GB2175855A

GB2175855A - Aqueous solution and gaseous-aqueous combination for etching copper

Info

Publication number: GB2175855A
Application number: GB08615381A
Authority: GB
Inventors: Norvell John Nelson
Original assignee: Psi Star; PSI Star Inc
Current assignee: Psi Star; PSI Star Inc
Priority date: 1983-06-06
Filing date: 1986-06-24
Publication date: 1986-12-10
Also published as: GB8413067D0; DE3421064A1; DE3420309A1; GB2143778A; GB2175855B; CA1221896A; FR2547834A1; GB8615381D0; GB2143778B

Abstract

For etching copper as in the manufacture of printed circuit boards an aqueous solution comprising NO2 or HNO3, a dissolved copper salt, and an additive which serves to inhibit undercutting of the copper as it is etched is provided. Alternatively a combination of gaseous NO2 and an aqueous solution of an additive which serves to inhibit undercutting for etching copper is provided.

Description

1 GB2175855A 1

SPECIFICATION

Aqueous solution and gaseous-aqueous combination for etching copper This invention relates generally to the etching of copper, and more particularly to an aqueous 5 solution and to a gaseous-aqueous combination for removing copper 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 10 compared to the wet etching processes currently in use. It uses a simpler chemistry having fewer prodess variables, it is less corrosive and thus permits the use of standard materials for processing equipment, and it 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 commer- 15 cialization. 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 is that a rapid and uncontrolled reaction between N02 and the organic solvent mixture is energetically favored, although the reaction of metals and NO, in organic solvents has been studied extensively without mishap.

In addition, the reaction of copper metal with N02 is highly exothermic, producing an excess of 25 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 incrases 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 with nitrogen dioxide in the manufacture of printed circuit boards. This discovery is surprising and unexpected because N02 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 can be removed rapidly from circuit board without damage to either the photoresist or the substrate, using an aqueous solution of either NO, or HNO, as an oxidant.

Our co-pending patent application Ni. 8413067 discloses a new and 9improved 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 etching processes heretofore provided and which is inexpensive and easy to carry out.

The present inventiion provides an aqueous etching solution and a combination of gaseous NO, and an aqueous solution for carrying out the processes described in our co-pending patent application No. 8413067.

Accordingly one embodiment of the present invention provides an aqueous solution for etching copper comprising NO, or HNO,, a dissolved copper salt and an additive which serves to inhibit 50 undercutting or etching of the copper in a direction parallel to its surface.

An alterantive embodiment of the present invention provides a combination for etching copper comprising gaseous NO, and an aqueous solution of an additive which serves to inhibit under cutting of the copper It is possible, in either embodiment of the invention to include additionally a surfactant. 55 In the etching process which occurs when the etching solution or combination described hereinbefore is contacted with copper, the oxidation of copper takes place according to the following reaction:

HO 60 30+8HNO3 3Cu(N03)2+2NO+4H,0 (1) which does not proceed in the absence of a suitable catalyst.

Since the reaction of N02with water superimposed the relatively complex nitric acid forming chemistry upon the desired copper etching chemistry, it might be helpful in understanding the 65 GB2175855A 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:

3N02=H,O;F.t2HNO,+NO, (2) which is the sum of at least three independent steps represented by the following equations:

N204;--NOI+NO,- (3) NO,H20:u-tH'+HONO (4) 10 HONO+NO2:p_+HNO, (5) It is important to note that all of these processes, including reaction (2), are readily reversible.

The formation of nitric acid is favored by high NO, pressures while it is retarded and even 15 destroyed by gaseous NO.

When the combination of gaseous N02 and the aqueous solution of the additive which serve to inhibit undercurrint, i.e. the second embodiment of the invention, is used for etching a copper foil circuit board, the copper foil circuit board is covered with a thin film of the aqueous solution (e.g.,.025" or less for a 1/2 OZ./ft2 laminate) prior to exposure to the N02. The circuit board is 20 exposed to the N02 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 withb nitrate as the only anion, the board can be made free of all residue by a simple water wash because there 25 are no Cu(1) species present.

A surfactant may be employed to lower the surface tension and increase the viscosity of the medium so that a uniform thin film can be obtained as it may be difficult otherwise to form a film. The additives to inhibit undercutting 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 carpoxymethyl-cellulose such as Hercules 12M31. Suitable surfactants include DuPont surfactants Zonyl FSC, Zonyl FSN, Zonyl FSP and Zonyl FSK, 3M Flourad surfactant FC-135. Sherex Adogen 477, and hexadecyltri methyl-ammonium bromide.

The use of the combination of gaseous NO, and the aqueous solution of additive to inhibit undercutting of the present invention for etching copper is illustrated below in the following examples. They also show the influence of the polymer additive on the rate and course of the 40 reaction:

Example 1

A 3"X4" 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 3.9g of a 0.7% solution of 45 Dow Chemical Separan CP-7HS polymer in water. This composite was then exposed to gaseous NO. at 23'C for 2 minutes. After an additional minute, the reaction mixture was washed from the board with a simple water rinse. It was found that over 0.8g of Cu had been removed, essentially 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 0.8 g 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.Og of a 0.35% solution of Separan CP-7HS in water. This sample was exposed to N02 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.

3 GB2175855A 3 Example 4

The process of Example 1 was repeated, using a similar test panel. This example was coated with a 4.1g layer of a 1% solution of Hercules 12M31 carboxymethyl- cellulose in water. This sample was exposed to N02 for 2 minutes, and the reaction layer was removed with a water rinse after an additional minute. It was found that 0.3g of Cu was removed fromthe test area, about 40% of the exposed copper. While the etching of the pattern was incomplete, there was virtually no undercutting of the lines protected by the photoresist.

When. the aqueous solution of nitrogen dioxide or nitric acid, a copper salt and the undercutt ing inhibitor, ie. the first embodiment of the invention, is used to etch copper the resulting process looks even more like a nitric acid etching process given the facile equilibria relating H20, N02 and HNO,, 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 HN03 concentration, should oxidize the copper without destroying the organic components.

Fairly concentrated nitric acid is required for reaction with copper because pure HNO, 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 I (or NO, in extremely concentrated HNO) arising from equations (3), (4) and (5). From these equa 2 tions, it can be seen that high acid (H 1) concentration will promote high NO I concentrations, driving equation (4) backward. This indicates that a H20, N02, HNO, 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 25 salt such as Cu(N03)21 the copper reaction product. Copper (11) nitrate is extremely soluble in water; about 380g of the salt Cu(N03)2.3H20will dissolve in 100ml of water at 40C. Any other salt of copper which is soluble in HNO, can likewise be used. Suitable salts include CuSO,, copper (11) tetraf luo rob orate, CUC12 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 30 with HN03' By using concentrated solutions of Cu(NO,)2 in water as the reaction solvent, patterns can be rapidly etched in copper laminate foils without damage to resists or substrates, using either N02 or HN03 as the oxidant with both spray and immersion techniques. The added N02 (N201) or HN03 is the sole source of oxidizing power in the system. Hence, facile control of the etching parameters is readily obtained. In contrast to other substrate is indifferent to the reaction medium (concentrated aqueous Cu(N03)2) in the absence of added oxidizer.

Due to the rapid interconversion between NO, and HNO, in the system chemistry, it is possible to use solutions of HNO, for copper oxidation. HN03 is less expensive than pure N02 and is already in water solution, thus saving the expense of removing the heat of reaction of N02with 40 water. This water is absorbed by the reacting copper and is retained in the hydrated salt.

The use of the aqueous solution, the first embodiment of the invention, is illustrated in the following examples, for the etching of copper patterns in circuit board foil:

Example 5

A 3"X4" board having a 1/2 OZ./ft2 copper laminate with a resist pattern formed from Kodak 752 Microresist was sprayed with a mixture of 10cc of 90% HNO, and 50cc of about 0.7% solution of Separan CP-7HS in water, diluted with about 225cc of a 40% by weight solution of Cu(N03)2 at 3035'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 50 damage to the resist or substrate.

Example 6

A 3"X4" board with a resist pattern formed with Dynachem Laminar-ML film was sprayed with the solution of Exampole 1 after aging for 24 hours and replenshing with 40cc of 90% 55 HNO,. The board was sprayed with this solution at a rapid rate and was substantially cleared of all Cu in the pattern area after 150cc 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 25cc of a 1.4% solution of Separan CP-7HS in water. Again, the copper was substantially removed from the pattern area after use of 150cc of solution. No resist or substrate damage was observed, and no undercutting of the resist pattern was noted.

GB2175855A 4 Example 8

The solution of Example 7 was replenished with 5cc of 90% HNO,' and 20cc of 1.4% Separan CP-7HS after the solution had aged 24 hours. A 3"X4" board having a resist pattern formed from Dynchem 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 25C. There 5 was no resist or substrate damage evident, and there was no noticeable undercutting of the resist pattern.

Similar results were obtained when NO, was substituted for HNO, in the Cu(NOJ, solution.

The use of aqueous solution of HNO,, a copper salt and an undercutting inhibitor is advan tageous in that it requires very few chemical components and, thus, affords relatively simple process control. The process is particularly suitable for use with thicker copper foils, i.e. foils thicker than 1/2 OZ.ft2. 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 insoluble sludge. Copper is removed as pure Cu(NO3),.3H20which has some market value itself. If a mixture of Cu(NOJ2 and CuSO, is 15 used as a buffer, the CuSO,.5H,O 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(N03)21 it can be precipitated from solution by cooling the solution to around 10'C or by heating it to around 50"C.

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 9

A solution of 3 liters of Cu(N03)2 in water, specific gravity 1.50 at 20'C, 1 liter of 70% HN031 500cc of 85% H,PO,, 15cc of 3M Fluorad FC-135 (a cationic fluorocarbon surfactant), 10cc of DuPont Zonyl FSP (a fluorocarbon phosphate), and 150cc of a 1.1% solution of Reen 520 (a 35 Hercules polyacrylamide) was heated to 40'C and used to etch a 4"X6" 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

1. An aqueous solution for etching copper comprising NO, or HNO,,, a dissolved copper salt, and an additive which serves to inhibit undercutting of the copper as it is etched.

2. An aqueous solution as claimed in Claim 1 which additionally includes a surfactant.

3. An aqueous solution as claimed in Claim 2 wherein the surfactant comprises a polymer. 45

4. An aqueous solution as claimed in any one of the preceding claims wherein the undercutt ing inhibitor additive comprises a polymer.

5. An aqueous solution as claimed in Claim 4 wherein the polymer is a water-soluble polya crylamide, a carboxymethyl-celiulose or poly(acrylic acid).

6. An aqueous solution as claimed in Claim 5-wherein the polymer comprises a cationic 50 water-soluble polyacrylamide.

7. An aqueous solution as claimed in any one of the preceding claims wherein the additive includes a phosphate.

8. An aqueous solution as claimed in any one of the preceding claims wherein the additive includes phosphoric acid and another phosphate.

9. A combination for etching copper comprising gaseous N02 and an aqueous solution of an additive which serves to inhibit undercutting of the copper at it is etched.

10. A combination as claimed in Claim 9 wherein the aqueous solution additionally includes a surfactant.

11. A combination as claimed in Claim 10 wherein the surfactant comprises a polymer. 60

12. A combination as claimed in any one of Claims 9 to 11 wherein the undercutting inhibitor additive comprises a polymer.

13. A combination as claimed in Claim 12 wherein the polymer is a watersoluble polyacry lamide, a carboxymethyl-cellulose or poly(acrylic acid).

14. A combination as claimed in Claim 13 wherein the polymer comprises a cationic water- 65 GB2175855A 5 soluble polyacrylamide.

15. A combination as claimed in any one of Claims 9 to 14 wherein the additive includes a phosphate.

16. A combination as claimed in any one of Claims 9 to 15 wherein the additive includes 5 phosphoric acid and another phosphate.

17. An aqueous solution as claimed in Claim 1 substantially as hereinbefore described with reference to any one of Examles 5 to 9.

18. A combination as claimed in Claim 9 substantially as hereinbefore described with reference to any one of Examples 1 to 4.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1986, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.