BL4A2

Fabrication of Electrode Holes on the Supported Membrane Biosensor
Substrates by Femto-Second Laser Patterning Followed by SR Etching
M.M. Rahman1, R. Sasaki2, H. Nagai2, M. Yoshida2, M. Aoyama3, T. Yano3, H. Uno3, R. Tero3,
Y. Nonogaki1,3 , T. Urisu 1,3
1
Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan
2
AISHIN SEIKI Co., Ltd. 17-1 Kojiritsuki, Hitotsugi-cho, Kariya 448-0003, Japan
3
Institute for Molecular Science , NINS, Myodaiji, Okazaki 444-8585, Japan
used because of its unique features of high spatial
resolution, extremely high material selectivity
between CoSi2 and SiO2, low damage, and clean
etching atmosphere. There are lot of advantages suchas process simplicity compared with the electron
beam (EB) lithography and no contamination bybeam elements compared with focused ion beam
patterning. Then SR etching using SF6 (0.05 torr) +O2 (0.002 torr) reaction was used for the etching of
SiO2 using the Co contact mask. Etching stopped at
the thin electrode (CoSi2) surface. After the removalof Co mask with 0.1 M HNO3 aq, AFM images
showed that the surface was very flat (Rz=1.5 nm) as
shown in Fig. 1.

Introduction
The supported membrane is a single lipid bilayer
supported on a solid surface, and is useful as artificial
cell membranes for the study of biological reactionsof membrane proteins. We are developing supported
membrane biosensors from the interest in developing
the new research tool of the cell membrane surface
reactions, and these devices are interesting also fromthe view point of application to the large scale
screening method for the new medicine development.
In the present work, we have fabricated very flat Si
substrates with buried-electrodes having the structureof SiO2/CoSi2/Si(100)/CoSi2/Au by the thin film
deposition using of sputtering and the electrode holefabrication using the femto-second laser patterning
followed by synchrotron radiation (SR) etching.

1µm
SiO2
CoSi2

1µm
Si

CoSi2
Au

6 mm
Fig. 2 A schematic illustration for the biosensor
substrate.

Fig. 1 AFM image (20x20 µm2) and the crosssection of the SiO2 surface electrode holefabricated by femto-second laser and SR etching.

Results and Discussion
Co thin film (200nm) was deposited on the
SiO2/CoSi2/Si(100)/CoSi2/Au substrates and small
hole patterns were fabricated by the femto second
laser pulses. These hole patterns were used as thecontact mask of the SR-stimulated etching. SR was

116

BL4B

X-Ray Magnetic Circular Dichroism Study on Cu Capping on Ultrathin Ni
Films Grown on Clean and O-Adsorbed Cu(001) Surfaces

T. Nakagawa, H. Watanabe, T. Yokoyama
Department of Molecular Structure, Institute for Molecular Science, Okazaki 444-8585 JapanXMCD and MOKE measurements, respectively. It isclearly found that these quantities behave very
similar.

Ultrathin Ni/Cu(001) films are well known to show
unique magnetic properties. The spin reorientationtransition (SRT) occurs twice by varying the Ni
thickness; at <9 ML (monolayer) the easy axis is inplane, at 9-40 ML it is perpendicular, and at >40 ML
it is again in plane. In this work, we have studied the
Cu capping effect on the Ni films grown on clean andO-preadsorbed Cu(001) by means of in-laboratory
magneto-optical Kerr effect (MOKE) and UVSOR
x-ray magnetic circular dichroism (XMCD) methods.

Experiments
Ni was deposited on clean (1u1) and O-preadsorbed (22u2)R45q Cu(001) at room temperaturein ultrahigh vacuum chambers. The thickness wasmonitored with the RHEED oscillations. Cu wassubsequently deposited on Ni/Cu(001). Ni L-edge
XMCD was taken at BL4B [1] at ~100 K by applying
the external magnetic of r1000 G.

Results
In the polar and longitudinal MOKE measurementsof Ni/Cu(001), we found that the SRT occurs at thecritical Ni thickness of 9.0 ML before Cu capping and
7.0 ML after 2 ML Cu capping, implying thestabilization of perpendicular magnetic anisotropy(PMA). On the contrary, for the Ni/O/Cu(001), theSRT occurs at the critical Ni thickness of 5.5 ML
before Cu capping and 7.0 ML after 2 ML Cu capping,indicating the instabilization of PMA.
In order to obtain structural information on thesetwo kinds of films, the Auger electron spectra and the
LEED pattern were observed. The oxygen atoms arefound to act as a surfactant and locate always at thesurface even after Ni and Cu deposition. Combining
the MOKE and Auger results, we can conclude thatthe surface oxygen atom stabilizes PMA more
effectively than the surface Cu atom.
For the understanding of such magnetic anisotropy,
we have performed Ni L-edge XMCD. Figure 1
shows the XMCD spectra. In the 5.5 ML Ni film on
clean Cu(001), the LIII-edge XMCD signals aregradually suppressed with the Cu capping, while inthe 11 ML Ni film, the XMCD intensity is almost
constant. This results indicate that with the Cu
capping the orbital magnetic moments are reduced
along the in-plane direction and not along the surface
normal direction. On the contrary, in the 4.8 ML Ni
film on O/Cu(001), the LIII-edge XMCD signals aregradually increased, implying the enhancement of theorbital magnetic moment.
Figure 2 shows the orbital magnetic moment (ml)and the hysteresis loss (BH) obtained the present

117

Fig. 1 Ni L-edge XMCD of 5.5 ML Ni on clean
Cu(001) (always in-plane magnetized; measured at
grazing x-ray incidence of 30q), 11 ML Ni on clean
Cu(001) (always perpendicularly magnetized;
measured at normal x-ray incidence), and 4.8 ML
Ni on O/Cu(001) (always in-plane magnetization;measured at grazing x-ray incidence of 30q).

Fig. 2Hystetesis loss and orbital magnetic
moments of the Cu-capped Ni films on clean andO-preadsorbed Cu(001) as a function of Cu
coverage.
[1] T. Nakagawa et al., UVSOR Activity Report
2003 (2004) 39.

BL4B

Spin Reorientation Transition in Ag-Covered Co Films Grown on Vicinal
Cu(001) Surface

T. Nakagawa, H. Watanabe, T. Yokoyama
Department of Molecular Structure, Institute for Molecular Science, Okazaki 444-8585 Japan
Magnetic thin films grown on vicinal surfaces
exhibit strong uniaxial magnetic anisotropy; themagnetic property should be essentially different
between the step-parallel and perpendicular directions.
In this work, in order to obtain microscopic
information on the spin reorientation transition in
Ag-deposited Co films on Cu(1 1 17), we performed
the longitudinal magneto-optical Kerr effect(L-MOKE) and the x-ray magnetic circular dichroism(XMCD) experiments.

Experiments
6 monolayer (ML) Co was deposited on clean Cu(11 17) at room temperature in ultrahigh vacuum
chambers. The Co thickness was monitored with the
RHEED oscillations. Ag was subsequently depositedon Co/Cu(1 1 17). Co L-edge XMCD was taken at
BL4B at a temperature of ~100 K and at grazing
x-ray incidence of 30q by applying the external
magnetic of r1000 G.

the XMCD variation. The x-intercept corresponds tothe easy axis, where 0q and –4.8q respectively mean
parallel to the physical plane and to the terrance plane.
The present finding in Fig. 3 concludes that the Ag
deposition induces also the out-of-plane rotation of
the easy axis from the physical surface plane (0q) to
the terrance plane (–4.8q).
Table 1 The results of the sum-rule analysis for 6
ML Co on Cu(1 1 17). The spin (ms) and orbital (ml)magnetic moments of Co are given.
msdirection
ml
(PB)
(PB)
Clean
// step
0.246
1.526
0.225
1.474
Astep
0.2 ML// step
0.200
1.468
Ag dep.
0.218
1.485
Astep

Results
Figure 1 shows the magnetic hysteresis loops of
L-MOKE. In the clean Co film, the magnetization
curve along the //step direction exhibits normal
rectangular shape, while that along the Astep
direction shows a double loop with zero remanence,
this implying the easy axis of the //step direction. On
the contrary, the reverse is true for the 0.2 ML
Ag-deposited Co film; the magnetic easy axischanges from //step to Astep, exhibiting clear spin
reorientation transition. This results is identical withthe previous report by Weber et al. [1].
Figure 2 shows the Co L-edge XMCD. The
intensities of the LIII- and LII-edge peaks exhibits
clear difference between the clean and Ag-deposited
Co films. The sum-rule results are given in Table 1.
Although the difference is rather small, one can findthat a larger orbital magnetic moments give themagnetic easy axis.
Moreover, we have determined the inclination
angle of the easy axis. Figure 3 shows the results ofFig. 2 Co L-edge XMCD of 6 ML Co/Cu(1 1 17)
at 100 K before and after 0.2 ML Ag deposition.

Fig. 3 Angle dependence of Co LIII-edge XMCD
intensity of 6 ML Co/Cu(1 1 17).
Fig. 1 L-MOKE hysteresis loops of (a) clean and(b) Ag-deposited 6 ML Co/Cu(1 1 17) at 100 K.

[1] Weber et al., Phys. Rev. B52 (1995) R14400.

118

BL4B

Drastic Magnetization Change Observed in NO Adsorption on Co/Cu(1 1 17)
T. Nakagawa, H. Watanabe, T. Yokoyama
Department of Molecular Structure, Institute for Molecular Science, Okazaki 444-8585 Japan
Co films. The sum-rule results are given in Table 1.
Although the difference is rather small, one can findthat in a clean Co film a larger orbital magnetic
moments give the magnetic easy axis, while theorbital magnetic moments are essentially the same
between the //step and step directions after NO
adsorption. This is consistent with the L-MOKE
results.

Magnetic thin films grown on vicinal surfaces
exhibit strong uniaxial magnetic anisotropy and theanisotropy could be modified by surface chemicaltreatments. In this work, we have investigated theeffect of NO adsorption on uniaxial Co films grownon Cu(1 1 17) by means of the longitudinalmagneto-optical Kerr effect (L-MOKE) and the x-ray
magnetic circular dichroism (XMCD) experiments.

Experiments

Table 1 The results of the sum-rule analysis for 6
ML Co on Cu(1 1 17). The spin (ms) and orbital (ml)magnetic moments of Co are given.
msdirection
ml
( B)
( B)
Clean
// step
0.256
1.459
0.224
1. 454step
0.5 ML// step
0.116
0.866
NO ads.
0.123
0. 879step

6 monolayer (ML) Co was deposited on clean Cu(11 17) at room temperature in ultrahigh vacuum
chambers. The Co thickness was monitored with the
RHEED oscillations. The Co film was exposed to 1 L
NO to give a saturated ~0.5 ML adsorption state of
NO/Co/Cu(1 1 17). Co L-edge XMCD was taken at
BL4B at a temperature of ~100 K and at grazing
x-ray incidence of 30 by applying the external
magnetic of 1000 G.

Results
Figure 1 shows the magnetic hysteresis loops of
L-MOKE. In the clean Co film, the magnetization
curve along the //step direction exhibits normal
rectangular shape, while that along thestep
direction shows a double loop with zero remanence,
this implying the presence of strong uniaxial
magnetic anisotropy and the easy axis of the //step
direction. However, drastic changes can be seen after
NO adsorption. The coercivity is reduced noticeably,
and the //step and step loops are completely
identical, this indicating the disappearance of theinherent uniaxial anisotropy and the appearance ofalmost fourfold symmetric magnetic anisotropy. Sucha change is much more drastic than the film on flatCu(001), as shown in Fig. 2.
Figure 2 shows the Co L-edge XMCD. The
intensities of the LIII- and LII-edge peaks exhibits
clear difference between the clean and NO-adsorbed

Fig. 2 L-MOKE hysteresis loops of clean (red)and NO-adsorbed (blue) 5 and 2 ML Co films on
the flat Cu(001) surface.

Fig. 1 L-MOKE hysteresis loops of clean andNO-adsorbed 6 ML Co films on Cu(1 1 17) at 100
K. The right figure is just the magnification along
the x axis.

Fig. 3 Co L-edge XMCD of 6 ML Co/Cu(1 1 17)
at 100 K before and after 0.5 ML NO adsorption.

119

BL4B

Enhancement of Magnetization of Fe/Cu(001) Induced by K Deposition

T. Nakagawa, H. Watanabe, T. Yokoyama
Department of Molecular Structure, Institute for Molecular Science, Okazaki 444-8585 Japan
It is interesting to investigate whether electron
donation to magnetic metal films induces theenhancement or suppression of magnetization. In thepresent work, we have studied the effect of K
adsorption on fct Fe grown on Cu(001) by means of
the polar magneto-optical Kerr effect (P-MOKE) andthe x-ray magnetic circular dichroism (XMCD)
methods.

Table 1 The results of the sum-rule analysis for 3
ML Fe and Co on Cu(001). 3d hole numbers and spinand orbital magnetic moments of Fe and Co are
given.
mlms
K dep.
3dholenumber
(ML)
(PB)
(PB)
3 ML Fe
0.0
3.40
2.29
0.24
/Cu(001)
0.1
3.27
2.40
0.24
0.2
3.00
1.91
0.17
3 ML
0.0
2.50
1.67
0.26
Co
0.1
2.37
1.58
0.26
/Cu(001)
0.2
2.27
1.50
0.29

Experiments
3 monolayer (ML) Fe was deposited on clean
Cu(001)-(1u1) at room temperature in ultrahighvacuum chambers. Special care was taken of goodvacuum because of easy oxidation of K. The
thickness was monitored with the RHEED
oscillations. K was subsequently deposited on
Fe/Cu(001). Fe L-edge XMCD was taken at BL4B
[1] at a temperature of ~100 K and at normal x-ray
incidence (the films shows perpendicular easy axis)
by applying the external magnetic of r1000 G. For
comparison, Co L-edge XMCD was examined for
K/Co(3ML)/Cu(001) at grazing x-ray incidence.

Results
Figure 1 shows the P-MOKE intensity as afunction of K coverage, which was measured during
K deposition. With the increase in the K coverage, the
P-MOKE intensity increases and at 0.1 ML K
deposition it is maximized. More deposition leads tothe suppression. The K coverage of 0.1 ML roughly
corresponds to the work function minimum.
Although the MOKE intensity is usually
proportional to the magnetization, it could not be thecase if the electronic structure changes drastically. In
order to confirm the enhanced magnetization and toobtain more direct information, we have performed
XMCD measurements. Figure 2 shows the Fe L-edge
XMCD. A small increase in the XMCD signals is
actually found at the K coverage of 0.1 ML.
The results of the sum-rule analysis are
summarized in Table 1, together with those of
K/Co/Cu(001). In both the Fe and Co cases, thenumber of 3d holes, which was estimated from theintensity of the white lines, is gradually reduced with
the K coverage, this exemplifying the electron
donation from K. In the Co case, the spin magnetic
moment decreases monotonically with the K
coverage. This finding is reasonable since themajority 3d band of fcc Co is fully occupied and the
donated electron is transferred to the minority 3dbands, leading to the reduction of the spin magnetic
moment of Co. On the contrary, the spin magnetic
moment of Fe is maximized at 0.1 ML K. This
verifies that the Fe spin magnetic moment isenhanced by a small amount of K deposition.

Fig. 1 P-MOKE intensity of 3 ML Fe/Cu(001) at
100 K as a function K deposition.

Fig. 2 Fe L-edge XMCD of 3 ML Fe/Cu(001)(perpendicularly magnetized) at 100 K for the K
coverages of 0.0, 0.1 and 0.2 ML.
[1] T. Nakagawa et al., UVSOR Activity Report
2003 (2004) 39.

120

BL5U

Absolute Asymmetric Synthesis of rac-Leucine in Ice at 21 K by Circularly
Polarized Synchrotron Radiation: a Terrestrial Proof of the Origin of
Homochirality in Biosphere
H. Nishino1, M. Hosaka2, M. Katoh2, Y. Inoue1
1
Entropy Control, ICORP, Japan Science and technology Agency, Toyonaka 560-0085 Japan
2
UVSOR facility, Institute for Molecular Science, Okazaki 444-8585 JapanThe homochirality in biosphere is an extreme caseof the entropy control in nature, while the origin of
biomolecular homochirality is one of the mostcontroversial issues in the chemical evolution on
Earth [1]. It is proposed that the enantiomeric
enrichment can be achieved by the circularly or
elliptically polarized synchrotron radiation from aneutron star. This idea is supported by the finding of
optically active amino acids in the organic mantle ofthe Murchison meteorite [2]. For this hypothesis, wehave already reported the absolute asymmetricsynthesis (AAS) of some aliphatic amino acids inaqueous solutions at varying pH (defined at 298 K),
by using left- and right-handed circularly polarized
light (l- and r-CPL) proceeded irrespective of thesolution pH through different mechanisms [3].
In outer space, amino acids are thought to befrozen in icy carbonaceous chondrite meteorite.
Recently, it was shown that a variety of racemic
amino acids are photochemically produced indeed
from a low-temperature matrix (12-15 K) of H2O,
CH3OH, CO2, CO, and NH3 (2:1:1:1:1), whichmimics the icy meteorite condition in outer space [4].
Hence, to fill the missing link of the origin of thehomochirality in biomolecules, it is essential toexamine whether the AAS of amino acids by CPL

proceeds in an ice matrix or not. In this study on the
AAS of leucine (Leu) in ice, we irradiated the icy
samples at 21 K (ice XI) and 81 K (ice Ih) with the
215 nm l- and r-CPL, generated by a helicalundulator installed in the electron storage ring
UVSOR-II.
We verified for the first time the occurrence of
AAS of Leu even at 21 K by using circular dichroism(CD) spectroscopy (Figure 1). Quantitative productanalyses of the irradiated samples revealed that,irrespective of the original pH (defined at 298 K),
ammonia and isovaleraldehyde are the only detectableproducts from icy Leu irradiated at 215 nm. This is
the
first
clear
observationthatthephotodecomposition of Leu does occur even in a 21 K
ice through photodeamination, which requires lessmolecular motions. In ice, HCl does not dissociate toan ion pair [H+ + Cl–], but exists as molecule at 77 K
[5]. Hence, irrespective of the original pH defined at
25 °C, Leu exists as a zwitterion at 21 K. This may
rationalize the very similar photochemical behaviorof Leu in ice of different original pHs of 1 and 7.
These results unambiguously complete the whole
scenario that, in outer space, simple starting materials,
such as H2O, CH3OH, CO2, CO, and NH3, areirradiated by polarized or unpolarized VUV-UV
irradiations to produce a mixture of racemic aminoacids, including some of the essential amino acids [4],
which
in
turn
are
enantioselectively
photodecomposed on the way to Earth by thecircularly polarized radiation from a neutron star or astar-forming area.

0.3

r-CPL 860 mA h
0.2[1] W. A. Bonner, Origins Life Evol. Biosphere 21(1991) 59.
[2] J. R. Cronin and S. Pizzarello, Science 275(1997) 951.
[3] H. Nishino, A. Kosaka, G. A. Hembury, F. Aoki, K.
Miyauchi, H. Shitomi, H. Onuki and Y. Inoue, J. Am.
Chem. Soc. 124 (2002) 11618.
[4] M. P. Bernstein, J. P. Dworkin, S. A. Sandford, G.
W. Cooper, L. J. Allamandola, Nature 416 (2002)
401. G. M. Munoz Caro, U. J. Meierhenrich, W. A.
Schutte, B. Barbler, A. A. Segovia, H. Rosenbauer, W.
H. P. Thiemann, A. Brack and J. M. Greenberg,
Nature 416 (2002) 403.
[5] H. Kang, T.-H. Shin, S. -C. Park, I. K. Kim and S.
-J. Han, J. Am. Chem. Soc. 122 (2000) 9842.

0T

T / mdeg

0.1

–0.1

before irradiation–0.2

l-CPL 860 mA h

–0.3
200

250

Wavelength / nm

Fig. 1 The final CD of rac-Leu [rac-Leu]0 = 6.5
mM, pH25ºC = 7) irradiated at 215 nm by CPL (21K). % Decomposition: r-CPL 860 mA h; 20.8%,l-CPL 860 mA h; 17.4%. Path length of CD cell:
0.2 cm.

121

BL7U

Undulator Radiation Induced Si-H Dissociationon H-Si(111) Surfaces Observed by STM

Y. Nonogaki, T. Urisu
Dept. Vaccum UV Photoscience, Institute for Molecular Science, Okazaki 444-8585 Japan
Synchrotron-radiation-stimulated reactions, such as
SR-etching and photon-stimulated desorption, havebeen attractive subjects from the view points of not
only engineering but also surface science, due to theirinteresting characteristics of unique material and site
selectivity, high spatial resolution and low damage.
We designed and constructed the undulator
beamline equipped with an STM observation systemat the UVSOR to investigate the excitation-energy
dependence of SR-stimulated reactions. The
undulator emits the quasi-monochromatized andtunable light of 50 - 120 eV, which is suitable for
excitation of the Si 2p (~100 eV). In this study, theirradiation effects and its excitation energy
dependence on the H-Si (111) surfaces were
investigatedbyin-situ
STM.
Althoughphoton-stimulated H desorptions from H-Si (111)
surfaces have been studied by detecting the desorbing
molecules[1,2], there is no report with the surface
morphology
studied
by
STM
after
the
photon-stimulated desorption.
The clean Si (111) surface was exposed to atomic
H at 350°C for 10 min, which gives sufficiently wide
1x1 areas. The surface was irradiated by the undulator
emission with irradiation dose of 5,000 and 10,000
mA sec (ring current x exposure time) at room
temperature. The undulator gap height was fixed at 17mm, 20 mm and 25 mm.
Figure 1 (a) shows a 10 nm x 10 nm STM image ofthe H-Si (111) surface obtained by the 10 min atomic
H exposure at 350°C. Islands and small protrusions(SPs) are visible on a flat surface with the atomic
corrugation, as reported in Refs. 3 and 4. The flat
surface with the atomic corrugation consists ofrest-atom monohydrides. According to Ref. 4, the SPs
are assigned to isolated adatom trihydrides or
rest-atoms with missing H. The SP density isobserved to be 1.5%ML.
Figure 1(b) shows the H-Si (111) surface after theundulator beam irradiation of 5,000 mA sec dose (1stharmonic radiation at ~84 eV). The number of theSPs significantly increased on the rest-atommonohydride surface from 1.4%ML to 4.5%ML. By
increasing the exposure to 10,000 mA sec, the SP
density increased to 12.8%ML, as shown in Fig. 1(c).
The distribution of the SPs seems to be random andindependent on the kind of the rest-atoms both on thefaulted or unfaulted half of the 7x7 structure. It is
most reasonable to assign the appeared SPs to therest-atoms with missing H. This means that therest-atom Si-H bond is easily dissociated by theirradiation of 84 eV photons.
The undulator gap height was changed toinvestigate the dependence of the SP density on theincident photon energy. The irradiation dose was
fixed at 5,000 mAsec. The peak energies of the 1st
harmonic radiation were changed from 68 eV to 113
eV by varying the gap height from 17 mm to 25 mm.
It is found that the SP densities observed in the 38
nm x 38 nm STM images were depend not on Si 2p
core electron transition threshold but on total photon
flux irradiated to the surface. From these results, it isconcluded that the main mechanism of the Hdesorption from the rest-atom monohydrides is the
direct valence electron excitations by the undulator
beam photons.

Fig. 1 10 nm x 10 nm STM images of H-Si (111)
surfaces after the undulator beam irradiations with theexposure of (a) 0 mAsec, (b) 5,000 mAsec and (c)10,000 mAsec. In (a), small protrusions (SPs) andadatom islands (AIs) are indicated by A and B,
respectively. It is observed that the SP density
significantly increases with increasing irradiation
dose. The appeared SPs are assigned to the rest-atoms
with missing H.
[1] M.L. Knotek and G.M. Loubriel, R.H. Stulen, C.E.
Parks, B.E. Koel, and Z. Hussain, Phys. Rev. B 26
(1982) 2292.
[2]H. Akazawa, M. Sugiyama, S. Maeyama, M.
Oshima and Y. Watanabe, Phys. Rev. B 57 (1998)
4883.
[3] J.J. Boland, Surf. Sci. 244 (1991) 1.
[4] F. Owman and P. Mårtensson, Surf. Sci. 324 (1995)
221.

122

BL8A

Synchrotron Radiation-Excited Etching of ZnTe
T. Tanaka, Y. Kume, K. Hayashida, K. Saito, M. Nishio, Q. Guo, H. Ogawa
Saga University, 1 Honjyo, Saga 840-8502, Japan

Synchrotron radiation (SR) is an ideal light source
for the photo-excited processes because of its highintensity, small divergence, and continuity of the
wavelength from the x-ray to the infrared. With
respect to the SR-excited etching, the materials for an
integrated circuit such as Si, SiO2, etc., have beenstudied actively [1]. However, few papers concerning
the etching of the compound semiconductor materials
such as III-V and II-VI semiconductors have been
published to date in spite of their importance in the
optoelectronic application. Since the single crystal
films of II-VI semiconductors, e.g. ZnTe, can begrown by SR-excited growth [2], the development of
the SR-excited etching is an important next step tomake the electronic devices. Recently, wedemonstrated the possibility of the SR-excited
etching of ZnTe using Ar gas [3]. In this study, we
have investigated the effect of the pressure upon theSR-excited etching process of ZnTe using Ar gas.

the energy lower than about 12 eV. Although the SR
was irradiated to the sample through the window for
105 mAǜmin at the pressure of 0.05 Torr under thesame bias condition, the ZnTe was not etched at all.
This indicates that the photons with energy less than
12 eV does not play important roles in the etchingprocess. The photoabsorption cross section of Ar isreported to be almost zero in the energy range below
the absorption edge of the LiF window, whereas it isquite large in the energy range between 12 eV and 50
eV [4]. The first ionization energy of Ar is 15.76 eV.
We suspect that the ionization of Ar is necessary inthis etching process. Since the mean free path of Ar is
estimated to be around 1 mm at the pressure of 0.05
Torr, a part of Ar ions extends to the outside of thebeam, resulting in the further expansion of the etchingarea. The increase of Ar gas pressure leads to theincrease of Ar+ ion density, and consequently, theetching rate is increased according to the gaspressure.

Experiments
The irradiation experiments were performed at abeam line, BL-8A, in UVSOR facility at IMS. The
critical energy of the SR from the bending magnet
field of the 750 MeV electron storage ring is 425 eV.
The undispersed SR was focused on a sample in areaction chamber using a Pt-coated toroidal mirror.
The light intensities on the sample measured using an
Au diode were determined as about 2×1016 photons/s
at a stored current of 100 mA. The Ar pressure waschanged between 0.02 and 0.5 Torr. The SR was
irradiated perpendicularly to the surface of ZnTe(100) at room temperature through a Ni mesh mask of
100 lines/in. and 80% transmittance. The Ni meshmask was put on the surface of the ZnTe directly. The
ZnTe and the Ni mesh were negatively biased against
the reaction chamber. The bias voltage was kept at–80 V in all experiments.

Etching rate (nm/A• imin)

20

15

10

5

0

–2

10

–1

10
Pressure (Torr)

Fig. 1 Dependence of the etching rate of ZnTe on
the pressure.
[1] D. C. Mancini, S. Varma, J. K. Simons, R. A.
Rosenberg, and P. A. Dowben, J. Vac. Sci. Technol. B
8 (1990) 1804.
[2] T. Tanaka, K. Hayashida, S. Wang, Q. Guo, M.
Nishio, and H. Ogawa, Nucl. Instr. And Meth. B 199(2003) 356.
[3] T. Tanaka, Y. Kume, S. Tokunaga, K. Hayashida,
M. Nishio, Q. Guo, and H. Ogawa, AIP series ofconference proceedings 705 (2004) 1154.
[4] W. F. Chan, G. Cooper, X. Guo, G. R. Burton, and C.
E. Brion, Physical Review A 46 (1992) 149.

Results and discussion
The pattern of the Ni mask was transferred to thesurface of the ZnTe exactly. However, the etched area
was found to extend beyond the calculated beam size.
It was also found that the etched depth depends on thephoton flux density. The maximum etched depth isabout 600 nm for 7×104 mAǜmin at 0.05 Torr. The
dependence of the etching rate of ZnTe on thepressure is shown in Fig. 1. The etching rate isincreased with increasing the pressure. In thisexperiment, the maximum etching rate is 16.7
nm/Amin, which is obtained at the pressure of 0.2
Torr.
In order to discuss the etching mechanism, we
have also carried out the experiments using LiF
window. LiF window pass through the photons with123