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Charge state fractions of Ni sputtered by Ne and Ne reflected from Ni
Nuclear Instruments and Methods North-Holland, Amsterdam
in Physics
CHARGE STATE FRACTIONS W. ECKSTEIN, Max-Planck-Institut
Research
507
B14 (1986) 507-514
OF Ni SPUTTERED
BY Ne AND Ne REFLECTED
FROM Ni
H.J. BARTH and E. MUHLING ftir Plasmaphysik,
Euratom Association,
D-8046 Garching/Minchen,
FRG
Charge state fractions of sputtered Ni and reflected Ne particles are determined from the measurement of energy distributions in different charge states for non-normal incidence in the energy range from 1 to 13 keV. It has been found that the charge state fractions depend on the type of collision. Monte Carlo simulations (TRIM.SP) were used to determine the ionization efficiency for the neutral Ni atoms in the N, stripping cell.
1. Introduction The charge state fractions are of primary interest in understanding the formation of different charge states at surfaces for reflected [l] as well as sputtered particles [2]. Some progress has been made in recent years in the theoretical approach [3], but it is still difficult to get quantitative agreement with experimental results. The formation of ions is also important in practical applications as in plasma-wall interaction [4] (because only neutrals can penetrate into the discharge) and in SIMS [2] for example. In SIMS usually all the sputtered ions are collected by ion-optical means. In this paper only the ions emitted in a small solid angle are analyzed. The bombardment at non-normal incidence is chosen to have the possibility to detect particles sputtered in one binary collision.
2. The experimental
sputtered particles (independent of their charge state) in a specific solid angle for the comparison with experimental data. As described below these calculated data were used to determine the ionization efficiency of the stripping cell for neutral Ni atoms. The justification for this use of calculated results comes from the fact that in most cases investigated the calculated data give good quantitative agreement with experimental results as shown previously [6,7] and again here in fig. 1.
4. Experimental results Typical measured energy distributions of positively and negatively charged particles as well as neutral particles are shown in fig. 2 for the bombardment of a
setup
The apparatus BOMBARDON used for these measurements is described in ref. 151. In short, the ion beam is mass-analyzed with an angular resolution of about 0.5”. The reflected and sputtered ions are energyanalyzed in a cylindrical condenser and detected with a channeltron. The reflected and sputtered neutrals are ionized in a N, stripping cell. The energy resolution of the detection device is AE/E = 0.008 and the angular resolution is 0.25” with a corresponding solid angle of 5.5 x 1O-5 sr. For a comparison with calculated energy distributions the measured spectra have to be corrected for the constant energy resolution AE/E of the condenser.
102 3. The calculation The Monte Carlo program TRIM.SP (version TRSPIC) [6] was applied to get energy distributions of 0168-583X/86/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
103
lo4
E,. INCIDENT ENERGY (eV)
B.V.
Fig. 1. Sputtering yield vs the incident energy at normal incidence for the bombardment of Ni by Ni: 0 calculated data, + experimental data (171, x experimental data [18]. V. SCATTERING
SO8
W. Eckstein et al. / Charge
clean Ni surface with 9 keV Ne+ and Ar+ ions at an angle of incidence a! = 70” (with respect to the surface normal). The emission angle of reflected and sputtered
9.0 keV Ne+d Ni f
I
2s
H
NE++
r
statefractionsof
Ni sputtered
INCIDENT PARTICLE
by Ne
SURFACE NORMAL
I
SPUTTERED PARTICLE
a= 70”. P= 60°. *= 50° 1 1 I
1 I
Ni Ne”*(Ni)
Fig. 3. Definitions of the different angles (incident angle IX. polar exit angle p, scattering (recoil) angle 9, azimuthal exit angle cp).
E. ENERGY 9.0 keV Ar+-
u
1
2
Ni
3
(keV)
a= 70”.
4
particles is j3 = 6W (giving a total scattering or recoil angle 6 = 500). (A definition of the different angles is given in fig. 3.) The distributions of positively charged particles show peaks due to reflected ions (Ne+, Ne2+), sputtered particles (Ni*, Ni’+) and desorbed particles (Hf, D*). The spectra of neutrals also show peaks due to reflected particles (Ne’), sputtered particles (Nia) and desorbed particles (Ho, Do). The spectra of negatively charged particles exhibit only peaks due to sputtered particles (Ni-) and desorbed particles (H-, D-). ,The only impurity seems to be hydrogen, which is hard to detect with other techniques such as ISS or AES. The absence of 0 and C peaks in the distributions of negatively charged particles shows that these species must be well below a 1,400 of a monolayer at the surface. All three energy distributions show large backgrounds beside the peaks mentioned. In the case of the Ne bombardment the particles above 6 keV must be Ne particles, because it is impossible to transfer more energy to Ni particles. Below 6 keV the background in the positive and neutral spectra consists of both reflected Ne particles and sputtered Ni particles, whereas in the negative spectrum the background is exclusively formed by sputtered Ni ions. Only at low energies desorbed
p= 60°.
5
E. ENERGY IkeV)
6
3=50°
7
6
Fig. 2. Measured distributions of positive (+), neutral (0) and negative (-) particles for the bombardment of Ni at an incident angle a = 70”. The polar exit angle is ~3= 60” and the scattering (recoil) angle is 9 = 50”. Single element names indicate sputtered or desorbed species, Ne (Ni) indicates Ne refiected from Ni. (a) 9 keV Ne’ bombardment. (b) 9 keV Ar+ bombardment. QS and QD indicate quasisingle and quasidouble elastic scattering of Ar from Ni.
W. Eckstein et al. / Charge state fructions of NI sputtered br, Ne
hydrogen and deuterium may give a small contribution to the background. The reason that the negative spectrum consists only of sputtered and desorbed particles is the nonexistence of negative noble gas ions [8] (He is an exception, it has a negative ion [9]).
5. Calculated
results
For later need, a few results from computer simulation should be shown here. In fig. 4 the calculated angular distributions (in all directions) for sputtered Ni and reflected Ne particles are shown. The sputtered Ni distributions exhibit a maximum at (‘p = O”, /? = 25”), whereas the distribution of the reflected Ne is centered around the specular direction (‘p = O”, /3 = 75”). ‘p is the azimuthal emission angle (fig. 3) and ‘p = 0” is in
9keV Ne-Ni;
509
the plane of incidence, defined by the incident beam and the normal of the target surface. From the distribution of sputtered Ni it can be seen that the angular distributions do not change for azimuthal angles 0” < ‘p < 15”. Therefore this angular range has been chosen to calculate the energy distributions for different exit angles (see fig. 5). To get reasonable statistics the azimuthal angular range cannot be made smaller. For the comparison with experimental energy distributions, which have been measured in the plane of incidence (cp = O”), the energy distributions are horizontal cuts through the contour line plot at the corresponding polar exit angle /3. The calculated energy spectra of sputtered Ni differ from those of reflected Ne in that the reflected Ne is concentrated at high energies and large angles /3, whereas the sputtered Ni increases with decreasing energy concentrated at low emission angles fi.
0r=70O
SPUTTERED
PARTICLES
REFLECTED
PARTICLES
60
a e
b1
a
2
ci
CZlO
80 60
Cp, AZIMUTHAL
ANGLE
Fig. 4. Contour line plots of angular distributions in the whole space for Ni bombarded The lines connect equal intensities per solid angle. c is the distance of adjacent contour sputtered
Ni, (b) reflected
by 9 keV Net at an Incident angle a = 70”. lines (c is proportional to the intensity). (a)
Ne. V. SCATTERING
W.
510
Ecksrrrn
9keV Ne-Ni;
er al.
/
Charge .xrure frac-trons o/ ,VI .cputleredb! Xr
a=70°;Oo
SPUTTERED
< cp <15O
PARTICLES -1
c=s ’
---.
-I 1
I : i
REFLECTED u”!
bl 1.0 eli’“T’;I-p
PARTICLES --.
--
‘I-
c = 10
0.8 ’ ’ I o,6 ‘__pd.:,,,yL. p \ .p I,! pfl,
I 0.4 -
E/E,,RELATlVE Fig. 5. Contour line
plots of
angular energy correlations
ENERGY into a forward direction (0” i ‘p < 15”) for the bombardment
Ne at an incident angle a = 70”. The lines connect points of equal intensities per solid angle and energy interval. gives an energy distribution intensity). (a) Sputtered
at a specific polar exit angle p. c is the distance of adjacent
contour
of Ni by 9 keV A horizontal
lines (c is proportional
cut
to the
Ni. (b) reflected Ne.
6. Discussion
Without an additional mass analysis it is hard to separate the reflected from the sputtered particles and it is therefore difficult to determine charge state fractions. But two things can be done. For the Ne bombardment the charge state fractions at the high energy end can easily be determined as usually [S.lO,ll] because that part of the energy distributions consists only of backscattered Ne particles. The other case for which charge state fractions can be determined (with more effort) are the peak regions in the energy distributions (below 6 keV in fig. 2 for the Ne bombardment). This is easily done for the desorbed hydrogen, because an ionization efficiency curve for neutral hydrogen in the stripping cell exists as in the case of Ne [12]. For the sputtered Ni
peaks. which originate from sputtering in one binary collision, a method of determining an ionization efficiency of neutral Ni atoms in the stripping cell is described in the next section. 6. I. Dererminution
of the ionizurion efficiency /or Ni
Assuming the TRIM.SP results to be correct. one can equate the integral I over the Ni peaks in the calculated distribution to the sum of integrals J’ over the Ni peaks in the experimental distribution, I=J*/y+J++J-+J’+. where the integral J ’ is corrected for the ionization efficiency y. All J’ are measured and I is calculated. Hence y is the only unknown quantity in the equation
W. Eckstern et ul. / Charge
o
Ar+---NI
i
l
Nd-NNi
‘;
statefracttons
of Ni sputtered by Ne
511
in y between Ne and Ar bombardment. Due to uncertainties in the accuracy of the TRIMSP results as well as in the determination of the absolute solid angles, the total error is believed to be larger than indicated by the error bars. An absolute error of about a factor of 2 is offered as a reasonable guess.
II
6.2. Charged
fractions
If y is known, the charge state fractions, calculated from the equation
0
10
20
30
E. ENERGY OF Ni
40
50
6.0
70
ATOMS IkeVl
Fig. 6. The ionization efficiency, y, of Ni atoms in the N, filled stripping sputtered
cell vs the energy of the Ni atoms. by Ne+ and Ar+ bombardment.
The Ni atoms are
and can thus be determined. The quantities J’ and I must be related to the same solid angle and to the same incident dose. The background in the experimental distributions consists mainly of backscattered rare gas atoms and ions and of Ni particles sputtered in multiple collisions and has to be subtracted from both the experimental and calculated peaks. The experimental distributions must also be corrected for the constant energy resolution. The calculations for one set of initial conditions give the energy dist~butions for all exit angles. By varying the exit angle in the measurement the ionization efficiency y can be obtained as a function of energy. The result is presented in fig. 6, where y is given for Ni particles sputtered by Ne and Ar, respectively, versus the Ni energy. The relatively large errors, indicated by vertical bars, originate mainly from the relatively poor statistics of the calculated distributions. The two sets of points obtained for Ne and Ar allow one to draw a mean efficiency curve. It is observed that the points for the Ne bombardment lie systematically higher, suggesting the possibility that the excitation of the Ni atoms is higher in Ne collisions, thus leading to a slightly larger stripping cross section in the gas cell. If the sputtered Ni atoms are in the ground state on reaching the stripping cell, there should be no difference
Q’, can be
where i stands for 0, +, 2 + and -, and S is the intensity at each energy interval. For the reflected Ne the charge state fraction can be determined point by point from a set of spectra as in fig. 2. One result is shown in fig. 7, where the charged fraction, q+, is plotted versus the energy of reflected Ne. The charged fraction shows two peaks. The one at lower energy is due to the quasisingle scattering peak, the one at higher energies is due to the quasidouble scattering peak. From several sets of energy distributions one can determine the dependence of the maximum charged fractions as a function of energy. This is shown in fig. 8. The charged fraction, q+, of the quasisingle scattering is nearly independent of the energy and its absolute value is in good
9 keV Ne*-,
QS
Ni
z
CL = 70”
x
QD x
x *
3
L
5
6
7
8
9
E. ENERGY OFs BACKSCATTERED Ne fkeV) Fig. 7. Positive charged fraction of Ne reflected from Ni vs the energy of backscattered Ne for 9 keV Ne* bombardment of Ni at an angle of incidence a = 70”.
V. SCATTERING
512
W. Eckstein
et al. / Charge statefractrons
of Ni sputtered
by Ne
The charge state fractions of the desorbed hydrogen are investigated in detail in ref. [14]. The same procedure as for Ne can be applied for sputtered Ni to get the charge state fractions versus the
2
a
9
keV
Ne+--
Ni
a =?a”
0
a
10
5
E,
ENERGY
OF
BACKSCATTERED
15
Ne
(k&I
Fig. 8. Maximum positive charged fractions, n+ and T’+. of Ne vs the energy of backscattered Ne particles. 4- 15 keV Ne+ bombardment of Ni at an angle of incidence fy = 70”. QS and QD correspond to the quasisingle and quasidouble scattering peaks, respectively.
agreement with earlier data [lo]. The charged fraction of the quasidouble scattering peak is about 5% at energies below 7 keV and increases to about the same values as for the quasisingle scattering at higher energies. Even in the n2i fraction the quasidouble scattering can be seen at energies above 10 keV. The n2+ fraction is below 1% at energies below 5 keV and increases to a few percent above 5 keV. Also for Ne the charged fractions depend on the exit angle. The scattering events leading to the so-called single collision peak or QS-peak are assumed to be mainly one main collision but with additional soft small angle collisions. From computer simulations [13] it is known that other collision events leading to the same exit energy and exit angle are possible. The contribution of other collision events depends strongly on the surface structure. It can be assumed from the literature [l) that different collision events at the surface produce different charge state fractions. In our experiment the exact surface structure is not known, so that nothing can be said about the contribution of different collision events in the QS-peak. The charged fractions given in figs. 7 and 8 are therefore mean values for the mixture of different processes as in the earlier literature [lO,ll]. From these general considerations it seems impossible to distinguish contributions from different processes. But by a careful look at fig. 2 it may be possible to distinguish at the maximum (7 keV) two contributions: the single collisions and other processes. These energy distributions then give at the maximum n+= 0.22 for the binary collisions and n+= 0.11 for the sum of other collisions at the same exit energy.
E, ENERGY OF SPUTT. Ni (keV I 9 keV
Ne*a = 700
Ni
0 .60’
,F
I
1
0.5
I
/ s
OO
I
1
I
I
2
II
I
3
I
I
L
I
s
E, ENERGY OF SPUTT. Ni ( keV 1 Fig. 9. Charged
fractions, n, of Ni sputtered by Ne vs the energy E of sputtered Ni (nor corrected for reflected Ne particles). 9 keV Ne+ bombardment of Ni at an angte of incidence a = 70’. The emission angle (detector) is at /? = 60°. (a) positive charge state fraction ?I+, (b) negative charge state fraction n-.
W. Eckstein Edal. / Charge state/ractums
energy of sputtered Ni for one incident condition (as in fig. 2). Here one has to assume that the intensity of reflected Ne is so small that it does not influence the result. This is only approximately true as can be seen from fig. 2. The result is shown in fig. 9, where the positive (fig. 9a) and negative (fig. 9b) charge state fractions are plotted versus the energy of sputtered Ni. The main result which can be seen from fig. 9 is that both charge state fractions of particles sputtered in one binary collision with Ne have a value about twice as high as the other sputtered Ni. Due to the uncertainty about the Ne background, the absolute values of the positive charge state fraction are less accurate than those of the negative fraction. The large increase of q+ directly after the Ni peak is due to the appearance of the Ne2+ peak at 3.5 keV (fig. 2a), for which q+ is not corrected. A more accurate procedure can be applied to determine the mean charge state fractions of Ni sputtered in a binary collision (the peak regions in fig. 2). In that case the integrated peak areas, where the background is subtracted, are used to calculate the charge state fractions. The result for the different charge state fractions versus the energy of the sputtered Ni is shown in fig. 10.
*
0---*-
‘, +..-a
of Ni sputtered by Ne
513
The most obvious result is the dominant role of the neutrali fraction, more than 90% in the energy range below 5 keV. The neutral fraction increases to lower energies. The singly charged positive fraction increases with increasing energy and it is more than an order of magnitude larger than the doubly charged fraction. The same is true for the negative fraction which has a less steep slope than the positive fractions. Most argon data are collected at slightly different angles (a, &?) than those for Ne. The positive charged fractions of Ni for Ar bombardment at the same exit angles and emission energy show a systematic trend to be lower than for Ne bombardment. This fact is a strong indication that the interaction of the bombarding ion with the target atom influences the resulting charge state fraction. Also the fact that the charged fractions in the binary collision (sputtering) peaks deviate from the values at lower and higher energies point in the same direction. In other words, the charged fraction is not only a surface effect but also an atomic collision effect. The doubly charged fraction n2* for Ar bombardment was at the detection limit, which was also true for Ne bombardment below 6 keV incident energy. The charge state fractions show a strong dependence on the polar exit angle /? [15]. This fact and the low charge state fractions at lower energies explain why Ni ions (sputtered in a binary collision) have not been seen in an earlier publication [16].
= 70°. p = 60”
7. Conclusions Charge state fractions of Ne reflected from Ni and of Ni sputtered by Ne and Ar bombardment have been determined. In the charge state fractions of sputtered and reflected particles a dependence on the type of collision has been observed. The reflected Ne data show reasonable agreement with earlier data. It has been shown that computer simulation can be used to determine ionization efficiencies of neutral Ni in a N, stripping cell. The appearance of both reflected and sputtered species in the measured energy distributions provides the problem how to separate the different species to determine the charged fractions over the whole energy range. An additional magnetic analysis or time of flight analysis will solve this problem. Note addedin proof: The efficiency curve in fig. 6 is erroneously a factor of 2 too high, so that all charged fractions in fig. 10 are also too large by the same factor of 2.
10
2.0
3.0
6.0
E. ENERGY OF SPUTTERED
50 Ni (keV)
Fig. 10. Charged fractions of positive and negative Ni ions sputtered in one binary collision by Ne+ and Ar+ bombardment.
Reference [l] W. Eckstein. Springer Series in Chemical Physics, vol. 17, eds., E. Taglauer and W. Heiland (Springer, Berlin, Heidelberg,
1981) p. 157. V. SCA’ITERING
514
W. Eckstein et al. / Charge
[2] Secondary Ion Mass Spectrometry SIMS III, eds., A. Benninghoven et al. (Springer, Berlin, 1982). [3] D.M. Newts, K. Makoshki, R. Brako and J.N.M. van Wunnik, Phys. Scripta T6 (1983) 5. [4] R. Behrisch and W. Eckstein, Physics of Plasma-Wall Interactions in Controlled Fusion, eds., D. Post and R. Behrisch (Pergamon, New York, 1985). [5] P.J. Schneider, W. Eckstein and H. Verbeek, Nucl. Instr. and Meth. 194 (1982) 387. [6] J.P. Biersack and W. Eckstein, Appl. Phys. A34 (1984) 73. [7] P.J. Schneider, W. Eckstein and H. Verbeek, Nucl Instr. and Meth. B2 (1984) 655. [S] B.M. Smirnow, Negative Ions (McGraw-Hill, New York, 1982). [9] P.J. Schneider, W. Eckstein and H. Verbeek, Nucl. Instr. and Meth. B2 (1984) 525.
statefractmnsof Ni sputtered by Ne [lo] T.M. Buck, G.H. Wheatley and L.K. Verheij, Surf. Sci. 90 (1979) 635. [ll] S.B. Luitjens, A.J. Algra, E.P.Th.M. Suurmeijer and A.L. Boers, Surf. Sci. 99 (1980) 631 and 652. 112) H. Verbeek, W. Eckstein and F.E.P. Matschke, J. Phys. El0 (1977) 944. [13] D.P. Jackson, W. Heiland and E. TagIauer, Phys. Rev. B24 (1981) 4198. [14] E. Miihling, Thesis, Munich (1985). 1151 H.J. Barth, E. Mahling and W. Eckstein, Appl. Surf. Sci. 22/23 (1985) 136. [16] P.J. Schneider, W. Eckstein and H. Verbeek, Nucl. Instr. and Meth. 218 (1983) 713. [17] A. Fontell and E. Arminen, Can. J. Phys. 47 (1969) 2405. 1181 E. Hechtl, H.L. Bay and F. Bohdansky, Appl. Phys. 16 (1978) 147.
End of the Proceedings of the Workshop on Inelastic Ion-Surface Collisions
in Physics
CHARGE STATE FRACTIONS W. ECKSTEIN, Max-Planck-Institut
Research
507
B14 (1986) 507-514
OF Ni SPUTTERED
BY Ne AND Ne REFLECTED
FROM Ni
H.J. BARTH and E. MUHLING ftir Plasmaphysik,
Euratom Association,
D-8046 Garching/Minchen,
FRG
Charge state fractions of sputtered Ni and reflected Ne particles are determined from the measurement of energy distributions in different charge states for non-normal incidence in the energy range from 1 to 13 keV. It has been found that the charge state fractions depend on the type of collision. Monte Carlo simulations (TRIM.SP) were used to determine the ionization efficiency for the neutral Ni atoms in the N, stripping cell.
1. Introduction The charge state fractions are of primary interest in understanding the formation of different charge states at surfaces for reflected [l] as well as sputtered particles [2]. Some progress has been made in recent years in the theoretical approach [3], but it is still difficult to get quantitative agreement with experimental results. The formation of ions is also important in practical applications as in plasma-wall interaction [4] (because only neutrals can penetrate into the discharge) and in SIMS [2] for example. In SIMS usually all the sputtered ions are collected by ion-optical means. In this paper only the ions emitted in a small solid angle are analyzed. The bombardment at non-normal incidence is chosen to have the possibility to detect particles sputtered in one binary collision.
2. The experimental
sputtered particles (independent of their charge state) in a specific solid angle for the comparison with experimental data. As described below these calculated data were used to determine the ionization efficiency of the stripping cell for neutral Ni atoms. The justification for this use of calculated results comes from the fact that in most cases investigated the calculated data give good quantitative agreement with experimental results as shown previously [6,7] and again here in fig. 1.
4. Experimental results Typical measured energy distributions of positively and negatively charged particles as well as neutral particles are shown in fig. 2 for the bombardment of a
setup
The apparatus BOMBARDON used for these measurements is described in ref. 151. In short, the ion beam is mass-analyzed with an angular resolution of about 0.5”. The reflected and sputtered ions are energyanalyzed in a cylindrical condenser and detected with a channeltron. The reflected and sputtered neutrals are ionized in a N, stripping cell. The energy resolution of the detection device is AE/E = 0.008 and the angular resolution is 0.25” with a corresponding solid angle of 5.5 x 1O-5 sr. For a comparison with calculated energy distributions the measured spectra have to be corrected for the constant energy resolution AE/E of the condenser.
102 3. The calculation The Monte Carlo program TRIM.SP (version TRSPIC) [6] was applied to get energy distributions of 0168-583X/86/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
103
lo4
E,. INCIDENT ENERGY (eV)
B.V.
Fig. 1. Sputtering yield vs the incident energy at normal incidence for the bombardment of Ni by Ni: 0 calculated data, + experimental data (171, x experimental data [18]. V. SCATTERING
SO8
W. Eckstein et al. / Charge
clean Ni surface with 9 keV Ne+ and Ar+ ions at an angle of incidence a! = 70” (with respect to the surface normal). The emission angle of reflected and sputtered
9.0 keV Ne+d Ni f
I
2s
H
NE++
r
statefractionsof
Ni sputtered
INCIDENT PARTICLE
by Ne
SURFACE NORMAL
I
SPUTTERED PARTICLE
a= 70”. P= 60°. *= 50° 1 1 I
1 I
Ni Ne”*(Ni)
Fig. 3. Definitions of the different angles (incident angle IX. polar exit angle p, scattering (recoil) angle 9, azimuthal exit angle cp).
E. ENERGY 9.0 keV Ar+-
u
1
2
Ni
3
(keV)
a= 70”.
4
particles is j3 = 6W (giving a total scattering or recoil angle 6 = 500). (A definition of the different angles is given in fig. 3.) The distributions of positively charged particles show peaks due to reflected ions (Ne+, Ne2+), sputtered particles (Ni*, Ni’+) and desorbed particles (Hf, D*). The spectra of neutrals also show peaks due to reflected particles (Ne’), sputtered particles (Nia) and desorbed particles (Ho, Do). The spectra of negatively charged particles exhibit only peaks due to sputtered particles (Ni-) and desorbed particles (H-, D-). ,The only impurity seems to be hydrogen, which is hard to detect with other techniques such as ISS or AES. The absence of 0 and C peaks in the distributions of negatively charged particles shows that these species must be well below a 1,400 of a monolayer at the surface. All three energy distributions show large backgrounds beside the peaks mentioned. In the case of the Ne bombardment the particles above 6 keV must be Ne particles, because it is impossible to transfer more energy to Ni particles. Below 6 keV the background in the positive and neutral spectra consists of both reflected Ne particles and sputtered Ni particles, whereas in the negative spectrum the background is exclusively formed by sputtered Ni ions. Only at low energies desorbed
p= 60°.
5
E. ENERGY IkeV)
6
3=50°
7
6
Fig. 2. Measured distributions of positive (+), neutral (0) and negative (-) particles for the bombardment of Ni at an incident angle a = 70”. The polar exit angle is ~3= 60” and the scattering (recoil) angle is 9 = 50”. Single element names indicate sputtered or desorbed species, Ne (Ni) indicates Ne refiected from Ni. (a) 9 keV Ne’ bombardment. (b) 9 keV Ar+ bombardment. QS and QD indicate quasisingle and quasidouble elastic scattering of Ar from Ni.
W. Eckstein et al. / Charge state fructions of NI sputtered br, Ne
hydrogen and deuterium may give a small contribution to the background. The reason that the negative spectrum consists only of sputtered and desorbed particles is the nonexistence of negative noble gas ions [8] (He is an exception, it has a negative ion [9]).
5. Calculated
results
For later need, a few results from computer simulation should be shown here. In fig. 4 the calculated angular distributions (in all directions) for sputtered Ni and reflected Ne particles are shown. The sputtered Ni distributions exhibit a maximum at (‘p = O”, /? = 25”), whereas the distribution of the reflected Ne is centered around the specular direction (‘p = O”, /3 = 75”). ‘p is the azimuthal emission angle (fig. 3) and ‘p = 0” is in
9keV Ne-Ni;
509
the plane of incidence, defined by the incident beam and the normal of the target surface. From the distribution of sputtered Ni it can be seen that the angular distributions do not change for azimuthal angles 0” < ‘p < 15”. Therefore this angular range has been chosen to calculate the energy distributions for different exit angles (see fig. 5). To get reasonable statistics the azimuthal angular range cannot be made smaller. For the comparison with experimental energy distributions, which have been measured in the plane of incidence (cp = O”), the energy distributions are horizontal cuts through the contour line plot at the corresponding polar exit angle /3. The calculated energy spectra of sputtered Ni differ from those of reflected Ne in that the reflected Ne is concentrated at high energies and large angles /3, whereas the sputtered Ni increases with decreasing energy concentrated at low emission angles fi.
0r=70O
SPUTTERED
PARTICLES
REFLECTED
PARTICLES
60
a e
b1
a
2
ci
CZlO
80 60
Cp, AZIMUTHAL
ANGLE
Fig. 4. Contour line plots of angular distributions in the whole space for Ni bombarded The lines connect equal intensities per solid angle. c is the distance of adjacent contour sputtered
Ni, (b) reflected
by 9 keV Net at an Incident angle a = 70”. lines (c is proportional to the intensity). (a)
Ne. V. SCATTERING
W.
510
Ecksrrrn
9keV Ne-Ni;
er al.
/
Charge .xrure frac-trons o/ ,VI .cputleredb! Xr
a=70°;Oo
SPUTTERED
< cp <15O
PARTICLES -1
c=s ’
---.
-I 1
I : i
REFLECTED u”!
bl 1.0 eli’“T’;I-p
PARTICLES --.
--
‘I-
c = 10
0.8 ’ ’ I o,6 ‘__pd.:,,,yL. p \ .p I,! pfl,
I 0.4 -
E/E,,RELATlVE Fig. 5. Contour line
plots of
angular energy correlations
ENERGY into a forward direction (0” i ‘p < 15”) for the bombardment
Ne at an incident angle a = 70”. The lines connect points of equal intensities per solid angle and energy interval. gives an energy distribution intensity). (a) Sputtered
at a specific polar exit angle p. c is the distance of adjacent
contour
of Ni by 9 keV A horizontal
lines (c is proportional
cut
to the
Ni. (b) reflected Ne.
6. Discussion
Without an additional mass analysis it is hard to separate the reflected from the sputtered particles and it is therefore difficult to determine charge state fractions. But two things can be done. For the Ne bombardment the charge state fractions at the high energy end can easily be determined as usually [S.lO,ll] because that part of the energy distributions consists only of backscattered Ne particles. The other case for which charge state fractions can be determined (with more effort) are the peak regions in the energy distributions (below 6 keV in fig. 2 for the Ne bombardment). This is easily done for the desorbed hydrogen, because an ionization efficiency curve for neutral hydrogen in the stripping cell exists as in the case of Ne [12]. For the sputtered Ni
peaks. which originate from sputtering in one binary collision, a method of determining an ionization efficiency of neutral Ni atoms in the stripping cell is described in the next section. 6. I. Dererminution
of the ionizurion efficiency /or Ni
Assuming the TRIM.SP results to be correct. one can equate the integral I over the Ni peaks in the calculated distribution to the sum of integrals J’ over the Ni peaks in the experimental distribution, I=J*/y+J++J-+J’+. where the integral J ’ is corrected for the ionization efficiency y. All J’ are measured and I is calculated. Hence y is the only unknown quantity in the equation
W. Eckstern et ul. / Charge
o
Ar+---NI
i
l
Nd-NNi
‘;
statefracttons
of Ni sputtered by Ne
511
in y between Ne and Ar bombardment. Due to uncertainties in the accuracy of the TRIMSP results as well as in the determination of the absolute solid angles, the total error is believed to be larger than indicated by the error bars. An absolute error of about a factor of 2 is offered as a reasonable guess.
II
6.2. Charged
fractions
If y is known, the charge state fractions, calculated from the equation
0
10
20
30
E. ENERGY OF Ni
40
50
6.0
70
ATOMS IkeVl
Fig. 6. The ionization efficiency, y, of Ni atoms in the N, filled stripping sputtered
cell vs the energy of the Ni atoms. by Ne+ and Ar+ bombardment.
The Ni atoms are
and can thus be determined. The quantities J’ and I must be related to the same solid angle and to the same incident dose. The background in the experimental distributions consists mainly of backscattered rare gas atoms and ions and of Ni particles sputtered in multiple collisions and has to be subtracted from both the experimental and calculated peaks. The experimental distributions must also be corrected for the constant energy resolution. The calculations for one set of initial conditions give the energy dist~butions for all exit angles. By varying the exit angle in the measurement the ionization efficiency y can be obtained as a function of energy. The result is presented in fig. 6, where y is given for Ni particles sputtered by Ne and Ar, respectively, versus the Ni energy. The relatively large errors, indicated by vertical bars, originate mainly from the relatively poor statistics of the calculated distributions. The two sets of points obtained for Ne and Ar allow one to draw a mean efficiency curve. It is observed that the points for the Ne bombardment lie systematically higher, suggesting the possibility that the excitation of the Ni atoms is higher in Ne collisions, thus leading to a slightly larger stripping cross section in the gas cell. If the sputtered Ni atoms are in the ground state on reaching the stripping cell, there should be no difference
Q’, can be
where i stands for 0, +, 2 + and -, and S is the intensity at each energy interval. For the reflected Ne the charge state fraction can be determined point by point from a set of spectra as in fig. 2. One result is shown in fig. 7, where the charged fraction, q+, is plotted versus the energy of reflected Ne. The charged fraction shows two peaks. The one at lower energy is due to the quasisingle scattering peak, the one at higher energies is due to the quasidouble scattering peak. From several sets of energy distributions one can determine the dependence of the maximum charged fractions as a function of energy. This is shown in fig. 8. The charged fraction, q+, of the quasisingle scattering is nearly independent of the energy and its absolute value is in good
9 keV Ne*-,
QS
Ni
z
CL = 70”
x
QD x
x *
3
L
5
6
7
8
9
E. ENERGY OFs BACKSCATTERED Ne fkeV) Fig. 7. Positive charged fraction of Ne reflected from Ni vs the energy of backscattered Ne for 9 keV Ne* bombardment of Ni at an angle of incidence a = 70”.
V. SCATTERING
512
W. Eckstein
et al. / Charge statefractrons
of Ni sputtered
by Ne
The charge state fractions of the desorbed hydrogen are investigated in detail in ref. [14]. The same procedure as for Ne can be applied for sputtered Ni to get the charge state fractions versus the
2
a
9
keV
Ne+--
Ni
a =?a”
0
a
10
5
E,
ENERGY
OF
BACKSCATTERED
15
Ne
(k&I
Fig. 8. Maximum positive charged fractions, n+ and T’+. of Ne vs the energy of backscattered Ne particles. 4- 15 keV Ne+ bombardment of Ni at an angle of incidence fy = 70”. QS and QD correspond to the quasisingle and quasidouble scattering peaks, respectively.
agreement with earlier data [lo]. The charged fraction of the quasidouble scattering peak is about 5% at energies below 7 keV and increases to about the same values as for the quasisingle scattering at higher energies. Even in the n2i fraction the quasidouble scattering can be seen at energies above 10 keV. The n2+ fraction is below 1% at energies below 5 keV and increases to a few percent above 5 keV. Also for Ne the charged fractions depend on the exit angle. The scattering events leading to the so-called single collision peak or QS-peak are assumed to be mainly one main collision but with additional soft small angle collisions. From computer simulations [13] it is known that other collision events leading to the same exit energy and exit angle are possible. The contribution of other collision events depends strongly on the surface structure. It can be assumed from the literature [l) that different collision events at the surface produce different charge state fractions. In our experiment the exact surface structure is not known, so that nothing can be said about the contribution of different collision events in the QS-peak. The charged fractions given in figs. 7 and 8 are therefore mean values for the mixture of different processes as in the earlier literature [lO,ll]. From these general considerations it seems impossible to distinguish contributions from different processes. But by a careful look at fig. 2 it may be possible to distinguish at the maximum (7 keV) two contributions: the single collisions and other processes. These energy distributions then give at the maximum n+= 0.22 for the binary collisions and n+= 0.11 for the sum of other collisions at the same exit energy.
E, ENERGY OF SPUTT. Ni (keV I 9 keV
Ne*a = 700
Ni
0 .60’
,F
I
1
0.5
I
/ s
OO
I
1
I
I
2
II
I
3
I
I
L
I
s
E, ENERGY OF SPUTT. Ni ( keV 1 Fig. 9. Charged
fractions, n, of Ni sputtered by Ne vs the energy E of sputtered Ni (nor corrected for reflected Ne particles). 9 keV Ne+ bombardment of Ni at an angte of incidence a = 70’. The emission angle (detector) is at /? = 60°. (a) positive charge state fraction ?I+, (b) negative charge state fraction n-.
W. Eckstein Edal. / Charge state/ractums
energy of sputtered Ni for one incident condition (as in fig. 2). Here one has to assume that the intensity of reflected Ne is so small that it does not influence the result. This is only approximately true as can be seen from fig. 2. The result is shown in fig. 9, where the positive (fig. 9a) and negative (fig. 9b) charge state fractions are plotted versus the energy of sputtered Ni. The main result which can be seen from fig. 9 is that both charge state fractions of particles sputtered in one binary collision with Ne have a value about twice as high as the other sputtered Ni. Due to the uncertainty about the Ne background, the absolute values of the positive charge state fraction are less accurate than those of the negative fraction. The large increase of q+ directly after the Ni peak is due to the appearance of the Ne2+ peak at 3.5 keV (fig. 2a), for which q+ is not corrected. A more accurate procedure can be applied to determine the mean charge state fractions of Ni sputtered in a binary collision (the peak regions in fig. 2). In that case the integrated peak areas, where the background is subtracted, are used to calculate the charge state fractions. The result for the different charge state fractions versus the energy of the sputtered Ni is shown in fig. 10.
*
0---*-
‘, +..-a
of Ni sputtered by Ne
513
The most obvious result is the dominant role of the neutrali fraction, more than 90% in the energy range below 5 keV. The neutral fraction increases to lower energies. The singly charged positive fraction increases with increasing energy and it is more than an order of magnitude larger than the doubly charged fraction. The same is true for the negative fraction which has a less steep slope than the positive fractions. Most argon data are collected at slightly different angles (a, &?) than those for Ne. The positive charged fractions of Ni for Ar bombardment at the same exit angles and emission energy show a systematic trend to be lower than for Ne bombardment. This fact is a strong indication that the interaction of the bombarding ion with the target atom influences the resulting charge state fraction. Also the fact that the charged fractions in the binary collision (sputtering) peaks deviate from the values at lower and higher energies point in the same direction. In other words, the charged fraction is not only a surface effect but also an atomic collision effect. The doubly charged fraction n2* for Ar bombardment was at the detection limit, which was also true for Ne bombardment below 6 keV incident energy. The charge state fractions show a strong dependence on the polar exit angle /? [15]. This fact and the low charge state fractions at lower energies explain why Ni ions (sputtered in a binary collision) have not been seen in an earlier publication [16].
= 70°. p = 60”
7. Conclusions Charge state fractions of Ne reflected from Ni and of Ni sputtered by Ne and Ar bombardment have been determined. In the charge state fractions of sputtered and reflected particles a dependence on the type of collision has been observed. The reflected Ne data show reasonable agreement with earlier data. It has been shown that computer simulation can be used to determine ionization efficiencies of neutral Ni in a N, stripping cell. The appearance of both reflected and sputtered species in the measured energy distributions provides the problem how to separate the different species to determine the charged fractions over the whole energy range. An additional magnetic analysis or time of flight analysis will solve this problem. Note addedin proof: The efficiency curve in fig. 6 is erroneously a factor of 2 too high, so that all charged fractions in fig. 10 are also too large by the same factor of 2.
10
2.0
3.0
6.0
E. ENERGY OF SPUTTERED
50 Ni (keV)
Fig. 10. Charged fractions of positive and negative Ni ions sputtered in one binary collision by Ne+ and Ar+ bombardment.
Reference [l] W. Eckstein. Springer Series in Chemical Physics, vol. 17, eds., E. Taglauer and W. Heiland (Springer, Berlin, Heidelberg,
1981) p. 157. V. SCA’ITERING
514
W. Eckstein et al. / Charge
[2] Secondary Ion Mass Spectrometry SIMS III, eds., A. Benninghoven et al. (Springer, Berlin, 1982). [3] D.M. Newts, K. Makoshki, R. Brako and J.N.M. van Wunnik, Phys. Scripta T6 (1983) 5. [4] R. Behrisch and W. Eckstein, Physics of Plasma-Wall Interactions in Controlled Fusion, eds., D. Post and R. Behrisch (Pergamon, New York, 1985). [5] P.J. Schneider, W. Eckstein and H. Verbeek, Nucl. Instr. and Meth. 194 (1982) 387. [6] J.P. Biersack and W. Eckstein, Appl. Phys. A34 (1984) 73. [7] P.J. Schneider, W. Eckstein and H. Verbeek, Nucl Instr. and Meth. B2 (1984) 655. [S] B.M. Smirnow, Negative Ions (McGraw-Hill, New York, 1982). [9] P.J. Schneider, W. Eckstein and H. Verbeek, Nucl. Instr. and Meth. B2 (1984) 525.
statefractmnsof Ni sputtered by Ne [lo] T.M. Buck, G.H. Wheatley and L.K. Verheij, Surf. Sci. 90 (1979) 635. [ll] S.B. Luitjens, A.J. Algra, E.P.Th.M. Suurmeijer and A.L. Boers, Surf. Sci. 99 (1980) 631 and 652. 112) H. Verbeek, W. Eckstein and F.E.P. Matschke, J. Phys. El0 (1977) 944. [13] D.P. Jackson, W. Heiland and E. TagIauer, Phys. Rev. B24 (1981) 4198. [14] E. Miihling, Thesis, Munich (1985). 1151 H.J. Barth, E. Mahling and W. Eckstein, Appl. Surf. Sci. 22/23 (1985) 136. [16] P.J. Schneider, W. Eckstein and H. Verbeek, Nucl. Instr. and Meth. 218 (1983) 713. [17] A. Fontell and E. Arminen, Can. J. Phys. 47 (1969) 2405. 1181 E. Hechtl, H.L. Bay and F. Bohdansky, Appl. Phys. 16 (1978) 147.
End of the Proceedings of the Workshop on Inelastic Ion-Surface Collisions