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Characterizing copper surfaces using a polysulfide reagent
Surface Technology, 22 (1984) 39 - 50
39
C H A R A C T E R I Z I N G COPPER SURFACES USING A P O L Y S U L F I D E REAGENT RICHARD D. GRANATA, HYACINTH VEDAGE and HENRY LEIDHEISER, Jr. Department of Chemistry and Center for Surface and Coatings Research, Lehigh University, Bethlehem, PA (U.S.A.)
(Received October 26, 1983)
Summary Copper immersed in a 0.025 M Na2S4 solution at pH 11 turns black at a characteristic time t hat is controlled by the presence of films or contaminants on the co p pe r surface. The color change occurs coincident with an abrupt change in the corrosion potential. It is hypot hesi zed t hat the ratecontrolling step is the conversion of elemental copper a n d / o r copper(I) to the copper(II) valence state. The rate of this reaction is controlled by the chemical nature of the copper surface. The procedure shows promise for characterizing the cleanliness of c o p p e r surfaces.
1. I n t r o d u c t i o n Webers [1] presented a paper at the June 1980 Colloid and Surface Chemistry S y mp o s i um that described a reaction between copper and a polysulfide solution whose rate was a f unct i on o f the cleanliness o f the copper surface. The reaction was t e r m e d the " c l o c k r e a c t i o n " . We were intrigued by the observations r e por t ed by Webers and with encouragem ent from the International Copper Research Association we m o u n t e d a study t o understand and apply the p h e n o m e n o n . The work r e p o r t e d herein was undertaken in order to understand the p h e n o m e n o n . The clock reaction is carried o u t as follows. A copper or high copper c o n t e n t alloy is immersed in a 0.025 M solution of sodium polysulfide (Na:S4) adjusted to pH 11 with NaOH. After a period of time, the surface turns brown, then greenish and abruptly turns black. The time taken to turn black is a f u n ctio n of the chemical nature of the copper surface and the alloy composition. An amplification of the procedural details is given in ref. 2.
2. Results One o f o u r first concerns was the difficulty in some instances in determining the exact time at which the sample t urned black. Different observers 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
40
•
I.U I--
/ Brown
Greenish
- " -
Black
Z 1 LU
T
I-'-
Tp
0
-1'.0
0 POTENTIAL
IN VOLTS
VERSUS
SCE
Fig. 1. Potential vs. time curve for air-oxidized copper 110 alloy in the polysulfide reagent. The end point for the determination of the value of Tp is shown. The color changes corresponding to the curve inflections are also shown for an oxidized copper surface.
differed slightly in their characterization of the "time to blacken". It was soon recognized that measurements of the corrosion potential of the metal as a function of time provided a m e t h o d for determining a distinct end point. An example is given in Fig. 1 of the potential of copper, previously oxidized in air, as a function of time after immersion in the Na2S4 solution. Just before the copper turns black, its potential reaches a plateau at a specific time. This time period is termed T , . The time to darken to the black color is termed T d. Data will be reported as both Tp and Td since some of the earliest experiments were carried out without measurements of the potential.
2.1. Reproducibility of values of Tp and Td The information given in the tables that follow includes a single average value for Tp or Td. In order to show the reproducibility of the data, examples are given in Table 1 of a series of experiments in which the procedure TABLE 1 Data showing the reproducibility of Tp or T d in a series of identical experiments Experiment
Values o f Tp or T d in individual experiments (rain)
Average value o f Tp or T d (min)
Abraded Cu Oxidized Cu
0.03, 0.03, 0.03, 0.05 0.52, 0.64, 0.64, 0.70, 0.70, 0.72, 0.72, 0.76 0.84, 0.90, 0.96, 0.98, 1.00, 1.04, 1.04, 1.04, 1.10, 1.08 1.70, 1.80, 1.85, 2.00, 2.25, 2.3,0
0.035 0.68
Oxidized Cu Oxidized Cu exposed to inhibitor solution
1.00 2.00
41 was repeated several times. It will be noted that the data in each case clustered around an average value.
2.2. Effect of alloy composition Five copper alloys were studied: alloy 110 which is commercially pure copper; alloy 122 which is a phosphorized copper with a copper content of 99.9%; alloy 260 which is cartridge brass with an approximate composition of 70% Cu and 30% Zn; alloy 706 which contains approximately 90% Cu and 10% Ni with small amounts of iron and manganese; alloy 715 which contains approximately 70% Cu and 30% Ni with small amounts of iron and manganese. Good end points are obtained with alloys 110, 122 and 706, but alloy 715 does n o t give a black color even on long exposure to the polysulfide and alloy 260 reacts with the polysulfide to yield a white precipitate, probably zinc sulfide. The data for freshly abraded samples of three alloys are summarized in Table 2. TABLE 2 Values of Td for abraded copper alloys
Alloy
Treatment
Td (min)
110 122 706
Abraded Abraded Abraded
0.27 0.48 23.7
2.3. Effect o f surface films Films of various types were formed on the alloy 110 surface and the values of either Tp or Td were determined. Films were formed by oxidation and contamination in air at room temperature, by oxidation in air for various times at 102 °C, by immersion in NaOH solution, by immersion in trichloroethylene, by immersion in surfactant solution, by electrolytic treatment in 42% phosphoric acid in order to form phosphate films and by immersion in solutions of benzotriazole. These data together with reference data for abraded surfaces are summarized in Table 3. 2.4. Effect of surface area ratio of abraded to film-covered copper on Tp Surfaces of alloy 110 were filmed by oxidation at 102 °C for 2 h. Portions of the surface were then scratched or abraded to expose 2%, 25%, 50% and 100% abraded surface. A plot of log Tp versus the fraction of abraded surface is given in Fig. 2. 2.5. Rate of reaction as a function o f temperature Alloy 110 panels were oxidized at 102 °C in air for 3 h and values for Tp were determined as a function of the temperature of the Na2S4 bath in which the clock reaction was performed. Measurements were made over a
42 TABLE 3 Values of Tp and T d for samples of alloy 110 whose surface was filmed in various ways
Treatment Abraded
Tp (min) 0.20
Exposure to ambient air for 1 day for 4 days for 7 days for 14 days for 21 days Oxidation in air at 102 °C for 0.5 h for 1 h for 2 h for 3 h for 4 h for 6 h Immersion in pH 11.5 NaOH for 1 day for 2 days for 3 days Immersion in 0.02% sodium lauryl sulfate for 1 day for 2 days for 3 days
Coated with copper phosphate by electrolytic treatment in 42.5% phosphoric acid for 15 min for 30 min for 60 min
0.27 0.62 0.67 0.87 0.82 1.48
0.33 0.44 0.68 1.00 1.26 1.25 9.3 26.5 35.2
9.7 27.0 36.0
0.22 0.42 0.72
Immersion in trichloroethylene for 5 min for 116 h Oxidized in air for 3 h at 1G2 °C and immersed for 5 min in 0.25% benzotriazole solution at 60°C
T d (min)
0.48 1.09 2.00
0.96 2.5 3.75
t e m p e r a t u r e r a n g e f r o m 0 t o 5 0 °C a n d t h e r e s u l t s are p l o t t e d i n Fig. 3. F u l l circles r e p r e s e n t d a t a o b t a i n e d in a stirred s o l u t i o n . T h e fact t h a t the d a t a p o i n t s f o r t h e s t i r r e d a n d u n s t i r r e d s o l u t i o n s fall o n t h e s a m e l i n e is e v i d e n c e t h a t a d i f f u s i o n p r o c e s s i n t h e a q u e o u s p h a s e is n o t r a t e c o n t r o l l i n g . T h e a c t i v a t i o n e n e r g y f o r t h e p r o c e s s as c a l c u l a t e d f r o m t h e d a t a is 6 . 8 kcal m o l -~"
2.6. Tp as a function o f oxide film thickness T h e i n f o r m a t i o n s u m m a r i z e d i n T a b l e 3 i n d i c a t e d t h a t t h e v a l u e s o f Tp i n c r e a s e d as a f u n c t i o n o f t h e t i m e o f o x i d a t i o n . Q u a n t i t a t i v e e x p e r i m e n t s
43
01 ) o
-05
_c
-J -1.0-
-1.5" O
012
014 Fraction
016
of C o p p e r
that
was
018
11.0
Abraded
Fig. 2. Data showing that the time to blackening of an oxidized copper surface in the polysulfide solution is decreased as the fraction of the surface that is abraded is increased.
2.2O
./
1.8A
o i
brr
14-
N
1.0 3.0
315 TEMPERATURE, 103/T (K)
410
Fig. 3. Arrhenius plot for the copper 110 alloy in the polysulfide reagent using Tp values for the rate dependence: o, unstirred solutions; e, stirred solutions.
44 20-
¢0
Z
"5
10
Z
Tr
I--
0
-1:0
-0:5
POTENTIAL IN V O L T S VERSUS SCE
Fig. 4. C h r o n o p o t e n t i o m e t r i c c u r v e f o r o x i d e o n a c o p p e r s u r f a c e . T h e m e t h o d f o r t h e d e t e r m i n a t i o n o f t h e value o f T r f r o m t h e c u r v e is s h o w n .
were th en p e r f o r m e d t o determine whether Tp was a function of the oxide film thickness on the copper. Alloy 110 specimens, 1 cm × 5 cm, were oxidized at 102 °C for increasing periods of time and the oxide film thickness was determined on 7.52 cm 2 of the oxidized surface by constantcurrent cathodic t r e a t m e n t at 200 pA cm -2 in an electrolyte consisting of 4.46 g o f boric acid per litre and 7.15 g of Na2B4OT" 10H20 per litre that was t h o r o u g h l y deaerated. A typical reduction current pl ot and the m e t h o d used in calculating the reduction time Tr are shown in Fig. 4. Data for the values of Tp as a function of the oxide film thickness as determined from the cathodic reduction experiments are summarized in Fig. 5. The p o in t shown for zero time, i.e. the abraded specimen, was n o t d e t e r min ed since it was assumed that the oxide film on the freshly abraded surface was very thin. The data given in Fig. 5 show that over the oxide film thickness range studied the values of Tp are a linear funct i on of the thickness o f the oxide on the surface.
2. 7. Electrochemical behavior of copper in polysulfide solution The anodic and cathodic polarization curves for abraded copper surfaces in the polysulfide m edi um are given in Fig. 6. The limiting current density in the cathodic polarization curve of the order of 104 pA cm -2 is high relative to the limiting c ur r e nt density of the order of 102 pA cm -2 for the oxygen reduction reaction in neutral NaC1 solution. The cathodicto-anodic p o r t i o n of the curve in the cathodic region exhibits a peak slightly negative to the corrosion potential. Simultaneously with the d e v e l o p m e n t of
45 1.5-
1.0.S c-. 0.5-
0
I
0
i
5
O x i d e Film T h i c k n e s s
1'0
15
2'0
as m i l l i c o u l o m b s / c m 2 R e q u i r e d
to R e d u c e
2~5 Oxide
Fig. 5. Data showing that the value of Tp increases with increases in the thickness of the oxide on the surface of copper. -0.4 -
u) o > .E
-0.~
g
c ~
-1.2
-1.6
i
10 ~
10 2 Current
i
10 3
i
10 4
i
10 s
in M i c r o e m p e r e s
Fig. 6. Anodic and cathodic polarization curves'for the 110 copper alloy in the polysulfide solution.
this peak the surface assumes the black color characteristic of cupric sulfide. The behavior of the copper in the polysulfide solution as a function of potential may be summarized as follows. Below - - 1 . 3 3 V the copper surface remains clean with some hydrogen evolution. Between - - 1 . 0 and - - 1 . 3 V the polysulfide reacts slowly with the copper surface and above - - 1 . 0 V the reaction is more rapid. These results suggest that polysulfide forms a reaction product on the copper surface at potentials slightly negative to the corrosion
46 p o t e n t i a l b u t t h a t at m o r e negative p o t e n t i a l s the p r o d u c t w h i c h is f o r m e d is r e m o v e d b y the h y d r o g e n w h i c h is p r o d u c e d at the c a t h o d e .
2.8. Comparison between the behavior of polysulfide and sulfide solutions A p p a r e n t l y the o x i d a t i o n - r e d u c t i o n n a t u r e o f the p o l y s u l f i d e r e a c t i o n is critical in achieving a sharp end p o i n t and in being sensitive to the n a t u r e o f the c o p p e r surface. The d a r k e n i n g r e a c t i o n in a 0.025 M s o l u t i o n o f Na2S requires a b o u t 75 min either for a freshly a b r a d e d surface or for o n e which had been o x i d i z e d in air. The final c o l o r is gray r a t h e r t h a n deep black and the r e a c t i o n p r o c e e d s t h r o u g h a series o f lighter t o n e colors. A n o t h e r m a j o r d i f f e r e n c e b e t w e e n the b e h a v i o r o f t h e p o l y s u l f i d e and the sulfide s o l u t i o n is seen in the potential. The p o t e n t i a l o f the c o p p e r in the Na2S s o l u t i o n stabilizes very rapidly in the vicinity o f - - 1 . 0 V (measured with reference to a s a t u r a t e d calomel electrode (SCE)) whereas the c o p p e r in the p o l y s u l f i d e s o l u t i o n changes with time in the m a n n e r s h o w n in Fig. 1. The p o t e n t i a l s o f c o p p e r in t h e sulfide and in the p o l y s u l f i d e solutions b e c o m e approxim a t e l y the same a f t e r 60 - 120 min.
2. 9. Chemical composition o f surface films These data are s u m m a r i z e d in Table 4 and were g e n e r a t e d by A u g e r s p e c t r o s c o p i c studies o f nine specimens t r e a t e d in various ways. The f o u r m a j o r elements p r e s e n t in the surface films are c o p p e r , sulfur, o x y g e n and TABLE 4 Summary of Auger surface analysis with ion etching
Specimen
Cu (abraded) After etch Cu (abraded)-Sn:After etch Cu (oxidized) After etch Cu (oxidized)-S42After etch Cu (oxidized)-Sn2After etch Cu (oxidized)-S42After etch Cu (NaOH) After etch Cu (NaOH)-S42After etch Cu (abraded)-S42- (scraped) After etch
Atomic concentration (%) of the following elements Cu
S
0
C
32 53 49 55 32 76 53 60 30 57 48 66 46 64 37 49 53 52
0.5 2.0 15.0 17.0 6.0 2.0 18.0 25.0 9.0 27.0 22.0 14.0 20.0 2.0 60.0 49.0 24.0 39.0
7.0 2.0 18.0 20.0 12.0 17.0 11.0 10.0 11.0 14.0 17.0 18.0 27.0 34.0 0.1 2.0 1.0 9.0
61 43 17 7 55 5 18 6 50 2 13 2 7 0.2 3 0.2 21 0.1
47
carbon. The major fraction of the carbon present probably originates from contamination by vapors in the analytical chamber since superficial argon ion sputtering reduces the carbon content of the surface to an insignificant value in the majority of instances. The atomic percentages given in Table 4 were calculated from the peak-to-peak height of the major Auger transition for the element corrected for the relative intensity of the transition. These calculations must be considered to be approximate but were considered sufficiently accurate for the types of conclusions drawn from the data.
3. Discussion A hypothesis for the clock reaction will be presented and then the experimental evidence that supports this hypothesis will be discussed. The black color, which is the end point of the reaction, is representative of cupric sulfide or, if a thick oxide is present on the surface before exposure to polysulfide, a mixture of cupric oxide and cupric sulfide. The formation of the sulfide is a conventional corrosion reaction with an electrochemical mechanism. The anodic half-reaction is either of the following two reactions: Cu -- 2e- = Cu 2÷ or Cu+ -- e- = Cu2+ The first reaction dominates if there is no significant oxide on the surface and the second reaction dominates if the copper has appreciable oxide covering the entire surface, The cathodic half-reaction is $42- + 6 e - = 4 S 2or an intermediate reaction such as $42 - + 4 e - =$2 2- +2S 2The overall reaction is thus either of the following:
3 C u + $ 4 2 - = 3 C u S + S 2or aCu20 + bS42- = (CuO)x(CuS)y + zS 2The rate at which the reaction occurs is determined by the rate at which the anodic half-reaction can occur. The anodic reaction is rate limiting. When contaminants other than oxide are present on the surface, the surface is shielded and the anodic reaction is impeded by steric effects. When an oxide film, from 20 /~ to several hundred ~tngstrbms in thickness, is present on the surface, the rate of the anodic reaction is determined by the defect density (cracks, fissures, pores etc.) which increases with time as the oxide is slowly converted to the sulfide or the mixed oxide-sulfide.
48 When the defect density is such that sufficient numbers o f cuprous ions or substrate co p p er ions can participate in the reaction, the reaction proceeds rapidly and the black color abruptly forms. Each of the critical features of the hypothesis will now be discussed.
3.1. The black color, which is the end point o f the reaction, is representative o f cupric sulfide or, if a thick oxide is present on the surface before exposure to polysulfide, a mixture o f cupric oxide and cupric sulfide. The data summarized in Table 4 represent chemical analyses of the surface film and are given in terms of the atomic percentages of the constituents. These percentages are approximate only and were obtained by correcting the raw peak-to-peak height data for the relative intensities of the transitions for the different elements. The t e n d e n c y towards the f or m a t i on of the cupric form of the p r o d u c t on the co p p er surface after exposure to the polysulfide solution is shown by the data obtained from the samples that were exposed to NaOH solution and those that were oxidized before exposure to the polysulfide. The data considered are those values obtained after argon ion etching to remove the carbon co n tamin a t i on and the very o u t e r layers of the surface product. The ratio [ C u ] / ( [ S ] + [O]) for the samples exposed to NaOH was 1.76 and after exposure to the polysulfide reagent the ratio decreased to 0.96. The same trend is observed in the oxidized samples before and after exposure to polysulfide. The ratio [ C u] / ( [ S] + [O]) was 3.99 for the oxidized sample and after exposure t o the polysulfide this ratio decreased to the range 1.37 2.06. F o r the NaOH and the air oxidation it is clear that the copper-toanion ratio decreases after exposure to the polysulfide. We take these data to be convincing evidence t hat the average valence state of the c o p p e r increases after exposure to the polysulfide or, stated in a more direct form, a significant fraction o f the copper ions in the surface film is in the divalent valence state after exposure to the polysulfide solution. It is clear also from the data in Table 4 t hat the film on the surface of copper after oxidation and after exposure to the polysulfide is a m i xt ur e of oxide and sulfide. 3.2. The anodic reaction involves the conversion of copper metal atoms to cupric ions .and/or cuprous ions to cupric ions. The data summarized in Table 4 are also p e r t i n e n t to this statement. The data obtained with samples oxidized by exposure to NaOH are most revealing in this connection. After exposure to NaOH f or 3 days, the surface is discolored and an oxide film in the interference color range is present on the surface. Auger analysis of the [ C u ] / ( [ S ] + [O]) ratio is 1.00 prior to any argon ion etching. This ratio is indicative o f the fact t hat the c o p p e r ions at the very o u t e r surface which are exposed to the oxidizing envi r onm ent are in the cupric state. Superficial argon ion etching increases this ratio to 1.76, indicative of the fact that the majority o f co p p e r ions in the surface film are in the m onoval ent state. Exposure to the polysulfide solution results in the f o r m a t i o n of the black color and the [ C u ] / ( [ S ] + [O]) ratio decreases to 0.96. Thus, the bulk of
49 the oxide formed in NaOH is cuprous oxide before exposure to the polysulfide and is cupric sulfide after exposure to the polysulfide. 3.3. The cathodic reaction is the conversion o f polysulfide to sulfide ions. This statement remains conjectural for the present. It is known t h a t in the pH region in which the polysulfide solution was stored (pH 11 - 12) the predominant species is the divalent tetrasulfur ion $42- [3]. It is thus probable that this is the active species. The limiting current densities as determined from the cathodic polarization curves are much lower than would be expected on the basis of a 0.025 M solution of $42- but the surface reaction that follows the electron reaction may be the rate-limiting step. Although the pathway by which the sulfide ions are formed in the cathodic step is conjectural, there is no d o u b t that the final product is the sulfide ion in the film that forms on the surface. 3.4. The anodic half-reaction is the rate-controlling step in the overall corrosion reaction. The major evidence for this statement is included in the data summarized in Fig. 2. In the experiments summarized in this figure, the copper panel is first oxidized and then a portion of the oxide is removed by scratching or abrasion to expose an active copper surface. The time required to develop the plateau just prior to the formation of the black color (see Fig. 1) is a function of the anodic-to-cathodic surface area ratio where it is assumed that the bare metal serves as the anode and the oxide-covered copper serves as the cathode. The rate of formation of the black color increases as the anodic-to-cathodic surface area ratio is increased. A very practical consequence of this statement is that the values of T d and T v must be determined on samples whose cut ends or cut edges are shielded from the polysulfide solution. We have had great success in eliminating the perturbing effects of cut ends or edges by coating the freshly exposed copper with a product known as Miccroshield manufactured by Michigan Chrome and Chemical Company. This work has concentrated on generating data which assist in developing a hypothesis for the nature of the sensitivity of the rate of sulfidization of the copper to the presence of contaminants or oxide films on the surface. The procedure shows promise as a quality control test for appraising the cleanliness of copper surfaces prior to a processing step. The m e t h o d has been applied to characterizing copper surfaces, processed in various ways, prior to the application of a photoresist coating.
Acknowledgment This work was supported by the International Copper Research Association. We are grateful to Dr. George Cypher of this organization for recognizing the significance of the polysulfide reaction to the copper industry and for his encouragement of our efforts.
50
References I V. J. Webers, Clock reaction characterizes copper surfaces. American Chemical Society Division o f Colloid and Surface and Surface Chemistry Syrup., Lehigh University, Bethlehem, Abstract 68. 2 R. D. Granata, H. Vedage and H. Leidheiser, Jr., Met. Finish., 3 W. Giggenbach, Inorg. Chem., 11 (1972)1201.
Presented in part at the Chemistry 54th Colloid PA, June 15 - 18, 1980, to be published.
39
C H A R A C T E R I Z I N G COPPER SURFACES USING A P O L Y S U L F I D E REAGENT RICHARD D. GRANATA, HYACINTH VEDAGE and HENRY LEIDHEISER, Jr. Department of Chemistry and Center for Surface and Coatings Research, Lehigh University, Bethlehem, PA (U.S.A.)
(Received October 26, 1983)
Summary Copper immersed in a 0.025 M Na2S4 solution at pH 11 turns black at a characteristic time t hat is controlled by the presence of films or contaminants on the co p pe r surface. The color change occurs coincident with an abrupt change in the corrosion potential. It is hypot hesi zed t hat the ratecontrolling step is the conversion of elemental copper a n d / o r copper(I) to the copper(II) valence state. The rate of this reaction is controlled by the chemical nature of the copper surface. The procedure shows promise for characterizing the cleanliness of c o p p e r surfaces.
1. I n t r o d u c t i o n Webers [1] presented a paper at the June 1980 Colloid and Surface Chemistry S y mp o s i um that described a reaction between copper and a polysulfide solution whose rate was a f unct i on o f the cleanliness o f the copper surface. The reaction was t e r m e d the " c l o c k r e a c t i o n " . We were intrigued by the observations r e por t ed by Webers and with encouragem ent from the International Copper Research Association we m o u n t e d a study t o understand and apply the p h e n o m e n o n . The work r e p o r t e d herein was undertaken in order to understand the p h e n o m e n o n . The clock reaction is carried o u t as follows. A copper or high copper c o n t e n t alloy is immersed in a 0.025 M solution of sodium polysulfide (Na:S4) adjusted to pH 11 with NaOH. After a period of time, the surface turns brown, then greenish and abruptly turns black. The time taken to turn black is a f u n ctio n of the chemical nature of the copper surface and the alloy composition. An amplification of the procedural details is given in ref. 2.
2. Results One o f o u r first concerns was the difficulty in some instances in determining the exact time at which the sample t urned black. Different observers 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
40
•
I.U I--
/ Brown
Greenish
- " -
Black
Z 1 LU
T
I-'-
Tp
0
-1'.0
0 POTENTIAL
IN VOLTS
VERSUS
SCE
Fig. 1. Potential vs. time curve for air-oxidized copper 110 alloy in the polysulfide reagent. The end point for the determination of the value of Tp is shown. The color changes corresponding to the curve inflections are also shown for an oxidized copper surface.
differed slightly in their characterization of the "time to blacken". It was soon recognized that measurements of the corrosion potential of the metal as a function of time provided a m e t h o d for determining a distinct end point. An example is given in Fig. 1 of the potential of copper, previously oxidized in air, as a function of time after immersion in the Na2S4 solution. Just before the copper turns black, its potential reaches a plateau at a specific time. This time period is termed T , . The time to darken to the black color is termed T d. Data will be reported as both Tp and Td since some of the earliest experiments were carried out without measurements of the potential.
2.1. Reproducibility of values of Tp and Td The information given in the tables that follow includes a single average value for Tp or Td. In order to show the reproducibility of the data, examples are given in Table 1 of a series of experiments in which the procedure TABLE 1 Data showing the reproducibility of Tp or T d in a series of identical experiments Experiment
Values o f Tp or T d in individual experiments (rain)
Average value o f Tp or T d (min)
Abraded Cu Oxidized Cu
0.03, 0.03, 0.03, 0.05 0.52, 0.64, 0.64, 0.70, 0.70, 0.72, 0.72, 0.76 0.84, 0.90, 0.96, 0.98, 1.00, 1.04, 1.04, 1.04, 1.10, 1.08 1.70, 1.80, 1.85, 2.00, 2.25, 2.3,0
0.035 0.68
Oxidized Cu Oxidized Cu exposed to inhibitor solution
1.00 2.00
41 was repeated several times. It will be noted that the data in each case clustered around an average value.
2.2. Effect of alloy composition Five copper alloys were studied: alloy 110 which is commercially pure copper; alloy 122 which is a phosphorized copper with a copper content of 99.9%; alloy 260 which is cartridge brass with an approximate composition of 70% Cu and 30% Zn; alloy 706 which contains approximately 90% Cu and 10% Ni with small amounts of iron and manganese; alloy 715 which contains approximately 70% Cu and 30% Ni with small amounts of iron and manganese. Good end points are obtained with alloys 110, 122 and 706, but alloy 715 does n o t give a black color even on long exposure to the polysulfide and alloy 260 reacts with the polysulfide to yield a white precipitate, probably zinc sulfide. The data for freshly abraded samples of three alloys are summarized in Table 2. TABLE 2 Values of Td for abraded copper alloys
Alloy
Treatment
Td (min)
110 122 706
Abraded Abraded Abraded
0.27 0.48 23.7
2.3. Effect o f surface films Films of various types were formed on the alloy 110 surface and the values of either Tp or Td were determined. Films were formed by oxidation and contamination in air at room temperature, by oxidation in air for various times at 102 °C, by immersion in NaOH solution, by immersion in trichloroethylene, by immersion in surfactant solution, by electrolytic treatment in 42% phosphoric acid in order to form phosphate films and by immersion in solutions of benzotriazole. These data together with reference data for abraded surfaces are summarized in Table 3. 2.4. Effect of surface area ratio of abraded to film-covered copper on Tp Surfaces of alloy 110 were filmed by oxidation at 102 °C for 2 h. Portions of the surface were then scratched or abraded to expose 2%, 25%, 50% and 100% abraded surface. A plot of log Tp versus the fraction of abraded surface is given in Fig. 2. 2.5. Rate of reaction as a function o f temperature Alloy 110 panels were oxidized at 102 °C in air for 3 h and values for Tp were determined as a function of the temperature of the Na2S4 bath in which the clock reaction was performed. Measurements were made over a
42 TABLE 3 Values of Tp and T d for samples of alloy 110 whose surface was filmed in various ways
Treatment Abraded
Tp (min) 0.20
Exposure to ambient air for 1 day for 4 days for 7 days for 14 days for 21 days Oxidation in air at 102 °C for 0.5 h for 1 h for 2 h for 3 h for 4 h for 6 h Immersion in pH 11.5 NaOH for 1 day for 2 days for 3 days Immersion in 0.02% sodium lauryl sulfate for 1 day for 2 days for 3 days
Coated with copper phosphate by electrolytic treatment in 42.5% phosphoric acid for 15 min for 30 min for 60 min
0.27 0.62 0.67 0.87 0.82 1.48
0.33 0.44 0.68 1.00 1.26 1.25 9.3 26.5 35.2
9.7 27.0 36.0
0.22 0.42 0.72
Immersion in trichloroethylene for 5 min for 116 h Oxidized in air for 3 h at 1G2 °C and immersed for 5 min in 0.25% benzotriazole solution at 60°C
T d (min)
0.48 1.09 2.00
0.96 2.5 3.75
t e m p e r a t u r e r a n g e f r o m 0 t o 5 0 °C a n d t h e r e s u l t s are p l o t t e d i n Fig. 3. F u l l circles r e p r e s e n t d a t a o b t a i n e d in a stirred s o l u t i o n . T h e fact t h a t the d a t a p o i n t s f o r t h e s t i r r e d a n d u n s t i r r e d s o l u t i o n s fall o n t h e s a m e l i n e is e v i d e n c e t h a t a d i f f u s i o n p r o c e s s i n t h e a q u e o u s p h a s e is n o t r a t e c o n t r o l l i n g . T h e a c t i v a t i o n e n e r g y f o r t h e p r o c e s s as c a l c u l a t e d f r o m t h e d a t a is 6 . 8 kcal m o l -~"
2.6. Tp as a function o f oxide film thickness T h e i n f o r m a t i o n s u m m a r i z e d i n T a b l e 3 i n d i c a t e d t h a t t h e v a l u e s o f Tp i n c r e a s e d as a f u n c t i o n o f t h e t i m e o f o x i d a t i o n . Q u a n t i t a t i v e e x p e r i m e n t s
43
01 ) o
-05
_c
-J -1.0-
-1.5" O
012
014 Fraction
016
of C o p p e r
that
was
018
11.0
Abraded
Fig. 2. Data showing that the time to blackening of an oxidized copper surface in the polysulfide solution is decreased as the fraction of the surface that is abraded is increased.
2.2O
./
1.8A
o i
brr
14-
N
1.0 3.0
315 TEMPERATURE, 103/T (K)
410
Fig. 3. Arrhenius plot for the copper 110 alloy in the polysulfide reagent using Tp values for the rate dependence: o, unstirred solutions; e, stirred solutions.
44 20-
¢0
Z
"5
10
Z
Tr
I--
0
-1:0
-0:5
POTENTIAL IN V O L T S VERSUS SCE
Fig. 4. C h r o n o p o t e n t i o m e t r i c c u r v e f o r o x i d e o n a c o p p e r s u r f a c e . T h e m e t h o d f o r t h e d e t e r m i n a t i o n o f t h e value o f T r f r o m t h e c u r v e is s h o w n .
were th en p e r f o r m e d t o determine whether Tp was a function of the oxide film thickness on the copper. Alloy 110 specimens, 1 cm × 5 cm, were oxidized at 102 °C for increasing periods of time and the oxide film thickness was determined on 7.52 cm 2 of the oxidized surface by constantcurrent cathodic t r e a t m e n t at 200 pA cm -2 in an electrolyte consisting of 4.46 g o f boric acid per litre and 7.15 g of Na2B4OT" 10H20 per litre that was t h o r o u g h l y deaerated. A typical reduction current pl ot and the m e t h o d used in calculating the reduction time Tr are shown in Fig. 4. Data for the values of Tp as a function of the oxide film thickness as determined from the cathodic reduction experiments are summarized in Fig. 5. The p o in t shown for zero time, i.e. the abraded specimen, was n o t d e t e r min ed since it was assumed that the oxide film on the freshly abraded surface was very thin. The data given in Fig. 5 show that over the oxide film thickness range studied the values of Tp are a linear funct i on of the thickness o f the oxide on the surface.
2. 7. Electrochemical behavior of copper in polysulfide solution The anodic and cathodic polarization curves for abraded copper surfaces in the polysulfide m edi um are given in Fig. 6. The limiting current density in the cathodic polarization curve of the order of 104 pA cm -2 is high relative to the limiting c ur r e nt density of the order of 102 pA cm -2 for the oxygen reduction reaction in neutral NaC1 solution. The cathodicto-anodic p o r t i o n of the curve in the cathodic region exhibits a peak slightly negative to the corrosion potential. Simultaneously with the d e v e l o p m e n t of
45 1.5-
1.0.S c-. 0.5-
0
I
0
i
5
O x i d e Film T h i c k n e s s
1'0
15
2'0
as m i l l i c o u l o m b s / c m 2 R e q u i r e d
to R e d u c e
2~5 Oxide
Fig. 5. Data showing that the value of Tp increases with increases in the thickness of the oxide on the surface of copper. -0.4 -
u) o > .E
-0.~
g
c ~
-1.2
-1.6
i
10 ~
10 2 Current
i
10 3
i
10 4
i
10 s
in M i c r o e m p e r e s
Fig. 6. Anodic and cathodic polarization curves'for the 110 copper alloy in the polysulfide solution.
this peak the surface assumes the black color characteristic of cupric sulfide. The behavior of the copper in the polysulfide solution as a function of potential may be summarized as follows. Below - - 1 . 3 3 V the copper surface remains clean with some hydrogen evolution. Between - - 1 . 0 and - - 1 . 3 V the polysulfide reacts slowly with the copper surface and above - - 1 . 0 V the reaction is more rapid. These results suggest that polysulfide forms a reaction product on the copper surface at potentials slightly negative to the corrosion
46 p o t e n t i a l b u t t h a t at m o r e negative p o t e n t i a l s the p r o d u c t w h i c h is f o r m e d is r e m o v e d b y the h y d r o g e n w h i c h is p r o d u c e d at the c a t h o d e .
2.8. Comparison between the behavior of polysulfide and sulfide solutions A p p a r e n t l y the o x i d a t i o n - r e d u c t i o n n a t u r e o f the p o l y s u l f i d e r e a c t i o n is critical in achieving a sharp end p o i n t and in being sensitive to the n a t u r e o f the c o p p e r surface. The d a r k e n i n g r e a c t i o n in a 0.025 M s o l u t i o n o f Na2S requires a b o u t 75 min either for a freshly a b r a d e d surface or for o n e which had been o x i d i z e d in air. The final c o l o r is gray r a t h e r t h a n deep black and the r e a c t i o n p r o c e e d s t h r o u g h a series o f lighter t o n e colors. A n o t h e r m a j o r d i f f e r e n c e b e t w e e n the b e h a v i o r o f t h e p o l y s u l f i d e and the sulfide s o l u t i o n is seen in the potential. The p o t e n t i a l o f the c o p p e r in the Na2S s o l u t i o n stabilizes very rapidly in the vicinity o f - - 1 . 0 V (measured with reference to a s a t u r a t e d calomel electrode (SCE)) whereas the c o p p e r in the p o l y s u l f i d e s o l u t i o n changes with time in the m a n n e r s h o w n in Fig. 1. The p o t e n t i a l s o f c o p p e r in t h e sulfide and in the p o l y s u l f i d e solutions b e c o m e approxim a t e l y the same a f t e r 60 - 120 min.
2. 9. Chemical composition o f surface films These data are s u m m a r i z e d in Table 4 and were g e n e r a t e d by A u g e r s p e c t r o s c o p i c studies o f nine specimens t r e a t e d in various ways. The f o u r m a j o r elements p r e s e n t in the surface films are c o p p e r , sulfur, o x y g e n and TABLE 4 Summary of Auger surface analysis with ion etching
Specimen
Cu (abraded) After etch Cu (abraded)-Sn:After etch Cu (oxidized) After etch Cu (oxidized)-S42After etch Cu (oxidized)-Sn2After etch Cu (oxidized)-S42After etch Cu (NaOH) After etch Cu (NaOH)-S42After etch Cu (abraded)-S42- (scraped) After etch
Atomic concentration (%) of the following elements Cu
S
0
C
32 53 49 55 32 76 53 60 30 57 48 66 46 64 37 49 53 52
0.5 2.0 15.0 17.0 6.0 2.0 18.0 25.0 9.0 27.0 22.0 14.0 20.0 2.0 60.0 49.0 24.0 39.0
7.0 2.0 18.0 20.0 12.0 17.0 11.0 10.0 11.0 14.0 17.0 18.0 27.0 34.0 0.1 2.0 1.0 9.0
61 43 17 7 55 5 18 6 50 2 13 2 7 0.2 3 0.2 21 0.1
47
carbon. The major fraction of the carbon present probably originates from contamination by vapors in the analytical chamber since superficial argon ion sputtering reduces the carbon content of the surface to an insignificant value in the majority of instances. The atomic percentages given in Table 4 were calculated from the peak-to-peak height of the major Auger transition for the element corrected for the relative intensity of the transition. These calculations must be considered to be approximate but were considered sufficiently accurate for the types of conclusions drawn from the data.
3. Discussion A hypothesis for the clock reaction will be presented and then the experimental evidence that supports this hypothesis will be discussed. The black color, which is the end point of the reaction, is representative of cupric sulfide or, if a thick oxide is present on the surface before exposure to polysulfide, a mixture of cupric oxide and cupric sulfide. The formation of the sulfide is a conventional corrosion reaction with an electrochemical mechanism. The anodic half-reaction is either of the following two reactions: Cu -- 2e- = Cu 2÷ or Cu+ -- e- = Cu2+ The first reaction dominates if there is no significant oxide on the surface and the second reaction dominates if the copper has appreciable oxide covering the entire surface, The cathodic half-reaction is $42- + 6 e - = 4 S 2or an intermediate reaction such as $42 - + 4 e - =$2 2- +2S 2The overall reaction is thus either of the following:
3 C u + $ 4 2 - = 3 C u S + S 2or aCu20 + bS42- = (CuO)x(CuS)y + zS 2The rate at which the reaction occurs is determined by the rate at which the anodic half-reaction can occur. The anodic reaction is rate limiting. When contaminants other than oxide are present on the surface, the surface is shielded and the anodic reaction is impeded by steric effects. When an oxide film, from 20 /~ to several hundred ~tngstrbms in thickness, is present on the surface, the rate of the anodic reaction is determined by the defect density (cracks, fissures, pores etc.) which increases with time as the oxide is slowly converted to the sulfide or the mixed oxide-sulfide.
48 When the defect density is such that sufficient numbers o f cuprous ions or substrate co p p er ions can participate in the reaction, the reaction proceeds rapidly and the black color abruptly forms. Each of the critical features of the hypothesis will now be discussed.
3.1. The black color, which is the end point o f the reaction, is representative o f cupric sulfide or, if a thick oxide is present on the surface before exposure to polysulfide, a mixture o f cupric oxide and cupric sulfide. The data summarized in Table 4 represent chemical analyses of the surface film and are given in terms of the atomic percentages of the constituents. These percentages are approximate only and were obtained by correcting the raw peak-to-peak height data for the relative intensities of the transitions for the different elements. The t e n d e n c y towards the f or m a t i on of the cupric form of the p r o d u c t on the co p p er surface after exposure to the polysulfide solution is shown by the data obtained from the samples that were exposed to NaOH solution and those that were oxidized before exposure to the polysulfide. The data considered are those values obtained after argon ion etching to remove the carbon co n tamin a t i on and the very o u t e r layers of the surface product. The ratio [ C u ] / ( [ S ] + [O]) for the samples exposed to NaOH was 1.76 and after exposure to the polysulfide reagent the ratio decreased to 0.96. The same trend is observed in the oxidized samples before and after exposure to polysulfide. The ratio [ C u] / ( [ S] + [O]) was 3.99 for the oxidized sample and after exposure t o the polysulfide this ratio decreased to the range 1.37 2.06. F o r the NaOH and the air oxidation it is clear that the copper-toanion ratio decreases after exposure to the polysulfide. We take these data to be convincing evidence t hat the average valence state of the c o p p e r increases after exposure to the polysulfide or, stated in a more direct form, a significant fraction o f the copper ions in the surface film is in the divalent valence state after exposure to the polysulfide solution. It is clear also from the data in Table 4 t hat the film on the surface of copper after oxidation and after exposure to the polysulfide is a m i xt ur e of oxide and sulfide. 3.2. The anodic reaction involves the conversion of copper metal atoms to cupric ions .and/or cuprous ions to cupric ions. The data summarized in Table 4 are also p e r t i n e n t to this statement. The data obtained with samples oxidized by exposure to NaOH are most revealing in this connection. After exposure to NaOH f or 3 days, the surface is discolored and an oxide film in the interference color range is present on the surface. Auger analysis of the [ C u ] / ( [ S ] + [O]) ratio is 1.00 prior to any argon ion etching. This ratio is indicative o f the fact t hat the c o p p e r ions at the very o u t e r surface which are exposed to the oxidizing envi r onm ent are in the cupric state. Superficial argon ion etching increases this ratio to 1.76, indicative of the fact that the majority o f co p p e r ions in the surface film are in the m onoval ent state. Exposure to the polysulfide solution results in the f o r m a t i o n of the black color and the [ C u ] / ( [ S ] + [O]) ratio decreases to 0.96. Thus, the bulk of
49 the oxide formed in NaOH is cuprous oxide before exposure to the polysulfide and is cupric sulfide after exposure to the polysulfide. 3.3. The cathodic reaction is the conversion o f polysulfide to sulfide ions. This statement remains conjectural for the present. It is known t h a t in the pH region in which the polysulfide solution was stored (pH 11 - 12) the predominant species is the divalent tetrasulfur ion $42- [3]. It is thus probable that this is the active species. The limiting current densities as determined from the cathodic polarization curves are much lower than would be expected on the basis of a 0.025 M solution of $42- but the surface reaction that follows the electron reaction may be the rate-limiting step. Although the pathway by which the sulfide ions are formed in the cathodic step is conjectural, there is no d o u b t that the final product is the sulfide ion in the film that forms on the surface. 3.4. The anodic half-reaction is the rate-controlling step in the overall corrosion reaction. The major evidence for this statement is included in the data summarized in Fig. 2. In the experiments summarized in this figure, the copper panel is first oxidized and then a portion of the oxide is removed by scratching or abrasion to expose an active copper surface. The time required to develop the plateau just prior to the formation of the black color (see Fig. 1) is a function of the anodic-to-cathodic surface area ratio where it is assumed that the bare metal serves as the anode and the oxide-covered copper serves as the cathode. The rate of formation of the black color increases as the anodic-to-cathodic surface area ratio is increased. A very practical consequence of this statement is that the values of T d and T v must be determined on samples whose cut ends or cut edges are shielded from the polysulfide solution. We have had great success in eliminating the perturbing effects of cut ends or edges by coating the freshly exposed copper with a product known as Miccroshield manufactured by Michigan Chrome and Chemical Company. This work has concentrated on generating data which assist in developing a hypothesis for the nature of the sensitivity of the rate of sulfidization of the copper to the presence of contaminants or oxide films on the surface. The procedure shows promise as a quality control test for appraising the cleanliness of copper surfaces prior to a processing step. The m e t h o d has been applied to characterizing copper surfaces, processed in various ways, prior to the application of a photoresist coating.
Acknowledgment This work was supported by the International Copper Research Association. We are grateful to Dr. George Cypher of this organization for recognizing the significance of the polysulfide reaction to the copper industry and for his encouragement of our efforts.
50
References I V. J. Webers, Clock reaction characterizes copper surfaces. American Chemical Society Division o f Colloid and Surface and Surface Chemistry Syrup., Lehigh University, Bethlehem, Abstract 68. 2 R. D. Granata, H. Vedage and H. Leidheiser, Jr., Met. Finish., 3 W. Giggenbach, Inorg. Chem., 11 (1972)1201.
Presented in part at the Chemistry 54th Colloid PA, June 15 - 18, 1980, to be published.