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The characterization of the oxidation of Fe by variable exit angle ultrasoft x-ray fluorescence spectroscopy
Applied Surface North-Holland
Science 52 (1991) 199-203
applied surface science
The characterization of the oxidation of Fe by variable exit angle ultrasoft X-ray fluorescence spectroscopy T. Scimeca NTT Interdisciplinary Research Laboratories, 3-9-11, Midori-cho. Musashino-shi, Tokyo 180, Japan
and G. Andermann Department of Chemistv. Received
13 January
Unillersi@ of Hawaii, Honolulu, HI 96822, USA
1989: accepted
for publication
16 July 1991
The oxidation of Fe has been characterized qualitatively by variable exit angle ultrasoft X-ray fluorescence (LJSXFSJ. The results indicate that the LB/L 01 intensity ratio can be used to evaluate the extent of Fe oxidation oxidation is believed to penetrate to a certain degree within Fe such that discrete Fe oxide layers are not formed.
1. Introduction The variable exit angle ultrasoft X-ray fluorescence spectroscopy (USXFS) technique has been used as a nondestructive depth profiling tool for studying thin films which may or may not have undergone chemical modification [l-3]. Moreover, this technique can be utilized in the presence of contamination provided one uses an appropriate internal standard. This technique has been applied to characterizing the oxidation of Ni [l] and Cu [ 1,2]. The oxidation of Fe, as examined by the variable exit angle USXFS technique, is discussed in this paper. Utilizing electron excitation, Fischer [4] has found that for 3d transition metals, the LP/La intensity ratio depends on the chemical state of the 3d transition metal. The reason for this spectral change is believed to be a result of a Coster-Kronig process that is discussed in greater detail by Wassdahl et al. [5]. While the origin of the 3d transition metal L spectral changes is of fundamental importance, the focus here is on 0169-4332/91/$03.50
0 1991 - Elsevier
Science
Publishers
spectroscopy and that the
how this intensity ratio can be analytically exploited to characterize a chemically modified sample. The LP/La technique was first used in an analytical way to characterize oxidized metals by Holliday [6]. Depth profiling was achieved by simplying varying the energy of the electrons which were used to excite the sample. In this report, sample depth profiling is achieved by varying the sample exit angle. The L/3/La technique has been discussed in detail elsewhere [1,2]. Nevertheless, it should be mentioned that the LP/La technique is a general technique in that any two spectral points that may be characteristic of the chemical state of the emitting atom can be used to determine the chemical nature of the material. For convenience, the La and Lp peaks are chosen in the intensity ratio. Furthermore, if the X-rays from these two characteristic points are close in energy and far from a contaminant absorption edge, each La (Lp) point acts as an internal standard for the other.
B.V. All rights reserved
2. Experimental The variable exit angle USXFS instrument used in this experiment has been described in detail elsewhere [7]. A Henke tube with a Cu anode was used as the excitation source, while the gas proportional detector was operated in a single channel mode. The instrumental intensity variations in the experiment were monitored with a pressed Fe,O, powder held in a two-position variable angle sample holder. The sample chamber pressure was approximately 10p5 Torr. The slit widths for the primary and secondary slits were 30 and 30 pm, respectively, which was sufficient in clearly resolving the Fe Lp and Ln lines. The commercially obtained Fe samples [8] were quite pure. Prior to oxidation these samples were rough cut with a vertical band saw, smoothed on a lathe, cleaned, lapped to a smooth finish with diamond paste, and finally cleaned again with acetone and methanol. The atmospheric oxidation of Fe was done on a hot plate at 550 o F. The treated Fe samples are listed in table 1. After oxidation, the sample was carefully placed in a watch glass and cooled on top of a liquid nitrogen dewar for several minutes. The sample was then marked and was always placed in the same orientation in the sample holder to minimize inhomogeneous sample oxidation effects. The Fe sample
Table
I
(a) Figure 2 oxidations Sample
Oxid.
Temp. ,”
number
time (min)
(“F)
I 2 3
5.0 35.0 117
Color
540
Brown-yellow
545
Light blue
5so
Light blue
Color
(h) Figure 3 oxidations Sample
Oxid.
Temp.
number
time (min)
(“F)
I
20
5so
Dark-light
2
IS
555
YellowPhlue
3
IO
550
Yellow-blue
1
I0
550
Yellow~purple
5
70
550
Light blue
‘I’ Oxidation
temperature.
blue
I IO
5’
@, (degrees)
Fig. I. I‘he experimental ;L function
of exit
Theoretical
Fc LIJ/Lo
assuming diwete
Fc L/3/l.tu
angle
Fe ,04
(or pure
intensity ratlcl plotted a\ Fe.
Fc,O,
and
Fc:O,.
intensity ratio curves arc alw plotted oxide overlayers of varying thickne\\ over Fc.
was then placed sis.
in the sample
chamber
for analy-
3. Results and discussion The L~/L~Y intensity ratio for unoxidized Fe. Fe,O, and Fc,O, are plotted in fig. 1. The other five curves are theoretically generated intensity ratio curves based upon an Fe,O, thin film overlaying a Fe substrate for different Fe,O, thicknesscs (SO-300 A). No theoretical calculations were made involving an Fc,O, oxide thin film overlaying a Fe substrate since the LB/La intensity ratio characteristics for the two Fe oxides is approximately the same. The d&ails of these calculations are discussed in greater detail elsewhere [2]. Nevertheless, it should bc mentioned that these curves are calculated with the assumption that the Fe oxide overlayer is discrete and that the oxide layer does not penetrate into the Fe substrate. The gradual decrease in the L/J/La intensity ratio for Fe, FeZO, and Fe,O, as the exit angle is decreased is partially attributed to self-absorption [4,Yl. The rapid decrease in the intensity ratio at exit angles < 2 o for Fe,O, is believed to be an anomalous dispersion effect and is discussed in greater detail below. For this reason, the theoret-
T. Scimecu,
G. Andermann
0
irr 9
0.5-
!% =
04-
5 z -
03-
z ;' s
ozOl-
z O-
‘% Fig. 2. Fe Lp/Lcu
intensity ratio plotted
angle. Oxidation
procedure
ofthe
/ Churucterizution
as a function of exit
described in text.
ically calculated curves in fig. I are not extended to very low exit angles. The first set of experiments were performed on the unoxidized Fe sample which was first characterized by the LP/La method, oxidized and subsequently characterized by the LP/LLu technique. This oxidation/ characterization step was repeated on the same Fe sample two more times. The results for this type of oxidation experimental procedure are plotted in fig. 2. The Lp/La intensity ratio overall increases as the Fe sample is oxidized and this ratio also increases slightly as the exit angle approaches 2 O. However, upon comparing the experimental data for the different oxidations with the theoretically generated curves in fig. 1, one observes that the theoretical LP/La intensity ratio increases more drastically than the experimentally observed ratio as the exit angle decreases. The discrepancy between the discrete model calculations and the experimental data suggests that in the oxidation process, the Fe oxide layer penetrates to a certain extent into the Fe substrate. This is in contrast with our earlier oxidation study on Cu [2]. One can also imagine an oxidation where Fe,O, uniformly mixes with Fe. In this case, the experimental LP/La intensity ratio would simply be a weighted average intensity ratio of Fe and Fe,O, with the weighting factor determined by the relative concentrations of Fe and Fe,O, in the sample. This type of uniform mixing is not
oxidution
of Fe by
20 1
USXFS
observed in the experimental data since the LP/La intensity ratio increases to some extent as the exit angle decreases (up to 2 o ). Thus, we observe that the oxidation of Fe does not result in either discrete Fe oxide overlayers or homogeneous mixing of the Fe oxide with the Fe. This result indicates that the Fe is oxidized in such a way that the distribution of Fe,O, lies somewhere between these two extreme oxidation models presented here. The decrease in the Lp/Lcy intensity ratio below 2 o requires additional discussion. The decrease in this intensity ratio at low exit angles might be attributed to angle-dependent matrix absorption. However, standard X-ray fluorescence calculations [lo] show matrix absorption effects depend only linearly on the exit angle, even at low sample exit angles. Another possibility is that the sample, either through the oxidation process [l 11or through an exposure to X-rays [2] has a reduced surface overlayer. One way of determining whether the sample was reduced as a result of X-ray irradiation is to oxidize each sample once and then characterize it by the LP/La intensity ratio. Thus, each oxidation in this procedure represents an oxidation of a different Fe sample. The results of this experimental procedure are plotted in fig. 3. One can see the experimental results are qualitatively the same for the two different oxidation procedures. This demonstrates that the oxidation process is
06 0 9 F
05
G E
04
? f
03
6 i
0.2
Q
2
2
0.1 0
, 0
Fig. 3. Fe Lp/La
I
I
IO"
5'
I
15
8, intensity ratio plotted as a function of exit
angle. Oxidation
procedure
described in text.
not dramatically influenced by the oxidation procedure. In addition, the drastic intensity ratio decrease at sample exit angles below 2 o persists for both oxidation procedures. Another factor which may influence this ratio at low exit angle is the refraction of the LCY and Lp X-rays [12]. Since these X-rays arc near the Fe L absorption edge (the anomalous dispersion region), the index of refraction can change rather drastically as a function of photon energy. Rcfraction index effects may change the L/~/LcY intensity ratio since the detected X-ray exit angle (external exit angle) may be different from the X-ray exit angle within the sample (internal exit angle). More importantly, while the external and internal X-ray exit angles may be different. the difference between the internal exit angle and the external exit angle may also change for both the Lcu and Lp X-ray lines as a function of angle. As the depth from which the X-rays emerge within the sample depends on this internal exit angle, the Lp/Lcy intensity ratio will also depend on this internal exit angle. To evaluate this refractive index effect, one can measure the absolute LCY and L/3 X-ray intensity for Fe as a function of the sample external exit angle for both the La and L/3 X-rays, as is shown in fig. 4. Since the angular dependence near the external exit angle of zero is essentially the same for both the Lru and L/3 X-rays, refractive index effects arising from Fe
can be ruled out. However, since the L~/LcY intensity ratio for Fc,O, does decrease slightly at low sample exit angles. refractive index effects may arise when Fe is oxidized. The rcaxon why the LP/La intensity ratio for Fe,O, does not decrease as drastically as the intensity ratio fog the oxidized Fe samples may have to do with the surface roughness of the Fe,O, powder, which would have the effect of smearing the sample exit angle and any sharp spectral changes associated with the sample exit angle. While this intensity decrcasc is believed to be a result of rcfractivc index cffccta, more work on the optical properties for Fe and several Fe compounds tc.g. Fe?(~), and Fe,O,) needs to bc done before this can be asccrtaincd.
4. Conclusion This Fc study has shown that one can use the variable exit angle USXFS technique to observe changes in the L/3/Ln intensity ratio. While this tcchniquc may be hampered to a certain extent by complicating effects at very low exit angles. a qualitative analysis on the oxidation of Fc has been done. The results show that the depth distribution of Fe,O, in Fe is complicated and will require further analysis with distribution models which lie between the two extreme distribution models presented here.
References
[?I T. Scimeca. Ci. Andermann Sci. 37
[4] D.W.
Fischer. .I. Appl.
[5] N. Waswlahl. Nyholm.
,rnd W.K.
Kuhn.
Phys. 30 (lY64)
201S.
/\ppl.
Surl.
(1080)167.
J.E. Ruhcnsson,
S. (‘ramm.
N.
G. Bray. J. Rindstcdt.
Martensson
R.
and J. Nordgren.
Phy\. Rev. H. preprint.
0"
[6] J.E. Holliday.
,
1
04'
0.2"
06
Karras.
Anal.
F. Burknrd.
Spectrosc.
for Fe L/3 and Lo.
Metalsmart,
Great
Lt-tt. Ih (10x3) Neck.
NY.
I6 (IYOO) 53.
R. Kim. F. Fujiwara
( ri [X] The purity of the Fe is YY.c)Y)
@N Fig. 4. Zero angle determination
Adv. X-Ray
[7] Ci. Andrrmann.
and M.
891.
and it
is oht;rlncd from
T Scimeca, G. Andermunn
/ Charucturizution of the oxidution
[9] C. Bonnelle, Ann. Phys. 1 (1966) 439. [IO] E.P. Bertin, Principles of X-Ray Spectrometric Analysis. 2nd ed. (Plenum, New York, 1981). [I 11 H. Uhlig and R. Revie, Corrosion and Corrosion Control (Wiley, New York, 19X5) p. 201.
of Fe
by USXFS
[I21 L. Kaihola and J. Bremer, 14 (1981) L43.
203
J. Phys. C (Solid State Phys.)
Science 52 (1991) 199-203
applied surface science
The characterization of the oxidation of Fe by variable exit angle ultrasoft X-ray fluorescence spectroscopy T. Scimeca NTT Interdisciplinary Research Laboratories, 3-9-11, Midori-cho. Musashino-shi, Tokyo 180, Japan
and G. Andermann Department of Chemistv. Received
13 January
Unillersi@ of Hawaii, Honolulu, HI 96822, USA
1989: accepted
for publication
16 July 1991
The oxidation of Fe has been characterized qualitatively by variable exit angle ultrasoft X-ray fluorescence (LJSXFSJ. The results indicate that the LB/L 01 intensity ratio can be used to evaluate the extent of Fe oxidation oxidation is believed to penetrate to a certain degree within Fe such that discrete Fe oxide layers are not formed.
1. Introduction The variable exit angle ultrasoft X-ray fluorescence spectroscopy (USXFS) technique has been used as a nondestructive depth profiling tool for studying thin films which may or may not have undergone chemical modification [l-3]. Moreover, this technique can be utilized in the presence of contamination provided one uses an appropriate internal standard. This technique has been applied to characterizing the oxidation of Ni [l] and Cu [ 1,2]. The oxidation of Fe, as examined by the variable exit angle USXFS technique, is discussed in this paper. Utilizing electron excitation, Fischer [4] has found that for 3d transition metals, the LP/La intensity ratio depends on the chemical state of the 3d transition metal. The reason for this spectral change is believed to be a result of a Coster-Kronig process that is discussed in greater detail by Wassdahl et al. [5]. While the origin of the 3d transition metal L spectral changes is of fundamental importance, the focus here is on 0169-4332/91/$03.50
0 1991 - Elsevier
Science
Publishers
spectroscopy and that the
how this intensity ratio can be analytically exploited to characterize a chemically modified sample. The LP/La technique was first used in an analytical way to characterize oxidized metals by Holliday [6]. Depth profiling was achieved by simplying varying the energy of the electrons which were used to excite the sample. In this report, sample depth profiling is achieved by varying the sample exit angle. The L/3/La technique has been discussed in detail elsewhere [1,2]. Nevertheless, it should be mentioned that the LP/La technique is a general technique in that any two spectral points that may be characteristic of the chemical state of the emitting atom can be used to determine the chemical nature of the material. For convenience, the La and Lp peaks are chosen in the intensity ratio. Furthermore, if the X-rays from these two characteristic points are close in energy and far from a contaminant absorption edge, each La (Lp) point acts as an internal standard for the other.
B.V. All rights reserved
2. Experimental The variable exit angle USXFS instrument used in this experiment has been described in detail elsewhere [7]. A Henke tube with a Cu anode was used as the excitation source, while the gas proportional detector was operated in a single channel mode. The instrumental intensity variations in the experiment were monitored with a pressed Fe,O, powder held in a two-position variable angle sample holder. The sample chamber pressure was approximately 10p5 Torr. The slit widths for the primary and secondary slits were 30 and 30 pm, respectively, which was sufficient in clearly resolving the Fe Lp and Ln lines. The commercially obtained Fe samples [8] were quite pure. Prior to oxidation these samples were rough cut with a vertical band saw, smoothed on a lathe, cleaned, lapped to a smooth finish with diamond paste, and finally cleaned again with acetone and methanol. The atmospheric oxidation of Fe was done on a hot plate at 550 o F. The treated Fe samples are listed in table 1. After oxidation, the sample was carefully placed in a watch glass and cooled on top of a liquid nitrogen dewar for several minutes. The sample was then marked and was always placed in the same orientation in the sample holder to minimize inhomogeneous sample oxidation effects. The Fe sample
Table
I
(a) Figure 2 oxidations Sample
Oxid.
Temp. ,”
number
time (min)
(“F)
I 2 3
5.0 35.0 117
Color
540
Brown-yellow
545
Light blue
5so
Light blue
Color
(h) Figure 3 oxidations Sample
Oxid.
Temp.
number
time (min)
(“F)
I
20
5so
Dark-light
2
IS
555
YellowPhlue
3
IO
550
Yellow-blue
1
I0
550
Yellow~purple
5
70
550
Light blue
‘I’ Oxidation
temperature.
blue
I IO
5’
@, (degrees)
Fig. I. I‘he experimental ;L function
of exit
Theoretical
Fc LIJ/Lo
assuming diwete
Fc L/3/l.tu
angle
Fe ,04
(or pure
intensity ratlcl plotted a\ Fe.
Fc,O,
and
Fc:O,.
intensity ratio curves arc alw plotted oxide overlayers of varying thickne\\ over Fc.
was then placed sis.
in the sample
chamber
for analy-
3. Results and discussion The L~/L~Y intensity ratio for unoxidized Fe. Fe,O, and Fc,O, are plotted in fig. 1. The other five curves are theoretically generated intensity ratio curves based upon an Fe,O, thin film overlaying a Fe substrate for different Fe,O, thicknesscs (SO-300 A). No theoretical calculations were made involving an Fc,O, oxide thin film overlaying a Fe substrate since the LB/La intensity ratio characteristics for the two Fe oxides is approximately the same. The d&ails of these calculations are discussed in greater detail elsewhere [2]. Nevertheless, it should bc mentioned that these curves are calculated with the assumption that the Fe oxide overlayer is discrete and that the oxide layer does not penetrate into the Fe substrate. The gradual decrease in the L/J/La intensity ratio for Fe, FeZO, and Fe,O, as the exit angle is decreased is partially attributed to self-absorption [4,Yl. The rapid decrease in the intensity ratio at exit angles < 2 o for Fe,O, is believed to be an anomalous dispersion effect and is discussed in greater detail below. For this reason, the theoret-
T. Scimecu,
G. Andermann
0
irr 9
0.5-
!% =
04-
5 z -
03-
z ;' s
ozOl-
z O-
‘% Fig. 2. Fe Lp/Lcu
intensity ratio plotted
angle. Oxidation
procedure
ofthe
/ Churucterizution
as a function of exit
described in text.
ically calculated curves in fig. I are not extended to very low exit angles. The first set of experiments were performed on the unoxidized Fe sample which was first characterized by the LP/La method, oxidized and subsequently characterized by the LP/LLu technique. This oxidation/ characterization step was repeated on the same Fe sample two more times. The results for this type of oxidation experimental procedure are plotted in fig. 2. The Lp/La intensity ratio overall increases as the Fe sample is oxidized and this ratio also increases slightly as the exit angle approaches 2 O. However, upon comparing the experimental data for the different oxidations with the theoretically generated curves in fig. 1, one observes that the theoretical LP/La intensity ratio increases more drastically than the experimentally observed ratio as the exit angle decreases. The discrepancy between the discrete model calculations and the experimental data suggests that in the oxidation process, the Fe oxide layer penetrates to a certain extent into the Fe substrate. This is in contrast with our earlier oxidation study on Cu [2]. One can also imagine an oxidation where Fe,O, uniformly mixes with Fe. In this case, the experimental LP/La intensity ratio would simply be a weighted average intensity ratio of Fe and Fe,O, with the weighting factor determined by the relative concentrations of Fe and Fe,O, in the sample. This type of uniform mixing is not
oxidution
of Fe by
20 1
USXFS
observed in the experimental data since the LP/La intensity ratio increases to some extent as the exit angle decreases (up to 2 o ). Thus, we observe that the oxidation of Fe does not result in either discrete Fe oxide overlayers or homogeneous mixing of the Fe oxide with the Fe. This result indicates that the Fe is oxidized in such a way that the distribution of Fe,O, lies somewhere between these two extreme oxidation models presented here. The decrease in the Lp/Lcy intensity ratio below 2 o requires additional discussion. The decrease in this intensity ratio at low exit angles might be attributed to angle-dependent matrix absorption. However, standard X-ray fluorescence calculations [lo] show matrix absorption effects depend only linearly on the exit angle, even at low sample exit angles. Another possibility is that the sample, either through the oxidation process [l 11or through an exposure to X-rays [2] has a reduced surface overlayer. One way of determining whether the sample was reduced as a result of X-ray irradiation is to oxidize each sample once and then characterize it by the LP/La intensity ratio. Thus, each oxidation in this procedure represents an oxidation of a different Fe sample. The results of this experimental procedure are plotted in fig. 3. One can see the experimental results are qualitatively the same for the two different oxidation procedures. This demonstrates that the oxidation process is
06 0 9 F
05
G E
04
? f
03
6 i
0.2
Q
2
2
0.1 0
, 0
Fig. 3. Fe Lp/La
I
I
IO"
5'
I
15
8, intensity ratio plotted as a function of exit
angle. Oxidation
procedure
described in text.
not dramatically influenced by the oxidation procedure. In addition, the drastic intensity ratio decrease at sample exit angles below 2 o persists for both oxidation procedures. Another factor which may influence this ratio at low exit angle is the refraction of the LCY and Lp X-rays [12]. Since these X-rays arc near the Fe L absorption edge (the anomalous dispersion region), the index of refraction can change rather drastically as a function of photon energy. Rcfraction index effects may change the L/~/LcY intensity ratio since the detected X-ray exit angle (external exit angle) may be different from the X-ray exit angle within the sample (internal exit angle). More importantly, while the external and internal X-ray exit angles may be different. the difference between the internal exit angle and the external exit angle may also change for both the Lcu and Lp X-ray lines as a function of angle. As the depth from which the X-rays emerge within the sample depends on this internal exit angle, the Lp/Lcy intensity ratio will also depend on this internal exit angle. To evaluate this refractive index effect, one can measure the absolute LCY and L/3 X-ray intensity for Fe as a function of the sample external exit angle for both the La and L/3 X-rays, as is shown in fig. 4. Since the angular dependence near the external exit angle of zero is essentially the same for both the Lru and L/3 X-rays, refractive index effects arising from Fe
can be ruled out. However, since the L~/LcY intensity ratio for Fc,O, does decrease slightly at low sample exit angles. refractive index effects may arise when Fe is oxidized. The rcaxon why the LP/La intensity ratio for Fe,O, does not decrease as drastically as the intensity ratio fog the oxidized Fe samples may have to do with the surface roughness of the Fe,O, powder, which would have the effect of smearing the sample exit angle and any sharp spectral changes associated with the sample exit angle. While this intensity decrcasc is believed to be a result of rcfractivc index cffccta, more work on the optical properties for Fe and several Fe compounds tc.g. Fe?(~), and Fe,O,) needs to bc done before this can be asccrtaincd.
4. Conclusion This Fc study has shown that one can use the variable exit angle USXFS technique to observe changes in the L/3/Ln intensity ratio. While this tcchniquc may be hampered to a certain extent by complicating effects at very low exit angles. a qualitative analysis on the oxidation of Fc has been done. The results show that the depth distribution of Fe,O, in Fe is complicated and will require further analysis with distribution models which lie between the two extreme distribution models presented here.
References
[?I T. Scimeca. Ci. Andermann Sci. 37
[4] D.W.
Fischer. .I. Appl.
[5] N. Waswlahl. Nyholm.
,rnd W.K.
Kuhn.
Phys. 30 (lY64)
201S.
/\ppl.
Surl.
(1080)167.
J.E. Ruhcnsson,
S. (‘ramm.
N.
G. Bray. J. Rindstcdt.
Martensson
R.
and J. Nordgren.
Phy\. Rev. H. preprint.
0"
[6] J.E. Holliday.
,
1
04'
0.2"
06
Karras.
Anal.
F. Burknrd.
Spectrosc.
for Fe L/3 and Lo.
Metalsmart,
Great
Lt-tt. Ih (10x3) Neck.
NY.
I6 (IYOO) 53.
R. Kim. F. Fujiwara
( ri [X] The purity of the Fe is YY.c)Y)
@N Fig. 4. Zero angle determination
Adv. X-Ray
[7] Ci. Andrrmann.
and M.
891.
and it
is oht;rlncd from
T Scimeca, G. Andermunn
/ Charucturizution of the oxidution
[9] C. Bonnelle, Ann. Phys. 1 (1966) 439. [IO] E.P. Bertin, Principles of X-Ray Spectrometric Analysis. 2nd ed. (Plenum, New York, 1981). [I 11 H. Uhlig and R. Revie, Corrosion and Corrosion Control (Wiley, New York, 19X5) p. 201.
of Fe
by USXFS
[I21 L. Kaihola and J. Bremer, 14 (1981) L43.
203
J. Phys. C (Solid State Phys.)