Surface modification of chemically vapor deposited diamond for producing adherent thick and thin film metallizations for electronic packaging

Article abstract of DOI:10.1063/1.116365

In order to make chemically vapor deposited diamond (CVDD) substrates amenable to surface metallization without significant reduction in thermal conductivity, very thin alumina films were grown on diamond by vapor deposition of Al in oxygen atmosphere, followed by very low-temperature annealing in oxygen. A Cr interlayer initially deposited on CVDD prior to alumina deposition was found to lead to excellent adhesion between the substrate and the deposit. The alumina films were characterized by scanning and transmission electron microscopy as well as Auger electron spectroscopy.

Full text of DOI:10.1063/1.116365

Surface modification of chemically vapor deposited diamond for producing adherent
thick and thin film metallizations for electronic packaging
E. S. K. Menon and I. Dutta
Citation: Applied Physics Letters 68, 2951 (1996); doi: 10.1063/1.116365
View online: http://dx.doi.org/10.1063/1.116365
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Surface modification of chemically vapor deposited diamond for producing
adherent thick and thin film metallizations for electronic packaging
E. S. K. Menona) and I. Dutta
Department of Mechanical Engineering, Naval Postgraduate School, Monterey, California 93943
͑Received 18 December 1995; accepted for publication 16 March 1996͒
In order to make chemically vapor deposited diamond ͑CVDD͒ substrates amenable to surface
metallization without significant reduction in thermal conductivity, very thin alumina films were
grown on diamond by vapor deposition of Al in oxygen atmosphere, followed by very
low-temperature annealing in oxygen. A Cr interlayer initially deposited on CVDD prior to alumina
deposition was found to lead to excellent adhesion between the substrate and the deposit. The
alumina films were characterized by scanning and transmission electron microscopy as well as
Auger electron spectroscopy. © 1996 American Institute of Physics. ͓S0003-6951͑96͒02621-6͔
Diamond is an ideal substrate material for electronic
packaging because of its excellent thermal conductivity, high
electrical resistivity, and low dielectric constant.1 However,
diamond is difficult to metallize because of two reasons: ͑1͒
The surface of diamond is relatively inert, making it difficult
for common, non-carbide forming, metallizations with high
electrical conductivity, e.g., Au or Cu, to adhere to it; and ͑2͒
its thermal expansion coefficient is very small (ϳ1–2
ϫ10Ϫ6/K), resulting in large interfacial residual stresses be-
tween the substrate and the metallization. As a result, tradi-
tional approaches2,3 to metallize diamond have utilized less
electrically conductive, but carbide-forming metals ͑W, Mo,
Nb, or Ti͒ with smaller thermal expansion mismatch with
diamond than Au or Cu, as either ͑1͒ the final metallization,
or ͑2͒ an interlayer between Au or Cu metallizations and
diamond. The first approach results in a metallization with
less than ideal electrical properties, whereas the second ap-
proach requires the interlayer to be patterned identically to
the final metallization, which in practice precludes this ap-
proach from being used for conventional thick film pro-
cesses. This letter discusses a novel method to circumvent
these limitations by producing a very thin, well-bonded layer
of an electrically insulating material of good dielectric
strength ͑alumina͒ on chemically vapor deposited diamond
͑CVDD͒ substrate, such that the thermal conductivity of the
treated substrate is not degraded by more than 5% relative to
that of the untreated substrate. This allows the treated dia-
mond surface to be metallized using all conventional thin
and thick film processes currently in use industry-wide for
alumina packages.
dimensional heat transfer calculation reveals that the
through-thickness thermal conductivity of a 200 ␮m thick
diamond of Kϭ1500 W/m K drops by less than 3% on being
coated with a 100 nm thick layer of alumina.
In the present work, an alumina layer was produced on a
CVDD substrate using a 20–50 nm thick carbide-forming
metal ͑Cr͒ as an adhesion-enhancing interlayer. Cr was de-
posited on CVDD by vacuum evaporation at a base pressure
of 10Ϫ7 Torr. Subsequently, a 100–200 nm layer of alumina
was deposited by reactive evaporation of Al and 99.99%
pure oxygen at a total chamber pressure of 10Ϫ4 Torr. The
substrate was maintained at 575–675 K during deposition. A
postdeposition annealing treatment ͑600 K for 24 h͒ was
carried out in oxygen atmosphere after backfilling the depo-
sition chamber with oxygen to a pressure of ϳ500 Torr. This
approach produced a continuous, fine grained, alumina film
which adhered strongly to the CVDD substrate. Attempts in
separating the films from the substrates by using a replicating
acetate tape ͑and a conventional scotch tape͒ were unsuc-
cessful and the alumina film could not be removed, demon-
strating that the films are quite adherent to the substrate.
The choice of alumina as a surface layer to render
CVDD metallizable was based on the following consider-
ations.
Alumina
has
high
electrical
resistivity
(108 –109 ⍀ cm), low dielectric constant ͑8.5͒, a coefficient
of thermal expansion (7ϫ10Ϫ6/K) between those of dia-
mond and most metals, is extremely stable chemically and is
easily metallizable using currently used techniques. Al-
though alumina has a low thermal conductivity ͑K ϳ25
W/m K͒, its deleterious effects on the thermal conductivity
of the package can be minimized by keeping the alumina
layer thickness very small. For instance, a simple one-
FIG. 1. Microstructure of the alumina film deposited on a 20 nm Cr inter-
layer. This film was deposited on cleaved single-crystal sodium chloride,
which was subsequently dissolved in water to separate the film from the
substrate. Bright field image shows fine 3–10 nm sized grains. The corre-
sponding SADP, shown as an inset, reveals that the film consists of numer-
ous ultrafine, misoriented ␥-Al2O3 crystallites. The sharp rings correspond
to the following Miller indices starting from the innermost: ͑220͒, ͑311͒,
͑400͒, ͑511͒, and ͑440͒.
a͒
Electronic mail: skmenon@nps.navy.mil
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Appl. Phys. Lett. 68 (21), 20 May 1996 0003-6951/96/68(21)/2951/2/$10.00 © 1996 American Institute of Physics 2951
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FIG. 2. Appearance of 125 nm thick surface alumina layer produced on a
CVDD substrate with a 50 nm thick Cr interlayer.
FIG. 4. Auger depth profile through an ϳ120 nm alumina film deposited on
a 50 nm Cr interlayer on a CVDD substrate. For reference, thicknesses
where known, are indicated along with sputter time on the abscissa.
Films for TEM studies were produced on a freshly cleaved
͑100͒ surface of NaCl under conditions identical to those
used for deposition on CVDD, so that the films could be
easily separated from the substrate for TEM observations.
The film was then characterized by transmission electron mi-
croscopy ͑TEM͒, energy dispersive x-ray spectroscopy
͑EDS͒, scanning electron microscopy ͑SEM͒, and Auger
electron spectroscopy ͑AES͒.
Al2O3 standard,5 confirming that the film is alumina. Spectra
collected after sputtering to different depths of the film were
found to be identical, suggesting that a uniform layer of alu-
mina was present on the CVDD samples studied here.
Figure 4 shows an Auger depth profile through the alu-
mina and Cr films deposited on CVDD. The Al/O ratio is
observed to be relatively constant until about 100 nm from
the surface ͑sputter time of ϳ5 min͒, in agreement with the
TEM results, which showed the layer to be ␥-Al2O3. Signifi-
cant interdiffusion is observed to have occurred between Cr
and Al/O. Furthermore, Cr and C from the underlying
CVDD are observed to have interdiffused, and appear to
have reacted as well. The prominent O signal near the Cr/C
interface is presumed to be due to the diffusion of O ad-
sorbed on the CVDD surface into the Cr layer. The interdif-
fusion zones between Al2O3 and Cr and between Cr and C
result in strong chemical bonding between Al2O3, Cr, and the
CVDD layers, and is responsible for the excellent adhesion
of the Al2O3 film to the substrate.
Figure 1 shows a bright field TEM image of the alumina
film deposited on a Cr interlayer and the corresponding se-
lected area diffraction pattern ͑SADP͒ is shown as an inset. It
is observed that the film is nanocrystalline, with the crystal-
lite size ranging from 3 to 10 nm. The sharp, somewhat
spotty, rings in the SADP correspond exactly to reflections
from ␥-Al2O3 and confirm that the film is largely
crystalline,4 with negligible amorphous component. The na-
ture of the alumina films was further confirmed by EDS and
AES studies. Figure 2 shows a SEM image of the surface of
the alumina film produced on CVDD ͑deposited on silicon͒
using the above technique. It is observed that some of the
surface crystallites have grown to about 0.2–0.3 ␮m due to
rapid surface diffusion during the postdeposition heat treat-
ment, although the underlying layer was nanocrystalline
͑Fig. 1͒. Figure 3, which is an Auger electron spectrum from
the surface of the film, clearly shows two peaks, one corre-
sponding to oxygen at 504 eV, and the other corresponding
to aluminum at 1384 eV. The detailed structure of the Al
peaks in Fig. 3 was found to be identical to that from a pure
Thus, the above surface treatment yields a very thin,
ultrafine grained, adherent alumina surface layer on CVDD
substrate. Since the surface layer is alumina, which is widely
used as a substrate material for electronic packaging, the
CVDD substrate, treated as above, is easily metallizable by a
number of thin and thin film techniques, using existing tech-
nology.
This work was supported by a grant from the Naval
Surface Warfare Center, Crane Division, with K. G. Beasley
as contract monitor. The authors also wish to express their
gratitude to Dr. J. Sosniak of Naval Air Warfare Center,
Aircraft Division, Indianapolis, for supplying the CVDD
substrates.
1 D. J. Pickrell, P. J. Santini, and F. M. Kimock, Proceedings of the Inter-
national Symposium on Microelectronics ͑The Microelectronics Society,
1993͒, Vol. 2105, p. 405.
2 C. D. Iacovangelo and E. C. Jerabek, Proceedings of the International
Symposium on Microelectronics, 1993 ͑The Microelectronics Society,
1993͒, Vol. 2105, p. 132.
3 C. D. Iacovangelo, P. J. DiConza, E. C. Jerabek, and K. P. Zarnoch,
Mater. Res. Soc. Symp. Proc. 337, 401 ͑1994͒.
FIG. 3. Auger electron spectrum from the surface of the film deposited on
CVDD with a Cr interlayer, clearly showing two peaks—one corresponding
to oxygen at 504 eV, and the other corresponding to aluminum at 1384 eV.
Details of the Al peak are consistent with the Al peak from a pure Al2O3
standard ͑Ref. 5͒ and is distinct from the peak from unreacted aluminum.
4 JCPDS Powder Diffraction File, Card No. 10-425.
5 L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach, and R. E.
Weber, Handbook of Auger Electron Spectroscopy ͑Physical Electronics,
1976͒.
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2952 Appl. Phys. Lett., Vol. 68, No. 21, 20 May 1996 E. S. K. Menon and I. Dutta
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