WO2010051831A1

WO2010051831A1 - New organo-metallic wet coating method to enhance electro-migration resistance

Info

Publication number: WO2010051831A1
Application number: PCT/EP2008/064925
Authority: WO
Inventors: Dominique Suhr; Isabelle Bispo
Original assignee: Alchimer
Priority date: 2008-11-04
Filing date: 2008-11-04
Publication date: 2010-05-14
Also published as: TW201018745A

Abstract

The present invention relates to a method for preparing a multilayer composite device, characterized in that it comprises the steps of: - forming an intermediate layer on the surface of a composite material, said surface of composite material having at least one area made of dielectric material and one area made of copper or copper alloy, said intermediate layer being formed by bringing said surface of composite material into contact with a coating solution containing a metallic salt, said metallic salt enabling a selective formation of the intermediate layer only on the area made of copper or copper alloy, and then - forming a dielectric encapsulating layer by plasma enhanced chemical vapor deposition (PECVD), said plasma comprising reducing species for transforming the metallic salt into the corresponding metal at the zero oxidation state.

Description

NEW ORGANO-METALLIC WET COATING METHOD TO ENHANCE ELECTRO-MIGRATION RESISTANCE

The present invention generally concerns a method for preparing a multilayer composite device having enhanced electromigration resistance, in particular when used as semiconductor devices.

BACKGROUND OF THE INVENTION

Multilevel interconnects consist in a network of lines (formed into trenches) of a conducting or semi-conducting material, that are used to distribute various signals as well as power and ground to different areas of an integrated circuit. In order to improve real estate-efficiency, these lines are stacked in several levels separated by a dielectric material and these levels are connected to each other through vertical apertures called vias. Copper has become widely adopted to form multilevel interconnects required in today's ultra large scale integration (ULSI) semiconductor devices, due to its lower resistivity compared to aluminum and its improved electromigration resistance.

Lines and vias are usually formed using the damascene process sequence (See for example S.Wolf: "Silicon processing for the VLSI Era", Vol.4, (2002), p.671 -687) in which, at each level of the interconnect system, features are etched in the dielectric material and subsequently filled with conducting or semi-conducting material, such as copper, before being planarized. A simplified version of this sequence can be described as follows: - dry etching of the dielectric material to form trenches and/or vias; deposition (conventionally by physical vapor deposition - PVD) of a Cu diffusion barrier (like TaNATa, TiN or Ti) since copper is a fast diffuser through dielectrics and can reach the underlying transistors built into the silicon, causing device failures; - deposition of a "seed layer" of copper, conventionally by PVD; electrochemical deposition of copper to fill the vias and trenches; planahzation by chemical-mechanical polishing (CMP) to leave inlaid copper lines; that is, the surface of copper at the same level as the surface of the surrounding dielectric; - deposition over the inlaid copper lines of a dielectric encapsulation layer (generally SiCN, SiN, SiC, SiOCN or SiON deposited by plasma- enhanced chemical vapor deposition - PECVD) which serves as a copper diffusion barrier as well as an etch-stop layer (ESL) during patterning and etching of the overlying inter-metal dielectric material; deposition of the next level of inter-metal dielectric material (generally a material having a low dielectric permittivity, below 4.0). As device integration density increases, the width of lines, vias and other features on the circuits decreases. As a result, the cross-section of lines and vias decreases and current densities carried through copper lines increase.

The increase in current density enhances the electromigration phenomenon in copper interconnects. Electromigration can be described by the displacement of metal atoms in interconnect lines in the direction of the current flow.

As a result of the motion of copper atoms, vacancies and then voids are formed in certain areas of copper lines, causing reliability issues and, with time, complete failures of the interconnect system and subsequently of the integrated circuit itself.

It has been shown (See for example C.K.Hu et al., Microelectronics and Reliability, Vol.46, Issues 2-4, (2006), p.213-231 ) that in copper lines, electromigration primarily proceeds through surface diffusion of copper atoms or ions as the surface diffusion coefficient of copper is higher than its self-diffusion coefficient. Surface diffusion of copper occurs preferentially along the weakest interface of a copper line where loosely bound copper atoms have more mobility.

In copper lines, the weakest interface is the top surface of the line in contact with the ESL (See for example T. C. Wang et al., Thin Solid Films, 498 (2006), p.36-42).

It is therefore desirable to enhance the adhesion strength between copper and the ESL in order to limit surface diffusion of copper, and to improve electromigration resistance and reliability of the interconnect system.

Several techniques have been proposed to achieve better adhesion of ESL to copper and better electromigration resistance.

Plasma treatments in reducing atmosphere such as ammonia or hydrogen have been employed (see for example US patents 6946401 , 6764952 and 6593660) to reduce copper oxides and eliminate other contaminants present on the copper surface, thus resulting in better adhesion of the ESL. However, the physical and directional nature of the treatment induces some copper sputtering over the surrounding dielectric areas resulting in a risk of higher line-to-line leakage. Some of the top surface of copper is removed during the process, resulting in increased line resistance which decreases the interconnect performance.

Silicidation of the copper surface has been proposed to enhance adhesion with Si-based ESL (see for example US patents 6492266, 6977218 and 5447887). During the silicidation process, some copper from the line is consumed to form the more resistive copper suicide. However, this results in increased line resistance.

Doping copper with other metals such as Ag or Zr, either in the bulk of the line during the electrochemical filling step, or in a localized manner on the top surface of copper lines, has also been suggested to enhance resistance to electromigration (see for example US patents 6387806 and 6268291 ). In this case, the dopants will act either as physical copper diffusion blockers or will assist in creating larger copper grains to eliminate copper diffusion pathways.

Once again, the use of dopants which have lower electrical conductivity compared to copper will increase line resistance.

The use of selective and conductive capping layers over copper lines such as CoWP layers deposited by an electroless deposition process has also been proposed (see for example US patents 6893959 and 6902605 and US application number 2005/0266673). In this case, copper surface diffusion is limited by metal-to-metal bonding. Such capping layers may also possess copper diffusion barrier properties, thus theoretically eliminating the need for a

SiCN layer.

However, their integration in the dual damascene process sequence without the use of ESLs is not straightforward; in particular when via misalignments have to be taken into account.

Furthermore, selective deposition is very difficult to maintain when line density increases. This causes increased line-to-line leakage and, in some cases, creates line-to-line shorts.

A solution to improve selectivity in high density areas was described (see PCT application published under the reference WO 2005/098087), such method consisting in coating the surface of the copper with a solution comprising an organo-metallic compound, which has the specificity of both enabling a covalent grafting on the conducting or semi-conducting layer of the composite material, and catalyzing the further electroless growth of a metallic layer onto such organo-metallic layer. The metallic layer formed selectively onto the areas of conducting or semi-conducting material enhanced the electromigration resistance. In a recent past, another solution to enhance electromigration resistance appeared (see for example US application published under the reference US 2007/0262449), consisting in putting an organic layer on the copper surface via chemical grafting before forming an ESL layer. The organic coating could contain, for example, silane derivatives playing several roles like enhancing the adhesion and the dielectric behavior of the following ESL layer. Moreover the use of these compounds, in well controlled conditions, can lead to a conformal and selective coating on copper. Thus the electromigration resistance could be increased. The aim of the present invention is to provide an alternative method for preparing a multilayer composite device having enhanced electromigration resistance.

In particular, a goal of the present invention is to provide a method for preparing a multilayer composite device having enhanced electromigration resistance which is simple to perform, with fewer steps, and which is easily adapted to industrial constraints.

SUMMARY OF THE INVENTION

To this end we propose a method for preparing a multilayer composite device, characterized in that it comprises the steps of:

- forming an intermediate layer on the surface of a composite material, said surface of composite material having at least one area made of dielectric material and one area made of copper or copper alloy, said intermediate layer being formed by bringing said surface of composite material into contact with a coating solution containing a metallic salt, said metallic salt enabling a selective formation of the intermediate layer only on the area made of copper or copper alloy, and then

- forming a dielectric encapsulating layer by plasma enhanced chemical vapor deposition (PECVD), said plasma comprising reducing species for transforming the metallic salt into the corresponding metal at the zero oxidation state.

Such a method for preparing a multilayer composite device is particularly advantageous as there is no need of further steps for activation of the organo- metallic intermediate layer as this activation is done by the plasma treatment necessary for forming the dielectric encapsulating layer. Consequently, it is very simple to perform, in particular in industrial conditions. Further, as it will be more apparent in the following description, the resistance to electromigration is highly enhanced with such a method.

Preferred but non limiting aspects of such method are the following: - the intermediate layer is formed by covalent grafting enabled by the metallic salt of the coating solution, said metallic salt being prepared with compounds of formula [Form.1]:

Where: • A is a diazonium N≡N group enabling the metallic salt to be selectively bound to copper or copper alloy and further enabling initiation of the formation of the intermediate layer by polymerisation,

• R1 , R2, R3 are three substituents for enabling complexation by the metallic ions, said substituents being chosen, independently from each other, among H, COOH and corresponding carboxylate salts, NH₂ and corresponding ammonium salts, (CH₂)_mCOOH and corresponding carboxylate salts, (CH₂)_mNH2 and corresponding ammonium salts, with 1 < m < 4, • Me is a transition metal chosen among Ni(II), Co(II) or Co(III), the oxidation state representing the oxidation state of the metal from the initial salt introduced when preparing the coating solution,

• L is a ligand which can be a water molecule, Cl^", OH^", SO₄ ^2", N- methyl-pyrrolidone (NMP), Dimethyl-formamide (DMF), acetonitrile, acetate, acetylacetonate, NO3^" or a binding moiety of the diazonium salt,

• n is an integer comprised between -4 and +3 representing the global charge of the metal complex,

• X is a counter ion for the diazonium salt chosen among sulfate, tetrafluororate, triflate, or nitrate,

• Y is a counter ion for the metal complex chosen among sulfate, tetrafluororate, nitrate, acetylacetonate, acetate, or chloride. - the metallic salt is an aryl diazonium salt where R1 =R3=H, R2=(CH₂)- COOH mixed with Co(II);

- the diazonium salt is in a concentration comprised between 5 and 50 mM, and preferably in a concentration of about 10 mM, within a solution using N-methyl-pyrrolidone (NMP), Dimethyl-formamide (DMF), acetonitrile or a mixture thereof as solvent;

- the coating solution comprises tetrafluoroborate para(acetic acid) phenylene diazonium salt of aniline mixed with Co(II) sulfate hexahydrate, where the molar ratio between the diazonium salt and the Co(II) being from 0.2 to 10, and preferably 0.5 to 2;

- the step of forming the intermediate layer comprises immersing, spraying or spin-coating said coating solution onto said surface of composite material under ultrasonic activation;

- the ultrasonic activation is performed during an activation time lower or equal to 10 minutes, preferably during an activation time comprised between 1 and 5 minutes, and even more preferably during an activation time of about 2 minutes;

- the reducing species of the plasma are selected among the group consisting of NH₃, H₂ or a mixture thereof; - the step of forming a dielectric encapsulating layer on the intermediate layer consists in depositing SiCN, SiN, SiC, SiOCN or SiON by plasma enhanced chemical vapor deposition (PECVD);

- the encapsulating layer is made of silicon nitride (SiN) deposited by plasma enhanced chemical vapor deposition (PECVD) using a silane (SiH₄) with ammonia (NH₃) chemistry;

- the composite material is a semiconductor device;

- The method of claim 11 , wherein the composite material is a multilayer interconnect system.

Another aspect of the invention resides in a multilayer composite device comprising :

- a composite material comprising a surface having at least one area made of dielectric material and one area made of copper or copper alloy,

- an intermediate organo-metallic layer comprising Cobalt at the zero oxidation state, said intermediate organo-metallic layer being formed selectively on the area made of copper or copper alloy, and having a thickness of less than 30nm, and - a dielectric encapsulating layer covering both the intermediate organo- metallic layer and the area made of dielectric material of the composite material.

According to a preferred but non limiting aspect of such device, the encapsulating layer is made of SiCN, SiN, SiC, SiOCN or SiON.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become clear from the following description which is only given for illustrative purposes and is in no way limitating, and should be read with reference to the attached drawings on which:

- Figure 1 is an AFM topographic image of Sp2Co coating before plasma;

- Figure 2 is an AFM phase image of Sp2Co coating before plasma; - Figure 3 is an AFM topographic image of Sp2Co coating after NH₃ plasma;

- Figure 4 is an AFM phase image of Sp2Co coating after NH₃ plasma;

- Figure 5 is a TOF-Sims profile of Sp2Co coating before NH₃ plasma;

- Figure 6 is a TOF-Sims profile of Sp2Co coating after NH₃ plasma; - Figure 7 is a graph illustrating the leakage current measured for a

"reference" vehicle test without Sp2Co coating;

- Figure 8 is a graph illustrating the leakage current measured for a vehicle test coated with Sp2Co;

- Figure 9 is a graph illustrating the line resistance measured for a "reference" vehicle test without Sp2Co coating;

- Figure 10 is a graph illustrating the line resistance measured for a vehicle test coated with Sp2Co;

DETAILED DESCRIPTION OF THE INVENTION Lines and vias are usually formed using the damascene process sequence in which, at each level of the interconnect system, features are etched in the dielectric material and subsequently filled with conducting or semiconducting material, such as copper or copper alloy, before being planahzed. The increase in current density enhances the electromigration phenomenon in copper interconnects, causing reliability issues and, with time, complete failures of the interconnect system and subsequently of the integrated circuit itself. In copper lines, the weakest interface has been identified to be the top surface of the line in contact with the etch-stop layer (ESL), and the purpose of the present invention is to enhance electromigration resistance at this interface, adding an organo-metallic layer between copper and ESL layer which acts as a "glue layer".

Diazonium salts have proved to be very effective organic products to chemically graft on metals, for example on copper or copper alloy (see in particular the US application published under the reference US 2007/0262449).

We chose to combine an organic precursor, like diazonium salt, with a transition metal, such as Ni(II), Co(II) or Co(III) to create an organo-metallic species. After such organo-metallic layer is selectively coated on the area made of copper or copper alloy, the reducing gas used in the plasma for ESL layer deposition, for example NH₃ or H₂, reduces the metallic ions into the corresponding metal at the zero oxidation state. For instance, the cobalt present in the organo-metallic layer to the zero oxidation state can act as an electromigration resistance enhancer. The reducing gas used in the pre-plasma treatment - if any - before effective ESL layer deposition may start reduction of the reduction process of the metallic ions.

One embodiment of the method for preparing a multi layer composite device, such as a semiconductor device, may thus comprise the following steps.

The first step (A) consists in forming a capping layer on the surface of a composite material having at least one area made of a dielectric material and at least one area made of copper or copper alloy, by bringing said surface of said composite material into contact with a coating solution comprising a metallic salt.

Such coating solution could be a solution comprising a diazonium salt of aniline (which corresponds to the organic precursor) mixed with a metal salt. The compounds forming such coating solution may have the following formula [Form.1]:

[Form.1] Where:

- A is a diazonium N≡N group enabling the metallic salt to be selectively bound to copper or copper alloy and further enabling initiation of the formation of the intermediate layer by polymerisation, - R1 , R2, R3 are three substituents for enabling complexation by the metallic ions, said substituents being chosen, independently from each other, among H, COOH and corresponding carboxylate salts, NH₂ and corresponding ammonium salts, (CH₂)_mCOOH and corresponding carboxylate salts, (CH₂V₁NH₂ and corresponding ammonium salts, with 1 < m < 4,

- Me is a transition metal chosen among Ni(II), Co(II) or Co(III), the oxidation state representing the oxidation state of the metal from the initial salt introduced when preparing the coating solution,

- L is a ligand which can be a water molecule, Cl^", OH^", SO₄ ^2", N-methyl- pyrrolidone (NMP), Dimethyl-formamide (DMF), acetonithle, acetate, acetylacetonate, NO₃ ^" or a binding moiety of the diazonium salt,

- n is an integer comprised between -4 and +3 representing the global charge of the metal complex,

- X is a counter ion for the diazonium salt chosen among sulfate, tetrafluororate, triflate, or nitrate,

- Y is a counter ion for the metal complex chosen among sulfate, tetrafluororate, nitrate, acetylacetonate, acetate, or chloride.

The skilled person will easily understand that the L ligands are not obligatory all identical and monodentate, strictly defined in chemical nature and position around the metal atom since they are dependant of the nature of the solvent, the way of preparation, etc.

Advantageously, the solution further contains an acid such as sulfuric acid to stabilize the diazonium compounds.

With such a preparation, the metal cation can complex the binding moiety (carboxylic or amine) of the diazonium salt.

In the following sequences, the name Sp2 will refer to the diazonium salt only where A= N≡N group, R1 =H, R2=CH₂COOH, R3=H, X=tetrafluoroborate and Sp2Co will refer to the corresponding mixtures of the diazonium salt Sp2 with the cobalt(ll) salt where L is water and Y is sulfate . The second step (B) consists in forming an encapsulating dielectric layer over the surface of the composite material obtained in step (A). Such overlayer consists of a Si-containing dielectric Cu-ESL and/or copper diffusion barrier. It has been discovered that the above described diazonium salts mixed with cobalt cations, i) shows spontaneous reaction and strong interaction (summarized as grafting) with metallic copper and allows stabilization of Cu or Cu alloy interface with respect to electromigration. This leads to an increasing of electromigration resistance; ii) provides a high compatibility with further process steps, in particular a very good adhesion to PECVD deposited Si-containing copper or copper alloy diffusion barriers and/or ESL. Furthermore these compounds exhibit good thermal stability ensuring integration with subsequent steps.

The intermediate organo-metallic layer deposition is advantageously performed by immersing the wafer surface in the above described coating solution, which is preferably submitted to ultrasonic frequencies. The ultrasonic activation is generally performed during an activation time lower or equal to 10 minutes, preferably during an activation time comprised between 1 and 5 minutes, and even more preferably during an activation time of about 2 minutes

After exposure to the coating solution, the wafer surface is rinsed (with DMF, NMP or de-ionized water) and dried, before proceeding to the standard ESL deposition process.

The resulting organo-metallic layers, with thicknesses inferior to 30 nanometers (nm), and generally inferior to 20 nm, exhibit good thermal and plasma resistance to insure compatibility with ESL deposition.

According to one particular embodiment, excellent results have been obtained with NMP or DMF solutions containing the in-situ generated mixture of diazonium salt of aniline and cobalt cation in a concentration of diazonium salt between 5 and 50 mM, and preferably about 10 mM, the molar ratio between the diazonium salt and the Co(II) being from 0.2 to 10, and preferably 0.5 to 2.

In this respect, it is noted that the process of the invention is not only usable with today's industry standard for ESL/Barrier which is PECVD SiCN but also with SiN, SiC, SiOCN or SiON as well as with known types of Cu diffusion barrier.

In one preferred embodiment of the invention, the composite material consists in a silicon work piece coated with a silicon dioxide layer or a Si- containing low k dielectric layer containing copper or copper alloy lines. The intermediate organo-metallic layer is preferably formed after a chemical- mechanical polishing (CMP) step. More advantageously, the composite material is a semiconductor device, in particular a multi layer interconnect system of an integrated circuit.

According to another aspect, the present invention concerns the multilayer composite device obtainable by the method described above. This device according to the present invention, therefore generally comprises an organo-metallic layer grafted on the surface of copper or copper alloy area of the composite material, this intermediate organo-metallic layer and the dielectric area of the composite material being coated with a dielectric Cu- ESL and/or a copper diffusion barrier. More specifically, this semiconductor device comprises:

- a dielectric layer having one or more trenches;

- copper or copper alloy lines formed in said trenches;

- an organo-metallic layer formed on the copper or copper alloy lines thus formed and covered by an overlayer consisting of a dielectric Cu-ESL and/or copper diffusion barrier;

- a silicon dioxide layer or Si-containing low k dielectric layer formed over said ESL and/or copper diffusion barrier.

The present invention will now be illustrated by the following non-limiting examples in which the method according to the invention is used to prepare copper interconnect structures for semiconductor devices.

Example 1 : description of the complete sequence to achieve the final stack from post-CMP samples

1.1. Non pattern substrates: The substrate used in this example consisted of a silicon work piece, coated with 400nm of silicon dioxide deposited by plasma-enhanced chemical vapor deposition (PECVD). The silicon dioxide layer was coated with 15nm of tantalum nitride (TaN) deposited by physical vapor deposition (PVD), on top of which 10nm of tantalum (Ta) was also deposited by PVD. Copper seed layer (100nm) was deposited by PVD on top of the Ta layer, followed by copper electroplating.

Chemical mechanical polishing (CMP) was then applied to the surface to reduce the total copper thickness to 500nm.

This substrate was cleaned firstly by immersion during 10 seconds in a solution containing 0.1 % wt. of sulfuric acid, and then by immersion during 10 seconds in a solution containing 2.5% wt. of citric acid. The substrate was then rinsed with de-ionized water and dried with nitrogen. Coating solution was prepared with 75mg of the tetrafluoroborate para(acetic acid) diazonium salt of aniline (3.10^"4 mol) added to 140 mg of the cobalt(ll) sulfate hexahydrate (5.10^"4 mol) in 50 ml_ of DMF. Such Sp2Co coating solution was stirred during at least 72 h before being used. After cleaning, the substrate was immersed in the coating solution described above during 2 minutes under ultra sonic conditions, rinsed with DMF, and dried with nitrogen.

AFM and TOF-SIMS characterizations of organo-metallic layer grafted are given in examples 2 and 3. A first pre-plasma was then applied on the sample during 1 minute under reducing atmosphere NH₃ at 300⁰C.

Characterizations of the grafted layer after pre-plasma are given in examples 2 and 3.

Following this pre-plasma procedure, 80nm of SiN were deposited in the same plasma conditions.

XPS characterizations of grafted layer after SiN deposition are given in example 4.

Subsequent adhesion improvement characterized by microscratching is summarized in example 5.

1.2. Pattern substrates :

The substrate used in this example consisted of a silicon work piece, coated with 400nm of silicon dioxide deposited by plasma-enhanced chemical vapor deposition (PECVD). Silicon dioxide was selectively etched to create trenches of 0.16μm width. Patterned surface was coated with 15nm of tantalum nitride (TaN) deposited by physical vapor deposition (PVD), on top of which 10nm of tantalum (Ta) was also deposited by PVD. Copper seed layer (40nm) was deposited by PVD on top of the Ta layer, followed by copper gap filling of trenches by electroplating. Chemical mechanical polishing (CMP) was then applied to the surface to remove excess of copper.

This substrate was cleaned firstly by immersion during 10 seconds in a solution containing 0.1 % wt. of sulfuric acid, and then by immersion during 10 seconds in a solution containing 2.5% wt. of citric acid. The substrate was then rinsed with de-ionized water and dried with nitrogen.

Coating solution was prepared with 75mg of the tetrafluoroborate para(acetic acid) diazonium salt of aniline (3.10^"4 mol) added to 140 mg of the cobalt(ll) sulfate hexahydrate (5.10^"4 mol) in 50 ml_ of DMF. Such Sp2Co coating solution was stirred during at least 72 h before being used.

After cleaning, the substrate was immersed in the coating solution described above during 2 minutes under ultra sonic conditions, rinsed with DMF, and dried with nitrogen.

A first pre-plasma was then applied on the sample during 1 minute under reducing atmosphere NH₃ at 300⁰C.

Following this pre-plasma procedure, 80nm of SiN were deposited in the same plasma conditions. Parametric electrical values (example 6) and electromigration resistance

(example 7) were evaluated in 0.16μm width trenches.

Example 2 : AFM characterization before and after pre-plasma

AFM (atomic force microscopy) analysis was done on non-pattern substrates made according to example 1 .1 , without SiN deposition step.

AFM was performed with the molecular imaging PicoSPM. AFM standard conditions data were commercial pyramidal Si tip (mounted on 225μm long single beam cantilever with a resonance frequency of approximately 75 kHz and a spring constant of about 3 N.m^"1). AFM images were recorded using the retrace signal. The scan rate was in the range of 0.25 Hz with a scanning density of 512 lines/frame.

Topographic mode gave us information on the physical aspect of the surface (roughness, homogeneity...).

Phase mode gave us information on the hardness of the deposit and therefore an insight of the chemical nature of the latter.

AFM topographic image of the Sp2Co coating before plasma (Figure 1 ) shows the hilly aspect of the polymer on the surface.

AFM phase image of the Sp2Co coating before plasma (Figure 2) suggests that there is a planar deposit of polymer, itself covered by the hills of the same polymer.

AFM topographic image of the Sp2Co coating after NH₃ plasma (Figure 3) shows the plasma smoothing effect on the polymeric surface. The grain joints are highlighted and have a height of around 5 to 20 nm. Interestingly they seem then to be more covered than the flat parts of the surface. AFM phase image of the Sp2Co coating after NH₃ plasma (Figure 4) is in relationship with the precedent image. The uniformity of the colour indicates that we have only one type of material on the surface which is the organo- metallic layer, being constituted of the polymer layer based on the diazonium compound mixed with the cobalt salt.

Example 3 : TOF-SIMS characterization of orqano-metallic layer before and after pre-plasma

TOF-SIMS (Time-Of-Flight Secondary Ion Mass Spectroscopy) analysis was performed on non-pattern substrates made according to example 1 .1 , without SiN deposition step.

TOF-SIMS analysis was done in IONTOF IV equipment (primary ions Au⁺, 25kV, 150x150μm²).

Etching sources for positive depth profile was O₂ ⁺ (from 500 V to 2KV,

500x500μm²), and for negative depth profile it was Cs⁺ (from 500V to 2KV,

500x500μm²).

TOF-SIMS depth profile presented on Figures 5 and 6 shows the counts of specific fragments of the coated films of Sp2Co on a copper wafer before

(Figure 5) and after (Figure 6) NH₃ plasma treatment.

If we follow the profiles of two fragments, CH₂OCo and CoO, two different shapes are obtained before the plasma treatment. The first fragment, which can be attributed to the polymeric part, is laying on the outside of the system. Our interpretation is that Co(II) is able to complex the carboxylate moieties and stays on the external layer. Interestingly the shape of the CoO fragment is not identical and indicates that a part of Co(II) has migrated into the copper oxide since the fragment CuO resembles to the CoO one. After the NH₃ plasma treatment the mixture of Co(II) and Cu(II) oxides goes deeper into the bulk copper. On the surface, the polymer (CH₂OCo fragment) is still present indicating its good resistance to the plasma treatment. The presence of oxides on the surface is explained by the air break between the NH₃ plasma and the TOF-SIMS analysis.

Since TOF-SIMS does not tell us if we have metallic cobalt, we made XPS analysis (see example 4).

Example 4 : XPS characterization of orqano-metallic grafting before and after pre-plasma, and after SiN deposition

XPS analysis was done on non-pattern substrates made according to example 1 .1 . XPS was performed in ESCALAB VG XL 220 equipment with a monochromator (Al source 1486.6 eV), 1 mm² of spot size, detector at 90° and all band energies calibrated on carbon 285 eV. Table 1 shows the XPS spectrum of Sp2Co coated on a copper wafer before and after pre plasma, and also after SiN coating by PECVD.

Table 1: XPS before and after treatment

* : x,y and z are between 0 and 4

XPS analysis before pre plasma confirms the presence, as seen in the TOF-SIMS analysis (example 3), of at least two types of cobalt. One incorporated in an oxide-hydroxide matrix and the other one linked to the sulfate anion. After pre plasma, the cobalt linked to the sulfate disappears. The overall amount of metals yields one undetermined product, the former cobalt oxide- hydroxide matrix and cobalt at the zero oxidation state. The low value from the latter one is certainly due to the non protection of the surface after pre plasma, oxidation taking place between the pre plasma process and the XPS analysis. This suggestion is corroborated by the 100% of Co(O) obtained when there is no airbrake between the pre plasma and the SiN coating.

Example 5: Adhesion characterization by microscratching test of the complete stack Adhesion measurement was done on non-pattern substrates made according to example 1.1 with an organo-metallic layer (Sp2Co), compared to a "reference" sample without any organo-metallic coating before SiN deposition, and compared also to a substrate with only organic layer before SiN deposition (Sp2). Adhesion of the complete stack was evaluated by measuring the critical force of the SiN layer in a microscratching test with the Nanoindenteur XP^R (1. Bispo and al., Microelectron. Eng., Vol. 83, Issue 11 -12, (2006), p. 2088).

The adhesion is known to be an indicator of the electromigration resistance (see for example M.W. Lane and al., J. Applied Physics, Vol. 93, (2003), p.1417). The microscratching test with the Nanoindenteur XP^R gave the following results. Table 2: adhesion results

Adhesion values are a mean of four measurements. The efficiency of Sp2Co compound is obvious compared to the other cases. While "reference" and Sp2 samples were measurable, Sp2Co compound had a resistance to scratch superior to the range of the apparatus (superior to 6 mN).

Example 6: parametric electrical results Parametric electrical tests were performed on pattern substrates made according to example 1.2 (Sp2Co), and compared to a "reference" sample without any organo-metallic coating before SiN deposition.

These structures were evaluated for line-to-line leakage and line resistance, and presented in cumulative graphs (Figures 7 - 10) after measurement in several trenches. These tests are required before electromigration (see example 7) to detect possible defects in electrical continuity (line resistance) and also to check level of contaminants between lines (leakage current).

Both samples, "reference" and treated according to the invention, exhibited similar performances, showing the very low impact of the organo- metallic coating on parametric electrical measurements.

Example 7 : electromigration results

Electromigration tests were performed on pattern substrates made according to example 1.2, and compared to a "reference" sample without any organo-metallic coating before SiN deposition.

After parametric electrical tests (see example 6 above), structures were diced in different dies and packaged using a procedure well known by people skilled in the art. The packaged dies were submitted to electromigration testing. The conditions were: oven temperature set at 310⁰C, current density set at 5,5.10⁵ A/cm², temperature 110⁰C (Ea = 0.8 eV, n=2). The failure definition was 20% of deviation from initial resistance.

Lifetime of the packaged dies is presented in Table 3, with MTTF (mean time to failure) criteria which represents time duration to kill 50% of dies, and lifetime t50% value which is extrapolated from MTTF and electromigration conditions.

Table 3: Electro-migration results

Lifetime of the dies coated with Sp2Co was significantly higher than the "reference" dies (gain was close to 50% in lifetime). As expected, there is a strong relationship between adhesion measurements, performed in example 5, and these electromigration performances.

Claims

1. A method for preparing a multilayer composite device, characterized in that it comprises the steps of:

2. The method of claim 1 , wherein the intermediate layer is formed by covalent grafting enabled by the metallic salt of the coating solution, said metallic salt being prepared with compounds of formula [Form.1]:

Where: - A is a diazonium N≡N group enabling the metallic salt to be selectively bound to copper or copper alloy and further enabling initiation of the formation of the intermediate layer by polymerisation,

- R1 , R2, R3 are three substituents for enabling complexation by the metallic ions, said substituents being chosen, independently from each other, among H, COOH and corresponding carboxylate salts, NH₂ and corresponding ammonium salts, (CH₂)_mCOOH and corresponding carboxylate salts, (CH₂V₁NH₂ and corresponding ammonium salts, with 1 < m < 4,

- Me is a transition metal chosen among Ni(II), Co(II) or Co(III), the oxidation state representing the oxidation state of the metal from the initial salt introduced when preparing the coating solution, - L is a ligand which can be a water molecule, Cl^", OH^", SO₄ ^2", N-methyl- pyrrolidone (NMP), Dimethyl-formamide (DMF), acetonitrile, acetate, acetylacetonate, NO₃ ^" or a binding moiety of the diazonium salt,

3. The method of claim 2, wherein the metallic salt is an aryl diazonium salt where R1 =R3=H, R2=(CH₂)-COOH mixed with Co(II).

4. The method of claim 3, wherein the diazonium salt is in a concentration comprised between 5 and 50 mM, and preferably in a concentration of about 1 O mM, within a solution using N-methyl-pyrrolidone (NMP), Dimethyl- formamide (DMF), acetonitrile or a mixture thereof as solvent.

5. The method of claim 4, wherein the coating solution comprises tetrafluoroborate para(acetic acid) phenylene diazonium salt of aniline mixed with Co(II) sulfate hexahydrate, where the molar ratio between the diazonium salt and the Co(II) being from 0.2 to 10, and preferably 0.5 to 2.

6. The method of any of claims 1 to 5, wherein the step of forming the intermediate layer comprises immersing, spraying or spin-coating said coating solution onto said surface of composite material under ultrasonic activation.

7. The method of claim 6, wherein the ultrasonic activation is performed during an activation time lower or equal to 10 minutes, preferably during an activation time comprised between 1 and 5 minutes, and even more preferably during an activation time of about 2 minutes.

8. The method of any of claims 1 to 7, wherein the reducing species of the plasma are selected among the group consisting of NH₃, H₂ or a mixture thereof.

9. The method of any of claims 1 to 8, wherein the step of forming a dielectric encapsulating layer on the intermediate layer consists in depositing SiCN, SiN, SiC, SiOCN or SiON by plasma enhanced chemical vapor deposition (PECVD).

10. The method of any of claims 1 to 9, wherein the encapsulating layer is made of silicon nitride (SiN) deposited by plasma enhanced chemical vapor deposition (PECVD) using a silane (SiH₄) with ammonia (NH₃) chemistry.

11. The method of any of claims 1 to 10, wherein the composite material is a semiconductor device.

12. The method of claim 11 , wherein the composite material is a multilayer interconnect system.

13. A multilayer composite device comprising :

14. The device of claim 13, wherein the encapsulating layer is made of SiCN, SiN, SiC, SiOCN or SiON.