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Analysis of energy distributions for molecular and cluster secondary ions
Surface Science 123 (1982) L717-L720 North-Holland Publishing Company
L717
SURFACE SCIENCE LETTERS ANALYSIS OF ENERGY DISTRIBUTIONS CLUSTER SECONDARY IONS
FOR MOLECULAR AND
S. K A T O , M. M O H R I a n d T. Y A M A S H I N A Department of Nuclear Engineering, Hokkaido University, Sapporo, Japan
Received 19 January 1982, accepted for publication 20 September 1982
Energy distributions of positive ion species sputtered from various target materials have been studied in terms of the exponent of N of E - u in the high energy part. The N value of physically sputtered ion species had a linear relationship extending over a wide range of mass numbers for both homo- and hetero-nuclear molecules, while a trunk-branch relationship with the parent ion species was observed for chemically sputtered ion species. The same significant relationship was discovered for the rearranged experimental data obtained by others.
Energy analysis of various s p u t t e r e d particles from the first wall m a t e r i a l s has recently been a great concern in the s t u d y of p l a s m a - w a l l interactions for n u c l e a r fusion devices [1]. L o w - Z C o m p o u n d s are k n o w n to be p r o m i s i n g first wall materials. T w o different s p u t t e r i n g processes have been o b s e r v e d usually when energetic h y d r o g e n isotopes b o m b a r d the l o w - Z c o m p o u n d s . T h e y are the physical s p u t t e r i n g process which p r o d u c e s particles of the solid target c o m p o n e n t s only, a n d the chemical s p u t t e r i n g process which p r o d u c e s particles of the chemical p r o d u c t s from the reaction between the i r r a d i a t i n g h y d r o g e n i s o t o p e s a n d the target c o n s t i t u e n t a t o m s or molecules [2]. W i t h use of an ion m i c r o p r o b e mass a n a l y z e r we m e a s u r e d energy d i s t r i b u t i o n s of s e c o n d a r y ion species with the m o l e c u l a r form of S i / C , , D + s p u t t e r e d from a p o l y c r y s t a l l i n e silicon c a r b i d e surface i r r a d i a t e d with 10 keV d e u t e r i u m ions at various target t e m p e r a t u r e s [3,4]. T h e i n c i d e n t angle 0 i of a p r i m a r y ion b e a m a n d the d e t e c t i o n angle 0d of a s e c o n d a r y ion b e a m were set at 45 ° to the target surface n o r m a l . T h e v a c u u m of the target c h a m b e r was m a i n t a i n e d below 1 x 10 -5 Pa d u r i n g irradiation. D e t a i l s of the e x p e r i m e n t a l a p p a r a t u s , m e t h o d s a n d results were d e s c r i b e d elsewhere [3-5]. E n e r g y d i s t r i b u t i o n s of s e c o n d a r y ion species are k n o w n to be expressed as a function o f E - N for the higher energy p a r t of them were E is the energy of the s e c o n d a r y ion a n d N is the fitting p a r a m e t e r [6-9]. A significant relationship was o b s e r v e d between the N value of E - N a n d the mass n u m b e r of m o l e c u l a r ions as shown in fig. 1. It was f o u n d that the N value of the p h y s i c a l l y s p u t t e r e d p r o d u c t s of S i t C ,+ is linear with the mass n u m b e r , 0039-6028/82/0000-0000/$02.75
© 1982 N o r t h - H o l l a n d
S. Kato et al. / Energ}' distributions for secondary ions
L718
IOkeV Deuterium I o n s - - - S i C ( 8 4 0 ° C ) - - P o s i t i v e
- 4
o'1
l
--6
~" " ~ '
oI~
-
blL'2U'
ll'l:l,bl2bUn3)
s+,ct
'
- 0~"~"
I
• Physical Spullering Products
-8 -I0
\
Ions
•
0
.
.
Chemicol Sputtering .
I0
.
.
.
20
.
.
.
30
.
.
.
40
Products .
.
.
,50
.
60
I
70
~
80
90
I00
J
I fO
m/e
Fig. 1. Relation between N of energy distributions for various sputtered ion species from silicon carbide and their mass numbers.
whereas the N value of the chemically sputtered products of Si]C,,D, + forms branches from the points of the parent physically sputtered ion species. The relationship between the N value and the mass number for the physically sputtered ion species can be expressed by the following equation, N = 0.054( role ) + constant.
( 1)
However, the N value of the chemically sputtered ion species was found to be always larger than that of the physically sputtered ion species and increasing as the number of deuterium atoms joining to the parent molecular ions increased as shown in the figure. Several authors have also reported the N value of energy distributions in different ion-target systems. Rudat and Morrison measured energy distributions of various positive ions sputtered from 31 pure elements and two compounds by a 5.5 keV Oz+ beam with 0~ and 0a of 73 ° and 0 ° to the target surface normal, respectively [8]. However, they did not analyze their experimental data in such a way as described above. Fig. 2 shows the rearranged plottings based upon their data obtained from the pure materials of carbon, aluminum, titanium and nickel, respectively. In the figure, the secondary ion species combined with oxygen atoms are the chemically sputtered products and those without oxygen atoms are the physically sputtered products. The N value of the physically sputtered ion species shows a linear relationship with the mass number, and that of the chemically sputtered ion species deviates from the straight line to have a trunk-branch relationship which is a very similar relationship to fig. 1. Staudenmaier measured energy distributions of cluster ions sputtered from polycrystalline tungsten targets during irradiation of 150 keV He, Ne, Ar, Kr, Xe and Cu ions with 0+ and 0d of 20 ° and 40 °, respectively. He found that the
S. Kato et aL / Energy distributions for secondary ions 55keY
A I , Ti and N i ~ P o s i t i v e
O~C,
L719
Ions
Ni +
-2
Ni 2
-3
C'~
AI2*
Ti I
T,,Oa" -5
Ni30~'~
" + AI20 0
50
IO0
150
200
m/e
Fig. 2. Relation between N of energy distributions for various sputtered ion species from carbon, a l u m i n u m titanium and nickel, and their mass numbers. D a t a of N were rearranged as a function of mass n u m b e r o b t a i n e d from Rudat and M o r r i s o n [8].
N value of the various cluster ions was independent of primary ion species and W ÷ was closely proportional to E -°'5, W2+ to E -3 and W3+ to E -4. K6nnen et al. proposed a recombination model for the emission of clusters formed by sputtering and obtained an equation which roughly predicted a linear relation for N versus number of constituent atoms in the cluster. They applied their model to the data obtained by Staudenmaier for W ÷, W2+ and W3+ ion emission and showed an agreement within an experimental error [10]. On the other hand, we interpreted the N value of respective ions of the same experimental data with the mass number of the clusters and obtained a straight line as shown in fig. 3.
150 keY He*, Ne*, At% Kr ~, Xe* 0nd Cu÷ ~ W(2500C) ~ Positive Ions 0 -I -2 z
-3
-4 -5 -6
w1 0
184
368 rn/e
552
Fig. 3. Relation between N o f energy distributions for sputtered ions from tungsten and their mass numbers. D a t a o f N were rearranged as a function o f mass number obtained f r o m Staudenmaier
[9l.
L720
S. Kato et aL / Energy distributions for secondary ions
Figs. 1, 2 and 3 show a remarkable linear relationship between the N value a n d the mass n u m b e r for the physically sputtered ions over wide-ranging mass n u m b e r s for not only h o m o n u c l e a r molecules b u t also heteronuclear molecules. We can now deduce a general relationship between the N value a n d the mass n u m b e r of molecular a n d cluster ions produced via physical sputtering through the following equation, N ~ x(m/e)
+ constant,
(2)
where X is the slope of the line which d e p e n d s on the target materials but does n o t d e p e n d on incident ion species. However, e x p l a n a t i o n s of these significant relations have not been successful yet, particularly for the chemically sputtered p r o d u c t s which markedly deviate from eq. (2) to show a b r a n c h relationship against physically sputtered products. F u r t h e r experimental a n d theoretical works will be required to explain adequately the present results a n d to u n d e r s t a n d the m e c h a n i s m of ionization processes of sputtered particles and the difference between physical a n d chemical sputtering processes.
References [1] A.R. Krauss and P.B. Wright, J. Nucl. Mater. 89 (1980) 229. [2] G.M. McCracken and P.E. Post, Nucl. Fusion 19 (1979) 889. [3] M. Mohri, K. Watanabe, T. Yamashina, H. Doi and K. Hayakawa, J. Nucl. Mater. 75 (1978) 309. [4] M. Mohri, S. Kato, S. Shinada, K. Watanabe and T. Yamashina, J. Nucl. Mater. 93/94 (1980) 692. [5] S. Kato, T. Satake, M. Mohri and T. Yamashina, J. Nucl. Mater. 103/104 (1981) 351. [6] Z. Jurela, Radiation Effects 19 (1973) 175. [7] A.R. Krauss and D.M. Gruen, Nucl. Instr. Methods 149 (1978) 547. [8] M.A. Rudat and G.H. Morrison, Surface Sci. 82 (1979) 549. [9] G. Staudenm~iier,Radiation Effects 13 (1972) 87. [10] G.P. K6nnen, A. Tip and A.E. de Vries, Radiation Effects 26 (1975) 23.
L717
SURFACE SCIENCE LETTERS ANALYSIS OF ENERGY DISTRIBUTIONS CLUSTER SECONDARY IONS
FOR MOLECULAR AND
S. K A T O , M. M O H R I a n d T. Y A M A S H I N A Department of Nuclear Engineering, Hokkaido University, Sapporo, Japan
Received 19 January 1982, accepted for publication 20 September 1982
Energy distributions of positive ion species sputtered from various target materials have been studied in terms of the exponent of N of E - u in the high energy part. The N value of physically sputtered ion species had a linear relationship extending over a wide range of mass numbers for both homo- and hetero-nuclear molecules, while a trunk-branch relationship with the parent ion species was observed for chemically sputtered ion species. The same significant relationship was discovered for the rearranged experimental data obtained by others.
Energy analysis of various s p u t t e r e d particles from the first wall m a t e r i a l s has recently been a great concern in the s t u d y of p l a s m a - w a l l interactions for n u c l e a r fusion devices [1]. L o w - Z C o m p o u n d s are k n o w n to be p r o m i s i n g first wall materials. T w o different s p u t t e r i n g processes have been o b s e r v e d usually when energetic h y d r o g e n isotopes b o m b a r d the l o w - Z c o m p o u n d s . T h e y are the physical s p u t t e r i n g process which p r o d u c e s particles of the solid target c o m p o n e n t s only, a n d the chemical s p u t t e r i n g process which p r o d u c e s particles of the chemical p r o d u c t s from the reaction between the i r r a d i a t i n g h y d r o g e n i s o t o p e s a n d the target c o n s t i t u e n t a t o m s or molecules [2]. W i t h use of an ion m i c r o p r o b e mass a n a l y z e r we m e a s u r e d energy d i s t r i b u t i o n s of s e c o n d a r y ion species with the m o l e c u l a r form of S i / C , , D + s p u t t e r e d from a p o l y c r y s t a l l i n e silicon c a r b i d e surface i r r a d i a t e d with 10 keV d e u t e r i u m ions at various target t e m p e r a t u r e s [3,4]. T h e i n c i d e n t angle 0 i of a p r i m a r y ion b e a m a n d the d e t e c t i o n angle 0d of a s e c o n d a r y ion b e a m were set at 45 ° to the target surface n o r m a l . T h e v a c u u m of the target c h a m b e r was m a i n t a i n e d below 1 x 10 -5 Pa d u r i n g irradiation. D e t a i l s of the e x p e r i m e n t a l a p p a r a t u s , m e t h o d s a n d results were d e s c r i b e d elsewhere [3-5]. E n e r g y d i s t r i b u t i o n s of s e c o n d a r y ion species are k n o w n to be expressed as a function o f E - N for the higher energy p a r t of them were E is the energy of the s e c o n d a r y ion a n d N is the fitting p a r a m e t e r [6-9]. A significant relationship was o b s e r v e d between the N value of E - N a n d the mass n u m b e r of m o l e c u l a r ions as shown in fig. 1. It was f o u n d that the N value of the p h y s i c a l l y s p u t t e r e d p r o d u c t s of S i t C ,+ is linear with the mass n u m b e r , 0039-6028/82/0000-0000/$02.75
© 1982 N o r t h - H o l l a n d
S. Kato et al. / Energ}' distributions for secondary ions
L718
IOkeV Deuterium I o n s - - - S i C ( 8 4 0 ° C ) - - P o s i t i v e
- 4
o'1
l
--6
~" " ~ '
oI~
-
blL'2U'
ll'l:l,bl2bUn3)
s+,ct
'
- 0~"~"
I
• Physical Spullering Products
-8 -I0
\
Ions
•
0
.
.
Chemicol Sputtering .
I0
.
.
.
20
.
.
.
30
.
.
.
40
Products .
.
.
,50
.
60
I
70
~
80
90
I00
J
I fO
m/e
Fig. 1. Relation between N of energy distributions for various sputtered ion species from silicon carbide and their mass numbers.
whereas the N value of the chemically sputtered products of Si]C,,D, + forms branches from the points of the parent physically sputtered ion species. The relationship between the N value and the mass number for the physically sputtered ion species can be expressed by the following equation, N = 0.054( role ) + constant.
( 1)
However, the N value of the chemically sputtered ion species was found to be always larger than that of the physically sputtered ion species and increasing as the number of deuterium atoms joining to the parent molecular ions increased as shown in the figure. Several authors have also reported the N value of energy distributions in different ion-target systems. Rudat and Morrison measured energy distributions of various positive ions sputtered from 31 pure elements and two compounds by a 5.5 keV Oz+ beam with 0~ and 0a of 73 ° and 0 ° to the target surface normal, respectively [8]. However, they did not analyze their experimental data in such a way as described above. Fig. 2 shows the rearranged plottings based upon their data obtained from the pure materials of carbon, aluminum, titanium and nickel, respectively. In the figure, the secondary ion species combined with oxygen atoms are the chemically sputtered products and those without oxygen atoms are the physically sputtered products. The N value of the physically sputtered ion species shows a linear relationship with the mass number, and that of the chemically sputtered ion species deviates from the straight line to have a trunk-branch relationship which is a very similar relationship to fig. 1. Staudenmaier measured energy distributions of cluster ions sputtered from polycrystalline tungsten targets during irradiation of 150 keV He, Ne, Ar, Kr, Xe and Cu ions with 0+ and 0d of 20 ° and 40 °, respectively. He found that the
S. Kato et aL / Energy distributions for secondary ions 55keY
A I , Ti and N i ~ P o s i t i v e
O~C,
L719
Ions
Ni +
-2
Ni 2
-3
C'~
AI2*
Ti I
T,,Oa" -5
Ni30~'~
" + AI20 0
50
IO0
150
200
m/e
Fig. 2. Relation between N of energy distributions for various sputtered ion species from carbon, a l u m i n u m titanium and nickel, and their mass numbers. D a t a of N were rearranged as a function of mass n u m b e r o b t a i n e d from Rudat and M o r r i s o n [8].
N value of the various cluster ions was independent of primary ion species and W ÷ was closely proportional to E -°'5, W2+ to E -3 and W3+ to E -4. K6nnen et al. proposed a recombination model for the emission of clusters formed by sputtering and obtained an equation which roughly predicted a linear relation for N versus number of constituent atoms in the cluster. They applied their model to the data obtained by Staudenmaier for W ÷, W2+ and W3+ ion emission and showed an agreement within an experimental error [10]. On the other hand, we interpreted the N value of respective ions of the same experimental data with the mass number of the clusters and obtained a straight line as shown in fig. 3.
150 keY He*, Ne*, At% Kr ~, Xe* 0nd Cu÷ ~ W(2500C) ~ Positive Ions 0 -I -2 z
-3
-4 -5 -6
w1 0
184
368 rn/e
552
Fig. 3. Relation between N o f energy distributions for sputtered ions from tungsten and their mass numbers. D a t a o f N were rearranged as a function o f mass number obtained f r o m Staudenmaier
[9l.
L720
S. Kato et aL / Energy distributions for secondary ions
Figs. 1, 2 and 3 show a remarkable linear relationship between the N value a n d the mass n u m b e r for the physically sputtered ions over wide-ranging mass n u m b e r s for not only h o m o n u c l e a r molecules b u t also heteronuclear molecules. We can now deduce a general relationship between the N value a n d the mass n u m b e r of molecular a n d cluster ions produced via physical sputtering through the following equation, N ~ x(m/e)
+ constant,
(2)
where X is the slope of the line which d e p e n d s on the target materials but does n o t d e p e n d on incident ion species. However, e x p l a n a t i o n s of these significant relations have not been successful yet, particularly for the chemically sputtered p r o d u c t s which markedly deviate from eq. (2) to show a b r a n c h relationship against physically sputtered products. F u r t h e r experimental a n d theoretical works will be required to explain adequately the present results a n d to u n d e r s t a n d the m e c h a n i s m of ionization processes of sputtered particles and the difference between physical a n d chemical sputtering processes.
References [1] A.R. Krauss and P.B. Wright, J. Nucl. Mater. 89 (1980) 229. [2] G.M. McCracken and P.E. Post, Nucl. Fusion 19 (1979) 889. [3] M. Mohri, K. Watanabe, T. Yamashina, H. Doi and K. Hayakawa, J. Nucl. Mater. 75 (1978) 309. [4] M. Mohri, S. Kato, S. Shinada, K. Watanabe and T. Yamashina, J. Nucl. Mater. 93/94 (1980) 692. [5] S. Kato, T. Satake, M. Mohri and T. Yamashina, J. Nucl. Mater. 103/104 (1981) 351. [6] Z. Jurela, Radiation Effects 19 (1973) 175. [7] A.R. Krauss and D.M. Gruen, Nucl. Instr. Methods 149 (1978) 547. [8] M.A. Rudat and G.H. Morrison, Surface Sci. 82 (1979) 549. [9] G. Staudenm~iier,Radiation Effects 13 (1972) 87. [10] G.P. K6nnen, A. Tip and A.E. de Vries, Radiation Effects 26 (1975) 23.