REDUCTION OF PLASMA INDUCED DAMAGE IN CRYOGENIC
ETCHING OF LOW-K MATERIALS
T. Tillocher1, F. Leroy1, L. Zhang2, P. Lefaucheux1, K. Yatsuda3, K. Maekawa4, C. Dussarrat5, J.-F. de
Marneffe3, M. Baklanov3 and R. Dussart1
1
GREMI, CNRS/Université d’Orléans, Orléans, France
2
IMEC, Leuven, Belgium
3
Tokyo Electron Ltd., Tokyo, Japan
4
TEL Technology Center, America, LLC, Albany, USA
5
Air Liquide Laboratories, Tsukuba, Japan
e-mail of contact author: thomas.tillocher@univ-orleans.fr
Topic: Etching and surface processing
Abstract— Etching porous low-k materials is
very challenging because of plasma induced
damage. Passivation, effective in SF6 plasma
when the substrate is cooled to cryogenic
temperature, can reduce damage. This can
also be achieved with condensation of the
initial chemistry (SF6 and FC gas). The choice
of the FC gas determines the process
temperature.
I.
INTRODUCTION
Porous Organosilicate Glasses (p-OSG) are low-k
materials that have been introduced to reduce the RC delay
and power consumption in advanced interconnects.
However, due to a significant porosity and the pore size,
integration of such materials is limited by Plasma Induced
Damage (PID). It has been demonstrated that porous OSG
are very sensitive to radicals, ions and photons. In
particular, interconnected pores promote diffusion of
radicals, which further increase PID. This results in carbon
depletion and material structure modification which
eventually leads to the increase of the k-value and leakage
current. Several solutions have been proposed to reduce
PID: late porogen removal approach [1], post-damage
repair process [2] and post-porosity plasma protection
(P4)[3].
We propose different approaches. The first relies on
passivation of the pores, when p-OSG is etched with SF6 at
cryogenic temperature [4,5]. The second is based on the
condensation of the initial reactants, a SF6/FC gas mixture.
Appropriate selection of the FC gas gives control on the
required substrate temperature [5].
II.
EXPERIMENTAL
An ICP etching tool with a diffusion chamber has been
used for all experiments. The substrate holder, located at
the bottom of the diffusion chamber, is cooled with a liquid
nitrogen circulation. Temperature can be regulated between
-150°C and 40°C. Helium backside cooling enables good
thermal contact between the chuck and the wafer.
The material used in this work was OSG with k=2.2
spin-on deposited on (100) silicon wafers. Porosity is 37%
and pore radius is 1.4 nm. Samples were 4x4 cm² coupons
glued on SiO2 carrier wafer with a thermal paste.
Substrates were introduced in the reactor via a loadlock.
After etch, substrates were kept in the loadlock under N2
atmosphere for warming-up without moisture uptake.
Thickness and refractive index were monitored inprocess by in-situ ellipsometry. Post-plasma damage was
evaluated using an FTIR in transmission mode. The
concept of “Equivalent Damage Layer” (EDL) was
introduced to quantify the carbon depletion.
In parallel to etch experiments, desorption mass
spectrometry was performed to analyze desorbed species
during the warming-up of etched low-k surfaces. This helps
to figure out which mechanisms are involved in the
protection process of the material.
III.
RESULTS
A. Cryoetching with SF6 plasma
This process takes advantage of a SiOxFy passivation
layer that can be formed when the substrate is cooled down
to a cryogenic temperature, typically below -100°C.
The principle is directly inspired from the standard
cryogenic process used for silicon deep etching. In this last
case, an SF6/O2 plasma interacts with a silicon wafer
cooled down to a low temperature of typically -100°C. This
leads to the formation of a SiOxFy passivation layer on the
feature sidewalls, which prevents lateral etching [6]. This
protection mechanism should help to decrease PID on lowk materials.
Etch tests were performed in pure a SF6 plasma. O2 is not
not necessary for passivation since OSG already contains O
bonded to Si (Si-O-Si bonds). Processes with and without
self-bias voltage (-120V) were compared. The first case
simulates what would happen at feature bottom while the
second represents the sidewalls during feature etching. SF6
flow was 50 sccm, pressure 3 Pa and source power 500W.
Both the etch rate and the normalized EDL are represented
for -120°C, -50°C, 20°C in Fig. 1. These results clearly
show a significant reduction of the EDL at -120°C both
with and without self-bias voltage. The EDL is much
higher at -50°C and 20°C than at -120°C, even without
bias, which shows that a very low temperature provides a
real benefit. The role of the source power has been
investigated between 200W and 1500W (not represented).
Both etch rate and EDL increase with source power. 500W
power corresponds to an optimum where the EDL is the
lowest and the etch rate is close to zero without any ion
bombardment.
voltage is. Consequently, with C4F8 condensation, the
process becomes less damaging [5].
Figure 2. OSG 2.2 etch rate and normalized EDL versus substrate
temperature and self-bias voltage (-135V and 0V) with C4F8 condensation
followed by an SF6/C4F8 etch plasma.
Figure 1. OSG 2.2 etch rate and normalized EDL as function of the
substrate temperature and self-bias voltage after an SF6 etch plasma.
In-situ ellipsometry has revealed that part of etch byproducts remains in the film. A post-etch annealing (350°C,
under N2, 15 min) has been subsequently used to desorb the
trapped etch products, without any additional damage, as
confirmed with FTIR.
Desorption mass spectrometry was performed on 8x8
cm² OSG samples glued on a SiO2 carrier wafer. After a 2
min etch process, the temperature was gradually increased
from -120°C to 20°C. The desorbed species were detected
by a mass spectrometer mounted on the diffusion chamber.
This revealed in particular a significant desorption of SiF4 at
-60°C, which is related to passivation. This suggests that a
passivation layer is present on the low-k material.
Desorption of CxFy molecules is also observed at -50°C,
which shows that fluorocarbon species may play a role in
the protection mechanism of the low-k material. Therefore,
it can be assumed that a carbon-containing passivation layer
is formed in the pores and prevents diffusion of free radicals
in the bulk material.
B. Cryoetching with FC-based plasma
As CxFy molecules are involved in the passivation of
low-k materials, C4F8 has been injected in the plasma to
further reduce PID. In-situ ellipsometry has revealed an
interesting phenomenon: at -120°C, C4F8 gas effectively
condenses in the pores of the low-k layer. The isobar plot
of the film refractive index as a function of the temperature
draws a hysteresis cycle. It increases, during condensation,
from its initial value (1.32) to a maximum at -120°C (1.42)
and then, remains stable. During desorption, the refractive
index starts to decrease only for a temperature above 90°C. The hysteresis is also observed in the case of a
constant temperature and varying pressure [5].
C4F8/SF6 plasma parameters are: total pressure 3Pa,
500W source power, -135V self-bias voltage, -120°C. The
plasma is followed by an ex-situ annealing (350°C, under
N2, 15 min). The resulting etch rate and normalized EDL
are plotted and compared to the pure SF6 process with or
without self-bias voltage in Fig.2. Both EDL and etch rate
are lower with SF6/C4F8 plasma whatever the self-bias
Another FC gas has been investigated as a candidate for
damage-free etching. The experimental study of its
condensation dynamics has shown that a low-damage
process can be run at a higher substrate temperature, -80°C.
Therefore, the choice of the FC determine the temperature
necessary for a damage free process. In addition, the SF6/FC
gas flow ratio tunes the C/F ratio, which an extra knob for
process control.
IV.
CONCLUSION
Plasma Induced Damage, which is a major issue in
etching of porous low-k materials, can be greatly reduced
with a cryogenic process. A passivation layer is believed to
form into the pores with SF6 plasma at cryogenic
temperature of the substrate. This prevents carbon
depletion in the bulk material.
FC gas can be added during plasma etching. It condenses
into the pores and prevents the diffusion of radicals
responsible for PID. This helps to further reduce carbon
depletion. Post-etch annealing must be performed to fully
desorb etch by-products or condensates and recover a
surface state close to its pristine state.
Consequently, cryogenic etching can be considered as a
low damaging process of interest for porous low-k
materials.
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