WO2007075063A1

WO2007075063A1 - Cobalt-based alloy electroless plating solution and electroless plating method using the same

Info

Publication number: WO2007075063A1
Application number: PCT/KR2006/005834
Authority: WO
Inventors: Sang-Chul Lee; Min-Kyoun Kim; Min-Jin Ko
Original assignee: Lg Chem, Ltd.
Priority date: 2005-12-29
Filing date: 2006-12-28
Publication date: 2007-07-05
Also published as: CN101336309B; KR100859259B1; JP4861436B2; US20070160857A1; TW200724716A; US7758681B2; TWI332999B; JP2009522445A; CN101336309A; KR20070070688A

Abstract

The present invention provides a cobalt-based alloy electroless plating solution comprising a cobalt precursor, a tungsten precursor, a phosphorus precursor, a reducing agent, a complexing agent, a pH regulator and a stabilizer, in which the reducing agent is dimethylamine borane (DMAB) or borohydride and the stabilizer is one or more compounds selected from a group consisting of imidazole, thiazole, triazole, disulfide and their derivatives; and an electroless plating method using the cobalt-based alloy electroless plating solution, as well as a thin film prepared by the same. According to the present invention, the cobalt-based alloy electroless plating solution is stable enough for long-term reuse and prevents deterioration of metal thin-film quality by inhibiting the formation of a precipitate. The present invention further provides an electroless plating method using the cobalt-based alloy electroless plating solution, and a cobalt-based alloy thin film prepared by the same.

Description

COBALT-BASED ALLOY ELECTROLESS PLATING SOLUTION

AND ELECTROLESS PLATING METHOD USING THE SAME

Technical Field

The present invention relates to a cobalt-based alloy electroless plating solution

and an electroless plating method using the same, and more precisely, a cobalt-based

alloy electroless plating solution which is stable enough to be reused several times and

prevents degradation of metal thin-film quality due to formation of a precipitate; and an

electroless plating method characterized by immersion in, or spraying of, the same.

Background Art

With recent increases in the packing density of semiconductor devices,

conventional aluminum wiring material must be replaced with copper to reduce signal

retardation and to improve electromigration resistance. However, when copper is used

as a wiring material, the copper is diffused into an interlayer insulating film (for example,

a silicon oxide layer) which defines the wiring. To solve this problem, a diffusion

barrier layer (formed on the side wall and the bottom of copper wiring) and a capping

layer (formed on the upper part of copper wiring) are formed between copper wires, and

an interlayer insulating film is applied to prevent direct contact. A silicon nitride layer has been used up to now as a capping layer for copper

wiring. However, in addition to poor adhesion to copper, the silicon nitride layer has a

different thermal expansion coefficient to the interlayer insulating film formed on the

upper part of the capping layer, for example a silicon oxide layer, so mechanical stress is

concentrated between the capping layer and the interlayer insulating film, resulting in

the separation of the capping layer (silicon nitride film) from the upper part of the

copper wiring. When the capping layer is separated from the copper wiring, the

diffusion of copper into the interlayer insulating film will not be inhibited. Owing to

the huge difference in permittivity of the silicon nitride film, the volume of parasitic

capacitance increases, which causes retardation of the driving speed of a semiconductor

device by RC retardation.

Thus, a cobalt-based alloy has been proposed as an alternative, which seems to

have excellent adhesion to copper wiring and low permittivity and prevents the diffusion

of copper into an interlayer insulating film. The cobalt-based alloy contains cobalt as a

major component and additionally includes metals such as tungsten, boron, phosphorus,

etc. To form a cobalt-based alloy thin film selectively on the upper part of copper

wiring, electroless plating has been proposed.

Electroless plating is a method of forming a metal thin-film by reducing a metal

ion using an electron generated through oxidation of a reducing agent on the surface of a

catalyst substrate without any externally supplied electrons. This method has an advantage of forming a metal thin-film specifically on a target area, which is activated

by catalyst, of an entire substrate. However, by containing a reducing agent the plating

solution becomes unstable, according to the plating conditions, and thus autolysis occurs.

Autolysis indicates that a metal ion is reduced not on the surface of a catalyst substrate,

but in the plating solution, thus forming a precipitate. This autolysis brings about the

loss of metal particles, resulting in a decrease in the durability of the solution (shortening

the life of the solution) and a decrease in the quality of the metal thin-film due to the

formation of a precipitate in the solution.

To apply a cobalt-based alloy on the upper part of copper wiring by electroless

plating, dimethylamine borane (DMAB), which is easily oxidized on the surface of

copper, has to be used as a reducing agent considering the low catalytic activity of

copper, and the temperature for the process has to be high. However, in this case the

electroless plating solution becomes chemically unstable, so autolysis occurs easily.

Disclosure of the Invention

In order to solve the above problems, the present invention provides a cobalt-

based alloy electroless plating solution which is stable enough to inhibit autolysis and to

be reused several times, and is able to prevent the deterioration of metal thin-film quality

due to the formation of a precipitate in the solution.

The present invention also provides an electroless plating method characterized by immersion in, or spraying of, the cobalt-based alloy electroless plating solution, and a

thin film prepared using the same.

An embodiment of the present invention provides a cobalt-based alloy

electroless plating solution comprising: a cobalt precursor, a tungsten precursor, a

phosphorus precursor, a reducing agent, a complexing agent, a pH regulator and a

stabilizer, in which the reducing agent is dimethylamine borane (DMAB) or borohydride

and the stabilizer is one or more compounds selected from a group consisting of

imidazole, thiazole, triazole, disulfide and their derivatives.

Another embodiment of the present invention provides an electroless plating

method characterized by immersion in, or spraying of, the cobalt-based alloy electroless

plating solution, and a thin film prepared using the same.

Hereinafter, the present invention is described in detail.

Conventional reducing agents, including hypophosphite, are not appropriate for

forming a capping layer on the upper part of copper wiring by cobalt-based alloy

electroless plating because the conventional reducing agents are not easily oxidized on

the surface of copper, making plating difficult. Thus dimethylamine borane (DMAB)

or borohydride, which is easily oxidized on the surface of copper, is required as a

reducing agent to form a capping layer. However, using DMAB or borohydride as a

reducing agent increases the chances of autolysis since plating with the reducing agent has to be performed at high temperature, which decreases the chemical stability.

Therefore, the present invention tries to inhibit autolysis by adding a stabilizer to the

cobalt-based alloy electroless plating solution.

The cobalt-based alloy electroless plating solution of the present invention is

comprised of: a cobalt precursor, a tungsten precursor, a phosphorus precursor, a

reducing agent, a complexing agent, a pH regulator and a stabilizer, in which the

reducing agent is dimethylamine borane (DMAB) or borohydride and the stabilizer is

one or more compounds selected from a group consisting of imidazole, thiazole, triazole,

disulfide and their derivatives.

The cobalt precursor is one or more compounds selected from a group

consisting of cobalt sulfate, cobalt chloride and cobalt ammonium sulphate. Among

these compounds, cobalt sulfate heptahydrate is preferred. The preferable content of

the cobalt precursor is 0.5 ~ 5.0 g/L considering reaction speed and plating time.

The tungsten precursor is one or more compounds selected from a group

consisting of ammonium tungstate, sodium tungstate and tetramethyl ammonium

tungstate, and among these ammonium tungstate is preferred. The content of the

tungsten precursor may be controlled to regulate the composition of a capping layer, and

the preferable content of the tungsten precursor is 0.1 ~ 1.0 g/L.

The phosphorus precursor is one or more compounds selected from a group

consisting of ammonium hypophosphite, ammonium dihydrogen phosphate and phosphoric acid, and among these compounds ammonium dihydrogen phosphate is

preferred. The content of the phosphorus precursor can be controlled to regulate the

composition of a capping layer, and the preferable content of the phosphorus precursor is

1.0 ~ 5.0 g/L.

The reducing agent is a compound that provides an electron necessary for the

reduction of a metal ion by oxidation. The reducing agent herein is dimethylamine

borane (DMAB) or borohydride. Considering the reaction speed and plating time, as

well as the stability of the plating solution, the preferable content of the reducing agent is

0.5 ~ 10.0 g/L, and 3.0 ~ 5.0 g/L is more preferred.

The complexing agent is a compound that forms a complex with a metal ion in

an electroless plating solution to stabilize the metal ion, which can be one or more

compounds selected from a group consisting of citric acid, ammonium citrate, sodium

citrate, tetramethyl ammonium citrate and ethylene diamine tetraacetic acid (EDTA).

Among these compounds, citric acid (anhydrous) is preferred. The preferable content

of the complexing agent is 3.0 ~ 15.0 g/L.

The pH regulator plays a role in regulating hydroxylation of an electroless

plating solution to maintain the proper pH for the reaction, and can be one or more

compounds selected from a group consisting of potassium hydroxide (KOH),

ammonium hydroxide and tetramethyl ammonium hydroxide (TMAH). Among these

compounds tetramethyl ammonium hydroxide (TMAH) is preferred. The preferable content of the pH regulator is 10 ~ 40 mL/L.

The stabilizer forms a complex with a metal ion to suppress the generation of a

metal particle, or is absorbed onto the surface of a metal particle to inhibit the growth of

the metal particle in an electroless plating solution, resulting in the enhancement of the

stability of the electroless plating solution.

The stabilizer included in an electroless plating solution plays a role in

inhibiting autolysis at high temperature and the long term maintenance of the properties

of the solution so the solution is stable, minimizes the deceleration of electroless plating

reaction speed, and thereby forms a cobalt-based alloy thin film on a copper thin film.

The stabilizer is one or more compounds selected from a group consisting of

imidazole, thiazole, triazole, disulfide and their derivatives. To minimize the

deceleration of the plating speed, 4,5-dithiaoctane-l,8-disulfonic acid (SPS), 3-(2-

benzothiazolethio)-l -propane sulfonic acid, N,N-dimethyl dithiocarbamic acid(3-

sulfopropyl)ester (DPS) or 3-mercapto-l-propanesulfonate (MPSA) is preferably used.

The preferable content of the stabilizer in a cobalt-based alloy electroless plating

solution is 0.001 mg/L ~ 1 g/L.

The preferable pH of the cobalt-based alloy electroless plating solution is 8 ~ 10.

The electroless plating method of the present invention is characterized by

immersion in, or spraying of, the cobalt-based alloy electroless plating solution.

In the process of semiconductor wiring, copper wiring is electroplated on the damascene structure formed by etching. The surface of the copper formed as described

is planarized so it is smooth. However, if the surface of copper on which electroless

plating is performed is oxidized or includes impurities, the electroless plating will not be

completed satisfactorily. Therefore, copper oxide or impurities have to be eliminated

by a semiconductor cleaning process before the electroless plating. Therefore, the

electroless plating method of the present invention can additionally include a pre-

treatment process of cleaning the copper substrate on which electroless plating will be

performed after planarizing.

The electroless plating method of the present invention is achieved by either

dipping a substrate which will be the base for a capping layer in an electroless plating

solution for a required amount of time, or spraying an electroless plating solution on a

substrate that will be the base for a capping layer.

The electroless plating method of the present invention using the cobalt-based

alloy electroless plating solution containing a stabilizer might reduce the electroless

plating speed. Therefore, it is preferred to quickly form a cobalt-based alloy thin film

on the copper thin film while keeping the solution stable and minimizing the

deceleration of the electroless plating speed.

The electroless plating reaction temperature is a crucial factor that affects the

stability of the electroless plating solution and the plating speed. The higher the

temperature goes, the lower the stability is and the faster the plating speed becomes. On the contrary, the lower the temperature goes, the higher the stability is and the slower

the plating speed becomes.

According to the electroless plating method of the present invention, the

preferable temperature for forming a capping layer on a substrate with an electroless

plating solution is 15 ~ 95 ^°C , and more preferably 70 ~ 90 ^°C .

The duration of the electroless plating process depends on the thickness of the

cobalt-based alloy thin film. That is, according to the thickness that the cobalt-based

alloy thin film will be prepared at, the electroless plating time will be within one hour, or

preferably within 10 minutes, and more preferably within 2 minutes.

According to the electroless plating method of the present invention, the

thickness of a cobalt-based alloy thin film formed on a substrate can be regulated. The

preferable thickness of a cobalt-based alloy thin film is up to 100 nm, and more

preferably up to 10 lira.

Brief Description of the Drawings

The application of the preferred embodiments of the present invention is best

understood with reference to the accompanying drawings, wherein:

Fig. 1 is a TEM photograph of the cobalt-based alloy thin film formed by using

the cobalt-based alloy electroless plating solution containing SPS of Example 1. Fig. 2 is a TEM photograph of the cobalt-based alloy thin film formed by using

the cobalt-based alloy electroless plating solution containing 3-(2-benzothiazolethio)-l-

propane sulfonic acid of Example 2.

Best Mode for Carrying Out the Invention

Practical and presently preferred embodiments of the present invention are

illustrated as shown in the following examples.

However, it will be appreciated that those skilled in the art, on consideration

of this disclosure, may make modifications and improvements within the spirit and

scope of the present invention.

[Examples]

Example 1

< Preparation of a cobalt-based alloy electroless plating solution >

0.01 M of cobalt sulfate heptahydrate, 0.04 M of citric acid, 0.5 g/L of

ammonium tungstate, 0.06 M of DMAB and 0.03 M of dihydrogen phosphate were

mixed, and the pH of the mixture was adjusted to 9 by using TMAH. 0.01 g/L of SPS

was added thereto as a stabilizer to prepare a cobalt-based alloy electroless plating

solution with improved stability.

The prepared electroless plating solution was heated in water at 95 ^"C . 30 minutes later the temperature of the solution reached 90 ^°C , and the solution was stable

' for over 12 hours.

< Preparation of a cobalt-based alloy electroless plating thin film >

A planarized copper wiring substrate was prepared for the cobalt-based alloy

electroless plating. The prepared substrate was immersed in ammonia solution (1:200)

for 30 seconds to eliminate copper oxides generated on the surface of the substrate.

The substrate was then washed with distilled water to eliminate any remaining impurities.

The prepared substrate was immersed for 1 minute in the cobalt-based alloy

electroless plating solution, which was standing at 90 ^°C, thus completing the cobalt-

based alloy electroless plating method.

Fig. 1 is a TEM photograph of the prepared electroless plating thin film. In Fig.

1, (a) illustrates the cobalt-based alloy thin film formed by the electroless plating and (b)

illustrates the copper thin film. As shown in Fig. 1, a 40 run thick electroless plating

thin film with excellent surface properties was prepared by using the stable cobalt-based

alloy electroless plating solution.

Example 2

< Preparation of a cobalt-based alloy electroless plating solution >

A cobalt-based alloy electroless plating solution was prepared in the same

manner as described in Example 1, except that 0.01 g/L of 3-(2-benzothiazolethio)-l-

propane sulfonic acid was added as a stabilizer. The prepared electroless plating solution was heated in water at 84 ^°C . 30

minutes later the temperature of the solution reached 80⁰C, and the solution was stable

for over 12 hours.

< Preparation of a cobalt-based alloy electroless plating thin film >

An experiment was performed in the same manner as described in Example 1 ,

except that the cobalt-based alloy electroless plating solution prepared in Example 2 was

used and it was kept at 80 "C .

Fig. 2 is a TEM photograph of the prepared electroless plating thin film. In Fig.

2, (a) illustrates the cobalt-based alloy thin film formed by electroless plating, (b)

indicates the copper thin film, (c) indicates the diffusion barrier layer and (d) illustrates

the silicon wafer substrate. As shown in Fig. 2, a 37 ran thick electroless plating thin

film with excellent surface properties was prepared by using the stable cobalt-based

alloy electroless plating solution.

Comparative Example 1

< Preparation of a cobalt-based alloy electroless plating solution >

A cobalt-based alloy electroless plating solution was prepared in the same

manner as described in Example 1, except the stabilizer used in Example 1 was excluded.

The prepared electroless plating solution was heated in water at 95 ^°C . After 20

minutes from the start of heating, air bubbles began to generate in the solution, autolysis

was observed, and a gray precipitate was generated from the reaction. < Preparation of a cobalt-based alloy electroless plating thin film >

An experiment was performed in the same manner as described in Example 1,

except the cobalt-based alloy electroless plating solution prepared in Comparative

Example 1 was used and it was kept at 90 ^°C .

As a result, a gray precipitate resulting from autolysis was found in the bottom

of the reactor and a thin film was not formed.

Comparative Example 2

< Preparation of a cobalt-based alloy electroless plating solution >

A cobalt-based alloy electroless plating solution was prepared in the same

The prepared electroless plating solution was heated in water at 84 ^°C . After 20

was observed, and a gray precipitate was generated from the reaction.

< Preparation of a cobalt-based alloy electroless plating thin film >

An experiment was performed in the same manner as described in Example 1,

Example 2 was used and it was kept at 80 °C .

of the reactor, and a thin film was not formed. Industrial Applicability

As explained hereinbefore, the present invention provides a cobalt-based alloy

electroless plating solution having excellent stability that is able to be reused many times,

and does not form a precipitate thereby preventing the deterioration of metal thin-film

properties. In addition, the present invention provides an electroless plating method

using the above solution and a cobalt-based alloy thin film using the said method.

Those skilled in the art will appreciate that the conceptions and specific

embodiments disclosed in the foregoing description may be readily utilized as a

basis for modifying or designing other embodiments for carrying out the same

purposes of the present invention. Those skilled in the art will also appreciate that

such equivalent embodiments do not depart from the spirit and scope of the

invention as set forth in the appended claims.

Claims

1. A cobalt-based alloy electroless plating solution comprising: a cobalt

precursor, a tungsten precursor, a phosphorus precursor, a reducing agent, a complexing

agent, a pH regulator and a stabilizer, in which the reducing agent is dimethylamine

borane (DMAB) or borohydride and the stabilizer is one or more compounds selected

from a group consisting of imidazole, thiazole, triazole, disulfide and their derivatives.

2. The cobalt-based alloy electroless plating solution according to claim 1,

wherein the cobalt precursor is one or more compounds selected from a group consisting

of cobalt sulfate, cobalt chloride and cobalt ammonium sulphate.

3. The cobalt-based alloy electroless plating solution according to claim 1,

wherein the tungsten precursor is one or more compounds selected from a group

consisting of ammonium tungstate, sodium tungstate and tetramethyl ammonium

tungstate.

4. The cobalt-based alloy electroless plating solution according to claim 1,

wherein the phosphorus precursor is one or more compounds selected from a group

consisting of ammonium hypophosphite, ammonium dihydrogen phosphate and

phosphoric acid.

5. The cobalt-based alloy electroless plating solution according to claim 1,

wherein the complexing agent is one or more compounds selected from a group

consisting of citric acid, ammonium citrate, sodium citrate, tetramethyl ammonium

citrate and ethylene diamine tetraacetic acid (EDTA).

6. The cobalt-based alloy electroless plating solution according to claim 1,

wherein the pH regulator is one or more compounds selected from a group consisting of

potassium hydroxide (KOH), ammonium hydroxide and tetramethyl ammonium

hydroxide (TMAH).

7. The cobalt-based alloy electroless plating solution according to claim 1,

wherein the pH of the cobalt-based alloy electroless plating solution is 8 ~ 10.

8. The cobalt-based alloy electroless plating solution according to claim 1,

wherein the stabilizer is selected from a group consisting of 4,5-dithiaoctane-l,8-

disulfonic acid (SPS), 3 -(2-benzothiazolethio)-l -propane sulfonic acid, N,N-dimethyl

dithiocarbamic acid(3-sulfopropyl)ester (DPS) and 3-mercapto-l-propanesulfonate

(MPSA).

9. The cobalt-based alloy electroless plating solution according to claim 1,

wherein the content of the stabilizer in the cobalt-based alloy electroless plating solution

is 0.001 mg/L ~ l g/L.

10. An electroless plating method characterized by using the cobalt-based alloy

electroless plating solution of claim 1.

11. The electroless plating method according to claim 10, wherein the

electroless plating is characterized by immersing a substrate, upon which a capping layer

will be formed, in the cobalt-based alloy electroless plating solution to form a capping

layer.

12. The electroless plating method according to claim 10, wherein the

electroless plating is characterized by spraying the cobalt-based alloy electroless plating

solution on a substrate upon which a capping layer will be formed.

13. The electroless plating method according to claim 10, wherein the

electroless plating is performed with the cobalt-based alloy electroless plating solution at

15 ~ 95°C .

14. The electroless plating method according to claim 10, wherein the duration

of the electroless plating is up to one hour.

15. The electroless plating method according to claim 10, wherein the

electroless plating method additionally includes a pre-treatment step of cleaning the

copper surface after a planarizing process.

16. The electroless plating method according to claim 10, wherein the thickness

of the cobalt-based alloy thin film formed by the electroless plating is up to 100 ran.

17. A cobalt-based alloy thin film prepared by the electroless plating method of

claim 10.