Correlation of Surface and Film Chemistry with
Mechanical Properties in Interconnects
Ying Zhou*§, Guanghai Xu§, Tracey Scherban§, Jihperng Leu\ Grant Kloster^
and Chih-I
^Materials Technology Laboratories, Logic Technology Development, Intel Corporation,
Hillsboro, OR 97124, USA
^Components Research, Logic Technology Development, Intel Corporation,
Hillsboro, OR 97124, USA
Abstract. As the industry moves towards using low k dielectrics in advanced interconnects, interfacial adhesion and film
mechanical properties play increasingly important roles in yield and reliability. Improving these mechanical properties
requires a better understanding of the surface, interface and film chemistry. Time-of-Flight Secondary Ion Mass
Spectrometry (TOF-SIMS) was used to examine the surface molecular structure of organosilicate and organic polymer
based low k interlayer dielectrics (ILDs). Strong correlations were established between surface group functionality,
reactivity of the TiN precursors with the ILDs during Atomic Layer Deposition (ALD), and adhesion between the copper
barrier and the ILDs. Hydrogen plasma treatment significantly changed the surface chemical structure of the polymer ILD
through hydrogenation. Correspondingly, degradation of barrier/polymer adhesion and thermal stability was observed. For
the organosilicate ILD, the modulation of film chemistry, primarily the siloxane structures, corresponds with modulus,
hardness, cohesive strength and dielectric constant change. The mechanisms of such a correlation will be discussed.
INTRODUCTION
Integrating low k dielectric materials in advanced
interconnects poses unprecedented challenges due to
significant differences in film material properties
between SiO2 and low k dielectrics. To provide value
added information for the materials integration, there
is an increasing need to enhance collaboration
between
different
disciplines
of
materials
characterization.
For example, the film bulk
chemistry has a significar< impact on dielectric
constant, modulus, hardness and cohesive strength.
Additionally, the surface chemistry influences the
surface wetting properties, interface adhesion, and
surface reactivity. A molecular-level understanding of
low k dielectrics is required to enable the integration
of these materials with the complex interactions found
in semiconductor manufacturing processes.
Time of Flight Secondary Ion Mass Spectrometry
(TOF-SIMS) [1] is a promising technique for the
characterization of interconnects.
This technique
applies a sub-nanosecond pulse of ions bombarding
the surface, ejecting ions from the surface of the
material of interest. TOF-SIMS is one of the most
surface sensitive analysis techniques, because the
majority of the ions are generated from the top 510A. Due to the very low dose of incoming ions used
in "static" mode, molecular fragments can be detected.
The combination of surface sensitivity and molecular
specificity provides powerful insight to the surface
functional groups. By adding a second ion beam with
substantially higher current for depth profiling, TOFSIMS also can provide the elemental distribution
gradient in the film. The depth profile complements
bulk chemical analysis techniques such as FTIR and
solid state NMR.
In this work, we will discuss the impact of film and
surface chemistry on atomic layer deposition (ALD)
nucleation activity, barrier/ILD interfacial adhesion,
and ILD mechanical properties. An organosilicate and
a polymer are discussed as examples of two distinctly
different ILD materials.
The results clearly
demonstrate the need for a greater understanding of
material surface and film chemistries as the
complexity of interconnects continues to increase.
EXPERIMENTAL
Time of Secondary Ion mass spectra were collected
on a Cameca ION-TOF 300mm spectrometer. The
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
455
Due to the lack of polar functional groups like SiOH
on the surface, the organosilicate film is hydrophobic.
When the surface was exposed to ALD TiN precursor
analysis ion beam was a 25keV Ga beam with a
600ps pulse width @ IpA current. The mass
resolution was > 10,000 at 100 amu with a reflectron
type analyzer. The depth profile was accomplished
with a pulsed Cs ion beam at !KeV@10nA rastering
larger than analysis areas between the analysis cycles.
Temperature dependent TOF-SIMS was carried out in
a sample preparation chamber with a vacuum of IE-7
torr.
FTIR data were collected on a Nicolet Magna-IR 860
spectrometer (32 scans averaged, 4 cm"1 resolution),
using transmission mode. In order to reduce effects
due to film thickness variations, peak area ratios are
reported using the Si-O peak area as a denominator in
all cases.
The interfacial adhesion was obtained with a 4- point
bend method described elsewhere [2]. Hardness and
elastic modulus data were collected using an MTS
NanoXP nanoindenter with a Berkovich diamond tip.
Fifteen indents, spaced at 100 um, were made on each
sample. Samples were loaded in continuous stiffness
mode. The hardness and modulus were calculated
using a contact depth range of 200-400 nm for
hardness and 50-150 nm for modulus to avoid
substrate interactions. A Poisson's ratio of 0.25 was
assumed.
10
Si
4.0
3.5
-
3.0
-
2.5
-
2.0
-
1.5
-
1.0
-
.
SiCH3
OSiCH3
0.5
0.0
-
,
TTTTTTtJ
I,
,,l
30
Si(CH3)2
.
i . ....... . I M H l l . M . M . h . l l . I.... | '
40
60
50
70
Mass
Figure 1. A positive ion mass spectrum of the CVD
organosilicate in the low mass range.
10
10
RESULTS AND DISCUSSION
10
(1) Surface Chemistry on Atomic Layer
Deposition (ALD) Activity
100
200
400
300
500
600
Mass
While the adhesion of Cu barrier to ILD is controlled
by many parameters [3], the initial reactivity between
the barrier precursor and ILD during atomic layer
deposition can shed some light on the bonding
information of the metals to the ILD. We have
examined the ALD activity on two representative low
k ILDs: a CVD organosilicate and a polymer. Figure
1 shows a positive ion mass spectrum of the CVD
organosilicate. The spectrum resembles that of
siloxanes [4]. The dominant fragments are Si and
SiCH3.
In addition, di- and tri-methyl silane
fragments Si(CH3)2 and Si(CH3)3 are presented as
well, indicating various bonding configuration of Si.
This was confirmed by solid state NMR. At higher
mass range (Figure 2), siloxane polymer fragments are
prominent.
In contrast to the spectrum of an oxide (Figure 3)
which shows dominant SiOH signal, the SiOH ion
intensity is substantially weaker than SiCH3 in the
spectrum of the CVD organosilicate (Figure 1),
indicating the majority of the Si on the surface is
terminated by CH3 with little oxide exposed.
456
Figure 2. A positive ion mass spectrum of the CVD
organosilicate in the high mass range.
x10 °
1.6
1.4
1 1.0
1 0.8
0.6
0.4
0.2
0.0
20
30
40
50
60
70
Mass
80
Figure 3. A positive ion mass spectrum of an oxide.
TiCl4/NH3, there was very little evidence of Ti
deposition. The Ti was below the detection limit of
XPS at about 0.1 at% after a number of deposition
cycles. Even for TOF-SIMS which has detection
For the polymer ILD, the positive ion mass spectrum
shows fragmentation patterns that can be summarized
with a formula CnHm where m/n ratio is substantially
less than 1 (Figure 6). For example, the dominant
peaks for fragments containing 4 carbon atoms are
C4H2 (50amu) and C4H3(51amu), rather than the
alkane peaks of C4H7(55amu) and C4H9(57amu). In
the higher mass range, polymer fragments with the
general formula CnH(1.3) dominate the spectrum. The
fragmentation pattern observed on the polymer ILD is
representative of aromatic polymers [4].
sensitivity of ppm for Ti, the Ti signal was relatively
weak as shown in Figure 4. In contrast, TiCl4/NH3
reacted readily on an oxide, generating 50A TiN with
the same number of deposition cycles, as is shown in
Figure 5. This difference can be summarized by the
following equations:
TiCl4
TiCl4
Si-OH
Si-CH3
-» ClxTi-O-Si + HC1
-> No reaction
(1)
(2)
Si-CH3 does not react with TiCl4 like SiOH, as C-H
bond is covalent and significantly more stable than
O-H bond in the SiOH.
2.0
x10 >
s
1.5
, 1.5
-
•- 1.0
-
0.5
-
-
£
(0
1
SiCHS
0.5
-
0.0
30
0.0
-rmrrf*
50
|hii|lllt|fl f|llVl|llll|llH|llll|MM|ll!lHlil|
30
20
40
40
50
60
70
Mass
80
Figure 4. A positive ion mass spectrum of "5C)A" ALD TiN
on the CVD organosilicate.
1.4
1.2
X10°
J-0
1.4
-
1.2
-
Ti
| 0.8
:
0.4
TiO
_
-
0.2
-
ll
||
| ,I I
,,|l,l
20
0.6
-
NH4
0.4
0.0
-
£•1.0
fo.8
c
~ 0.6
,,,,,,
1
ill II
40
[• # :
60
0.2
0.0
80
100
120
140
160
180
200
Figure 6. A positive ion mass spectrum of the polymer ILD
in the low and high mass range.
80
Mass
Some siloxane related fragments were observed in
the TOF-SIMS spectrum as well (Figure 6). However,
XPS detected less than lat% Si on the surface.
Therefore, these species will not have measurable
impact on the surface chemistry and will not be
discussed further.
Unlike the CVD organosilicate, a significant amount
of Ti nucleation on the polymer ILD film without any
surface modification was observed during ALD of
TiN (Figure 7).
In this case, the nucleation
Figure 5. A positive ion mass spectrum of "5C)A" ALD TiN
on an oxide.
Once the surface was treated with an O2 plasma
that converted the surface to an oxide with a spectrum
similar to figure 3, substantial Ti nucleation was
observed, resulting in a spectrum similar to Figure 5
and demonstrating surface SiOH groups are key to
nucleation of chloride based ALD TiN precursors on
Si based ILDs [5].
457
mechanism is different from the one depicted in
equation (1). Instead, the likely reaction pathway
between TiCl4 and
the polymer is through
coordination of d orbitals of Ti with the n electrons of
the aromatic rings. Coordination of d orbitals with n
electrons is a fairly common mechanism for forming
organometallics between transition metals and
aromatics. A textbook example is Cr(t|6-C6H6)2 in
which all six n electrons are shared with the metal(Cr)
[6].
point bend measured that the adhesion between a
CVD Ti based barrier and the polymer was in the
range of 40 J/m2 (Figure 8), substantially higher than
that on SiO2. The film remained intact through
various torture tests including thermal anneal and
CMP. However, the barrier on H2 plasma pretreated
polymer showed measurable adhesion degradation,
reducing adhesion strength from 40 J/m2 to 20 J/m2
(Figure 8).
The mechanism of this adhesion degradation can be
explained by the chemical structure change induced
by H2 plasma as shown in Figure 9. For various
fragment lengths, the dominant peaks shifted from
CnH(i_3) to CnH(3_7) upon H2 plasma treatment. This
shift indicates increased H content in the hydrocarbon.
6.0
*5-°
c 4.0
x10
c
~ 3.0
1.2
2.0
1.0
0.0
20
n,.^..,4, ,.1,.,;
40
60
1.0
-
-
I0'8
-
-
Io6
80
Mass
0.4
Figure 7. A positive ion mass spectrum of ALD TiN on the
polymer ILD.
j
SiOH
"l
30
(2) Adhesion between barrier and polymer
ILD
C4l 17
SJCH3
-
0.2
0.0
,l
1
1
C5H7
C6H5 -
j-1--ri-rr llll|llfl|llll + > l ( h l l j t l l l |
4C
50
60
70
80
mass
Figure 9. A positive ion mass spectrum of H2 plasma
treated polymer ILD.
The examples discussed in section (1) illustrate that
surface chemistry plays a key role in initial deposition
and bonding of the barrier to ILDs. The bonding
strength between barrier and the ILD is also directly
H2 reduction
C3H5
Figure 10. Reaction pathways of aromatics during
hydrogenation.
None
There are many possible hydrogenation pathways,
such as hydrogenating aromatics into cyclic
alkanes/alkenes,
opening
rings
into
linear
alkanes/alkenes, and fragmenting longer carbon chains
(hydrogenolysis) as depicted in figure 10. While it is
not possible to separate these reaction pathways
unambiguously from the mass spectra, the data
conclusively show that H2 plasma reduced the surface
aromatic structures.
Preclean type
Figure 8. Adhesion between Cu barrier and the polymer
ILD.
reflected in the adhesion strength. The model of
interaction between Ti and the aromatic polymer
appears to be applicable to CVD barriers as well. 4-
458
The change of film chemistry has several potential
consequences. First, the reactivity between the metals
and alkanes is substantially lower. We can use ALD
of TiN on the CVD organosilicate as an example. The
CH3 in Si-CH3 is fairly stable in terms of reactivity
toward metals, similar to that of CH3 in alkanes CH3(CH2)n-CH3. Elevated reaction temperature or plasma
condition would be needed to initiate the bonding
formation. Second, the transition metal bonding with
the aromatic ring n electrons that existed on untreated
polymer is changed to sigma bonding with alkanes,
reducing the coordination number of C to Ti. In
addition, hydrogenation could reduce crosslinking in
the polymer film by breaking some of the bonds
(hydrogenolysis), thereby degrading cohesive strength
of the ILD in the modified region. The combination
of all these effects might explain the degraded
adhesion between the barrier and the polymer upon H2
plasma treatment.
The adhesion was degraded even more drastically at
elevated temperatures on H2 plasma treated polymer.
While the barrier/ILD interface remained intact after a
400C anneal on polymer films that were not exposed
to plasma, delamination was observed on H2 plasma
treated films. Temperature dependent TOF-SIMS
(Figure 11) clearly
detected that
the ratio
thermal stability of the treated film. Further support
for this model is found in the fact that delamination
was reduced or eliminated by annealing the H2 plasma
treated film to 200-400C prior to barrier deposition.
(3)The correlation between film chemistry
of the organosilicates and mechanical
properties
Generally, the parameters impacting dielectric
constant and mechanical strength not only include
chemical structure, but also porosity. Using the CVD
organosilicate as an example, we have examined the
relationship between the film chemistry and
mechanical properties as well dielectric constant.
4 -
o
X
15
25
35
45
3
Si-CH3/Si-0(x10 )
Figure 12. Correlation between hardness and SiCH3 in the
CVD organosilicate.
40
30
s.
O
20
UJ
H2 plasma
H2 plasma+
300C
None
10
Figure 11. Impact of thermal anneal on H2 plasma treated
polymer.
15
25
35
45
3
Si-CH 3 /Si-O (x10 )
between aromatics and alkanes/alkene increased
significantly upon annealing at 300C, moving toward
the structure of an untreated film. The increased ratio
indicates decomposition and desorption of the
hydrogenated species, transforming the surface back
to the original film structure. The reduced
crosslinking, molecular weight, and bond strength
between the C atoms likely account for the poorer
Figure 13. Correlation between elastic modulus and SiCH3
in the CVD organosilicate.
Figures 12 and 13 show that the higher level of
SiCH3 measured in FTIR directly correlates with
lower modulus and hardness. Since the modulus
459
When the organosilicate is subjected to various
chemical or beam treatments, chemistry changes of
the film can be detected with TOF-SIMS depth
profiling. Figure 15 shows a depth profile of the
organosilicate post e-beam treatment with varying
energy. C depletion through e-beam induced
decomposition of Si-CH3 structure was observed. The
depth of the C depletion strongly depended on the
beam energy. Consistent with the mechanism
discussed above, modulus and hardness increased as
the C reduced in the film (Table 1). However, the
dielectric constant generally increases as well.
Therefore, the challenge for low k organosilicate lies
in improving mechanical properties without degrading
the dielectric constant significantly.
correlates linearly with cohesive strength for the CVD
organosilicate [7], a higher SiCH3 concentration
correspondingly correlates to a lower cohesive
strength. This is because that Si-CH3 represents a
terminal group as opposed to a bridging Si-O group.
Therefore it does not contribute to the overall bond
strength or cohesive strength. Replacement of Si-CH3
by Si-O increases the crosslinking between Si atoms
as an O atom connects between Si atoms, thus
increasing the mechanical strength and film density as
expected. However, the Si-O bond is more polar and
also prone to moisture absorption, contributing to k
increase at lower SiCH3/SiO ratios as shown in Figure
14.
Table 1. The impact of E-beam treatment on elastic
modulus and hardness of the CVD organosilicate
3.8
3.4 -I
T r e a t m en t
C o n d i t i o ns
3.0
2.6
15
25
35
45
Figure 14. Correlation between dielectric constant k and
SiCH3 in the CVD organosilicate.
100
150
Depth
U V-treated
8.5 ± 0.9
1.7 ± 0.1
22.8 ± 0.4
3.1 ± 0.1
12.8 ± 0.4
1.9 ± 0.1
e-beam
treated
8 keV
e-bea m
treated
3 keV
Si-CHj/Si-O(x1oP)
50
H ardness
(GPa)
U ntreated
Elastic
M od u l u s
(GPa)
9.9 ± 0.1
1.8 ± 0.1
CONCLUSIONS
We have demonstrated that the reactivity
of
TiCVNH3 with the ILDs strongly depends on the
surface chemistry during ALD of TiN. Ti does not
nucleate on the organosilicate with Si-CH3 terminated
surface unless it is converted to oxide. The reaction of
TiCl4 with aromatic polymers has a distinctly different
reaction pathway, possibly through coordination of Ti
with the 71 electrons in aromatics. H2 plasma
converted the aromatic structure into alkanes/alkenes,
leading to degraded adhesion between barrier and
polymer ILD and thermal stability. The mechanical
properties of the organosilicate such as modulus,
hardness and cohesive strength were found to strongly
depend on the film chemistry. Converting SiCH3 into
oxides through various chemical or energetic beam
treatment improves the mechanical strength generally
at the expense of increasing dielectric constant.
These results demonstrate
the criticality of
understanding interface and film chemistry during
integration of low k ILDs in advanced interconnects.
200
<nm>
Figure 15. C depth profile for an UV and E-beam treated
CVD organosilicate
460
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