- Email: [email protected]
Some observations on sample sputtering in a glow discharge
Spectrochimics Acts, Vol.29B,
pp. 73 to 77. PergamonPm-s 1974. Printed in Northern Ireland
Some observations on sample sputtering in a glow discharge H. Jiio~~ and F. BLUY National Physical Research Laboratory, C.S.I.R., Pretoria, Republic of South Africa (Received 19 November1973) Abstract-Cross-sections through burn-spots on brass and gold samples produced by a glow discharge lamp have been investigated with the aid of a scanning electron microscope. These samples contain lead in the form of inclusions. Ion bombardment of the lead-rich sample in the lamp attacks these inclusionsto a lesserdegreethan it does the matrix and cones are formed on the sample surface during sputtering. An explanation for this effect is given and the analytical aspects are discussed. 1.
INTRODUCTION
CATHODICsputtering by ion bombardment in a low pressure gas discharge is a phenomenon that has been known and studied for more than a century. A survey of numerous publications dealing with this subject is given by KAMINSKY[l]. Hollow cathode lamps, where use is made of cathodic sputtering, are widely used as spectrochemical light sources. Cathodic sputtering was exploited by Grimm in 1968 for rapid analyses of conductive samples in the glow discharge lamp [2]. The glow discharge lamp has been the subject of several investigations [3, 41 and has since 1968 been applied to various analytical problems [5-81. The following facts must be considered in the case of cathodic sputtering in the glow discharge lamp: (i) The target is bombarded by ions of different masses (originsting from the discharge gas and the sample material); the ions have a wide energy range because they origin&e from various parts of the cathode fall, and there is a wide angular distribution of the impact due to collisions. (ii) The sample is inhomogeneous, both physically (multi-crystalline) and chemically (alloy, inclusions). 2. EXPERIMENTAL The burn-spots produced in an argon atmosphere by a glow discharge lamp (RSV, Germany) were investigated with the aid of a scanning electron miscroscope of the type JEOL JSM-U3 equipped with a 2 x 400 channel energy-dispersive X-ray analyser (EDAX International, Inc.). The spectra emitted by the lamp were recorded with a 2 m grating spectrometer (Ebert mount, RSV) using an on-line computer (General Data, Corporation “Nova 1200”). [l] M. KAMINSKY, Atomic and Ior& Impact Phenomena on Metal Surface. Springer, Berlin, Heidelberg, New York (1965). [2] W. GRIMM,Spectrochim. Acta 23B, 443 (1968). [3] M. DOGAN,I(. LA&WAand H. MASSMANN,Spectrochim. Acta 26B, 631 (1971). [4] P. W. J. M. BOWMANS,And. Chem. 44, 1291 (1972). [5] M. DOGAN,K. LAQUAand H. MASSMANX,Spectrochim. Acta 27B, 65 (1972). [6] S. EL ALFY, K. LAQUAand H. MASSMANN,2. Anal. Chem. 285, 1 (1973). [7] H. J~QER, Anal. Chim. Acta M, 57 (1972). [S] H. J&ER, AnaL Chim. Acta 60, 303 (1972). 73
H. J&ER
74
and F. BLUM
A variety of gold and brass samples were exposed to sputtering iu the glow discharge lamp. Cross-sectional cuts were made through the burn-spots. The sample was then embedded in Araldite for side-on observation and polished with diamond powders (0.25 pm final). 3.
RESULTS
Preliminary investigations of the burn-spots and of their cross-sections under a binooular microscope revealed three facts: (i) The sample removal depends on crystal structure and orientation of the sample. (ii) There are no signs of any surface melting. (iii) The sputtered surface is dotted with cones if the sample contains inclusions. Further investigations were then carried out with the scanning electron microscope. Secondary and backscattered electron images of these cross-se&ions were recorded and spot analyses of different zones were performed. Figures la and 2a show electron ~~ro~aphs of cross-sections through the burn-spots of a brass and a gold sample, respectively. Composition of the samples and discharge conditions are given in Table 1. Both electron micrographs show scattered inclusions in the matrix (maximum size &-10 ,um) and cones formed on the surface of the sample. Inclusions can be found at the tops of the cones. The cone on the right-hand side in Fig. 2a is either not cut through its center or the top has already disappeared. The steady bombardment by ions will repeatedly form new cones while others are removed. The average composition of the inclusion as obtained by spot analysis is given in Table 2. Two different types of inclusions were found in the brass sample, viz., type 1 with high lead concentration and type 2 with high zinc concentration.
The tops of the cones in the gold and brass samples show very high lead concentrations. The composition of the tops of the cones corresponds to that of the inclusions as is given in Table 2. Analyses from the top of the cones into the matrices (Figs. lb and 2b) show the concentration distribution of some characteristic elements: (i) Lead concentration in the brass sample decreases from 80-90 % at the top to 0 % at the base of cones, while the copper concentration increases from 10-20 % Table 1. Accepted oomposition of samples and discharge conditions Sample Brass-alloy Gold-alloy
Composition, a-t % 84.79 % 5.22 % 32.52 % 4.53 %
Cu, 4.96 % Zn Sn, 4.79 % Pb Au, IO-52% Ag Cu, 2.43 % Pb
Discharge conditions 7oov,
18OmA,
0.93 kPa4
300 Vb
60 mA”, 0.67 kPaa
E+Pressuremeasured in the pumping system. b Pulsed mode, voltage and current are average indiwtions of the meters.
Exposure time 90 sex.?
6Osec
Depth
in sample,
pm
(b) Fig. I. Sputtered brass sample. a: electron micrograph of cross-section through burn-spot; b: Pb and Cu concentrations as functions of depth in sample (indicated by arrow). 74
.
60
.-5
+ t? +
/, t/ 0
40
0
I
Depth
1 Pbl
I
I
5
15
IO
in sample,
(b
20
pm
)
Fig. 2. Sputtered gold sample. a: electron micrograph of cross-section throw burn-spot; b: Au, Pb and Cu concentrations as functions of depth in sam (indicated by arrow).
Fig. 3. Electron micrograph of cross-section through cone where the top is broken off.
Someobservations
on sample sputtering in a glow discharge
Table 2. Average composition Composition
Sample Brass-alloy Gold-alloy
76
of inclusions
of inclusions, wt %
10% cu (type 1) 90 % Pb, 70% Zn, 20% Cu, 10% Pb (type 2) 20-70 % Au, 4-8 % Cu 20-70 % Pb,
to 90 %. The copper concentration in the matrix is 90 % and the lead concentration can be between 0 and 0.9 % as the detection limit for lead in brass is O-9%. (ii) The lead and copper concentration in the gold sample decreases from about 30 % and 20 %, respectively, at the top to 0 % at the base of cones, while gold increases from about 50 % to 90 %. The gold concentration in the matrix is 90 % and the lead concentration can be as high as l-2 % which is the detection limit of lead in gold. The composition of the sample close to the surface on the planes between cones and at the lower sides of cones corresponds to that of the matrix. 4. DISCUSSION It has been shown that the formation of cones during the sputtering of the sample is caused by the presence of inclusions with high lead concentrations. These inclusions are more resistant than the matrix to sputtering. The second type of inclusions in the brass sample containing high concentrations of zinc was never found on the top of the cones and seems to be similarly affected as the matrix by sputtering. Since lead is sputtered at the same rate as gold or copper as shown in Table 3, it cannot be present in the inclusions in the metallic state. It is known that oxide layers on surfaces of aluminium and magnesium samples can prevent the sputtering of the elements in the glow discharge lamp. Accordingly it can be assumed that the lead inclusions are either present as oxides or are surrounded by oxide layers. It can further be seen in Fig. 2a that each cone is surrounded by a circular trough. This could have been caused by a distortion of the electrical field in the vicinity of the cones due to a space charge. Such a distortion around an inclusion will cause concentration of the field at a certain distance from the space charge, resulting in increased bombardment of the target in a circular zone around the base of cones. Table 3. Sputtering rates of pure elements Element Au Ag cu Zn Pb
mg/min
Particles/min
3.3
0.10 x
3.6 1.1 3.4 4.9
0.20 0.10 0.31 0.12
x x x x
1020 1020 1020 1020 1020
Sputtering conditions8 1,2kPa 1,2 kPa I,2 kPa 1,2 kPa 1,2 kPa
400V 400V 400V 400V 400V
71mA 122mA 122mA 140mA 125mA
8 The pulsed mode of the discharge has been applied to prevent lead from melting and current and voltage values are average indioations of the meters.
H. J&ER
76
and F. BL~M
Obviously, the height of the cone will increase as long as the material of the inclusion remains on top of it. After the removal of the lead-rich material the cone will gradually be reduced in size because the bombardment is then no longer obstructed. The removal of the top may be by sputtering, but there are indications on some electron micrographs that the top may break off as soon as the cone reaches a certain height as is indicated in Fig. 3. 5. ANALYTICALASPECTS Burn-off curves resulting from different discharge conditions and recorded with a direct-reading spectrometer revealed in all cases a significant deviation of the lead intensity from the general trend of the other elements. As a typical example, the burn-off curves for copper, zinc, lead and tin of a brass sample (NBS 1105: 63.72 % Cu, 34.03 % Zn, 2.01% Pb, 0.20 % Sn) are shown in Fig. 4. Each measurement was obtained by integrating the intensity over 1.6 set at intervals of O-4 sec. The main elements copper and zinc and the trace element tin show an initially steep and then gradual increase in intensity, whereas the lead intensity reaches a maximum after 20 set and declines to a minimum after 70 set, after which it behaves similarly to the other elements. The initial intensity peak of lead might be caused by the fact that the inclusions at the surface were cut during the sample preparation and that the lead therefore is no longer covered by a protective oxide layer. The lead is therefore affected by the ion bombardment during this initial
IO .
0.
t
I
40
60 Time,
I
120
I
160
200
set
Fig. 4. Burn-off curves of brass sample (NW 1106). Sputteringoonditions: 1 kV, 120mA, 0.8 kPa.
Some
observationson samplesputteringin a glow discharge
77
sputtering period in the same way as the other elements of the matrix, and the sputtering rates as given in Table 3 are relevant. After the removal of the surface layer, the lead inclusions show resistance to ion bombardment as these inclusions were not affected by the sample preparation. An equilibrium is reached after 80 set between the lengthening and the removal of the cones. The lead thereafter follows the same trend as the other elements. The time taken for equilibrium to be reached is very dependent on the discharge conditions. The statistical fluctuations in the burn-off curves are determined by fluctuations of the photomultiplier signal rather than by the distribution of the elements in the sample. The greater fluctuations in the tin curve (0.20 % Sn) are due to the high gain setting of the photomultiplier. The good reproducibility of the results indicates that the sample area subjected to sputtering (50 mm2) was sufficient to give a representative sample. 6. CONCLUSIONS The results presented in this paper indicate that satisfactory calibration curves are possible for all elements if a rather long preburn time is used. The analytical results for lead in brass and gold obtained thus far have been good and the results will be reported elsewhere [9]. [91 H. J&ER, Anal. C&m. Acta (submitted for publication).
pp. 73 to 77. PergamonPm-s 1974. Printed in Northern Ireland
Some observations on sample sputtering in a glow discharge H. Jiio~~ and F. BLUY National Physical Research Laboratory, C.S.I.R., Pretoria, Republic of South Africa (Received 19 November1973) Abstract-Cross-sections through burn-spots on brass and gold samples produced by a glow discharge lamp have been investigated with the aid of a scanning electron microscope. These samples contain lead in the form of inclusions. Ion bombardment of the lead-rich sample in the lamp attacks these inclusionsto a lesserdegreethan it does the matrix and cones are formed on the sample surface during sputtering. An explanation for this effect is given and the analytical aspects are discussed. 1.
INTRODUCTION
CATHODICsputtering by ion bombardment in a low pressure gas discharge is a phenomenon that has been known and studied for more than a century. A survey of numerous publications dealing with this subject is given by KAMINSKY[l]. Hollow cathode lamps, where use is made of cathodic sputtering, are widely used as spectrochemical light sources. Cathodic sputtering was exploited by Grimm in 1968 for rapid analyses of conductive samples in the glow discharge lamp [2]. The glow discharge lamp has been the subject of several investigations [3, 41 and has since 1968 been applied to various analytical problems [5-81. The following facts must be considered in the case of cathodic sputtering in the glow discharge lamp: (i) The target is bombarded by ions of different masses (originsting from the discharge gas and the sample material); the ions have a wide energy range because they origin&e from various parts of the cathode fall, and there is a wide angular distribution of the impact due to collisions. (ii) The sample is inhomogeneous, both physically (multi-crystalline) and chemically (alloy, inclusions). 2. EXPERIMENTAL The burn-spots produced in an argon atmosphere by a glow discharge lamp (RSV, Germany) were investigated with the aid of a scanning electron miscroscope of the type JEOL JSM-U3 equipped with a 2 x 400 channel energy-dispersive X-ray analyser (EDAX International, Inc.). The spectra emitted by the lamp were recorded with a 2 m grating spectrometer (Ebert mount, RSV) using an on-line computer (General Data, Corporation “Nova 1200”). [l] M. KAMINSKY, Atomic and Ior& Impact Phenomena on Metal Surface. Springer, Berlin, Heidelberg, New York (1965). [2] W. GRIMM,Spectrochim. Acta 23B, 443 (1968). [3] M. DOGAN,I(. LA&WAand H. MASSMANN,Spectrochim. Acta 26B, 631 (1971). [4] P. W. J. M. BOWMANS,And. Chem. 44, 1291 (1972). [5] M. DOGAN,K. LAQUAand H. MASSMANX,Spectrochim. Acta 27B, 65 (1972). [6] S. EL ALFY, K. LAQUAand H. MASSMANN,2. Anal. Chem. 285, 1 (1973). [7] H. J~QER, Anal. Chim. Acta M, 57 (1972). [S] H. J&ER, AnaL Chim. Acta 60, 303 (1972). 73
H. J&ER
74
and F. BLUM
A variety of gold and brass samples were exposed to sputtering iu the glow discharge lamp. Cross-sectional cuts were made through the burn-spots. The sample was then embedded in Araldite for side-on observation and polished with diamond powders (0.25 pm final). 3.
RESULTS
Preliminary investigations of the burn-spots and of their cross-sections under a binooular microscope revealed three facts: (i) The sample removal depends on crystal structure and orientation of the sample. (ii) There are no signs of any surface melting. (iii) The sputtered surface is dotted with cones if the sample contains inclusions. Further investigations were then carried out with the scanning electron microscope. Secondary and backscattered electron images of these cross-se&ions were recorded and spot analyses of different zones were performed. Figures la and 2a show electron ~~ro~aphs of cross-sections through the burn-spots of a brass and a gold sample, respectively. Composition of the samples and discharge conditions are given in Table 1. Both electron micrographs show scattered inclusions in the matrix (maximum size &-10 ,um) and cones formed on the surface of the sample. Inclusions can be found at the tops of the cones. The cone on the right-hand side in Fig. 2a is either not cut through its center or the top has already disappeared. The steady bombardment by ions will repeatedly form new cones while others are removed. The average composition of the inclusion as obtained by spot analysis is given in Table 2. Two different types of inclusions were found in the brass sample, viz., type 1 with high lead concentration and type 2 with high zinc concentration.
The tops of the cones in the gold and brass samples show very high lead concentrations. The composition of the tops of the cones corresponds to that of the inclusions as is given in Table 2. Analyses from the top of the cones into the matrices (Figs. lb and 2b) show the concentration distribution of some characteristic elements: (i) Lead concentration in the brass sample decreases from 80-90 % at the top to 0 % at the base of cones, while the copper concentration increases from 10-20 % Table 1. Accepted oomposition of samples and discharge conditions Sample Brass-alloy Gold-alloy
Composition, a-t % 84.79 % 5.22 % 32.52 % 4.53 %
Cu, 4.96 % Zn Sn, 4.79 % Pb Au, IO-52% Ag Cu, 2.43 % Pb
Discharge conditions 7oov,
18OmA,
0.93 kPa4
300 Vb
60 mA”, 0.67 kPaa
E+Pressuremeasured in the pumping system. b Pulsed mode, voltage and current are average indiwtions of the meters.
Exposure time 90 sex.?
6Osec
Depth
in sample,
pm
(b) Fig. I. Sputtered brass sample. a: electron micrograph of cross-section through burn-spot; b: Pb and Cu concentrations as functions of depth in sample (indicated by arrow). 74
.
60
.-5
+ t? +
/, t/ 0
40
0
I
Depth
1 Pbl
I
I
5
15
IO
in sample,
(b
20
pm
)
Fig. 2. Sputtered gold sample. a: electron micrograph of cross-section throw burn-spot; b: Au, Pb and Cu concentrations as functions of depth in sam (indicated by arrow).
Fig. 3. Electron micrograph of cross-section through cone where the top is broken off.
Someobservations
on sample sputtering in a glow discharge
Table 2. Average composition Composition
Sample Brass-alloy Gold-alloy
76
of inclusions
of inclusions, wt %
10% cu (type 1) 90 % Pb, 70% Zn, 20% Cu, 10% Pb (type 2) 20-70 % Au, 4-8 % Cu 20-70 % Pb,
to 90 %. The copper concentration in the matrix is 90 % and the lead concentration can be between 0 and 0.9 % as the detection limit for lead in brass is O-9%. (ii) The lead and copper concentration in the gold sample decreases from about 30 % and 20 %, respectively, at the top to 0 % at the base of cones, while gold increases from about 50 % to 90 %. The gold concentration in the matrix is 90 % and the lead concentration can be as high as l-2 % which is the detection limit of lead in gold. The composition of the sample close to the surface on the planes between cones and at the lower sides of cones corresponds to that of the matrix. 4. DISCUSSION It has been shown that the formation of cones during the sputtering of the sample is caused by the presence of inclusions with high lead concentrations. These inclusions are more resistant than the matrix to sputtering. The second type of inclusions in the brass sample containing high concentrations of zinc was never found on the top of the cones and seems to be similarly affected as the matrix by sputtering. Since lead is sputtered at the same rate as gold or copper as shown in Table 3, it cannot be present in the inclusions in the metallic state. It is known that oxide layers on surfaces of aluminium and magnesium samples can prevent the sputtering of the elements in the glow discharge lamp. Accordingly it can be assumed that the lead inclusions are either present as oxides or are surrounded by oxide layers. It can further be seen in Fig. 2a that each cone is surrounded by a circular trough. This could have been caused by a distortion of the electrical field in the vicinity of the cones due to a space charge. Such a distortion around an inclusion will cause concentration of the field at a certain distance from the space charge, resulting in increased bombardment of the target in a circular zone around the base of cones. Table 3. Sputtering rates of pure elements Element Au Ag cu Zn Pb
mg/min
Particles/min
3.3
0.10 x
3.6 1.1 3.4 4.9
0.20 0.10 0.31 0.12
x x x x
1020 1020 1020 1020 1020
Sputtering conditions8 1,2kPa 1,2 kPa I,2 kPa 1,2 kPa 1,2 kPa
400V 400V 400V 400V 400V
71mA 122mA 122mA 140mA 125mA
8 The pulsed mode of the discharge has been applied to prevent lead from melting and current and voltage values are average indioations of the meters.
H. J&ER
76
and F. BL~M
Obviously, the height of the cone will increase as long as the material of the inclusion remains on top of it. After the removal of the lead-rich material the cone will gradually be reduced in size because the bombardment is then no longer obstructed. The removal of the top may be by sputtering, but there are indications on some electron micrographs that the top may break off as soon as the cone reaches a certain height as is indicated in Fig. 3. 5. ANALYTICALASPECTS Burn-off curves resulting from different discharge conditions and recorded with a direct-reading spectrometer revealed in all cases a significant deviation of the lead intensity from the general trend of the other elements. As a typical example, the burn-off curves for copper, zinc, lead and tin of a brass sample (NBS 1105: 63.72 % Cu, 34.03 % Zn, 2.01% Pb, 0.20 % Sn) are shown in Fig. 4. Each measurement was obtained by integrating the intensity over 1.6 set at intervals of O-4 sec. The main elements copper and zinc and the trace element tin show an initially steep and then gradual increase in intensity, whereas the lead intensity reaches a maximum after 20 set and declines to a minimum after 70 set, after which it behaves similarly to the other elements. The initial intensity peak of lead might be caused by the fact that the inclusions at the surface were cut during the sample preparation and that the lead therefore is no longer covered by a protective oxide layer. The lead is therefore affected by the ion bombardment during this initial
IO .
0.
t
I
40
60 Time,
I
120
I
160
200
set
Fig. 4. Burn-off curves of brass sample (NW 1106). Sputteringoonditions: 1 kV, 120mA, 0.8 kPa.
Some
observationson samplesputteringin a glow discharge
77
sputtering period in the same way as the other elements of the matrix, and the sputtering rates as given in Table 3 are relevant. After the removal of the surface layer, the lead inclusions show resistance to ion bombardment as these inclusions were not affected by the sample preparation. An equilibrium is reached after 80 set between the lengthening and the removal of the cones. The lead thereafter follows the same trend as the other elements. The time taken for equilibrium to be reached is very dependent on the discharge conditions. The statistical fluctuations in the burn-off curves are determined by fluctuations of the photomultiplier signal rather than by the distribution of the elements in the sample. The greater fluctuations in the tin curve (0.20 % Sn) are due to the high gain setting of the photomultiplier. The good reproducibility of the results indicates that the sample area subjected to sputtering (50 mm2) was sufficient to give a representative sample. 6. CONCLUSIONS The results presented in this paper indicate that satisfactory calibration curves are possible for all elements if a rather long preburn time is used. The analytical results for lead in brass and gold obtained thus far have been good and the results will be reported elsewhere [9]. [91 H. J&ER, Anal. C&m. Acta (submitted for publication).