Development of the sputtering yields of ArF photoresist after the onset ofargon ion bombardmentTakuya Takeuchi, Carles Corbella, Simon Grosse-Kreul, Achim von Keudell, Kenji Ishikawa et al. Citation: J. Appl. Phys. 113, 014306 (2013); doi: 10.1063/1.4772996 View online: http://dx.doi.org/10.1063/1.4772996 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v113/i1 Published by the American Institute of Physics. Related ArticlesRevisiting the mechanisms involved in Line Width Roughness smoothing of 193nm photoresist patterns duringHBr plasma treatment J. Appl. Phys. 113, 013302 (2013) Modeling of feature profile evolution for ion etching J. Appl. Phys. 113, 014305 (2013) The penetration limit of poly(4-vinyl phenol) thin films for etching via holes by inkjet printing Appl. Phys. Lett. 101, 253302 (2012) The penetration limit of poly(4-vinyl phenol) thin films for etching via holes by inkjet printing APL: Org. Electron. Photonics 5, 270 (2012) TiO2 nanoparticles and silicon nanowires hybrid device: Role of interface on electrical, dielectric, andphotodetection properties Appl. Phys. Lett. 101, 253104 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
Development of the sputtering yields of ArF photoresist after the onsetof argon ion bombardment
Takuya Takeuchi,1,a) Carles Corbella,2 Simon Grosse-Kreul,2 Achim von Keudell,2
Kenji Ishikawa,1 Hiroki Kondo,1 Keigo Takeda,1 Makoto Sekine,1 and Masaru Hori11Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan2Research Group Reactive Plasmas, Ruhr-Universit€at at Bochum, Bochum 44780, Germany
(Received 8 October 2012; accepted 6 December 2012; published online 4 January 2013)
Modification of an advanced ArF excimer lithographic photoresist by 400 eV Ar ion irradiation
was observed in situ in real time using both infrared spectroscopy and a quartz microbalance
sensor. The photoresist sputtering yields had a characteristic behavior; the sputtering yields were
higher than unity at the beginning, until an ion dose of 2� 1016 ions cm�2. Thereafter, the yields
decreased immediately to almost zero and remained constant with the yield at zero until a dose of
approximately 4� 1016 ions cm�2 was reached. At larger doses, the yields increased again and
reached a steady-state value of approximately 0.6. This development of the sputtering yield after
the onset of ion bombardment is explained by an ion-induced modification of the photoresist that
includes preferential sputtering of individual groups, argon ion implantation and the generation
of voids. All these effects must be taken into account to assess line-edge-roughness on a
photoresist subjected to highly energetic ion irradiation. VC 2013 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4772996]
I. INTRODUCTION
In the fabrication of semiconductor devices, the litho-
graphic pattern transfer of masks into underlying materials by
plasma etching is the basis for sophisticated nano-scale proc-
essing. To reduce the critical dimensions, wavelengths in vac-
uum ultraviolet (i.e., ArF excimer laser light source 193 nm)
or below are necessary to overcome the diffraction limit. Con-
sequently, the photoresist material must also be sensitive to
vacuum ultraviolet (VUV), and methacrylates have been cho-
sen in the past as a major candidate for ArF excimer laser
photolithography. However, methacrylates have poor plasma
etch resistance1,2 which leads to deformation of the etched
feature, such as distortion, bowing and line edge roughness
(LER) on the sidewalls of the etched patterns.3,4
To reduce those deviations, many researchers have
investigated photoresist modification during plasma etch-
ing.5,6 However, the mechanisms of plasma photoresist inter-
action are still unclear with respect to the individual roles of
not only ions, radicals, and VUV/UV but also byproducts
produced from the sample surface.
To separate each effect, experiments employing particle
beams have been valuable to simplify the complex chemistry
of plasmas containing hydrogen, fluorine, and fluorocar-
bons.7–9 Thus, the individual contributions of gas phase and
surface chemistry can be isolated for identification. Moreover,
the effects of ions can be characterized by direct control of
the bombardment energy. In addition, the experimental results
of beam studies can support simulations of plasma processes,
and this has been the basis for the development of the plasma
etching models for silicon and silicon dioxide.10–13 Previous
beam studies on photoresist modification have fundamentally
focused on the mechanism of surface roughness formation as
a function of ion energy, on exposure to VUV, and also of the
substrate temperature.14–16 It was concluded that energetic ion
irradiation plays a key role in the deformation of the photore-
sist. However, the particular nature of the ion-induced change
in the photoresist structure during the plasma etching process
has yet to be clarified.
In this work, experiments were conducted using particle
beams to examine the etching process of a methacryl resin
photoresist as used for ArF excimer laser lithography. The
experiment employed a quantified energetic argon ion beam
to reveal the fundamental modification process of the photo-
resist. Sputtering yields (SY; sputtering rate normalized to
the ion dose rate) of the photoresist were evaluated in real-
time by quartz crystal microbalance (QCM) measurements.
In addition, the chemical changes in the photoresist films
were characterized using in situ Fourier transform infrared
spectroscopy (FTIR).
II. EXPERIMENT
Figure 1 shows the experimental setup of the in situ Ar
ion beam system.17 This system is consisted of a chamber
evacuated with a turbo molecular drag pump to yield a base
pressure of 5� 10�6 Pa. Three instruments were mounted at
the center of the chamber: (1) a sample stage, (2) a Faraday
cup, and (3) a QCM sensor head (Inficon Co., Ltd.). An ion
beam source was facing the QCM. The Faraday cup mounted
on the sample holder was used to measure the ion current den-
sity. This system was also equipped with two KBr infrared-
transparent windows and contained a load lock chamber for
the introduction of samples without exposure to the air.
The ion beam was generated using a commercial plasma
ion source (Tectra GmbH, Gen2 Plasma Source),18 which
generates plasmas by electron cyclotron resonance (ECR). Aa)electronic mail: [email protected].
0021-8979/2013/113(1)/014306/6/$30.00 VC 2013 American Institute of Physics113, 014306-1
JOURNAL OF APPLIED PHYSICS 113, 014306 (2013)
pure argon gas flow at a rate of 1.0 sccm was introduced into
the source through a gas inlet port, and the pressure in the
chamber was usually increased to 2.9� 10�2 Pa. The argon
plasma was generated by applying microwave (2.45 GHz)
power to an antenna. The source contained two grid electro-
des in front of an exit aperture with a diameter of 10 mm.
The first grid defines the plasma potential and thus deter-
mines the ion energy with respect to the grounded substrate,
while the second is negatively based with respect to the first
grid to extract the ions. The distance between the exit aper-
ture and sample position was 90 mm. A sample within an
area of approximately 20 mm in diameter could be uniformly
irradiated using this setup.
The argon ion beam is produced by an ECR plasma in
direct line-of-sight from plasma to sample. Thereby, all films
are inherently exposed to VUV photons at 105 nm and
106 nm in the emission spectrum of an argon ECR plasma.19
An absolute quantification of these VUV photon fluxes is
extremely difficult, because standard calibration sources for
a VUV spectrometer in that wavelength region are basically
missing besides large scale synchrotron sources. This is
beyond the scope of this paper and we limit ourselves to
experiments conducted at a constant plasma density assuring
a constant VUV photon background flux irrespective of the
energy of the extracted ion beam.
Samples were base polymers of the ArF excimer photoli-
thographic resist (Tokyo Ohka Kogyo Co., Ltd., TARF-P6111
ME), with the chemical structure shown in Fig. 2.20 The poly-
meric material comprises a copolymer of the following four
kinds of methacrylates: (1) tert-butylmethacrylate (tBuMA),
(2) methyl methacrylate (MMA), (3) methacrylic acid (MAA),
and (4) 2-naphthylmethacrylate (2NpMA). The compositional
ratio of these four units was not disclosed by the supplier.
Films of the photoresist were spin-coated on the substrate.
For the QCM measurements, photoresist films approxi-
mately 200 nm thick were coated on aluminum-coated quartz
crystals. For the FTIR measurements, approximately 50 nm
samples thick deposited on oxidized silicon substrates with
an aluminum backside coating were used. These substrates
are designed as optical cavity substrate (OCS) to enhance the
infrared sensitivity.21
The SY of the photoresist is defined as the number of
carbon atoms removed from the photoresist per incident ion
and is given by:
SY ¼ mcA
MD; (1)
where m is the mass removed per second, c is the number of
carbon atoms in one molecule of photoresist base polymer, Ais the Avogadro number, M is the molecular mass of photo-
resist, and D is the current of incident ions. D is calculated
from the ion current density measured from the Faraday cup.
A typical ion flux of 1.75� 1014 ions cm�2 s�1 under an
acceleration voltage of 400 V and an extraction voltage of
0 V was used. The pressure of the main chamber was kept at
3.9� 10�2 Pa during the process for an argon flow rate of
1.0 sccm.
The QCM measurements were performed every 0.5 s
during all experiments. A FTIR spectrum was accumulated
for 30 s. Each of the sampling periods for QCM and FTIR
measurements corresponds to ion doses of 8.75� 1013 and
5.25� 1015 ions cm�2, respectively. These two methods ena-
ble in situ, real-time measurements of the sputtering yield,
and measurement of the chemical changes of the photoresist.
The modification of the films could be analyzed for dif-
ferent treatment times at a constant ion dose or for different
ion doses at constant treatment time. Thereby, any explicit
flux dependence of the ion-induced damage could be identi-
fied. Such dependence is, however, not expected since any
non-linear effects in sputtering require competing time con-
stants with respect to the picosecond evolution of a collision
cascade. Such effects are known in nuclear fusion for hydro-
carbon sputtering at extremely high ion flux densities of
1020 cm�2 s�1. In our system, the fluxes are 6 orders of mag-
nitude below, so that any direct and explicit flux dependence
is not anticipated. Therefore, we kept the representation of
the data as a plot of etch rate vs. ion dose.
III. RESULTS AND DISCUSSION
A. SY dependence on the ion dose
The weight change of the photoresist coated on the
quartz crystal was measured in real time during Ar ion bom-
bardment with an ion energy of 400 eV. Figures 3(a) and
3(b) show the weight loss and SY of the photoresist as a
function of ion dose from 0 to 1� 1017 ions cm�2, respec-
tively. Interestingly, a dependence of the sputtering process
on the ion dose was observed, which can be divided into
FIG. 1. Ion beam apparatus composed of (1) a substrate holder that can be
replaced by a QCM and Faraday cup, (2) ECR ion beam source, and (3), (4)
windows for infrared light.
FIG. 2. Chemical structure of the ArF photoresist.
014306-2 Takeuchi et al. J. Appl. Phys. 113, 014306 (2013)
three phases: (phase I) a high weight loss is observed with
SYs higher than unity until a dose of approximately
1.0� 1016 ions cm�2; (phase II) the weight loss is almost
negligible between a dose of 1.0� 1016 and 6.0� 1016 ions
cm�2, which implies an almost zero SY, although the highly
energetic ions are still bombarding the surface; (phase III)the weight loss increases again and the SYs reached a
steady-state beyond a dose of 6.0� 1016 ions cm�2.
During phase I, the extremely high SYs may be caused
by the ion-induced spontaneous evaporation of volatile spe-
cies from the photoresist or by efficient chemical sputtering.
Physical sputtering at normal incidence and at low ion ener-
gies generally results in smaller yields than unity due to the
limited back scattering in the collision cascades. Thus, we
consider that the high SYs in phase I are caused by the re-
moval of residual solvents or by chemical sputtering of
weakly connected parts of the photoresist network.
However, the plasma ion source is also an intense emit-
ter of VUV/UV radiation. The effect of the ions may be lim-
ited to a region at the topmost surface of photoresist, but the
VUV/UV light may reach to greater depths in the photore-
sist. As a result, the entire film may be modified by VUV/
UV emitted from the argon plasma.
In phase II, the SY drops to almost zero at an ion dose of
2� 1016 ions cm�2. The decrease of the SY may be caused by
the lack of low etch-resistance part of the photoresist, which
is almost removed during phase I. Furthermore, the effect of
the photoresist cross-linking should also be considered. The
photoresist may be completely modified due to cross-linking
between the polymer chains, which would result in a low
SY.22 However, if the decrease of SY is caused by an increase
of etch-resistance, the SY should already reach a low steady
state during phase II. Instead, a SY of almost zero is observed
in phase II. This phenomenon of a zero SY may be explained
by the hypothesis that a part of the impinging argon ions are
dynamically trapped inside the photoresist and thereby com-
pensate the mass loss of the photoresist caused by physical
sputtering. Argon ion implantation with an ion dose of
6.0� 1016 ions cm�2 reached in phase II would be sufficient
to counterbalance the mass removal of the photoresist.
This ion dose can be compared with standard plasma
etching systems with a typical plasma density in front of the
wafer surface in the order of 1011 cm�3, which yields an ion
flux in the order of 1016 ions cm�2 s�1. Consequently, the
ion dose of 6.0� 1016 ions cm�2 is reached in plasma sys-
tems within a few seconds, which is too short to be detected
in most plasma experiments.
In phase III, at ion doses above 6.0� 1016 ions cm�2,
the SYs saturate at a steady state value of approximately 0.6.
This yield is typical for organic materials, such as the photo-
resist under investigation.23 For inorganic materials, lower
yields for physical sputtering ranging between 0.1 and 0.3 at
energies of a few hundred volts have been reported.24
Many researchers have discussed the effects of physical
ion bombardment onto organic materials. The initial decrease
of the SYs was interpreted as a hardening of the organic mate-
rial by cross-linking and/or the formation of graphitic struc-
tures. Under ion bombardment, radicals may be generated in
the photoresist, which then cross-link and form a more inter-
connected network. The preferential sputtering of hydrogen
also leads to unsaturated carbons that cause graphitization.
Such cross-linking is believed to cause the reduction of the
SYs in phase II.
B. Surface modified layer
We postulate a characteristic surface structure to explain
the transient behavior of the SYs. This is investigated using
FTIR analysis of the C@O and C-Hx bonds of the methacry-
late groups. In situ FTIR measurements were conducted in
real time under identical conditions as those for the QCM
measurements.
Figure 4 shows a series of spectra for infrared reflec-
tance R normalized to the reflectance R0 for the pristine pho-
toresist. An increase of R/R0 indicates that the concentration
of groups corresponding to a characteristic peak decreases in
the film due to erosion. The peaks of C@O groups appear at
1720.2 and 1733.7 cm�1. As the ion dose increases, the peak
intensities increase (Fig. 4(a)), which indicates the removal
of C@O groups in the methacrylates. Figure 4(b) shows that
the intensities of the C@O peaks increase almost monotoni-
cally with the ion dose.
FIG. 3. (a) Removed mass and (b) SY of ArF photoresist for 400 eV Arþ as
a function of the ion dose.
014306-3 Takeuchi et al. J. Appl. Phys. 113, 014306 (2013)
In contrast, the peak at 1793.5 cm�1 increases almost
instantaneously at the beginning of beam exposure and then
gradually reaches a steady state. This peak may be assigned to
MAA or 2NpMA groups according to the quantum chemical
simulation.25 The initial strong change of the 1793.5 cm�1
peak occurs simultaneously with the weight loss in phase I.
We assume that the C@O groups are also degraded due to
VUV illumination from the ECR ion source.15,26–28 This
VUV can penetrate into any polymer to a depth of around
100 nm, which is sufficient to completely modify the 50 nm
layers. To test this assumption, we exposed the samples to the
VUV of the ECR plasma source only by blocking ions with
the extraction grid. Figure 5 shows a comparison of the evolu-
tion of C@O peaks during exposure to VUV alone, and to
both VUV and 400 eV ions as a function of time. The results
indicate that the C@O bonds are broken by VUV alone,
although the change is smaller than that caused by exposure
to both VUV and ions. We conclude that during phase I,
C@O groups are etched by VUV illumination and the high
SY is mainly caused by desorption of C¼O groups.
Different gradients of peak change at 1793.5 cm�1
between phases I and II were observed. Most of the C@O
bonds related to 1793.5 cm�1 are broken mainly by VUV dur-
ing phase I. The thickness of the modified layer by incident
ions is considered to be less than 10 nm under conditions with
energies of some hundred eV.29 Thus, ions gradually sputter
the photoresist material from the surface. In phase II, residual
C@O bonds are sputtered by Ar ions, as with other groups
such as C-O and C-Hx, which results in a low-gradient
change. Therefore, the removal of C@O bonds is caused
mainly by incident Ar ions during phases II and III.
Figures 6 and 7 show the spectral changes in the regions
for the C-O and C-Hx groups, respectively. The evolutions of
C-O and C-Hx bonds have similar tendencies, although the
variation of the C-O groups is smaller than that exhibited by
the C-Hx bonds. The changes in C-Hx are in good agreement
FIG. 5. Comparison of IR peak intensity variations of C@O during exposure
to VUV alone and to both VUV and ions.
FIG. 6. Dependence of CAO (a) IR absorption spectra and (b) peak height
variations on the ion dose.
FIG. 4. Dependence of C@O (a) IR absorption spectra and (b) peak height
variations on the ion dose.
014306-4 Takeuchi et al. J. Appl. Phys. 113, 014306 (2013)
with the weight loss measured by QCM during phase I: at
the onset of ion exposure, the peaks immediately increase
until an ion dose of 1.0� 1016 ions cm�2 is reached during
phase I. The peaks then continue to increase during phase II
at a modest rate with respect to the ion dose. Finally, the
modification of the photoresist reaches a steady state in
phase III as the ion dose exceeds 6� 1016 ions cm�2. Appa-
rently, the decrease of the C-Hx groups agrees well with the
weight loss of the photoresist.
Comparing the weight loss in Fig. 3 with the change in
C-Hx groups in Fig. 7(b) reveals a slight difference espe-
cially in phase II. The C-Hx peaks continue to increase
slowly although the weight loss vanishes corresponding to
the zero SY in phase II. Therefore, we postulate that the zero
SY measured with the QCM is caused by a balance between
the removal of the photoresist (observed with FTIR and
QCM) by sputtering and argon ion implantation (observed
with QCM only). We assume that incident Ar ions are
trapped within the top layer of the photoresist, which causes
a lowering of the apparent SY measured with the QCM. At
the same time, the ion implantation may weaken the photore-
sist structure by the formation of bubbles or voids.30,31 Thus,
the increase of the SY in phase III may be caused by satura-
tion of argon ion implantation combined with an ion-induced
weakening of the photoresist surface.
C. Model for ArF photoresist sputtering by Ar plasma
Figure 8 shows a schematic model for photoresist sputter-
ing by Ar ions. Sputtering progresses through a sequence of
phases, depending on the ion dose. At the beginning, many
C@O bonds are present in the pristine photoresist, as indicated
in Fig. 8(a). These three phases are distinguished as:
Phase I (Fig. 8(b)): At the onset of sputtering up to an
ion dose of 1.0� 1016 ions cm�2, sputtering is governed by
the following processes: C@O bonds are easily broken by
VUV or by a synergetic effect from both VUV and ion bom-
bardment. The SY is very high due to the removal of broken
C@O groups and any residual solvent.
In phase II, ion-induced cross-linking and graphitization
occurs, which results in a higher etch resistance and incident
FIG. 7. Dependence of C-Hx (a) IR absorption spectra and (b) peak height
variations on the ion dose.
FIG. 8. Proposed mechanism for Ar
plasma sputtering of the photoresist: (a)
before the process, (b) component of
increasing SY during phase I, (c) com-
ponent of decreasing SY during phaseII, (d) during phase III.
014306-5 Takeuchi et al. J. Appl. Phys. 113, 014306 (2013)
Ar ions are partially trapped in the photoresist, as shown in
Fig. 8(c). This ion implantation may counterbalance the
mass-loss due to continuous sputtering of the photoresist.
The net SY reaches almost zero.
Between phases II and III, as shown in Fig. 8(d), Ar ion
trapping reaches a steady state, which results in a concentra-
tion maximum of voids or bubbles and a surface modified
layer with a weakened structure. The SY increases between
phases II and III until a steady state is reached with a SY of
0.6 in phase III. Some of the bombarding Ar ions sputter the
photoresist, and others penetrate into the modified layer and
continually cause new bubble formation at the boundary
between the already-modified and the unmodified layers.
Our experiments may be compared with plasma induced
roughening experiments in the literature by Oerhlein
et al.,32–34 who present a two-step mechanism to explain the
roughness evolution of plasma-treated thick photoresist
films: First, a densified layer is created by the ion bombard-
ment and the underlying polymer is altered by VUV radia-
tion. Second, the damage changes the mechanical properties
of the photoresist and any stress in the surface layers induces
the underlying polymer to buckle. This effect is most severe,
if any surface heating during plasma treatment occurs caus-
ing the polymer temperature to rise above its glass transition
temperature. The experiments of Oehrlein et al. are plasma
experiment using rather high ion fluxes, where the formation
of the damage surface layer is very fast. Our experiments are
complementary to this, because our ion fluxes are smaller
and we can follow the very beginning of the ion beam treat-
ment of very thin photoresist layers more accurately. It will
be interesting to evaluate in the forthcoming works, how the
proposed argon void/bubble formation according to our
results may affect the mechanical properties of the stressed
surface layers as being the driving force for roughness devel-
opment in plasma treated photoresists.
IV. CONCLUSIONS
We investigated the in-situ real-time modification of the
photoresist by Ar ion beam extracted from an ECR plasma
source. The structural changes of the photoresist were meas-
ured using FTIR and the SY was measured using a QCM.
The data analysis revealed three phases; phase I at the begin-
ning of the process, C@O bonds are broken by incident ions
and by VUV from the plasma ion source until an ion dose of
2.0� 1016 ions cm�2 is reached. In phase II, Ar ion implan-
tation may compensate the mass loss due to sputtering,
which results in an apparent net zero SY. After saturation of
argon ion implantation between 2.0� 1016 and 6.0� 1016
ions cm22, the SY increases again and reaches a constant
value of 0.6 carbon/ion.
ACKNOWLEDGMENTS
This work was supported by the International Training
Program promoted by the Japan Society for the Promotion of
Science. The beam experiment was funded by the SFB-TR
87 project C7 of the German Science Foundation.
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