Full text of DOI:10.1063/1.114834

Structural properties of low temperature plasma enhanced chemical vapor deposited
silicon oxide films using disilane and nitrous oxide
Juho Song and G. S. Lee
Citation: Applied Physics Letters 67, 2986 (1995); doi: 10.1063/1.114834
View online: http://dx.doi.org/10.1063/1.114834
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Structural properties of low temperature plasma enhanced chemical vapor
deposited silicon oxide films using disilane and nitrous oxide
Juho Songa) and G. S. Lee
Solid State Laboratory, Department of Electrical and Computer Engineering, Louisiana State University,
Baton Rouge, Louisiana 70803
͑
Received 4 August 1995; accepted for publication 20 September 1995͒
The structural properties of low temperature plasma enhanced chemical vapor deposited SiO films
2
using Si H and N O have been studied. It is observed that the degree of compaction of as-deposited
2
6
2
SiO films, upon subsequent annealing, increases up to 4%. The shift of Si-O-Si stretching peak
2
Ϫ1
wave number of the as-deposited SiO 2 films (⌬␻ϭϪ20 cm ) compared to the undensified
SiO films is attributed to 9.4% increase in the film density, resulting in smaller Si-O-Si bridging
2
bond angle of 138°. It is also believed that the high temperature annealing results in the reduction
of hydroxyl containing species in the film and in turn drives the dielectric constant towards that of
thermal SiO films. © 1995 American Institute of Physics.
2
A great deal of research on the low temperature deposi-
ried out in conventional tube furnace flowing N at different
2
tion of silicon oxide ͑SiO ) films has been reported for ap-
2
temperatures for 30 min. The vibrational properties in the
Ϫ1
plications toward primary insulator layers in the microelec-
tronic industry. One of the more preferred low temperature
deposition techniques is plasma enhanced chemical vapor
deposition ͑PECVD͒, where the plasma dissociation of the
reactant gas molecules enables the deposition temperature to
be further decreased compared to other thermal CVD tech-
4
00–4000 cm wave number range were observed using a
Perkin Elmer Model 1600 Fourier transform infrared spec-
Ϫ1
trophotometer with a resolution of 4 cm . A bare silicon
wafer was used for background subtraction purposes. Al gate
Ϫ3
2
MOS capacitors with predefined area of 2.6ϫ10 cm were
fabricated using a standard photolithography technique to in-
niques. Silane ͑SiH ) as the silicon source and nitrous oxide
4
vestigate the dielectric constant of the SiO films annealed at
2
͑
N O͒ as the oxidant source are typically used in the PECVD
2
different temperatures. Forming gas anneal with 5% H in
2
1
–4
process.
Recently, low temperature SiO deposition pro-
2
N ambient at 400 °C for 30 min was carried out as the
2
cess with disilane ͑Si H ) instead of SiH as the silicon
2
6
4
postmetallization anneal ͑PMA͒ prior to capacitance-voltage
(C–V) measurements.
5
,6
source has been reported, indicating that the SiO films can
2
be deposited even at room temperature due to the high reac-
In Fig. 1, degree of compaction and chemical etch rates
in the P-etching solution are plotted as a function of the
postdeposition annealing temperature. The degree of com-
paction increases with increasing the annealing temperature.
The maximum compaction, as a result of high temperature
annealing, obtained in this study was about 4% when the
tivity of Si H . In this letter, we present the dependence of
2
6
the structural properties of SiO films prepared by low tem-
2
perature PECVD using Si H 6 and N O on the post-
2
2
deposition annealing process.
Chemically polished, 4 in. diameter boron doped silicon
wafers with ͑100͒ orientation and 5–15 ⍀ cm resistivity
were used as the substrates. Deposition of 100 nm thick
SiO films was made in Plasma Therm VII-70 PECVD reac-
2
tor with 2.54 cm electrode spacing and 13.56 MHz operating
rf frequency. The deposition process parameters were main-
tained the same throughout this study. The substrate tempera-
ture was set at 120 °C while the top electrode temperature
was 60 °C. A 140 sccm gas mixture of 4.8% Si H in He and
2
6
pure N O was introduced into the process chamber while the
2
gas flow ratio of N O to Si H was fixed at 50 in order to
2
2
6
ensure the stoichiometry of the SiO films. The process pres-
2
sure and rf power were 700 mTorr and 50 W, respectively.
These conditions gave a deposition rate of 12.5 nm/min and
thickness uniformity of within Ϯ3% across 4 in. wafers. The
conventional RCA cleaning followed by 100:1 parts by vol-
ume of DI water-HF ͑48%͒ dipping was used as predeposi-
tion cleaning procedure. The thickness of the deposited
SiO films was measured using an Applied Materials Ellip-
someter II. The postdeposition annealing processes were car-
FIG. 1. Percentage degree of compaction ͑᭹͒ and etch rate in the P-etching
solution ͑ᮀ͒ of the SiO2 films as a function of postdeposition annealing
temperature. Experimental data at 25 °C are for the as-deposited film. The
circle corresponds to etch rate of the thermal SiO2 films grown at 1000
°C, 0.15 nm/s.
2
a͒Electronic mail: song@max.ee.lsu.edu
2
986
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Appl. Phys. Lett. 67 (20), 13 November 1995
0003-6951/95/67(20)/2986/3/$6.00
© 1995 American Institute of Physics
1

FIG. 3. Si–O–Si asymmetric stretching peak wave number ͑᭹͒ and its
relative peak intensity with respect to the as-deposited SiO films ͑ᮀ͒ as a
2
function of postdeposition annealing temperature.
of the stretching peak increases with annealing temperature
and is up to 2.5 times higher for films annealed at 1100 °C
FIG. 2. Infrared transmission spectra of the SiO films annealed at different
temperatures. Three characteristic peaks of Si–O–Si vibrational motion are
marked.
2
compared with as-deposited films.
12,13
Previous studies
had reported that the position of the
stretching peak angular frequency (␻) is related to the film
Ϫ1
2
as-deposited SiO2 films were annealed at 1100°C. This
density ͑␳͒, d␻/d␳ϭϪ93 g cm , and to the mean Si-O-Si
Ϫ1
3
matches with the results reported for SiH based PECVD
bridging bond angle (␪), d␪/d␳ϭϪ28 °g
cm . The
4
1
,7
SiO2 . The chemical etch rate was obtained by dipping the
changes in the film density ͑⌬␳/␳͒ and ␪ calculated using
these relationships are plotted in Fig. 4. The Si–O–Si
stretching peak wave number of 100 nm thick thermal SiO2
films grown at 1000 °C in our laboratory is located at 1076
cm and is used as the reference wave number for the un-
densified SiO2 films. Assuming that ␳ and ␪ of the undensi-
films in the P-etching solution, that is, 15 HF ͑48%͒:10
HNO ͑70%͒:300 DI water. The etch rate of the as-deposited
3
SiO films was found to be 0.81 nm/s. As the annealing
2
Ϫ1
temperature increases, the etch rate decreases. The etch rate
ratio of the as-deposited to the annealed SiO2 films at
3
12,14
1100 °C was 7.4. Higher etch rates obtained indicate that the
fied amorphous SiO 2 films are 2.2 g/cm and 144 °,
the
as-deposited SiO2 films may have strained bonds, mi-
cropores, and impurities in the network. The stress induced
cracking which is known to occur during high temperature
processing, particularly in films prepared at low tempera-
tures, has not been observed even at 1100 °C.
film density and Si–O–Si bridging bond angle of the as-
deposited SiO2 films are calculated to be 2.4 g/cm and
138 °. This results in 9.4% densification of the as-deposited
SiO2 films compared to the undensified SiO2 films. It is be-
lieved that the high temperature annealing favors the relax-
3
Infrared transmission spectra of the SiO films annealed
2
at different temperatures are shown in Fig. 2. Three charac-
Ϫ1
Ϫ1
teristic peaks located at 1075 cm , 800 cm , and 450
Ϫ1
cm
corresponding to Si–O–Si asymmetric stretching,
bending, and rocking motion, respectively, are evident.
8,9
Also from the figure, transmission intensities of the hydrogen
1
0
Ϫ1
Ϫ1
containing bonds such as Si–H ͑2270 cm , 880 cm ),
Ϫ1
Ϫ1
Si–OH ͑3620 cm ), H O ͑1620 cm ) are below the spec-
2
11
trophotometer’s detection level. It is claimed that the detec-
tion limit for a 100 nm film is five times higher than the
2
0
.5–1 at. % limit estimated for 500 nm films. This indicates
less than 5 at. % of bonded hydrogen content in the films
studied.
The Si–O–Si stretching vibration mode is commonly
used to study the structural property of the SiO films. The
2
Si–O–Si stretching peak wave number and its relative peak
intensity as a function of the annealing temperature are plot-
ted in Fig. 3. As the annealing temperature increases, the
stretching peak wave number increases: for example, from
FIG. 4. Changes in the SiO film density ⌬␳/␳ ͑᭹͒ and Si–O–Si bridging
bond angle, ␪ ͑ᮀ͒ as a function of postdeposition annealing temperature. It
2
Ϫ1
Ϫ1
1
056 cm for as-deposited SiO films to 1077 cm for
2
is assumed that ␳ and ␪ of the undensified amorphous SiO films are 2.2
2
3
o
SiO films annealed at 1100 °C. Also, the relative intensity
g/cm and 144 , respectively.
2
Appl. Phys. Lett., Vol. 67, No. 20, 13 November 1995
J. Song and G. S. Lee
2987
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1

the dielectric constant. This may be due to hydrogen related
passivation17 known to occur during forming gas anneal.
More detailed study is in progress to understand the bearing
of these results on the electrical properties of the SiO films.
2
The results reported in this letter indicate that the struc-
tural properties of the PECVD SiO2 films deposited at
1
20 °C using Si H and N O are not significantly different
2 6 2
from the conventional SiH based SiO films deposited at
4
2
2
50–350 °C. It is concluded that PECVD deposition of
SiO films using Si H and N O can be a promising tech-
2
2
6
2
nique toward making available a low temperature process for
device fabrication where such conditions might be crucial.
This work was partially supported by the Council on
Research at Louisiana State University. The authors grate-
fully acknowledge technical support provided by Golden
Hwaung.
FIG. 5. Dependence of dielectric constant of SiO films annealed at different
2
temperatures on postmetallization annealing ͑PMA͒. The PMA was carried
out in 5% H in N ambient at 400 °C for 30 min.
2
2
1
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2
2
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change in dielectric constant was also investigated using
C–V measurement and is shown in Fig. 5. The dielectric
constant is calculated from the measured values of the ca-
pacitance in the accumulation region of MOS devices with
SiO2 films, annealed at different temperatures prior to Al
evaporation, as dielectric layers. Increase in annealing tem-
perature caused the reduction of dielectric constant of the
4
5
6
7
8
9
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2
deposited SiO films was 5.88 whereas that of the SiO films
12
2
2
13
annealed at 1100 °C was 4.16. Dielectric constant has been
known to be proportional to the OH content in the film,16
thereby suggesting the presence of reduced number OH
14
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16
2
17
was also found that the PMA resulted in further reduction of
2
988
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Appl. Phys. Lett., Vol. 67, No. 20, 13 November 1995
J. Song and G. S. Lee
1