DEVICE BASED ON ALKALI METAL NIOBATE COMPRISING A BARRIER LAYER AND MANUFACTURING PROCESS

Abstract

A piezoelectric device includes at least one upper layer of piezoelectric material based on alkali metal niobate and one lower layer of metal located above a substrate, wherein it comprises a barrier layer of material that is a barrier to the diffusion of alkali metals into the metal and that is inert to the alkali metals of the niobite, the barrier material layer being located between the lower layer of metal and the upper layer of piezoelectric material. A process for producing the device is also provided.

Claims

1. A piezoelectric device comprising at least one upper layer of piezoelectric material based on alkali metal niobate and one lower layer of metal located above a substrate, wherein it comprises a barrier layer of material that is a barrier to the diffusion of alkali metals into said metal and that is inert to the alkali metals of said niobate, said barrier material layer being located between the lower layer of metal and the upper layer of piezoelectric material, said diffusion barrier material being a conductive oxide or a conductive nitride.

2. The device according to claim 1, wherein the metal is Pt or Mo or Ti.

3. The device according to claim 1, wherein the oxide comprises a Pt or Ru or Ir metal and preferably RuO.sub.2.

4. The device according to claim 1, wherein said diffusion barrier material is a nitride that may be TiN or WN or TaN and preferably TiN.

5. The device according to claim 1, wherein the thickness of the barrier layer is greater than several tens of nanometres and preferably of the order of around a hundred nanometres.

6. The device according to claim 1, comprising, between the surface of the substrate and said lower layer of metal, when this metal is a noble metal, a layer for attachment of said noble metal, for example made of TiO.sub.2.

7. The device according to claim 6, wherein the thickness of the lower layer of metal, which may be a noble metal, being greater than several tens of nanometres, the thickness of the attachment layer is of the order of a few nanometres.

8. The device according to claim 1, comprising a conductive upper layer above the layer of piezoelectric material.

9. A process for manufacturing a device according to claim 1, comprising the following steps: depositing a lower layer of metal, which may be a noble metal, on a substrate; producing a barrier layer on the surface of said lower layer of metal; depositing a layer of a piezoelectric material on the surface of said barrier layer by sputtering from a target.

10. The manufacturing process according to claim 9, wherein the sputtering is carried out at a temperature above 300 C., preferably at 500 C.

11. The manufacturing process according to claim 9, comprising at least one sputtering step carried out at a pressure of a few mTorr in order to deposit the piezoelectric material.

12. The manufacturing process according to claim 11, comprising a first sputtering step carried out at a first RF power, and a second sputtering step carried out with a second RF power higher than the first power, in order to deposit the piezoelectric material.

13. The manufacturing process according to claim 9, comprising the deposition of an attachment layer on the surface of the substrate, prior to the deposition of the lower layer of metal.

14. The manufacturing process according to claim 9, comprising carrying out a deposition of upper layer of metal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The invention will be better understood and other advantages will become apparent on reading the following description, which is given in a non-limiting manner, and by virtue of the figures wherein:

[0055] FIG. 1 depicts a transducer-type device according to the known art;

[0056] FIGS. 2a and 2b depict a surface acoustic wave device according to the known art;

[0057] FIG. 3 illustrates a photograph of a stack of KNN on the surface of a Pt layer according to the known art;

[0058] FIG. 4 illustrates the crystallographic peaks of a layer of KNN on a layer of Pt (111) according to the known art;

[0059] FIG. 5 illustrates the crystallographic peaks of a layer of KNN deposited on a Pt metal showing various crystalline orientations of KNN and various phases, according to the known art;

[0060] FIG. 6 depicts the formation of a layer of Li.sub.xNbO.sub.y from the sputtering of a target;

[0061] FIG. 7 depicts the diffusion of alkali metals into the layer of metal and the formation of various phases other than Li.sub.xNbO.sub.y;

[0062] FIG. 8 illustrates a first example of a device according to the invention comprising a barrier layer according to the invention;

[0063] FIG. 9 illustrates a second example of a device according to the invention;

[0064] FIG. 10 illustrates an example of an MIM device according to the invention.

DETAILED DESCRIPTION

[0065] Generally, the device of the invention comprises, on the surface of a substrate, a conductive layer of metal, on which it is sought to deposit a good-quality layer of alkali metal piezoelectric material, through the intermediary of a barrier layer.

[0066] FIG. 8 depicts a first example of a device of the invention, showing a substrate 100, a layer of noble metal 200, a barrier layer 300, a layer of piezoelectric material 400.

[0067] The stack, described in detail below, is deposited on a substrate suitable for the intended application and which may be a sapphire, MgO, glass or else silicon substrate or preferably an Si substrate covered with an insulating layer (preferably a layer of SiO.sub.2).

[0068] Before the deposition of the piezoelectric material, the metallic bottom electrode (i.e. the layer 200) is first deposited, followed by the deposition of the barrier layer 300. To form the bottom electrode of one of the aforementioned devices, use may preferably be made of Pt or Mo.

[0069] For the barrier material of the barrier layer, it is possible to use any material having the role of: [0070] preventing alkali metals (Li, Na, K) from reacting with the bottom electrode; and [0071] preventing these same alkali metals from diffusing towards the bottom electrode.

[0072] Since these barrier materials must be chemically inert with respect to the alkali metals, use is preferably made of oxides or nitrides.

[0073] In field of conductive nitrides and oxides, use may be made of oxides of noble metals (Pt, Ru, Ir) and preferably of RuO.sub.2.

[0074] In field of nitrides, it is possible to use TiN, WN, TaN and preferably TiN.

[0075] The use of an insulating barrier is also possible. However, so as not to degrade the response of the piezoelectric material, it is preferable to use a barrier which is also piezoelectric, such as aluminium nitride (AlN), for example.

[0076] It may be advantageous to provide an attachment layer that promotes the attachment of the metal layer to the substrate.

[0077] FIG. 9 illustrates, for this purpose, a second example of a device of the invention comprising, on a substrate 100:

[0078] an attachment layer 210;

[0079] a metal layer 200;

[0080] a barrier layer of barrier material 300;

[0081] a layer of piezoelectric material 400.

[0082] FIG. 10 illustrates the same stack as the one described previously and represented in FIG. 9, supplemented by an upper electrode that makes it possible to produce an MIM-type device, i.e. the following stack:

[0083] a substrate 100;

[0084] an attachment layer 210;

[0085] a metal layer 200;

[0086] a barrier layer of barrier material 300;

[0087] a layer of piezoelectric material 400;

[0088] a top electrode 500.

[0089] To produce the aforementioned device illustrated in FIG. 9, the steps of a standard process are the following:

[0090] Step 1:

[0091] To improve the adhesion of this Pt layer on the substrate, an attachment layer made of TiO.sub.2, for example, a few nm thick (5 nm) is deposited beforehand.

[0092] Step 2:

[0093] The deposition of a Pt electrode having a thickness of a few tens of nm, for example 100 nm, on the surface of said attachment layer is carried out.

[0094] Step 3:

[0095] On the Pt layer, the deposition of the barrier layer of barrier material, which may be RuO.sub.2, is carried out. In order to be effective against the diffusion of the alkali metals, the thickness of this barrier layer must be at least 80 nm (preferably 100 nm) for a deposition temperature of the piezoelectric material of between 500 C. and 700 C.

[0096] Step 4:

[0097] The piezoelectric material (LiNbO.sub.3 or K.sub.xNa.sub.1-xNbO.sub.3) is deposited by sputtering using a target corresponding to the material to be deposited. The deposition is carried out at a temperature above 300 C., preferably at 500 C. For the sputtering, use is made of an Ar:O.sub.2 mixture at a pressure of a few mTorr and preferably 5 mTorr. The Ar:O.sub.2 ratio may be adjusted in order to obtain the desired deposition velocity while limiting the amount of oxygen vacancies in the piezoelectric material. This Ar:O.sub.2 ratio is preferably around 4:1.

[0098] For the sputtering machine used notably within the context of a 200 mm Si wafer, the RF power is a few hundred watts, typically 500 W.

[0099] Another variant consists in depositing the piezoelectric material by sputtering using two RF powers. It is advantageous to begin the deposition of the first layers at a lower RF power (200 W, for example), then to increase the RF power to 500 W after 15 minutes.