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Diagnostics of impurities in ion beams by carbon probes
DIAGNOSTICS OF IMPURITIES IN ION BEAMS BY CARBON PROBES A. POSPIESZCZYK,
H.L. BAY and B. SCHWEER
Instirur fiir Plasmaphysik der Kemforschungsanlage
Jiilich GmbH,
Association Eurarom-KFA,
Postfach 1913, D-5170
Jiilich, Fed. Rep. of Germany
Beam analysis of a 10 keV hydrogen and helium beam produced by a duopigatron source were carried out by sampling the contaminants in successive shots in a pyrolytic carbon probe. The subsequent analysis of the probe by Auger electron spectroscopy and Rutherford backscattering spectroscopy is a simple way of obtaining an estimate of the impurity concentration in the beam. In our case it was found that the beam contained less than 1% of metal impurities.
1. Introduction A serious problem in the application of hydrogen- and helium-ion beams can arise if a nonanalysed beam has to be used for sputtering processes and neutral injection, for example. This can be the case if the diameter of the beam is too large, which would require expensive deflection systems for an ion beam, or if the beam already consists of neutral particles. In this case a knowledge of the percentage of heavy impurities in the beam is especially valuable because already a few percent of metals generally dominate sputtering effects or lead to an intolerably high concentration of impurities in a fusion plasma. Up to now data concerning the impurity concentration in the type of source which we used are very rare. Therefore, in this paper a relatively simple method will be presented which can easily yield an estimate of the beam composition. 2. Method When a solid is bombarded with energetic atoms the particles will be implanted in the solid and after a certain time, when the number of particles deposited is balanced by an equal number of particles leaving the surface, a stationary profile will develop, which then is independent of erosion time and reflects -for instance at the surface-the composition of the beam. For simplicity let us consider a uniform stream of particles of flux q normal to a surface, which produces steady erosion and causes the surface
Journal of Nuclear iUat&als 93 & 94 (1980) 396-401@
to retreat at a constant speed o from an initial position. Thus, u = (X qiYi)/py where p is the atomic density of the solid, qi are the fluxes of the constituents of the beam with Z qi = q, and Y, the corresponding sputtering yields. In a “pure” hydrogen or helium beam the erosion speed will be determined by qHYH and qHeYHe,respectively, but care is needed if the beam contains heavy impurities. If ci(x, t) denotes the instantaneous concentration of one implanted species per unit volume, where x denotes the depth below the actual (moving) surface, the corresponding differential equation is aci _
at-
a2Ci
D
axZ+'ax
aq +Q,(X),
where D is the diffusion constant and Q,(x) the depth distribution function of the implanted atoms. Eq. (1) has already been solved by a general mathematical treatment in [l, 21, but for simplicity let us consider the case D = 0 which is justified for heavy metals in carbon. Then eq. (1) reduces to &i = o at
aci
ax+ Qi(x)*
The solution of (2) is straightforward by ci(Xyt) =
I
Qi[x(t) + UP]dt’,
and is given
(3)
0
with c(x, 0) = 0.
North-Holland
Publishing Company
3%
A. Pospieszczyk et al. I Diagnostics of impurities in ion beams
397
If one assumes as a source function
(yy] Qi(X)=t& exp[-;
(4)
with
-10
where x0 denotes the range and A the half-width (straggling) of the distribution, the solution of (3) is
Ci(X,t)=~(erf[Ut~~xo]-e~[~]].(5) At the surface (x = 0) the stationary concentration c(0,~) = qJu will already be reached within an accuracy of 5% when the argument of the first error function is about 1.5, which leads to the condition vt =2A+x,.
(6)
This means, if D = 0, that equilibrium would be achieved when the beam had sputtered through the target at least to the projected range of this species (note that for all cases x0/A > 1 holds) plus about twice the straggling length. In the case where the equilibrium has not been reached, the saturation concentration may be obtained at a depth x = x&vt
(7)
when the surface has been eroded to a depth ut>4A.
(8)
This will happen if x$A & 1. In this case the saturation concentration may be found by depth profiling of the probe. This is demonstrated in fig. 1 where the actual surface for a given condition has to be drawn at the respective value of (x - x&A, for x = 0. The factor q/r, in eq. (5) is a measure of the magnitude of the concentration at equilibrium conditions. To obtain the impurity concentration in the beam we find
-9
-8
-7
-6
-5
4
-3
-2
-1
0
1 -%-9
Fig. 1. Development of a stationary profile at positions x/A x0/A with ur/A as parameter. The equilibrium value, which will be reached at the surface after an erosion of u/A = 2+x,44 is indicated. The actual surface for a specific case has to be drawn at -x0/A.
and we obtain Pb=PSY,
(9)
with pS = c/p, the fraction of implanted atoms this is the quantity which is normally measured by AES -and pb = qi/qb, the fraction of impurities in the beam, as the final result. If one assumes a sputtering yield Y of about lob3 in the case of a 10 keV hydrogen beam and a relative detection limit of lo-* for AES, a lower limit of about 10ppm impurity concentration in the beam should be reached. At the moment it seems that just hydrogen beams offer a unique possibility, because of their low Y-values, to detect very low impurity concentrations. However, low sputtering yields may cause high impurity concentrations in the probe, which may then change the sputtering yield and the erosion rate v drastically; in this case care must be taken in the interpretation of the results. 3. Experiment The measurements were carried out at the ion source which was used for experiments described in ref. [3]. The source is of the duopigatron type and was operated in a pulsed mode with maximal pulse lengths of 10 s. At the target, about 1 m from source, a beam with a FWHM of 7 cm was delivered, the large dimension of which was excellent to simulate a plane geometry for the source of sputtered particles. This large dimension was also one reason why no deflection sys-
A. Pospieszczyk et al. / Diagnostics of impurities in ion beams
398
tern for beam analysis could be inserted. This forced us to try different methods for the determination of impurity concentrations. The extraction grids were made of copper; the same was true for the beam collimator about 0.3 m from the source. The beam line itself was built from stainless steel parts and some stainless steel parts were also used inside the source, the filament of which was a 1 mm thick Ta-wire. The pyrolytic carbon probe had a diameter of 1 cm, a thickness of 1 mm, and was mounted on a water-cooled holder with silver screws. The normal position for sampling was in the centre of the beam. The subsequent analysis of the probe was performed by AES including depth profiling with argon and partially by RBS for comparison. 4. Results for the hydrogen beam For the experiments with a hydrogen beam at 10 keV the PyC-target was bombarded with 40 shots of 10-s length and a current density of 4mA/cni* at the position of the target. Therefore, during this time about 6 nm of the surface would have been eroded if one assumes a value of Yu = 2 X low3 for a 10 keV-hydrogen on carbon [4]. According to the values of x0 and A for the respective elements (table 1) and the conditions above, one can derive, using eq. (6), that certainly no stationary concentration at the surface was achieved. For iron and copper, however,
Table 1 Projected ranges x0, straggling A, and x0/A in carbon at 10 keV particle energy according to [5] Projectile
xdnm)
Atnm)
xo/A
N
26 22 9 9
6.3 4.5 0.9 0.8
3.3 4.0 10 11
0 Fe CU
saturation values should have been reached inside the probe at x = xo - 2A (eqs. (7) and (8)). The results from AES are shown in fig. 2a. The spectrum for x = 0 indicates that copper and iron from the wall of the beam line and/or the source was deposited on the probe. To decide whether their origin was from energetic beam particles or not, depth profiling of the probe by 1 keV Ar-ions was performed (fig. 2b). The sputtering time was converted into a distance scale by assuming Y = 0.8 for 1 keV Ar on carbon. Although this scale should be treated with caution because of the large uncertainties of the sputtering coefficients, one can say that iron and also probably a part of the copper was implanted to about x0 at high energies, whereas less oxygen (probably H20) reached the probe in an energetic form. Sulfur, for example, was certainly a pure surface contamination. The results for all elements seen are summarized in table 2,
a
% rebtlve
Ar C
c Ix&O)
024% 676%
N
375%
Fe cu
216% llL%
concentration
w o
YT)
200
300
ur)
500
603
700
800
QX)Energy
IV1
2
1
6
8
10
12
1L depthInm1
Fig. 2. AES results for a carbon probe in a 10 keV hydrogen beam: (a) Auger spectrum at the surface; (b) depth profile obtained by Ar-sputtering and conversion into a distance scale.
A. Pospieszczyk et al. I Diagnostics of impurities in ion beams
Table2 Surface, bulk, and beam concentrations 10 keV hydrogen beam
of impurities for a
Element
Surface concentration (x = O)(%)
Bulk concentration (x = 4 nm)(%)
Beam concentration (%)
N 0 S Cl Fe cu
3.7 3.35 2.0 0.22 2.2 1.9
4.0 0.82 0.07 0.12 1.4 1.6
0.008 0.002
0.003 0.003
columns 1 and 2. It should especially be noted here that no tantal - the filament material -could be found during all probe measurements. In order to obtain the actual beam composition one has to multiply all values by Y = 2 x 10m3 according to eq. (9) (table 2, column 3). The result is that an upper limit of 40ppm of iron and copper was present as a beam contamination and therefore only hydrogen is responsible for sputtering. The situation with regard to the nitrogen found might differ a little from the other constituents. Since it was not present on the probe before the bombardment with the energetic beam, it might well have been implanted during the sampling. On the other hand, neither condition (6) nor (8) was fulfilled; also, the assumption of no diffusion could not be justified in this case. Possibly its concentration is in the 10% range, which would result in a 0.02% nitrogen impurity of the beam. The most probable explanation for the origin of the nitrogen could be found in an air leak of the source. In order to clear this situation this cgas was definitively added to the helium gas feed in the following experiments with a helium beam. 5. Results for the helium beam For the experiments with a helium beam at 10 keV the PyC-target was bombarded with 25 shots of 10-s length and a current density of 2 mA/cm* at the position of the target. Therefore (with YHc= 10-l for a 10 keV helium beam on
399
carbon [4]) 80 nm of the surface have been eroded and both conditions (6) and (8) were sufficiently fulfilled. In addition l%, 2.5%, and 5% nitrogen was added to the helium gas feed of the source. Since YN= 4X 10-l for 10 keV nitrogen on carbon, the fraction of nitrogen sputtering in comparison to helium is only 20% in the last case and therefore can be tolerated. In order to have an independent test of the AES method, the first two targets were first analysed by Rutherford backscattering (RBS). This is a very efficient means of detecting high mass numbers in an absolute scale with a much lower detection limit than AES, whereas the latter is favoured at low masses. On the other hand the masses of copper and iron cannot clearly be distinguished from each other in RBS. In this case the value given has to be treated as the sum of both elements. The results for both targets are given in table 3, column 4. To compare the absolute numbers from RBS with the relative AES scale, the respective surface concentrations were converted into relative volume concentrations by using the projected ranges from table 1 and p = 1 x 10’6/(cm2 *nm) for carbon (column 6). Another independent test is possible by comparing the absolute numbers of particles in the beam (3 x 10” for the bombardment time above) with the absolute numbers from RBS. In this case one must still take into consideration the simultaneous loss of implanted particles by erosion. Therefore, a correction factor of the form 80 nm/xo has to be applied. The results are also listed in table 3, column 5. The results from AES for one probe are shown in fig. 3a. Although again the linear distance scale should be treated with caution (Y = 1.5 for 2 keV Ar on carbon was used), the implantation seems to be qualitatively in agreement for bombardment with energetic 10 keV nitrogen, iron, and (possibly) oxygen particles. The relative concentrations for all three probes are also summarized in table 3, columns 1 and 2. The application of the sputtering yield (Y = 10-l) to obtain the respective values for the beam composition has been performed in the same way as before (section 4) (table 3, columns 3 and 7).
A. Pospieszcryk et al. I Diagnosticsof impuritiesin ion beams
400
Table 3 Surface, bulk, and beam concentrations of impurities for a 10 keV helium beam Carbon probe Element
RBS
AES (1) Surface concentration (x = OX%)
No. 1 N (1% Nr added) 0 Fe (Cu)
(2) Bulk concentration (“~)
(3) (4) Beam Atoms concentration cm* * 10” (%)
(5) Atoms&l. Atoms,,,ul (%)
(6) Atoms,,l Atoms,m (%)
(7) Beam concentration (%)
7.5 4.1 0.4
5.7 0.8 0.6
0.57 0.08 0.06
22 12 7.9
2.9 1.8 2.1
9 5.4 7
0.9 0.54 0.7
N 0 Fe (Cu)
4 10 cu.5
10.4 1.5 co.1
1.04 0.15 co.01
38 8.4 8.4
4.9 1.0 2.2
15 4 7.5
1.5 0.4 0.75
No. 3 N (5% Nr added) 0 Fe (Cu)
1.4 3.3 co.1
14 1.5 0.4
1.4 0.15 0.04
No. 2 (2.5% Nr added)
Again the situation for nitrogen does not seem to be well described under the assumption of no diffusion. In this case nitrogen might have diffused further in during the bombardment or out during the time between bombardment and analysis of the probe (some weeks). This behaviour seems to be indicated by the nitrogen depth profile in fig. 3b. Therefore, the respective values for the concentration at the implantation
r? e : ;j
200x
3
Cl “Ox
100x
2
& 0
SI k_;,
v
c
r
l-
ts 0
length were corrected by factors of 1.0, 1.6, and 2.9 for probes 1, 2, and 3, respectively, which should cancel this effect. As a result, one can say that the expected increase in nitrogen concentration in the beam by the addition of nitrogen to the helium gas feed can indeed by observed. With regard to the absolute numbers, the satisfactory for agreement is qualitatively nitrogen and oxygen, whereas the discrepancy
I. loo
I. 200
C
8.. 300
Loo
Fe ~01 %
915%
0 s
33%
I.
8..
Cl <005% ,Si 29%
1 L%
500
600
I I v7COEneqy[VI
4 0
, 5
, l0
, 15
I 20
, 25
I I30 35 depth [nm]
Fig. 3. AES results for carbon probe no. 3 in a 10 keV helium beam: (a) Auger spectrum at the surface; (b) depth profile obtained by Ar-sputtering and conversion into a distance scale.
A. Pospieszczyket al. / Diagnostics of impuritiesin ion beams
for the metals lies within one order of magnitude. The reason for this is not yet understood, but one can say that the impurity concentration in the beam for metals should be in the range of less than 1%. 6. Diseussfon and conclusion If one compares the results for the hydrogen and the helium beam there seems to be a strong difference in the respective concentrations of the different impurities. At the moment we are not quite sure if this effect is real, and should be attributed to the different gases used, or if it is purely accidental, possibly favoured by the complete dismantling and reconstruction of source and beam line between both measurements. Higher sputtering inside the source in the case of the He operation could have led, for example, to a more contaminated beam. Another possible explanation for this discrepancy might be found in the fact that in the case of a hydrogen beam too a high surface concentration could have been built up which could have changed the sputtering conditions drastically (see section 2). If one assumes the same concentration of metal contamination in both beams as a very pessimistic case, the accumulation of metals in the probe might well have reached a level of 50% for the hydrogen beam. This would have changed the erosion rates completely. In general, impurity-dominated effects can only be neglected in our case if the percentage of heavy
401
impurities in the beam is not more than 0.05% in the hydrogen and 5% in the helium beam. On the other hand one is justified in saying that the general idea of beam analysis by sampling the contaminants during adequate bombardment times by a pyrolytic carbon probe and the subsequent analysis of the probe by AES technique is a simple method for detecting low concentrations of contaminants in atoms beams. The analysis showed that our hydrogen beam contained possibly less than 0.1% and the helium beam less than 1% impurities. Therefore, the contaminants were not able to contribute definitively to the sputtering processes and falsify the results obtained, Acknowledgements
We would like to thank H. Repp for valuable mathematical assistance, J. Kirschner (IGVKFA) for the performance of the AES measurements, and J. Roth (MPI-Garching) for the performance of the RBS measurements. References [l] J.P. Biersack, Radiation EfTects 19 (1973) 249. [2] R. Collim and G. Carter, Radiation Effects 26 (1975) 181. [3] A. Elbem, E. Hink and B. Schweer, J. Nucl. Mater. 76/77 (1978) 143. [4] J. Bohdamky, H.L. Bay and W. Ottenberger, J. Nucl. Mater. 76177 (1978) 163. [S] K.B. Winterbon, Ion Implantation Range and Energy Deposition Distributions, Vol. 2, Low Incident Ion Energies (Plenum, New York, 1975).
H.L. BAY and B. SCHWEER
Instirur fiir Plasmaphysik der Kemforschungsanlage
Jiilich GmbH,
Association Eurarom-KFA,
Postfach 1913, D-5170
Jiilich, Fed. Rep. of Germany
Beam analysis of a 10 keV hydrogen and helium beam produced by a duopigatron source were carried out by sampling the contaminants in successive shots in a pyrolytic carbon probe. The subsequent analysis of the probe by Auger electron spectroscopy and Rutherford backscattering spectroscopy is a simple way of obtaining an estimate of the impurity concentration in the beam. In our case it was found that the beam contained less than 1% of metal impurities.
1. Introduction A serious problem in the application of hydrogen- and helium-ion beams can arise if a nonanalysed beam has to be used for sputtering processes and neutral injection, for example. This can be the case if the diameter of the beam is too large, which would require expensive deflection systems for an ion beam, or if the beam already consists of neutral particles. In this case a knowledge of the percentage of heavy impurities in the beam is especially valuable because already a few percent of metals generally dominate sputtering effects or lead to an intolerably high concentration of impurities in a fusion plasma. Up to now data concerning the impurity concentration in the type of source which we used are very rare. Therefore, in this paper a relatively simple method will be presented which can easily yield an estimate of the beam composition. 2. Method When a solid is bombarded with energetic atoms the particles will be implanted in the solid and after a certain time, when the number of particles deposited is balanced by an equal number of particles leaving the surface, a stationary profile will develop, which then is independent of erosion time and reflects -for instance at the surface-the composition of the beam. For simplicity let us consider a uniform stream of particles of flux q normal to a surface, which produces steady erosion and causes the surface
Journal of Nuclear iUat&als 93 & 94 (1980) 396-401@
to retreat at a constant speed o from an initial position. Thus, u = (X qiYi)/py where p is the atomic density of the solid, qi are the fluxes of the constituents of the beam with Z qi = q, and Y, the corresponding sputtering yields. In a “pure” hydrogen or helium beam the erosion speed will be determined by qHYH and qHeYHe,respectively, but care is needed if the beam contains heavy impurities. If ci(x, t) denotes the instantaneous concentration of one implanted species per unit volume, where x denotes the depth below the actual (moving) surface, the corresponding differential equation is aci _
at-
a2Ci
D
axZ+'ax
aq +Q,(X),
where D is the diffusion constant and Q,(x) the depth distribution function of the implanted atoms. Eq. (1) has already been solved by a general mathematical treatment in [l, 21, but for simplicity let us consider the case D = 0 which is justified for heavy metals in carbon. Then eq. (1) reduces to &i = o at
aci
ax+ Qi(x)*
The solution of (2) is straightforward by ci(Xyt) =
I
Qi[x(t) + UP]dt’,
and is given
(3)
0
with c(x, 0) = 0.
North-Holland
Publishing Company
3%
A. Pospieszczyk et al. I Diagnostics of impurities in ion beams
397
If one assumes as a source function
(yy] Qi(X)=t& exp[-;
(4)
with
-10
where x0 denotes the range and A the half-width (straggling) of the distribution, the solution of (3) is
Ci(X,t)=~(erf[Ut~~xo]-e~[~]].(5) At the surface (x = 0) the stationary concentration c(0,~) = qJu will already be reached within an accuracy of 5% when the argument of the first error function is about 1.5, which leads to the condition vt =2A+x,.
(6)
This means, if D = 0, that equilibrium would be achieved when the beam had sputtered through the target at least to the projected range of this species (note that for all cases x0/A > 1 holds) plus about twice the straggling length. In the case where the equilibrium has not been reached, the saturation concentration may be obtained at a depth x = x&vt
(7)
when the surface has been eroded to a depth ut>4A.
(8)
This will happen if x$A & 1. In this case the saturation concentration may be found by depth profiling of the probe. This is demonstrated in fig. 1 where the actual surface for a given condition has to be drawn at the respective value of (x - x&A, for x = 0. The factor q/r, in eq. (5) is a measure of the magnitude of the concentration at equilibrium conditions. To obtain the impurity concentration in the beam we find
-9
-8
-7
-6
-5
4
-3
-2
-1
0
1 -%-9
Fig. 1. Development of a stationary profile at positions x/A x0/A with ur/A as parameter. The equilibrium value, which will be reached at the surface after an erosion of u/A = 2+x,44 is indicated. The actual surface for a specific case has to be drawn at -x0/A.
and we obtain Pb=PSY,
(9)
with pS = c/p, the fraction of implanted atoms this is the quantity which is normally measured by AES -and pb = qi/qb, the fraction of impurities in the beam, as the final result. If one assumes a sputtering yield Y of about lob3 in the case of a 10 keV hydrogen beam and a relative detection limit of lo-* for AES, a lower limit of about 10ppm impurity concentration in the beam should be reached. At the moment it seems that just hydrogen beams offer a unique possibility, because of their low Y-values, to detect very low impurity concentrations. However, low sputtering yields may cause high impurity concentrations in the probe, which may then change the sputtering yield and the erosion rate v drastically; in this case care must be taken in the interpretation of the results. 3. Experiment The measurements were carried out at the ion source which was used for experiments described in ref. [3]. The source is of the duopigatron type and was operated in a pulsed mode with maximal pulse lengths of 10 s. At the target, about 1 m from source, a beam with a FWHM of 7 cm was delivered, the large dimension of which was excellent to simulate a plane geometry for the source of sputtered particles. This large dimension was also one reason why no deflection sys-
A. Pospieszczyk et al. / Diagnostics of impurities in ion beams
398
tern for beam analysis could be inserted. This forced us to try different methods for the determination of impurity concentrations. The extraction grids were made of copper; the same was true for the beam collimator about 0.3 m from the source. The beam line itself was built from stainless steel parts and some stainless steel parts were also used inside the source, the filament of which was a 1 mm thick Ta-wire. The pyrolytic carbon probe had a diameter of 1 cm, a thickness of 1 mm, and was mounted on a water-cooled holder with silver screws. The normal position for sampling was in the centre of the beam. The subsequent analysis of the probe was performed by AES including depth profiling with argon and partially by RBS for comparison. 4. Results for the hydrogen beam For the experiments with a hydrogen beam at 10 keV the PyC-target was bombarded with 40 shots of 10-s length and a current density of 4mA/cni* at the position of the target. Therefore, during this time about 6 nm of the surface would have been eroded if one assumes a value of Yu = 2 X low3 for a 10 keV-hydrogen on carbon [4]. According to the values of x0 and A for the respective elements (table 1) and the conditions above, one can derive, using eq. (6), that certainly no stationary concentration at the surface was achieved. For iron and copper, however,
Table 1 Projected ranges x0, straggling A, and x0/A in carbon at 10 keV particle energy according to [5] Projectile
xdnm)
Atnm)
xo/A
N
26 22 9 9
6.3 4.5 0.9 0.8
3.3 4.0 10 11
0 Fe CU
saturation values should have been reached inside the probe at x = xo - 2A (eqs. (7) and (8)). The results from AES are shown in fig. 2a. The spectrum for x = 0 indicates that copper and iron from the wall of the beam line and/or the source was deposited on the probe. To decide whether their origin was from energetic beam particles or not, depth profiling of the probe by 1 keV Ar-ions was performed (fig. 2b). The sputtering time was converted into a distance scale by assuming Y = 0.8 for 1 keV Ar on carbon. Although this scale should be treated with caution because of the large uncertainties of the sputtering coefficients, one can say that iron and also probably a part of the copper was implanted to about x0 at high energies, whereas less oxygen (probably H20) reached the probe in an energetic form. Sulfur, for example, was certainly a pure surface contamination. The results for all elements seen are summarized in table 2,
a
% rebtlve
Ar C
c Ix&O)
024% 676%
N
375%
Fe cu
216% llL%
concentration
w o
YT)
200
300
ur)
500
603
700
800
QX)Energy
IV1
2
1
6
8
10
12
1L depthInm1
Fig. 2. AES results for a carbon probe in a 10 keV hydrogen beam: (a) Auger spectrum at the surface; (b) depth profile obtained by Ar-sputtering and conversion into a distance scale.
A. Pospieszczyk et al. I Diagnostics of impurities in ion beams
Table2 Surface, bulk, and beam concentrations 10 keV hydrogen beam
of impurities for a
Element
Surface concentration (x = O)(%)
Bulk concentration (x = 4 nm)(%)
Beam concentration (%)
N 0 S Cl Fe cu
3.7 3.35 2.0 0.22 2.2 1.9
4.0 0.82 0.07 0.12 1.4 1.6
0.008 0.002
0.003 0.003
columns 1 and 2. It should especially be noted here that no tantal - the filament material -could be found during all probe measurements. In order to obtain the actual beam composition one has to multiply all values by Y = 2 x 10m3 according to eq. (9) (table 2, column 3). The result is that an upper limit of 40ppm of iron and copper was present as a beam contamination and therefore only hydrogen is responsible for sputtering. The situation with regard to the nitrogen found might differ a little from the other constituents. Since it was not present on the probe before the bombardment with the energetic beam, it might well have been implanted during the sampling. On the other hand, neither condition (6) nor (8) was fulfilled; also, the assumption of no diffusion could not be justified in this case. Possibly its concentration is in the 10% range, which would result in a 0.02% nitrogen impurity of the beam. The most probable explanation for the origin of the nitrogen could be found in an air leak of the source. In order to clear this situation this cgas was definitively added to the helium gas feed in the following experiments with a helium beam. 5. Results for the helium beam For the experiments with a helium beam at 10 keV the PyC-target was bombarded with 25 shots of 10-s length and a current density of 2 mA/cm* at the position of the target. Therefore (with YHc= 10-l for a 10 keV helium beam on
399
carbon [4]) 80 nm of the surface have been eroded and both conditions (6) and (8) were sufficiently fulfilled. In addition l%, 2.5%, and 5% nitrogen was added to the helium gas feed of the source. Since YN= 4X 10-l for 10 keV nitrogen on carbon, the fraction of nitrogen sputtering in comparison to helium is only 20% in the last case and therefore can be tolerated. In order to have an independent test of the AES method, the first two targets were first analysed by Rutherford backscattering (RBS). This is a very efficient means of detecting high mass numbers in an absolute scale with a much lower detection limit than AES, whereas the latter is favoured at low masses. On the other hand the masses of copper and iron cannot clearly be distinguished from each other in RBS. In this case the value given has to be treated as the sum of both elements. The results for both targets are given in table 3, column 4. To compare the absolute numbers from RBS with the relative AES scale, the respective surface concentrations were converted into relative volume concentrations by using the projected ranges from table 1 and p = 1 x 10’6/(cm2 *nm) for carbon (column 6). Another independent test is possible by comparing the absolute numbers of particles in the beam (3 x 10” for the bombardment time above) with the absolute numbers from RBS. In this case one must still take into consideration the simultaneous loss of implanted particles by erosion. Therefore, a correction factor of the form 80 nm/xo has to be applied. The results are also listed in table 3, column 5. The results from AES for one probe are shown in fig. 3a. Although again the linear distance scale should be treated with caution (Y = 1.5 for 2 keV Ar on carbon was used), the implantation seems to be qualitatively in agreement for bombardment with energetic 10 keV nitrogen, iron, and (possibly) oxygen particles. The relative concentrations for all three probes are also summarized in table 3, columns 1 and 2. The application of the sputtering yield (Y = 10-l) to obtain the respective values for the beam composition has been performed in the same way as before (section 4) (table 3, columns 3 and 7).
A. Pospieszcryk et al. I Diagnosticsof impuritiesin ion beams
400
Table 3 Surface, bulk, and beam concentrations of impurities for a 10 keV helium beam Carbon probe Element
RBS
AES (1) Surface concentration (x = OX%)
No. 1 N (1% Nr added) 0 Fe (Cu)
(2) Bulk concentration (“~)
(3) (4) Beam Atoms concentration cm* * 10” (%)
(5) Atoms&l. Atoms,,,ul (%)
(6) Atoms,,l Atoms,m (%)
(7) Beam concentration (%)
7.5 4.1 0.4
5.7 0.8 0.6
0.57 0.08 0.06
22 12 7.9
2.9 1.8 2.1
9 5.4 7
0.9 0.54 0.7
N 0 Fe (Cu)
4 10 cu.5
10.4 1.5 co.1
1.04 0.15 co.01
38 8.4 8.4
4.9 1.0 2.2
15 4 7.5
1.5 0.4 0.75
No. 3 N (5% Nr added) 0 Fe (Cu)
1.4 3.3 co.1
14 1.5 0.4
1.4 0.15 0.04
No. 2 (2.5% Nr added)
Again the situation for nitrogen does not seem to be well described under the assumption of no diffusion. In this case nitrogen might have diffused further in during the bombardment or out during the time between bombardment and analysis of the probe (some weeks). This behaviour seems to be indicated by the nitrogen depth profile in fig. 3b. Therefore, the respective values for the concentration at the implantation
r? e : ;j
200x
3
Cl “Ox
100x
2
& 0
SI k_;,
v
c
r
l-
ts 0
length were corrected by factors of 1.0, 1.6, and 2.9 for probes 1, 2, and 3, respectively, which should cancel this effect. As a result, one can say that the expected increase in nitrogen concentration in the beam by the addition of nitrogen to the helium gas feed can indeed by observed. With regard to the absolute numbers, the satisfactory for agreement is qualitatively nitrogen and oxygen, whereas the discrepancy
I. loo
I. 200
C
8.. 300
Loo
Fe ~01 %
915%
0 s
33%
I.
8..
Cl <005% ,Si 29%
1 L%
500
600
I I v7COEneqy[VI
4 0
, 5
, l0
, 15
I 20
, 25
I I30 35 depth [nm]
Fig. 3. AES results for carbon probe no. 3 in a 10 keV helium beam: (a) Auger spectrum at the surface; (b) depth profile obtained by Ar-sputtering and conversion into a distance scale.
A. Pospieszczyket al. / Diagnostics of impuritiesin ion beams
for the metals lies within one order of magnitude. The reason for this is not yet understood, but one can say that the impurity concentration in the beam for metals should be in the range of less than 1%. 6. Diseussfon and conclusion If one compares the results for the hydrogen and the helium beam there seems to be a strong difference in the respective concentrations of the different impurities. At the moment we are not quite sure if this effect is real, and should be attributed to the different gases used, or if it is purely accidental, possibly favoured by the complete dismantling and reconstruction of source and beam line between both measurements. Higher sputtering inside the source in the case of the He operation could have led, for example, to a more contaminated beam. Another possible explanation for this discrepancy might be found in the fact that in the case of a hydrogen beam too a high surface concentration could have been built up which could have changed the sputtering conditions drastically (see section 2). If one assumes the same concentration of metal contamination in both beams as a very pessimistic case, the accumulation of metals in the probe might well have reached a level of 50% for the hydrogen beam. This would have changed the erosion rates completely. In general, impurity-dominated effects can only be neglected in our case if the percentage of heavy
401
impurities in the beam is not more than 0.05% in the hydrogen and 5% in the helium beam. On the other hand one is justified in saying that the general idea of beam analysis by sampling the contaminants during adequate bombardment times by a pyrolytic carbon probe and the subsequent analysis of the probe by AES technique is a simple method for detecting low concentrations of contaminants in atoms beams. The analysis showed that our hydrogen beam contained possibly less than 0.1% and the helium beam less than 1% impurities. Therefore, the contaminants were not able to contribute definitively to the sputtering processes and falsify the results obtained, Acknowledgements
We would like to thank H. Repp for valuable mathematical assistance, J. Kirschner (IGVKFA) for the performance of the AES measurements, and J. Roth (MPI-Garching) for the performance of the RBS measurements. References [l] J.P. Biersack, Radiation EfTects 19 (1973) 249. [2] R. Collim and G. Carter, Radiation Effects 26 (1975) 181. [3] A. Elbem, E. Hink and B. Schweer, J. Nucl. Mater. 76/77 (1978) 143. [4] J. Bohdamky, H.L. Bay and W. Ottenberger, J. Nucl. Mater. 76177 (1978) 163. [S] K.B. Winterbon, Ion Implantation Range and Energy Deposition Distributions, Vol. 2, Low Incident Ion Energies (Plenum, New York, 1975).