CA1050865A - Alkaline compositions and process for etching copper - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Alkaline etchant solutions for dissolving copper at a controlled relative slow rate, the solution comprising cupric ions as oxidant for metallic copper, sulfamate ions for accelerating appreciably the etching of the copper, and ammonium ions for main-taining the pH of the etchant on the alkaline side of pH and also for complexing dissolved ionic copper. The etchant solutions are especially well adapted for selective etching away of unwanted copper in the manufacture of additive circuit boards.

Description

~05~865 This invention relates to etching copper and more es-pecially to new and improved alkaline etchant solutions and pro-cesses for dissolving copper.
United States Patent 3,505,135 discloses the steady ~tate etching of copper cladded printed circuit boards utilizing an ammonium persulfate ~olution having an initial predetermined molar concentration as etchant. Copper is removed from solution at the rate in which it is dissolved therein and etching reagent consumed during the etching proces~ is replaced at an equivalent rate.
United States Patent 3,705,061 relates of a continuous redox pro-cess for dissolving copper. An alkaline etch solution containing cupric ions as oxidizer is used to oxidize and thereby dissolve copper with attendant reduction of the cupric ions to the cuprous state. A portion of the etch solution is continuously removed from ~he system, and a source of oxygen and an ammonia based re-plenishing solution are added to the etch solution to recons~itute it. The replenishing solution requirements of the system are determined by continuously monitoring the specific gravity of the etch solution and the pH with respect to ammonium content. United States Patent 3,466,208 discloses alkaline ammoniacal chlorite solutions for dissolving copper and containing one or both of ammonium chloride and ammonium nitrate in full or partial substi-tition of ammonium bi~arbonate used ln a prior system. United States Patent 3,677,950 discloses a chemical etchant solution for dissolving copper from printed circuit boards and consisting of an a~ueous ammoniacal solution of a normally acidic oxidizer such as ammonium persulfate, cupric chloride or cupric sulfate, the solu-tion having an alkaline pH up to about ll. United States Patent 3,650,957 relates to etching solutions for stripping copper from substrates and containing a source of cupric ions as an oxidant, one or more complexing agents for the cupric ions which is capable of forming a soluble copper (II) complex at solution p~, a source o~ bromide or chloride ions, and molybdenum, tungsten or vanadium .. .

9865 ~ ~
ions for retarding attack on tin and tin-lead solder plate, the solutions havin~ a pH between about 4 and 13. United States Patent 3,650,958 discloses etchant solutions for copper containing a source of cupric ions, and one or more non-fuming complexing agent~
for the cupric ions, the complexing agent b~3ing capable o forming a soluble copper (II) complex at ~olution pH. United States Patent 3,650,959 discloses etchant solutions for copper, containing a source of cupric ions, one or more non-fuming complexing agents ~ for cupric ions which is capable of forming a soluble copper (II~
; 10 complex at solution pH, chloride or bromide ionsl and nitrate ions in amount sufficient to prevent build-up of a film on the surface of the copper being etched. United States Patent 3,367,874 per-tains to acid baths for dissolving nickel, copper, zinc, cobalt or iron and containing concentrated nitric aaid, and a mixture of additives capable of providing urea in the bath and capable of forming sulfamate ions in the bath. United States Patent 3,537,895 discloses cleaning-pickling solutions for removing copper oxides ~` from copper, the solutions being aqueous solutions containing sulfuric acid, sulfamic acid, or an equivalent acid salt thereof for dissolving cupric oxide from the surface of a workpiece, hydro-gen peroxide, and an organic stabilizing compound such as an organic compound from the class of benzoic, glycolic or propionic acids, glycerine, ethylene and propylene glycols.
The prior art etchan~ solutions are in general not suit-able for ekching away copper in the ma~ufacture of additive-type - printed circui~ boaxds. The reasons for this is the prior art etchant solutions etch copper at too fast a rate and the etching rate fluctuates excessively. In additive type prinked circuit board manufacture a thin copper film is elec~rolessly deposited over the entire dielectric board from an elec~roless copper plat-ing solution, as contrasted with the relatively thick copper foil bonded to the board in subtractive-type printed circuit board manufackure. In ~electively etching away unwanted copper in addi-~0508~5 tive-type printed circuit board manufacture, it i8 important to etch away the unwanted copper at a rela~ively slow rate to avoid etching away the entire circuit, which occurls with u~e of the prior art etchant solutions having the fa3t or high speed etching rate.
Furthermore, use of the prior art etchant solutions having the fast etching rate for etching copper in additive-type circuit board manufacture results in undesirable undercutting o~ ~he circuit lines.
One object of the invention is ~o provide new and im-proved etchant solution for dissolving copper.
Another ob]ect is to provide new etchant solution~ e~-pecially adapted for use in selectively~etching away copper in additive-type printed circuit board manufacture.
A further object is to provide new etchant soIutions characterized by etching copper at a desired controlled ~low rate.
A further object is to provide new etchant solution~
capable of etching away unwanted copper in the preparation o~
additive-type printed circuit boards without any substantial ,:
, undercutting of the circuit lines or pattern.
; 20 Still another object is to provide a new and improved process for dissolving or etching away copper.
Additi~nal objects and advantages will be readily apparent as the invention is hereinafter described in more detail.
The aforementioned objects and advantages are attained in accordance with the present invention by an alkaline etchant solution comprising essentially sulfamate ions for accelerating appreciably the etching or dissolving of the copper, cupric ion~
as an oxidant for the copper, and ammoniu~ ions for maintaining ;~
the solution pH on the alkaline æide of p~ 7.
The etchant solutions of this invention i~ characterized by the highly desirable property that its ~tripping rate for stripping or etching copper can be either increased or decrea-~ed (although still faster than in the ab3ence of the sulfamate ion~) 105(~865 at any given copper content of the solution merel~ by var~ing the sulfamate ion concentration of ~he solution. Thu~ at any given copper concentration of the solution, the rate of stripping or dissolving copper can be increased merely by increasing the con-centration of sulfamate ions in the solution, or the stripping rate for copper can be lowered ~imply by having a lower concentra-tion of sulfamate ions ,n the solution albeit still an appreciably faster stripping rate than in the absence of the sulfamate ions.
The cupric ions are supplied in the etchant solutions of this invention by cupric carbonate or cupric ammonium sulfate ~Cu-(NH~)2(S04)2). Only these two cupric salts of the plurality of -~copper salts tested resulted in the desired slow rate o etching the copper by the etc~ant solution.
Cupric chloride, cupric nitrate or cupric acetate were ~ound to be unsatis~actory in the etchant solutions due to re~ult-ing in the etchant solution etching away the copper at an undesir-able fast rate, which was attributed to the anion of these cupric salts. Cupric sulfate was also found to be unsatisfactory in the etchant ~olution due to forming a slu~ge in the etching machine after only a limited tlme, and resulting in clogging of the spray nozzles of the machine.
The sulamate ion are supplied in the etchant 301utions ; o thi~ invention by any suitable source of sulfamate ion. Exem-plary of the sulfamate ion source is sulfamic acid, ammonium sulfamate, and an alkali metal sulfamate, e.g. sodium or potassium suLfamate.
Any suitable source of ammonium ion i9 utilizable in the e~hant solutlons herein. As exemplary, ammonium hydroxide, ammonlum sulfamate, ammonium chloride, ammonium ni~ra~e, ammonium phospha~e, ammonium carbonate, ammonium citrate, and NH3 ga~ which forms in situ in the etchant solution NH4+ ions are utili~able.
The sulfamate ions (calculated as OSOaN~) are present in the etchant solutions of this invention in an effect:Lve amount, ~05~365 ~
which is sufficient to accelera~e appreciably ~he etching or dis-solving of the copper. The sulfamate ion~ may be present in the etchant solutions herein in an effective amount of at leas~ 0.2 mole per liter. The amount of sulfamate ions utili~ed in tha etchant solutions herein should be insufficient to result in a~
uncontrollable, fast etching rate, and a fast etching rate which results in significant undercutting of the circuit lines or pattern in the preparation of additive-~ype printed circuit boards.
The cupric ions may be present in the etchant 301utions herein in an amount (calculated as Cu~ of 0.2 mole per liter to ~ -saturation.
The ready-to-use e~chant solutions o$ this invention will u~ually contain the constituents in amounts wI~hin the following proportion ranges: ;
Sulfamate ions (calculated as OS02N~
from about 0.3 to 1.0 mole per li~er Cupric ions (calculated ~g Cu)--from about 0.3 to 1.0 mole per liter Ammonium ions (calculated a~ NH4)--sufficient to maintain pH in the range of about 8 to about 12.
The etchant solutions herein are u~ually supplied as a concentrate which is adapted to be mixed together with an aqueaus liquid, such as waterl to ~orm the ready-to-use etchànt ~olution.
A source of ammonium ion may also be mixed together with the con-centrate solution and water. The concentrate solutio~s are u~ually aqueous solutions containing cupric ions, sulfamate ions and ammonium ions. The etchant concentrate ~olution~ of course contain a les~er amvunt of water than do the ready to-use etchant solution~
The concentrate solution is mixed togethex with water by the usqr to form the ready-to-use etchant solution typically in a volume ratio of concentrate solution to water of about 1:3 respectively.
The etchant concentrate solutlons will usually contain the ~onstituents in amounts within the following proporti.on ranges:

... . .

SulEamate ions (calculated a~ OSO2NH )--from about 0.3 to 3.0 moles per l~ter Cupric ions ~calculated as Cu)--from about 0.3 to 3.0 moles per liter Ammonium ions ~calculated as NH4)--from about 1 to 18 moles per liter The kemperature of the etchant solutions herein during the dissolution or etching of the copper can be room temp~ra~ure or elevated temperatures up to about 135F. The etching rate in- ;
creaqes with increasing solution tempera~ure. ~emperatures of the etchant solution much above 135F. should be avoided, due to expelling ammonia at Ruch higher temperatures.
The ionic copper concentration of the etchant solution during the etohing process can be determined by any suitable means ; or method. Thus the determination of the ionic copper aoncentra-tion, such as the cupric ion concentration can be effected by batch or manual means or method or by continuou or automatlc means or method. One suitable means and method for the oontinuous deter-mination of the ionic copper concentration of the et~hant solution is that disclosed and claimed in United States Patent 3,70$/061 and involvlng sensing the specific gravity of the solution, to generate a signal when the concentration of i~nic copper reaches a predeter-mined maximum. Exemplary of a batch or manual means and method for determining the ionlc copper concentration is titration.
When required, the sulfamate ions are replenished in ,. ~
the etchant solutions herein. The replenishment of the sulfamate ionic in the solution is conveniently effected by adding a re-plenisher solution containing the sulfama~e ions, ammonium ions and water to the etchant solution. The replenisher solution i5 ,: , exemplified by an aqueous solution of the ollowing compo~ition:
Sulfamlc acid.............. ~....... 290 g/l ;~ Ammonium hydroxide......... O....... S00 g/l H2o... ~............. q......... ~........ to 1 liter ) The designation "g/l" herein means grams per liter. ;The addition ~0~0~6S
of this replenisher concentrate al~o serves to maintal~ the pH o~
the solution within the desired alkaline pH range due to the ammonium ion~
- The followlng example~ of ready-to-use et~hant ~olutlon~
of this inven~ion for dl~solving copper are illu~tratlv~ only:
: EXAMPLE I
Sulfamic acid (calculated a~ sulfamate, i-e- OSO2NH2)~ ~---......... 0.3 mole/liter Cuprlc carbonate ~calculated as Cu),........................ 0.3 mole/lltor - ~mmonium hydroxid~............. sufficient to malntai~
p~ in th2 range of about ~i ; 11-12 -~ -H2O............................ to 1 liter ~ :
- EXAMP~E~
Sulfamic aaid (calaulated as sulfamat~, ;
i-e- OSO2NH2) -----......... 0,5 mole/liter , Cupric carbonate (calculated a3 CU)D~ O~6 mole/liter A~unonium hydroxide............ ~ufflcien~ to maintain -; pH in the range of about ; 11-12 H~O.............. ,................................. to 1 liter EXAMPLE III
Sulfamia acid ~calculated a~ sul~amate, 2N~2) .. ~ ...... ~ ......................... 1. 0 mole/liter Cupric carbonate ~calculated as Cu)............. ........ .... ....................... l.0 mole/liter Ammonium hydroxide............. 3ufficient ~o maintain p~ in the range of about . H2O............................ to 1 liter EXAMPL~ IV
:; Sulfamic a~id ~calcula~ed a~ ~ul~mata, i.e.
02NH~ ) ~ D ~ O ~ 5 mole/lit~r :~
Cupric carbonate tcalculated a Cu)....... ~........................................ 0.3 mole/liter ~mmonium hydxoxide................................. sufi~ient to mainta~n pH in ~he range o~ about :~
`~ 40 11-12 ~:

: H2O....... O................................... to 1 lit~r .

~(~8~;S
The following examples of etchant concentrate solutions o~ this invention, which are adapted to be mixed togcther with water to form the ready-to-usc concentrate solutions, are illu~
~ra~ive only.
EXAMPLE V
Sulfamic acid (calculated as sulfamate, i.a.
O2NH2) -- -~ ............. 0.6 mole/liter Cupric carbonate (calculated as Cu~......................... 0.~ mola/liter Ammonium hydroxide........ ...... 300 g/l H2O............................ to 1 liter This aqueous concentrate solution is mixed together wi~h wat~r in the volume ratio of 1:1 re~pectively to form the ready-to-use etchant solution. Ammonium hydroxide is also added to adjust the pH of the resulting solution to within the pH range of about 11 ~o :;
12.
E~AMPLE VI
Sulfamic acid (calculated as sulfamate, i.e. OSO~NH2).......... 0.6 mole/liter Cupric carbonate (calculated as Cu)..... ~................................. 0.8 mole/liter Ammonium hydroxide........................ 300 g/l H2O....................................... to 1 liker This a~ueous concentrate solution is mixed together with wate~ in the volume ratio o 1:1 respectively to form the ready-to-u~e etchant solution. Ammonium hydroxide is also added to adjust the pH of the resulting solution to within ths pH range of about:ll to .l2.
EXAMPhE VII
Sulfamic acid ~calculated as sulfamate, OSO2NH2) o ---~ 0.9 mole/liter Cupric carbonate (calculated as Cu~.. O........ O.......... .......... O.g mole/liter Ammonium hydroxide....... .......... 500 g~l H2O............................... to 1 liter This concentrate solution is mixed together with water ln the 1~5~865 volume ratio of 1-3 respectively to form the ready-to-use etchant solution. Ammonium hydroxide is al90 added to the solution mixture to adjust the pH of the resulting solution to within the pH range of about 11 to 12.
EXAMPLE VIII
Sulfamic acid (calculated as sulfamate, i.e.
OS02NH2).................... 3.0 moles/liter Cupric carbonate (calculated as Cu)................ 0........ O.~ moles/liter Ammonium hydroxide.... ,........ 500 g/l H20....................... ..... to make 1 liter This aqueous concentrate solution is mixed together with water in the volume ratio of 1:3 respectively to form the ready-to-u~e ~-etchant solution. Ammonium hydroxide is also added to adjust the p~ of the resulting solution to withln the pH range of about 11 to 12.
The following tests were carried out:
The sulfamate ion concentration was varied by varying the sulfamic acid concentration in a plurality of solutions each initially containing 3 oz./gal. cupric ions (calculated as Cu) and NH40H suficient to adjust the pH to 11. The etchant solutions were each at a temperature of 75~C., and the solutions were used to etch copper ~rom ABS panels. The copper was immersed in the solutions in separate beakers. The following results were o~tain- ;
edo TABLE I
Etching Rate Sulfamic Acid Minutes to Etch 25 Concentratlon Micro-inches of Cu 0.1 M No etching (blackened Cu) 0.3 M 5.0 0.5 M 3.5 1.0 M 2.5

2.0 M 2.0 The test results of Table I show that at a sulfamic acid concen-tration of 0.3 moles, the fastest etching rate occurred, and that at progressivaly higher sulfamic concentrations up to 2.0 mole , _g_ .

~ ~ . . . .

~ L~5086~
that etching of ~he copper occurred but at a progressively slower rate~
The temperature of a plurality of solutions was varied to determine the effects on the rate o~ etching copper~ A plural-ity of separate solutions were made up init:Lally containing 3 oz./-gal. Cu metal as CuCO3-Cu(OH)2, 0.3 M sulfamic acid and 150 g/l NH40H. Each solution had a p~ of 11. The ~opper wa~ immer~ed in the solutions in separate beakers in carrying out the tests. The following results were obtained.
TABLE II
Temperature Etching Rate of Minutes to Etch 25 Solution Micro-inches Q~ Cu 80F. 4.25 90F. 3,75 100F. 3~75 lZ0F.
140F. ~oss of NH40H
20 The test results of Table II show that the best etching rate occur-red at room temperature or 70F., and that satisfactory e~ching `
rates were attained at elevated temperatures above 70F. The te~t results of Table II also show tha~ a temperature of 140F. resulted in loss of ~H40H from the etchant bath, which was undesirable.
A plurality of etchant solutions containing different concentrations of cupric ions and sulfamic acid ~s set ~orth in th~
,I table of test results which ~ollows and also NH40H in amount suffi-cient to adjust the pH of each solution to about 11.0, were utiliz-;:
ed to etch copper from glas~-epoxy printed circuit boards in a con ventional etching machine. The etchan~ solutions were each at a temperature of 75F. during the spray application to the copper in the etching machine. The following results were obtained, with the results being given hereafter in terms of the micro-inche~ of copper etched per minute: ;

. ~ . --10--~. ~

-:l~)S~ 365 Sulfamic Acid Content ofCu Metal Content of Etchant Solution Etchant Solution(oz. Cu/~al of Etchant Solution)

3 4 5 6 7 9 0.3 M 100 110 115 120 115 lQ0 0.5 M 145 160 170 165 160 1~'5 0.7 M 170 190 230 220 205 140 1.0 M 200 4S0 600 650 600 1~l0 Cupric carbonate is the preerred source of cupric ions.
Sulfamic acid and ammonium sulfamat:e are the preferred source of sulfamate ion.
Ammonium hydroxide is the preferred source of ammonium ions.
The p~ o the etchant solutions is preferably in the pH range of ab~ut 11 to about 12.
Preferably the ready-to-use etchant solutions herein contain the following constituents in amounts within the proportion ranges hereafter cet forth:
Sulfamate ions (calculated as OS02NH2)--from ahout 0~3 to 1.0 mole per liter Cupric ions (calculated as Cu3--from about 0.3 to 1.0 mole per liter Ammonium ions (calculated a (NH )--sufficient to maintain pH in ~he range of about 11 to about 12 Especially preferred ready-to-use copper etchant solu-tions o~ this invention contain the ~ollowing constituents in amounts within the proportion ranges hereafter sek forth:
Sulfamic acid.............. ~.......... 0.3 molar to 1.0 molar Cupric ions (calculated as Cu).. 22.5 to 67.5 g/1 Ammonium hydroxide.............. l50 to 180 g/l Such solution has a pH in the range of about 11 to about 12.
The etchant solution is preferably at a temperature in the range of about 70F. to about 135F. during the etching.
Although the etchant solutions of thi~ invention are eminently adapted fox selectively etching copper in khe produc-tion of additive-type printed circuit boards, these et:chan~ solu-l~S~ ;5 solution~ are al~o utilizable in ~he preparation of ~ubtractive~ ;
type printed circuit board~ or for ~bher copper etchlng or dis-~olving purposes.

:

: `
ol2--. .

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An alkaline copper etchant solution comprising cupric ions supplied by cupric carbonate or cupric ammonium sulfate as an oxidant for copper, sulfamate ions for accelerating the etching of the copper, and ammonium ions for maintaining the pH of the etchant solution on the akaline side of pH 7, said cupric ions being present in an amount of at least 0.2 mole per liter and said sulfamate ions (calculated as OSO2NH2) being present in an amount of at least 0.2 mole per liter but insufficient to result in an uncontrollable fast etching rate.

2. The etchant solution of claim 1, wherein the pH
is maintained in the range of about 8 to about 12.

3. The etchant solution of claim 2, wherein the pH
is maintained in the range of about 11 to about 12.

4. The etchant solution or claim 1, wherein the source of sulfamate ions in the solution is at least one sulfamate ion source selected from the group consisting of sulfamic acid and ammonium sulfamate.

5. The etchant solution of claim 2, wherein the source of sulfamate ions in the solution is sulfamic acid.

6. The etchant solution of claim 2, wherein the sulfamate ions (calculated as OSO2NH2) are present in an effective amount in the range from about 0.3 to 1.0 mole per liter.

7. The etchant solution of claim 1, wherein the con-stituents are present in amounts within the proportion ranges hereafter set forth:

Sulfamate ions (calculated as OS02NH2) --from about 0.3 to 1.0 mole per liter Cupric ions (calculated as Cu) --from about 0.3 to 1.0 mole per liter Ammonium ions (calculated as NH4) --sufficient to maintain pH in the range of about 8 to about 12

8. A process for dissolving copper from a copper-containing substrate which comprises contacting the copper-containing substrate with the etchant solution of claim 1, and maintaining the copper-containing substrate in contact with the etchant solution until the copper is dissolved in said solution.

9. The process of claim 8, wherein the temperature of the etchant solution is in the range of room temperature to about 135°F.

10. The process of claim 8, wherein the ionic copper concentration of the etchant solution is determined during the process.

11. The process of claim 8, wherein the pH of the etchant solution is maintained in the pH range of about 8 to about 12 by addition of a source of ammonium ions to the solution.

12. The process of claim 11, wherein the ammonium ion source is ammonium hydroxide.