carbon, soot, etc. it is found that the nucleation density on C70 is equivalent to that of diamond seeds themselves. On the other hand, diamond nucleation of C6 ...
AD-A264 634 OFFICE OF NAVAL RESEARCH Grant # N0001489J 1848 R&T Code 413u002
Technical Report No. 12 Diamond Nucleation on Surfaces Using Carbon Clusters by
D T IC ELECanE SELECTE
prepared for publication in the
MAY 1 81993
A
R.J. Meilunas, R.P.H.
D
Journal of Materials Research
Northwestern University Dept. of Materials Science and Engineering, Evanston, IL 60208
May 1993
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Technical S•. FUNDING NUMBERS
Diamond Nucleation on Surfaces Using CarbIon Clusters
G NOOOI489J1848 R&T code 4 13uO02
6. AUTHOR(S)
R.J. Meilunas,
R.P.H. Chang B. PERFORMING ORGANIZATION REPORT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
Materials Science & Engineering Dept. Northwestern University 2225 Sheridan Road 60208 Evanston, IL
Technical Report #12
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"13. ABSTRACT
(Maximum 200 words)
Thin solid films of Ce and C7 have been used as nucleation layers for the growth of diamond thin films on a variety of substrate surfaces, including metal, insulator and semiconductors. Compared to other forms of carbon, such as graphite, amorphous carbon, soot, etc. it is found that the nucleation density on Ca is equivalent to that of diamond seeds themselves. On the other hand, diamond nucleation of C. is less favorable. We argue from our experiments that the reason for C- to have such favorable nucleating properties is its chemical stability and geometry. A working model is proposed to explain the nucleation of diamond on solid C0 films. Application of this work extending to the growth of diamond on a wide range of substrates is also discussed.
14, SUBJECT TERMS
Diamond Nucleation;
carbon clusters
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Diamond Nucleation on Surfaces Using Carbon Clusters R.J. Meilunas* and R.P.H. Chang Dept. of Materials Science and Engineering 60208 Northwestern University, Evanston, IL ABSTRACT Thin aolid films of C6 layers
for the
growth of
substrate surfaces,
and C7 diamond
is
soot,
etc.
thin
films
on
a variety
of
including metal, insulator and semiconductors.
Compared to other forms of carbon, carbon,
have been used as nucleation
it
is
such as graphite,
amorphous
found that the nucleation density on C70
equivalent to that of diamond seeds themselves.
On the other
hand, diamond nucleation of C6 is less favorable. We argue from our experiments
that
the
nucleating properties
reason is
its
for
C70
to
have
such
favorable
chemical stability and geometry.
A
working model is proposed to explain the nucleation of diamond on solid C7. films. Application of this work extending to the growth of diamond on a wide range of substrates is also discussed. Accesio!', Fri, NTIS CRA&I D IC TA3 Undnrounced
E
Ju~thcation
By.Dist. ibnti,,•
A
I
zI , '3
e
I. Introduction
The success in growing thin diamond films using chemical vapor deposition properties
methods of
potentially
for
diamond
unique
the
in
interest
stimulated
has
technological
new
applications. As an example, diamond has the combined properties of good
electrical
high
insulation,
well suited for use in
dielectric constant which make it
packaging and multi-chip module technologies. wide
and
optical
diamond
of
bandgap
device
The extreme hardness
would
provide
also
an
optical
oxcellent protective coating material for a variety of
applications. The chemical inertness and hardness of diamond films would also find wide utilization as a protective against corrosion and wear
low
and
conductivity
thermal
thin
coating
in the cutting tool and metal working
industries. A
number
different
to deposit
developed problem
of
deposition
diamond
thin
have
techniques
films.'I 2 -3 "4-5
A fundamental
most deposition methods is
encountered in
been
the limited
understanding and controllability of diamond nucleation on nondiamond substrates.
the most common method to obtain
At present,
substantial nucleation of diamond on non-diamond substrates is pretreatment
step.
accomplished by
This
simply
paste before placing it type of diamond seeding,
pretreatment
procedure
is
a
regularly
polishing the sample with diamond grit or in the deposition chamber.
Without some
not enough diamond crystallites nucleate 2
on
the surface of most materials in a reasonable amount of time to
eventually
grow
and
coalesce
into
continuous
films.
Such
pretreatments are also not easily adapted towards coating large or non-planar surface areas. In
this paper we provide a more comprehensive discussion on
the nucleation method described technique
in an earlier publication
eliminates the need for surface abrasion
6
. This
and diamond
seeding by usirg thin layers of fullerene films sublimated onto various non-diamond substrates. The thin fullerene layer produces a large number of nucleation sites suitable for the formation of continuous diamond films. The specificity of the type of fullerene is found to aid in elucidating the mechanism for diamond nucleation in
low pressure CVD.
I1.
Background
Detailed abrasion
and
experimental seeding
with
studies of the effects diamond
grit
have
of
been
substrate reported.
7
Recently, using high resolution transmission electron microscopy, Iijima et al a have demonstrated the effects of substrates on diamond nucleation. extensive cleaning
polishing the Si
They observed that even after
of the diamond grit polished Si substrates,
small "diamond dust " particles of nanometer size remain on the Si surface. The dust particles act as seeds for diamond nucleation on the substrate.
This represents more of a form of homoepitaxy on
isolated, randomly oriented,
single crystal diamond than any type
of heteroepitaxy. Earlier reports of enhancing diamond nucleation on unpolished, limited and
non-diamond substrates by various methods have been
inconclusive about nucleation mechanisms. These include enhancement like carbon films 9
with the use of diamond plasmas 11
fluorinated
and
derivatives 13
thin
,
even
dc biasing 10
,
overlayers 12
metal
with
However,
.
fingerprints 14
perylene
P
the
exception of dc biasing, none of the above methods has produced as significant a degree of nucleation enhancement as that obtained by of polishing with diamond grit.
the simple method
studies have proposed possible diamond
Various theoretical
forming precursors on which to nucleate
compounds as possible diamond cluster embryos included adamantane, dodecahedrane.
cage
or "seeds" which
hexacyclopentadecane and
tetracyclododecane,
The adamantane
Matsumoto and
hydrocarbon
several
to identify
were the first
Matsui 15
diamond.
represents
CIOH 16 ,
molecule,
the
-smallest combination of carbon atoms which possess the diamond unit structure (three six membered rings in a chair coordination).
The
and
were
represent
compounds
other proposed
as
the
precursors
twinned to the
diamond five
embryos twinned
fold
diamond
microcrystals which are found prevalent in CVD diamond thin films. From
simple
atomic
generate the diamond lattice compounds by simple
comparisons,
structure
hydrogen
from
the
above
abstraction
thermodynamic
molecules to
unstable in the
quite
4
hydrocarbon
followed
considerations
addition. Nevertheless, be
one can easily
harsh
by
cage carbon
show these
environment used
( > 600C
),
reducing
such
low
the
high
in
environment used to grow CVD
finds that the favorable enthalpies of formation
Stein 17
diamond.
stable
hydrocarbons are not
molecular weight temperature
find that
and Stein 17
et al. 16
of Godleski
equilibrium calculations
Thermodynamic
films.
to grow diamond
of such diamond forming precursors as adamantane,
diadamantane and
cyclohexane are overwhelmed by unfavorable entropies of reactions Above this temperature,
above 500 K.
the corresponding graphite
precursors (polycyclic aromatics) are found both theoretically and experimentally
to
be
more
stable.
addition,
In
below
this
temperature calculations show these diamond-like molecules to be unstable with respect to dissociation into methane and other small molecular weight gas species. Angus et al.
18-19
have,
in turn, proposed various saturated
multiple fused ring compounds ( with structures representative of nascent parallel -precursors.
twin planes of diamond
Such
"graphite
precursor"
hydrocarbon flame and combustion literature
)
as possible
as
described 20,
21,
diamond in
the
are stable in
the high temperature reducing atmosphere of CVD diamond growth. no published reports have appeared of successfully using
However,
such aromatics to nucleate diamond. On the contrary, Frenklach and Wang
22 ,
in a detailed chemical kinetics model of a hot filament CVD
reactor, find that in the operating parameters typical for diamond deposition,
atomic
hydrogen
suppresses
the
formation
of
such
aromatics. More
recently,
Shen et al. 2 5
have modelled diamond
based
precursors
a
employing
finite cluster
approximation
approach.
This theoretical model investigates carbon cluster fragments of the diamond lattice which are saturated by hydrogen atoms,
so called
adamantane based or polymantane derived carbon clusters. Ab initio quantum
mechanical
are
computations
used
to
the
investigate
convergence behavior of the properties of the clusters toward the properties
bulk diamond.
of
clusters of up to only is
C3H.
Although
stable polymantane
based
reference
are calculated and little
made to the stability of such molecules in the CVD environment approach to the problem has much
used to grow diamond films, the
relevance to the results described in this report. Several
detailed
studies
on
the
nucleation
mechanism
of
diamond on silicon and metal surfaces have been reported recently. Stoner et al 24 have made an in depth study of diamond nucleation on silicon using a biasing technique developed by Yugo et al 10 in which a negative potential is applied to the substrate with respect -to the plasma to enhance carbon ion bombardment of the surface during the nucleation stage. The carbon ions are found to initially form an amorphous silicon carbide layer on the substrate surface. However,
continued carbon ion bombardment results in preferential
etching of the silicon from the Si-C layer and/or continued high flux of carbon
to the surface.
This excess carbon is
postulated
for diamond nucleation.
The
molecular or structural arrangement of such carbon sites is
not
to form small
"clusters" favorable
described. Belton and Schmieg 2 have studied the nucleation of diamond on 6
platinum and nickel substrates using hot filament chemical vapor
electron
combination
X-ray
of
electron energy loss spectroscopy, and
photoelectron spectroscopy, low energy
a
experiments,
these
In
information.
structural
site
nucleation
elucidated
have
and
deposition
was
diffraction
used
the
to examine
time These
evolution of surface carbon species during diamond growth.
results indicate that diamond nucleates on platinum and nickel by
They postulate
surfaces.
scratched
carbon
graphitic
of
pre-deposition
on mechanically
precursors that defect
sites
in
graphite deposits contain the nucleation sites for diamond. al
26
these Ong et
have observed similar results from nucleation and growth of
diamond crystals on single-crystal copper surfaces implanted with carbon ions. This study shows that during diamond growth on carbon microwave plasma CVD,
implanted copper using
a polycrystalline
graphite film initially forms on the copper surface. This graphitic layer is
found to
enhance
nucleation of diamond on copper.
the
-This study concludes with a simple lattice model for diamond growth on graphite which is di.d
From conditions on is
dimod
pazuillel to grate
one
can
postulate
several
necessary
must be met to help enhance diamond nucleation
(1) a structured, pure
carbon source on the
necessary to act as a diamond seed. Pure carbon,
from
surface simple
thermodynamic equilibrium considerations 21 would be inherently more stable than any hydrocarbon molecule at typical CVD diamond growth conditions;
(2)
the need for the structured carbon source to be 7
both air stable to
facilitate application and durable enough to
withstand to a degree the environment in which plasma enhanced CVD of diamond takes place; source
to have steps
sites;
and
(4)
(3)
or
the need for the structured carbon
ledges to serve as diamond
finally,
to have a means of
nucleation
initiating diamond
nucleation from the gas phase during the CVD process. Advances by Kraetschmer et al
27
in
scale quantities of air stable pure C6
the synthesis of gram
and C.
and other
even
larger fullerenes now enable the practical experimental study of the utility
of carbon clusters as diamond nucleating agents.
addition to fulfilling the
requirements
In
stipulated above for a
good diamond nucleating agent, C., and C0 clusters are unique from other types of carbon precursors previously studied major respects.
in two other
The fullerenes are a highly ordered allotrope of
pure carbon which is
easily formed into thin solid films by
simple low temperature sublimation process.
a
Unlike many proposed
-hydrocarbon based diamond precursors, these cluster solid films are also found to be chemically stable up to the temperatures at which diamond is
found to grow.
in
C70
temperature approaching 600 0 C,
which is
limit for CVD diamond synthesis. results
of
attempts
to
enhance
substrates with the use of
IZI.
particular,
has a sublimation
a typical low temperature
We report in the
nucleation
this paper the of
C6 and C. fullerene films.
RZXeDliontal Procedure
8
diamond
on
Debcription of Microwave Plasma CVD Reactor
A.
A microwave enhanced plasma chemical vapor deposition system was designed ane constructed for the purpose of depositing high purity diamord thin films. A schematic of the deposition system is The microwave energy operating at a frequency
shown in Figure 1.
directed to the CVD reactor
of 2.45 GHz is waveguide,
through a circular
then launched into the chamber through a quartz window.
The substrate holder consists of a quartz tube with an annular cap, is
which either
used to keep the sample in graphite,
quartz,
particular experiment.
or
boron
place. nitride
made of
The cap is
on
depending
the
In this configuration the sample iq floating
with respect to the plasma when no bias potential is applied. Bias with respect to the plasma is made via platinum foil in mechanical contact with the back of the sample. B.
C6 and C70 Separation and Their Properties
Purified forms of C6,
C7,,,their various derivatives and other
-higher number carbon clusters were prepared by Shengzhong Liu in the Department of Chemistry at Northwestern University. C6. and C. were obtained from carbon soot, prepared by graphite evaporation in a helium atmosphere
26,29
. The clusters were first
recovered from
the soot by soxlet extraction. Separation of the C6 and C7 clusters was achieved
by dispersing the extracted material
column of active alumina, hexane,
and eluting with hexane,
and finally 20% toluene in hexane 3
on top of a 5% toluene in
. The purity of the
resulting C6 and C. fractions was confirmed by electron impact mass spectroscopy,
which
showed the
C7. fraction 9
slightly
(
