METHOD FOR MANUFACTURING AN ELECTRO-ACOUSTIC RESONATOR AND ELECTRO-ACOUSTIC RESONATOR DEVICE

Abstract

A seed layer (210) of a noble metal is formed by electrochemical deposition on a metal electrode (111) disposed on a dielectric layer (110,310). The noble metal seed layer allows the deposition of a highly textured piezoelectric layer (320) on the metal electrode.

Claims

1. A method for manufacturing an electro-acoustic resonator, comprising: providing a workpiece comprising a dielectric layer; forming a metal electrode on the dielectric layer of the workpiece; providing a solution containing a salt of a noble metal; immersing the workpiece having the metal electrode disposed thereon into the solution to deposit a layer of the noble metal on the metal electrode; forming a piezoelectric layer on the metal electrode.

2. The method of claim 1, wherein immersing the workpiece into the solution comprises performing an electro-chemical plating process to deposit the layer of the noble metal on the metal electrode.

3. The method of claim 1, wherein providing a workpiece comprises providing a bragg mirror layer stack including a dielectric layer at its surface.

4. The method of claim 3, wherein the dielectric layer comprises a layer of silicon oxide or silicon dioxide.

5. The method of claim 1, wherein the metal of the metal electrode comprises at least one of tungsten, molybdenum, titanium, aluminum and copper.

6. The method of claim 1, wherein the step of forming a metal electrode comprises forming a metal electrode of a metal selected from one of tungsten, molybdenum, titanium, aluminum and a composition of aluminum and copper.

7. The method of any of claims 1, wherein the noble metal comprises at least one of platinum, palladium, ruthenium and nickel.

8. The method of claim 1, wherein the salt of the noble metal comprises at least one of sodium hexachloroplatinate (II) or Na.sub.2PtCl.sub.6, potassium hexachloroplatinate (II) or K.sub.2PtCl.sub.6, sodium tetrachloropalladate (II) or Na.sub.2PdCl.sub.4, potassium tetrachloropalladate (II) or K.sub.2PdCl.sub.4, potassium hexachloropalladate (IV) or K.sub.2PdCl.sub.6, ruthenium (III) chloride hydrate or RuCl.sub.3.3H.sub.2O, nickel (II) chloride and nickel (II) sulfate.

9. The method of claim 1, wherein the solution further contains hydrazine or another reducing agent.

10. The method of claim 1, comprising selectively depositing a layer of the noble metal on the metal electrode and not depositing a layer of the noble metal on the surface of the dielectric layer.

11. The method of claim 1, wherein forming a piezoelectric layer comprises forming an aluminum nitride layer or an aluminum scandium nitride layer on the layer of the noble metal.

12. The method of claim 1, wherein forming a piezoelectric layer comprises forming an aluminum nitride layer or an aluminum scandium nitride layer on the layer of the noble metal, wherein the aluminum scandium nitride layer comprises more than 5 at-% or more than 10 at-% of scandium or between 10 at-% and 40 at% of scandium.

13. The method of claim 1, comprising: providing a substrate comprising one of a bragg mirror layer stack including a top layer of silicon dioxide and a substrate layer having a top layer of silicon dioxide; forming a metal layer on the layer of silicon dioxide comprising one of tungsten and molybdenum and stucturing the metal layer to form an electrode; then applying a platinum salt solution or a palladium salt solution to the substrate; then forming an aluminum scandium nitride layer having a scandium contents of at least 10 at-% on the electrode layer; forming another electrode layer on the aluminum scandium nitride layer to form another electrode.

14. The method of claim 1, wherein the piezoelectric layer is formed on the metal electrode covered with the layer of noble metal.

15. An electro-acoustic resonator device, comprising: a dielectric substrate; an electrode disposed on the dielectric substrate; a layer of a noble metal disposed on the electrode; a layer of a piezoelectric material disposed on the layer of a noble metal.

16. The electro-acoustic resonator device of claim 15, wherein the electrode is disposed on a top side of the dielectric substrate, the layer of noble metal fully covers a top side of the electrode facing away from the substrate and side surfaces of the electrode running transversely to the top side of the electrode, regions of the top side of the dielectric substrate are free of the layer of a noble metal.

17. The electro-acoustic resonator device of claim 15, comprising: a silicon dioxide substrate layer; an electrode layer of one of molybdenum and tungsten disposed on the silicon dioxide substrate layer; a seed layer of one of platinum and palladium disposed on the electrode layer; a layer of aluminum scandium nitride disposed on the seed layer, the layer of aluminum scandium nitride comprising at least 10 at-% of scandium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In the drawings:

[0028] FIG. 1 shows a cross-section of a workpiece;

[0029] FIG. 2 shows the workpiece after the electrochemical forming of a noble metal seed layer on the bottom electrode layer;

[0030] FIG. 3 shows a cross-section of a BAW resonator of the SMR type; and

[0031] FIG. 4 shows a cross-section of a BAW resonator of the FBAR type.

DETAILED DESCRIPTION OF EMBODIMENTS

[0032] 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.

[0033] Turning now to FIG. 1, a workpiece is provided of which the topmost portion is depicted. Layer 110 is the top layer of the workpiece comprising a dielectric layer. Dielectric layer 110 may be a silicon oxide layer such as silicon dioxide. Other dielectric oxide layers are also useful. Layer 110 may be the top layer of a Bragg mirror structure. An electrode layer 111 is formed on the dielectric layer 110. Electrode 111 forms the bottom electrode of a BAW resonator. Electrode 111 may be made of tungsten or molybdenum. Alternatively, electrode 111 may be made of titanium, aluminum or a composition of aluminum and copper. Electrode 111 is grown on the surface of dielectric 110 and structured to achieve suitable size and shape of the bottom electrode.

[0034] Turning now to FIG. 2, the workpiece of FIG. 1 is immersed into a solution of a noble metal salt such as Na.sub.2PtCl.sub.6 or Na.sub.2PdCl.sub.4. Other metal salts useful to provide the solution are K.sub.2PtCl.sub.6, K.sub.2PdCl.sub.4, K.sub.2PdCl.sub.6, RuCl.sub.3.3H.sub.2O, nickel (II) chloride, and nickel (II) sulfate. An electrochemical process takes place in which metal ions S.sup.+ such as ions of platinum, palladium, ruthenium or nickel deposit on the top and sidewall surface of electrode 111. At the same time, metal ions M.sup.+ migrate out from electrode 111 and dissolve in the electrochemical solution.

[0035] According to the electrochemical working principle, the metal ions S.sup.+ in the electrochemical solution are more noble than the metal ions M.sup.+ in the electrode 111. The metallized areas of the electrodes such as 111 are separated by dielectric areas of dielectric layer 110 such as areas 112. By immersing the workpiece with the structured electrodes into the solution containing a noble metal salt, the electrochemical displacement reaction takes place. The less noble metal from the electrode M.sup.+ such as tungsten, molybdenum, titanium, aluminum or copper goes into solution while the more noble metal S.sup.+ dissolved in the solution such as platinum, palladium, ruthenium or nickel is deposited on the electrode as a thin layer 210. No deposition will occur on the surface of the top dielectric layer 110 of the workpiece in areas 112 as these areas are dielectric and are already in an oxidized state such as silicon dioxide. The deposition of the noble metal S.sup.+ is self-limiting when no more of the native metal from the electrode M.sup.+ is exposed to the solution. The deposited seed layer 210 fully covers the surface of the original metal electrode 111. The electrochemical process in the noble metal salt solution selectively deposits the noble metal on the metal electrode so that a structuring of the noble metal layer including a photolithography step is not required.

[0036] The deposition can be accelerated or assisted by adding a reducing agent such as hydrazine, N.sub.2H.sub.4, to the solution. The hydrazine will facilitate the reduction of the metal of the metal electrode in that hydrazine dissociates to nitrogen N.sub.2 providing electrons for the reduction of metal:

N.sub.2H.sub.4-->N.sub.2+4H.sup.++4e.sup.−

[0037] Turning now to FIG. 3, a cross-section of a SMR BAW resonator is shown after additional process steps. The noble metal layer 210 serves as a seed layer for the subsequent deposition of a piezoelectric layer 320 to enable a textured nucleation of the piezoelectric material. Piezoelectric layer 320 may be a crystalline, columnar layer of aluminum nitride or aluminum scandium nitride. The content of aluminum scandium nitride may be more than 5 at-%, preferably more than 10 at-%, specifically between 10 at-% and 40 at-%. It is believed that the lattice structure of the local metal seed layer 210 is similar to the lattice structure of the piezoelectric layer 320 so that it enables a good nucleation of the piezoelectric layer to achieve a highly textured layer 320. The noble metal, such as platinum or palladium, may have a catalytic effect on the dissociation of nitrogen present in the precursor gas that enables the piezoelectric layer deposition. As a result, the piezoelectric layer 320 is highly textured and highly crystalline, allowing a high electro-acoustic coupling within the resonator. Further deposited on piezoelectric layer 320 is a top electrode layer 321 that may be made of the same materials as original bottom electrode layer 111.

[0038] The SMR BAW resonator depicted in FIG. 3 comprises further a Bragg mirror layer stack 300 on which the electrode sandwich 111, 210, 320, 321 is disposed. Bragg mirror layer stack 300 is formed on a carrier substrate 311. The Bragg mirror 300 includes a sequence of acoustically hard and acoustically soft layers which may be made of, for example, tungsten and silicon dioxide. A variety of other metal and dielectric materials suitable to form a Bragg mirror are also useful. For example, layers 312, 314, 316 may be acoustically hard layers such as tungsten layers, and layers 313, 315, 310 may be acoustically soft layers such as silicon dioxide layers. Specifically, the top layer of the Bragg mirror 310 is a dielectric layer such as silicon dioxide. Bragg mirror 300 has the function to prevent the acoustic energy from escaping into the substrate. The energy is reflected back into the piezoelectric layer 320.

[0039] FIG. 4 shows another type of electro-acoustic resonator such as an FBAR BAW resonator. The electrode stack of layers 111, 210, 320, 321 is the same as shown for the SMR BAW type of FIG. 3. The originating workpiece 410 includes a carrier substrate 411 on which a dielectric top layer 412 is disposed on which the bottom electrode 111 is arranged. The carrier layer 111 may be a crystalline silicon, and the dielectric layer 412 may be silicon dioxide. According to the FBAR working principle, a cavity 413 is arranged opposite the electro-acoustic active area of the layer stack of top and bottom electrodes and the piezoelectric layer sandwiched therebetween. Cavity 413 is filled with ambient air that performs the function of confining the acoustic energy within piezoelectric layer 320.

[0040] In conclusion, an electrochemical deposition of a seed layer enables a deposition of a highly textured, crystalline piezoelectric layer for SMR and FBAR BAW devices. The crystallographic alignment of the piezoelectric film is enhanced. The electrochemical deposition of a noble metal material on the bottom electrode serves as a seed layer favoring higher alignment of a deposited piezoelectric material layer. The described process may be specifically useful when the piezoelectric layer is an aluminum scandium nitride layer having a scandium concentration of about more than 10 at-%.

[0041] This patent application claims the priority of the German patent application 10 2018 126 804.1, the disclosure content of which is hereby incorporated by reference.

[0042] 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 spirit 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.