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Auger electron spectroscopy (AES) studies of argon ion induced desorption of carbon from tantalum
L550
Surface Science 109 (1981) L5506L554 North~olland Publishin& Company
SURFACE SCIENCE LETTERS AUGER ELECTRON SPECTROSCOPY (AES) STUDIES OF ARGON ION EDUCED
D~SO~TION
OF CARBON FROM TANTALUS
J.L. PENA * Physics Department, Surface Studies Laboratory, Wisconsin 53201, USA
Received
University of Wisconsin, Milwaukee,
7 April 1981
AES studies carbon adlayer was unvarying surface normal measured under
of argon ion induced desorption of carbon from tantalum were performed. The was allowed to adsorb from a well characterized residual gas atmosphere, that within 20% The argon ions impact on the surface at an angle of 60” from the with energies between 0.2 to 1.0 keV. The total desorption cross section values these conditions are 0.07- 1.1 X 10-r 5 cm*.
Argon ion bombardment has been extensively used as a means of cleaning solid surfaces, particularly during AES analysis. As is well known, carbon is one of the most common contaminants found in surface studies and it can be desorbed during ion bombardment. However, because of the very process of the cleaning technique, very few systematic studies have been carried out on the ion stimulated desorption (sputtering) process itself. In recent years more work has been done in such directions [l] because the particle stimulated desorption of adsorbed species on the first walls of fusion devices was recognized to be one of the major sources of plasma contamination [2]. These problems are considered in the selection and testing of materials used in fusion machines; among the refractory metals, tantalum has been used as a shield [3]. In this paper total desorption cross sections (TDCS) for C (a,) on tantalum by argon ion bombardment are reported. The possible consequences of having a prebombarded surface in the desorption experiments are discussed. The experimental set-up and the measuring method have already been described elsewhere [4]. The sample used was a tantaium sheet provided by Alfa Products. It was cleaned in an ultrasonic bath using acetone and methanol as grease dissolvents, and afterwards was introduced into the vacuum system and cleaned with argon ion bombardment before the first experiment. For the desorption process and cleaning of the sample a differential pressure ion gun with low current density (ie - 1 X lo-’ A cm-‘) and a low system pressure (in the upper 10M9Torr range) was used. The changes in the carbon signal due to the ion bombardment were detected by means of AES. The carbon adlayer was allowed to adsorb on the clean tantalum * Permanent
address:
CIEA IPN Fisica, Ap. Postal
14-740,
Mexico
0039~6028/8 1/OOOO-0000/$02 SO 0 198 1 North-Holland
14 DF, Mexico.
XL. PeFa / AES studies
ofargonion induced demrption
of C fromTa
L551
surface from the residual gas atmosphere (base pressure in the low lo-” Torr range) that, during experiments, showed partial pressure analysis of -90% argon and 10% for other gases, such as H20, HaI C&I,, CH4, and CUfNa. This condition remained constant within 20%. This variation might account for some of the scatter to be seen later in fig. 3. For best stability and experimental control the ion gun was run continuously, thus insuring that the sample was exposed to the same atmosphere for each experiment. However, in some experiments, the time of exposure to the residual gas atmosphere was longer leading to multilayer adsorption, as will be discussed further. A glancing incidence electron gun with a current density of -10 ~.rA/crn’ was used. Care was taken to insure that there were no electron beam effects. After the desorption experiment, because of its nature, the tantalum surface was cleaned for the next experiment. However, because of knock-on effects some carbon atoms, as high as 10% of the initial coverage [S], can be recoil implanted in the bulk of the metal substrate. In those cases where the sample did not show a reasonable cleanliness, as determined by AES, a PHI ion gun with higher current density was used for sputtering (1e - 30 PA cm-*, E = 1 keV, t - 30 min, argon pressure of -5 X 10W5Torr). AESmonitoring of the surface did not show readsorption; at least during the time expended in the experiments. Ion beam profiles were obtained with a movable Faraday cup and they were constant and uniform in the region of AES analysis. The carbon desorption experimental results can be divided into two typical cases, as figs. 1 and 2 show. The data points are the Auger peak-to-peak amplitudes (A,,) in relative units (RU) with fnst order sensitivity factor corrections, plotted
(1KeV)Ar’dTa
+
C
ic? = 5.6 x IO” ions cm-* set”’ 0 c = 2.7
0-i 0
2
4
x 1cl-‘6 cm2
6
3
* 10
ION DOSE [IONS cni2 x 1O’5] Fig. 1. Plot of Auger peak-to-peak relative values with sensitivity factor corrections versus the ion dose. The solid line is the fitting af eq. (1) (see text) for the carbon desorption. In this case the argon ion energy was 1.0 keV with current density of 5.6 X IO” ions cm-’ sY1. The measured value for oc is 2.7 X IO-‘6 cm2.
J.L. PeKa / AES studies of argon ion induced
L552
(0.5
KeV)
A;’
*To
t
desorption
of C from
Ta
C
0.8+
0 0
I
2
ION
3
DOSE
4
[IONS
5
cm-’
6
++ 7
xIO’~I
Fig. 2. Plot of Auger peak-to-peak relative values with sensitivity factor corrections versus the ion dose. The solid line is the fitting of eq. (1) (see text) for the carbon desorption. In this case the argon ion energy was 0.5 keV with current density of 5.6 X 10’ 1 ions cmW2s-l. The measured value for oC is 7 X lo-l6 cm*.
versus ion d,ose (ion dose E i,,t, where iO is the ion current density and t is time of ion bombardment). In both cases Ta and C signals account for more than 90% of the surface composition, but in fig. 2, the carbon adlayer is higher by a factor of two than in fig. 1. Those differences can be produced since we allow the adlayer to be adsorbed from the residual gas atmosphere for different lengths of time. Oxygen and nitrogen were detected in small amounts, giving a contribution of less than -6%. The oxygen decayed during ion bombardment at approximately the same rate as did the carbon. Because the oxygen signal to noise was quite low, cross section values were considered less reliable. Another difference is the background value; it is higher in fig. 2. When this condition prevails, the sample is cleaned with the PHI ion gun, then the adlayer is allowed to adsorb for the next experiment. The desorption of carbon from the tantalum surface under argon bombardment was investigated using the ion-induced desorption method [4,6]. The Auger signal from the adsorbate decreases exponentially as a function of the bombarding time: N(t) = No exp[-(ai0t)l
+ N, ,
where iot is the ion dose, u is the TDCS, No is the initial value of the adsorbed atom, N(t) is the number of the remaining adsorbed atoms at time t, and N, is the equilibrium value. We used a computer program which performs a nonlinear least squares fit of eq. (I) to our data points, and as is shown in figs. 1 and 2 (by solid lines), is in good agreement with the data. In the desorption experiments plotted in figs. 1 and 2, ion energies of 1 and 0.5 keV respectively and current density of 5.6 X 10” ions cm-* s- ’ in both cases were used. The calculated values for (I, were
J.L. Peii2 / AES studies of argon ion induced desorption of C from Ta
Is53
2.7 X lo-l6 and 7 X lo-l6 cm2, respectively. It was reasonable to expect an increase in the TDCS with higher ion energy because the sputtered atom flux from the substrate is larger. However the calculated value for TDCS is higher for the lower energy (0.5 keV). This result can be possibly related to the fact of having more than one monolayer adsorbed. r
!
16’58r 64
1
Ar+--mTa
2-
60’ +
FROM
t C NORMAL :
+
+
6-
+
+
+
: +
b”
+
+
+
2+ +
ICP+ 0
0.2 ION
+
0.4 ENERGY
0.6
0.8
1.0
[KeV]
Fig. 3. A plot of the total desorption of the argon ion beam.
cross section
of C from Ta, oc, as a function
of the energy
In fig. 3 the calculated values for TDCS of C on Ta under argon ion bombardment with an impact angle of 60” from normal are plotted for the range of incident ion energies of 0.2 to 1 .O keV. The experiments were performed on the same sample. TDCS values measured when the adlayer had higher content of carbon, i.e. more than one monolayer adsorbed, as in the case of fig. 2, are in general larger. It has been reported [7] that multilayer adsorption exhibits higher desorption cross sections. The values for TDCS reported in the present work are in the order of magnitude found in the literature [7]. However, as can be observed in fig. 3, the scatter of the data is very high. This behavior can be possibly related to two factors. One, when the TDCS are calculated from the desorption experiments where the carbon signal changes less than one order of magnitude, for example in fig. 1, the experimental error is larger. The second factor is related with the experimental use of prebombarded surfaces. Recently Taglauer et al. [8] have reported that ion stimulated desorption of oxygen from heavily damaged Cu surfaces due to ion bombardment (ion dose -10” ions cm-*) have shown drastic changes in the expected exponential decay [eq. (I)]. After the ion desorption experiments, they analyzed [8] the surface by means of scanning electron microscopy (SEM), finding a cone formation. They indicated [8], as a possible explanation for the complex changes in surface coverage with ion dose, an increase in the surface roughness (cone foimation)
Is54
J.L. Peiia / AES studies of argon ion induced desorption of C from Ta
leading to not well defined ion impact angle and to changes in the adsorbate binding energy. SEM analysis of our samples after the ion bombardment experiments did not show the cone formation; only slight changes in the roughness of the surfaces were observed. This roughness could still account for the scatter of the data in fig. 3. Another factor which may account for differing TDCS from one experiment to the next is that there are several molecules which are likely sources of the C AES signal. In addition we did not find changes in the exponential decay [eq. (l)]; such a result can be understood since our total ion dose was one order of magnitude lower than that in the experiments of Taglauer and Heiland [8]. The author gratefully acknowledges J.W. Dieball for taking the measurements and D. Lichtman for useful discussions. Also the support of this work by the Department of Energy, Division of Magnetic Fusion Energy, is acknowledged, under contract EY-76-S-02-242.5. The author received a CONACYT (Mexico) fellowship.
References [l] E. Taglauer, U. Beitat, G. Marin and W. Heiland, J. Nucl. Mater. 63 (1976) 193. [2] G.M. McCracken and P.E. Stott, Nucl. Fusion 19 (1979) 889. [3] D. Hildebrandt, W. Hintze, B. Juttner, M. Laux, .I. Lingertat, P. Pech, H.D. Reinder, Strusny and H. Wolff, J. Nucl. Mater. 93/94 (1980) 310. [4] J.L. Pena, J. Dieball and D. Lichtman, J. Vacuum Sci. Technol., to be published. [5] E. Taglauer, W. Heiland and R.J. MacDonald, Surface Sci. 90 (1979) 661. [6] E. Taglauer, G. Marin and W. Heiland, Appl. Phys. 13 (1977) 47. [7] E. Taglauer, W. Heiland and J. Onsgaard, Nucl. Instr. Methods 168 (1980) 571. [8] E. Taglauer and W. Heiland, J. Nucl. Mater. 93/94 (1980) 823.
H.
Surface Science 109 (1981) L5506L554 North~olland Publishin& Company
SURFACE SCIENCE LETTERS AUGER ELECTRON SPECTROSCOPY (AES) STUDIES OF ARGON ION EDUCED
D~SO~TION
OF CARBON FROM TANTALUS
J.L. PENA * Physics Department, Surface Studies Laboratory, Wisconsin 53201, USA
Received
University of Wisconsin, Milwaukee,
7 April 1981
AES studies carbon adlayer was unvarying surface normal measured under
of argon ion induced desorption of carbon from tantalum were performed. The was allowed to adsorb from a well characterized residual gas atmosphere, that within 20% The argon ions impact on the surface at an angle of 60” from the with energies between 0.2 to 1.0 keV. The total desorption cross section values these conditions are 0.07- 1.1 X 10-r 5 cm*.
Argon ion bombardment has been extensively used as a means of cleaning solid surfaces, particularly during AES analysis. As is well known, carbon is one of the most common contaminants found in surface studies and it can be desorbed during ion bombardment. However, because of the very process of the cleaning technique, very few systematic studies have been carried out on the ion stimulated desorption (sputtering) process itself. In recent years more work has been done in such directions [l] because the particle stimulated desorption of adsorbed species on the first walls of fusion devices was recognized to be one of the major sources of plasma contamination [2]. These problems are considered in the selection and testing of materials used in fusion machines; among the refractory metals, tantalum has been used as a shield [3]. In this paper total desorption cross sections (TDCS) for C (a,) on tantalum by argon ion bombardment are reported. The possible consequences of having a prebombarded surface in the desorption experiments are discussed. The experimental set-up and the measuring method have already been described elsewhere [4]. The sample used was a tantaium sheet provided by Alfa Products. It was cleaned in an ultrasonic bath using acetone and methanol as grease dissolvents, and afterwards was introduced into the vacuum system and cleaned with argon ion bombardment before the first experiment. For the desorption process and cleaning of the sample a differential pressure ion gun with low current density (ie - 1 X lo-’ A cm-‘) and a low system pressure (in the upper 10M9Torr range) was used. The changes in the carbon signal due to the ion bombardment were detected by means of AES. The carbon adlayer was allowed to adsorb on the clean tantalum * Permanent
address:
CIEA IPN Fisica, Ap. Postal
14-740,
Mexico
0039~6028/8 1/OOOO-0000/$02 SO 0 198 1 North-Holland
14 DF, Mexico.
XL. PeFa / AES studies
ofargonion induced demrption
of C fromTa
L551
surface from the residual gas atmosphere (base pressure in the low lo-” Torr range) that, during experiments, showed partial pressure analysis of -90% argon and 10% for other gases, such as H20, HaI C&I,, CH4, and CUfNa. This condition remained constant within 20%. This variation might account for some of the scatter to be seen later in fig. 3. For best stability and experimental control the ion gun was run continuously, thus insuring that the sample was exposed to the same atmosphere for each experiment. However, in some experiments, the time of exposure to the residual gas atmosphere was longer leading to multilayer adsorption, as will be discussed further. A glancing incidence electron gun with a current density of -10 ~.rA/crn’ was used. Care was taken to insure that there were no electron beam effects. After the desorption experiment, because of its nature, the tantalum surface was cleaned for the next experiment. However, because of knock-on effects some carbon atoms, as high as 10% of the initial coverage [S], can be recoil implanted in the bulk of the metal substrate. In those cases where the sample did not show a reasonable cleanliness, as determined by AES, a PHI ion gun with higher current density was used for sputtering (1e - 30 PA cm-*, E = 1 keV, t - 30 min, argon pressure of -5 X 10W5Torr). AESmonitoring of the surface did not show readsorption; at least during the time expended in the experiments. Ion beam profiles were obtained with a movable Faraday cup and they were constant and uniform in the region of AES analysis. The carbon desorption experimental results can be divided into two typical cases, as figs. 1 and 2 show. The data points are the Auger peak-to-peak amplitudes (A,,) in relative units (RU) with fnst order sensitivity factor corrections, plotted
(1KeV)Ar’dTa
+
C
ic? = 5.6 x IO” ions cm-* set”’ 0 c = 2.7
0-i 0
2
4
x 1cl-‘6 cm2
6
3
* 10
ION DOSE [IONS cni2 x 1O’5] Fig. 1. Plot of Auger peak-to-peak relative values with sensitivity factor corrections versus the ion dose. The solid line is the fitting af eq. (1) (see text) for the carbon desorption. In this case the argon ion energy was 1.0 keV with current density of 5.6 X IO” ions cm-’ sY1. The measured value for oc is 2.7 X IO-‘6 cm2.
J.L. PeKa / AES studies of argon ion induced
L552
(0.5
KeV)
A;’
*To
t
desorption
of C from
Ta
C
0.8+
0 0
I
2
ION
3
DOSE
4
[IONS
5
cm-’
6
++ 7
xIO’~I
Fig. 2. Plot of Auger peak-to-peak relative values with sensitivity factor corrections versus the ion dose. The solid line is the fitting of eq. (1) (see text) for the carbon desorption. In this case the argon ion energy was 0.5 keV with current density of 5.6 X 10’ 1 ions cmW2s-l. The measured value for oC is 7 X lo-l6 cm*.
versus ion d,ose (ion dose E i,,t, where iO is the ion current density and t is time of ion bombardment). In both cases Ta and C signals account for more than 90% of the surface composition, but in fig. 2, the carbon adlayer is higher by a factor of two than in fig. 1. Those differences can be produced since we allow the adlayer to be adsorbed from the residual gas atmosphere for different lengths of time. Oxygen and nitrogen were detected in small amounts, giving a contribution of less than -6%. The oxygen decayed during ion bombardment at approximately the same rate as did the carbon. Because the oxygen signal to noise was quite low, cross section values were considered less reliable. Another difference is the background value; it is higher in fig. 2. When this condition prevails, the sample is cleaned with the PHI ion gun, then the adlayer is allowed to adsorb for the next experiment. The desorption of carbon from the tantalum surface under argon bombardment was investigated using the ion-induced desorption method [4,6]. The Auger signal from the adsorbate decreases exponentially as a function of the bombarding time: N(t) = No exp[-(ai0t)l
+ N, ,
where iot is the ion dose, u is the TDCS, No is the initial value of the adsorbed atom, N(t) is the number of the remaining adsorbed atoms at time t, and N, is the equilibrium value. We used a computer program which performs a nonlinear least squares fit of eq. (I) to our data points, and as is shown in figs. 1 and 2 (by solid lines), is in good agreement with the data. In the desorption experiments plotted in figs. 1 and 2, ion energies of 1 and 0.5 keV respectively and current density of 5.6 X 10” ions cm-* s- ’ in both cases were used. The calculated values for (I, were
J.L. Peii2 / AES studies of argon ion induced desorption of C from Ta
Is53
2.7 X lo-l6 and 7 X lo-l6 cm2, respectively. It was reasonable to expect an increase in the TDCS with higher ion energy because the sputtered atom flux from the substrate is larger. However the calculated value for TDCS is higher for the lower energy (0.5 keV). This result can be possibly related to the fact of having more than one monolayer adsorbed. r
!
16’58r 64
1
Ar+--mTa
2-
60’ +
FROM
t C NORMAL :
+
+
6-
+
+
+
: +
b”
+
+
+
2+ +
ICP+ 0
0.2 ION
+
0.4 ENERGY
0.6
0.8
1.0
[KeV]
Fig. 3. A plot of the total desorption of the argon ion beam.
cross section
of C from Ta, oc, as a function
of the energy
In fig. 3 the calculated values for TDCS of C on Ta under argon ion bombardment with an impact angle of 60” from normal are plotted for the range of incident ion energies of 0.2 to 1 .O keV. The experiments were performed on the same sample. TDCS values measured when the adlayer had higher content of carbon, i.e. more than one monolayer adsorbed, as in the case of fig. 2, are in general larger. It has been reported [7] that multilayer adsorption exhibits higher desorption cross sections. The values for TDCS reported in the present work are in the order of magnitude found in the literature [7]. However, as can be observed in fig. 3, the scatter of the data is very high. This behavior can be possibly related to two factors. One, when the TDCS are calculated from the desorption experiments where the carbon signal changes less than one order of magnitude, for example in fig. 1, the experimental error is larger. The second factor is related with the experimental use of prebombarded surfaces. Recently Taglauer et al. [8] have reported that ion stimulated desorption of oxygen from heavily damaged Cu surfaces due to ion bombardment (ion dose -10” ions cm-*) have shown drastic changes in the expected exponential decay [eq. (I)]. After the ion desorption experiments, they analyzed [8] the surface by means of scanning electron microscopy (SEM), finding a cone formation. They indicated [8], as a possible explanation for the complex changes in surface coverage with ion dose, an increase in the surface roughness (cone foimation)
Is54
J.L. Peiia / AES studies of argon ion induced desorption of C from Ta
leading to not well defined ion impact angle and to changes in the adsorbate binding energy. SEM analysis of our samples after the ion bombardment experiments did not show the cone formation; only slight changes in the roughness of the surfaces were observed. This roughness could still account for the scatter of the data in fig. 3. Another factor which may account for differing TDCS from one experiment to the next is that there are several molecules which are likely sources of the C AES signal. In addition we did not find changes in the exponential decay [eq. (l)]; such a result can be understood since our total ion dose was one order of magnitude lower than that in the experiments of Taglauer and Heiland [8]. The author gratefully acknowledges J.W. Dieball for taking the measurements and D. Lichtman for useful discussions. Also the support of this work by the Department of Energy, Division of Magnetic Fusion Energy, is acknowledged, under contract EY-76-S-02-242.5. The author received a CONACYT (Mexico) fellowship.
References [l] E. Taglauer, U. Beitat, G. Marin and W. Heiland, J. Nucl. Mater. 63 (1976) 193. [2] G.M. McCracken and P.E. Stott, Nucl. Fusion 19 (1979) 889. [3] D. Hildebrandt, W. Hintze, B. Juttner, M. Laux, .I. Lingertat, P. Pech, H.D. Reinder, Strusny and H. Wolff, J. Nucl. Mater. 93/94 (1980) 310. [4] J.L. Pena, J. Dieball and D. Lichtman, J. Vacuum Sci. Technol., to be published. [5] E. Taglauer, W. Heiland and R.J. MacDonald, Surface Sci. 90 (1979) 661. [6] E. Taglauer, G. Marin and W. Heiland, Appl. Phys. 13 (1977) 47. [7] E. Taglauer, W. Heiland and J. Onsgaard, Nucl. Instr. Methods 168 (1980) 571. [8] E. Taglauer and W. Heiland, J. Nucl. Mater. 93/94 (1980) 823.
H.