Abstract:
Disclosed is a device that includes a crystalline substrate and a patterned aluminum-based material layer disposed onto the crystalline substrate. The patterned aluminum-based material layer has a titanium-alloyed surface. A titanium-based material layer is disposed over select portions of the titanium-alloyed surface. In an exemplary embodiment, the patterned aluminum-based material layer forms a pair of interdigitated transducers to provide a surface wave acoustic (SAW) device. The SAW device of the present disclosure is usable to realize SAW-based filters for wireless communication equipment.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional patent application Ser. No. 62/360,558, filed Jul. 11, 2016, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure is directed to improving performance characteristics of acoustic wave devices. 
       BACKGROUND 
       [0003]    In acoustic wave device technology a first metal layer is a patterned aluminum-based material layer known as an interdigital transducer (IDT) metal layer. Typically, an IDT metal layer comprises aluminum (Al) and a relatively smaller amount of titanium (Ti). The aluminum dominates the composition of the IDT metal layer because it has a higher ratio of conductivity to mass than titanium. The conductivity-to-mass ratio is a relatively critical characteristic for acoustic wave devices because a higher conductivity-to-mass ratio yields a lower insertion loss for radio frequency filters constructed from acoustic wave devices. 
         [0004]    While aluminum is a desirable component of IDT metal, aluminum is not ideal because aluminum oxidizes readily when exposed to oxygen in air to form aluminum oxide. However, the oxidation of aluminum is typically self-limiting such that the formation of the oxide layer prevents additional oxidation of the aluminum metal making up the IDT metal layer. The oxide layer becomes detrimental at locations on the acoustic wave device where it is necessary to connect additional metal layers to the IDT metal layer. 
         [0005]    In lithium tantalate surface acoustic wave (LTSAW) technology used to make a subset of acoustic wave devices, a second metal layer most often in contact with the IDT metal layer is an under bump metallurgy (UBM) metal. This second metal layer is deposited to form appropriate circuit connections for the acoustic wave device and is composed of Ti/Al/Ti. However, an aluminum oxide (AIO) layer formed on the IDT layer is insulating and is chemically and physically robust such that etching vias to connect the UBM to the IDT metal layer is not effective at removing the AIO layer. In addition to the robustness of the AIO layer, LTSAW process flow requires exposure of the aluminum IDT metal layer to ambient air during operations prior to UBM evaporation, which allows for re-oxidation of aluminum in any areas exposed by removal of the AIO during processing. The insulating nature of the AIO layer prevents consistent, low direct current (DC) contact resistance between UBM and the IDT metal layer. This AIO insulating layer also increases insertion loss by limiting conductivity between UBM and IDT layers. The lack of DC contact makes process control monitoring difficult because most process control monitoring is performed by DC measurement of test structures. The AIO between the UBM and the IDT metal layer also adds additional capacitance to radio frequency test structures, which can complicate parameter extraction and modeling. In addition to negative effects on electrical characteristics of acoustic devices, the AIO layer can also affect the mechanical properties of acoustic devices. The AIO layer prevents metal-to-metal contact between the IDT metal layer and the UBM. Therefore, adhesion between the IDT metal layer and the UBM is reduced. This reduction in adhesion reduces shear strength between the IDT metal layer and the UBM. As such, the robustness of acoustic devices during assembly is reduced by mechanical stresses of assembly. Thus, there is a need for acoustic devices that do not have AIO layers formed between the UBM and IDT layers. 
       SUMMARY 
       [0006]    Disclosed is a device that includes a crystalline substrate and a patterned aluminum-based material layer disposed onto the crystalline substrate. The patterned aluminum-based material layer has a titanium-alloyed surface. A titanium-based material layer is disposed over select portions of the titanium-alloyed surface. 
         [0007]    In an exemplary embodiment, the patterned aluminum-based material layer forms a pair of interdigitated transducers to provide a surface acoustic wave (SAW) device. The SAW device of the present disclosure is usable to realize SAW-based filters for wireless communication equipment. 
         [0008]    Another aspect of the disclosure provides a method of fabricating the device. In general, the method includes a process step of disposing a patterned aluminum-based material layer onto a crystalline substrate. Another process step includes disposing a titanium-based material layer over the patterned aluminum-based material layer. Yet another process step includes selectively etching away portions of the titanium based material layer to leave an exposed titanium-alloyed surface on the portions of the patterned aluminum-based material layer. 
         [0009]    In at least one exemplary embodiment another step includes disposing an under bump metallurgy layer comprising a metal directly in contact with the portions of the titanium-alloyed surface. The resulting contact resistivity is in the range of 1×10 −8  ohm (Ω)/cm 2  and 1×10 −7  Ω/cm 2 . 
         [0010]    Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0011]    The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
           [0012]      FIG. 1  is a perspective drawing of an acoustic device of the present disclosure. 
           [0013]      FIG. 2  is a cross-sectional view of a completed portion of one embodiment of the acoustic device of the  FIG. 1 . 
           [0014]      FIG. 3  is a cross-sectional view of a result of beginning steps completed for fabricating the portion of the embodiment of the acoustic device depicted in  FIG. 2 . 
           [0015]      FIG. 4  is a cross-sectional view of a result of a step that involves disposing a crossover pattern that will in later steps provide support for a bridging portion of an under bump metallurgy (UBM) layer. 
           [0016]      FIG. 5  is a cross-sectional view of a result of another step that involves disposing the passivation layer onto the titanium-based material layer, the crossover pattern, and exposed sections of the substrate. 
           [0017]      FIG. 6  is a cross-sectional view of a result of another step that involves disposing a photoresist mask over portions of the patterned aluminum-based material layer and titanium-based material layer. 
           [0018]      FIG. 7  is a cross-sectional view of a result of another step that involves etching away portions of the passivation layer and titanium-based material layer to leave regions of the titanium-alloyed surface exposed. 
           [0019]      FIG. 8  is a cross-sectional view of a result of another step that involves disposing the UBM layer onto the exposed titanium-alloyed surface and the crossover pattern. 
           [0020]      FIG. 9  is a cross-sectional view of a result of a final step that involves lifting off the photoresist mask and unwanted portions of the UBM layer. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
         [0022]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0023]    It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
         [0024]    Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. 
         [0025]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0026]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0027]      FIG. 1  is a perspective drawing of the acoustic wave device  10  of the present disclosure. The acoustic wave device  10  includes a crystalline substrate  12 , such as lithium tantalate (LiTaO 3 ) or lithium niobate (LiNbO 3 ), onto which an interdigital transducer layer that is a patterned aluminum-based material layer  14  is disposed. In the exemplary embodiment of  FIG. 1 , the patterned aluminum-based material layer  14  has a pattern of interdigitated fingers. Additional elements such as acoustic reflectors  16  are included on acoustic devices such as surface acoustic wave (SAW) resonators. An exemplary acoustic wave device  10  is depicted as a SAW resonator that is typically coupled with other SAW resonators to form SAW filters that are used in wireless products such as smart phones. 
         [0028]      FIG. 2  is a cross-sectional view of a completed portion of one embodiment of the acoustic wave device  10 . As in  FIG. 1 , the patterned aluminum-based material layer  14  is disposed onto the crystalline substrate  12 . In at least one embodiment the patterned aluminum-based material layer  14  is at least 99% aluminum. In other embodiments, the patterned aluminum-based material layer  14  is an aluminum alloy. In some embodiments, the aluminum alloy is an aluminum/copper alloy. 
         [0029]    In addition, the patterned aluminum-based material layer  14  has a titanium-alloyed surface  18  that is formed during fabrication of the acoustic wave device  10  as portions of a titanium-based material layer  20  are etched away. The remaining portions of the titanium-based material layer  20  are protected from etching by a photoresist during subsequent processing. 
         [0030]    The titanium-based material layer  20  is disposed between 10% and 90% of the titanium-alloyed surface  18 . In yet other embodiments, the titanium-based material layer  20  is disposed between 10% and 75% of the titanium-alloyed surface  18 . In yet other embodiments, the titanium-based material layer  20  is disposed between 10% and 50% of the titanium-alloyed surface  18 . In at least one embodiment, the titanium-based material layer  20  is 99% titanium. In other embodiments, the titanium-based material layer  20  is a titanium alloy. In at least some embodiments, the titanium alloy making up the titanium-based material layer  20  is titanium/aluminum. 
         [0031]    In some embodiments, the titanium-based material layer  20  has a thickness that is in the range of 20 angstroms (Å) to 50 Å. In other embodiments, the titanium-based material layer  20  has a thickness that is in the range of 50 Å to 70 Å. In yet other embodiments, the titanium-based material layer  20  has a thickness that is in the range of 70 Å to 100 Å. In still yet other embodiments, the titanium-based material layer  20  has a thickness that is in the range of 100 Å to 500 Å. 
         [0032]    An under bump metallurgy (UBM) layer  24  is made up of a metal disposed directly onto portions of the titanium-alloyed surface  18  of the patterned aluminum-based material layer  14  not covered by the titanium-based material layer  20 . In some locations a crossover pattern  26  provides support for a bridging portion  28  of the UBM layer  24 . Contact resistivity between the metal of the UBM layer  24  and the titanium-alloyed surface is in the range of 1×10 −8  and 1×10 −7  ohm (Ω)/cm 2 . 
         [0033]      FIG. 3  is a cross-sectional view of a result of beginning steps completed for fabricating the portion of the embodiment of the acoustic wave device  10  depicted in  FIG. 2 . A first step is disposing the patterned aluminum-based material layer  14  onto the crystalline substrate  12  (step  100 ). A next step is disposing the titanium-based material layer  20  directly onto the patterned aluminum-based material layer  14  (step  102 ). The thickness of the titanium-based material layer  20  is controlled throughout the deposition step such that the titanium-based material layer  20  has an ultimate thickness that is selectively in the range of 20 Å to 500 Å. 
         [0034]      FIG. 4  is a cross-sectional view of a result of a next step that involves disposing the crossover pattern  26  that in later steps provides support for the bridging portion  28  of the UBM layer  24  (step  104 ). In an exemplary embodiment, the crossover pattern is made of photo definable polymer. Other suitable materials for the crossover pattern include, but are not limited to, silicon oxide, silicon oxynitride, and other dielectrics with a permeability below 5. 
         [0035]      FIG. 5  is a cross-sectional view of a result of another step that involves disposing the passivation layer  22  onto the titanium-based material layer  20 , the crossover pattern  26 , and exposed sections of the crystalline substrate  12  (step  106 ). In some embodiments, the passivation layer  22  is made of SiO 2  and in other embodiments the passivation layer is made of SiN or a bilayer of SiO 2  and SiN. 
         [0036]      FIG. 6  is a cross-sectional view of a result of another step that involves disposing a photoresist mask  30  over portions of the patterned aluminum-based material layer  14  and titanium-based material layer  20  (step  108 ). The photoresist mask  30  defines interconnect metal regions that are occupied by the UBM layer  24  in subsequent steps. 
         [0037]      FIG. 7  is a cross-sectional view of a result of another step that involves etching away portions of the passivation layer  22  and the titanium-based material layer  20  to leave regions of the titanium-alloyed surface  18  exposed (step  110 ). The passivation layer  22  is also etched from the crossover pattern  26  during the etching process. Techniques for etching away portions of the titanium-based material layer  20  are those commonly employed in passivation etching processes. 
         [0038]      FIG. 8  is a cross-sectional view of a result of another step that involves disposing the UBM layer  24  onto the exposed titanium-alloyed surface  18  and the crossover pattern  26  (step  112 ). The UBM layer  24  is typically made of metals such as a titanium and aluminum or titanium and gold. Contact resistivity between the metal of the UBM layer  24  and the titanium-alloyed surface is in the range of 1×10 −8  and 1×10 −7  Ω/cm 2 . 
         [0039]      FIG. 9  is a cross-sectional view of a result of a final step that involves lifting off the photoresist mask  30  ( FIG. 6 ) and unwanted portions of the UBM layer  24  (step  114 ). At this point, the acoustic wave device  10  ( FIG. 1 ) is complete and ready for testing. The acoustic wave device  10  typically has an absolute value of insertion loss that is no more than 2 dB. However, depending on the ultimate function of the acoustic wave device  10 , whether it is employed as a filter component or a sensor component determines the boundaries of insertion loss, which can range from 0.1 dB to 10 dB. 
         [0040]    Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.