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Trapping of atmospheric oxygen on metal surfaces under Ar+ ion bombardmeNT
609
Surface Science 221 (1989) 609-618 North-Holland, Amsterdam
TRAPPING OF ATMOSPHERIC OXYGEN ON METAL SURFACES UNDER Ar + ION BOMBARDMENT Y. BABA, T.A. SASAKI Department of ChemMy, Japan Atomic Energy Research Institute, Zbaraki-ken 319-11, Japan
Tokai-mura,
and I. TAKANO Department
of Electrical Engineering, Kogakuin
University, Shinjuku-ku,
Tokyo 160, Japan
Received 23 March 1989; accepted for publication 19 May 1989
Various metals were bombarded with 8 keV Ar+ ions in an oxygen atmosphere at a pressure of 1 x 10F3 Pa. The chemical changes on the surface were analyzed using XPS. It was observed that titanium easily incorporates oxygen. This results in the formation of titanium dioxide with a nearty stoichiometric composition. The oxygen-~app~g was also confirmed for other metal targets. It was proven that the metals with a Gibbs energy (AG) for oxide formation lower than - 80 kcaI/mol easily trap oxygen, while those with a AG value higher than - 60 k&f/m01 do not. We conclude from our observations that the trapping of atmospheric oxygen under Ar+ ion bombardment can be attributed to chemical reactions at the metal-gas interface rather than to physical processes such as knock-on collisions.
1. Introduction Bomb~dment of a compound with energetic ions generally induces a decomposition of the compound. For example, the bombardment of a metal oxide with rare-gas ions leads to the reduction of the oxide due to preferential sputtering of the oxygen atoms. Such decomposition reactions essentially occur through physical processes, which are determined by the mass and energy of the bombing ions, the mass of the target, and so forth ]l]_ There have been some thermochemical approaches to explain the mechanism of decomposition reactions at ion-bombarded surfaces [2-61. A widely accepted theory is the localized thermal equihbrium (LTE) model, proposed by Andersen et al. [3,4]. The products of the decomposition reaction in the ionbombarded surface are fairly well estimated by this model, supposing that the reaction occurs in the thermal equilibrium region around the track of the ion 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holl~d Physics ~b~s~g Division)
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Y. Baba et al. / Atmosphericoxygen on metal surfaces
at quite high temperatures [8,9]. This thermochemical treatment has been so far also applied to the sputtering of oxides by rare-gas ion bomb~dment
[6,9,101.
In contrast to decomposition of compounds by ion bombardment, there have been a few approaches concerning thermochemical explanations regarding synthesis reactions induced by ion beams. It has been observed recently from experiments that the bombardment of some metals with rare-gas ions in an oxygen or nitrogen atmosphere promotes the incorporation of gases in the surface layer of the metals [ll-211. From a physical point of view, these phenomena have been attributed primarily to the formation of defects by the ion bomb~dment [lS]. Radiation enhanced diffusion [22] may further promote the incorporation. The present paper deals with the trapping of atmospheric oxygen on metal surfaces under Ar+ ion bombardment from a thermochemical point of view. The mechanism of the incorporation reaction will be discussed, mainly based on the the~odyn~c~ parameters of the metal oxides. 2. Experimental The targets used were high-purity metals (> 99.9%) of 10 mm 0 X 0.2 mm. Titanium dioxide with stoic~omet~c composition, prepared by oxidizing a titanium surface, was used as standard. The metals were polished with l/4 pm diamond paste before introduction into the vacuum chamber. The metal was annealed at 600” C in vacuum, and sputtered with 8 keV Ar+ ions until surface oxygen and carbon contaminations were completely removed. Then, the sample was bombarded with 8 keV Ar+ ions, produced in a duoplasmatron ion source, at room temperature. The bombarding angle was 25” from the surface normal The typical flux of the Ar+ beam was 2.5 X 1013 atoms cm-’ s-l (4 pA/cm2), monitored with a Faraday cup. The partial pressure of argon during the bomb~dment was 8 x lop5 Pa. During the bombardment, high-purity oxygen was introduced into the bombarding chamber up to a pressure of 1 X 10m3 Pa. The chemical states of the surfaces and the Ols/Me (Me = metal) ratios were measured by XPS at a pressure of 3 X lo-’ Pa, using Mg Ka X-ray (1253.6 eV) as the excitation source. The bring energy was calibrated using line of metallic gold, assumed to be 84.0 eV. the 4f,, 3. Results
Fig. 1 shows the XPS spectra in the Ti2p region for titanium before (a) and after (c) Ar+ ion bombardment in an oxygen atmosphere for 4 min. For
Y. Baba et al. / Atmospheric
Binding
oxygen on metal surfaces
energy
611
(eV)
Fig. 1. XPS spectra in the Ti2p region: (a) titanium metals, (b) titanium metal exposed to oxygen at a pressure of 1 x 10e3 Pa for 4 min, and (c) titanium metal bombarded with 8 keV Ar+ ions of 4 PA/cm2 in oxygen at a pressure of 1 X 10m3 Pa for 4 min.
the spectrum for the oxygen-adsorbed sample prepared without Ar+ ion bombardment is also shown as spectrum b. Although an appreciable amount of oxygen is adsorbed on titanium without bombardment, the metallic phase can still be observed in spectrum b. On the other hand, the metallic phase completely disappears after bombardment with Ar+ ions (spectrum c). The binding energy of the most intense peak in spectrum c is consistent with that of standard TiO,. The time dependence of the surface O/Ti ratio determined by XPS, both with and without Ar+ ion bombardments, are shown in fig. 2. The O/Ti ratio is normalized so that the 0 ls/Ti 2p ratio of standard TiO, is 2.0. In the case of the oxygen-adsorbed sample, the O/Ti ratio saturates to about 1.3 after 4 min. In this region, the rate of adsorption and desorption becomes equal. The ratio for the Ar+ ion bombarded sample saturates to 1.96. The difference from 2.0 is due to the existence of a small amount of Ti,O,. In order to investigate the depth profile of the oxide layers, titanium samples oxidized for 4 min with (a) and without (b) Ar+-ion bombardment were sputtered with 8 keV Ar+ ions of 10 PA/cm’. The fluence dependences of the 0 ls/Ti2p ratio are indicated in fig. 3. The 0 ls/Ti2p ratios exponencomparison,
Y. Baba et al. / Atmospheric oxygen on metal surfaces
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/
I
/
I
OlTi 2.0
I
ratio
I
of bulk TiOp
-___________-__---__---------_-_ -
I
I
I
I
I
I
2
4
6
8
10
12
Time
(0)
14
(rnin)
Pig. 2. Time dependences of surface O/Ti ratio measured by XPS for titanium; (a) titanium bombarded with 8 keV Ar+ ions at 4 PA/cm* in oxygen at a pressure of 1 X 10e3 Pa, and (b) titanium exposed to oxygen at a pressure of 1 x 10m3 Pa without bombardment.
r
1
I
I
I
I
I
o
1
2
3
4
5
6
Fluence
( otoms /cm2 !
7
( xfo’5)
Fig. 3. Changes in the Ols/Ti2p ratio in XPS for titanium samples sputtered with 8 keV Ar+ ions of 10 pA/cm*; (a) titanium metal bombarded with 8 keV Ar+ ions of 4 pA/cm’ in oxygen at a pressure of 1 x10-’ Pa for 4 min, and (b) titanium exposed to oxygen at a pressure of 1 X 10e3 Pa for 4 mm without bombardment.
Y. Baba et al. / Atmospheric
oxygen on metal surfaces
613
tially decrease with the fluence of Ar+ ions. It is observed that the oxide layer prepared with Ar+-ion bombardment extends deeper in the target than the one without bombardment. Estimated from the sputtering yield of TiO1 by the 8 keV Arf ion bombardment [24] and the density of TiO,, the approximate thicknesses of the oxide layers are determined to be 5.0 nm for line a and 0.8 nm for line b. The range, perpendicular to the surface, of a 8 keV Ar+ ion in titanium, is calculated to be 6.1 nm using TRIM code [25]. The result suggests that the oxide layer produced by the Ar+ ion bombardment is formed around the track of the implanted Ar+ ions. For the purpose of sputter cleaning, the surface of titanium was bombarded with Ar+ ions. Thus, there is already an appreciable amount of defects in the surface before exposure to oxygen. However, the amount of surface oxide and its thickness produced by the adsorption is much smaller than those produced by Ar+ ion bombardment. Therefore, promotion of oxidation is caused by simultaneous bombardment during the oxygen exposure. This finding indicates that the defects or excited states, having a short life time, play important roles in the oxidation reaction. 3.2. Trapping of oxygen on various metals under Ar ’ ion bombardment Promotion of oxidation reactions is also observed for other transition-metal targets, as listed in table 1. In this table, the surface O/Me ratios for the samples exposed to oxygen for 4 min are indicated, because the ratios become almost saturated after 4 min exposure. Among the metals examined, promotions of the oxidation reactions for copper, silver and gold are negligibly small, but appreciable promotion is observed for the other metals.
4. Discussion The stability of the reaction product is one of the essential factors to discuss the gas trapping process. Such stability is inferred from the Gibbs energy of formation (AG) of the compounds of target metals and atmospheric gases. In the case of titanium target, it has been observed that atmospheric nitrogen is trapped under the same conditions as in the present experiment [20]. For comparison, trapping of hydrogen and carbon was investigated using hydrogen and methane, respectively, as atmospheric gases under the same experimental conditions. However, the formation of titanium hydride or carbide was not observed. The AG values of the titanium compounds, TiH,, TiN, TiO, and TIC are -37, -93, -229 and -46 kcal/mol, respectively [26]. The different trapping behavior of these gases suggests that an increase in the
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Table 1 Saturated ratios of Ols/(MeZp, Target
Ti V Cr Fe Ni cu Y Zr Nb MO Ag W Au
oxygen on metal surfaces
3d or 4f)
Main product
Saturated Line
With bombardment
Without bombardment
Theoretical ratio
TiO,
Ols/Ti2p Ols/VZp 0 ls/Cr 2p Ols/FeSp Ols/Ni2p Ols/Cu2p Ols,‘Y3d Ols/Zr3d Ols/‘Nb3d 0 ls/Mo3d 0 ls/Ag3d Ols/W4f Ols/Au4f
0.71 0.55 0.36 0.26 0.095 0.005 0.60 0.63 0.56 0.32 0.004 0.13 0.005
0.48 0.30 0.15 0.22 0.077 0.005 0.56 0.41 0.19 0.10 0.003 0.072 0.000
0.72 0.74 0.37 0.27 0.14 0.12 0.69 0.78 0.84 0.88 0.079 0.83 0.24
v205 Cr203
Fe203 NiO cue w3
ZrO, NbzOs MOO, AgzO wo3 Au2o3
0 ls/(Me2p,
3d or 4f) ratio
The metal targets were exposed to 1 X lo-’ Pa 0, for 4 min with and without keV Ar+ ions at a fluence of 4 nA/cm2. The theoretical ratios are calculated tion cross-sections [23].
bombardment of 8 using photoioniza-
stability of the reaction product leads to a high trapping efficiency. This relation will be discussed more quantitatively for the trapping of oxygen. In the case of chemical adsorption, the rate of oxide formation is simply expressed as: dx/dt = k,(l
-x)
- k2x,
(1)
where x (0 s x s 1) is the surface coverage of the oxide, t (s) the time of oxygen exposure, and k, and k, (s-l) the rate constants of adsorption and desorption, respectively. The ratio of k,/k, is proportional to the sticking coefficient of oxygen at the metal surface. Note that eq. (1) only expresses the average value in the surface region, detected by XPS, i.e., about 1.5 nm from the surface. By solving eq. (l), we can obtain the oxide coverage at t:
(2) At the steady state, the saturated value of x is expressed by setting t + co in eq. (2): x(a)
= W(k
+ kz).
(3)
Although chemical adsorption of oxygen on a metal surface is essentially based on the chemical reactivity of the metal with oxygen, it also depends on the crystal structure of the surface, the surface roughness, and so forth. In the
615
Y. Baba et al. / Atmospheric oxygen on metal surfaces
‘,Oj TiO2
t -i r” G
80 ‘t cz
0.6 -
0.4 -
0.2-
.
1 NiO .
$02
c’$3
w5
.
Nb205 .
AG/y
(kcol/mol)
Fig. 4. Relation between saturated ratio of O/Me for metals exposed to oxygen at a pressure of 1 x lo-' Pa and Gibbs free energy of oxide formation at 298 K. The O/Me ratio is represented as the Ols/(MeZp, 3d or 4f) value in XPS divided by the theoretical ratio calculated using photoionization cross sections. The Gibbs free energy of formation is divided by y, assuming that the composition of the oxide is Me,O,,.
case of the thermal process, the reaction of the oxide formation is expressed in the following equation: xMe+(y/2)O,+Me,O,+AG.
(4)
The value of AG is related to the pressure of oxygen as: AG = -RT
lr1(1/[0~]~“)
= (y/2)RT
ln[Oz].
(5)
In order to compare the chemical reactivity of various metals with oxygen at the same pressure, we may compare the AG/y values of the oxides. Fig. 4 shows the relation between the saturated elemental surface O/Me ratio and the Gibbs energy of oxide formation, AG/y, for various metals. The O/Me ratio is obtained by dividing the observed 0 ls/(Me 2p, 3d or 4f) value by that calculated using the theoretical photoionization cross sections [23]. Oxygen hardly adsorbs on the metals with a AC/y value of the oxide larger than - 50 kcal/mol. When bombarding with Ar+ ions during oxygen exposure, we assume that the rate constant of oxide formation increases to k, + a (a 2 0), where (Y(s-l) is the number of oxides produced by Ar+ ion bombardment, depending on the
616
Y. Baba et al. f Atmospheric
oxygen on metal surfaces
flux of the Ar+ ion beam. It should be noted that the rate constant of decomposition of oxides also increases to k, + j3 (j3 2 0) due to the preferential sputtering of oxygen atoms. The value of j3 is related to the sputtering yield Y and the flux of the incident ion f as /3 = Yf. Thus, the eq. (3) can be rewritten as: x(co)=(k,+a)/(kl+a+k2+j3).
(6)
If the promotion of the oxidation reaction by Arc ions is observed, it is obvious that the value of ar is much larger than that of /3 for those metals. In addition, the sputtering yields of the present oxides by 8 keV Ar+ range from 0.2 to 1.7 [25]. This means that the large variation of the saturated O/Me ratio for various metals is mainly due to the difference in the value of (Y. With regard to the physical process, the promotion of oxide formation by Ar+ ion bombardment can be primarily attributed to knock-on collision by Ar+ ions on the surface. However, the contribution of the chemical reaction must be considered in order to interpret the large difference in the reaction rate of oxide formation between various metals. Kim et al. have found the relation between sputtering behavior and thermodynamical parameters for various oxides sputtered by rare-gas ions, assuming that the sputtering occurs in the thermal equilibrium region [9]. They have indicated that the standard Gibbs energy of formation of an oxide, AG, is related to the reduction of the oxide. In our case, both the formation and d~mposition occur simultaneously. The relation between the saturated value of the surface O/Me ratio and AG/y for various metals bombarded with Ar+ ions is shown in fig. 5. Compared with the result for oxygen-adsorbed samples (fig. 4), the relation between two values is clearly observed. There exists a threshold at about - 70 kcal/mol. Oxides with a AC/y value higher than -60 kcal/mol are hardly produced under the present condition, while most of the ratios of those with a AC/y value lower than - 80 kcal/mol are close to 1. Regarding the decomposition of oxides, the threshold of AG of the oxide for the reduction by Ar+ ion bombardment is in the range from - 60 to - 118 kcal/mol[9], In this case, the threshold of thermodynamical parameters exists, because the sputtering occurs in the local thermal ~ui~b~urn region produced around the track of the Arf ions, and we can assume that the sputtering occurs through thermal reactions at quite high temperatures. The present results for the threshold of oxide formation, ranging from -60 kcal/mol to - 80 kcal/mol, are close to those for reduction. This finding suggests that the formation of compounds by Ar+ ion bombardment also occurs through a thermochemical reaction near the thermal equilibrium region. Therefore, it can be concluded that the trapping of atmospheric gases in the surface induced by ion bombardment occurs through chemical reaotions at quite high temperatures near the track of the bombarding ions, rather than through physical
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Y. Baba et al. / Atmospheric oxygen on metal surfaces
o.2[ ,,, !180 -160 -140
(
-120 -100
lx/y
:, -80
$“p -60
-40
Ikp2*J -20
0
20
( kcol/mol)
Fig. 5. Relation between saturated ratio of O/Me for metals bombarded with 8 keV Ar+ ions in oxygen at a pressure of 1 x 10e3 Pa and Gibbs free energy of oxide formation at 298 K. The O/Me ratio is represented as the Ols/(MeZp, 3d or 4f) value in XPS divided by the theoretical ratio calculated using photoionization cross sections. The Gibbs free energy of formation is divided by y, assuming that the composition of the oxide is MexO,.
processes such as knock-on collisions. The obtained relation between the thermochemical parameter of the reaction product and the trapping of atmospheric gas will be useful to estimate the extent of the gas trapping reaction by ion bombardment for other gas-solid systems.
5. Summary (1) Titanium bombarded with 8 keV Ar+ ions easily traps atmospheric oxygen, and nearly stoichiometric titanium dioxide was produced in the surface layer. The bombardment with AI-+ ions appreciably promotes the formation of the oxides, compared with that by oxygen adsorption. (2) Atmospheric oxygen was trapped by the Ar+ ion bombardment in metals with AC/y values lower than -80 kcal/mol, but not in those with AC/y values higher than - 60 kcal/mol. (3) The relation between the oxide formation and the thermodynamical parameter suggests that the trapping of atmospheric oxygen is attributed to a chemical reaction rather than to physical processes such as knock-on collisions by Ar+ ion bombardment.
Y. Baba et al. / Atmospheric
618
oxygen on metal surfaces
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
P. Sigmund, Phys. Rev. 184 (1969) 383. Z. Jurela, Intern. J. Mass Spectrom. Ion Phys. 12 (1973) 33. C.A. Andersen and J.R. Hinthome, Anal. Chem. 45 (1973) 1421. C.A. Andersen, in: Proc. Workshop on SIMS and IMMA, NBS Special Publication 427 (1975) 79. R. Kelly, Radiation Effects 32 (1977) 91. R. Kelly, Surface Sci. 90 (1979) 280. T. Ishitani, H. Tamura and T. Kondo, Anal. Chem. 47 (1975) 1294. D. Newbury, US-Japan Joint Seminar, Honolulu, Hawai, 12-17 October 1975. K.S. Kim, W.E. Baitinger, J.W. Amy and N. Winograd, J. Electron Spectrosc. Related Phenomena 5 (1974) 351. K.S. Kim, W.E. Baitinger and N. Winograd, Surface Sci. 55 (1976) 285. K. Wittmaack, Surface Sci. 68 (1978) 118. J. Hrbek, Thin Solid Films 42 (1977) 185. W.O. Hofer, H.L. Bay and P.J. Martin, J. Nucl. Mater. 76/77 (1978) 156. G.I. Grigorov, I.N. Martev and K.K. Tzatzov, Compt. Rend. Acad. Bulgare Sci. 32 (1979) 1069. W. Wach and K. Wittmaack, J. Appl. Phys. 52 (1981) 3341. G.I. Grigorov, I.N. Martev and K.K. Tzatzov, Thin Solid Films 76 (1981) 269. H. Gechsner and A. Wucher, Appl. Surface Sci. 10 (1982) 342. M. Saidoh, J. Nucl. Mater. 128/129 (1984) 540. M. Saidoh, H. Gnaser and W.O. Hofer, Nucl. Instr. Methods Phys. Res. B 28 (1987) 540. Y. Baba, T.A. Sasaki and I. Takano, J. Vacuum Sci. Technol. A 6 (1988) 2945. Y. Baba and T.A. Sasaki, J. Mater. Res., in press. P.S. Ho, Surface Sci. 72 (1978) 253. J.H. Scofield, J. Electron Spectrosc. Related Phenomena 8 (1976) 129. J. Bohdansky, J. Nucl. Mater. 93/94 (1980) 44. J.P. Biersack and L.C. Haggmark, Nucl. Instr. Methods 174 (1980) 257. I. Bartin and 0. Knacke, Thermochemical Properties of Inorganic Substances (Springer, Berlin, 1973).
Surface Science 221 (1989) 609-618 North-Holland, Amsterdam
TRAPPING OF ATMOSPHERIC OXYGEN ON METAL SURFACES UNDER Ar + ION BOMBARDMENT Y. BABA, T.A. SASAKI Department of ChemMy, Japan Atomic Energy Research Institute, Zbaraki-ken 319-11, Japan
Tokai-mura,
and I. TAKANO Department
of Electrical Engineering, Kogakuin
University, Shinjuku-ku,
Tokyo 160, Japan
Received 23 March 1989; accepted for publication 19 May 1989
Various metals were bombarded with 8 keV Ar+ ions in an oxygen atmosphere at a pressure of 1 x 10F3 Pa. The chemical changes on the surface were analyzed using XPS. It was observed that titanium easily incorporates oxygen. This results in the formation of titanium dioxide with a nearty stoichiometric composition. The oxygen-~app~g was also confirmed for other metal targets. It was proven that the metals with a Gibbs energy (AG) for oxide formation lower than - 80 kcaI/mol easily trap oxygen, while those with a AG value higher than - 60 k&f/m01 do not. We conclude from our observations that the trapping of atmospheric oxygen under Ar+ ion bombardment can be attributed to chemical reactions at the metal-gas interface rather than to physical processes such as knock-on collisions.
1. Introduction Bomb~dment of a compound with energetic ions generally induces a decomposition of the compound. For example, the bombardment of a metal oxide with rare-gas ions leads to the reduction of the oxide due to preferential sputtering of the oxygen atoms. Such decomposition reactions essentially occur through physical processes, which are determined by the mass and energy of the bombing ions, the mass of the target, and so forth ]l]_ There have been some thermochemical approaches to explain the mechanism of decomposition reactions at ion-bombarded surfaces [2-61. A widely accepted theory is the localized thermal equihbrium (LTE) model, proposed by Andersen et al. [3,4]. The products of the decomposition reaction in the ionbombarded surface are fairly well estimated by this model, supposing that the reaction occurs in the thermal equilibrium region around the track of the ion 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holl~d Physics ~b~s~g Division)
610
Y. Baba et al. / Atmosphericoxygen on metal surfaces
at quite high temperatures [8,9]. This thermochemical treatment has been so far also applied to the sputtering of oxides by rare-gas ion bomb~dment
[6,9,101.
In contrast to decomposition of compounds by ion bombardment, there have been a few approaches concerning thermochemical explanations regarding synthesis reactions induced by ion beams. It has been observed recently from experiments that the bombardment of some metals with rare-gas ions in an oxygen or nitrogen atmosphere promotes the incorporation of gases in the surface layer of the metals [ll-211. From a physical point of view, these phenomena have been attributed primarily to the formation of defects by the ion bomb~dment [lS]. Radiation enhanced diffusion [22] may further promote the incorporation. The present paper deals with the trapping of atmospheric oxygen on metal surfaces under Ar+ ion bombardment from a thermochemical point of view. The mechanism of the incorporation reaction will be discussed, mainly based on the the~odyn~c~ parameters of the metal oxides. 2. Experimental The targets used were high-purity metals (> 99.9%) of 10 mm 0 X 0.2 mm. Titanium dioxide with stoic~omet~c composition, prepared by oxidizing a titanium surface, was used as standard. The metals were polished with l/4 pm diamond paste before introduction into the vacuum chamber. The metal was annealed at 600” C in vacuum, and sputtered with 8 keV Ar+ ions until surface oxygen and carbon contaminations were completely removed. Then, the sample was bombarded with 8 keV Ar+ ions, produced in a duoplasmatron ion source, at room temperature. The bombarding angle was 25” from the surface normal The typical flux of the Ar+ beam was 2.5 X 1013 atoms cm-’ s-l (4 pA/cm2), monitored with a Faraday cup. The partial pressure of argon during the bomb~dment was 8 x lop5 Pa. During the bombardment, high-purity oxygen was introduced into the bombarding chamber up to a pressure of 1 X 10m3 Pa. The chemical states of the surfaces and the Ols/Me (Me = metal) ratios were measured by XPS at a pressure of 3 X lo-’ Pa, using Mg Ka X-ray (1253.6 eV) as the excitation source. The bring energy was calibrated using line of metallic gold, assumed to be 84.0 eV. the 4f,, 3. Results
Fig. 1 shows the XPS spectra in the Ti2p region for titanium before (a) and after (c) Ar+ ion bombardment in an oxygen atmosphere for 4 min. For
Y. Baba et al. / Atmospheric
Binding
oxygen on metal surfaces
energy
611
(eV)
Fig. 1. XPS spectra in the Ti2p region: (a) titanium metals, (b) titanium metal exposed to oxygen at a pressure of 1 x 10e3 Pa for 4 min, and (c) titanium metal bombarded with 8 keV Ar+ ions of 4 PA/cm2 in oxygen at a pressure of 1 X 10m3 Pa for 4 min.
the spectrum for the oxygen-adsorbed sample prepared without Ar+ ion bombardment is also shown as spectrum b. Although an appreciable amount of oxygen is adsorbed on titanium without bombardment, the metallic phase can still be observed in spectrum b. On the other hand, the metallic phase completely disappears after bombardment with Ar+ ions (spectrum c). The binding energy of the most intense peak in spectrum c is consistent with that of standard TiO,. The time dependence of the surface O/Ti ratio determined by XPS, both with and without Ar+ ion bombardments, are shown in fig. 2. The O/Ti ratio is normalized so that the 0 ls/Ti 2p ratio of standard TiO, is 2.0. In the case of the oxygen-adsorbed sample, the O/Ti ratio saturates to about 1.3 after 4 min. In this region, the rate of adsorption and desorption becomes equal. The ratio for the Ar+ ion bombarded sample saturates to 1.96. The difference from 2.0 is due to the existence of a small amount of Ti,O,. In order to investigate the depth profile of the oxide layers, titanium samples oxidized for 4 min with (a) and without (b) Ar+-ion bombardment were sputtered with 8 keV Ar+ ions of 10 PA/cm’. The fluence dependences of the 0 ls/Ti2p ratio are indicated in fig. 3. The 0 ls/Ti2p ratios exponencomparison,
Y. Baba et al. / Atmospheric oxygen on metal surfaces
612
/
I
/
I
OlTi 2.0
I
ratio
I
of bulk TiOp
-___________-__---__---------_-_ -
I
I
I
I
I
I
2
4
6
8
10
12
Time
(0)
14
(rnin)
Pig. 2. Time dependences of surface O/Ti ratio measured by XPS for titanium; (a) titanium bombarded with 8 keV Ar+ ions at 4 PA/cm* in oxygen at a pressure of 1 X 10e3 Pa, and (b) titanium exposed to oxygen at a pressure of 1 x 10m3 Pa without bombardment.
r
1
I
I
I
I
I
o
1
2
3
4
5
6
Fluence
( otoms /cm2 !
7
( xfo’5)
Fig. 3. Changes in the Ols/Ti2p ratio in XPS for titanium samples sputtered with 8 keV Ar+ ions of 10 pA/cm*; (a) titanium metal bombarded with 8 keV Ar+ ions of 4 pA/cm’ in oxygen at a pressure of 1 x10-’ Pa for 4 min, and (b) titanium exposed to oxygen at a pressure of 1 X 10e3 Pa for 4 mm without bombardment.
Y. Baba et al. / Atmospheric
oxygen on metal surfaces
613
tially decrease with the fluence of Ar+ ions. It is observed that the oxide layer prepared with Ar+-ion bombardment extends deeper in the target than the one without bombardment. Estimated from the sputtering yield of TiO1 by the 8 keV Arf ion bombardment [24] and the density of TiO,, the approximate thicknesses of the oxide layers are determined to be 5.0 nm for line a and 0.8 nm for line b. The range, perpendicular to the surface, of a 8 keV Ar+ ion in titanium, is calculated to be 6.1 nm using TRIM code [25]. The result suggests that the oxide layer produced by the Ar+ ion bombardment is formed around the track of the implanted Ar+ ions. For the purpose of sputter cleaning, the surface of titanium was bombarded with Ar+ ions. Thus, there is already an appreciable amount of defects in the surface before exposure to oxygen. However, the amount of surface oxide and its thickness produced by the adsorption is much smaller than those produced by Ar+ ion bombardment. Therefore, promotion of oxidation is caused by simultaneous bombardment during the oxygen exposure. This finding indicates that the defects or excited states, having a short life time, play important roles in the oxidation reaction. 3.2. Trapping of oxygen on various metals under Ar ’ ion bombardment Promotion of oxidation reactions is also observed for other transition-metal targets, as listed in table 1. In this table, the surface O/Me ratios for the samples exposed to oxygen for 4 min are indicated, because the ratios become almost saturated after 4 min exposure. Among the metals examined, promotions of the oxidation reactions for copper, silver and gold are negligibly small, but appreciable promotion is observed for the other metals.
4. Discussion The stability of the reaction product is one of the essential factors to discuss the gas trapping process. Such stability is inferred from the Gibbs energy of formation (AG) of the compounds of target metals and atmospheric gases. In the case of titanium target, it has been observed that atmospheric nitrogen is trapped under the same conditions as in the present experiment [20]. For comparison, trapping of hydrogen and carbon was investigated using hydrogen and methane, respectively, as atmospheric gases under the same experimental conditions. However, the formation of titanium hydride or carbide was not observed. The AG values of the titanium compounds, TiH,, TiN, TiO, and TIC are -37, -93, -229 and -46 kcal/mol, respectively [26]. The different trapping behavior of these gases suggests that an increase in the
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Y. Baba et al. / Atmospheric
Table 1 Saturated ratios of Ols/(MeZp, Target
Ti V Cr Fe Ni cu Y Zr Nb MO Ag W Au
oxygen on metal surfaces
3d or 4f)
Main product
Saturated Line
With bombardment
Without bombardment
Theoretical ratio
TiO,
Ols/Ti2p Ols/VZp 0 ls/Cr 2p Ols/FeSp Ols/Ni2p Ols/Cu2p Ols,‘Y3d Ols/Zr3d Ols/‘Nb3d 0 ls/Mo3d 0 ls/Ag3d Ols/W4f Ols/Au4f
0.71 0.55 0.36 0.26 0.095 0.005 0.60 0.63 0.56 0.32 0.004 0.13 0.005
0.48 0.30 0.15 0.22 0.077 0.005 0.56 0.41 0.19 0.10 0.003 0.072 0.000
0.72 0.74 0.37 0.27 0.14 0.12 0.69 0.78 0.84 0.88 0.079 0.83 0.24
v205 Cr203
Fe203 NiO cue w3
ZrO, NbzOs MOO, AgzO wo3 Au2o3
0 ls/(Me2p,
3d or 4f) ratio
The metal targets were exposed to 1 X lo-’ Pa 0, for 4 min with and without keV Ar+ ions at a fluence of 4 nA/cm2. The theoretical ratios are calculated tion cross-sections [23].
bombardment of 8 using photoioniza-
stability of the reaction product leads to a high trapping efficiency. This relation will be discussed more quantitatively for the trapping of oxygen. In the case of chemical adsorption, the rate of oxide formation is simply expressed as: dx/dt = k,(l
-x)
- k2x,
(1)
where x (0 s x s 1) is the surface coverage of the oxide, t (s) the time of oxygen exposure, and k, and k, (s-l) the rate constants of adsorption and desorption, respectively. The ratio of k,/k, is proportional to the sticking coefficient of oxygen at the metal surface. Note that eq. (1) only expresses the average value in the surface region, detected by XPS, i.e., about 1.5 nm from the surface. By solving eq. (l), we can obtain the oxide coverage at t:
(2) At the steady state, the saturated value of x is expressed by setting t + co in eq. (2): x(a)
= W(k
+ kz).
(3)
Although chemical adsorption of oxygen on a metal surface is essentially based on the chemical reactivity of the metal with oxygen, it also depends on the crystal structure of the surface, the surface roughness, and so forth. In the
615
Y. Baba et al. / Atmospheric oxygen on metal surfaces
‘,Oj TiO2
t -i r” G
80 ‘t cz
0.6 -
0.4 -
0.2-
.
1 NiO .
$02
c’$3
w5
.
Nb205 .
AG/y
(kcol/mol)
Fig. 4. Relation between saturated ratio of O/Me for metals exposed to oxygen at a pressure of 1 x lo-' Pa and Gibbs free energy of oxide formation at 298 K. The O/Me ratio is represented as the Ols/(MeZp, 3d or 4f) value in XPS divided by the theoretical ratio calculated using photoionization cross sections. The Gibbs free energy of formation is divided by y, assuming that the composition of the oxide is Me,O,,.
case of the thermal process, the reaction of the oxide formation is expressed in the following equation: xMe+(y/2)O,+Me,O,+AG.
(4)
The value of AG is related to the pressure of oxygen as: AG = -RT
lr1(1/[0~]~“)
= (y/2)RT
ln[Oz].
(5)
In order to compare the chemical reactivity of various metals with oxygen at the same pressure, we may compare the AG/y values of the oxides. Fig. 4 shows the relation between the saturated elemental surface O/Me ratio and the Gibbs energy of oxide formation, AG/y, for various metals. The O/Me ratio is obtained by dividing the observed 0 ls/(Me 2p, 3d or 4f) value by that calculated using the theoretical photoionization cross sections [23]. Oxygen hardly adsorbs on the metals with a AC/y value of the oxide larger than - 50 kcal/mol. When bombarding with Ar+ ions during oxygen exposure, we assume that the rate constant of oxide formation increases to k, + a (a 2 0), where (Y(s-l) is the number of oxides produced by Ar+ ion bombardment, depending on the
616
Y. Baba et al. f Atmospheric
oxygen on metal surfaces
flux of the Ar+ ion beam. It should be noted that the rate constant of decomposition of oxides also increases to k, + j3 (j3 2 0) due to the preferential sputtering of oxygen atoms. The value of j3 is related to the sputtering yield Y and the flux of the incident ion f as /3 = Yf. Thus, the eq. (3) can be rewritten as: x(co)=(k,+a)/(kl+a+k2+j3).
(6)
If the promotion of the oxidation reaction by Arc ions is observed, it is obvious that the value of ar is much larger than that of /3 for those metals. In addition, the sputtering yields of the present oxides by 8 keV Ar+ range from 0.2 to 1.7 [25]. This means that the large variation of the saturated O/Me ratio for various metals is mainly due to the difference in the value of (Y. With regard to the physical process, the promotion of oxide formation by Ar+ ion bombardment can be primarily attributed to knock-on collision by Ar+ ions on the surface. However, the contribution of the chemical reaction must be considered in order to interpret the large difference in the reaction rate of oxide formation between various metals. Kim et al. have found the relation between sputtering behavior and thermodynamical parameters for various oxides sputtered by rare-gas ions, assuming that the sputtering occurs in the thermal equilibrium region [9]. They have indicated that the standard Gibbs energy of formation of an oxide, AG, is related to the reduction of the oxide. In our case, both the formation and d~mposition occur simultaneously. The relation between the saturated value of the surface O/Me ratio and AG/y for various metals bombarded with Ar+ ions is shown in fig. 5. Compared with the result for oxygen-adsorbed samples (fig. 4), the relation between two values is clearly observed. There exists a threshold at about - 70 kcal/mol. Oxides with a AC/y value higher than -60 kcal/mol are hardly produced under the present condition, while most of the ratios of those with a AC/y value lower than - 80 kcal/mol are close to 1. Regarding the decomposition of oxides, the threshold of AG of the oxide for the reduction by Ar+ ion bombardment is in the range from - 60 to - 118 kcal/mol[9], In this case, the threshold of thermodynamical parameters exists, because the sputtering occurs in the local thermal ~ui~b~urn region produced around the track of the Arf ions, and we can assume that the sputtering occurs through thermal reactions at quite high temperatures. The present results for the threshold of oxide formation, ranging from -60 kcal/mol to - 80 kcal/mol, are close to those for reduction. This finding suggests that the formation of compounds by Ar+ ion bombardment also occurs through a thermochemical reaction near the thermal equilibrium region. Therefore, it can be concluded that the trapping of atmospheric gases in the surface induced by ion bombardment occurs through chemical reaotions at quite high temperatures near the track of the bombarding ions, rather than through physical
617
Y. Baba et al. / Atmospheric oxygen on metal surfaces
o.2[ ,,, !180 -160 -140
(
-120 -100
lx/y
:, -80
$“p -60
-40
Ikp2*J -20
0
20
( kcol/mol)
Fig. 5. Relation between saturated ratio of O/Me for metals bombarded with 8 keV Ar+ ions in oxygen at a pressure of 1 x 10e3 Pa and Gibbs free energy of oxide formation at 298 K. The O/Me ratio is represented as the Ols/(MeZp, 3d or 4f) value in XPS divided by the theoretical ratio calculated using photoionization cross sections. The Gibbs free energy of formation is divided by y, assuming that the composition of the oxide is MexO,.
processes such as knock-on collisions. The obtained relation between the thermochemical parameter of the reaction product and the trapping of atmospheric gas will be useful to estimate the extent of the gas trapping reaction by ion bombardment for other gas-solid systems.
5. Summary (1) Titanium bombarded with 8 keV Ar+ ions easily traps atmospheric oxygen, and nearly stoichiometric titanium dioxide was produced in the surface layer. The bombardment with AI-+ ions appreciably promotes the formation of the oxides, compared with that by oxygen adsorption. (2) Atmospheric oxygen was trapped by the Ar+ ion bombardment in metals with AC/y values lower than -80 kcal/mol, but not in those with AC/y values higher than - 60 kcal/mol. (3) The relation between the oxide formation and the thermodynamical parameter suggests that the trapping of atmospheric oxygen is attributed to a chemical reaction rather than to physical processes such as knock-on collisions by Ar+ ion bombardment.
Y. Baba et al. / Atmospheric
618
oxygen on metal surfaces
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