JP2024075182A - Etching solution set, etching method, and method for forming conductive pattern - Google Patents

Abstract

An etching solution set capable of selectively etching a metal to be treated, such as a Ni--Cr alloy, is provided.
[Solution] The etching solution set includes a first liquid containing hydrochloric acid and a thio compound, and a second liquid containing hydrochloric acid and a nitrous acid source. The thio compound contained in the first liquid is a compound having S-H or S=C and having 7 or less carbon atoms and at least one functional group selected from the group consisting of an amino group, an imino group, a carboxy group, a carbonyl group, and a hydroxyl group, and the concentration of the thio compound in the first liquid is 0.5 to 30 wt %. The second liquid has a nitrous acid source concentration of 0.1 to 30 mM, a hydrochloric acid concentration of 8 to 30 wt %, and a sulfuric acid concentration of 4.5 wt % or less.
[Selected Figure] Figure 1

Description

The present invention relates to an etching solution set used for etching a metal selected from Ni, Cr, Ni-Cr alloy, and Pd, and an etching method and a method for forming a conductor pattern using the etching solution set.

For flexible printed wiring boards that require fine-pitch circuits, two-layer copper-clad laminates are used, in which a copper layer is formed on a polyimide film as an insulating substrate without a resin adhesive layer. In particular, a method in which a base layer is formed on a polyimide film and a copper layer is formed by electrolytic plating only on the parts that constitute the wiring on the base layer (semi-additive method) is advantageous for forming fine wiring.

When copper wiring is formed by the semi-additive method, the underlayer functions as a power supply layer (seed layer) for forming a copper layer by electrolytic plating, and is formed, for example, by electroless copper plating using Pd as a catalyst. Also, when the underlayer is made of Ni, Cr, or a Ni-Cr alloy, it acts as a power supply layer and also acts as an adhesive layer that improves adhesion between the insulating substrate, such as a polyimide film, and the copper wiring.

After forming a copper layer on the underlayer, the underlayer where the copper layer is not formed is etched away to form wiring consisting of the copper layer and underlayer. To etch the underlayer, an etching solution containing ferric chloride as the main component is generally used, but because the etching rate of the metal that constitutes the underlayer is low, there is a problem that the copper layer dissolves while the underlayer is being etched, resulting in a reduction in the height and width of the wiring.

As a method for etching an underlayer such as a Ni-Cr layer, Patent Documents 1 and 2 propose using an acid-based aqueous solution containing nitric acid or nitrous acid. Patent Document 1 uses an acid-based etching solution containing nitrous acid and a specific thio compound. Patent Document 2 discloses an etching solution set containing a first solution and a second solution, and describes that the etching rate of the underlayer is increased by removing the oxide film on the surface of the underlayer with a first solution containing hydrochloric acid and a thio compound, and then performing etching with a second solution containing hydrochloric acid, nitric acid, and cupric ions.

JP 2006-229196 A JP 2005-154899 A

The etching solutions of Patent Documents 1 and 2 have a high etching rate for the metal to be treated, such as the Ni-Cr alloy that constitutes the base layer, and can etch away the base layer in a short time, thereby suppressing etching of the copper layer of the substrate to be treated. However, because they cannot completely suppress copper dissolution, the concentration of copper (copper ions) dissolved in the solution increases when etching is performed continuously on multiple substrates to be treated (when the etching solution is used continuously).

Cupric ions act as an oxidizing agent for metallic copper and have the effect of increasing the etching rate of copper, so as the copper ion concentration in the etching solution increases, the etching rate of copper increases. In addition, in systems that use nitric acid, such as those in Patent Document 2, nitrogen oxides (NOx) are generated in the presence of copper ions, which also increases the etching rate of copper.

When the copper ion concentration or NOx concentration in the liquid rises and the copper etching rate increases, the etching characteristics cannot be maintained and the etching liquid must be replaced, which leads to reduced process efficiency and increased costs. For this reason, there is a demand for etching liquids that change only slightly in etching characteristics even when used continuously, and that require less frequent liquid replacement (improved running performance).

One embodiment of the present invention is an etching solution set used for etching a metal selected from Ni, Cr, Ni-Cr alloys, and Pd. The etching solution set includes a first solution containing hydrochloric acid and a thio compound, and a second solution containing hydrochloric acid and a nitrous acid source.

The thio compound contained in the first liquid is a compound having S-H or S=C and having at least one functional group selected from the group consisting of amino, imino, carboxy, carbonyl, and hydroxyl groups, and has a carbon number of 7 or less, and the concentration of the thio compound in the first liquid is 0.5 to 30% by weight. The second liquid has a nitrite source concentration of 0.1 to 30 mM, a hydrochloric acid concentration of 8 to 30% by weight, and a sulfuric acid concentration of 4.5% by weight or less.

The metal (the metal to be treated) selected from Ni, Cr, Ni-Cr alloy, and Pd is etched by successively contacting the first liquid and the second liquid. For example, the substrate to be treated may be immersed in the first liquid to bring the surface of the metal to be treated into contact with the first liquid, and then the substrate to be treated may be immersed in the second liquid to bring the surface of the metal to be treated into contact with the second liquid.

The substrate to be treated may be one having a metal to be treated on an insulating substrate. The substrate to be treated may also be one having both a metal to be treated and a copper layer.

An example of the substrate to be treated is a structure having an unpatterned underlayer on an insulating substrate, and a patterned copper layer on the unpatterned underlayer. The underlayer contains a metal to be treated selected from Ni, Cr, Ni-Cr alloy, and Pd, and this underlayer is etched using an etching solution set. That is, the underlayer exposed between the copper layers of the substrate to be treated is etched by sequentially contacting the first liquid and the second liquid of the etching solution set with the underlayer. By etching the underlayer exposed between the copper layers, a conductor pattern is formed on the insulating substrate, which sequentially includes a patterned underlayer and a copper layer.

When etching multiple substrates to be treated continuously using an etching solution set, a replenisher solution may be added to the first solution and/or the second solution to adjust the composition for the purpose of maintaining the etching characteristics, etc. For example, by adding a replenisher solution containing a nitrite source to the second solution, the etching rate of the metal to be treated by the second solution can be maintained at or above a predetermined value.

When adding a replenishment liquid to the second liquid, the replenishment liquid may be added so that the etching rate of the metal to be treated by the second liquid is maintained at or above a predetermined value. In addition, the replenishment liquid may be added so that the concentration of the nitrite source in the second liquid is maintained within a predetermined range.

By using the etching solution set of the present invention, it is possible to quickly etch metals to be treated, such as Ni-Cr alloys, and therefore to suppress excessive etching of copper and other metals on the substrate to be treated. Furthermore, even when the etching solution is used continuously, the change in etching characteristics is small and the etching rate of copper is unlikely to increase, so the etching solution does not need to be replaced frequently, which is advantageous for improving process efficiency and reducing costs.

1 is a cross-sectional view showing an example of the configuration of a substrate to be processed; 1 is a cross-sectional view of a printed wiring board having a conductor pattern on an insulating substrate. FIG. 1 is a schematic diagram showing an example of a process for forming a substrate to be processed;

The present invention relates to an etching solution set used for etching at least one metal selected from Ni, Cr, a Ni-Cr alloy, and Pd (hereinafter, sometimes referred to as "metal to be treated"). The etching solution set includes a first liquid and a second liquid. In one embodiment, the etching solution set is used for selective etching of the metal to be treated on a substrate to be treated where the metal to be treated and a copper layer coexist.

Figure 1 is a cross-sectional view showing an example of a substrate to be treated. The substrate to be treated shown in Figure 1 has a Ni-Cr alloy layer as an underlayer 20 on a polyimide film as an insulating substrate 10, and a copper conductor pattern 31 is formed on the underlayer 20. The underlayer 20 exposed between the conductor patterns 31 of the substrate to be treated (regions 5 where no conductor pattern is formed) is contacted with the first and second liquids of the etching solution set in order, whereby the underlayer is etched away, and a printed wiring board is obtained in which a conductor pattern 3 consisting of a patterned copper layer 31 and underlayer 21 is formed on the insulating substrate 10, as shown in Figure 2.

[Composition of Etching Solution]
The etching solution set includes a first solution and a second solution. The first solution is an aqueous solution containing hydrochloric acid and a thio compound, and the second solution is an aqueous solution containing hydrochloric acid and a nitrite source (nitrous acid or nitrite ions).

The metal being treated, such as a Ni-Cr alloy, is primarily etched by the action of the hydrochloric acid and nitrous acid in the second liquid, and the thio compounds carried over from the first liquid to the second liquid promote the etching of the metal being treated while inhibiting the etching of copper.

<First liquid>
(hydrochloric acid)
The hydrochloric acid in the first liquid has the effect of removing an oxide film formed on the surface of the metal to be treated and enhancing the etching property of the metal to be treated by the second liquid. In addition, when the copper of the substrate to be treated is etched and dissolved in the liquid, the hydrochloric acid has the effect of enhancing the solubility of the complex formed by the thio compound and copper described later. The hydrochloric acid concentration in the first liquid is preferably 0.5 to 30% by weight. If the hydrochloric acid concentration is excessively low, the removal of the oxide film becomes insufficient, the etching rate of the metal to be treated by the second liquid decreases, or when etching treatment of a plurality of substrates to be treated is performed continuously without changing the etching liquid, the liquid may become turbid or precipitate due to insoluble matter of the copper-thio compound complex. If the hydrochloric acid concentration is excessively high, it becomes difficult to keep the concentration constant. In terms of the removability (cleaning ability) of the oxide film and the solubility of the copper-thio compound, there is no particular difference as long as the hydrochloric acid concentration in the first liquid is within the above range.

When the substrate to be treated after being treated with the first liquid is immersed in the second liquid, the first liquid adhering to the surface of the substrate to be treated is carried over into the second liquid. From the viewpoint of suppressing fluctuations in the composition of the second liquid and maintaining constant etching characteristics when etching is performed continuously on a plurality of substrates to be treated without replacing the etching liquid, it is preferable that the difference between the hydrochloric acid concentrations of the first liquid and the second liquid is small. The hydrochloric acid concentration of the first liquid is preferably 0.3 to 2 times, and more preferably 0.5 to 1.5 times, the hydrochloric acid concentration of the second liquid. The hydrochloric acid concentration of the first liquid may be 3 to 30% by weight, 5 to 25% by weight, 7 to 23% by weight, or 10 to 20% by weight.

(Other acids)
The first liquid may contain an acid other than hydrochloric acid, such as inorganic acids, including sulfuric acid, nitric acid, nitrous acid, and phosphoric acid, and various organic acids.

If the first liquid contains nitric acid, when the copper of the substrate to be treated is etched and dissolved in the liquid, nitrogen oxides (NOx) are generated, and the generated NOx has the effect of increasing the etching rate of copper. If the copper ion concentration in the liquid increases with copper etching, the etching rate of copper further increases. In addition, if the first liquid contains nitric acid, the thio compound described below is easily decomposed, and its concentration decreases over time. From the viewpoint of suppressing the etching of copper and the decrease in the concentration of the thio compound, it is preferable that the first liquid does not substantially contain nitric acid. The nitric acid concentration in the first liquid is preferably 1% by weight or less, more preferably 0.5% by weight or less, even more preferably 0.1% by weight or less, and may be 0.05% by weight or less or 0.01% by weight or less. It is particularly preferable that the first liquid does not contain nitric acid.

If the first liquid contains nitrous acid, the thio compound described below is easily decomposed, and its concentration decreases over time. From the viewpoint of suppressing the decrease in the concentration of the thio compound, it is preferable that the first liquid does not substantially contain a nitrite source (nitrous acid or nitrite salt). The nitrite concentration in the first liquid is preferably 0.001% by weight or less, more preferably 0.0005% by weight or less, and even more preferably 0.0001% by weight or less. It is particularly preferable that the first liquid does not contain a nitrite source.

If the first liquid contains sulfuric acid, the first liquid adhering to the surface of the substrate to be treated is carried over into the second liquid, increasing the sulfuric acid concentration in the second liquid. As described below, if the sulfuric acid concentration in the second liquid is high, the nitrite source concentration in the second liquid is likely to decrease over time, and the liquid stability may be poor. From the viewpoint of suppressing the carryover of sulfuric acid into the second liquid, it is preferable that the first liquid has a low sulfuric acid concentration. The sulfuric acid concentration in the first liquid is preferably 4.5% by weight or less, and may be 4% by weight or less, 3% by weight or less, 2% by weight or less, 1% by weight or less, 0.5% by weight or less, or 0.1% by weight or less. It is particularly preferable that the first liquid does not contain sulfuric acid.

(Thio compounds)
The thio compound contained in the first liquid is a compound having 7 or less carbon atoms, which has -SH or S=C, and has one or more functional groups selected from the group consisting of an amino group, an imino group, a carboxy group, a carbonyl group, and a hydroxyl group. The carbon number of the thio compound is preferably 1 to 5. The molecular weight of the thio compound is preferably 60 to 300, and may be 70 to 200.

Specific examples of thio compounds having an amino group include thiourea, thiourea dioxide, N-methylthiourea, 1,3-dimethylthiourea, and 1,3-diethylthiourea. Specific examples of thio compounds having an imino group include ethylenethiourea. Specific examples of thio compounds having a carboxy group include sulfur atom-containing carboxylic acids such as thiosalicylic acid, thioglycolic acid, β-mercaptopropionic acid, 2-mercaptopropionic acid, 2,2'-thiodiglycolic acid, thiomalic acid, mercaptosuccinic acid, cysteine, and cystine, and salts thereof (sodium salts, potassium salts, calcium salts, ammonium salts, etc.). Specific examples of thio compounds having a carbonyl group include 2-thiobarbituric acid. Specific examples of thio compounds having a sulfo group include 2-mercaptoethanesulfonic acid, 3-mercapto-1-propanesulfonic acid, 2-mercaptobenzenesulfonic acid, and salts thereof. Specific examples of thio compounds having a hydroxyl group include sulfur atom-containing alcohols such as thioglycerol.

When the copper on the substrate is etched by the action of the acid and dissolved into the liquid, the copper ions act as an oxidizing agent, accelerating the etching of the copper on the substrate. The thio compound contained in the first liquid has the effect of suppressing the etching of the copper on the substrate by chelating and capturing the copper ions.

When the substrate to be treated after being treated with the first liquid is immersed in the second liquid, the first liquid adhering to the surface of the substrate to be treated is carried over into the second liquid. Therefore, the thio compound contained in the first liquid also contributes to suppressing the etching of copper in the second liquid. Furthermore, the thio compound carried over from the first liquid to the second liquid also contributes to improving the etching properties of the metal to be treated in the second liquid.

The content of the above thio compound in the first liquid is 0.5% by weight or more. If the content of the thio compound is small, the thio compound is likely to be consumed by copper dissolved in the first liquid when etching is performed continuously on multiple substrates to be treated (when the etching solution is used continuously). When the concentration of the thio compound in the first liquid decreases with continuous use of the etching solution, the etching rate of copper in the first liquid increases. Furthermore, as the concentration of the thio compound in the first liquid decreases, the amount of thio compound carried over to the second liquid decreases, and therefore the etching rate of copper in the second liquid tends to increase.

If the thio compound concentration in the first liquid is 0.5% by weight or more, the concentration of the thio compound is sufficiently high compared to the copper ions dissolved in the first liquid, so that the copper ions are easily chelated by the thio compound, and etching of copper in the first liquid is suppressed. Because etching of copper by the first liquid is suppressed, consumption of the thio compound is small (the concentration of the thio compound is less likely to decrease), and fluctuations in composition are small. In addition, the action of the thio compound carried over from the first liquid to the second liquid promotes etching of the metal to be treated in the second liquid, and suppresses etching of copper by the second liquid.

The thio compound concentration in the first liquid is preferably 0.7% by weight or more, more preferably 1.0% by weight or more, and may be 1.3% by weight or more, 1.5% by weight or more, or 1.7% by weight or more. The higher the thio compound concentration in the first liquid, the greater the effect of inhibiting copper etching. In addition, the higher the thio compound concentration in the first liquid, the smaller the change in thio compound concentration (decrease in concentration) due to continuous use of the etching liquid, and therefore the less frequent the thio compound needs to be replenished to the first liquid or the first liquid needs to be replaced, which is advantageous for improving process efficiency and reducing costs.

The concentration of the thio compound in the first liquid is preferably 30% by weight or less, more preferably 20% by weight or less, even more preferably 10% by weight or less, and may be 7% by weight or less or 5% by weight or less. If the concentration of the thio compound in the first liquid is excessively high, the amount of the thio compound carried over to the second liquid may be excessive, which may inhibit etching of the metal to be treated in the second liquid.

<Second liquid>
(hydrochloric acid)
The hydrochloric acid in the second liquid, together with the nitrous acid source described below, has the effect of dissolving and etching the metal to be treated. The concentration of hydrochloric acid in the second liquid is preferably 8 to 30% by weight, more preferably 10 to 25% by weight, and even more preferably 12 to 20% by weight. If the hydrochloric acid concentration is too low, the etching rate of the metal to be treated tends to decrease, and if the hydrochloric acid concentration is too high, the etching rate of copper tends to increase.

(Nitrite source)
The nitrite source in the second liquid has the effect of dissolving and etching the metal to be treated. The nitrite source is nitrous acid and nitrite ions, and by blending nitrous acid or a nitrite salt, the nitrite or nitrite ions are present in the second liquid. Examples of the nitrite salt include sodium nitrite, lithium nitrite, potassium nitrite, calcium nitrite, and barium nitrite.

The nitrite source concentration in the second liquid (total concentration of nitrous acid and nitrite ions) is 0.1 to 30 mM, preferably 0.15 to 25 mM, more preferably 0.2 to 20 mM, even more preferably 0.3 to 17 mM, and may be 0.4 to 15 mM, 0.5 to 12 mM, or 0.6 to 10 mM. If the nitrite source concentration is too low, the etching rate of the metal being treated tends to decrease, and if the nitrite source concentration is too high, the etching rate of copper tends to increase.

In an acidic aqueous solution, the nitrite source exists mostly in the form of nitrous acid, but nitrous acid as a free acid is less stable, and the concentration of the nitrite source in the second liquid may decrease over time. Therefore, it is preferable to prepare the second liquid by mixing nitrite salts, etc., so that the nitrite source concentration is within the above range immediately before using the etching solution. In addition, if the nitrite source concentration decreases during use of the etching solution, it is preferable to replenish nitrite salts, etc., to maintain the nitrite source concentration in the second liquid within the above range.

The nitrite source may be added to the second liquid as a solid or liquid (solution). Since the nitrite source is stable as nitrite ions in a neutral or weakly alkaline solution, it is preferable to add a nitrite salt as a neutral or weakly alkaline aqueous solution to the second liquid to replenish the nitrite source. When an aqueous solution of nitrite salt is added to the second liquid as a replenishment liquid, the concentration of the nitrite salt in the replenishment liquid is, for example, about 0.1 to 75% by weight, and may be 1 to 70% by weight, 5 to 60% by weight, or 10 to 50% by weight.

Although nitrite has low stability in an acidic aqueous solution, if the concentration is 30 mM or less, the decrease in concentration is gradual and can be replenished by adding nitrite salts, etc. Also, as described below, if sulfuric acid is not used as the acid in the second liquid, or if sulfuric acid is used but its concentration is low, the decrease in nitrite can be suppressed. Furthermore, since the nitrite source can be quantified by spectroscopy, etc., concentration control is easy.

When nitric acid is used as the etching component of the second liquid, when the copper of the substrate being treated is etched and dissolved in the liquid, nitrogen oxides (NOx) are generated, and the generated NOx has the effect of increasing the etching rate of copper. If the copper ion concentration in the liquid increases as copper is etched, the copper etching rate will increase further, and the etching performance cannot be maintained, so the etching liquid must be replaced frequently.

On the other hand, when nitrous acid is used as an etching component of the second liquid, NOx is unlikely to be generated even in the presence of copper ions. Therefore, in the present invention, in which a nitrous acid source is used in the second liquid, etching performance can be stably maintained and the liquid needs to be changed less frequently, which is advantageous in terms of process efficiency and cost.

In addition, as the etching solution is used, the thio compounds are continuously introduced from the first solution to the second solution, which also contributes to stabilizing the etching characteristics (suppressing an increase in the copper etching rate). As mentioned above, the thio compounds improve the etching properties of the metal being treated and suppress the etching of copper.

In the presence of a nitrite source, the thio compound is easily decomposed, and its concentration decreases over time. Therefore, in a one-liquid etching solution in which a nitrite source and a thio compound coexist, the concentrations of the nitrite source and the thio compound tend to decrease over time, and it is necessary to manage the concentrations of each separately and replenish them so that they remain within the specified range.

On the other hand, when using the etching solution set of the present invention in which the first liquid contains a thio compound and the second liquid contains a nitrite source, the first liquid adhering to the surface of the substrate to be treated is carried into the second liquid, supplying the thio compound to the second liquid, so that the concentration of the thio compound in the second liquid is kept substantially constant. Therefore, the second liquid only needs to be managed so that the concentration of the nitrite source is within a predetermined range as described above, and the liquid can be managed more easily than a one-liquid type etching solution.

(Other acids)
The second liquid may contain an acid other than hydrochloric acid and nitrous acid, such as inorganic acids such as sulfuric acid, nitric acid, and phosphoric acid, and various organic acids.

If the second liquid contains nitric acid, when the copper of the substrate to be treated is etched and dissolved in the liquid, nitrogen oxides (NOx) are generated from the nitric acid, and the generated NOx has the effect of increasing the etching rate of copper. From the viewpoint of suppressing copper etching, it is preferable that the second liquid does not substantially contain nitric acid. The nitric acid concentration in the second liquid is preferably 1 wt% or less, more preferably 0.5 wt% or less, even more preferably 0.1 wt% or less, and may be 0.05 wt% or less or 0.01 wt% or less. It is particularly preferable that the second liquid does not contain nitric acid.

The second liquid may contain sulfuric acid as an acid, but if the sulfuric acid concentration is high, the nitrite source concentration of the second liquid is likely to decrease over time, and the liquid stability may be poor. Therefore, the sulfuric acid concentration of the second liquid is preferably 4.5% by weight or less, and may be 4% by weight or less, 3% by weight or less, 2% by weight or less, 1% by weight or less, 0.5% by weight or less, or 0.1% by weight or less. The second liquid may not contain sulfuric acid.

When nitric acid is used as the component for etching the metal being treated, sulfuric acid is used to convert the nitric acid into nitrous acid, thereby increasing the etching efficiency. On the other hand, when the etching component is nitrous acid, the etching rate of the metal being treated can be sufficiently increased without using sulfuric acid.

<Preparation of first and second liquids>
The first and second liquids can be prepared by dissolving the above components in water. The water is preferably water from which ionic substances and impurities have been removed. Specifically, it is preferable to use ion-exchanged water, pure water, ultrapure water, etc.

The first and second liquids may further contain various additives as necessary. For example, a surfactant may be used as an additive to improve wettability and prevent copper erosion.

Examples of surfactants include cationic surfactants, anionic surfactants, amphoteric surfactants, and nonionic surfactants. When the first liquid and/or the second liquid contain a surfactant, the concentration thereof may be 0.001 to 5% by weight, 0.01 to 4% by weight, or 0.1 to 3% by weight.

The first liquid and/or the second liquid may contain additives such as antifoaming agents and rust inhibitors.

When etching is performed using an etching apparatus, all components of each of the first and second liquids may be prepared to have a predetermined composition and then supplied to the etching apparatus, or each component may be supplied to the etching apparatus separately and then mixed within the etching apparatus to prepare the predetermined composition. Alternatively, a solution in which some components of the first and second liquids are premixed may be supplied to the etching apparatus, and then other components may be added, and the components may be mixed within the etching apparatus to prepare the predetermined composition.

As mentioned above, the concentration of nitrous acid in an acidic solution tends to decrease over time. For this reason, it is preferable to supply the components other than the nitrous acid source to the etching device in advance, and then add the nitrous acid source immediately before use to adjust the composition of the second liquid.

[Etching of treated metal using etching solution set]
The metal to be treated is etched by contacting the surface of the metal to be treated with the first liquid and then the second liquid. Methods for contacting the metal to be treated with the first liquid and the second liquid include immersion, spraying, bar coating, and the like.

From the viewpoint of suppressing copper etching by carrying the first liquid adhering to the surface of the substrate to be treated into the second liquid, thereby causing the second liquid to contain a thio compound, the immersion method is preferred as a method for contacting the first and second liquids with the metal to be treated. That is, a preferred method is to immerse the substrate to be treated in the first liquid, treat it in the first liquid, and then immerse the substrate to be treated removed from the first liquid in the second liquid. It is preferred that the substrate to be treated removed from the first liquid is immersed directly in the second liquid without cleaning the surface.

The temperature of the first and second liquids and the treatment time with the first and second liquids are not particularly limited and may be set appropriately depending on the type of metal to be etched and the amount of etching (thickness), etc. The temperature of the first and second liquids is, for example, about 20 to 65°C, and the treatment time is about 5 to 60 seconds.

When an etching solution is used continuously, the composition of the solution may change over time, causing a change in the etching properties (etching rate). In order to maintain a constant etching property, a refill solution may be added to the first solution and/or the second solution. In particular, since the concentration of the nitrite source in the second solution decreases over time, it is preferable to refill the second solution with the nitrite source. As described above, a nitrite aqueous solution is preferable as a refill solution for refilling the nitrite source to the second solution, and the nitrite concentration of the refill solution is, for example, about 0.1 to 75% by weight, and may be 1 to 70% by weight, 5 to 60% by weight, or 10 to 50% by weight.

When replenishing the nitrite source by adding a replenishment liquid to the second liquid, the replenishment liquid may be added so that the nitrite source concentration is maintained within the aforementioned range. In addition, the etching rate of the metal to be treated may be monitored, and the replenishment liquid may be added so that the etching rate is maintained at or above a predetermined value.

As described above, the second liquid does not contain sulfuric acid or has a low sulfuric acid concentration, so that the decrease in the concentration of the nitrite source in the second liquid is suppressed. When the etching liquid set of the present invention is used, the change in the concentration of the nitrite source is small, so that the change in the etching rate is suppressed. In addition, the amount of nitrite source to be replenished to the second liquid and the frequency of replenishment can be reduced, simplifying the process management when continuously etching the metal to be treated.

The first liquid may have a decreased concentration due to consumption (oxidation) of the thio compound by the copper ions dissolved in the liquid. In order to maintain the thio compound concentration in the first liquid constant, an aqueous solution of the thio compound may be added to the first liquid as a replenishment liquid. As described above, if the thio compound concentration in the first liquid is increased in advance, the copper dissolved in the first liquid is appropriately chelated, suppressing dissolution of copper in the substrate to be treated, thereby reducing consumption of the thio compound by copper and suppressing a decrease in the concentration of the thio compound. Therefore, when using the etching liquid set of the present invention, it is possible to reduce the frequency of replenishing the thio compound to the first liquid, and simplify the process management when continuously etching the metal to be treated.

[Formation of Conductive Pattern]
By using the above etching solution set, it is possible to efficiently etch the metal to be treated while suppressing dissolution of copper. By applying the above etching solution set to a substrate on which the metal to be treated and a copper layer coexist, the metal to be treated on the substrate to be treated can be selectively etched to form a conductor pattern. Examples of the conductor pattern include a wiring pattern, a land pattern, or a combination of these.

As an example of a substrate on which the metal to be treated and a copper layer coexist, there is a configuration in which an insulating substrate 10 is provided with a base layer 20 of the metal to be treated, and a conductor pattern 31 is provided on top of that, as shown in FIG. 1.

Figures 3A to 3E are schematic diagrams showing an example of a process for forming the substrate shown in Figure 1. First, a base layer 20 of the metal to be treated is formed on an insulating substrate 10 (Figure 3A).

The insulating material of the insulating substrate 10 may be a thermoplastic resin such as AS resin, ABS resin, fluororesin, polyamide, polyethylene, polyethylene terephthalate, polyvinylidene chloride, polyvinyl chloride, polycarbonate, polystyrene, polysulfone, polypropylene, or liquid crystal polymer; or a thermosetting resin such as epoxy resin, phenolic resin, polyimide, polyurethane, bismaleimide-triazine resin, or modified polyphenylene ether. These resins may be reinforced with glass fiber, aramid fiber, or the like. The insulating substrate 10 may be a ceramic material or glass. In a flexible printed wiring board, a polyimide film is preferably used as the insulating substrate 10.

The material of the underlayer 20 is Ni, Cr or a Ni-Cr alloy. When the underlayer is a Ni-Cr layer, the atomic ratio of Ni to Cr is not particularly limited. Examples of Ni-Cr alloys include those with a Ni/Cr mass ratio of 6/1, 7/1 or 3/1. The thickness of the underlayer 20 is about 5 to 500 nm. The underlayer 20 is formed by a method such as sputtering, vapor deposition or electroless plating.

A plating resist layer 40 is formed on the underlayer 20 (FIG. 3B), and the plating resist layer 40 is patterned by photolithography or the like to form a resist pattern 41 (FIG. 3C).

By using this substrate and performing electrolytic copper plating while supplying power to the underlayer 20, a copper layer 31 is deposited only in the areas that form the conductor pattern (Figure 3D). The thickness of the copper layer 31 is, for example, about 1 to 30 μm.

After the copper layer 31 is formed, the plating resist is removed to obtain a substrate in which the underlayer 20 is exposed between the patterned copper layers 31 (regions 5 where the copper layer 31 is not formed) (FIGS. 3E and 1). The substrate is then treated in turn with the first and second liquids of the etching solution set, whereby the underlayer 20 in region 5 is etched away, and a printed wiring board is obtained in which the conductor pattern 3 consisting of the patterned copper layer 31 and underlayer 21 is formed on the insulating substrate 10, as shown in FIG. 2.

The above process is a method for forming a conductor pattern by the so-called semi-additive method, but the conductor pattern may also be formed by the subtractive method. In the subtractive method, after forming an underlayer 20 on an insulating substrate 10, an electrolytic copper plating layer is formed on the underlayer, and the copper layer in the portion constituting the conductor pattern is covered with an etching resist. The copper layer in the area not covered with the etching resist is etched using a copper etching solution, and then the resist is peeled off to obtain a substrate to be processed in which the underlayer 20 is exposed between the patterned copper layers 31. By treating this substrate to be processed sequentially with the first and second solutions of the etching solution set, the underlayer 20 in the area 5 is etched and removed, and a printed wiring board is obtained in which a conductor pattern 3 consisting of a patterned copper layer 31 and underlayer 21 is formed on the insulating substrate 10, as shown in FIG. 2.

In both the semi-additive and subtractive methods, the substrate is successively contacted with the first and second liquids, which suppresses copper etching while selectively etching away the underlayer in areas where no copper layer is formed. This ensures insulation between conductor patterns with little change to the shape of the copper layer that constitutes the conductor pattern.

In the method of the present invention, which uses a first liquid containing hydrochloric acid and a thio compound and a second liquid containing hydrochloric acid and a nitrous acid source, there is little variation in the liquid composition even when multiple substrates are treated continuously. In addition, even if the copper ion concentration in the liquid increases due to continuous use of the liquid, the copper etching rate is unlikely to increase, so the liquid needs to be replaced less frequently and the method has excellent running properties.

The above etching solution set also has excellent etching properties for Pd. Pd is used as a catalyst for electroless copper plating in the manufacture of printed wiring boards. When the electroless copper plating layer is etched away, if the Pd catalyst remains on the surface of the insulating substrate, the electrical insulation between patterns decreases. Furthermore, the remaining Pd catalyst can cause gold to deposit in unnecessary areas during the gold plating process in the subsequent process. By using the above etching solution set, it is possible to selectively etch away the Pd remaining on the surface of the insulating substrate while suppressing the etching of the copper layer that constitutes the conductor pattern.

The present invention will be described in more detail below based on examples and comparative examples, but the present invention should not be interpreted as being limited to the following examples.

[Preparation of Etching Solution]
<First liquid>
A first liquid was prepared by mixing hydrochloric acid and a thio compound so as to obtain the concentration (wt%) shown in Table 1. The balance in Table 1 is water (the same applies to the second liquid). The abbreviations of the thio compounds are as follows. Note that propionic acid (PA) used in Comparative Example 3 is not a thio compound.
ATG: Ammonium thioglycolate (molecular weight: 109.15)
TGA: thioglycolic acid (molecular weight: 92.11)
TG: thioglycerol (molecular weight: 108.16)
TU: Thiourea (molecular weight: 76.12)
RCoM: 2-mercaptoethanesulfonic acid (molecular weight: 142.19)
MPA: β-mercaptopropionic acid (molecular weight: 106.14)
DETU: Diethylthiourea (molecular weight: 132.23)
ETU: Ethylenethiourea (molecular weight: 102.16)
PA: Propionic acid

<Second liquid>
A second liquid was prepared by mixing hydrochloric acid and a nitrite (sodium nitrite or calcium nitrite) to the concentration (wt%) shown in Table 1. In Comparative Examples 4 and 6, nitric acid and sulfuric acid were mixed without mixing a nitrite. Sulfuric acid was also mixed in Examples 2 to 4 and Comparative Example 2. In Comparative Example 2, 0.5 wt% of β-mercaptopropionic acid was mixed as a thio compound. The nitrite and nitric acid were mixed to the concentrations shown in Table 1 just before using the etching solution. The nitrite concentration (mM) in Table 1 is the molar concentration of the nitrite source, and in the example in which calcium nitrite was used as the nitrite, the value shown is twice the molar concentration of calcium nitrite.

[evaluation]
<Etching properties of Ni-Cr alloy>
A 20 nm thick Ni-Cr alloy film (Ni:Cr atomic ratio 88:12) was formed on one side of a 50 μm thick polyimide film by sputtering. This sample was cut into a 40 mm x 40 mm square, immersed in the first liquid (50°C) for 30 seconds, and then immersed in the second liquid (50°C) for 30 seconds to dissolve the Ni-Cr alloy film. After washing and drying the sample, it was observed at 50x magnification using a digital microscope (Keyence "VHX-5000") to confirm the presence or absence of etching residues of the Ni-Cr alloy film. Those that did not have any residues were rated as having good etching properties (◯), and those that had any residues were rated as having poor etching properties (×).

<Copper etching rate>
A double-sided copper-clad laminate (40×40 mm, plate thickness 0.2 mm, copper thickness 35 μm) manufactured by Risho Kogyo was immersed in the first liquid (50° C.) for 30 seconds, and then immersed in the second liquid (50° C.) for 30 seconds. The double-sided copper-clad laminate was then washed with water and dried, and the copper etching rate was calculated from the weight change (amount of copper etched).

<Etching properties in the presence of copper ions>
The double-sided copper-clad laminate was immersed in the second liquid (50° C.) and copper was dissolved until the copper concentration in the liquid reached 0.05, 0.2, 0.35, 1.0, or 2.0 g/L. Thereafter, the amount of the nitrite source in the liquid was quantified by the Griess method (described later), and if the concentration of the nitrite source had decreased, a nitrite was added so that the concentration of the nitrite source was the same as that of the blended composition (immediately after preparation) in Table 1. In Comparative Example 4 and Comparative Example 6, since no nitrite was used, the amount of the nitrite source was not quantified and no nitrite was added.

As the second liquid, a solution prepared using the above method to have a specified copper concentration (a model liquid reproducing a state in which copper from the substrate to be treated is dissolved in the second liquid as the etching liquid is used and a nitrite source is replenished) was used to evaluate the etching properties of the Ni-Cr alloy and the etching rate of copper in the same manner as above.

The formulations of the first and second liquids in the examples and comparative examples, as well as the evaluation results of the etching properties of Ni-Cr alloys and the etching rate of copper, are shown in Table 1. In Table 1, when the copper concentration of the second liquid is 0, this is the evaluation result when the first and second liquids were used immediately after preparation.

The etching solution set of the embodiment had excellent Ni-Cr removability and a low copper etching rate, and excellent etching selectivity, in the initial formulation (when the copper concentration was 0). Even when the copper concentration in the solution was increased to 2.0 g/L, the Ni-Cr removability was maintained, and the copper etching rate was less than 0.35 μm/min, maintaining excellent etching selectivity.

The etching solution set of the embodiment does not impair the removability of Ni-Cr even when the copper concentration in the solution increases due to continuous processing of the substrate to be processed, and maintains a low copper etching rate, demonstrating excellent runnability.

In Comparative Example 1, in which treatment with the second liquid containing hydrochloric acid and nitrite was performed without treatment with the first liquid, Ni-Cr residue was observed after 30 seconds of immersion treatment, and the etching properties of the treated metal were poor. In Comparative Example 3, in which treatment with the first liquid containing no thio compounds was performed followed by treatment with the second liquid, the etching properties of Ni-Cr were also poor, similar to Comparative Example 1.

From these results, it is believed that in the etching solution set of the Example, the thio compound in the first solution is carried over to the second solution, improving the etching of Ni-Cr in the second solution and suppressing the etching of copper, whereas in Comparative Examples 1 and 3, the thio compound is not supplied to the second solution, and therefore the etching of Ni-Cr is poor. Furthermore, Comparative Example 3 showed a higher copper etching rate than Comparative Example 1. In Comparative Example 3, the absence of a thio compound and the high concentration of the nitrite source in the second solution are believed to be the causes of the increased copper etching rate.

Comparative Example 5, in which the first liquid has a low thio compound concentration, had excellent Ni-Cr removal properties, a low copper etching rate, and excellent etching selectivity in the initial formulation (when the copper concentration was 0), and even when the copper concentration was high, the increase in the copper etching rate was suppressed. However, when the copper concentration was 2.0 g/L, the Ni-Cr etching properties decreased. In Comparative Example 5, the thio compound concentration in the first liquid decreased with continuous use of the etching solution, and when etching was continued until the copper concentration in the second liquid reached 2.0 g/L, it is believed that the Ni-Cr etching properties decreased because the thio compound concentration required for etching Ni-Cr could not be maintained (for the residual rate of thio compounds and etching properties, see Examples 12 to 14 and Comparative Examples 10 and 11 described below).

In Comparative Example 2, in which treatment with the second liquid containing a thio compound in addition to hydrochloric acid and nitrite was performed without treatment with the first liquid, the initial formulation (when the copper concentration was 0) showed excellent Ni-Cr removability, a low copper etching rate, and excellent etching selectivity, but when the copper concentration was 1.0 g/L or more, the Ni-Cr removability decreased and the copper etching rate also increased significantly. In Comparative Example 2, the thio compound in the second liquid was consumed as copper was dissolved, and the thio compound was not supplied from the first liquid, so the Ni-Cr removability decreased due to the decrease in the thio compound, and the copper etching inhibition effect was lost, and the copper etching rate increased. Note that the second liquid in Comparative Example 2 has a sufficiently large thio compound concentration in the initial formulation, but since the thio compound and the nitrite source coexist in the second liquid, the decomposition of the thio compound is easily promoted, and it is believed that the consumption of the thio compound due to the dissolution of copper, as well as the decrease in the thio compound concentration due to the decomposition of the thio compound are involved in the decrease in Ni-Cr removability and the increase in the copper etching rate.

Comparative Example 7, in which the second liquid had a low concentration of the nitrite source, had poor Ni-Cr removal. Comparative Example 8, in which the second liquid had a high concentration of the nitrite source, had a high copper etching rate even in the initial formulation in which the copper concentration was 0, and had poor etching selectivity for the treated metal.

In Comparative Examples 4 and 6, in which the second liquid did not contain a nitrite source but contained nitric acid instead, the etching ability of Ni-Cr was poor in the initial formulation, but when the copper concentration was 0.05 g/L, the etching ability of Ni-Cr improved. When the copper concentration was even higher, the etching ability of Ni-Cr decreased again. When the copper concentration was 2.0 g/L, the etching ability of Ni-Cr improved again, but the copper etching rate also increased.

In Comparative Examples 4 and 6, the etching ability of Ni-Cr was poor because the second liquid did not contain a nitrite source in the initial formulation and copper ions were not present, making it difficult to generate nitrite from nitric acid. When the copper concentration of the second liquid was 0.05 g/L, the copper ions in the second liquid acted catalytically to convert nitric acid to nitrite, which is thought to have improved the etching ability of Ni-Cr. When the copper concentration was further increased, the copper etching rate increased and most of the nitrite generated from nitric acid was consumed in etching copper, which is thought to have reduced the etching ability of Ni-Cr. When the copper concentration was still higher, the amount of nitrite generated from nitric acid was greater than the consumption of nitrite by etching, and the etching ability of Ni-Cr was thought to have increased again due to the increased etching power of copper ions.

When nitric acid is used as the second liquid, as in Comparative Examples 4 and 6, the amount of nitrous acid produced and consumed changes significantly due to changes in composition that accompany continuous use of the liquid, and this changes the etching characteristics, resulting in a lack of stability and problems with running performance.

[Study on the effect of thio compound concentration in the first liquid]
<Residual rate of thio compounds>
The first and second liquids were prepared in the same manner as in Example 1, except that the type and amount of the thio compound were changed as shown in Table 2. Cupric oxide was added to the first liquid so that the concentration was 0.025 wt% (copper concentration: 0.2 g/L), and cupric oxide was added to the second liquid so that the concentration was 0.25 wt% (copper concentration: 2 g/L). After adding the cupric oxide, the first and second liquids were stirred at room temperature for 5 hours. The concentration of the thio compound in the first liquid after adding the cupric oxide and stirring for 5 hours was quantified by iodometric titration, and the remaining rate relative to the initial concentration was calculated.

<Copper etching rate>
The copper etching rate was measured in the same manner as above using the first liquid and the second liquid (model liquids reproducing the state in which copper on the substrate to be treated is dissolved in the second liquid as the etching liquid is used) in which cupric oxide was mixed and stirred.

Table 2 shows the formulations of the first and second liquids in Examples 1, 12 to 14 and Comparative Examples 10 and 11, as well as the copper etching rates using the first and second liquids after mixing copper oxide and stirring for 5 hours, and the residual percentage of thio compounds in the first liquid.

As shown in Table 2, the higher the initial concentration of the thio compound in the first liquid, the higher the remaining rate of the thio compound after 5 hours, and the lower the copper etching rate when treated with the second liquid containing added copper after treatment with the first liquid. When the thio compound concentration was less than 0.5 wt%, the decrease in the remaining rate of the thio compound and the increase in the copper etching rate were significant.

When the concentration of thio compounds in the first liquid is low, the thio compounds are easily consumed in the presence of copper ions, and it is believed that the amount of thio compounds carried over from the first liquid to the second liquid is small, resulting in a high copper etching rate in the second liquid. From these results, it can be seen that the higher the concentration of thio compounds in the first liquid, the less the decrease in thio compound concentration and the less likely the copper etching rate to increase, resulting in excellent stability of etching characteristics and running performance.

[Study on sulfuric acid concentration in second liquid]
<Residual rate of nitrite source>
In preparing the second liquid in Examples 1 to 4, sodium nitrite was added so that the concentration would be 0.006% by weight (nitrite source concentration: 0.87 mM) after heating to 50°C. The concentration of the nitrite source in the liquid was measured immediately after the addition of sodium nitrite and after standing for 1 hour in an open system at 50°C, and the residual rate of the nitrite source was calculated. For Comparative Examples 12 to 14 shown in Table 3, the residual rate of the nitrite source after standing for 1 hour at 50°C was also determined in the same manner as in Examples 1 to 4.

The remaining amount of nitrite source was quantified by the Griess method. Griess reagent (a reagent containing sulfanilamide and ortho-(1-naphthyl)ethylenediamine) was added to the sampled specimen and left at room temperature for 30 minutes, after which the absorbance of the specimen was measured at 520-550 nm using a spectrophotometer. The concentration of the nitrite source was calculated from the absorbance of the sample based on a calibration curve created using standard specimens with known nitrite ion concentrations.

When Griess reagent is added to a sample containing a nitrite source, the sample turns reddish purple, the color deepens over time, and the absorbance at 520 to 550 nm increases. After adding Griess reagent, the absorbance remains constant for 30 minutes to several hours, after which the color fades and the absorbance decreases.

<Etching property>
The second liquid was prepared by leaving the liquid at 50° C. for 1 hour (a model liquid reproducing a state in which the concentration of the nitrite source was reduced with the use of the etching liquid), and the Ni—Cr removability was evaluated and the copper etching rate was measured in the same manner as described above.

Table 3 shows the blends of the first and second liquids in Examples 1 to 4 and Comparative Examples 12 to 14, as well as the remaining percentage of the nitrite source in the second liquid one hour after blending the nitrite salt into the second liquid, and the etching properties (Ni-Cr removability and copper etching rate) when using this second liquid.

The higher the sulfuric acid concentration in the second liquid, the lower the remaining rate of the nitrite source after one hour. In Comparative Examples 12 to 14, where the sulfuric acid was 5% by weight or more, the remaining rate of the nitrite source after one hour was below 40%, and when this was used as the second liquid, the Ni-Cr removal performance was poor.

When the sulfuric acid concentration of the second liquid is high, good etching properties can be obtained if the second liquid is used immediately after preparation, but the rate at which the nitrite source is reduced is high, so the removability (etching rate) of Ni-Cr tends to decrease, and frequent addition of the nitrite source is necessary to maintain etching properties.

From these results, it can be said that a low sulfuric acid concentration in the second liquid is preferable in order to suppress the decrease in the concentration of the nitrite source in the second liquid and to reduce the frequency of adding the nitrite source using a replenishment liquid and changing the liquid. Furthermore, from the results of each of the above examples and comparative examples, it can be seen that an etching liquid set in which the second liquid contains a nitrite source exhibits excellent etching characteristics and running properties even when it does not contain sulfuric acid.

REFERENCE SIGNS LIST 10: insulating substrate 20, 21: undercoat layer 31: copper layer 3: conductor pattern 40, 41: plating resist

Claims (10)

An etching solution set comprising: a first solution comprising hydrochloric acid and a thio compound; and a second solution comprising hydrochloric acid and a nitrous acid source,
the thio compound is a compound having S—H or S═C, and having one or more functional groups selected from the group consisting of an amino group, an imino group, a carboxy group, a carbonyl group, and a hydroxyl group, and having 7 or less carbon atoms;
The concentration of the thio compound in the first liquid is 0.5 to 30% by weight,
The concentration of the nitrite source in the second liquid is 0.1 to 30 mM;
The second liquid has a hydrochloric acid concentration of 8 to 30% by weight and a sulfuric acid concentration of 4.5% by weight or less.
Etching solution set. A method for etching a metal to be treated selected from Ni, Cr, a Ni-Cr alloy, and Pd, using the etching solution set according to claim 1, comprising the steps of:
An etching method comprising contacting the first liquid with a surface of a metal to be treated, and then contacting the second liquid with the surface of the metal to be treated. a substrate to be treated comprising the metal to be treated on an insulating substrate is immersed in the first liquid, thereby bringing the first liquid into contact with a surface of the metal to be treated;
3. The etching method according to claim 2, wherein the second liquid is brought into contact with the surface of the metal to be treated by immersing the substrate to be treated in the second liquid. The etching method according to claim 3, wherein the substrate to be treated has both the metal to be treated and a copper layer. An etching method in which the first liquid and the second liquid are repeatedly used,
5. The etching method according to claim 2, further comprising adding a replenishment liquid containing a nitrite source to the second liquid, thereby maintaining an etching rate of the metal to be treated by the second liquid at a predetermined value or higher. An etching method in which the first liquid and the second liquid are repeatedly used,
5. The etching method according to claim 2, further comprising adding a replenishment solution containing a nitrite source to the second solution so that a concentration of the nitrite source in the second solution is maintained within a range of 0.1 to 30 mM. A method for forming a conductor pattern, which forms a conductor pattern having a patterned underlayer and a copper layer in that order on an insulating substrate, comprising the steps of:
A substrate to be processed is prepared, which includes an insulating substrate, an unpatterned underlayer, and a patterned copper layer in this order, the underlayer containing a metal to be processed selected from Ni, Cr, a Ni--Cr alloy, and Pd;
the first liquid and the second liquid of the etching solution set according to claim 1 are successively brought into contact with the underlying layer exposed between the copper layers of the substrate to be processed, thereby etching the underlying layer.
A method for forming a conductor pattern. The substrate to be treated is immersed in the first liquid to bring the first liquid into contact with the surface of the metal to be treated;
8. The method for forming a conductive pattern according to claim 7, wherein the second liquid is brought into contact with the surface of the metal to be treated by immersing the substrate in the second liquid. The method for forming a conductor pattern according to claim 7 or 8, further comprising adding a replenishment liquid containing a nitrite source to the second liquid, and continuously processing a plurality of the substrates to be processed while maintaining the etching rate of the metal to be processed by the second liquid at a predetermined value or higher. 9. The method for forming a conductive pattern according to claim 7, wherein a plurality of the substrates are successively processed while adding a replenishment solution containing a nitrite source to the second liquid so that a concentration of the nitrite source in the second liquid is maintained in a range of 0.1 to 30 mM.

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