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The effect of oxides on PIXE measurements
Nuclear Instruments and Methods North-Holland, Amsterdam
in Physics
Research
269
B12 (1985) 269-272
THE EFFECT OF OXIDES ON PIXE MEASUREMENTS
*
J. RICKARDS Institute de Fkica, Universidad National Autbnoma de Mc?xico, Ap. Postal 20 - 364, 01000 Mkxico D. F., Mkxico Received
11 March
1985 and in revised form 20 April 1985
The presence of oxides on metallic samples used for PIXE analysis affects quantitative measurements. This effect has been calculated based on thick target analysis on layers of different compositions. The ratio of oxidized metal yield to clean metal yield is seen to depend on proton energy and on oxide thickness. Calculations are presented for oxides of Al, Si, Fe, and Cu, and applied to experimental data on Si. The method may be applied to measuring thicknesses of oxides of known stoichiometry.
1. Introduction Thick target PIXE analysis has received considerably less attention than the usual thin target work [l], although the basic effects have been studied and the principal advantages and drawbacks pointed out [2-61. However, when low energy proton beams are used, even very thin samples must be considered thick, since the bombarding particle range is small. Another fact is that the stopping power is close to the maximum in the dE/dx vs E curve, so a great deal of energy is deposited even in thin films, making them fragile and unstable. It is therefore advantageous to apply thick target analysis at low proton energies. In this case thin deposits or oxides on the sample are expected to affect quantitative measurements in an important way. The present work uses known procedures to calculate and demonstrate how oxides reduce the intensity of X-rays reaching the detector.
2. Calculation of X-ray yields
is calculated in small steps as the particle traverses the sequence of layers. It is governed by the proton energy loss, giving an average energy per step, the K-shell ionization cross section, the fluorescence yield and branching ratio, and the outgoing photon absorption (at an angle 8, from the normal to the target). The width of the steps can also be chosen. When the proton has reached its full range, the X-rays from all the steps are summed for each element. Proton straggling is not taken into account since its effect is expected to be small. The method of ref. [4] was followed except for the calculation of the stopping power because oxygen is outside of the range of values proposed for the coefficients of the semiempirical expression. It is also noted that the lighter the element, the larger is its contribution to the stopping power. Fig. 2 shows the stopping power of protons in oxygen taken from the tables of Northcliffe and Schilling [7] (marked NS) and from Ziegler [8] (marked Z). At low energies the difference is considerable. Ziegler’s values, which are adjusted to experimental data, do not go below 200 keV. Instead the stopping power was calculated with the expression of Montenegro et al. [9], which has no adjustable parameters and may
The geometry shown in fig. 1 was used to write a computer program that calculates thick target yields according to the method of Reuter et al. [4]. Protons impinge on a flat surface at an angle 8i to the normal, and as they travel into the target produce X-rays and lose energy. The composition of the first layer, assumed homogeneous, may be changed and admits up to five different elements. After going through layer 1 it enters layer 2 whose composition is also variable. The thicknesses of the layers are variable; up to three different layers may be used. The proton induced K, X-ray yield * Work carried de Ciencia
out with partial y Tecnologla grant
support of Consejo PCCB-020348.
National
0168-583X/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Fig. 1. The geometry used for the calculations. oxide; layer 2 is the pure metal.
Layer
1 is the
J. Rickark
270
/ Effect of oxides on PZXE
et al. [lo]. The mass attenuation those of Theisen
coefficients
used are
and Vollath [ll].
oxygen
3. Results of the cakdation
Proton
Energy (MeV)
Fig. 2. The stopping power of oxygen for protons. The solid curve MCV taken from Montenegro et al. [9] was used for the calculations. The dashed curve Z is taken from Ziegler [8] and the dotted curve NS from Northcliffe and Schilling [7].
easily be incorporated into the program. The curve obtained (marked MCV in fig. 2) is closer to the experimental points and goes to lower energy. Comparison of the MCV stopping powers with those of NS and 2 for other elements calculated in this work {Al and Cu) are shown in fig. 3. The method proposed in ref. (41 for calculating the cross section was used, the values obtained being in close (- 1%) agreement with the table given by Garcia
Energy
Calculations were carried out for oxides of four different elements (Al, Si, Fe and Cu) with the following procedure. The first layer is the oxide, of variable thickness, with the weight concentration of oxygen and the metal corresponding to the oxide in question. The second layer is the pure metal, with essentially infinite thickness. The quantity R is defined as the ratio of the metal X-rays produced from an oxidized sample to those produced by an unoxidized sample. It was calculated for energies between 0.1 and 5 MeV, with different thicknesses of the oxide. It is always less than unity. Fig. 4 shows R vs proton energy for Al,O,. Each curve corresponds to a different oxide thickness, the value of which is given in mg/cm2. The general trend for the lower energies is that for a given thickness, R approaches unity with increasing energy. At the higher energies R levels off to a given value with increasing energy. The lower envelope corresponds to the proton stopping in layer 1, all oxide. Silicon (fig. 5) shows a similar behavior. Two different oxides of iron, Fe0 and Fe,O, are shown in fig. 6. They both behave very much the same, but differ from Al and Si in that the lower envelope rises with increasing energy. For copper, CuO and Cu ,O were calculated; their qualitative behavior is similar to the iron oxides (fig. 7).
10
05
Proton
and experiment
(MeV)
Fig. 3. Stopping power of Al and Cu for protons. The solid curve is MCV [9], the dashed curve is Z [S], and the dotted curve is NS 171.
Proton
Energy
(MeVf
Fig. 4. The ratio R calculated for AlsO, vs proton energy from 0.1 to 5 Mev, for different oxide thicknesses. The oxide thickness in mg/cm2 is shown next to each curve.
J. Rickards
271
/ Effect of oxides on PIXE
R
1.0
0.9
20 30
0.8
0.7
I 0,2
0.6)
_-
0,2
0.4
Proton
0,6
0.6
1
Energy
2
3
r
I
I
0.2
0.4
Proton Fig. 6. The ratio R calculated stoichiometries.
I,,
0.6
0.6
Energy
1
1
I 0.6
I I I I 0.8 I
Energy
Fig. 7. The ratio R calculated ent stoichiometries.
I 2
I 3
I 45
(MeV)
for two copper
oxides of differ-
the experimental
Two facts cause the qualitative difference between the lighter (Al, Si) and the heavier (Fe, Cu) elements. On the one hand the K-shell ionization cross section (fig. 8) is essentially flat for Al from 2 to 5 MeV, whereas for Cu it increases fivefold. But more important is the absorption of the outgoing X-rays. Fig. 9 shows the outgoing X-ray transmission T vs depth in mg/cm2 for two contrasting cases, Al and Cu, and their respec-
0.5
I 0.4
Proton
45
(MeV)
Fig. 5. Same as fig. 4, but for SiO,, showing points obtained for an oxidized sample.
I
I
1
I
2
3
45
I
(MeV)
for two iron oxides of different
tive oxides, corresponding to 5 MeV proton bombarding energy. For Al the X-ray production is dominated by the first few mg/cm’, making the effect of the oxide large and more or less independent of proton energy,
‘O-” 7
I o-”
L
I Proton
Fig. 8. K-shell ionization energy.
I
I
2
3 Energy
I
4
5
(MeV)
cross sections of Al and Cu vs proton
272
J. Rickurds / Effect
of oxides on PIXE this case by the experiment. Besides, an estimate of the thickness of the SiO, can be made, giving appro~mately 0.03 mg/cm2. This method may be used to measure the thickness of oxides using PIXE, if one assumes a certain stoichiometry for the oxide. The curves of figs. 4 to 7 can be used to find out what proton bombarding energy one must use depending on the oxide thickness expected, in order to have ma~mum sensitivity. Reuter and Smith [12] used PIXE to measure thin film thicknesses at low energies. Here we extend the method to higher energies and propose the ratio method.
4. Conclusion The ratio method is shown to be useful in determining the effect of oxides in PIXE measurements. The sensitivity of the ratio to oxide thickness was calculated for proton energies 0.1 to 5 MeV and the trend of the curves explained mostly by the sample outgoing X-ray transmission. The method can be used to measure oxide thicknesses when the stoichiometry is known.
The author wishes to acknowledge the technical support of Mr. Karim Lopez. Depth
(rng/crn~~
Fig. 9. Transmission of the outgoing X-rays produced in two metals (AI and Cu) and their oxides.
References
explaining the shape of the curves of fig. 4. The mass attenuation coefficient of 0 for Al X-rays is greater than that of Al for Al X-rays, so the Al,O, tr~s~ssion curve is below the Al curve. In Cu and its oxides the transmission is much higher for Cu X-rays, the tenth-value layer being above 30 mg/cm*. The oxide transmission curve is above the Cu curve, and the tendency is for the oxide to be less important as proton energy increases, as shown in fig. 7. In a simple experiment, clean and oxidized silicon were bombarded with the proton beam of a 700 kV Van de Graaff accelerator, at three different bombarding energies, 300, 400 and 500 keV. In order to minimize the possible effect of secondary electrons, a +90 V potential was applied to the target. For each energy three successive runs were made for the clean sample and three for the oxidized one. Reproducibility was usually better than 1%. the averages of the runs were used to calculate R; the points obtained are plotted on fig. 5. The trend of the calculated curve is corroborated in
(11 J.L. Campbell and J.A. Cookson, Nucl. Instr. and Meth. B3 (1984) 185. [2] J.L. Campbell, J.A. Cookson and H. Paul, Nucl. Instr. and Meth. 212 (1983) 427. [3] S.A.E. Johansson and T.B. Johansson, Nucl. Instr. and Meth. 137 (1976) 473. [4] W. Reuter, A. Lurio, F. Cardone and J.F. Ziegler, J. Appl. Phys. 46 (1975) 3194. [5] E. Clayton, Nucl. Instr. and Meth. 191 (1981) 567. [6] MS. Ahlberg, Nucl. Instr. and Meth. 142 (1977) 61. [7] L.C. No~hcliffe and R.F. Schilhng, Nucl. Data Tables A7 (1970) 233. [S] J.F. Ziegler, Handbook of Stopping Cross-Sections for Energetic Ions in All Elements (Pergamon, New York, 1980). [9] E.C. Montenegro, S.A. Cruz and C. Vargas-Aburto, Phys. Lett. 92A (1982) 195. [lo] I.D. Garcia, R.J. Former and T.M. Kavanagh, Rev. Mod. Pbys. 45 (1973) 111. [II] R. Theisen and D. Vollath, Tables of X-Ray Mass Attenuation Coefficients (Verlag Stahleisen M.B.H., Dusseldorf, 1967). [12] F.W. Reuter and H.P. Smith, J. Appl. Phys. 43 (1972) 4228.
in Physics
Research
269
B12 (1985) 269-272
THE EFFECT OF OXIDES ON PIXE MEASUREMENTS
*
J. RICKARDS Institute de Fkica, Universidad National Autbnoma de Mc?xico, Ap. Postal 20 - 364, 01000 Mkxico D. F., Mkxico Received
11 March
1985 and in revised form 20 April 1985
The presence of oxides on metallic samples used for PIXE analysis affects quantitative measurements. This effect has been calculated based on thick target analysis on layers of different compositions. The ratio of oxidized metal yield to clean metal yield is seen to depend on proton energy and on oxide thickness. Calculations are presented for oxides of Al, Si, Fe, and Cu, and applied to experimental data on Si. The method may be applied to measuring thicknesses of oxides of known stoichiometry.
1. Introduction Thick target PIXE analysis has received considerably less attention than the usual thin target work [l], although the basic effects have been studied and the principal advantages and drawbacks pointed out [2-61. However, when low energy proton beams are used, even very thin samples must be considered thick, since the bombarding particle range is small. Another fact is that the stopping power is close to the maximum in the dE/dx vs E curve, so a great deal of energy is deposited even in thin films, making them fragile and unstable. It is therefore advantageous to apply thick target analysis at low proton energies. In this case thin deposits or oxides on the sample are expected to affect quantitative measurements in an important way. The present work uses known procedures to calculate and demonstrate how oxides reduce the intensity of X-rays reaching the detector.
2. Calculation of X-ray yields
is calculated in small steps as the particle traverses the sequence of layers. It is governed by the proton energy loss, giving an average energy per step, the K-shell ionization cross section, the fluorescence yield and branching ratio, and the outgoing photon absorption (at an angle 8, from the normal to the target). The width of the steps can also be chosen. When the proton has reached its full range, the X-rays from all the steps are summed for each element. Proton straggling is not taken into account since its effect is expected to be small. The method of ref. [4] was followed except for the calculation of the stopping power because oxygen is outside of the range of values proposed for the coefficients of the semiempirical expression. It is also noted that the lighter the element, the larger is its contribution to the stopping power. Fig. 2 shows the stopping power of protons in oxygen taken from the tables of Northcliffe and Schilling [7] (marked NS) and from Ziegler [8] (marked Z). At low energies the difference is considerable. Ziegler’s values, which are adjusted to experimental data, do not go below 200 keV. Instead the stopping power was calculated with the expression of Montenegro et al. [9], which has no adjustable parameters and may
The geometry shown in fig. 1 was used to write a computer program that calculates thick target yields according to the method of Reuter et al. [4]. Protons impinge on a flat surface at an angle 8i to the normal, and as they travel into the target produce X-rays and lose energy. The composition of the first layer, assumed homogeneous, may be changed and admits up to five different elements. After going through layer 1 it enters layer 2 whose composition is also variable. The thicknesses of the layers are variable; up to three different layers may be used. The proton induced K, X-ray yield * Work carried de Ciencia
out with partial y Tecnologla grant
support of Consejo PCCB-020348.
National
0168-583X/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Fig. 1. The geometry used for the calculations. oxide; layer 2 is the pure metal.
Layer
1 is the
J. Rickark
270
/ Effect of oxides on PZXE
et al. [lo]. The mass attenuation those of Theisen
coefficients
used are
and Vollath [ll].
oxygen
3. Results of the cakdation
Proton
Energy (MeV)
Fig. 2. The stopping power of oxygen for protons. The solid curve MCV taken from Montenegro et al. [9] was used for the calculations. The dashed curve Z is taken from Ziegler [8] and the dotted curve NS from Northcliffe and Schilling [7].
easily be incorporated into the program. The curve obtained (marked MCV in fig. 2) is closer to the experimental points and goes to lower energy. Comparison of the MCV stopping powers with those of NS and 2 for other elements calculated in this work {Al and Cu) are shown in fig. 3. The method proposed in ref. (41 for calculating the cross section was used, the values obtained being in close (- 1%) agreement with the table given by Garcia
Energy
Calculations were carried out for oxides of four different elements (Al, Si, Fe and Cu) with the following procedure. The first layer is the oxide, of variable thickness, with the weight concentration of oxygen and the metal corresponding to the oxide in question. The second layer is the pure metal, with essentially infinite thickness. The quantity R is defined as the ratio of the metal X-rays produced from an oxidized sample to those produced by an unoxidized sample. It was calculated for energies between 0.1 and 5 MeV, with different thicknesses of the oxide. It is always less than unity. Fig. 4 shows R vs proton energy for Al,O,. Each curve corresponds to a different oxide thickness, the value of which is given in mg/cm2. The general trend for the lower energies is that for a given thickness, R approaches unity with increasing energy. At the higher energies R levels off to a given value with increasing energy. The lower envelope corresponds to the proton stopping in layer 1, all oxide. Silicon (fig. 5) shows a similar behavior. Two different oxides of iron, Fe0 and Fe,O, are shown in fig. 6. They both behave very much the same, but differ from Al and Si in that the lower envelope rises with increasing energy. For copper, CuO and Cu ,O were calculated; their qualitative behavior is similar to the iron oxides (fig. 7).
10
05
Proton
and experiment
(MeV)
Fig. 3. Stopping power of Al and Cu for protons. The solid curve is MCV [9], the dashed curve is Z [S], and the dotted curve is NS 171.
Proton
Energy
(MeVf
Fig. 4. The ratio R calculated for AlsO, vs proton energy from 0.1 to 5 Mev, for different oxide thicknesses. The oxide thickness in mg/cm2 is shown next to each curve.
J. Rickards
271
/ Effect of oxides on PIXE
R
1.0
0.9
20 30
0.8
0.7
I 0,2
0.6)
_-
0,2
0.4
Proton
0,6
0.6
1
Energy
2
3
r
I
I
0.2
0.4
Proton Fig. 6. The ratio R calculated stoichiometries.
I,,
0.6
0.6
Energy
1
1
I 0.6
I I I I 0.8 I
Energy
Fig. 7. The ratio R calculated ent stoichiometries.
I 2
I 3
I 45
(MeV)
for two copper
oxides of differ-
the experimental
Two facts cause the qualitative difference between the lighter (Al, Si) and the heavier (Fe, Cu) elements. On the one hand the K-shell ionization cross section (fig. 8) is essentially flat for Al from 2 to 5 MeV, whereas for Cu it increases fivefold. But more important is the absorption of the outgoing X-rays. Fig. 9 shows the outgoing X-ray transmission T vs depth in mg/cm2 for two contrasting cases, Al and Cu, and their respec-
0.5
I 0.4
Proton
45
(MeV)
Fig. 5. Same as fig. 4, but for SiO,, showing points obtained for an oxidized sample.
I
I
1
I
2
3
45
I
(MeV)
for two iron oxides of different
tive oxides, corresponding to 5 MeV proton bombarding energy. For Al the X-ray production is dominated by the first few mg/cm’, making the effect of the oxide large and more or less independent of proton energy,
‘O-” 7
I o-”
L
I Proton
Fig. 8. K-shell ionization energy.
I
I
2
3 Energy
I
4
5
(MeV)
cross sections of Al and Cu vs proton
272
J. Rickurds / Effect
of oxides on PIXE this case by the experiment. Besides, an estimate of the thickness of the SiO, can be made, giving appro~mately 0.03 mg/cm2. This method may be used to measure the thickness of oxides using PIXE, if one assumes a certain stoichiometry for the oxide. The curves of figs. 4 to 7 can be used to find out what proton bombarding energy one must use depending on the oxide thickness expected, in order to have ma~mum sensitivity. Reuter and Smith [12] used PIXE to measure thin film thicknesses at low energies. Here we extend the method to higher energies and propose the ratio method.
4. Conclusion The ratio method is shown to be useful in determining the effect of oxides in PIXE measurements. The sensitivity of the ratio to oxide thickness was calculated for proton energies 0.1 to 5 MeV and the trend of the curves explained mostly by the sample outgoing X-ray transmission. The method can be used to measure oxide thicknesses when the stoichiometry is known.
The author wishes to acknowledge the technical support of Mr. Karim Lopez. Depth
(rng/crn~~
Fig. 9. Transmission of the outgoing X-rays produced in two metals (AI and Cu) and their oxides.
References
explaining the shape of the curves of fig. 4. The mass attenuation coefficient of 0 for Al X-rays is greater than that of Al for Al X-rays, so the Al,O, tr~s~ssion curve is below the Al curve. In Cu and its oxides the transmission is much higher for Cu X-rays, the tenth-value layer being above 30 mg/cm*. The oxide transmission curve is above the Cu curve, and the tendency is for the oxide to be less important as proton energy increases, as shown in fig. 7. In a simple experiment, clean and oxidized silicon were bombarded with the proton beam of a 700 kV Van de Graaff accelerator, at three different bombarding energies, 300, 400 and 500 keV. In order to minimize the possible effect of secondary electrons, a +90 V potential was applied to the target. For each energy three successive runs were made for the clean sample and three for the oxidized one. Reproducibility was usually better than 1%. the averages of the runs were used to calculate R; the points obtained are plotted on fig. 5. The trend of the calculated curve is corroborated in
(11 J.L. Campbell and J.A. Cookson, Nucl. Instr. and Meth. B3 (1984) 185. [2] J.L. Campbell, J.A. Cookson and H. Paul, Nucl. Instr. and Meth. 212 (1983) 427. [3] S.A.E. Johansson and T.B. Johansson, Nucl. Instr. and Meth. 137 (1976) 473. [4] W. Reuter, A. Lurio, F. Cardone and J.F. Ziegler, J. Appl. Phys. 46 (1975) 3194. [5] E. Clayton, Nucl. Instr. and Meth. 191 (1981) 567. [6] MS. Ahlberg, Nucl. Instr. and Meth. 142 (1977) 61. [7] L.C. No~hcliffe and R.F. Schilhng, Nucl. Data Tables A7 (1970) 233. [S] J.F. Ziegler, Handbook of Stopping Cross-Sections for Energetic Ions in All Elements (Pergamon, New York, 1980). [9] E.C. Montenegro, S.A. Cruz and C. Vargas-Aburto, Phys. Lett. 92A (1982) 195. [lo] I.D. Garcia, R.J. Former and T.M. Kavanagh, Rev. Mod. Pbys. 45 (1973) 111. [II] R. Theisen and D. Vollath, Tables of X-Ray Mass Attenuation Coefficients (Verlag Stahleisen M.B.H., Dusseldorf, 1967). [12] F.W. Reuter and H.P. Smith, J. Appl. Phys. 43 (1972) 4228.