Surface and Coatings Technology 163 – 164 (2003) 318–322
Structures and properties of BON and multilayered TiNyBON thin films
prepared by PAMOCVD method
D.-C. Lim*, G.C. Chen, S.-B. Lee, J.-H. Boo*
Department of Chemistry and Institute of Basic Science, SungKyunKwan University, Suwon 440-746, South Korea
Abstract
Hard multilayered TiNyBON thin films have been deposited on steel and copper substrates in the deposition temperature range
of RT;300 8C by low frequency RF derived plasma assisted MOCVD. Trimethylborate and tetrakisdimethylaminotitanium
precursors were used to grow multilayered TiNyBON thin films. We used Ar gas for a plasma source and N2 gas as a reactive
and additional nitrogen source. In this study, we have mainly investigated the relationship between hardness and structures of the
coating layers by the effects of deposition parameters such as deposition time and substrate temperature. We found that the
microhardness strongly depended on film thickness and structures of the bilayer films. In addition, the sequence of deposition
procedure for the TiNyBON bilayers highly affected the hardness. The maximum hardness obtained from TiNyBON bilayer was
30 GPa.
䊚 2002 Elsevier Science B.V. All rights reserved.
Keywords: BON and TiNyBON thin films; Low frequency RF-PAMOCVD; High hardness
1. Introduction
Covalent material, such as B, C, N, Si, have been
used to make hard materials. Especially the material,
composed of boron (B) with other light elements such
as nitrogen (N), carbon (C), oxygen (O), etc., are
interesting, because of their excellent properties for
superhardness w1x, insulation w2x and nonlinear optical
behavior w3x. For example, the hardness of diamond is
80–100 GPa and that of cubic boron nitride is 50 GPa.
However, diamond is etched by oxygen in the air at
high temperature, which limits its usefulness in hightemperature applications. Cubic boron nitride is widely
viewed as the ideal material for cutting tools. But,
synthesizing coatings of cubic boron nitride have proved
to be very difficult. So, recently new hard materials
based on nanocomposite, composed of grains separated
by grain boundaries, superlattices which consist of
alternating layers of two different materials were developed w4x. The hardness of a mutilayer coating can be
enhanced over the average of two components (e.g.
*Corresponding author. Fax: q82-31-290-7075.
E-mail addresses: djlove99@hanmail.net (D.-C. Lim),
jhboo@chem.skku.ac.kr (J.-H. Boo).
TiNyVN, 56 GPa; TiNyNbN, 51 GPa; TiNyCNx, 45–
55 GPa) w5–11x.
Based on these backgrounds, high hardness superlattice needs to have a large difference in the shear moduli
of the two components, and thus the superlattice period
should ideally be less than 10 nm. Also the component
materials in a superlattice should be immiscible so that
the interfaces are sharp and the materials remain stable
at high temperature w12,13x. In this article, therefore, we
would like to study other factors such as interlayer
structure, surface morphologies and film thickness, that
influence on high hardness. Generally speaking, multilayered superhard materials observed up to now consist
of two different hard materials. But we fabricated BONy
TiN multilayered thin films by a combination of soft
BON film and hard TiN film, expecting that the soft
material can reduce the internal stress of the hard film
resulting in a new material formation with high hardness.
2. Experimental
The procedure of BON and BONyTiN fabrications
was done in a set of parallel plate electrode discharged
system. The plasma was derived by low frequency RF
with 100 kHz and 500 W power. In the case of high
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 6 3 6 - 9
D.-C. Lim et al. / Surface and Coatings Technology 163 – 164 (2003) 318–322
frequency, 13.56 MHz, a high ratio of gas-phase molecule dissociation was expected and resulted in the multideposit fabrication of multi-element compounds. To
avoid these disadvantages, we used low frequency RF
rather than that of high frequency.
The plasma source gas was Ar in which the flux was
150 sccm. N2 gas was used as nitrogen source gas with
20 sccm flux. In the deposition chamber there were two
parallel gas distribution rings. Trimethylborate,
(OCH3)3B (TMB) was used as a boron and oxygen
source by a gas distribution ring with 2 sccm flux
w(CH3)2Nx4Ti
and
tetrakisdimethylaminotitanium,
(TDMAT) was used as a titanium nitride source by
another gas distribution ring with 5 sccm flux. The
general working pressure was 600 mTorr–1 Torr in the
chamber. Steel (s45c) and copper plates were used as
substrates. Before they were installed on the substrate
heater in the deposition chamber, they underwent
degreasing and drying in a vacuum. The substrate
temperature measured by a Chromel–Alumel thermocouple was maintained in the range of room temperature
to 300 8C. The deposition time was lasted from 80 to
160 min. The details of experimental set-up and deposition procedures have already been reported in our
previous papers w14,15x. The as-grown films were analyzed with XRD, SEM, AFM, and a Knoop microhardness tester. In this study, we have mainly investigated
interlayer structure, surface morphologies and film thickness, which influence on high hardness of BONyTiN
multilayer.
3. Results and discussion
Fig. 1 shows the dependence of hardness and thickness and the SEM morphologies of BON films fabricated
on a steel plate at 300 8C and 500 W RF power by a
low frequency (100–300 kHz) RF derived PAMOCVD
method with the TMB precursor and N2 plasma. The
microhardness test for the as-grown films was carried
out by using a Knoop indentor under the following
condition: 50 g load; 10 s loading time; 5 s holding
time; 5 s unloading time. When the thickness of BON
film is less than 1 mm (in zone I), the film hardness
decreased with increasing the film thickness. More than
1 mm, the hardness firstly increases with the thickness
(in zone II), then arrived at a thickness independent
value which is approximately 10 GPa. As the hardness
of BON film is not high, the change of hardness in zone
I may be ascribed to the reduction of substrate effect. It
is smooth and continuous in zones I and III, but becomes
rough and discontinuous in zone II (see the inserted
SEM images for each zone). The changes of morphology
indicate the changes of film structure. Thus the variation
of hardness with the thickness is due to the substrate
effect and change of film structure. Other properties of
BON thin film have been published in Refs. w14,15x.
319
Fig. 1. The dependence of hardness on film thickness for BON thin
films. The insets show the SEM images of each zone.
In the case of simple TiN film deposition, highly
oriented TiN thin films along the w1 1 1x direction are
observed on copper substrates under the same deposition
condition as BON film deposition. Generally the hardness value of commercial TiN products is approximately
20–25 GPa. But the hardness of as-grown TiN film is
approximately 17 GPa in this study. This is due to the
surface roughness. The AFM image shown in Fig. 4a
proved the rough surface of the as-grown TiN film.
Fig. 2a shows the variety of film hardness with the
different kinds of thin films. We calibrated the film
hardness using a Knoop micro-hardness tester and
changed the measured value to the unit of GPa. We
obtained the hardness of steel to be 7 GPa, while these
of BON and TiN single layers were 12 and 17 GPa,
respectively. We experimented two cases based on these
data. The first experiment was the deposition of TiN
thin film onto the deposited soft BON (TiNyBONy
substrate), and the second was the deposition of BON
thin film onto the deposited TiN thin film (BONyTiNy
substrate). We obtained higher hardness in both of these
experiments compared to the single layer thin films such
as TiN and BON. Especially, we could measure the
excellent hardness value in the case of BONyTiN thin
film to be over 30 GPa. Also the hardness of TiNyBON
thin film has a higher value than the single layer thin
films. However, the hardness of TiNyBON film always
has a lower value than that of BONyTiN films. This
indicates that the deposition sequence for bilayer formation will be one of important factors to influence the
film hardness, and a multilayered film has a merit of
enhancing the hardness rather than the simple deposits.
The reason why the TiNyBON thin film had the higher
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D.-C. Lim et al. / Surface and Coatings Technology 163 – 164 (2003) 318–322
Fig. 4 shows AFM images of TiN and BONyTiN
films. We could clearly see the TiN columnar structure
in Fig. 4a, but we could not see this structure when
BONyTiN films were deposited. This can be explained
as follows. Firstly, amorphous BON filled up the gaps
of TiN columnar structure and formed grains itself (Fig.
4b and c). This is the main reason of hardness enhancement. At 60 min, the deposition of BON onto the TiN
coated layer shown in Fig. 4c, a BONyTiN bilayer with
the maximum hardness had a continuous grain size, and
this grain dispersed on the whole, resulting in the
reduction of internal stress of film itself. In Fig. 4d,
however, lumps of BON were made due to BON islands
or cluster formations. Because of this property, the
hardness of BONyTiN films is decreased again, reflecting the softness of BON itself.
Conclusively, it was found from this result that when
a multilayer film was formed by a combination of
Fig. 2. (a) The variety of film hardness with of different kinds of thin
films. (b) The variation of hardness of BONyTiN thin films as a
function of deposition time (thickness).
hardness is that the film hardness is just increased with
increasing film thickness and by nano-crystalline formation between interlayers. The other reason is that the
TiN film made higher adhesion than BON onto the
substrates, resulting in the reduction of film stress. The
detailed reason why the BONyTiN film has a high
hardness will be discussed later on.
Fig. 2b shows the variation of hardness of BONyTiN
thin films as a function of deposition time (thickness)
of the BON film deposited on a TiN coated layer. With
increasing deposition time of BON thin film, the hardness value of BONyTiN thin film is increased. However,
when the deposition time of BON thin film is over 60
min, the hardness value of BONyTiN thin film is
suddenly decreased. The reason of this result was due
to not a characteristic of interlayer of the TiN layer but
soft property of BON layer. In the SEM images shown
in Fig. 3, we could see smooth morphology of BON
(60 min)yTiN thin film with the maximum hardness
value (Fig. 3b). But there were defects in the BON (80
min)yTiN film in Fig. 3c. These defects come from ion
bombardment of plasma. We thus confirmed this result
through AFM images again.
Fig. 3. SEM images of (a) TiN (80 min); (b) BON (60 min)yTiN
(80 min) and (c) BON (80 min)yTiN (80 min) thin films.
D.-C. Lim et al. / Surface and Coatings Technology 163 – 164 (2003) 318–322
321
Fig. 4. AFM images of (a) TiN (80 min); (b) BON (20 min)yTiN (80 min); (c) BON (60 min)yTiN (80 min) and (d) BON (80 min)yTiN (80
min) thin films.
amorphous BON and columnar structured TiN. The
properties of interlayer of multilayer film, film thickness,
and grain size will thus be the heart of enhancement of
hardness. To ensure this phenomenon in detail, a crosssectional TEM experiment as well as TED and EDX is
now going on.
4. Conclusions
A BON film with a hardness of 10 GPa was able to
be grown by low frequency RF plasma assisted MOCVD
with TMB and N2 plasma. Highly oriented, crack-free
TiN (1 1 1) films with a hardness of 20–25 GPa could
also be deposited with TDMAT under the same deposition condition of BON. When multilayered films consisting of BON layer and TiN layer were deposited, a
new film with a much higher hardness can be obtained
compared to the single layer thin films. Especially, we
could measure the excellent hardness value in the case
of BONyTiN multilayered film to be 30 GPa. Also the
hardness of TiN thin film deposited onto a BON coated
substrate was obtained with a more higher value than
the single layer thin films. The reason why the BONy
TiN thin film had the higher hardness is that amorphous
BON will firstly fill up the gaps of TiN columnar
structures and then formed continuous grains itself,
resulting in the reduction of the internal stress of film.
Acknowledgments
Supports of this research by the 2000 BSRI project
(Grant No. 2000-015-PP0195) of Korea Research Foun-
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D.-C. Lim et al. / Surface and Coatings Technology 163 – 164 (2003) 318–322
dation and by the Center for Advanced Plasma Surface
Technology at the Sungkyunkwan University are gratefully acknowledged. This work was also supported by
the BK21 project of the Ministry of Education, Korea
and by the Korean–German Plasma Center project of
the Ministry of Science and Technology, Korea.
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