezoelectric resonator. 
         FIGS. 2-5  illustrate exemplary fabrication steps used to fabricate the top electrode of the piezoelectric resonator in  FIG. 1 . 
         FIG. 6  illustrates a cross-section side view of a first embodiment of the piezoelectric resonator. 
         FIGS. 7-10  illustrate exemplary fabrication steps used to fabricate the top electrode of the piezoelectric resonator in  FIG. 6 . 
         FIG. 11  illustrates the results after a subsequent deposition of a passivation layer on the conventional piezoelectric resonator of  FIG. 5 . 
         FIG. 12  illustrates an interconnect metal layer added to the piezoelectric resonator of  FIG. 10 . 
         FIG. 13  illustrates the results after a subsequent etch of an interconnect metal layer on the conventional piezoelectric resonator of  FIG. 11 . 
         FIG. 14  illustrates an exemplary configuration of the piezoelectric resonator including a bi-layer interconnect metal layer. 
     
    
    
     Embodiments of the piezoelectric resonator are described relative to the several views of the drawings. Where appropriate and only where identical elements are disclosed and shown in more than one drawing, the same reference numeral will be used to represent such identical elements. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 6  illustrates a cross-section side view of a first embodiment of the piezoelectric resonator. The piezoelectric resonator includes an acoustic mirror or acoustic reflector  120  formed on a substrate  110  preferably by a deposition process. A bottom electrode  130  is formed on the acoustic reflector  120  preferably by a deposition process. A piezoelectric layer  140  is formed on the bottom electrode  130  preferably by a deposition process. A top electrode  150  is formed on the piezoelectric layer  140  preferably by a deposition process. In this first embodiment, the top electrode  150  is configured as a bi-layer electrode, including a top metal layer  154  formed on a bottom metal layer  152  preferably by a deposition process. The top metal layer  154  is formed relative to the bottom metal layer  152  such that a passivation layer  156  completely covers any exposed portion of both the top metal layer  154  and the bottom metal layer  152 . In some embodiments, the outer edges of the top metal layer  154  is recessed compared to the bottom metal layer  152 , for example forming a step-like pattern, as shown in  FIG. 6 . In such a configuration, the passivation layer  156  protects against etch undercut of the bi-layer during subsequent etches. 
     In some embodiments, the bottom metal layer  152  is made of tungsten or a tungsten alloy, such as titanium tungsten (TiW), and the top metal layer  154  is made of aluminum. TiW is a preferred material for the bottom metal layer  152  because of its acoustic properties. Alternative materials for the bottom metal layer  152  include, but are not limited to, refractory metals such as ruthenium, molybdenum, platinum, osmium, iridium, rhenium, or other acoustically hard materials to optimize the acoustic resonator performance. Aluminum is preferred for the top metal layer  154  because of its high electrical conductivity and for its ability to act as an etch stop during subsequent etch process steps. Alternative materials for the top metal layer  154  include, but are not limited to, an aluminum alloy, gold, platinum, or a specific alloy composition that has a high selectivity from subsequent etch steps. In some embodiments, the passivation layer  156  is silicon nitride. Alternatively, conventional material or composite material that provides a non-reactive layer over the top electrode  150  can be used as the passivation layer. 
       FIGS. 7-10  illustrate exemplary fabrication steps used to fabricate the top electrode of the piezoelectric resonator in  FIG. 6 . As illustrated in  FIG. 7 , the bottom metal layer  152  is deposited on the piezoelectric layer  140 . The top metal layer  154  is then deposited on the bottom metal layer  152 . The top metal layer  154  is then coated with a photoresist layer  158 . The photoresist layer  158  is then patterned and etched. The top metal layer  154  is then patterned and etched while maintaining the patterned photoresist layer  158 , as illustrated in  FIG. 8 . In some embodiments, the top metal layer  154  is wet etched, leading to an undercut below the photoresist layer  158 . 
     The bottom metal layer  152  is then patterned and etched, as illustrated in  FIG. 9 . In some embodiments, the bottom metal layer  152  is anisotropically etched with a dry plasma process. The patterned photoresist layer  158  is then removed, and the passivation layer  156  is deposited over the top metal layer  154  and the bottom metal layer  152 , as illustrated in  FIG. 10 . The passivation layer  156  covers any exposed portions of the top metal layer  154  and the bottom metal layer  152 . 
     For comparative purposes,  FIG. 11  illustrates the results after a subsequent deposition of a passivation layer  58  on the conventional piezoelectric resonator of  FIG. 5 . As shown in  FIG. 11 , the passivation layer  58  does not cover all portions of the top metal layer  54  and the bottom metal layer  52  due to the etch undercut portions  56 . As a result, portions of both the top metal layer  54  and the bottom metal layer  52  are exposed to subsequent etches. 
       FIG. 12  illustrates an interconnect metal layer  160  added to the piezoelectric resonator of  FIG. 10 . The interconnect metal layer  160  is fabricated by patterning and etching the passivation layer  156  for appropriate contact point with the top electrode  150 . The interconnect metal layer  160  is then formed by depositing a metal layer on the exposed portion of the top electrode  150  and the remaining passivation layer  156 . The deposited metal layer is then patterned and etched to form the interconnect metal layer  160 . In some embodiments, the interconnect metal layer  160  is titanium tungsten (TiW). Alternatively, the interconnect metal layer  160  is tungsten, molybdenum, or any material that is selectively etched relative to the passivation layer  156 . For example, a TiW interconnect metal layer is removed selectively from a silicon nitride passivation layer by a peroxide based wet etch. The interconnect metal layer  160  can also comprise of a bi-layer, for example TiW/AlCu or TiW/Cu. In this example, the AlCu or the Cu is for low resistance interconnect, while the TiW can be etched with fine selectivity to the passivation layer  156 . In the case where the bi-layer comprises TiW/Cu, the Cu material can be plated upon the TiW layer, with TiW used as a seed conductive layer for the plate-up process. Subsequently, the TiW layer is etched in the field, where the Cu layer has not been electroplated.  FIG. 14  illustrates an exemplary configuration of the piezoelectric resonator including a bi-layer interconnect metal layer  160 ′. The interconnect metal layer  160 ′ includes a first interconnect metal layer  162  and a second interconnect metal layer  164 . The relative thickness of the first interconnect metal layer  162  shown in  FIG. 14  is for exemplary purposes only. The piezoelectric resonators shown in  FIGS. 6 and 12  allow for the stacking of films to create a top electrode that prevents any etch undercut. The interconnect metal layer  160  can be used to build a filter out of a fixed number of resonators. 
     For comparative purposes,  FIG. 13  illustrates the results after a subsequent etch of an interconnect metal layer  60  on the conventional piezoelectric resonator of  FIG. 11 . As is shown in  FIG. 13 , etching the interconnect metal layer  60  further undercuts the top metal layer  54  by removing an additional portion  62  of the exposed bottom metal layer  52 . In contrast, the bottom metal layer  152  of the piezoelectric resonator in  FIG. 12  is completely isolated from etching of the interconnect metal layer  160 . 
     The piezoelectric resonator described above in relation to  FIGS. 6-10  and  12  are directed to a bi-layer, top electrode configuration. Alternatively, an extra layer can be integrated between the top metal layer and the bottom metal layer, for example a shunt load layer. 
     The piezoelectric resonator fabrication methods described above can be optimized to reduce lateral spurious modes in the resonator device without additional processing steps. Spurious modes are reduced by uniquely tailored resonator loading at the resonator perimeter. Examples of such resonator loading techniques are described in U.S. Pat. No. 6,812,619, which is hereby incorporated by reference. 
     It is understood by those skilled in the art that the terms depositing, patterning, and etching used above are intended as general descriptive terms used in the fabrication processes. The fabrication steps described above can be performed using any conventional fabrication methods capable of forming, patterning, and etching the layers described. 
     The piezoelectric resonators are used as reference oscillators, stand-alone filters, and also as building blocks for RF filters. Such RF filters can be used to replace SAW devices. The piezoelectric devices can also be used in all RF stages of products needing filtering, either band filtering, or channel filtering. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such references, herein, to specific embodiments and details thereof are not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications can be made in the embodiments chosen for illustration without departing from the spirit and scope of the invention.