- Email: [email protected]
The initial oxidation of aluminum thin films at room temperature
SURFACE
SCIENCE 30 (1972) 263-279 0 North-Holland
THE INITIAL OXIDATION
OF ALUMINUM
Publishing Co.
THIN FILMS AT ROOM
TEMPERATURE Wm. H. KRUEGER* School
of Metallurgy
and S. R. POLLACK
and Materials Science University Pennsylvania 19104, U.S.A.
of Pennsylvania
Philadelphia,
Received 5 October 1971; revised manuscript received 8 December 1971 Thin film and ultra-high-vacuum techniques have been utilized in fabricating uncontaminated, atomically clean, aluminum surfaces in order to characterize the initial low temperature, low pressure oxidation kinetics of aluminum. The kinetics were followed gravimetrically by the quartz crystal microbalance technique and the aluminum-aluminum oxide surface was examined by utilizing electron microscopy and scanning electron microscopy techniques. It has been found that the kinetics of oxygen uptake associated with the initial oxidation stage cannot be. adequately described by present chemisorption theory. An oxygen uptake model based on an “incorporationchemisorption” transition is proposed to explain the “stable layer” associated with the aluminum oxygen reaction at room temperature. The kinetics of uptake associated with the incorporation reaction is characterized as a zero order reaction with an oxygen sticking coefficient of approximately 0.03. In contrast, the kinetics of the chemisorption range cannot distinguish between dissociative chemisorption or immobile molecular adsorption on adjacent vacant sites. The proposed “incorporationchemisorption” model is consistent with low energy electron diffraction, resistivity and contact potential difference measurements previously reported in the literature and removes the apparent ambiguities regarding the interpretation of the initial oxygen uptake measurements.
1. Introduction It has been only very recently that a few independent studies have been undertaken to elucidate the processes involved in the initial stage of the low temperature aluminum-oxygen oxidation reaction. These studies have been performed by Huber and Kirkl), Jona2), and Roberts and Wellss). The conclusions obtained by these authors from their respective measurements have lead to some controversy as to the mechanism of the initial stage of oxidation. Based on the fact that above an exposure of 2.3 x 10e6 torr-min an aluminum surface at room temperature becomes comparatively non-reactive as judged by weight gain and resistivity changes, Huber and Kirk have *Submitted in partial fulfillment of the requirements and Materials Science, University of Pennsylvania. 263
for the Ph. D. degree of Metallurgy
264
W. H. KRUEGER
AND
S. R. POLLACK
suggested that this behavior is indicative of the completion of a chemisorbed layer of oxygen atoms and that this layer occurs before subsequent oxidation proceeds. Simultaneously measured contact potentials indicated a sharp minimum of only 0.05kO.02 eV. Therefore, Huber and Kirk have made the further suggestion that the newly created surface might be described as a surface composed of alternating ~lnrni~~rn-oxygen dipoles. However, more recently LEED work by Jona has indicated that clean (I IO), (100) and (111) surfaces of aluminum exposed to low oxygen pressures give rise to an amorphous layer of oxide as defined by low energy electron diffraction theory. Roberts and Wells have reported data which they believe are at variance with Huber and Kirk’s suggestion that an initial stable confi~ration occurs before subsequent oxidation. The former have exclusively monitored the contact potential of an aluminum surface during its interaction with oxygen at various temperatures. These authors have observed that upon oxygen exposure the contact potential of an aluminum surface held at - 196°C immediately increases and remains stable whereas for a surface held at - 183°C the contact potential increases to a lesser degree and then slowly decreases, Roberts and Wells believe that this contact potential behavior is proof that even at - 183°C the chemisorbed layer is unstable and that significant i~eo~oration or oxidation occurs at submonolayer coverage. They argue that in increasing the temperature of the film from - 196 “C to - 183°C sufficient energy has been provided to allow relatively facile entry of oxygen into the sub-surface region and hence the decrease in the work function. Based on these low temperature studies Roberts and Wells conclude that nothing distinctive should occur upon the initial exposure of an aluminum surface to oxygen at room temperature and that there should not be a stable chemisorbed layer phenomena as reported by Huber and Kirk. In this work, accurate mass adsorption measurements of oxygen by ultrahigh vacuum prepared aluminum thin films held at room temperature have been made, Various chemisorption models were investigated to analyze both the oxygen weight gain versus time dependence and the first time derivative of the oxygen weight gain versus the time of exposure. It has been found that the kinetics of the oxygen uptake associated with the initial oxidation stage cannot be adequately described by present chemisorption theories. An transition is proposed to explain the “incorporation-chemisorption” rapidly achieved “stable layer” associated with the room temperature aluminum oxygen reaction. 2. Theory On theoretical grounds our present understanding
of the initial reaction
INITIAL OXIDATION OF Al THTN FILMS
265
between a metal surface and a gas phase can be considered only as a starting point. In general, the dominating factor which accounts for the rapid decrease in the rate of the reaction is not known. Following Trapnell the current theory of chemisorption may be expressed as 4,
dNa __ = KPaf(B)exp dt
where dN,/dt is the rate of atoms adsorbed per unit area per unit time; * is the rate of collision of molecules with a surface per unit area per unit time. This quantity is obtained from kinetic gas theory where P is the pressure, m the mass of the particle in the gas phase, k is the ISoltzmann factor, T the temperature in “K, E(B) is the activation energy, d is a “probability factor” or “condensation coefficient”, f(8) is the probability that a gas molecule collides with a vacant site and S(0) is the sticking coefficient. Current theory of chemisorption has assumed that the adsorbed species remain on the surface and thus the rate of uptake must immediately decrease with uptake. However, the present initial oxidation study strongly indicates that the assumption that the adsorbed species remains on the surface is not necessarily valid. For this reason a further distinction will be made with regard to the initial oxidation stage. The initial oxidation stage will be subsequently referred to as either a “normal” chemisorption reaction or a ehemisorption process with incorporation. The precise meaning of these terms will now be discussed. ~=Pl~2~~kT~
2.1. “NORMAL" CHEMISORPTION The term “normal” chemisorption will be used here when referring to the additional requirements on eq. (l), such that the total number of vacant sites is fixed and that significant adsorption can only occur upon these sites. The first requirement is simply that Na+N,=No, (2) where N, is the number of sites/cm2 occupied by adsorbed particles, NVis the number of sites vacant/cm’, and NOis the total number of sites/cm’ initially available. The second requirement is met by describing the f(f3) term in eq. (1) in terms of fractional coverage where e = NJ&.
(3)
Thus a coverage of unity refers to the completion of the monolayer whereby each available site is occupied by an adsorbed particle.
266
W.H.
KRUEGER
AND
S.R.POLLACK
I3y %ormal” chemjso~jon we preclude the ~oss~b~Iity of the ~e~erat~o~ of fresh adsorbent sites via a rapid incorporation mechanism, or if an incorporation mechanism does occur, the resultant new site remains inactive in the subsequent chemisorption. Thus, a site is either active for an adsorption occurrence or it is completely de-activated by a previous adsorption. For “normal” chemisorption at least two separate types of chemisorption may be treated. In the first, the molecule is adsorbed on a single site. Although it is felt by most authors that this process does not occur, a molecule occupying a single surface site would lead to a f 1 -a) dependence for j’(4). In the second type of chemiso~tion~ the molecule dissociates into two radicals each of which occupies one site. If it is assumed that the resultant two radicals spread out through forces of repulsion and there is no ~~teract~o~ between the radicals of the adsorbed molecule, thef(8) term is simply (1 - 8) 2.
INITIAL OXIDATION OF Al THIN FILMS
267
measurement since an atom adsorbed on the surface or incorporated into the lattice yields the same increment in weight uptake. Work function measurements done simultaneously with weight gain measurements should be able to verify the above transition from a chemisorption with incorporation range to a “normal” chemisorption range. The present ‘“chemisorption-incorporation” model &lies upon the recent ideas of Mott and Fehlnere) in describing the initial ypid oxidation of metals at low tem~ratures. The model that they have suggested involves the image force on the chemisorbed ion and a place exchange mechanism to account for the rapid rate of oxidation. They suggest that the activation energy required for the movement of an ion into the metal is substantially reduced by this image force pulling the ion toward the metal. Assuming the activation energy for place exchange is approximately equal to the relative strength of the average metal bond on the surface, the image force may or may not be sufficient to break the bond and allow the exchange mechanism to occur. If the image force is sufficient, then rapid oxidation will occur. The extent of further rapid oxidation will then the determined by be relative bond strength of the stable compound as compared to the image force on the ion. 3. Experimental procedures 3.1. FABRICATION SampIe preparation and mass measurements were carried out in a stainless steel bakeable high vacuum system7) capable of operation at 1 x 10-r’ tori-. A vacuum gas analyzer was permanently fixed to the vacuum chamber for residual gas analysis. Mass measurements were carried out using a dual quartz crystal oscillator microbalance*). One of the crystals is used as a reference crystal while the other serves as a substrate for the aluminum film and subsequent chemisorption measurements. Gold was deposited on one side of each crystal for purposes of electrical contacts. The microbalance head was imbedded in a 1.5 kg copper heat sink. This procedure was required to maintain temperature stability. An accuracy of 1_ 1 Hz was obtained during the time of the mass measurements. This uncertainty in the frequency shift corresponds to a mass sensitivity of f 5 x 10M9g/cm’. Assuming the freshly deposited aluminum had a surface roughness of two, a monolayer of oxygen atoms on each available site would weigh approximately 7.0 x IO-* g/cm’. Thus the sensitivity of + 1 Hz would correspond to slightly better than rt one-tenth of a monolayer coverage. This accuracy was obtained only after the adoption of a special two-fold technique for selecting the quartz crystals. First, adjacent crystals
268
W. H.
KRUEGER AND
S. R. POLLACK
from the same as-cut batch were used in pairs. This insured that the reference crystal and substrate crystal had the same temperature coefficients and orientation. Secondly, sufficient gold was deposited on each crystal so that after deposition of the fresh aluminum onto the substrate crystal, the difference in resonance frequency was between 5 and 10 kc. Although, the manufacturer of the microbalance unit states that the crystals may be used over a 100 kc range; for chemisorption measurements, meaningful data could be obtained only over the narrower frequency range. High purity aluminum (0.99999 pure) was evaporated from a 0.020 inch thick tungsten evaporation boat in order to form a fresh aluminum surface on both the quartz substrate and a Corning 7059 glass substrate. The pressure in the vacuum system prior to evaporation was approximately 2 x lo-” torr. During the evaporation the pressure in the system increased to 5 x lo-* torr then decreased to a stable pressure of 5 x lo-’ torr. Upon shutting the power off the pressure dropped to approximately 2 x lo-’ torr but in l-2 min it fell to about 5 x 1O-1o torr and was still decreasing. By the time approximately 20 min had elapsed the pressure was again near 2 x lo-” torr and the microbaIan~e output was a constant frequency. At this point pure oxygen* was leaked into the system to the desired pressure through a servooperated leak valve. The mass gain of the films was monitored continuously by the use of a digital frequency counter. 3.2. ALUMINIUMSURFACE ROUGHNESS Since the aluminum films prepared in this study are polycrystalline, it is essential to assess the actual surface area exposed to the oxidizing atmosphere. This information was obtained by the use of scanning electron microscopy and extraction replica techniques in conjunction with electron microscopy. Scanning electron micrographs of aluminum surfaces deposited on quartz substrates showed no distinctive difference to micrographs of the cleaved quartz substrates. Extraction replicas of the aluminum films produced by initially evaporating a thin layer of carbon, shadowing with platinum, and dissolving the underlying aluminum films which had been deposited on 7040 Dow Corning glass substrates and quartz crystal substrates exhibited no significant surface roughness at magnification as high as 240000. Typical micrographs obtained from the scanning electron microscope and extraction replicas are shown in figs. 1 and 2, respectively. It is believed that the replicating techniques used in this work gave a resolution of about 40-50 A. Thus, roughness on a finer scale could not be directly observed. However, Swaine and Plumbs) have shown that aluminum *Medical grade 0% was passed through a dry ice and acetone cooled trap to remove water vapor.
INI’MAL
OXIDATION
OF
Al TRIN
FILMS
269
films deposited at room temperature at normal incidence on glass substrates have a roughness factor between 1.OOand 1.15, Based on Swaine and Plumb’s surface measurements and the similarity of extraction replicas from both the quartz substrates and glass substrates of this work, it is believed that the roughness of the aluminum films is mainly due to the roughness of the intial quartz substrate.
Fig. 1. Scanningelectron micrograph of aluminumthin film surface (magnification10000). 4. Results snd discussion
The typical initial “dry Uzf’ uptake is shown in figs. 3 and 4. In fig. 3 the mass adsorption in terms of weight gain and frequency shift is plotted versus the logarithm of the oxidation time. The difference in magnitude of the final mass uptake of the two curves in fig. 3 is due to a difference in the initial surface roughness of quartz substrates chosen from different batches. In fig, 4 the weight gain due to adsorption of oxygen by aluminum films is plotted far two different low “dry Oa” pressures. In fig. 5 the sticking coefficient of the adsorption of oxygen is plotted as a function of total oxygen weight gain. From the mass data obtained at these low pressures of “dry 02*’ exposure,
Fig. 2.
175
Extraction replica micrograph of aluminum thin film surface (magnification 24ptoW).
am )rpms/cm2
150
lr5
100
075
,050
,025
*
i
aalafr?
La”A~lJ Minutrs
Fig. 3.
Chemisorption
”
of oxygen by aluminum thin films deposited having different surface rnugbness.
100
an quartz crystals
INITIAL
OXIDATION
OF
Al
THIN
271
FILMS
two conclusions may be reached with regard to the initiai room temperature oxidation. (1) The rery initialuptake of oxygen by a “clean” aluminum film exposed to low oxygen pressure (5 x 10 -6 and 5 x lo-’ torr 0,) indicates that the weight of uptake varies linearly with both time and pressure. (This dependence is shown clearly in the insert of fig. 4. The dashed line in the insert
5
LO
15
20
25
30
Minutes Fig. 4.
Oxygen chemisorption
by aluminum thin films at two different pressures.
corresponds to the predicted initial slope at 5 x 10b6 torr using the data at 5 x IF7 torr and assuming a linear dependence on pressure. Only one data point could be obtained for the initial uptake at 5 x lo-” torr since the reaction proceeded very rapidly.) (2) Over the above mentioned pressure range an abrupt change in rate of uptake occurs and the degree of abruptness strongly indicates the completion of a %table layer.” Huber and Kirk have suggested that the arbitrary extrapolations in fig, 3 correspond to the completion of a stable monolayer of chemisorbed oxygen atoms. That is, the frequency shift of 16.5 Hz (in the top curve of fig. 1) corresponds to the weight of one oxygen atom per available site on the aIumin~m surface. An average density of adsorption sites of 1.13 x 101‘/cm2 (based on an equal distribution of the three main crystallographic planes) along with the value of 16.5 Hz as the monolayer of uptake implies a surface roughness of 2.7. This value appears to be consistent with a 2-3 surface
272
W. H. KRUEGER
AND
S. R. POLLACK
roughness indicated by the scanning micrographs as shown in fig. 1. Thus, it does seem reasonable to analyze the “layer effect” in terms of a chemisorption process characterized by a rapid slow down in oxygen uptake as the number of available adsorption sites are depleted. Explicit in attempting to describe the oxygen uptake in terms of a chemisorption process is the fact that the rate limiting mechanism in the weight gain kinetics is the attachment of the oxygen molecules to the surface. Sticking
Coefflclent
P=5xlO-‘Torr
,001
r
vs. ‘Dry
Op”
Weight 23*
Gain
O2
.
0 1030;
Thick
5OObin
0
10306
Thick
SOOb/Min
i) *
l
of
c
0
Fig. 5.
Sticking coefficient of oxygen as a fuction of total oxygen weight gain.
A computer program was written to facilitate the calculation of the first derivative of the weight gain versus time, since in most cases it provides a more sensitive test of the functional dependence in chemisorption than the direct weight gain itself. Thus two quantitative requirements must be satisfied in the analysis of the oxygen adsorption data : (‘I) The chemisorption model must describe the oxygen uptake versus time curves. (2) The chemisorption model should describe the rate of oxygen uptake versus total uptake. The second requirement is met if the model describes either the sticking
coefhcient versus 8 de~nden~e d@/dt versus B dependence. 4.2. PROPOSBD MODEL
FOR
or equivaI~~~y in the case of aluminum the
Al-0 REACTrON
It has been found that the alumin~m~xy~e~ chemisor~tion data at room temperature could not be adequately described by the current theory of chemisorption if we demanded that the model describe both the uptake versus time and the rate of uptake versus time requirements. The major difliculty in trying to obtain a ~h~misorption model which accurately describes the data has been the fact #hat the initial rate of oxygen uptake does not decrease as rapidly as expected from current theory. A detailed analysis of various current chemisorption models has been found to fail in describing the present data, They will be briefly mentioned before proposing a model which most accurately describes the adsorption data observed for aluminum. “Normal’” homogeneous chemis~~tio~ models which have been explicitly demanstrated to fail in describing the data included: (I) First and second order, con-activated adsorption models. (2) Activated adsorption models including an f(e) dependence of (I- 0) or (1 -Q2, and an activation energy dependent upon coverage. It has been found that the previous analysis of the oxygen uptake of alumi~~rn thin films utilizing the Elovich equation has been performed incorrectly and that the correct application of this phenomenologica~ equation does not realistically describe the oxygen-aluminum reaction at room temperature. A discussion of the application of the Elovich equation in the ease ~F~l~rnin~rn thin films can be found eIsewherelQ). It has also been found that the “normal” heterogeneous ~hemis~rpti~~ model envisioned by TrapneIl is incapable of describing the adsorption of oxygen on the aluminum films. It is admitted that perhaps other “normal”’ heterogeneous models may be postulated to describe the mass uptake data. However, at the present time there are two reasons why this approach does not appear reasonable for the room temperature data. (1) Weight gain measurements for the films prepared in this study were reproducible and are also in excellent agreement with the mass measurements by Huber and Kirk. If the surface was heterogeneous with a significant number of special sites, such as kinks or ledges, one would expect diKerent mass measurements as a result af either the thickness of the metal film or conditions of fabrication. (2) Contact potential measurements of Huber and Kirk indicate that oxygen does not remain on the surface during the initial range of uptake, since the contact potential of the Al-0 surface does not immediately increase,
274
W. H. KRUEGER
AND
S. R. POLLACK
For the above reasons the approach of describing the data in terms of a simple, one step process described by “normal” chemisorption has been abandoned in favor of the present model. The model proposed for describing the initial oxidation involves a transition from an initial rapid incorporation range to a much slower “normal” chemisorption range which ultimately leads to the observed ‘“stabie layer effect”. The ~n~or~orat~on range is best described by the following rate equation 2K’SFN* dnr, - = -__lll..l--= constant ’ ~~~~kT~~
cv
dt
where dN,/dt is the number of oxygen atoms adsorbed per cm’ per min, is the number of molecules hitting per cm’ per min, S is the sticking coefficient, K’ is a constant dependent on the equilibrium between physisorbed oxygen and gas phase oxygen, and N, is the number of cation sites available for adsorption. It is proposed that the initial rate of oxygen rate of oxygen uptake is not hnited by the depletion of available cation sites. An atom&tic model suggested to account for the constant level of adsorption sites, N,, is a rapid piace exchange as shown in fig. 6.
P/(2nmkT)t
0
M
M
=
M
M
M
0
0
M
M
M
M
M
M
M
0
M
M
M “--+
M
M
M
M Fig. 6.
M
Model for incorporation.
The above, ~~~arentIy non-activated, incorporation occurs r~~~o~~~ over the entire surface during the ‘“incorporation range”. It is seen from the above model that the adsorption of an oxygen atom does not remove an adsorption site since the place exchange creates another fresh cation site. The newly generated cation site is assumed to have essentially the same adsorption characteristics but it cannot undergo another subsequent place exchange. The reason for this is the fact that this adsorption site is no longer bonded by just AI-AI bonds but an Al-0 bond formed by the previous exchange. Thus, incorporation via place exchange will continue to occur, generating new adsorption sites, untif a fresh stabihzed surface of aluminum is formed. At the completion of the stable surface, the subsequent chemisorption should be described in terms of “normaf” chemisorption theory since fresh sites will no longer be generated.
INITIAL
OXIDATION
OF
Al
THIN
FILMS
215
Fehlner and Motts) have suggested that the driving force for the place exchange mechanism is the simple electrostatic image force pulling the easily ionized oxygen atom into the metal lattice. These authors have proposed that place exchange is apparently non-activated since the image force on the oxygen ion is approximately as large as the average metal-metal bond existing on the surface atom. It is admitted that this picture is quite naive since screening by the electrons has been neglected in their estimation of the magnitude of this image force. However, the assumption of the place exchange mechanism regardless of its driving force will be shown to be reasonable in describing the present data. In order to describe the oxygen uptake in terms of the proposed model it is necessary to redefine the meaning of the extrapolated point on the curves in fig. 3. The intersection of the extrapolated curves are indicative of the “stable layer” but they do not correspond to one monolayer of coverage as suggested by Huber and Kirk l). Assuming that the initial constant rate of uptake in fig. 5 is due to chemisorption with incorporation, the data has been replotted in fig. 7 where the abrupt change in rate of uptake versus total uptake has been assumed to correspond to the transition from chemisorption with incorporation to “normal” chemisorption. It is seen that approximately 1.58 x 10” atoms of oxygen are rapidly adsorbed as a result of incorporation before the rate of uptake abruptly decreases. If these 1.58 x IO” oxygen atoms have undergone a rapid incorporation, the newly created surface, as shown in fig. 6, can be referred to as having zero coverage. Accordingly, the various theoretical curves in fig. 7 correspond to the subsequent “normal” chemisorption models. The total oxygen uptake of the films can be expressed in terms of 6 if it is understood that this quantity is the sum of oxygen atoms chemisorbed and incorporated plus the oxygen atoms adsorbed through the “normal” chemisorption stage. As would be expected, it is readily seen in fig. 7 that an adsorption model based on the adsorption of one 0, molecule per site does not describe the rapid decrease in the rate over the “normal” chemisorption range. The reason that the rate of uptake for this model is much greater than the observed data is the fact that a monolayer of 0, molecules is equivalent to two monolayers of oxygen atoms. Thus, the predicted stable layer of O2 molecules does not occur until an additional uptake of two monolayers of oxygen atoms are added to the previous incorporated uptake. The two additional curves correspond to the rate of uptake predicted by a zero interaction dissociative adsorption model and a model of immobile molecular adsorption on pairs of adjacent sites as proposed by Roberts5). It can be seen from fig. 7 that both of these models reasonably describe the
276
W. H. KRUEGER
AND
S. R. POLLACK
rapid fall off in uptake characteristic of the “layer effect”. The mobile layer, dissociative adsorption model predicts a greater amount of uptake since a full monolayer of uptake should occur in addition to the incorporated oxygen atoms. The model based on the adsorption of molecules on pairs of adjacent sites predicts an additional coverage of only 0.92 0 since approximately 8% of the surface will contain non-adjacent bare sites. Both theoretical models fail to describe the experimental data at high surface coverage
7V
Oxygen
atoms
1.26 I
1.68
I
‘he!$$$ptiok&~ Incorporation I
dt
adsorbed 2.10
I
I
Fig. 7.
Comparison
.6%
I
2.94
,
= 0.35 (p (8) (after
Dissociative interaction atoms.
ld5fcm2
I
1 3.36
t
“Normal” Chemtsorption
Immgbile adsgrgtion on 0 locent sit s.
+’
(r( 2.62
Roberts)
of molecule
adsorption, no between adsorbed
i
1
i
.86 1.06 1.28 1.46 Monolayers of oxygen atoms
r, , I.68
of the experimental rate of oxygen uptake by aluminum thin films with various chemisorption models.
since they do not take into account the effect of the repulsive forces between the ionized oxygen atoms as they are packed more densely on the surface. However, the degree of fit over a large portion of the “normal” chemisorption range is quite remarkable and also indicates that approximately 60-70x of a monolayer of oxygen atoms is adsorbed before the repulsive forces must be considered.
INITIAL. OXiDATlON OF
Al THIN
FILMS
277
From the present data it is not possible to determine which of the above models is the true adsorption model. At high coverages the rate of uptake associated with the undissociative adsorption model appears to be closer to the observed data but it is felt that a distinction cannot be made between the two models on this basis. The kinetics of uptake versus time predicted by these non-activated models is shown in fig. 8 along with the experimental data. The curve corresponding to the immobile molecular adsorption model crosses the mobile curve at @= I .5, since this model predicts only 0.92 oxygen coverage on the surface in addition to the incorporated oxygen. I3 I.8
c
,5.4-’
po2=
5x
to“’ torr 23VJ
; k"Normol
”
Chemisorpti~
Range
(8 ZO.75)
P
Chemis~rption with incorporation
2
i
I,
4
6
t 8
M TtME
I
,
,
,
Range 18 =O-,751 J
,
12 14 16 I8 20 22 (minutes)
Fig. 8. Comparison of oxygen uptake by aluminum thin films with model based on a transition from a chemisorption with incorporation range to a c‘normal” cbemisorption range,
If the above description of the “stable layer effect” is correct, then the arbitrarily extrapolated curves in fig. 3 correspond not to a monolayer of coverage as suggested by Huber and KirkI) but rather closer to approximately 1.5 fayers of oxygen atoms. The rapid slowdown in uptake indicated by these extrapolated curves is due to an initial inco~oration which stabilizes the Af surface folfowed by a “normal” chemisorption process, Based on
278
W.H.KRUEGER
AND S.R.POLLACK
the experimental and theoretical fit in fig. 8 approximately three-fourths of a monolayer of oxygen is rapidly incorporated into the second layer of the surface and an additional three-fourths is “normally” chemisorbed before the surface becomes relatively unreactive towards further adsorption. If the extrapolated point of 16.5 Hz corresponds to 1.5 9 and the average density of sites is the same for both the original cation sites and generated sites, the surface roughness of the films is calculated as 1.9. The proposed model involving a transition from incorporation to “normal” chemisorption is not inconsistent with the LEED work of Jonas). The net effect of oxygen exposure will be a slow deterioration of the original aluminum diffraction spots as the initial oxygen atoms first randomly incorporate into the sub-surface and the final stage of the ~hemisorption generally weakens the intensity of the remaining diffraction peak. Thus the resultant structure of the “stable layer” would be best described as amorphous. The contact potential behavior observed by Huber and Kirk is also understood in terms of the proposed model of chemisorption. A small decrease in the contact potential will be expected for the initial oxygen exposure since the oxygen will be rapidly incorporated beneath the fresh cation sites and thus create a positive surface potential. As the normal chemisorption process begins to play a more dominant role in the adsorption, the contact potential will begin to increase since the ele~tronegativity of the surface oxygen will produce a negative surface potential. Finally, the metal surface will begin to take on the properties of the more stabfe oxide and the contact potential will continue to increase with further exposure as observed by Huber and Kirk. The proposed model also helps in understanding the resistivity data of Huber and Kirk. The sharp break in the resistivity should occur before the break in the mass uptake curve since it would be expected that the incorporated oxygen atoms would have a greater effect on the resistivity than the subsequentIy adsorbed atoms on the surface. The contact potential measurements of Roberts and Wells3) at - 196°C and - 183°C suggest that the incorporation process is activated. However, the energy associated with this step is su~~iently small, that at room temperature it will not be significant. At room temperature rapid incorporation occurs until the surface becomes stabilized for “normal” chemisorption to occur. In summary it appears that the rapid decrease in oxygen uptake by aluminum films at low pressures (5 x 10m7 torr to 5 x lO-‘j torr) is indicative of the completion of a stable surface configuration. The kinetics of oxygen uptake can be described remarkably well in terms of the proposed “incortransition. The initial incorporation range is poration-chemisorption”
INITIAL
OXIDATION
OF
Al
THIN
FILMS
279
characterized by a zero order reaction with an oxygen sticking coefficient of approximately 0.03. The “normal” range of chemisorption, where the surface is characterized as stable with respect to the incorporation mechanism, can be described in terms of either a dissociative chemisorption or an immobile molecular adsorption on adjacent vacant sites. It is believed that the exact mechanism accounting for the rapid fall off occurring in the “normal” range cannot be determined from the present study. The proposed incorporation-chemisorption model qualitatively accounts for the previously observed resistivity and contact potential measurements and is not inconsistent with recent low energy electron diffraction studies. Acknowledgements This paper is a publication of the Laboratory for Research on the Structure of Matter, University of Pennsylvania. The authors gratefully acknowledge the support from contract DAHCl567C0215 of the Advanced Research Projects Agency, Office of the Secretary of Defense, for a Fellowship for one of the authors. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)
E. E. Huber, Jr. and C. T. Kirk, Jr., Surface Sci. 5 (1966) 447. F. Jona, J. Phys. Chem. Solids28 (1967) 2155. M. W. Roberts and B. R. Wells, Surface Sci. 15(1969) 325. D. 0. Hayward and B. M. W. Trapnell, Chemisorption, 2nd ed. (Butter-worth, London, 1964)p.91. J. K. Roberts, Proc. Cambridge Phil. Sot. 34 (1938) 399. F. P. Fehlner and N. F. Mott, Oxidation of Metals 2 (1970) 59. Varian Associates. Westinghouse Scientific Instruments. J. W. Swaine and R. C. Plumb, J. Appl. Phys. 33 (1962) p. 2378. Wm. H. Krueger and S. R. Pollack, Surface Sci. 30 (1972) 280.
SCIENCE 30 (1972) 263-279 0 North-Holland
THE INITIAL OXIDATION
OF ALUMINUM
Publishing Co.
THIN FILMS AT ROOM
TEMPERATURE Wm. H. KRUEGER* School
of Metallurgy
and S. R. POLLACK
and Materials Science University Pennsylvania 19104, U.S.A.
of Pennsylvania
Philadelphia,
Received 5 October 1971; revised manuscript received 8 December 1971 Thin film and ultra-high-vacuum techniques have been utilized in fabricating uncontaminated, atomically clean, aluminum surfaces in order to characterize the initial low temperature, low pressure oxidation kinetics of aluminum. The kinetics were followed gravimetrically by the quartz crystal microbalance technique and the aluminum-aluminum oxide surface was examined by utilizing electron microscopy and scanning electron microscopy techniques. It has been found that the kinetics of oxygen uptake associated with the initial oxidation stage cannot be. adequately described by present chemisorption theory. An oxygen uptake model based on an “incorporationchemisorption” transition is proposed to explain the “stable layer” associated with the aluminum oxygen reaction at room temperature. The kinetics of uptake associated with the incorporation reaction is characterized as a zero order reaction with an oxygen sticking coefficient of approximately 0.03. In contrast, the kinetics of the chemisorption range cannot distinguish between dissociative chemisorption or immobile molecular adsorption on adjacent vacant sites. The proposed “incorporationchemisorption” model is consistent with low energy electron diffraction, resistivity and contact potential difference measurements previously reported in the literature and removes the apparent ambiguities regarding the interpretation of the initial oxygen uptake measurements.
1. Introduction It has been only very recently that a few independent studies have been undertaken to elucidate the processes involved in the initial stage of the low temperature aluminum-oxygen oxidation reaction. These studies have been performed by Huber and Kirkl), Jona2), and Roberts and Wellss). The conclusions obtained by these authors from their respective measurements have lead to some controversy as to the mechanism of the initial stage of oxidation. Based on the fact that above an exposure of 2.3 x 10e6 torr-min an aluminum surface at room temperature becomes comparatively non-reactive as judged by weight gain and resistivity changes, Huber and Kirk have *Submitted in partial fulfillment of the requirements and Materials Science, University of Pennsylvania. 263
for the Ph. D. degree of Metallurgy
264
W. H. KRUEGER
AND
S. R. POLLACK
suggested that this behavior is indicative of the completion of a chemisorbed layer of oxygen atoms and that this layer occurs before subsequent oxidation proceeds. Simultaneously measured contact potentials indicated a sharp minimum of only 0.05kO.02 eV. Therefore, Huber and Kirk have made the further suggestion that the newly created surface might be described as a surface composed of alternating ~lnrni~~rn-oxygen dipoles. However, more recently LEED work by Jona has indicated that clean (I IO), (100) and (111) surfaces of aluminum exposed to low oxygen pressures give rise to an amorphous layer of oxide as defined by low energy electron diffraction theory. Roberts and Wells have reported data which they believe are at variance with Huber and Kirk’s suggestion that an initial stable confi~ration occurs before subsequent oxidation. The former have exclusively monitored the contact potential of an aluminum surface during its interaction with oxygen at various temperatures. These authors have observed that upon oxygen exposure the contact potential of an aluminum surface held at - 196°C immediately increases and remains stable whereas for a surface held at - 183°C the contact potential increases to a lesser degree and then slowly decreases, Roberts and Wells believe that this contact potential behavior is proof that even at - 183°C the chemisorbed layer is unstable and that significant i~eo~oration or oxidation occurs at submonolayer coverage. They argue that in increasing the temperature of the film from - 196 “C to - 183°C sufficient energy has been provided to allow relatively facile entry of oxygen into the sub-surface region and hence the decrease in the work function. Based on these low temperature studies Roberts and Wells conclude that nothing distinctive should occur upon the initial exposure of an aluminum surface to oxygen at room temperature and that there should not be a stable chemisorbed layer phenomena as reported by Huber and Kirk. In this work, accurate mass adsorption measurements of oxygen by ultrahigh vacuum prepared aluminum thin films held at room temperature have been made, Various chemisorption models were investigated to analyze both the oxygen weight gain versus time dependence and the first time derivative of the oxygen weight gain versus the time of exposure. It has been found that the kinetics of the oxygen uptake associated with the initial oxidation stage cannot be adequately described by present chemisorption theories. An transition is proposed to explain the “incorporation-chemisorption” rapidly achieved “stable layer” associated with the room temperature aluminum oxygen reaction. 2. Theory On theoretical grounds our present understanding
of the initial reaction
INITIAL OXIDATION OF Al THTN FILMS
265
between a metal surface and a gas phase can be considered only as a starting point. In general, the dominating factor which accounts for the rapid decrease in the rate of the reaction is not known. Following Trapnell the current theory of chemisorption may be expressed as 4,
dNa __ = KPaf(B)exp dt
where dN,/dt is the rate of atoms adsorbed per unit area per unit time; * is the rate of collision of molecules with a surface per unit area per unit time. This quantity is obtained from kinetic gas theory where P is the pressure, m the mass of the particle in the gas phase, k is the ISoltzmann factor, T the temperature in “K, E(B) is the activation energy, d is a “probability factor” or “condensation coefficient”, f(8) is the probability that a gas molecule collides with a vacant site and S(0) is the sticking coefficient. Current theory of chemisorption has assumed that the adsorbed species remain on the surface and thus the rate of uptake must immediately decrease with uptake. However, the present initial oxidation study strongly indicates that the assumption that the adsorbed species remains on the surface is not necessarily valid. For this reason a further distinction will be made with regard to the initial oxidation stage. The initial oxidation stage will be subsequently referred to as either a “normal” chemisorption reaction or a ehemisorption process with incorporation. The precise meaning of these terms will now be discussed. ~=Pl~2~~kT~
2.1. “NORMAL" CHEMISORPTION The term “normal” chemisorption will be used here when referring to the additional requirements on eq. (l), such that the total number of vacant sites is fixed and that significant adsorption can only occur upon these sites. The first requirement is simply that Na+N,=No, (2) where N, is the number of sites/cm2 occupied by adsorbed particles, NVis the number of sites vacant/cm’, and NOis the total number of sites/cm’ initially available. The second requirement is met by describing the f(f3) term in eq. (1) in terms of fractional coverage where e = NJ&.
(3)
Thus a coverage of unity refers to the completion of the monolayer whereby each available site is occupied by an adsorbed particle.
266
W.H.
KRUEGER
AND
S.R.POLLACK
I3y %ormal” chemjso~jon we preclude the ~oss~b~Iity of the ~e~erat~o~ of fresh adsorbent sites via a rapid incorporation mechanism, or if an incorporation mechanism does occur, the resultant new site remains inactive in the subsequent chemisorption. Thus, a site is either active for an adsorption occurrence or it is completely de-activated by a previous adsorption. For “normal” chemisorption at least two separate types of chemisorption may be treated. In the first, the molecule is adsorbed on a single site. Although it is felt by most authors that this process does not occur, a molecule occupying a single surface site would lead to a f 1 -a) dependence for j’(4). In the second type of chemiso~tion~ the molecule dissociates into two radicals each of which occupies one site. If it is assumed that the resultant two radicals spread out through forces of repulsion and there is no ~~teract~o~ between the radicals of the adsorbed molecule, thef(8) term is simply (1 - 8) 2.
INITIAL OXIDATION OF Al THIN FILMS
267
measurement since an atom adsorbed on the surface or incorporated into the lattice yields the same increment in weight uptake. Work function measurements done simultaneously with weight gain measurements should be able to verify the above transition from a chemisorption with incorporation range to a “normal” chemisorption range. The present ‘“chemisorption-incorporation” model &lies upon the recent ideas of Mott and Fehlnere) in describing the initial ypid oxidation of metals at low tem~ratures. The model that they have suggested involves the image force on the chemisorbed ion and a place exchange mechanism to account for the rapid rate of oxidation. They suggest that the activation energy required for the movement of an ion into the metal is substantially reduced by this image force pulling the ion toward the metal. Assuming the activation energy for place exchange is approximately equal to the relative strength of the average metal bond on the surface, the image force may or may not be sufficient to break the bond and allow the exchange mechanism to occur. If the image force is sufficient, then rapid oxidation will occur. The extent of further rapid oxidation will then the determined by be relative bond strength of the stable compound as compared to the image force on the ion. 3. Experimental procedures 3.1. FABRICATION SampIe preparation and mass measurements were carried out in a stainless steel bakeable high vacuum system7) capable of operation at 1 x 10-r’ tori-. A vacuum gas analyzer was permanently fixed to the vacuum chamber for residual gas analysis. Mass measurements were carried out using a dual quartz crystal oscillator microbalance*). One of the crystals is used as a reference crystal while the other serves as a substrate for the aluminum film and subsequent chemisorption measurements. Gold was deposited on one side of each crystal for purposes of electrical contacts. The microbalance head was imbedded in a 1.5 kg copper heat sink. This procedure was required to maintain temperature stability. An accuracy of 1_ 1 Hz was obtained during the time of the mass measurements. This uncertainty in the frequency shift corresponds to a mass sensitivity of f 5 x 10M9g/cm’. Assuming the freshly deposited aluminum had a surface roughness of two, a monolayer of oxygen atoms on each available site would weigh approximately 7.0 x IO-* g/cm’. Thus the sensitivity of + 1 Hz would correspond to slightly better than rt one-tenth of a monolayer coverage. This accuracy was obtained only after the adoption of a special two-fold technique for selecting the quartz crystals. First, adjacent crystals
268
W. H.
KRUEGER AND
S. R. POLLACK
from the same as-cut batch were used in pairs. This insured that the reference crystal and substrate crystal had the same temperature coefficients and orientation. Secondly, sufficient gold was deposited on each crystal so that after deposition of the fresh aluminum onto the substrate crystal, the difference in resonance frequency was between 5 and 10 kc. Although, the manufacturer of the microbalance unit states that the crystals may be used over a 100 kc range; for chemisorption measurements, meaningful data could be obtained only over the narrower frequency range. High purity aluminum (0.99999 pure) was evaporated from a 0.020 inch thick tungsten evaporation boat in order to form a fresh aluminum surface on both the quartz substrate and a Corning 7059 glass substrate. The pressure in the vacuum system prior to evaporation was approximately 2 x lo-” torr. During the evaporation the pressure in the system increased to 5 x lo-* torr then decreased to a stable pressure of 5 x lo-’ torr. Upon shutting the power off the pressure dropped to approximately 2 x lo-’ torr but in l-2 min it fell to about 5 x 1O-1o torr and was still decreasing. By the time approximately 20 min had elapsed the pressure was again near 2 x lo-” torr and the microbaIan~e output was a constant frequency. At this point pure oxygen* was leaked into the system to the desired pressure through a servooperated leak valve. The mass gain of the films was monitored continuously by the use of a digital frequency counter. 3.2. ALUMINIUMSURFACE ROUGHNESS Since the aluminum films prepared in this study are polycrystalline, it is essential to assess the actual surface area exposed to the oxidizing atmosphere. This information was obtained by the use of scanning electron microscopy and extraction replica techniques in conjunction with electron microscopy. Scanning electron micrographs of aluminum surfaces deposited on quartz substrates showed no distinctive difference to micrographs of the cleaved quartz substrates. Extraction replicas of the aluminum films produced by initially evaporating a thin layer of carbon, shadowing with platinum, and dissolving the underlying aluminum films which had been deposited on 7040 Dow Corning glass substrates and quartz crystal substrates exhibited no significant surface roughness at magnification as high as 240000. Typical micrographs obtained from the scanning electron microscope and extraction replicas are shown in figs. 1 and 2, respectively. It is believed that the replicating techniques used in this work gave a resolution of about 40-50 A. Thus, roughness on a finer scale could not be directly observed. However, Swaine and Plumbs) have shown that aluminum *Medical grade 0% was passed through a dry ice and acetone cooled trap to remove water vapor.
INI’MAL
OXIDATION
OF
Al TRIN
FILMS
269
films deposited at room temperature at normal incidence on glass substrates have a roughness factor between 1.OOand 1.15, Based on Swaine and Plumb’s surface measurements and the similarity of extraction replicas from both the quartz substrates and glass substrates of this work, it is believed that the roughness of the aluminum films is mainly due to the roughness of the intial quartz substrate.
Fig. 1. Scanningelectron micrograph of aluminumthin film surface (magnification10000). 4. Results snd discussion
The typical initial “dry Uzf’ uptake is shown in figs. 3 and 4. In fig. 3 the mass adsorption in terms of weight gain and frequency shift is plotted versus the logarithm of the oxidation time. The difference in magnitude of the final mass uptake of the two curves in fig. 3 is due to a difference in the initial surface roughness of quartz substrates chosen from different batches. In fig, 4 the weight gain due to adsorption of oxygen by aluminum films is plotted far two different low “dry Oa” pressures. In fig. 5 the sticking coefficient of the adsorption of oxygen is plotted as a function of total oxygen weight gain. From the mass data obtained at these low pressures of “dry 02*’ exposure,
Fig. 2.
175
Extraction replica micrograph of aluminum thin film surface (magnification 24ptoW).
am )rpms/cm2
150
lr5
100
075
,050
,025
*
i
aalafr?
La”A~lJ Minutrs
Fig. 3.
Chemisorption
”
of oxygen by aluminum thin films deposited having different surface rnugbness.
100
an quartz crystals
INITIAL
OXIDATION
OF
Al
THIN
271
FILMS
two conclusions may be reached with regard to the initiai room temperature oxidation. (1) The rery initialuptake of oxygen by a “clean” aluminum film exposed to low oxygen pressure (5 x 10 -6 and 5 x lo-’ torr 0,) indicates that the weight of uptake varies linearly with both time and pressure. (This dependence is shown clearly in the insert of fig. 4. The dashed line in the insert
5
LO
15
20
25
30
Minutes Fig. 4.
Oxygen chemisorption
by aluminum thin films at two different pressures.
corresponds to the predicted initial slope at 5 x 10b6 torr using the data at 5 x IF7 torr and assuming a linear dependence on pressure. Only one data point could be obtained for the initial uptake at 5 x lo-” torr since the reaction proceeded very rapidly.) (2) Over the above mentioned pressure range an abrupt change in rate of uptake occurs and the degree of abruptness strongly indicates the completion of a %table layer.” Huber and Kirk have suggested that the arbitrary extrapolations in fig, 3 correspond to the completion of a stable monolayer of chemisorbed oxygen atoms. That is, the frequency shift of 16.5 Hz (in the top curve of fig. 1) corresponds to the weight of one oxygen atom per available site on the aIumin~m surface. An average density of adsorption sites of 1.13 x 101‘/cm2 (based on an equal distribution of the three main crystallographic planes) along with the value of 16.5 Hz as the monolayer of uptake implies a surface roughness of 2.7. This value appears to be consistent with a 2-3 surface
272
W. H. KRUEGER
AND
S. R. POLLACK
roughness indicated by the scanning micrographs as shown in fig. 1. Thus, it does seem reasonable to analyze the “layer effect” in terms of a chemisorption process characterized by a rapid slow down in oxygen uptake as the number of available adsorption sites are depleted. Explicit in attempting to describe the oxygen uptake in terms of a chemisorption process is the fact that the rate limiting mechanism in the weight gain kinetics is the attachment of the oxygen molecules to the surface. Sticking
Coefflclent
P=5xlO-‘Torr
,001
r
vs. ‘Dry
Op”
Weight 23*
Gain
O2
.
0 1030;
Thick
5OObin
0
10306
Thick
SOOb/Min
i) *
l
of
c
0
Fig. 5.
Sticking coefficient of oxygen as a fuction of total oxygen weight gain.
A computer program was written to facilitate the calculation of the first derivative of the weight gain versus time, since in most cases it provides a more sensitive test of the functional dependence in chemisorption than the direct weight gain itself. Thus two quantitative requirements must be satisfied in the analysis of the oxygen adsorption data : (‘I) The chemisorption model must describe the oxygen uptake versus time curves. (2) The chemisorption model should describe the rate of oxygen uptake versus total uptake. The second requirement is met if the model describes either the sticking
coefhcient versus 8 de~nden~e d@/dt versus B dependence. 4.2. PROPOSBD MODEL
FOR
or equivaI~~~y in the case of aluminum the
Al-0 REACTrON
It has been found that the alumin~m~xy~e~ chemisor~tion data at room temperature could not be adequately described by the current theory of chemisorption if we demanded that the model describe both the uptake versus time and the rate of uptake versus time requirements. The major difliculty in trying to obtain a ~h~misorption model which accurately describes the data has been the fact #hat the initial rate of oxygen uptake does not decrease as rapidly as expected from current theory. A detailed analysis of various current chemisorption models has been found to fail in describing the present data, They will be briefly mentioned before proposing a model which most accurately describes the adsorption data observed for aluminum. “Normal’” homogeneous chemis~~tio~ models which have been explicitly demanstrated to fail in describing the data included: (I) First and second order, con-activated adsorption models. (2) Activated adsorption models including an f(e) dependence of (I- 0) or (1 -Q2, and an activation energy dependent upon coverage. It has been found that the previous analysis of the oxygen uptake of alumi~~rn thin films utilizing the Elovich equation has been performed incorrectly and that the correct application of this phenomenologica~ equation does not realistically describe the oxygen-aluminum reaction at room temperature. A discussion of the application of the Elovich equation in the ease ~F~l~rnin~rn thin films can be found eIsewherelQ). It has also been found that the “normal” heterogeneous ~hemis~rpti~~ model envisioned by TrapneIl is incapable of describing the adsorption of oxygen on the aluminum films. It is admitted that perhaps other “normal”’ heterogeneous models may be postulated to describe the mass uptake data. However, at the present time there are two reasons why this approach does not appear reasonable for the room temperature data. (1) Weight gain measurements for the films prepared in this study were reproducible and are also in excellent agreement with the mass measurements by Huber and Kirk. If the surface was heterogeneous with a significant number of special sites, such as kinks or ledges, one would expect diKerent mass measurements as a result af either the thickness of the metal film or conditions of fabrication. (2) Contact potential measurements of Huber and Kirk indicate that oxygen does not remain on the surface during the initial range of uptake, since the contact potential of the Al-0 surface does not immediately increase,
274
W. H. KRUEGER
AND
S. R. POLLACK
For the above reasons the approach of describing the data in terms of a simple, one step process described by “normal” chemisorption has been abandoned in favor of the present model. The model proposed for describing the initial oxidation involves a transition from an initial rapid incorporation range to a much slower “normal” chemisorption range which ultimately leads to the observed ‘“stabie layer effect”. The ~n~or~orat~on range is best described by the following rate equation 2K’SFN* dnr, - = -__lll..l--= constant ’ ~~~~kT~~
cv
dt
where dN,/dt is the number of oxygen atoms adsorbed per cm’ per min, is the number of molecules hitting per cm’ per min, S is the sticking coefficient, K’ is a constant dependent on the equilibrium between physisorbed oxygen and gas phase oxygen, and N, is the number of cation sites available for adsorption. It is proposed that the initial rate of oxygen rate of oxygen uptake is not hnited by the depletion of available cation sites. An atom&tic model suggested to account for the constant level of adsorption sites, N,, is a rapid piace exchange as shown in fig. 6.
P/(2nmkT)t
0
M
M
=
M
M
M
0
0
M
M
M
M
M
M
M
0
M
M
M “--+
M
M
M
M Fig. 6.
M
Model for incorporation.
The above, ~~~arentIy non-activated, incorporation occurs r~~~o~~~ over the entire surface during the ‘“incorporation range”. It is seen from the above model that the adsorption of an oxygen atom does not remove an adsorption site since the place exchange creates another fresh cation site. The newly generated cation site is assumed to have essentially the same adsorption characteristics but it cannot undergo another subsequent place exchange. The reason for this is the fact that this adsorption site is no longer bonded by just AI-AI bonds but an Al-0 bond formed by the previous exchange. Thus, incorporation via place exchange will continue to occur, generating new adsorption sites, untif a fresh stabihzed surface of aluminum is formed. At the completion of the stable surface, the subsequent chemisorption should be described in terms of “normaf” chemisorption theory since fresh sites will no longer be generated.
INITIAL
OXIDATION
OF
Al
THIN
FILMS
215
Fehlner and Motts) have suggested that the driving force for the place exchange mechanism is the simple electrostatic image force pulling the easily ionized oxygen atom into the metal lattice. These authors have proposed that place exchange is apparently non-activated since the image force on the oxygen ion is approximately as large as the average metal-metal bond existing on the surface atom. It is admitted that this picture is quite naive since screening by the electrons has been neglected in their estimation of the magnitude of this image force. However, the assumption of the place exchange mechanism regardless of its driving force will be shown to be reasonable in describing the present data. In order to describe the oxygen uptake in terms of the proposed model it is necessary to redefine the meaning of the extrapolated point on the curves in fig. 3. The intersection of the extrapolated curves are indicative of the “stable layer” but they do not correspond to one monolayer of coverage as suggested by Huber and Kirk l). Assuming that the initial constant rate of uptake in fig. 5 is due to chemisorption with incorporation, the data has been replotted in fig. 7 where the abrupt change in rate of uptake versus total uptake has been assumed to correspond to the transition from chemisorption with incorporation to “normal” chemisorption. It is seen that approximately 1.58 x 10” atoms of oxygen are rapidly adsorbed as a result of incorporation before the rate of uptake abruptly decreases. If these 1.58 x IO” oxygen atoms have undergone a rapid incorporation, the newly created surface, as shown in fig. 6, can be referred to as having zero coverage. Accordingly, the various theoretical curves in fig. 7 correspond to the subsequent “normal” chemisorption models. The total oxygen uptake of the films can be expressed in terms of 6 if it is understood that this quantity is the sum of oxygen atoms chemisorbed and incorporated plus the oxygen atoms adsorbed through the “normal” chemisorption stage. As would be expected, it is readily seen in fig. 7 that an adsorption model based on the adsorption of one 0, molecule per site does not describe the rapid decrease in the rate over the “normal” chemisorption range. The reason that the rate of uptake for this model is much greater than the observed data is the fact that a monolayer of 0, molecules is equivalent to two monolayers of oxygen atoms. Thus, the predicted stable layer of O2 molecules does not occur until an additional uptake of two monolayers of oxygen atoms are added to the previous incorporated uptake. The two additional curves correspond to the rate of uptake predicted by a zero interaction dissociative adsorption model and a model of immobile molecular adsorption on pairs of adjacent sites as proposed by Roberts5). It can be seen from fig. 7 that both of these models reasonably describe the
276
W. H. KRUEGER
AND
S. R. POLLACK
rapid fall off in uptake characteristic of the “layer effect”. The mobile layer, dissociative adsorption model predicts a greater amount of uptake since a full monolayer of uptake should occur in addition to the incorporated oxygen atoms. The model based on the adsorption of molecules on pairs of adjacent sites predicts an additional coverage of only 0.92 0 since approximately 8% of the surface will contain non-adjacent bare sites. Both theoretical models fail to describe the experimental data at high surface coverage
7V
Oxygen
atoms
1.26 I
1.68
I
‘he!$$$ptiok&~ Incorporation I
dt
adsorbed 2.10
I
I
Fig. 7.
Comparison
.6%
I
2.94
,
= 0.35 (p (8) (after
Dissociative interaction atoms.
ld5fcm2
I
1 3.36
t
“Normal” Chemtsorption
Immgbile adsgrgtion on 0 locent sit s.
+’
(r( 2.62
Roberts)
of molecule
adsorption, no between adsorbed
i
1
i
.86 1.06 1.28 1.46 Monolayers of oxygen atoms
r, , I.68
of the experimental rate of oxygen uptake by aluminum thin films with various chemisorption models.
since they do not take into account the effect of the repulsive forces between the ionized oxygen atoms as they are packed more densely on the surface. However, the degree of fit over a large portion of the “normal” chemisorption range is quite remarkable and also indicates that approximately 60-70x of a monolayer of oxygen atoms is adsorbed before the repulsive forces must be considered.
INITIAL. OXiDATlON OF
Al THIN
FILMS
277
From the present data it is not possible to determine which of the above models is the true adsorption model. At high coverages the rate of uptake associated with the undissociative adsorption model appears to be closer to the observed data but it is felt that a distinction cannot be made between the two models on this basis. The kinetics of uptake versus time predicted by these non-activated models is shown in fig. 8 along with the experimental data. The curve corresponding to the immobile molecular adsorption model crosses the mobile curve at @= I .5, since this model predicts only 0.92 oxygen coverage on the surface in addition to the incorporated oxygen. I3 I.8
c
,5.4-’
po2=
5x
to“’ torr 23VJ
; k"Normol
”
Chemisorpti~
Range
(8 ZO.75)
P
Chemis~rption with incorporation
2
i
I,
4
6
t 8
M TtME
I
,
,
,
Range 18 =O-,751 J
,
12 14 16 I8 20 22 (minutes)
Fig. 8. Comparison of oxygen uptake by aluminum thin films with model based on a transition from a chemisorption with incorporation range to a c‘normal” cbemisorption range,
If the above description of the “stable layer effect” is correct, then the arbitrarily extrapolated curves in fig. 3 correspond not to a monolayer of coverage as suggested by Huber and KirkI) but rather closer to approximately 1.5 fayers of oxygen atoms. The rapid slowdown in uptake indicated by these extrapolated curves is due to an initial inco~oration which stabilizes the Af surface folfowed by a “normal” chemisorption process, Based on
278
W.H.KRUEGER
AND S.R.POLLACK
the experimental and theoretical fit in fig. 8 approximately three-fourths of a monolayer of oxygen is rapidly incorporated into the second layer of the surface and an additional three-fourths is “normally” chemisorbed before the surface becomes relatively unreactive towards further adsorption. If the extrapolated point of 16.5 Hz corresponds to 1.5 9 and the average density of sites is the same for both the original cation sites and generated sites, the surface roughness of the films is calculated as 1.9. The proposed model involving a transition from incorporation to “normal” chemisorption is not inconsistent with the LEED work of Jonas). The net effect of oxygen exposure will be a slow deterioration of the original aluminum diffraction spots as the initial oxygen atoms first randomly incorporate into the sub-surface and the final stage of the ~hemisorption generally weakens the intensity of the remaining diffraction peak. Thus the resultant structure of the “stable layer” would be best described as amorphous. The contact potential behavior observed by Huber and Kirk is also understood in terms of the proposed model of chemisorption. A small decrease in the contact potential will be expected for the initial oxygen exposure since the oxygen will be rapidly incorporated beneath the fresh cation sites and thus create a positive surface potential. As the normal chemisorption process begins to play a more dominant role in the adsorption, the contact potential will begin to increase since the ele~tronegativity of the surface oxygen will produce a negative surface potential. Finally, the metal surface will begin to take on the properties of the more stabfe oxide and the contact potential will continue to increase with further exposure as observed by Huber and Kirk. The proposed model also helps in understanding the resistivity data of Huber and Kirk. The sharp break in the resistivity should occur before the break in the mass uptake curve since it would be expected that the incorporated oxygen atoms would have a greater effect on the resistivity than the subsequentIy adsorbed atoms on the surface. The contact potential measurements of Roberts and Wells3) at - 196°C and - 183°C suggest that the incorporation process is activated. However, the energy associated with this step is su~~iently small, that at room temperature it will not be significant. At room temperature rapid incorporation occurs until the surface becomes stabilized for “normal” chemisorption to occur. In summary it appears that the rapid decrease in oxygen uptake by aluminum films at low pressures (5 x 10m7 torr to 5 x lO-‘j torr) is indicative of the completion of a stable surface configuration. The kinetics of oxygen uptake can be described remarkably well in terms of the proposed “incortransition. The initial incorporation range is poration-chemisorption”
INITIAL
OXIDATION
OF
Al
THIN
FILMS
279
characterized by a zero order reaction with an oxygen sticking coefficient of approximately 0.03. The “normal” range of chemisorption, where the surface is characterized as stable with respect to the incorporation mechanism, can be described in terms of either a dissociative chemisorption or an immobile molecular adsorption on adjacent vacant sites. It is believed that the exact mechanism accounting for the rapid fall off occurring in the “normal” range cannot be determined from the present study. The proposed incorporation-chemisorption model qualitatively accounts for the previously observed resistivity and contact potential measurements and is not inconsistent with recent low energy electron diffraction studies. Acknowledgements This paper is a publication of the Laboratory for Research on the Structure of Matter, University of Pennsylvania. The authors gratefully acknowledge the support from contract DAHCl567C0215 of the Advanced Research Projects Agency, Office of the Secretary of Defense, for a Fellowship for one of the authors. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)
E. E. Huber, Jr. and C. T. Kirk, Jr., Surface Sci. 5 (1966) 447. F. Jona, J. Phys. Chem. Solids28 (1967) 2155. M. W. Roberts and B. R. Wells, Surface Sci. 15(1969) 325. D. 0. Hayward and B. M. W. Trapnell, Chemisorption, 2nd ed. (Butter-worth, London, 1964)p.91. J. K. Roberts, Proc. Cambridge Phil. Sot. 34 (1938) 399. F. P. Fehlner and N. F. Mott, Oxidation of Metals 2 (1970) 59. Varian Associates. Westinghouse Scientific Instruments. J. W. Swaine and R. C. Plumb, J. Appl. Phys. 33 (1962) p. 2378. Wm. H. Krueger and S. R. Pollack, Surface Sci. 30 (1972) 280.