GB2143778A - Etching copper and other metals - Google Patents

Abstract

A nitrogen dioxide process for etching copper and other metals, using water as a catalyst/solvent is disclosed. In one embodiment, a film of water is formed on the surface of the metal, and the water-covered metal is exposed to gaseous NO2 to dissolve the metal. In another embodiment, the metal is exposed to an aqueous solution of NO2 or HNO3 in water, either by immersion or by spraying, to remove the metal. A polymer additive and a nitrate of the metal being etched may be present.

Description

SPECIFICATION Aqueous process for etching copper and other metals This invention relates generally to the etching of metals, and more particularly to a process for removing copper and other metals in the manufacture of printed circuit boards.

Our co-pending Patent Application No 8333553 describes a process for etching patterns in laminated copper foils using a gaseous nitrogen dioxide oxidant and an organic catalyst/solvent. This approach dramatically simplifies the chemical etching step in printed circuit board production compared to the wet etching processes currently in use. It uses a simpler chemistry having fewer process variables, it is less corrosive and thus permits the use of standard materials for processing equipment, and it produces less pollution and yields a single pure oxidized copper specie which is readily disposed of.

However, notwithstanding these substantial advantages, the process has been found to have certain limitations and disadvantages which might limit its large scale development and commercialization. For each mole of copper reacted, 2/3 mole of gaseous NO are produced, and this causes considerable bubbling and foaming in the catalyst/solvent layer which interferes with the reaction by separating the reacting layer from the copper. This, in turn, leads to non-uniform specific area reaction rates, since areas having more exposed copper also have more vigorous gasing, thus reducing the copper removal in these areas.

This system also has somewhat of a thermodynamic explosion potential in that a rapid and uncontrolled reaction between NO2 and the organic solvent mixture is energetically favored, although the reaction of metals and NO2 in organic solvents has been studied extensively without mishap.

In addition, the reaction of copper metal with NO2 is highly exothermic, producing an excess of 80 kilocalories per mole of copper reacted. The reaction is somewhat adiabatic, that is, the heat of reaction is primarily absorbed by the system. Hence, the temperature of the board increases rapidly during the reaction, and this limits the thickness of the copper foil which can be removed. Under conditions explored to date, the thickest foil which can be removed by the adiabatic gas reaction appears to be about 1/2 oz./ft2, or a thickness of about .007" (18 microns). Attempts to etch thicker foils produce disappointing results in that either the reaction film goes dry giving an incomplete etch or the substrate overheats, damaging the resist which destroys the pattern.

It has now been found, somewhat surprisingly, that these problems can be overcome and that even more improved results can be obtained by utilizing water as a catalyst/solvent in the etching of copper and other metals with nitrogen dioxide in the manufacture of printed circuit boards. This discovery is surprising and unexpected because NO2 reacts with water to produce nitric acid which tends to attack both photoresist and substrate materials, two fatal problems in the manufacture of circuit boards. However, it has been found that by proper control of the process conditions, copper and other metal can be removed rapidly from circuit boards without damage to either the photoresist or the substrate, using an aqueous solution of either NO2 or HNO3 as an oxidant.

We have now developed a new and improved process for etching copper and other metals in the manufacture of printed circuit boards and in other applications which overcomes the limitations and disadvantages of copper etcing processes heretofore provided and which is inexpensive and easy to carry out.

Accordingly, the present invention provides a process for etching copper which process comprises the steps of: forming a fim of water on the surface of the copper, and exposing the water-covered copper to gaseous NO2 for a time sufficient to dissolve the copper. In one disclosed embodiment, a film of water is formed on the surface of the metal, and the water-coated metal is exposed to gaseous NO2 to dissolve the metal. In another disclosed embodiment, the metal is exposed to an aqueous solution of NO2 or HNO3 and water, either by immersion or by spraying to remove the metal. In either embodiment, a polymer additive can be employed to prevent undercutting or etching of the metal in a direction parallel to its surface.

In the etching process of the invention, the oxidation of copper takes place according to the following reaction: H20 3Cu + 8HNO3 < 3Cu(NO3)2 +2NO +4H2O (1) which does not proceed in the absence of a suitable catalyst.

Since the reaction of NO2 with water superimposes the relatively complex nitric acid forming chemistry upon the desired copper etching chemistry, it may be helpful in understanding the invention to consider some aspects of the nitric acid chemistry. The overall process by which nitric acid is formed is given in the following equation: 3NO2 + H2O > 2HNO3 + NO, (2) which is the sum of at least three independent steps represented by the following equations: N2O4# oz NO* + NO3# (3) NO + H2O > H+ + HONO (4) HONO + NOP #F# NO + HNO3 (5) It is important to note that all these processes, including reaction (2), are readily reversible.The formation of nitric acid is favored by high NO2 pressures while it is retarded and even destroyed by gaseous NO.

With gaseous NO2 and a copper foil circuit board (with resist patterns), the invention is carried out by covering the board with a thin film of water (e.g., .025" or less for a 1/2 oz./ft2 laminate) prior to exposure to the NO2. The circuit board is exposed to the NO2 at room temperature, and the etching process is found to begin almost immediately and to be substantially complete within 2-3 minutes. At the end of the reaction, the board is covered with a thin film of concentrated copper nitrate which can be readily removed by a number of methods. Since the Cu product is in aqueous solution with nitrate as the only anion, the board can be made free of all residue by a simple water wash because there are no Cu (I) species present.Since it is difficult to form a uniform thin film with pure water, a polymer additive with a surfactant co-promoter is employed to lower the surface tension and increase the viscosity of the medium so that a uniform thin film can be obtained. The additives are selected to promote the anisotropic etching of copper foil so that the desired pattern can be obtained with little or no undercutting of the copper. Suitable polymers include water soluble polyacrylamides such as the cationic Dow Chemical Separan CP-7HS and Hercules Reten 210, the neutral Hercules Reten 520 and the anionic Dow Chemical MG 700. Also included are water soluble poly (acrylic acids) such as Aldrich Chemical #18, 127-7 and Rohm and Haas Acrysol A5 and a carboxymethylcellulose such as Hercules 12M31.Suitable surfactants include DuPont surfactants Zonyl FSC, Zonyl FSN, Zonyl FSP and Zonyl FSK, 3M Fluorad surfactant FC-135, Sherex Adogen 477, and hexadecyltrimethyiammonium bromide.

The following examples demonstrate the use of a water film to catalyze the etching of copper by gaseous NO2, and they also show the influence of the polymer additive on the rate and course of the reaction: Example 1 A 3" x 4" board with a laminated layer of 1/2 oz. copper per ft2 having a test pattern of imaged Dynachem Laminar-ML dry film photoresist was coated with 3.99 of a 0.7% solution of Dow Chemical Separan CP-7HS polymer in water. This composite was then exposed to gaseous NO2 at 23 C for 2 minutes. After an additonal minute, the reaction mixture was washed from the board with a simple water rinse. It was found that over 0.89 of Cu had been removed, esentially all of the exposed Cu in the test pattern area, with virtually no undercutting of the pattern defined by the photoresist.

Example 2 The process of Example 1 was repeated using a test sample having an etch pattern defined by an 4 micron thick layer of Kodak 752 Microresist. Under the same reaction conditions, over 0.8g of Cu was removed in the area defined by the test pattern. The etching action was such that lines having substantially vertical sides were produced, although this sample was slightly undercut compared to that in Example 1.

Example 3 The process of Example 1 was repeated using a similar test panel. This test sample was covered with 4.0g of a 0.35% solution of Separan CP-7HS in water. This sample was exposed to NO2 for 1 1/2 minutes, and the reaction layer was removed by a water rinse after an additional 30 seconds. Again, 0.89 of Cu was removed, substantially all the exposed copper in the test pattern area, with virtually no observed undercutting of the lines delineated by the test pattern.

Example 4 The process of Example 1 was repeated, using a similar test panel. This sample was coated with a 4.1 g layer of a 1% solution of Hercules 12M31 carboxymethyI#celluIose in water. This sample was exposed to NO2 for 2 minutes, and the reaction layer was removed with a water rinse after an additional minute. It was found that 0.39 of Cu was removed from the test area, about 40% of the exposed copper. While the etching of the pattern was incomplete, there was virtualy no undercutting of the lines protected by the photoresist.

With nitrogen dioxide in an aqueous solution, the process looks even more like a nitric acid etching process given the facile equilibria relating H2O, NO2 and HNO3, i.e. equations (2)-(5). Nitric acid is a good oxidant and a strong acid, and it is destructive of organic materials such as photoresist and glass epoxy circuit boards. A system with good oxidizing power and attenuated acidity, i.e. lower HNO3 concentration, should oxidize the copper without destroying the organic components.

Fairly concentrated nitric acid is required for reaction with copper because pure HNO3 in itself is unreactive toward copper. for the reaction of equation (1) to occur, the nitric acid must contain some dissolved nitrogen oxides. Hence, the reaction probably proceeds through NO+ (or NO2* in extremely concentrated HNO3) arising from equations (3), (4) and (5). From these equations, it can be seen that high acid (H+) concentrations will promote high NO concentrations, driving equation (4) backward. This indicates that a H2O, NO2, HNO3 system should be highly acidic in order to be reasonably reactive toward copper.

Unfortunately, such a system will also be reactive toward organic materials.

It has been found that the NO* concentration can be maximized without high acidity by reducing the water concentration and its chemical potential. This is achieved by using a copper salt such as Cu (NO3)2, the copper reaction product. Copper (II) nitrate is extremely soluble in water; about 380g of the salt Cu (NO3)23H2O will dissolve in 100 ml of water at 40 C. Any other salt of copper which is soluble in HNO3 can likewise be used. Suitable salts include CuS04, copper (11) tetrafluoroborate, CuCl2 and combinations thereof. With any of these salts, it is the copper ion Cu''which removes the water molecule from solution and makes it possible to etch with HNO3.

By using concentrated solutions of Cu(NO3)2 in water as the reaction solvent, patterns can be rapidly etched in copper laminate foils without damage to resists or substrates, using either NO2 or HNO3 as the oxidant with both spray and immersion techniques. The added NO2(N2O4) or HNO3 is the sole source of oxidizing power in the system. Hence, facile control of the etching parameters is readily obtained. In contrast to other commercial wet etching processes, the copper substrate is indifferent to the reaction medium (concentrated aqueous Cu(NO3)2) in the absence of added oxidizer.

Due to the rapid interconversion between NO2 and HNO3 in the system chemistry, it is posible to use solutions of HNO3 for copper oxidation. HNO3 is less expensive than pure NO2 and is already in water solution, thus saving the expense of removing the heat of reaction of NO2 with water. This water is absorbed by the reacting copper and is retained in the hydrated salt.

Operation of this process for the etching of copper patterns in circuit board foil is illustrated in the following examples: Example 5 A 3" x 4" board having a 1/2 oz./ft2 copper laminate with a resist pattern formed from Kodak 752 Microresist was sprayed with a mixture of 10 cc of 90% HNO3 and 50 cc of about 0.7% solution of Separan CP-7HS in water, diluted with about 225 cc of a 40% by weight solution of Cu(NO3)2 at 30-35 C. The solution was recycled once. At the end of this time substantially all of the copper was removed from the pattern area with no undercutting of the resist pattern and no damage to the resist or substrate.

Example 6 A 3" x 4" board with a resist pattern formed with Dynachem Laminar-ML film was sprayed with the solution of Example 1 after aging for 24 hours and replenishing with 40 cc of 90% HNO3. The board was sprayed with this solution at a rapid rate and was substantialy cleared of all Cu in the pattern area after 150 cc of solution was used. Again, no resist or substrate damage was noted. The pattern lines were very slightly undercut.

Example 7 The process of Example 6 was repeated with an identical sample except that the same reaction solution was diluted with 25 cc of a 1.4% solution of Separan CP-7HS in water. Again, the copper was substantially removed from the pattern area after use of 150 cc of solution. No resist or substrate damage was observed, and no undercutting of the resist pattern was noted.

Example 8 The solution of Example 7 was replenished with 5 cc of 90% HNO3 and 20 cc of 1.4% Separan CP-7HS after the solution had aged 24 hours. A 3" x 4" board having a resist pattern formed from Dynachem Laminar-ML film was immersed in this solution with agitation. The copper unprotected by the resist pattern was completely removed in less than 2 minutes at 25 C. There was no resist or substrate damage evident, and there was no noticeable undercutting of the resist pattern.

Similar results were obtained when NO2 was substituted for HNO3 in the Cu (NO3)2 solution.

This process is advantageous in that it requires very few chemical components and, thus, affords relatively simple process control. It is particularly suitable for use with thicker copper foils, i.e. foils thicker than 1/2 oz1ft2. Corrosion problems are minimized, and special materials such as titanium are not required for the processing equipment. The chemistry is clean, and the reaction product is extremely stable with no Cu (I) compounds to generate an insolubel sludge. Copper is removed as pure Cu (NO3)23H20 which has some market value itself. If a mixture of Cu (NO3)2 and CuS04 is used as a buffer, the CuSO4#5H2O will precipitate first. The process can be operated as a closed system, thus reducing pollution problems.The process consumes only low cost nitric acid, which is a readily available major commodity chemical, and it runs under mild conditions, i.e. low temperatures. When the etching solution becomes too concentrated in Cu (NO3)2, it can be precipitated from solution by cooling the solution to around 1 00C or by heating it to around 5000.

Other metals which can be etched by this process include vanadium, manganese, iron, cobalt, nickel, palladium, and alloys of these metals such as constantan and Monel. With each of these metals, Cu (NO3)2 can be employed as in the etching of copper to maintain control of the reaction. Alternatively, a nitrate of the metal being etched can be utilized instead of Cu (NO3)2. Thus, for example, Ni (NO3)2 and Mn (NO3)2 can be used in the etching of nickel and manganese, respectively, and Ni (NO3)2 can also be use in the etching of nickel alloys.

Example 9 - Nickel A solution of 500 cc of 50% nickel nitrate (Ni (NO3)2),125 cc of 70% nitric acid (H NO3) and 20 cc of 7% Separan CP-7HS (Dow Chemical polyacrylamide) heated to 4500 was used to etch a 1" x 6" piece of nickel foil. About 0.37g of Ni was removed in 10 minutes; at 500C 0.289 of Ni was removed in 5 minutes.

Example 10-Nickel A solution of 4 liters of 50% Cu (NO3)2 in water, 1.2 liters of 70% HNO3 and 200 cc of a 0.9% solution of Separan MG 700 (Dow Chemical polyacrylamide) was heated to 4500 and used to etch a 1" x 6" piece of nickel foil. About 0.36g of Ni was removed in 1 minute.

Example 11- Constantan The solution, of Example 10 above, was used to etch a 41/2" long piece of 20 gauge constantan (a Ni/Cu alloy) wire. About 0.0689 of material was etched in 1 minute, an additional 2 minutes of reaction removed another 0.1379 of material.

In etching copper circuit boards protected with a lead-tin solder resist, appreciable chemical reaction of the resist can be prevented by the addition of a small amount of phosphoric acid or any other phosphate to the etching solutions disclosed herein, e.g. nitric acid and copper nitrate with a polymer and surfactant. With these additives, there is virtually no undercutting of the solder etch mask, and this offers a significant improvement over existing etch processes used for the production of plated-copper circuit boards.

Moreover, it has been found that the addition of a fluorocarbon phosphate such as Zonyl FSP detergent to the phosphoric acid makes the solder even less reactive, and it is believed that the surface of the resist becomes covered with a lead phosphate.

Example 12 A solution of 3 liters of Cu (NO3)2 in water, specific gravity 1.50 at 2000, 1 liter of 70% HNO3,500 cc of 85% H3PO4, 15 cc of 3M Fluorad FC-135 (a cationic fluorocarbon surfactant), 10 cc of DuPont Zonyl FSP (a fluorocarbon phosphate), and 150 cc of a 1.1% solution of Reten 520 (a Hercules polyacrylamide) was heated to 4000 and used to etch a 4" x 6" panel of copper laminate having a solder etch resist pattern. The 1.4 mil layer of copper not covered by the solder pattern was removed in 3 minutes. Examination of a cross-section of this pattern showed that the copper was removed without noticeable undercutting of the solder resist pattern.

Claims (34)

1. A process for etching copper which process comprises the steps of: forming a film of water on the surface of the copper, and exposing the water-covered copper to gaseous NO2 for a time sufficient to dissolve the copper.

2. A process as claimed in Claim 1 wherein the film of water is formed on the copper by applying a mixture of water and an additive which serves both as a surfactant and as an inhibitor of etching in a direction parallel to the surface of the copper.

3. A process as claimed in Claim 2 wherein the additive comprises a polymer.

4. A process as claimed in Claim 3 wherein the polymer is a water-soluble polyacrylamide, a carboxymethylcellulose or poly(acrylic acid).

5. A process as claimed in Claim 3 wherein the polymer comprises a cationic water-soluble polyacrylamide.

6. A process as claimed in Claim 2 wherein the additive is a cationic surfactant.

7. A process as claimed in Claim 10, Claim 2 wherein a lead-tin solder mask is formed over a portion of the copper, and a phosphate is included in the applied mixture.

8. A process as claimed in Claim 7 wherein the mixture includes both phosphoric acid and another phosphate.

9. A process for removing copper from a printed circuit board having a substrate and etch resist material covering a portion of the copper to be retained, which process comprises exposing the board to an aqueous solution of NO2 or HNO3 in water containing a sufficient quantity of a dissolved copper salt to prevent attack of the substrate or the etch resist material for a time sufficient to dissolve the exposed copper.

10. A process as claimed in Claim 9 wherein the aqueous solution includes a polymer additive which prevents undercutting of the copper as it is etched.

11. A process as claimed in Claim 10 wherein the polymer is a water-soluble polyacrylamide, a carboxymethylcellulose, or poly(acrylic acid).

12. A process as claimed in Claim 10 wherein the polymer comprises a cationic water-soluble polyacrylamide.

13. A process as claimed in any one of Claims 9 to 12 wherein the aqueous solution comprises 0.5-2.0 parts by weight of 10%-100% HNO3 in water, 2.5-10 parts by weight of 0.1-1.0% solution of a cationic water-soluble acrylamide polymer in water, and 11 -44% parts by weight of a 10-60% solution of Cu (NO3)2 in water.

14. A process as claimed in Claim 9 wherein the copper salt is Cu (NO3)2, CuS04, copper (II) tetrafluoroborate, CuCI2 or a combination thereof.

15. A process as claimed in any one of Claims 9 to 14 wherein a lead-tin solder mask is applied to a portion of the copper, and the aqueous solution includes a phosphate.

16. A process as claimed in Claim 15 wherein the aqueous solution contains phosphoric acid and another phosphate.

17. A catalyst/solvent comprising a mixture of water and an additive which serves as a surfactant and as an inhibitor of etching in a direction parallel to the surface of the copper for use in the etching of copper with gaseous NO2.

18. A catalyst/solvent as claimed in Claim 17 wherein the additive comprises a polymer.

19. A catalyst/solvent as claimed in Claim 18 wherein the polymer is a water-soluble polyacrylamide, a carboxymethyl-cellulose or poly(acrylic acid).

20. A catalyst/solvent as claimed in Claim 18 wherein the polymer comprises a cationic water-soluble polyacrylamide.

21. A catalyst/solvent as claimed in Claim 17 wherein the additive includes a phosphate.

22. A catalyst/solvent as claimed in Claim 17 wherein the additive includes phosphoric acid and another phosphate.

23. A process for etching a metal which comprises exposing the metal to an aqueous solution of NO2 or HNO3.

24. A process as claimed in Claim 23 wherein the metal is copper, vanadium, manganese, iron, cobalt, nickel, palladium, or an alloy thereof.

25. A process as claimed in Claim 23 wherein the nitrate contained in the solution is Cu (NO3)2.

26. A process as claimed in Claim 23 wherein the nitrate contained in the solution is a nitrate of the metal being etched.

27. A process as claimed in Claim 23 wherein a lead-tin solder mask is formed on the metal, and the aqueous solution contains a phosphate.

28. A process as claimed in Claim 27 wherein the aqueous solution contains phosphoric acid and another phosphate.

29. A process as claimed in Claim 1 substantially as hereinbefore described with reference to any one of Examples 1 to 4.

30. A process as claimed in Claim 9 substantially as hereinbefore described with reference to any one of Examples 5 to 8.

31. A process as claimed in Claim 23 substantially as hereinbefore described with reference to any one of Examples 9 to 11.

32. A copper substrate whenever etched by a process as claimed in any one of Claims 1 to 8 or 29.

33. A printed circuit board whenever treated by a process as claimed in any one of Claims 9 to 16 or 30.

34. A metal whenever etched by a process as claimed in any one of Claims 23 to 28 or 31.