WO2009121901A1

WO2009121901A1 - Layer system for electrodes

Info

Publication number: WO2009121901A1
Application number: PCT/EP2009/053862
Authority: WO
Inventors: Siegfried Menzel
Original assignee: Leibniz-Institut Für Festkörper- Und Werkstoffforschung Dresden E.V.
Priority date: 2008-04-04
Filing date: 2009-04-01
Publication date: 2009-10-08
Also published as: DE102008001000B4; DE102008001000A1

Abstract

The invention is concerned with the field of material sciences and relates to a layer system for electrodes which can be used for example in frequency filters, sensors or actuators. The object of the present invention is to specify a layer system for electrodes having improved power compatibility and lifetime. The object is achieved by means of a layer system for electrodes, comprising a piezoelectric substrate with strip structures that are composed of a composite material and are situated on or embedded in the substrate, said composite material comprising a metallic matrix material with at least one incorporation phase composed of carbon nanostructures. The object is furthermore achieved by means of a method in which catalyst material is applied on a seed layer, this is subjected to a temperature increase and carbon nanostructures are subsequently produced and the interspaces between the carbon nanostructures filled by a metallic matrix material, and/or in which particles are applied to a substrate, which particles lead to carbon nanostructures, and the interspaces between the carbon nanostructures are subsequently filled by a metallic matrix material.

Description

Layer system for electrodes

The invention relates to the field of materials science and relates to a layer system for electrodes, in particular for interdigital transducers for surface acoustic wave devices, and especially for interdigital transducers for surface acoustic wave devices, which can be used for example as a frequency filter, sensors or actuators, but also for electrodes of other micro-electro-mechanical systems.

Surface acoustic wave (SAW) devices generally consist of a piezoelectric substrate or piezoelectric thin film on a non-piezoelectric substrate and one or more interdigital transducers (IDTs) for coupling and decoupling the electrical energy, as well as corresponding ones Contact areas (pads). In the operation of the surface acoustic wave devices at high temperatures and / or high SAW performance, stress-induced failures of SAW devices may result from migration effects (akustomigration), chipping (metal interface substrate failure) or cracking in the substrate itself , All of this limits the power compatibility, the reliability, as well as the lifetime of the components.

Another problem with SAW devices is that of contacting the IDT, which requires adherence between a contact element (e.g., Al or Au wire) and the pads. For components with operating frequencies in the GHz range, these pads must also be thickened accordingly (increasing the layer thickness of the metallization in the pad area).

Interdigital transducers for SAW devices are often fabricated from either Al-based or Cu-based thin-film systems (metallization) with typical dimensions of 50 to 500 nm total thickness with or without a primer layer (usually Ti). Conventional deposition methods are electron beam evaporation or magnetron sputtering. These thin-film systems are deposited either as a single layer or as alloys or as multilayer systems. The structuring of the metallization layers takes place either via lift-off technology or via etching techniques with the aid of a mask. Due to a given relationship between migration and the mechanical properties of the metallization, the performance compatibility and consequently the lifetime of the SAW devices can be increased by improved thermomechanical properties of the metallization.

Metal matrix composite materials with embedded carbon nanotubes (CNT-MMC) have become known in recent years in the form of compact powder materials, the specially investigated CNT-Cu sintered materials having markedly improved mechanical properties [Kim, KT. et al .: Mater. Be. and Eng. A430 (2006), p. 27; Cha, SI et al.: Adv. Mater. 17 (2005) p. 13772.3]. However, such materials are due to their structure and their powder metallurgy production not suitable for metallization for the production of SAW devices, which with thin-film techniques and with Finger widths of the interdigital transducer must be made in the sub-micron range.

Further, electrochemical processes are known for the production of metallic thin films, e.g. Cu, Ni, Ag, or similar. Specifically, a method for the electrochemical deposition of CNT-Cu composites (US 2007/0199826 A1, US 2007/0158619 A1, US 7226531 B2) is known. It has been shown in principle that after functionalization of the CNTs by means of chemical methods, CNT-Cu composite layers can be produced on substrates having improved mechanical and tribological properties by means of electroplating.

According to US Pat. No. 6,709,562 B1, a method for producing interconnects buried in an insulator for microelectronic circuits is known.

3

Further, according to US 7226531 B2, the production of a connecting wire ("interconnection wire") made of a composite material of CNTs and metal on flexible substrates known.

It is also known that nanostructures, and in particular carbon nanotubes, can exhibit extraordinary properties [Dresselhaus, M.S., Dresselhaus, G., Avouris, P. (Eds.): Carbon Nanotubes, Synthesis, Structure, Properties, and Applications. Springer-V. Berlin (2001)].

Disadvantages of the known solutions for metallizations for interdigital transducers for SAW components consist, above all, in the still inadequate power stability and service life with regard to acoustomigration and mechanically caused flaking or failure of the interface between metallization and substrate, in particular for power filters, optical applications and for actuators.

The object of the present invention is to specify a layer system for electrodes with improved power compatibility and lifetime. The object is achieved by the invention specified in the claims. Advantageous embodiments are the subject of the dependent claims.

The layer system for electrodes according to the invention consists of a piezoelectric substrate with strip structures embedded thereon or embedded in the substrate made of a composite material which consists of a metallic matrix material having at least one intercalation phase of carbon nanostructures.

Advantageously, the layer system is used for interdigital transducers for surface acoustic wave devices.

Further advantageously, the composite material on amorphous or nanocrystalline structure.

Also available as a metallic matrix material is a fcc matehal having a high conductivity.

And also advantageously, as the metallic matrix material, Al, Cu, Ag, Au, Pt or Ru are present.

It is also advantageous if the metallic matrix material to 50.1 to 99.9 VoI.-% is included.

And it is also advantageous if the intercalation phase consists of carbon nanotubes, carbon nanowires, fullerenes or graphenes, wherein advantageously the carbon nanotubes are single-walled (single wall CNT) or multi-walled (multiwall CNT), and / or the intercalation phase is contained in a directional arrangement in the matrix material and / or as storage phase completely filled carbon nanotubes or filled fullerenes are present or a combination of such and / or as a storage phase, a combination of filled carbon nanotubes or filled fullerenes with unfilled carbon nanotubes or unfilled fullerenes or graphenes or combination of such. It is also advantageous if the carbon nanostructures are carbon nanostructures produced by catalytic CVD processes.

It is also advantageous if, in addition to the storage phase, further globular, tubular, rod-shaped or planar structures are contained in the matrix matehal.

It is also advantageous if the carbon nanostructures of the intercalation phase are bonded to the substrate in a material-locking and / or force-locking manner, wherein carbon nanostructures of the intercalation phase, which are arranged in a directionally advantageous manner, are all connected in one material to the substrate in a cohesive and / or force-locking manner.

It is likewise advantageous if a buffer layer is present between the substrate and the layer system.

It is furthermore advantageous if a cover layer is applied to the layer system.

It is also advantageous if the strip structures are completely or almost completely embedded in the substrate.

In the method according to the invention for producing a layer system for electrodes, particles or a layer of a catalyst material are applied either to a seed layer located on a substrate, this layer composite is exposed to a temperature increase and subsequently carbon nanostructures are produced by means of plasma-assisted processes, and subsequently at least the interspaces between the carbon nanostructures filled in a metallic matrix material and / or in which are applied to a substrate with or without already existing layers by means of plasma-assisted processes particles that lead during or after deposition to the formation of carbon nanostructures and subsequently the spaces between the carbon nanostructures are filled by a metallic matrix material , Advantageously, the seed layer material used is Cu or Ru or TaN.

Further advantageously, Fe, Ni and / or Co are used as the catalyst material.

And also advantageously used as filling material for the carbon nanotubes Fe, Ni and / or Co.

Also advantageously, a barrier layer is applied between substrate and seed layer.

It is also advantageous if CH ₄ -GaS is used in the plasma-assisted process, wherein advantageously still another gas is used for diluting the C content, which is advantageously used as Ar or He.

It is also advantageous if Al, Cu, Ag or Au are used as the metallic matrix material or alloys or multilayers thereof.

It is likewise advantageous if the nanostructures are provided with an intermediate layer, which is preferably applied by means of atomic layer deposition (ALD), advantageously the intermediate layer consisting of a metal or an oxide or nitride such as Ru, Ta, Ti, TaN, TiN, Hf oxide or Zr oxide or multilayers or alloys thereof and is used as such.

With the solution according to the invention, it is possible for the first time to use a composite material as material for electrodes and in particular for the finger electrodes of the interdigital transducers of surface acoustic wave components. The use of composite materials based on a metal or a metal alloy or metal multilayers in conjunction with nanostructures as storage phase as thin-film systems as a material for finger electrodes of the interdigital transducer is not yet known. In particular, is not known the Use of carbon nanostructured metal matrix composites and in particular of carbon nanostructured Cu or Al composites.

Of particular interest in connection with nanocomposite layers for the o.g. Applications as a layer system for interdigital transducers for surface acoustic wave devices are strictly ordered 0-, 1-, or 2-dimensional carbon nanostructures, such as fullerenes, single-walled or multi-walled carbon nanotubes (CNTs), or carbon nanowires or graphenes. CNTs can be made of different quality, as single-walled CNT (SWCNT) or multi-walled (multiwalled CNT = MWCNT) carbon nanotubes either via laser-arc, arc-technique or CVD with or without plasma assist. Especially the so-called catalytic CVD (cCVD method) is suitable for the production of larger amounts of CNT. The properties of the produced CNTs are dependent on the manufacturing process. CNTs prepared via cCVD are predominantly MWCNTs in the diameter range between 10 nm and 100 nm. The outer sheaths usually have many structural defects or are partially or completely amorphous. Also, the CNTs can be purified, functionalized or filled with various materials during or after preparation.

Defect - free CNTs can have an extremely high Young ^' s modulus (up to about 1.2 TPa) and a ballistic electron transport as well as a high heat conduction.

With the inventive solutions for coating systems for interdigital transducer for surface acoustic wave elements, the previous disadvantages of the known solutions for interdigital transducer for the SAW devices can be eliminated or at least be significantly improved, in particular the homogeneous distribution of the ^CNTs within the inventive coating system improves and the arrangement thereof can be controlled selectively.

Furthermore, the viscosity of the layer material can be adjusted in a certain range and / or kept at a certain value, whereby the insertion loss of the device can be positively influenced. According to the invention, layer systems with at least one intercalation phase (composite layer system) of zero (fullerenes), single (carbon nanotubes, carbon nanowires), or two-dimensional (graphene) carbon nanostructures can be used for producing power-compatible metallizations for electrodes and in particular for interdigital transducers for SAW components a method of thin film deposition (preferably electrochemical deposition (electroplating)) can be produced. For improved wetting with the matrix material, the CNTs are functionalized or provided with at least one intermediate layer which, after growth, is applied to the carbon modifications, preferably with atomic layer deposition (ALD).

The production of the layer systems for electrodes according to the invention and in particular for interdigital transducers for surface acoustic wave components can, for example, be carried out by applying catalyst particles (for example Fe, Ni, Co or others) to a substrate. This is preferably done in a continuous flow plant, either directly using the catalyst particles, e.g. be applied via a sputtering technique, from a supersaturated vapor, or in a continuous system, first a thin layer (1 -5 nm thickness) of the catalyst material via one of the thin-film techniques (sputtering, evaporation, CVD) is deposited on the surface of the substrate and a subsequent Heat treatment, the formation of the catalyst particles by agglomeration effects. The particle size is preferably between 5 and 30 nm, depending on the process parameters.

Subsequently, the coated substrate is exposed in a C-containing atmosphere via thermally activated processes or thermal plasma activated processes with carbon nanostructures, e.g. Carbon nanotubes, coated. These carbon nanotubes in this case have a preferential direction with respect to the surface of the substrate.

In a subsequent process, the carbon nanostructures are functionalized (formation of OH groups) and / or coated via ALD with at least one adhesion promoter layer (eg TaN or Ru), and then coated with the metallic matrix material by means of a deposition process (preferably electroplating), so that the spaces between the Carbon nanostructures are partially or completely filled and the matrix material serves as a sheath for the carbon nanostructures. Thus, a composite layer is obtained which comprises aligned carbon nanostructures in a matrix material in which the carbon nanostructures are incorporated in a length or over a thickness which corresponds at least to the thickness of the layer. In addition, unregulated incorporation of the carbon nanostructures in the matrix material is possible when the carbon nanostructures are deposited together with the metal layer. For this purpose, it is advantageous to add the carbon nanostructures to the electrolyte for the electrochemical deposition.

Furthermore, it is possible to combine both manufacturing processes, so that the

Layer system contains areas with directionally arranged carbon nanostructures and at the same time other areas in which the carbon nanostructures are arranged non-directionally.

It is also possible to combine areas of the layer system with directional and / or non-directional intercalation phases with areas without intercalation phase.

In addition to the electroplating, the application of at least the matrix layer can also be effected by other methods, for example electrolessly by means of electroless plating.

Furthermore, the coating of a substrate with the layer system according to the invention can take place and subsequently the substrate can be removed.

The layer systems according to the invention can also be used for electrodes for micromirror systems, for microelectromechanical pressure sensors or actuators, for piezoelectromechanical control elements and there also shows improved power compatibility and service life.

The invention will be explained in more detail using an exemplary embodiment. Show

Fig. 1 is a schematic representation of the layer system according to the invention in

Cross section with directionally arranged carbon nanotubes in the form of carbon nanotubes and

Fig. 2 is a schematic representation of the layer system according to the invention in

Cross section with undirected carbon nanotubes in the form of carbon nanotubes and

3 shows examples of transmission electron microscopic images a) CNT-Cu layer system with a layer thickness of 1.95 μm, b) with a thin Ta (N) layer by means of ALD-coated CNT before Cu electroplating

example 1

The production of the interdigital transducers for SAW components from the layer system according to the invention is illustrated using the example of the damascene technique.

A quartz wafer provided with a resist mask is patterned by means of reactive ion etching into finger-shaped arrangements, the interdigital transducers, of dimensions (finger widths) of 3.76 μm, and the remaining lacquer is peeled off. Subsequently, this structured arrangement is provided with a 5 nm thick layer of TaN as a barrier layer and subsequently with a 50 nm thick Cu layer as a seed layer. These layers are sputtered on. Thereafter, on the Cu layer, Ni catalyst particles having an average size of 20 nm and having a density of 1000-5000 particles / μm ^{3 are} deposited by a supersaturated steam sputtering method. In the following plasma-assisted CVD process in the temperature range between 300 ^° C. and 500 ^° C. using CH ₄ , single- and multi-walled carbon nanotubes with an average length of 1-3 μm grow. Subsequently, using electroplating, copper is deposited on the surface and around the carbon nanotubes using sulfuric acid electrolytes, so that the arrangement of CNTs is completely covered. By chemical-mechanical polishing (CMP) with a standard Cu suspension, the resulting layer system is structured. Characterized buried Interdigitalwandler- finger structures are Cu-layer CNT obtained from a substantially unidirectional with embedded CNT ^'s.

With a layer thickness of 1.95 μm of the CNT-Cu layer, the electrical resistance is 1.73 μOhmcm.

These interdigital transducers show a significantly higher power compatibility and lifetime compared to Cu layers without CNT storage phases.

Example 2

A lacquer-masked wafer made of LiNbOs is patterned by means of reactive ion etching into finger-shaped arrangements of 0.5 μm width and a similar spacing (interdigital transducer for filter structures in the GHz range) and the residual lacquer is peeled off. Subsequently, this structured arrangement is provided with a 3 nm thick layer of Ta-Si-N as a barrier layer and subsequently with a 50 nm thick Cu layer as a seed layer. These layers are sputtered on. Thereafter, chemically functionalized carbon nanotubes with a narrow diameter distribution with a maximum at 20 nm diameter and a length between 3 and 5 microns are placed in a Cu electrolyte and circulated the bath. In a further process step, the coating with the Cu electrolyte takes place via electroplating.

These interdigital transducers also show a significantly higher power compatibility and lifetime in comparison to Cu layers without CNT storage phases. LIST OF REFERENCE NUMBERS

1 substrate

2 intermediate layer

3 germ layer

4 storage phase

5 metallic matrix material

Claims

claims

1. Layer system for electrodes, consisting of a piezoelectric substrate with or embedded in the substrate strip structures of a composite material, which consists of a metallic matrix material having at least one storage phase of carbon nanostructures.

2. Layer system according to claim 1, wherein the layer system is used for interdigital transducers for surface acoustic wave devices.

3. Layer system according to claim 1, wherein the composite material has amorphous or nanocrystalline structure.

4. Layer system according to claim 1, in which a fcc material with a high conductivity is present as the metallic matrix material.

5. Layer system according to claim 4, in which is present as the metallic matrix material Al, Cu, Ag, Au, Pt or Ru.

6. Layer system according to claim 1, wherein the metallic matrix material to 50.1 to 99.9 vol .-% is contained.

7. Layer system according to claim 1, wherein the intercalation phase consists of carbon nanotubes, carbon nanowires, fullerenes or graphenes.

8 layer system according to claim 7, wherein the carbon nanotubes are formed single-walled (single wall CNT) or multi-walled (multiwall CNT).

9. Layer system according to claim 7, wherein the intercalation phase is contained in a directional arrangement in the matrix material. 11. The layer system according to claim 7, in which the storage phase is completely filled carbon nanotubes or filled fullerenes or a combination of such.

11. A layer system according to claim 7, wherein as a storage phase, a combination of filled carbon nanotubes or filled fullerenes with unfilled carbon nanotubes or unfilled fullerenes or graphenes are present or combination of such.

The layered system of claim 1, wherein the carbon nanostructures are carbon nanostructures produced by catalytic CVD processes.

13 layer system according to claim 1, wherein in addition to the storage phase further globular, tubular, rod-shaped or planar structures are contained in the matrix material.

14. Layer system according to claim 1, wherein the carbon nanostructures of the storage phase are connected in a material-locking and / or force-locking manner to the substrate.

15. The layer system according to claim 14, wherein directionally arranged carbon nanostructures of the intercalation phase are all materially and / or non-positively connected at one end to the substrate.

16. Layer system according to claim 1, wherein a buffer layer is present between the substrate and the layer system.

17. Layer system according to claim 1, wherein a cover layer is applied to the layer system.

18. Layer system according to claim 1, wherein the strip structures are completely or almost completely embedded in the substrate.

19. A method for producing a layer system for electrodes according to any one of claims 1 to 18, wherein applied either on a substrate located on a seed particle or a layer of a catalyst material, this layer composite exposed to a temperature increase and subsequently produced by means of plasma-enhanced carbon nanostructures, and Subsequently, at least the interstices between the carbon nanostructures are filled by a metallic matrix material and / or are applied to a substrate with or without already existing layers by means of plasma-assisted processes particles that lead to the formation of carbon nanostructures during or after application and subsequently the interstices between the carbon nanostructures filled by a metallic matrix material.

20. The method of claim 19, wherein Cu or Ru or TaN is used as the seed layer material.

21. The method according to claim 19, wherein the catalyst material Fe, Ni and / or Co are used.

22. The method of claim 19, are used as the filling material for the carbon nanotubes Fe, Ni and / or Co.

23. The method of claim 19, wherein between the substrate and seed layer, a barrier layer is applied.

24. The method of claim 19, wherein in the plasma-assisted process CH ₄ - gas is used.

25. The method of claim 24, wherein a further gas is used to dilute the C content.

26. The method of claim 25, wherein Ar or He is used.

27. The method of claim 19, wherein as the metallic matrix material Al, Cu, Ag or Au are used or alloys or multilayers thereof.

28. The method of claim 19, wherein the nanostructures are provided with an intermediate layer, which is preferably applied by means of atomic layer deposition (ALD).

29. The method of claim 28, wherein as the intermediate layer, a metal or an oxide or nitride, such as Ru, Ta, Ti, TaN, TiN, Hf oxide or Zr oxide or multilayers or alloys thereof are used.