The mixing of Ti, Cr, and Ni thin films with SiO_2 by low‐temperature (−196–25 °C) irradiation with 290 keV Xe has been investigated. Comparison of the morphology of the intermixed region and the dose dependences of net metal transport into SiO_2 reveals that long range motion and phase formation probably occur as separate and sequential processes. Kinetic limitations suppress chemical effects in these systems during the initial transport process. Chemical interactions influence the subsequent phase formation

Banwell, T.

Grunthaner, P. J.

Nicolet, M-A.

Sands, T.

English

Caltech Authors - Main

Downloaded 22 Dec 2005 to 131.215.225.9. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
Chemical effects in ion mixing of a ternary system {metaJ .. Si02) 
T. Banwe!l and M-A. Nicolet 
California lnstitute of Technology. Pasadena. California 91125 
T. Sands 
Bell Communications Research, Inc., Red Bank, New Jersey 07701-7020 
P. J. Grunthaner 
Jet Propulsion Laboratory, Pasadena, California 91109 
(Received 31 July 1986; accepted for publication 12 January 1987) 
The mixing ofTi, Cr. and Ni thin films with Si01 by low-temperature (- 196-25 "C) 
irradiation with 290 keY Xe has been investigated. Comparison of the morphology of the 
intermixed region and the dose dependences of net metal transport into Si02 reveals that long 
range motion and phase formation probably occur as separate and sequential processes. 
Kinetic limitations suppress chemical effects in these systems during the initial transport 
process. Chemical interactions influence the subsequent phase formation. 
A number of mechanisms are known to contribute to 
atomic transport in ion mixing. !-4 Recoil implantation from 
single high-energy collisions can be recognized from its lin-
ear fluence dependence. 2 A very large number of target 
atoms experience short range displacements induced by the 
cascade of higher generation collisions. The motion within a 
cascade is predominately isotropic, although the anisotropic 
flux of secondary recoils may also be significant. Electronic 
interactions introduce chemical effects after the average par-
tide energy has diminished to about 1 eV. 3 Several recent 
studies have characterized the correlation between the ex-
tent of ion-irradiation-induced mixing and thermochemical 
properties for binary metal-metal systems.3- 5 Much of the 
mixing in metal-metal systems apparently occurs in this 
thermalized regime. 6 Chemical effects can bias the random 
walk motion within this regime and produce a Darken effect. 
Johnson eta!. have proposed for these systems that the me-
tallurgical phases are formed after the modified concentra-
tion profile is established. 3 Rapid quenching imposes kinetic 
constraints which may favor formation of metastable 
phases. 
This letter summarizes results from our studies of ion 
mixing in Ti/Si02, Cr/Si02, and Ni/Si02 bilayers induced 
by 290 ke V Xe irradiation at 25 •c and - 196 ·c. Thermo-
dynamically, these three systems are quite distinct. There 
are many favorable reactions of Ti with Si02 . 7 Segregated 
TiSi2 and Ti20 1 layers were the observed reaction products 
for thermal annealing above 670 "C. The case of Cr is inter-
mediate; a few of the reactions for Cr forming Cr20 3 are 
favorable but kinetically inhibited. 7 ·8 The reactions of Ni 
with Si02 are all unfavorable. 7 We compare here the amount 
of metal transport into the Si02 with the morphology of the 
intermixed region produced by the ion irradiation. 
A set of Ti, Cr, and Ni samples on -1 11m thermally 
grown Si02 was prepared by eiectron beam evaporation with 
a range of film thicknesses chosen for each metal. 290 keY 
Xe irradiations were performed at - 196° or 25 "C with a 
prescribed fluence ¢of l-15X 1015 cm- 2 . The projected 
range ofXe in these metals is 34,ug!cm2 with 11.5fig/cm2 
standard deviation. 9 The unreacted metal was selectively re-
moved after the irradiation by chemicai etching, as reported 
previously. ~,IO.I 1 The distribution of metal in the Si02 was 
measured using 2 MeV He Rutherford backscattering spec-
trometry (RBS) with a 1-10 mm~ beam spot. High-resolu-
tion cross-sectional transmission electron microscopy 
( TEM) was used to determine the structure and morpholo-
gy of the intermixed region. The TEM samples were cleaved 
and epoxied face to face prior to thinning. X-ray photoelec-
tron spectrof.copy ( XPS) was employed to identify changes 
in chemical states. High-resolution cross-sectional TEM im-
ages of samples irradiated at 25 cc after removal of the free 
metal are shown in Fig. 1 for (a) Ti and (b) Cr with 
-
(aj 
I20nm 
(b) 
120nm 
20nm 
(c) 
FIG. !. High-resolution images of irradiated (a) Ti, (b) Cr, and (c) Ni 
samples after removing free-metal layer. 
574 AppL Phys. Lett. 50 (1 0), 9 March i 987 0003-6951/87/100571-03$01.00 @ 1987 American Institute of Physics 571 
Downloaded 22 Dec 2005 to 131.215.225.9. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
¢;=lOX 10 15 Xe em- 2 , and (c) Ni with¢;= 6X 1015 Xe 
cm- 2• The initial metal thickness was 23 ,ug/cm2• Arrows 
identify the Si02 interface. The dark clusters in (c) corre-
spond to a Ni phase. Most of the apparent Ni is located 
within a 5-nm region centered 5 nm below the Si02 interface. 
This Ni is contained in crystalline clusters which show a 
lattice spacing of0.241 ± 0.004 nm in high-resclution imag-
ing. A single Ni 2p112 line was observed at 853.6 eV binding 
energy using XPS. These results imply that cubic NiO is the 
only Ni phase presenL The Cr samples show no evidence of 
crystalline phase formation or clustering. TEM would prob-
ably reveal crystalline phases if they were present. TEM is 
not sensitive, however, to the Cr distribution if it is disor-
dered since the electron scattering factor for Cr is not signifi-
cantly different from that for Si. The Ti sample in Fig. 1 (a) 
exhibits considerable structure. There are crystalline clus-
ters at the interface which show a lattice spacing of 
0.33 ± 0.01 nm. This value corresponds to a strong reflec-
tion in several titanium oxides, although it is also possible 
that they are TiSi2 • The Ti 2p312 peak was identified with a 
459-eV binding energy, also suggesting oxide formation. 
There is, in addition, a band of amorphous or partially crys-
talline material centered at a depth of 10 nm, well beyond the 
electron escape depth, which probably contains Ti. These 
results clearly demonstrate that the detailed morphology of 
the intermixed region is strongly affected by the chemistry of 
the system. 
A different perspective is given by Fig. 2 which shows 
the RBS average metal concentration [1111 plotted logarith-
mically versus depth x into the Si02 for¢= lOX 10[5 Xe 
em - 2 at 25 oC. Recall that the depth profile inferred from a 
RBS spectrum is an average over the area of the sample ex-
posed to the He beam of the actual distribution of atoms in 
the sample at a depth corresponding to a given energy loss. 
The profiles in Fig. 2 consist of two distinct parts. Most of 
the metal is contained in a resolution-limited peak near the 
interface. In contrast to the Ni bearing region, which clearly 
lies below the interface, the average distribution ofTi and Cr 
may extend to the surface monotonically. In each case, a 
signiflcant discontinuity in the metal distribution is main-
tained across the metal-Si02 interface which is not expected 
for regular diffusion" The tails extending beyond ~ 40 nm 
result from recoil implantation. x.l 1 Both the peak and the tail 
were seen in the RBS spectra for all of the irradiated samples, 
although with differing proportions. The RBS profiles of 
samples irradiated at - 196 oc were indistinguishable from 
the 25 oc samples. 
The areal density of M atoms incorporated in the Si02 , 
[M L (¢;) = f[M ](r,b,x)dx, can be calculated from the pro-
files such as shown in Fig. 2 for different doses c/1. It is found 
for each sample configuration that [Nf ls =a,,.t<fJ 1 + wMc/1 112 . 
This was deduced from a linear relationship observed 
between¢; li2 [.M L and ¢; 1 / 2 over the investigated ftuence 
range. The ¢ 1 component arises from anisotropic processes 
while random walks generated by overlapping cascades in-
troduce a¢; 112 dependence. For a given initial film areal den-
sity, the aM are similar among Ti, Cr, and Ni. This is expect-
ed if recoil implantation is the dominant anisotropic process 
since these metals are collisionally similar. The mixing as so-
572 Appl. Phys. Lett, Vol. 50, No. 10,9 March 1987 
"" 'E 21 
u 
: I 0 
~ 201 
l'"c- t-- + I i-;·-., ~~ .\ 
I /) \ 
21 r J \ 
I ! I ',. I I \. j 
r.! 1 ····.,. 1 
Ll ·' 20 ~-- --~·~~J 
0 40 80 120 
DEPTH (nm) 
HG. 2. RBS depth proliles of (a) Ti, (b) Cr, and (c) Ni in Si07 produced 
by lOX 1015 em- 'Xe irradiation. The free metal was previously removed. 
ciated with caM correlates linearly with the energy deposited 
in the metal per incident Xe per unit depth (FD) at the inter-
face.s There is, however, no grouping of the data points to 
suggest a chemical trend. 
Over the dose range investigated, the number of metal 
atoms incorporated in the Si02 follows the same description 
for all three metals. We conclude from this common behav-
ior that in this dose range the process responsible for the 
metal incorporation into the Si02 is probably the same for all 
three metals, even though the individual chemistry of these 
metals with Si02 is different. This contrasts with the mixing 
behavior of transition metal bilayers, where the ion mixing at 
low temperatures (i.e., in the absence of radiation enhance-
ments) correlates with chemical mixing enthalpies.3 •4 The 
different chemical affinities are expressed in the final mor-
phology (Fig, 1), not in the average spatial distribution of 
the metal in Si02 (Fig. 2) or its dose dependence. The domi-
nant processes governing mixing in Ni!Si02 were tentatively 
identified by a series of parametric investigations. 8 For 
¢J > ~ 10 15 em - 2 , the concentration of Ni near the interface 
is probably determined by the competition between second-
Banwell et at. 572 
Downloaded 22 Dec 2005 to 131.215.225.9. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
ary recoil injection of Ni into the Si02 and difffusion of Ni 
back to the Ni-Si02 interface. The injection could have a 
linear FD dependence.H· 12 
Our observations only partly concur with other reports. 
Farlow eta!. 13 find little mixing for Cr on Si02 under experi-
mental conditions similar to ours, but report substantial 
mixing forTi on Si02 • In contrast, we observe similar mixing 
for both Cr and Ti at room temperature and below. The 
reasons for this discrepancy are unclear. 
Additional discrepancies arise when comparing pub-
lished results for Cr at elevated temperatures. Farlow et al. 13 
see a decrease in the mixing of Cr on Si02, while earlier work 
reports the opposite. 10 Nickel intermixes less at elevated 
temperatures than at room temperature, while the converse 
is true forTi. 10 This is readily understood as an expression of 
the chemical driving force in the system when kinetic limita-
tions disappear. 13 In the case of Cr, the chemical driving 
forces are weak and the outcome may be dominated by sec-
ond-order effects. This is suggested by the apparent discrep-
ancy in the mixing of Cr at elevated temperatures. 
The results reported here forTi, Cr, and Ni show that at 
room temperature kinetic constraints impede the influence 
of chemical driving forces. White et al. 14 show that Nb mixes 
more readily with Si02 at room temperature than Ti, Cr, and 
Ni do. In the Nb case, the kinetic constraints are evidently 
relaxed. We believe that the kinetic constraints are associat-
ed with the evolutkm of a cascade. A dense cascade can last 
long enough to allow for significant chemical interaction. 
In summary, it is found that at room temperature and 
below, the net transportation of metal into the Si02 is similar 
forTi, Cr, and Ni. For these systems, the amount of inter-
mixing does not correlate with the metal's reactivity. This 
suggests that there are kinetic limitations associated with a 
high activation energy for the relatively complicated ex-
change reactions, as occurring in these ternary systems, and 
573 Appl. Phys. Lett, Vol. 50, No. I 0. 9 March 1987 
the short duration of the cascade. At low temperatures, the 
metal depth distribution is probably established before 
chemistry is important. The limitations can be circumvented 
at elevated irradiation temperatures where the mixing rate 
shows a positive correlation with reactivity. 10 
This work was financially supported in part by the Of-
fice of Naval Research under contract No. N00014-84-K-
0275 and by the National Science Foundation under grant 
No. DMR-8421119. T. Ban well thanks IBM for a fellowship 
during this work. 
'J. W. Mayer, B. Y Tsaur, S. S. Lau. and L. S. Hung, NucL Instrum. Meth-
ods 182/183, I ( 1981 ) . 
2P. Sigmund and A. Gras-Marti, Nucl. Instrum. Methods 182/183, 25 
(1981 ). 
'W. L. Johnson, Y.-T. Cheng, M. VanRossum, and M-A. Nicolet, Nuc!. 
Instrum. Methods B 718. 657 ( 1985). 
4 Y.-T. Cheng, M. VanRossum, W. L. Johnson, and M-A. Nicolet, App\. 
Phys. LetL 45, 185 { 1984). 
'F. D'Hcurle, J. E. E. Baglin, and G. J. Clark, J. Appl. Phys. 57, 1426 
(1985). 
"H. M. l'aine and R. S. Averback, Nucl. Instrum. Methods B 7/8, 666 
( 1985). 
R. Prctorius, J. M. Harris. and M-A. Nicolet, Solid State Electron. 21, 6o7 
( 1978). 
"T. C. Banwell, Ph.D. Thesis, California Institute of Technology, 1986. 
"K. B. Winte.rbon, [on Implantation Range and Energy Depostion Distribu-
tions (IFI Plenum, New York, 1975). 
"'T. Banwell, 8-X. Liu, I. Go!ecki, and M-A. Nicolet, Nu<.:l. Instrum. 
Methods209/210, 125 (1983). 
11T. C. Banwdl, M-A. Nkolet, P. J. Gnmthaner, and T. Sands, in Energy 
Beam-Solid Interactions and Transient Thermal Processing 1984, Mater. 
Res. Soc. Symp. Proc., edited by D. K. Biegelscn, G. A. Rozgonyi, and C. 
V. Shank (MRS, Pittsburgh, 1984), Vol. 35, p. 545. 
12G. Falcone and A. Oliva, Appl. Phys. Lett. 42, 41 ( 1983 ). 
'-'G. C. Farlow, B. R. Appleton, LA. Boatner, C. J. McHargue, C. W. 
White, G. J. Clark, and J. E. E. Eaglin, :Water. Res. Soc. 5"ymp. Proc .. 
edited by D. K. Biegclsen, G. A. Rozgonyi, and C. V. Shank (MRS, Pitts-
burgh, 1985), Vol. 45, p. 137. 
14C. W. White, G. C. Farlow, J. Narayau, G. J. Clark, and J. E. Raglin, 
Mater. Lett. 2, 367 (1984). 
BanweJI et ai 573 


Chemical effects in ion mixing of a ternary system (metal-SiO_2)

https://authors.library.caltech.edu/32022/1/BANapl87.pdf

Chemical effects in ion mixing of a ternary system (metal-SiO_2)

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main