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Study of the changes in the silicide surface composition under argon ion bombardment
Surface Science 251/252 North-Holland
159
(1991) 159-164
Study of the changes in the silicide surface composition ion bombardment V.I. Zaporozchenko,
S.S. Vojtusik,
M.G.
Stepanova
All-Union Research Center for Surface and Vacuum Investigation
and
(VNICPV),
A.I.
under argon
Zagorenko
2 Andreeuskaja
nab., Moscow 117334, USSR
Received 1 October 1990; accepted for publication 1 February 1991
AES depth profiling and ARAES were used to study the concentration profiles of altered layers in CoSi, and FeSi, systems under Ar+ ion bombardment with l-4 keV energy and an ion incidence angle of 0 O-70 “. The concentration profiles were observed to have nonmonotonic character along the altered layer depth. Data for the “silicon-metal” sputtering yield ratio were obtained. The experimental data were compared with results of a model calculation.
1. Introduction Solid surface sputtering by ions of inert gases is widely used to solve technological and analytical problems. Ion sputtering of multi-component systems usually leads to changes in the surface composition. This effect is not desirable and it is necessary to take it into account when profiling solids by Auger-electron spectroscopy or other surface analysis methods. The change in surface composition is caused, according to ref. [l], by preferential sputtering of one of the components, radiation-induced segregation, etc. This is why it is rather difficult to think of a theoretical model which can access quantitative changes in surface composition. To understand more fully the processes which influence the changes in surface
composition further investigation on altered layer formation depending on the incident ion energy, fluence, and angle of incidence are obligatory. The purpose of the present paper is to investigate the altered layers of CoSi(lOO), Co,Si(lOO), CoSi, (loo), FeSi(lOO), Fe,Si,(lOO) formed after argon ion bombardment in the l-4 keV energy range.
2. Experimental Experimental determination of the dependence of sputtering yield ratios of the components on dose and energy were carried out in an “ADES400” spectrometer with angular resolution and a “PHI-590” scanning Auger microprobe. The partial pressures of the chemically active residual
Table 1 Preferential sputtering data for CoSi, and FeSi, Yield ratio Y,/ Ya
System A-B
Experimental results
Co,Si CoSi CoSi z FeSi FesSi, a) Y,,/Y,
Numerical calculation results
Low energy line
High energy line
Y,o/ Yao@
Ref. [3]
Eq. (2)
0.90 0.85 0.40
0.75 0.75 0.62 0.72 0.61
2.1 2.1 2.1 2.3 2.3
0.88 0.88 0.88 0.89 0.89
0.83 0.87 0.90 0.80 0.84
_
are the sputtering yield ratios of the pure elements.
0039-6028/91/$03.50
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
K I. Zaporozchenko
160
et al. / Changes in the silicide surjace composition
gases during the experiments equalled lo-’ Pa and were controlled by mass-spectrometry. To measure surfaces non-affected by ion bombardment we scraped the sample by a sapphire needle directly in the analysis chamber. The experiment software provided quick measurement of the Auger line intensities in narrow energy windows depending on sputtering parameters and mean results (after consideration of several analysed spatial points).
3. Rest&s and discussion To determine the sputtering yield ratio of the components we used the formula suggested in ref. VI: Y&/Y;; = I,I;/I&
(1)
under Ar ion bombardment
where Yi, Yi are the sputtering yields of components A and B respectively: IA. I, are the Auger peak amplitudes of elements A and B measured on the scraped surface; i;, 16 are the Auger peak amplitudes measured on the sputtered surface after obtaining steady state. Comparison of the composition in the scrape to the volume composition was carried out by ion profiling. The composition in the scrape was determined by considering several points. It was shown that under sputtering the composition transforms according to an exponential law with a time constant of 300 s, calculated using the model given in ref. [3] for an Ar’ ion current density of 0.5 PA/cm’. The ratios of the sputtering yields of CoSi. and FeSi, measured for the low- and high-energy Auger peaks are given in table 1. To compare values in table 1 we also present sputtering yield
-
low
Y
r*** *
Ar
Y **#***M
, lkev
I high
Fig. 1. Low (*
AES
energy
**********
*******
9
Ar.4keV
enerqy
I
Ar,lkeV --
AES
) and high ( + ) energyexperimental
Co/Si peak-to-peak ratio versus sputtering energy of 1.4 and 1 keV at an incidence angle of 60 o
time for CoSi,
by Ar’
ions with an
V.I. Zaporozchenko
et al. / Changes in the siiicide surface composition under Ar ion bombardment
ratios, obtained using sputtering yields of pure elements [4] and the equations given in ref. [5] together with an empirical formula suggested by the authors: Y + G/G> r, -yB = I+ Y(C,/Ca)
3.18(U~/~) 2.18 + (U,,‘U,)”
(2)
’
where
MA, M, are the atom masses of the components; U/j, U, are surface binding energies of the compo-
nents; C,, C, the bulk concentration of the components in the compound. As follows from table 1, the best agreement between experimental data and numerical calculations was obtained using Sigmund’s formula and eq. (2). Use of the low and high-energy Auger lines to determine the yield ratio of the components according to formula (1) gives different results (see table 1). This difference can be explained by the Auger electron escape depth and consequently by various analysis depths of the near-surface layer. Analysis of the results for Co,Si and CoSi points out that there exist an altered layer nonmonotonic profile.
I
0
10
20
30
40
Considering all the samples we determined the dependence of the Auger peak intensity ratios on the fluence with variation of the Ar+ ion energy. In fig. 1 we present such a dependence for CoSi,. As it follows from fig. 1 variation of the Ar+ ion energy from 1 to 4 keV causes a sharp change in the surface composition on all the samples and a new steady state is observed. The initial part of the curve is well described by the model of ref. (61. But this model does not describe the changes in composition, observed upon variation of the Ar+ ion energy from 4 to 1 keV. We can explain this effect by ion mixing and by the fact that the composition profile of an altered layer differs from the exponential law. Numerical calculations were made for the altered layer composition of CoSi during its steady state after sputtering. The calculations allow us to determine depth dependencies for atomic concentrations of cobalt N,(x) and silicon i’$(x), satisfying the equation [7]: - &J,(x)
(0) and calculated
+ v&v&)
- &(X) + I,(x)
= 0,
(3) with boundary conditions J,(O) = 0, N,(ce) = N,‘, where cy= 1, 2, N,” is the initial atomic concentra-
50
ANGLE OF INCIDENCE Fig. 2. Experimental
161
( * ) data for the cobalt concentration ion bombardment.
60
70
80
(DEG.) in CoSi 2 as a function
of incidence
angle for 4 keV Ar +
162
V.I. Zaporozchenko
et al. / Changes in the silicide surface composition
under Ar ton bombardment
angle dependence of ion sputtering (fig. 2). At the same time the calculations show that the depth concentration dependencies of the target components vary considerably due to variation in their diffusi~ty ratio showing extrema in the dependence C(x) (figs. 3 and 4). The position of the extremum in fig. 4 is correlated with the experimental depth-profiles of the altered layer obtained for 1 keV Arf ions (fig. 1) and the ion mean penetration depth into the target:
tion of the components, S,(x) is the number of (Y atoms knocked out off the equilibrium state in a unit volume at depth x per unit time, I’ is the sputter erosion velocity of the surface, and J,(x) is the diffusion flux density of (r atoms. The functions S,(X) and 1,(x) were obtained by means of a combined solution of eq. (3) and a system of master equations for atomic collision cascades with fixed atom path length between two collisions. Ion-target and target-target interactions upon collision were described by a power potential u(r) Z = (2,“‘” + - T-m, where m = 3.97 + 2,60 Z-i/r 2:‘” + Zj/3)3/2. Binding of the target atoms was taken into account by introducing the energy of a bulk displacement of target components U,, so that U, lX=0=O.5 U, lX,0. This model gives a satisfactory description of the experimental data. This fact can be proved by the similarity of the experimental and calculated
X--E2mtos 0
e0
where E. and i?, are the energy and angle of incidence, respectively. The depth position of this characteristic extremum at 1 keV ion energy sputtering with an incidence angle of 60 ’ is approximately 2.5 nm. This is 1.75 times less than the depth position of the similar maximum (4.5 nm) at 4 keV ion energy sputtering. Such con-
7
-
DSi/DCo=10
----
DSiiDC.=3
...... DSi/DCo=l
!I
5
10
15 DEPTH
Fig. 3. Results of model calculations for the composition of the CoSi, (normal incidence) for Us,/&, = 0.5: with various diffusivity ratios. I(.
).
20
(NM)
altered
layer during
Ds,/DCO = lO(-
Ar + ion sputtering ). D,,/D,,
= X-
with 1 keV energy -
-1.
Ds,/DC,
=
V.I. Zaporozchenko
et al. / Changes in the silicide surface composition under Ar ion bombardment
5
Fig. 4. Results
of model calculations
10
15
DEPTH (NM) for the composition of the CoSi, altered (60 D incidence angle).
centration behaviour is caused by variation in the atomic target density during ion bombardment [7] and ion mixing. To study the composition profile of near-surface layers (1 nm) we used angle resolved Auger electron spectroscopy with low- and high-energy Auger electrons. We took measurements of Auger-emission angle dependencies for Co(MVV), ~~L~~) and Si(LVV) for low- and high-energy Auger peaks with characteristic energies of 53, 775 and 92 eV respectively. Auger peak intensities were registered for electron take-off angles of 0 “-70” with 10” interval. To remove the angular dependence caused by the apparatus, the measured intensities for C&i, were normalized with respect to those for pure elements taken similarly. Such no~alization made it possible to determine Co concentration profiles on low- and high-energy lines and also Si profiles on low-energy line independently. The obtained normalized angle dependencies were used
163
20 layer during Ar + ion sputtering
with 4 keV energy
to calculate the concentration profiles using the technique suggested in ref. [8]. The electron mean free paths were calculated according to formula of Seah and Dench [9]. The obtained altered layer profile is given in fig. 5 and shows that the nearsurface layer is enriched by Co with a maximum concentration at 1 nm depth. Little enrichment of the Si external surface layer can be explained by Si radiation-induced segregation. We can prove this by the fact that the surface composition did not depend on temperature within the 100-300 K range.
4. S~rn~ Experimental and theoretical study of surface altered layers of transition metal silicides proved the fact of nonmonotonic character of the depth distribution of the component up to 10 nm. The
164
V.I. Zapororchenko
et al. / Changes in the silicide surface composition
under Ar ion bombardment
Si(92eV)
n ”
.!
I
DEPTH (NM1 using ARAES data [Co(LMM) ( -----)). Co(MVV) Fig. 5. Calculated profile of the surface altered layer of Co%,, obtained at an incidence angle of 60 O. (- ~ -) and Si(LVV) ( . )I. The altered layer is formed after 4 keV Ar + ion bombardment
concentration profiles in the altered layers are determined by parameters of the incident ion beam (energy and ion incidence angle) and solid characteristics such as diffusivities and atom binding energy. The obtained data explain the great variety of available data on preferential sputtering of solids and prove the fact that, using such data, it is oecessary to reproduce both sputtering conditions and the employed methods with identical analysis depth. References [I] N.Q. Lam, Surf. Interface Anal. 12 (1988) 65. [2] G. Betz, Surf. Sci. 92 (1980) 238.
131 P.S. Ho. J.E. Lewis, H.S. Wildman and J.K. Howard. Surf. Sci. 57 (1976) 393. et al.. At. Data Nucl. Data Tables 31 (1984) [41 N. Matsunami [51 P. Sigmund, in: Topics of Applied Behrisch (Springer, Berlin, 1981). 161 G. Be&. M. Opitz and P. Braun. 182,‘183 (1981, 63. Rad. Eff. Defects 171 M.G. Stepanova. SE. Borodjansky, 181 Y.G. Abashkin, Dementjev, Poverkhnost 10 ( 1989) [91 M.P. Seah and W.A. Dench. Surf.
Physics. Nucl.
Vol. 47. Ed. R.
Instrum.
Methods
Solids, submitted. L.A. Vasiljev and A.P. 71. Interface Anat. 1 (1979)
159
(1991) 159-164
Study of the changes in the silicide surface composition ion bombardment V.I. Zaporozchenko,
S.S. Vojtusik,
M.G.
Stepanova
All-Union Research Center for Surface and Vacuum Investigation
and
(VNICPV),
A.I.
under argon
Zagorenko
2 Andreeuskaja
nab., Moscow 117334, USSR
Received 1 October 1990; accepted for publication 1 February 1991
AES depth profiling and ARAES were used to study the concentration profiles of altered layers in CoSi, and FeSi, systems under Ar+ ion bombardment with l-4 keV energy and an ion incidence angle of 0 O-70 “. The concentration profiles were observed to have nonmonotonic character along the altered layer depth. Data for the “silicon-metal” sputtering yield ratio were obtained. The experimental data were compared with results of a model calculation.
1. Introduction Solid surface sputtering by ions of inert gases is widely used to solve technological and analytical problems. Ion sputtering of multi-component systems usually leads to changes in the surface composition. This effect is not desirable and it is necessary to take it into account when profiling solids by Auger-electron spectroscopy or other surface analysis methods. The change in surface composition is caused, according to ref. [l], by preferential sputtering of one of the components, radiation-induced segregation, etc. This is why it is rather difficult to think of a theoretical model which can access quantitative changes in surface composition. To understand more fully the processes which influence the changes in surface
composition further investigation on altered layer formation depending on the incident ion energy, fluence, and angle of incidence are obligatory. The purpose of the present paper is to investigate the altered layers of CoSi(lOO), Co,Si(lOO), CoSi, (loo), FeSi(lOO), Fe,Si,(lOO) formed after argon ion bombardment in the l-4 keV energy range.
2. Experimental Experimental determination of the dependence of sputtering yield ratios of the components on dose and energy were carried out in an “ADES400” spectrometer with angular resolution and a “PHI-590” scanning Auger microprobe. The partial pressures of the chemically active residual
Table 1 Preferential sputtering data for CoSi, and FeSi, Yield ratio Y,/ Ya
System A-B
Experimental results
Co,Si CoSi CoSi z FeSi FesSi, a) Y,,/Y,
Numerical calculation results
Low energy line
High energy line
Y,o/ Yao@
Ref. [3]
Eq. (2)
0.90 0.85 0.40
0.75 0.75 0.62 0.72 0.61
2.1 2.1 2.1 2.3 2.3
0.88 0.88 0.88 0.89 0.89
0.83 0.87 0.90 0.80 0.84
_
are the sputtering yield ratios of the pure elements.
0039-6028/91/$03.50
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
K I. Zaporozchenko
160
et al. / Changes in the silicide surjace composition
gases during the experiments equalled lo-’ Pa and were controlled by mass-spectrometry. To measure surfaces non-affected by ion bombardment we scraped the sample by a sapphire needle directly in the analysis chamber. The experiment software provided quick measurement of the Auger line intensities in narrow energy windows depending on sputtering parameters and mean results (after consideration of several analysed spatial points).
3. Rest&s and discussion To determine the sputtering yield ratio of the components we used the formula suggested in ref. VI: Y&/Y;; = I,I;/I&
(1)
under Ar ion bombardment
where Yi, Yi are the sputtering yields of components A and B respectively: IA. I, are the Auger peak amplitudes of elements A and B measured on the scraped surface; i;, 16 are the Auger peak amplitudes measured on the sputtered surface after obtaining steady state. Comparison of the composition in the scrape to the volume composition was carried out by ion profiling. The composition in the scrape was determined by considering several points. It was shown that under sputtering the composition transforms according to an exponential law with a time constant of 300 s, calculated using the model given in ref. [3] for an Ar’ ion current density of 0.5 PA/cm’. The ratios of the sputtering yields of CoSi. and FeSi, measured for the low- and high-energy Auger peaks are given in table 1. To compare values in table 1 we also present sputtering yield
-
low
Y
r*** *
Ar
Y **#***M
, lkev
I high
Fig. 1. Low (*
AES
energy
**********
*******
9
Ar.4keV
enerqy
I
Ar,lkeV --
AES
) and high ( + ) energyexperimental
Co/Si peak-to-peak ratio versus sputtering energy of 1.4 and 1 keV at an incidence angle of 60 o
time for CoSi,
by Ar’
ions with an
V.I. Zaporozchenko
et al. / Changes in the siiicide surface composition under Ar ion bombardment
ratios, obtained using sputtering yields of pure elements [4] and the equations given in ref. [5] together with an empirical formula suggested by the authors: Y + G/G> r, -yB = I+ Y(C,/Ca)
3.18(U~/~) 2.18 + (U,,‘U,)”
(2)
’
where
MA, M, are the atom masses of the components; U/j, U, are surface binding energies of the compo-
nents; C,, C, the bulk concentration of the components in the compound. As follows from table 1, the best agreement between experimental data and numerical calculations was obtained using Sigmund’s formula and eq. (2). Use of the low and high-energy Auger lines to determine the yield ratio of the components according to formula (1) gives different results (see table 1). This difference can be explained by the Auger electron escape depth and consequently by various analysis depths of the near-surface layer. Analysis of the results for Co,Si and CoSi points out that there exist an altered layer nonmonotonic profile.
I
0
10
20
30
40
Considering all the samples we determined the dependence of the Auger peak intensity ratios on the fluence with variation of the Ar+ ion energy. In fig. 1 we present such a dependence for CoSi,. As it follows from fig. 1 variation of the Ar+ ion energy from 1 to 4 keV causes a sharp change in the surface composition on all the samples and a new steady state is observed. The initial part of the curve is well described by the model of ref. (61. But this model does not describe the changes in composition, observed upon variation of the Ar+ ion energy from 4 to 1 keV. We can explain this effect by ion mixing and by the fact that the composition profile of an altered layer differs from the exponential law. Numerical calculations were made for the altered layer composition of CoSi during its steady state after sputtering. The calculations allow us to determine depth dependencies for atomic concentrations of cobalt N,(x) and silicon i’$(x), satisfying the equation [7]: - &J,(x)
(0) and calculated
+ v&v&)
- &(X) + I,(x)
= 0,
(3) with boundary conditions J,(O) = 0, N,(ce) = N,‘, where cy= 1, 2, N,” is the initial atomic concentra-
50
ANGLE OF INCIDENCE Fig. 2. Experimental
161
( * ) data for the cobalt concentration ion bombardment.
60
70
80
(DEG.) in CoSi 2 as a function
of incidence
angle for 4 keV Ar +
162
V.I. Zaporozchenko
et al. / Changes in the silicide surface composition
under Ar ton bombardment
angle dependence of ion sputtering (fig. 2). At the same time the calculations show that the depth concentration dependencies of the target components vary considerably due to variation in their diffusi~ty ratio showing extrema in the dependence C(x) (figs. 3 and 4). The position of the extremum in fig. 4 is correlated with the experimental depth-profiles of the altered layer obtained for 1 keV Arf ions (fig. 1) and the ion mean penetration depth into the target:
tion of the components, S,(x) is the number of (Y atoms knocked out off the equilibrium state in a unit volume at depth x per unit time, I’ is the sputter erosion velocity of the surface, and J,(x) is the diffusion flux density of (r atoms. The functions S,(X) and 1,(x) were obtained by means of a combined solution of eq. (3) and a system of master equations for atomic collision cascades with fixed atom path length between two collisions. Ion-target and target-target interactions upon collision were described by a power potential u(r) Z = (2,“‘” + - T-m, where m = 3.97 + 2,60 Z-i/r 2:‘” + Zj/3)3/2. Binding of the target atoms was taken into account by introducing the energy of a bulk displacement of target components U,, so that U, lX=0=O.5 U, lX,0. This model gives a satisfactory description of the experimental data. This fact can be proved by the similarity of the experimental and calculated
X--E2mtos 0
e0
where E. and i?, are the energy and angle of incidence, respectively. The depth position of this characteristic extremum at 1 keV ion energy sputtering with an incidence angle of 60 ’ is approximately 2.5 nm. This is 1.75 times less than the depth position of the similar maximum (4.5 nm) at 4 keV ion energy sputtering. Such con-
7
-
DSi/DCo=10
----
DSiiDC.=3
...... DSi/DCo=l
!I
5
10
15 DEPTH
Fig. 3. Results of model calculations for the composition of the CoSi, (normal incidence) for Us,/&, = 0.5: with various diffusivity ratios. I(.
).
20
(NM)
altered
layer during
Ds,/DCO = lO(-
Ar + ion sputtering ). D,,/D,,
= X-
with 1 keV energy -
-1.
Ds,/DC,
=
V.I. Zaporozchenko
et al. / Changes in the silicide surface composition under Ar ion bombardment
5
Fig. 4. Results
of model calculations
10
15
DEPTH (NM) for the composition of the CoSi, altered (60 D incidence angle).
centration behaviour is caused by variation in the atomic target density during ion bombardment [7] and ion mixing. To study the composition profile of near-surface layers (1 nm) we used angle resolved Auger electron spectroscopy with low- and high-energy Auger electrons. We took measurements of Auger-emission angle dependencies for Co(MVV), ~~L~~) and Si(LVV) for low- and high-energy Auger peaks with characteristic energies of 53, 775 and 92 eV respectively. Auger peak intensities were registered for electron take-off angles of 0 “-70” with 10” interval. To remove the angular dependence caused by the apparatus, the measured intensities for C&i, were normalized with respect to those for pure elements taken similarly. Such no~alization made it possible to determine Co concentration profiles on low- and high-energy lines and also Si profiles on low-energy line independently. The obtained normalized angle dependencies were used
163
20 layer during Ar + ion sputtering
with 4 keV energy
to calculate the concentration profiles using the technique suggested in ref. [8]. The electron mean free paths were calculated according to formula of Seah and Dench [9]. The obtained altered layer profile is given in fig. 5 and shows that the nearsurface layer is enriched by Co with a maximum concentration at 1 nm depth. Little enrichment of the Si external surface layer can be explained by Si radiation-induced segregation. We can prove this by the fact that the surface composition did not depend on temperature within the 100-300 K range.
4. S~rn~ Experimental and theoretical study of surface altered layers of transition metal silicides proved the fact of nonmonotonic character of the depth distribution of the component up to 10 nm. The
164
V.I. Zapororchenko
et al. / Changes in the silicide surface composition
under Ar ion bombardment
Si(92eV)
n ”
.!
I
DEPTH (NM1 using ARAES data [Co(LMM) ( -----)). Co(MVV) Fig. 5. Calculated profile of the surface altered layer of Co%,, obtained at an incidence angle of 60 O. (- ~ -) and Si(LVV) ( . )I. The altered layer is formed after 4 keV Ar + ion bombardment
concentration profiles in the altered layers are determined by parameters of the incident ion beam (energy and ion incidence angle) and solid characteristics such as diffusivities and atom binding energy. The obtained data explain the great variety of available data on preferential sputtering of solids and prove the fact that, using such data, it is oecessary to reproduce both sputtering conditions and the employed methods with identical analysis depth. References [I] N.Q. Lam, Surf. Interface Anal. 12 (1988) 65. [2] G. Betz, Surf. Sci. 92 (1980) 238.
131 P.S. Ho. J.E. Lewis, H.S. Wildman and J.K. Howard. Surf. Sci. 57 (1976) 393. et al.. At. Data Nucl. Data Tables 31 (1984) [41 N. Matsunami [51 P. Sigmund, in: Topics of Applied Behrisch (Springer, Berlin, 1981). 161 G. Be&. M. Opitz and P. Braun. 182,‘183 (1981, 63. Rad. Eff. Defects 171 M.G. Stepanova. SE. Borodjansky, 181 Y.G. Abashkin, Dementjev, Poverkhnost 10 ( 1989) [91 M.P. Seah and W.A. Dench. Surf.
Physics. Nucl.
Vol. 47. Ed. R.
Instrum.
Methods
Solids, submitted. L.A. Vasiljev and A.P. 71. Interface Anat. 1 (1979)