Aluminum oxide film remover and method for surface treatment of aluminum or aluminum alloy

Abstract

Disclosed herein is an aluminum oxide film remover for removing an oxide film on the surface of aluminum or aluminum alloy, which comprises silver ions and/or copper ions, a solubilizing agent for silver ions and/or copper ions, and a quaternary ammonium hydroxide compound, and has a pH value of 10 to 13.5. A method for surface treatment of aluminum or aluminum alloy is also disclosed, which comprises immersing a workpiece having aluminum or aluminum alloy at least on the surface thereof in the aluminum oxide film remover, and depositing the silver and/or copper contained in the remover on the surface of aluminum or aluminum alloy while removing the aluminum oxide film.

Claims

1. A method for surface treatment of aluminum or aluminum alloy, comprising: immersing a workpiece having aluminum or aluminum alloy at least on the surface thereof in the aluminum oxide film remover comprising: a silver ion, a solubilizing agent for the silver ion, and a quaternary ammonium hydroxide compound having an alkyl group and/or hydroxyalkyl group of 1 to 4 carbon atoms, said remover having a pH value of 10 to 13.5; and depositing the silver contained in the remover on the surface of aluminum or aluminum alloy while removing the aluminum oxide film, wherein the surface of the aluminum or aluminum alloy comprises a (100) plane.

2. The method for surface treatment of aluminum or aluminum alloy of claim 1, wherein the workpiece is one which has an aluminum film or an aluminum alloy film formed on the surface of a non-aluminum material.

3. The method for surface treatment of aluminum or aluminum alloy of claim 1, wherein the step of depositing silver is followed by an additional step of removing the deposited metal by an acid solution having oxidizing properties.

4. The method for surface treatment of aluminum or aluminum alloy of claim 3, wherein the step of removing the deposited metal by an acid solution having oxidizing properties is followed by a step of subjecting the aluminum or aluminum alloy to a zinc substitution treatment or a palladium treatment, and a subsequent step of plating.

5. The method for surface treatment of aluminum or aluminum alloy of claim 3, wherein the step of removing the deposited metal by an acid solution having oxidizing properties is followed by a step of plating directly on the aluminum or aluminum alloy.

6. The method for surface treatment of aluminum or aluminum alloy of claim 1, wherein the aluminum oxide film remover further comprises a surfactant.

7. The method for surface treatment of aluminum or aluminum alloy of claim 1, wherein the aluminum oxide film remover further comprises zinc ions.

8. The method for surface treatment of aluminum or aluminum alloy of claim 1, wherein the step of depositing silver is followed by an additional step of forming a plating layer thereon.

9. The method for surface treatment of aluminum or aluminum alloy of claim 1, wherein the solubilizing agent for the silver ion comprises a hydantoin compound, a barbituric acid compound or an imide compound.

10. A method for surface treatment of aluminum or aluminum alloy, comprising: immersing a workpiece having aluminum or aluminum alloy at least on the surface thereof in the aluminum oxide film remover comprising: a copper ion, a zinc ion, a solubilizing agent for the copper ion, and a quaternary ammonium hydroxide compound having an alkyl group and/or hydroxyalkyl group of 1 to 4 carbon atoms, said remover having a pH value of 10 to 13.5; and depositing the copper contained in the remover on the surface of aluminum or aluminum alloy while removing the aluminum oxide film, wherein the surface of the aluminum or aluminum alloy comprises a (100) plane.

11. The method for surface treatment of aluminum or aluminum alloy of claim 10, wherein the workpiece is one which has an aluminum film or an aluminum alloy film formed on the surface of a non-aluminum material.

12. The method for surface treatment of aluminum or aluminum alloy of claim 10, wherein the step of depositing copper is followed by an additional step of forming a plating layer thereon.

13. The method for surface treatment of aluminum or aluminum alloy of claim 10, wherein the step of depositing copper is followed by an additional step of removing the deposited metal by an acid solution having oxidizing properties.

14. The method for surface treatment of aluminum or aluminum alloy of claim 13, wherein the step of removing the deposited metal by an acid solution having oxidizing properties is followed by a step of subjecting the aluminum or aluminum alloy to a zinc substitution treatment or a palladium treatment, and a subsequent step of plating.

15. The method for surface treatment of aluminum or aluminum alloy of claim 13, wherein the step of removing the deposited metal by an acid solution having oxidizing properties is followed by a step of plating directly on the aluminum or aluminum alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic sectional view showing the method in which the alkaline remover of the present invention removes an aluminum oxide film on the surface of aluminum or aluminum alloy.

(2) FIG. 2 is a schematic sectional view showing the method in which the conventional alkaline remover removes an aluminum oxide film on the surface of aluminum or aluminum alloy.

DESCRIPTION OF THE EMBODIMENT

(3) The following is a detailed description of the present invention.

(4) The aluminum oxide film remover according to the present invention contains silver ions and/or copper ions, a solubilizing agent of the silver ions and/or copper ions, and a quaternary ammonium hydroxide compound, having a pH value of 10 to 13.5.

(5) The aluminum oxide film remover according to the present invention contains silver ions and/or copper ions, which prepare an aluminum surface that easily undergo zinc substitution after treatment therewith. This is because the step of removing oxide film causes silver and/or copper to deposit on the aluminum surface and the subsequent step for removing silver and/or copper causes a surface with fine roughness to expose itself.

(6) Examples of the compounds providing silver ions include, but are not limited to, silver nitrate, silver chloride, silver bromide, silver iodide, silver acetate, silver carbonate, silver vanadate, silver sulfate, silver thiocyanate, silver tetrafluoroborate, silver p-toluenesulfonate, silver trifluoroacetate, and silver trifluoromethanesulfonate. Examples of the compounds providing copper ions include, but are not limited to, copper(II) acetate, copper(II) nitrate, copper(I) iodide, copper(I) chloride, copper(II) chloride, copper(I) oxide, copper(II) oxide, copper(II) sulfate, copper(I) sulfide, copper(II) sulfide, copper(I) thiocyanate, copper(II) tetrafluoroborate, copper(II) pyrophosphate, and copper(II) formate. These silver and/or copper compounds may be used alone or in combination with two or more.

(7) The silver ions and/or copper ions are not specifically restricted in concentration; however, the concentration is preferably 0.1 to 5,000 ppm, more preferably 1 to 2,000 ppm. A concentration lower than 0.1 ppm may be not enough to remove oxide film completely, with residual oxide film causing poor plating. A concentration higher than 5,000 ppm may lead to a decrease in bath stability.

(8) The remover of the present invention also contains a solubilizing agent (or a complexing agent) that solubilizes the silver ions and/or copper ions contained therein. This agent is not specifically restricted, but may be an ordinary complexing agent or a chelating agent. Examples of the solubilizing agent includes aminocarboxylic acids and salts thereof such as ethylenediaminetetraacetic acid, nitrilotriacetic acid, hydroxyethylethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, and polyaminocarboxylic acid; phosphonic acids and salts thereof such as 1-hydroxyethylidenebisphosphonic acid (HEDP), aminotrimethylphosphonic acid, and ethylenediaminetetramethylphosphonic acid; amines and salts thereof such as ethylenediamine, diethylenetriamine, and triethylenetetramine; hydantoin compounds; barbituric acid compounds; and imide compounds. Of these, as solubilizing agents for silver ions, hydantoin compounds and barbituric acid compounds are preferred especially from the standpoint of bath stability. As solubilizing agents for copper ions, ethylenediaminetetraacetic acid and hydroxyethylethylenediaminetriacetic acid are preferred. They may be used alone or in combination with two or more.

(9) The solubilizing agent contained in the remover of the present invention is not specifically restricted in concentrations. The concentration is preferably 0.01 to 50 g/L, more preferably 0.1 to 30 g/L. A concentration lower than 0.1 g/L may lead to a decrease in bath stability, and a concentration higher than 50 g/L may cause a poor appearance of plating.

(10) The remover of the present invention contains a quaternary ammonium hydroxide compound as an alkaline compound. The quaternary ammonium hydroxide compound acts to reduce attackability to aluminum or aluminum alloy because it is slow in etching an aluminum oxide film compared with alkali metal hydroxide.

(11) The quaternary ammonium hydroxide compound is preferably one which has an alkyl group and/or hydroxyalkyl group of 1 to 4 carbon atoms. Typical examples of the compounds include tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, trimethyl(2-hydroxyethyl)ammonium hydroxide (or choline), and triethyl(2-hydroxyethyl)ammonium hydroxide, but are not limited thereto. Preferable of these examples are tetramethylammonium hydroxide (TMAH) and trimethyl(2-hydroxyethyl)ammonium hydroxide (or choline) in the viewpoints of removal effectiveness of oxide films, stability, and cost.

(12) The quaternary ammonium hydroxide compound is added in an amount sufficient to make the remover have a pH value of 10 to 13.5, preferably 11 to 13, as specified.

(13) The remover of the present invention may contain a surfactant that imparts wettability thereto. Examples of the surfactants include, but are not limited to, nonionic ones such as polyethylene glycol, polyoxyethylene ether, polyoxyethylene alkyl ether, and a polyoxyethyleneoxy-propylene block copolymer; anionic ones such as a fatty acid sodium salt, sodium alkylsulfate, and sodium alkylethersulfate; and cationic ones such as an alkyltrimethylammonium salt and a dialkyldimethylammonium salt. Of these, nonionic and anionic surfactants are preferred for uniform treatment. They may be used alone or in combination with two or more.

(14) When using polyethylene glycol as the surfactant, its molecular weight is not particularly limited, and is normally not less than 100, preferably not less than 200, and is normally not more than 20,000, preferably not more than 6,000. With an excessively high molecular weight, it may be poor in solubility, and with an excessively low molecular weight, it may not impart wettability as desired. Incidentally, polyethylene glycol may be commercially available, and its molecular weight may be measured according to the method specified in Japanese Pharmacopoeia.

(15) The surfactant in the remover is not specifically restricted in concentration. Its concentration is normally not lower than 1 ppm (mg/L), preferably not lower than 10 ppm (mg/L), and is normally not higher than 5,000 ppm (mg/L), preferably not higher than 2,000 ppm (mg/L). With an excessively low concentration, it may not impart wettability as desired, and with an excessively high concentration, it may cause the deposited metal to deposit on other members than aluminum or aluminum alloy.

(16) The remover of the present invention may preferably be prepared in the form of aqueous solution for safety in operation. It is also possible to use water-soluble organic solvents such as methanol, ethanol and isopropyl alcohol (IPA), and mixed solvents with water. These solvents may be used alone or in combination with two or more.

(17) The remover of the present invention has a pH value of 10 to 13.5, preferably 11 to 13. With its pH value adjusted to alkaline pH value, the remover easily attacks an aluminum oxide film and achieves a treatment in a short time. If the remover has a pH value lower than 10, a dissolution rate of an oxide film is remarkably low. With its pH value higher than 13.5, the remover dissolves an oxide film too fast to control.

(18) The remover of the present invention may optionally contain zinc ions in order to increase the compactness of a zincate film formed in the subsequent zinc substitution treatment. The compounds providing zinc ions include, but are not limited to, zinc nitrate, zinc chloride, zinc oxide, zinc gluconate, zinc citrate, zinc sulfate, zinc phosphate, zinc salicylate, zinc tartrate, zinc tetrafluoroborate, zinc thiocyanate, zinc p-toluenesulfonate, zinc bromide, zinc acetate, and zinc pyrophosphate. When zinc ions are incorporated, the concentration thereof is preferably 0.01 to 50 g/L, more preferably 0.1 to 10 g/L. A concentration lower than 0.01 g/L may not contribute to a compactness of the zincate film formed in the subsequent zinc substitution step. A concentration higher than 50 g/L may cause a poor appearance of plating.

(19) The remover mentioned above is used for surface treatment in the following way. First, it is applied to a workpiece of aluminum or aluminum alloy for surface treatment, so that silver and/or copper derived from the silver or copper compound contained in the remover substitutionally deposits on the surface of aluminum or aluminum alloy. After this step, the deposited silver and/or copper can be removed by means of an acid solution having oxidizing properties. A plating process may be performed directly on the deposited silver and/or copper or directly on the aluminum or aluminum alloy on which the deposited silver and/or copper have been removed. Alternatively, after removing the deposited silver and/or copper, a zinc substitution treatment or palladium treatment may be performed prior to a plating process.

(20) The workpiece having aluminum or aluminum alloy may be immersed in the remover in any condition without specific restrictions. The condition for immersion may be properly established according to the thickness of the aluminum oxide film and the like. The immersion time is normally not less than 10 seconds, preferably not less 30 seconds, and is normally not more than 10 minutes, preferably not more than 5 minutes. Immersion in an excessively short time may not permit substitution to take place, resulting in incomplete removal of the oxide film. Immersion in an excessively long time may allow the remover to infiltrate through interstices of the substituted metal, resulting in dissolution of aluminum or aluminum alloy.

(21) The immersion temperature is not specifically restricted. It is normally not lower than 25 C., preferably not lower than 30 C., and is normally not higher than 100 C., preferably not higher than 95 C. With an excessively low immersion temperature, the remover may not dissolve the oxide film. With an excessively high immersion temperature, the remover may attack other materials other than aluminum or aluminum alloy. The immersion treatment may preferably be accompanied by stirring or shaking for uniform treatment.

(22) In the present invention, a workpiece which has aluminum or aluminum alloy at least on the surface thereof may be one which is formed entirely from aluminum or aluminum alloy, or one which is composed of a non-aluminum material, such as silicon and FRA (base material for printed circuit boards), having its surface entirely or partly covered with aluminum or aluminum alloy. The aluminum or aluminum alloy may be in any form, for example, a blank material, a rolled material, a casting material, a film, or the like. An aluminum film or an aluminum alloy film may be formed on the surface of a non-aluminum material by any methods, preferably by vapor plating such as vacuum deposition, sputtering, ion plating, or the like.

(23) The film which is subjected to a surface treatment by the method of the present invention generally has a thickness of not less than 0.5 m, preferably not less than 5 m, so that the aluminum ground or the aluminum alloy ground remains reliably after a treatment with the remover. The upper limit of the film thickness is generally not more than 100 m, but is not limited thereto. The remover of the present invention can be effectively applied to any film having a thickness of not more than 5 m because it hardly attacks the aluminum ground or aluminum alloy ground, to which a conventional remover could not be applied because of the problem that the basis material would be too thin after the treatment.

(24) The film mentioned above is not specifically restricted in its composition so long as it is of aluminum or aluminum alloy. Examples of aluminum alloys include AlSi (containing 0.5 to 1.0% by weight of Si) and AlCu (containing 0.5 to 1.0% by weight of Cu). Such films can be preferably subjected to a surface treatment by the method according to the present invention.

(25) The substituted metal mentioned above may be removed prior to a post-treatment. An acid solution having oxidizing properties may be used to dissolve the substituted metal in terms of reducing reactivity to aluminum or aluminum alloy as a ground. Preferable examples of the acid solution having oxidizing properties include oxidizing acids, such as nitric acid, and aqueous solutions thereof, which may contain iron nitrate, cerium(IV) sulfate, ammonium metavanadate, ammonium molybdate or the like. Also, preferable examples include non-oxidizing acids, such as sulfuric acid and hydrochloric acid, and aqueous solutions thereof containing one or more types of oxidizing agents such as hydrogen peroxide, sodium persulfate, ammonium persulfate and potassium persulfate. In the latter case, the acid acts to dissolve the substituted metal and the oxidizing agent acts to reduce reactivity to aluminum or aluminum alloy as a ground. Incidentally, preferred among the oxidizing agents is hydrogen peroxide, which is composed of hydrogen and oxygen and changes into water upon reduction. Sodium persulfate and potassium persulfate are also preferred on account of their stability and good handling properties.

(26) When nitric acid is use as an acid (and an oxidizing agent), the concentration of nitric acid in the solution (aqueous solution) is normally not less than 200 mL/L, preferably not less than 300 mL/L, and is normally not more than 1,000 mL/L, preferably not more than 700 mL/L. With an excessively small amount of nitric acid, the acid solution may be too poor in oxidizing power to end reaction. The amount of 1,000 mL/L means that the solution is composed entirely of nitric acid.

(27) When the solution contains an oxidizing agent, the concentration thereof is normally not less than 50 g/L, preferably not less than 75 g/L, and is normally not more than 500 g/L, preferably not more than 300 g/L. With an excessively small amount of the oxidizing agent, the acid solution is too poor in oxidizing power to end reaction. On the other hand, an excessively large amount of the oxidizing agent may lead to an economically disadvantage. The concentration of acids such as hydrochloric acid and sulfuric acid used with an oxidizing agent is normally not less than 10 g/L, preferably not less than 15 g/L, and is normally not more than 500 g/L, preferably not more than 300 g/L. An excessively low concentration of acid may hardly dissolve the substituted metal. An excessively high concentration of acid may attack other materials than aluminum or aluminum alloy. The acid solution may preferably contain a non-oxidizing acid; however, it may contain an oxidizing acid such as nitric acid or it may contain a mixture of an oxidizing acid and a non-oxidizing acid.

(28) The dissolution treatment may take any length of time, say 5 to 300 seconds. The temperature for dissolution ranges from 10 to 40 C., for example. During the dissolution treatment, the workpiece for plating may be stationary or shaking, with the solution being optionally agitated.

(29) The surface treatment with the remover of the present invention may be followed by plating, with an intermediate step placed between them to form the substituted metal on the surface of aluminum or aluminum alloy of the workpiece. Plating may be performed directly on the substituted metal or it may be performed after the substituted metal has been removed. In the latter case in which an oxide film is completely absent on the surface of aluminum or aluminum alloy, for example, electroless nickel plating may be performed so that aluminum as the ground material is directly substituted by nickel. Alternatively, after the substituted metal has been removed and then the surface of the workpiece is activated by zinc substitution treatment or palladium treatment, plating may be performed. Such an activating treatment preferably include zinc substitution treatment, particularly alkaline zinc substitution treatment, which permits a zinc film to be formed on the surface of aluminum or aluminum alloy, thereby allowing the plating film to firmly adhere.

(30) The zinc substitution treatment denotes a treatment in which zinc is substitutionally deposited on the surface using a zincate-containing solution. In an alkaline zinc substitution treatment, an alkaline zincate-containing solution is used. An acidic zinc substitution treatment denotes a treatment in which zinc is substitutionally deposited on the surface using an acidic zincate-containing solution. These treatments may be performed by well-known methods. The palladium treatment denotes a treatment in which palladium is substitutionally deposited on the surface using a solution containing a palladium salt, which may be performed by well-known methods.

(31) In the field of semiconductor devices, the above-mentioned treatment for forming a zinc film is preferably carried out as a pretreatment in order to activate the surface of aluminum thin film electrodes patterned on a wafer. The surface activation permits bumps to be formed stably by nickel plating. Although the zinc substitution treatment is liable to attack the aluminum ground or the aluminum alloy ground, the aluminum thin film electrodes is maximally protected against corrosion by using the remover of the present invention. Therefore, even though the aluminum ground or the aluminum alloy ground is somewhat attacked by the zinc substitution treatment, the aluminum thin film electrodes is sure to remain after the zinc substitution treatment.

(32) The zinc substitution treatment may preferably be carried out once or twice, although a single treatment may be satisfactory. The remover of the present invention permits complete zinc substitution by a single treatment unlike the conventional remover which merely permits coarse zinc substitution by a single treatment. In the ensuing nickel plating processing, a good nickel film can be formed.

(33) The surface treatment by the method of the present invention is followed by plating in any method which is not specifically restricted. Either electric plating or electroless plating may be employed.

(34) Since electroless plating consumes less energy than electric plating, it particularly needs a good pretreatment to form a good plating layer. The method according to the present invention, in which impurities such as an aluminum oxide film are completely removed, provides a firmly adhering plating layer by electroless plating.

(35) In addition, electroless plating is free of problems involved in electroplating, such as need for wiring, troublesome equipment assembling, inability to increase the density of the plating, and inability to form uniform plating film due to noise.

(36) The plating metal may be selected from Cu, Ni, Au, and the like according to applications. Two or more plating layers may be formed.

EXAMPLES

(37) The present invention will be described below in more detail with reference to the following Examples and Comparative Examples, which are not intended to restrict the scope thereof.

Examples 1 to 6 and Comparative Examples 1 to 7

(38) Samples of the remover were prepared according to the formulation shown in Tables 1 (Examples 1 to 6) and 2 (Comparative Examples 1 to 7). In the remover was immersed a silicon board covered by sputtering with a polycrystalline aluminum layer having a thickness of 5 m and having crystal orientation planes (111) and (100), at 60 C. for 60 seconds. Incidentally, each remover was adjusted to pH 12.8. Subsequently, the workpiece was immersed in an aqueous solution of nitric acid (500 mL/L) at 25 C. for 30 seconds, thereby dissolving and removing the metal which had substitutionally deposited on the aluminum layer of the workpiece in the step of immersion in the remover. Further, the workpiece was subjected to a single zinc substitution treatment by immersion in an alkaline zincate solution. Finally, the workpiece was plated by electroless nickel plating to form a nickel film having a thickness of 1 m, and then displacement plating was performed to form a gold film having a thickness of 0.05 m thereon.

(39) The resulting plated products were evaluated on the appearance of the plating films formed on the (111) plane and the (100) plane, respectively. In this case, the electroless nickel plating film was formed in a small thickness, and the gold film was further formed thereon. Thus, when the oxide film remained unremoved, nickel and gold were not deposited thereon and the non-plated area was left as a hole (white color). The absence of plating film (or the presence of residual oxide film) was determined by examining the white holes by contrast with gold color. The results are shown in Tables 1 and 2.

(40) TABLE-US-00001 TABLE 1 Example Amount of ingredients in 1 L of water 1 2 3 4 5 6 Remover Alkali (base) TMAH 50 50 50 50 50 50 (g/L) NaOH Solubilizing hydroxyethylethylene- 10 10 15 agent diaminetriacetic acid (g/L) Nitrilotriacetic acid 10 10 15 5,5-dimethylhydantoin 1 1 Succinimide 1 1 Barbituric acid 1 1 Metal ion Ag (silver acetate) 0.01 0.01 1 1 (g/L) Cu (copper sulfate) 0.1 1 Pd (palladium tetramine chloride) Ni (nickel sulfate) Zn (zinc oxide) 3 3 Mo (ammonium molybdate) Surfactant Polyoxyethylene ether 5 1 5 1 5 1 (g/L) pH 12.8 12.8 12.8 12.8 12.8 12.8 Appearance of plating (111) plane (: good, x: poor) (100) plane Observation under microscope (111) plane (: without holes, (100) plane x: with holes

(41) TABLE-US-00002 TABLE 2 Comparative Example Amount of ingredients in 1 L of water 1 2 3 4 5 6 7 Remover Alkali (base) TMAH 50 50 50 50 (g/L) NaOH 50 50 50 Solubilizing hydroxyethylethylene- 10 10 10 10 10 agent diaminetriacetic acid (g/L) Nitrilotriacetic acid 10 10 5,5-dimethylhydantoin 1 Succinimide Barbituric acid 1 Metal ion Ag (silver acetate) 1 1 (g/L) Cu (copper sulfate) 1 Pd (palladium tetramine 1 chloride) Ni (nickel sulfate) 1 Zn (zinc oxide) 1 Mo (ammonium molybdate) 1 Surfactant Polyoxyethylene ether 5 5 5 5 5 5 5 (g/L) pH 12.8 12.8 12.8 12.8 12.8 12.8 12.8 Appearance of plating (111) plane (: good, x: poor) (100) plane x x x x x x x Observation under microscope (111) plane (: without holes, (100) plane x x x x x x x x: with holes

(42) Japanese Patent Application Nos. 2010-142787 and 2011-106928 are incorporated herein by reference.

(43) Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.