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The effect of deposition temperature on the properties of TiN diffusion barriers prepared by
atomic layer chemical vapor deposition
Hsyi-En Cheng, Wen-Jen Lee, and C.-M. Hsu
Department of Electrical Engineering, Southern Taiwan University of Technology, Tainan
710, Taiwan
Phone: +886-6-2533131 ext. 3331 Fax: +886-6-2537461
E-mail: [email protected]
Abstract
TiN films were grown on p-type Si (100) substrates and SiO2/Si substrates by a modified
atomic-layer chemical vapor deposition (ALCVD) cycle using TiCl4 and NH3 as precursors.
The effects of deposition temperature on growth rate, film resistivity, microstructure, and
diffusion barrier properties of TiN films were investigated. The results show that the grown
films are all polycrystalline with (200) preferred orientation and that the (200) texture
is stronger at high deposition temperatures. The growth rate is about 0.03 nm per
deposition cycle, and almost independent of deposition temperature. The resistivity,
however, exponentially decreases with increasing deposition temperature. The chlorine
impurity concentration measured by Auger electron spectrometry is lower than 1 at.% for
o
films grown at deposition temperatures above 350 C. Atomic force microscopy analysis
reveals that the surface quality is good with root mean square roughness values below
0.9 nm. Various Cu/ALCVD TiN/Si samples were annealed at temperatures between 500 oC
and 800 oC in vacuum ambient of 6.67×10-4 Pa for 1 h to evaluate the performance of TiN
barriers. It is found that the failure temperature of TiN barriers is related to the deposition
o
o
o
o
temperature. The failure temperatures are 600 C, 600-650 C, 700 C, and 750 C for the TiN
o
o
o
o
films deposited at 300 C, 350-400 C, 450 C, and 500 C, respectively. The formation of
voids at some week spots after annealing suggests that Cl residues are responsible for the
cause of early failure of ALCVD TiN barriers.
1. Introduction
Titanium nitride (TiN) is widely used as adhesion layer and diffusion barrier for W, Al,
and
Cu
metallization.
It
is
also
a
potential
direct
metal-gate
material
for
metal-oxide-semiconductor (MOS) devices [1-4]. However, the current deposition techniques
such as ionized metal plasma sputtering, low pressure chemical vapor deposition (LPCVD),
and plasma enhanced chemical vapor deposition are becoming inadequate to meet the
requirements in better film quality and higher step coverage of deep contacts and via trenches
for sub-90 nm technology and beyond. Atomic layer chemical vapor deposition (ALCVD) is
one of the most promising deposition methods for future generation IC fabrication due to its
excellent uniformity, accurate thickness control, low deposition temperature, and almost
100% step coverage [5]. Therefore TiN films grown by ALCVD have attracted considerable
attention [6-12]. Previous studies [10,11] have shown that the chlorine residues in TiN films
grown by ALCVD at deposition temperature of 400oC were as low as 1.0 %, which is lower
than those of other LPCVD methods grown at high temperature of 650oC [2]. D.-G. Park et al.
[1] has shown the leakage current of a MOS capacitor gated with ALCVD-TiN is remarkably
lower than that with sputter-deposited TiN and poly-Si gate. Although these experiments
have demonstrated the advantages of ALCVD TiN over conventional CVD TiN, the
properties of TiN diffusion barrier by ALCVD is however not yet correlated with deposition
temperature. In this study, a modified ALCVD cycle with 6-steps was adopted and the effects
of deposition temperature on growth rate, film resistivity, microstructure, and diffusion
barrier properties of TiN films were investigated. The relationship between the failure
temperature of Cu/TiN/Si samples and the deposition temperature of TiN films was studied.
2. Experimental details
ALCVD TiN films were grown on two kinds of substrates: p-type Si (100) substrates
with a resistivity of 0.003-0.005 Ω-cm, and SiO2/Si substrates in which the SiO2 was
thermally grown by a wet oxidation process with a thickness of 1.5 m. The films grown on
Si were for the characteristics of growth and diffusion barrier properties, and the films grown
on SiO2/Si for the analysis of sheet resistance. The ALCVD tool is a hot-wall system with a
quartz deposition chamber. The precursors of TiCl4 and NH3 were alternatively introduced
into the ALCVD chamber through time controlled solenoid valves. A modified ALCVD cycle
with 6-steps, which was different from the previous 4-steps per cycle [9-12], was adopted for
this experiment. The 6-steps cycle adds a pump-down step between the reactant pulse and Ar
purge steps to improve the removal efficiency of residual reactants and by-products. The time
for each step is 3, 2, 5, 2.5, 2.5, and 5 s respectively for TiCl4 reactant, pump-down, Ar purge,
NH3 reactant, pump-down, and Ar purge steps. 1000 cycles in total were conducted for each
sample. The process pressure is between 13 and 107 Pa in the step of reactant pulses. The
flow rates of TiCl4 and NH3, determined by the reservoir temperature and the gas flow meter
respectively, were 0.11 cc/pulse and 6.4 cc/pulse. The substrates were cleaned in an
ultrasonic bath sequentially using acetone, methanol and de-ionized water for 10 min, and
then dried with N2 gas before introducing into the vacuum chamber. Prior to deposition, the
chamber was evacuated to a base pressure of 0.27 Pa. To investigate the effect of deposition
temperature on the TiN film properties, five types of samples were prepared at various
substrate temperatures of 300, 350, 400, 450, and 500 oC. The choice of 300 oC as the lowest
deposition temperature is based on that the lowest Gibb’s free energy required for the
reaction 2TiCl4 + 4NH3 → 2TiN + 8HCl + N2 + 2H2 appears at 280 oC as shown in Fig. 1.
The TiN films grown on Si were then capped with a copper layer of 200 nm by E-beam
evaporation. The synthesized Cu/TiN/Si structure was annealed at temperatures between 500
o
C and 800 oC with an interval of 50 oC in the vacuum ambient of 6.67×10-4 Pa for 1 h to
investigate the diffusion barrier ability of TiN.
The thickness of the TiN films was measured by an Alpha-step profilometer, and the
sheet resistance by a four-point probe. The film resistivity was then calculated from film
thickness and sheet resistance. The film structure was determined by a RIGAKU D/MAX
3.V X-ray diffraction (XRD) with voltage of 40 kV and current of 30 mA at a wavelength of
1.5418 Å. The cross-sectional microstructure was observed by a Philips Tecnai G2 F20
tunneling electron microscope (TEM) at working voltage of 200 kV. The TEM samples were
prepared by a series of thinning processes. Firstly, the sliced TiN/Si samples were mounted
using epoxy, and then polished by abrasive papers. This was followed by Ar ion milling of
the samples to an approximate thickness for TEM observation. The surface morphology was
characterized by a Philips XL-40FEG scanning electron microscope (SEM) with an
accelerating voltage of 15 kV. The surface roughness was examined by an atomic force
microscope (Seiko HV-300) in tapping mode. A fast scan rate was used firstly for finding a
non-contaminated area and then a slow scan rate of 1.0 Hz for measuring the
root-mean-square (RMS) roughness over a scanning area of 5μm×5μm. The average RMS
value was taken from 3 measurements in different locations with an error less than
10%. Impurities in the films were analyzed by a Fison Microlab 310D Auger electron
spectrometer (AES) with incident electron energy of 10 keV. The depth profiles were studied
by sequential sputtering of 3 keV Ar+ beams at current of 8 mAcm-2.
3. Results and discussion
In conventional CVD TiN, the deposition temperature is one of the main factors
affecting the film growth rate. For ALCVD TiN, the film growth rate is less sensitive
to deposition temperature. Fig. 2 shows the growth rate of TiN films on SiO 2 /Si with
the deposition temperatures between 300 oC and 500 oC. The growth rate is about 0.031
nm per deposition cycle, and almost not affected by deposition temperature. The
growth rate of TiN films on p-type Si(100) was found to be the same as those on
SiO2 /Si. This identical growth rate indicates that the reaction is not a kinetic control. It
is a self-limiting reaction caused by the saturated surface adsorption of reactants.
However, the growth rate is far below the theoretical value of 1 monolayer of 0.42 nm
per cycle. The small growth rate has also been reported for the TaCl 5 -based Ta, TaN
and Ta2 O5 films grown by ALCVD [13-15]. A steric hindrance model proposed by M.
Ylilammi [15] based on the geometry of the reactant molecules and the density of the
adsorption sites on the surface reasonably explains the low growth rate. The size of the
reactant molecules of TiCl 4 is too big to achieve a chemisorbed monolayer density
equal to the Ti atomic density in the final solid material. Nevertheless, a wide process
window exists on thickness control of ALCVD TiN.
Fig. 3 shows the SEM images of ALCVD TiN on Si at various deposition
temperatures. It can be seen that the surface morphology is too smooth to be resolved
clearly. Nevertheless, small domes can still be observed and they uniformly distribute
in the film. The microstructure of these TiN films was examined by x -ray diffraction
and TEM. Fig. 4 shows the XRD spectra of TiN films at various deposition
temperatures. The TiN films are all polycrystalline with (200) preferred orientation
and the (200) texture is stronger for the films grown at higher deposition temperatures.
Fig. 5 shows the cross-sectional micrographs of TiN film grown at deposition
temperature of 500 oC by TEM. The grains in TiN films exhibit columnar with
diameter of about 18 nm. The dome diameter measured from SEM images in Fig. 3 is
consistent with the TEM result, indicating that a dome means a grain in the films. Fig.
6 shows the surface roughness of TiN films as a function of the deposition temperature.
The RMS values of roughness are all below 0.9 nm, showing a good surface quality
for all the ALCVD TiN films.
The resistivity of these films is shown in Fig. 7. Unlike growth rate, the resistivity
strongly depends on the deposition temperature. It decreases exponentially with
increasing deposition temperature. The Arrhenius plot implies that the resistivity is
related to a reaction determined by a kinetic process. In general, the resistivity is
affected by impurities, grain size, texture, and thickness of the thin films. The film
thickness, preferred orientation, and surface quality described above do not provide an
evidence for this dependence. Fig. 8 shows the AES depth profiles of ALCVD TiN
o
o
films grown at 300 C and 350 C. Although some researches have shown that the
dependence of resistivity on deposition conditions is the same as that for Cl impurity
[10,13], the Cl impurity in the TiN films in this study is low as compared with the oxygen
impurity and even below the detection limit (1 at.%) of AES for films grown at deposition
o
temperature above 350 C. Therefore the Cl impurity should not be the main dominating
factor. The oxygen impurity due to the adsorption of residual H 2 O gas in the chamber
is also not a dominating factor because oxygen content is similar for each TiN film.
Therefore the grain size is considered as the main effect for the strong dependence of
resistivity on deposition temperature. Fig. 3 and 5 clearly show the grain sizes increase
with increasing deposition temperature. The lower Cl residues compared with previous
study [10] may be due to the added pump-down step that improves the efficiency of
desorption of HCl by-products.
The diffusion barrier properties of these TiN films were studied by monitoring the
changes in sheet resistance (Rs), XRD spectra, and surface morphologies of Cu/TiN/Si
samples at various conditions of the vacuum heat treatment. Fig. 9 shows the variation in the
o
Rs with the annealing temperature. The Rs value of Cu/TiN(300 C)/Si increases abruptly
o
o
after annealing at 650 C for 1 h, but the Rs value of Cu/TiN(500 C)/Si does not increase
o
until 800 C. Apparently the deposition temperature of ALCVD TiN is a sensitive parameter
for the ability of diffusion barrier against Cu. In order to determine the cause of the sharp
change in Rs, surface morphologies of various Cu/ALCVD TiN/Si samples were inspected
by SEM. Fig 10 shows the dependence of surface morphologies on deposition temperature at
various annealing temperatures and Fig. 11a and b are the enlarged images of Fig. 10a1 and
e4 for further observation on the morphology variation at the beginning of the sharp Rs
change. The results show some voids were formed firstly on copper surface before the sharp
increase in Rs and then crystals of Cu compounds grow at higher annealing temperature. The
voids formation suggests that the diffusion velocity of Cu through TiN film is higher than
that of Si through TiN film. Comparing the Rs value with the surface morphologies, the sharp
increase in Rs is due to a large quantity of crystals formation. To identify the phase of these
o
o
crystals, XRD measurements for the Cu/TiN(300 C)/Si and Cu/TiN(500 C)/Si samples were
conducted at various annealing temperatures and the results are shown in Fig. 12. Two new
o
peaks related with Cu3Si phase are observed at 2 = 44.7 and 45.4o for the Cu/TiN(300 C)/Si
sample after annealing at 700
o
C, whereas no additional peaks appear for the
o
o
Cu/TiN(500 C)/Si sample even annealed at 800 C. The lack of Cu3Si diffraction peak in the
o
Cu/TiN(500 C)/Si sample is due to the fact that the Cu3Si phases only form at some week
spots. In summary, the annealing temperature for voids formation is lower than those for Rs
increase and for Cu3Si diffraction peak appearance. In other words, the inspection for the
failure temperature of TiN diffusion barriers by SEM imaging method is more suitable than
the Rs method and XRD method. We use the starting point that voids form as the failure
o
temperature of TiN barriers, and the failure temperatures are concluded as follows: 600 C for
o
o
o
o
TiN films deposited at 300 C, 600-650 C for TiN films deposited at 350 C and 400 C, 700
o
o
o
o
C for TiN films deposited at 450 C, and 750 C for TiN films deposited at 500 C.
o
It is noteworthy that the distribution of voids on the surface of Cu/TiN(300 C)/Si sample
o
is more uniform than that of Cu/TiN(500 C)/Si sample. The voids appearing only at some
o
week spots for Cu/TiN(500 C)/Si sample implies that the grain boundary should not be a
dominating factor on the failure of diffusion barriers because the grain size observed on SEM
pictures is uniform. Therefore the Cl residues are suggested to be the cause resulting in the
early failure of ALCVD TiN. The Cl residues in the TiN films are less stable and help to
generate defects for Cu diffusion. A uniform distribution of voids on the surface of
o
Cu/TiN(300 C)/Si sample is then reasonable because the Cl impurity content in the TiN films
o
o
grown at 300 C is higher. At a higher deposition temperature such as 500 C, the Cl impurity
content is too low to yield defects and so the diffusion paths for Cu migrants. Therefore the
failure temperature is higher at high deposition temperatures and the voids only happen at
some weak spots. The high failure temperature of the TiN films grown by the 6-steps cycle
ALCVD is expected because the pump-down steps enhance the desorption of HCl
by-products.
4. Conclusions
TiN films were grown on p-type Si (100) and SiO2/Si substrates by a modified ALCVD
technique with a 6-steps cycle. The as-deposited TiN films are all polycrystalline with a
(200) preferred orientation and the (200) texture is stronger for the films grown at high
deposition temperatures. The growth rate is about 0.03 nm per deposition cycle, and is
found independent of the deposition temperature. The resistivity, however, decreases
with increasing deposition temperature exponentially. The AES depth profiles show
the chlorine residues in ALCVD TiN films are below 1 at.% for the films grown at deposition
o
temperature above 350 C. The surface quality is good and the RMS values of roughness
are all below 0.9 nm. The observations from SEM images show that the failure temperature
o
of TiN barriers is related to the deposition temperature. The failure temperatures are 600 C,
o
o
o
o
o
o
600-650 C, 700 C, and 750 C for the TiN films deposited at 300 C, 350-400 C, 450 C,
o
and 500 C, respectively. The Cl residues are suggested as the cause resulting in the early
failure of ALCVD TiN barriers.
Acknowledgements
This study was supported by National Science Council (NSC92-2216-E-218-006), Taiwan,
and is gratefully acknowledged.
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Figure captions
Fig. 1 The dependence of reaction free energy on temperature for TiN formation from
reaction 2TiCl4 + 4NH3 → 2TiN + 8HCl + N2 + 2H2 [16].
Fig. 2 The growth rate of TiN films on SiO 2 /Si substrates as a function of deposition
temperatures.
Fig. 3 The SEM images of TiN films grown on Si at each indicated deposition
temperature.
Fig. 4 The XRD spectra of TiN films grown on Si at each indicate d deposition
temperature.
Fig. 5 The cross-sectional micrographs of TiN film grown on Si at deposition
temperature of 500 oC by TEM with (a) bright field and (b) dark field.
Fig. 6 The surface roughness of TiN films grown on Si as a function of deposition
temperatures.
Fig. 7 The resistivity of TiN films grown on SiO 2 /Si as a function of deposition
temperatures. Insert shows the resistivity on a log scale as a function of 1/T.
Fig. 8 The AES depth profiles of TiN films grown on Si at deposition temperatures of
o
o
300 C and 350 C.
Fig. 9 Sheet resistance change of Cu/TiN/Si samples with TiN films deposited at 300 oC,
350 oC, 400 oC, 450 oC, and 500 oC as a function of post-annealing temperatures.
Fig. 10 The SEM images of Cu/TiN/Si samples with TiN films deposited at 300 oC (a1-a3),
350 oC (b1-b3), 400 oC (c1-c4), 450 oC (d1-d5), and 500 oC (e1-e5) after 1h
post-annealing at various annealing temperatures.
Fig. 11 The enlarged SEM images of Fig. 10a1 (a) and Fig. 10e4 (b).
o
o
Fig. 12 The XRD patterns of Cu/TiN(300 C)/Si (a) and Cu/TiN(500 C)/Si (b) samples after
1h post-annealing at each indicated temperature.
Reaction Free Energy (kJ/mole)
Fig. 1
150
2TiCl4 + 4NH3 = 2TiN + 8HCl + N2 + 2H2
100
50
0
-50
-100
-150
400
450
500
550
600
Reaction Temp. (K)
650
700
Fig. 2
Growth Rate (nm/cycle)
0.06
0.04
0.02
0.00
300
350
400
450
o
Deposition Temp. ( C)
500
Fig. 3
Fig. 4
TiN(200)
TiN(111)
TiN(220)
o
Intensity (a. u.)
500 C
TiN(311)
o
450 C
o
400 C
o
350 C
o
300 C
30
35
40
45
50
55
60
2 (deg.)
65
70
75
80
Fig. 5
Fig. 6
1.4
Roughness, Rms (nm)
1.2
1.0
0.82
0.8
0.71
0.70
350
400
0.68
0.6
0.4
0.49
0.2
0.0
300
450
o
Deposition Temp. ( C )
500
Fig. 7
Fig. 8
1200000
(a)
o
TiN(300 C)/p-Si
Counts/eV/Sec
1000000
N+Ti
800000
600000
Si
400000
Ti
200000
Cl
O
C
0
0
100
200
300
400
500
600
Etch Time (s)
1200000
(b)
o
TiN(350 C)/p-Si
Counts/eV/Sec
1000000
800000
N+Ti
600000
Si
400000
Ti
200000
O
C
0
0
100
200
300
400
Etch Time (s)
500
600
Fig. 9
0.30
Sheet Resistance ( /sq)
0.28
o
Cu/TiN(300 C)/Si
o
Cu/TiN(350 C)/Si
o
Cu/TiN(400 C)/Si
o
Cu/TiN(450 C)/Si
o
Cu/TiN(500 C)/Si
0.26
0.24
0.22
0.20
0.18
0.16
as-deposited
0.14
0.12
0.10
300
400
500
600
o
Anneal Temp. ( C)
700
800
Fig. 10
Fig. 11
(a)
2 m
(b)
5 m
Fig. 12
(a)
o
Cu/TiN(300 C)/Si
Cu(111)
Cu(220)
Intensity (a. u.)
Cu3Si
Cu(200)
TiN(111)
o
700 C
o
650 C
o
600 C
30
40
50
60
70
80
2 (deg.)
Cu(111)
(b)
o
Cu/TiN(500 C)/Si
Intensity (a. u.)
Cu(200)
Cu(220)
TiN(111)
o
800 C
o
750 C
o
700 C
o
650 C
30
40
50
60
2 (deg.)
70
80