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Sputtering on cobalt with noble gas ions
Nuclear Instruments and Methods 209/210 (1983) 543-548 North-Holland Publishing Company
SPUTTERING
543
ON COBALT WITH NOBLE GAS IONS
L. S A R H O L T - K R I S T E N S E N ,
A. J O H A N S E N and E. J O H N S O N
Physics Laboratory' 11, H.C. Orsted Institute, Universitetsparken 5, D K 2100, Denmark
V.S. C H E R N Y S H Moscow State University, University of Moscow, USSR
Single crystals of cobalt have been bombarded with 80 keV Ar + ions and with 80 keV and 200 keV Xe + ions in the (0001) direction of the hcp phase and the ( 111 ) direction of the fcc phase. The sputtering yield has been measured as function of target temperature (20°C-500°C), showing a reduction in sputtering yield for 80 keV Ar + ions and 200 keV Xe + ions, when the crystal structure changes from hcp to fcc. In contrast to this, bombardment with 80 keV Xe + ions results in an increase in sputtering yield as the phase transition is passed. Sputtering yields for ( 111 ) nickel are in agreement with the sputtering yields for fcc cobalt indicating normal behaviour of the fcc cobalt phase. The higher sputtering yield of (0001) cobalt for certain combinations of ion mass and energy may then be ascribed to disorder induced partly by martensitic phase transformation, partly by radiation damage.
1. Introduction T h e s p u t t e r i n g of p o l y c r y s t a l l i n e m a t e r i a l s has been the object of a vast a m o u n t of e x p e r i m e n t a l a n d theoretical investigations [1,2], while less effort has been d e v o t e d to s p u t t e r i n g of single crystals. The m a i n e m p h a s i s has been given to d e t e r m i n a t i o n s of the a n g u l a r d e p e n d e n c e of the s p u t t e r i n g yield while total s p u t t e r i n g yields have o n l y been investigated for very limited i o n - t a r g e t c o m b i n a t i o n s , m a i n l y in the low keV energy range [3]. Theoretical t r e a t m e n t s of single crystal sputtering yields are m o s t l y b a s e d on the c o n c e p t of t r a n s p a r a n c y , originally f o r m u l a t e d b y O n d e r l i n d e n [4] a n d treated in recent review articles b y R o s e n d a h l [3] a n d R o b i n s o n [5]. A c c o r d i n g to the t r a n s p a r a n c y m o d e l the i n c o m i n g b e a m is d i v i d e d into two c o m p o n e n t s ; the r a n d o m (or n o n - c h a n neled) beam, which causes s p u t t e r i n g of a t o m s from the surface layers a n d the aligned (or channeled) b e a m , which does not c o n t r i b u t e to sputtering. T h e n o n - c h a n n e l e d c o m p o n e n t of the i n c o m ing b e a m can be c a l c u l a t e d from L i n d h a r d ' s channeling theory [6]. T h e present w o r k describes an investigation of the d e p e n d e n c e of s p u t t e r i n g yield on t e m p e r a t u r e f r o m single crystals a n d polycrystals of c o b a l t b o m b a r d e d with n o b l e gas ions. E x p e r i m e n t s have been carried out b e l o w as well as a b o v e the phase 0167-5087/83/0000-0000/$03.00
c h a n g e at - 415°C where cobalt in a martensitic t r a n s f o r m a t i o n changes its structure from hexagonal close p a c k e d (hcp) to face centred cubic (fcc) [7]. As sputtering yields of polycrystals as well as single crystals are generally i n d e p e n d e n t of temp e r a t u r e [3], except at t e m p e r a t u r e s very close to the melting p o i n t [8], c o b a l t offers a rare possibility of studying the influence on sputtering yields of different crystal structures for the s a m e material, the m o r e so, as c o b a l t like most other metals is believed to be o n l y slightly affected b y d i s o r d e r [9,10,11] in spite of the d a m a g e a t t a i n e d at the high fluences used in the experiments.
2. Experimental The e x p e r i m e n t a l set up has earlier been described in detail [12,13]. Discs from 99.99% c o b a l t a n d nickel single crystals were s p a r k cut with (0001) a n d ( 111 ) normals, respectively. M e c h a n i cal, v i b r a t i o n a l a n d electrolytic polishing results in crystals of high p e r f e c t i o n with very little RBS-disorder. The crystals were o r i e n t e d a n d a n a l y s e d using 400 keV H e 2+ ions, and s u b s e q u e n t l y b o m b a r d e d with 80 keV A r + or Xe + ions or 200 keV Xe + ions to fluences up to 10 23 m - z , using b e a m fluxes up to 2 × 1019 m - 2 . s -1. F o r m e a s u r e m e n t s of single crystal s p u t t e r i n g
© 1983 N o r t h - H o l l a n d
v. SPUTTERING
544
L. Sarholt- Kristensen et al. / Sputtering on cobalt
yields, hcp cobalt was b o m b a r d e d in the (0001} direction a n d fcc c o b a l t and nickel in the ~111) direction. R a n d o m sputtering yields were o b t a i n e d by tilting the crystals - 10 ° off n o r m a l incidence d u r i n g b o m b a r d m e n t a n d rotating them a r o u n d the surface n o r m a l through a region c o n t a i n i n g no m a j o r axis or planes. S p u t t e r i n g yields were o b t a i n e d b y measuring the weight-loss of the crystals ( 1 - 1 0 0 /~g) on a M e t t l e r M E 22 (or more recently a Mettler U M 3) m i c r o b a l a n c e with an accuracy better than 0.5/~g. Scanning m i c r o s c o p y investigations have shown [12] that surface t o p o g r a p h y does not develop on c o b a l t single crystals sputtered with A r ÷ ions. F r o m test experiments it was established that a c o b a l t single crystal could be used for at least three experiments, before repolishing was required. Because of a " m e m o r y effect" [14] consecutive sputtering experiments on the same crystal were always p e r f o r m e d at increasing temperatures. Using a d o u b l e alignment RBS-analysis [14] it has f u r t h e r m o r e been verified that the phase transition of the u n b o m b a r d e d c o b a l t crystal takes p l a c e at - 415°C, in a c c o r d a n c e with other results
[
I
t
i
80 keY Ar" ~ ...........
I
~ ...............
r I. . . . .
a
[
eo
x
P
L 0
-100
bombardrnentl
o nickel <111>
x random
bombardment
~, nickel random ___[ 100
• polycrysta[[ine J [ 200 300
r J J
[I L,O0
I 500
C°
Fig. l. Sputtering yields of 80 keV Ar + on cobalt and nickel (for reference). The size of marks in the figure represents the experimental uncertainties. Y atoms/ion
I
.... ~ 80 keY Xe + ~
I Co
,
i •
\
--•-'t"
t0
• channeled i
Sputtering yields as function of t e m p e r a t u r e were measured for 80 keV A r + (fig. 1), 80 keV Xe + (fig. 2) and 200 keV Xe + (fig. 3). In each case, the sputtering yields of ~111) nickel have been measured, too, because of the close resemblance between fcc c o b a l t a n d nickel in physical and chemical p a r a m e t e r s . A c c o r d i n g l y the ~111 sputtering yields of fcc cobalt a n d nickel should be essentially equal. This is c o n f i r m e d by figs. 1-3, indicating that the fcc phase of c o b a l t behaves " n o r m a l l y " . C o n t r a r y to this the (0001) sputtering yield for 80 keV A r + show a n o m a l o u s behaviour (fig. 1). W h i l e the hcp sputtering yield at a n d below room t e m p e r a t u r e is at the same level as fcc sputtering yiel& it increases linearly from - 20°C to - 100°C, where it stabilises at a level, only 20% less than the sputtering yield of polycrystalline cobalt. A similar behaviour, although less p r o n o u n c e d , is seen in fig. 3 for 200 keV Xe + ions. In c o n t r a s t to the 80 keV A r + and 200 keV Xe + sputtering yields, the sputtering yields for 80 keV Xe + (fig. 2) are all lower than the sputtering yield of the fcc phase.
I
Co
• chonneted
[71.
3. Results and discussion
[
f lOmsliOn
O nickel <111>
l 0--0
bombardment
x random
bombardment
• po(ycrystaltine ~ ..... I 200 300
J ............ 100
I /,00
T C°
Fig. 2. Sputtering yields of 80 keV Xe + on cobalt and nickel (for reference). The size of the marks in the figure represents the experimental uncertainties, The dashed lines for polycrystalline and random sputtering yields have only been drawn to guide the eye. ato~ms/ion
1
I 200 keY
-/_/1-
S
I Xe* ~
]
Co
/ j - A ~ ,~
//
_ . - . . . ~ / . . x4.\
J • channeled o nickel <111> _1 100
x random
bombardment
bombardment
• polyc rystoiline I L 200 300
I Z,O0
T C°
Fig. 3. Sputtering yields of 200 keV Xe + on cobalt and nickel (for reference). The size of the marks in the figure represents the experimental uncertainties. The dashed lines for polycrystalline and random sputtering yields have only been drawn to guide the eye.
L. Sarholt-Kristensen et al. / Sputtering on cobalt
According to the transparancy theory of Onderlinden [4], the single crystal sputtering yield Y; .... > in a channeled direction can be related to the sputtering yield Y of the structureless medium through y< ...... > = ~tX,,,i,,Y,
(1)
where Xm~n is the nonchanneled fraction of the sputter beam in the ( u v w ) direction and ~/ is a fitting parameter. Xmi, can be calculated for the unbombarded crystal using empirical, partly temperature dependent channeling models [ 15,16]. At the fairly high energies used in these experiments both Ypoly and Yrandom should be good approximations to Y. In accordance with earlier experiments (for references see [3]) we have chosen to use Ypo~y, the more so as preliminary experiments with 80 keV Ar ÷ on Co have shown, that surface topography only influences the polycrystalline sputtering yields, when fluences exceed 10 24 m -2,
The fitting parameter ~ can then be calculated as function of temperature from the ratio between the measured single crystal sputtering yield and the polycrystalline yield at the appropriate temperature, divided by the associated calculated minim u m yield Xmi, (fig. 4). Sputtering experiments have shown that ~/ depends on ion mass and energy, and the crystallographic orientation [3,5]. ~ values between 1.2 and 2.0 have been found to be good fits at low energy (a few keV) sputtering as well as for high energy Ar ÷ sputtering on (111) fcc Cu [17,3]. The values obtained in our experiments for the fcc
545
cobalt phase are in close agreement with those results. The hcp phase, on the other hand, shows quite different behaviour of the ~ values. Firstly, all the r/ values are well above 2 (in accordance with Ar ÷ sputtering on Cu in open directions [17,3]), and secondly, the 71 value depends strongly on the combination of ion mass and energy, and for Ar + on the temperature, too. The high values of ~ are either due to underestimation of the minimum yield Xm~n a n d / o r the polycrystalline sputtering yields, Ypo~y. The latter, however, agrees well with theory [1,2] and, for Ar ÷, with other experiments [18]. Although some discrepancies do exist between the experimental Ypoly and Yrandomfor Xe + bombardments, raising some doubt about the best assignment to Y, the differences are too small to account for the high values. The explanation of the ~ values for the hcp cobalt phase should then be ascribed to differences between Xm~n values calculated from theory and values representative of the hcp cobalt crystals sputtered to fluences - 1023 m - 2 . At low fluences ( - 1 0 21 m 2) the sputtering yield of (0001) Co with 80 keV Ar + ions at room temperature is 1.0 [14] assuming that none of the implanted material is sputtered off. Y<000~> equal to 1.0 is in agreement with the calculated Xm~,, when an ~ value equal to 1.8 is used. Furthermore, angular scans across the unbombarded (0001) channel, using 400 keV He 2+ ions yield Xmin values in close agreement with calculations [13]. This is presumably also the case for Ar + and Xe +. At higher fluences the agreement between calculated and experimental minimum yields for 400 keV He z+ becomes worse [13]. A measure of the RBS-disorder can be obtained by
i .....
.... -
-
80keV
Ar +
2 0 0 key Xe + 80key
D
Xe +
T 100
200
300
400
C°
Fig. 4. The fitting parameter ~/ calculated for 20°C, 100°C, 250°C, 350°C and 470°C. Smooth lines have been drawn through the points.
o,-
r . , - Yv
'
(2)
Yd', Yv and Yr being the yields in the RBS-spectra of the bombarded crystal, the virgin crystal and the random crystal, respectively. The yields are taken at the same position, just behind the surface peak. /)re I is shown in fig. 5 as a function of fluence for Ar + bombarded crystals. At low fluences, neither the hcp structure, nor the fcc structure is damaged to any appreciable extent. At a fluence equal to a few times 102° m 2 severe damage starts to build up in the hcp structure at 250°C, and the disorder saturates at a level equal V. S P U T T E R I N G
516
1.~
L. Sarholt-Kristensen et al. / Sputtering on cobalt
¥o' -Yv Yr
"Disorder"versus fluence 80 key Ar+--~,,-Co • ok 20°C • at 250°C • at 470°C
10 O.a , i II
OG OA
/ z//
0.; ..... 1019
• .......
x. . . . . . . . . . . .
~
/
/
• .''~"
1011
i0 zO
1022
Fluence ( ions I m 2)
Fig. 5. The relative disorder /)re~equal to the minimum yield for 400 keV He2+ of the argon bombarded crystal minus the minimum yield of the virgin crystal versus fluence. Smooth lines to guide the eye have been drawn through the experimental points. to 0.8 (closely equal to the value for a random RBS-analysis). Similar disorder measurements for room temperature sputtering performed in the fluence range equal to 1 0 2 1 - 1 0 22 m 2 (fig. 5), show that the disorder in the hcp crystal is slightly larger than in the fcc crystal. The high sputtering yields of 80 keV Ar + on hcp cobalt at high fluences must then be ascribed to a heavily disordered crystal structure, resulting in high minimum yields Xmin' In the context of the transparancy model, any discrepancies between the calculated and real Xm~n values will express itself in high values of the fitting parameter 7According to fig. 4, Xe ÷ bombardments, too, result in high values of r/ for the hcp phase, from which it is anticipated that considerable disorder is created also by Xe ÷ ions in the hcp phase. The dependence of sputtering yield on temperature for channeled bombardment with Ar ÷ ions (fig. 1) strongly indicates that the disorder may be associated with the phase change. Furthermore, the hcp ~ fcc phase change, as reflected in sputtering yield, occurs at a temperature - 5 0 ° C below the commonly accepted value of - 415°C [7]. This lowering of the phase transition temperature is not due to beam-heating, as demonstrated in low flux sputtering experiments [14], but may reflect the possibility of inducing martensitic phase transformations by ion implantation [19-21]. Ion induced lowering of the phase transition temperature can be discussed in the context of the
energy cascade - more particular its size and its energy density. To stabilize the size of the new phase embryos, transformed volumes should, according to Nabarro [22], be large enough to facilitate break-away from the matrix. This critical size is for the transformation in cobalt of the order of a few tens of nanometer. It is furthermore to be expected that the energy density must exceed a certain minimum value, presumably, considerably lower than the sublimation energy. If ion induced phase transformations contribute to the disorder created during sputtering, as strongly indicated by fig. 1, 80 keV Ar ÷ ions on Co should fulfill both the above mentioned requirements. The average damage depth (xD) calculated [23] for 80 keV Ar + ions on Co is - 25 nm and thus comparable to Nabarro's critical size. According to Sigmund [24] the energy density 00 in the cascade can be calculated for different ion energies as function of incident ion mass in a cobalt target (fig. 6). The dashed line included in fig. 6 gives those combinations of ion mass and energy, which result in the same (XD) as for argon. Points above the dashed line have smaller (xD) values and points below larger (xD) values than 80 keV Ar +. 200 keV Xe + ions on Co have a somewhat smaller energy cascade with a somewhat higher energy density, whereas for 80 keV Xe + ions on Co, the energy cascade is considerably smaller, and the energy density an order of magnitude higher. Fig. 2 shows in agreement with this that the change in sputtering yield for 80 keV Xe + ions tends to happen closer to the real phase transition temperature than for 80 keV Ar + ions. Furthermore, the hcp--, fcc phase change in that case increases the sputtering yield, and this is qualitatively in agreement with the low transparancy of the (111) direction compared to the @001) direction. Although 80 keV Xe + sputtering presents the lowest ~ values for the hcp structure, they are still rather high, compared to fcc target material, indicating that even in this case the hcp structure is partially disordered by ion bombardment. The high values of the (0001) high fluence sputtering yields of Ar + and Xe ÷ ions in cobalt may then be ascribed to formation of excessive disorder, partly resulting from an implantation induced phase transformation, premature with respect to temperature, and partly due to radiation damage. The main contribution to 80 keV Ar + and
L. Sarholt-Kristensen et a L / Sputtering on cobalt
10pOoatom
i
Beside the single crystal sputtering yields, the polycrystalline and random yields in figs. 2 and 3 attract attention, too. Whereas the polycrystalline and random yields are in good agreement for 80 keV Ar + ions, 80 keV Xe + bombardments give random yields which are substantially higher than the polycrystalline yields, whilst the opposite applies to 200 keV Xe ÷ bombardments. Remarkable, too, is the oscillating behaviour for both random and polycrystalline sputtering yields of 200 keV Xe ÷ bombardments near the phase transition temperature. At present, no adequate physical explanation can be given and further experiments are certainly needed.
Target material:CO
////S
/;// ,~
'
4. Conclusion
i
0
547
100
2C)0
MI
Fig. 6. The energy density calculated according to [24] versus the ion mass of the impinging ions, when cobalt is used as target.
200 keV Xe ÷ ion induced disorder seems to come from the phase transformation, which seems to be less pronounced for 80 keV Xe ÷ ion bombardment. Preliminary experiments on 80 keV Ar ÷ b o m b a r d e d polycrystalline cobalt specimens investigated by transmission electron microscopy [14], have shown that there are no signs of amorphization, in accordance with similar results from nickel implanted with argon [25]. The observed microstructure, which is insensitive to the implantation temperature, consists of very dense and virtually unresolvable distributions of defect clusters and dislocation tangles. Superimposed on this defect structure are dense distributions of argon bubbles, a few tens of nanometers in diameter, which can be observed in phase contrast. Amorphization is presumably suppressed due to formation of the bubbles. The fluences used in these experiments were 102~ m 2 i.e. considerably lower than those used for single crystal sputtering experiments. Additional experiments at higher fluences and a detailed combined analysis of defect microstructures and related electron diffraction patterns have to be carried out in order to outline the reason for the easily damaged hcp phase.
It has been shown that the hcp phase of cobalt is heavily disordered by ion bombardment with Ar ÷ and Xe ÷ ions. Part of the disorder is presumably connected with the martensitic phase transition which may be induced by the ion bombardment. For 80 keV Xe ÷ bombardments, however, it is more likely that the disorder in the hcp structure can be explained from formation of damage, which will also contribute to the disorder created by 80 keV Ar ÷ and 200 keV Xe ÷ ions. Further experiments should concentrate, by means of transmission electron microscopy, on investigations of the nature of damage produced by ion bombardment in the hcp cobalt phase. Furthermore, the importance of energy density and energy cascade dimension, may be assessed by using suitable combinations of projectile energy and mass. By using a fitting parameter ~ equal to about 1.5 the transparancy model predicts fairly well the (111) sputtering yields of 80 keV Ar +, 80 keV Xe ÷ and 200 keV Xe ÷, as well as the low fluence (0001) sputtering yield of 80 keV Ar ÷ at room temperature. For high fluences, ~ values well above 2 have to be used for the open (0001) direction in the hcp phase in order to fit the predictions of the transparancy model to the experimental sputtering yields. A similar result was obtained for copper sputtered in open directions with argon ions at similar energy and fluence [17,3]. In the hcp phase of cobalt the high values of can be ascribed to the sensitivity of 77 on disorder. V. SPUTTERING
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L. Sarholt-Kristensen et al. / Sputtering on cobalt
7/ then becomes dependent on quantities such as ion mass, ion energy and fluence, making the use of the transparancy model questionable.
Support from the Danish Natural Sciences Research Council is gratefully acknowledged.
References [1] H.H. Andersen and H.L. Bay in Sputtering by Particle Bombardment I, Topics in Applied Physics 47, ed., R. Behrisch (Springer, Heidelberg, 1981) ch. 4 and refs. therein. [2] P. Sigmund in Sputtering by Particle Bombardment I, Topics in Applied Physics 47, ed., R. Behrisch (Springer, Heidelberg, 1981) ch. 2 and refs. therein. [3] H.E. Roosendaal in Sputtering by Particle Bombardment I, Topics in Applied Physics 47, ed., R. Behrisch (Springer, Heidelberg, 1981), ch. 5 and refs. therein. [4] D. Onderlinden, Can. J. Phys. 46 (1968) 739. [5] M.T. Robinson in Sputtering by Particle Bombardment I, Topics in Applied Physics 47, ed., R. Behrisch (Springer, Heidelberg, 1981), ch. 3 and refs. therein. [6] J. Lindhard, K. Dan. Vidensk. Selsk., Mat. Fys. Medd. 34 (1965) no. 14. [7] R. Alzenz, B.W. Lee, A. Ignatiev and M.A. van Hove, Solid State Comm. 25 (1978) 641 and refs. therein. [8] R.S. Nelson, The Observation of Atomic Collisions in Crystalline Solids (North-Holland, Amsterdam, 1968). [9] R.S. Nelson in Channeling, Theory Observation and Applications, ed., D.V. Morgan (Wiley, New York, 1973), ch. IX.
[10] E. Bogh, Proc. Cairo Solid State Conf., Interaction of Radiation with Solids, ed., A. Bishay (Plenum, New York, 1969) p. 361. [11] P.P. Pronko, J. Bcttiger, J.A. Davies and J.B. Mitchell. Rad. Effects 21 (1974) 25. [12] V.S. Chernysh, A. Johansen and L. Sarholt-Kristensen Rad. Effects Lett. 57 (1980) 119. [13] V.S. Chernysh, A. Johansen and L. Sarholt-Kristensen Proc. V Int. Conf. on Ion Beam Analysis, Sydney, eds., J.R. Bird and G.J. Clark (North-Holland, Amsterdam, 1981) p. 253. [14] A. Johansen, E. Johnson, L. Sarholt-Kristensen and V.S. Chernysh, Proc. Nat. Conf. on Interaction of Atomic Particles with Solids, Minsk, (1981) p. 18, in Russian, to be published in English. [15] J.H. Barrett, Phys. Rev. B3 (1971) 1527. [16] B.R. Appleton and G. Foti, Ion Beam Handbook for Material Analysis, eds., J.W. Mayer and E. Rimini (Academic Press, New York, 1977) ch. 3. [17] T.W. Snouse and E.C. Haughney, J. Appl. Phys. 37 (1966) 700. [18] C. Fert, N. Colombie, B. Fagot and Pham van Chuong, Le Bombardemend Ionique (CNRS, 1961) 67. [19] M. Marinov and D. Dobrev, Thin Solid Films 42 (1977) 265. [20] E. Johnson, U. Littmark, A. Johansen and C. Christodoulides, Phil. Mag. A45 (1982) 803. [21] L. Sarholt-Kristensen, E. Johnson and A. Johansen, these Proceedings, p. 289. [22] F.R.N. Nabarro, Proc. Roy. Soc. A175 (1940) 519. [23] K.B. Winterbon, P. Sigmund and J.B. Sanders, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 37 no. 14 (1970). [24] P. Sigmund, Appl. Phys. Lett. 25 (1974) 169. [25] A. Ali, W.A. Grant and P.J. Grundy, Phil. Mag. B37 (1978) 353.
SPUTTERING
543
ON COBALT WITH NOBLE GAS IONS
L. S A R H O L T - K R I S T E N S E N ,
A. J O H A N S E N and E. J O H N S O N
Physics Laboratory' 11, H.C. Orsted Institute, Universitetsparken 5, D K 2100, Denmark
V.S. C H E R N Y S H Moscow State University, University of Moscow, USSR
Single crystals of cobalt have been bombarded with 80 keV Ar + ions and with 80 keV and 200 keV Xe + ions in the (0001) direction of the hcp phase and the ( 111 ) direction of the fcc phase. The sputtering yield has been measured as function of target temperature (20°C-500°C), showing a reduction in sputtering yield for 80 keV Ar + ions and 200 keV Xe + ions, when the crystal structure changes from hcp to fcc. In contrast to this, bombardment with 80 keV Xe + ions results in an increase in sputtering yield as the phase transition is passed. Sputtering yields for ( 111 ) nickel are in agreement with the sputtering yields for fcc cobalt indicating normal behaviour of the fcc cobalt phase. The higher sputtering yield of (0001) cobalt for certain combinations of ion mass and energy may then be ascribed to disorder induced partly by martensitic phase transformation, partly by radiation damage.
1. Introduction T h e s p u t t e r i n g of p o l y c r y s t a l l i n e m a t e r i a l s has been the object of a vast a m o u n t of e x p e r i m e n t a l a n d theoretical investigations [1,2], while less effort has been d e v o t e d to s p u t t e r i n g of single crystals. The m a i n e m p h a s i s has been given to d e t e r m i n a t i o n s of the a n g u l a r d e p e n d e n c e of the s p u t t e r i n g yield while total s p u t t e r i n g yields have o n l y been investigated for very limited i o n - t a r g e t c o m b i n a t i o n s , m a i n l y in the low keV energy range [3]. Theoretical t r e a t m e n t s of single crystal sputtering yields are m o s t l y b a s e d on the c o n c e p t of t r a n s p a r a n c y , originally f o r m u l a t e d b y O n d e r l i n d e n [4] a n d treated in recent review articles b y R o s e n d a h l [3] a n d R o b i n s o n [5]. A c c o r d i n g to the t r a n s p a r a n c y m o d e l the i n c o m i n g b e a m is d i v i d e d into two c o m p o n e n t s ; the r a n d o m (or n o n - c h a n neled) beam, which causes s p u t t e r i n g of a t o m s from the surface layers a n d the aligned (or channeled) b e a m , which does not c o n t r i b u t e to sputtering. T h e n o n - c h a n n e l e d c o m p o n e n t of the i n c o m ing b e a m can be c a l c u l a t e d from L i n d h a r d ' s channeling theory [6]. T h e present w o r k describes an investigation of the d e p e n d e n c e of s p u t t e r i n g yield on t e m p e r a t u r e f r o m single crystals a n d polycrystals of c o b a l t b o m b a r d e d with n o b l e gas ions. E x p e r i m e n t s have been carried out b e l o w as well as a b o v e the phase 0167-5087/83/0000-0000/$03.00
c h a n g e at - 415°C where cobalt in a martensitic t r a n s f o r m a t i o n changes its structure from hexagonal close p a c k e d (hcp) to face centred cubic (fcc) [7]. As sputtering yields of polycrystals as well as single crystals are generally i n d e p e n d e n t of temp e r a t u r e [3], except at t e m p e r a t u r e s very close to the melting p o i n t [8], c o b a l t offers a rare possibility of studying the influence on sputtering yields of different crystal structures for the s a m e material, the m o r e so, as c o b a l t like most other metals is believed to be o n l y slightly affected b y d i s o r d e r [9,10,11] in spite of the d a m a g e a t t a i n e d at the high fluences used in the experiments.
2. Experimental The e x p e r i m e n t a l set up has earlier been described in detail [12,13]. Discs from 99.99% c o b a l t a n d nickel single crystals were s p a r k cut with (0001) a n d ( 111 ) normals, respectively. M e c h a n i cal, v i b r a t i o n a l a n d electrolytic polishing results in crystals of high p e r f e c t i o n with very little RBS-disorder. The crystals were o r i e n t e d a n d a n a l y s e d using 400 keV H e 2+ ions, and s u b s e q u e n t l y b o m b a r d e d with 80 keV A r + or Xe + ions or 200 keV Xe + ions to fluences up to 10 23 m - z , using b e a m fluxes up to 2 × 1019 m - 2 . s -1. F o r m e a s u r e m e n t s of single crystal s p u t t e r i n g
© 1983 N o r t h - H o l l a n d
v. SPUTTERING
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L. Sarholt- Kristensen et al. / Sputtering on cobalt
yields, hcp cobalt was b o m b a r d e d in the (0001} direction a n d fcc c o b a l t and nickel in the ~111) direction. R a n d o m sputtering yields were o b t a i n e d by tilting the crystals - 10 ° off n o r m a l incidence d u r i n g b o m b a r d m e n t a n d rotating them a r o u n d the surface n o r m a l through a region c o n t a i n i n g no m a j o r axis or planes. S p u t t e r i n g yields were o b t a i n e d b y measuring the weight-loss of the crystals ( 1 - 1 0 0 /~g) on a M e t t l e r M E 22 (or more recently a Mettler U M 3) m i c r o b a l a n c e with an accuracy better than 0.5/~g. Scanning m i c r o s c o p y investigations have shown [12] that surface t o p o g r a p h y does not develop on c o b a l t single crystals sputtered with A r ÷ ions. F r o m test experiments it was established that a c o b a l t single crystal could be used for at least three experiments, before repolishing was required. Because of a " m e m o r y effect" [14] consecutive sputtering experiments on the same crystal were always p e r f o r m e d at increasing temperatures. Using a d o u b l e alignment RBS-analysis [14] it has f u r t h e r m o r e been verified that the phase transition of the u n b o m b a r d e d c o b a l t crystal takes p l a c e at - 415°C, in a c c o r d a n c e with other results
[
I
t
i
80 keY Ar" ~ ...........
I
~ ...............
r I. . . . .
a
[
eo
x
P
L 0
-100
bombardrnentl
o nickel <111>
x random
bombardment
~, nickel random ___[ 100
• polycrysta[[ine J [ 200 300
r J J
[I L,O0
I 500
C°
Fig. l. Sputtering yields of 80 keV Ar + on cobalt and nickel (for reference). The size of marks in the figure represents the experimental uncertainties. Y atoms/ion
I
.... ~ 80 keY Xe + ~
I Co
,
i •
\
--•-'t"
t0
• channeled i
Sputtering yields as function of t e m p e r a t u r e were measured for 80 keV A r + (fig. 1), 80 keV Xe + (fig. 2) and 200 keV Xe + (fig. 3). In each case, the sputtering yields of ~111) nickel have been measured, too, because of the close resemblance between fcc c o b a l t a n d nickel in physical and chemical p a r a m e t e r s . A c c o r d i n g l y the ~111 sputtering yields of fcc cobalt a n d nickel should be essentially equal. This is c o n f i r m e d by figs. 1-3, indicating that the fcc phase of c o b a l t behaves " n o r m a l l y " . C o n t r a r y to this the (0001) sputtering yield for 80 keV A r + show a n o m a l o u s behaviour (fig. 1). W h i l e the hcp sputtering yield at a n d below room t e m p e r a t u r e is at the same level as fcc sputtering yiel& it increases linearly from - 20°C to - 100°C, where it stabilises at a level, only 20% less than the sputtering yield of polycrystalline cobalt. A similar behaviour, although less p r o n o u n c e d , is seen in fig. 3 for 200 keV Xe + ions. In c o n t r a s t to the 80 keV A r + and 200 keV Xe + sputtering yields, the sputtering yields for 80 keV Xe + (fig. 2) are all lower than the sputtering yield of the fcc phase.
I
Co
• chonneted
[71.
3. Results and discussion
[
f lOmsliOn
O nickel <111>
l 0--0
bombardment
x random
bombardment
• po(ycrystaltine ~ ..... I 200 300
J ............ 100
I /,00
T C°
Fig. 2. Sputtering yields of 80 keV Xe + on cobalt and nickel (for reference). The size of the marks in the figure represents the experimental uncertainties, The dashed lines for polycrystalline and random sputtering yields have only been drawn to guide the eye. ato~ms/ion
1
I 200 keY
-/_/1-
S
I Xe* ~
]
Co
/ j - A ~ ,~
//
_ . - . . . ~ / . . x4.\
J • channeled o nickel <111> _1 100
x random
bombardment
bombardment
• polyc rystoiline I L 200 300
I Z,O0
T C°
Fig. 3. Sputtering yields of 200 keV Xe + on cobalt and nickel (for reference). The size of the marks in the figure represents the experimental uncertainties. The dashed lines for polycrystalline and random sputtering yields have only been drawn to guide the eye.
L. Sarholt-Kristensen et al. / Sputtering on cobalt
According to the transparancy theory of Onderlinden [4], the single crystal sputtering yield Y; .... > in a channeled direction can be related to the sputtering yield Y of the structureless medium through y< ...... > = ~tX,,,i,,Y,
(1)
where Xm~n is the nonchanneled fraction of the sputter beam in the ( u v w ) direction and ~/ is a fitting parameter. Xmi, can be calculated for the unbombarded crystal using empirical, partly temperature dependent channeling models [ 15,16]. At the fairly high energies used in these experiments both Ypoly and Yrandom should be good approximations to Y. In accordance with earlier experiments (for references see [3]) we have chosen to use Ypo~y, the more so as preliminary experiments with 80 keV Ar ÷ on Co have shown, that surface topography only influences the polycrystalline sputtering yields, when fluences exceed 10 24 m -2,
The fitting parameter ~ can then be calculated as function of temperature from the ratio between the measured single crystal sputtering yield and the polycrystalline yield at the appropriate temperature, divided by the associated calculated minim u m yield Xmi, (fig. 4). Sputtering experiments have shown that ~/ depends on ion mass and energy, and the crystallographic orientation [3,5]. ~ values between 1.2 and 2.0 have been found to be good fits at low energy (a few keV) sputtering as well as for high energy Ar ÷ sputtering on (111) fcc Cu [17,3]. The values obtained in our experiments for the fcc
545
cobalt phase are in close agreement with those results. The hcp phase, on the other hand, shows quite different behaviour of the ~ values. Firstly, all the r/ values are well above 2 (in accordance with Ar ÷ sputtering on Cu in open directions [17,3]), and secondly, the 71 value depends strongly on the combination of ion mass and energy, and for Ar + on the temperature, too. The high values of ~ are either due to underestimation of the minimum yield Xm~n a n d / o r the polycrystalline sputtering yields, Ypo~y. The latter, however, agrees well with theory [1,2] and, for Ar ÷, with other experiments [18]. Although some discrepancies do exist between the experimental Ypoly and Yrandomfor Xe + bombardments, raising some doubt about the best assignment to Y, the differences are too small to account for the high values. The explanation of the ~ values for the hcp cobalt phase should then be ascribed to differences between Xm~n values calculated from theory and values representative of the hcp cobalt crystals sputtered to fluences - 1023 m - 2 . At low fluences ( - 1 0 21 m 2) the sputtering yield of (0001) Co with 80 keV Ar + ions at room temperature is 1.0 [14] assuming that none of the implanted material is sputtered off. Y<000~> equal to 1.0 is in agreement with the calculated Xm~,, when an ~ value equal to 1.8 is used. Furthermore, angular scans across the unbombarded (0001) channel, using 400 keV He 2+ ions yield Xmin values in close agreement with calculations [13]. This is presumably also the case for Ar + and Xe +. At higher fluences the agreement between calculated and experimental minimum yields for 400 keV He z+ becomes worse [13]. A measure of the RBS-disorder can be obtained by
i .....
.... -
-
80keV
Ar +
2 0 0 key Xe + 80key
D
Xe +
T 100
200
300
400
C°
Fig. 4. The fitting parameter ~/ calculated for 20°C, 100°C, 250°C, 350°C and 470°C. Smooth lines have been drawn through the points.
o,-
r . , - Yv
'
(2)
Yd', Yv and Yr being the yields in the RBS-spectra of the bombarded crystal, the virgin crystal and the random crystal, respectively. The yields are taken at the same position, just behind the surface peak. /)re I is shown in fig. 5 as a function of fluence for Ar + bombarded crystals. At low fluences, neither the hcp structure, nor the fcc structure is damaged to any appreciable extent. At a fluence equal to a few times 102° m 2 severe damage starts to build up in the hcp structure at 250°C, and the disorder saturates at a level equal V. S P U T T E R I N G
516
1.~
L. Sarholt-Kristensen et al. / Sputtering on cobalt
¥o' -Yv Yr
"Disorder"versus fluence 80 key Ar+--~,,-Co • ok 20°C • at 250°C • at 470°C
10 O.a , i II
OG OA
/ z//
0.; ..... 1019
• .......
x. . . . . . . . . . . .
~
/
/
• .''~"
1011
i0 zO
1022
Fluence ( ions I m 2)
Fig. 5. The relative disorder /)re~equal to the minimum yield for 400 keV He2+ of the argon bombarded crystal minus the minimum yield of the virgin crystal versus fluence. Smooth lines to guide the eye have been drawn through the experimental points. to 0.8 (closely equal to the value for a random RBS-analysis). Similar disorder measurements for room temperature sputtering performed in the fluence range equal to 1 0 2 1 - 1 0 22 m 2 (fig. 5), show that the disorder in the hcp crystal is slightly larger than in the fcc crystal. The high sputtering yields of 80 keV Ar + on hcp cobalt at high fluences must then be ascribed to a heavily disordered crystal structure, resulting in high minimum yields Xmin' In the context of the transparancy model, any discrepancies between the calculated and real Xm~n values will express itself in high values of the fitting parameter 7According to fig. 4, Xe ÷ bombardments, too, result in high values of r/ for the hcp phase, from which it is anticipated that considerable disorder is created also by Xe ÷ ions in the hcp phase. The dependence of sputtering yield on temperature for channeled bombardment with Ar ÷ ions (fig. 1) strongly indicates that the disorder may be associated with the phase change. Furthermore, the hcp ~ fcc phase change, as reflected in sputtering yield, occurs at a temperature - 5 0 ° C below the commonly accepted value of - 415°C [7]. This lowering of the phase transition temperature is not due to beam-heating, as demonstrated in low flux sputtering experiments [14], but may reflect the possibility of inducing martensitic phase transformations by ion implantation [19-21]. Ion induced lowering of the phase transition temperature can be discussed in the context of the
energy cascade - more particular its size and its energy density. To stabilize the size of the new phase embryos, transformed volumes should, according to Nabarro [22], be large enough to facilitate break-away from the matrix. This critical size is for the transformation in cobalt of the order of a few tens of nanometer. It is furthermore to be expected that the energy density must exceed a certain minimum value, presumably, considerably lower than the sublimation energy. If ion induced phase transformations contribute to the disorder created during sputtering, as strongly indicated by fig. 1, 80 keV Ar ÷ ions on Co should fulfill both the above mentioned requirements. The average damage depth (xD) calculated [23] for 80 keV Ar + ions on Co is - 25 nm and thus comparable to Nabarro's critical size. According to Sigmund [24] the energy density 00 in the cascade can be calculated for different ion energies as function of incident ion mass in a cobalt target (fig. 6). The dashed line included in fig. 6 gives those combinations of ion mass and energy, which result in the same (XD) as for argon. Points above the dashed line have smaller (xD) values and points below larger (xD) values than 80 keV Ar +. 200 keV Xe + ions on Co have a somewhat smaller energy cascade with a somewhat higher energy density, whereas for 80 keV Xe + ions on Co, the energy cascade is considerably smaller, and the energy density an order of magnitude higher. Fig. 2 shows in agreement with this that the change in sputtering yield for 80 keV Xe + ions tends to happen closer to the real phase transition temperature than for 80 keV Ar + ions. Furthermore, the hcp--, fcc phase change in that case increases the sputtering yield, and this is qualitatively in agreement with the low transparancy of the (111) direction compared to the @001) direction. Although 80 keV Xe + sputtering presents the lowest ~ values for the hcp structure, they are still rather high, compared to fcc target material, indicating that even in this case the hcp structure is partially disordered by ion bombardment. The high values of the (0001) high fluence sputtering yields of Ar + and Xe ÷ ions in cobalt may then be ascribed to formation of excessive disorder, partly resulting from an implantation induced phase transformation, premature with respect to temperature, and partly due to radiation damage. The main contribution to 80 keV Ar + and
L. Sarholt-Kristensen et a L / Sputtering on cobalt
10pOoatom
i
Beside the single crystal sputtering yields, the polycrystalline and random yields in figs. 2 and 3 attract attention, too. Whereas the polycrystalline and random yields are in good agreement for 80 keV Ar + ions, 80 keV Xe + bombardments give random yields which are substantially higher than the polycrystalline yields, whilst the opposite applies to 200 keV Xe ÷ bombardments. Remarkable, too, is the oscillating behaviour for both random and polycrystalline sputtering yields of 200 keV Xe ÷ bombardments near the phase transition temperature. At present, no adequate physical explanation can be given and further experiments are certainly needed.
Target material:CO
////S
/;// ,~
'
4. Conclusion
i
0
547
100
2C)0
MI
Fig. 6. The energy density calculated according to [24] versus the ion mass of the impinging ions, when cobalt is used as target.
200 keV Xe ÷ ion induced disorder seems to come from the phase transformation, which seems to be less pronounced for 80 keV Xe ÷ ion bombardment. Preliminary experiments on 80 keV Ar ÷ b o m b a r d e d polycrystalline cobalt specimens investigated by transmission electron microscopy [14], have shown that there are no signs of amorphization, in accordance with similar results from nickel implanted with argon [25]. The observed microstructure, which is insensitive to the implantation temperature, consists of very dense and virtually unresolvable distributions of defect clusters and dislocation tangles. Superimposed on this defect structure are dense distributions of argon bubbles, a few tens of nanometers in diameter, which can be observed in phase contrast. Amorphization is presumably suppressed due to formation of the bubbles. The fluences used in these experiments were 102~ m 2 i.e. considerably lower than those used for single crystal sputtering experiments. Additional experiments at higher fluences and a detailed combined analysis of defect microstructures and related electron diffraction patterns have to be carried out in order to outline the reason for the easily damaged hcp phase.
It has been shown that the hcp phase of cobalt is heavily disordered by ion bombardment with Ar ÷ and Xe ÷ ions. Part of the disorder is presumably connected with the martensitic phase transition which may be induced by the ion bombardment. For 80 keV Xe ÷ bombardments, however, it is more likely that the disorder in the hcp structure can be explained from formation of damage, which will also contribute to the disorder created by 80 keV Ar ÷ and 200 keV Xe ÷ ions. Further experiments should concentrate, by means of transmission electron microscopy, on investigations of the nature of damage produced by ion bombardment in the hcp cobalt phase. Furthermore, the importance of energy density and energy cascade dimension, may be assessed by using suitable combinations of projectile energy and mass. By using a fitting parameter ~ equal to about 1.5 the transparancy model predicts fairly well the (111) sputtering yields of 80 keV Ar +, 80 keV Xe ÷ and 200 keV Xe ÷, as well as the low fluence (0001) sputtering yield of 80 keV Ar ÷ at room temperature. For high fluences, ~ values well above 2 have to be used for the open (0001) direction in the hcp phase in order to fit the predictions of the transparancy model to the experimental sputtering yields. A similar result was obtained for copper sputtered in open directions with argon ions at similar energy and fluence [17,3]. In the hcp phase of cobalt the high values of can be ascribed to the sensitivity of 77 on disorder. V. SPUTTERING
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L. Sarholt-Kristensen et al. / Sputtering on cobalt
7/ then becomes dependent on quantities such as ion mass, ion energy and fluence, making the use of the transparancy model questionable.
Support from the Danish Natural Sciences Research Council is gratefully acknowledged.
References [1] H.H. Andersen and H.L. Bay in Sputtering by Particle Bombardment I, Topics in Applied Physics 47, ed., R. Behrisch (Springer, Heidelberg, 1981) ch. 4 and refs. therein. [2] P. Sigmund in Sputtering by Particle Bombardment I, Topics in Applied Physics 47, ed., R. Behrisch (Springer, Heidelberg, 1981) ch. 2 and refs. therein. [3] H.E. Roosendaal in Sputtering by Particle Bombardment I, Topics in Applied Physics 47, ed., R. Behrisch (Springer, Heidelberg, 1981), ch. 5 and refs. therein. [4] D. Onderlinden, Can. J. Phys. 46 (1968) 739. [5] M.T. Robinson in Sputtering by Particle Bombardment I, Topics in Applied Physics 47, ed., R. Behrisch (Springer, Heidelberg, 1981), ch. 3 and refs. therein. [6] J. Lindhard, K. Dan. Vidensk. Selsk., Mat. Fys. Medd. 34 (1965) no. 14. [7] R. Alzenz, B.W. Lee, A. Ignatiev and M.A. van Hove, Solid State Comm. 25 (1978) 641 and refs. therein. [8] R.S. Nelson, The Observation of Atomic Collisions in Crystalline Solids (North-Holland, Amsterdam, 1968). [9] R.S. Nelson in Channeling, Theory Observation and Applications, ed., D.V. Morgan (Wiley, New York, 1973), ch. IX.
[10] E. Bogh, Proc. Cairo Solid State Conf., Interaction of Radiation with Solids, ed., A. Bishay (Plenum, New York, 1969) p. 361. [11] P.P. Pronko, J. Bcttiger, J.A. Davies and J.B. Mitchell. Rad. Effects 21 (1974) 25. [12] V.S. Chernysh, A. Johansen and L. Sarholt-Kristensen Rad. Effects Lett. 57 (1980) 119. [13] V.S. Chernysh, A. Johansen and L. Sarholt-Kristensen Proc. V Int. Conf. on Ion Beam Analysis, Sydney, eds., J.R. Bird and G.J. Clark (North-Holland, Amsterdam, 1981) p. 253. [14] A. Johansen, E. Johnson, L. Sarholt-Kristensen and V.S. Chernysh, Proc. Nat. Conf. on Interaction of Atomic Particles with Solids, Minsk, (1981) p. 18, in Russian, to be published in English. [15] J.H. Barrett, Phys. Rev. B3 (1971) 1527. [16] B.R. Appleton and G. Foti, Ion Beam Handbook for Material Analysis, eds., J.W. Mayer and E. Rimini (Academic Press, New York, 1977) ch. 3. [17] T.W. Snouse and E.C. Haughney, J. Appl. Phys. 37 (1966) 700. [18] C. Fert, N. Colombie, B. Fagot and Pham van Chuong, Le Bombardemend Ionique (CNRS, 1961) 67. [19] M. Marinov and D. Dobrev, Thin Solid Films 42 (1977) 265. [20] E. Johnson, U. Littmark, A. Johansen and C. Christodoulides, Phil. Mag. A45 (1982) 803. [21] L. Sarholt-Kristensen, E. Johnson and A. Johansen, these Proceedings, p. 289. [22] F.R.N. Nabarro, Proc. Roy. Soc. A175 (1940) 519. [23] K.B. Winterbon, P. Sigmund and J.B. Sanders, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 37 no. 14 (1970). [24] P. Sigmund, Appl. Phys. Lett. 25 (1974) 169. [25] A. Ali, W.A. Grant and P.J. Grundy, Phil. Mag. B37 (1978) 353.