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A study on the plasma‑treated surfaces ofMgO(100) and quartz substrates by infraredmultiple‑angle incidence resolution spectrometry

Author(s)Nagao, Yuki

Citation

e‑Journal of Surface Science and Nanotechnology,
10: 229‑233

Issue Date

2012‑06‑16

Type

Journal Article

Text version

publisher

URL

http://hdl.handle.net/10119/12149

RightsCopyright (C) 2012 日本表面科学会. Yuki Nagao, e‑Journal of Surface Science and Nanotechnology,
10, 2012, 229‑233.
http://dx.doi.org/10.1380/ejssnt.2012.229

Description

Japan Advanced Institute of Science and Technology

e-Journal of Surface Science and Nanotechnology

16 June 2012

Regular Papere-J. Surf. Sci. Nanotech. Vol. 10 (2012) 229-233

A Study on the Plasma-Treated Surfaces of MgO(100) and Quartz Substrates byInfrared Multiple-Angle Incidence Resolution Spectrometry
Yuki Nagao∗
Division of Chemistry, Graduate School of Science,
Kyoto University, Kitashirakawa Oiwake-cho, Kyoto 606-8502, Japan(Received 18 December 2011; Accepted 7 April 2012; Published 16 June 2012)
In this study, contact angle measurements and an infrared p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) technique were performed on MgO(100) and quartz substrates. The contact angle and IR
spectra of the in-plane and out-of-plane vibration modes depended on the cleaning methods. The surface of theas-received substrates was contaminated in MgO(100) and quartz substrates. As determined by contact angleand p-MAIR spectra analysis, plasma treatment resulted in a relatively clean and superhydrophilic surface underatmospheric conditions. [DOI: 10.1380/ejssnt.2012.229]
Keywords: Plasma processing; Amorphous surfaces; Coatings; Wetting; Infrared absorption spectroscopyI.

INTRODUCTION

Technologies and scientiﬁc studies that employ thinﬁlms are attracting signiﬁcant attention. Recently, many
functional thin ﬁlms have been investigated and developed [1–19]. Atoms or molecules at the surface of a material experience a diﬀerent environment from those in thebulk of the material, and thus have diﬀerent free energies, electronic states, reactivities, mobilities, and structures [17]. Most thin ﬁlms require a clean substrate anda clean substrate surface. Therefore, an understandingof the surface state is important in experiments that employ substrates. There are many methods available forcleaning surface of the substrates. Often surfactants areapplied leading to negative eﬀects on the environment of
the substrate surface [20]. Small particles on the surface are harder to be removed. For example, submicronparticles often adhere to surfaces via Van der Waals andcapillary forces, which are proportional to their diametersd [21]. The importance of cleaning the substrate for fabricating electroluminescent devices was highligted by Soet al. [22]. Recently a plasma treatment was used as pretreatment to fabricate a self-assembled monolayer (SAM)
on the substrate in the ﬁeld of micro electro mechanicalsystems (MEMS).
Recently an infrared p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) technique wasdeveloped by Hasegawa [23–26]. Infrared p-MAIRS canprovide the in-plane (IP) and out-of-plane (OP) molecularvibration data simultaneously from an identical substrate.
This technique is recognized as a powerful spectroscopic
tool for revealing molecular orientation in thin ﬁlms (evenif the sample is amorphous). In this paper, the surface
wettability and p-MAIR spectra were investigated in order to determine the surface state of the MgO(100) andquartz substrates. As reﬂection-absorption spectrometry
could not be performed, it was diﬃcult to characterize thesurface cleanness of the infrared transparent substrate.
Only IP molecular vibration modes were obtained by measuring infrared transparent spectra. The aim of this study∗ Corresponding author:
ynagao@jaist.ac.jp; Present address:
School of Materials Science, Japan Advanced Institute of Scienceand Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292 JapanFIG. 1: Photographs of the top and side views of water
droplets on the surface of MgO(100) substrate. These images
show the contact angles of a water sample for the as-received
substrate (a), after washing with wet acetone (b), and after
washing with dry acetone (c), after cleaning by plasma treatment (d).

is to investigate the contaminants on the surface by using
p-MAIR spectra.

II.

EXPERIMENTAL

The commercially available substrates investigated inthis paper were MgO(100) and quartz substrates. The
sizes of MgO(100) and quartz substrates used were
15×15×0.5 mm and 17 mmϕ×1 mm, respectively. Two
substrates were mirror polished. Wet acetone and dry ace-c 2012 The Surface Science Society of Japan (http://www.sssj.org/ejssnt)
ISSN 1348-0391 ⃝229

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FIG. 2: Photographs of the top and side views of water
droplets on the surface of quartz substrate. These images show
the contact angles of a water sample for the as-received substrate (a), after washing with wet acetone (b), and after washing with dry acetone (c), after cleaning by plasma treatment(d).

tone were used to wash these substrates. In this paper, theterm “wet acetone” is used to describe an already opened
bottle of acetone, which includes a little atmospheric water vapor. As a result, few small water droplets remainedon the surface when the substrate was washed with wetacetone. In contrast, when the substrate was washed withdry acetone, no droplets were observed on the surface.
Each substrate was washed for 15 min using an ultrasonic cleaner and dried completely with an air gun in aclean booth. To check the surface hydrophilicity of each
substrate including the as-received substrates, the contact
angles were measured using water droplets. The measurements were performed in the following order. The contact
angle of the substrate was measured, ﬁrst, in as-received
condition (Figs. 1(a) and 2(a)), second, after washing with
wet acetone for 15 min (Figs. 1(b) and 2(b)), third, after washing with dry acetone for 15 min (Figs. 1(c) and2(c)), and ﬁnally after cleaning the substrate by plasma
treatment (Figs. 1(d) and 2(d)). The plasma treatment
was performed using a vacuum plasma system (CUTEMP, FEMTO SCIENCE). Air was used as the inlet gas,
and the power supply was operated at 50 kHz. The substrate was activated at the optimum conditions (70 W,75 mTorr, 10 sec).
To investigate the contamination on the surface of thesubstrates after every washing or cleaning, infrared pMAIRS was performed. The p-MAIRS measurements
were performed on a Thermo Fisher Scientiﬁc Nicolet 6700 FT-IR spectrometer equipped with a mercurycadmium-telluride (MCT) detector. Single-beam spectra
were collected from 38◦ to 8◦ at 6◦ steps in the rangeof the angle of incidence. The aperture was fully opened(size of 150) and a metal plate with small pores was placed
230

FIG. 3: Infrared p-MAIR spectra of the IP vibration mode in
the MgO(100) substrate after acquiring the single-beam spectra measured from 38◦ to 8◦ at 6◦ steps. The single-beamspectrum of the as-received MgO(100) substrate was used as a
background spectrum. The spectra shown were obtained after
washing with wet acetone (b), after washing with dry acetone(c), and after cleaning by plasma treatment (d). For clarity,
the curves were oﬀset along the Y-axis.

in the light path of the incident beam to prevent saturation. To obtain the p-polarized light, a ZnSe polarizer wasused. The p-MAIRS analysis of the collected spectra wasautomatically performed by p-MAIRS analyzer software(Thermo Fisher Scientiﬁc). The as-received substrate wasused as the reference data.

III.

RESULTS AND DISCUSSIONS

Figure 1 shows photographs of the top and side views ofwater droplets on the surface of the MgO(100) substrate.
The contact angle on the as-received substrate was 82◦ ,as shown in Fig. 1(a). Therefore the surface of the asreceived MgO(100) was in a hydrophobic state. On theother hand, the contact angles on the substrates washedwith wet acetone (Fig. 1(b)), dry acetone (Fig. 1(c)), andplasma treatment (Fig. 1(d)) were 46◦ , 28◦ , and less than10◦ , respectively. Therefore, the contact angle strongly
depended on the cleaning method. The surface cleanedby the plasma treatment was found to be superhydrophilic
under atmospheric conditions. From these results, it wasassumed that some contaminants (wax or aliphatic oiletc.) remained on the surface after mirror polishing of
the substrate. When the as-received MgO(100) substrate
was cleaned only by plasma treatment without washingwith dry acetone, the surface was found to be superhydrophilic under these conditions as well.
Figure 2 shows photographs of the top and side views

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FIG. 4: Infrared p-MAIR spectra of the OP vibration mode in
the MgO(100) substrate after acquiring the single-beam spectra measured from 38◦ to 8◦ at 6◦ steps. The single-beamspectrum of the as-received MgO(100) substrate was used as abackground spectrum. The spectra shown were obtained after
washing with wet acetone (b), after washing with dry acetone(c), and after cleaning by plasma treatment (d). For clarity,
the curves were oﬀset along the Y-axis.

FIG. 5: Infrared p-MAIR spectra of the IP vibration modein the quartz substrate after acquiring the single-beam spectra measured from 38◦ to 8◦ at 6◦ steps. The single-beamspectrum of the as-received quartz substrate was used as abackground spectrum. The spectra shown were obtained after
washing with wet acetone (b), after washing with dry acetone(c), and after cleaning by plasma treatment (d). For clarity,
the curves were oﬀset along the Y-axis.

of water droplets on the surface of the quartz substrate.
The contact angle on the as-received substrate was 39◦
as shown in Fig. 2(a). This contact angle was less thanthat of the MgO(100) substrate. The contact angle depended on the state of the as-received substrate. The
contact angles on the quartz substrates washed with wetacetone (Fig. 2(b)), dry acetone (Fig. 2(c)), and plasma
treatment (Fig. 2(d)) were 34◦ , 11◦ , and less than 10◦ ,respectively. The contact angle also strongly dependedon the cleaning method. It was assumed that the difference between contact angles is due to varying degrees
of hydrocarbon contamination on the surface. When theas-received quartz substrate was cleaned only by plasma
treatment without washing it with dry acetone, the surface was found to be superhydrophilic under these atmospheric conditions. From the view point of wettability of
two substrates, plasma treatment is a more useful cleaning procedure.
To investigate the contaminants on the surface ofMgO(100) substrate, measurements of the p-MAIR spectra were carried out. Figure 3 shows the IP mode spectra of MgO(100) substrate after each treatment. The
single-beam spectrum of the as received MgO(100) substrate was used as a background spectrum. For clarity,
curves were oﬀset along the Y-axis. The CH2 vibration
modes were observed in the wavenumber range of 28003000 cm−1 . The absorbances at 2919 and 2850 cm−1 were
due to the typical asymmetric (va ) and symmetric (vs ) vibrations of CH2 groups of the hydrocarbon, respectively.
The absorbance of the CH2 vibration modes decreased toa greater degree by washing with wet acetone, dry acetone, and cleaning by the plasma treatment. These results suggest that some hydrocarbons on the surface of theMgO(100) substrate were removed by cleaning the surface. The largest decrease in the absorbance was observedin the MgO(100) substrate at 2919 cm−1 after cleaning byplasma treatment. Therefore, the plasma treatment canremove the hydrocarbon contaminant from the surface.
Figure 4 shows the OP mode spectra of MgO(100) after
each treatment. It was observed that the absorbance due
to CH2 vibration modes decreased to a greater degree bywashing and cleaning. The largest decrease in absorbancewas observed at 2921 cm−1 after cleaning by plasma treatment. Therefore, it was conﬁrmed that plasma treatmentof the MgO(100) substrate was useful for cleaning thesurface. Infrared p-MAIRS reveals the information of theIP and OP molecular vibrations in an identical substrate.
Hence, the molecular orientation could be calculated by
the following equation [23].
(√)
2IIP
ϕ = tan−1.
(1)
IOP
Here ϕ is the orientation angle from the surface normal
and 2IIP and IOP are the decreased band intensities ofIP and OP spectra, respectively. The factor of 2 in IIP
has been explained in Ref. [23]. From Fig. 4, the orientation angles of the transition dipoles for the modes (va andvs ) for the CH2 group were calculated to be 54◦ and 58◦ ,respectively, from the surface normal. Since the hydrocar-

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Nagao, et al.

Volume 10 (2012)

Relative Absorbance

bcd0.001 Abs.

3000 2950 2900 2850 2800

Wavenumber / cm-1FIG. 6: Infrared p-MAIR spectra of the OP vibration modein the quartz substrate after acquiring the single-beam spectra measured from 38◦ to 8◦ at 6◦ steps. The single-beamspectrum of the as-received quartz substrate was used as abackground spectrum. The spectra shown were obtained after
washing with wet acetone (b), after washing with dry acetone(c), and after cleaning by plasma treatment (d). For clarity,
the curves were oﬀset along the Y-axis.

bon chain comprised of all-trans-zigzag conformers due tothe band locations, the tilt angle of the hydrocarbon chain
was evaluated by using the following equation [27].
cos2 α + cos2 β + cos2 γ = 1.

(2)

Here α and β are the orientation angles of the transition
dipole moment of the respective modes va and vs for theCH2 group. The tilt angle of the molecular axis of thehydrocarbon chain from the surface normal (γ) was 52◦ .
This tilt angle suggested that the structure of the contaminant was in a completely disordered state [28].
Figure 5 shows the IP mode spectra of the quartz substrate after each treatment. The decrease in the absorbance due to the CH2 vibration modes were observed in
the wavenumber range of 2800-3000 cm−1 , which implied
the elimination of the contaminants. The contact angle on
the quartz substrate washed with dry acetone (Fig. 2(c))
decreased (11◦ ). This result was consistent with the observed IP mode spectrum. The degree of the decreasing
absorbance after the plasma treatment was comparable tothat of dry acetone washing. These results inferred that[1] K. L. Chopra, S. Major, and D. K. Pandya, Thin Solid
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the contaminants could be almost completely removed byusing dry acetone. This result was diﬀerent from thatof the MgO(100) substrate, where the hydrophilic stateon the surface was more improved and became superhydrophilic when plasma treatment was performed. Therefore, in case of the quartz substrate, the results of the pMAIRS inferred that the contaminants could be removed
with dry acetone.
Figure 6 shows the OP mode spectra of the quartz substrate after each treatment. The absorbance of the CH2vibration modes decreased, and the width of the decreasing band broadened compared to that of the MgO(100)substrate. This result suggested that the hydrocarbon
chain did not comprise all-trans-zigzag conformers. The
hydrocarbon contaminant exhibited diﬀerent structureson the surface of the MgO(100) compared to the quartz
substrate. The decrease of relative absorbance in thequartz substrate was found to be small compared to thatin the MgO(100) substrate. The degree of contamination was relatively small in this quartz substrate and it
strongly depended on the supplier of the substrates. (Thesupplier was not shown here.) When wet acetone was used
for cleaning the substrates, an additional eﬀort was required to remove contaminants from the surface for both
MgO(100) and quartz substrates.

IV.

CONCLUSION

To investigate the surface state of MgO(100) and quartz
substrates, the contact angle measurements and infraredp-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) technique were performed on the substrates. I found that the contact angle and IR spectra of
the in-plane and out-of-plane vibration modes depended
on the cleaning methods. The surface of the as-received
substrates was contaminated by hydrocarbons on bothMgO(100) and quartz substrates. A plasma treatment
resulted in a relatively clean and superhydrophilic surface
under atmospheric conditions for the two substrates. Although, dry acetone was useful for cleaning the surface,
wet acetone required additional steps to remove the contaminants from the surfaces of both MgO(100) and quartz
substrates.

Acknowledgments

The author would like to thank Ms. M. Urano for all her
assistance. This work was supported by the Japan Society
for the Promotion of Science (JSPS) through the Funding
Program for Next Generation World-Leading Researchers(NEXT Program), which was initiated by the Council for
Science and Technology Policy (CSTP).

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