Epitaxial growth of aluminum on aluminum nitride compounds

Abstract

The apparatus and associated method relate to forming an epitaxial layer (14') of aluminum on an aluminum nitride compound (12'). The aluminum is epitaxially grown on the crystalline aluminum nitride compound by maintaining the temperature of the crystalline aluminum nitride compound below a cluster effective temperature threshold in a vacuum chamber. The crystalline aluminum nitride compound is then exposed to atoms of elemental aluminum for a predetermined duration. The aluminum is epitaxially grown in this manner for a predetermined duration to produce a layer of epitaxial aluminum of a predetermined thickness. Such epitaxially grown single crystalline aluminum has a lower resistivity than polycrystalline aluminum. The resulting structure is used in a BAW resonator (10'), with the epitaxial layer (14') of aluminum functioning as the top electrode and the crystalline aluminum nitride compound (12') functioning as the piezoelectric layer. The bottom aluminum electrode (16') may be polycrystalline.

Description

Background Art

[0001] Bulk acoustic wave (BAW) resonators are used for high-frequency filtering of signals. Various structures and materials are used to manufacture BAW resonators. These structures are generally formed from a thin layer of piezoelectric material sandwiched between two electrically conductive contacts. These structures are then acoustically isolated from the surrounding medium, such as any substrate. The impedance / frequency relationship of such BAW resonators may have distinct resonance characteristics. Such impedance resonances may result in precise frequency selection for filtering electrical signals. However, these resonance characteristics depend on various metrics of a particular BAW resonator structure, such as, for example, each of the thicknesses of the piezoelectric material, the contact material, and the layer. Thinner contact layers advantageously facilitate better control and performance of these BAW resonator structures.

Summary of the Invention

[0002] Apparatus and related methods relate to forming an epitaxial layer of aluminum on a single-crystalline aluminum nitride compound layer. The method of doing so includes loading a wafer having a single-crystalline aluminum nitride compound layer into a vacuum chamber in which the temperature and atmosphere are controlled. The method further includes maintaining the temperature of the wafer below a cluster effective temperature threshold. The method also includes exposing the wafer to atoms of elemental aluminum for a predetermined duration within the vacuum chamber.

[0003] Some embodiments relate to a bulk acoustic wave (BAW) resonator structure. The BAW resonator structure includes a single-crystalline aluminum nitride-compound layer. The BAW resonator structure further includes a first contact layer bonded to a first surface of the single-crystalline aluminum nitride-compound layer, the first contact layer being a single-crystalline aluminum contact layer crystallographically bonded to the single-crystalline aluminum nitride-compound layer. The BAW resonator structure also includes a second contact layer bonded to a second surface of the single-crystalline aluminum nitride-compound layer.

Brief Description of the Drawings

[0004] [Figure 1] This is a cross-sectional view of a bulk acoustic wave (BAW) resonator structure in which epitaxially grown aluminum grows against an aluminum nitride compound in a vacuum chamber. [Figure 2] This is a graph of the thickness of the piezoelectric aluminum nitride-compound layer against the simulated figure of merit (FOM) for a BAW resonator. [Figure 3] This is a graph of the resistance of aluminum against thickness for both sputtered and epitaxially grown aluminum. [Figure 4] Figures A and B are plan views of the surface of aluminum deposited below room temperature and the surface of aluminum deposited at room temperature. [Figure 5] This is a flowchart for a method of epitaxially growing aluminum on an aluminum nitride compound. [Figure 6] Figures A and B are cross-sectional views of embodiments of BAW resonator structures at various manufacturing stages. [Modes for carrying out the invention]

[0005] The apparatus and associated methods relate to forming an epitaxial layer of aluminum against an aluminum nitride compound. Aluminum is grown epitaxially against a single-crystal aluminum nitride compound by maintaining the temperature of the single-crystal aluminum nitride compound below a cluster effective temperature threshold in a vacuum chamber. The single-crystal aluminum nitride compound is then exposed to atoms of elemental aluminum for a predetermined duration. The aluminum is thus grown epitaxially for a predetermined duration to produce a layer of epitaxial aluminum of a predetermined thickness. Such epitaxially grown single-crystal (i.e., single-crystal) aluminum has a lower resistivity than polycrystalline aluminum. Because the resistivity of epitaxially grown aluminum is lower than that of polycrystalline aluminum, a single-crystal aluminum contact layer can be thinner than a polycrystalline contact layer of equal resistance. Such a thin contact layer can facilitate better control and performance of bulk elastic wave (BAW) resonator structures.

[0006] Figure 1 is a cross-sectional view of a bulk acoustic wave (BAW) resonator structure in which epitaxially grown aluminum is grown against a piezoelectric aluminum nitride compound in a vacuum chamber. The use of a vacuum chamber promotes high-quality epitaxial growth of aluminum because it prevents unintended oxidation of such aluminum layers. In Figure 1, the BAW resonator structure 10 includes a single-crystal aluminum nitride compound layer 12 sandwiched between a first contact layer 14 and a second contact layer 16. Various types of aluminum nitride compounds can be used in the aluminum nitride compound layer 12. For example, scandium-aluminum nitride (Sc) can be used in the aluminum nitride compound layer 12. x Al 1-x N) or yttrium-aluminum nitride (Y x Al 1-x N) can be used. Sc x Al 1-xThe subscript x used in the formula for N can be, for example, 0 to 0.50, 0.15 to 0.45, or 0.40 to 0.43. Here, the piezoelectric response changes in response to changes in the relative composition index x, as is known in this art (see, for example, Akiyama et al. Appl. Phys. Lett. 95 162107 (2009)).

[0007] The first contact layer 14 is a layer of single-crystal aluminum crystallographically bonded to the first surface 18 of the single-crystal aluminum nitride-compound layer 12. The crystallographic bonding of the first contact layer 14 to the single-crystal aluminum nitride-compound layer 12 results from the epitaxial growth of the first contact layer 14 on the crystallographic structure of the single-crystal aluminum nitride-compound layer 12 relative to the single-crystal aluminum material. Assuming that the transverse lattice dimensions are equivalent (e.g., relevant) to those of the underlying single-crystal material, such epitaxial growth of one material on a single-crystal material results in the grown material. Similarly, such epitaxial growth also results in a crystallographic relationship between the grown material and the single-crystal surface material of the substrate.

[0008] The second contact layer 16 is bonded to the second surface 20 of the single-crystal aluminum nitride-compound layer 12. In some embodiments, the second contact layer 16 is polycrystalline aluminum, while in other embodiments, the second contact layer 16 is single-crystal aluminum. In these latter embodiments, the aluminum for both the first contact layer 14 and the second contact layer 16 can be grown epitaxially on the single-crystal aluminum nitride-compound layer 12. In principle, the first contact layer 14, the aluminum nitride-compound layer 12, and the second contact layer 16 are all grown sequentially, and in practice, the first contact layer 14 and / or the second contact layer 16 are typically grown at separate times. In such cases, an etching step is performed to remove part / all of the substrate material under the aluminum nitride-compound layer 12 before the second contact layer 16 is created, thereby enabling epitaxial growth on the exposed surface of the single-crystal aluminum nitride-compound layer 12. As shown below, epitaxially grown aluminum contact layers can have lower bulk resistivity than conventionally deposited polycrystalline aluminum contact layers.

[0009] Figure 2 is a graph of the thickness of the piezoelectric aluminum nitride-compound layer against the simulated figure of merit (FOM) for a BAW resonator. In Figure 2, graph 22 has a horizontal axis 24, a first vertical axis 26, a second vertical axis 28, a contact-thickness / resonator-thickness relationship 32, and a figure of merit (FOM) / resonator-thickness relationship 34. The horizontal axis 24 shows the thickness of the piezoelectric aluminum nitride-compound layer 12 (as shown in Figure 1) as measured between the interior of the surface of the first contact layer 14 and the interior of the surface of the second contact layer 16. The first vertical axis 26 shows the thickness of each of the aluminum in the first contact layer 14 and the second contact layer 16 grown / deposited on the single-crystal aluminum nitride-compound layer 12. The contact-thickness / resonator-thickness relationship 32 represents the relationship between the respective thicknesses of the first contact layer 14 and the second contact layer 16 and the thickness of the aluminum nitride-compound layer 12. As shown by the contact-thickness / resonator-thickness relationship 32, the thickness of the aluminum nitride-compound layer can increase with respect to a given center frequency as the thickness of the contact layer decreases.

[0010] The second vertical axis 26 is,

number

[0011] Figure 3 is a graph of the thickness of both sputtered and epitaxially grown aluminum against the resistivity of aluminum. In Figure 3, graph 38 includes a horizontal axis 40, a vertical axis 42, an electrical-resistivity / thickness relationship 44, and electrical-resistivity data 46 / thickness data 48. The horizontal axis 40 represents the thickness of the first contact layer 14 (as shown in Figure 1). The vertical axis 42 represents the resistivity of the first contact layer 14. The electrical-resistivity / thickness relationship 44 represents the relationship between the electrical resistivity (e.g., measured in ohms) and thickness (e.g., measured in nm) of a sputtered layer of polycrystalline aluminum (data from A. Karoui 2011 ECS Trans. 41 21). As shown in the electrical-resistivity / thickness relationship 44, the electrical resistivity is approximately inversely proportional to the thickness of the sputtered layer of polycrystalline aluminum. The electrical resistivity data 46 and thickness data 48 are data points representing the relationship between the electrical resistivity and thickness of an epitaxially grown layer of single-crystal aluminum used for the first contact layer 14. As shown in Figure 3, epitaxially grown single-crystal aluminum has a much lower electrical resistivity than sputter-deposited polycrystalline aluminum. Such low electrical resistivity allows for the use of a thinner first contact layer without being affected by the resulting sheet resistance. Conversely, while the same thickness can be used for the first contact layer, epitaxially grown single-crystal aluminum exhibits a much reduced sheet resistance. The resulting resistivity of the single-crystal aluminum used for the first contact layer 14 is close to the resistivity of the bulk resistivity of aluminum (approximately 2.65 × 10⁻⁶). -8 It should be noted that (Ω·m). In some embodiments, the resistivity of the single-crystal aluminum used for the first contact layer 14 can be less than 150%, 135%, 120%, or 110% of the bulk resistivity of aluminum.

[0012] Figures 4A and 4B are plan views of the surface of aluminum deposited below room temperature and the surface of aluminum deposited at room temperature, respectively. Figure 4A shows an atomic force microscope (AFM) image of the surface of the first contact layer 14 (as shown in Figure 1) for aluminum epitaxially grown at a temperature below room temperature. The display in Figure 4A shows a microscopically flat surface with no indication of aluminum clusters. X-ray diffraction studies have confirmed that such aluminum epitaxially grown below room temperature is indeed a single crystal. Figure 4B shows an image of the surface of the first contact layer 14 (as shown in Figure 1) for aluminum epitaxially grown at room temperature. The display in Figure 4B (and the magnified view shown in Figure 4B above) shows cluster formation, which can indicate polycrystalline aluminum formation (with higher electrical resistivity) or electrical discontinuities between clusters. The surface mobility of aluminum is high even at room temperature. Therefore, at room temperature, adsorbed aluminum atoms tend to cluster with each other before they find support sites to form crystallographic bonds with the underlying material. Thus, room temperature is the cluster-effective temperature. A cluster-effective threshold temperature exists; above this temperature, adsorbed aluminum atoms support cluster formation on crystallographic bonds, and below this temperature, adsorbed aluminum atoms support crystallographic bonds on cluster formation. Because room temperature is above such a cluster-effective temperature threshold, attempts to epitaxially grow aluminum in aluminum nitride compounds typically result in polycrystalline aluminum deposition, not single-crystal aluminum formation. Conversely, attempts to epitaxially grow aluminum in aluminum nitride compounds below the cluster-effective temperature threshold can be successful.

[0013] Figure 5 is a flowchart of a method for epitaxially growing single-crystal aluminum on an aluminum nitride compound. In Figure 5, the flowchart illustrates method 50 for forming an epitaxial layer of aluminum on an aluminum nitride compound. Method 50 begins in step 52, in which a wafer having a single-crystal surface of an aluminum nitride compound is loaded into a vacuum chamber for molecular beam epitaxy. Then, in step 56, the wafer can optionally be cleaned under vacuum via various techniques known in the art, such as thermal degassing, plasma cleaning, gallium flash cleaning, or other approaches. In step 66, the substrate temperature is reduced below the clustering temperature threshold. The temperature maintained is lower than 0°C, or the temperature maintained is below -25°C. Various methods for cooling the wafer can be performed in step 66. When the substrate temperature decreases to a temperature below the cluster formation temperature threshold, the method proceeds to step 68, in which elemental aluminum is introduced into the chamber (e.g., via an evaporation cell). In step 68, the aluminum is epitaxially grown on the single crystal surface of the aluminum nitride compound. The method remains in step 68 until the epitaxially grown aluminum layer reaches a predetermined thickness. In step 72, the substrate temperature is increased to room temperature. Finally, in step 74, the wafer is Unload from the room And with that, method 50 is finished.

[0014] Figures 6A-6B are cross-sectional views of embodiments of BAW resonator structures at various manufacturing stages. In Figure 6A, the BAW resonator structure 10' is formed on a substrate 76 having a single-crystal surface 78 of a single-crystal aluminum nitride-compound layer 12'. On the single-crystal surface 78 is a first contact layer 14' composed of a single-crystal aluminum layer. Subsequently, in Figure 6B, the lower surface of the substrate 76 is partially removed, for example, through mechanical etching or chemical etching, to provide access to the lower single-crystal surface 80. Then, a second contact layer 16' is formed on the lower single-crystal surface 80, either of polycrystalline aluminum or single-crystal aluminum. In this way, for example, the sandwich structure of the BAW resonator structure 10' can be formed within a specific region of the substrate 76.

[0015] While the present invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various modifications can be made and equivalents can be substituted for those elements without departing from the scope of the invention. In addition, numerous modifications can be made to adapt the teachings of the invention to specific situations or materials without departing from their essential scope. Thus, the present invention is not limited to the specific embodiments disclosed, but is intended to include all embodiments that fall within the scope of the appended claims.

Claims

1. A method for forming an aluminum epitaxial layer on a single-crystal aluminum nitride compound layer, A step of loading a wafer containing a single-crystal aluminum nitride compound layer into a vacuum chamber where both temperature and environment are controlled, wherein the single-crystal aluminum nitride compound layer is positioned directly on the wafer, The steps include maintaining the wafer temperature at a temperature lower than the cluster favoring temperature threshold, The steps include: exposing the wafer to atoms of elemental aluminum in the vacuum chamber for a predetermined period of time; Includes, The method wherein the temperature maintained is below -25°C.

2. The method according to claim 1, further comprising the step of unloading the wafer from the vacuum chamber.

3. The method according to claim 1, wherein the single-crystal aluminum nitride compound layer is a piezoelectric layer.

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