Epitaxial growth of aluminum on aluminum nitride compound

Abstract

To provide a bulk acoustic wave (BAW) resonator structure.SOLUTION: A 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. 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 and exposing it to elemental aluminum atoms for a predetermined duration. The aluminum is epitaxially grown in this manner for a predetermined duration to produce an epitaxial aluminum layer of a predetermined thickness. Such epitaxially grown single-crystal aluminum has a lower resistivity than polycrystalline aluminum.SELECTED DRAWING: Figure 1

Description

Background Art

[0001] Bulk acoustic wave (BAW) resonators are used for high-frequency filtering of signals. Various structures and materials are used to fabricate 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 accurate frequency selection for filtering electrical signals. However, these resonance characteristics depend on various metrics of a particular BAW resonator structure, such as the piezoelectric material, contact material, and thickness of each 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. A method of doing so includes loading a wafer having a single-crystalline aluminum nitride compound layer into a vacuum chamber where 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

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DETAILED DESCRIPTION OF THE INVENTION

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

[0006] FIG. 1 is a cross-sectional view of a bulk acoustic wave (BAW) resonator structure in which epitaxially grown aluminum grows on a piezoelectric aluminum nitride compound within a vacuum chamber. The use of the vacuum chamber facilitates high-quality epitaxial growth of aluminum because unintentional oxidation of such an aluminum layer is thereby prevented. In FIG. 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. In the aluminum nitride-compound layer 12, various types of aluminum nitride compounds can be used. For example, in the aluminum nitride-compound layer 12, scandium-aluminum nitride (Sc 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 the art (see, for example, Akiyama et al. Appl. Phys. Lett. 95 162107 (2009)).

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

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

[0009] Figure 2 is a graph of the thickness of the piezoelectric aluminum nitride - compound layer versus 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 indicates the thickness of the piezoelectric aluminum nitride - compound layer 12 (as represented in FIG. 1) as measured between the inside of the surface of the first contact layer 14 and the inside of the surface of the second contact layer 16. The first vertical axis 26 indicates the thickness of aluminum in each of 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 thickness of each of the first contact layer 14 and the second contact layer 16 and the thickness of the aluminum nitride - compound layer 12. As indicated by the contact - thickness / resonator - thickness relationship 32, as the thickness of the contact layer decreases, the thickness of the aluminum nitride - compound layer can increase for a given center frequency.

[0010] The second vertical axis 26

Number

[0011] Figure 3 is a graph of thickness for both sputter-deposited aluminum and epitaxially grown aluminum with respect to 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 indicates the thickness of the first contact layer 14 (as represented in Figure 1). The vertical axis 42 indicates the resistivity of the first contact layer 14. The electrical-resistivity / thickness relationship 44 represents the relationship between the electrical resistivity (as measured, for example, in ohm-meters) and the thickness (as measured, for example, in nm) of a sputter-deposited 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 sputter-deposited layer of polycrystalline aluminum. The electrical-resistivity data 46 and thickness data 48 are data points representing the relationship between the electrical resistivity and the thickness of an epitaxially grown layer of single-crystalline aluminum used for the first contact layer 14. As represented in Figure 3, the epitaxially grown single-crystalline aluminum has a much lower electrical resistivity than the sputter-deposited polycrystalline aluminum. Such a low electrical resistivity can allow a thinner first contact layer to be used without being affected by the resulting sheet resistance. Conversely, the same thickness can be used for the first contact layer, and the epitaxially grown single-crystalline aluminum exhibits a much-reduced sheet resistance. It should be noted that the resulting resistivity of the single-crystalline aluminum used for the first contact layer 14 is close to the resistivity of the bulk resistivity of aluminum (about 2.65×10 -8 Ω·m). In some embodiments, the resistivity of the single-crystalline 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 - 4B are respectively plan views of the surface of aluminum deposited below room temperature and the surface of aluminum deposited at room temperature. Figure 4A represents an atomic force microscope (AFM) image of the surface of the first contact layer 14 (as represented in FIG. 1) for aluminum grown epitaxially at a temperature below room temperature. The display of Figure 4A shows a flat surface as viewed by the microscope, without any indication of aluminum clusters. X-ray diffraction studies have confirmed that such aluminum grown epitaxially below room temperature is indeed a single crystal. Figure 4B represents an image of the surface of the first contact layer 14 (as represented in FIG. 1) for aluminum grown epitaxially at room temperature. The display of Figure 4B (and the enlarged view shown in the inserted Figure 4B above) shows cluster formation, which can indicate polycrystalline aluminum formation (having a higher electrical resistivity) or electrical discontinuities between the clusters. The surface mobility of aluminum is high even at room temperature. Thus, at room temperature, aluminum adatoms tend to cluster with each other before they find the support sites that form crystallographic bonds with the underlying material. Therefore, room temperature is the cluster-enabled temperature. There exists a cluster-effective threshold temperature, above which aluminum adatoms support cluster formation in terms of crystallographic bonding, and below which aluminum adatoms support crystallographic bonding in terms of cluster formation. Due to room temperature being above such a cluster-effective temperature threshold, attempts to grow aluminum epitaxially on aluminum nitride compounds typically result in polycrystalline aluminum deposition and do not result in single crystal aluminum formation. Conversely, attempts to grow aluminum epitaxially on aluminum nitride compounds can succeed below the cluster-effective temperature threshold.

[0013] FIG. 5 is a flowchart of a method for epitaxially growing single crystal aluminum on an aluminum nitride compound. In FIG. 5, the flowchart represented describes a method 50 for forming an epitaxial layer of aluminum on an aluminum nitride compound. Method 50 begins at step 52, where a wafer having a single crystal surface of an aluminum nitride compound is loaded into a vacuum chamber for molecular beam epitaxy. Next, at step 56, the wafer can optionally be cleaned in vacuo via various techniques known in the art, such as thermal degassing, plasma cleaning, gallium flash cleaning, or other approaches. At step 66, the temperature of the substrate is decreased below a cluster formation temperature threshold. The temperature maintained is lower than zero degrees Celsius. Alternatively, the temperature maintained is less than -25 degrees Celsius. Various methods for cooling the wafer can be performed at step 66. When the temperature of the substrate has decreased to a temperature below the cluster formation temperature threshold, the method proceeds to step 68, where elemental aluminum is introduced into the chamber (e.g., via an evaporation cell). At step 68, aluminum grows epitaxially on the single crystal surface of the aluminum nitride compound. The method remains at step 68 until the epitaxially grown aluminum layer reaches a predetermined thickness. At step 72, the temperature of the substrate is increased to room temperature. Finally, at step 74, the wafer is unloaded from the chamber and method 50 ends.

[0014] Figures 6A-6B are cross-sectional views of embodiments of a BAW resonator structure at various stages of manufacture. In Figure 6A, a 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. Then, in Figure 6B, the lower surface of the substrate 76 is partially removed, such as through mechanical etching or chemical etching, to provide access to a lower single crystal surface 80. Next, a second contact layer 16' is formed on the lower single crystal surface 80, either by polycrystalline aluminum or single crystal aluminum. In this way, for example, a sandwich structure of the BAW resonator structure 10' can be formed within a specific region of the substrate 76.

[0015] Although the invention has been described with reference to exemplary embodiments (s), those skilled in the art will understand that various changes can be made without departing from the scope of the invention and equivalents can be substituted for those elements. In addition, numerous modifications can be made to adapt a particular situation or material to the teachings of the invention without departing from their essential scope. Accordingly, the invention is not limited to the specific embodiments (s) disclosed, but the invention is intended to cover all embodiments falling within the scope of the appended claims.

Claims

1. A bulk acoustic wave (BAW) resonator structure comprising: a single-crystalline aluminum nitride compound layer; 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; a second contact layer bonded to a second surface of the single-crystalline aluminum nitride compound layer; The BAW resonator structure having the above.

2. The BAW resonator structure according to claim 1, wherein the first contact layer has a thickness smaller than the critical thickness limit.

3. The BAW resonator structure according to claim 1, wherein the first contact layer has a thickness of less than 40 nm.

4. The BAW resonator structure according to claim 1, wherein the first contact layer has a thickness of less than 30 nm.

5. The aluminum nitride compound layer is made of scandium ( ｘ )-aluminum( １－ｘ )-nitride (Sc ｘ Al １－ｘ 10. The BAW resonator structure of claim 1, comprising:

6. The BAW resonator structure according to claim 5, wherein x is between 0.00 and 0.

50.

7. The BAW resonator structure according to claim 6, wherein x is between 0.15 and 0.

45.

8. The BAW resonator structure according to claim 7, wherein x is between 0.40 and 0.

43.

9. The BAW resonator structure according to claim 1, wherein the second contact is a polycrystalline aluminum contact layer.

10. The BAW resonator structure according to claim 9, wherein a second thickness of the second contact is greater than a first thickness of the first contact.

11. The BAW resonator structure according to claim 1, wherein the single-crystalline aluminum nitride compound layer is a piezoelectric layer.

12. The BAW resonator structure according to claim 1, wherein a resistivity of the first contact layer is less than 150% of a bulk resistivity of aluminum.

13. The BAW resonator structure according to claim 1, wherein a resistivity of the first contact layer is less than 135% of a bulk resistivity of aluminum.

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