JOURNAL OF MATERIALS SCIENCE LETTERS 22, 2003, 817– 819
Stress relaxation-induced phase transformation in chromium
nitride films
HONG-YING CHEN, FU-HSING LU
∗
Department of Materials Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402,
Taiwan, Republic of China
E-mail: fhlu@dragon.nchu.edu.tw
Chromium nitride (CrN) films have been widely used
as tribological coatings and also in the electronics in-
dustry, because of their high hardness (Hv 1090) and
low resistivity (640 µ-cm) [1]. Although the oxida-
tion of CrN films has been investigated extensively, the
phase transformation of CrN to Cr
2
N has seldom been
studied [2–6]. Lai and Wu [2] stated that the Cr
2
N phase
was generated in CrN films annealed at 1150
◦
CinN
2
and vacuum (pO
2
= 1.3 × 10
−3
Pa) conditions. H´eau
et al. [3] indicated that Cr
2
N was present in CrN films
annealed above 327
◦
C in vacuum (pO
2
= 10
−3
Pa).
Almer et al. [4] reported that CrN transformed to Cr
2
N
between 450
◦
C and 550
◦
C in Ar, and interpreted the
phase transformation as tending toward the equilibrium
CrN-Cr
2
N phase fraction within the nitrogen-deficient
CrN phase. Hsieh et al. [5] showed that the Cr
2
N phase
appeared in CrN films over temperatures from 500
◦
Cto
800
◦
C. The authors’ previous work [6] proposed that a
non-thermodynamic factor governs the phase transfor-
mation of CrN in a low temperature range. This work
determines the residual stresses of the films at differ-
ent temperatures and then correlates these values to
phase transformation. More experimental evidence is
also presented to validate the proposed mechanism of
phase transformation at low temperatures.
(100) p-type Si wafers (Toshiba Ceramics Co., Ltd.)
were used as substrates. CrN films were deposited di-
rectly onto the substrates by cathodic arc plasma de-
position. Before deposition, the chamber was pumped
down to a pressure of 6.7 × 10
−3
Pa. The bias and the
current of the substrate were maintained at −150 V
and 60 A, respectively, under a pN
2
of 3 Pa during
deposition. The deposition time was 30 min and the
corresponding thickness of the films was about 1 µm.
After deposition, the films were annealed at temper-
atures between 450
◦
C and 900
◦
C under a reducing
atmosphere of N
2
/H
2
= 9 for 2 h, in a gas-tight tube
furnace equipped with an oxygen sensor (15% CaO-
doped ZrO
2
) that was used to monitor and ensure a low
oxygen level in the flowing gas.
Changes in the crystal structure of the films af-
ter annealing, were examined by X-ray diffraction
(MacScience MXP3, λ
Cu,K
α
= 0.154 nm) operated at
40 kV and 30 mA. The collection interval was 0.02
◦
(2θ mode) and the scanning rate was 5
◦
/min. The resid-
ual stress was measured by the scanning laser curvature
∗
Author to whom all correspondence should be addressed.
method. A He-Ne laser source with a power of 7 mW
and a wavelength of 632.8 nm was used. The curvature
of the surface of each film was recorded 100 times. The
corresponding residual stress was then calculated from
the curvature using Stoney’s relation [7], as described
elsewhere [6].
Fig. 1 displays the X-ray diffraction results for CrN
films annealed in N
2
/H
2
= 9 over the temperature range
from 450
◦
C to 900
◦
C. As shown in the figure, the CrN
phase (JCPDS #11-0065) was obtained over the whole
temperature range. Additionally, the Cr
2
N (JCPDS
#35-0803) (111) diffraction peak appeared between
500
◦
C and 650
◦
C but disappeared at 700
◦
C. Notably,
no chromium silicide was formed over the whole tem-
perature range used in this research.
The chemical reaction of the phase transformation
can be expressed by considering the thermodynamic
data summarized in the literature [8].
2CrN = Cr
2
N +
1
2
N
2
G [kJ/mole] = G
◦
+ RT ln
pN
1/2
2
= (105.2 − 0.0775 × T ) + RT ln
pN
1/2
2
where G represents the change in the Gibbs free en-
ergy for this transformation; G
◦
is the standard Gibbs
free energy; R is the gas constant; T is the absolute tem-
perature [K], and pN
2
is the partial pressure of nitrogen.
InaN
2
/H
2
= 9 atmosphere (pN
2
= 0.9 atm), the cal-
culated temperature at which CrN is in thermodynamic
equilibrium with C
2
N is 1077
◦
C. Hence, thermody-
namics do not explain the formation of Cr
2
N between
500
◦
C and 650
◦
C. A non-thermodynamic factor must
govern this phase transformation.
Fig. 2 plots the measured curvatures and the calcu-
lated residual stresses of as-deposited CrN films and
the films annealed at 450
◦
C, 500
◦
C, and 900
◦
C. The
as-deposited CrN films exhibit a compressive stress
of 2.38 ± 0.01 Gpa, which is similar to the values of
−2.0–−8.8 GPa reported in the literature [6, 9–12]. The
obtained residual stresses of the annealed CrN films
are −1.77 ± 0.01 (450
◦
C), −1.15 ± 0.01 (500
◦
C) and
−0.28 ± 0.02 GPa (900
◦
C). Apparently, the residual
stresses were relaxed rapidly as the annealing tem-
perature increased. The stress relaxation was further
0261–8028
C
2003 Kluwer Academic Publishers
817