I
E. 8. SAUBESTRE' Sylvania Research Laboratories, Division of Sylvania Electric Products, Inc., Bayside, N. Y.
Copper Etching in Ferric Chloride Study of the chemistry of ferric chloride etchants shows that cupric ion is as important as ferric ion in etching copper. The slowing down of etching rate with age is explained
As 1898, Peters (76) found that the oxidation potential of the ferrous E A R L Y AS
ion in chloride solutions depends on the amount of free chloride present. This was later verified (4, 78) and ascribed to complex formation between ferric and chloride ions :
anions is that ferric ion in 12,M hydrogen chloride solutions is extractable lvith ether (7: P ) . .ilso, ferric chloride in the gaseous state is dimeric-i.e., a bridged structure occurs (75) :
c1 C1 c1 \ / \ /
+
Fe
Fe(H?0)6++ = Fe(Hz0)6+++ e E a = -0.771 volt
+
+
Fe(HzO)6+t el- = Fe(H20)6C1++ HzO f e E o = -0.682 volt T h e relative concentration of various ion species in chloride solutions can be de termined from the followins equilibria :
+ C1-
FeC13 (aq.) = Fe(H20)4Clz+
K FeCI3 (aq.)
=
=
10
+
Fe(H20)6C1++ 2CIK = 2.3
FeCI3 (as.)= Fe(HzO),+++
+ 3C1-
Fe
'I'hus. each ferric ion is surrounded by a tetrahedron of chloride ions. the tetrahedra sharing a n edge. Such data favor existence of FeC1,- complexions. Ho\z.ever. the FeCI,- ion is of no importance except \\.hen the ferric ion concentration is very small compared to that of the free chloride ion (73). I n etching solutions? this occurs only when the bath is nearly exhausted, so rhat neglect of FeCli introduces only
minor errors. According to stability by Bjerrum, constants determined Sch\zartzenbach: and Sillen (>?). for the 3.75.14 FeCl3 solutions considered heloiv, only 0.03.21 FeCl; is present. Even if enough hy-drogen chloride ivere added to make the solution 7.\I in chloride. only 0.14.11 FeC'l, \\.odd form. T h e ferrous ion does n u t complex with the chloride ion to an!. great extent ( d . 76. 7 8 ) . T h u s . tlic ferrous ion \vi11 be considered to be Fe' H 2 0 j s - ~ - . T h e nature of cupric ion i n chloride solutions is in doubt, but experimental evidence indicates that C : u C l - and CuCI,-exist (10. 19). \-et x-ray data show that alkali metal salts of cupric chloride complexes exist as lattice compounds of the simple salts-^e.g.: K2CuC14.2H20( 6 . 9. 7.1). Con-
K = 2.2
Thus, the predominant ionic species in a ferric chloride solution is F e ( H z 0 ) 4 C12+, with lesser amounts of Fe(H20)jC1++. T h e various ferric ionic species hydrolyze extensively, and the importance of this is discussed belo\+, Because thermodynamic data on FeC12+ are not well establis5ed. the better studied F e C l + + ion is used in the calculations. Ca!cuiations based on the assumption that the E'eCI2+ ion predominates shoived almsst identical re sults to those presented here. Ferric ion may also form anionic complexes with the chloride ion. Fluoride complexes: such as FeFB-3are well knoizn. but the chloride complexes are not nearly so well characterized. Ferric ion in chloride solutions is rapidly and extensively raken u p by anion eschange resins, thus permitting puri. fication of chloride solutions 1 7 7 . 72). However, substantial take-up by such resins requires a t least 6 M hydrogen chloride (72). Further evidence for Present address, Enthone, Inc., New Haven 8, Conn.
288
INDUSTRIAL AND ENGINEERING CHEMISTRY
I I , _ -
. I "
vcrseij, x-ray data on CsCuCla suggest the existence of a copper chloride complex ( 2 7 ) . I n this report, data on copper chloride complexes published by Bjerrum ( 2 ) have been adopted. This assumes that no cations are present other than copper and hydrogen, and that c U c 1 3 - predominates over CuC14--. Then, Cu(H?O)d-'
+ 3C1-
=
3H20
+
Cu(HzO)C13K = 2.3 X lo-'
Solutions in Tvhich the chloride content is high compared to that of the copper ion are pertinent here. thus, most of the cupric ion is present in the form of Cu(H2O)C13-. Specifically, for the conditions considered: the cupric ion is 90Yc complexed up to 2Syo exhaustion of the barh. over SOT, complexed u p to 50Yc exhaustion. and over 75C;; complexed u p to 'SrC exhaustion. Cuprous ion does not exist in the free state to any great extent because of the insolubility of its salts:
+ 2H?0
CUCI (SI
+ 5H?O =
CU?O
+ 2Cu(HzO)z+ +
C U ( H ~ O ) ~ +C1K = 3.2 X 10-7
20H-
K
=
1.4 X 10-'5
In chloride solutions, therefore, it must e: ist in complex form :
+ 21320 + c1- =
cuc1 ( s )
Cu(HZO)?Cl?- K = 6.5 X l o - '
OF THEORETICAL
Figure 2.
Ferric ion is most important in copper etching in fresh solutions
action is of no importance. and oxidation of the cuprous ion to cupric ion may occur through reduction of the i'erric ion to ferrous ion :
+
Fe(H20)5C1+T Cu(H?O)?CI?- = Fe(H?O)6'Cu(H?O)ClsE o = 0.121 volt h-= 112
+
Oxidation-Reduction Reactions I n a chloride solution, the cuprous ion is stable with respect to disproportionation into copper and cupric ion:
-
Cu(H20)2Cl?-
+
CU 2C12H20 - e E o = 0.208 volt
+
+
Cu(H,Oj2Cl2- C1- = Cu(H20)CIZH2O $- e E" = -0.561 volt Adding,
+
+
2Cu(H?Oj?CI?- CU Cu(H*O)C13CI3H20 E" = -0.353 volt
+
Thus, no cupric ion is formed directly a t the surface of the copper to an). significaiir degree. O n the other hand. any cupric ion formed in the bulk of the solution is capable of dissolving copper. Thus, copper is simultaneously dissolved by the following two reactions. involving the ferric ion and the cupric ion :
+
+ + +
Cu ( s ) Fe(H20)jC1++ C13HzO = Cu(H?O;lyCI?- F e ( H 2 0 ) s + E" = 0.474 volt K = 1.07 X 108 CU ( s )
+ CU(H2O)Clj- + C1- + 3 H r 0 = 2C~(HgO)gC19E a = 0.353 volt K = 9.6 x 105
O n the other hand, in the bulk of the solution. \vhere copper metal is not present. the disproportionation re-
EXHAUSTION VALUE)
Composition of Ferric Chloride Etch vs. Time I n calculating the composition of ferric chloride etches as a function of solution age, ir is assumed t h a t :
At the surface of the copper, copper is oxidized to cuprous ion: cupric ion is reduced to cuprous ion, and ferric ion is reduced to ferrous ion. I n the bulk of the solution, cuprous ion is oxidized to cupric ion, and ferric ion is reduced to ferrous ion. Equilibrium is independently maintained in the preceding tLvo reactions. Cupric ion is present largely as Cu(H20)C13-; cuprous ion is preseni largely as Cu(HzO)zCl?-; ferric ion is present largely as Fe(H?O):CIT' or Fe(H20)4CI:' (either assumption leads to nearly identical results; the graphs are drawn in terms of Fe(H20)&1-+); ferrous ion is present largely as Fe-
5OC; exhaustion of the bath has been reached. and falls slowly thereafter. T h e cuprous ion content is negligible until the bath is over SOYo exhausted. Figure 2 shows that although ferric ion plays the main role in copper etching in fresh solutions, the cupric ion plays a role from the beginning. and becomes more important than the ferric ion in copper etching lvhen the bath is 35y0 or more exhausted. After the bath has become 50% exhausted, the etching reaction is essentially one ofcupric ion reduction. Rate of Etching S o t too much theoretical work has been done on the rate of copper etching in feriic chloride. although some studies have been made (7. / 7 . 2 0 ) . T h e etching process follows the rate equation of a monomolecular reaction and the diffusion of reacting species a t the copper surface is the rate-controlling step. In determining the rate of etching as a function of the age of the bath, it is assumed that copper is oxidized bq ferric ion and cupric ion in the same ratio as their equilibrium concentrations in the bulk of the solution. Thus. for the reaction :
and is coynpletely dissociated into Fe(HIO)jCIAfand C1-. O n the basis of the above. the results in Figure 1 are obtained. T h e ferric ion content decreases rapidly (and almost linearly) u p to about SOYc of the theoretical exhaustion of the bath. and decreases very slo\vly thereafter. T h e ferrous ion content rises inversely to the decrease of ferric ion content. T h e cupric ion content rises until about
ivhere A-1 is the rate constant of the forivard reaction, ,and k i f is the rate constant of the rrverse reaction. -As the equilibrium constanr I; = kl k l ' ,
Siniilarl)-. for tlie reaction : VOL. 51, NO. 3
MARCH 1959
289
The cuprous chloride complex displays some tendency to precipitate as cuprous oxide:
+
2C~(Hz0)&1*-= C U ~ O
40
+
l
o DATA
FROM
(STANFORD
SMCHAlKlN
RESEARCH
TOTAL
INSTITUTE)
AMOUNT
f% OC
OF COPPER
THEORETICAL EXHAUSTION
DISSOLVED VALUE)
Figure 3. Etching r a t e in a 3.75M ferric chloride etch is a function of the d e g r e e of exhaustion of the b a t h cu --dccu(II) -= dt
If K1 = 10s and K , = lo6 (see reactions above for equilibria), then the reverse reactions are negligible. Further, under comparable conditions, cupric chloride etching of copper proceeds at about 60YGof the rate of ferric chloride etching of copper. Thus: the above reactions siiiiplif>-to :
If it is assumed that it takes 7 minutes to etch 2 ounces of copper when the bath is 15% depleted ( 5 ) , Figure 3 is obtained. There is satisfactory agreement with the experimental data. Hydrolysis
Literature Cited If. as assumed earlier? the initial etching solution contains 3.75.21 ferric chloride, (1) Baars, B., Ornstein, L. S., J. E/ircmdcpsitors’ Tech. Sac. 13, 15 (1937). then by the above equation. H(H?O)+ (2) Bjerrum, Jannik, &I. Danske Virien= 0.8 X lo-?. Other ferric ion comskab. Seiskab M a i . - f y s . M e d d . 21, No. 4 plexes d o not contribute significantly (1943). to the hydrolysis reaction : (3 i Bjerrum, J., Schwartzenbach, G., Sillen, L. G., “Stability Constants,” Part 11, Fe(HZOjjCl+- 3 H 2 0 = Chemical Society (London), Spec, Publ. Fe(H:0)4(OH)I2H(H,O)+ C1No. 7 , 1958, 131 pp. (,41Carter, S. R., Clews, F. H., J . Chem. K = 2.3 x 10-9 SOL.125, 1880 (1924). (51 Chaikin, S. \V., others, “Study of I h e equilibrium for precipitation of Effects and Control of Surface Conferric hydroxide then becomes : taminants on Dielectric Materials,” Stanford Research Institute, llenlo Fe(H?O)sCl+L 3 0 H - = Park, Calif., 1958. Fe(H?Oj,(OH), 2 H 2 0 C1(6) Chrobak, L.. Z. Krist. 88, 35 (1934);. K = 5 X 1035 (7) Dodson, R. L V , , Forney, G. J., Swift, E. H.. J . A m . C‘hem Sac. 5 8 . 2573 (19361. Bh- coinparison ivith the ferric ion, (8 Friebman, H. L., Ibid., 74, 5 (1 952). ’ (9) Harker, D., 2.Krist. 93, 136 (19361. the ferrous ion does not hydrolyze (10) Helmholz, L.: Kruh, R. F.: J . .4m. extensively : Chem. SOL. 74, 11’6 (1952). (11 Kraus, K . A: in “Trace Analysis,” Fe(H.O);-2H?O = p. 34, Wiley, New York. 1957. Fe(HzO)4(OH)? 2 H ( H , O ) + (12) Kraus, K. A,, Moore, G. E., J . .h. K = 5.3 x 10-14 Ciiem. Sac. 72, 5792 (1950). (13) Myers, R. J., hletzler, D. E., Sxvift. T h e cupric ion displays a fair degree E. H . , I6id., 72, 3’67 (1950). of hydrolysis: j14) Keuhaus, A , , 2.Krisi. 97, 28 (193’). (15) Palmer, K . .J , Elliot, N., J . .4m. 2Cu(H?O)C138H?O = Chem. Sac. 6 0 , 1832 (1938). i l G ) Peters, Z . Pliysik 26, 193 (18981. Cun(H?O)s(OH)n’2H(H20)+ + (17) Pletenev, S. A , Pavlov, S . .4., J . 6Cl.-1ppI, Chem. (C.S.S.R.i 10, 1957 1193-~. K = 2.5 x 10-8 (18) Popoff, S., Kunz, A . H.: J . .-1~1i. Ciiem. Sac. 51, 382 (19291. ;It the maximum, the cupric ion content (19) Rossi, C . Strocchi, P. I f . , Gciiz. in the ferric chloride etch is 1.6,21; c h m . ita/. 7 8 , 725 (19481. this is equivalent to H(H20)+= 1.3 X (20) Schaffer, R., Jl’inkler, J., Vaaler, L., Deubner, R., “Ferric Chloride Etching lo-‘. Thus, although the cupric ion of Copper for Photoenyravings,” Photodisplays some hydrolysis, it does not engravers Research, Inc., Columbus, contribute much to the solution in Ohio, 1949. comparison to the ferric ion until the (21 I Wells; A. F., J . Chem. Soc. 1947, 1662. latter has been largely used u p ( a t RECEIVED for review June 5, 1958 or above 50% exhaustion of the bath). ACCEPTED December 9: 1958
+
+ 2H20 =
INDUSTRIAL AND ENGINEERINGCHEMISTRY
+
+
+
+
+
+
+
T h e age of a ferric chloride etch effects the mechanism and rate of copper etching. Hydrolysis of the ions present is of importance in establishing the p H of the solution and the point a t which precipitates may form on the work being etched, thus interfering with the etching reaction, and also leading to possible later contamination problems. T h e ferric ion hydrolyzes extensively : Fe(HIO)jCI+C
+
Fe(H?O)j(OH)++ H(H,O)+ CIK = 1.24 X 10-4
Hoivever, the cuprous ion content of the etching solution is so small that precipitation of cuprous oxide is not a problem. Even a t the maximum value of cuprous ion (1.7M), the content of H(H2O)+ from cuprous hydrolysis would b e 2 . 7 X 10-6. Thus, the p H of the etchant is established by the extensively hydrol>-zed ferric ion. From the data of Figure 1 it appears that precipitation of ferric hydroxide will occur when the bad1 has been 407, exhausted. Such precipitation can be avoided by adding 0.l2Mhydrogen chloride to the solution. O n the other hand, this also points u p the problem encountered in rinsing of objects which have been etched in ferric chloride. I n the rinse solution, the p H will be relatively high, and anv residual ferric ion !vi11 &en precipitate ferric hydroxide, \vhich is relatiL-ely difficult to remove Thus, rinse waiers should always be acidified to p H 4 or less.
u ~ ~ ) * / ~ i )
T h e amount of copper dissolved is the sum of the two above rate reactionsi.e.,
290
+
+ Cu(I1) = 2Cu(I), ~ Z ( C C ~( I 6I )c
+
2H(HzO)+ 4C1HzO K = 1.3 x 10-9
+
+
