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Optical transmittance and microgravimetric studies of the oxidation of 〈100〉 single crystal films of copper
Surface Science 49 (1975) 529-536 0 North-Holland Publishing Company
OPTICAL TRANSMITTANCE
OF THE OXIDATION
AND MICROGRAVIMETRIC
STUDIES
OF (100) SINGLE CRYSTAL FILMS OF COPPER
EC. CLARKE, Jr. * and A.W. CZANDERNA Department of’Physics and Institute oj’colloid and Surface Science, Clarkson CoIleRr o.f Technology, Potsdam. New York 13676, U.S.A.
Received
18 October
1974; revised manuscript
received
29 January
1975
Thin single crystal copper films have been grown and oxidized on (100) faces of cleaved sodium chloride discs suspended from a vacuum ultramicrobalance. Optical transmittance measurements between 400-800 nm and electron microscopic investigations were also used to characterize the oxidation process. Polycrystalline copper films grown at room temperature are substantially the same as those grown previously on glass sub strates. Single crystalline growth at 325 “C on rock salt produces a characteristic transmittance curve due to the “island” nature of the films. These curves compare favorably with other previously published results. Single crystal copper films oxidized to CuOee7 at temperatures of 117-159 “C in 100 Torr of oxygen for films less than 500 i% thick. For films 378 to 1000 A thick, compositions of CuOo,s2 to CuOu.e2 were obtained to the existence between 123-176 “C. The oxidation to less than CuO o,e7 is attributed of islands in these films which are thicker than the average film thickness, and require higher temperatures or thinner films to permit oxidation to CuOo,e7 before the nucleation of CuO sets in.
1. Introduction
The oxidation of thin copper films has, for some time, been an object of experimental investigation [l-4]. In particular, Wieder and Czandera have shown [5] that Cu0u,67, a gross defect structure of Cu,O, can be obtained by the oxidation of polycrystalline copper films deposited at room temperature on glass. This oxide results from oxidation between 110 “C and 200 “C for copper thicknesses up to 108.5 A. The present study was undertaken to attempt to grow a single crystal of this oxide to study its crystallographic properties. It was hoped that the oxidation of single crystal copper films grown epitaxially on (100) rock salt substrates at elevated temperatures would yield single crystals of the oxide. However, these elevated deposition temperatures substantially altered the physical characteristics of the film, such as particle size, continuity, etc. [6], and it was thus necessary to * Present
address:
Wadhams
Hall Seminary
College.
Ogdensburg,
New York 13669,
U.S.A.
530
I:‘.(;. Clarke, A. W. CzarlderrzalOxidation of’single crystal copper films
demonstrate that Cu00,67 could indeed be grown on such films. Although CUO,,.~~ was not obtained in single crystalline form [7], much insight into both the propcrties of single crystal films of copper and its oxides, and the morphology of their oxdiation has been obtained. This study employed microgravimetric, optical transmittance and electron diffraction and microscopic techniques. Results obtained with electron microscopy and diffraction and also some optical transmittance measurements have been previously reported [7,8]. In this paper the microgravimetric results are presented in detail to demonst ate the limits of temperature and thickness over which CLIO~.~~ can be attained upon oxidizing (100) single crystal films of copper grown on rock sZllt.
2. Experimental Copper thin films were vacuum evaporated at 325 “C onto a cleaved (100) disc of sodium chloride suspended from a microbalance. The films were subsequently oxidized in 100 Torr of oxygen at various temperatures. During oxidation, changes in mass and optical transmittance were continuously and simultaneously measured. The ultramicrogravimetric measurements allowed an accurate determination of the mass of deposited copper and by inference the film thickness, assuming continuous films and bulk density. These assumptions, adequate for films grown on glass at room temperature, had to be reexamined for films grown at higher temperatures due to the changes in the film structure 191. The optical measurement gave insight into these structural changes and, by comparison with parallel studies in other vacuum chambers. the degree of its single crystallinity [8]. The gravimetric and optical transmittance apparatus, previously described [5.10,1 I] was improved [8] for the present study. An ultramicrobalance of the pivot type with an automated sensibility of 0.2 pg was housed in a bakeable Pyrex system which was pumped by a 15 Q/s ion pump capable of achieving base pressures of 3 X 10mm8Torr in the balance housing and evaporation chamber without baking. After baking the system, it was possible to deposit a 500 a film while maintaining a base pressure below 2 X 10d6 Torr. The components of the optical transmittance apparatus used for scanning the wavelength region 400.--800 nm have been described in detail [ Ill. The transmittance curve was used as an in situ indicator of the degree of single crystallinity of the film, particle size, continuity and stages of oxidation. This was possible due to extensive studies made on an auxiliary system which duplicated the temperature. pressure and physical characteristics of the balance chamber. Films were removed from the auxiliary system at various stages of deposition and oxidation and examined optically and with an electron microscope. It was possible to use the transmittance curve as a “fingerprint” of many physical and crystallographic parameters ofthe film [5,12-1-l.
E.G. Clarke, A. W. Czanderm/Oxidation
of single crystal copper films
531
Standard epitaxial growth techniques were used to obtain single crystal copper films [6]. After cleavage in air the sodium chloride discs were suspended from the microbalance by a quartz fiber drawn through a 0.8 mm hole drilled through the disc with a low speed drill. After re-evacuation, deposition was made at 325 “C and the thickness of the film was inferred from the mass of the deposited copper as determined by the microbalance. The films were oxidized by heating the sample in oxygen from room temperature to an elevated temperature to complete the oxidation. This procedure was used to minimize the error in the mass measurement [IO]. The reaction was normally complete after 9 to 12 hr although most of the mass uptake was complete after 4 to 5 hr.
3. Results and discussion The polycrystalline films grown by Wieder and Czanderna at room temperature on glass were continuous with a particle size of about 50 a [5]. The lack of voids in these films allowed the use of a parallel layer model for the growth of the oxide [IS]. Copper films grown at room temperature on (100) rock salt displayed a diffraction pattern which contained only rings without enhancement, indicating a polycrystalline film. Electron micrographs demonstrated that the films lacked voids and were composed of small (-500 a) particles, In fig. 1, the optical transmittance of a polycrystalline film grown at room temperature on glass (a) is compared with one grown on rock salt at room temperature (b). They are virtually identical except for slight overall amplitude change due to film thickness differences. These results indicate that there were no unwanted physical alterations in the films due to the change from glass to sodium chloride substrates. However, considerable alteration in the physical structure of the film did result by depositing at 325 “C to obtain epitaxially grown single crystals. Electron diffraction indicated that these films were single crystals with the characteristic spot pattern [7]. However, electron micrographs indicated that the films had taken on an “island” structure with considerable intergranular void area. The island width, typically tens of thousands of angstroms, is dependent on film thickness [ 171. The transmittance spectrum strongly reflects these structural changes (fig. 1c). The maxima at about 575 nm has disappeared and is replaced by a strong resonance minimum near 620 nm as predicted by Petrov [ 181 and Rasigni and Rouard [ 191. This minimum is characteristic of the colloidal sols described by Petrov, indicating that the copper islands are acting to some extent as isolated resonators. Pertinent data on 12 representative films prepared for this study are given in table 1. The first six were successfully oxidized to CUO~,~~ between temperatures of 117 “C to 198 “C and thicknesses of 198 a to 505 8. Initially, all of these copper films were single crystalline with the exception of Number 6. This film was deposited on a rock salt substrate which had been etched at 400 “C to remove all water vapor from the surface. This procedure precludes single crystal growth although it
Clarke, A. W. Czandcrna/Oxidation
532
01
I
400
500
I 600 WAVELENOTH
of sin,&, cq~stal copper films
I (nIrd
700
I
Fig. 1. The optical transmittance of (100) copper films deposited at room temperature on glass (a) and rock salt (b) and deposited at 325 “C on rock salt (c). Film (a) is 250 A in thickness; films (b) and (c) are 200 A. in thickness.
Table 1 Thin films of copper Film No.
Oxidation temperature
grown
on sodium
chloride
Mass
Thickness
(fig)
(A)
substrates *
oxidized
at various
temperatures
x in CuO,
Comments
(“C) 1 2 3 4 5
117 126 134 1Sl 159
221 398 558 449 534
198 359 505 404 482
0.66 0.68 0.67 0.66 0.66
Single crystal films oxidized to C’uOo,e7
6
198
522
470
0.67
I 8
123 124
526 420
473 378
0.52 0.62
9 10 11
160 176 176
804 992 1100 -.__
725 894 1000
0.60 0.60 0.61
Polycrystalline film oxidized to CuOrrh7 Single crystal films not oxidized to T apparentcu00.67; ly too low Single crystal films apparently too thick to reach CuOu,e7
* Void areas ncglcctcd.
E.G. Clarke, A. W. Czanderna/Oxidation of single crystal copper films
533
FILM 80 -
TIME
(mln)
Fig. 2. The oxidation curves for three (100) single crystal copper films approximately 500 A in thickness oxidized at various temperatures. Film 5: 482A, 159°C; Film 3: 505 A, 134 ‘C; Film 7: 473 A, 123 “C.
does produce the island growth associated with an elevated deposition temperature. Thus, this film was crystallographically polycrystalline but physically resembled single crystal films 1 through 5. No difficulty was experienced in oxidizing this film to CUOO.67. The oxidation curves of three films approximately 500 a thick heated in 100 Torr of oxygen are shown in fig. 2. Typically, a 500 A film has 556 pg of copper which requires 14Opg of oxygen uptake to produce a CuO film. Therefore a film oxidized to an x (in CuO,) of 0.67 would require 93 pg of oxygen. It is apparent from the figure that the process is thermally activated, i.e., films oxidized at higher temperatures reached their maximum mass gain faster than those oxidized at lower temperatures. This is similar to the initial results of Wieder and Czanderna [S] and Hapase et al. [20] on polycrystalline films. Comparison of the rates of oxidation for films grown at room temperature on glass and the single crystal films indicated that the latter reached their maximum mass uptake faster than the polycrystalline films. This result is not surprising due to the island nature of the single crystal films. The interparticle regions present extra copper surfaces for oxidation. Indeed, as has been previously reported [7], these void regions are rapidly filled with oxide before reaching an x of 0.3. Thereafter, oxidation resembles the polycrystalline situation where the parallel layer growth
534
E.G. Clarke, A. W. Czanderna/Oxidation
of single crystal copper ,films
S (SD5A)
17
)
100
200 TIME (mln)
500
Fig. 3. The oxidation curves for four (100) single crystal copper films of various thicknesses oxidized at various temperatures. Film 10: 894 A, 176 “C; Film 3: 505 A, 134 “C; Film 2: 359 A, 126 “C; Film 1: 198 A, 117 “C.
model is applicable. In a representative case, film 1 had virtually completed its oxygen uptake within 200 minutes whereas a similar polycrystalline film (200 A; oxidation temperature 122 “C) oxidized by Wieder and Czanderna [S] took more than 500 min to reach a similar stage of oxidation. In fig. 3, the oxidation curves are given for four representative films of varying thicknesses oxidized at various temperatures. It is evident from table 1 that oxidation to CUO~,~~ was not obtained for film thicknesses greater than 505 A. Wieder and Czanderna [5] oxidized polycrystalline films on glass to an x of 0.67 for films up to 1085 A in thickness. It should be remembered, however, that the thickness estimate is an average based on the known mass and assumes continuous films with bulk density. The single crystal films in this study were not continuous and for thicker films it was possible that the void area was large enough to cause significant error in the estimate, resulting in particles with an average thickness greater than the calculated value. For example, if we assume the void area is 20% of the total film area, then a deposit whose calculated thickness is 800 A, in reality contains a distribution of particles with a mean height of 1000 A. Thus, it is not surprising that the maximum film thickness of oxidation to CUO~.~~ is higher for polycrystalline films than for single crystal films. This effect is not based on any crystallographic differences but merely the physical characteristics of the films such as continuity, distribution of particle heights, void areas, etc. Apparently, it is the existence of voids which precluded the growth of a single
535
E. ti. Clarke, A. W. CzandernajOxidatiorz of single crystal copper films
crystal of CUO~.~~. Since oxide growth takes place in these regions initially, where high index crystal planes of copper are exposed and high defect concentrations favor nucleation, it is presumed that several orientations of oxide are present before the well oriented copper islands themselves are oxidized [3]. Thus, although the resulting oxide is highly oriented it is not a single crystal. The transmission electron diffraction results [7] did not disclose any detectable lines of CuO in any of the CUO~.~~ films. This is similar to the results for polycrystalline films [5], where any CuO lines that might have been present were undetectable after X-ray diffraction exposures of up to I7 hr. As is well known, CuO is the thermodynamically stable oxide at temperatures of 100 to 200 ‘C, but the rate of formation of CuO from the Cu,O structure below 200 “C is extremely slow because of the activation energy of the reaction, 3 CUO~.~~ t i 0, + 3 CuO. Although CuO diffraction lines are not detectable by conventional electron and X-ray diffraction techniques used in this and previous studies, the presence of a surface phase of a few atomic layers of CuO cannot be excluded from the overall uncertainty of the gravimetric, diffraction and transmittance results. In table I, some unexplained failures to obtain CUO~.~~ are indicated. Films 7 and 8, apparently within the required temperature and thickness range, only reached x values of 0.52 and 0.62 respectively. As in the study of polycrystalline films [5], the electron diffraction results [7] show that only Cu,O and Cu lines are present for partially oxidized films below 176 “C. Thus, compositions below CuOu.67 result from an outer layer of the gross defect structure, CUO~,~~ over a copper interior with an intervening layer of Cu,O of undetermined thickness. In the case of CUO~,~~, copper lines were found in the electron diffraction patterns, but not from CUO~.~~_~,~~, which is similar to the results of Wieder and Czanderna [5].
200
cu 0, x qO.67
2
180
/
L i 160
3
L
'oolOO
,/
I
I
300
500
/
0”
I THICKNESS
TOO
po
PI
I so0
I
00
(A)
Fig. 4. Plot of the relationship between oxidizing temperature and original film thickness for (100) single crystal copper films oxidized to CuO,, x < 0.67. Triangles: films oxidized to an x = 0.67; circles: films oxidized to anx < 0.67.
536
E.G. Clarke, A. W. Czandcrna/Oxidation of’siugle crystal
copper films
In fig. 4, the relationship between film thickness and oxidation temperature is indicated. The thickness-temperature relationship from 140 to 200 “C could have been anticipated from the previous work [5]. Below 140 OC, however, the plot in fig. 4 suggests that oxidation to CUO~,~~ is dependent upon these two variables in a way not previously suspected. The dotted line roughly separates the regions where the temperature and thickness conditions determine if CuO, 67 will or will not be obtained. There is obviously not enough data yet to precisely define these regions.
Acknowledgement The authors gratefully acknowledge Atomic Energy Commission.
support for this work by the United States
References [l] S. Shirai, J. Phys. Sot. Japan 2 (1947) 81. [2] L.O. Brockway and A.P. Rowe, in: Fundamentals of Gas Surface Interactions, Eds. H. Saltsburg, J.N. Smith and M. Rodgers (Academic Press, New York, 1967) p. 147. [3] J. Jardinier-Offergeld and 1:. Bouillon, J. Vacuum Sci. Technol. 8 (1972) 770. [4] R.W. Vook and C.T. Horng, Thin Solid P’ilms 18 (1973) 295. [5] H. Wieder and A.W. Czanderna, J. Phys. Chem. 66 (1962) 816. [6] A.W. Czanderna, B.W. Brennan and E.G. Clarke Jr., Phys. Status Solidi (a) 8 (1971) K75. [7] E.G. Clarke Jr. and A.W. Czanderna, Thin Solid Films 12 (1972) 443. [8] A.W. Czanderna and E.G. Clarke Jr., in: Progress in Vacuum Microbalance Techniques, Vol. 2, Ed. S.C. Bevan (Heyden, London, 1973) p. 9. [9] E.G. Clarke Jr., Ph.D. Thesis, Clarkson College, 1973. [lo] C. Angel1 and A.W. Czanderna, in: Ultra Micro Weight Determination in Controlled Environments, Eds. S.P. Wolsky and E.J. Zdanuk (WileyyInterscience, New York, 1969) p. 287. [ 111 A.W. Czanderna and H. Wieder, in: Vacuum Microbalance Techniques, Vol. 2, Ed. R.F. Walker (Plenum, New York, 1962) p. 147. [ 121 H. Wieder and A.W. Czanderna, J. Chem. Phys. 39 (1963) 483. (131 A.W. Czandernd and H. Wieder, J. Chem. Phys. 35 (1961) 2259. [ 141 H. Wieder and A.W. Czandernd, J. Appl. Phys. 37 (1966) 184. [ 151 A.W. Czanderna and F.L. Boyko, J. Vacuum Sci. Technol. 6 (1969) 746. [ 161 A.W. Czanderna and F.L. Boyko, J. Vacuum Sci. Technol. 9 (1972) 393. [ 171 B.W. Brennan, Masters Thesis, Clarkson College of Technology, New York, 1971. [18] Y.I. Petrov, Opt. Spectrosc. 27 (1969) 359. [19] G.R. Rasigniand P. Rouard, J. Opt. Sot. Am. 53 (1963)604. [20] M.G. Hapase, M.K. Gharpurey and A.B. Biswas, Surface Sci. 9 (1968) 87.
OPTICAL TRANSMITTANCE
OF THE OXIDATION
AND MICROGRAVIMETRIC
STUDIES
OF (100) SINGLE CRYSTAL FILMS OF COPPER
EC. CLARKE, Jr. * and A.W. CZANDERNA Department of’Physics and Institute oj’colloid and Surface Science, Clarkson CoIleRr o.f Technology, Potsdam. New York 13676, U.S.A.
Received
18 October
1974; revised manuscript
received
29 January
1975
Thin single crystal copper films have been grown and oxidized on (100) faces of cleaved sodium chloride discs suspended from a vacuum ultramicrobalance. Optical transmittance measurements between 400-800 nm and electron microscopic investigations were also used to characterize the oxidation process. Polycrystalline copper films grown at room temperature are substantially the same as those grown previously on glass sub strates. Single crystalline growth at 325 “C on rock salt produces a characteristic transmittance curve due to the “island” nature of the films. These curves compare favorably with other previously published results. Single crystal copper films oxidized to CuOee7 at temperatures of 117-159 “C in 100 Torr of oxygen for films less than 500 i% thick. For films 378 to 1000 A thick, compositions of CuOo,s2 to CuOu.e2 were obtained to the existence between 123-176 “C. The oxidation to less than CuO o,e7 is attributed of islands in these films which are thicker than the average film thickness, and require higher temperatures or thinner films to permit oxidation to CuOo,e7 before the nucleation of CuO sets in.
1. Introduction
The oxidation of thin copper films has, for some time, been an object of experimental investigation [l-4]. In particular, Wieder and Czandera have shown [5] that Cu0u,67, a gross defect structure of Cu,O, can be obtained by the oxidation of polycrystalline copper films deposited at room temperature on glass. This oxide results from oxidation between 110 “C and 200 “C for copper thicknesses up to 108.5 A. The present study was undertaken to attempt to grow a single crystal of this oxide to study its crystallographic properties. It was hoped that the oxidation of single crystal copper films grown epitaxially on (100) rock salt substrates at elevated temperatures would yield single crystals of the oxide. However, these elevated deposition temperatures substantially altered the physical characteristics of the film, such as particle size, continuity, etc. [6], and it was thus necessary to * Present
address:
Wadhams
Hall Seminary
College.
Ogdensburg,
New York 13669,
U.S.A.
530
I:‘.(;. Clarke, A. W. CzarlderrzalOxidation of’single crystal copper films
demonstrate that Cu00,67 could indeed be grown on such films. Although CUO,,.~~ was not obtained in single crystalline form [7], much insight into both the propcrties of single crystal films of copper and its oxides, and the morphology of their oxdiation has been obtained. This study employed microgravimetric, optical transmittance and electron diffraction and microscopic techniques. Results obtained with electron microscopy and diffraction and also some optical transmittance measurements have been previously reported [7,8]. In this paper the microgravimetric results are presented in detail to demonst ate the limits of temperature and thickness over which CLIO~.~~ can be attained upon oxidizing (100) single crystal films of copper grown on rock sZllt.
2. Experimental Copper thin films were vacuum evaporated at 325 “C onto a cleaved (100) disc of sodium chloride suspended from a microbalance. The films were subsequently oxidized in 100 Torr of oxygen at various temperatures. During oxidation, changes in mass and optical transmittance were continuously and simultaneously measured. The ultramicrogravimetric measurements allowed an accurate determination of the mass of deposited copper and by inference the film thickness, assuming continuous films and bulk density. These assumptions, adequate for films grown on glass at room temperature, had to be reexamined for films grown at higher temperatures due to the changes in the film structure 191. The optical measurement gave insight into these structural changes and, by comparison with parallel studies in other vacuum chambers. the degree of its single crystallinity [8]. The gravimetric and optical transmittance apparatus, previously described [5.10,1 I] was improved [8] for the present study. An ultramicrobalance of the pivot type with an automated sensibility of 0.2 pg was housed in a bakeable Pyrex system which was pumped by a 15 Q/s ion pump capable of achieving base pressures of 3 X 10mm8Torr in the balance housing and evaporation chamber without baking. After baking the system, it was possible to deposit a 500 a film while maintaining a base pressure below 2 X 10d6 Torr. The components of the optical transmittance apparatus used for scanning the wavelength region 400.--800 nm have been described in detail [ Ill. The transmittance curve was used as an in situ indicator of the degree of single crystallinity of the film, particle size, continuity and stages of oxidation. This was possible due to extensive studies made on an auxiliary system which duplicated the temperature. pressure and physical characteristics of the balance chamber. Films were removed from the auxiliary system at various stages of deposition and oxidation and examined optically and with an electron microscope. It was possible to use the transmittance curve as a “fingerprint” of many physical and crystallographic parameters ofthe film [5,12-1-l.
E.G. Clarke, A. W. Czanderm/Oxidation
of single crystal copper films
531
Standard epitaxial growth techniques were used to obtain single crystal copper films [6]. After cleavage in air the sodium chloride discs were suspended from the microbalance by a quartz fiber drawn through a 0.8 mm hole drilled through the disc with a low speed drill. After re-evacuation, deposition was made at 325 “C and the thickness of the film was inferred from the mass of the deposited copper as determined by the microbalance. The films were oxidized by heating the sample in oxygen from room temperature to an elevated temperature to complete the oxidation. This procedure was used to minimize the error in the mass measurement [IO]. The reaction was normally complete after 9 to 12 hr although most of the mass uptake was complete after 4 to 5 hr.
3. Results and discussion The polycrystalline films grown by Wieder and Czanderna at room temperature on glass were continuous with a particle size of about 50 a [5]. The lack of voids in these films allowed the use of a parallel layer model for the growth of the oxide [IS]. Copper films grown at room temperature on (100) rock salt displayed a diffraction pattern which contained only rings without enhancement, indicating a polycrystalline film. Electron micrographs demonstrated that the films lacked voids and were composed of small (-500 a) particles, In fig. 1, the optical transmittance of a polycrystalline film grown at room temperature on glass (a) is compared with one grown on rock salt at room temperature (b). They are virtually identical except for slight overall amplitude change due to film thickness differences. These results indicate that there were no unwanted physical alterations in the films due to the change from glass to sodium chloride substrates. However, considerable alteration in the physical structure of the film did result by depositing at 325 “C to obtain epitaxially grown single crystals. Electron diffraction indicated that these films were single crystals with the characteristic spot pattern [7]. However, electron micrographs indicated that the films had taken on an “island” structure with considerable intergranular void area. The island width, typically tens of thousands of angstroms, is dependent on film thickness [ 171. The transmittance spectrum strongly reflects these structural changes (fig. 1c). The maxima at about 575 nm has disappeared and is replaced by a strong resonance minimum near 620 nm as predicted by Petrov [ 181 and Rasigni and Rouard [ 191. This minimum is characteristic of the colloidal sols described by Petrov, indicating that the copper islands are acting to some extent as isolated resonators. Pertinent data on 12 representative films prepared for this study are given in table 1. The first six were successfully oxidized to CUO~,~~ between temperatures of 117 “C to 198 “C and thicknesses of 198 a to 505 8. Initially, all of these copper films were single crystalline with the exception of Number 6. This film was deposited on a rock salt substrate which had been etched at 400 “C to remove all water vapor from the surface. This procedure precludes single crystal growth although it
Clarke, A. W. Czandcrna/Oxidation
532
01
I
400
500
I 600 WAVELENOTH
of sin,&, cq~stal copper films
I (nIrd
700
I
Fig. 1. The optical transmittance of (100) copper films deposited at room temperature on glass (a) and rock salt (b) and deposited at 325 “C on rock salt (c). Film (a) is 250 A in thickness; films (b) and (c) are 200 A. in thickness.
Table 1 Thin films of copper Film No.
Oxidation temperature
grown
on sodium
chloride
Mass
Thickness
(fig)
(A)
substrates *
oxidized
at various
temperatures
x in CuO,
Comments
(“C) 1 2 3 4 5
117 126 134 1Sl 159
221 398 558 449 534
198 359 505 404 482
0.66 0.68 0.67 0.66 0.66
Single crystal films oxidized to C’uOo,e7
6
198
522
470
0.67
I 8
123 124
526 420
473 378
0.52 0.62
9 10 11
160 176 176
804 992 1100 -.__
725 894 1000
0.60 0.60 0.61
Polycrystalline film oxidized to CuOrrh7 Single crystal films not oxidized to T apparentcu00.67; ly too low Single crystal films apparently too thick to reach CuOu,e7
* Void areas ncglcctcd.
E.G. Clarke, A. W. Czanderna/Oxidation of single crystal copper films
533
FILM 80 -
TIME
(mln)
Fig. 2. The oxidation curves for three (100) single crystal copper films approximately 500 A in thickness oxidized at various temperatures. Film 5: 482A, 159°C; Film 3: 505 A, 134 ‘C; Film 7: 473 A, 123 “C.
does produce the island growth associated with an elevated deposition temperature. Thus, this film was crystallographically polycrystalline but physically resembled single crystal films 1 through 5. No difficulty was experienced in oxidizing this film to CUOO.67. The oxidation curves of three films approximately 500 a thick heated in 100 Torr of oxygen are shown in fig. 2. Typically, a 500 A film has 556 pg of copper which requires 14Opg of oxygen uptake to produce a CuO film. Therefore a film oxidized to an x (in CuO,) of 0.67 would require 93 pg of oxygen. It is apparent from the figure that the process is thermally activated, i.e., films oxidized at higher temperatures reached their maximum mass gain faster than those oxidized at lower temperatures. This is similar to the initial results of Wieder and Czanderna [S] and Hapase et al. [20] on polycrystalline films. Comparison of the rates of oxidation for films grown at room temperature on glass and the single crystal films indicated that the latter reached their maximum mass uptake faster than the polycrystalline films. This result is not surprising due to the island nature of the single crystal films. The interparticle regions present extra copper surfaces for oxidation. Indeed, as has been previously reported [7], these void regions are rapidly filled with oxide before reaching an x of 0.3. Thereafter, oxidation resembles the polycrystalline situation where the parallel layer growth
534
E.G. Clarke, A. W. Czanderna/Oxidation
of single crystal copper ,films
S (SD5A)
17
)
100
200 TIME (mln)
500
Fig. 3. The oxidation curves for four (100) single crystal copper films of various thicknesses oxidized at various temperatures. Film 10: 894 A, 176 “C; Film 3: 505 A, 134 “C; Film 2: 359 A, 126 “C; Film 1: 198 A, 117 “C.
model is applicable. In a representative case, film 1 had virtually completed its oxygen uptake within 200 minutes whereas a similar polycrystalline film (200 A; oxidation temperature 122 “C) oxidized by Wieder and Czanderna [S] took more than 500 min to reach a similar stage of oxidation. In fig. 3, the oxidation curves are given for four representative films of varying thicknesses oxidized at various temperatures. It is evident from table 1 that oxidation to CUO~,~~ was not obtained for film thicknesses greater than 505 A. Wieder and Czanderna [5] oxidized polycrystalline films on glass to an x of 0.67 for films up to 1085 A in thickness. It should be remembered, however, that the thickness estimate is an average based on the known mass and assumes continuous films with bulk density. The single crystal films in this study were not continuous and for thicker films it was possible that the void area was large enough to cause significant error in the estimate, resulting in particles with an average thickness greater than the calculated value. For example, if we assume the void area is 20% of the total film area, then a deposit whose calculated thickness is 800 A, in reality contains a distribution of particles with a mean height of 1000 A. Thus, it is not surprising that the maximum film thickness of oxidation to CUO~.~~ is higher for polycrystalline films than for single crystal films. This effect is not based on any crystallographic differences but merely the physical characteristics of the films such as continuity, distribution of particle heights, void areas, etc. Apparently, it is the existence of voids which precluded the growth of a single
535
E. ti. Clarke, A. W. CzandernajOxidatiorz of single crystal copper films
crystal of CUO~.~~. Since oxide growth takes place in these regions initially, where high index crystal planes of copper are exposed and high defect concentrations favor nucleation, it is presumed that several orientations of oxide are present before the well oriented copper islands themselves are oxidized [3]. Thus, although the resulting oxide is highly oriented it is not a single crystal. The transmission electron diffraction results [7] did not disclose any detectable lines of CuO in any of the CUO~.~~ films. This is similar to the results for polycrystalline films [5], where any CuO lines that might have been present were undetectable after X-ray diffraction exposures of up to I7 hr. As is well known, CuO is the thermodynamically stable oxide at temperatures of 100 to 200 ‘C, but the rate of formation of CuO from the Cu,O structure below 200 “C is extremely slow because of the activation energy of the reaction, 3 CUO~.~~ t i 0, + 3 CuO. Although CuO diffraction lines are not detectable by conventional electron and X-ray diffraction techniques used in this and previous studies, the presence of a surface phase of a few atomic layers of CuO cannot be excluded from the overall uncertainty of the gravimetric, diffraction and transmittance results. In table I, some unexplained failures to obtain CUO~.~~ are indicated. Films 7 and 8, apparently within the required temperature and thickness range, only reached x values of 0.52 and 0.62 respectively. As in the study of polycrystalline films [5], the electron diffraction results [7] show that only Cu,O and Cu lines are present for partially oxidized films below 176 “C. Thus, compositions below CuOu.67 result from an outer layer of the gross defect structure, CUO~,~~ over a copper interior with an intervening layer of Cu,O of undetermined thickness. In the case of CUO~,~~, copper lines were found in the electron diffraction patterns, but not from CUO~.~~_~,~~, which is similar to the results of Wieder and Czanderna [5].
200
cu 0, x qO.67
2
180
/
L i 160
3
L
'oolOO
,/
I
I
300
500
/
0”
I THICKNESS
TOO
po
PI
I so0
I
00
(A)
Fig. 4. Plot of the relationship between oxidizing temperature and original film thickness for (100) single crystal copper films oxidized to CuO,, x < 0.67. Triangles: films oxidized to an x = 0.67; circles: films oxidized to anx < 0.67.
536
E.G. Clarke, A. W. Czandcrna/Oxidation of’siugle crystal
copper films
In fig. 4, the relationship between film thickness and oxidation temperature is indicated. The thickness-temperature relationship from 140 to 200 “C could have been anticipated from the previous work [5]. Below 140 OC, however, the plot in fig. 4 suggests that oxidation to CUO~,~~ is dependent upon these two variables in a way not previously suspected. The dotted line roughly separates the regions where the temperature and thickness conditions determine if CuO, 67 will or will not be obtained. There is obviously not enough data yet to precisely define these regions.
Acknowledgement The authors gratefully acknowledge Atomic Energy Commission.
support for this work by the United States
References [l] S. Shirai, J. Phys. Sot. Japan 2 (1947) 81. [2] L.O. Brockway and A.P. Rowe, in: Fundamentals of Gas Surface Interactions, Eds. H. Saltsburg, J.N. Smith and M. Rodgers (Academic Press, New York, 1967) p. 147. [3] J. Jardinier-Offergeld and 1:. Bouillon, J. Vacuum Sci. Technol. 8 (1972) 770. [4] R.W. Vook and C.T. Horng, Thin Solid P’ilms 18 (1973) 295. [5] H. Wieder and A.W. Czanderna, J. Phys. Chem. 66 (1962) 816. [6] A.W. Czanderna, B.W. Brennan and E.G. Clarke Jr., Phys. Status Solidi (a) 8 (1971) K75. [7] E.G. Clarke Jr. and A.W. Czanderna, Thin Solid Films 12 (1972) 443. [8] A.W. Czanderna and E.G. Clarke Jr., in: Progress in Vacuum Microbalance Techniques, Vol. 2, Ed. S.C. Bevan (Heyden, London, 1973) p. 9. [9] E.G. Clarke Jr., Ph.D. Thesis, Clarkson College, 1973. [lo] C. Angel1 and A.W. Czanderna, in: Ultra Micro Weight Determination in Controlled Environments, Eds. S.P. Wolsky and E.J. Zdanuk (WileyyInterscience, New York, 1969) p. 287. [ 111 A.W. Czanderna and H. Wieder, in: Vacuum Microbalance Techniques, Vol. 2, Ed. R.F. Walker (Plenum, New York, 1962) p. 147. [ 121 H. Wieder and A.W. Czanderna, J. Chem. Phys. 39 (1963) 483. (131 A.W. Czandernd and H. Wieder, J. Chem. Phys. 35 (1961) 2259. [ 141 H. Wieder and A.W. Czandernd, J. Appl. Phys. 37 (1966) 184. [ 151 A.W. Czanderna and F.L. Boyko, J. Vacuum Sci. Technol. 6 (1969) 746. [ 161 A.W. Czanderna and F.L. Boyko, J. Vacuum Sci. Technol. 9 (1972) 393. [ 171 B.W. Brennan, Masters Thesis, Clarkson College of Technology, New York, 1971. [18] Y.I. Petrov, Opt. Spectrosc. 27 (1969) 359. [19] G.R. Rasigniand P. Rouard, J. Opt. Sot. Am. 53 (1963)604. [20] M.G. Hapase, M.K. Gharpurey and A.B. Biswas, Surface Sci. 9 (1968) 87.