Measurements of sputtering yields and composition profiles have been carried out using backscattering spectrometry for samples of CrSi_2 on Si irradiated with 200‐keV Xe ions. When the CrSi_2 layer is thinner than the ion range, the sputtering yield ratio of Si to Cr increases from 3.5 for room‐temperature irradiation to 65 at 290 °C. For a thick sample, the corresponding increase is from 2.4 to 4.0. only. These changes are explained in terms of a rise in the Si surface concentration at 290 °C. The driving force for this process seems to be the establishment of stoichiometric CrSi_2 compound. Transport of Si to the surface is by ion mixing in the thin sample and thermal diffusion through the thick layer

Fernandez, R.

Nicolet, M-A.

Shreter, U.

English

Caltech Authors - Main

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Temperature-dependent ion mixing and diffusion during sputtering of thin 
films of CrSi2 on silicon 
U. Shreter, R. Fernandez,a> and M-A. Nicolet 
California Insitute of Technology, Pasadena. California 91125 
(Received 10 March 1983; accepted for publication 12 May 1983) 
Measurements of sputtering yields and composition profiles have been carried out using 
backscattering spectrometry for samples of CrSi2 on Si irradiated with 200-ke V Xe ions. When the 
CrSi2 layer is thinner than the ion range, the sputtering yield ratio ofSi to Cr increases from 3. 5 for 
room-temperature irradiation to 65 at 290 ·c. For a thick sample, the corresponding increase is 
from 2.4 to 4.0. only. These changes are explained in terms of a rise in the Si surface concentration 
at 290 ·c. The driving force for this process seems to be the establishment of stoichiometric CrSi2 
compound. Transport ofSi to the surface is by ion mixing in the thin sample and thermal diffusion 
through the thick layer. 
PACS numbers: 79.20.Nc, 68.60. + q, 64.75. + g 
Evidence for the activation of fast transport processes 
in solid compound films was recently found in ion mixing 
experiments with some metal silicides (NbSi2, 1 CrSi2, 2 
Ni2Si3). In those experiments, the atomic mixing of metal 
layers with a silicon substrate was enhanced greatly when 
the irradiation was carried out at elevated temperatures. The 
possibility that the same processes might also affect sputter-
ing from these silicides motivated the present investigation. 
In this letter we present results of measurements of par-
tial sputtering yields and compositional changes in samples 
of CrSi2 sputtered by high-energy xe+ ions at room tem-
perature and at 290 ·c. 
Samples were prepared by successively vacuum-eva-
porating layers ofSi and Cr on Si02 substrates and annealing 
them at 500 •c to form CrSi2• For all samples, the amount of 
Si deposited was more than that needed to consume all of the 
Cr in the reaction, with the result that a layer ofunreacted Si 
remained between the silicide and the substrate. One set of 
samples consisted of370 A ofCrSi2 (denoted ta) on 1000 A of 
Si and the other of 1800 A of CrSi2 (denoted tb) on 580 A of 
Si. A 1 em X 1 em area of each sample was irradiated with 
200-keV Xe+ ions to a dose of 5 X 1016 atoms/cm2• These 
ions have a projected range of about 450 A in CrSi2, which is 
larger than ta, but -4 times less than tb. A band of carbon 
foil was placed in a semicircle in front of each sample to 
collect the sputtered material. The samples and carbon col-
lectors were analyzed by backscattering (BS) spectrometry of 
1.5-MeV 4 He+ ions. The sputtering yields ofSi and Cr were 
determined both from the amounts of material collected on 
the carbon foils (assuming axial symmetry) and from the loss 
of the two elements from the irradiated portions of the sam-
ples. Composition profiles were obtained from the relative 
heights ofSi and Cr signals in the BS spectra (for more details 
see Ref. 4). 
Table I summarizes the sputtering yield calculations. 
Here the average sputtering yield of element A is defined by 
SA = total number of A atoms sputtered, divided by the 
total number of incoming ions. In all cases, data obtained 
"
1 Present address: Jet Propulsion Laboratory, California Institute of Tech-
nology, Pasadena, California. 
from the collector and from the sample agreed to within 
experimental uncertainties. 
Figure 1 presents the Cr concentration profiles calcu-
lated from the BS spectra for the two sample configurations 
after sputtering. The fractional atomic concentration of Cr 
at the surface ( C ~r) as obtained by extrapolating these pro-
files to zero depth is given in Table I. 
From Table I we see that the sputtering yields ratios 
S5; /Scr increase with temperature for both sample configu-
rations. However, the effect is very much larger in the thin 
sample. As the table shows, this temperature effect is due 
mainly to the sharp reduction of the sputtering yield of Cr. 
From Fig. 1 we also see that there are major differences in 
the profiles of the two samples after irradiation at the same 
conditions. One obvious conclusion from these differences is 
that the large drop in Cr yield from the thin sample at the 
high temperature is not the result of an external cause like 
excessive beam heating or surface contamination (oxygen, 
carbon) since such an effect, if present, should influence both 
thin and thick samples alike. 
We now analyze the results and show first that our me-
sured surface concentrations are consistent with the notion 
that the sputtering process itself is temperature independent 
and is not responsible for the observed effects. 
TABLE I. Summary of experimental results. 
Sample 370 A CrSi 2 1800 A CrSi 2 
Temperature 
(•q 20 290 20 290 
Ss, 2.8 ± 0.2 3.3 ± 0.3 2.4 ± 0.3 3.2 ± 0.3 
Sc, 0.8 ± 0.2 0.05 ± 0.05 1.0 ± 0.2 0.8 ± 0.2 
Ss.fSc, 3.5 65 2.4 4.0 
C' a Cc 0.33 0.14 0.45 0.33 
c~r b 0.40 0.05 0.49 0.27 
• Extrapolated from profiles of Fig. I. 
b Estimated values assuming the sputtering yield is proportional to the sur-
face concentration. See text for details. 
247 Appl. Phys. Lett. 43 (3), 1 August 1983 0003-6951/83/150247-03$01.00 © 1983 American Institute of Physics 247 
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o.5o.r----.....!.__ ___ __..l __ +--... 
j ••• 
0.40~ •• 
STOICHIOMETRIC CrSi2--, 
! 
z • •• 
0 • • • • 
~ ORIGINAL THICKNESS 
u 0.10 ·THICK' . 
, THIN f CrS 12 
0 
0 250 
DEPTH 
THICK o' 
THIN •J RT 
THICK o-:. 290oc THIN 0 I 
OF I Rp ~ 450A 
-~,I 
~ 
500 1800 
(,8..) 
FIG. I. Depth profiles of the Cr concentration in the irradiated samples. 
The profiles were calculated from the relative heights of the Si and Cr sig-
nals in the backscattering spectra. The depth scale was calculated for stoi-
chiometric CrSi,. 
Instantaneous sputtering yield ratios are given by 
Ss; Ys; c~i 
Sc, Yc, C' Cr 
Ys; 1 ~ c~, ( 1) 
Yc, C' Cr 
where Y5 , and Yc, are sputtering probabilities (which, for 
single element targets are identical to the sputtering yields 
S ). The value of Ys; I Y cr at room temperature is known to be 
2.3 from measurements of steady-state sputtering of 1500 A 
CrSi2 by 100-ke V Xe + ions. 4 When we substitute this value, 
and the average sputtering yields from Table I in Eq. ( 1 ), we 
can estimate the expected surface concentrations of Cr, C ~r, 
which are listed in the last row of the table. These values are 
in general agreement with the measured concentrations C~, 
(see Table I). The remaining differences can arise from sever-
al sources: ( 1) by using average sputtering yields in Eq. ( 1 ), we 
actually estimate the average surface composition during 
sputtering while C ~r is the measured final composition; (2) 
the depth resolution in our measurements is about 80 A, so 
that changes in composition on a smaller scale are smoothed 
out and cannot be measured accurately; (3) the sputtering 
probabilities and their ratio may depend (weakly5 ) on the Xe 
energy. From the generally good agreement of this estimate, 
we conclude that the sputtering probability ratio Y5JYc, is 
constant in all the cases studied here. The inference is that 
the sputtering mechanism does not change significantly with 
temperature either. 
Since all the changes in sputtering yields can be attri-
buted to changes in surface composition, we now consider 
the temperature-dependent processes that determine this 
composition. At elevated temperatures, the bulk composi-
tions are quite uniform. This fact indicates that chemical 
driving forces (formation of equilibrium phases) and the nec-
essary atomic mobility are determining factors. A similar 
conclusion is reached for the ion mixing of Si and Cr bi-
248 Appl. Phys. Lett., Vol. 43, No. 3, 1 August 1983 
layers, where the intermixing rate at 270 oc is much higher 
than at room temperature and the mixed layer consists of 
polycrystalline CrSi2 .2 Also, the rate of mixing is thermally 
activated which proves that the mixing process extends in 
time much beyond the duration of an individual collision 
cascade ( ~ 10~ 12 s).n The mixing may thus be viewed as an 
equilibration process of a volume of excited material that 
surrounds the track of a penetrating particle and extends to a 
depth at which the density of deposited energy is apprecia-
ble. Applied to the present experiment, this means that when 
the ion range approaches or penetrates into the underlying 
silicon layer, Si is mixed into the silicide. As the mixed mate-
rial relaxes to the equilibrium state appropriate to the sample 
temperature, this excess silicon must segregate out to grain 
boundaries of the CrSi2 phase and to the surface. For the thin 
sample, we can estimate the amount of excess- Si in the bulk 
as~ 7 at . %from Fig. 1. If the size ofCrSi 2 grains is compar-
able to the thickness of the layer ( ~400 A), this excess Si 
corresponds to a layer of ~50 A around the grains. This 
estimate is consistent with the measured profile (see Fig. l ). 
In electron diffraction measurements on the thin sam-
ple after irradiation at 290 °C, we find that it contains poly-
crystalline CrSi2 . This result agrees with the concept of 
phase separation following the mixing of Si in the silicide, 
however, it does not supply conclusive evidence that Si preci-
pitation to grain boundaries takes place. 
For the thick sample, the amount of Si mixed into the 
silicide from the underlying layer is expected to be much 
smaller and, apparently, is just enough to compensate for the 
preferential sputtering of Si from the surface. The result is 
that the composition remains nearly stoichiometric (Fig. I). 
The fact that stoichiometric conditions are established 
continuously in our samples at 290 oc means that steady-
state sputtering, where the ratio of sputtered atoms equals 
the bulk composition, can never be reached since that re-
quires the surface to become rich in Cr (the lower sputtering 
probability elemene). In this respect, our results differ from 
previous experiments on temperature effects in sputtering. 
Such experiments were performed with alloys that form 
complete series of solid solutions (Ni-Cu, H-lo Au-Pd, 10, Ag-
Au 10). Since there are no stable compounds in these alloys, 
there is no driving force for maintaining uniform composi-
tions. Indeed, steady-state sputtering could be reached in all 
those cases. The only effect of elevated temperature was to 
increase the (radiation-enhanced) diffusion coefficient and 
thereby also the time needed to reach steady-state and the 
depth of the altered layer. 
In a recent study, Dumke eta!. 11 found that the sputter-
ing yield ratio of In to Ga from the eutectic alloy increased 
by a factor of ~ 30 when the temperature was raised above 
the melting point. This rise is explained as the result of ther-
mal segregation of In to the outermost atomic layer in the 
liquid. It is interesting to note that in the present work, a 
comparable change was observed by raising the sample tem-
perature only to ~0.3 of the melting point. The possibility 
that the sputtering yield of one element in a thin compound 
film will be drastically reduced and a surface layer will be 
retained, may have an important implication for cases where 
sputter erosion is used to obtain depth composition profiles. 
Shreter, Fernandez, and Nicolet 248 
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This work was performed under the Benevolent U. R. 
Fund of the Bohmische Physical Society (B. M. Ullrich). The 
authors also acknowledge the partial financial support for 
the operation of the ion implantation system by the U. S. 
Department of Energy through an agreement with the Na-
tional Aeronautics and Space Administration and moni-
tored by the Jet Propulsion Laboratory, California Institute 
of Technology (D. Bickler). 
'S. Matteson. J. Roth, and M-A. Nicolet, Radial. Elf. 42,217 (19791. 
'J. W. Mayer, B. Y. Tsaur, S. S. Lau, and L. S. Hung, Nucl. Instrum. Meth-
ods 182/183. I 1 1981 ). 
'L. S. Wielunski, B. M. Paine, B. X. Liu, C-D. Lien, and M-A. Nicolet, 
Phys. Status Solidi A 72,399 (1982). 
"R. Fernandez, U. Shreter, and M-A. Nicolet. Nucl. Instrum. Methods (in 
press). 
'P. Sigmund. Phys. Rev. 184, 383 (1969). 
"S. Matteson and M-A. Nicolet. in Annual Repieu· of Mataia/s Science, 
edited by R. A. Huggins (Annual Review of Materials Science, Palo Alto, 
19831. Vol. 13, p. 339. 
'P. S. Ho, Surf. Sci. 72, 253 (1978). 
'N. Q. Lam and H. Wiedersich, in Metastable Materials Formation by Jon 
Implantation, edited by S. T. Picraux and W. 1. Choyke (Elsevier, New 
York, 1982), p. 35. 
"H. Shimizu, M. Ono. N. Koyama, andY. Ishida. J. Appl. Phys. 53, 3044 
(1982) 
'"D. G. Swartzfager, S. B. Ziemecki, and M. J. Kelley. J. Vac. Sci. Techno!. 
19. 185 (1981). 
"M. F. Dumke. T. A. Tomhrello, R. A. Weiler, R. M. Hansley. and E. H. 
Cirlin. Surf. Sci. (in press). 
Growth of metastable Cu1 -xCrx solid solutions by ion mixing during bias-
sputter deposition 
S. M. Shin, M. A. Ray, J. M. Rigsbee, and J. E. Greene 
Department of Metallurgy and the Materials Research Laboratory, University of Illinois at Urbana-
Champaign, Urbana, Illinois 61801 
(Received 25 March 1983; accepted for publication 13 May 1983) 
The Cu-Cr system exhibits a miscibility gap in the liquid and has essentially no solid solubility. 
However, using low energy, < 160 eV, Ar+ ion bombardment of the growing film to promote 
ballistic collisional mixing, metastable single phase face-centered-cubic Cu-rich and body-
centered-cubic Cr-rich solid solutions have been grown on amorphous glass substrates at 75 oc. 
The films were typically 1 pm thick with average grain sizes of90 and 110 nm, respectively, and a 
( 111) preferred orientation. Annealing studies combined with x-ray and electron diffraction 
analyses showed that the films have good thermal stability against phase separation at 
temperatures up to 400 oc. The metastable to equilibrium phase transformation occurred with the 
precipitation of an essentially pure second phase, rather than through a continuous series of 
metastable states, due to structural constraints. 
PACS numbers: 68.55. + b, 64.75. + g, 64.60.My,81.40.Cd 
The Cu-Cr system exhibits a miscibility gap in the liq-
uid phase and essentially no solid solubility at room tem-
perature.1 Even at the eutectic temperature, 1075 oc, the sol-
ubility of Cr in fcc Cu is < 0.8 at. %. There is, however, a 
strong interest in producing metastable single phase Cu-Cr 
solid solutions, especially on the Cu-rich side where the ad-
dition of Cr would be expected to increase the corrosion re-
sistance with the possibility of producing "stainless" Cu-
based alloys. 
Several investigators2•3 have shown that high energy, 
100-300 keY, inert ion bombardment of thin film/substrate 
couples can lead to the formation of randomly mixed amor-
phous alloys through recoil implantation and cascade mix-
ing processes. Such alloys can then transform to metastable 
crystalline phases upon subsequent annealing. However, 
Greene and coworkers4- 7 have demonstrated that crystalline 
metastable phases can be formed directly by using low-ener-
gy ( < 300 e V) inert ion bombardment of the growing film 
during deposition. Single crystal metastable alloys such as 
(InSb)l -xBix,4 (GaSh),_ XGex·'·h and (GaAs)l XGex, 7 
have been grown at elevated temperatures on single crystal 
GaAs substrates. These "ion-mixed" films do not require 
further post-deposition ion bombardment or annealing and 
exhibit good thermal and temporal stability. 
This letter presents the initial results of an investigation 
into the growth and microstructure of metastable polycrys-
talline Cu 1 _ x Crx solid solutions. Both fcc Cu-rich and bee 
Cr-rich single phase alloys were grown on glass substrates 
using low-energy Ar+ ion bombardment mixing during de-
position. Films grown in the absence of ion bombardment 
were two-phase. The thermal stability of the ion-mixed sin-
gle phase metastable alloys as well as the kinetics of the me-
tastable to equilibrium phase transformation have been stud-
ied using x-ray and electron diffraction. 
The films were grown by rf bias-sputter deposition. 
Two 10-cm-diam two-phase targets, one Cu-rich and one 
Cr-rich, were fabricated from 99.999% pure Cu and 99.9% 
pure Cr sheets. The Cu substrate plate was physically and 
249 Appl. Phys. Lett. 43 (3), 1 August 1983 0003-6951 /83/150249-03$01.00 @ 1983 American Institute of Physics 249 


Temperature-dependent ion mixing and diffusion during sputtering of thin films of CrSi_2 on silicon

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Temperature-dependent ion mixing and diffusion during sputtering of thin films of CrSi_2 on silicon

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