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Sputtering of titanium carbide coatings under D+ and 4He+ ion bombardment: Total sputter yields and angular distribution
Thin Solid l"ilm.~, 83 (1981) 107--114
107
METALLURGICAL AND PROTECTIVE COATINGS
S P U T T E R I N G O F T I T A N I U M C A R B I D E C O A T I N G S U N D E R D ÷ AND 4He+ ION B O M B A R D M E N T : T O T A L S P U T T E R YIELDS A N D ANGULAR DISTRIBUTION * M. KAMINSKY AND R. NIELSEN A rgonm" National Laboratory, Argonne, IL 60439 (U.S.A. j (Received April 6, 1981 : accepted April 8, 1981)
The release of sputtered materials from chemically vapor-deposited titanium carbide coatings on Poco graphite substrates was studied for surfaces which had been mechanically polished prior to irradiation or left in their as-deposited state. These coatings were irradiated with mass-analyzed D + and 4 He + ions with energies ranging from 1.5 to 60 keV at ambient temperature and normal incidence. The equivalent titanium and carbon thicknesses of the sputter deposits were determined by the Rutherford backscattering technique. The experimentally determined total sputtering yields for both polished and unpolished titanium carbide coatings were observed to decrease as the D + and 4He + ion energies increase above 3 keV. This trend is in agreement with theoretical predictions. The total yields for the unpolished surfaces are higher than those for the polished surfaces. The relevance of these data to the fusion program will be discussed.
1. INTRODUCTION
The control of plasma impurity release from the surfaces of first-wall components of fusion devices is necessary to obtain high plasma temperatures and densities for sufficiently long confinement times. The release of plasma contaminants with a high atomic number Z from first-wall components under radiation from plasmas and neutral beam injectors can cause undesirable plasma power losses due to electromagnetic radiation 1-3. For sufficiently high impurity concentrations the ignition of fusion reactions can become impossible. For this reason the use of low to medium Z materials which have low impurity release yields and minimal surface erosion yields appear desirable for plasma device operations 2-5. Titanium carbide (TIC) and titanium diboride (TiB2) are candidate coating materials which have been considered for this purpose 4-6. These coatings can be chemically vapor deposited onto many substrate materials (e.g. stainless steel, copper and graphite) and have low to medium Z constituents (Z B = 5, Z c = 6 and Zxi = 22). In order to evaluate the suitability of TiC and TiB 2 coatings for first-wall component applications, it is * Paper presented at the International Conference on Metallurgical Coatings, San Francisco, CA, U.S.A., April 6 - 10, 198 I. 0040-6090/81/0000-0000:S02.50
~C, Elsevier Sequoia/Printed in The Netherlands
108
M. KAMINSKY, R. NIEI,SI!N
important to determine the effects of D" and *He ' irradiations on these coatings. We have recently reported on the sputtering and the surface erosion ofTiB 2 coatings under I ) and '~He ~ irradiations for energies ranging from 3 to 15 keV and from 5 to 120 keV respectively " ". The primary aims of the present study are to determine the total sputtering yield and the angular distribution of sputtered species for D" and "*He- irradiations of TiC with energies ranging from 1.5 to 60 kcV. 2. EXPERIMI!NTAI. I*RO(.'Iil)URI'I
2.1. Target and collector preparatiotl The chemically vapor-deposited TiC coatings on Poco AXF5Q graphite substrates were produced by Ultramet Corp.. Pacoima. CA 91231: the:, were obtained from D. Mattox. Sandia National Laboratories, Albuquerque. NM. The thickness of the coatings ranged from 10 to 20 lain. One set of TiC-coated surfaces was mechanically polished with 0.1 p.m diamond paste compound as an abrasive, whereas another set was left in the as-deposited condition. Both sets of coatings were degreased in ultrasonic baths of trichloroethylene, acetone, triply distilled water and methanol before insertion into the high vacuum. The coating surfaces were characterized by scanning electron microscopy ( S E M ) a n d scanning Auger microscopy (SAM} both before and after irradiation. Figures l(al and l(bl show scanning electron micrographs of surfaces of TiC coatings on Poco graphite in the as-deposited condition and after mechanical polishing respectively. The asdeposited sample is rough and has an average grain size of 9.3 ~.tm. SAM analysis of the surfaces of as-deposited samples revealed oxygen, silicon and chlorine impurities (resulting from the coating procedurej which were subsequently eliminated by removing a few atom layers by sputtering with 2 keY argon ions.
(al (hi Fig. 1. Scanning electron rnicrographs of two t~pcs of surla.ces of chemically vapor-dcpositcd TiC coatings before irradiation: (al as-deposited surface" lbl polished surface.
For the collection of sputtered material, physically vapor-deposited aluminum foils were used which had an equivalent thickness of about 50 ~.tg cnl 2. The collectors so prepared were characterized with Auger electron spectroscopy {AES) prior to use.
SPUTTERING OF
TiC
COATINGS UNDER
D ÷ A N D '*He +
BOMBARDMENT
109
2.2. Ion irradiations The irradiations were carried out with collimated and mass-analyzed D ÷ and 4He* ion beams striking the targets at normal incidence. A low energy accelerator system was used for ion beam energies ranging from approximately 1.5 to 20 keV 9.10 and another accelerator system was used for energies ranging from approximately 20 to 60 keV 11. In order to collect the sputtered material, the aluminum foils were placed on a holder and positioned at a distance of 4.5 mm from the target surface, between the last incident beam collimator and the target. The collector had a 3 mm hole to allow the passage of the incident beam and was protected by two apertures, each of which had a beam-spot-defining hole of 2 mm diameter. Another aperture, placed between the target and the collector, was biased with respect to the target to suppress secondary electron emission from the target and to allow accurate measurements of the ion beam current on the target. This aperture had a large slot to allow the sputtered material to reach the collector. The ion beam current on the target was monitored and integrated to obtain the total dose for irradiation. The doses varied from approximately 2 x 1020 to 8 x 1020 ions cm-2. All irradiations were performed at ambient temperatures, estimated to be near room temperature. During the irradiations the pressure in the target chamber ranged from 1.2 x 1 0 - 6 t o 2.5 × 1 0 - 6 Pa.
2.3. Analysis oj'chemicalcompositionand thickness of sputter deposit For a determination of the total sputtering yield it was necessary to determine the total mass M of material which had been sputtered from the irradiated spot for a given dose. For ions impinging on the surface, with normal incidence, the emitted mass M is given by 12
M=2nr2pt
(
l+(R/I)2-(r/l)2 !
[[1-(R/I)2+(r/I)2}2+4(R/I)2]
i~
)-' (1)
where r is the beam spot radius on target, R is the distance of a point on the deposit measured from the center of the deposit spot, I is the distance between the target and collector (located parallel to each other) and pt is the equivalent thickness of the deposit (p being the deposit density). It should be pointed out that eqn. (i) was derived under the assumption that every sputtered particle that strikes the collector surface will stick. For the collection of sputtered particles this assumption appears to be reasonable in the light of recent experimental evidence' 3 (i.e. a large fraction of such particles have kinetic energies much higher than thermal energy). It follows from eqn. (1) that the deposit thickness will not be uniform over the deposit area. Rutherford backscattering (RBS) spectrometry was used to determine the function p t(R) along a line which passed through the center of the deposit. For the RBS technique a 1.25 MeV H * beam of 0.3 mm diameter was used to strike the deposit, and the energies of the backscattered protons were analyzed at a laboratory scattering angle of 154 °. Figure 2 is an RBS spectrum of the sputter deposit at a particular point R. We can readily distinguish the broad aluminum peak (from the 50 lag cm -2 aluminum collector), the titanium peak, the carbon peak and the nitrogen and oxygen peaks due to contamination. The agreement between the calculated and experimental values of the onset energies is very good, as indicated in Fig. 2. A quantitative correlation between the area A under the peak for each of the
1
lO
M. K A M I N S K Y .
R. NIEI.SEN
elements titanium and carbon and the atom densities for these elements (the equivalent thickness pt) at point R was obtained by calibration with titanium and carbon standards of known atomic densities. The standards were prepared by physical vapor deposition of known amounts of titanium and carbon (with equivalent thicknesses of 5. 10. 25, 50 and 100 monolaycrs onto the aluminum collector foils. The vapor deposit thicknesses were determined with a Kronos crystal monitor (model Q M 321 ). The deviation in A from the average value obtained from four repeat runs for a given thickness of the standard was found to be + 10",,. In addition, for the carbon calibration, thin carbon foils of known thickness were used. A linear calibration curve of the average values o f A rer.sus pt w a s obtained with a mean deviation of + 10'!,,. and on extrapolation this cttrvc went through ,-1 = 0 at pt = 0 for both the titanium and the carbon standards. 3. RI!SUI. IS
3. I. Spatial distribution q/.sputter deposit Measurements of the function t*tlR) were made with a very high spatial rcsolution (Ar)e/r z of 0.02",, whcrc Ar is the proton beam radius and r the deposit spot radius (about 95". of the deposit was in the area rtr-'). Figurc 3 allows a comparison of the function (pt(R))t~ c calculated from cqn. (IJ with the experimentally determined values of this function for TIC,_"coatings irradiated with 8.0 keV Hc" ions. The agreement between experimental and theoretical values is good and indicates that thc angular distribution of the sputtered particlcs follows a cosine distribution (implicit in the derivation of eqn. (I)) for this set of irradiation parameters. Similarly, good agreement was observed tbr I)- ion irntdiations for the energy range studied. e 5
w, d ~ : ) v:
4000
L
"
"
o 3000
.
d
~ :;
b(~'-
~
_i
,j
(j
.
#0
5 I2 I
I000
;
•
•
I.
~: I /
EC: 0.91
EO, 0 . 9 9 EN= 0 . 9 6
BACKSCATTERED
Erl: 116 EA, : 1.09
N* ION ENERGY
,b
6 31S-ANCE
6
+
1 IC
R ~rnr~)
E (MeV)
I"ig. 2. A n R B S spectrum of a sputter deposit (resulting from an 8 keV "SHe' irradiation of a polished "FiC surface) on a 5 0 lag c m -' aluminum substrate. The spectrum was obtained with a 1.25 M e V H " ion beam for a laboratory scattering angle of 154 . E o E N, E o, EAI a n d E l , arc the calculated onset energies for carbon, nitrogen, oxygen, aluminum and titanium respectively. F i g . 3. Variation in the TiC deposit thickness (resulting from an 8 kcV "~[le" irradiation of a polished T i C surface) as a function of the distance R as measured from the center of the deposit spot ( R = ()1 a l o n g a radial direction: - -, calculated using eqn. ( 1 ) : @ . experimentally determined values.
SPUTTERINGOF TiC COATINGSUNDER D + AND '*He ÷ BOMBARDMENT
111
3.2. Determination of sputtering yields A determination of the functions (p t(R))Ti and (p t(R})c from the RBS analysis allowed an estimate of the average emitted masses M for titanium and carbon according to eqn. (1). From these values we can determine the differential sputtering yields for carbon or titanium. For example, the differential sputtering yield for titanium is given by Slq -
MTi
NA
mTi f T i N i
where Mrl is the emitted titanium mass (eqn. (1)), mTi is the atomic weight of titanium, N A is Avogadro's number, Ni is the total number of ions impinging on a target and fl~N~ is the fraction of the total number of ions which strike titanium atoms; a similar expression can be obtained for carbon. It is assumed that fvi +fc = 1. It should be pointed out that the AES analysis of TiC samples before irradiation revealed that the titanium and carbon concentrations in surface regions had stoichiometric proportions yielding TiCo.97 within the experimental accuracy of approximately 15'~i and that other surface impurity concentrations (e.g. carbon and o/ oxygen) did not exceed 10/o. For a determination of the differential sputtering yields for carbon and titanium, the beam fraction values were chosen to be fTi = 1 and fc = ½ respectively. The total sputtering yield for TiC is then determined from the sum of the titanium and carbon masses emitted from the irradiated spot divided by the total number of ions striking this spot. In Table I the experimentally determined total sputtering yield values for the TiC surfaces in the polished and the as-deposited state for the D ÷ irradiations are summarized, and Table I1 gives similar data for the 4He+ ion irradiations. These tables also include values which have been calculated with the use of the semiempirical formula proposed by Smith ~4. The major parameters used in these calculations are listed in the footnotes to Tables I and II. 4. DISCUSSION The results summarized in Tables I and II reveal the following trends: the yield values decrease with increasing io.n energy for the energy range 3-60 keV for both D ÷ and 4He+ ions. The values for 1.5 keV deuterons are smaller than those for 3.0 keV deuterons. The general trend of the observed energy dependence of the yields can be understood on the basis of the energy dependence of the dominant collision processes acting between the incident ions and the lattice atoms. At the lower energies the collisions are influenced by the weak screening of the nuclei of the lattice particles, while at the higher energies the Rutherford collisions dominate 15. The experimentally determined yield values for the rough TiC surfaces in the as-deposited state are about 15')/o-100'yolarger than those for the smoother polished surfaces. This result suggests that for this type of rough surface the enhancement in sputtering (e.g. that resulting from angles of incidence other than normal for a large fraction of ions) is larger than the possible reduction due to redeposition of sputtered particles. We have reported previously a similar trend for as-deposited and polished TiB 2 surfaces 9. For energies lower than 8 keV it is possible to compare our data with some of those obtained by Bohdansky et al.16 for as-deposited TiC. In contrast with our
112 TABLE IOIAL
M. KAMINSKY. R. NIELSEN I S P l : I ll. R l N ( i Y I F I . D S I O R [ ) "
Projectile energy (keV~
IRRADIA]ION
{)I. "['i("
Experimental total yield ralues Imolecules ion - ~ )
Cah'uhm,d .vieht ralues " (molecules ion ~1
~Is-deposited "l iC
Polished TiC"
1.5
(2.2 ~0.1lx 10 "
{l.l+-0.1)x 10 "~
3.5x 10 '
3.0 5.(1 8.0 14.0 20.0 40.0 60.0
(3.2+0.11xI0 : {2.6"-0. I i x I0 -" (l.7ZO. IIxl0 ' (l.4f0.]Ixl0 " (0.8±0. Ii×]0 ' (0.4a-0.1)x10 ' i0.4u20. I)× I0--'
(2.2"0. I)xI0 -" (2.1"-0.11x 10 ~ (1.5_+.0.]ix10 2 (0.9+_0.1)×10 " ( 0 . 7 + 0 . 1 1 x 1 0 -" (0.3-+0.1)x ] 0 - " [0.2"-0. I ) x I0 -"
2.2×10 1 5 , I0 1.2x10 I).7,< I0 0 . 6 x I0 0 . 3 x 10 0.2,~10
: ": : -' -' -" :
" Reference 14. eqn. (7). was used for these calculations. The following values ~cre chosen for the major parameters: surface binding energy I{, = 2.3 eV: effective charge Z ~ , t T i ( ' l = 14: "~l.,,[Ti('l = 2 9 . 9 6 . TABLE II r()TAI
S P U I I E R I N ( . i "lll:l. DS I O R 41"Ie* IRRAI)I.',,TI()N
Projectih' energy (keVl
()!
TiC
,4s-deposited 7 iC
Poll.shed Ti("
(I.6., -~0.()5) × 10 (1.2, ± 0 . 0 5 ) x l O
{I.5, L0.05)× 10-
3.0 8.0
1.5
("alculated yield ralm,s" (molecules ion ~1
Experimental total yieht ralues Imoleculesion ~l
(0.8., + - 0 . 0 5 ) x I0
(().g ----0.05} x l O
14
(0.7,, +-0.051 x 10
(0.5 -+0.05) × 10
20 40 60
(0.4,, +-0.05) x I0 (0.3,, +0.051 x I0 (0.1- +-0.05) × I0
(0.2,, ~-0.051 x I 0
(0.4 o +0.05) x I0 (0. I - - - 0 . 0 5 l x I0
]
1.6 * 10 1.4 x IO 0.8 × 10 0.5 x 10 0.3,, × 10 0.2~ x 1() 0.1 a × 10
" Reference 14, eqn. (7), was used for these calculations. The following values were chosen for the major parameters: surface binding energ~ [i, = 2.3 eV'effectivc charge Z.,,(TiCI = 14: X/ u(l'iC - 2996. R B S a n a l y s i s o f s p u t t e r d e p o s i t s , B o h d a n s k y et al. used the target w e i g h t loss m e t h o d to d e t e r m i n e the s p u t t e r yields. A direct c o m p a r i s o n o f the yields S is o n l y p o s s i b l e at o n e e n e r g y , 8 keV" B o h d a n s k y e t a / . o b t a i n e d S D = 1.4 x 10 2 t n o l e c u l e s ion t a n d S m . = 6 . 1 x l 0 - - ' m o l e c u l e s ion i. we o b t a i n e d S D ( a s - d c p o s i t e d T i C ) = 1 . 7 x 10 - 2 t n o l e c u l e s ion ~. SD(polished T I C ) = 1.5x 10 2 m o l e c u l e s ion - l . Su~(as-deposited T i C ) = 8 . 4 x 10 2 m o l e c u l e s ion ~ a n d Sm,(polished T i C ) = 8.1 x 10 - 2 m o l e c u l e s i o n - ' F i g u r e 4 illustrates the e n e r g y d e p e n d e n c e of the e x p e r i m e n t a l l y d e t e r m i n e d t o t a l s p u t t e r yields for D ~ a n d 4He+ ion i r r a d i a t i o n s o f p o l i s h e d T i C samples. It a l l o w s a c o m p a r i s o n o f these yields with t h o s e w h i c h h a v e been c a l c u l a t e d o n the basis of S m i t h ' s s e m i e m p i r i c a l e x p r e s s i o n l'~ ( F i g . 4 . full curves). It s h o u l d be n o t e d that for the 8 k e V "*He' i r r a d i a t i o n the c a l c u l a t e d v a l u e was titted to the e x p e r i m e n t a l v a l u e by a d j u s t i n g the v a l u e of the surface b i n d i n g energy. T h i s v a l u e of the b i n d i n g e n e r g y was t h e n used for the c a l c u l a t i o n of the S ( E ) c u r v e s for l) * a n d 4 H e * i r r a d i a t i o n s . T h e g o o d a g r e e m e n t b e t w e e n the c a l c u l a t e d a n d m e a s u r e d yields
SPUTTERING OF
TiC COATINGS UNDER D + AND CHe + BOMBARDMENT
1 13
for energies above 20 keV can readily be seen. For energies from 1.5 to 20 keV we see good agreement for helium irradiations but not for deuteron irradiations. Both our experimental data and those of Bohdansky et al. 16 indicate that the yield values decrease with increasing energy below a deuteron energy of about 3 keV, while the calculated value at 1.5 keV is still larger than that calculated for 3 keV.
-5 10
-
hI62
id
j
2
, ,,,,,,I , , ,,,,,,I 4 6 I0 20 40 60 I00 ION ENERGY (WeV)
Fig. 4. The total sputtering yields for polished TiC surfaces as a function of projectile energy: e , 4He +, polished TiC, present data; &, 4He ", TiC, data from ref. 16; O, D * , polished TiC, present data: ~., D ~, TiC, data from ref. 16; - - - - , calculated using Smith's semiempirical expression ~" and by fitting the calculated value typical for 8 keV *He- to the experimental value (through the choice of the value for the surface binding energy).
For the operation of plasma devices these sputtering results for TiC coatings are encouraging. For example, the yield values for these coatings are lower than those for an uncoated surface of 304 LN stainless steel (the vacuum vessel materials for the tokamak fusion test reactor). For the polished TiC target under 4He+ irradiations over the energy range 3-15 keV, the yield values are smaller than those for 304 LN stainless steel 17 (e.g. for 8 keY 4He + irradiations, S(TiC) -- (8.1 +0.5)x 10 -2 atom ion-1 whereas S(304 LN stainless steel) = (11.3 +0.5)x 10-2 atoms ion-'). A comparison of yield values for these materials for deuteron irradiations also shows lower yields for polished TiC coatings. From the point of view of permissible plasma impurity concentration the gain is even larger because of the difference in the effective atomic numbers of TiC and 304 LN stainless steel. These encouraging results suggest that the use of TiC coatings on first-wall components may help to decrease plasma radiation losses and to improve plasma stability. ACKNOWLEDGMENTS
We would like to thank Mr. R. Chamberlin, Jr., for his assistance in taking the RBS spectra, Mr. P. Zschack for target preparations and Dr. A. S. Rao for taking some of the scanning electron micrographs. We are grateful to Dr. L. Goodman, Argonne National Laboratory, for carefully reading this manuscript. The work was performed under the auspices of the Office of Fusion Energy, U.S. Department of Energy.
I 14
M. KAMINSKY, R. NIELSEN
REFERENCES I 2 3
4 5
6 7 8 9 l0 11 12 13 14 15 16 17
M. Kaminsky, IEEE Trans. ,Vml. Sci., 1~¢(1971) 208. M. Kaminsky, Plasma Physics and ('ontrolled ,\.'uch'ar I.'t~ton Research. Vol. I1, International Atomic Energy Agency. Vienna. 1975, p. 287. W . M . Stacey, Jr., et al.. Pro~. 2rid "l~Jpi(al Meet. on the' T~'chnolog.v ol ('ontrolh'd Nuclear l"u,~ion, Vol. I. in L'SERDA (onL l'uhl. 760935-P1 (1976) 21 (U.S Energy Research and l)evclopment Administration). D . M . Mattox. Thin Solht Fihn.~. 63 (1979) 213. D . M . Mattox. A. W. Mullendore, H. O. Pierson and I). J. Sharp. J..Vml. ~htt~'r.. ,~'5 ,~6 (1979) 1127. H.O. Pierson, E. Randich and D. M. Mattox, J. L¢,.~-('ommon 3h't.. 67(1979) 381. M. Kaminsky and S. K. I)as, J. ,Vm'l. lhttcr.. ,~'5 86 (1979) 1095. S.K. Das and M. Kaminsk~,, "Ibm Solid FTbn~. 63 (1979) 269. M. Kaminsky, S. K. l,am and K. Moy. Thin Solid t.Thn~. 73 (1980) 9 I. S.K. L a m a n d M. Kaminsky, J..¥uc/. :~tater..S9(1980) 205. M. Kaminsky. S. K. Das. R. Ekern and D. C. Hess. In.~t. Phv.~. ("onl. Set. 3,'¢(1978) 305. R. (Jl~.ng. in L. Maissel and R. (Jlang (eds.), thmdhook ~1 lhin 1"Th~ l~,¢hnoloev. Mc(.ira~,~.-Hill. New York, 1970, pp. I 3. M. R. Wcller and T. A. Tombrello, ( 1 7 R¢T. B.,II'-7, August 1979 (Calilornia Institute of l'cchnology). D . L . Smith, J. :\:m'l. :~tater.. 75 (1978) 20. M. Kaminsky, ,4 tomic attd hmi~" bttpa('! Phenomena on Metal Su(l~wc.~, Springer, Heidelberg, 1965, pp. 222fl. J. Bohdansky, H. L. Bay and W. Ottenbergcr, J. Nm.I. ,~4awr.. 76 77(19781 163. M. Kaminsky, Thin Solid Fihnv. 7311)(1980)91.
107
METALLURGICAL AND PROTECTIVE COATINGS
S P U T T E R I N G O F T I T A N I U M C A R B I D E C O A T I N G S U N D E R D ÷ AND 4He+ ION B O M B A R D M E N T : T O T A L S P U T T E R YIELDS A N D ANGULAR DISTRIBUTION * M. KAMINSKY AND R. NIELSEN A rgonm" National Laboratory, Argonne, IL 60439 (U.S.A. j (Received April 6, 1981 : accepted April 8, 1981)
The release of sputtered materials from chemically vapor-deposited titanium carbide coatings on Poco graphite substrates was studied for surfaces which had been mechanically polished prior to irradiation or left in their as-deposited state. These coatings were irradiated with mass-analyzed D + and 4 He + ions with energies ranging from 1.5 to 60 keV at ambient temperature and normal incidence. The equivalent titanium and carbon thicknesses of the sputter deposits were determined by the Rutherford backscattering technique. The experimentally determined total sputtering yields for both polished and unpolished titanium carbide coatings were observed to decrease as the D + and 4He + ion energies increase above 3 keV. This trend is in agreement with theoretical predictions. The total yields for the unpolished surfaces are higher than those for the polished surfaces. The relevance of these data to the fusion program will be discussed.
1. INTRODUCTION
The control of plasma impurity release from the surfaces of first-wall components of fusion devices is necessary to obtain high plasma temperatures and densities for sufficiently long confinement times. The release of plasma contaminants with a high atomic number Z from first-wall components under radiation from plasmas and neutral beam injectors can cause undesirable plasma power losses due to electromagnetic radiation 1-3. For sufficiently high impurity concentrations the ignition of fusion reactions can become impossible. For this reason the use of low to medium Z materials which have low impurity release yields and minimal surface erosion yields appear desirable for plasma device operations 2-5. Titanium carbide (TIC) and titanium diboride (TiB2) are candidate coating materials which have been considered for this purpose 4-6. These coatings can be chemically vapor deposited onto many substrate materials (e.g. stainless steel, copper and graphite) and have low to medium Z constituents (Z B = 5, Z c = 6 and Zxi = 22). In order to evaluate the suitability of TiC and TiB 2 coatings for first-wall component applications, it is * Paper presented at the International Conference on Metallurgical Coatings, San Francisco, CA, U.S.A., April 6 - 10, 198 I. 0040-6090/81/0000-0000:S02.50
~C, Elsevier Sequoia/Printed in The Netherlands
108
M. KAMINSKY, R. NIEI,SI!N
important to determine the effects of D" and *He ' irradiations on these coatings. We have recently reported on the sputtering and the surface erosion ofTiB 2 coatings under I ) and '~He ~ irradiations for energies ranging from 3 to 15 keV and from 5 to 120 keV respectively " ". The primary aims of the present study are to determine the total sputtering yield and the angular distribution of sputtered species for D" and "*He- irradiations of TiC with energies ranging from 1.5 to 60 kcV. 2. EXPERIMI!NTAI. I*RO(.'Iil)URI'I
2.1. Target and collector preparatiotl The chemically vapor-deposited TiC coatings on Poco AXF5Q graphite substrates were produced by Ultramet Corp.. Pacoima. CA 91231: the:, were obtained from D. Mattox. Sandia National Laboratories, Albuquerque. NM. The thickness of the coatings ranged from 10 to 20 lain. One set of TiC-coated surfaces was mechanically polished with 0.1 p.m diamond paste compound as an abrasive, whereas another set was left in the as-deposited condition. Both sets of coatings were degreased in ultrasonic baths of trichloroethylene, acetone, triply distilled water and methanol before insertion into the high vacuum. The coating surfaces were characterized by scanning electron microscopy ( S E M ) a n d scanning Auger microscopy (SAM} both before and after irradiation. Figures l(al and l(bl show scanning electron micrographs of surfaces of TiC coatings on Poco graphite in the as-deposited condition and after mechanical polishing respectively. The asdeposited sample is rough and has an average grain size of 9.3 ~.tm. SAM analysis of the surfaces of as-deposited samples revealed oxygen, silicon and chlorine impurities (resulting from the coating procedurej which were subsequently eliminated by removing a few atom layers by sputtering with 2 keY argon ions.
(al (hi Fig. 1. Scanning electron rnicrographs of two t~pcs of surla.ces of chemically vapor-dcpositcd TiC coatings before irradiation: (al as-deposited surface" lbl polished surface.
For the collection of sputtered material, physically vapor-deposited aluminum foils were used which had an equivalent thickness of about 50 ~.tg cnl 2. The collectors so prepared were characterized with Auger electron spectroscopy {AES) prior to use.
SPUTTERING OF
TiC
COATINGS UNDER
D ÷ A N D '*He +
BOMBARDMENT
109
2.2. Ion irradiations The irradiations were carried out with collimated and mass-analyzed D ÷ and 4He* ion beams striking the targets at normal incidence. A low energy accelerator system was used for ion beam energies ranging from approximately 1.5 to 20 keV 9.10 and another accelerator system was used for energies ranging from approximately 20 to 60 keV 11. In order to collect the sputtered material, the aluminum foils were placed on a holder and positioned at a distance of 4.5 mm from the target surface, between the last incident beam collimator and the target. The collector had a 3 mm hole to allow the passage of the incident beam and was protected by two apertures, each of which had a beam-spot-defining hole of 2 mm diameter. Another aperture, placed between the target and the collector, was biased with respect to the target to suppress secondary electron emission from the target and to allow accurate measurements of the ion beam current on the target. This aperture had a large slot to allow the sputtered material to reach the collector. The ion beam current on the target was monitored and integrated to obtain the total dose for irradiation. The doses varied from approximately 2 x 1020 to 8 x 1020 ions cm-2. All irradiations were performed at ambient temperatures, estimated to be near room temperature. During the irradiations the pressure in the target chamber ranged from 1.2 x 1 0 - 6 t o 2.5 × 1 0 - 6 Pa.
2.3. Analysis oj'chemicalcompositionand thickness of sputter deposit For a determination of the total sputtering yield it was necessary to determine the total mass M of material which had been sputtered from the irradiated spot for a given dose. For ions impinging on the surface, with normal incidence, the emitted mass M is given by 12
M=2nr2pt
(
l+(R/I)2-(r/l)2 !
[[1-(R/I)2+(r/I)2}2+4(R/I)2]
i~
)-' (1)
where r is the beam spot radius on target, R is the distance of a point on the deposit measured from the center of the deposit spot, I is the distance between the target and collector (located parallel to each other) and pt is the equivalent thickness of the deposit (p being the deposit density). It should be pointed out that eqn. (i) was derived under the assumption that every sputtered particle that strikes the collector surface will stick. For the collection of sputtered particles this assumption appears to be reasonable in the light of recent experimental evidence' 3 (i.e. a large fraction of such particles have kinetic energies much higher than thermal energy). It follows from eqn. (1) that the deposit thickness will not be uniform over the deposit area. Rutherford backscattering (RBS) spectrometry was used to determine the function p t(R) along a line which passed through the center of the deposit. For the RBS technique a 1.25 MeV H * beam of 0.3 mm diameter was used to strike the deposit, and the energies of the backscattered protons were analyzed at a laboratory scattering angle of 154 °. Figure 2 is an RBS spectrum of the sputter deposit at a particular point R. We can readily distinguish the broad aluminum peak (from the 50 lag cm -2 aluminum collector), the titanium peak, the carbon peak and the nitrogen and oxygen peaks due to contamination. The agreement between the calculated and experimental values of the onset energies is very good, as indicated in Fig. 2. A quantitative correlation between the area A under the peak for each of the
1
lO
M. K A M I N S K Y .
R. NIEI.SEN
elements titanium and carbon and the atom densities for these elements (the equivalent thickness pt) at point R was obtained by calibration with titanium and carbon standards of known atomic densities. The standards were prepared by physical vapor deposition of known amounts of titanium and carbon (with equivalent thicknesses of 5. 10. 25, 50 and 100 monolaycrs onto the aluminum collector foils. The vapor deposit thicknesses were determined with a Kronos crystal monitor (model Q M 321 ). The deviation in A from the average value obtained from four repeat runs for a given thickness of the standard was found to be + 10",,. In addition, for the carbon calibration, thin carbon foils of known thickness were used. A linear calibration curve of the average values o f A rer.sus pt w a s obtained with a mean deviation of + 10'!,,. and on extrapolation this cttrvc went through ,-1 = 0 at pt = 0 for both the titanium and the carbon standards. 3. RI!SUI. IS
3. I. Spatial distribution q/.sputter deposit Measurements of the function t*tlR) were made with a very high spatial rcsolution (Ar)e/r z of 0.02",, whcrc Ar is the proton beam radius and r the deposit spot radius (about 95". of the deposit was in the area rtr-'). Figurc 3 allows a comparison of the function (pt(R))t~ c calculated from cqn. (IJ with the experimentally determined values of this function for TIC,_"coatings irradiated with 8.0 keV Hc" ions. The agreement between experimental and theoretical values is good and indicates that thc angular distribution of the sputtered particlcs follows a cosine distribution (implicit in the derivation of eqn. (I)) for this set of irradiation parameters. Similarly, good agreement was observed tbr I)- ion irntdiations for the energy range studied. e 5
w, d ~ : ) v:
4000
L
"
"
o 3000
.
d
~ :;
b(~'-
~
_i
,j
(j
.
#0
5 I2 I
I000
;
•
•
I.
~: I /
EC: 0.91
EO, 0 . 9 9 EN= 0 . 9 6
BACKSCATTERED
Erl: 116 EA, : 1.09
N* ION ENERGY
,b
6 31S-ANCE
6
+
1 IC
R ~rnr~)
E (MeV)
I"ig. 2. A n R B S spectrum of a sputter deposit (resulting from an 8 keV "SHe' irradiation of a polished "FiC surface) on a 5 0 lag c m -' aluminum substrate. The spectrum was obtained with a 1.25 M e V H " ion beam for a laboratory scattering angle of 154 . E o E N, E o, EAI a n d E l , arc the calculated onset energies for carbon, nitrogen, oxygen, aluminum and titanium respectively. F i g . 3. Variation in the TiC deposit thickness (resulting from an 8 kcV "~[le" irradiation of a polished T i C surface) as a function of the distance R as measured from the center of the deposit spot ( R = ()1 a l o n g a radial direction: - -, calculated using eqn. ( 1 ) : @ . experimentally determined values.
SPUTTERINGOF TiC COATINGSUNDER D + AND '*He ÷ BOMBARDMENT
111
3.2. Determination of sputtering yields A determination of the functions (p t(R))Ti and (p t(R})c from the RBS analysis allowed an estimate of the average emitted masses M for titanium and carbon according to eqn. (1). From these values we can determine the differential sputtering yields for carbon or titanium. For example, the differential sputtering yield for titanium is given by Slq -
MTi
NA
mTi f T i N i
where Mrl is the emitted titanium mass (eqn. (1)), mTi is the atomic weight of titanium, N A is Avogadro's number, Ni is the total number of ions impinging on a target and fl~N~ is the fraction of the total number of ions which strike titanium atoms; a similar expression can be obtained for carbon. It is assumed that fvi +fc = 1. It should be pointed out that the AES analysis of TiC samples before irradiation revealed that the titanium and carbon concentrations in surface regions had stoichiometric proportions yielding TiCo.97 within the experimental accuracy of approximately 15'~i and that other surface impurity concentrations (e.g. carbon and o/ oxygen) did not exceed 10/o. For a determination of the differential sputtering yields for carbon and titanium, the beam fraction values were chosen to be fTi = 1 and fc = ½ respectively. The total sputtering yield for TiC is then determined from the sum of the titanium and carbon masses emitted from the irradiated spot divided by the total number of ions striking this spot. In Table I the experimentally determined total sputtering yield values for the TiC surfaces in the polished and the as-deposited state for the D ÷ irradiations are summarized, and Table I1 gives similar data for the 4He+ ion irradiations. These tables also include values which have been calculated with the use of the semiempirical formula proposed by Smith ~4. The major parameters used in these calculations are listed in the footnotes to Tables I and II. 4. DISCUSSION The results summarized in Tables I and II reveal the following trends: the yield values decrease with increasing io.n energy for the energy range 3-60 keV for both D ÷ and 4He+ ions. The values for 1.5 keV deuterons are smaller than those for 3.0 keV deuterons. The general trend of the observed energy dependence of the yields can be understood on the basis of the energy dependence of the dominant collision processes acting between the incident ions and the lattice atoms. At the lower energies the collisions are influenced by the weak screening of the nuclei of the lattice particles, while at the higher energies the Rutherford collisions dominate 15. The experimentally determined yield values for the rough TiC surfaces in the as-deposited state are about 15')/o-100'yolarger than those for the smoother polished surfaces. This result suggests that for this type of rough surface the enhancement in sputtering (e.g. that resulting from angles of incidence other than normal for a large fraction of ions) is larger than the possible reduction due to redeposition of sputtered particles. We have reported previously a similar trend for as-deposited and polished TiB 2 surfaces 9. For energies lower than 8 keV it is possible to compare our data with some of those obtained by Bohdansky et al.16 for as-deposited TiC. In contrast with our
112 TABLE IOIAL
M. KAMINSKY. R. NIELSEN I S P l : I ll. R l N ( i Y I F I . D S I O R [ ) "
Projectile energy (keV~
IRRADIA]ION
{)I. "['i("
Experimental total yield ralues Imolecules ion - ~ )
Cah'uhm,d .vieht ralues " (molecules ion ~1
~Is-deposited "l iC
Polished TiC"
1.5
(2.2 ~0.1lx 10 "
{l.l+-0.1)x 10 "~
3.5x 10 '
3.0 5.(1 8.0 14.0 20.0 40.0 60.0
(3.2+0.11xI0 : {2.6"-0. I i x I0 -" (l.7ZO. IIxl0 ' (l.4f0.]Ixl0 " (0.8±0. Ii×]0 ' (0.4a-0.1)x10 ' i0.4u20. I)× I0--'
(2.2"0. I)xI0 -" (2.1"-0.11x 10 ~ (1.5_+.0.]ix10 2 (0.9+_0.1)×10 " ( 0 . 7 + 0 . 1 1 x 1 0 -" (0.3-+0.1)x ] 0 - " [0.2"-0. I ) x I0 -"
2.2×10 1 5 , I0 1.2x10 I).7,< I0 0 . 6 x I0 0 . 3 x 10 0.2,~10
: ": : -' -' -" :
" Reference 14. eqn. (7). was used for these calculations. The following values ~cre chosen for the major parameters: surface binding energy I{, = 2.3 eV: effective charge Z ~ , t T i ( ' l = 14: "~l.,,[Ti('l = 2 9 . 9 6 . TABLE II r()TAI
S P U I I E R I N ( . i "lll:l. DS I O R 41"Ie* IRRAI)I.',,TI()N
Projectih' energy (keVl
()!
TiC
,4s-deposited 7 iC
Poll.shed Ti("
(I.6., -~0.()5) × 10 (1.2, ± 0 . 0 5 ) x l O
{I.5, L0.05)× 10-
3.0 8.0
1.5
("alculated yield ralm,s" (molecules ion ~1
Experimental total yieht ralues Imoleculesion ~l
(0.8., + - 0 . 0 5 ) x I0
(().g ----0.05} x l O
14
(0.7,, +-0.051 x 10
(0.5 -+0.05) × 10
20 40 60
(0.4,, +-0.05) x I0 (0.3,, +0.051 x I0 (0.1- +-0.05) × I0
(0.2,, ~-0.051 x I 0
(0.4 o +0.05) x I0 (0. I - - - 0 . 0 5 l x I0
]
1.6 * 10 1.4 x IO 0.8 × 10 0.5 x 10 0.3,, × 10 0.2~ x 1() 0.1 a × 10
" Reference 14, eqn. (7), was used for these calculations. The following values were chosen for the major parameters: surface binding energ~ [i, = 2.3 eV'effectivc charge Z.,,(TiCI = 14: X/ u(l'iC - 2996. R B S a n a l y s i s o f s p u t t e r d e p o s i t s , B o h d a n s k y et al. used the target w e i g h t loss m e t h o d to d e t e r m i n e the s p u t t e r yields. A direct c o m p a r i s o n o f the yields S is o n l y p o s s i b l e at o n e e n e r g y , 8 keV" B o h d a n s k y e t a / . o b t a i n e d S D = 1.4 x 10 2 t n o l e c u l e s ion t a n d S m . = 6 . 1 x l 0 - - ' m o l e c u l e s ion i. we o b t a i n e d S D ( a s - d c p o s i t e d T i C ) = 1 . 7 x 10 - 2 t n o l e c u l e s ion ~. SD(polished T I C ) = 1.5x 10 2 m o l e c u l e s ion - l . Su~(as-deposited T i C ) = 8 . 4 x 10 2 m o l e c u l e s ion ~ a n d Sm,(polished T i C ) = 8.1 x 10 - 2 m o l e c u l e s i o n - ' F i g u r e 4 illustrates the e n e r g y d e p e n d e n c e of the e x p e r i m e n t a l l y d e t e r m i n e d t o t a l s p u t t e r yields for D ~ a n d 4He+ ion i r r a d i a t i o n s o f p o l i s h e d T i C samples. It a l l o w s a c o m p a r i s o n o f these yields with t h o s e w h i c h h a v e been c a l c u l a t e d o n the basis of S m i t h ' s s e m i e m p i r i c a l e x p r e s s i o n l'~ ( F i g . 4 . full curves). It s h o u l d be n o t e d that for the 8 k e V "*He' i r r a d i a t i o n the c a l c u l a t e d v a l u e was titted to the e x p e r i m e n t a l v a l u e by a d j u s t i n g the v a l u e of the surface b i n d i n g energy. T h i s v a l u e of the b i n d i n g e n e r g y was t h e n used for the c a l c u l a t i o n of the S ( E ) c u r v e s for l) * a n d 4 H e * i r r a d i a t i o n s . T h e g o o d a g r e e m e n t b e t w e e n the c a l c u l a t e d a n d m e a s u r e d yields
SPUTTERING OF
TiC COATINGS UNDER D + AND CHe + BOMBARDMENT
1 13
for energies above 20 keV can readily be seen. For energies from 1.5 to 20 keV we see good agreement for helium irradiations but not for deuteron irradiations. Both our experimental data and those of Bohdansky et al. 16 indicate that the yield values decrease with increasing energy below a deuteron energy of about 3 keV, while the calculated value at 1.5 keV is still larger than that calculated for 3 keV.
-5 10
-
hI62
id
j
2
, ,,,,,,I , , ,,,,,,I 4 6 I0 20 40 60 I00 ION ENERGY (WeV)
Fig. 4. The total sputtering yields for polished TiC surfaces as a function of projectile energy: e , 4He +, polished TiC, present data; &, 4He ", TiC, data from ref. 16; O, D * , polished TiC, present data: ~., D ~, TiC, data from ref. 16; - - - - , calculated using Smith's semiempirical expression ~" and by fitting the calculated value typical for 8 keV *He- to the experimental value (through the choice of the value for the surface binding energy).
For the operation of plasma devices these sputtering results for TiC coatings are encouraging. For example, the yield values for these coatings are lower than those for an uncoated surface of 304 LN stainless steel (the vacuum vessel materials for the tokamak fusion test reactor). For the polished TiC target under 4He+ irradiations over the energy range 3-15 keV, the yield values are smaller than those for 304 LN stainless steel 17 (e.g. for 8 keY 4He + irradiations, S(TiC) -- (8.1 +0.5)x 10 -2 atom ion-1 whereas S(304 LN stainless steel) = (11.3 +0.5)x 10-2 atoms ion-'). A comparison of yield values for these materials for deuteron irradiations also shows lower yields for polished TiC coatings. From the point of view of permissible plasma impurity concentration the gain is even larger because of the difference in the effective atomic numbers of TiC and 304 LN stainless steel. These encouraging results suggest that the use of TiC coatings on first-wall components may help to decrease plasma radiation losses and to improve plasma stability. ACKNOWLEDGMENTS
We would like to thank Mr. R. Chamberlin, Jr., for his assistance in taking the RBS spectra, Mr. P. Zschack for target preparations and Dr. A. S. Rao for taking some of the scanning electron micrographs. We are grateful to Dr. L. Goodman, Argonne National Laboratory, for carefully reading this manuscript. The work was performed under the auspices of the Office of Fusion Energy, U.S. Department of Energy.
I 14
M. KAMINSKY, R. NIELSEN
REFERENCES I 2 3
4 5
6 7 8 9 l0 11 12 13 14 15 16 17
M. Kaminsky, IEEE Trans. ,Vml. Sci., 1~¢(1971) 208. M. Kaminsky, Plasma Physics and ('ontrolled ,\.'uch'ar I.'t~ton Research. Vol. I1, International Atomic Energy Agency. Vienna. 1975, p. 287. W . M . Stacey, Jr., et al.. Pro~. 2rid "l~Jpi(al Meet. on the' T~'chnolog.v ol ('ontrolh'd Nuclear l"u,~ion, Vol. I. in L'SERDA (onL l'uhl. 760935-P1 (1976) 21 (U.S Energy Research and l)evclopment Administration). D . M . Mattox. Thin Solht Fihn.~. 63 (1979) 213. D . M . Mattox. A. W. Mullendore, H. O. Pierson and I). J. Sharp. J..Vml. ~htt~'r.. ,~'5 ,~6 (1979) 1127. H.O. Pierson, E. Randich and D. M. Mattox, J. L¢,.~-('ommon 3h't.. 67(1979) 381. M. Kaminsky and S. K. I)as, J. ,Vm'l. lhttcr.. ,~'5 86 (1979) 1095. S.K. Das and M. Kaminsk~,, "Ibm Solid FTbn~. 63 (1979) 269. M. Kaminsky, S. K. l,am and K. Moy. Thin Solid t.Thn~. 73 (1980) 9 I. S.K. L a m a n d M. Kaminsky, J..¥uc/. :~tater..S9(1980) 205. M. Kaminsky. S. K. Das. R. Ekern and D. C. Hess. In.~t. Phv.~. ("onl. Set. 3,'¢(1978) 305. R. (Jl~.ng. in L. Maissel and R. (Jlang (eds.), thmdhook ~1 lhin 1"Th~ l~,¢hnoloev. Mc(.ira~,~.-Hill. New York, 1970, pp. I 3. M. R. Wcller and T. A. Tombrello, ( 1 7 R¢T. B.,II'-7, August 1979 (Calilornia Institute of l'cchnology). D . L . Smith, J. :\:m'l. :~tater.. 75 (1978) 20. M. Kaminsky, ,4 tomic attd hmi~" bttpa('! Phenomena on Metal Su(l~wc.~, Springer, Heidelberg, 1965, pp. 222fl. J. Bohdansky, H. L. Bay and W. Ottenbergcr, J. Nm.I. ,~4awr.. 76 77(19781 163. M. Kaminsky, Thin Solid Fihnv. 7311)(1980)91.