REDUCTION OF CuOo.67 IN Hz
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The Reduction of CuOa,, in Hydrogen
by A. W. Czanderna Union Carbide Corporation, Chemicals Division, Research and Development Department, South Charleston, West Virginia (Received M a y 1.4,1966)
The oxidation of a 500-A.copper film h oxygen and its reduction in hydrogen has been studied with gravimetric and optical techniques. The composition cu00.67 can be reduced to copper in hydrogen a t 25" without appreciably altering the film uniformity. The data obtained were used to conclude that the reduction process is a nucleation and growth phenomenon.
Introduction Numerous studies of the nucleation and growth of metal film and of the oxidation of metal films have been made.'I2 However, little is known about the effect of cyclic oxidation and reduction on the properties of films. Considerable information was accumulated during previous studies on the oxidation of evaporated copper films to Cu00.67a and on the reduction of CuOS4 Kinetic and optical transmission data also have been obtained during the oxidation of agglomerated copper films.5 It seemed desirable to study the effect of the oxidation of Cu to CUOO.~~ in oxygen and the reduction of CuOo.6.itoCu in hydrogen on the propertiesofthefilm.
are necessary to remove the final 4% of oxygen from the film.
Results and Discussion The mass loss during reduction of cu00.67 at 25" is plotted as a function of time in curve I in Figure 1. The sigmoidal shape was obtained in each reduction cycle. The shape of this curve is typical for a nucleation, growth, and depletion mechanism. In the induction period, which varied from 1 hr. at 125" to 70 hr. at 25O, the average thickness of the copper nuclei formed on the C U O ~was . ~ ~20-30 A. The transition to the growth of the nuclei was extremely abrupt when nucleation was completed at temperatures of about 70" or more. This is shown by the change in the rate of Experimental Section mass loss at A in curve I1 (Figure 1). The rate of mass loss was approximately linear from a copper mole A uniform copper film, 500 A. thick, was evaporated of 0.2 to 0.6 a t all reduction temperatures. onto a Pyrex glass substrate and oxidized to C U O ~ . ~ , . fraction ~ The activation energy calculated for this mole fraction The optical transmission of the film was measured from range was 12 i= 2 kcal./mole. The rate-determining 400 to 800 mp while the mass change was simultaneously step during the growth of the nuclei, suggested by the monitored with a microbalance during evaporation, magnitude of the activation energy, could be the disoxidation, and also during subsequent reduction. A sociation of a copper-oxygen-hydrogen surface complex detailed description of the apparatus employed has or the diffusion of copper on a CuOo.6, surface. The been reported.6 Except as noted below, the oxidation was accomplished by heating the film from room temperature to 143" in 100 torr of oxygen. The film was (1) G . Haas, Ed., "Physics of Thin Films," Vol. 1, Academic Press Inc., New York, N. Y., 1963. reduced in 100 torr of hydrogen a t temperatures ranging ( 2 ) C. A. Neugebauer, J. W. Newkirk, and D. A. Vermilyea, Ed., from 25 to 125". To permit detailed transmission "Structure and Properties of Thin Films," John Wiley and Sons, studies of the film during reduction, it was found conInc., New York, N. Y., 1959. (3) H. Wieder and A. W. Czanderna, J . Phys. Chem., 66, 816 (1962). venient to reduce the furnace temperature in one of the (4) H. Wieder and A. W. Czanderna, J . Chem. Phys., 35, 2259 reduction cycles as soon as the rate of reduction be(1961). came rapid. Six complete oxidation-reduction cycles (5) A. W. Czanderna and H. Wieder, unpublished. were carried out on the film. After the sixth reduction, (6) A. W. Czanderna and H. Wieder, "Vacuum Microbalance Techit was established that temperatures exceeding 125" niques," Vol. 11, Plenum Press, Inc., New York, N. Y., 1962, p. 147. Volume 69, Number 10 October 1366
A. W. CZANDERNA
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The optical transmission data for C U O ~obtained .~~ after each oxidation cycle and for Cu after each reduction cycle are shown in Figures 3 and 4. The data in Figure 3 are characteristic curves that have been obtained many times for C U O ~ . ~The ~ . ~apparent shift of the absorption edge and the increased transmission at the longer wave lengths from the first cycle to the subsequent oxidation cycles probably result from annealing of the film. For example, the as-evaporated film is annealed a t room temperature, while the reduced films are annealed at several different higher temperatures. The transmission curves obtained for the reduced copper film of composition C U O ~are . ~ comparable ~ to those observed for a partially oxidized copper film.3 However, the transmission at either extreme of the wave length spectrum exhibits deviations which are characteristic of the first stages of agglomeration of a copper film.6 Again, nearly all changes in the trans-
100 90
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2op OXIDATION CWXE Na
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Figure 2. The reoxidation of copper films after reduction in hydrogen.
depletion tail is less gradual than has been observed for other systems.&? The mass gain of the film during reoxidation to CUOO.~~ after each reduction cycle is shown in Figure 2. The reproducibility of the oxidation curve up to the saturation limit is within experimental error for all of these data. If important changes in the uniformity of the film had occurred, the oxidation rate would have been slower, particularly in the saturation stages and in later cycles. As can be seen, six reduction cycles produced no effect in the oxidation rate. Furthermore, the difference between the rate of the sixth oxidation, carried out at 127", and those carried out at 143' is the same as has been observed for freshly evaporated films.* The Joumal of Physical Chemistry
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5z
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a a
+ 30
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WAVELENGTH (mp) Figure 3. The effect of oxidation-reduction cycling on the optical transmission spectrum of cuo0.67. (7) W. D. Bond and W. E. Clark, Oak Ridge National Laboratory, Report No. ORNL-2815, March 16, 1960. (8) A. W.Csanderna, unpublished.
REDUCTION OF C U O ~IN. ~H2 ~
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I
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I
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WAVELENGM (mp) Figure 4. The effect of oxidation-reduction cycling on the optical transmission of a copper a m . (E is for the evaporated film.)
mission spectrum occurred in the first two reduction 0400 500 600 700 800 cycles. Optical examination (at 2000X) of a similar WAVELENGTH (mp) film subjected to oxidation reduction treatment showed spectra obtained 5. Optical no “gaps” in tjhe reduced film. If all of the increase during the reduction of cuoo.s,. Initial in transmission resulted from formation of hills and temperature 85’; final temperature 50”. trough in a continuous film, the film thicknem must vary between 300 and 700 A. to account for the transcause the long induction period indicates a nucleation mission observed. If this were true, the thickest and growth mechanism for the reduction. Each of the regions then would have a measurable effect on the rates nuclei formed on the surface serves as a scattering cenof reoxidations which was not evident in the reoxidater and therefore has different loss characteristics from tion curves. Hence, it seems more probable that the the metal. The optical constants are only valid for difference in transmission of the reduced copper film films greater than 200 8. thick in the case of copper and and the evaporated copper film results either from a greater than 600 8. thick for CuOo.~.Therefore, it change in the extent of annealing of the film orfrom would not be expected that quantitatively accurate rethe small oxygen content of the film being closer to sults would be obtained from these optical constants in either the substrate-oxide or the air-oxide interface. the initiation and termination stages of the reduction The transmission data obtained during the sixth process. The calculated curves follow qualitatively . ~ shown ~ in reduction cycle from C u 0 0 . ~to C U O ~are the shape of the experimental curves shown in Figure 5. Figure 5. It is of interest to analyze these curves Of greater importance is that the simplified expression as though the reduction process consists of the forma(1) can be used to fit the experimental curves. tion of aarallel smooth layers of copper _ _ and CUOO.S~ at all stages. The transmission of the system airT = exp [- ( d l add I (1) copper-CuOo.sr.Pyrex glass could then be calculated Here, T is the over-all per cent transmission at any from the mass data at each stage of the reduction from wave length, cy1 and a2are the absorption coefficients of the optical constants for copper and Cu00.d‘ and the
+
usual eauations. lo The transmission, shown by the ex-
Volunte 69 Number IO Oetober 1066
3610
copper and of CuOO.67at that wave length, and tl and tz are the respective thicknesses of the two species. In this formula (l),it is assumed that the entire loss in transmission through the sample may be accounted for by absorption within it. In actuality, however, there are initial effects which must be c~nsidered.~Reflections at both copper interfaces and the scattering from the copper nuolei would be expected, and both of these effects might be more pronounced as the thickness of the copper layer increases. Since it would be difficult to obtain an analytical expression for these effects in a detailed treatment, an effective absorption coefficient which includes the reflection and scattering losses was calculated in terms of the simplified model of eq. 1. It was found convenient to solve for a1 and c y 2 in eq. 1 using the measured transmission and the values of tl and tz calculated from the mass data for curves corresponding to C U O ~and . ~ CuOo.13. ~ These values of a1 and az were used with the values of tl and tz computed from the mass data to obtain transmission curves for the intermediate stages of reduction. The resultant calculated curves superimposed perfectly on all of the curves taken and shown in Figure 5 . This demonstrates that no deviation from the simplified treatment exists at any stage of the reduction. This is in contrast to the reduction of CUO.~Thus, after the initial copper layer is formed, any subsequent stage of the reduction of the sample can be considered as made up of parallel layers of copper and CuOO.67. This again seems to be indicative that the uniformity of the original copper film has not been drastically altered by the cyclic oxidation and reduction treatment. It further implies that the size
The Journal of Physieal Chemistry
,4.W. CZANDERNA
of the copper nuclei must be relatively small and the number of nuclei very large. After the seventh oxidation, the cu00.67 was oxidized to CuO at higher temperatures and reduced t o copper in hydrogen as an additional check on the mass data. The transmission spectrum after this reduction showed severe agglomeration of the film had occurred as has been previously reported.* It is interesting to compare the results obtained on the reduction of cu00.67 with those of CuO. Where even the mildest reduction of CuO produces severe agglomeration of the film,comparable thermal reduction of CuOO.67repeated several times does not produce changes in the film uniformity which are detectable with our measuring techniques. i t is suspected that this may be because the crystal structures of Cu00.67 and copper are similar, and distances between the copper in them are not drastically different. It may be possible for a large number of small nuclei to be formed by an epitaxial process on C U O ~in. contrast ~~ to CuO where the copper crystal structure must be formed in the nuclei during growth. It is evident from the agglomeration of the reduced CuO films which takes place that the copper nuclei formed are larger and less numerous than in Cu00.67. It would seem very appropriate to confirm these preliminary conclusions by a study of the nucleation and growth of copper on cu00.67 and on CuO by selected-area electron microscopy or some other suitable technique.
Acknowledgments. The author is grateful to Drs. Harold Wieder, J. H. Block, and A. W. Smith for their helpful comments about this work.
