Sputtering and secondary ion yields of TiAl alloys subjected to oxygen ion bombardment

491

Surface Science 140 (1984) 491-498 North-Holland, Amsterdam

SPUTTERING AND SECONDARY ION YIELDS SUBJECTED TO OXYGEN ION BOMBARDMENT K. INOUE

OF Ti-AI

ALLOYS

and Y. TAGA

Toyota Central Research and Development Laboratories, Inc., Nagakute - rho, Aichi ken, 480 11, Japan Received

16 September

1983; accepted

for publication

8 February

1984

Total sputtering and secondary ion yields of single phase Ti-Al alloys containing O-35 at% Al under oxygen ion bombardment were measured for thin film targets with known thicknesses and compositions. It was found that the total sputtering yield initially decreased with increasing Al content and became constant beyond 20 at% Al. On the other hand, the secondary ion yield of Al roughly increases with increasing Al content, while that of Ti shows a complementary decrease. Their variations, however, show a large flucutation with Al content. Detailed observations revealed that the secondary 0, ion yield also exhibited the same pattern of fluctuation as that of Al and Ti. The modified degrees of ionization of both Al and Ti in the consideration for the secondary oxygen ion yield indicated an exponential dependence upon the alloy composition. The trends of the modified degrees of ionization revealed that the matrix effect due to the alloy composition affected more strongly on Ti than on Al.

1. Introduction The secondary ion mass spectrometry (SIMS) technique has been successfully applied to a wide variety of materials [l-3], but quantitative analysis has long been hampered by lack of understanding of the factors governing the sputtering and the secondary ion emission processes. In practical SIMS analysis, an 0: beam becomes very popular, because it enhances and stabilizes the positive secondary ion yield [4]. In our previous study [S], we determined the yields of sputtering and secondary ion emission of pure metals under 0: bombardment by the so-called depth profiling technique of multi-layer targets with a known thickness. The result revealed that the sputtering yield of metals varied linearly with the energy transfer factor in the classical head-on collision model with a negative slope, and a linear relationship was found between the ionization potential and a modified degree of ionization, which can be expressed by the ratio of the secondary ion yield and the secondary 0: current. For pure metals, therefore, the present authors have found a unified explanation governing the sputtering and the secondary ion emission. On the other hand, it is well known that SIMS signals are sensitive to the 0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

K. Inoue,

492

Y. Taga / Sputtering and SI yields of Ti - Al

so-called matrix effect, i.e., the local chemical environment of the target atoms [6]. To develop a quantitative SIMS analysis, it has been necessary to understand the factors governing the matrix effect, which may strongly influence the process of sputtering and secondary ion emission of alloys. In this study, we determined the yields of sputtering and secondary ion in the same way as the emission of Ti-Al alloys under 0: bombardment measurement for pure metals. The degree of ionization derived from the above two yields was also discussed.

2. Experimental About 2000 A thick Ti-Al alloy films of various compositions were deposited onto Si wafer substrates by the magnetron sputtering technique. Two separate magnetron sputtering sources in the same vacuum chamber were used for simultaneous deposition of the components. Compositions of Ti-Al alloy films were controlled by changing the input powers on the sputtering sources of Ti and Al. Quantitative determination of the thickness and composition of the films was performed by electron probe microanalysis (EPMA) [5,7]. A typical example is shown in fig. 1 which indicates a comparison between EPMA and chemical analysis for Al contents in Ti-Al films. Good agreement was found between the two analyses, so it was confirmed that the compositions determined by EPMA were valid. The crystal structure of the samples was determined by X-ray diffraction technique and was found to correspond to the solid solution. The lattice

Ti-AI

4.8

12.8 0

50

25

”

at % Al (Chemical Analysis)

Fig. 1. Comparison analysis.

of Al atomic

Fig. 2. Lattice constants

of Ti-AI

concentrations

in Ti-AI

alloy films as a function

40

20 at %Al

films between

of Al atomic

EPMA

concentration.

and chemical

K. inoue, Y. Taga / Sputtering and SI yields of Ti - AI

493

constants of Ti-Al films obtained in this study were shown in fig. 2 with those determined by Clark et al. [8] for bulk Ti-Al alloys for comparison. Good agreement was found between the two lattice constants. This result revealed that the samples formed solid solutions. The lattice constants were also used to calculate the number of atoms per cm3. The lattice constants of the samples which were not performed by X-ray diffraction were determined by the results of Clark et al. The yields of sputtering and positive secondary ion emission were simultaneously measured by the depth analysis technique. The total sputtering yield of each alloy film was determined by the time to sputter away the alloy film, which was measured by the depth analysis, the primary current and the number of atoms contained in the sputtered volume. Measurement of the secondary ion current was performed with an ion microprobe mass analyser (Applied Research Laboratories). The mass filtered 0: ions struck the target at 18.5 keV at normal incidence and were rastered to provide uniform sputtering over an area of 70 pm x 70 pm. The primary ion beam current and diameter were 10 nA and 10 pm, respectively. Electronic gating was used to limit the data acquisition on secondary ion current to the central part (5%) of the crater, to avoid crater edge effects. The chamber pressure was maintained at 10m5 Pa during ion bombardment. The positive ion yield was finally determined after isotope correction of the measured secondary ion current. It is well known that in SIMS study small changes in the experimental conditions are very much influential on secondary ion emission. Therefore, reproducibility was certified three times and each measurement shown in the following figures denotes the average data.

3. Results The total sputterng yield Y is shown in fig. 3 as a function of Al atomic concentration. As shown in the figure, the total sputtering yield initially decreased with increasing Al content and became constant beyond 20 at% Al. In fig. 3, the broken line indicates a straight line connecting sputtering yields of pure Al and Ti. It was found that the total sputtering yield of single phase Ti-Al alloy did not change monotonously with the alloy composition. The secondary ion yields 1, are shown in fig. 4 as a function of Al atomic concentration. It was found that the secondary ion yield of Al increases with Al content, while that of Ti shows a complementary decrease. While their variations show a large fluctuation with Al content, the results indicate that the secondary ion yields of Al and Ti change primarily in accordance with the alloy composition. The secondary oxygen ion yields are shown in fig. 5. The secondary ion yields of 0 and 0, show the same decrease with Al content,

K. Inoue, Y Taga / Spurrering and SI yields of TI -Al Ti-AI

Ti-AI

0

25 at % Al

Fig. 3. Total sputterng yield Y of Ti-AI function of AI atomic concentration. Fig. 4. Secondary

0-

50

0

25 at%Al

50

alloy and the term X derived from Sigmund’s

ion yields of Al and Ti as a function

of Al atomic

theory as a

concentration

which results in constant yield ratio of 0,/O irrespective of the alloy composition. Comparison between figs. 4 and 5 reveals that the fluctuation pattern of the secondary ion yields of Al and Ti are very similar to the increase and decrease of the secondary oxygen ion yields. This result confirms that the oxygen on the sputtered surface provides a considerable effect on the secondary ion emission [9].

25

50

at % Al

Fig. 5. Secondary concentration.

0 and 0, ion yields and their ratio from Ti-Al

alloy as a function

of Al atomic

K. Inoue, Y. Taga / Sputtering and SI yields 01 Ti - AI

495

4. Discussion

4. I. Sputtering

yield

We recently reported [5] that irrespective of the chemical nature of the primary ion species, the sputtering process of pure metals under oxygen ion bombardment could be simply explained in terms of the classical atomic collision model and be subjected to Sigmund’s theory [lo]. According to Sigmund’s theory, the sputtering yield is given by: Y = O.O42aS,( E)/U,,

(1)

where (Yis a factor depending on the target mass (M,)/primary ion mass (M,) ratio, S,(E) is the elastic stopping cross section when E is the energy of the primary ion, and U, is the surface binding energy of the sample, which is usually expressed by the sublimation energy. (x and S,(E) contain the target mass M, and the target atomic number Z,. While V, in an alloy under oxygen ion bombardment cannot be simply obtained with the present knowledge, (Y and S,,(E) of alloy targets can be derived from calculation by assuming M, and Z, of an alloy to be replaced by the average values of the pure alloys, weighted according to the respective atomic concentrations. Then we try to discuss the sputtering yield by means of the X which can be obtained by the calculation and expressed by the relation X= O.O42olS,,( E),

(2) where we used an experimentally derived value of (Yas given by Andersen and Bay [ll] and S,,(E) was derived from calculations [12] based on the Thomas-Fermi model. As shown in fig. 3, the behaviour of the sputtering yield does not agree well with that of the X values. These results suggest that the sputtering process of Ti-Al alloys cannot be simply explained in terms of Sigmund’s theory unless we take the surface binding energy of an alloy into consideration. Unfortunately, no sputtering yield data of Ti-Al alloys have been reported. On the other hand, Betz [13] and Meyer and Wehner [14] reported that the yields of single phase alloys changed monotonously with the alloy compositions, but Gnaser et al. [15] observed the existence of a minimum in the yield of the single phase Au-Pd alloy system. By taking other authors’ data into account together with ours, it is concluded that the sputtering behaviour of alloys is not particularly attributed to the solubility, and the surface binding energy is a strong factor governing the sputtering process of alloys in the same manner as the case of pure metals. 4.2. Degree of ionization The degree of ionization P,

=Z,/(N,e),

of an element

M, PM, is given by: (3)

K. Innow, Y. Taga / Sputtering and SI yields of TI - AI

496

where IM = secondary ion yield of an element M, N, = number of sputtered atoms of an element M, and e = electron charge. According to the preferential sputtering model by Saeki and Shimizu [16], the composition of sputtered atoms from an alloy is equal to that of the alloy target. So, N, is obtained by the relation: eN,

= I,YC,

,

(4)

where I, = primary current, Y = total sputtering yield and C, = concentration of an element M. From eqs. (3) and (4), PM is obtained by the relation PM = W(I,YC,).

(5)

The degrees of ionization p,, and Pr, are shown in fig. 6 as a function of Al atomic concentration. As shown in the figure, the degrees of ionization of both Al and Ti indicated the trend of increase with increasing Al content, but they varied with a large fluctuation. Pivin et al. [17,18] and Yu and Reuter [19] studied secondary ion emission from binary alloys under Ar+ bombardment in oxygen ambience and 0: bombardment, respectively. Pivin et al. supposed the empirical formula for the degree of ionization and Yu and Reuter supposed the rule between the degree of ionization and the energy of the oxide bond. But we cannot discuss our result in terms of an agreement with the above authors’ suppositions, since the degrees of ionization obtained in this study did not indicate a clear relationship with the alloy composition, as shown in fig. 6. From fig. 5 which illustrates the behaviour of the secondary oxygen ion yield with Al content, it is found that the degrees of ionization fluctuated very similarly to the behaviour of the secondary oxygen ion yield. This result suggests that the oxygen on the sputtered surface provides a considerable effect

,o-2

I

lo-“0

,

25

Ti-Al,

50

at % Al Fig. 6. Degrees of ionization and modified degrees of ionization O2 ion yield as a function of Al atomic concentration.

in consideration

for the secondary

K. Inoue, Y. Taga / Sputtering and SI yields of Ti- AI

491

on the ionization. We therefore tried to modify the degree of ionization by dividing by the secondary oxygen ion yield in the same manner as the modification which we recently reported [5] for the degree of ionization of pure metals. In the case of pure metals, a difference was found between the modification by the secondary 0, and 0 ion yields. In this work, however, the two oxygen ion yields changed similarly with Al content and, as shown in fig. 5, the ratio between them was practically constant with Al content. No difference was therefore found between the modified degrees of ionization in consideration for two secondary oxygen ion yields, qualitatively. The modified degrees of ionization in consideration for the secondary 0, ion yield are solely shown in fig. 6. It is found that the modified degrees of ionization of both Al and Ti show an exponential dependence upon Al atomic concentration; namely, the relationship was derived for single phase Ti-Al alloy: log(

P/IO,

> = wk

+ k2’

where j3 = degree of ionization, Zo, = secondary 0, ion yield, C,, = Al atomic concentration, and k, and k, are constant. The gradients k, of Al and Ti in eq. (6) indicate the degrees of matrix effect due to the alloy composition and were estimated to be 1.0 and 2.0, respectively. It was found that the matrix effect due to the composition of the Ti-Al alloy affected Ti more strongly than Al.

5. Conclusion The yields of sputtering and positive secondary ion emission under oxygen ion bombardment were simultaneously measured for single phase Ti-Al alloy films by the depth analysis technique. And the degrees of ionization were derived from the above two yields. We have shown that the total sputtering yield did not change monotonously with the alloy compostion and that the sputtering behaviour may be particularly related to the surface binding energy. The modified degrees of ionization of both Al and Ti in consideration for the secondary oxygen ion yield indicated an exponential dependence upon the alloy composition. The trends of the modified degrees of ionization revealed that the matrix effect due to the alloy composition affected Ti more strongly than Al.

Reference [l] H. Lieble, J. Vacuum Sci. Technol. 12 (1975) 385. [2] H.W. Werner, Surface Sci. 47 (1975) 301. [3] A. Benninghoven, Surface Sci. 53 (1975) 596.

498

[4] [S] [6] [7] [8] (91 [lo] [ll] [12]

[13] [14] [IS] [16] [17] (1X] 1191

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