CN116607132A

CN116607132A - Plating bath composition for electroless gold plating and method of depositing gold layer

Info

Publication number: CN116607132A
Application number: CN202310280778.1A
Authority: CN
Inventors: 罗伯特·施普伦曼; 克里斯蒂安·内特利希; 萨布里纳·格伦诺瓦; 德米特罗·沃隆申; 鲍里斯·亚历山大·詹森; 东尼·劳坦
Original assignee: Atotech Deutschland GmbH and Co KG
Current assignee: Atotech Deutschland GmbH and Co KG
Priority date: 2015-09-21
Filing date: 2016-09-16
Publication date: 2023-08-18
Also published as: EP3144413A1; KR20180044923A; CN108026642A; WO2017050662A1; US20200232099A1; EP3144413B1; TWI709663B; JP2018532046A; TW201720955A; JP6930966B2

Abstract

The present invention relates to plating bath compositions for electroless gold plating and methods of depositing gold layers. In particular, the present invention relates to an aqueous electroless gold plating bath comprising at least one source of gold ions and at least one reducing agent for the gold ions, characterized in that the aqueous electroless gold plating bath comprises at least one ethylenediamine derivative of formula (I) as plating enhancer compound, wherein the residue R ¹ And R is ² Containing 2 to 12 carbon atoms and selected from branched alkyl, unbranched alkyl, cycloalkyl, or a combination thereof, each of whichResidue R ¹ And R is ² The same or different. The aqueous electroless gold plating bath is suitable for providing a soft gold layer that can be used in wire bonding and soldering applications required for electronic components.

Description

Plating bath composition for electroless gold plating and method of depositing gold layer

The present patent application is a divisional application of the invention patent application with international application number PCT/EP2016/072053, international application date of 2016, 9/16, application number 201680052427.3 entering the national stage of china, entitled "plating bath composition for electroless gold plating and method of depositing gold layer".

Technical Field

The present invention relates to plating bath compositions for electroless gold plating and methods of depositing gold layers. In particular, the present invention relates to an aqueous electroless gold plating bath composition for electroless gold layer plating onto a substrate and a method of depositing gold. The plating bath is particularly suitable for the manufacture of printed circuit boards, IC substrates, semiconductor devices, interposers (interposers) made of glass, and the like.

Background

Gold layers are of greatest interest in the electronic component manufacturing and semiconductor industry. Gold layers are often used as solderable and/or wirebondable surfaces in the manufacture of printed circuit boards, IC substrates, semiconductor devices, and the like. Typically, they are used as a final surface treatment (finish) prior to bonding and wire bonding. In order to provide an electrical connection between a copper wire and a wire bonded thereto that has sufficient electrical conductivity and robustness while providing good strength for wire bonding, there are a variety of layer assemblies conventionally used in the art. Among these are electroless nickel electroless gold (ENIG), electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), direct Immersion Gold (DIG), electroless palladium immersion gold (epid), and Electroless Palladium Autocatalytic Gold (EPAG). Although these techniques have been established for some time, many challenges remain unresolved. Such challenges are corrosion of the nickel layer placed between the gold and copper wire (nickel corrosion) and insufficient stability of the gold plating bath, which is highly undesirable due to the cost of the plating bath. Also, it is highly desirable to deposit a gold layer at a plating rate sufficient to run the manufacturing process economically. Another desirable property of the gold layer is that the optical appearance should be lemon yellow, as discoloration of the gold layer is unacceptable.

Due to the small size of current electronic components, it is not possible to use electrolytic processes that require electrical connection to a substrate. Thus, an electroless metal deposition process (electroless plating) is used. Electroless plating generally describes a method of reducing metal ions without using an external current source. Plating processes using external current sources are generally described as electrolytic plating or galvanic plating methods. The nonmetallic surfaces may be pretreated to render them receptive to or catalyze the metal deposition. All or selected portions of the surface may be suitably pretreated. The main components of the electroless metal bath are metal salts, reducing agents and, as optional ingredients, complexing agents, pH adjusting agents and additives such as stabilizers. Complexing agents (also known in the art as chelating agents) are used to chelate the metal to be deposited and prevent the metal from precipitating from the solution (i.e., as a hydroxide, etc.). The chelating metal renders the metal available for use by a reducing agent that converts the metal ions into a metallic form.

Another form of metal deposition is dip plating. Dip plating is another metal deposition that uses neither an external current source nor a chemical reducing agent. The mechanism relies on the substitution of metal from the underlying substrate with metal ions present in the immersion plating solution. This is a significant disadvantage of dip plating because depositing thicker layers is often limited by the porosity of the layers.

In most cases, electroless gold plating baths use one or two types of electroless plating. Immersion plating may occur even if a reducing agent has been added to the plating bath, although the proportion is significantly reduced.

Electroless plating in the context of the present invention is to be understood as (mainly) autocatalytic deposition by means of a chemical reducing agent (herein referred to as "reducing agent").

US 2012/019005 A1 discloses an electroless gold plating bath comprising a water-soluble gold compound and an alkylene diamine, a dialkylene triamine, and the like. However, such gold plating solutions lack sufficient stability and plating rate and are therefore unsuitable for use in industrial processes (see example 4).

US 2008/0138807 A1 reports the use of aldehyde compounds as reducing agents and N-substituted ethylenediamine derivatives such as N ¹ ,N ² -dimethylethylenediamine and N ¹ ,N ² Electroless gold plating bath of bis- (hydroxymethyl) ethylenediamine. However, the plating bath described therein still lacks plating rate and stability (see example 4). In order to meet the current industry requirements, it is often sufficient that the gold plating bath has a plating rate of 150nm/h or more, preferably 200nm/h or more or desirably 250nm/h or more.

Object of the Invention

It is an object of the present invention to provide an aqueous electroless gold plating bath composition from which a gold layer can be deposited at a sufficient plating rate and a method for the purpose. It is another object of the present invention to provide an aqueous electroless gold plating bath that has sufficient stability and can be used for a long period of time.

Yet another object is that the gold layer formed does not exhibit discoloration.

Disclosure of Invention

These objects are solved by the aqueous electroless gold plating bath according to the invention, which comprises at least one source of gold ions and at least one reducing agent for the gold ions, and which is characterized in that the plating bath comprises at least one ethylenediamine derivative of formula (I) as plating enhancer compound

Wherein residue R ¹ And R is ² Containing 2 to 12 carbon atoms and selected from branched alkyl, unbranched alkyl, cycloalkyl or a combination thereof, wherein each residue R ¹ And R is ² The same or different.

These objects are additionally solved by a method for depositing a gold layer from the above-mentioned plating bath and the use of the above-mentioned plating enhancer compound in a gold plating bath comprising at least one source of gold ions and at least one reducing agent for gold ions.

Drawings

Fig. 1 shows a test substrate having a number of copper pads to be plated thereon. Also depicted are 10 different points (circles labeled 1 through 10) of measured layer thickness.

Detailed Description

The ethylenediamine derivatives of formula (I) will be referred to herein as plating enhancer compounds.

Plating enhancer compounds of formula (I)

With residue R ¹ And R is ² Comprising 2 to 12 carbon atoms and selected from branched alkyl, unbranched alkyl, cycloalkyl or a combination thereof, wherein each residue R ¹ And R is ² The same or different.

The amine moiety in the plating enhancer compound of formula (I) is a secondary amine moiety. The inventors found that R ¹ And R is ² The corresponding diamine, or derivative thereof, which is a methyl residue, does not allow for sufficient plating rates nor for a gold plating bath to be sufficiently stable (see example 4).

In a preferred embodiment of the present invention, residue R of the plating enhancer compound of formula (I) ¹ And R is ² Containing 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms, even more preferably 2 to 4 carbon atoms.

In another preferred embodiment of the present inventionResidues R in formula (I) ¹ And R is ² Are identical. In a further preferred embodiment of the invention, the alkyl residue R in formula (I) ¹ And R is ² No terminal hydroxyl moiety (-OH) was contained, as the inventors found that the terminal hydroxyl moiety bound thereto is detrimental to the stability of the plating bath (see example 4). In a further preferred embodiment of the invention, the residue R in formula (I) ¹ And R is ² No terminal primary amino moiety is contained, as the inventors found that the terminal amino moiety bound thereto is also detrimental to the stability of the plating bath (see example 4). In another more preferred embodiment of the invention, residue R ¹ And R is ² Without any additional amino moieties and/or any hydroxyl moieties. Even more preferably the alkyl residue is free of substituents and consists of carbon and hydrogen atoms only.

It is particularly preferred to select the plating enhancer compound from the following options: n (N) ¹ ,N ² -diethyl ethane-1, 2-diamine, N ¹ ,N ² -dipropylethane-1, 2-diamine, N ¹ ,N ² -di-isopropyl ethane-1, 2-diamine, N ¹ ,N ² -dibutyl ethane-1, 2-diamine, N ¹ ,N ² -di-iso-butylethane-1, 2-diamine, N ¹ ,N ² -di-tert-butylethane-1, 2-diamine, N ¹ ,N ² Dipentylethane-1, 2-diamine, N ¹ ,N ² -di-iso-amyl ethane-1, 2-diamine, N ¹ ,N ² -di-sec-amyl ethane-1, 2-diamine, N ¹ ,N ² -di-tert-amyl ethane-1, 2-diamine, N ¹ ,N ² -di-neopentyl ethane-1, 2-diamine, N ¹ ,N ² Dihexylethane-1, 2-diamine, N ¹ ,N ² -di- (1-methylpentyl) ethane-1, 2-diamine, N ¹ ,N ² -di- (2-methylpentyl) ethane-1, 2-diamine, N ¹ ,N ² -di- (3-methylpentyl) ethane-1, 2-diamine, N ¹ ,N ² -di- (4-methylpentyl) ethane-1, 2-diamine, N ¹ ,N ² -di- (1, 1-dimethylbutyl) ethane-1, 2-diamine, N ¹ ,N ² -di- (1, 2-dimethylbutyl) ethane-1, 2-diamine, N ¹ ,N ² -di- (1, 3-di)Methyl butyl) ethane-1, 2-diamine, N ¹ ,N ² -di- (2, 2-dimethylbutyl) ethane-1, 2-diamine, N ¹ ,N ² -di- (2, 3-dimethylbutyl) ethane-1, 2-diamine and N ¹ ,N ² -di- (3, 3-dimethylbutyl) ethane-1, 2-diamine.

Most preferably R ¹ And R is ² Is a branched alkyl residue having 3 to 6 carbon atoms. Surprisingly, it has been found that when branched alkyl residues having 3 to 6 carbon atoms are used as R ¹ And R is ² When a combination of a high plating rate with an even more improved bath stability is obtained (see example 5).

In the aqueous electroless gold plating bath according to the invention, the concentration of the at least one plating enhancer compound of formula (I) preferably ranges from 0.001 to 1mol/L, more preferably from 10 to 100mmol/L, even more preferably from 25 to 75mmol/L. If more than one plating enhancer compound is contained in the aqueous electroless gold plating bath of the invention, the concentration is based on the total mass of all plating enhancer compounds.

The synonymous designation of the aqueous electroless gold plating bath of the invention is an aqueous solution. The term "aqueous solution" means that the primary liquid medium in solution as a solvent is water. Other liquids that are miscible with water, such as alcohols, and other polar organic liquids that are miscible with water, may be added. In principle, the aqueous solution comprises more than 50% by weight of water.

The electroless plating bath of the invention may be prepared by dissolving all components in an aqueous liquid medium, preferably water.

The aqueous electroless gold plating bath of the invention comprises at least one source of gold ions. The gold ion may be in Au ⁺ 、Au ³⁺ Or both oxidation states. The source of gold ions may be any water-soluble gold salt having the oxidation state. Preferably, the gold ion source is selected from gold cyanide, gold ammonium cyanide, gold (I) alkali metal cyanide, including gold (I) potassium cyanide, gold (I) sodium cyanide, trisodium gold disulfite, tripotassium gold disulfite and triammonium gold disulfite, gold thiosulfate, gold thiocyanate, gold sulfate, gold chloride, and gold bromide. Preferably, the gold ion source is gold (I) alkali metal cyanide and is capable ofAdded to the aqueous plating bath in the form of a solution containing the salt. The preferred range of gold ion concentration in the aqueous electroless gold plating bath of the invention is 0.1 to 10g/L, more preferably 0.3 to 6g/L.

The aqueous electroless gold plating bath further comprises at least one reducing agent for gold ions. The reducing agent for gold ions is preferably selected from aliphatic aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde, α -methylpentanal, β -methylpentanal, γ -methylpentanal, etc.; aliphatic dialdehydes such as glyoxal, succinaldehyde, and the like; aliphatic unsaturated aldehydes such as crotonaldehyde and the like; aromatic aldehydes such as benzaldehyde, o-nitrobenzaldehyde, m-nitrobenzaldehyde, p-nitrobenzaldehyde, o-tolualdehyde, m-tolualdehyde, p-tolualdehyde, o-hydroxybenzaldehyde, m-hydroxybenzaldehyde, p-hydroxybenzaldehyde, phenylacetaldehyde and the like; sugars having aldehyde groups (-CHO), such as glucose, galactose, mannose, ribose, maltose, lactose, and the like; and precursors of formaldehyde such as urotropin, 1, 3-bis (hydroxymethyl) -5, 5-dimethylimidazolidine-2, 4-dione (DMDM-hydantoin), paraformaldehyde, glyoxylic acid, sources of glyoxylic acid, and glycolic acid. The term "source of glyoxylic acid" includes glyoxylic acid and all compounds which can be converted to glyoxylic acid in aqueous solution. In aqueous solution, the aldehyde-containing acid is in equilibrium with its hydrate. A suitable source of glyoxylic acid is a dihaloacetic acid, such as dichloroacetic acid, which will hydrolyze in an aqueous medium to the glyoxylic acid hydrate. An alternative source of glyoxylic acid is the bisulfite adduct, but also hydrolyzable esters or other acid derivatives. The bisulfite adduct may be added to the aqueous electroless gold plating bath of the invention or formed in situ. The bisulfite adducts may be made from glyoxylate and any of bisulfite, sulfite, or metabisulfite. Formaldehyde, a source of glyoxylate and glyoxylate are preferred, most preferred is formaldehyde.

The concentration of the at least one reducing agent for gold ions is preferably in the range of 0.0001 to 0.5mol/L, more preferably 0.001 to 0.3mol/L, even more preferably 0.005 to 0.12mol/L.

Without being bound by theory, the inventors have discovered a significant inventive technique, namely, the reaction product of a specific ethyleneamine derivative, such as triethylenetetramine, and a reducing agent for gold ions, such as formaldehyde (or its oxidation product formic acid), with the consequence of causing precipitation and reducing plating rates. Typical reaction products are, for example, the corresponding aminal, enamine and amide derivatives. Thus, the extent of the possibility of unwanted reaction product formation is preferably limited by selecting the molar ratio of the plating enhancer compound of formula (I) to the reducing agent for gold ions in the aqueous electroless gold plating bath of the invention to a range of 0.5 to 9, preferably 0.8 to 3.0, more preferably 1.0 to 2.0 (see example 6). If more than one plating enhancer compound of formula (I) and/or more than one reducing agent for gold ions are used in the aqueous electroless gold plating bath of the invention, this ratio is calculated based on the total mass of all corresponding individual compounds.

The aqueous electroless gold plating bath of the invention optionally further comprises at least one complexing agent. The optional at least one complexing agent present in the aqueous electroless gold plating bath of the invention is preferably selected from carboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, aminophosphonic acids, or salts of the foregoing. The optional at least one complexing agent acts as a complexing agent for gold ions as well as for metal ions, such as nickel ions or copper ions, dissolved from the substrate during plating. Preferred carboxylic acids are for example oxalic acid or salts thereof. Preferred hydroxycarboxylic acids are, for example, tartaric acid, citric acid, lactic acid, malic acid, gluconic acid and salts of the foregoing. Preferred aminocarboxylic acids are, for example, glycine, cysteine, methionine and salts of the foregoing. Preferred aminophosphonic acids are nitrilotris (methylphosphonic acid) (commonly abbreviated as ATMP), diethylenetriamine penta (methylphosphonic acid) (commonly abbreviated as DTPMP) and ethylenediamine tetra (methylenephosphonic acid) (commonly abbreviated as EDTMP). In all cases, sodium, potassium and ammonium salts of the compounds are also suitable. The concentration of the optional at least one complexing agent preferably ranges from 0.1 to 50g/L, more preferably from 0.5 to 30g/L.

More preferably, the aqueous electroless gold plating bath of the invention comprises two different complexing agents and/or salts thereof, for example a hydroxycarboxylic acid or salt thereof and an aminocarboxylic acid or salt thereof.

The aqueous electroless gold plating bath of the invention optionally comprises a crystal modifier selected from thallium ions, arsenic ions, selenium ions, and lead ions. Such a crystal modifier is preferably added to the aqueous electroless gold plating bath of the invention in a concentration range of 0.00001 to 0.1 g/L. Useful sources of the ions may be their water-soluble salts, such as the corresponding nitrates, sulfates and halides.

The aqueous electroless gold plating bath of the invention optionally comprises at least one stabilizer selected from the group consisting of cyanide ion sources, hydantoins and alkyl derivatives thereof such as alkyl hydantoins and dialkyl hydantoins, wherein the alkyl residues herein include C ₁ To C ₈ Alkyl (preferably methyl) which may be cyclic and/or alicyclic, branched or unbranched, sulfur compounds such as 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, mercaptoacetic acid, 3- (2-benzothiazolylthio) -1-propanesulfonic acid, mercaptosuccinic acid, thiosulfate, thioglycol, thiourea, thiomalic acid and the like, and aromatic nitrogen compounds such as benzotriazole, 1,2, 4-aminotriazole and the like. Suitable sources of cyanide ions may be any soluble cyanide salt, such as alkali metal cyanide including sodium cyanide, potassium cyanide.

The concentration of the optional stabilizer may be selected according to its chemical structure and may be determined by anyone with routine experimentation as known in the art. The concentration of the optional stabilizer is preferably in the range of 0.0000001 to 0.2mol/L, more preferably 0.000001 to 0.1mol/L. Such stabilizers are routinely added to electroless gold plating baths to improve their service life and prevent precipitation.

In a preferred embodiment, two or more stabilizers are used. More preferably, a source of cyanide ions in a concentration of 0.0003 to 5mmol/L and one or more hydantoins and alkyl derivatives thereof in a concentration of 10 to 100mmol/L and/or sulfur compounds in a concentration of 0.000001 to 0.05mol/L are selected.

In another preferred embodiment of the invention, the aqueous electroless gold plating bath of the invention is free of an intentionally added second source of reducible metal ions (usually present in the technical raw materials, excluding trace impurities) so that a pure gold deposit is formed. The pure gold deposit is soft and ductile and is particularly suitable for wire bonding and soldering. Trace amounts of impurities are understood to mean that the compound is present in the technical feedstock at 1% by weight or less.

The preferred range of pH of the aqueous electroless gold plating bath of the invention is 5 to 9, more preferably 6 to 8, even more preferably 6.5 to 7.5. The target pH is adjusted by using, for example, an acid such as phosphoric acid or a base such as sodium hydroxide or potassium hydroxide. Continuous control and adjustment of the pH during plating is advantageous and therefore preferred, as this also improves plating bath life.

Ethylenediamine derivatives of formula (I)

With residue R ¹ And R is ² The residues comprise 2 to 12 carbon atoms and are selected from branched alkyl, unbranched alkyl, cycloalkyl or a combination thereof, wherein each residue R ¹ And R is ² The ethylenediamine derivatives of formula (I), either identical or different, are used in aqueous gold plating baths to adjust the plating rate and improve its stability. Such aqueous gold plating baths may be electroless gold plating baths, including immersion type gold plating baths, autocatalytic gold plating baths, and gold plating baths using a mixture of autocatalytic and immersion type plating, as well as electrolytic plating baths.

Preferably, the plating enhancer compound is used in an electroless plating bath, which preferably comprises at least one source of gold ions and at least one reducing agent for gold ions.

A method of depositing a gold layer onto a substrate, the method comprising the steps of, in order

(i) Providing a substrate;

(ii) Contacting at least a portion of the surface of the substrate with the aqueous Jin Duyu of the invention described above;

and thereby depositing a gold layer on at least a portion of the surface of the substrate.

Such contact is preferably achieved by immersing the substrate or the at least part of the surface of the substrate in the plating bath or by spraying the plating bath onto the substrate or onto the at least part of the surface of the substrate.

The at least a portion of the surface of the substrate is preferably composed of a metal or metal alloy, and then gold is deposited on the at least a portion of the surface of the substrate composed of a metal or metal alloy selected from nickel, nickel alloys, such as nickel-phosphorus alloys, nickel-boron alloys, cobalt, cobalt alloys, such as cobalt phosphorus alloys, cobalt molybdenum boron phosphorus alloys, cobalt tungsten boron phosphorus alloys, palladium alloys, such as palladium phosphorus alloys, palladium boron alloys, copper and copper alloys, and gold or gold alloys. The aqueous electroless gold plating bath of the invention can be used to deposit a gold layer on a gold substrate and can be used to thicken an existing gold layer, for example, obtained from an immersion gold plating bath.

The substrate may be pretreated prior to plating, as is known in the art. Such pretreatment includes a cleaning step with solvents and/or surfactants to remove most of the organic contaminants, an etching step with acids and optionally oxidizing or reducing agents to remove oxides, and an activation step. The latter is to deposit noble metals on the surface or a portion thereof to make it more receptive to plating. Such noble metal may be palladium, which may be deposited as a salt, which is then reduced to elemental palladium on the surface. Or it may be deposited in colloidal form and-where appropriate-an acceleration step with an acid such as hydrochloric acid to remove any protective colloids such as tin colloids. Such an activation layer is typically not a discrete layer, but rather an aggregation of palladium in an island-like structure. However, in the context of the present invention, the active layer is considered to be a metal substrate.

The temperature of the aqueous electroless gold plating bath of the invention is preferably in the range of 30 to 95 ℃, more preferably 70 to 90 ℃, even more preferably 75 to 85 ℃, still even more preferably 77 to 84 ℃ during plating. The plating time is preferably in the range of 1 to 60 minutes, more preferably in the range of 5 to 30 minutes. However, if thinner or thicker deposits are desired, the plating time can be outside the above ranges and adjusted accordingly.

The components used during plating are preferably replenished continuously or at specific intervals. Such components are in particular the gold ion source, the reducing agent for gold ions, the at least one stabilizer and the plating enhancer compound. The pH can also be adjusted continuously or at intervals, if necessary.

The aqueous electroless gold plating bath of the invention can be used in horizontal plating equipment, vertical plating equipment, and plating equipment.

An advantage of the present invention is the improved stability of the aqueous electroless gold plating bath of the invention compared to Jin Duyu (see example 5) known in the art. Stability is herein understood to be the life of the plating bath before the compounds precipitate ("educt") from the bath to render the plating bath useless for plating purposes.

Another advantage is that the aqueous gold plating bath of the present invention allows for a sufficient plating rate (deposit thickness of the plated metal layer over time) of 250nm/h or more (see examples 1 to 3 and 5). Most of the slightly stable plating baths known in the prior art are not capable of achieving a sufficient plating rate.

Thus, a unique feature of the aqueous electroless gold plating bath of the invention is to provide a highly stable gold plating bath with sufficient plating rate to enable a more economically viable gold plating process.

The aqueous gold plating bath according to the invention forms a uniform gold deposit with very small layer thickness deviations. The standard deviation of the gold layer thickness is below 10% or even below 8%. Such small deviations can be advantageously achieved even when plating on a variety of substrates having different dimensions.

The following non-limiting examples further illustrate the invention.

Examples

General procedure

CLN、/>ME、/>Pre Dip、/>Aktivator and +.>ACT V3 STD is a product available from Atotech Deutschland GmbH (etot de national company). The gold ion source is K [ Au (CN) in each case ₂ ]。

In all experiments were used with many having 0.25 to 49mm on both sides ² A range of copper pad printed circuit test boards of different sizes were used as substrates. They are cleaned and etched prior to activation with palladium. Palladium is then deposited on the copper surface and then a gold layer is plated thereon. Different shims at the measured layer thickness are shown in fig. 1. Each shim has the following areas: 1:0.25mm ² ，2：0.52mm ² ，3：0.68mm ² ，4：0.97mm ² ，5：1.33mm ² ，6：1.35mm ² ，7：3.3mm ² ，8：6.7mm ² ，9：25mm ² ，10：49mm ² 。

Determination of thickness of metal deposit and plating rate

Deposit thickness was measured at 10 copper shims on each side of the test plate. The copper shims were chosen to have different dimensions and were used to determine layer thickness by XRF using an XRF instrument fischer recipe XDV-SDD (Helmut Fischer GmbH (hermote fischer, germany). By assuming a layered structure of the deposit, the layer thickness can be calculated from such XRF data. The plating rate is calculated by dividing the resulting layer thickness by the time required to obtain the layer thickness.

Layer thickness uniformity was determined as standard deviation of the average thickness value.

Example 1 (invention):N ¹ ,N ² diisopropylethane-1, 2-diamine as plating enhancer compound

Jin Duyu containing the following components was prepared by dissolving all the components in water:

the substrate was subjected to the following treatment steps (table 1) by immersing the substrate in each solution with the given parameters:

table 1: process steps for gold plating

After this process step, the thickness of each metal layer is measured. Plating rates were calculated as described above.

Table 2: containing N ¹ ,N ² Layer thickness and plating rate of gold plating bath of diisopropylethane-1, 2-diamine

	Layer thickness	Standard deviation of	Plating rate
				Palladium layer	74nm	444nm/h
Gold layer	103nm	5.6％	309nm/h

The gold layer is lemon yellow. Moreover, the plating rate is very high, well above the desired minimum of 250 nm/h. The layer thickness distribution is also very uniform with a deviation of only 5.6%.

¹ ² Example 2 (invention): n, N-dipropylethane-1, 2-diamine as plating enhancer compound

The process described in example 1 was repeated with a gold plating bath containing 50mmol/L N ¹ ,N ² -dipropylethane-1, 2-diamine instead of 50mmol/L N ¹ ,N ² -diisopropylethane-1, 2-diamine. The results are summarized in the following table:

table 3: containing N ¹ ,N ² Layer thickness and plating rate of gold plating bath of dipropylethane-1, 2-diamine

	Layer thickness	Standard deviation of	Plating rate
				Palladium layer	82nm	492nm/h
Gold layer	101nm	6.6％	303nm/h

The gold layer is lemon yellow. Moreover, the plating rate is very high, above the desired minimum of 250 nm/h. The layer thickness distribution is also very uniform with a deviation of only 6.6%.

¹ ² Example 3 (invention): n, N-diethyl ethane-1, 2-diamine as plating enhancer compound

The process described in example 1 was repeated, wherein the gold plating bath contained N ¹ ,N ² -diethyl ethane-1, 2-diamine instead of N ¹ ,N ² Diisopropylethane-1, 2-diamine, but at the same concentration. The results are summarized in the following table:

table 4: containing N ¹ ,N ² Layer thickness and plating rate of gold plating bath of diethyl ethane-1, 2-diamine

	Layer thickness	Standard deviation of	Plating rate
				Palladium layer	86nm	516nm/h
Gold layer	108nm	6.4％	348nm/h

The gold layer is lemon yellow. Moreover, the plating rate is very high, clearly above the desired minimum of 250 nm/h. The layer thickness distribution is also very uniform with a deviation of only 6.4%.

Example 4 (comparative): using other amines

The process described in example 1 was repeated, wherein the gold plating bath contained other compounds as listed in Table 5 in place of N ¹ ,N ² -diisopropylethane-1, 2-diamine. The results of 20 minutes gold plating are summarized in the table:

table 5: for comparison of layer thickness and plating rate of compounds in gold plating baths

A variety of compounds containing an amino moiety were tested. In those cases where the respective plating rates were too low to meet the current industry requirements for 250nm/h plating rates, the stability test was omitted.

Compound A comprises only tertiary amine moieties and does not carry any alkyl residues R ¹ And R is ² . When the plating enhancer compound in the gold plating bath is replaced with this compound, hardly any gold plating occurs. The gold layer is also very non-uniform with a standard deviation of 58% of the layer thickness.

Compound B is an alkylene diamine derivative (having only methyl residues) comprising only primary and tertiary amino moieties. When such compounds are used in place of plating enhancer compounds in gold plating baths, gold plating is slow. The gold layer is also very non-uniform with a standard deviation of 53% of the layer thickness.

Compounds C and D are alkanolamines having only a tertiary amino moiety or only one secondary amino moiety. Gold plating is slow when these compounds are used in place of the plating enhancer compounds in the gold plating bath. The gold layer was also very heterogeneous, with a layer thickness standard deviation of 24% for compound C and 33% for compound D.

Compounds E and F do not contain any alkyl residues of sufficient length and plating is slow when these compounds are used in place of the plating enhancer compounds in gold plating baths. Compounds E and F have a similar structure to the plating enhancer compound of formula (I), but they either have no alkyl residues at all or are short. In the case of compound E, the gold layer was not uniform in thickness, the standard deviation was 14.4%, and the deviation of compound F was 6.4%.

Compound G has two terminal hydroxyl moieties. When this compound is used instead of the plating enhancer compound in the gold plating bath, the plating rate is high, but the stability of the gold plating bath is insufficient. In less than one day, the gold plating bath is irreversibly degraded and cannot be used for gold plating. The standard deviation of the gold layer thickness was 6.3%.

Compound H has two terminal primary amino moieties. When this compound is used instead of the plating enhancer compound in the gold plating bath, the plating rate is sufficiently high, but the stability of the gold plating bath is poor. Within 3 hours, the gold plating bath irreversibly deteriorated. The standard deviation of the gold layer thickness was 8.5%.

In summary, comparative compounds a to F were not able to give gold baths containing these compounds a sufficient plating rate. Plating rates are always even below 200nm/h and are therefore inadequate to meet today's industrial requirements.

The comparative compounds G and H gave adequate plating rates as additives, but the stability of the respective gold plating baths was unsatisfactory.

Example 5 (invention): stability and lifetime of gold plating baths

Gold was deposited on the substrate for a long time using the gold plating baths of examples 1 to 3. The stability of the gold plating bath and plating rate were monitored over time. If precipitation occurs, the solution is filtered and reused. During the experiment, the pH was measured daily, if necessary with KOH and/or H ₃ PO ₄ Adjust to 7.1. During plating, the gold ion source, the cyanide ion source and the plating are continuously supplementedApplying an enhancer compound.

Table 6 provides information regarding the stability of gold plating baths containing different plating enhancer compounds. The plating bath was inspected visually directly after construction (day 0) and on a daily basis for one week. During this test, the Jin Duyu was also used to deposit gold on the substrate daily. These results are summarized in table 7. The values given in the table are the deposit thicknesses in nanometers obtained after plating for 20 minutes.

Table 6: visual inspection Jin Duyu over time while in use

Table 7: thickness of gold layer after 20 minutes of deposition from gold plating bath containing plating enhancer compound on different days

Although in the linear plating enhancer compound N ¹ ,N ² -diethyl ethane-1, 2-diamine and N ¹ ,N ² In the case of dipropylethane-1, 2-diamine, a slight precipitation occurs, but the plating bath is still able to deposit a gold layer without any reduction in the plating rate. Branched plating enhancer compound, N ¹ ,N ² Diisopropylethane-1, 2-diamine showed no precipitation for 7 days and provided good plating rates throughout the test period. It is therefore inferred that the plating enhancer compound having a branched alkyl residue results in improved bath stability.

Example 6 (invention): ratio of plating enhancer compound to reducing agent for gold ion

jin Duyu by KOH/H ₃ PO ₄ The pH was adjusted to 7.1. The substrate was subjected to the treatment as described in table 1, wherein the electroless gold plating step was performed for only 10 minutes.

This process was repeated several times with different gold plating baths containing increasing amounts of plating enhancer compound while the amount of reducing agent for the gold ions was maintained at the same level. The results are provided in table 8.

Table 8: ratio of plating enhancer compound to reducing agent for gold ions.

It can be seen that the highest plating rates can be obtained if the molar ratio of plating enhancer compound to reducing agent for gold ions is between 1 or 2 to 1. As the amount of plating enhancer compound further increases, the plating rate decreases.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims.

Claims

1. An aqueous electroless gold plating bath comprising at least one source of gold ions and at least one reducing agent for gold ions, characterized in that the aqueous electroless gold plating bath comprises at least one ethylenediamine derivative according to formula (I) as plating enhancer compound

Wherein the residue R in formula (I) ¹ And R is ² Is a branched alkyl residue having 3 carbon atoms, wherein each residue R ¹ And R is ² The same; the at least one ion is used for gold ionsIs selected from the group consisting of aliphatic aldehydes, aliphatic dialdehydes, aliphatic unsaturated aldehydes, aromatic aldehydes, and sugars having aldehyde groups; wherein the molar ratio of the plating enhancer compound to the reducing agent according to formula (I) is from 1.0 to 2.0.

2. The aqueous electroless gold plating bath according to claim 1, characterized in that the residue R in formula (I) ¹ And R is ² Without any additional amino moieties and/or any hydroxyl moieties.

3. The aqueous electroless gold plating bath according to claim 1 or 2, characterized in that the concentration of the at least one plating enhancer compound according to formula (I) ranges from 0.001 to 1mol/L.

4. An aqueous electroless gold plating bath according to claim 3, characterized in that the concentration of the at least one plating enhancer compound according to formula (I) ranges from 10mmol/L to 100mmol/L.

5. The aqueous electroless gold plating bath according to claim 1 or 2, characterized in that the pH of the aqueous electroless gold plating bath is in the range of 5 to 9.

6. The aqueous electroless gold plating bath according to claim 1 or 2, characterized in that the gold ion concentration ranges from 0.1g/L to 10g/L.

7. The aqueous electroless gold plating bath according to claim 1 or 2, characterized in that the aqueous electroless gold plating bath further comprises at least one complexing agent selected from the group consisting of carboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, aminophosphonic acids or salts of the foregoing.

8. A method for depositing a gold layer onto a substrate, the method comprising the steps of, in order

(i) A substrate is provided and a substrate is provided,

(ii) Contacting at least a portion of the surface of the substrate with an aqueous electroless gold plating bath according to any of claims 1 to 7,

and thereby depositing a gold layer on the at least a portion of the surface of the substrate.

9. The method for depositing a gold layer onto a substrate of claim 8 wherein the at least a portion of the surface is comprised of a metal or metal alloy and then gold is deposited onto the at least a portion of the surface comprised of a metal or metal alloy selected from the group consisting of nickel, nickel alloys, cobalt alloys, palladium alloys, copper and copper alloys and gold or gold alloys.

10. The use of ethylenediamine derivatives according to formula (I) as plating enhancer compounds in aqueous gold plating baths,

the ethylenediamine derivative has a residue R ¹ And R is ² Residues R in formula (I) ¹ And R is ² Is a branched alkyl residue having 3 carbon atoms, wherein each residue R ¹ And R is ² Likewise, the aqueous gold plating bath comprises at least one source of gold ions and at least one reducing agent for gold ions selected from the group consisting of aliphatic aldehydes, aliphatic dialdehydes, aliphatic unsaturated aldehydes, aromatic aldehydes, and sugars having aldehyde groups; wherein the molar ratio of the plating enhancer compound to the reducing agent according to formula (I) is from 1.0 to 2.0.