22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Low-k OSG damage and etching by F atoms at lowered temperatures
S. Zyryanov1,2, K. Kurchikov1,2, D. Lopaev1, Yu. Mankelevich1, A. Palov1, T. Rakhimova1, E. Voronina1, N. Novikova3
and M. Baklanov4
1
Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, RU-119991 Moscow, Russia
2
Faculty of Physics, Moscow State University, Moscow, Russia
3
Institute for Spectroscopy of Russian Academy of Sciences, Troitsk, Russia
4
IMEC vzw, BE-3001 Leuven-Heverlee, Belgium
Abstract: Damage and etching by F atoms was studied in ICP downstream reactor at
temperatures from +15 to -30°C for low-k OSG films. It is shown that OSG exposure to
fluorine atoms leads to film fluorination, H abstraction reactions from –CH 3 groups with
formation of –CH x F y species and film etching. Activation energy for H atom replacement
by F atom in -CH 3 was estimated using the experimental data.
Keywords: low-k SiOCH materials, low-k damage, etching, fluorine atoms
1. Introduction
Ultra low-k dielectric materials are the key component
of the microchips with structure size of 20 nm and below.
Their decreased dielectric constant compared to SiO 2
allows decreasing the signal propagation delay as well as
preventing the interconnect cross-talk and power
dissipation.
Most of the modern low-k materials are porous organosilicate glasses (OSG). They are deposited on the
substrate by means of PECVD and spin-on technologies.
Typical pore radius is 1-2.5 nm with porosity from 20 to
50% while the greater pore radius is typical for highporosity films.
One of the main problems of low-k material processing
in microchip manufacturing process is plasma damage
[1]. During plasma treatment such as reactive ion etching
the low-k film structure may be seriously modified and
owing to that dielectric constant increases. The film
becomes hydrophilic, which leads to the increase of k
value due to water absorption as soon as the film is
exposed to the atmosphere.
Chemically active radicals [2-4], energetic positive ions
[5,6] and VUV photons [7-10] are the reasons for low-k
material plasma damage. Their interaction with low-k
material can be both separate and synergetic. For example
radicals can be adsorbed on the pore walls and then
chemical reactions can be stimulated by VUV radiation
and ions from plasma.
To understand the damage mechanisms the impacts of
radicals, ions and VUV photons have to be studied
separately. This work is devoted to study low-k OSG
damage and etching by F atoms [11,12] at the lowered
temperatures. Temperature variation allows estimating the
activation energy of chemical reactions that lead to low-k
film damage during plasma treatment.
2. Experiment
Low-k films of five different types were used in this
study. Their parameters are represented in Table 1. CVD1
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sample is a previous generation low-k film, now used in
industry and so well studied. It is used in this study as a
reference sample. ALK B is a PECVD ULK material.
SOG1.8 – SOG2.2 are spin-on ULK films with different
pore radius and porosity. This set of the films is quite
adequate to understand the dependency of the damage and
etching on pore geometry.
Table 1.Low-k films.
Name
CVD1
ALK B
SOG2.2
SOG2.0
SOG1.8
Type
PECVD
PECVD
SOG
SOG
SOG
Porosity (%)
24
40
39
44
51
Pore radius (nm)
0.8
1.5
2.1
2.6
2.7
Thckness (nm)
175
270
218
217
214
k value
2.5
2.1
2.2
2.0
1.8
SF6, 100 mTorr
13.56 MHz ICP
plasma 200 W
Grid to stop
plasma
Downstream
zone
F
Screen to stop
VUV emission
Peltier cooled
sample holder
Pump
1
3. Low-k film damage
The internal structure of the low-k film consists of the
interconnected pores, formed by the Si-O-Si framework.
Pore walls are covered by –CH 3 bonded to surface Si
atoms. Therefore the damage during fluorine atom
treatment is mainly connected with the partial extraction
of the methyl groups from the pore volume as well as the
replacement of hydrogen atoms by fluorine atoms. These
effects can be seen in FTIR spectra of the film by
measuring the decrease of the Si-CH 3 peak at 1275 cm-1.
The dynamics of the –CH 3 density in low-k film during
the treatment at various temperatures is shown in Fig 2. It
can be clearly seen that the reaction rate is lowered with
decreasing temperature. The higher porosity and pore
radius promote the deeper penetration of fluorine atoms
inside the film and therefore the remaining fraction of SiCH 3 chemical bonds in SOG2.0 with higher porosity is
significantly lower than for CVD1 at the same F atom
dose.
According to XPS data shown in Fig. 3 methyl groups
are not removed as a whole. Instead they are transformed
from Si-CH 3 to Si-CH 2 F and then to Si-CHF 2 before
being extracted.
The activation energy for H abstraction reaction was
estimated comparing the Si-CH 3 depletion rate for
different temperatures. This comparison is shown in Fig.
4. The Arrhenius plot (dashed line) gives an estimation
2
1.2
Fraction of Si-CH3
CVD1
1.0
0.8
-30C
-15C
0.6
0C
+15C
0.4
0.2
0.0
0
1000
2000
3000
4000
5000
6000
F exposure time, s (F dose)
1.2
SOG2.0
Fraction of Si-CH3
All of the samples, listed above were treated by fluorine
atoms in the downstream ICP chamber, shown in Fig. 1.
The fluorine atom source was 13.56 MHz, 200 W ICP
discharge in SF 6 at 100 mTorr. The plasma region was
separated from the downstream region by the stainless
steel grid with a small cell of 70x70 μm to prevent plasma
penetration to treated samples. A metal screen was also
used to avoid VUV light effect on the samples. As a result
low-k films were treated only by chemically active
fluorine atoms. Treatment time varied from 225 to 7200 s.
Low-k samples were located on the Peltier-cooled
holder that allowed temperature variation from +15 to 30°C. To prevent water absorption during cooling and
heating of the samples these procedures were carried out
in the chamber in SF 6 flow.
After the exposure to fluorine atom the treated and
pristine samples were analysed by using FTIR
spectrometer, spectroscopic ellipsometer, EDX and XPS
diagnostics. FTIR data was used to measure the remaining
integral fraction of Si-CH 3 chemical bonds in the low-k
film as the indicator of the film damage. Ellipsometry was
used to measure the film thickness and refractive index to
analyse both film damage and etching rate. EDX data was
used to measure integral density of C, F, and O atoms in
the film while XPS was used to analyse various chemical
bonds appearing on the surface during phases of damage
and etching.
for activation energy in Si-CH 3 reaction with F atoms:
~1500 K.
1.0
0.8
-30C
0.6
-15C
0.4
0C
0.2
+15C
0.0
0
1000
2000
3000
4000
5000
6000
F exposure time, s (F dose)
Fig. 2.Si-CH 3 depletion by F atoms
28000
XPS (7200 s), a.u.
Fig. 1.ICP downstream reactor
CF2 CF3
ALK B
24000
F(1s)
F-C
20000
exper.data
approximation
F-Si
16000
12000
8000
4000
0
680
682
684
686
688
690
692
694
696
Binding energy, eV
Fig. 3.XPS spectrum for F on ALK B surface after
7200s treatment by F atoms
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[Si-CH3]treated/[Si-CH3]pristine depletion rate
Fig. 5.C, O and F atom density dependency on treatment
time (with Si-CH 3 dynamics for comparison)
The etching process has two distinct phases. In the first
phase the film thickness is approximately constant while
in the second phase it starts to decrease linearly.
1
0.1
Ea=1500+/-200 K
CVD1 (24%)
ALK B (40%)
SOG2.2 (39%)
SOG2.0 (44%)
SOG1.8 (51%)
0.01
1.05
1.10
1.15
1.20
1.25
300/T, K-1
Fig. 4.Rates of Si-CH 3 depletion in low-k films for
different temperatures
4. Low-k film etching
However the reaction with –CH 3 groups is not the only
effect of fluorine atoms on the low-k films. According to
EDX data shown in Fig. 5 the initial step of treatment is
the fluorination. It occurs even at small F doses when
fluorine atom density in the film does not change
significantly until the thickness of the film starts to
decrease due to etching.
Fig. 6. Evolution of the film thickness (reduced to 200
nm) of OSG films with F dose
The dynamics of film thickness is shown in Fig 6. and
can be explained in the following way. Fluorine atoms
penetrate in the film bulk with fluorine atom density in
the top layer of the film being higher due to loss in
chemical reactions. Pore walls are etched by fluorine
while the etching rate in the top layer is higher due to
higher fluorine atom density. When pore walls are
completely etched in the uppermost layer of the film, the
thickness of the film starts to decrease providing some
effective etch rate. This effective etch rate depends on the
pore wall etch rate as well as the fluorine atom density
profile in the film bulk. This mechanism is illustrated by
Fig. 7.
F dose = Fd
F collisions
with walls
1.2
SOG2.0
F
etched
pores
300
1.0
0.8
200
O
0.6
F
C
0.4
0.2
0.0
F atoms (a.u.)
Portion of Si-CH3; C and O
F
100
SiOx
pristine
pores
Si-CH3
0
1x1020
2x1020
3x1020
2
F atoms dose (at/cm )
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0
Fig. 7. Porous low-k film etching by F atoms
3
It is worth noting that the etching rate similarly to the
damage rate decreases with decreasing temperature.
5. Conclusion
Fluorine atom impact on OSG low-k films leads to few
effects: fluorination, damage and etching. Fluorination is
fastest process leading to formation of surface fluorine
complexes on pore walls. Damage is mostly caused by H
abstraction reactions generating –CH x F y species (instead
of –CH 2 and –CH 3 ) on the pore surface. Etching takes
place not only at the surface of the film but also in the
film bulk. Therefore the effective etch rate is higher for
films with the deeper fluorine atom penetration. The
higher porosity provides the higher area of open surface
pores and thereby promotes the higher rates of damage
and etching. Both damage and etching rates drop with
decreasing temperature that indicates existence of
activation energies. The obtained data allowed estimating
activation energy in Si-CH 3 reaction with F atoms: ~1500
K. It shows that the low damage by fluorine atoms can be
possible only at the temperature close to cryogenic.
6. Acknowledgements
This research is supported by SRC program Contract
2012-KJ-2280, RFBR, research project No. 14-02-31599
mol-a and by Optec grant № 28/2014/71-Msk.
4
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