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Electronic sputtering of nanometric cluster ions of gold from ultradispersed targets of gold by fission fragments; masses and energies of cluster ions
Nuclear Instruments and Methods in Physics Research B 122 (19971329-334
Besm Intsractiens wlth MaterlaIr 6 Atoms ELSEVIER
Electronic sputtering of nanometric cluster ions of gold from ultradispersed targets of gold by fission fragments; masses and energies of cluster ions I. Baranov a**,S. Bogdanov a, A. Novikov a, V. Obnorskii a, B. Kozlov b a V.G. Khlopin Radium Institute, Z-Murinskij Ave., 194021 St. Petersburg, Russia b A.F. loffe Physical Technical Institute. Polytechnicheskaya str., 194021 St. Petersburg, Russia
Abstract Sputtering of superheavy negative cluster ions of gold from ultradispersed targets - gold islet films (islet sizes being ratio (m/s) of the desorbed particles within the range 3-20 nm) by “*Cf fission fragments is studied. Mass-to-charge I X !05-S X IO7 amu/e as we!! as their initial energies are measured by means of a special mass-spectrograph. It is established that desorbed cluster ions have initial kinetic energies of tens-hundreds eV per charge unit. With this, small cluster ions have larger initial energies per charge unit than larger cluster ions. It is also shown that for - 70% cluster ions desorbed from the target with islets sized 7.5-15 nm m/q is 6 X 105-4X lo6 amu/e which is on average S-IO times smaller than the mass values for the islets. The results are discussed.
1. Introduction
2. Experimental
It was first shown in 1992 that sputtering of gold islet films (islet sizes being S-20 nm) by 252Cf fission fragments (FF) results in the formation of gold negative cluster ions with m/q 1 X 105-1 X lo7 amu/e, i.e. having sizes of the same order as the islets on the irradiated targets [ !I. It was also shown earlier that sputtering-desotption from such ultradispersed targets occurs as a result of electronic stopping of the FF impinging on the target matter [2]. Thus, studying this extraordinary clusterisation effect is connected with finding a conversion mechanism of electron excitation energy, produced by an ion in the sample, into the energy of movement of the whole desorbed particle. The size effect plays an important role in this process. In [3] it was established that electronic sputtering of metails (gold) by ions (FF) takes place only when the metal islet size is smaller than some threshold value (15-20 nm). Evidently, below the threshold processes leading to cluster desorption from an irradiated islet layer may substantialy depend on the grain size. Thus, a matter of great interest is measuring the energy distributions of desorbed metal cluster ions for targets with different size distributions of islets as we!! as finding the connection between mass distributions of islets on the targets and those of cluster ions resulted from sputtering of the same target by FF.
2.1. Targets The technique of making ultradispersed-islet targets of gold and their characterization by a transmission electron microscope is described in Ref. [3]. In the present work two gold islet targets were used with considerably different size and mass distributions of islets. Fig. la, b shows size distributions of the islets for these two targets. For target A the mean islet size is 11 nm, 70% islets having sizes 7.5-15 nm. For target B these parameters are 4.4 nm and 3.2-5.6 nm. Fig. la, b also shows mass values for gold islets which were calculated with help of me measured diameter assuming a spherical islet shape. 2.2. Mass-spectrograph A special mass-spectrograph was used for measuring of cluster ions within the range !05-lo7 amu/e. Its principles of operation were described in Ref. [l]. The scheme of the modernized mass-spectrograph is given on Fig. 2. A cluster ion desorbed by FF from the islet target is accelerated by the potential applied to the target. In the static mode a constant negative accelerating potential CL,,, is applied to the target. In this case the ion kinetic energy per charge unit after the accelerating gap will be
m/q
w/q l
Corresponding author.
0168-583X/97/$17.00
technique
= w,/q
where W,/q
+ u,, . is the axial component
(1) of the initial energy of
Copyright 0 I997 Elsevier Science B.V. All rights resewed
P/I SO168-583X9(96)00657-X
1. CLUSTER BEAMS
330
1. Burunov et (11./ Nud. Instr. und Meth. in Phys. Res. B I22 (I 997) 329-334 Islet Mass(Sphere), am"
9,5x1 0’
2,6x106
a Target A
1.2~10~
works as an energy analyser, in the dynamic mode as a mass analyser. The particles in the mass-spectrograph are analysed and collected in vacuum _ 3 X IO-’ Torr. The amounts of gold atoms on the collector are determined by neutron activation analysis [3]. Comparing the distribution of gold gathered along the collector with the mass or energy scale of the instrument, we determine the W,/q or m/q distribution of the desorbed particles (more exactly, of the amounts of gold they contain). To reduce influence of the initial energy and angular spread of the cluster ions on the mass scale and the resolution of the instrument they should be accelerated significantly. At present, working in the dynamic mode we use the generator with the amplitude of accelerating pulses 5-6 kV and the time of pulse evolution 30-100 /.Ls, the accelerating distance being 10 mm. In the static mode the value of the accelerating potential is determined by the order of W,/q vaIues analysed. Fig. 3 shows the instrument functions of resolution obtained by computer simulation for certain conditions of static (Fig. 3a) and dynamic (Fig. 3b) experiments. During calculations the following parameters were taken into account: the size of the target area emitting the clusters (1 mm), the initial angular distribution of clusters ( - exp( - 2.2 I@ 1) [2,5]), energy spread (50-140 eV, to be used for calculation of mass resolution, Fig. 3b) and scattering on the grids [6]. AS it is seen from Fig. 3 the width of monolines (W,/q = const or m/q = const) at the half of the hight AL. = 3-4 mm, which provides for the spatial resolution
3,3x10'
--
I i
I
I.
I
2.5
5,0
.
I,
I
I
,,.,.I,
7,5 10.0 12,5 15.0 17,5 20.0 22.5 Islet Size, nm
0.40
.,
.,
.,
.,
9,5x10d b.Tar@
.
,I,.
7,6x105
2.6~10~
B
t 0,25 t 0,20 0.15
i
t
0,lO
R,=(L-L,)/AL.
O-05 1
t
I
0.00
1.25
0.00 i
2.50
3,75
islet
500
6.25
7,50
6,75
Size, nm
Fig. I. (a, b) Islet size distributions
(4)
from I to 10 (L, is the coordinate corresponding to the zero value of W,/q or m/q). As in the static mode the energy scale of the instrument is linear, then the energy resolution
__
for targets A and B.
Rw =
we/q
W’o/q)
the ion per charge unit. In the dynamic mode of operation of the mass-spectrograph, a cluster ion is accelerated by a negative potential linearly increasing with time. In this case the energy gained by the ion in the accelerating gap is determined by its mass-to-charge ratio (m/q) [I] W/q
= a3m
+ b,
(2)
where a and b are constants determined by the parameters of the accelerating field and the mean initial energies of the particles desorbed. While going through the electrostatic mirror, particles with different W/q are spatially separated and then collected at different points of the collector with coordinates L determined as L(W/q)
= c x w/q,
(3)
where c is a constant determined by the parameters of the reflecting field. Thus, in the static mode the instrument
is equal to R,. Due to cubic character of the mass scale of the instrument working in the dynamic mode, its mass resolution can be estimated as
R”=
m/q
____ A(m/q)
= R,/3.
As it is seen from Fig. 3a, b and Eqs. (4)-(6) the ranges of W,/q and m/q which can be analysed by the mass-spectrograph are limited at the bottom. In the dynamic mode the instrument is used for the analysis of particles with m/q > 1 x IO5 amu/e. In the static mode if it is adjusted for the particles with W,/q of tens-hundreds Volt, the low energy component (units Volt) cannot be resolved. By applying a retarding potential to grid electrode El (see Fig. 2), discrimination of low energy (W,/q) or low mass (m/q) particles is achieved. This electrode is made of the grid with square cells (the period of the grid
I. Bararwv et al. /Nucl.
331
Instr. and Meth. in Phys. Res. B 122 (1997) 329-334
start detector
z
R 1 -
0
(U retled.)
Fig. 2. Scheme of the mass-spectrograph. (R) electrostatic mirror, (Ci) generator of accelerating pulses (to be replaced in static mode by a source of constant voltage), (El) grid electrode providing low level discrimination of W,/q or m/q.
013
Wdq=ZOV
5 4@3-
d b
!Ta
ml
143
187
26
31
36
41
2s = 500 pm, the radius of rods r = 18 pm). The distances to the neighbour grounded electrodes d, = d, = d = 5 mm. While applying a retarding potential U,, to electrode El an effective potential in the centre of the grid cell U& is lower. Basing on [7] it can be estimated as
\
3m-
-4
1
6
11
16
21
46
3. Experimental
Collector Length, mm
results
m&. amuk 8 ’
700 . 600-
b.
-4
1
I * I ’
6
11
z&l@
1,4x10* 4.2d08 1 . I . I . I . I ., 1 . 6xld Z.ExlO’5,4x10’ -
16
21
26
31
36
41
46
Collector Length, mm
Fig. 3. Simulated functions of resolution of the mass-spectrograph, workmg: (a) m stanc mode (fJ,,,= 170 V, field of reflector is 12.3 V/mm); (b) in dynamic mode (amplitude and duration of accelerating pulse are 5.2 kV and 50 MS, field of reflector is 131 V/mm, initial energy spread is taken as W, /q = 50- 140 V).
3.1. Measuring initial energies of cluster ions (W,/qJ Fig. 4a, b shows distributions of gold atoms along the mass-spectrograph collector length obtained in the static experiments with targets A and B respectively. The distributions corresponding to the experiments with the grounded electrode El (see Fig. 2) are not shaded. The shaded distributions were obtained while applying some retarding potential Uret.> U,,, to electrode El. In this case it is only the particles having W,/q more than the difference CJ&UX,, that reach the collector (c!& - an effective potential in the centres of the grid cells). This results in a decrease of the amount of gold on the collector, shifting the whole distribution towards larger values of the collector coordinates due to larger values of W,/q. The energy scales of the instrument are given on the top axes of Fig. 4a, b.
1. CLUSTER BEAMS
I. Baranm er al. /Nucl. Instr. and Meth. in Phys. Res. B 122 (1997) 329-334
332
Wdq>V 7
~,.08
I,.,I,I,I,,.,.,I,.,I
e 5
a.(Target A)
0.07
013
56
100
143
It should be noted that earlier we have already reported some data concerning the energies of the charged particles desorbed by FF from gold islet targets. By means of the retarding potential method it was shown that some part of desorbed gold particles (_ 30%) leave the target, having energies up to 10 eV per charge unit [5]. Values obtained in the present work are on average l-2 orders of magnitude higher. Evidently, the main reason of this difference is that in [5] using relatively low retarding potentials the high energy charged particles were mistakenly regarded as a neutral component.
187
U acC =-170v U ,-
=-310v
.is
Collector
rE EO.08,.
i
8,
.
b.(TargetB)
t
I
3.2. Measuring m /q with islet masses
Length, mm
0
107
II.
I,
W&z
V
276
444
I
1,
613 I,
781 I
',
“WI =oy
of cluster ions and comparing them
Target A was used for measuring m/q of cluster ions in the dynamic mode of operation of the mass-spectrograph. Fig. Sa shows the distribution of gold atoms along the collector length, obtained in the dynnamic experiment in question. The mass scale calculation (top axis Fig. 5a), besides the analyser parameters (accelerating impulse amplitude and duration -5.2 kV, 50 ~CLS respectively, reflec-
Cluaer Ion m/q, amulemd
42r106
1 4r10e
jl,,,j L
d
Collector
Length, mm
Fig. 4. Distributions of gold atoms along the collector length obtained in the static experiments with targets A and B. The length scales have been converted to cluster ion initial anergy scales (top ax&s). The unshaded distributions are obtained with the grounded electrode El (see Fig. 2), the shaded distributions while applying a retarding potential (I,, to electrode El (for (a) C/L,,-- (I,,,= 97 V, for(b) U:,,,- C&,=‘i28 V). 1
6
The quota of gold atoms, corresponding to particles with W,/q larger than U&,.-U,,,,, could be estimated as the ratio of the amounts of atoms collected in the experiments tiith the retarding potential and with full transmission. But it is evident that such a ratio will be an underestimation due to the non-equivalent conditions of flight of the particles in the experiments in question, as well as for particles with different W,/q. While processing the data we took into consideration the function of spatial resolution of the instrument and transparency of the analysing system to particles with different initial energies. It was established from the distributions 4, b (non-shaded histograms) that for target A the mean value of the axial component of the initial energy of the desorbed cluster ions (W,/q),, is 96 V. - 70% of the whole gold desorbed in the form of negative cluster ions correspond to W,/q = 50- 140 V. For target B these characteristics are 216 V and 100-320 V respectively.
11
16
Iklet Mass
5 E 3
21
26
31
38
41
46
Collector Length, mm
z
0.0
5.0x106 I
5x10*
1.0x10' I
(Hemisphere).amu 1.5X10' I
2.0x10' I
2.5x10' I .
3,0x10'
b. Islets
B
0~ 0.0
'I 1.0x10'
'I'# 2.0x10'
l&t
'C 3.0110
4,ox10'
” 5.G7.10'
6.0x10'
Mass (Sphere), a””
Fig. 5. (a) Distribution of gold atoms along the collector length obtained in the dinamic experiment with target A. The length scale has been converted to a cluster ion m/q scale (top axis); (b) Dependence of gold atom content on the islet mass for target A.
I. Baranou et al./Nucl.
Instr. and Meth. in Phys. Res. B 122 (1997) 329-334
tor field 13 1 V/mm) took also into consideration the mean value of the initial energy (WO/q)mem = 96 V which was
determined as a result of the static experiment. A retarding potential of - 1.4 kV was applied to electrode El (see Fig. 2) which provided the low level discrimination of m/q < 1.5 X 10’ amu/e. Taking into consideration the function of resolution and transparency of the system to particles with different m/q, it was established from distribution Sa that the mean value of m/q of cluster ions desorbed from the given target is 2 X lo6 amu/e, 70% of the whole amount of gold flying in the form of cluster ions with m/q 6 x lo’-4 x lo6 amu/e. In order to compare m/q of cluster ions with islet mass values the islet size distribution (Fig. la) was converted into the islet mass distribution (see Fig. 5b). The relative content of gold atoms instead of the relative number of islets was plotted on the ordinate axis. The mean mass of an islet proved to be 1.8 X IO’ amu, 70% of gold corresponding to islets with masses 6 X 106-3 X IO’ amu, if they are regarded as spheres. These values are 9 X lo6 amu and 3 X 106-1.5 X 10’ amu respectively, if the islets are regarded as hemispheres. Thus, m/q of cluster ions
desorbed by FF from the gold islet target sized 7.5-15 nm appears to be * 5-10 times smaller than islet masses.
4. Discussion This work presents two important results.
(1) A substantial part of the desorbed cluster ions ( - 70% or more) have extremely high start energy - about tens-hundreds of eV per their charge unit. With this, cluster ions desorbed from targets with smaller islets have higher values of the initial energy per charge unit. (2) m/q of cluster ions desorbed from the target with islets sized 7.5- 15.0 nm are 6 X lo’-4 X lo6 amu/e and appear to be on average 5-10 times smaller than the islet masses on the target. The result concerning the initial energies changes considerably our representations of the mechanism of the process studied. It was supposed earlier that energy transfer from the electrons of an islet excited by FF to atoms of a crystal lattice is performed after their thermalisation through electron-phonon interactions, mainly in the islet surface layer, the islet leaving the substrate as a result of its thermal expansion [2]. However, estimations show that if islet leaving the substrate is connected with their thermal expansion then the kinetic energy of clusters can only be about several eV. But the experiment shows that initial energies of the desorbed cluster ions reach tens-hundreds eV. This indicates a very rapid nonequilibrium process of hot electron energy conversion into the kinetic energy of the desorbed particle. Apparently the representation of the first stage of the energy transfer process - from hot electrons to the surface atoms of islets - do not require any changes. They are
333
logically connected with the size effect which is an experimental fact [3,4]. However explanation of the final stage that of the islet leaving the substrate needs new aproaches. With this, more than two times initial kinetic energy increase for islets sized 4.4 nm (target B) than for islets sized 11 nm (target A) may be connected with the increase of energy deposited into these islets by FF per atom: 5 25 eV/atom and 5 eV/atom for targets B and A respectively (the mean range of FF in an islet is equal to its radius, the electron stopping power for FF being _ 30 keV/nm). As for the observed shift between mass distribution of islets and m/q distribution of cluster ions, it can have several reasons. (a) All the processes in the irradiated layer between the moment of FF passing through the islet and the moment of islet desorption can strongly depend on the islet size. As it was shown ealier [3,8], islets with the size close to the size threshold of the electronic sputtering have lower probability to be desorbed by FF than smaller ones. And on the contrary, in [9] it was established that one high-energetic projectile can remove up to several islets from the targets with very small islets (- 4 nm). Thus it may occur that mass (not m/q) distribution of clusters desorbed is also shifted relatively to the islet mass distribution. (b) An islet, on leaving the substrate, can have on its surface several elementary charges. This reduces efficient value of m/q of the formed cluster ion. This reason seems to be the most probable. It agrees well with very high yield (tens %) of charged gold particles sputtered by FF from islet layers 12,101. However, the comparision of m/q distribution of the cluster ions and islet mass distribution must not be used as a direct method of the cluster ion charge state determination. (c) Cluster fragmentation resulting in the formation of lighter particles must not be completely excluded. However, this process cannot be the main one, since it would contradict narrow angular distributions of sputtered gold observed earlier [2,5]. In conclusion we would like to resume, that electronic sputtering of islet targets by multiply-charged ions (FF) is a new method of producing nanometric cluster ions in free state [I]. The experiments described in this work are the first to characterize beams of such specific objects.
Acknowledgements This work has been supported in part by the Intemational Science Foundation and Russian Government, Grants RISOOO, RlS300,
199&1995.
References
[I] LA. Baranov. A.C. Novikov, V.V. Obnorskii. S.O. Tsepelevich, B.N. Kozlov and 1.1.pilyugin, Nucl. Instr. and Me& B 6.5(1992) 177. I. CLUSTERBEAMS
334
I. Baranov et al./Nucl.
[2] LA. Baranov,
[3] [4] [5] [6]
Insa. and Meth. in Phys. Res. B 122 (1997) 329-334
V.V. Obnorskii and $0. Tsepelevich, Nucl. Instr. and Meth. B 35 (1988) 140. LA. Baranov and V.V. Obnorskii, Sov. At. Energy 54 (1983) 192. I.A. Baranov, Yu.V. Martynenko, S.O. Tsepelevich and Yu.N. Yavlinskii, Sov. Phys. Usp. 31 (1988) 1015. LA. Baranov and S.O. Tsepelevich, Sov. At. Energy 61 ( 1986) 809. O.V. Konstantinov, B.A. Mamyrin, L.E. Shchebelina and V.G. Shchebelin, Sov. Phys. Tech. Phys. 31 (1986) 629.
[7] O.V. Konstantinov, L.E. Shchebelina and V.G. Shchebelin, Sov. Phys. Tech. Phys. 29 (1984) 1228. [S] I.A. Baranov and V.V. Obnorskii, Radiat. Eff. 79 (1983) 1. [9] H. Andersen, H. Knudsen and P. Petersen, J. Appl. Phys. 49 (1978) 5638. [IO] LA. Baranov, S.O. Tsepelevich, and V.V. Obnorskii, Sov. At. Energy 60 (1986) 85.
Besm Intsractiens wlth MaterlaIr 6 Atoms ELSEVIER
Electronic sputtering of nanometric cluster ions of gold from ultradispersed targets of gold by fission fragments; masses and energies of cluster ions I. Baranov a**,S. Bogdanov a, A. Novikov a, V. Obnorskii a, B. Kozlov b a V.G. Khlopin Radium Institute, Z-Murinskij Ave., 194021 St. Petersburg, Russia b A.F. loffe Physical Technical Institute. Polytechnicheskaya str., 194021 St. Petersburg, Russia
Abstract Sputtering of superheavy negative cluster ions of gold from ultradispersed targets - gold islet films (islet sizes being ratio (m/s) of the desorbed particles within the range 3-20 nm) by “*Cf fission fragments is studied. Mass-to-charge I X !05-S X IO7 amu/e as we!! as their initial energies are measured by means of a special mass-spectrograph. It is established that desorbed cluster ions have initial kinetic energies of tens-hundreds eV per charge unit. With this, small cluster ions have larger initial energies per charge unit than larger cluster ions. It is also shown that for - 70% cluster ions desorbed from the target with islets sized 7.5-15 nm m/q is 6 X 105-4X lo6 amu/e which is on average S-IO times smaller than the mass values for the islets. The results are discussed.
1. Introduction
2. Experimental
It was first shown in 1992 that sputtering of gold islet films (islet sizes being S-20 nm) by 252Cf fission fragments (FF) results in the formation of gold negative cluster ions with m/q 1 X 105-1 X lo7 amu/e, i.e. having sizes of the same order as the islets on the irradiated targets [ !I. It was also shown earlier that sputtering-desotption from such ultradispersed targets occurs as a result of electronic stopping of the FF impinging on the target matter [2]. Thus, studying this extraordinary clusterisation effect is connected with finding a conversion mechanism of electron excitation energy, produced by an ion in the sample, into the energy of movement of the whole desorbed particle. The size effect plays an important role in this process. In [3] it was established that electronic sputtering of metails (gold) by ions (FF) takes place only when the metal islet size is smaller than some threshold value (15-20 nm). Evidently, below the threshold processes leading to cluster desorption from an irradiated islet layer may substantialy depend on the grain size. Thus, a matter of great interest is measuring the energy distributions of desorbed metal cluster ions for targets with different size distributions of islets as we!! as finding the connection between mass distributions of islets on the targets and those of cluster ions resulted from sputtering of the same target by FF.
2.1. Targets The technique of making ultradispersed-islet targets of gold and their characterization by a transmission electron microscope is described in Ref. [3]. In the present work two gold islet targets were used with considerably different size and mass distributions of islets. Fig. la, b shows size distributions of the islets for these two targets. For target A the mean islet size is 11 nm, 70% islets having sizes 7.5-15 nm. For target B these parameters are 4.4 nm and 3.2-5.6 nm. Fig. la, b also shows mass values for gold islets which were calculated with help of me measured diameter assuming a spherical islet shape. 2.2. Mass-spectrograph A special mass-spectrograph was used for measuring of cluster ions within the range !05-lo7 amu/e. Its principles of operation were described in Ref. [l]. The scheme of the modernized mass-spectrograph is given on Fig. 2. A cluster ion desorbed by FF from the islet target is accelerated by the potential applied to the target. In the static mode a constant negative accelerating potential CL,,, is applied to the target. In this case the ion kinetic energy per charge unit after the accelerating gap will be
m/q
w/q l
Corresponding author.
0168-583X/97/$17.00
technique
= w,/q
where W,/q
+ u,, . is the axial component
(1) of the initial energy of
Copyright 0 I997 Elsevier Science B.V. All rights resewed
P/I SO168-583X9(96)00657-X
1. CLUSTER BEAMS
330
1. Burunov et (11./ Nud. Instr. und Meth. in Phys. Res. B I22 (I 997) 329-334 Islet Mass(Sphere), am"
9,5x1 0’
2,6x106
a Target A
1.2~10~
works as an energy analyser, in the dynamic mode as a mass analyser. The particles in the mass-spectrograph are analysed and collected in vacuum _ 3 X IO-’ Torr. The amounts of gold atoms on the collector are determined by neutron activation analysis [3]. Comparing the distribution of gold gathered along the collector with the mass or energy scale of the instrument, we determine the W,/q or m/q distribution of the desorbed particles (more exactly, of the amounts of gold they contain). To reduce influence of the initial energy and angular spread of the cluster ions on the mass scale and the resolution of the instrument they should be accelerated significantly. At present, working in the dynamic mode we use the generator with the amplitude of accelerating pulses 5-6 kV and the time of pulse evolution 30-100 /.Ls, the accelerating distance being 10 mm. In the static mode the value of the accelerating potential is determined by the order of W,/q vaIues analysed. Fig. 3 shows the instrument functions of resolution obtained by computer simulation for certain conditions of static (Fig. 3a) and dynamic (Fig. 3b) experiments. During calculations the following parameters were taken into account: the size of the target area emitting the clusters (1 mm), the initial angular distribution of clusters ( - exp( - 2.2 I@ 1) [2,5]), energy spread (50-140 eV, to be used for calculation of mass resolution, Fig. 3b) and scattering on the grids [6]. AS it is seen from Fig. 3 the width of monolines (W,/q = const or m/q = const) at the half of the hight AL. = 3-4 mm, which provides for the spatial resolution
3,3x10'
--
I i
I
I.
I
2.5
5,0
.
I,
I
I
,,.,.I,
7,5 10.0 12,5 15.0 17,5 20.0 22.5 Islet Size, nm
0.40
.,
.,
.,
.,
9,5x10d b.Tar@
.
,I,.
7,6x105
2.6~10~
B
t 0,25 t 0,20 0.15
i
t
0,lO
R,=(L-L,)/AL.
O-05 1
t
I
0.00
1.25
0.00 i
2.50
3,75
islet
500
6.25
7,50
6,75
Size, nm
Fig. I. (a, b) Islet size distributions
(4)
from I to 10 (L, is the coordinate corresponding to the zero value of W,/q or m/q). As in the static mode the energy scale of the instrument is linear, then the energy resolution
__
for targets A and B.
Rw =
we/q
W’o/q)
the ion per charge unit. In the dynamic mode of operation of the mass-spectrograph, a cluster ion is accelerated by a negative potential linearly increasing with time. In this case the energy gained by the ion in the accelerating gap is determined by its mass-to-charge ratio (m/q) [I] W/q
= a3m
+ b,
(2)
where a and b are constants determined by the parameters of the accelerating field and the mean initial energies of the particles desorbed. While going through the electrostatic mirror, particles with different W/q are spatially separated and then collected at different points of the collector with coordinates L determined as L(W/q)
= c x w/q,
(3)
where c is a constant determined by the parameters of the reflecting field. Thus, in the static mode the instrument
is equal to R,. Due to cubic character of the mass scale of the instrument working in the dynamic mode, its mass resolution can be estimated as
R”=
m/q
____ A(m/q)
= R,/3.
As it is seen from Fig. 3a, b and Eqs. (4)-(6) the ranges of W,/q and m/q which can be analysed by the mass-spectrograph are limited at the bottom. In the dynamic mode the instrument is used for the analysis of particles with m/q > 1 x IO5 amu/e. In the static mode if it is adjusted for the particles with W,/q of tens-hundreds Volt, the low energy component (units Volt) cannot be resolved. By applying a retarding potential to grid electrode El (see Fig. 2), discrimination of low energy (W,/q) or low mass (m/q) particles is achieved. This electrode is made of the grid with square cells (the period of the grid
I. Bararwv et al. /Nucl.
331
Instr. and Meth. in Phys. Res. B 122 (1997) 329-334
start detector
z
R 1 -
0
(U retled.)
Fig. 2. Scheme of the mass-spectrograph. (R) electrostatic mirror, (Ci) generator of accelerating pulses (to be replaced in static mode by a source of constant voltage), (El) grid electrode providing low level discrimination of W,/q or m/q.
013
Wdq=ZOV
5 4@3-
d b
!Ta
ml
143
187
26
31
36
41
2s = 500 pm, the radius of rods r = 18 pm). The distances to the neighbour grounded electrodes d, = d, = d = 5 mm. While applying a retarding potential U,, to electrode El an effective potential in the centre of the grid cell U& is lower. Basing on [7] it can be estimated as
\
3m-
-4
1
6
11
16
21
46
3. Experimental
Collector Length, mm
results
m&. amuk 8 ’
700 . 600-
b.
-4
1
I * I ’
6
11
z&l@
1,4x10* 4.2d08 1 . I . I . I . I ., 1 . 6xld Z.ExlO’5,4x10’ -
16
21
26
31
36
41
46
Collector Length, mm
Fig. 3. Simulated functions of resolution of the mass-spectrograph, workmg: (a) m stanc mode (fJ,,,= 170 V, field of reflector is 12.3 V/mm); (b) in dynamic mode (amplitude and duration of accelerating pulse are 5.2 kV and 50 MS, field of reflector is 131 V/mm, initial energy spread is taken as W, /q = 50- 140 V).
3.1. Measuring initial energies of cluster ions (W,/qJ Fig. 4a, b shows distributions of gold atoms along the mass-spectrograph collector length obtained in the static experiments with targets A and B respectively. The distributions corresponding to the experiments with the grounded electrode El (see Fig. 2) are not shaded. The shaded distributions were obtained while applying some retarding potential Uret.> U,,, to electrode El. In this case it is only the particles having W,/q more than the difference CJ&UX,, that reach the collector (c!& - an effective potential in the centres of the grid cells). This results in a decrease of the amount of gold on the collector, shifting the whole distribution towards larger values of the collector coordinates due to larger values of W,/q. The energy scales of the instrument are given on the top axes of Fig. 4a, b.
1. CLUSTER BEAMS
I. Baranm er al. /Nucl. Instr. and Meth. in Phys. Res. B 122 (1997) 329-334
332
Wdq>V 7
~,.08
I,.,I,I,I,,.,.,I,.,I
e 5
a.(Target A)
0.07
013
56
100
143
It should be noted that earlier we have already reported some data concerning the energies of the charged particles desorbed by FF from gold islet targets. By means of the retarding potential method it was shown that some part of desorbed gold particles (_ 30%) leave the target, having energies up to 10 eV per charge unit [5]. Values obtained in the present work are on average l-2 orders of magnitude higher. Evidently, the main reason of this difference is that in [5] using relatively low retarding potentials the high energy charged particles were mistakenly regarded as a neutral component.
187
U acC =-170v U ,-
=-310v
.is
Collector
rE EO.08,.
i
8,
.
b.(TargetB)
t
I
3.2. Measuring m /q with islet masses
Length, mm
0
107
II.
I,
W&z
V
276
444
I
1,
613 I,
781 I
',
“WI =oy
of cluster ions and comparing them
Target A was used for measuring m/q of cluster ions in the dynamic mode of operation of the mass-spectrograph. Fig. Sa shows the distribution of gold atoms along the collector length, obtained in the dynnamic experiment in question. The mass scale calculation (top axis Fig. 5a), besides the analyser parameters (accelerating impulse amplitude and duration -5.2 kV, 50 ~CLS respectively, reflec-
Cluaer Ion m/q, amulemd
42r106
1 4r10e
jl,,,j L
d
Collector
Length, mm
Fig. 4. Distributions of gold atoms along the collector length obtained in the static experiments with targets A and B. The length scales have been converted to cluster ion initial anergy scales (top ax&s). The unshaded distributions are obtained with the grounded electrode El (see Fig. 2), the shaded distributions while applying a retarding potential (I,, to electrode El (for (a) C/L,,-- (I,,,= 97 V, for(b) U:,,,- C&,=‘i28 V). 1
6
The quota of gold atoms, corresponding to particles with W,/q larger than U&,.-U,,,,, could be estimated as the ratio of the amounts of atoms collected in the experiments tiith the retarding potential and with full transmission. But it is evident that such a ratio will be an underestimation due to the non-equivalent conditions of flight of the particles in the experiments in question, as well as for particles with different W,/q. While processing the data we took into consideration the function of spatial resolution of the instrument and transparency of the analysing system to particles with different initial energies. It was established from the distributions 4, b (non-shaded histograms) that for target A the mean value of the axial component of the initial energy of the desorbed cluster ions (W,/q),, is 96 V. - 70% of the whole gold desorbed in the form of negative cluster ions correspond to W,/q = 50- 140 V. For target B these characteristics are 216 V and 100-320 V respectively.
11
16
Iklet Mass
5 E 3
21
26
31
38
41
46
Collector Length, mm
z
0.0
5.0x106 I
5x10*
1.0x10' I
(Hemisphere).amu 1.5X10' I
2.0x10' I
2.5x10' I .
3,0x10'
b. Islets
B
0~ 0.0
'I 1.0x10'
'I'# 2.0x10'
l&t
'C 3.0110
4,ox10'
” 5.G7.10'
6.0x10'
Mass (Sphere), a””
Fig. 5. (a) Distribution of gold atoms along the collector length obtained in the dinamic experiment with target A. The length scale has been converted to a cluster ion m/q scale (top axis); (b) Dependence of gold atom content on the islet mass for target A.
I. Baranou et al./Nucl.
Instr. and Meth. in Phys. Res. B 122 (1997) 329-334
tor field 13 1 V/mm) took also into consideration the mean value of the initial energy (WO/q)mem = 96 V which was
determined as a result of the static experiment. A retarding potential of - 1.4 kV was applied to electrode El (see Fig. 2) which provided the low level discrimination of m/q < 1.5 X 10’ amu/e. Taking into consideration the function of resolution and transparency of the system to particles with different m/q, it was established from distribution Sa that the mean value of m/q of cluster ions desorbed from the given target is 2 X lo6 amu/e, 70% of the whole amount of gold flying in the form of cluster ions with m/q 6 x lo’-4 x lo6 amu/e. In order to compare m/q of cluster ions with islet mass values the islet size distribution (Fig. la) was converted into the islet mass distribution (see Fig. 5b). The relative content of gold atoms instead of the relative number of islets was plotted on the ordinate axis. The mean mass of an islet proved to be 1.8 X IO’ amu, 70% of gold corresponding to islets with masses 6 X 106-3 X IO’ amu, if they are regarded as spheres. These values are 9 X lo6 amu and 3 X 106-1.5 X 10’ amu respectively, if the islets are regarded as hemispheres. Thus, m/q of cluster ions
desorbed by FF from the gold islet target sized 7.5-15 nm appears to be * 5-10 times smaller than islet masses.
4. Discussion This work presents two important results.
(1) A substantial part of the desorbed cluster ions ( - 70% or more) have extremely high start energy - about tens-hundreds of eV per their charge unit. With this, cluster ions desorbed from targets with smaller islets have higher values of the initial energy per charge unit. (2) m/q of cluster ions desorbed from the target with islets sized 7.5- 15.0 nm are 6 X lo’-4 X lo6 amu/e and appear to be on average 5-10 times smaller than the islet masses on the target. The result concerning the initial energies changes considerably our representations of the mechanism of the process studied. It was supposed earlier that energy transfer from the electrons of an islet excited by FF to atoms of a crystal lattice is performed after their thermalisation through electron-phonon interactions, mainly in the islet surface layer, the islet leaving the substrate as a result of its thermal expansion [2]. However, estimations show that if islet leaving the substrate is connected with their thermal expansion then the kinetic energy of clusters can only be about several eV. But the experiment shows that initial energies of the desorbed cluster ions reach tens-hundreds eV. This indicates a very rapid nonequilibrium process of hot electron energy conversion into the kinetic energy of the desorbed particle. Apparently the representation of the first stage of the energy transfer process - from hot electrons to the surface atoms of islets - do not require any changes. They are
333
logically connected with the size effect which is an experimental fact [3,4]. However explanation of the final stage that of the islet leaving the substrate needs new aproaches. With this, more than two times initial kinetic energy increase for islets sized 4.4 nm (target B) than for islets sized 11 nm (target A) may be connected with the increase of energy deposited into these islets by FF per atom: 5 25 eV/atom and 5 eV/atom for targets B and A respectively (the mean range of FF in an islet is equal to its radius, the electron stopping power for FF being _ 30 keV/nm). As for the observed shift between mass distribution of islets and m/q distribution of cluster ions, it can have several reasons. (a) All the processes in the irradiated layer between the moment of FF passing through the islet and the moment of islet desorption can strongly depend on the islet size. As it was shown ealier [3,8], islets with the size close to the size threshold of the electronic sputtering have lower probability to be desorbed by FF than smaller ones. And on the contrary, in [9] it was established that one high-energetic projectile can remove up to several islets from the targets with very small islets (- 4 nm). Thus it may occur that mass (not m/q) distribution of clusters desorbed is also shifted relatively to the islet mass distribution. (b) An islet, on leaving the substrate, can have on its surface several elementary charges. This reduces efficient value of m/q of the formed cluster ion. This reason seems to be the most probable. It agrees well with very high yield (tens %) of charged gold particles sputtered by FF from islet layers 12,101. However, the comparision of m/q distribution of the cluster ions and islet mass distribution must not be used as a direct method of the cluster ion charge state determination. (c) Cluster fragmentation resulting in the formation of lighter particles must not be completely excluded. However, this process cannot be the main one, since it would contradict narrow angular distributions of sputtered gold observed earlier [2,5]. In conclusion we would like to resume, that electronic sputtering of islet targets by multiply-charged ions (FF) is a new method of producing nanometric cluster ions in free state [I]. The experiments described in this work are the first to characterize beams of such specific objects.
Acknowledgements This work has been supported in part by the Intemational Science Foundation and Russian Government, Grants RISOOO, RlS300,
199&1995.
References
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I. Baranov et al./Nucl.
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Insa. and Meth. in Phys. Res. B 122 (1997) 329-334
V.V. Obnorskii and $0. Tsepelevich, Nucl. Instr. and Meth. B 35 (1988) 140. LA. Baranov and V.V. Obnorskii, Sov. At. Energy 54 (1983) 192. I.A. Baranov, Yu.V. Martynenko, S.O. Tsepelevich and Yu.N. Yavlinskii, Sov. Phys. Usp. 31 (1988) 1015. LA. Baranov and S.O. Tsepelevich, Sov. At. Energy 61 ( 1986) 809. O.V. Konstantinov, B.A. Mamyrin, L.E. Shchebelina and V.G. Shchebelin, Sov. Phys. Tech. Phys. 31 (1986) 629.
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