(3) J. Gaude and J. Lang, C. R. Acad. Sci., Ser. C, 271, 510 (1970). (4) J. Gaude, p. L'Haridon, Y . Laurent, and J. Lang, Rev. Chirn. Miner.: 8, 287 (1971). (5) K. H. Linke and K. Schroedter, Z.Naturforsch., B, 26, 736 (1971). (6) J. Gaude and J. Lang, C. R. Acad. ScL, SerC, 274, 521 (1972). (7) R. J. Hynek and J. A . Neien, Anal. Chern., 35, 1655-7 (1963).
Our method is also suitable for measuring most gases, and do& not involve any expensive equipment. lt can, therefore, be applied extensively.
LITERATURE CITED Hantzpergue, These de 3eme cycle, U.E.R. Sciences et Techniques, Universite d'Angers, 49000 Angers (1973). (2) C. Healy and A. Parker, U.K.At. Energy Auth., Res. Group, AERE-R (1) J. J.
RECEIVED for review April 26, 1974. Accepted October 29, 1974.
6491, 13 pp (1970).
ESCA Study of Sputtered Platinum Films G. Michael Bancroft,' Ian Adams, and Leighton L. Coatsworth Department of Chemistry, University of Western Ontario, London, Ontario
C. David Bennewitz and James D. Brown Faculty of Engineering Science, Materials Science Group, University of Western Ontario, London, Ontario
William D. Westwood Bell Northern Research Laboratories, Ottawa, Ontario
The need for a more stable thin film resistor material for micro-circuits, especially hybrid circuits, has led to the investigation of films reactively sputtered from a platinum cathode in argon-oxygen mixtures ( 1 ) . It has been observed that the resistivity and temperature coefficient of resistance (TCR) of these films reach values typical of cermet films (Le., mixtures of metal and insulator phases) as the oxygen concentration in the sputtering discharge is increased from 0 to 100%. X-Ray analysis indicates that besides fcc Pt, there is a second phase present in the films which is isostructural with PdO and is probably PtO, as predicted by Moore and Pauling ( 2 ) .The PtO phase acts as an insulating phase and is responsible for the cermet-like behavior of the films. Recent measurements of the oxygen content and TCR of thin film samples has shown that the TCR decreases and becomes negative as the oxygen to platinum ratio in the samples increases. In samples with large negative TCR values, the ratio is greater than 1.0 which, of course, is not possible if PtO is the only oxide of platinum present. However, reflection electron diffraction patterns of these samples show only the presence of Pt and PtO which agrees with the results of previous X-ray analysis. The failure of both X-ray and electron diffraction to identify this second oxide phase indicates that it has an amorphous structure. In this paper, we report X-ray photoelectron studies on these Pt films. The use of ESCA for such surface studies has been well documented (3, 4 ) . The results show that Pt02 is present in the films as well as PtO and Pt, and the calculated O/Pt ratios from the ESCA areas are in reasonable agreement with the weight loss values. Our assignment of peaks to PtO is very useful in reassigning peaks in previous ESCA studies of electrolyzed Pt surfaces (5, 6 ) . The qualitative and quantitative uses of detailed computation in such complex ESCA spectra are discussed.
strates were cut into strips before sputtering to enable the preparation of several samples for each set of sputtering conditions. Each strip was weighed before and after film deposition to determine the weight of film deposited. A deposition time of 60 min was used for each sample. Determination of TCR and Oxygen Content. The TCR of each sample was calculated from four-point probe measurements of film resistance a t room and liquid nitrogen temperatures (8). The oxygen content was determined by heating the samples in air a t 850 "C and measuring their change in weight with a microbalance; thermogravimetric analysis of films containing PtO ( I ) and Pt02 (9, 1 0 ) indicate that, at this temperature, the weight change is due to the loss of all oxygen in the film. A value for the oxygen to platinum ratio in each film was then calculated using the initial film weight and the weight change during decomposition. Details of the sample preparation, X-ray diffraction, electron diffraction, thermogravimetric analyses, and TCR values are given in a forthcoming paper (11 ). ESCA Procedure and Computation. The spectra were record ed using a McPherson ESCA 36 spectrometer and a Mg anode. The narrowest line width obtainable (as determined from the Ag. 3d5/2 peak of an evaporated silver film) was 0.88 eV. Binding energies were calibrated by the Au evaporation technique, and taking the Au 4f;/2 peak to be at 84.0 e\'. The P t 4f7/2 peak for Pt metal was a t 71.30 eV (f0.05). X-Ray powers were normally 120 watts. and the vacuum in the sample chamber was in the Torr range. A computer program written by one of us (LLC) was used to deconvolute the spectra. This program obtains a weighted least squares fit from the data points by means of an iterative GaussNewton procedure ( 1 2 ) .Each spectrum was fitted with an analytical function consisting of a sum of Gauss-Lorentz shape functions and a linear base-line correction. The Gauss-Lorentz shape function has four parameters which describe each peak: the peak position, the full width a t half height, the height, and the Gauss-fraction (GFAC) which determines the fraction of the Gaussian component in the fitted peak shape. The program allows for each parameter to be constrained at some given initial vaiue for any number of iterations up to the maximum allowed for convergence. x2, the sum of the squares of the deviations divided by the variance V of the count C are computed, and are useful in comparing the relative "goodness of fit" of different fits to the same spectrum.
EXPERIMENTAL
RESULTS Computation and Assignment. Four typical ESCA spectra of the Pt 4f region are shown in Figure 1. The plots include the original data (after subtraction of the linear base line), the calculated fit, and each individual peak. The
Sample Preparation. Samples were prepared by sputtering onto 2- X 2-inch alumina substrates in a dc diode system ( 7 ) a t a voltage of 3 kV and various argon-oxygen atmospheres. The sub-
' Author to whom correspondence should be addressed. 586
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
2823 N
QX -
218-
z
3
.
ED0
w
: >
8-
3
z
-
3-
1
BlN3IhUG EhERGY/ELECT93N L3L'S
Figure 1. Pt 4f ESCA spectra of sputtered Pt films ( a )A l . ( b ) E l , ( c )A3, ( d )A4. Peaks A and A' are due to Pt metal, B and E' are due to PtO, and C and C' are due to PtOn
Table I. ESCA D a t a for Sputtered Pt Films Pr Peaks ( A and A') Sample
x2
Binding energy
A1
490
...
A7121
-
P t o Peaks (B and B')
Binding energy
A7/zlA5/2'
P O 2 Peaks (C and C ' ) Binding energy
A ~ I ~ I A O~/ PIt r ~ a t i o~b
...
72.40' 1.21 74.03 1.18 1.25 75.70' 77.44 300 ... ... 72.42 1.19 73.82 1.11 1.23 B1 75.67 77.10 ... ... 72.20 1.24 74.10 1.19 1.19 A2 516 75.50 77.40 A3 710 71.30' 1.78 72.40' 1.22 74.20' 1.15 1.10 74.60' 75.70' 77.50' 1.17 74.20' 1.09 0.81 A4 903 71.30' 1.41 72.40' 74.60' 75.70' 77.50' 0.68 A5 988 71.30' 1.31 72.40' 1.16 74.20' 1.00 74.60' 75.70' 77.50' Area of 4f?z peak divided by area of 4f5,*peak. b Estimated error = &0.10. c Peak position constrained a t the given value. The Gauss fraction for all peaks was constrained a t 0.5, while the widths of A and A', B and B', and C and C ( were constrained a t 1.50, 1.80, and 1.80eV, respectively.
length of the bars in the spectra are given by f 2 c,where c is the square root of the count. The peaks in the spectra are assigned as follows: A and A' to Pt metal, B and B' to PtO, and C and C' to Pt02. The derived binding energies and ratio of the Pt 4f7/2 to Pt 4f512 peak areas are given in Table I. The x2 values and O/Pt ratios are also recorded in Table I, while the TCR values and bulk O/Pt ratios are given in Table 11. The peak positions in Table I for Pt metal and peaks B and B' [previously assigned as Pt(OHl2 (6)]are in good agreement with those
Table 11. TCR Values and Bulk O / P t Ratios Oxygen to platinvm ratio Sample
TCR, ppm/'C
Bulk
ESCA
A1 A2 A3 A4 A5
- 10,900
1.34 1.24 1.02 0.69 0.47
1.25 1.19 1.10 0.81 0.68
-2,500 -540 -160 3 50
-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
587
given previously (4-6). This assignment of peaks B and B' to PtO is based on the X-ray diffraction evidence ( I , 1 1 ) that this is the major oxide component in all films. Our peak positions for PtO2 are in reasonable agreement with those given recently [Pt 4f7/2 = 74.4 eV ( 1 3 ) ] ,but are more than 0.4 eV lower than other reported values (4-6). Cahen et al. have noted that PtO2 is susceptible to surface charging (13)and these higher values may be a result of charging shifts. The spin-orbit coupling of 3.3 eV is in good agreement with the values given previously, and the ratio of the areas of the 4f7/2 to 4f5/2 peaks is Consistently in agreement with values normally found for this ratio (theoretical ratio is 1.33). The O/Pt ratio was calculated from the sum of the areas ( A ) of the Pt 4f7,2 and Pt 4f5/2 peaks using the formula
Before discussing these results in more detail, some comment on the method of fitting such complex spectra seems desirable. The Gauss fraction was constrained a t 0.5 for all peaks in all spectra. This value is typical of those computed for single peak spectra on our instrument. The positions of the Pt metal 4f7/2 and 4f5/2 peaks were found to be at 71.30 eV and 74.60 eV, respectively (with respect to gold 4f7/2 binding energy defined as 84.0 eV), while the width of the 4f;,2 peak, the more symmetric of the two Pt peaks, was 1.50 eV. The Pt metal peaks (A and A') and widths were always constrained at these values in subsequent spectra of mixtures of Pt and Pt oxides. As noted recently for Pd metal ( 1 4 ) , the asymmetry of such metal peaks is due to hole-conduction band interaction, and is not due to an appreciable oxide layer. This asymmetry, and nonlinear base line, always gives rise to large x 2 values for spectra of Pt metal, and spectra containing Pt metal (samples A3, A4, and A5). Samples Al, A2, and B1 all had bulk O p t ratios substantially greater than unity, and their peak maxima were shifted to higher binding energies than the Pt peaks for the pure metal. The X-ray diffraction data indicated that the main component in these films was PtO but there had to be another oxyplatinum species present to account for O/Pt > 1. Thus two sets of peaks (B and B', C and C') were fitted to these spectra. Lowest x2 values were obtained with widths of 1.80 eV and, for samples B1 and A2, convergence occurred without having to constrain peak positions. The peak positions for these two spectra are in reasonably good agreement and the ratio of the areas (always unconstrained) are consistently about 1.2. For A l , the PtO peaks had to be constrained a t the value computed for A2 and B1 in order that convergence occurred. For A3, A4, and A5, the peaks broadened, and the maxima decreased from 72.4 eV and 75.7 eV indicating an appreciable Pt metal component. Thus, these spectra were fitted to three sets of doublets. All peaks positions, widths, and GFAC were constrained a t previously observed values, except for peaks C and C'. Area constraints again were not used. The latter peaks were constrained at slightly larger values than those observed in A l , B1, and A2 (74.0 eV and 77.3 eV), because better x 2 values were obtained with the positions at 74.20 eV and 77.50 eV. Because of the poor fits in the high binding energy region, there is obviously a rather large error associated with these peak positions. The ratios of the 4f712 to 4f5/2 areas are still surprisingly consistent considering the complexity of the spectra. 588
ANALYTICAL CHEMISTRY, VOL. 47, N O . 3, M A R C H 1975
The oxygen core-level spectra of the samples were always monitored. Clearly one would expect a direct relationship between the 0 1s peaks and the oxygen content of the sputtered film. However, in practice, the situation is considerably more complicated, presumably due to the presence of adsorbed oxygen containing species. At least three types of oxygen were readily distinguishable, but we are unable to comment at present upon their nature.
DISCUSSION Our assignment of Pt and PtO2 peaks is consistent with previous ESCA work (4-6). The assignment of peaks B and B' to PtO is novel and is confirmed by the previous X-ray diffraction results and the O/Pt ratios (vide infra). It is also interesting to note that the Pt 4f7/2 peak shifts (between Pt metal and its two oxides) observed here are entirely consistent with the shifts observed recently (15) for Pd 3d5/2 levels in Pd metal and its corresponding oxides. Thus, Kim et al. (14) found Pd, PdO and PdO2 peaks at 335.0,336.3, and 337.9 eV (relative separations of 0, 1.3, and 2.9 eV) respectively, compared to our values for Pt 4f7/2 peaks in Pt, PtO, and PtO2 peaks at 71.3, 72.4, and 74.2 eV (relative separations of 0, 1.1, and 2.9 eV). Therefore the peaks at 72.5 and 75.8 eV observed previously by Allen et al. (6) and assigned to Pt(OH)z could be due to PtO. Like Allen et al. (6), we have found no evidence for the PtO(ads) species observed by Kim ( 5 ) . The trend in O/Pt ratios derived from ESCA is in very good agreement with the bulk values (Table 11) observed earlier, and the absolute values (with the exception of A5) are within the estimated error of f O . l O . The larger ESCA O/Pt ratios for samples A4 and 5 are undoubtedly due to the inherent asymmetry of the Pt peaks (14) which enhances the apparent oxide content of the film. This agreement in O/Pt ratios confirms our assignment of peaks given previously. In particular, our assignment of PtO2 which was not observed (11 ) by X-ray diffraction, electron diffraction, or thermogravimetric analysis explains the O/Pt ratios of greater than one. Our evidence strongly suggests that any other Pt oxygen species such as Pt,04 ( 1 3 )is present in very small amounts. ACKNOWLEDGMENT The authors are very grateful to the National Research Council, and N. Winograd for helpful comments.
LITERATURE CITED (1) W. D. Westwood and C. D. Bennewitz, J. Appl. Phys.45, 2313 (1974). (2) W. J. Moore, Jr., and L. Pauling, J. Amer. Chem. SOC.,63, 1392 (1941). (3) K. Siegbahn, "ESCA, Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy," Almqvist and Wiksells, Uppsala, 1967. (4) W. N. Delgass, T. R. Hughes, and C. S. Fadley, Catal. Rev., 4, 179 (1970). (5) K. S. Kim, N. Winograd. and R. E. Davis, J. Amer. Chem. SOC., 93, 6296 (1971). (6) G. C. Allen, P. M. Tucker, A. Capon, and R. Parsons, Nectronal. Chem. hterfacial Necochem.. 50, 335 (1974). (7) W. D.Westwood and R. Boynton, J. Appl. Phys., 43, 2691 (1972). (8) W. D. Westwood and N. Waterhouse. J. Appl. Phys., 42, 2946 (1971). (9) 0. Muller, Ph.D. Thesis, The Pennsylvania State University, 1968. (10) R. D. Shannon, Solidstate Commun.,6, 139 (1968). (1 I)W. D. Westwood, C. D. Bennewitz, and J. D. Brown, J. Appl. Phys., in press. (12) R. I. Jennrich and P. F. Sampson. Technometrics, I O , 63 (1968). (13) D. Cahen, J. A. Ibers, and J. B. Wagner, horg. Chem., 13, ,1377 (1974). (14) S. Hufner, G. K. Wertheim, and D. N. E. Buchanan, Chem. Phys. Lett.. 24, 527 (1974). (15) K. S. Kim, A. F. Gossmann. and N. Winograd. Anal. Chem., 46, 197 (1974).
RECEIVEDfor review August 9,1974. Accepted October 14, 1974.
