DEVICE HAVING A TITANIUM-ALLOYED SURFACE

[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.

[0002] The present disclosure is directed to improving performance characteristics of acoustic wave devices.

[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.

[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)/cm² and 1×10⁻⁷ Ω/cm².

[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.

[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 (LiTaO₃) or lithium niobate (LiNbO₃), 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)/cm².

[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 SiO₂ and in other embodiments the passivation layer is made of SiN or a bilayer of SiO₂ 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/cm².

[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.