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Velocity measurements of sputtered particles using the Laser-Doppler method
Nuclear Instruments and Methods 170 (1980) 287- 293 © North-Holland Publishing Company
287
VELOCITY MEASUREMENTS OF SPUTTERED PARTICLES USING THE LASER-DOPPLER METHOD W. HUSINSKY *, R. BRUCKMULLER and P. BLUM
Institut ffir Allgemeine Physik, Technical University Vienna, Vienna, Austria The development of tunable single frequency dye lasers has enabled the realization of a Doppler-Shift-Laser-Spectrometer (DSLS) for the detection and energy analysis of neutral sputtered particles. It is based on the Doppler-shifted resonance excitation by monochromatic radiation. The particle beam to be measured is intersected, by two laser beams at 90 ° and 30 ° respectively. The laser is tuned over a resonance line of the species to be investigated. The sharp non-Doppler-shifted 90 ° spectrum is used as zero marking on the velocity axis. The intensity of the Doppler-shifted 30 ° fluorescence spectrum is directly proportional to the particle intensity in the corresponding velocity interval. A high detection sensitivity of about 50 particles/cm3 and a velocity resolution of 50 m/s can be achieved. With the DSLS it is possible to investigate one particular kind of sputtered atoms with high resolution over a wide energy region. Results for different targets bombarded with rare gas ions are presented. The obtained spectra allow to determine the influence of slow thermal processes, thermal spikes and collision cascades to the sputtering process. A comparison with results obtained with time-of-flight experiments is given.
1. Introduction During the last years an increasing interest arose in the detection and energy analysis of neutral particles. Particular interest in a universally applicable tool for neutral particles exists in surface physics. Many experiments in the large field o f atomic collisions with solids as for instance sputtering or scattering involve the problem o f the energy analysis o f neutral particles. In the following paper a velocity spectrometer for neutral particles based on the Doppler-shifted resonance excitation o f atoms is described (DSLS) [1,2]. In the first part a detailed description o f the L a s e r Doppler m e t h o d and the spectrometer developed in our laboratory is given. Its features and advantages compared with other methods will be discussed. A similar spectrometer using a pulsed dye laser system [3] will also be shortly described. In the second part some results obtained with the spectrometer in sputter experiments will be discussed. A comparison with similar experiments using a correlation-time-of-flight m e t h o d will be given [4].
* Temporary address: ORNL, Solid State Division, Oak Ridge, Tenn. 37830, U.S.A.
2. The optical m e t h o d for the detection and energy analysis o f neutrals
2.1. DSL-spectrometer In general, two steps are performed for the energy analysis o f materials: a) the detection o f the neutrals, b) the velocity analysis. Depending on the type o f particles there exists quite a lot o f proven methods for the detection of neutrals, such as for instance surface ionization, ionization by electrons, magnetic resonance techniques, radioactive tracers or others. Combined with a time of flight apparatus or with a velocity selector these detection methods can be used for velocity measurements of neutrals [ 4 - 6 ] . Compared to the methods mentioned above resonance excitation by photons has an important advantage as cross sections for optical excitation in the visible region are o f the order o f 10 -9 c m 2. In comparison to the cross sections o f ionization by electrons o f 1 0 - 1 5 - - 1 0 -17 c m 2 they are rather high. Furthermore, using the optical m e t h o d and making use o f the Doppler-effect the particle velocity can be measured directly. Fig. 1 gives a schematic view o f the DSL spectrometer using a c.w. tunable dye laser. Suppose that a particle beam is traveling on the same axis VII. HIGH DENSITY CASCADES
288
W. Husinsky et aL / Velocity measurements of sputtered particles
4.0
I
r
I
>w "-J
r=tO -8
w=
ORNL-DWG 79-15065 I __ " 1.107 a 2.107 • 4"107 " 5 11d • 9.10 7
IA A , , ,
~o.5Frl
,,
o
7---.
H ~11
I~ A "
_
9-t0 8
,
r
.
~
_
i O
Fig. 1. Schematic view of the DSLS spectrometer using a c.w. tunable dye laser system.
as the ion beam indicated in the figure. The particle beam is produced by ion bombardment of a target. The particle beam itself is collimated by diaphragms. Its maximum aperature is about 1°. This particle beam is now intersected by a laser beam. For the further discussion we have to distinguish between two cases; first that the intersecting angle is 90 °. In this case the optical excitation is Doppler-free. If the frequency of the laser coincides with one transition frequency of the particles investigated, they are excited, independently of their velocity. After excitation there are two possibilities to return to one of the lower states; either by induced or by spontaneous emission. The induced emission is a coherent process in the direction of the laser beam. The spontaneous emission which is an isotropic, incoherent process can be registered easily by a photomultiplier detection system. The spontaneously emitted intensity is directly proportional to the number of particles in the excited state. Fig. 2 shows the time dependent population of an upper state for different laser intensities saturation of the upper level is obtained. This is of great advantage because in this case the fluorescence signal is independent of laser intensity fluctuations. Typical saturation values are for instance for the Na-D2 line about 0.2 W/cm 2. This result is obtained by calculation as well as by experiment [1 ].
0
tO
20
30 TIME (nsec)
40
Fig. 2. Time dependent evolution of the population of the upper state after excitation by a monochromatic radiation. The life time for spontaneous emission of the transition is 10 -8 s corresponding to the value for many dipole transitions. The population is plotted for some cases of increasing
laser power w (in arbitrary units).
For the excitation by the laser beam intersecting under 90 ° we thus obtain a sharp fluorescence peak at exactly the transition frequency Uo of the particles. This peak is an exact mark for zero velocity. For excitation by the second laser beam intersecting under 0 = 30 ° the situation is different. In this geometry the particles 'see" a different frequency due to the Doppler effect
+ os0) In order to excite particles with velocity u the laser must be detuned by the amount (u/c) cos 0. The velocity resolution is primarily given by the line width of the laser and the transition used. In sputter experiments we expect velocities from some 100 m/s for thermal particles up to some 100 km/s for particles in the eV range. The corresponding Doppler shifts are some I00 MHz up to 100 GHz. A maximum laser bandwidth of some ten MNz is therefore allowed in such an experiment. Only the development of tunable dye lasers has enabled the development of a DSLS. In table 1 the specifications of the DSLS developed in our laboratory are summarized. Up to now a DSLS based on a c.w. single mode, tunable dye laser which has the advantage of an excellent velocity resolution due to its small bandwidth of about 50 MHz has been discussed. Alas, nowadays c.w. single mode dye lasers still have a
W. Husinsky et al. / Velocity measurements o f sputtered particles
289
Table 1 Specifications o f DSL-spectrometer with c.w.-dye laser.
Velocity range: (0 = 30°C) Vmax: frequency scan: ~ f = 400 G H z ~ Ao = 300 km/s, zXE = 10 keV (Na) Vmin: laser bandwidth: zXf= 60 MHz absorption linewidth: ~ f - ~ 10-5 0 MHz ~ ,Sv = 70 m/s, ,SE -~ 1 meV (Na) Velocity resolution: Low energies: laser b a n d w i d t h : zXf -~ 60 MHz absorption linewidth: Af--~ 10-50 MHz High energies: particle b e a m divergence: 2c~ = 0.6°C
zXv-~ 70 m/s av/v = 0.5 % ~E/E = 1% (Na)
Limits o f detectability: Signal-to-noise ratio: -~ 10 theoretical: p h o t o n counting: -~10 -3 particles/cm 3 analog lock-in: 1 - 10 particles/cm 3 experimental lock-in: < 102 particles/cm 3
restricted tuning range, but for many materials the resonance transitions from the ground state lie in the UV region. A possible solution is to use pulsed dye lasers with high output power and frequency doubling. Thus a tunable laser radiation in the UV region is obtainable. The main disadvantage of such a system is the higher bandwidth in the range of some GHz. A DSL spectrometer using a pulsed dye laser has been developed in Jiilich by Elbern et al. [3]. The velocity resolution of this spectrometer is about 900 m/s. It was intended for measuring the fluxes in a Tokamak experiment where only the mean velocity was of importance. It should be mentioned that since the development of c.w. ring lasers with intra-cavity frequency doubling a great step towards a c.w. DSLS in the UV region has been made.
involved in the transition are plotted. Due to the interaction between the magnetic moment of the electrons the two terms split up into two, resp. four terms according to the momentum addition 3/2 and
3p2p3¢~
i
62,5MHz
~
1772MHz
2 36,1 MHz 1 16,4 MHz 0
o~
oo
2.2. Influence of hyperfine structure A detailed knowledge of the hyperfine structure of the transition line used is necessary. In many cases, the hyperfine splitting is in the range of some hundred MHz to some GHz and can be resolved by the DSLS. If the Doppler-shifts are greater than the hyperfine splitting a superposition of identical spectra results. As an example we will consider the case of the Na-D2 transition line. In fig. 3 the two terms 3s2S1/2 and 3p2p3/2
3S2
Sl/2
1 Fig. 3. Hyperfine-split term scheme o f the Na-D 2 transition line. VII. HIGH DENSITY CASCADES
W. Husinsky et al. / Velocity measurements of sputtered particles
290
O R N L - OWG 7 9 - # 5 0 6 6
I ooo
I
I
152
I
I
20keV RARE GAS IONS-NoCI 416 6 s27Fo._~,,. 4 f e 6 s 6 p
15a
800
rF~
\ L/
147 149
o :=
600
I\ I\~
I Ne + 2 Ar <"
3 Kr + 4 Xe÷
4
400 )48 149
147
149
147
150
X
O
. o
5
I
to t5 VELOCITY (km/$ec)
20
Fig. 5. Velocity spectra of Na sputtered from a NaCI target with different rare gas ions. The ion energy is 20 keV. The lowest curve is for Ne + (and He +) bombardment. With the increasing ion mass of Ar + and Kr + the contribution due to random cascade collisions becomes more relevant. For Xe ÷ bombardment the collisional contribution is dominant. 635MHz t
Fig. 4. Fluorescence signals of different isotopes obtained in the case of a Doppler-free excitation of the transition line at
562.602 nm of Sm.
1/2, resp. 3/2 and 3/2. Taking into account the selection rule ~ = 0, -+1, six hyperfine transitions are possible as indicated in the term scheme. Due to the small bandwidth of the laser nearly all lines can be resolved if a Doppler-free excitation geometry is used (such as for the 90 ° beam). In a frequency scale generally used for velocity measurements in sputter experiments only the two hyperfine lines with a splitting of 1772 MHz are resolved. The existence of these two transition lines is of importance since the corresponding velocity spectra might overlap in a certain area. A mathematic unfolding procedure must be performed in such a case [7,8]. A more complicated case is shown in fig. 4 for Sm where the existence of many isotopes cause many hyperfine lines.
3. Velocity spectra of sputtered particles obtained
with the DSLS Extended investigations using the DSLS have been performed with sodium halide targets. Similar experiments have also been done by the group of De Vries using a time-of-flight-correlation technique. Thus, a good comparison of the results can be made. Using the DSLS we were restricted to investigate
sputtered Na from NaC1, NaBr and Nal targets. In this case, the sputter mechanisms are rather complicated as many different processes may contribute to sputtering. The features and capabilities of the DSLS can be very well demonstrated on behalf of these investigations. The excellent velocity resolution for very low velocities is a great help in these experiments, as in many cases a dominant part of sputtered particles has thermal energies [2,9,10]. Furthermore, it is a great advantage that the excitation is particle selective. Therefore, we can investigate one particular kind of sputtered species without special supplementary, often difficult, aids. An illustrative picture of the influence of different mechanisms to the sputtering of alkali-halides can be obtained by regarding velocity spectra of Na sputtered from NaCI for bombardment with different rare gas ions. As example the velocity spectra for He +, Ne ÷, Ar +, Kr ÷, and Xe ÷ bombardment of NaC1 are given in fig. 5. The ion energy was 20 keV. Let us regard the two extremes, He ÷ and Xe ÷ bombardments: For the He ÷ the energy spectrum shows a thermal shape and can be very well fitted with a MaxwellBoltzmann distribution with a temperature of about 20°C. For Xe ÷ still a great thermal contribution is found, but now a higher energy tail up to some ten eVs is found. From electron sputter experiments with halides it is known that sputtering of the halide can take place as a consequence of Vk-center production [2,11,12]. Directly connected with this the alkali
W. Husinsky et al. / Velocity measurements o f sputtered particles
291
uL
=:.
+
NACL
KR
e¢.ii
_J
u
N
....J
B ID
L~
IZENEFIO¥(EV) ZI
Fig. 6. Fitting of the measured energy spectrum of Na sputtered from a NaC1 target by 20 keV Kr÷ ions (*) using the theoretical shapes for thermal evaporation (~x),thermal spikes (o), and random cascade collisions (o).
atoms can evaporate from the surface when they are present in excess. A similar process is possible for ion bombardment. As the energy lost in inelastic collisions decreases with the ion mass this process must become less relevant for higher ion masses. As a consequence, thermal spike effects and collisional effects increase. In fig. 6 it is shown that the measured energy spectrum can be very well fitted with the theoretically expected shapes for random collisions, thermal spikes and thermal evaporation. There is a good correspondance between our measurements and those of Overeijnder et al. using a correlation-time-of-flight method [12]. The main discrepancy between our DSLS- and the time-of-flight measurements of alkali-halides lie in the form of the thermal distribution of the evaporated atoms. The DSLS measurements yield a E . e - E / k T distribution, while the time-of-flight method yield a v / f t • e - e / k T distribution [10]. After presenting some results observed with alkali-halides, some investigations of metal sputtering are discussed. For this purpose, the DSLS was used for sputter experi-
ments of Sm and Na targets bombarded with different ions. Contrary to the alkali-halides, for metal sputtering a dominant contribution from collision cascades is expected for the ion energies we have used [13,14]. A more detailed investigation and comparison of the collisional part with theoretical models is, therefore, possible [15]. For Na also thermal contributions are expected as the vapor pressure for Na is rather high even at room temperature. In fig. 7 the resulting velocity spectra for this case obtained with the DSLS are summarized. For this purpose, a Na target was heated up to about 200°C and cooled down afterwards. The velocity spectra were recorded meanwhile. The heating procedure was started after exposure of the target to 02. At room temperature, a dominant collisional contribution is found. After the heating procedure an additional small thermal peak is also present. For high temperatures an increasing thermal peak is registrated. Furthermore, it is interesting that the oxide layer after 02 inlet seems to disfavor thermally sputtered particles. The corresponding value o f the binding energy is slightly higher than after heating. What seems to be a remarkable VII. HIGH DENSITY CASCADES
W. Husinsky et al. / Velocity measurements of sputtered particles
292
velocity
km s"
velocity
k m s-'
Pig. 7. Velocity spectra of Na sputtered from a Na target by 20 keV Ar ÷ ions. (a) After exposure to oxygen the target was heated up to about 200°C. With increasing temperature a thermal peak begins to grow. (b) The target is cooled down from about 200°C to room temperature. Even at room temperature a small thermal peak is clearly visible. In (a) and (b) the spectra are normalized to the same contribution due to collision cascades which have the maximum at 2000 m/s.
t~
I
I • 0
I
me,su,oa
result o f these i n v e s t i g a t i o n s is t h e fact t h a t the collisional c o n t r i b u t i o n s h o w s a m a x i m u m at very low energies a r o u n d zero. This fact is also f o u n d in s p u t t e r e x p e r i m e n t s o f S m targets [ 1 5 ] . l f w e assume t h a t an 1 / ( E + E B ) 2 r e l a t i o n is r e l e v a n t for collision cascades we can explain t h e m e a s u r e d s p e c t r u m w i t h a m a x i m u m a r o u n d zero energy. F o r c o m p a r i s o n we have also c o n s i d e r e d t h e E / ( E + EB) 3 s h a p e w h i c h is usually used for t h e e x p l a n a t i o n o f the collision cas-
Flux
COllilionll 10ikll rml
lo0
ft. t
0.01
1
I
O. t
t
I tO = eV
too
Fig. 8. Energy spectrum of sputtered from a Na target by 20 keV Ar ÷ ions. Random cascades and thermal evaporation are dominant. A small thermal spike contribution might be fitted between 0.2 and 0.5 eV. The target temperature in this case was about 100°C.
W. Husinsky et al. / Velocity measurements of sputtered particles
cade mechanism. Time-of-flight measurements of Ag sputtered from a Ag target bombarded with Xe ÷ ions performed by Szymonski [16] also showed a deviation from the E/(E + EB) 3 shape for low energies. This deviation is explained by a spike contribution. It should be mentioned that we could explain the Sm as well as the Na spectra assuming an I/(E +/:-)z distribution as we show for the case of Na targets in fig. 8 without spike contributions.
4. Conclusion On behalf of the measurements presented above it was demonstrated that a DSL-spectrometer is an excellent tool for neutral particle investigations in sputter experiments. Its excellent velocity resolution, its high detection sensitivity, and its particle selectivity allow a detailed investigation of sputter processes even in complicated situations as for alkali halides. In addition, it allows to investigate the velocity distribution for atoms in different excitation states. With the availability of cw. ring lasers with frequency doubling the DSLS will be a universal tool for neutral particle analysis.
293
References [1] D. Hammer, E. Benes, P. Blum and W. Husinsky, Rev. Sci. Instr. 47 (1976) 1178. [2] W. Husinsky, R. BruckmOllcr, P. Blum, F. Vichb6ck, D. Hammer and E. Bencs, J. Appl. Phys. 48 (1977) 4734. [3] A. Elbern, E. Hintz and B. Schweer, J. Nucl. Mat. 76 and 77 (1978) 143. [4] C.A. Visscr, J. Wolleswinkel and J. Los, J. Phys. E 3 (1970) 77. [5] D. Hammer, Vacuum 28 (1978) 107. [6] P. Hucks, G. Stoecklin, E. Victzke and K. Vogelbruch, J. Nucl. Mat. 76 and 77 (1978) 136. [71 W. Husinsky, Thesis, Technical University Vienna, 1977 (unpublished). [8] P. Blum, Thesis, Technical University Vienna, 1977 (unpublished). [9] G.K. Koennen, Ph.D. thesis, FOM-lnstitut, Amsterdam, 1974 (unpublished). [10] W. Husinsky and R. Bruckmi.iller, Surface Sci. 80 (1979) 637. [11] D. Pooley, Proc. Phys. Soc. (London) 87 (1966) 245. [12] H. Overeijnder, A. Haring and A.E. de Vries, Rad. Effects, 37 (1978) 205. [13] R. Kelly, Rad. Effects 32 (1977) 91. [14] R. Kelly, Surface Sci., in press (1979). [15] R. Bruckm011er, W. Husinsky and P. Blum, to be published in Rad. Effects. [16] M. Szymonski and A.E. de Vries, Phys. Lett. 36A (1977) 359.
The support of this work by the Austrian "Fonds zur F6rderung der wissenschaftlichen Forschung" is gratefully acknowledged (project Nr. 3605). We also want to express our thanks to Prof. F. Viehb6ck for his interest and support.
VII. HIGtt DENSITY CASCADES
287
VELOCITY MEASUREMENTS OF SPUTTERED PARTICLES USING THE LASER-DOPPLER METHOD W. HUSINSKY *, R. BRUCKMULLER and P. BLUM
Institut ffir Allgemeine Physik, Technical University Vienna, Vienna, Austria The development of tunable single frequency dye lasers has enabled the realization of a Doppler-Shift-Laser-Spectrometer (DSLS) for the detection and energy analysis of neutral sputtered particles. It is based on the Doppler-shifted resonance excitation by monochromatic radiation. The particle beam to be measured is intersected, by two laser beams at 90 ° and 30 ° respectively. The laser is tuned over a resonance line of the species to be investigated. The sharp non-Doppler-shifted 90 ° spectrum is used as zero marking on the velocity axis. The intensity of the Doppler-shifted 30 ° fluorescence spectrum is directly proportional to the particle intensity in the corresponding velocity interval. A high detection sensitivity of about 50 particles/cm3 and a velocity resolution of 50 m/s can be achieved. With the DSLS it is possible to investigate one particular kind of sputtered atoms with high resolution over a wide energy region. Results for different targets bombarded with rare gas ions are presented. The obtained spectra allow to determine the influence of slow thermal processes, thermal spikes and collision cascades to the sputtering process. A comparison with results obtained with time-of-flight experiments is given.
1. Introduction During the last years an increasing interest arose in the detection and energy analysis of neutral particles. Particular interest in a universally applicable tool for neutral particles exists in surface physics. Many experiments in the large field o f atomic collisions with solids as for instance sputtering or scattering involve the problem o f the energy analysis o f neutral particles. In the following paper a velocity spectrometer for neutral particles based on the Doppler-shifted resonance excitation o f atoms is described (DSLS) [1,2]. In the first part a detailed description o f the L a s e r Doppler m e t h o d and the spectrometer developed in our laboratory is given. Its features and advantages compared with other methods will be discussed. A similar spectrometer using a pulsed dye laser system [3] will also be shortly described. In the second part some results obtained with the spectrometer in sputter experiments will be discussed. A comparison with similar experiments using a correlation-time-of-flight m e t h o d will be given [4].
* Temporary address: ORNL, Solid State Division, Oak Ridge, Tenn. 37830, U.S.A.
2. The optical m e t h o d for the detection and energy analysis o f neutrals
2.1. DSL-spectrometer In general, two steps are performed for the energy analysis o f materials: a) the detection o f the neutrals, b) the velocity analysis. Depending on the type o f particles there exists quite a lot o f proven methods for the detection of neutrals, such as for instance surface ionization, ionization by electrons, magnetic resonance techniques, radioactive tracers or others. Combined with a time of flight apparatus or with a velocity selector these detection methods can be used for velocity measurements of neutrals [ 4 - 6 ] . Compared to the methods mentioned above resonance excitation by photons has an important advantage as cross sections for optical excitation in the visible region are o f the order o f 10 -9 c m 2. In comparison to the cross sections o f ionization by electrons o f 1 0 - 1 5 - - 1 0 -17 c m 2 they are rather high. Furthermore, using the optical m e t h o d and making use o f the Doppler-effect the particle velocity can be measured directly. Fig. 1 gives a schematic view o f the DSL spectrometer using a c.w. tunable dye laser. Suppose that a particle beam is traveling on the same axis VII. HIGH DENSITY CASCADES
288
W. Husinsky et aL / Velocity measurements of sputtered particles
4.0
I
r
I
>w "-J
r=tO -8
w=
ORNL-DWG 79-15065 I __ " 1.107 a 2.107 • 4"107 " 5 11d • 9.10 7
IA A , , ,
~o.5Frl
,,
o
7---.
H ~11
I~ A "
_
9-t0 8
,
r
.
~
_
i O
Fig. 1. Schematic view of the DSLS spectrometer using a c.w. tunable dye laser system.
as the ion beam indicated in the figure. The particle beam is produced by ion bombardment of a target. The particle beam itself is collimated by diaphragms. Its maximum aperature is about 1°. This particle beam is now intersected by a laser beam. For the further discussion we have to distinguish between two cases; first that the intersecting angle is 90 °. In this case the optical excitation is Doppler-free. If the frequency of the laser coincides with one transition frequency of the particles investigated, they are excited, independently of their velocity. After excitation there are two possibilities to return to one of the lower states; either by induced or by spontaneous emission. The induced emission is a coherent process in the direction of the laser beam. The spontaneous emission which is an isotropic, incoherent process can be registered easily by a photomultiplier detection system. The spontaneously emitted intensity is directly proportional to the number of particles in the excited state. Fig. 2 shows the time dependent population of an upper state for different laser intensities saturation of the upper level is obtained. This is of great advantage because in this case the fluorescence signal is independent of laser intensity fluctuations. Typical saturation values are for instance for the Na-D2 line about 0.2 W/cm 2. This result is obtained by calculation as well as by experiment [1 ].
0
tO
20
30 TIME (nsec)
40
Fig. 2. Time dependent evolution of the population of the upper state after excitation by a monochromatic radiation. The life time for spontaneous emission of the transition is 10 -8 s corresponding to the value for many dipole transitions. The population is plotted for some cases of increasing
laser power w (in arbitrary units).
For the excitation by the laser beam intersecting under 90 ° we thus obtain a sharp fluorescence peak at exactly the transition frequency Uo of the particles. This peak is an exact mark for zero velocity. For excitation by the second laser beam intersecting under 0 = 30 ° the situation is different. In this geometry the particles 'see" a different frequency due to the Doppler effect
+ os0) In order to excite particles with velocity u the laser must be detuned by the amount (u/c) cos 0. The velocity resolution is primarily given by the line width of the laser and the transition used. In sputter experiments we expect velocities from some 100 m/s for thermal particles up to some 100 km/s for particles in the eV range. The corresponding Doppler shifts are some I00 MHz up to 100 GHz. A maximum laser bandwidth of some ten MNz is therefore allowed in such an experiment. Only the development of tunable dye lasers has enabled the development of a DSLS. In table 1 the specifications of the DSLS developed in our laboratory are summarized. Up to now a DSLS based on a c.w. single mode, tunable dye laser which has the advantage of an excellent velocity resolution due to its small bandwidth of about 50 MHz has been discussed. Alas, nowadays c.w. single mode dye lasers still have a
W. Husinsky et al. / Velocity measurements o f sputtered particles
289
Table 1 Specifications o f DSL-spectrometer with c.w.-dye laser.
Velocity range: (0 = 30°C) Vmax: frequency scan: ~ f = 400 G H z ~ Ao = 300 km/s, zXE = 10 keV (Na) Vmin: laser bandwidth: zXf= 60 MHz absorption linewidth: ~ f - ~ 10-5 0 MHz ~ ,Sv = 70 m/s, ,SE -~ 1 meV (Na) Velocity resolution: Low energies: laser b a n d w i d t h : zXf -~ 60 MHz absorption linewidth: Af--~ 10-50 MHz High energies: particle b e a m divergence: 2c~ = 0.6°C
zXv-~ 70 m/s av/v = 0.5 % ~E/E = 1% (Na)
Limits o f detectability: Signal-to-noise ratio: -~ 10 theoretical: p h o t o n counting: -~10 -3 particles/cm 3 analog lock-in: 1 - 10 particles/cm 3 experimental lock-in: < 102 particles/cm 3
restricted tuning range, but for many materials the resonance transitions from the ground state lie in the UV region. A possible solution is to use pulsed dye lasers with high output power and frequency doubling. Thus a tunable laser radiation in the UV region is obtainable. The main disadvantage of such a system is the higher bandwidth in the range of some GHz. A DSL spectrometer using a pulsed dye laser has been developed in Jiilich by Elbern et al. [3]. The velocity resolution of this spectrometer is about 900 m/s. It was intended for measuring the fluxes in a Tokamak experiment where only the mean velocity was of importance. It should be mentioned that since the development of c.w. ring lasers with intra-cavity frequency doubling a great step towards a c.w. DSLS in the UV region has been made.
involved in the transition are plotted. Due to the interaction between the magnetic moment of the electrons the two terms split up into two, resp. four terms according to the momentum addition 3/2 and
3p2p3¢~
i
62,5MHz
~
1772MHz
2 36,1 MHz 1 16,4 MHz 0
o~
oo
2.2. Influence of hyperfine structure A detailed knowledge of the hyperfine structure of the transition line used is necessary. In many cases, the hyperfine splitting is in the range of some hundred MHz to some GHz and can be resolved by the DSLS. If the Doppler-shifts are greater than the hyperfine splitting a superposition of identical spectra results. As an example we will consider the case of the Na-D2 transition line. In fig. 3 the two terms 3s2S1/2 and 3p2p3/2
3S2
Sl/2
1 Fig. 3. Hyperfine-split term scheme o f the Na-D 2 transition line. VII. HIGH DENSITY CASCADES
W. Husinsky et al. / Velocity measurements of sputtered particles
290
O R N L - OWG 7 9 - # 5 0 6 6
I ooo
I
I
152
I
I
20keV RARE GAS IONS-NoCI 416 6 s27Fo._~,,. 4 f e 6 s 6 p
15a
800
rF~
\ L/
147 149
o :=
600
I\ I\~
I Ne + 2 Ar <"
3 Kr + 4 Xe÷
4
400 )48 149
147
149
147
150
X
O
. o
5
I
to t5 VELOCITY (km/$ec)
20
Fig. 5. Velocity spectra of Na sputtered from a NaCI target with different rare gas ions. The ion energy is 20 keV. The lowest curve is for Ne + (and He +) bombardment. With the increasing ion mass of Ar + and Kr + the contribution due to random cascade collisions becomes more relevant. For Xe ÷ bombardment the collisional contribution is dominant. 635MHz t
Fig. 4. Fluorescence signals of different isotopes obtained in the case of a Doppler-free excitation of the transition line at
562.602 nm of Sm.
1/2, resp. 3/2 and 3/2. Taking into account the selection rule ~ = 0, -+1, six hyperfine transitions are possible as indicated in the term scheme. Due to the small bandwidth of the laser nearly all lines can be resolved if a Doppler-free excitation geometry is used (such as for the 90 ° beam). In a frequency scale generally used for velocity measurements in sputter experiments only the two hyperfine lines with a splitting of 1772 MHz are resolved. The existence of these two transition lines is of importance since the corresponding velocity spectra might overlap in a certain area. A mathematic unfolding procedure must be performed in such a case [7,8]. A more complicated case is shown in fig. 4 for Sm where the existence of many isotopes cause many hyperfine lines.
3. Velocity spectra of sputtered particles obtained
with the DSLS Extended investigations using the DSLS have been performed with sodium halide targets. Similar experiments have also been done by the group of De Vries using a time-of-flight-correlation technique. Thus, a good comparison of the results can be made. Using the DSLS we were restricted to investigate
sputtered Na from NaC1, NaBr and Nal targets. In this case, the sputter mechanisms are rather complicated as many different processes may contribute to sputtering. The features and capabilities of the DSLS can be very well demonstrated on behalf of these investigations. The excellent velocity resolution for very low velocities is a great help in these experiments, as in many cases a dominant part of sputtered particles has thermal energies [2,9,10]. Furthermore, it is a great advantage that the excitation is particle selective. Therefore, we can investigate one particular kind of sputtered species without special supplementary, often difficult, aids. An illustrative picture of the influence of different mechanisms to the sputtering of alkali-halides can be obtained by regarding velocity spectra of Na sputtered from NaCI for bombardment with different rare gas ions. As example the velocity spectra for He +, Ne ÷, Ar +, Kr ÷, and Xe ÷ bombardment of NaC1 are given in fig. 5. The ion energy was 20 keV. Let us regard the two extremes, He ÷ and Xe ÷ bombardments: For the He ÷ the energy spectrum shows a thermal shape and can be very well fitted with a MaxwellBoltzmann distribution with a temperature of about 20°C. For Xe ÷ still a great thermal contribution is found, but now a higher energy tail up to some ten eVs is found. From electron sputter experiments with halides it is known that sputtering of the halide can take place as a consequence of Vk-center production [2,11,12]. Directly connected with this the alkali
W. Husinsky et al. / Velocity measurements o f sputtered particles
291
uL
=:.
+
NACL
KR
e¢.ii
_J
u
N
....J
B ID
L~
IZENEFIO¥(EV) ZI
Fig. 6. Fitting of the measured energy spectrum of Na sputtered from a NaC1 target by 20 keV Kr÷ ions (*) using the theoretical shapes for thermal evaporation (~x),thermal spikes (o), and random cascade collisions (o).
atoms can evaporate from the surface when they are present in excess. A similar process is possible for ion bombardment. As the energy lost in inelastic collisions decreases with the ion mass this process must become less relevant for higher ion masses. As a consequence, thermal spike effects and collisional effects increase. In fig. 6 it is shown that the measured energy spectrum can be very well fitted with the theoretically expected shapes for random collisions, thermal spikes and thermal evaporation. There is a good correspondance between our measurements and those of Overeijnder et al. using a correlation-time-of-flight method [12]. The main discrepancy between our DSLS- and the time-of-flight measurements of alkali-halides lie in the form of the thermal distribution of the evaporated atoms. The DSLS measurements yield a E . e - E / k T distribution, while the time-of-flight method yield a v / f t • e - e / k T distribution [10]. After presenting some results observed with alkali-halides, some investigations of metal sputtering are discussed. For this purpose, the DSLS was used for sputter experi-
ments of Sm and Na targets bombarded with different ions. Contrary to the alkali-halides, for metal sputtering a dominant contribution from collision cascades is expected for the ion energies we have used [13,14]. A more detailed investigation and comparison of the collisional part with theoretical models is, therefore, possible [15]. For Na also thermal contributions are expected as the vapor pressure for Na is rather high even at room temperature. In fig. 7 the resulting velocity spectra for this case obtained with the DSLS are summarized. For this purpose, a Na target was heated up to about 200°C and cooled down afterwards. The velocity spectra were recorded meanwhile. The heating procedure was started after exposure of the target to 02. At room temperature, a dominant collisional contribution is found. After the heating procedure an additional small thermal peak is also present. For high temperatures an increasing thermal peak is registrated. Furthermore, it is interesting that the oxide layer after 02 inlet seems to disfavor thermally sputtered particles. The corresponding value o f the binding energy is slightly higher than after heating. What seems to be a remarkable VII. HIGH DENSITY CASCADES
W. Husinsky et al. / Velocity measurements of sputtered particles
292
velocity
km s"
velocity
k m s-'
Pig. 7. Velocity spectra of Na sputtered from a Na target by 20 keV Ar ÷ ions. (a) After exposure to oxygen the target was heated up to about 200°C. With increasing temperature a thermal peak begins to grow. (b) The target is cooled down from about 200°C to room temperature. Even at room temperature a small thermal peak is clearly visible. In (a) and (b) the spectra are normalized to the same contribution due to collision cascades which have the maximum at 2000 m/s.
t~
I
I • 0
I
me,su,oa
result o f these i n v e s t i g a t i o n s is t h e fact t h a t the collisional c o n t r i b u t i o n s h o w s a m a x i m u m at very low energies a r o u n d zero. This fact is also f o u n d in s p u t t e r e x p e r i m e n t s o f S m targets [ 1 5 ] . l f w e assume t h a t an 1 / ( E + E B ) 2 r e l a t i o n is r e l e v a n t for collision cascades we can explain t h e m e a s u r e d s p e c t r u m w i t h a m a x i m u m a r o u n d zero energy. F o r c o m p a r i s o n we have also c o n s i d e r e d t h e E / ( E + EB) 3 s h a p e w h i c h is usually used for t h e e x p l a n a t i o n o f the collision cas-
Flux
COllilionll 10ikll rml
lo0
ft. t
0.01
1
I
O. t
t
I tO = eV
too
Fig. 8. Energy spectrum of sputtered from a Na target by 20 keV Ar ÷ ions. Random cascades and thermal evaporation are dominant. A small thermal spike contribution might be fitted between 0.2 and 0.5 eV. The target temperature in this case was about 100°C.
W. Husinsky et al. / Velocity measurements of sputtered particles
cade mechanism. Time-of-flight measurements of Ag sputtered from a Ag target bombarded with Xe ÷ ions performed by Szymonski [16] also showed a deviation from the E/(E + EB) 3 shape for low energies. This deviation is explained by a spike contribution. It should be mentioned that we could explain the Sm as well as the Na spectra assuming an I/(E +/:-)z distribution as we show for the case of Na targets in fig. 8 without spike contributions.
4. Conclusion On behalf of the measurements presented above it was demonstrated that a DSL-spectrometer is an excellent tool for neutral particle investigations in sputter experiments. Its excellent velocity resolution, its high detection sensitivity, and its particle selectivity allow a detailed investigation of sputter processes even in complicated situations as for alkali halides. In addition, it allows to investigate the velocity distribution for atoms in different excitation states. With the availability of cw. ring lasers with frequency doubling the DSLS will be a universal tool for neutral particle analysis.
293
References [1] D. Hammer, E. Benes, P. Blum and W. Husinsky, Rev. Sci. Instr. 47 (1976) 1178. [2] W. Husinsky, R. BruckmOllcr, P. Blum, F. Vichb6ck, D. Hammer and E. Bencs, J. Appl. Phys. 48 (1977) 4734. [3] A. Elbern, E. Hintz and B. Schweer, J. Nucl. Mat. 76 and 77 (1978) 143. [4] C.A. Visscr, J. Wolleswinkel and J. Los, J. Phys. E 3 (1970) 77. [5] D. Hammer, Vacuum 28 (1978) 107. [6] P. Hucks, G. Stoecklin, E. Victzke and K. Vogelbruch, J. Nucl. Mat. 76 and 77 (1978) 136. [71 W. Husinsky, Thesis, Technical University Vienna, 1977 (unpublished). [8] P. Blum, Thesis, Technical University Vienna, 1977 (unpublished). [9] G.K. Koennen, Ph.D. thesis, FOM-lnstitut, Amsterdam, 1974 (unpublished). [10] W. Husinsky and R. Bruckmi.iller, Surface Sci. 80 (1979) 637. [11] D. Pooley, Proc. Phys. Soc. (London) 87 (1966) 245. [12] H. Overeijnder, A. Haring and A.E. de Vries, Rad. Effects, 37 (1978) 205. [13] R. Kelly, Rad. Effects 32 (1977) 91. [14] R. Kelly, Surface Sci., in press (1979). [15] R. Bruckm011er, W. Husinsky and P. Blum, to be published in Rad. Effects. [16] M. Szymonski and A.E. de Vries, Phys. Lett. 36A (1977) 359.
The support of this work by the Austrian "Fonds zur F6rderung der wissenschaftlichen Forschung" is gratefully acknowledged (project Nr. 3605). We also want to express our thanks to Prof. F. Viehb6ck for his interest and support.
VII. HIGtt DENSITY CASCADES