ELECTRO-ACOUSTIC RESONATOR AND METHOD OF FORMING THEREOF

Abstract

An electro-acoustic resonator comprises a piezoelectric substrate on which an electrode structure is disposed. The electrode structure comprises a metal layer of aluminum and copper, a barrier layer forming a barrier against the diffusion of copper and another metal layer disposed on the barrier layer comprising aluminum. An AlCu intermetallic phase formed after an anneal is restricted to the portion beneath the barrier layer so that Galvano-corrosion of the electrode structure is avoided.

Claims

1. An electro-acoustic resonator, comprising: a substrate, the substrate having piezoelectric properties; an electrode structure disposed on the substrate, the electrode structure comprising: a metal layer, the metal layer comprising aluminum and copper; a barrier layer disposed on the metal layer to form a barrier against the diffusion of copper; and another metal layer disposed on the barrier layer, the other metal layer comprising aluminum.

2. The electro-acoustic resonator of claim 1, wherein the metal layer comprises grains of an intermetallic phase comprising aluminum and copper.

3. The electro-acoustic resonator of any of claims 1 to 2, wherein the barrier layer comprises a metal or a metal nitride.

4. The electro-acoustic resonator of any of claims 1 to 3, wherein the barrier layer comprises at least one of titanium, chromium, cobalt, tantal, tungsten, a nitride of one of titanium, tantal and tungsten.

5. The electro-acoustic resonator of any of claims 1 to 4, wherein the thickness of the metal layer is at least half of the thickness of the other metal layer.

6. The electro-acoustic resonator of any of claims 1 to 5, wherein, within the metal layer, the mass of copper is equal to the mass of aluminum.

7. The electro-acoustic resonator of any of claims 1 to 6, wherein the thickness of the metal layer is larger than the thickness of the other metal layer, wherein the thickness of the other metal layer is in the range of 15 nm to 30 nm, preferably in the range of 20 nm to 25 nm.

8. The electro-acoustic resonator of any of claims 1 to 7, wherein the substrate comprises lithium tantalate or lithium niobate.

9. The electro-acoustic resonator of any of claims 1 to 8, further comprising a seed layer disposed between the substrate and the metal layer, the seed layer comprising at least one of titanium and chromium.

10. The electro-acoustic resonator of any of claims 1 to 8, further comprising a seed layer disposed between the substrate and the metal layer, the seed layer comprising a metal, wherein the metal has a hardness of at least 1 Gigapascal.

11. The electro-acoustic resonator of claim 10, wherein the seed layer comprises a metal having one of a body centered cubic crystal structure and a hexagonal closed packed crystal structure.

12. The electro-acoustic resonator of claim 10 or 11, wherein the seed layer comprises a metal selected from the group consisting of chromium, cobalt, niobium, molybdenum and tungsten.

13. The electro-acoustic resonator of any of claims 1 to 12, wherein the electro-acoustic resonator is a surface acoustic wave resonator and the electrode structure forms an interdigital transducer arrangement.

14. A method of forming an electro-acoustic resonator, comprising the steps of: providing a substrate made of a piezoelectric material; forming a copper layer on the piezoelectric material; forming an aluminium layer on the copper layer; forming a barrier layer on the aluminum layer, the barrier layer configured to form a barrier against the diffusion of copper; forming another aluminum layer on the barrier layer; and annealing the formed structure to enable a diffusion of copper from the copper layer into the aluminum layer.

15. The method according to claim 14, comprising forming the aluminum layer with the same mass as the copper layer and forming a barrier layer comprising at least one of titanium, chromium, cobalt, tantal, tungsten, a nitride of one of titanium, tantal and tungsten.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In the drawings:

[0032] FIG. 1 shows a top view on a SAW resonator;

[0033] FIG. 2 shows two versions of acoustic reflectors;

[0034] FIG. 3 shows a cross-sectional view of a portion of a conventional SAW resonator;

[0035] FIG. 4 shows a cross-sectional view of a SAW resonator according to a first embodiment before an annealing step;

[0036] FIG. 5 shows a cross-sectional view of a SAW resonator according to the first embodiment after the annealing step;

[0037] FIG. 6 shows a cross-sectional view of a SAW resonator according to a second embodiment before an annealing step; and

[0038] FIG. 7 shows a cross-sectional view of a SAW resonator according to the second embodiment after the annealing step.

DETAIL DESCRIPTION OF EMBODIMENTS

[0039] The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings showing embodiments of the disclosure. The disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the disclosure will fully convey the scope of the disclosure to those skilled in the art. The drawings are not necessarily drawn to scale but are configured to clearly illustrate the disclosure.

[0040] Turning now to FIG. 1, a top view on an exemplary embodiment of a SAW resonator is shown. The depicted structure comprises an interdigital transducer (IDT) disposed on top of a piezoelectric substrate. The IDT is formed by two electrodes 110, 120, each comprising interleaved fingers 111, 112 and 121, 122, respectively. The shape of the fingers and their length and distance is selected such that the resonator meets expected electrical characteristics such as resonance frequency, Q-factor etc. The resonator may be part of an electronic RF filter, for example, in a mobile communication device such as a smartphone. In practice, the IDT may comprise several hundreds, e.g., about 300 fingers.

[0041] In operation, an electrical signal is supplied to the terminals of the IDT structure and generates an acoustic resonating wave within the piezoelectric substrate. In order to avoid leaking of the acoustic wave from the IDT structure, acoustic reflectors 131, 133 are provided adjacent to the side portions of the IDT structure. Examples for the reflectors 131, 133 are shown in FIG. 2, and a cross-section through two adjacent fingers of the electrode pair of the IDT along line A-A is depicted in FIG. 3.

[0042] Turning now to FIG. 2, depending on the electronic requirements, the reflectors may be open-circuited as shown at 210 or short-circuited as shown at 220. Each reflector includes parallel disposed fingers such as 211, 212. The principles explained in this disclosure with regard to the electrode structures of the IDTs apply to the reflectors as well.

[0043] Turning now to FIG. 3, the cross-section shows substrate 310 and two adjacent fingers 121, 112 of the two electrodes of the IDT along ling A-A of FIG. 1. The substrate 310 has piezoelectric properties and any suitable piezoelectric material may be used. In accordance with the present disclosure, the substrate 310 is made of lithium tantalate (LiTaO.sub.3) or lithium niobate (LiNbO.sub.3). The conventional finger 121 comprises a seed layer 321 of titanium (Ti) on which a thin copper (Cu) layer is formed. One single aluminum (Al) layer is deposited on the copper layer followed by an anneal step. FIG. 3 shows the resulting structure after the anneal step. The cross-section demonstrates that the copper diffused through the aluminum thereby forming an AlCu-intermetallic phase primarily comprising Al.sub.2Cu grains such as 322, 323, 324 embedded in the remaining unreacted Al grains 331, 332. As can be gathered from FIG. 3, grain 323 reaches the top surface of finger 121, and grain 324 reaches the sidewall surface of finger 112. Al.sub.2Cu grain 323 is in direct contact with the Al material 331 at the top surface of finger 121 so that this structure has a corrosive potential if subjected to corrosive media such as cleaning and rinsing fluids and especially alkaline or acid media. In particular, a problematic situation occurs when the structure shown in FIG. 3 is treated in a photolithography process that uses basic or alkaline solvents for the photodeveloper. Having grains of noble and less noble metals in contact with each other subjected to a basic photodeveloper will lead to a Galvano-corrosive chemical reaction that destroys the less noble metal that is the pure aluminum grains at the surface of the IDT fingers.

[0044] FIG. 4 shows a cross-section through a portion of a SAW resonator of a first embodiment at an intermediate step during its production. During manufacturing, the piezoelectric substrate 310 is provided, made of one of the above-described piezoelectric materials. On the surface of the substrate 310 a layer of an adhesion promoter or a seed layer 430 is formed which may be a layer of titanium. Thereon formed is a layer of copper 431 and thereon formed is a first layer of aluminum 440. While aluminum has high electrical conductivity, the addition of copper increases the acoustic hardness of the electrode. On top of the first aluminum layer 440, a barrier layer 450 is deposited. The barrier material for layer 450 is selected such that it forms a barrier against the diffusion of copper. The barrier layer 450 ensures that no copper will diffuse beyond barrier layer 450.

[0045] Barrier layer 450 confines the copper in the region between the adhesion layer 430 and the barrier layer 450. On top of barrier layer 450, a second aluminum layer 441 is formed that provides good electrical conductivity. The thickness of layer 441 may be about two times the thickness of layer 440. The layer stack is suitably structured to realize the shape of the electrodes of the IDT as shown in FIG. 1 or the shape of the reflectors as shown in FIG. 2. After deposition and structuring the workpiece as shown in FIG. 4, an anneal step is performed.

[0046] FIG. 5 shows a cross-section of the resulting structure after the anneal step. As can be gathered from FIG. 5, the originally deposited copper and aluminum layers 431, 440 are transformed to a new layer 535 that includes grains of an intermetallic AlCu phase, primarily grains of Al.sub.2Cu. The anneal step is performed at a temperature of about 270 C. for a sufficiently long time so that the copper from original layer 431 diffuses through the original first aluminum layer 440 and reacts with the aluminum to the intermetallic Al.sub.2Cu phase. This reaction is continued until all available Cu from layer 431 is transformed into the Al.sub.2Cu phase 535 so that the diffusion process reaches saturation. The mass of Cu and the thickness of the Al layer 440 are selected such that no unreacted aluminum from original aluminum layer 440 remains underneath the barrier layer 450. No Al grains remain in the Al.sub.2Cu layer 535 so that layer 535 consists substantially only of Al.sub.2Cu grains with no unreacted Al grains. This requires that the deposited mass of copper in layer 431 is substantially the same as the mass of aluminum in layer 440. The relation of mass of the Cu layer 431 and the Al layer 440 is selected to 1:1. Especially, no copper passes through barrier layer 450 so that no copper from layer 431 reaches the second aluminum layer 441.

[0047] The sidewalls of layer 535 include only Al.sub.2Cu grains without any remaining Al grains so that these sidewalls will not be affected by Galvano-corrosion. Specifically, aggressive fluids such as basic or alkaline solvents of photodevelopers will not affect or deteriorate the Al.sub.2Cu layer 535. As an advantage, the structure shown in FIG. 5 can be further treated with photolithography steps so that the shown fingers of the IDT electrodes can be trimmed. Trimming can be made to correct manufacturing variations of electrical parameters and/or to fine-tune a precise resonance frequency of the SAW resonator. The yield of the production process is increased in that a trimming by photolithography is possible.

[0048] As another advantage, it is to be noted that the generated Al.sub.2Cu grains are confined to the portion of the electrode finger underneath barrier layer 450 and inbetween barrier layer 450 and substrate 310 or adhesion promoter 430. The portion of the electrode close to the surface of the piezoelectric substrate 310 is subjected to considerable mechanical stress during the acoustic operation of the device when compared to the portion of the electrode distant from the substrate 310 such as the second aluminum layer 441. Because the addition of copper to the aluminum material generates the mechanically harder Al.sub.2Cu grains and the concentration of the Al.sub.2Cu grains is increased in the region close to the substrate, the power durability of the resulting SAW resonator is increased.

[0049] The thickness of Al.sub.2Cu layer 535 and the thickness of Al layer 441 is selected such that the acoustic properties are maintained. According to the embodiment of FIGS. 4 and 5, the layers are deposited such with respect to barrier layer 450 that, after the anneal, the thickness of the Al.sub.2Cu layer 535 is about half of the thickness of the aluminum layer 441.

[0050] In other words, the thickness of the aluminum layer 441 is about two times the thickness of the Al.sub.2Cu layer 535. Furthermore, considering the combined thickness of layers 441 and 535 or the combined thickness of deposited layers 441 and 440, the barrier layer 450 is positioned at a height of one third () of the combined thickness. This ensures that the IDT electrode design achieves acoustic and electrical characteristics according to the knowledge and experience of the design engineer resulting from conventional electrode designs. Provided that the thickness of the aluminum layer 441 is two times the thickness of the Al.sub.2Cu layer 535 and that the mass of Cu and the mass of Al have a relation of about 1:1 in layer 535, layer 535 contains only Al.sub.2Cu grains and no pure Al grains so that the electrode is highly corrosion resistant and power durable.

[0051] The materials useful for the barrier layer comprise such metals or metal nitrides that form a barrier against the diffusion of copper. Suitable metals are titanium, chromium, cobalt, tantalum, tungsten of which the barrier layer 450 can be formed. The barrier layer 450 can be one layer of at least one of said metals or a sandwich of one or more layers of said metals. Furthermore, nitrides of titanium, tantalum or tungsten are possible as one layer or as a sandwich of two or more layers of said nitrides. Also a sandwich of a metal layer with a metal nitride layer is possible.

[0052] According to a second embodiment depicted in FIGS. 6 and 7, the first aluminum layer 640 has a substantially larger thickness than the second aluminum layer 641. The barrier layer 650 is disposed between the two aluminum layers 640, 641. The overall height or thickness of the electrode is determined such that the desired acoustic resonating properties are achieved. The thickness of the second top aluminum layer 641 may be relatively thin, in the range of 15 nm to 30 nm. Preferably, the second aluminum layer has a thickness of between 20 nm to 25 nm. A copper layer 631 is disposed underneath the first aluminum layer 640, and a seed layer 630 is disposed underneath the copper layer 631.

[0053] Turning now to FIG. 7, a cross-sectional view of the electrodes is shown after an anneal step. The copper from original layer 631 diffused through the first aluminum layer 640 and formed Al.sub.2Cu grains 736. The mass of copper in copper layer 631 is in the range of 2 to 9 weight-% compared to the above-disposed aluminum layer 640 so that, after the anneal, unreacted aluminum grains 737 are still present in the first metal layer 735 below the barrier layer 640. Al.sub.2Cu grains may reach the sidewall surfaces of the electrode 621 in connection with Al grains. However, the barrier layer 650 prevents the copper from diffusing into the top aluminum layer 641 so that no Al.sub.2Cu grains reach the top surface of the electrode. The embodiment depicted in FIG. 7 is substantially corrosion-resistant in that the top surface of the electrode 621 includes only pure Al grains and no Al.sub.2Cu grains. The latter are confined to the portion below the barrier layer 650. On the other hand, the lower electrode includes Al.sub.2Cu grains in connection with Al grains. This combination exhibits sufficient acoustic hardness for certain ranges of resonance frequencies.

[0054] The adhesion layer or seed layer 430, 630 may be made of titanium. As an alternative, the material of the seed layer 430, 630 is selected from one of chromium, cobalt, niobium, molybdenum and tungsten. In the following, it is assumed that seed layer 430, 630 is made of chromium. The seed layer 430, 630 of chromium has a thickness of between 10 nm to 20 nm, preferably 15 nm. The overall height of the electrode reaching from the bottom of the seed layer or the top surface of the piezoelectric substrate to the top surface of the electrode is between 120 nm and 400 nm depending on the frequency range and the field of application of the SAW resonator. A relatively thin layer of silicon nitride (not shown) covers the electrode. The silicon nitride layer has a thickness of between 3 nm to 7 nm, preferably 5 nm. The chromium seed layer has a relatively high Vickers hardness of 1.06 GPa and a moderate density of 7.19 g/cm.sup.3. Accordingly, the chromium seed layer is relatively hard and may be dimensioned relatively thick. This increases the acoustic stability of the AlCu portion of the electrode, especially in the bottom portion which is in contact with or close to the seed layer and especially at the corner portions of the electrode close to the sidewall surface portions of the electrode near the seed layer. Compared to conventional systems, the height of the electrode is to be reduced thereby counterbalancing the mass added by the chromium layer. This does not noticeably increase the electrical resistance of the IDT or affect the resonator frequency.

[0055] The improved power durability of the electrode avoids defects and cracks that may be generated or may propagate in the aluminum grains or in the aluminum copper grains or along the grain boundaries so that the setup resonance frequency of the SAW resonator is maintained over its lifetime.

[0056] It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosure as laid down in the appended claims. Since modifications, combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirt and substance of the disclosure may occur to the persons skilled in the art, the disclosure should be construed to include everything within the scope of the appended claims.