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Auger-ion sputter profiling of some homogeneous binary systems
Surface Science 194 (1988) L127-L134 North-Holland, Amsterdam
L127
S U R F A C E SCIENCE LETTERS
AUGER-ION SPUTFER PROFILING OF SOME HOMOGENEOUS BINARY SYSTEMS QU Zhe and XIE Tian-Sheng Institute of Metal Research, Academia Sinica, Shenyang, People's Rep. of China Received 25 June 1987; accepted for publication 21 October 1987
Component depth distributions of some homogeneous binary systems have been measured by Auger analysis using sputter depth profiling under several sputter ion energies. The resulting sputtering curves can hardly reflect the original depth distribution. Some of the curves show that the components change monotonouslyfrom their bulk concentration with ion fluencewhile others show complex changes in the early stages of profiling. The steady-state concentrations predicted by an equation based on preferential sputtering arguments are in fairly good agreement with the experimental values. A proposition has been put forward to restore the damaged distribution curves.
Ion sputtering combined with Auger analysis, as a conventional composition versus depth profiling technique, has been applied extensively to determine the element diatribution of multicomponent systems. It has been regarded that such measurements have the advantage of good depth resolution and high sensitivity, and therefore can reveal the distribution in more detail. Many studies show that the distribution curves of elements in homogeneous alloys measured by this technique usually deviate more or less from the true distributio~ curves due to preferential sputtering. The species with the higher sputtering yield is expected to exhibit a sputter depth profile decreasing in concentration monotonously with increasing depth [1,2]. However, in some cases the sputter profile shows complex changes which have been simply attributed to the original distribution of the elements in the testing system or taken to be caused by the pre-treatment of the specimen, as for example ion bombardment, prior to profiling [3]. In order to learn how and to what extent the systems have suffered from ion sputtering, we have studied changes in the distribution of the surface composition of some homogeneous binary, systems under 0.5-3.0 keV Ar ion sputtering with ion current densities of 1-3 ~ A / c m ° and normat incidence, which are commonly chosen in ion sput~.er profiling. The binary systems measured in the present study are listed in table 1. These alloys form solid solutions over the whole concentration range. 0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L128
Qu Zhe, Xie Tian-Sheng / A uger-ion sputter profiling of binary systems
Table 1 Systems and concentrations (in at%) used in the present study Cu-Ni
Cu-Pd
Au-Pd
Cu-Au
Ni-Au
21.2-78.8 40.9-59.1 50.0-50.0
51.7-48.3
55.0-45.0
52.4-47.6
59.0-47.0
The samples were carefully prepared by melting pure metals in an induction furnace under Ar protection atmosphere, fully stirred and quenched in water. Then the ingots were homogenized at higher temperatures, forged to 2 mm thick blanks, annealed at lower temperatures and finally ground and polished to 1 mm thick specimens. Electron micro-probe measurements were employed to check the composition and homogeneity of the specimens. The samples were put into a vacuum chamber. After the vacuum reached 2 × 10 - 9 Torr or better the surfaces were scraped in situ by a specially designed device with a tip coated by titanium nitride just before the profiling measurements [4]. By this technique the contaminated surface layer was removed and a homogeneous binary specimen was prepared [5]. The specimen surface co,nposition was monitored by Auger testing after each scraping until no foreign element could be detected. The surface analysis for each ion sputtering interval was carded out by using an Auger electron spectrometer (Riber LAS 300). The electron beam size was about 200 ~tm, primary energy 3.0 keV, modulation voltage 2 Vp_p, beam current 5 #A; dN/dE-E spectra were recorded. The peak heights of Cu 920 and 60 eV, Ni 848 and 61 eV, Pd 330 eV, Au 239 and 69 eV were measured to determine the concentrations of the corresponding elements. Fhe component depth distribution cur~'es obtained with hio~ energy and lower energy transitions have almost the same feature. For simplicity only those curves measured with low energy peaks were presented. The concentration of the elements in each sputtering depth was calculated based on the scraped surfaces which were referred to as the bulk. Fig. 1 shows the profiling curves of C u - N i alloy with three different contents of Ni sputtered by 2.0 keV Ar +. The concentrations of Ni in the three specimens increased with sputtering depth as expected from the sputtering yields of Cu (4.3 atoms/ion) and Ni (3 atoms/ion). The steady-state concentrations analysed by Auger electron spectroscopy were fairly coincident with the calculated results obtained by using the following equation [6]: cF, =
+
),
)
where Cs is the steady-state concentration of species A on the surlace; Cau, C~ are the bulk concentrations of species A and B respectively; YA and YB are the sputtering yields on the pure dements A and B respectively. The calculated
Qu Zhe, Xie Tian-Sheng / A uger-ion sputter profiling of binary systems .~ ~'
Depth (atom ao
L129
layer) 8o
i
"
u
,,,~,,¢...e
F
80
o
o
Q 0
c
Z
O0 0
0
'
0
g
0
0 Sputtering
20 time rain
10
Fig. 1. Profiling curves of three homogeneous C u - N i alloys with Cu concentration of 0.21 (high), 0.41 (middle) and 0.5 (low), sputtered by 2.0 keV .4I +.
and measured steady-state concentrations of Ni for the three specimens are listed in table 2. The original element distribution is denoted by a dashed line parallel to the abscissa in all the cases. The profiling curves of component Ni dramatically deviated )rom the true distribution in the beginning, then slow)y reached the steady-state level. This kind of behaviour is conunoaty encountered in ion sputter profiling measurements of mulficomponent systems and is very difficult to distinguish from the situation "~heze ~he concentration gradient in the original distribution does exist. Table 2 Calculated and measured steady-state concentrations of specimens sputtered with 2.0 keV a~- ions System
Bulk conc. (at%)
Steady-state concentration (at%) Calculated
Measured
Cu-Ni
21-79
26-84
17-83
Cu-.Pd Au-Pd Au-Cu Au-Ni
50-50 52-48 55-45 48-52 41-59
41-59 48-52 52-48 48-52 33-67
4~.-_ 8 47-53 52-48 47-53 29-71
Qu Zhe, Xie Tian-Sheng / Auger-ion sputterprofiling of binary systems
L130
Depth
60
(atom
tayer)
/+5
90
135
|
I
I
0.5KEY Cu 0.52 Pd 0.4 8
KeV 55 ¢3
o~""~/
"o I1.
2KeY
t4m
o
o
3KeV
°,~
0
50
¢,j I= o ¢,,,,) O u
45~
=i
m
o i
0
10
I
,.
i
20 30 Sputtering Time (rain)
~0
Fig. 2. Composition-sputtering time curves of a homogeneous C u - P d alloy sputtered by argon ions of different energies.
Fig. 2 shows ~he ion profiling curves for a Cuo.52Pdo.48 ahoy. The sputter ion energies were 0.5, 1.0, 2.0 and 3.0 keV respectively• A rather complicated sputter depth profile was observed. For low ion fluences the Pd concentration drops down a little then rises up until it reaches steady-state conditions. At steady state, the Pd concentration can be higher than or nearly equal to its buLk concentration depending on the bombarding ion energy. The final concentration under 3.0 keV ion bombardment is roughly equal to ff,e bulk concentration, while for sputtering with 0.5, 1.0 and 2.0 keV Ar + ions enrichment was observed. The degree of enrichment increases with decreasing sputter ion energy. A "spike" appeared in each profiling curve in the nearsteady-state concentration of Pd calculated with )~a = 3.6 for 2.0 keV Ar + sputtering was 0.52 and the experimentai value was 0.53; the agreement between the two is again good. Fig. 3 shows the sputter profiling cur~es for a Au0.ssPd0.a5 alloy. These curves can be roughly divided into two categories; one comprises those curves
Qu Zhe, Xie Tian-Sheng / A uger-ion sputter profiling of binary systems
L131
Dept[, (atom layer) L~5 '
~ I P,_
90
!
.
.
.
.
.
.
.
135
t
.
.
.
.
.
.
I
Au 0.55 Pd 0.45
"
""
0,5KEY -
._
"6 54
.
-
"
°
IKeV
8
2KeY
Z5 8
.o
o
o
3KeY i,n 50
0
- ' ...... 10
'
20 Sputtering
30
....
40
Time (rain)
Fig. 3. Ion sputtering curves of a homogeneous A u - P d alloy sputtered by argon ions with various
energies.
that are obtained from sputtering by ions with low energies (0.5 and 1.0 keV), which leads to a maximum in the concentration of Au in the near-surface region, followed by a descent to reach the steady-state concentration; the other those from sputtering by higher energy ions, which leads to a valley in the beginning of sputtering, followed by an ascent to reach the steady-state concentration. The steady-state concentrations might be equal to, higher or lower than the bulk concentration depending on the ion energy. The calculated steady-state concentration of Au in P d - A u alloy sputtered with 2.0 keV ions, showed a perfect agreement with the experimental result (table 2). The ion sputter profiling curves of the Cu0.szAuo.as alloy are shown in fig. 4. The profULng curves sputtered whh ion having different energy display almost the same shape. All showed a minimum in the near-surface region, but lower energy ions were sharper and those sputtered with higher energy ions were smoother. The behaviour of the cur~,es obtained here is quite sirrfilar to that observed in a pre-bombarded A u - C u alloy whh the same profiling technique [3]. The steady-state concentration decreased on increasing the sputter ion energy. For the 2.0 keV ion sputtering curve the steady-state
Qu Zhe, Xie Tian-Sheng / Auger-ion sputterprofiling of binary systems
L132
Depth (atom l a y e r ) ~,
I
,~ 50
I
-
50
100
I
I
150
Cu 0.524 Au 0.476
•
-~ 0.5 KeV
"
0 1-
•
__e/f
.o_
o (:
-
~
'
"
. ....
--
i
KeY
2KeY
u
tO
,,
~5,
~
-
3KeY
U
°I 40 0
i
10
I
20 Sputtering Time(rain)
!
30
40
Fig. 4. Changes of the concentration with sputtering time in a homogeneous Cu-Au alloy sputtered with argon ions with various energies.
concentration of Au showed a reasonably good agreement with the calculated data (table 2), The data from table 2 show that the component with the lower sputtering yield is enriched in the surface layer when steady-state conditions are reached even though there may be several effects involved in the sputtering process. The steady-state concentration can be predicted by using eq. (1) which was derived based simply on the assumption that the sputtering yield of each component in an alloy has the same value as for the pure dement. The data listed in the third column of table 2 were calculated by taking YAu= 4.3, Ycu= 4.3, Ypd = 3.6 and YNi = 3.0 (atoms/ion) [7]. The excellent agreement between calculated and measured values implies that the sputtering yield is an intrinsic nature of the element which governs the final sputtering concentration. The effects of alloying on it is trivial at least for the systems and concentration ranges we studied here. Other mechanisms might be operative in the sputtering process but they did not exert any significant effects on the steady-state concentration. However, the transient concentration measured by ion sputter profiling showed unexpected changes: in most of the cases ~ "spike" in the early stage of sputtering was created and its amplitude and steepness depen, ls on the
Qu Zhe, Xie Tian-Sheng / Auger-ion sputterprofiling of binary systems
L133
system and ion energy. Similar phenomena have been observed in some pre-bombarded homogeneous binary systems and have been taken as evidence of enhanced segregation induced by pre-bombardment. The present study shows that this profiling technique might bring about a depression or a hill in the resulting distribution curve and this was not necessarily true in the initial distribution. However, this feature was usually accepted as a true distribution of the element in specimens before ion sputtering, because it has been generally regarded ,hat the ion sputtering curve is a reflection of the real distribution of elements. The sputtering curves drawn in figs. 1 - 4 show that a significant "distortion" introduced by ion sputtering is inevitable. Any conclusion derived from a sputtering curve without any calibration may lead to a wrong interpretation of the initial element distribution or of the mechanisms of ion sputtering. Such transient behaviour of the sputtering curves is difficult to understand on the basis of the individual processes involved in ion bombardment only. However, it can be rationalized by considering a combination of several different processes. Preferential sputtering is one of the phenomena often found to occur whenever a multicomponent system is subjected to ion bombardment. Due to different partial sputtering yields on the constituent elements, one of the elements will deplete gradually with sputtering time until the steady-state build-up (fig. 5, dashed line). During ion bombardment, ions disp;ace many atoms in the lattice in the course of their collision cascade, creating vacancies, interstitials, implanted ions and other defects, all of which can enhance atom diffusion. As a consequence of enhanced diffusion rates, one of the compo-
w0J
,m
°.."" ".o
<
............
"~
"" --. . . . .
(U
.a.} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b)
r.. o
|
i
i
S~ttering
Time
Fig. 5. Schematic illustration of the formation of the maxima/mirfima on a profiling curve o i a homogeneous binary system A-B. The sputtering y/eld YA> YB. Curve a due to surface segregation and removal. Curve b due to preferential sputtering. Curve c combination of cur,re a and curve b.
L134
Qu Zhe, Xie Tian.Sheng / Auger-ion sputterprofiling of binary systems
nents may segregate to the free surface and develop a subsurface depleted zone accordingly [8,9]. Therefore., haifizdly the segregating element piled up rapidly on the surface before slowing down (fig. 5, dotted line) due to that the enriched surface was removed and the depleted zone moved toward the surface. If the element preferentially sputterexl is the same element as the segregated one, the final measured surface concentration changes are obtained as a superposition of the segregation curve on the preferential sputtering curve (fig. 5, full line). The shape of the profiling curve is determined by the segregation rate and by the preferential sputtering rate. If two dements have the similar sputtering yield (Au-Cu system, 2 keV ion) then the shape of the profiling curve will be dominated by the segregation curve, if the difference between the sputtering yields of the two components is large then the profiling curve will be shaped mainly by the preferential sputtering process and the segregation contribution to the curve, if any, v,511be neutralized (the cases of Ni-Cu alloy and Au-Cu alloy with 3 keV ion), because the behaviour of segregation and preferential sputtering is strongly influenced by sputtering ion energy. Subsequently the profiling curves will vary, according to the sputtering conditions. The measured dement distribution function F ( M ) is actually a convolution of the real distribution function F(R) and the "distortion" function F(D). In order to "restore" the damage brought about by ion sputtering, a deconvolution treatment of the measured function F ( M ) is necessary to be carded out with F(D) which can be found in a well designed distribution system (for example a homogeneous binary system), or simply compare the measured curve of an ur&nown specimen with that of a standard specimen with the sirnilar composition under the same measurement conditions. The latter treatment, however, is somewhat arbitrary but it can at least minimize the artifacts created by the ion sputter-Auger analysis technique. The authors are grateful to the National Science Foundation of China for financial support of this work. The authors would like to thank the Department of Ceramics, the Umversity of Leeds, for help in the preparation of this paper.
References [1] [2] [3] [4] [5] [6] [7]
G. Bete, Surface ScL 92 (1950) 283. H.H. Tompkins, J. Vacuum ScL Tectmol. 16 (1979) 778. R.S. Li, T. Kosl-~kawa and K. Goto, Surface Sci. 121 (I982) L561. Qu Zhe, Shun Yu-Zhen a_ad Tu Li-Shua, Anal. Chem. 14 (!986) 775. W. F~Srber, G. Betz arid P. Braun, Nucl. Instr. Methods 132 (1976) 351. Qu Zhe, Surface Sci. 161 (1985)L549. R, Behfisch, Ed., Sputtering by Panicle Bombardment I, Physical Sputtering of Single Element Solids (Springer, Berlin, 1981). [8] R. Kelly, Surface Interface ?real. 7 (1985) 1. [9] N.Q. Lain and H. Wiedersich, NucL Instr. Methods B18 (1987) 471.
L127
S U R F A C E SCIENCE LETTERS
AUGER-ION SPUTFER PROFILING OF SOME HOMOGENEOUS BINARY SYSTEMS QU Zhe and XIE Tian-Sheng Institute of Metal Research, Academia Sinica, Shenyang, People's Rep. of China Received 25 June 1987; accepted for publication 21 October 1987
Component depth distributions of some homogeneous binary systems have been measured by Auger analysis using sputter depth profiling under several sputter ion energies. The resulting sputtering curves can hardly reflect the original depth distribution. Some of the curves show that the components change monotonouslyfrom their bulk concentration with ion fluencewhile others show complex changes in the early stages of profiling. The steady-state concentrations predicted by an equation based on preferential sputtering arguments are in fairly good agreement with the experimental values. A proposition has been put forward to restore the damaged distribution curves.
Ion sputtering combined with Auger analysis, as a conventional composition versus depth profiling technique, has been applied extensively to determine the element diatribution of multicomponent systems. It has been regarded that such measurements have the advantage of good depth resolution and high sensitivity, and therefore can reveal the distribution in more detail. Many studies show that the distribution curves of elements in homogeneous alloys measured by this technique usually deviate more or less from the true distributio~ curves due to preferential sputtering. The species with the higher sputtering yield is expected to exhibit a sputter depth profile decreasing in concentration monotonously with increasing depth [1,2]. However, in some cases the sputter profile shows complex changes which have been simply attributed to the original distribution of the elements in the testing system or taken to be caused by the pre-treatment of the specimen, as for example ion bombardment, prior to profiling [3]. In order to learn how and to what extent the systems have suffered from ion sputtering, we have studied changes in the distribution of the surface composition of some homogeneous binary, systems under 0.5-3.0 keV Ar ion sputtering with ion current densities of 1-3 ~ A / c m ° and normat incidence, which are commonly chosen in ion sput~.er profiling. The binary systems measured in the present study are listed in table 1. These alloys form solid solutions over the whole concentration range. 0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L128
Qu Zhe, Xie Tian-Sheng / A uger-ion sputter profiling of binary systems
Table 1 Systems and concentrations (in at%) used in the present study Cu-Ni
Cu-Pd
Au-Pd
Cu-Au
Ni-Au
21.2-78.8 40.9-59.1 50.0-50.0
51.7-48.3
55.0-45.0
52.4-47.6
59.0-47.0
The samples were carefully prepared by melting pure metals in an induction furnace under Ar protection atmosphere, fully stirred and quenched in water. Then the ingots were homogenized at higher temperatures, forged to 2 mm thick blanks, annealed at lower temperatures and finally ground and polished to 1 mm thick specimens. Electron micro-probe measurements were employed to check the composition and homogeneity of the specimens. The samples were put into a vacuum chamber. After the vacuum reached 2 × 10 - 9 Torr or better the surfaces were scraped in situ by a specially designed device with a tip coated by titanium nitride just before the profiling measurements [4]. By this technique the contaminated surface layer was removed and a homogeneous binary specimen was prepared [5]. The specimen surface co,nposition was monitored by Auger testing after each scraping until no foreign element could be detected. The surface analysis for each ion sputtering interval was carded out by using an Auger electron spectrometer (Riber LAS 300). The electron beam size was about 200 ~tm, primary energy 3.0 keV, modulation voltage 2 Vp_p, beam current 5 #A; dN/dE-E spectra were recorded. The peak heights of Cu 920 and 60 eV, Ni 848 and 61 eV, Pd 330 eV, Au 239 and 69 eV were measured to determine the concentrations of the corresponding elements. Fhe component depth distribution cur~'es obtained with hio~ energy and lower energy transitions have almost the same feature. For simplicity only those curves measured with low energy peaks were presented. The concentration of the elements in each sputtering depth was calculated based on the scraped surfaces which were referred to as the bulk. Fig. 1 shows the profiling curves of C u - N i alloy with three different contents of Ni sputtered by 2.0 keV Ar +. The concentrations of Ni in the three specimens increased with sputtering depth as expected from the sputtering yields of Cu (4.3 atoms/ion) and Ni (3 atoms/ion). The steady-state concentrations analysed by Auger electron spectroscopy were fairly coincident with the calculated results obtained by using the following equation [6]: cF, =
+
),
)
where Cs is the steady-state concentration of species A on the surlace; Cau, C~ are the bulk concentrations of species A and B respectively; YA and YB are the sputtering yields on the pure dements A and B respectively. The calculated
Qu Zhe, Xie Tian-Sheng / A uger-ion sputter profiling of binary systems .~ ~'
Depth (atom ao
L129
layer) 8o
i
"
u
,,,~,,¢...e
F
80
o
o
Q 0
c
Z
O0 0
0
'
0
g
0
0 Sputtering
20 time rain
10
Fig. 1. Profiling curves of three homogeneous C u - N i alloys with Cu concentration of 0.21 (high), 0.41 (middle) and 0.5 (low), sputtered by 2.0 keV .4I +.
and measured steady-state concentrations of Ni for the three specimens are listed in table 2. The original element distribution is denoted by a dashed line parallel to the abscissa in all the cases. The profiling curves of component Ni dramatically deviated )rom the true distribution in the beginning, then slow)y reached the steady-state level. This kind of behaviour is conunoaty encountered in ion sputter profiling measurements of mulficomponent systems and is very difficult to distinguish from the situation "~heze ~he concentration gradient in the original distribution does exist. Table 2 Calculated and measured steady-state concentrations of specimens sputtered with 2.0 keV a~- ions System
Bulk conc. (at%)
Steady-state concentration (at%) Calculated
Measured
Cu-Ni
21-79
26-84
17-83
Cu-.Pd Au-Pd Au-Cu Au-Ni
50-50 52-48 55-45 48-52 41-59
41-59 48-52 52-48 48-52 33-67
4~.-_ 8 47-53 52-48 47-53 29-71
Qu Zhe, Xie Tian-Sheng / Auger-ion sputterprofiling of binary systems
L130
Depth
60
(atom
tayer)
/+5
90
135
|
I
I
0.5KEY Cu 0.52 Pd 0.4 8
KeV 55 ¢3
o~""~/
"o I1.
2KeY
t4m
o
o
3KeV
°,~
0
50
¢,j I= o ¢,,,,) O u
45~
=i
m
o i
0
10
I
,.
i
20 30 Sputtering Time (rain)
~0
Fig. 2. Composition-sputtering time curves of a homogeneous C u - P d alloy sputtered by argon ions of different energies.
Fig. 2 shows ~he ion profiling curves for a Cuo.52Pdo.48 ahoy. The sputter ion energies were 0.5, 1.0, 2.0 and 3.0 keV respectively• A rather complicated sputter depth profile was observed. For low ion fluences the Pd concentration drops down a little then rises up until it reaches steady-state conditions. At steady state, the Pd concentration can be higher than or nearly equal to its buLk concentration depending on the bombarding ion energy. The final concentration under 3.0 keV ion bombardment is roughly equal to ff,e bulk concentration, while for sputtering with 0.5, 1.0 and 2.0 keV Ar + ions enrichment was observed. The degree of enrichment increases with decreasing sputter ion energy. A "spike" appeared in each profiling curve in the nearsteady-state concentration of Pd calculated with )~a = 3.6 for 2.0 keV Ar + sputtering was 0.52 and the experimentai value was 0.53; the agreement between the two is again good. Fig. 3 shows the sputter profiling cur~es for a Au0.ssPd0.a5 alloy. These curves can be roughly divided into two categories; one comprises those curves
Qu Zhe, Xie Tian-Sheng / A uger-ion sputter profiling of binary systems
L131
Dept[, (atom layer) L~5 '
~ I P,_
90
!
.
.
.
.
.
.
.
135
t
.
.
.
.
.
.
I
Au 0.55 Pd 0.45
"
""
0,5KEY -
._
"6 54
.
-
"
°
IKeV
8
2KeY
Z5 8
.o
o
o
3KeY i,n 50
0
- ' ...... 10
'
20 Sputtering
30
....
40
Time (rain)
Fig. 3. Ion sputtering curves of a homogeneous A u - P d alloy sputtered by argon ions with various
energies.
that are obtained from sputtering by ions with low energies (0.5 and 1.0 keV), which leads to a maximum in the concentration of Au in the near-surface region, followed by a descent to reach the steady-state concentration; the other those from sputtering by higher energy ions, which leads to a valley in the beginning of sputtering, followed by an ascent to reach the steady-state concentration. The steady-state concentrations might be equal to, higher or lower than the bulk concentration depending on the ion energy. The calculated steady-state concentration of Au in P d - A u alloy sputtered with 2.0 keV ions, showed a perfect agreement with the experimental result (table 2). The ion sputter profiling curves of the Cu0.szAuo.as alloy are shown in fig. 4. The profULng curves sputtered whh ion having different energy display almost the same shape. All showed a minimum in the near-surface region, but lower energy ions were sharper and those sputtered with higher energy ions were smoother. The behaviour of the cur~,es obtained here is quite sirrfilar to that observed in a pre-bombarded A u - C u alloy whh the same profiling technique [3]. The steady-state concentration decreased on increasing the sputter ion energy. For the 2.0 keV ion sputtering curve the steady-state
Qu Zhe, Xie Tian-Sheng / Auger-ion sputterprofiling of binary systems
L132
Depth (atom l a y e r ) ~,
I
,~ 50
I
-
50
100
I
I
150
Cu 0.524 Au 0.476
•
-~ 0.5 KeV
"
0 1-
•
__e/f
.o_
o (:
-
~
'
"
. ....
--
i
KeY
2KeY
u
tO
,,
~5,
~
-
3KeY
U
°I 40 0
i
10
I
20 Sputtering Time(rain)
!
30
40
Fig. 4. Changes of the concentration with sputtering time in a homogeneous Cu-Au alloy sputtered with argon ions with various energies.
concentration of Au showed a reasonably good agreement with the calculated data (table 2), The data from table 2 show that the component with the lower sputtering yield is enriched in the surface layer when steady-state conditions are reached even though there may be several effects involved in the sputtering process. The steady-state concentration can be predicted by using eq. (1) which was derived based simply on the assumption that the sputtering yield of each component in an alloy has the same value as for the pure dement. The data listed in the third column of table 2 were calculated by taking YAu= 4.3, Ycu= 4.3, Ypd = 3.6 and YNi = 3.0 (atoms/ion) [7]. The excellent agreement between calculated and measured values implies that the sputtering yield is an intrinsic nature of the element which governs the final sputtering concentration. The effects of alloying on it is trivial at least for the systems and concentration ranges we studied here. Other mechanisms might be operative in the sputtering process but they did not exert any significant effects on the steady-state concentration. However, the transient concentration measured by ion sputter profiling showed unexpected changes: in most of the cases ~ "spike" in the early stage of sputtering was created and its amplitude and steepness depen, ls on the
Qu Zhe, Xie Tian-Sheng / Auger-ion sputterprofiling of binary systems
L133
system and ion energy. Similar phenomena have been observed in some pre-bombarded homogeneous binary systems and have been taken as evidence of enhanced segregation induced by pre-bombardment. The present study shows that this profiling technique might bring about a depression or a hill in the resulting distribution curve and this was not necessarily true in the initial distribution. However, this feature was usually accepted as a true distribution of the element in specimens before ion sputtering, because it has been generally regarded ,hat the ion sputtering curve is a reflection of the real distribution of elements. The sputtering curves drawn in figs. 1 - 4 show that a significant "distortion" introduced by ion sputtering is inevitable. Any conclusion derived from a sputtering curve without any calibration may lead to a wrong interpretation of the initial element distribution or of the mechanisms of ion sputtering. Such transient behaviour of the sputtering curves is difficult to understand on the basis of the individual processes involved in ion bombardment only. However, it can be rationalized by considering a combination of several different processes. Preferential sputtering is one of the phenomena often found to occur whenever a multicomponent system is subjected to ion bombardment. Due to different partial sputtering yields on the constituent elements, one of the elements will deplete gradually with sputtering time until the steady-state build-up (fig. 5, dashed line). During ion bombardment, ions disp;ace many atoms in the lattice in the course of their collision cascade, creating vacancies, interstitials, implanted ions and other defects, all of which can enhance atom diffusion. As a consequence of enhanced diffusion rates, one of the compo-
w0J
,m
°.."" ".o
<
............
"~
"" --. . . . .
(U
.a.} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b)
r.. o
|
i
i
S~ttering
Time
Fig. 5. Schematic illustration of the formation of the maxima/mirfima on a profiling curve o i a homogeneous binary system A-B. The sputtering y/eld YA> YB. Curve a due to surface segregation and removal. Curve b due to preferential sputtering. Curve c combination of cur,re a and curve b.
L134
Qu Zhe, Xie Tian.Sheng / Auger-ion sputterprofiling of binary systems
nents may segregate to the free surface and develop a subsurface depleted zone accordingly [8,9]. Therefore., haifizdly the segregating element piled up rapidly on the surface before slowing down (fig. 5, dotted line) due to that the enriched surface was removed and the depleted zone moved toward the surface. If the element preferentially sputterexl is the same element as the segregated one, the final measured surface concentration changes are obtained as a superposition of the segregation curve on the preferential sputtering curve (fig. 5, full line). The shape of the profiling curve is determined by the segregation rate and by the preferential sputtering rate. If two dements have the similar sputtering yield (Au-Cu system, 2 keV ion) then the shape of the profiling curve will be dominated by the segregation curve, if the difference between the sputtering yields of the two components is large then the profiling curve will be shaped mainly by the preferential sputtering process and the segregation contribution to the curve, if any, v,511be neutralized (the cases of Ni-Cu alloy and Au-Cu alloy with 3 keV ion), because the behaviour of segregation and preferential sputtering is strongly influenced by sputtering ion energy. Subsequently the profiling curves will vary, according to the sputtering conditions. The measured dement distribution function F ( M ) is actually a convolution of the real distribution function F(R) and the "distortion" function F(D). In order to "restore" the damage brought about by ion sputtering, a deconvolution treatment of the measured function F ( M ) is necessary to be carded out with F(D) which can be found in a well designed distribution system (for example a homogeneous binary system), or simply compare the measured curve of an ur&nown specimen with that of a standard specimen with the sirnilar composition under the same measurement conditions. The latter treatment, however, is somewhat arbitrary but it can at least minimize the artifacts created by the ion sputter-Auger analysis technique. The authors are grateful to the National Science Foundation of China for financial support of this work. The authors would like to thank the Department of Ceramics, the Umversity of Leeds, for help in the preparation of this paper.
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