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Anal. Chem. 1984, 56,412-416
Analysis of Binary Alloy Surfaces by Low Wattage Glow Discharge Emission Spectrometry K a z u a k i Wagatsuma a n d Kichinosuke Hirokawa*
The Research Institute for Iron, Steel, and Other Metals, Tohoku University, Sendai, Japan
The surfaces of a few binary alloys, Ag-Cu, CU-NI, and NI-Co alloys, were Investigated by low wattage glow discharge emisslon spectrometry. It was found that the characteristics of sputtering for each alloy obtained In this work corresponded to the earller works by other techniques for surface analysis such as Auger electron spectroscopy and secondary Ion mass spectrometry. This method can be extended to the study of real surfaces wlthout ultrahigh vacuum conditions and some advantages were found comparing to the discharge at the abnormal glow reglon whlch is not low wattage in this study.
Considerable interest has recently converged on the examination of real surfaces because of their wide influence on the mechanical and/or chemical properties of materials in the ambient atmosphere. Various methods have been introduced for the study of surface analysis, but the main aim of these has been focused into the research of clean or well-defined surfaces. Glow discharge spectrometry, especially Grimm glow discharge spectrometry (GDS) (I), has some advantages for the study of real surfaces. In the GDS, the photons emitted from atoms in glow discharge plasma are detected and the sample introduction into the plasma is based upon cathode sputtering at ambient pressure, so the ultrahigh vacuum (UHV) system is unnecessary in contrast to Auger electron spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS) equipped with an ion etching gun and other techniques for the surface study. Surface contamination, adsorbed water, oxide films, and others seriously influence experiments in UHV. However, these problems are not significant in GDS because they can be easily removed by predischarge. An abnormal glow discharge region (2) has been employed in earlier works of GDS (3-5). By use of this region, enough intensity of atomic emission can be observed due to the relatively high primary ion density. However, as the sputtering rate is rather high, the resolution of depth profie is poor (6)and much deposit is heaped around the anode, and it is difficult to control the plasma conditions accurately. On the other hand, in a low power glow discharge: normal glow discharge, or the transient region from normal to abnormal glow discharge, spectrum intensity drops abruptly, and its mild sputtering condition conversely suits the measurements for surface analysis. We measured and analyzed the emission spectra of several binary alloy systems, using this mild sputtering condition, so-called low wattage glow discharge spectrometry. In this paper, the results on typical binary alloys, Ag-Cu, Cu-Ni, and Ni-Co alloys, are reported and the availability of the method for surface analysis is discussed. THEORETICAL SECTION If the thermodynamic equilibrium plasma is assumed and the self-absorption can be ignored, the intensity ratio (za/zb) of resonance lines for two different elements (a and b) is given by z a / z b = (sa/Sb) (Ab/ Xa) (Aa/Ab) (zfla/zaNb) X (Ra/gb) exp[(Eb - Ea)/kT] (1) 0003-2700/84/0356-0412$01.50/0
where S is the instrumental function (mainly dependent on A), X is the wavelength of a spectral line, A is the einstein transition probability for the spontaneous emission, N is the atomic density in the plasma, 2 is the partition function of a energy level, g is the statistical weight of each transition, E is the energy level of a excited state, and T i s the excitation temperature. If E, is approximately equal to E b , eq 1 is simplified into eq 2 because exp[(Eb - Ea)/kT] becomes unity. za/zb
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(Sa/Sb) (Ab/ A,) (&/Ab) (Zb/za) k?A/gB) (Na/Nb) = R(Na/Nb) R = constant (2) In abnormal glow discharge mode, the effect of self-absorption is small compared to the other emission spectrometric light source (7). In order to estimate the influence of selfabsorption in the low wattage glow discharge mode, it is useful to examine the intensity ratio of two resonance lines of the same element. If the ratio of these intensities is independently constant upon the input power, self-absorption is not significant. In Cu-52.4 at. % Ni, for example, the normalized intensity for two copper resonance lines (Cu 1324.7 nm and 327.4 nm) and two pairs of normalized intensities for three nickel lines (Ni 1339.3 nm, 341.5 nm, and 352.5 nm) are almost constant in the range of 3-10 W of the input power as shown in Figure 1. Similar phenomena are observed in the case of Ag-Cu or Ni-Co alloys. Therefore, in the low wattage GDS, if these emission lines are employed (see Table 11),the contribution of self-absorption to the intensity ratio can be neglected. Equation 2 is independent of the excitation temperature and shows that the intensity ratio is directly proportional to the atomic ratio. The selection of spectral lines according to the condition of eq 2 makes it possible to obtain information on the sputtered atom composition. Low pressure and low power plasmas such as the low wattage GDS do not satisfy the condition of the thermodynamic equilibrium and Boltzmann distribution (8,9).In this case, the electron temperature ( T J is much higher than the gas temperature and the electron density in plasma is also a important factor. However, if Ea is nearly equal to E b and T, is higher, it is thought that the distribution function among excited states becomes constant and an equation like eq 2 may be realized. za/zb
= R'(Na/Nb)
R' = constant
(3)
The emission intensities obtained are considered with these equations.
EXPERIMENTAL SECTION The emission spectrum was recorded with a Hitachi 808 visible and ultraviolet spectrometer at a spectral band-pass of 0.1 nm. The structure of our glow lamp was similar to the original model by W. Grimm (IO). The lamp was evacuated to 4.0-1.3 Pa and then argon gas was introduced (special grade for vacuum spectrometry) until the proposed pressure. Argon gas pressure of 4.0 X lo2 Pa was selected as the operating condition. A Kikusui Electronics Corp. Model PAD 1K-0.2L power supply was employed with a current constant mode. 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
Table I. Alloy Compositions and Their Structures Ag, at. % Ag/Cu structure Ag-Cu 74.7 2.95 hypoeutectic 59.9 1.49 eutectic 31.6 0.462 hypereutectic Cu, at. % Cu/Ni structure Cu-Ni 18.7 0.233 all proportional 28.3 0.394 homogeneous 37.3 0.595 solid solution
Ni-Co
47.6 58.2 68.3 78.9
0.908 1.39 2.16 3.74
Ni, at. %
Ni/Co
20.1 39.6 61.2 80.2
0.265 0.654 1.58 4.05
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RESULTS AND DISCUSSION Selection of Emission Lines. Compared to the high-energy excitation sources, the emission spectra in the low wattage glow discharge are rather simple, consisting almost entirely of atomic resonance lines and fill gas lines. Therefore, the selection of emission lines is relatively easy. As already described, to simplify the interpretation of an observed intensity ratio (to utilize eq. 3), it is necessary to select a pair of emission lines so that their excited states may have almost the same energy levels. It is also significant to distinguish the proposed lines of sputtered materials from emission lines of gas since the density of atmosphere gas atoms is much higher than that of removed atoms. In the Ag-Cu system, four resonance lines (Cu I 324.7 nm, Cu 1327.4 nm, Ag I 328.1 nm, and Ag I 338.2 nm), which have nearly same energy levels (see Table 11) and have no gas emission interferences, are able to be selected as analytical
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The series of Ag-Cu, Cu-Ni, and Ni-Co alloys were prepared with 99.99% Ag, 99.99% Cu, 99.9% Ni, and 99.2% Co, respectively, in an induction furnace with Ar atmosphere. Silver-copper and Cu-Ni alloys were rolled to about 1.0 mm thickness and annealed at 600 "C for 10-12 hr with 10-2-10-3 Pa. Nickel-cobalt alloys were cut in the thickness of about 20 mm and annealed Pa. The compsotions of these alloys at 800 OC for 18 h with are summarized in Table I. The surfaces were polished with waterproof emery papers (no. 6Wno. 1500) and finished to mirror-faces with emery cloth. To remove the contaminations and the surface oxide, predischarge was employed with the following conditions: (a) Ag-Cu, for less than 1 min at 5-10 W (370 V, 13.5 mA to 450 V, 22.5 mA), (b) Cu-Ni, for 3-5 min at 10-15 W (450 V, 22.5 mA to 490 V, 30.5 mA), (c) Ni-Co, for 3-5 min at 15-20 W (500 V, 30.0 mA to 550 V, 36.5 mA).
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Figure 1. The normalized intensitles for two emission lines of the same element In Cu-52.4 at. % Ni: (a) Ni I 339.3 nml(Ni I 339.3 nm Ni I 341.5 nm), (b) Ni I 352.5 nm/(Ni I 352.5 nm Ni I 341.5 nm), (c) Cu I 327.4 nm/(Cu I 327.4 nm Cu I 324.7 nm).
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lines. Two intense nickel resonance lines (Ni 1341.5 nm and Ni I 352.5 nm) are satisfactory and, in the case of Cu-Ni alloy, Cu 1327.4 nm is better because Ni 1324.8 nm may have an influence upon the intensity of Cu I 324.7 nm, particularly when copper concentration is low. As the positions of nickel emission lines overlap with those of cobalt in most cases, it is difficult to find a suitable pair of intense emission lines which fit the condition already stated. However, the following lines, Co 1340.5 nm, Co 1345.5 nm, Co I 350.2 nm, and Ni I 361.9 nm, which have no interferences and have approximately the same energy levels (see Table 111, are available. Table I1 shows the selected emission lines of each element in three different alloys (11, 12). Selection of Input Power. Sputtering rate is controlled mainly by the ion species, the kinetic energy, and the current density of primary ions (13). Generally speaking, at gas pressures where the mean free path of the primary ion is large compared to the lamp dimensions, the bombarding ion energy is nearly equal to the voltage applied between the cathode and anode (14),and if the secondary electrons created by collisions can be ignored, the current density of primary ions is approximately equal to the measured current density. However, in this study, as gas pressure is rather high, the kinetic energy and the current density of primary ions cannot be monitored directly. On the other hand, the supplied power indirectly controls not only these parameters but the plasma charac-
Table 11. Selected Emission Lines of Each Element and Their Assignments alloys wavelength assignment Ag-Cu Ag 1 3 3 8 . 2 nm 'P,,, (3.66 eV)-'S,,, (0.00 eV) Ag 1 3 2 8 . 1 nm 'P,,, (3.78 eV)-2Sliz(0.00 eV) Cu 1 3 2 7 . 4 nm 'P,,, (3.79 eV)-zS!i2(0.00 eV) Cu 1 3 2 4 . 7 nm 'P,,, ( 3 . 8 2 eV)-'S,,, (0.00 eV) Cu-Ni Cu 1 3 2 7 . 4 nm 'P,,, (3.79 eV)-'S,,, (0.00 eV) Ni I 339.3 nm 3D, (3.67 eV)-,D, ( 0 . 0 3 eV) Ni 1 3 4 1 . 5 nm ,F, ( 3 . 6 6 eV)-,D, (0.03 eV) Ni I 352.5 nm 'P, ( 3 . 5 4 eV)-,D, (0.03 eV) 'F, (3.85 eV)-'D, (0.42 eV) Ni-Co Ni 1 3 6 1 . 9 nm Co 1 3 4 5 . 5 nm 345.52 nm 4Dl,, (3.81 eV)-4F3,z( 0 . 2 2 e V ) 345.35 nm 4Dl,,, 9 i z (4.02 eV)-4F,,2 ( 0 . 4 3 e V ) Co 1 3 4 0 . 5 nm 4F,,, (4.07 eV)-4F9iz(0.43 eV) Co 1 3 5 0 . 2 nm 4D,,, (3.96 eV)-4F9,z( 0 . 4 3 e V )
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