ELECTROLESS RUTHENIUM PLATING BATH

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2024-165154 filed on Sep. 24, 2024, the entire disclosure of which is incorporated herein by reference.

BACKGROUND

The present disclosure relates to electroless ruthenium plating baths.

Copper has high electrical conductivity. It also has excellent physical properties, such as strong bonding through thermocompression, and excellent chemical properties, such as high resistance to oxidation and chemicals. For these reasons, copper has been widely used in the electronics industry, for example, in interconnects on printed circuit boards and in mounting portions and terminal portions of integrated circuit (IC) packages.

With the miniaturization of semiconductor circuits, copper interconnects have also become finer. As a result, the current density in a copper interconnect increases. This high current density can cause electromigration, which is a phenomenon in which copper atoms migrate when a high-density current flows through the copper interconnect. Electromigration leads to the formation of voids in the copper interconnect, which can result in disconnection and circuit failure.

In recent years, ruthenium has attracted attention as a next-generation interconnect material to replace copper. Compared with copper, ruthenium has a higher allowable current density and greater resistance to electromigration. Ruthenium is expected to be used not only in the interconnects of the semiconductor circuits, but also in thin films (cap metals) formed on copper interconnects, and in liner layers formed on barrier metals to enable uniform growth of copper seed films when copper interconnects are deposited by electrolytic plating.

If electroless ruthenium plating could be applied in the process of forming interconnects of semiconductor circuits etc., no external power supply would be required. Ruthenium could then be selectively deposited simply by immersion treatment. For this reason, electroless plating baths containing ruthenium have been proposed.

For example, electroless ruthenium plating baths have been proposed that contain a ruthenium source, a polyamino polycarboxylic acid as a complexing agent, sodium borohydride (NaBH₄) as a reducing agent, and hydroxylamine sulfate as a stabilizer (see, for example, Japanese Unexamined Patent Application Publication No. 2012-508819).

Electroless ruthenium plating baths have also been proposed that contain a ruthenium source and an amine borane compound, such as dimethylamine borane (DMAB), as a reducing agent (see, for example, Japanese Unexamined Patent Application Publication No. S61-39235).

SUMMARY

In the plating bath described in Japanese Unexamined Patent Application Publication No. 2012-508819, ruthenium exists in multiple valence states, making it difficult to deposit ruthenium as a metal and thus resulting in poor depositability. In addition, sodium borohydride is used as a reducing agent. However, since sodium borohydride is highly reactive, bath decomposition and deposition outside the desired pattern are likely to occur, making the bath difficult to handle.

In the plating bath described in Japanese Unexamined Patent Application Publication No. S61-39235, a common reducing agent, namely an amine borane compound, is used. However, amine borane compounds have relatively weak reducing ability. Accordingly, only films containing a large amount of impurity oxygen are obtained due to the codeposition of unreduced oxides (ruthenium oxide). When such films are subjected to annealing, the oxygen is released as gas, causing the films to shrink.

In view of the above, an object of the present disclosure is to provide an electroless ruthenium plating bath that uses hydrazines as the reducing agent. The plating bath of the present disclosure can improve bath stability, provides excellent ruthenium depositability, and reduces film shrinkage during annealing.

In order to achieve the above object, an electroless ruthenium plating bath according to the present disclosure contains at least a ruthenium compound, a reducing agent, and a stabilizer. The reducing agent is hydrazines. The stabilizer includes a hydroxylamine compound and an organic compound having a hydroxyl group. The hydroxylamine compound is either or both of hydroxylamine sulfate and hydroxylamine chloride. The organic compound having a hydroxyl group is at least one selected from the group consisting of gluconolactone, sorbitol, mannitol, and citric acid monohydrate.

According to the present disclosure, since hydrazines having a high reducing ability are used as the reducing agent, a ruthenium plating film with a low impurity oxygen content can be obtained, and shrinkage of the film during annealing can be suppressed.

In addition, since a hydroxylamine compound and an organic compound having a hydroxyl group are used in combination as the stabilizer, bath stability is improved and ruthenium depositability is also enhanced.

DETAILED DESCRIPTION

An electroless ruthenium plating bath of the present disclosure will now be described.

Electroless Ruthenium Plating Bath

The electroless ruthenium plating bath of the present disclosure contains at least a ruthenium compound, a reducing agent, and a stabilizer.

Ruthenium Compound

The ruthenium compound serves as a source of ruthenium ions for obtaining ruthenium plating. Any water-soluble ruthenium compound may be used. Examples of the ruthenium compound include inorganic water-soluble ruthenium salts such as ruthenium chloride, ruthenium sulfate, and ruthenium nitrate. These ruthenium compounds may be used individually or in combination of two or more.

Ruthenium has multiple valence states, and the deposition amount of ruthenium in the plating bath varies depending on the valence state. From the standpoint of ensuring plating bath stability, it is suitable that the valence state of ruthenium be trivalent or tetravalent.

Accordingly, suitable examples of the ruthenium compound include ruthenium chloride (III) and ruthenium nitrate (III), which contain trivalent ruthenium, and ruthenium chloride (IV) and ruthenium sulfate (IV), which contain tetravalent ruthenium.

The concentration of the ruthenium compound (that is, ruthenium ions) in the electroless ruthenium plating bath is not particularly limited. However, if the ruthenium ion concentration is too low, the deposition rate of the plated film may be significantly reduced. Therefore, the ruthenium io concentration is suitably 0.01 g/L or more, and more suitably 0.1 g/L or more. On the other hand, if the ruthenium ion concentration is too high, bath decomposition due to excessive reaction may occur. Therefore, the ruthenium ion concentration is more suitably 10 g/L or less.

The ruthenium ion concentration can be measured by atomic absorption spectrometry (AAS) using an atomic absorption spectrophotometer.

Reducing Agent

The reducing agent serves to reduce the ruthenium compound, which is the source of ruthenium ions, and thus deposit ruthenium in the electroless plating bath. In the electroless ruthenium plating bath of the present disclosure, hydrazines are used as the reducing agent.

Examples of the hydrazines include hydrazine monohydrate, hydrazine dihydrochloride, and hydrazinium sulfate. From the standpoint of high solubility and the absence of a need for neutralization, it is suitable to use hydrazine monohydrate. These hydrazines may be used individually or in combination of two or more.

The concentration of the hydrazines in the plating bath is suitably 0.03 mol/L or more and 1.32 mol/L or less. More specifically, when hydrazine monohydrate (80%) is used, the concentration is suitably 2 mL/L or more and 80 mL/L or less. When hydrazine dihydrochloride is used, the concentration is suitably 3 g/L or more and 138 g/L or less. When hydrazinium sulfate is used, the concentration is suitably 4 g/L or more and 171 g/L or less. When the concentration is less than 0.03 mol/L, deposition of ruthenium may become difficult. In general, the plating rate increases in proportion to the concentration of the reducing agent. However, when the concentration of the reducing agent exceeds 1.32 mol/L, the plating rate does not improve significantly in proportion to the concentration of the reducing agent, and therefore, bath stability may decrease.

Because the electroless ruthenium plating bath of the present disclosure uses hydrazines with a high reducing ability as the reducing agent, the residual amount of unreduced oxides (ruthenium oxide) is reduced. Accordingly, a ruthenium plating film with a low impurity oxygen content can be obtained. The volumetric shrinkage ratio due to oxygen released from the ruthenium plating film during annealing is reduced, and the shrinkage of the ruthenium plating film during annealing can be suppressed.

In the present disclosure, since the ruthenium plating film contains a small amount of impurity oxygen, the impurity oxygen can be easily released from the ruthenium plating film during annealing. As a result, a film equivalent to a pure ruthenium plating film can be obtained, and the resistivity of the film after annealing can be reduced to 30 μΩ·cm or less. A low-resistance ruthenium plating film can thus be achieved.

Stabilizer

The stabilizer mainly serves as a complexing agent that stabilizes the solubility of ruthenium in the electroless ruthenium plating bath. In the electroless ruthenium plating bath of the present disclosure, a hydroxylamine compound is used together with an organic compound having a hydroxyl group as the stabilizer.

In the electroless ruthenium plating bath, hydroxylamine compounds contribute to bath stability by forming metal complexes with ruthenium ions. Examples of such hydroxylamine compounds include hydroxylamine sulfate and hydroxylamine chloride. These hydroxylamine compounds may be used individually or in combination of two.

The concentration of the hydroxylamine compound in the electroless ruthenium plating bath is suitably 1 g/L or more, more suitably 2 g/L or more, since too low a concentration may reduce bath stability and cause bath decomposition. The concentration of the hydroxylamine compound in the electroless ruthenium plating bath is also suitably 10 g/L or less, more suitably 8 g/L or less, since too high a concentration may lead to excessive bath stability and reduced ruthenium depositability.

Organic compounds having a hydroxyl group serve as a second complexing agent when used together with the hydroxylamine compound in the electroless ruthenium plating bath, and contribute to bath stability by forming metal complexes with ruthenium ions.

Examples of such organic compounds having a hydroxyl group include gluconolactone, sorbitol, mannitol, and citric acid monohydrate. These organic compounds having a hydroxyl group may be used individually or in combination of two or more.

The concentration of the organic compound having a hydroxyl group in the electroless ruthenium plating bath is suitably 1 g/L or more, since too low a concentration may reduce bath stability. The concentration the organic compound having a hydroxyl group in the electroless ruthenium plating bath is suitably 20 g/L or less, more suitably 10 g/L or less, since too high a concentration may lead to excessive bath stability and reduced ruthenium depositability.

Hydroxylamine compounds contribute more to bath stability than organic compounds having a hydroxyl group. Therefore, if a hydroxylamine compound alone is used as the stabilizer, bath stability may become excessive, and the deposition range of ruthenium relative to the amount of the hydroxylamine compound added becomes narrower, which may result in reduced ruthenium depositability.

Accordingly, in the electroless ruthenium plating bath of the present disclosure, a combination of a hydroxylamine compound and an organic compound having a hydroxyl group, which contributes less to bath stability than the hydroxylamine compound, is used as the stabilizer. In an electroless ruthenium plating bath that uses hydrazines as the reducing agent to suppress shrinkage of the ruthenium film, the use of this combination as the stabilizer improves bath stability (i.e., reduces the likelihood of excessive bath stability) and achieves excellent depositability.

In particular, the electroless ruthenium plating bath of the present disclosure provides excellent depositability even in fine regions where plating reactions are difficult to initiate (for example, in portions to be plated having a plating area on the order of several tens of square nanometers).

Deposition Rate Adjusting Agent

A deposition rate adjusting agent is added to smoothly remove underlying oxides etc., and serves to increase the deposition rate of ruthenium.

Various chelating agents can be used as the deposition rate adjusting agent. Examples include nitrogen- or phosphorus-containing compounds such as ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), 1,3-diamino-2-hydroxypropanetetraacetic acid (DTPA-OH), hydroxyethylethylenediaminetriacetic acid (HEDTA), triethylenetetraminehexaacetic acid (TTHA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CyDTA), and ethylenediamine tetra(methylenephosphonic acid) (EDTMP). Other examples include citric acid, malic acid, gluconic acid, lactic acid, malonic acid, fumaric acid, maleic acid, tartaric acid, acetic acid, succinic acid, oxalic acid, glycolic acid, and formic acid. Further examples include amino acid compounds such as glycine, alanine, aspartic acid, glutamic acid, iminodiacetic acid, leucine, isoleucine, lysine, tryptophan, valine, histidine, arginine, serine, and tyrosine. These deposition rate adjusting agents may be used individually or in combination of two or more.

The concentration of the deposition rate adjusting agent in the electroless ruthenium plating bath is suitably 1 g/L or more, more suitably 2 g/L or more, since too low a concentration may reduce the deposition rate of ruthenium and result in prolonged plating treatment. The concentration of the deposition rate adjusting agent is also suitably 60 g/L or less, more suitably 20 g/L or less, since too high a concentration increases costs due to excessive addition.

Other Components

In order to further promote the deposition of ruthenium, the electroless ruthenium plating bath of the present disclosure may also contain an amine borane compound as a second reducing agent in addition to the hydrazines described above.

Examples of the amine borane compound include dimethylamine borane (DMAB), trimethylamine borane (TMAB), morpholine borane, picoline borane, pyridine borane, diethylaniline borane, and ammonia borane. Dimethylamine borane, trimethylamine borane, and morpholine borane are suitable due to their wide availability and ease of procurement. Ammonia borane is suitable due to its relatively high reactivity. These amine borane compounds may be used individually or in combination of two or more.

When an amine borane compound is used as a reducing agent in addition to the hydrazines in the electroless ruthenium plating bath of the present disclosure, the concentration of the amine borane compound in the plating bath is suitably 0.1 g/L or more and 5 g/L or less. When the concentration is less than 0.1 g/L, ruthenium deposition may not be sufficiently promoted. When the concentration exceeds 5 g/L, bath decomposition due to excessive reaction may occur.

pH

The pH of the electroless ruthenium plating bath of the present disclosure is suitably from 11 to 14, more suitably from 12 to 14.

When the pH is less than 11, deposition of ruthenium may be insufficient.

The pH of the plating bath can be adjusted using a pH adjusting agent. Examples of the pH adjusting agent include sodium hydroxide, potassium hydroxide, aqueous ammonia, tetramethylammonium hydroxide, sulfuric acid, hydrochloric acid, citric acid, boric acid, phosphoric acid, monocarboxylic acids, and dicarboxylic acids. These pH adjusting agents may be used individually or in combination of two or more.

Temperature of Plating Bath

The temperature of the plating bath is not particularly limited, but is suitably from 45° C. to 85° C., more suitably from 50° C. to 75° C. When the temperature of the plating bath is less than 45° C., the plating bath may be inactivated and ruthenium deposition may become insufficient. When the temperature exceeds 85° C., the plating bath may become excessively activated and bath decomposition may occur.

Object to be Plated

The type of object to be plated in the electroless ruthenium plating bath of the present disclosure is not particularly limited. Examples include objects that are conventionally subjected to ruthenium plating treatment (such as interconnects on printed circuit boards, and mounting portions or terminal portions of IC packages).

The electroless ruthenium plating bath of the present disclosure is particularly suitable for forming ruthenium plating films constituting interconnects of semiconductor circuits, thin films (cap metals) formed on copper interconnects, or liner layers formed on barrier metals to enable uniform growth of copper seed films when copper interconnects are deposited by electrolytic plating.

Electroless Ruthenium Plating Treatment

By bringing the object to be plated into contact with the electroless ruthenium plating bath of the present disclosure and subjecting the object to electroless ruthenium plating treatment, it is possible to form, for example, ruthenium plating films constituting the interconnect portions, thin films (cap metals), or liner layers described above. The temperature during the electroless ruthenium plating treatment is controlled to the temperature of the electroless ruthenium plating bath described above.

The duration of the electroless ruthenium plating treatment is not particularly limited and may be set as appropriate so as to achieve a desired film thickness. More specifically, the duration may be, for example, about 30 seconds to about 15 hours.

The thickness of the ruthenium plating film may be set as appropriate according to desired properties, and is usually about 0.001 μm to about 1.0 μm.

EXAMPLES

The present disclosure will now be described in further detail based on Examples and Comparative Examples. However, the present disclosure is not limited to the following Examples.

Examples 1 to 37, Comparative Examples 1 to 6

Preparation of Plating Baths

The plating baths of Examples 1 to 37 and Comparative Examples 1 to 6 were each prepared by mixing and stirring 200 mL of deionized water with a ruthenium compound (ruthenium salt), a combination of a hydroxylamine compound and an organic compound having a hydroxyl group as a stabilizer, a reducing agent, a deposition rate adjusting agent, a pH adjusting agent, and a second reducing agent at the concentrations shown in Tables 2 to 6.

An aqueous solution of tetramethylammonium hydroxide (25%), used as the pH adjusting agent, was added in an appropriate amount (mL) to the plating baths of Examples 1 to 37 and Comparative Examples 1 to 6 until the pH of the plating baths reached predetermined values (i.e., the values shown in Tables 2 to 6).

As shown in Tables 2 to 6, the temperature of each plating bath (plating treatment temperature) was set to 45° C. to 85° C., and the pH of each plating bath was set to 11.0 to 13.3.

Pretreatment

Before electroless plating treatment, the substrate was subjected to pretreatment steps 1, 2 shown in Table 1. The substrate was washed with deionized water between the steps.

Step 1: The substrate (a silicon (Si) wafer provided with a ruthenium thin film (thickness: 10 nm) as a base by sputtering) was degreased and cleaned using MCL-12 (product name: EPITHAS (registered trademark) MCL-12, manufactured by C. Uyemura & Co., Ltd.).

Step 2: The surface of the substrate was then activated using MRU-30 (product name: EPITHAS (registered trademark) MRU-30, manufactured by C. Uyemura & Co., Ltd.).

	TABLE 1
	Treatment	Treatment	Treatment
	Solution	Temperature	Time (min)

Pretreatment	1	Degreasing	MCL-12	40° C.	3
Steps		and Cleaning
	2	Activation	MRU-30	60° C.	10

Electroless Ruthenium Plating Treatment

Thereafter, the pretreated substrate was immersed for 7 minutes in each of the plating baths of Examples 1 to 37 and Comparative Examples 1 to 6 shown in Tables 2 to 6, thereby forming a ruthenium plating film on the surface of the substrate (on the ruthenium thin film).

Measurement of Thickness of Ruthenium Plating Film Before Annealing

Cross-sectional preparation and observation were performed using a focused ion beam apparatus (product name: MI-4050, manufactured by Hitachi High-Tech Corporation), and the thickness (nm) of the ruthenium plating film formed on the surface of the substrate was measured.

More specifically, in the cross-sectional observation, the total thickness (nm) of all the films including the above base (10 nm-thick ruthenium thin film), that is, the total thickness (nm) of the base and the ruthenium plating film formed on the base, was measured, and the measured value was defined as the “thickness of the ruthenium plating film before annealing.” The results are shown in Tables 2 to 6.

Measurement of Oxygen Content in Ruthenium Plating Film

Subsequently, the ruthenium plating film formed on the surface of the substrate was analyzed in the depth direction using an Auger electron spectrometer (product name: JAMP-9500F, manufactured by JEOL Ltd.). The oxygen content in the ruthenium plating film formed on the surface of the substrate was measured based on the ratio (at. %) of ruthenium to oxygen at an etching depth equivalent to 57.6 nm in SiO₂. An oxygen content of 14 at. % or less was evaluated as being low and satisfactory. The results are shown in Tables 2 to 6.

Evaluation of Plating Bath Stability

After the electroless ruthenium plating treatment, the ruthenium plating bath was visually observed to determine whether ruthenium particles had precipitated. The stability of the plating bath was evaluated according to the following criteria. The results are shown in Tables 2 to 6.

• • No precipitation of ruthenium particles was observed even 3 hours after plating: Excellent
  • A very small amount of ruthenium particles was generated 3 hours after plating: Good
  • A small amount of ruthenium particles was generated 3 hours after plating: Fair
  • A large amount of ruthenium particles was generated 3 hours after plating: Poor
    Measurement of Thickness of Ruthenium Plating Film after Annealing

The substrate subjected to the above electroless ruthenium plating treatment was annealed using a reduction reflow apparatus (product name: VSS-300-EP, manufactured by Unitemp GmbH) at 400° C. for 40 minutes in a formic acid atmosphere.

After annealing, cross-sectional preparation and observation were performed using a focused ion beam apparatus (MI-4050, manufactured by Hitachi High-Tech Corporation), and the thickness (nm) of the ruthenium plating film formed on the surface of the substrate was measured.

More specifically, in the cross-sectional observation, the total thickness (nm) of all the films including the above base (ruthenium thin film) (i.e., the total thickness (nm) of the base and the ruthenium plating film on the base after annealing) was measured. The measured value was defined as the “thickness of the ruthenium plating film after annealing.” The results are shown in Tables 2 to 6.

Calculation of Resistivity of Ruthenium Plating Film after Annealing

Subsequently, the sheet resistance of the ruthenium film after annealing was measured using a four-point probe meter (product name: Napson RT-70V, manufactured by Napson Corporation). Based on this sheet resistance value and the thickness (nm) of the ruthenium plating film after annealing (i.e., the total thickness of the base and the ruthenium plating film on the base after annealing), the resistivity (μΩ·cm) of the ruthenium plating film after annealing was calculated using the following equation (1). The results are shown in Tables 2 to 6.

Resistivity of ruthenium plating film (μΩ·cm)=(Sheet resistance (Q/Q)×Thickness of ruthenium plating film after annealing (nm))/10  (1)

The resistivity of a pure ruthenium plating film is about 7.6 μΩ·cm. Accordingly, when the resistivity of the ruthenium plating film after annealing is 30 μΩ·cm or less, it is considered that most of the impurity oxygen has been released from the ruthenium plating film formed on the substrate surface by annealing. In this case, the ruthenium plating film is regarded as equivalent to a pure ruthenium plating film and as having the same low resistivity as a pure ruthenium plating film.

Calculation of Film Shrinkage Ratio after Annealing

Next, the film shrinkage ratio (%) after annealing was calculated using the following equation (2). When the film shrinkage ratio is 20% or less, the film shrinkage is evaluated as being small and satisfactory. The results are shown in Tables 2 to 6.

	{1 − [(Thickness (nm) of ruthenium plating film after
	annealing)/(Thickness (nm) of ruthenium plating
	film before annealing)]} × 100 (2)

In Comparative Examples 1, 3, 4, and 6, ruthenium was not deposited on the surface of the substrate. Therefore, measurement of the thickness of the ruthenium plating film before annealing, measurement of the oxygen content in the ruthenium plating film, calculation of the resistivity of the ruthenium plating film after annealing, measurement of the thickness of the ruthenium plating film after annealing, and calculation of the film shrinkage ratio after annealing could not be performed.

TABLE 2
			Plating Bath Composition
			Examples

			1	2	3	4	5	6
Ruthenium Salts	Ruthenium Chloride (IV)	g/L	0.1	0.5	1	2	10
	(as Ruthenium)
	Ruthenium Sulfate (IV)	g/L						1
	(as Ruthenium)
	Ruthenium Nitrate (III)	g/L
	(as Ruthenium)
	Ruthenium Chloride (III)	g/L
	(as Ruthenium)
Hydroxylamine	Hydroxylamine Sulfate	g/L
Compounds	Hydroxylamine Chloride	g/L	5	5	5	5	5	5
Organic Compounds	Gluconolactone	g/L	5	5	5	5	5	5
Having a Hydroxyl	Sorbitol	g/L
Group	Mannitol	g/L
	Citric Acid Monohydrate	g/L
Deposition Rate	DTPA-OH•4H	g/L	10	10	10	10	10	10
Adjusting Agent
Reducing Agents	Hydrazine Monohydrate (80%)	mL/L	10	10	10	10	10	10
	Hydrazine Dihydrochloride	g/L
	Hydrazinium Sulfate	g/L
Second Reducing	Ammonia Borane	g/L
Agents	Dimethylamine Borane	g/L

pH Adjusting

Tetramethylammonium

mL/L

added in an approprite amount until pH reached the predetermined value

Agent

Hydroxide Solution (25%)

pH	13.3	13.3	13.3	13.3	13.3	13.3
Treatment Temperature (° C.)	65	65	65	65	65	65

Evaluation	Thickness of Ruthenium Plating	nm	25	100	162	152	142	153
	Film Before Annealing
	Oxygen Content of Ruthenium	at. %	10.2	7.8	8.3	10.5	8.3	9.3
	Plating Film
	Thickness of Ruthenium Plating	nm	23	95	154	149	140	140
	Film After Annealing
	Film Shrinkage Ratio	%	8.0	5.0	4.9	2.0	1.4	8.5
	Resistivity of Ruthenium Plating	μΩ · cm	16.1	18.2	15.3	16.2	17.1	17.5
	Film After Annealing

	Plating Bath Stability	Excellent	Excellent	Excellent	Excellent	Good	Excellent

			Plating Bath Composition
			Examples

			7	8	9	10
Ruthenium Salts	Ruthenium Chloride (IV)	g/L			1	1
	(as Ruthenium)
	Ruthenium Sulfate (IV)	g/L
	(as Ruthenium)
	Ruthenium Nitrate (III)	g/L	1
	(as Ruthenium)
	Ruthenium Chloride (III)	g/L		1
	(as Ruthenium)
Hydroxylamine	Hydroxylamine Sulfate	g/L	5	5
Compounds	Hydroxylamine Chloride	g/L			1.5	10
Organic Compounds	Gluconolactone	g/L	5	5	5	5
Having a Hydroxyl	Sorbitol	g/L
Group	Mannitol	g/L
	Citric Acid Monohydrate	g/L
Deposition Rate	DTPA-OH•4H	g/L	10	10	10	10
Adjusting Agent
Reducing Agents	Hydrazine Monohydrate (80%)	mL/L	10	10	10	10
	Hydrazine Dihydrochloride	g/L
	Hydrazinium Sulfate	g/L
Second Reducing	Ammonia Borane	g/L
Agents	Dimethylamine Borane	g/L

pH Adjusting

Tetramethylammonium

mL/L

added in an approprite amount until pH reached the predetermined value

Agent

Hydroxide Solution (25%)

pH	13.3	13.3	13.3	13.3
Treatment Temperature (° C.)	65	65	65	65

Evaluation	Thickness of Ruthenium Plating	nm	123	153	143	75
	Film Before Annealing
	Oxygen Content of Ruthenium	at. %	11.5	9.3	8.9	10.2
	Plating Film
	Thickness of Ruthenium Plating	nm	110	143	132	72
	Film After Annealing
	Film Shrinkage Ratio	%	10.6	6.5	7.7	4.0
	Resistivity of Ruthenium Plating	μΩ · cm	20.1	25.5	16.2	19.8
	Film After Annealing

	Plating Bath Stability	Excellent	Excellent	Excellent	Excellent

TABLE 3
			Plating Bath Composition
			Examples

			11	12	13	14	15	16
Ruthenium Salts	Ruthenium Chloride (IV)	g/L	1	1	1	1	1	1
	(as Ruthenium)
	Ruthenium Sulfate (IV)	g/L
	(as Ruthenium)
	Ruthenium Nitrate (III)	g/L
	(as Ruthenium)
	Ruthenium Chloride (III)	g/L
	(as Ruthenium)
Hydroxylamine	Hydroxylamine Sulfate	g/L
Compounds	Hydroxylamine Chloride	g/L	5	5	5	5	5	5
Organic Compounds	Gluconolactone	g/L	1	10	20
Having a Hydroxyl	Sorbitol	g/L				1	5	10
Group	Mannitol	g/L
	Citric Acid Monohydrate	g/L
Deposition Rate	DTPA-OH•4H	g/L	10	10	10	10	10	10
Adjusting Agent
Reducing Agents	Hydrazine Monohydrate (80%)	mL/L	10	10	10	10	10	10
	Hydrazine Dihydrochloride	g/L
	Hydrazinium Sulfate	g/L
Second Reducing	Ammonia Borane	g/L
Agents	Dimethylamine Borane	g/L

pH Adjusting

Tetramethylammonium

mL/L

added in an approprite amount until pH reached the predetermined value

Agent

Hydroxide Solution (25%)

pH	13.3	13.3	13.3	13.3	13.3	13.3
Treatment Temperature (° C.)	65	65	65	65	65	65

Evaluation	Thickness of Ruthenium Plating	nm	280	124	71	243	219	145
	Film Before Annealing
	Oxygen Content of Ruthenium	at. %	9.6	9.2	9.1	9.2	8.5	9.2
	Plating Film
	Thickness of Ruthenium Plating	nm	260	120	70	220	210	132
	Film After Annealing
	Film Shrinkage Ratio	%	7.1	3.2	1.4	9.5	4.1	9.0
	Resistivity of Ruthenium Plating	μΩ · cm	17.2	16.4	17.9	18.2	16.2	18.1
	Film After Annealing

	Plating Bath Stability	Excellent	Excellent	Excellent	Excellent	Excellent	Excellent

			Plating Bath Composition
			Examples

			17	18	19
Ruthenium Salts	Ruthenium Chloride (IV)	g/L	1	1	1
	(as Ruthenium)
	Ruthenium Sulfate (IV)	g/L
	(as Ruthenium)
	Ruthenium Nitrate (III)	g/L
	(as Ruthenium)
	Ruthenium Chloride (III)	g/L
	(as Ruthenium)
Hydroxylamine	Hydroxylamine Sulfate	g/L
Compounds	Hydroxylamine Chloride	g/L	5	5	5
Organic Compounds	Gluconolactone	g/L
Having a Hydroxyl	Sorbitol	g/L	20
Group	Mannitol	g/L		1	5
	Citric Acid Monohydrate	g/L
Deposition Rate	DTPA-OH•4H	g/L	10	10	10
Adjusting Agent
Reducing Agents	Hydrazine Monohydrate (80%)	mL/L	10	10	10
	Hydrazine Dihydrochloride	g/L
	Hydrazinium Sulfate	g/L
Second Reducing	Ammonia Borane	g/L
Agents	Dimethylamine Borane	g/L

pH Adjusting

Tetramethylammonium

mL/L

added in an approprite amount until pH reached the predetermined value

Agent

Hydroxide Solution (25%)

pH	13.3	13.3	13.3
Treatment Temperature (° C.)	65	65	65

Evaluation	Thickness of Ruthenium Plating	nm	77	290	272
	Film Before Annealing
	Oxygen Content of Ruthenium	at. %	8.1	10.1	8.1
	Plating Film
	Thickness of Ruthenium Plating	nm	76	279	268
	Film After Annealing
	Film Shrinkage Ratio	%	1.3	3.8	1.5
	Resistivity of Ruthenium Plating	μΩ · cm	19.2	18	17.6
	Film After Annealing

	Plating Bath Stability	Excellent	Excellent	Excellent

	TABLE 4
	Plating Bath Composition
	Examples

	20	21	22	23	24	25	26	27	28

Ruthenium Salts	Ruthenium Chloride (IV)	g/L	1	1	1	1	1	1	1	1	1
	(as Ruthenium)
	Ruthenium Sulfate (IV)	g/L
	(as Ruthenium)
	Ruthenium Nitrate (III)	g/L
	(as Ruthenium)
	Ruthenium Chloride (II)	g/L
	(as Ruthenium)
Hydroxylamine	Hydroxylamine Sulfate	g/L
Compounds	Hydroxylamine Chloride	g/L	5	5	5	5	5	5	5	5	5
Organic Compounds	Gluconolactone	g/L							5	5	5
Having a Hydroxyl	Sorbitol	g/L
Group	Mannitol	g/L	10	20
	Citric Acid Monohydrate	g/L			1	5	10	20
Deposition Rate	DTPA-OH•4H	g/L	10	10	10	10	10	10	10	10	10
Adjusting Agent
Reducing Agents	Hydrazine Monohydrate (80%)	mL/L	10	10	10	10	10	10	2	5	80
	Hydrazine Dihydrochloride	g/L
	Hydrazinium Sulfate	g/L
Second Reducing	Ammonia Borane	g/L
Agents	Dimethylamine Borane	g/L

pH Adjusting

Tetramethylammonium

mL/L

added in an approprite amount until pH reached the predetermined value

Agent

Hydroxide Solution (25%)

pH	13.3	13.3	13.3	13.3	13.3	13.3	13.3	13.3	13.3
Treatment Temperature (° C.)	65	65	65	65	65	65	65	65	65

Evaluation	Thickness of Ruthenium Plating	nm	171	60	300	263	214	30	32.1	78.7	75
	Film Before Annealing
	Oxygen Content of Ruthenium	at. %	7.1	9.9	8.1	7.1	9.5	9.9	9.1	9.2	8.1
	Plating Film
	Thickness of Ruthenium Plating	nm	160	50	288	250	198	28	29.8	72.1	68
	Film After Annealing
	Film Shrinkage Ratio	%	6.4	16.7	4.0	4.9	7.5	6.7	7.2	8.4	9.3
	Resistivity of Ruthenium Plating	μΩ · cm	15.3	18.1	16.4	18.2	15.2	17.3	16.1	18.2	19.1
	Film After Annealing

	Plating Bath Stability	Excel-	Excel-	Excel-	Excel-	Excel-	Excel-	Excel-	Excel-	Good
		lent	lent	lent	lent	lent	lent	lent	lent

	TABLE 5
	Plating Bath Composition
	Examples

	29	30	31	32	33	34	35	36	37

Ruthenium Salts	Ruthenium Chloride (IV)	g/L	1	1	1	1	1	1	1	1	1
	(as Ruthenium)
	Ruthenium Sulfate (IV)	g/L
	(as Ruthenium)
	Ruthenium Nitrate (II)	g/L
	(as Ruthenium)
	Ruthenium Chloride (III)	g/L
	(as Ruthenium)
Hydroxylamine	Hydroxylamine Sulfate	g/L
Compounds	Hydroxylamine Chloride	g/L	5	5	5	5	5	5	5	5	5
Organic Compounds	Gluconolactone	g/L	5	5	5	5	5	5	5	5	5
Having a Hydroxyl	Sorbitol	g/L
Group	Mannitol	g/L
	Citric Acid Monohydrate	g/L
Deposition Rate	DTPA-OH•4H	g/L	10	10	10	10	10	10	10	10	10
Adjusting Agent
Reducing Agents	Hydrazine Monohydrate (80%)	mL/L			10	10	10	10	10	10	10
	Hydrazine Dihydrochloride	g/L	17
	Hydrazinium Sulfate	g/L		21
Second Reducing	Ammonia Borane	g/L				1	2		2
Agents	Dimethylamine Borane	g/L			1

pH Adjusting

Tetramethylammonium

mL/L

added in an approprite amount until pH reached the predetermined value

Agent

Hydroxide Solution (25%)

pH	13.3	13.3	13.3	13.3	13.3	13.3	13.3	11	13.3
Treatment Temperature (° C.)	65	65	65	65	65	45	45	65	85

Evaluation	Thickness of Ruthenium Plating	nm	153	160	172	185	230	22.1	80	23.1	278
	Film Before Annealing
	Oxygen Content of Ruthenium	at. %	8.4	9.1	8.5	8.4	8.2	8.1	8.6	9.1	7.1
	Plating Film
	Thickness of Ruthenium Plating	nm	140	153	168	179	221	20	76	20	267
	Film After Annealing
	Film Shrinkage Ratio	%	8.5	4.4	2.3	3.2	3.9	9.5	5.0	13.4	4.0
	Resistivity of Ruthenium Plating	μΩ · cm	16.1	16.3	15.8	15.6	16.3	19.2	18.5	18.3	18.2
	Film After Annealing

	Plating Bath Stability	Excel-	Excel-	Excel-	Excel-	Excel-	Excel-	Excel-	Excel-	Good
		lent	lent	lent	lent	lent	lent	lent	lent

	TABLE 6
	Plating Bath Composition
	Comparative Examples

	1	2	3	4	5	6

Ruthenium Salts	Ruthenium Chloride (IV)	g/L	1	1	1	1	1	1
	(as Ruthenium)
	Ruthenium Sulfate (IV)	g/L
	(as Ruthenium)
	Ruthenium Nitrate (II)	g/L
	(as Ruthenium)
	Ruthenium Chloride (III)	g/L
	(as Ruthenium)
Hydroxylamine	Hydroxylamine Sulfate	g/L
Compounds	Hydroxylamine Chloride	g/L		5		5	5	5
	Diethylhydroxylamine	g/L			5
Organic Compounds	Gluconolactone	g/L	5		5		5	5
Having a Hydroxyl	Sorbitol	g/L
Group	Mannitol	g/L
	Citric Acid Monohydrate	g/L
	Glutaric Acid	g/L				20
Deposition Rate	DTPA-OH•4H	g/L	10	10	10	10	10	10
Adjusting Agent
Reducing Agents	Hydrazine Monohydrate (80%)	mL/L	10	10	10	10
	Hydrazine Dihydrochloride	g/L
	Hydrazinium Sulfate	g/L
	Sodium Borohydride	g/L						10
Second Reducing	Ammonia Borane	g/L
Agents	Dimethylamine Borane	g/L					1

pH Adjusting	Tetramethylammonium	mL/L	added in an approprite amount until pH reached the
Agent	Hydroxide Solution (25%)		predetermined value

pH	13.3	13.3	13.3	13.3	13.3	13.3
Treatment Temperature (° C.)	65	65	65	65	65	65

Evaluation	Thickness of Ruthenium Plating	nm	Not	325	Not	Not	140	Not
	Film Before Annealing		deposited		deposited	deposited		deposited
	Oxygen Content of Ruthenium	at. %		7.6			18.0
	Plating Film
	Thickness of Ruthenium Plating	nm		298			82
	Film After Annealing
	Film Shrinkage Ratio	%		8.3			41.4
	Resistivity of Ruthenium Plating	μΩ · cm		16.8			16.4
	Film After Annealing

	Plating Bath Stability	Poor	Poor	Poor	Poor	Excellent	Poor

As shown in Tables 2 to 5, Examples 1 to 37, in which hydrazines were used as the reducing agent, produced ruthenium plating films with a low impurity oxygen content and suppressed film shrinkage during annealing. In addition, Examples 1 to 37, in which a hydroxylamine compound (either or both of hydroxylamine sulfate and hydroxylamine chloride) was used in combination with an organic compound having a hydroxyl group (at least one selected from the group consisting of gluconolactone, sorbitol, mannitol, and citric acid monohydrate) as the stabilizer, exhibited improved bath stability and improved ruthenium depositability.

As shown in FIG. 6, Comparative Examples 1, 2, and 4, in which a hydroxylamine compound and an organic compound having a hydroxyl group were not used in combination as the stabilizer, exhibited poor bath stability.

Comparative Example 3, in which diethylhydroxylamine was used as the hydroxylamine compound rather than hydroxylamine sulfate or hydroxylamine chloride, exhibited poor bath stability.

Comparative Example 5, in which dimethylamine borane was used as the reducing agent instead of hydrazines, produced a ruthenium plating film with a high oxygen content and a large film shrinkage ratio during annealing.

Comparative Example 6, in which sodium borohydride was used as the reducing agent instead of hydrazines, exhibited poor bath stability even though a hydroxylamine compound and an organic compound having a hydroxyl group were used in combination as the stabilizer.

The electroless ruthenium plating bath of the present disclosure is particularly suitable as a plating bath for forming ruthenium plating films constituting interconnects of semiconductor circuits, thin films (cap metals) formed on copper interconnects, or liner layers formed on barrier metals to enable uniform growth of copper seed films when copper interconnects are deposited by electrolytic plating.