Measurement of differential sputtering yields by laser fluorescence spectroscopy

Nuclear Instruments and Methods North-Holland, Amsterdam

in Physics

Research

B33 (1988) 511-514

MEASUREMENT OF DIFFERENTIAL SPUTTERING BY LASER FLUORESCENCE SPECTROSCOPY Y. MATSUDA,

K. YAMAGUCHI,

T. KAJIWARA,

511

YIELDS

K. MURAOKA

and M. AKAZAKI

of Energy Conversion, Kyushu University, Kasuga Fukuoka 816, Japan

Department

C. HONDA, T. OKADA and M. MAEDA Department

of Electrical Engineering.

Kyushu

University, Hakozaki

Fukuoka

812, Japan

Y. YAMAMURA Department

of Applied Physics, Okayama

University of Science, Ridaicho

Okayama

700, Japan

E. KAWATOH Department

of Applied Physics, Osaka University, Suita Osaka 565, Japan

J. FUJITA Institute

of Plasma Physics, Nagoya

University

The technique of laser fluorescence bombardment by observing the transient

Chikwa

Nagoya 464, Japan

spectroscopy (LFS) was exploited change of the velocity distribution

for the study of surface phenomena during ion-beam functions from pure iron and iron-oxide targets. It was shown that the surface binding energy of compound as well as pure materials could thus be measured. In addition, the LFS technique was expanded to the VUV region by observing the fluorescence signal of carbon atoms sputtered from a graphite target.

1. Introduction Information d*Y/dO

2. Experimental about

d v, namely

the

differential

the angular

and

sputtering velocity

yields distribu-

tions of sputtered particles, is important not only for understanding fundamental sputtering processes but also for plasma surface interactions in industrial applications and in nuclear fusion devices. Recent advances of the technique of laser fluorescence spectroscopy (LFS) have enabled such detailed measurements to be made [l]. Previously, we measured the angular distributions of sputtered iron (Fe) and titanium (Ti) atoms [2]. As a result, effects of the surface collisions and insufficiently developed collision cascades on the angular distribution were elucidated. However, there are two areas of further expansion of LFS into sputtering studies, namely the instantaneous measurements of a velocity distribution function using the already established rapid-frequencyscan dye laser (RAFS) [3,4], and the detection of light elements, whose resonance wavelengths are in the vacuum ultraviolet (VUV). It is the purpose of this paper to report results of such detailed measurements of differential sputtering yields, and to discuss their implications for surface physics. 0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

For the LFS measurements of velocity-distribution functions of sputtered metal atoms such as Fe and Fe,O,, the second harmonic of RAFS was used as a laser radiation source [5,6]. The experimental setup is shown in Fig. 1. Two types of ion guns were used in the present experiment, namely a simple electron-bombardment type (A) with a maximum current of 60 PA at 3 kV, and a duoPIGatron type (B) with a maximum current of 1 mA at 7 kV. In all these measurements, the working gas was argon (Ar). During the ion source operation, Ar pressures in the sputtering chamber were kept at 2 X 1O-5 and 4 x 10e4 Torr for (A) and (B), respectively. The bombarded diameters on the target surface were 6 and 4 mm FWHM, for (A) and (B), respectively. The laser beam at the wavelength around 302.06 nm (a5D4-y5Di) was incident normal to the target surface, making an angle of 45” to the ion beam axis. The fluorescence around 382.04 nm (y5Di-a5F4) was detected from sputtered neutral Fe atoms within a cylinder defined by the laser beam diameter of 3 mm and a slit which allowed a view of a region from 10 mm to 26 VII. SPUITERING/SIMS

512

Y. Matsuda et al. / Measurement

of differential sputtering yields by laserfluorescence

DuoPlGatron ion source

Argas

spectroscopy

3. Results and discussion 3.1. Vekity distribution functions of Fe atoms sputtered from pure iron and iron-oxide targets

c

[tvlicroccmputer

/-+/xv plotter]

Fig. 1. Experimental setup for the measurement of velocity-distribution functions of Fe atoms sputtered from pure iron and iron-oxide targets.

the target surface. A photomultiplier detected the fluorescence signal from a direction perpendicular to both the ion and laser beam with a solid angle of 0.1 sr. This geometrical ~angement of scattering volume was chosen by taking the trade-off into account between ejection angle resolution, which favours smaller scattering volume at larger distance from the target, and SN ratio, which favours the exact opposite. Both the fluorescence and the laser intensity signals were recorded by a two-channel wave memory and processed by a microcomputer system. Each measured fluorescence signal profile was modified into a profile of a velocity distribution function by compensating for the change in laser power intensity during wavelength scanning. Saturation characteristics of the fluorescence by the excitation power were derived experimentally, and were compensated in the data processing. The transit-time broadening and powerbroadening effects [7,8] were neglected here because the laser spectral width was much larger than the transittime and power-broadened atomic linewidth. On the other hand, for the detection of sputtered carbon (C) atoms, VUV radiation by two-photon resonance four-wave sum mixing (4WM) in Mg vapour of an excimer-laser pumped dye-laser was used [9]. A graphite target was bombarded by the ion gun (B) at normal incidence, and the laser beam was incident parallel to the target surface at a distance of 5 mm. Fluorescences from a scattering volume of 20 mm in length in front of the bombarded area were directly detected by a solar blind photomultiplier placed 20 mm from the observation point without a collecting lens. Sputtered C atoms were excited by the VUV laser radiation at around a wavelength of 165 nm, and fluorescence at the same wavelength was observed. mm from

Fig. 2 shows a transient change in the velocity distribution functions of sputtered neutral ground-state iron atoms after ion bombardment was started, where the type (A) ion gun was used at 15 PA and 3 kV. Here, a pure iron target had been mechanically polished with a roughness of less than 1 pm in air and no further preparation was made before it was placed in the vacuum chamber. Each profile shown in the figure is an average of three successive data obtained at one second intervals. Laser fluorescence signals due to the backscattered laser light on the target surface appear in the figure as mirror images on the negative velocity side. It is easily seen that, as the irradiation time progressed, (1) the fluorescence-signal intensity increased, and (2) the profiles of the velocity distribution functions changed, namely, the profile curves gradually narrowed and their peaks shifted to a lower energy. The geometrical arrangements, shown in the inset of fig. 2, are mainly chosen due to the limited access through ports. Because the experiments were performed in the fully developed cascade region (relatively heavy argon ions impinging on metals at a sufficiently high energy of 3 keV), we could disregard contributions of primary knock-on atoms on sputtered particles.

Wavelength

1

Fig. 2. Transient change in vel~ity~ist~bution functions of sputtered Fe atoms state. Each profile is an average of three successive

data obtained

at one second intervals.

Y. Matsuda et al. / Measurement

of differential sputtering yiela5 by laser fluorescence

The determination of the surface binding energy U, was thoroughly discussed by Kelly [lo] for metals, oxides and halides, where U, in the Thompson formula [ll] was taken as a free parameter to fit the observed velocity distribution function. The procedure was adopted here in interpreting the data in fig. 2. The surface binding energies thus estimated are plotted against the ion dose in fig. 3. The surface binding energy was initially (10 + 1) eV, but it approached (4.3 k 0.4) eV for ion doses of more than 5 X 1019 ions/m2. The latter value of the surface binding energy is consistent with the sublimation energy of pure iron (4.3 eV). This fact reveals that oxide layers are completely removed after an ion dose of 5 x 1019 ions/m2. It is difficult to give the accurate number of monolayers removed by this ion dose, because molecular contribution to the sputtering yield becomes important for oxides. Taking measurements by Dullni [12] into account, we estimated the removal of five monolayers to be the lower limit. Because U, estimated for pure iron agrees well with the sublimation energy (4.3 ev), we estimate U, for iron oxides (U, at an ion dose of less than 1 X 1019 ions/m* in fig. 3) by the method proposed by Dullni [13]. He suggested calculating the binding energy from the tabulated formation enthalpy. For the present case, we calculated the formation energy per iron atom for iron oxides such as FeO, Fe,O, and Fe,O,, by using thermodynamic data of oxide formation reactions, sublimation of iron, and dissociation of an oxygen molecule, as follows: FeO(s)

--) Fe(g) + O(g) - 9.7 eV,

fFe,O,(s)

+ Fe(g) + $0(g)

:Fe,O,(s)

+ Fe(g)

(I)

- 11.7 eV,

(2)

+ GO(g) - 12.5 eV,

(3)

_. >a, 2

12

t 5 10

Fe203

2

6

4 Ion

dose

8 (x10’

10

12

ions /m2)

Fig. 3. Change in the equivalent surface binding energy versus ion dose.

target

5

0

10

Wavelength Velocity

1

( pm)

( km/s

)

Fig. 4. The velocity-distribution function of Fe atoms sputtered from a Fe,O, target. The dotted line shows a convoluiion curve of the Thompson formula calculated for U, = 9 eV and the Lorentzian laser spectra of 2 pm FWHM.

where s and g stand for solid and gas, respectively. The observed surface binding energy at a low ion dose (less than 1 x 1019 lons/m2) . clearly correlates with these calculated formation energies of iron oxide. In order to verify these arguments, a velocity-distribution function of sputtered iron atoms from a wellcharacterized iron oxide (Fe,O,) target was measured. Here, the ion gun (B) was used to obtain sufficient fluorescence signals even for this material with a much reduced sputtering yield. The results are shown in fig. 4. The dotted line in the figure shows a fitting curve, which was obtained by the convolution of the Thompson formula calculated for U, = 9 eV and the laser spectrum having a Lorentzian profile with 2 pm FWHM. While experimentally obtained values of U, for pure iron agree well with the calculated value (4.3 eV> as shown above, some differences between the experiment (8.5-10.5 eV) and calculations (12.5 eV from eq. (3)) are noticed for the iron oxide. For an explanation of the disagreement, we can suggest the possibility of surface enrichment of Fe on the Fe,O, surface by the ion-beam bombardment. However, further experiments are necessary to definitely identify the cause of the disagreement. 3.2. Detection

0

513

spectroscopy

of sputtered

C atoms

A fluorescence profile observed by sweeping the wavelength of the VUV laser is shown in fig. 5. Closed circles show the averages of ten shots, and error bars give their scatters. This experimental profile represents VII. SPUTTERING/SIMS

Y. Matsada et al. / Measurement

514

2

2.0

of differential sputtering yields by laser fluorescence

spectroscopy

from carbon atoms sputtered from a graphite target using the light source by 4WM.

Carbon

The authors wish to thank Prof. R. Shimizu and the referee for useful comments. They are also grateful to Messrs. T. Hayashida (now at Kyushu Matsushita Electric Company) and K. Suenaga for their technical help in the experiment. This work was supported in part by the Special Project Research on Ion Beam Interactions with Solids from the Japanese Ministry of Education, Science and Culture. -5

0 +5 Wavelength

+10

+15

( pm 1

Fig. 5. Fluorescence signal intensity of sputtered carbon atoms

versus wavelength.Left and right peaks correspond to 165.692 and 165.701 inn, respectively.Closed circles show averages of ten data and the error bars show their scatter. Three solid curves show two-Gaussian curves with respective spectral widthsof 4,6 and 8 pm. a convolution of a laser spectrum and line profiles of sputtered carbon atoms. Two resonance signals which correspond to 165.692 and 165.701 nm were observed. Experimental data were fitted with two-Gaussian curves with respective spectral widths of 4, 6 and 8 pm. From the curve fitting, the spectral width of the profile could be determined to be 5-8 pm. Large error bars were caused by the fact that the photoelectron number was at most 10 at the maximum. From the absolute calibration by Rayleigh scattering, the carbon density measured in the experiment was evaluated to be about 1014 me3. Details of the carbon detection will be described elsewhere in the near future [14].

4. summary Detailed measurements of velocity-distribution functions of sputtered iron atoms from pure iron and wellcharacterized iron-oxide (Fe,O,) targets revealed that such information could be used for obtaining surface binding energies of compound as well as pure materials. In addition, the LFS technique was expanded to the VUV region by the first observation of fluorescences

References PI K. Muraoka, M. Maeda, T. Okada, C. Honda, M. Hamamoto, K. Uchino, T. Kajiwara, Y. Matsuda, Y. Itsumi, M. Akazaki, T. Kawamoto, R. Kumazawa, S. Okamura, H.R. Garner and RFC-XX-M Group, J. Nucl. Mater. 128/129 (1984) 977. PI Y. Matsuda, Y. Yamamura, Y. Ueda, K. Uchino, K. Muraoka, M. Maeda and M. Akazaki, Jpn. J. Appl. Phys. 25 (1986) 8. [31 C. Honda, K. Nishimura, M. Maeda, K. Muraoka and M. Akazaki, J. Phys. D16 (1983) 1943. 141 C. Honda, M. Maeda, K. Nisbimura, K. Muraoka and M. Akazaki, Appl. Phys. B33 (1984) 171. (51 Y. Matsuda, Y. Yamamura, C. Honda, S. Matsubaguchi, M. Maeda, K. Muraoka and M. Akazaki, Jpn. J. Appl. Phys. 25 (1986) 1256. 161 Y. Matsuda, C. Honda, S. Matsubaguchi, T. Moroishi, M. Maeda, K. Muraoka and M. Akazaki, Jpn. J. Appl. Phys. 25 (1986) L182. 171 M.J. Peltin, R.B. Wright and D.M. Gruen, J. Chem. Phys. 74 (1981) 6448. PI R.B. Wright, M.J. Pellin and D.M. Gruen, Surf. Sci. 110 (1981) 151. [91 T. Okada, A. Nishio, S. Araki, M. Maeda, K. Muraoka and M. Akazaki, Rev. Laser Eng. 14 (1986) 136 in Japanese. WI R. Kelly, Nucl. Instr. and Meth. B18 (1987) 388. [111 P. Sigmund, Sputtering by Particle Bombardment, ed. R. Behrisch (Springer, Berlin, 1981). P21 E. Dullm, Appl. Phys. A38 (1985) 131. t131 E. Dulhri, Nucl. Instr. and Meth. B2 (1984) 610. (141 T. Kajiwara, Y. Matsuda, T. Okada, M. Maeda, K. Muraoka and M. Akazaki, Jpn. J. Appl. Phys., to be submitted.