A comparative study of the atomic-layer-deposited tungsten thin films as nucleation layers for W-plug deposition

Article abstract of DOI:10.1149/1.2222966

The properties of three different kinds of atomic-layer-deposited (ALD) W thin films were comparatively characterized and investigated as nucleation layers for the W-plug process of 70 nm design-rule dynamic random access memory. ALD-W (A) film was deposited using alternating exposures of WF6 and SiH ₄, and ALD-W (B) film was treated with B₂H₆ for 5 s prior to W ALD using WF₆ and SiH₄. Finally, ALD-W (C) film was deposited using alternating exposures of WF₆ and B ₂H₆. All the ALD-W films showed excellent step coverage at the contact with an aspect ratio of ～14, but their resistivities were as high as 125-145 μΩ cm at the thickness of 20 nm. High resistivities of ALD-W films are discussed on the basis of impurities cooperation such as Si and B, phase (body-centered-cubic α-W or primitive cubic β-W), crystallinity (crystalline or amorphous), and grain size. It was found that ALD-W (C) film formed an amorphous phase, which was stable until 900°C annealing. This is clearly different from ALD-W (A) and ALD-W (B) with polycrystalline grains of α-W and β-W, and β-W was transformed to α-W after 800°C annealing. The formation of amorphous W resulted in the formation of large-size grains of chemical-vapor-deposited W film deposited on ALD-W (C) and the reduction in the resistivity of W-plug stack. The integration results showed that the reduced resistivity of W-plug stack with ALD-W (C) provided a significantly lower resistance at the W bit line contact. Another advantage of the integration scheme with ALD-W (C) was its stable contact resistance at the ultrahigh aspect ratio (UHAR) contact even though the step coverage of the underlayer, TiN, was poor. It was also found that the B₂H₆ pretreatment was effective for obtaining the low and stable contact resistance at UHAR contact.

Full text of DOI:10.1149/1.2222966

Journal of The Electrochemical Society, 153 ͑10͒ G887-G893 ͑2006͒
G887
0
013-4651/2006/153͑10͒/G887/7/$20.00 © The Electrochemical Society
A Comparative Study of the Atomic-Layer-Deposited Tungsten
Thin Films as Nucleation Layers for W-Plug Deposition
a,
a
a
b,z
Soo-Hyun Kim, * Nohjung Kwak, Jinwoong Kim, and Hyunchul Sohn
a
Hynix Semiconductor, Incorporated, Research and Development Division, Ami-ri, Bubal-eub, Icheon-si,
Kyoungki-do 467-701, Korea
b
Department of Ceramic Engineering, Yonsei University, Seodaemun-gu, Seoul 120-749, Korea
The properties of three different kinds of atomic-layer-deposited ͑ALD͒ W thin films were comparatively characterized and
investigated as nucleation layers for the W-plug process of 70 nm design-rule dynamic random access memory. ALD-W ͑A͒ film
was deposited using alternating exposures of WF and SiH and ALD-W ͑B͒ film was treated with B H for 5 s prior to W ALD
6
4
2
6
using WF and SiH . Finally, ALD-W ͑C͒ film was deposited using alternating exposures of WF and B H . All the ALD-W films
6
4
6
2
6
showed excellent step coverage at the contact with an aspect ratio of ϳ14, but their resistivities were as high as 125–145 ␮⍀ cm
at the thickness of 20 nm. High resistivities of ALD-W films are discussed on the basis of impurities cooperation such as Si and
B, phase ͑body-centered-cubic ␣-W or primitive cubic ␤-W͒, crystallinity ͑crystalline or amorphous͒, and grain size. It was found
that ALD-W ͑C͒ film formed an amorphous phase, which was stable until 900°C annealing. This is clearly different from ALD-W
͑
A͒ and ALD-W ͑B͒ with polycrystalline grains of ␣-W and ␤-W, and ␤-W was transformed to ␣-W after 800°C annealing. The
formation of amorphous W resulted in the formation of large-size grains of chemical-vapor-deposited W film deposited on ALD-W
C͒ and the reduction in the resistivity of W-plug stack. The integration results showed that the reduced resistivity of W-plug stack
͑
with ALD-W ͑C͒ provided a significantly lower resistance at the W bit line contact. Another advantage of the integration scheme
with ALD-W ͑C͒ was its stable contact resistance at the ultrahigh aspect ratio ͑UHAR͒ contact even though the step coverage of
the underlayer, TiN, was poor. It was also found that the B H pretreatment was effective for obtaining the low and stable contact
2
6
resistance at UHAR contact.
2006 The Electrochemical Society. ͓DOI: 10.1149/1.2222966͔ All rights reserved.
©
Manuscript submitted March 20, 2006; revised manuscript received May 22, 2006. Available electronically July 26, 2006.
11
Atomic layer deposition ͑ALD͒ is one of the most promising
reported. The results showed that the B H pretreatment could
2 6
technologies related to preparing thin films for future generation
enhance the nucleation of ALD-W on SiO and also enhance the
step coverage of ALD-W films at UHAR contact.
2
1,2
microelectronic device fabrication. Several key features of ALD
include the capability of depositing ultrathin, highly uniform, and
high-purity film with near 100% step coverage in a complicated
geometry. This is desirable for the ever-shrinking device. ALD is
mainly applied in semiconductor devices, including Cu diffusion
barrier, high-permittivity ͑high-k͒ gate or capacitor dielectrics, and
metal gate or capacitor electrodes as well. One of the attractive
features about using ALD in microelectronic devices, which is the
main subject of this paper, is the nucleation layer deposition for
W-plug process at ultrahigh aspect ratio ͑UHAR͒ contact of ad-
vanced dynamic random access memory ͑DRAM͒ with a stacked
capacitor. The recent International Technology Roadmap for
While there has been much effort in developing ALD-W pro-
cesses, there have been very limited investigations on the electrical
properties at the UHAR contact of semiconductor devices. Thus, it
is still not easy to compare the electrical performances when the
various ALD-W films are integrated to the semiconductor device. To
evaluate the comparative performance of each of these films, there-
fore, it is necessary to test the same thickness of the film at the
contact with the same dimension. It is also critical to understand the
key film properties to affect the electrical performances at the
UHAR contact.
In this study, the properties of three kinds of ALD-W films were
comparatively characterized and tested as nucleation layers for
W-plug process of 70 nm design-rule DRAM. The systematic inves-
tigations on the film properties such as resistivity, composition, im-
purities, phase, and microstructure were carried out. In addition, the
properties of CVD-W films deposited on different ALD-W films
were also characterized. We found that the properties of CVD-W
films were much affected by the ALD-W nucleation layers, which in
turn has some effect on the electrical performance of each W-plug
scheme. As the potential applications of ALD-W films in microelec-
tronics also include the metal gate electrode, capacitor electrode for
memory device, or diffusion barrier for Cu metallization, further
studies need to be performed but are not discussed herein.
3
Semiconductors shows that a 16:1 aspect ratio ͑AR͒ contact needs
to be filled at 70 nm technology node DRAM contact and with 17:1
at 50 nm. Though a chemical-vapor-deposited ͑CVD͒ W film, gen-
erally deposited by the reaction of WF and SiH , has been success-
6
4
4
fully used as a nucleation layer, its limited conformality at UHAR
contact can induce potential problems such as the seam or void in
5
,6
final W-plug, leading to the degradation in contact resistance. In
this respect, the ALD-W process is expected to solve any potential
problems in connection with preparing ultrathin and conformal
nucleation layer for W-plug process.
W ALD is the most successful case among elemental metal ALD
processes. This is mainly due to the fact that W ALD is accom-
plished from the well-known good metal-containing precursor, WF6,
and good reducing agents, silane or borane compound. The success-
7
Experimental
ful deposition of ALD-W was first reported by Klaus et al. They
used a sequential supply of WF and disilane ͑Si H ͒ at the tem-
perature of lower than 325°C, which is low enough for semiconduc-
tor device fabrication. Later, it was reported that the ALD-W films
6
2
6
CVD-TiN film deposited using the reaction of TiCl and NH
4
3
at 580°C or periodically plasma-treated metallorganic CVD
MOCVD͒-TiN film deposited at 450°C using tetrakis-dimethyl-
amido-titanium ͕Ti͓N͑CH ͒ ͔ , TDMAT͖ as a precursor were used
͑
could be deposited by using diborane ͑B H ͒ as a reducing agent of
2
6
3
2 4
8
WF at 300°C. The ALD-W process using a very similar reducing
6
as substrates for W growth. The properties of ALD-W films to be
investigated in this study were almost same. Slight differences with
TiN films were observed in their roughness and texture, which are
generally dependent on the underlayer. Three different ALD W films
were prepared as nucleation layers for W-plug deposition using
commercial 200 mm chambers. Among them, two ALD-W films
agent to disilane, silane ͑SiH ͒, was also extensively investi-
4
9
-11
gated.
Recently, the effect of B H6 pretreatment on ALD-W
2
film deposited using an alternating supply of WF and SiH was
6
4
͓
ALD-W ͑A͒ and ALD-W ͑B͔͒ were deposited using a sequential
*
Electrochemical Society Active Member.
E-mail: hyunchul.sohn@yonsei.ac.kr
z
supply of WF and SiH as reactants and one ALD-W film ͓ALD-W
6
4
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Journal of The Electrochemical Society, 153 ͑10͒ G887-G893 ͑2006͒
͑
2
C͔͒ using WF and B H . The ALD-W ͑B͒ was pretreated with
6
2
6
11
00 sccm of B H for 5 s before ALD. The deposition temperature
2 6
for ALD-W films was 300°C.
The thicknesses of ALD-W films were determined from the
cross-sectional view transmission electron microscopy ͑XTEM͒
analysis. The film properties were analyzed with various tools and
compared with each other; sheet resistance by 4-point probe, surface
roughness by atomic force microscopy ͑AFM͒, composition and im-
purities content by X-ray photoelectron spectroscopy ͑XPS͒ or Au-
ger electron spectroscopy ͑AES͒ and secondary ion mass spectrom-
etry ͑SIMS͒, film crystallinity and phase by X-ray diffractometry
͑
XRD͒, and microstructure and phase by plan-view and cross-
sectional view TEM. The step coverage of ALD-W films were in-
vestigated at the contact hole ͑height ϳ3.52 ␮m and aspect ratio
ϳ15͒. Aspect ratio of the contact was determined by dividing the
contact height with its diameter at the top of the contact. Finally, the
thermal stability and phase transformation behavior were investi-
gated by sheet resistance measurement and XRD after high-
temperature annealing. The annealing was performed for 30 min at
the processing pressure of 5 ϫ 10−5 Torr. In order to understand the
effect of ALD-W layers on the properties of CVD-W films growing
on them, CVD-W films deposited by H2 reduction of WF6 at
ϳ390°C were deposited in situ after ALD-W depositions. Their
properties were also analyzed by 4-point probe, AFM, AES, and
plan-view scanning electron microscopy ͑SEM͒.
Figure 1. The step coverages of ALD-W films at UHAR contact ͑AR ϳ15͒:
a͒ ALD-W ͑A͒, ͑b͒ ALD-W ͑B͒, and ͑c͒ ALD-W ͑C͒. 5 nm thick
In order to study the electrical properties when the ALD-W film
was used as a nucleation layer for W-plug process, Si wafers were
processed through a 70 nm design-rule DRAM process flow. Prior
͑
TiCl -based CVD-TiN as a barrier layer was deposited prior to ALD-W
4
deposition.
to ALD-W depositions, TiN films ͑TiCl -based CVD-TiN and peri-
4
odically plasma-treated MOCVD-TiN͒ were deposited as glue/
barrier layers. Then, ALD-W films were deposited and following
this, W-plug processes were performed in situ. Blanket W deposition
and etch-back were used to fill the contacts followed by metal line
deposition. Finally, metal line patterning was accomplished. The
contact resistance was measured using a contact chain structure con-
necting the W bit line and metal line with two different contact sizes
and aspect ratios.
SiH -based ALD-W films ͑Fig. 3a and b͒. It is thought that Si from
4
the decomposition of SiH at the SiH pulsing step is incorporated in
4
4
1
2
SiH -based ALD-W films. Herner et al. reported that a monolayer
4
Si was formed by SiH pretreatment at ϳ410°C used in CVD-W
4
plug process. So, a successive SiH pulsing step in ALD could be
4
the reason for Si incorporation. For comparison, the results of
CVD-W nucleation layer deposited using WF and SiH showed no
6
4
6
Si impurities. The XPS depth profile of B H -based ALD-W film
Results and Discussion
2
6
͑
Fig. 3c͒ showed a B content of ϳ5 atom%, which indicates that a
11
Properties of ALD-W films.— All the ALD-W films showed ex-
small amount of B H was thermally decomposed at 300°C and
incorporated into the film. Thus, the Si and B impurities can be one
of the reasons for the increase of the resistivities of ALD-W films
2 6
cellent step coverage irrespective of B H pretreatment and reducing
2
6
agents of WF at UHAR contact holes ͑AR ϳ15͒ as shown by the
XTEM images in Fig. 1. Here, prior to ALD-W depositions,
TiCl -based CVD-TiN films were deposited as barrier layers. The
excellent step coverage of the nucleation layers ensured excellent
6
6
,13
investigated in this study by impurity scattering.
Although no
4
F was detected in XPS analysis, in order to more sensitively detect
F impurities SIMS depth profiling was performed ͑Fig. 4͒. F con-
centration was much lower in B H -based ALD-W film ͑ϳ1.2
filling of W-plug. However, the B H6 pretreatment enhanced the
2
2
6
1
7
3
step coverage of ALD-W film deposited using WF and SiH , espe-
cially when the step coverage of underlayer TiN was very poor.
ϫ 10 /cm ͒ as compared to those of SiH -based ALD-W films
6
4
4
11
20
3
͑
1 ϳ 5.4 ϫ 10 /cm ͒. This indicates that B H pulsing is more
2 6
Resistivity of metal film is an important consideration because it
represents the overall films properties. Figure 2 shows the resistivi-
ties of ALD-W films as a function of film thickness. All the ALD-W
films showed high resistivities of 125–145 ␮⍀ cm at the thickness
of ϳ20 nm. For comparison, the resistivity of CVD-W nucleation
effective in reducing WF absorbed on the surface and making pure
x
metal W film. The CVD-W film deposited using WF and SiH also
6
4
6
layer with the thickness of ϳ18 nm was ϳ25 ␮⍀ cm. Interest-
ingly, as the film thicknesses decreased, the resistivities of ALD-W
͑
A͒ and ALD-W ͑B͒ films prepared from SiH reduction of WF
4 6
significantly increased but the increase in the resistivity of ALD-W
film ͑C͒ film prepared from B H reduction increased only slightly.
2
6
Thus, B H -based ALD-W film has an advantage from the point of
2
6
view of continuous thickness scaling of nucleation layer. Relatively
higher film resistivities of ALD-W films can be explained by con-
sidering impurity incorporation, film phase, crystallinity, and grain
size, which are further discussed later.
The composition and impurities of ALD-W films were analyzed
using XPS depth profiling ͑Fig. 3͒. It is noteworthy that the results
of the XPS analysis do not show that any impurities such as O and
F were detected in ALD-W films irrespective of deposition pro-
cesses. Approximately 5–6 atom % of Si impurity was detected in
Figure 2. The resistivities of ALD-W films as function of film thicknesses.
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Journal of The Electrochemical Society, 153 ͑10͒ G887-G893 ͑2006͒
G889
Figure 5. Glazing angle XRD spectra of ALD-W films. The incidence angle
of X-ray was ϳ1°.
showed no specific peak, indicating that the film was amorphous or
an amorphous-like structure, which is known to cause the film re-
sistivity to increase.
To further determine the phase and microstructure, the films
were analyzed using TEM. The plan-view TEM bright-field ͑BF͒
images of 20 nm thick ALD-W ͑A͒ film ͑Fig. 6a͒ showed that
the film formed a polycrystalline structure with the grain size rang-
ing from ϳ9–17 nm and not an amorphous one. Indexing of se-
lected area diffraction pattern ͑SADP͒ showed a clear ring pattern of
bcc ␣-W. Diffraction rings, which can be indexed as ␤-W ͑200͒
Figure 3. XPS depth profiles of ALD-W films.
20
3
showed F content of ϳ3.5 ϫ 10 /cm ͑not shown here͒ and this
indicates that F content in ALD-W films of this study is not a major
factor in the increase of their resistivities.
͑
d = 2.52 Å͒ and ␤-W ͑211͒ ͑d = 2.08 Å͒, respectively, were ob-
served close to ␣-W ͑110͒ ͑d = 2.238 Å͒. The XTEM image of
ALD-W ͑A͒ ͑Fig. 6b͒ showed a crystalline microstructure with a
relatively rough film surface. The TEM analysis of ALD-W ͑B͒
showed a very similar plan-view image ͑Fig. 7a͒ to that of ALD-W
Figure 5 shows the glazing angle XRD patterns of ALD-W films.
The XRD patterns of both SiH -based ALD-W films show peaks
4
1
4-16
from A15 primitive cubic ␤-W
or ␤-W ͑200͒ ͓2␪ = 35.56°͔ and
͑
A͒. The sharp diffraction pattern from ␤-W phase matches well
͑
211͒ ͓2␪ = 43.96°͔ apart from the peaks from A2 body-centered-
with the XRD results. However, the grain size slightly increased to
ϳ11–21 nm. Slightly larger grains of ALD-W ͑B͒ than those of
ALD-W ͑A͒ partly explain its relatively lower resistivity ͑see Fig.
cubic ͑bcc͒ ␣-W. This indicates that the films are mixtures of ␣-W
and ␤-W. Comparing the XRD peaks of ALD-W ͑A͒ and ALD-W
͑
B͒, we found that the B H pretreatment caused ␤-W phase to form
2 6
2
͒. Another reason why the ALD-W ͑B͒ showed a lower resistivity
more favorably. ␤-W is known as a metastable phase and is reported
in spite of having the relatively larger amount of ␤-W phase com-
pared with ALD-W ͑A͒ is its lower roughness. The root-mean-
square ͑rms͒ roughness at ϳ20 nm thickness was ϳ1 nm for
ALD-W ͑B͒ and ϳ2 nm for ALD-W ͑A͒, respectively. The XTEM
image of ALD-W ͑B͒ ͑Fig. 7b͒ showed the relatively lower surface
roughness as compared to ALD-W ͑A͒.
to have significantly higher resistivity of as high as
16
1
00–300 ␮⍀ cm than the bulk resistivity of thermodynamically
stable ␣-W ͑5.6 ␮⍀ cm͒. Thus, the formation of high-resistivity
-W phase in SiH -based ALD-W films can be one of the main
␤
4
reasons for the increase of the film resistivity, which is clearly dif-
ferent from the results of CVD-W nucleation layer deposited from
Figure 8a shows the plan-view TEM BF images of 20 nm thick
B H -based ALD-W film ͓ALD-W ͑C͔͒. Compared to Fig. 6a and
6
WF and SiH showing the formation of ␣-W single phase. Inter-
6
4
2
6
estingly, the XRD result of B H -based ALD-W film ͓ALD-W ͑C͔͒
2
6
7a, a plan-view TEM image seems to be featureless, indicating that
the film forms the amorphous structure. This was confirmed by the
corresponding faint SADP. Thus, in TEM analysis, the amorphous
nature of B H -based ALD-W film has a clear advantage in terms of
2
6
barrier performance to WF diffusion occurring at the subsequent
6
plug-fill step compared to SiH -based ALD-W films. The XTEM
4
Figure 6. ͑a͒ Plan-view TEM BF image and ͑b͒ cross-sectional view TEM
Figure 4. SIMS depth profile of ALD-W films showing the content of F
BF image of ALD-W ͑A͒ deposited on TiCl -based CVD-TiN. The inset in
4
impurity.
͑a͒ shows the selected area electron diffraction pattern.
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G890
Journal of The Electrochemical Society, 153 ͑10͒ G887-G893 ͑2006͒
Figure 7. ͑a͒ Plan-view TEM BF image and ͑b͒ cross-sectional view TEM
BF image of ALD-W ͑B͒ deposited on TiCl -based CVD-TiN. The inset in
4
͑
a͒ shows the selected area electron diffraction pattern.
image of 20 nm thick ALD-W ͑C͒ film ͑Fig. 8b͒ showed very
smooth surface topography as compared to those of ALD-W ͑A͒ and
ALD-W ͑B͒.
The thermal stability and phase transformation behavior of
ALD-W films were investigated because their potential applications
include the electrode for gate or capacitor in semiconductor device
or diffusion barrier for Cu metallization. Figure 9a shows the change
in sheet resistances of ALD-W/TiN/SiO stacks as a function of
2
Figure 9. ͑a͒ Sheet resistance change of ALD-W/TiN/SiO film stacks, ͑b͒
2
annealing temperature. In the case of ALD-W ͑A͒ and ALD-W ͑B͒,
when the annealing temperature increased to 800°C, the sheet resis-
tance decreased significantly. XRD analysis with annealing tempera-
ture ͑Fig. 9b and c͒ showed that the peak intensity from ␣-W ͑110͒
nearby 2␪ = 40° increased and the full width at half maximum
XRD analysis of ALD-W͑A͒/TiN/SiO2 film stack, ͑c͒ ALD-W͑B͒/
TiN/SiO film stack, and ͑d͒ ALD-W͑C͒/TiN/SiO film stack as a function
2
2
of annealing temperature.
͑
fwhm͒ became narrower after annealing at 800°C. This indicated
typically deposited as a barrier layer. Thus, the total W-plug stack is
generally composed of plug-fill CVD-W/W nucleation layer/TiN. As
it is well known that the underlayer film has some effect on the
that the grain size slightly increased after annealing, resulting in the
decrease in their sheet resistances. XRD analysis also showed that
the peaks from ␤-W phase disappeared after annealing at 800°C. As
the annealing temperature increased to 900°C, the peak intensities
from ␣-W increased, which matches well with the sheet resistance
change shown in Fig. 9a. Thus, the significant decrease in sheet
resistance of ALD-W ͑A͒ and ALD-W ͑B͒ after annealing at 800
and 900°C is mainly due to the phase transformation from ␤-W with
high resistivity to ␣-W with low resistivity, not the increase in the
grain size. However, the remarkable change in sheet resistance of
properties of the film growing on it in the many thin film
17-19
systems,
it is necessary to understand how the ALD-W under-
layers affect the growing CVD-W films on them. In this study, 5 nm
thick TiCl -based CVD-TiN film was used as the barrier layer and a
4
plug-fill CVD-W film was deposited at ϳ390°C using H reduction
2
of WF .
6
Figure 10 shows the resistivities of total W-plug stacks of CVD-
W/ALD-W/CVD-TiN. Their resistivities are very important because
they can directly affect the contact resistance obtained by W-plug
process. Here, as the resistivities of CVD-TiN and ALD-W films are
much higher ͑ϳ600 ␮⍀ cm for CVD-TiN and 125–150 ␮⍀ cm for
the ALD-W͑C͒/TiN/SiO stack was not observed when annealing
2
continued to 900°C. This indicates that the structural change of
ALD-W ͑C͒ film that includes an amorphous phase is negligible. In
fact, XRD analysis ͑Fig. 9d͒ confirms that ALD-W ͑C͒ film kept its
amorphous state until annealing at 900°C.
Effect of ALD-W nucleation layers on the properties of CVD-W
films.— The traditional CVD W-plug process is composed of two
4
steps: W nucleation and W-plug fill. Prior to W nucleation, TiN is
Figure 10. The resistivity of film stack of CVD-W/ALD-W/CVD-TiN with
the thickness of ALD-W film and CVD-W film. TiN film 5 nm thick was
deposited using the reaction of TiCl and NH at 580°C and CVD-W was
Figure 8. ͑a͒ Plan-view TEM BF image and ͑b͒ cross-sectional view TEM
BF image of ALD-W ͑C͒ deposited on TiCl -based CVD-TiN. The inset in
4
4
3
͑
a͒ shows the selected area electron diffraction pattern.
prepared using H reduction of WF at ϳ390°C.
2 6
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Journal of The Electrochemical Society, 153 ͑10͒ G887-G893 ͑2006͒
G891
Figure 12. The distribution of contact resistances with ALD-W films at
the contact chains A connecting Al metal line with W bit line. Here, the
aspect ration of contact chain A was ϳ15. As a barrier layer, TiCl -based
4
CVD-TiN film ͑closed symbol͒ or MOCVD-TiN film ͑open symbol͒ were
used.
6
,13,20
roughness.
In fact, the rms roughness of CVD-W films depos-
ited on both SiH -based ALD-W nucleation layers was much
4
smaller ͑10–12 nm͒ than one on B H -based ALD-W nucleation
2
6
layer ͑ϳ20 nm͒ when the thickness of plug-fill CVD-W was
2
00 nm. Thus, much lower resistivity of film stack with B H -based
2
6
ALD-W film suggests that the microstructure characterized by the
grain size is the dominant factor that determines the film resistivity.
Figure 11 shows the plan-view SEM images of CVD-W films
͑
200 nm͒ deposited on different ALD-W films. It clearly showed
that CVD-W film deposited on ALD-W ͑C͒ has much larger-sized
grains than the ones deposited on both ALD-W ͑A͒ and ALD-W ͑B͒,
which contributes to the lower resistivity of the film stack with
ALD-W ͑C͒ as a nucleation layer.
Comparison of electrical performances of ALD-W films as nucle-
ation layers for W-plug process of DRAM.— In this section, the
contact resistances of integration schemes with ALD-W films as
nucleation layers for W-plug process were comparatively investi-
gated at 70 nm design-rule DRAM. Here, W-plug process was used
at the contact connecting the metal line and W bit line of DRAM
with the stacked capacitor. Figure 12 shows the contact resistance
distribution with different deposition processes of barrier and nucle-
ation layers at the contact chains structure with the aspect ratio of
Figure 11. Plan-view SEM images of CVD-W films ͑200 nm͒ deposited on
a͒ ALD-W ͑A͒, ͑b͒ ALD-W ͑B͒, and ͑C͒ ALD-W ͑C͒. The thickness of
ALD-W film is 20 nm.
͑
ϳ15 ͑contact chain A͒. When the TiCl -based CVD-TiN film was
4
used as barrier layer, we could obtain stable contact resistances for
all the integration schemes at such UHAR contact but the contact
resistances were different with nucleation processes. Figure 12
ALD-W films͒ and the thicknesses are much thinner ͑5 nm for
CVD-TiN and 5–20 nm for ALD-W films͒ than those of CVD-W
film, the resistivity of the total film stack is dominantly determined
by that from CVD-W film. Interestingly, the film stack of CVD-
W/B H -based ALD-W ͓ALD-W ͑C͔͒/CVD-TiN showed a much
2
6
lower resistivity compared to those with SiH -based ALD-W films
4
͓
ALD-W ͑A͒ and ALD-W ͑B͔͒, irrespective of the thickness of the
nucleation layer. Figure 10 also shows that the resistivities of film
stacks decrease with increasing CVD-W film thickness. However,
when the CVD-W film thickness exceeded 300 nm, the resistivity of
film stack was not further decreased. As mentioned earlier, as it is
the resistivity of CVD-W film that determines the resistivity of total
film stack, it is important to understand why the resistivity of
CVD-W film is different with W nucleation process.
Here, when we compare the resistivity of the film with the same
6,13,20
thickness, the thickness effect
can be ignored. As the deposi-
tion conditions for CVD-W film are the same, the impurities incor-
poration such as F and O can also be ignored when comparing
resistivity. In fact, we were able to confirm through the AES analysis
Figure 13. The distribution of contact resistances with ALD-W films at
the contact chains B connecting Al metal line with W bit line. Here, the
͑
not shown here͒ that there was no difference in the impurities con-
tent such F and O of CVD-W films deposited on different ALD-W
films. The AFM analysis also indicated that a lower resistivity of
film stack with B H -based ALD-W film is not related with the
aspect ration of contact chain A was ϳ17. As a barrier layer, TiCl -based
4
CVD-TiN film ͑closed symbol͒ or MOCVD-TiN film ͑open symbol͒ were
used.
2
6
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G892
Journal of The Electrochemical Society, 153 ͑10͒ G887-G893 ͑2006͒
cess for barrier layer ͑see Fig. 12 and 13͒ and its contact resistance
was significantly lower as compared with the integration schemes
with ALD-W film deposited by SiH reduction.
4
Figure 14 shows the effect of ALD-W film thickness on the
contact resistance. Here, TiCl -based CVD-TiN was used as a bar-
4
rier layer. From Fig. 14, it is clear that the contact resistance de-
creased as the thicknesses of ALD-W films decreased. This is due to
the decrease in cross-sectional area occupied by ALD-W film with a
higher resistivity ͑Fig. 2͒ and the increase of cross-sectional area
occupied by low-resistivity plug-fill CVD-W film ͑Fig. 10͒. The
minimum thickness of the nucleation layer evaluated as a nucleation
layer in this study was as thin as ϳ5 nm and the successful integra-
tion of this ultrathin nucleation layer indicates that ALD provides
uniform nucleation and accurate thickness control.
Figure 14. The distribution of contact resistances with different thicknesses
of ALD-W films at the contact chains A ͑AR ϳ15͒ connecting Al metal line
Conclusions
with W bit line. As a barrier layer, TiCl -based CVD-TiN film ͑closed sym-
4
bol͒ was used.
In summary a thin and conformal W nucleation layer is essential
for successfully filling the UHAR small plug as the device is ever-
shrinking. In this study, the properties of three kinds of ALD-W
films ͑two SiH -based ALD-W films and one B H -based ALD-W
4
2
6
clearly shows that the integration scheme with ALD-W ͑C͒ as a W
nucleation layer shows a much lower contact resistance than those
with ALD-W ͑A͒ or ALD-W ͑B͒. This is due to the much lower
resistivity of the total film stack consisting of W-plug, as shown by
Fig. 10, rather than the difference in the plug-filling capability.
Interestingly, when the MOCVD-TiN film was used as a barrier
layer, the integration scheme with ALD-W ͑A͒ showed higher
contact resistance and its distribution at 200 mm wafer became
poor. It is thought that the step coverage of the barrier layer, TiN,
film͒ were comparatively characterized and investigated as nucle-
ation layers for W-plug process of 70 nm design-rule DRAM.
Though all the ALD-W films had excellent step coverage irrespec-
tive of deposition processes, their resistivities were high, ranging
between ϳ125 and ϳ180 ␮⍀ cm with film thickness. High resis-
tivities of SiH -based ALD-W films are mainly due to the Si incor-
4
poration in the film and the formation of metastable ␤-W phase with
high resistivity. The high resistivity of B H -based ALD-W film is
2
6
due to the B incorporation and the formation of amorphous phase.
The formation of amorphous W film as a nucleation layer for
W-plug process has many advantages such as superior diffusion bar-
rier properties, less-sensitive thickness effect of film resistivity, and
the large-size grains formation growing on it, leading to lowering
the resistivity of the W-plug stack. The results demonstrated that the
integration scheme with B H -based ALD-W film showed a much
was degraded with MOCVD as compared to TiCl -based CVD. This
4
is possibly due to the fact that generally, the deposition for
MOCVD-TiN is performed in the diffusion-controlled regime to ob-
tain high-quality film. That the precursor is too bulky for MOCVD
to transport into the bottom of UHAR contact could be another
reason for poor step coverage. When the step coverage of TiN is not
sufficient at UHAR contact, the growth of ALD-W film is more
2
6
lower contact resistance at UHAR contact. The B H -based process
2
6
likely to occur on the interlayer dielectric material, SiO , and not
also has advantages in terms of nucleation on SiO . The enhanced
2
2
TiN, especially at the bottom of the contact. Meanwhile, it is known
that nucleation and growth of ALD-W film deposited using alternat-
nucleation on SiO by B H -based process provides a stable and low
2
2
6
contact resistance with MOCVD-TiN as the barrier layer with lim-
ited step coverage. By the same argument, the B H pretreatment
11,21
ing exposures of WF and SiH or Si H on SiO is difficult
and
6
4
2
6
2
2
6
this causes the degradation in step coverage of W nucleation layer
even though W is deposited by ALD. The degradation in the step
coverage of the nucleation layer again degrades W-plug filling, re-
sulting in the increase in contact resistance. It was recently reported
that the B H pretreatment enhanced the nucleation and growth of
prior to ALD-W formation using SiH reduction contributed to the
enhancement of the nucleation and assured stable and low contact
resistance at the UHAR contact in this study.
4
Hynix Semiconductor, Inc., assisted in meeting the publication costs of
this article.
2
6
11
ALD-W film on SiO surface. This could enhance the step cover-
2
age of ALD-W nucleation layer deposited by SiH reduction of WF
References
4
6
even though the step coverage of the underlayer, TiN, was not sat-
isfactory. In this study, the contact resistance with ALD-W ͑B͒
nucleation layer kept lower values, except for some positions of the
wafer when the MOCVD-TiN was used. This suggests that B H
1
. H. Kim, J. Vac. Sci. Technol. B, 21, 2231 ͑2003͒.
2. M. Leskelä and M. Ritala, Thin Solid Films, 409, 138 ͑2002͒.
3
4
. http://www.itrs.net/Links/2005ITRS/Interconnect2005.pdf
. J. E. J. Schmitz, Chemical Vapor Deposition of Tungsten and Tungsten Silicides,
Noyes Publications, Park Ridge, NJ ͑1991͒.
2
6
pretreatment provides some flexibility in terms of choosing the
method for depositing TiN underlayer for W-plug process. In terms
of W nucleation on SiO , B H reduction of WF could provide the
5
6
7
. T. Omstead, G. C. D’Couto, S.-H. Lee, P. Wongsenkaum, J. Collins, and K. Levy,
Solid State Technol., 51, 45 ͑2002͒.
. S.-H. Kim, E.-S. Hwang, S.-H. Pyi, H.-J. Sun, J.-W. Lee, J.-K. Kim, N. Kawk, H.
2
2
6
6
Sohn, and J. Kim, J. Electrochem. Soc., 152, C408 ͑2005͒.
most effective passway and this assured stable and low contact re-
sistance even when the MOCVD-TiN was used as the barrier layer.
Figure 13 shows the contact resistance distribution when the as-
pect ratio was further increased to ϳ17 ͓contact chain B͔. All the
integration schemes show stable and low contact resistances with
. J. W. Klaus, S. J. Ferro, and S. M. George, Thin Solid Films, 360, 145 ͑2000͒.
8. M. Yang, H. Chung, A. Yoon, H. Fang, A. Zhang, C. Knepfler, R. Jackson, J. S.
Byun, A. Mak, M. Eizenberg, M. Xi, M. Kori, and A. K. Sinha, in Conference
Proceedings ULSI XVII, p. 655, MRS, Warrendale, PA ͑2002͒.
9
. S.-H. Lee, L. Gonzalez, Josh Collins, K. Ashitani, and K. Levy, in Conference
Proceedings ULSI XVII, p. 649, MRS, Warrendale, PA ͑2002͒.
TiCl -based CVD-TiN, which is similar to Fig. 12. Even though
10. Y. Mizoguchi, K. Suzuki, M. Tachibana, and D. Abe, in Conference Proceedings
ULSI XVIII, p. 451, MRS, Warrendale, PA ͑2003͒.
1. S.-H. Kim, E.-S. Hwang, B.-M. Kim, J.-W. Lee, H.-J. Sun, T. E. Hong, J.-K. Kim,
H. Sohn, J. Kim, and T.-S. Yoon, Electrochem. Solid-State Lett., 8, C155 ͑2005͒.
2. S. B. Herner, S. A. Desai, A. Nak, and S. G. Ghanayem, Electrochem. Solid-State
Lett., 2, 398 ͑1999͒.
3. S. M. Rossnagel, J. Vac. Sci. Technol. B, 20, 2328 ͑2002͒.
4. A. K. Sinha, Prog. Mater. Sci., 15, 92 ͑1972͒.
5. Y. G. Shen, Y. W. Mai, Q. C. Zhang, D. R. Mckenzie, W. D. McFall, and W. E.
McBride, J. Appl. Phys., 87, 117 ͑2000͒.
4
B H pretreatment was done prior to ALD-W deposition ͓ALD-W
2
6
1
͑
B͔͒, the distribution of contact resistance was degraded with
MOCVD-TiN barrier layer and its values were higher at all the
1
positions of 200 mm wafer than those with TiCl -based CVD-TiN.
4
1
1
1
The contact resistance of the integration scheme with ALD-W ͑A͒
further increased and its distribution worsened with MOCVD-TiN.
However, with ALD-W ͑C͒ deposited by B H reduction, we could
2
6
obtain a stable contact resistance irrespective of the deposition pro-
16. P. M. Petroff, A. K. Sinha, T. T. Sheng, H. J. Levinstein, and F. B. Alexander, J.
Downloaded on 2015-06-20 to IP 129.215.17.190 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 153 ͑10͒ G887-G893 ͑2006͒
G893
Appl. Phys., 44, 2545 ͑1973͒.
J. Appl. Phys., 43, 8008 ͑2004͒.
1
1
7. J. M. E. Harper and K. P. Rodbell, J. Vac. Sci. Technol. B, 15, 763 ͑1997͒.
8. J.-M. Yang, I. S. Choi, Y. S. Kim, J.-C. Park, S.-M. Lee, T.-E. Hong, Y.-B. Park,
S.-Y. Lee, and I. W. Kim, J. Appl. Phys., 91, 9788 ͑2002͒.
20. T. S. Kuan, C. K. Inoki, G. S. Oehrlein, K. Ross, Y.-P. Zhao, G.-C. Wang, S. M.
Rossnagel, and C. Cabral, Mater. Res. Soc. Symp. Proc., 612, D7.1.1 ͑2000͒.
21. J. W. Elam, C. E. Nelson, R. K. Grubbs, and S. M. George, Thin Solid Films, 386,
41 ͑2001͒.
1
9. J.-W. Lee, J. K. Kim, S.-H. Kim, H.-J. Sun, H.-S. Yang, H. C. Sohn, and J. W. Kim,
Downloaded on 2015-06-20 to IP 129.215.17.190 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).