FTIR analysis of silicon dioxide thin film deposited by Metal organic-based
PECVD
B. Shokri 1, M. Abbasi Firouzjah 1, S. I. Hosseini 1
1
Laser & Plasma Institute, Shahid Beheshti University, Tehran, Iran
Abstract: In this study, the silicon dioxide was deposited on the silicon substrate
by metal-organic based plasma enhanced chemical vapor deposition (PECVD)
method at the low temperature. The metal-organic tetraethoxy-silane (TEOS) was
used as a silicon precursor in liquid state. In addition, oxygen and argon were
used as ambient gases. Effects of the working pressure and O2/TEOS pressure
ratio on the chemical bonding states of the deposited films were analyzed using
FTIR spectroscopy. It has been found that the impurities contents in the silicon
dioxide films have been lowered by decreasing the working pressure and increasing the O2/TEOS pressure ratio.
Keywords: Silicon dioxide; Thin film; Plasma; Metal-organic; PECVD; FTIR
1. General
Silicon dioxide films can be produced by different methods. Sol-gel [1] and liquid phase deposition [2] do have
limitations in terms of the minimal thickness of the films.
Sputtering [3] and chemical vapor deposition [4] allow
better control of the film thickness.
Pure silicon dioxide films were difficult to be deposited
at a low temperature. Films deposited at low temperatures
contain many impurities. High quality silicon dioxide
films have been extensively deposited at low temperatures
by plasma-enhanced CVD for the manufacture of MEMS
and optoelectronic devices [5-8].
Precursor for CVD experiments is selected based on the
specific properties of the layer it produces, costs, health
and safety and environmental effects. A large number of
precursors have been found and tested. Silicon compounds such as Si(CH3)4 (and higher) and Silane are extremely flammable and toxic. The silicon halides although
stable, are very corrosive by products such as HF or HCl
on contact with water. Consequently, the most attractive
compounds for silica deposition are the metal organic
materials [9], metal organic sources such as TMOS (tetramethoxy-silane: Si((OCH3)4) [10], TEOS (tetraethoxysilane) [11], HMDS (hexamethyldisilazane) [12], TMS
(tetramethyl-silane) [13], and HMDSO (hexamethyl-disiloxane) [14] deposited with an oxidizer such as
oxygen and ozone are used increasingly because they
result in the conformal thin films and allow the deposition
of a large variety of materials. Using low TEOS/O2 flow
ratio, high quality films could be deposited by PECVD at
room temperature [15-17]. PECVD of silicon dioxide
from organosilicon sources and oxygen plasma has been
extensively investigated. Tetraethoxy-silane (TEOS;
Si(OC2H5)4) is an organosilicon precursor which is widely
used, is safe, liquid form and chemically stable and it
causes superior conformality, low defect and stable film
properties [5-23].
In this paper, we study the effects of working pressure
and O2/TEOS pressure ratio on the chemical composition
of silicon dioxide film by Fourier Transform Infrared
Spectroscopy (FTIR) analysis.
2. Experiments
Silicon dioxide thin film deposition was performed on
silicon substrates in a 34 MHz capacitively coupled
PECVD system driven by a 180 watt RF power supply.
Liquid TEOS is heated at about 80 oC and then transported to a vaporizer, in which its flux was adjusted by a
liquid needle valve. TEOS vapor was carried with Ar as a
carrier gas and finally mixed with oxygen gas and introduced into the chamber (fig.1) through a nozzle near the
substrate. TEOS vapor transforming tubes were heated
using the heating band. This procedure is important to
prevent condensation. And also in order to clean and
warm up the precursor inlet port and substrate, Ar plasma
was applied before the deposition process. During the
deposition, a self dc-bias voltage was applied to the substrates.
Table 1 shows the deposition processes conditions applied for sample A and B. The plasma power was kept at
180 Watt. Both depositions were performed with a substrate temperature below 100o C and the O2/TEOS pressure ratio in amount of almost 1.9. In fact among sample A and B all deposition processes conditions were kept
constant except The working pressure. During deposition
the working pressure was about 1 and 0.1 mbar for sample A and B respectively.
The chemical composition of the films was examined
by Fourier Transform Infrared Spectroscopy in transmittance mode.
Table 1. Process parameters used in the silicon dioxide deposition processes
Parameters
Sample A
Sample B
2.0×10-3
8.0×10-2
Oxygen partial pressure (mbar)
3.2×10-2
Argon partial pressure (mbar)
6.2×10-2
TEOS partial pressure (mbar)
2.6×10-2
2.7×10-1
Base pressure
(mbar)
1.4×10-1
4.2×10-1
180
RF power (Watt)
Vaporizer temperature
o
( C)
Substrate temperature (o C)
Fraction of TEOS/O2
Working pressure (mbar)
180
~80
~80
<100
<100
1.9
1.9
~1
~0.1
working pressure effect on the film purity. As seen from
Table 1, in samples A and B all effective conditions were
equal (spatially the TEOS/O2 pressure ratio and RF power) except the working pressure.
The FTIR transmittance spectra of samples A and B are
shown in Fig. 3 and the assignment of the bands is shown
in Table 3. The characteristic bands almost at 1085, 800
and 460 cm-1 are correspond to the stretching, bending
and out of plane of Si-O bonds, respectively. The position and the shape of the main Si-O vibrational band at
1085 cm-1 shows a stoichiometric silicon dioxide
structure. Moreover some impurity vibrational bands
are seen in the FTIR spectra which were shown in Table 3, as it is observed from the spectra these are too
smaller than the main pick. Peaks in the spectral range
at around of 1600 cm-1 and 2300 cm-1 are attributed to
vibrations of carbon impurity atoms in the film. The
FTIR spectra for the deposited films also showed a
large amount of OH groups in the films at around of
2500cm-1. If annealing technique applies, these peaks
will be decrease. As whole by comparing the Fig. 3 with
Table 3 it is clear that increasing the working pressure
decreases the impurity contents in sample B. Fig. 3 shows
the spectrum of sample C in which the O2/TEOS content
is 1/4. Obviously it can be seen, decreasing the amount of
oxygen increases the ratio of impurity picks to main Si-O
picks. It could be attributed to the lack of oxygen content
in the reacting chamber to burn out the carbon impurities.
Table 2. Some possible reactions
Ref .
Fig.1 TEOS Vaporizr
3. Chemical reactions
The most common MOCVD reaction is the decomposition of tetraethoxy-silane (TEOS), shown in simplified
form as follows: [20]
Si(OC2H5)4
SiO2 + 4C2H4 + 2H2O
(1)
Si(OC2H5)4 + e
Si(OC2H5)3OCH 2 + CH3 + e
[24]
Si(OC2H5)4 + e
Si(OC2H 5)3O + C2H 5 + e
[24]
Si(OC2H5)4 + e
Si(OC2H 5)3 + OC2H 5 + e
[24]
Si(OC2H5)2 OCH3+ + OC2 H5 + CH2 + 2e
[25]
Si(OC2H5)4 + e
Si(OC2H 5)3OCH2+ + CH 3 + 2e
[25]
Si(OC2H5)4 + e
Si(OC2H 5)3O+ + C2H 5 + 2e
[25]
Si(OC2H5)4 + e
Si(OC2H 5)3(OH) + C2H4 + e
[26]
Si(OC2 H5)4 + e
o
This reaction normally takes place at 700 C and at low
pressure (< 1 mbar). But in plasma medium this reaction
takes place at lower temperature, even at lower than
100oC. Some other possible Chemical reactions are listed
in table 2.
4. Results
Fig. 2 shows XRD spectroscopy of sample A. This
spectrum shows that the SiO2 film hasn’t crystalloid form.
Therefore FTIR spectroscopy was used in order to reconnaissance the film material. In this study we examined the
Fig.2 XRD spectrum of sample A
Sample A
Sample B
Fig.3 FTIR spectra of sample A and B
Table 3: Assignment of the bands in the FTIR spectra of silicon
dioxide films
Sample A
Sample B
458 cm-1
461 cm-1
Si-O bending
800 cm-1
802 cm-1
Si-OH stretching
939 cm-1
932 cm-1
Si-O-Si stretching
1083 cm-1
1089 cm-1
C-O bending
1630 cm-1
1628 cm-1
Si-C stretching
2357 cm-1
_
Si-O out of plane
deformation
…-OH stretching
-1
3444 cm
3427 cm-1
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