Absorbance (a.u.)
New materials with nanoscale porosity are now being
available as a result of developments in synthetic
chemistry and materials science. These materials exhibit
many interesting properties such as extremely high
surface area and low density. There is growing interest in
microelectronics to use these new porous materials as
membranes to realize Micro-Electro-Mechanical Systems
(MEMS) or as ultra low dielectric constant material in
advanced interconnections. For these applications, one
challenge is to perform thin porous films layers (<1 m)
by using processes compatible with microelectronic
industry. In case of dielectric films, the deposition
techniques used are spin-coating or Plasma Enhanced
Chemical Vapour Deposition (PECVD). The first one
allows depositing many different films, by benefiting of
sol-gel science but the second one is usually preferred by
the microelectronic industry. In this work, several
synthesis ways were studied to deposit nanoporous
SiOCH layers (pore diameter lower than 5 nm) by spincoating. Then, the possibility to deposit porous films by
PECVD using these different sol-gel concepts was
investigated.
A first approach consists in depositing a dual-phase thin
film using silsesquioxane precursor for the matrix and a
sacrificial organic molecule (template or porogen
approach). The porogen is degraded during a subsequent
thermal treatment. A second approach is based on a
solution of nanocluster silica precursors which can be
crosslinked during a curing treatment. A third alternative
is to use a solution of partially crosslinked silsesquioxane
oligomers with terminal silicon atoms carrying a hydroxyl
group. This silanol can condensate during curing to give a
three-dimensional cross linked network and the porosity
can be controlled by adjusting the content of functional
end group and the ratio of cage to network structure
(polymer approach). All this technique has been
successfully tested to perform, by spin-coating, porous
SiOCH thin film with nanoscale porosity. A last possible
investigated was a foaming technique which allows
creating porosity by the use of gas nucleation in a
polymer matrix but this technique lead to macropores and
is not reported for thin films.
The possibility to deposit porous films by PECVD
using these different concepts was then investigated. This
work shows that the template approach can be performed
by using CVD technique. Indeed, it is possible to perform
porous thin films by co-depositing a matrix precursor
(organo-silane) and a sacrificial organic porogen,
followed by a post-treatment to remove the organic
porogen phase and create porosity. As in the case of a solgel approach, the thermal annealing causes a film
thickness decrease due to material crosslinking, structural
rearrangements and nanopores formation. Figure 1 shows
that numerous similarities can be observed between both
deposition techniques (similar matrix, hybrid and porous
Si-O-Si
Si-CH3
CHx
Matrix
C=O
Porogen
Hybrid film
Porous film
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Si-O-Si
Absorbance (a.u.)
V. Jousseaume1, L. Favennec2, A. Zenasni1, O. Gourhant2,
P. Maury2 and J.P. Simon3
.
(1) CEA-LETI, 17 rue des Martyrs, 38054 Grenoble
Cedex 9, France.
(2) ST-Microelectronics, 850 rue Jean Monnet, 38921
Crolles, France
(3) CNRS/INPG/UJF-LTPCM,BP 75, 38402 St Martin
d’Hères, France.
film signature). In the template approach, the porosity is
generally related to the porogen loading in the hybrid
film. This study shows that a porogen loading threshold
exists in the case of PECVD, limiting the amount of
porosity which can be created by this technique (<40%)
while in case of spin-coating, porosity larger than 50%
can be easily performed.
Matrix
Porogen
CH3
CHx
Si-CH3
C=O
Hybrid film
Porous film
4000 3500 3000 2500 2000 1500 1000
500
-1
Wavenumber (cm )
Figure 1: FTIR spectra for the template approach (matrix,
porogen, hybrid and porous film signatures) performed
a) by spin-coating and b) by PECVD
Concerning the alternatives to the template approach,
cyclic siloxane precursors (such as Deca-methyl-cyclopenta-siloxane) were tested to try reproducing the spincoating nanocluster approach by incorporating large
cages in the deposited thin film. In that case, even if soft
plasma conditions allow keeping intact the siloxane ring
in the film, no porosity is created. The polymer approach
allows obtaining by sol-gel porous SiOCH with porosity
rate between 0 and 50% but this concept is not
transferable to PECVD. Finally, this work shows that it’s
possible to perform a foaming in a PECVD SiOCH thin
film by using an H2 plasma treatment. H2 reacts with
methyl (from Si-CH3) to form methane which can
coalesce and allow the formation of nanopores even in
thin films.
6
5
Pore diameter (nm)
Nanoporous SiOCH thin films:
From sol gel to PECVD
4
3
2
1
1.20
1.25
1.30
Refractive Index
1.35
1.40
Figure 2. Pore diameter obtained by GiSAXS vs. film
refractive index (at 633nm)
Spin coating:  polymer  template  nanocluster
PECVD:  template foaming
Structural characterizations of all these materials show
that the mean pore diameter (fig. 2) is dependant on the
deposition technique used and on the porosity rate. Spincoated dielectrics show mean pore radius higher than 1.5
nm while PECVD films deposited using a template
approach presents a low mean pore radius (close to 1 nm).