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Atmospheric oxidation of a Nb–Zr alloy studied with XPS
Corrosion Science 46 (2004) 213–224 www.elsevier.com/locate/corsci
Atmospheric oxidation of a Nb–Zr alloy studied with XPS C.-O.A. Olsson, D. Landolt
*
Laboratoire de M etallurgie Chimique, Institut des Mat eriaux, Ecole Polytechnique F ed erale de Lausanne, MX-C Ecublens, CH-1015 Lausanne, Switzerland Received 20 March 2003; accepted 11 May 2003
Abstract The long term atmospheric oxidation of a Nb–Zr alloy was studied by XPS. Curve fit parameters were defined and strictly constrained to obtain a consistent evaluation of film thickness and chemistry. A formalism was introduced to distinguish between the water, hydroxide and oxide contributions to the surface oxide. Ageing experiments were carried out simultaneously exposing one lot of samples to a laboratory atmosphere for 10 months and a second lot to a dry atmosphere in a desiccator. 2003 Elsevier Ltd. All rights reserved. Keywords: Atmospheric corrosion; XPS; Zirconium; Niobium
1. Introduction The literature on film growth under atmospheric conditions is rather limited when compared to studies in solution. One reason for this is the increased complexity of the system studied under atmospheric conditions. In this case, it is no longer sufficient to study a metal–oxide–film interface, but a metal/oxide/thin liquid/gaseous interface, where the thickness of the liquid and the composition of the gaseous part of the system may vary significantly in time. An extensive introduction to atmospheric corrosion can be found in a recent monograph by Leygraf and Graedel [1]. The classical way of studying atmospheric corrosion on pure metals is to investigate the weight change as a function of time, while meticulously controlling or measuring
*
Corresponding author. Tel.: +41-21-693-2981; fax: +41-21-693-3946. E-mail address: dieter.landolt@epfl.ch (D. Landolt).
0010-938X/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0010-938X(03)00139-2
214
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Nomenclature Iifilm photoelectron intensity a spectrometer constant IRX incident X-ray intensity RSFmet relative sensitivity factor of element i i g common factor for spectrometer terms T ðEÞ transmission function ci concentration of element i, fraction or % r cross section for generation of photoelectrons k inelastic mean free path # XPS analysis angle, referenced to the sample surface a, b, c, x film geometry parameters
the changes in environment. In this way, Rice et al. [2] studied the corrosion rate of individual metals, e.g. Fe, Ni, Co, in indoor as well as outdoor environments. They measured weight loss curves over a year and a half and correlated those to pollutants in the atmosphere, e.g. chlorides and sulphides. A later investigation of similar type, but in different indoor environments, was performed by Johansson et al. [3,4], who used weight change and resistance measurements to study different foils in different indoor atmospheres. The relative corrosion rate of different metals was shown to vary widely with the composition of the surrounding atmosphere. With the continued miniaturization of mechanical and electronic devices, the need for information on the resistance to atmospheric corrosion takes new dimensions. The implications to semiconductor industry has been described by Frankel [5]. This puts new demands on the tools used for monitoring the corrosion rates. The aim with this paper is to study the film growth in a dry atmosphere and normal laboratory conditions of a Nb–Zr alloy with XPS. In the XPS analysis, particular attention was given to obtain a reproducible quantification of the long-term development of the oxide film, which was partitioned into oxide, hydroxide and water parts. This was then used to obtain thickness change curves for long term storage of the wires in a dry low pressure atmosphere (desiccator) and in laboratory atmosphere.
2. Theory There are a vast number of multi-layer models in the XPS literature. In this paper, it was of particular interest to evaluate the amount of water and hydroxide on the surface. For this purpose, a three layer formalism was developed, based on a procedure originally developed for mixed oxides on stainless steels by Brox, Elfstr€ om, and Olefjord [6,7]. There are also models available for the Nb–Zr system, cf. Bastl et al. [8]. They studied films formed at room temperature in UHV by exposure to
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215
200 L of oxygen, water and hydrogen peroxide (1 L ¼ 1 Langmuir ¼ 1 lTorr 1s). They identified a series of oxidation states between metallic and (IV) for zirconium and interpreted the results with a multi-layer model for the cations. In the present study, the bonds found were essentially Zr(IV) and Nb(V). Thus, the layered model was developed for the anionic species only. Another approach was taken by McCafferty and Wightman, who used a detailed layer model to study pure valve metal oxide samples [9]. They used a deconvolution method similar to the one presented in this paper to determine the amount of hydroxyl groups on the surface. Compared to their study, we found much lower amounts of C@O and C–O bonds in the carbon spectrum, thus no correction was performed for a carbon contribution to the oxygen peak. In addition, they used sputter depth profiling to gain additional information on the depth distribution of different species. For a long term study of very thin films, it was desired to avoid a calibration dependent technique such as sputter depth profiling. Thus. the method presented below uses only physical parameters that are independent of the day-today condition of the spectrometer. Even though the IMFP estimates are not known with high precision, they are not subject to calibration and give a solid base for comparative measurements. Compared with the Olefjord concept [6,7], a slight modification was introduced. Elemental relative sensitivity factors (RSFs) were used instead of yields established for a set of reference samples. Further, a formalism for dividing the oxide film into water, hydroxide and oxide parts was added to partition the contributions from the respective oxide, hydroxide and water parts of the oxide film. All intensities below are corrected for transmission. The X-ray induced photoelectron intensity for an oxide film can be written as Iifilm ¼ a IRX cfilm RSFi T ðEi Þ i
kfilm x=kfilm sin # i i met sin # 1 e ki
ð1Þ
where the ratio kfilm =kmet has been introduced to correct for the difference in inelastic mean free path of the film and the substrate. The RSF already contains the inelastic mean free path in the metal. Here, it is defined with rkmet Dmet . This approximation is better than it seems at first glance, since niobium and zirconium are diluted in similar ways in the oxide. The corresponding formula for the metal is met
Iimet ¼ a IRX cmet RSFi T ðEi Þ sin # ex=ki i
sin #
ð2Þ
For the metal, the relative sensitivity factor already contains the inelastic mean free path. When the oxide and metal intensities have been defined for the elements measured, it is convenient to assume an oxide film thickness, e.g. 2 nm. This estimate then serves as a start approximation for the calculation of the film composition: film
x=ki sin # Iifilm =kfilm Þ i RSFi ð1 e cfilm ¼ Pn film film i x=kfilm sin # =kj RSFj ð1 e j Þ j¼1 Ij
ð3Þ
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The corresponding formula can also be derived for the underlying metal. By forming the ratio between Eqs. (1) and (2), and rearranging, one finds the film thickness from xfilm ¼
kfilm i
met I film cmet i ki sin # ln 1 þ imet film film I i c i ki
! ð4Þ
Eq. (4) completes the loop. A loop of Eqs. (3) and (4) was iterated until the oxide film thickness is the same in two consecutive loops. In this treatment, the adventitious carbon contamination layer was disregarded, since it attenuates the film and metal intensities by the same factor, and consequently does not affect the film thickness estimate. For atmospheric corrosion studies, it is of particular interest to investigate the influence of humidity on the surface, since there is a strong correlation between humidity and corrosion rate. This relation has been established for several different metals [1]. To estimate the amount of water on the surface, a formalism was developed for dividing the oxide film into three sublayers by using the deconvolution of the O 1s peak. It is important to remember that most of the water adsorbed on the surface is desorbed during transfer into UHV. If a three layer model is introduced, cf. Fig. 1, the intensities from the three different layers can then be expressed as film
a=kO IH2 O ¼ g cH2 O kfilm O sin #ð1 e
IOH ¼ g cOH
kfilm O
sin #ðe
Iox ¼ g cox kfilm O sin #ðe
a=kfilm O sin #
b=kfilm O
sin #
sin #
e
e
Þ
b=kfilm O sin #
d=kfilm O
sin #
ð5a–cÞ
Þ
Þ
The variable c is the concentration of oxygen in the respective layer (mol/cm3 ) and g is a common factor containing spectrometer terms. The thickness of the oxide and hydroxide layers, x, is already calculated from the metal peak intensity ratios. As defined in Fig. 1, x ¼ d a. The equation system (5a–c) can now be solved analytically. It is convenient to regard the different intensity ratios
H2O
a b
OH-
d
x O2Met
Fig. 1. Three layer model from which the relative intensities can be calculated.
C.-O.A. Olsson, D. Landolt / Corrosion Science 46 (2004) 213–224 film
k1 ¼ k2 ¼
1 ea=kO e
a=kfilm O sin #
e
sin #
b=kfilm O sin #
¼
e film eb=kO sin #
a=kfilm O sin # film
eb=kO
sin #
film
eðaþxÞ=kO
sin #
cOH IH2 O cH2 O IOH cox IOH ¼ cOH Iox
From these ratios, thicknesses for each layer are obtained from " # x=kfilm O sin # 1 þ k e 2 b ¼ kfilm film O sin # ln 1 þ k2 þ k1 k2 ð1 ex=kO sin # Þ 1 þ k1 eb=k sin # film a ¼ kO sin # ln 1 þ k1
217
ð6a;bÞ
ð7a–cÞ
d ¼xþa For this calculation, it was assumed that the attenuation length in all three layers is the same. The main uncertainty in the layer division originates from the curve fit, i.e. the ratios between the water, hydroxide and oxide contributions.
3. Experimental 3.1. Samples Wire samples were analysed after a standard cleaning procedure. The bulk composition was 85%Nb and 15%Zr. The wires had a rectangular cross section with a width of about 100 lm. All measurements were performed on the wider side. After reception, the samples were stored in a desiccator for one month, whereafter they were separated into two lots––one of which was left in a loosely closed plastic box under laboratory conditions, the other was left in the desiccator. Before each measurement, the desiccator was brought up to air and the samples were taken out. Vacuum was obtained with a water pump within one hour after opening. The test was carried out from January to December 2002. 3.2. XPS analysis The XPS measurements were performed using a Kratos Axis Ultra spectrometer and a monochromated Al Ka X-ray source at 1486.6 eV. The base pressure during normal operation was 1 · 109 Torr. Charge neutralization was used for all measurements on oxides. The energy scale was calibrated by fixing the adventitious carbon peak at 285 eV. The analyser was operated in the fixed analyser transmission (FAT) mode at a pass energy of 20 eV. The diameter of the analysed area was about 100 lm, resulting in a total acquisition time of about 90 min per sample. Typically, five samples of each type were analysed to obtain satisfactory statistics. No sputtering was performed prior to analysis. Quantification was performed with sensitivity factors provided by the manufacturer.
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The wires were mounted on a custom design sample holder which allowed for simultaneous introduction of at least 10 wires with a surrounding vacuum. The holder was designed to avoid stray intensity from the sample holder and to facilitate optimisation of the signal. Curve fitting was performed using CasaXPS v. 2. Some charging was observed during the analysis. This was compensated for by shifting the adventitious carbon peak to 285 eV. Background subtraction was performed using the iterated Shirley method. For the zirconium and niobium metal peaks, synthetic peak shapes were defined from XPS spectra taken under similar conditions from sputter cleaned pure metal reference standards. The asymmetry parameters used follow the definitions of PHI-Multipak. Quantification of the oxide film was made with in house written software based on Matlab 6.0, using Eqs. (1)–(7). The peak fit parameters are crucial for the following quantification. It is of particular importance to mutually fix the half widths and peak positions of the O 1s peaks to obtain consistency in the three layer model for water adsorption. The area ratios for the 3/2 and 1/2 doublets were fixed to 50% throughout.
4. Results and discussion 4.1. XPS curve fitting on metal peaks for intensity separation The oxides studied in the ageing process were sufficiently thin to be analysed with the formalism given above. This procedure can be used for film thicknesses up to about 3k. For thicker films, other methods have to be applied, e.g. Tougaard nanostructure analysis [10] or sputter depth profiling [11]. The first step in the analysis is to define relevant curve fit parameters. This was done on pure niobium and zirconium foils which were sputter cleaned before the analysis. The curve fits obtained for the metallic species are shown in Fig. 2. The oxide films were slightly insulating during analysis, thus charge compensation was used throughout. The spectrum was brought back into position by defining the C–C peak at 285 eV. This gave a reproducible energy scale, cf. the standard deviations of the metal peaks in Table 1. To obtain a consistent fitting of the oxide peaks, their half widths were kept mutually fixed for each element. The uncertainty in this procedure is also given in Table 1. The error estimates are based on a standard deviation of 10 fits. An error estimate is only given for one peak in each group, since the other peaks were fitted together with fixed distances/half widths. It was also checked that there was a linear relation between the oxidation state number and the binding energy shift for the metal peaks. The contamination level was not increasing systematically in time. Sample surfaces were very clean throughout the series. The contribution from carbon, i.e. C–O and C@O bonds, to the oxygen peak was judged less than 5%, and thus considered negligible. It is evident that the total amount of water adsorbed on the surface will not remain under UHV conditions. However, it appears that the water remaining on the
C.-O.A. Olsson, D. Landolt / Corrosion Science 46 (2004) 213–224
219
Metallic Zr 3d
192
190
188
186
184
182
180
178
176
Binding Energy, eV
Metallic Nb 3d 5000
4000
3000
2000
1000
0 216
212
208
204
200
Binding Energy, eV Fig. 2. Curve fits of the Nb 3d and Zr 3d metal peaks. The metal peak shapes defined on the sputter cleaned metal is then later used in the curve fits of the oxides.
surface in UHV correlates with the surface condition immediately before introduction into the chamber. The validity of this assumption and the curve fit procedure was checked using a sample immersed in water immediately before introduction into UHV. The resulting O 1s spectrum is shown in Fig. 3. The spectrum was recorded after 15 h under vacuum at 1 · 109 Torr. Although most of the surface water is expected to desorb during introduction into UHV, the sample immersed in water showed a significantly higher water coverage than those stored in atmosphere. Still, , out of which the quantification of the XP spectrum showed a total thickness of 42 A , respectively. This should be the hydroxide and water parts constituted 7 and 5 A
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Table 1 Curve fit parameters for the peaks used for oxide layer quantification Element
Oxidation state
Position
FWHM
%Gauss
Tail height
Tail length
Zr Zr Zr Zr
met met IV IV
178.8 181.2 182.4 ± 0.1 184.8
0.85 0.98 1.2 ± 0.05 1.2
70 80 100 100
0.7 0.5 – –
17 17 – –
met met II II IV IV V V
201.7 ± 0.1 204.8 203.3 205.8 205.3 208.0 207.1 209.8
0.63 ± 0.03 0.76 1.24 ± 0.09 1.24 1.24 1.24 1.24 1.24
70 80 100 100 100 100 100 100
0.5 0.5 – – – – – –
14 12 – – – – – –
O2 OH H2 O
530.0 ± 0.1 531.4 533.4
1.33 ± 0.02 1.33 1.52
100 100 100
– – –
– – –
Nb Nb Nb Nb Nb Nb Nb Nb
2p3=2 2p1=2 2p3=2 2p1=2 2p3=2 2p1=2 2p3=2 2p1=2 2p3=2 2p1=2 2p3=2 2p1=2
O 1s O 1s O 1s
For each element, the most well defined peak was chosen to have a variable position, which is given within the uncertainty limits based on standard deviation from 10 measurements. For the oxidised metal peaks, the FWHM was fitted for the group as an entity. For the O 1s line, the water peak was fitted with a wider, but still fixed FWHM. The variation of half widths for each group is given only once as the standard deviation from 10 measurements. The metal peak shapes and asymmetries were fixed for spectra acquired on sputter cleaned pure metal surfaces for the respective elements.
O 1s
O OH
2-
-
H2O
540
535
530
525
Binding Energy, eV Fig. 3. XPS curve fit of sample immersed in water immediately before introduction for control of the water adsorption model.
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221
compared with the data obtained for the samples stored in laboratory atmosphere , and in desiccator that showed hydroxide and water layers of the order of 5 and 1 A respectively. The resulting curve fits for wires stored in atmosphere are shown in Fig. 4. For each ageing time, 3–5 identical samples were analysed and evaluated to obtain a reliable estimate of the thickness. Film thicknesses were then calculated according to the procedure given above, using the physical parameters obtained for bulk oxides that can be found in Table 2. 4.2. Ageing in atmosphere It was desired to study the influence of atmospheric exposure on the native wire oxide on the surface. Initially, samples were stored in a desiccator for one month. Nb 3d
Zr 3d ZrO2 Nb2 O5
Nb-met NbO2 NbO
Zr-met
190
188
186
184
182
180
178
176
220
216
212
Binding Energy, eV
208
204
200
196
Binding Energy, eV
O 1s O
OH
2-
-
H 2O
540
535
530
525
Binding Energy, eV
Fig. 4. XPS curve fits of the Zr 3d, Nb 3d and O 1s peaks used for determining the oxide layer thicknesses. The peak fit parameters can be found in Table 1.
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Table 2 Physical parameters for thickness determination in XPS Parameter
Value 19.2 A 19.5 A 25.1 A 25.5 A 23.1 A 16.5 A
kNb in Nb kZr in Nb kNb in Nb2 O5 kZr in Nb2 O5 kZr in ZrO2 kO in Nb2 O5 Bulk conc. Zr cO in Nb2 O5 cO in OH part cO in H2 O cmet;Zr in Zr cmet;Nb in Nb cox;Zr in ZrO2 cox;Nb in NbO2:5
15% 0.0840 mol/cm3 0.07 mol/cm3 0.0556 mol/cm3 0.0711 mol/cm3 0.0922 mol/cm3 0.0478 mol/cm3 0.0336 mol/cm3
IMFPs according to TPP [12]. The density values were taken from the respective bulk oxides.
A part of the lot was then taken out from the desiccator and stored in laboratory atmosphere. The oxide film thickness for the two different lots was estimated with XPS at time intervals ranging from 0 to 300 days (Table 3). The peak deconvolution used to find the metal and oxide intensities of the respective metal peaks is indicated in Fig. 4. A small but significant increase in the surface oxide film was observed for the samples aged in laboratory atmosphere. After 300 days the oxide on these thicker than for the samples aged in a dry environment. samples was about 10 A The total oxide film was decomposed into water, hydroxide and oxide parts according to the method described above. In addition, the film composition was also Table 3 Ageing experiments in desiccator and laboratory atmosphere Time days
Condition
Samples
Nb-ox cat%
Dfilm , A
DOH , A
DH2 O , A
1 27 62 62 90 90 160 160 229 229 299 299
As rec. Desiccator Desiccator Lab. atm. Desiccator Lab. atm. Desiccator Lab. atm. Desiccator Lab. atm. Desiccator Lab. atm.
3 3 3 3 4(1) 4(1) 4(1) 5 5 5 5 5
34 ± 9 42 ± 4 30 ± 9 43 ± 2 34 ± 6 36 ± 8 40 ± 5 27 ± 4 38 ± 10 35 ± 8 33 ± 14 48 ± 14
28.6 ± 1.0 29 ± 0.4 30.2 ± 0.9 32.2 ± 1 31.2 ± 0.6 35.7 ± 1.6 30.8 ± 0.5 39.1 ± 1.2 32.5 ± 0.6 40.2 ± 1.5 33.0 ± 0.8 42.1 ± 1.4
8.6 ± 2.2 4.3 ± 0.4 4 ± 0.3 3.7 ± 0.2 4.0 ± 0.2 4.4 ± 0.4 5.2 ± 0.3 5.0 ± 0.3 4.4 ± 0.3 4.7 ± 0.2 4.6 ± 0.4 4.6 ± 0.3
2.2 ± 0.7 1.7 ± 1.1 0.6 ± 0.4 0.5 ± 0.1 0.7 ± 0.1 0.7 ± 0.3 1.1 ± 0.2 1.4 ± 0.3 0.8 ± 0.2 0.8 ± 0.2 0.9 ± 0.3 0.8 ± 0.2
For the last two times, five samples from each environment were measured, and single outliers were discarded (number of discarded points is indicated in parentheses). The total film thickness is plotted in Fig. 5. Dfilm is equivalent to the total film thickness (oxide + hydroxide), not considering the water coverage.
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223
XPS Film Thickness 45 Laboratory Atmosphere
Film Thickness, Å
40 Separation date
35
Desiccator
30
25 0
50
100 150 200 Ageing Time, Days
250
300
Fig. 5. Results of ageing of pre-oxidised wires in laboratory and dry atmospheres. After 300 days, the thicker than those aged in the atmospherically aged samples were found to have an oxide layer, about 12 A desiccator.
estimated, cf. Table 1. From the decomposition of the oxide film, it is clear that all oxide growth is occurring in the oxide part, whereas the hydroxide and water parts remain constant. The XPS measurements also indicate that the oxide film composition is not homogeneous. The analysed spot size was about 100 lm diameter. The thickness changes observed in Fig. 5 are clearly occurring in the oxide film. The measured thickness change is about 1.2 nm in the oxide film during 10 months, whereafter it slows down. This change can be recalculated into a corrosion velocity of 3.75 mg/m2 /year in metallic equivalents. This is much lower than corrosion rates for silver and zinc plates stored in a research library with controlled environment. Such corrosion rates have been estimated to 100 and 150 mg/m2 /year, respectively [3]. With standard techniques such as gravimetry or cathodic reduction, the detection limit for atmospheric corrosion is normally around 10 mg/m2 /year.
5. Conclusions • XPS was shown to be a viable tool for the study of low temperature oxidation and alloys over prolonged time periods. The data analysis routine developed for this purpose permitted the reproducible quantification and partitioning of the oxide layer in oxide, hydroxide and water parts. • It is possible to obtain a reproducible estimate of the amount of water remaining on the surface under UHV conditions. This amount of water also reflects the amount of water present on the surface before introduction into UHV. • Film growth was located to the oxide part of the film; the hydroxide and water parts remained constant during the test period.
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• The oxide films of the Nb–Zr alloys showed a thickness increase of about 1 nm when compared to samples stored under low pressure in a desiccator.
References [1] C. Leygraf, T.E. Graedel, Atmospheric Corrosion, in: The Electrochemical Society Series, vol. 1, The Electrochemical Society, Pennington, NJ, 2000. [2] D.W. Rice, R.J. Cappell, W. Kinsolving, J.J. Laskowski, J. Electrochem. Soc. 127 (4) (1980) 891. [3] E. Johansson, C. Leygraf, B. Rendahl, Br. Corros. J. 33 (1) (1998) 59. [4] E. Johansson, C. Leygraf, Br. Corros. J. 34 (1) (1999) 27. [5] G.S. Frankel, in: P. Marcus, J. Oudar (Eds.), Corrosion Mechanisms in Theory and Practice, vol. 1, Marcel Dekker, New York, 1995, p. 641. [6] B. Brox, I. Olefjord, in: Stainless Steel 1984, Gothenburg, Sweden, The Institute of Metals, London, UK, 1984, p. 134. [7] I. Olefjord, B.-O. Elfstr€ om, Corrosion 38 (1) (1982) 46. [8] Z. Bastl, A.I. Senkevich, I. Spirovova, V. Vrtilkova, Surf. Interface Anal. 34 (2002) 477. [9] E. McCafferty, J.P. Wightman, Surf. Interface Anal. 26 (1998) 549. [10] S. Tougaard, Surf. Interface Anal. 26 (1998) 249. [11] C.-O.A. Olsson, D. Landolt, Electrochim. Acta (2003), in press. [12] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 21 (1993) 165.
Atmospheric oxidation of a Nb–Zr alloy studied with XPS C.-O.A. Olsson, D. Landolt
*
Laboratoire de M etallurgie Chimique, Institut des Mat eriaux, Ecole Polytechnique F ed erale de Lausanne, MX-C Ecublens, CH-1015 Lausanne, Switzerland Received 20 March 2003; accepted 11 May 2003
Abstract The long term atmospheric oxidation of a Nb–Zr alloy was studied by XPS. Curve fit parameters were defined and strictly constrained to obtain a consistent evaluation of film thickness and chemistry. A formalism was introduced to distinguish between the water, hydroxide and oxide contributions to the surface oxide. Ageing experiments were carried out simultaneously exposing one lot of samples to a laboratory atmosphere for 10 months and a second lot to a dry atmosphere in a desiccator. 2003 Elsevier Ltd. All rights reserved. Keywords: Atmospheric corrosion; XPS; Zirconium; Niobium
1. Introduction The literature on film growth under atmospheric conditions is rather limited when compared to studies in solution. One reason for this is the increased complexity of the system studied under atmospheric conditions. In this case, it is no longer sufficient to study a metal–oxide–film interface, but a metal/oxide/thin liquid/gaseous interface, where the thickness of the liquid and the composition of the gaseous part of the system may vary significantly in time. An extensive introduction to atmospheric corrosion can be found in a recent monograph by Leygraf and Graedel [1]. The classical way of studying atmospheric corrosion on pure metals is to investigate the weight change as a function of time, while meticulously controlling or measuring
*
Corresponding author. Tel.: +41-21-693-2981; fax: +41-21-693-3946. E-mail address: dieter.landolt@epfl.ch (D. Landolt).
0010-938X/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0010-938X(03)00139-2
214
C.-O.A. Olsson, D. Landolt / Corrosion Science 46 (2004) 213–224
Nomenclature Iifilm photoelectron intensity a spectrometer constant IRX incident X-ray intensity RSFmet relative sensitivity factor of element i i g common factor for spectrometer terms T ðEÞ transmission function ci concentration of element i, fraction or % r cross section for generation of photoelectrons k inelastic mean free path # XPS analysis angle, referenced to the sample surface a, b, c, x film geometry parameters
the changes in environment. In this way, Rice et al. [2] studied the corrosion rate of individual metals, e.g. Fe, Ni, Co, in indoor as well as outdoor environments. They measured weight loss curves over a year and a half and correlated those to pollutants in the atmosphere, e.g. chlorides and sulphides. A later investigation of similar type, but in different indoor environments, was performed by Johansson et al. [3,4], who used weight change and resistance measurements to study different foils in different indoor atmospheres. The relative corrosion rate of different metals was shown to vary widely with the composition of the surrounding atmosphere. With the continued miniaturization of mechanical and electronic devices, the need for information on the resistance to atmospheric corrosion takes new dimensions. The implications to semiconductor industry has been described by Frankel [5]. This puts new demands on the tools used for monitoring the corrosion rates. The aim with this paper is to study the film growth in a dry atmosphere and normal laboratory conditions of a Nb–Zr alloy with XPS. In the XPS analysis, particular attention was given to obtain a reproducible quantification of the long-term development of the oxide film, which was partitioned into oxide, hydroxide and water parts. This was then used to obtain thickness change curves for long term storage of the wires in a dry low pressure atmosphere (desiccator) and in laboratory atmosphere.
2. Theory There are a vast number of multi-layer models in the XPS literature. In this paper, it was of particular interest to evaluate the amount of water and hydroxide on the surface. For this purpose, a three layer formalism was developed, based on a procedure originally developed for mixed oxides on stainless steels by Brox, Elfstr€ om, and Olefjord [6,7]. There are also models available for the Nb–Zr system, cf. Bastl et al. [8]. They studied films formed at room temperature in UHV by exposure to
C.-O.A. Olsson, D. Landolt / Corrosion Science 46 (2004) 213–224
215
200 L of oxygen, water and hydrogen peroxide (1 L ¼ 1 Langmuir ¼ 1 lTorr 1s). They identified a series of oxidation states between metallic and (IV) for zirconium and interpreted the results with a multi-layer model for the cations. In the present study, the bonds found were essentially Zr(IV) and Nb(V). Thus, the layered model was developed for the anionic species only. Another approach was taken by McCafferty and Wightman, who used a detailed layer model to study pure valve metal oxide samples [9]. They used a deconvolution method similar to the one presented in this paper to determine the amount of hydroxyl groups on the surface. Compared to their study, we found much lower amounts of C@O and C–O bonds in the carbon spectrum, thus no correction was performed for a carbon contribution to the oxygen peak. In addition, they used sputter depth profiling to gain additional information on the depth distribution of different species. For a long term study of very thin films, it was desired to avoid a calibration dependent technique such as sputter depth profiling. Thus. the method presented below uses only physical parameters that are independent of the day-today condition of the spectrometer. Even though the IMFP estimates are not known with high precision, they are not subject to calibration and give a solid base for comparative measurements. Compared with the Olefjord concept [6,7], a slight modification was introduced. Elemental relative sensitivity factors (RSFs) were used instead of yields established for a set of reference samples. Further, a formalism for dividing the oxide film into water, hydroxide and oxide parts was added to partition the contributions from the respective oxide, hydroxide and water parts of the oxide film. All intensities below are corrected for transmission. The X-ray induced photoelectron intensity for an oxide film can be written as Iifilm ¼ a IRX cfilm RSFi T ðEi Þ i
kfilm x=kfilm sin # i i met sin # 1 e ki
ð1Þ
where the ratio kfilm =kmet has been introduced to correct for the difference in inelastic mean free path of the film and the substrate. The RSF already contains the inelastic mean free path in the metal. Here, it is defined with rkmet Dmet . This approximation is better than it seems at first glance, since niobium and zirconium are diluted in similar ways in the oxide. The corresponding formula for the metal is met
Iimet ¼ a IRX cmet RSFi T ðEi Þ sin # ex=ki i
sin #
ð2Þ
For the metal, the relative sensitivity factor already contains the inelastic mean free path. When the oxide and metal intensities have been defined for the elements measured, it is convenient to assume an oxide film thickness, e.g. 2 nm. This estimate then serves as a start approximation for the calculation of the film composition: film
x=ki sin # Iifilm =kfilm Þ i RSFi ð1 e cfilm ¼ Pn film film i x=kfilm sin # =kj RSFj ð1 e j Þ j¼1 Ij
ð3Þ
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The corresponding formula can also be derived for the underlying metal. By forming the ratio between Eqs. (1) and (2), and rearranging, one finds the film thickness from xfilm ¼
kfilm i
met I film cmet i ki sin # ln 1 þ imet film film I i c i ki
! ð4Þ
Eq. (4) completes the loop. A loop of Eqs. (3) and (4) was iterated until the oxide film thickness is the same in two consecutive loops. In this treatment, the adventitious carbon contamination layer was disregarded, since it attenuates the film and metal intensities by the same factor, and consequently does not affect the film thickness estimate. For atmospheric corrosion studies, it is of particular interest to investigate the influence of humidity on the surface, since there is a strong correlation between humidity and corrosion rate. This relation has been established for several different metals [1]. To estimate the amount of water on the surface, a formalism was developed for dividing the oxide film into three sublayers by using the deconvolution of the O 1s peak. It is important to remember that most of the water adsorbed on the surface is desorbed during transfer into UHV. If a three layer model is introduced, cf. Fig. 1, the intensities from the three different layers can then be expressed as film
a=kO IH2 O ¼ g cH2 O kfilm O sin #ð1 e
IOH ¼ g cOH
kfilm O
sin #ðe
Iox ¼ g cox kfilm O sin #ðe
a=kfilm O sin #
b=kfilm O
sin #
sin #
e
e
Þ
b=kfilm O sin #
d=kfilm O
sin #
ð5a–cÞ
Þ
Þ
The variable c is the concentration of oxygen in the respective layer (mol/cm3 ) and g is a common factor containing spectrometer terms. The thickness of the oxide and hydroxide layers, x, is already calculated from the metal peak intensity ratios. As defined in Fig. 1, x ¼ d a. The equation system (5a–c) can now be solved analytically. It is convenient to regard the different intensity ratios
H2O
a b
OH-
d
x O2Met
Fig. 1. Three layer model from which the relative intensities can be calculated.
C.-O.A. Olsson, D. Landolt / Corrosion Science 46 (2004) 213–224 film
k1 ¼ k2 ¼
1 ea=kO e
a=kfilm O sin #
e
sin #
b=kfilm O sin #
¼
e film eb=kO sin #
a=kfilm O sin # film
eb=kO
sin #
film
eðaþxÞ=kO
sin #
cOH IH2 O cH2 O IOH cox IOH ¼ cOH Iox
From these ratios, thicknesses for each layer are obtained from " # x=kfilm O sin # 1 þ k e 2 b ¼ kfilm film O sin # ln 1 þ k2 þ k1 k2 ð1 ex=kO sin # Þ 1 þ k1 eb=k sin # film a ¼ kO sin # ln 1 þ k1
217
ð6a;bÞ
ð7a–cÞ
d ¼xþa For this calculation, it was assumed that the attenuation length in all three layers is the same. The main uncertainty in the layer division originates from the curve fit, i.e. the ratios between the water, hydroxide and oxide contributions.
3. Experimental 3.1. Samples Wire samples were analysed after a standard cleaning procedure. The bulk composition was 85%Nb and 15%Zr. The wires had a rectangular cross section with a width of about 100 lm. All measurements were performed on the wider side. After reception, the samples were stored in a desiccator for one month, whereafter they were separated into two lots––one of which was left in a loosely closed plastic box under laboratory conditions, the other was left in the desiccator. Before each measurement, the desiccator was brought up to air and the samples were taken out. Vacuum was obtained with a water pump within one hour after opening. The test was carried out from January to December 2002. 3.2. XPS analysis The XPS measurements were performed using a Kratos Axis Ultra spectrometer and a monochromated Al Ka X-ray source at 1486.6 eV. The base pressure during normal operation was 1 · 109 Torr. Charge neutralization was used for all measurements on oxides. The energy scale was calibrated by fixing the adventitious carbon peak at 285 eV. The analyser was operated in the fixed analyser transmission (FAT) mode at a pass energy of 20 eV. The diameter of the analysed area was about 100 lm, resulting in a total acquisition time of about 90 min per sample. Typically, five samples of each type were analysed to obtain satisfactory statistics. No sputtering was performed prior to analysis. Quantification was performed with sensitivity factors provided by the manufacturer.
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The wires were mounted on a custom design sample holder which allowed for simultaneous introduction of at least 10 wires with a surrounding vacuum. The holder was designed to avoid stray intensity from the sample holder and to facilitate optimisation of the signal. Curve fitting was performed using CasaXPS v. 2. Some charging was observed during the analysis. This was compensated for by shifting the adventitious carbon peak to 285 eV. Background subtraction was performed using the iterated Shirley method. For the zirconium and niobium metal peaks, synthetic peak shapes were defined from XPS spectra taken under similar conditions from sputter cleaned pure metal reference standards. The asymmetry parameters used follow the definitions of PHI-Multipak. Quantification of the oxide film was made with in house written software based on Matlab 6.0, using Eqs. (1)–(7). The peak fit parameters are crucial for the following quantification. It is of particular importance to mutually fix the half widths and peak positions of the O 1s peaks to obtain consistency in the three layer model for water adsorption. The area ratios for the 3/2 and 1/2 doublets were fixed to 50% throughout.
4. Results and discussion 4.1. XPS curve fitting on metal peaks for intensity separation The oxides studied in the ageing process were sufficiently thin to be analysed with the formalism given above. This procedure can be used for film thicknesses up to about 3k. For thicker films, other methods have to be applied, e.g. Tougaard nanostructure analysis [10] or sputter depth profiling [11]. The first step in the analysis is to define relevant curve fit parameters. This was done on pure niobium and zirconium foils which were sputter cleaned before the analysis. The curve fits obtained for the metallic species are shown in Fig. 2. The oxide films were slightly insulating during analysis, thus charge compensation was used throughout. The spectrum was brought back into position by defining the C–C peak at 285 eV. This gave a reproducible energy scale, cf. the standard deviations of the metal peaks in Table 1. To obtain a consistent fitting of the oxide peaks, their half widths were kept mutually fixed for each element. The uncertainty in this procedure is also given in Table 1. The error estimates are based on a standard deviation of 10 fits. An error estimate is only given for one peak in each group, since the other peaks were fitted together with fixed distances/half widths. It was also checked that there was a linear relation between the oxidation state number and the binding energy shift for the metal peaks. The contamination level was not increasing systematically in time. Sample surfaces were very clean throughout the series. The contribution from carbon, i.e. C–O and C@O bonds, to the oxygen peak was judged less than 5%, and thus considered negligible. It is evident that the total amount of water adsorbed on the surface will not remain under UHV conditions. However, it appears that the water remaining on the
C.-O.A. Olsson, D. Landolt / Corrosion Science 46 (2004) 213–224
219
Metallic Zr 3d
192
190
188
186
184
182
180
178
176
Binding Energy, eV
Metallic Nb 3d 5000
4000
3000
2000
1000
0 216
212
208
204
200
Binding Energy, eV Fig. 2. Curve fits of the Nb 3d and Zr 3d metal peaks. The metal peak shapes defined on the sputter cleaned metal is then later used in the curve fits of the oxides.
surface in UHV correlates with the surface condition immediately before introduction into the chamber. The validity of this assumption and the curve fit procedure was checked using a sample immersed in water immediately before introduction into UHV. The resulting O 1s spectrum is shown in Fig. 3. The spectrum was recorded after 15 h under vacuum at 1 · 109 Torr. Although most of the surface water is expected to desorb during introduction into UHV, the sample immersed in water showed a significantly higher water coverage than those stored in atmosphere. Still, , out of which the quantification of the XP spectrum showed a total thickness of 42 A , respectively. This should be the hydroxide and water parts constituted 7 and 5 A
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C.-O.A. Olsson, D. Landolt / Corrosion Science 46 (2004) 213–224
Table 1 Curve fit parameters for the peaks used for oxide layer quantification Element
Oxidation state
Position
FWHM
%Gauss
Tail height
Tail length
Zr Zr Zr Zr
met met IV IV
178.8 181.2 182.4 ± 0.1 184.8
0.85 0.98 1.2 ± 0.05 1.2
70 80 100 100
0.7 0.5 – –
17 17 – –
met met II II IV IV V V
201.7 ± 0.1 204.8 203.3 205.8 205.3 208.0 207.1 209.8
0.63 ± 0.03 0.76 1.24 ± 0.09 1.24 1.24 1.24 1.24 1.24
70 80 100 100 100 100 100 100
0.5 0.5 – – – – – –
14 12 – – – – – –
O2 OH H2 O
530.0 ± 0.1 531.4 533.4
1.33 ± 0.02 1.33 1.52
100 100 100
– – –
– – –
Nb Nb Nb Nb Nb Nb Nb Nb
2p3=2 2p1=2 2p3=2 2p1=2 2p3=2 2p1=2 2p3=2 2p1=2 2p3=2 2p1=2 2p3=2 2p1=2
O 1s O 1s O 1s
For each element, the most well defined peak was chosen to have a variable position, which is given within the uncertainty limits based on standard deviation from 10 measurements. For the oxidised metal peaks, the FWHM was fitted for the group as an entity. For the O 1s line, the water peak was fitted with a wider, but still fixed FWHM. The variation of half widths for each group is given only once as the standard deviation from 10 measurements. The metal peak shapes and asymmetries were fixed for spectra acquired on sputter cleaned pure metal surfaces for the respective elements.
O 1s
O OH
2-
-
H2O
540
535
530
525
Binding Energy, eV Fig. 3. XPS curve fit of sample immersed in water immediately before introduction for control of the water adsorption model.
C.-O.A. Olsson, D. Landolt / Corrosion Science 46 (2004) 213–224
221
compared with the data obtained for the samples stored in laboratory atmosphere , and in desiccator that showed hydroxide and water layers of the order of 5 and 1 A respectively. The resulting curve fits for wires stored in atmosphere are shown in Fig. 4. For each ageing time, 3–5 identical samples were analysed and evaluated to obtain a reliable estimate of the thickness. Film thicknesses were then calculated according to the procedure given above, using the physical parameters obtained for bulk oxides that can be found in Table 2. 4.2. Ageing in atmosphere It was desired to study the influence of atmospheric exposure on the native wire oxide on the surface. Initially, samples were stored in a desiccator for one month. Nb 3d
Zr 3d ZrO2 Nb2 O5
Nb-met NbO2 NbO
Zr-met
190
188
186
184
182
180
178
176
220
216
212
Binding Energy, eV
208
204
200
196
Binding Energy, eV
O 1s O
OH
2-
-
H 2O
540
535
530
525
Binding Energy, eV
Fig. 4. XPS curve fits of the Zr 3d, Nb 3d and O 1s peaks used for determining the oxide layer thicknesses. The peak fit parameters can be found in Table 1.
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Table 2 Physical parameters for thickness determination in XPS Parameter
Value 19.2 A 19.5 A 25.1 A 25.5 A 23.1 A 16.5 A
kNb in Nb kZr in Nb kNb in Nb2 O5 kZr in Nb2 O5 kZr in ZrO2 kO in Nb2 O5 Bulk conc. Zr cO in Nb2 O5 cO in OH part cO in H2 O cmet;Zr in Zr cmet;Nb in Nb cox;Zr in ZrO2 cox;Nb in NbO2:5
15% 0.0840 mol/cm3 0.07 mol/cm3 0.0556 mol/cm3 0.0711 mol/cm3 0.0922 mol/cm3 0.0478 mol/cm3 0.0336 mol/cm3
IMFPs according to TPP [12]. The density values were taken from the respective bulk oxides.
A part of the lot was then taken out from the desiccator and stored in laboratory atmosphere. The oxide film thickness for the two different lots was estimated with XPS at time intervals ranging from 0 to 300 days (Table 3). The peak deconvolution used to find the metal and oxide intensities of the respective metal peaks is indicated in Fig. 4. A small but significant increase in the surface oxide film was observed for the samples aged in laboratory atmosphere. After 300 days the oxide on these thicker than for the samples aged in a dry environment. samples was about 10 A The total oxide film was decomposed into water, hydroxide and oxide parts according to the method described above. In addition, the film composition was also Table 3 Ageing experiments in desiccator and laboratory atmosphere Time days
Condition
Samples
Nb-ox cat%
Dfilm , A
DOH , A
DH2 O , A
1 27 62 62 90 90 160 160 229 229 299 299
As rec. Desiccator Desiccator Lab. atm. Desiccator Lab. atm. Desiccator Lab. atm. Desiccator Lab. atm. Desiccator Lab. atm.
3 3 3 3 4(1) 4(1) 4(1) 5 5 5 5 5
34 ± 9 42 ± 4 30 ± 9 43 ± 2 34 ± 6 36 ± 8 40 ± 5 27 ± 4 38 ± 10 35 ± 8 33 ± 14 48 ± 14
28.6 ± 1.0 29 ± 0.4 30.2 ± 0.9 32.2 ± 1 31.2 ± 0.6 35.7 ± 1.6 30.8 ± 0.5 39.1 ± 1.2 32.5 ± 0.6 40.2 ± 1.5 33.0 ± 0.8 42.1 ± 1.4
8.6 ± 2.2 4.3 ± 0.4 4 ± 0.3 3.7 ± 0.2 4.0 ± 0.2 4.4 ± 0.4 5.2 ± 0.3 5.0 ± 0.3 4.4 ± 0.3 4.7 ± 0.2 4.6 ± 0.4 4.6 ± 0.3
2.2 ± 0.7 1.7 ± 1.1 0.6 ± 0.4 0.5 ± 0.1 0.7 ± 0.1 0.7 ± 0.3 1.1 ± 0.2 1.4 ± 0.3 0.8 ± 0.2 0.8 ± 0.2 0.9 ± 0.3 0.8 ± 0.2
For the last two times, five samples from each environment were measured, and single outliers were discarded (number of discarded points is indicated in parentheses). The total film thickness is plotted in Fig. 5. Dfilm is equivalent to the total film thickness (oxide + hydroxide), not considering the water coverage.
C.-O.A. Olsson, D. Landolt / Corrosion Science 46 (2004) 213–224
223
XPS Film Thickness 45 Laboratory Atmosphere
Film Thickness, Å
40 Separation date
35
Desiccator
30
25 0
50
100 150 200 Ageing Time, Days
250
300
Fig. 5. Results of ageing of pre-oxidised wires in laboratory and dry atmospheres. After 300 days, the thicker than those aged in the atmospherically aged samples were found to have an oxide layer, about 12 A desiccator.
estimated, cf. Table 1. From the decomposition of the oxide film, it is clear that all oxide growth is occurring in the oxide part, whereas the hydroxide and water parts remain constant. The XPS measurements also indicate that the oxide film composition is not homogeneous. The analysed spot size was about 100 lm diameter. The thickness changes observed in Fig. 5 are clearly occurring in the oxide film. The measured thickness change is about 1.2 nm in the oxide film during 10 months, whereafter it slows down. This change can be recalculated into a corrosion velocity of 3.75 mg/m2 /year in metallic equivalents. This is much lower than corrosion rates for silver and zinc plates stored in a research library with controlled environment. Such corrosion rates have been estimated to 100 and 150 mg/m2 /year, respectively [3]. With standard techniques such as gravimetry or cathodic reduction, the detection limit for atmospheric corrosion is normally around 10 mg/m2 /year.
5. Conclusions • XPS was shown to be a viable tool for the study of low temperature oxidation and alloys over prolonged time periods. The data analysis routine developed for this purpose permitted the reproducible quantification and partitioning of the oxide layer in oxide, hydroxide and water parts. • It is possible to obtain a reproducible estimate of the amount of water remaining on the surface under UHV conditions. This amount of water also reflects the amount of water present on the surface before introduction into UHV. • Film growth was located to the oxide part of the film; the hydroxide and water parts remained constant during the test period.
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• The oxide films of the Nb–Zr alloys showed a thickness increase of about 1 nm when compared to samples stored under low pressure in a desiccator.
References [1] C. Leygraf, T.E. Graedel, Atmospheric Corrosion, in: The Electrochemical Society Series, vol. 1, The Electrochemical Society, Pennington, NJ, 2000. [2] D.W. Rice, R.J. Cappell, W. Kinsolving, J.J. Laskowski, J. Electrochem. Soc. 127 (4) (1980) 891. [3] E. Johansson, C. Leygraf, B. Rendahl, Br. Corros. J. 33 (1) (1998) 59. [4] E. Johansson, C. Leygraf, Br. Corros. J. 34 (1) (1999) 27. [5] G.S. Frankel, in: P. Marcus, J. Oudar (Eds.), Corrosion Mechanisms in Theory and Practice, vol. 1, Marcel Dekker, New York, 1995, p. 641. [6] B. Brox, I. Olefjord, in: Stainless Steel 1984, Gothenburg, Sweden, The Institute of Metals, London, UK, 1984, p. 134. [7] I. Olefjord, B.-O. Elfstr€ om, Corrosion 38 (1) (1982) 46. [8] Z. Bastl, A.I. Senkevich, I. Spirovova, V. Vrtilkova, Surf. Interface Anal. 34 (2002) 477. [9] E. McCafferty, J.P. Wightman, Surf. Interface Anal. 26 (1998) 549. [10] S. Tougaard, Surf. Interface Anal. 26 (1998) 249. [11] C.-O.A. Olsson, D. Landolt, Electrochim. Acta (2003), in press. [12] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 21 (1993) 165.