Characterization of Barrier Layer Phase and Morphology
As a Function of Differing Dielectric Substrate Conditions
by AFM and Grazing Angle XRD
Charles C. Wang, Gigi Lai, C. R. Brundle and Yuri Uritsky
Defect and Thin Films Characterization Laboratory, Applied Materials, Inc., Santa Clara, CA 95054, USA
Abstract. Tantalum (Ta) metal has emerged as one of the leading materials of choice for diffusion barrier applications in
Cu-damascene interconnects. For successful implementation, the microstructure, the electrical property and the surface
roughness of the barrier layer have to be controlled. To satisfy such needs, atomic force microscopy (AFM) and X-ray
diffraction (XRD) techniques have been employed to monitor the surface roughness and crystal phase of the PVD Ta thin
film deposited on low-£ dielectrics. XRD results show that the population of the two different Ta crystal phases, a and p,
depends on substrate material, surface pretreatment prior to thin film deposition and deposition conditions. AFM results
reveal that surface roughness of the thin barrier layer is mainly determined by the starting substrate surface roughness. Also,
AFM surface imaging discovered that each Ta crystal phase possesses a unique surface morphology. Therefore, the
population of the Ta crystal phases can be crudely quantified by performing image analysis on the AFM images. The Ta
phase population obtained from AFM images correlates very well with the XRD results.
surface conditioning to the surface roughness and the
crystal phase of Ta barrier layer. An important
discovery was that AFM surface imaging could also
detect the Ta crystal phases and that the AFM
determined crystal phase information correlated very
well with the results of XRD phase analysis.
INTRODUCTION
Due to their chemical and mechanical stability, Ta
metal and its compounds have become the material of
choice for diffusion barrier applications in 1C Cuinterconnects [1], However, the desirable diffusion
barrier properties of such materials depend on their
crystal structure, chemical composition as well as
surface roughness [1, 2]. Therefore the development
of metrology techniques for characterizing such
properties is critical for successful process
development and quality control.
EXPERIMENTAL
Ta can exist in two different phases, a (bcc), and P
(tetragonal). The a-phase is preferable because its
resistivity is approximately one tenth of that of the Pphase [1], and in addition study also had shown that
the a-phase could promote the growth of the preferred
(111) orientation in subsequent ECP Cu deposition [2].
When Ta is deposited on Si or SiO2 directly using
PVD process, it will form the less desirable p-phase
unless additional process steps are introduced [2, 3].
On the other hand, it was recently reported that the
introduction of low-& dielectrics has enabled the
formation of a-phase Ta directly on the dielectrics
surfaces [2]. The study reported here concerns mainly
the crystal phase determination by XRD and the
surface roughness measurement by AFM of the Ta
barrier layer deposited by PVD on several dielectric
materials.
The emphasis was on studying the
relationship of substrate bias during PVD deposition,
types of low-A; dielectrics and differing dielectrics
Table 1 summarizes the experimental conditions
used to prepare the Ta barrier samples. 25nm Ta thin
films were deposited on three different types of low-/:
dielectrics that were subjected to either no surface
pretreatment or one of the two preclean processes prior
to Ta deposition. Due to commercial considerations,
no detail description of the three low-/: dielectrics is
given here. The preclean process PT1 used pure Ar
plasma while the other preclean process PT2 used Ar
plasma together with a florinated gas. The substrate
bias during PVD deposition is expressed in a relative
scale based on the measurement of power to the
substrate. Sample #13 is a 25nm thick layer of Ta on
SiO2 and samples #14 and #15 are TaN/Ta
(5nm/20nm) bilayers on SiC>2. These three samples
are included in the study for baseline purposes. It was
pointed out in the introduction that, when Ta is
deposited directly on SiO2, p-phase Ta forms
spontaneously [2]. It has also been shown in the
literature that as Ta is deposited on a TaN surface, the
low-resistivity a-phase Ta is formed [1]. Since the
current study is concerned mainly with the crystal
phase and surface roughness of Ta on low-&
CP683, Characterization and Metrology for ULSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
© 2003 American Institute of Physics 0-7354-0152-7/03/$20.00
519
dielectrics, the data obtained from Ta films on SiO2
and TaN covered SiO2 will serve as references to
facilitate the interpretation of data from Ta films on
low-A: dielectrics.
geometry was adopted with the X-ray incident angle
(00 angle) relative to the sample surface fixed at 0.5°.
A parabolic mirror was used to collimate the incident
X-ray beam into a narrow parallel beam, while on the
detector side a parallel-plate collimator was used. A
Veeco Instruments Dimension 7000 AFM operated in
tapping mode was used for roughness measurement
and surface morphology imaging.
The XRD crystal phase analysis was carried out
using a Philips MRD XRD system. To enhance
sensitivity to the thin film layer, a glancing incident
TABLE 1. Summary of the Preparation Conditions of the PVD Ta Barrier Samples
Sample #
Dielectrics Surface Conditioning*
Type of Dielectrics
PTO
1
Low-A: 1
PTO
2
A: =3.0
PTl
3
PT2
4
PTO
5
Low-/: 2
PTO
6
k = 2.6
PTl
7
PT2
8
PTO
9
Low-/: 3
PTO
10
A: = 2.2
11
PTl
PT2
12
AS
deposited
13
SiO2
With a thin TaN layer
14
With a thin TaN layer
15
Substrate Bias
IX
OX
IX
IX
IX
ox
IX
IX
IX
ox
IX
IX
2X
2X
OX
PTO - No preclean treatment, PTl - preclean type I (use pure Ar plasma), PT2 - preclean type II (use Ar plasma
with a fluorinated gas).
we believe that Ta in the Ta/TaN bi-layer structures
studied here forms pure a-Ta phase. Because the
(3-Ta phase also has a strong diffraction peak (202)
occurring at 38°, it may be argued that some (J-Ta
phase may also be present. Nevertheless, judging by
the fact that no other strong (3-Ta diffraction peaks
were present, none of the TaN/Ta bilayers contained
significant quantity of (i-Ta.
RESULTS AND DISCUSSION
The two Ta/TaN bilayer samples prepared with
two different substrate bias conditions had very
similar XRD spectra. As shown in Fig. 1, both
spectra had four distinct diffraction peaks occurring
at the same 20 angles, 38°, 55°, 69° and 82°,
respectively. Only very small variations in peak
positions, relative peak intensities between the two
spectra were observed which may have been induced
by minor change in crystal orientations or textures
between the two films. Since the TaN under-layer is
considerably thinner than the Ta layer and it has very
weak diffraction peaks according to our previous
studies, all the XRD features should be contributed
by the Ta layer only. For comparison, the powder
diffraction spectrum of a-Ta phase obtained from
JCPDS-ICDD card file 04-0788 is plotted in discrete
squares in Fig. 1. It can be seen that the spectra from
both TaN/Ta bilayers match the reference a-Ta
phase spectrum closely in both relative intensities
and peak positions. Such close matches indicate that
the Ta films on both bilayer structures are a-Ta
phase with close to random grain orientations. The
finding agrees with what was reported in literature
that when Ta was deposited on TaN, the spontaneous
formation of pure a-phase occurred [1]. Therefore
30000-
(110)
ICDD 04-0788
• Experimental Data
20000
10000
FIGURE 1. XRD spectra of T/TaN bi-layer films on
SiO2. The peaks are labeled with the a-Ta phase
reflections and the spectra are offset from each other for
easy viewing. For reference, the powder diffraction
spectra of a-phase Ta obtained from JCPDS-ICDD card
files 04-0788 are plotted in discrete square dots.
520
The XRD spectrum of the single Ta layer on SiO2
showed entirely different characteristics from those
of the Ta/TaN bi-layers. See Fig. 2. Two broad
structures, each of which was composed of many
sub-peaks and shoulder-like structures, were
observed close to the 20 angles 38° and 65°,
respectively. According to literature, when Ta film
is deposited on SiO2, P~Ta forms spontaneously [2],
therefore, for comparison purpose the powder
diffraction spectra of P-phase Ta obtained from
JCPDS-ICDD card files 19-1290 and 25-1280 are
plotted in discrete square and triangular dots in Fig. 2
[5]. The intensities of the reference spectra are
scaled to the relative intensities tabulated in the card
files. It can
broad peak structures. In the case of the 25nm thick
Ta film, according to the results obtained using
Scherrer equation [5], the broadening of the X-ray
peaks should be at least 0.7° (in terms of 29 FWHM)
for the diffraction peaks appearing at 26 angles
larger than 38°. The thin film broadening effect can
be seen clearly from the TaN/Ta bilayer spectra
studied above, where the FWHM of even the
narrowest (110) peak is 1.3°. Because the (110),
(200) and (220) diffraction peaks of ct-Ta also
appear at 38°, 69° and 82°, where the p-Ta has peaks
too, one might argue that the cc-Ta could also be
present in the single layer Ta on SiO2. However,
because the (200) reflection of the a-Ta, which still
has substantial intensity, is missing in the spectrum
of the single layer Ta on SiO2, we may safely
identify that this Ta film has only the P-phase Ta.
For later discussion purpose, we designate the XRD
spectra from Ta/TaN bi-layers on SiO2 as Type I, and
that from Ta single layer on SiO2 as Type II.
12000
8000
As for Ta barrier layers deposited on the low-fc
dielectrics, the XRD spectra of some of them
resemble the Type II spectrum, while the spectra of
most others seem to be a mixture of Type I and Type
II. Only the Ta film deposited with IX substrate bias
on the low-£ 2 surface which did not subjected to any
pre-clean treatment has a spectrum that resembles the
Type I spectrum. Fig. 3 shows all four spectra
obtained from Ta barrier layer deposited on low-£ 2
and illustrates the presence of all three cases
described above.
4000
20
30
40
50
60
70
2ThetaAngle(°)
FIGURE 2. XRD spectra of Ta film on SiO2 deposited
using 2x bias. For reference, the powder diffraction
spectra of p-phase Ta obtained from JCPDS-ICDD card
files 19-1290 and 25-1280 are plotted in discrete square
and triangular dots.
be seen from Fig. 2 that there are discrepancies
between the two reference spectra: 1) the peaks at
58° and 82° in 19-1290 are missing in 25-1280, while
the peaks at 41°, 44°, 77° and 80° in 25-1280 are not
present in 19-1290 and 2) for peaks occurring in both
reference spectra, the corresponding peaks do not
necessarily have the same intensities. As for the
spectrum of the Ta film on SiO2, it seems to be a
combination of the two reference spectra, because its
broad peak structures bear some similarity to the
contours outlined by the discrete peaks in the
reference spectra and also the spectrum contains all
the peaks in both reference spectra, even those
mentioned above that occur only in one of the
reference spectrum but not in both. Such match
suggests that the Ta film on SiO2 is indeed composed
mainly of the P~Ta. The reason that the spectrum
has broad peak structures is that there are many
diffraction peaks from p-Ta that appear within 1° to
2° of each other near the 20 angles, 38° and 65°, and
the peak broadening due to thin film effect caused
the crowded individual diffraction peaks merged into
30000
15000-
40
50
60
80
2Th«UAngto(°)
FIGURE 3. XRD spectra of Ta films on low-/: 2. The
spectra are offset from each other for easy viewing. The
bottom spectrum shows only Type I features, while the top
two spectra show only Type II features. Only the second
spectrum from the bottom possesses features characteristic
of both the Type I spectrum and the Type II spectrum.
Because the Type I spectrum and the Type II
spectrum represent a-Ta and P~Ta, respectively, it
521
fitting case the one produces the best fit were chosen.
The fitting results are summarized in Table 3 column
4. It will be shown later that the XRD phase
synthesis results correlated very well with the AFM
morphology image analysis results.
was interesting to see if the spectra that seemed to be
a mixtures of these two extremes can be synthesized
or curve fitted using Type I and Type II spectra as
templates. The purpose of this exercise was to
reveal the concentration of each Ta phases in the
spectra. To perform such synthesis, one of the Type
I spectra from the Ta/TaN bi-layer on SiO2 was
mixed with the Type II spectrum from the Ta single
layer on SiO2. The proportion of each type is
adjusted until a best visual fit was obtained. Because
the two Type I spectra from the Ta/TaN bi-layers
have minor difference in relative intensities, for each
Table 2 lists the AFM root-mean-square (rms)
roughness results of the Ta films on all the dielectrics
here. It is evident that the roughness of the finished
film stack is mainly influenced by the roughness of
the surfaces of the as received starting dielectric
materials.
TABLE 2. rms Roughness of Ta Films on different Dielectrics (unit in nm).
Low-* 2
LOW-& 1
Deposition conditions
0.74
0.81
Nobias-PTO
IXbias-PTO
0.65
0.79
0.74
0.53
IXbias-PTl
0.52
1.34
lXbias-PT2
2X bias - single Ta layer
2X bias - TaN/Ta bilayer
OX bias - TaN/Ta bilayer
0.92
Bare dielectrics
0.71
Low-* 3
0.89
1.28
1.51
1.91
0.87
SiO2
0.20
0.29
0.28
0.20
FIGURE 4. AFM images of Ta thin films on SiO2: a) TaN/Ta bilayer with 2X bias, b) Ta single layer with 2X bias. The
TaN/Ta bilayer with OX bias has the same surface morphology as the TaN/Ta bilayer with 2X bias; hence its image
is not shown here.
contain mainly circular shaped grains. According to
the XRD results discussed above that the Ta metal in
all the Ta/TaN by-layer stacks has only the a-phase,
while the single Ta metal layer on SiC>2 possesses only
the p-phase, it seems that the grain morphology of the
Ta films on different substrates is governed by the
crystal phase of the Ta film and the crystal phase is in
turn influenced by substrate surface chemistry.
Fig. 4 shows the AFM images of the Ta films
deposited on the dielectric SiO2. The two Ta films
with the TaN underlayer have very similar
morphology. Irregular shaped protruding grains are
seen together with lamella-kind of structures between
these grains (Fig. 4a). On the contrary, the AFM
image of the single layer Ta metal deposited on SiO2
(Fig. 4b) has a smoother appearance that seems to
522
On the surfaces of the Ta barrier films deposited
on the low-fc dielectrics, interesting fine features were
discovered by the AFM imaging. These consisted of
a mixture of tiny circular shaped particles (100's A
with no pre-clean treatment, the density of the claw
marks is higher on Ta films deposited with 1 X bias
than those deposited with no bias. Comparing cases
with pre-clean with cases without pre-clean, pre-clean
in general caused decrease in the density of claw
marks.
One striking finding revealed by the examination
of the claw mark features in the AFM images is that
their population seems to be correlated with the
population of ct-Ta phase in the films as determined
by the XRD spectrum synthesis method. For example,
AFM images of the Ta films deposited on the low-k 2
dielectric using the deposition conditions, IX bias/PTO
and IX bias/PTl show approximately 100% and 0%
claw marks respectively and XRD results tabulated in
Table 3 also show that the former contains 100%
cc-Ta, while the latter has none. In order to obtain
more quantitative comparison results between the two
measurement techniques, the population of the clawmark features was quantified by applying image
analysis method to the AFM images. Since AFM
images are very noisy, it was difficult to use an
automated procedure to separate the two features.
Therefore, the claw marks were manually outlined to
generate binary images for feature counting. The
image analysis results are listed in Table 3 column 3
FIGURE 5. AFM image of Ta film deposited with OX bias
on low-& 2 subjected to no pre-clean (PTO). A low pass filter
was applied to eliminate the substrate roughness contribution
for easier viewing of the fine features.
diameter) and short strokes of lines resembling claw
marks. See Fig. 5. For the cases of all the dielectrics
TABLE 3. Comparison of AFM Image Analysis Results with XRD Spectra Decomposition Results._________
Substrate
Process Conditions
Population of Claw
Population of Type I Spectrum in
Marks from AFM (%)_______XRD Spectra (%)
29.9
31.3
IX Bias /PTO
22.2
No
Bias
/PTO
17.3
Low-Jt 1
0.2
IX Bias/PTl
0.0
lXBias/PT2
0.0
0.0
100.0
IX Bias /PTO
100.0
36.4
No Bias /PTO
25.5
Low-* 2
IX Bias/PTl
0.2
0.0
lXBias/PT2
0.5
0.0
IX Bias /PTO
37.7
31.6
No Bias /PTO
0.3
0.0
Low-£3
IX Bias/PTl
0.0
0.0
lXBias/PT2
17.4
15.6
Indeed, the comparison of AFM image analysis
results with XRD spectra synthesis results in Table 3
reveals that the population of the claw-mark features
in the AFM images of a particular sample is
proportional to the population of the Type I spectrum
(the XRD spectrum characteristic of the pure a-phase
Ta) in the corresponding XRD spectrum of that
sample. To further test the correlation of the AFM
data with the XRD data, X-Y scatter plot using AFM
results as the X-coordinates and XRD results as the Ycoordinates was produced in Fig. 6. The data could be
fitted with a straight line with correlation coefficient to
be 0.98. Considering the difference between the AFM
sampling area (1 \im square) and the XRD sampling
area (a couple inches square), the correlation is very
good. Such high correlation indicates that the clawmark features should be the ct-Ta phases that are
responsible for the Type I XRD spectra and the
523
features with circular grains are the P~Ta phases that
generate the Type II spectra.
Q
100.0% !
X
|
50.0% f
Linear Fit
Experimental
Data
I
0.0%
0.0%
50.0%
100.0%
population of claw marks by AFM
FIGURE 6. Plot to show the correlation of Type I XRD
spectra population with the population of claw marks in
AFM images.
Therefore, we are certain that the claw-mark features
in the AFM images are due to the a-phase Ta
formation, whereas the rest circular-grained structures
are manifestations of pure p-phase. Our results also
confirm what were reported in the literature that, on
low-k dielectrics, higher substrate bias promoted
a-phase Ta formation and, on the other hand,
substrate surface modification by plasma preclean
promoted P-phase growth [2].
The reason that, on the low-k dielectrics, the ct-Ta
phase assumes a claw mark structure, while the
P-phase has a smooth circular grain structure can be
explained by the morphology of the two different
crystal structures detect on the dielectric, SiO2. It can
be seen that the claw mark structures on the low-k
dielectrics bear great resemblance to the lamella-kind
structures on the oc-Ta in the Ta/TaN bilayer deposited
on the SiO2, and that the P~Ta on the low-k dielectrics
also have the similar appearance as the circular
grained P-phase Ta in Ta single layer on SiO2. It is
apparent that the substrate surface chemistry
determines the crystal phase of the Ta film deposited
on it for the case of SiO2 and the same must also be
true for the low-k dielectrics. What is interesting for
the latter type of dielectrics is that both Ta crystal
phases can coexist on the same surface, indicating
local variations of surface chemistry in the
microscopic scale and such variations can be
manipulated by low-k types, preclean conditions and4.
possibly substrate bias conditions.
CONCLUSIONS
AFM and XRD have been successfully used to study
the surface characteristics and crystal phases of Ta
barrier films deposited on low-A: dielectrics using Ta
and Ta/TaN films of the same thickness on SiO2 as
references. We have determined, by AFM, the
roughness of the Ta surface for all the conditions used
and have shown that it is dominated by the initial
roughness of the dielectric surfaces. When Ta film
was deposited on the dielectric, SiO2, XRD only
detected ct-Ta in Ta/TaN bi-layers and P~Ta in single
layer Ta; AFM images showed the former had
irregular shaped grains together with lamella-kind of
structures, while the latter revealed smoother circular
shaped grains. For the case of Ta on the twelve low-k
dielectric samples, XRD still detected the two crystal
phases oc-Ta and P~Ta and, on some of the samples,
both phases were found to coexist. By spectra
synthesis method using the spectra of the pure
a-phase and the pure P-phase from SiO2, we can
crudely determine their ratios present for the different
conditions studied. They vary quite significantly with
processing conditions. There are fine structures in the
AFM images of Ta films on the low-A: substrates.
These consist of a mixture of tiny circular shaped
grains (<10 nm diameter) and short strokes of lines
resembling claw marks. Image analyses were able to
quantify the surface density of each type of
morphological feature. By correlating the XRD and
AFM results we show that the claw marks represent
the a- phase and the circular grains the P-phase.
Knowing this, the AFM technique can then be used to
characterize the Ta films for roughness and
microstructure at high spatial resolution.
REFERENCES
1. D. Edelstein, C. Uzoh, C. Cabral, P. DeHaven, P.
Buchwalter, A. Simon, E. Cooney, S. Malhotra, D.
Klaus, H. Rathore, B. Agarwala, D. Nguyen, Proc.IITC,
San Francisco: IEEE, 2001, pp. 9-11.
2. H. Donohue, H Gris, J. C. Yeoh, K. Buchanan,
ProcJITC, San Francisco: IEEE, 2002, pp. 179-181.
3. K. Min, K. Chun, K. Kirn, J. Vac. Sci. Technol B14 (5),
3263-3269(1996)
4. PCPDFWIN Version 2.2, June 2001, JCPDS International Center for Diffraction Data.
5. Cullity, B. D., Elements of X-ray Diffraction, Boston,
MA: Addison-Wesley, 1978, pp. 281-285.
524