Journal of the Korean Physical Society, Vol. 53, No. 3, September 2008, pp. 16341637
Study on the Plasma-Polymer Thin Films Deposited by Using
PECVD and Application Tests for Low-k Insulator
S.-J.
Cho,
Bae
I.-S.
and J.-H.
Boo
Department of Chemistry and Center for Advanced Plasma Surface Technology, Sungkyunkwan University, Suwon 440-746
Y. S.
Park
and B.
Hong
School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746
(Received 12 December 2007)
Plasma-polymer pure ethylcyclohexane thin lms were deposited on Si(100) substrates at room
temperature by using PECVD (plasma enhanced chemical vapor deposition). Hydrogen and argon
gases were used as precursor bubbler and carrier gases, respectively. We also investigated the
electrical and the physical properties of the plasma-polymer thin lms at various deposition RF
powers and annealing temperatures. The as-grown and the annealed plasma-polymer thin lms
were analyzed by using FT-IR (Fourier transform infrared spectroscopy) and SEM (scanning
electron microscopy). The IR spectra showed that the plasma-polymer thin lms had totally
di erent chemical functionalities from those of the ethylcyclohexane precursor and that the
chemical functionalities of the thin lms changed with the RF power and annealing temperature.
From the SEM results, we determined the thicknesses of the thin lms before and after the
annealing, with the thickness shrinkage (%) being measured by using SEM cross-sectional images.
An impedance analyzer was used to measure the capacitance and from the electrical property
measurements, the lowest dielectric constant obtained was 1.71.
PACS numbers: 82.35.-x, 77.22.-d, 62.25.+g
Keywords: Plasma polymer, PECVD, Low-k, Electrical and physical properties
I. INTRODUCTION
There has been an increase of interest in the use of
glow discharges for the polymerization of a number of organic and organometallic compounds [1,2]. Plasma polymers are used as dielectric and optical coatings to inhibit
corrosion. The investigation of the optical properties of
polymer lms is of particular interest because of their
use in optical devices [1]. Among many chemical vapor
deposition (CVD) methods, the plasma-enhanced chemical vapor deposition (PECVD) process is a very ecient
method to produce homogeneous organic thin lms on
large area substrates, o ering good control over the lm
properties [3{7]. Plasma polymerization is known to be
a unique method of organic thin- lm deposition [8].
As the integration level and the speed of semiconductor devices are increased, the reduction in the resistance
capacitance (RC ) delays in the metallization is becoming more important than before. The prevalent problems
are propagation delay, cross-talk noise and power dissipation due to RC coupling, which becomes signi cant
due to increased wiring capacitance, especially the in
E-mail: jhboo@skku.edu; Fax: +82-31-290-7075
terline capacitance between the metal lines on the same
metal level [9, 10]. Whereas the resistance is a ected
by the conducting materials, the capacitance is mainly
determined by the dielectric materials [11]. Therefore,
thin lms with relatively low dielectric constants (k <
3.0) are under intense study due to their applications as
interlayer dielectrics for multilevel metallization of ultralarge-scale integrated (ULSI) semiconductor devices [12].
A low-k value is one of the most important requirements
for interlayer dielectrics.
In this study, we report our results for plasma-polymer
thin lms deposited on Si (100) substrates at room temperature by using an ethylcyclohexane precursor, for the
annealed thin lms and for PECVD as a method for use
in low dielectric constant device applications. In addition, the e ects of the plasma power and the annealing
temperature on the dielectric constants were studied.
II. EXPERIMENT
Silicon (100) wafers were cleaned by sonication with
acetone, methyl alcohol, distilled water and isopropyl
alcohol, along with in-situ Ar plasma bombardment at
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Study on the Plasma-Polymer Thin Films Deposited by Using { S.-J. Cho et
al.
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Fig. 1. Shrinkage percentage of the thin lms annealed at
200 { 500 C.
100 W for 15 min. The plasma-polymer thin lms were
deposited by using the PECVD method at room temperature. Ethylcyclohexane was utilized as the organic
precursor, was preheated at 60 C and was bubbled with
50 sccm of hydrogen gas, with an additional 50 sccm of
argon gas as the carrier gas. The deposition lasted 10
min to achieve a thickness of 300 nm [7]. The deposition pressure and temperature were 4.0 10 1 Torr and
room temperature, respectively.
Chemical bonding in the plasma-polymer thin lms
was investigated by using Fourier-transform infrared
spectroscopy (FT-IR). A MTS nano-indenter(R) was
used to measure the hardness and the Youngs modulus of these lms with measurements made by applying a constant strain rate to a diamond indenter tip
and then unloading the indenter while monitoring the
load versus total indenter displacement. The hardness
and the modulus were continuously determined from the
load-displacement curves by using the continuous sti ness method [13]. These lms were also investigated by
using a multi-frequency precision LCR meter (HP 4284b)
to measure the capacitance.
III. RESULTS AND DISCUSSION
Figure 1 shows the thickness shrinkages of the plasmapolymer thin lms at various annealing temperature.
With increasing annealing temperature, the thickness
was dramatically decreased by annealing. Plasmapolymerized ethylcyclohexane thin lms were thermally
stable up to 300 C, with no signi cant shrinkage in the
thickness and a shrinkage of below 3 % after annealing
at 200 and 300 C. However, a more obvious thickness
shrinkage of 10.2 % was observed for annealing at 500 C,
a result of the annealing promoting a degassing of internal and terminal CHx [14]. The thickness decreased dramatically when annealed at higher temperatures (over
Fig. 2. FT-IR spectra: (a) as-grown polymerized ethylcyclohexane thin lms with increasing RF power and (b) annealed thin lms deposited at RF powers under 20 W.
500 C) as a result of the signi cant thermal decomposition and the collapse of nano-voids or nano-pores in the
lms [15].
The bonding states of the ethylcyclohexane-plasmapolymer thin lms were analyzed by using FT-IR absorption spectra over a range of 4000 { 600 cm 1 as shown in
Figure 2(a) for plasma polymerized thin lms grown on
Si(100) substrates at various RF powers at room temperature. The spectra exhibited absorption peaks at
1030 { 1100 cm 1 , corresponding to SiO stretching vibration bands, along with bands with range of 1480 {
1560, 1700, 2850 { 2910 and 3300 cm 1 , corresponding
to CHx bending vibrations and C=C, CHx and OH
stretching vibration bands, respectively. The intensity
of the CHx (2910 cm 1 ) peak decreased with increasing RF power, which is also associated with the peak
intensity of C=C (1700 cm 1 ). The peak intensity of
C=C increased with decreasing peak intensity of CHx
(2910 cm 1 ) as the RF power was increased. Figure 2(b)
shows the IR spectra of the annealed thin lms deposited
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Journal of the Korean Physical Society, Vol. 53, No. 3, September 2008
Fig. 3. Dielectric constants: (a) as-grown polymerized
ethylcyclohexane thin lms and (b) annealed thin lms deposited at RF powers under 20 W.
Fig. 4. Hardness and Young's modulus; (a) as-grown polymerized ethylcyclohexane thin lms and (b) annealed thin
lms deposited at RF powers under 20 W.
at RF powers under 20 W. The peak intensity of CHx
decreased with increasing annealing temperature, as did
the C=C peak intensity. Also, the SiO (1030 { 1100
cm 1 ) and the OH (3300 cm 1 ) peaks disappeared after
annealing due to thermal desorption of absorbed water
on the surface of polymerized thin lms. Si-CH3 broad
peaks (1216 cm 1 ) appeared after annealing, indicating
that desorbed CHx reacted with Si under thermal conditions. From this, a higher RF power plasma generated
many radicals that increased the degree of cross-linked
polymerization in the thin lms. These results also indicated that ethylcyclohexane is totally ionized by the
plasma because the spectrum of liquid ethylcyclohexane
could not detected by using FT-IR [16, 17]. Polymerized thin lms have high degrees of cross-linking because
ethylcyclohexane molecules produce many radicals with
increasing RF power. Also, with increasing annealing
temperature, the terminal CHx undergo thermal desorption from the surface and from inside the polymerized
thin lms. We expect the dielectric constants of the thin
lms to increase because of the increasing C=C bond.
Figure 3 shows the changes in the dielectric constant
of the polymerized thin lms for various deposited RF
powers and annealing temperatures. The dielectric con-
stant increased with increasing RF power. The lowest
dielectric constant was 1.71 at a 20 W deposition RF
power (Figure 3(a)). Also, after annealing, the dielectric constant increased to above 2.9 (Figure 3(b)). From
this result, pin-hole-free lms are made with higher RF
power because, with to increasing RF power, the density
of the thin lms increases and with decreasing RF power,
the thin lms has many nano-voids or nano-pores, which
make for low dielectric constants. The nano-voids or
nano-pores of the thin lms are increased by the annealing e ect, so the dielectric constants are large increase
after annealing. From this result, ethylcyclohexaneplasma-polymer thin lms are electrically unstable under
annealing. The structure of the lms can be rearranged
by annealing, so annealed thin lms have completely different electrical properties.
The hardness and Young's modulus of the
ethylcyclohexane-plasma-polymer thin lms are shown
in Figure 4, showing their dramatic increase with
increasing RF power. High RF power makes for many
radicals, ions and molecular fragments and these make
high-density thin lms under high RF power. The hardness and Young's modulus decreased with increasing
annealing temperature. From the IR results, the main
Study on the Plasma-Polymer Thin Films Deposited by Using { S.-J. Cho et
cause for the decrease in the hardness and Young's
modulus was the degassing of the terminal and the
internal CHx , indicating that the main components
of the thin lms were degassed from the surface and
from inside the thin lms due to the lms being made
by using the PECVD method with ethylcyclohexane
(C8 H16 ). The variations the hardness and Young's
modulus with annealing temperature were consistent
with the intensity of CHx (2910 cm 1 ) peak for the
lms [11].
IV. CONCLUSIONS
Ethylcyclohexane-plasma-polymer thin lms were deposited at room temperature by using PECVD with
ethylcyclohexane as an organic precursor. The electrical,
mechanical, thermal and chemical properties of the polymerized thin lms, as well as the e ects of RF plasma
power and the annealing, were the focus of this study.
An increase in the RF power made many fragment radicals and ions in the reaction chamber. From the results,
the FT-IR spectra showed that the as-grown lms had
CHx , C=C and SiO functional groups, indicating that
the thin lm had a totally di erent structure from ethylcyclohexane. These results showed that plasma-polymer
thin lms at low plasma power had a non-polarized structure. CHx underwent thermal desorption from the surface and from inside the plasma-polymer thin lms. The
hardness and Young's modulus increased with both increasing RF power and annealing temperature. A higher
RF power makes chemically and mechanically stronger
bonds with each fragment molecules, ions and radicals.
Higher annealing temperatures cause shrinkage of the
thin lms by heat reconstruction. Annealing at high
temperatures was driven by signi cant thermal decomposition, resulting in dielectric constants increasing with
increasing RF power; thus, the dielectric constants increased with increasing annealing temperatures as they
are associated with the chemical and the mechanical
properties. Also, the lowest dielectric constant was 1.71
at a deposition RF power of 20 W and was highly in uenced by the deposition RF power. From all of these results, the optimum condition was a deposition RF power
of 20 W without annealing.
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al.
ACKNOWLEDGMENTS
Support of this research by the BK 21 project of
the Ministry of Education, Korea, is gratefully acknowledged. This work was also supported by the Center for
Advanced Plasma Surface Technology at Sungkyunkwan
University and the Korea Research Foundation grant
funded by the Korean Government (MOEFRD, KRF2005-005-J11902) and by the Korea Research Institute
of Standards and Science (KRISS).
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