Chem. Mater. 1992, 4 , 640-641
640
Plasma Oxidation of Copper-Silver Alloy Surfaces J. M. Knight, R. K. Wells, and J. P. S. Badyal* Department of Chemistry, Science Laboratories, Durham University, Durham DH1 3LE, UK Received November 20, 1991. Revised Manuscript Received February 10, 1992
Oxidation of a 20% Cu-80% Ag binary alloy surface by an oxygen plasma treatment has been examined by X-ray photoelectron spectroscopy (XPS). A mixed Cu(I1) + Ag(I)/Ag(II) oxide film was formed with a Cu:Ag ratio comparable to that found for the clean surface. However, the subsequent exposure of this oxide overlayer to an argon glow discharge resulted in a dramatic change in its composition to a highly Cu(1) rich oxide. This change in oxide stoichiometry can be explained in terms of the chemistry occurring at the plasma/alloy interface.
Introduction Surface oxidation of metals is an area of considerable technological and academic importance in terms of improvement of surface properties and in the understanding of oxidative degradation. Considerable effort in the past has gone into understanding reactions between molecular oxygen and metallic surfaces.'V2 Oxygen glow discharges are reported to be of a highly oxidizing nature. Such plasma environments are very complicated, and a whole range of gas-solid processes are believed to be present; these include the interaction of radicals, atoms and electronically excited molecules with the surface, neutralization, secondary electron emission, sputtering, ion-induced chemistry, electron-induced chemical reactions, and photochemi~try.~In this article, we investigate the surface chemistry occurring at plasma/metal alloy interfaces by X-ray photoelectron spectroscopy (XPS). Experimental Section A 1-mm-thickdisk of copper-silver alloy (20% Cu-80% Ag,
Johnson Matthey) was mechanically polished and subsequently rinsed in isopropyl alcohol. The high proportion of silver in the binary alloy gave it a silvery appearance. This was placed in an electrodeless flow reactor, which is similar to that described in a previous article: It was fitted with a gas inlet, a pirani gauge, a 47 L min-' two-stage rotary pump with a liquid nitrogen cold trap, and a matching network for inductive coupling of a 13.56MHz radio-frequency (rf) source. All joints were grease-free. The RF coils were positioned over a quartz substrate holder. Firstly the system was pumped down to 1 X Torr, and the walls of the reactor were allowed to outgas. A continuous flow of oxygen or argon (Research grade, BOC) was introduced into the reaction vessel at a pressure of 2 x lo-' Torr. Prior to igniting the plasma, the reactor was flushed with gas for 10 min. Sample heating due to the rf field was tested for by measuring the bulk alloy temperature immediately after extinguishing the glow discharge, this was never found to be more than 20 "C above room temperature. X-ray photoelectron spectra were acquired on a Kratos ES200 surface analysis instrument. Magnesium K a radiation was used as the excitation source with electron detection in the fixed analyzer transmission (FAT)mode. X P S measurements were taken with an electron takeoff angle of 30" from the surface normal. No evidence was obtained for radiation damage to the samples during the typical time scale involved in these experiments. Data accumulation and component peak analysis were performed with an IBM PC computer, using linear background subtraction. All binding energies are referenced to any adventitious hydrocarbon at 285.0 eve5
Results and Discussion The clean alloy surface gave strong XPS features cor~) responding to ita metallic componenta:'j C u ( 2 ~ ~=/ 933.2 eV and Ag(3dSI2)= 368.6 eV, Figures l a and 2a. As
* To whom correspondence should be addressed. 0897-4756/92/2804-0640$03.00/0
Table I. XPS Peak Area Ratios (Uncorrected for Sensitivity Factors) treatment C u ( 2 ~ ~ / ~ ) : A d 3 dO(~S):[CU(~PMZ) ~/~) + Ag(3d~~)l clean O2 plasma Ar plasma
0.40
0.37 20.7
0.01 0.27 0.12
expected from the bulk composition of the alloy, the Ag/~) (3d5/2)peak was far more intense than the C U ( ~ P ~signal. Previous workers have reported that the Ag component of a 20% Cu-80% Ag alloy exhibits a slight preference toward surface segregation.'" A very small amount of oxygen was also detected in the O(1s) region, Table I. An oxygen plasma is known to contain both positive and negative ions, atoms, ozone, and metastables of atomic and molecular oxygen, as well as electrons and a broad electromagnetic ~ p e c t r u m .Electron-molecule ~ reactions are responsible for ionization and dissociation within the glow discharge. Following plasma oxidation at 25 W for 10 min, the alloy surface darkened to a gray-black texture. The relative proportions of silver to copper changed only slightly from the value measured for the clean surface, / ~ eV and Ag(3d5,,) Table I. However, both C U ( ~ P=~934.7 = 368.5 eV peak intensities were attenuated. This decrease in concentration of metallic species at the surface, along with the appearance of a strong O(1s) signal can be attributed to oxide formation. A 1.5-eV shift in the Cu(2p3 2 ) peak toward higher binding energy, together with the emergence of a shakeup structure at -943 eV, is consistent with +2 valency for the copper ions in the surface oxide latti~e.~JOPronounced shakeup satellites have been reported to occur for Cu2+ (3d9)species, whereas they are known to be absent for Cu+ (3d'O) ions in an oxide overlayer.9 The slight negative shift in Ag(3d5/2) binding energy is consistent with the oxidation of silver metal to an oxide;6 however, it is difficult to decide by XPS whether Ag(1) or Ag(I1) ions are present. The extent of surface oxidation was much greater with the plasma turned on than that found for just a flow of O2 over the same length of time. (1) Wandelt, K. Surf. Sci. Rep. 1982, 2, 1. (2) Benndorf, C.; Cam, H.; Egert, B.; Seidel, H.; Thieme, F. J.Electron Spectrosc. Relat. Phenom. 1980, 19, 77. (3) Techniques and Applications of Plasma Chemistry; Hollahan, J. R., Bell, A. T., Eds., Wiley: New York, 1974. (4) Shard, A. G.; Munro, H. S.; Badyal, J. P. S. Polym. Commun. 1991, 32, 152. (5) Johansson, G.; Hedman, J.; Berndtason, A.; Klasson, M.; Nilsson, R. J. Electron Spectrosc. Relat. Phenom. 1973, 2, 295. (6) Romand, M.; Roubin, M.; Deloume, J. P. J. Electron Spectrosc. Relat. Phenom. 1978,13, 229. (7) Braun, P.; Farber, W. Surf. Sci. 1975, 47, 57. (8) Mukherjee, S.; Moran-Lopez, J. L. Surf. Sci. 1987, 189/190, 1135. (9) Larson, P. E. J.Electron Spectrosc. Relat. Phenom. 1974,4,213. (10) Kim, K. S. J. Electron Spectrosc. Relat. Phenom. 1974, 3, 217.
0 1992 American Chemical Society
Plasma Oxidation of Cu-Ag Alloy Surfaces
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Chem. Mater., Vol. 4, No. 3, 1992 641
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9 2 8 9 3 2 936 940 944 948 BINDING ENERGY (ev) Figure 1. Cu(2p32) XPS spectra of Ag-Cu alloy surface following (a) cleaning, (b) plasma treatment, and (c) exposure of (b) to Ar glow discharge.
d,
This enhanced rate of surface oxidation can be attributed to the highly reactive constituents of the glow discharge, e.g., atomic oxygen." Ar glow discharge treatment (25 W, 30 min) of the previously oxidized Cu-Ag alloy surface resulted in a substantial enrichment of copper species C ~ ( 2 p ~ / ~ ) : A g (3d6/,) = 20.7. Indeed, the disappearance of the shakeup region associated with Cu2+,Figure IC,and the decrease in Cu(2p3/,) binding energy to 933.6 eV are indicative of the formation of CU~O.~JO Furthermore, the surface had a bright yellow appearance, which is characteristic of Cu(1) oxide.12 Interaction of the Ar plasma with the oxide film should be dominated by ablative processes such as ion sputtering and electron-induced decomposition. Cu(1) oxide is known to be more stable than the corresponding Cu(I1) compound under reducing conditions,2J2and both Ag(1) and Ag(I1) oxides decompose quite ea~i1y.l~ Therefore the silver oxides must be undergoing preferential (11) Gousset, G.; Touzeau, M.; Vialle, M.; Ferreira, C. M.Plasma Chem. Plasma Process. 1989, 9, 189. (12) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley-Interscience: New York, 1988. (13) CRC Handbook of Chemistry and Physics, 63rd ed.; CRC Boca Raton, FL, 1982.
364 368 3 7 2 376 380 384
BINDING ENERGY (eV) Figure 2. Ag(3d5/2,3/2)XPS spectra of Ag-Cu alloy surface following (a) cleaning, (b) O2plasma treatment, and (c) exposure of (b) to Ar glow discharge. decomposition and ablation during exposure to the Ar plasma. This cannot be attributed to simply heating of the substrate by the glow discharge, since if this was the case, then the Preferential buildup of copper oxides would also have been expected for the O2plasma treatment. The extent of surface segregation in these experiments is far greater than that found for Ar ion sputtering of a 20% Cu-80% Ag alloy; where only a slight enrichment of copper a t the surface was observed, this was attributed to the higher sputter coefficient of silver.'
Conclusions To summarize, plasma oxidation of the binary Cu-Ag alloy produces a thin oxide overlayer, without a change in the relative proportions of Cu:Ag; the oxide film comprises of Cu(I1) and Ag(I)/Ag(II) ions. Subsequent exposure to an argon plasma results in a marked rise in concentration of Cu(1) ions and virtually the total loss of silver species a t the oxide surface. The former treatment is highly oxidizing in nature, whereas the argon glow discharge is responsible for ablation and reduction a t the plasma/oxide interface. Registry No. 29% CudO% Ag alloy, 39440-34-1; CuO, 1317-38-0; AgZO, 20667-12-3; Ago, 1301-96-8; CU~O,1317-39-1.
