Development of the sputtering yields of ArF photoresist … · Development of the sputtering yields...

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Development of the sputtering yields of ArF photoresist after the onset of argon ion bombardment Takuya 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 Articles Revisiting the mechanisms involved in Line Width Roughness smoothing of 193nm photoresist patterns during HBr 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, and photodetection 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

Transcript of Development of the sputtering yields of ArF photoresist … · Development of the sputtering yields...

Page 1: Development of the sputtering yields of ArF photoresist … · Development of the sputtering yields of ArF photoresist after the onset of argon ion bombardment Takuya Takeuchi, Carles

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

Page 2: Development of the sputtering yields of ArF photoresist … · Development of the sputtering yields of ArF photoresist after the onset of argon ion bombardment Takuya Takeuchi, Carles

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)

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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)

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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)

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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)

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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)

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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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