S-04-1
In situ Electron Spin Resonance Study of Plasma-Surface Interactions
電子スピン共鳴測定によるプラズマ表面相互作用のその場観察
Kenji Ishikawa, Naoya Sumi, Akihiko Kono, Hideo Horibe,Keigo Takeda, Hiroki Kondo, Makoto Sekine, and Masaru Hori
石川健治，鷲見直也，河野昭彦，堀邊英夫，竹田圭吾，近藤博基，関根誠，堀勝

Nagoya University, Kanazawa Institute of Technology
Furo-cho, Chikusa, Nagoya 464-8603, Japan;3-1, Yatsukaho, Hakusan, Kanazawa 924-0838 Japan名古屋大学 〒464-8603 名古屋市千種区不老町; 金沢工業大学 〒924-0838 石川県白山市八束穂3-1
During the plasma processes, simultaneous irradiation of electrons, ions, radicals, and photons is occurred
and therefore individual contributions of each of these species must be elucidated with atomic level. We
have focused on direct observation of the formation and destruction of dangling bonds for this purpose.
Actually the real time in situ electron spin resonance (ESR) technique was demonstrated elucidation ofplasma-surface interaction on polymer materials as examples for soft- and bio-materials. This opens a newexperimental approach to the microscopic understanding of chemical reactions on surfaces with gaseousradicals during plasma processes.

1. Introduction
Plasma discharge had been utilized in various
applications such as plasma-enhanced chemical
vapor deposition, plasma etching, plasma-induced
surface modification, plasma medicine, and so on.
Material-synthesis or chemical-functionalizationby employing plasmas has recently become ahot-topic in material science fields. Meanwhile,
to control those plasma induced damages and toimprove their process performance, chemical
reactions under plasma are needed to understandcomprehensively. During the plasma processes
simultaneous irradiation of electrons, ions,radicals, and photons is occurred. Therefore, theindividual contributions of each of these species
in the plasma must be elucidated.
Recently, the surface treatment of organic
materials using atmospheric pressure plasmas has
attracted much attention. For example, there isconsiderable interest in changing the surfacehydrophobicityofpoly(tetrafluoroethylene)(PTFE) or poly(methylmethacrylate)(PMMA)
using treatments such as UV radiation undervacuum at room temperature, oxygen plasma
exposure at low-pressure, or surface modification
using an atmospheric pressure He plasma. In
addition, not only photochemical processes withplasma-induced emission but also synergistic
photo- and radical processes play important rolesin the utilization of plasma.
To understand the dynamic changes in bonding
configurations during processing and surface
reactions, direct observation of the formation anddestruction of dangling bonds will provide
important information. Originally, Westenberg

reported that chemical reactions could beanalyzed using the electron spin resonance (ESR)
technique [1]. Previously Yamasaki et al.
succeeded in attempts measure dangling bonds insitu during silicon film growth using ESR [2].
Subsequently we have investigated the plasma
processes for plasma etching of dielectricmaterials and diamonds using in vacuo and in line
ESR technique [3]. Recently we have reported
synergistic formation of radicals by irradiation
with both vacuum ultraviolet and atomic
hydrogen by using a real-time in situ ESR [4].
In this study, we focused on in situ ESR
measurements
performed
duringplasmaprocesses using an ESR cavity and remote plasma
system. PTFE has C-F bonds with a relatively
high bond-energy of approximately 5 eV;therefore, it is difficult for photochemical
oxidation to occur. Using plasma discharge, C
dangling bond was generated during and after
surface modification. Dominant pathways undergiven conditions will be presented with atentative interpretation of their dynamic changes
which observed by ESR signals from both thesolid and gas phases.
2. Experimental
An ESR system was connected to a plasma
discharge system using a quartz tube with an innerdiameter of approximately 9 mm, as shown in Fig.
1. A microwave (2.45 GHz) power supply of 50 W
from a generator was used to generate plasma
inside of the quartz tube. Hydrogen gas was flowed
into the quartz tube at a flow rate of 50 sccm, andthe pressure was maintained at approximately 10 Pa

Plasma source
GasGlass tube
Φ10 mmdMicrowave(2.45 GHz)

Plasma emission
ESR cavity

RadicalPTFE

distance, dExhaust
ESR Magnet<Cross section> Microwave<ESR condition>
Microwave frequency
Microwave power
Modulation amplitude

Quartz tube

: 9.53 GHz: 20 mW: 2.0 GESR Cavity

PTFE
ESR Magnet
Magnetic field

Fig. 1. Schematic of the experimental setup

in the down-flow region.
ESR measurements were conducted using astandard X-band (9 GHz) spectrometer with amicrowave resonator. ESR spectra were recordedwith microwave power of 2 mW, a field modulationof 0.2 mT, and a modulation frequency of 100 kHz.
All experiments were performed at room
temperature. The quartz tube and polymer film
sample were inserted inside of the ESR cavity in the
down-flow region, typically 20 cm from the plasma
discharge. The sample used was a 0.1 mm thick
PTFE film. Both gaseous reactive species andsurface radicals on the PTFE film weresimultaneously detected in situ.
Individual contributions from reactive species ingas phase and plasma emission light have beenstudied in the manner using pallets for plasma
process evaluation (PAPE). Since the sample set atthe down-flow region, no effect of ion
bombardment was observed in this study because
the distance from the discharge was sufficiently
large and charged particles such as electrons andionswereimmediatelydiminishedbyrecombination. To limit interactions of plasma
emissions with the sample, a U-shaped quartz tubeand light shade were used as shown in Fig. 2.
Neutral reactive species such as atoms and radicals
in the gas-phase are relatively long-lived; therefore
a sufficient amount of neutral species weretransported to interact. In addition, inter-actions
RadicalLight (Plasma emissions)Lights

Shadowing platewith only plasma emissions were clarified using thetransparent MgF2 window, which can transmit
wavelengths above approximately 115 nm.
Individual roles for atomic H or plasma emission
When the sample was irradiated with only atomic
hydrogen, no C-DB was negligible to detect. The
lack of C-DB formation was due to the highbond-dissociation energy for C-F of approximately
5 eV.
For VUV irradiation, strong absorptionenergy of approximately 7.8 eV, which corresponds
to a VUV wave-length of 160 nm, was reported.
When the MgF2 window material was used, there
was formation of C-DB as shown in Fig. 4.
However, when the window material was changed
to a quartz window that cut the wavelengths to as
low as 250 nm, there was no formation of C-DB.
Notably, for irradiation with both atomic H andVUV, significant acceleration in the rate of C-DB
formation was observed at the beginning ofirradiation. This behavior can be interpreted thatatomic hydrogen may play a scavenging role inreaction with atomic fluorine which is released
from C-F bonds. This process is represented byd [ DB ]dtk (1 [ DB ])[ H ][ DB](1 e k[ H ]t ) (1)where k is rate coefficient. From this, synergistic
formation of surface C-DB occurred close to thesurface due to the scission of C-F bonds and wasaccelerated by VUV illumination and ruptured F
atom scavenging by H atom to form HF. Thissurface C-DB produced immediately changed toperoxy radicals when the treated PTFE films wereexposed to air. This suggests that high reactivity ofC-DB with oxy-gen and that the peroxy radical
species can contribute to enhancement of thesurface biocompatibility.
3. Conclusion
The in situ real time ESR technique wasdemonstrated as a new experimental approach tothe microscopic under-standing of chemical
reactions on surfaces with gaseous radicals during
plasma processes.
Acknowledgments
This work was supported in part by the knowledge
Cluster Initiative (Second Stage) of the Tokai Region
Nanotechnology Manufacturing Cluster.

RadicalTransparent window(MgF2 or Quartz)

Fig. 2. Irradiation for only radical (upper), and onlylight (lower)

References[1] A. A. Westenberg, J. Chem. Phys. 40 (1964) 3087.
[2] S. Yamasaki, et al., Appl. Phys. Lett. 70 (1997) 1137.
[3] K. Ishikawa, et al., Appl. Phys. Lett. 81 (2002) 1773.
[4] K. Ishikawa, et al., J. Phys. Chem. Lett. 2 (2011)1278.