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The analysis for light elements of hard gold electrodeposits
Nuclear Instruments and Methods in Physics Research 218 (1983) 555-558 North-Holland, Amsterdam
THE ANALYSIS FOR LIGHT ELEMENTS A.J. B E N T L E Y ,
L.G. E A R W A K E R ,
555
OF HARD GOLD ELECTRODEPOSITS
J.P.G. F A R R , J. P O U N T N E Y
a n d M. S A R E M I
UniversiO' of Birmingham, Birmingham BI 5 2TT. England
Examinations of gold-cobalt electrodeposits by Auger spectroscopy and prompt radiation analysis are repor.ted. Auger analyses indicate low C, N and O levels after surface erosion by Ar + sputtering. Nuclear (d, p) reactions indicate levels of C N comparable with reported values. A sputtering artefact is thought to account for the difference. The results are consistent with suggestions in the literature that inclusions dispersed in the bulk of the electrodeposit contain complex potassium cobalt cyanide.
1. Introduction The electrodeposition of gold is increasingly important in technology, both in solid-state device manufacture and in the provision of electrical contacts [1]. Whereas the former application requires "soft", or notionally pure, deposits, the latter usually involves hard, bright electroplate often containing up to 1% of cobalt. In both instances there is a need for better knowledge of the light element contents of the electrodeposits. These are difficult to obtain by conventional chemical analysis, particularly on small samples, so there are few published data [2]. Recent instrumental studies have concerned the structure of hard gold electrodeposits as revealed in the transmission electron microscope [3,4] and the application of SIMS [5] and M6ssbauer [6,7] techniques. The results of these studies can now be compared with Auger Spectroscopy analysis using Ar + sputter profiling and with prompt radiation analysis using (d, p) reactions.
2. Experimental Specimens of hard gold electroplate were obtained from typical proprietary electroplating baths (citratephosphate buffered to pH 4.9, with approximately 2 g 1 1 containing a variety of addition agents, the baths being used as recommended by the proprietors. Specimens were then detached from inert substrates and subjected to (a) conventional analysis (by atomic absorption) for their Co contents; (b) Auger spectroscopic analysis using Ar + bombardment (250 eV) to obtain a depth profile; and (c) prompt radiation analysis using the (d, p) reaction with incident 2 MeV radiation from the Birmingham Radiation Centre Dynamitron accelerator. Nuclear (d, p) reactions are particularly sensitive for the analysis of light elements in heavy element matrices 0167-5087/83/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
as reaction cross sections from the heavy elements are very strongly suppressed by the Coulomb barrier. In addition, the elements carbon and nitrogen are probably the easiest to detect and analyse by (d, p) reactions because the former has a very high cross section and the latter produces a proton group of very high energy which is free of interference from other reaction proton groups [8]. In order to extract absolute concentrations, spectra were compared with those from steels which contained known amounts of nitrogen. Concentrations for carbon and oxygen were extracted using the known (d, p) cross sections from C, N and O.
3. Results Auger analyses revealed heavy surface contamination with C, but this peak diminished after eroding some 250 by Ar + bombardment. Only surface O was observed and N levels were consistently low (often beneath the level of detection, lower than 1%). Surface traces of S and C1 were also observed on some of the samples. Depth profiles of C and N were obtained down to a maximum depth of 0.20/~m. A typical Auger spectrum is shown in fig. 1. Nuclear reaction analysis is capable of analysing C and N concentrations, non-destructively, to a depth of many microns. In fact it is difficult to sensibly analyse thin layers at the surface, For this reason only regions of the proton groups originating from well beneath the sample surface have been analysed. Measurements are average values for depths between 1.0-1.5/am, 5 - 6 / ~ m and 9-10 /~m. It was not possible to extract O concentrations at depth because of the strength of the carbon contribution. Only relative values for amounts of C and O at the target surface were obtained. A typical (d, p) spectrum from a gold electrodeposit is shown in fig. 2. The Auger and nuclear reaction measurements together with measured Co contents for a
556
A.J. Bentley et al. / Analysis for light elements
<
dN
Au S
Ar
Au
E
N
[
J
I
]
100
200
300
/+00
600
500
Elecfron Energy, E/eV
Fig. 1. Typical Auger electron spectrum of a sputter-profiled gold electrodeposit.
160
3200 "Standard"
steel
120
2400 • 12C(d,Po)
8O
1600 14
"i
800
k~
.'"
°
""=::d"
.\
z
""
"•" . -~o . """ " :
"."
~
7p
~
°
"
" "i~
~-
°
"
- ~ - " % ..
.-...~¢
1
40
* " " z : . ' , °•
.~.~.v
Gold electrodeposit ";512
;" i c(a'Po>
a -n 0 L.)
:
N ( d , po )
1
14N(d,Po)
2400
120
° .'--
--'¢
i.
° .al o
I
1600
.
80
~° °°
25-." 800
~.'. --.
40
: •
•
|
°.
,-
0 0
128
r,~,
.~
"
.~'.~_.
256
384
512
0
CHANNEL NUMBER Fig. 2. Typical (d, p) spectra from the gold electrodeposit that produced the spectrum in fig. 1 and a "'standard" steel sample
A.J. Bentley el aL / Analysis for light elements
557
Table 1 Analysis of gold electrodeposits (at.%) Specimen no.
Co content
Technique
1
0.50
Auger (d,p) (d, p) (d, p) Auger Auger Auger (d, p) (d,p) (d, p) Auger Auger Auger Auger Auger (d, p) (d, p) (d, p) Auger Auger Auger (d, p) (d, p) (d, p) Auger Auger Auger Auger (d, p) (d, p) (d, p) Auger Auger Auger Auger (d, p)
2
3
4
1.00
0.64
0.94
5
1.07
6
0.60
Approx. depth (/*m) 0.10 1.0 1.5 5-6 9 10 0 0.004 0.12 1.0-1.5 5-6 9 10 0 0.006 0.015 0.050 0.090 1.0 1.5 5-6 9 10 0 0.006 0.070 1.0-1.5 5 6 9-10 0 0.008 0.065 0.12 1.0-1.5 5 6 9 10 0 0.006 0.015 0.20 5-6
Concentration (at.%) Au C
O
N
S
CI
98
47 85 100
54 83 85 100 100
36 77 95
38 89 93 95
25 41 70 71
2.8 3.2 1.6 45 15
2.2 1.9
2.4 2.7 2.5 38 17 15
2.6 1.9
1.6 1.8 1.9 51 21 5 0.4 0.6 0.6 51 9 5 3 2.0 2.1 2.9 67 54 30 29 1.7
1.8 1.5
0.6 0.6 3 2 2 2 2.5 2.0
2.6
~') Blanks indicate concentrations below the useful range of the instrument.
selection of samples are summarised in table 1. Auger analysis for Co showed a surface enrichment that sometimes persisted to 500 A depth.
4. D i s c u s s i o n
4.1. Quantitativeness of the analyses Cobalt analyses were reproducible to 1% on standards; there was a variation of 5% between measurements of various 30 mg samples taken from a given gold specimen. Auger analyses were obtained by comparing peaks in
the differential emission spectra with standard curves for the respective elements given in the H a n d b o o k of Auger Spectroscopy [9]. These values, therefore, assume that the emissions were from the surface and neglect any matrix effects. The quantitativeness of the p r o m p t radiation analysis is more difficult to assess. Ultra-high vacua were not used in the specimen chamber, nor was ion cleaning or liquid nitrogen vapour trapping employed, and it is k n o w n that the ion beam b o m b a r d m e n t leads to a slow building of deposited organic material on the surface being analysed. For these reasons reliable data were available for carbon and nitrogen concentrations at a d e p t h of some microns under the surface only. These
558
A.J. Bentley et al. / Analysis for light elements
data, however, were a valuable c o m p l e m e n t to the Auger m e a s u r e m e n t s which were confined to the surface region. In order to be able to extract absolute concentrations, the p r o t o n spectra from gold were c o m p a r e d with those from an accurately standardised steel (N 0.162, C 0.04, S 0.016, P 0.032% by weight). The steel was in the form of turnings a n d the physical nature of the standards, i.e. of varying thickness and coherence, led to an absolute uncertainty of a b o u t +15%. However, repeated m e a s u r e m e n t s on given gold specimens were reproducible to better than _+ 5%. Analysis of the proton groups corresponding to surface C and O indicated that b o t h species were present in approximately equal quantities. This finding does not agree with the Auger measurements. However, the k n o w n c o n t a m i n a t i o n build-up on the surface during deuteron b o m b a r d m e n t should not be overlooked.
accuracy of the data, the equivalence shown in table I is very good. The order of magnitude of the C and N c o n t e n t s measured by (d, p) reactions agrees with analyses of similar electroplate reported in refs. 1 and 2. The strength of the C signal prevented the estimated of bulk O concentration by (d, p) reaction and the absence of O after ion profiling in the Auger spectra may be due to the lability of it surface compounds. However, it does seem likely that CoO or Co 0.0H if present in bulk [6] would have resulted in O Auger emissions from the surface when exposed by ion sputtering. Finally, the absence of N and the drastic lowering of C levels from the Auger spectra on progressively sputtered specimens are in agreement with a conclusion from SIMS analysis of gold electroplate [5] that, whatever their bulk concentrations, large chemically b o u n d cyanide complexes are not surface-stable.
4.2. Comparison of Auger and (d, p) analyses It is difficult to make a comparison between the Auger and (d, p) analyses for C a n d N as shown in table 1 because of the very different depths at which the analyses have been made. However, it is clear that the (d, p) results indicate consistently higher C and N concentrations at d e p t h in the samples. This could well be because of the much higher sensitivity of the nuclear technique and the fact that the concentrations recorded are near the limit of detection for Auger. Small changes in absolute calibration of either technique could account for the differences. A more likely possibility could be that C and N (and perhaps O) are lost from the inclusions in the electroplate during the surface erosion, i.e. there is an artefact in ion profiling due to the lability of the inclusions when these lie in the surface. P r o m p t radiation analysis, whereby the analytical i n f o r m a t i o n comes from a considerable depth of material, is not subject to this effect.
4.3. Comparison with other instrumental analyses It is, therefore, interesting to compare the present results, especially of nuclear reaction analyses, with those from the application MOssbauer spectroscopy to similar gold cobalt electrodeposits [6,7]. There is agreem e n t in the quantities of cobalt incorporated. F r o m M 6 s s b a u e r spectroscopy up to 70% of the cobalt is f o u n d to be substitutionally alloyed with gold. The r e m a i n d e r is held as particles (cf. Munier [10]) of complex cyanides [7,11]. Accepting that the inclusions observed t h r o u g h o u t the deposits contain cyanide complexes [e.g. K2Co(CN)6 ], it is reasonable that the C and N contents should be equivalent. Bearing in mind the
5. Conclusions A comparison of analyses by nuclear reactions and by Auger spectroscopy has revealed a significant artefact in ion-sputtering g o l d - c o b a l t electrodeposits. Results support the conclusion that inclusions in these deposits contain complex cyanides.
References [1] D.R. Mason and A. Blair, Trans. Inst. Met. Fin. 55 (1977) 141; Plating in the electronics industry, Amer. Electroplaters Soc. Vth Symp. (1975). [2] Ch.J. Raub and A. Knodler, Gold Bulletin 10 (1977): also literature cited in: J. Electrochem. Soc. 126 (1979) [391. [3] S. Nakahara and Y. Okinaka, J. Electrochem. Soc. 123 (1976) 1284. [4] S. Nakahara and Y. Okinaka, J. Electrochem. Soc. 128 (1981) 284. [5] R. Schubert, J. Electrochem. Soc. 128 (1981) 126. [6] H. Leidheiser, A. Vertes, M.L. Varsanyi and 1. CzakoNagy, J. Electrochem. Soc. 126 (1979) 391. [7] R.L. Cohen, F.B. Koch, L.N. Schoenberg and K.W. West, J. Electrochem. Soc. 126 (1979) 1608. [8] R.A. Jarjis, Internal report, Department of Physics, University of Manchester (1979). [9] L.E. Davis, N.C. McDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, eds., Handbook of Auger spectroscopy (Physics Electronics Industries Inc., Eden Prairie, Minnesota, 1976). [10] G.B. Munier, Plating 56 (1969) 1151. [11] Y. Okinawa, F.B. Koch, C. Wolowiuk and D.R. Blessington, J. Electrochem. Soc. 125 (1978) 1745.
THE ANALYSIS FOR LIGHT ELEMENTS A.J. B E N T L E Y ,
L.G. E A R W A K E R ,
555
OF HARD GOLD ELECTRODEPOSITS
J.P.G. F A R R , J. P O U N T N E Y
a n d M. S A R E M I
UniversiO' of Birmingham, Birmingham BI 5 2TT. England
Examinations of gold-cobalt electrodeposits by Auger spectroscopy and prompt radiation analysis are repor.ted. Auger analyses indicate low C, N and O levels after surface erosion by Ar + sputtering. Nuclear (d, p) reactions indicate levels of C N comparable with reported values. A sputtering artefact is thought to account for the difference. The results are consistent with suggestions in the literature that inclusions dispersed in the bulk of the electrodeposit contain complex potassium cobalt cyanide.
1. Introduction The electrodeposition of gold is increasingly important in technology, both in solid-state device manufacture and in the provision of electrical contacts [1]. Whereas the former application requires "soft", or notionally pure, deposits, the latter usually involves hard, bright electroplate often containing up to 1% of cobalt. In both instances there is a need for better knowledge of the light element contents of the electrodeposits. These are difficult to obtain by conventional chemical analysis, particularly on small samples, so there are few published data [2]. Recent instrumental studies have concerned the structure of hard gold electrodeposits as revealed in the transmission electron microscope [3,4] and the application of SIMS [5] and M6ssbauer [6,7] techniques. The results of these studies can now be compared with Auger Spectroscopy analysis using Ar + sputter profiling and with prompt radiation analysis using (d, p) reactions.
2. Experimental Specimens of hard gold electroplate were obtained from typical proprietary electroplating baths (citratephosphate buffered to pH 4.9, with approximately 2 g 1 1 containing a variety of addition agents, the baths being used as recommended by the proprietors. Specimens were then detached from inert substrates and subjected to (a) conventional analysis (by atomic absorption) for their Co contents; (b) Auger spectroscopic analysis using Ar + bombardment (250 eV) to obtain a depth profile; and (c) prompt radiation analysis using the (d, p) reaction with incident 2 MeV radiation from the Birmingham Radiation Centre Dynamitron accelerator. Nuclear (d, p) reactions are particularly sensitive for the analysis of light elements in heavy element matrices 0167-5087/83/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
as reaction cross sections from the heavy elements are very strongly suppressed by the Coulomb barrier. In addition, the elements carbon and nitrogen are probably the easiest to detect and analyse by (d, p) reactions because the former has a very high cross section and the latter produces a proton group of very high energy which is free of interference from other reaction proton groups [8]. In order to extract absolute concentrations, spectra were compared with those from steels which contained known amounts of nitrogen. Concentrations for carbon and oxygen were extracted using the known (d, p) cross sections from C, N and O.
3. Results Auger analyses revealed heavy surface contamination with C, but this peak diminished after eroding some 250 by Ar + bombardment. Only surface O was observed and N levels were consistently low (often beneath the level of detection, lower than 1%). Surface traces of S and C1 were also observed on some of the samples. Depth profiles of C and N were obtained down to a maximum depth of 0.20/~m. A typical Auger spectrum is shown in fig. 1. Nuclear reaction analysis is capable of analysing C and N concentrations, non-destructively, to a depth of many microns. In fact it is difficult to sensibly analyse thin layers at the surface, For this reason only regions of the proton groups originating from well beneath the sample surface have been analysed. Measurements are average values for depths between 1.0-1.5/am, 5 - 6 / ~ m and 9-10 /~m. It was not possible to extract O concentrations at depth because of the strength of the carbon contribution. Only relative values for amounts of C and O at the target surface were obtained. A typical (d, p) spectrum from a gold electrodeposit is shown in fig. 2. The Auger and nuclear reaction measurements together with measured Co contents for a
556
A.J. Bentley et al. / Analysis for light elements
<
dN
Au S
Ar
Au
E
N
[
J
I
]
100
200
300
/+00
600
500
Elecfron Energy, E/eV
Fig. 1. Typical Auger electron spectrum of a sputter-profiled gold electrodeposit.
160
3200 "Standard"
steel
120
2400 • 12C(d,Po)
8O
1600 14
"i
800
k~
.'"
°
""=::d"
.\
z
""
"•" . -~o . """ " :
"."
~
7p
~
°
"
" "i~
~-
°
"
- ~ - " % ..
.-...~¢
1
40
* " " z : . ' , °•
.~.~.v
Gold electrodeposit ";512
;" i c(a'Po>
a -n 0 L.)
:
N ( d , po )
1
14N(d,Po)
2400
120
° .'--
--'¢
i.
° .al o
I
1600
.
80
~° °°
25-." 800
~.'. --.
40
: •
•
|
°.
,-
0 0
128
r,~,
.~
"
.~'.~_.
256
384
512
0
CHANNEL NUMBER Fig. 2. Typical (d, p) spectra from the gold electrodeposit that produced the spectrum in fig. 1 and a "'standard" steel sample
A.J. Bentley el aL / Analysis for light elements
557
Table 1 Analysis of gold electrodeposits (at.%) Specimen no.
Co content
Technique
1
0.50
Auger (d,p) (d, p) (d, p) Auger Auger Auger (d, p) (d,p) (d, p) Auger Auger Auger Auger Auger (d, p) (d, p) (d, p) Auger Auger Auger (d, p) (d, p) (d, p) Auger Auger Auger Auger (d, p) (d, p) (d, p) Auger Auger Auger Auger (d, p)
2
3
4
1.00
0.64
0.94
5
1.07
6
0.60
Approx. depth (/*m) 0.10 1.0 1.5 5-6 9 10 0 0.004 0.12 1.0-1.5 5-6 9 10 0 0.006 0.015 0.050 0.090 1.0 1.5 5-6 9 10 0 0.006 0.070 1.0-1.5 5 6 9-10 0 0.008 0.065 0.12 1.0-1.5 5 6 9 10 0 0.006 0.015 0.20 5-6
Concentration (at.%) Au C
O
N
S
CI
98
47 85 100
54 83 85 100 100
36 77 95
38 89 93 95
25 41 70 71
2.8 3.2 1.6 45 15
2.2 1.9
2.4 2.7 2.5 38 17 15
2.6 1.9
1.6 1.8 1.9 51 21 5 0.4 0.6 0.6 51 9 5 3 2.0 2.1 2.9 67 54 30 29 1.7
1.8 1.5
0.6 0.6 3 2 2 2 2.5 2.0
2.6
~') Blanks indicate concentrations below the useful range of the instrument.
selection of samples are summarised in table 1. Auger analysis for Co showed a surface enrichment that sometimes persisted to 500 A depth.
4. D i s c u s s i o n
4.1. Quantitativeness of the analyses Cobalt analyses were reproducible to 1% on standards; there was a variation of 5% between measurements of various 30 mg samples taken from a given gold specimen. Auger analyses were obtained by comparing peaks in
the differential emission spectra with standard curves for the respective elements given in the H a n d b o o k of Auger Spectroscopy [9]. These values, therefore, assume that the emissions were from the surface and neglect any matrix effects. The quantitativeness of the p r o m p t radiation analysis is more difficult to assess. Ultra-high vacua were not used in the specimen chamber, nor was ion cleaning or liquid nitrogen vapour trapping employed, and it is k n o w n that the ion beam b o m b a r d m e n t leads to a slow building of deposited organic material on the surface being analysed. For these reasons reliable data were available for carbon and nitrogen concentrations at a d e p t h of some microns under the surface only. These
558
A.J. Bentley et al. / Analysis for light elements
data, however, were a valuable c o m p l e m e n t to the Auger m e a s u r e m e n t s which were confined to the surface region. In order to be able to extract absolute concentrations, the p r o t o n spectra from gold were c o m p a r e d with those from an accurately standardised steel (N 0.162, C 0.04, S 0.016, P 0.032% by weight). The steel was in the form of turnings a n d the physical nature of the standards, i.e. of varying thickness and coherence, led to an absolute uncertainty of a b o u t +15%. However, repeated m e a s u r e m e n t s on given gold specimens were reproducible to better than _+ 5%. Analysis of the proton groups corresponding to surface C and O indicated that b o t h species were present in approximately equal quantities. This finding does not agree with the Auger measurements. However, the k n o w n c o n t a m i n a t i o n build-up on the surface during deuteron b o m b a r d m e n t should not be overlooked.
accuracy of the data, the equivalence shown in table I is very good. The order of magnitude of the C and N c o n t e n t s measured by (d, p) reactions agrees with analyses of similar electroplate reported in refs. 1 and 2. The strength of the C signal prevented the estimated of bulk O concentration by (d, p) reaction and the absence of O after ion profiling in the Auger spectra may be due to the lability of it surface compounds. However, it does seem likely that CoO or Co 0.0H if present in bulk [6] would have resulted in O Auger emissions from the surface when exposed by ion sputtering. Finally, the absence of N and the drastic lowering of C levels from the Auger spectra on progressively sputtered specimens are in agreement with a conclusion from SIMS analysis of gold electroplate [5] that, whatever their bulk concentrations, large chemically b o u n d cyanide complexes are not surface-stable.
4.2. Comparison of Auger and (d, p) analyses It is difficult to make a comparison between the Auger and (d, p) analyses for C a n d N as shown in table 1 because of the very different depths at which the analyses have been made. However, it is clear that the (d, p) results indicate consistently higher C and N concentrations at d e p t h in the samples. This could well be because of the much higher sensitivity of the nuclear technique and the fact that the concentrations recorded are near the limit of detection for Auger. Small changes in absolute calibration of either technique could account for the differences. A more likely possibility could be that C and N (and perhaps O) are lost from the inclusions in the electroplate during the surface erosion, i.e. there is an artefact in ion profiling due to the lability of the inclusions when these lie in the surface. P r o m p t radiation analysis, whereby the analytical i n f o r m a t i o n comes from a considerable depth of material, is not subject to this effect.
4.3. Comparison with other instrumental analyses It is, therefore, interesting to compare the present results, especially of nuclear reaction analyses, with those from the application MOssbauer spectroscopy to similar gold cobalt electrodeposits [6,7]. There is agreem e n t in the quantities of cobalt incorporated. F r o m M 6 s s b a u e r spectroscopy up to 70% of the cobalt is f o u n d to be substitutionally alloyed with gold. The r e m a i n d e r is held as particles (cf. Munier [10]) of complex cyanides [7,11]. Accepting that the inclusions observed t h r o u g h o u t the deposits contain cyanide complexes [e.g. K2Co(CN)6 ], it is reasonable that the C and N contents should be equivalent. Bearing in mind the
5. Conclusions A comparison of analyses by nuclear reactions and by Auger spectroscopy has revealed a significant artefact in ion-sputtering g o l d - c o b a l t electrodeposits. Results support the conclusion that inclusions in these deposits contain complex cyanides.
References [1] D.R. Mason and A. Blair, Trans. Inst. Met. Fin. 55 (1977) 141; Plating in the electronics industry, Amer. Electroplaters Soc. Vth Symp. (1975). [2] Ch.J. Raub and A. Knodler, Gold Bulletin 10 (1977): also literature cited in: J. Electrochem. Soc. 126 (1979) [391. [3] S. Nakahara and Y. Okinaka, J. Electrochem. Soc. 123 (1976) 1284. [4] S. Nakahara and Y. Okinaka, J. Electrochem. Soc. 128 (1981) 284. [5] R. Schubert, J. Electrochem. Soc. 128 (1981) 126. [6] H. Leidheiser, A. Vertes, M.L. Varsanyi and 1. CzakoNagy, J. Electrochem. Soc. 126 (1979) 391. [7] R.L. Cohen, F.B. Koch, L.N. Schoenberg and K.W. West, J. Electrochem. Soc. 126 (1979) 1608. [8] R.A. Jarjis, Internal report, Department of Physics, University of Manchester (1979). [9] L.E. Davis, N.C. McDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, eds., Handbook of Auger spectroscopy (Physics Electronics Industries Inc., Eden Prairie, Minnesota, 1976). [10] G.B. Munier, Plating 56 (1969) 1151. [11] Y. Okinawa, F.B. Koch, C. Wolowiuk and D.R. Blessington, J. Electrochem. Soc. 125 (1978) 1745.