The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

THE POSSIBILITY OF THE ION BEAM MACHINING OF THE HIGHTEMPERATURE SUPERCONDUCTOR Bi2223
S. Watanabe, Y. Ohshiro, and T. Katayama, CNS, University of Tokyo, JapanT. Watanabe and T. Ikeda, RIKEN, Wako Saitama, Japan
Abstract
In this paper, while the previous examination result
about minute machining of a Bi2223 thick film is
described, the details of the possibility of ion beammachining and a beam irradiation system and an actualirradiation experiment plan are presented.

INTRODUCTION
In recent years, it is progressing that development of
the equipment to which high temperature superconductivity technology has been applied in the field ofan accelerator. For example, the application to the beam
current monitor in a beam transport system [1], ion beamstorage ring, etc. is known. Although Bi2-Sr2-Ca2-Cu3-Ox(Bi-2223) etc. has been used as a high temperature
superconductor in these equipments, since the hightemperature superconductor Bi2223 coated on a substrate
is various as a basic material of superconductivity
electronics, attention is attracted. When applying thisbasic material to an electronics circuit, circuit patterningwith resolution of sub µm and multi-layered structure of
the material are demanded. The current minute patterningtechnologies such as a focused ion beam (FIB) irradiation
is applied to deposit an only mono-layered metal (exNiobium) on the substrate for making low temperature
superconductivity electronics. Although the printing
method is investigated about circuit patterning of Bi2223,
this method can be patterned only in the stage of printing
which is one stage of the creation process of asuperconductor, and minute pattern machining is madedifficult from restrictions of heat contraction in the stageof calcinations etc. For this reason, it requests fordevelopment of the machining method in which
minimizing is possible.
This paper examined how to etch the pattern required
for the thick film of Bi2223 to the MgO substrate at
uniform thickness like a copper-plated print circuit board.
In order to form arbitrary patterns, it examined firstwhether the technique of wet etching adopted by thesemiconductor manufacturing process and dry etching
would be inapplicable. Since the chemistry substance as
etchant needed to be specified about wet etching, researchwhich directed its attention to the etching rate of Bi2223
film was performed. Consequently, although the goodresult was obtained and it was promising about wetetching, it turns out that contamination of the basis
material by etchant and the load to environment are large.
So we had started the study of ion beam machining ofBi2223 in the various ion energy ranges from 1 KeV to
100 keV.

ION BEAM ETCHING
The plasma etching method and the ion beam etching
method were examined for dry etching. It decided forplasma and the ion species of an ion beam to adopt the Ar
with many papers about the reaction with the substanceetched. About the plasma method, dry etching bycommercial ion-sputter was considered. The mostpromising method is a method of setting a sample to thenegative electrode of sputter, applying the Ar plasma ofpressure 0.02 Torr, and etching the surface. Figure 1
shows calculated sputtering process of 10 keV Ar+ ions inCu.

Fig. 1. Calculated sputtering process of 10 keV Ar+ ionsin Cu. [From F. Fujimoto, K. Komaki Ion BeamTechnology, (1995) 240]
It planned choosing the beam from the Hi ECR ion
source installed in the CNS about etching the sample by
the Ar ion beam, setting a sample to the focal plane ofbeam focusing equipment, and performing an irradiation
experiment. Ion current density enabled it, as for theirradiation size of a beam, for this plan to realize 100µA/cm2 by 10mm diameter. Moreover, the degree of vacuumof the vacuum chamber for irradiation was designed so
The introductorythat 10-8 Torr might be obtained.
mechanism of reactive gas is also examined so that ion
beam etching may be performed early, and the kind ofreactive gas was also examined to apply a reactive ionbeam etching (RIBE). Since an irradiation sample was anoxide, it was examined also about the difference in eachetching rate of a composition element (Bi, Sr, Ca, Cu, O).
Although multi-charge ion is taken out, decelerationelectrode is installed in the irradiation system, and it
enabled it to change the energy in this ion source.
Moreover, in order to perform comparison with the Ar ion
beam of single charge, the test by the commercial ion
beam milling equipment for SEM (Scanning ElectronMicroscope) sample creation is planned.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

ION SOURCE
We considered the Hi ECR ion source for ion beammachining. The Hi ECR has been constructed at CNS tostudy the optimum parameters for producing multicharged ions comparable to 14 GHz operations by using
10 GHz microwave source. The effort on experimental
study has been concentrated to how support gas,microwave frequency and the mirror coils field contributeto the production of multi-charged ions in ECR zone.

beam slit and Faraday-cup 1 (FC1) are put on an image
focal point of the BM. Ion species with selection of thecharge to mass ratio has been carried out with the slit andFC1. A wire monitor [3] can measure the beam profiles
in x-y directions. A screen monitor is also available toobserve the beam profile using a TV camera.
DC monitor,
RIBE & FC2

Table 1. Parameters of Hi ECR.
Mirror field
Side of gas-feed
Side of extraction
Hexapole magnet
Field on plasma chamber
Inner diameter
Mechanical length
Microwave power source
Frequency
Input power
Plasma chamber
Inner diameter
Ion species (typ)

8 kG
7 kG

Wire Monitor

Slit & FC1

10.4 kG
76 mm
145 mm10 GHz
88 W

BMQTM

70 mm
Arq+ q ≤ 9

The Hi ECR comprises nine solenoid coils (Mirrorcoils), a hexapole magnet (Halbach-type), a highfrequency RF feeder and a plasma chamber. To produce
multi-charged ions, we need high electron density.
Therefore electrons are confined by magnetic field
generated by the mirror coils. The parameters of Hi ECR
are tabulated in Table 1. In order to analyse charge-massratio e/m of ion beam, an analysing magnet with bending
angle of 90-degree and a Faraday cup were set
downstream of the Hi ECR. Producing ions with Ar gas
only carried out the beam experiment of Hi ECR operated
at 10 GHz. During the operation, the mass flow rate of Ar
gas was adjusted to maintain the pressure to 2 × 10-6 Torr.

Fig. 2. Beam envelopes of the Hi ECR ion source beamtransport line. The beam is guided from the left to theright. The RIBE instruments are placed at the end point ofbeam line.
Table 2. Beam diagnostic instrumentsSlit and Faraday cup 1
Cu, Slit =10 mm max.
Tungsten wire monitor
100 µm, Au coating
Screen monitor
71x50 mm2, 95%Al2O3+Cr
DC beam current monitor HTS-SQUID sensor type
Faraday cup 2
SUS304, 100 φ L=0.5 m

BEAM TRANSPORT LINE
We have constructed a beam transport line for a Hi
ECR ion source with a 10 GHz power source. The beamtransport line comprises an analyser magnet (BM) as
mentioned above and QFQDQF magnets (QTM). The
specifications of all the magnets are tabulated in thepublished paper [2]. Table 2 shows the beam diagnostics
instruments. There are three sets of vacuum pumping
system as shown in Table 3. We have designed the beam
optics associate with making a parallel beam at the end ofbeam line. The lattice structure and result of calculation
are shown in Fig. 2. A DC beam current monitor isinstalled in the end of beam line. The assumed beamemittances are the same to 200 πmmmrad in the verticaland the horizontal planes. In calculation of beamenvelopes, full-width of beam momentum spread iscalculated as 0%.
It is easy to make a focusing point at the exit of BM,
which is followed by the beam diagnostic devices. ATable 3. Vacuum pumping system
Hi ECR IS chamber
500 L/s TMP+RPAnalyzer magnet chamber 300 L/s TMP+RP
Beam monitor chamber
250 L/s TMP+RP
In order to confirm the beam optics we made ameasurement of Ar3+ beam at an extraction voltage of 10kV. The slit width is chosen to 10 mm. The measured
beam widths (FWHM) are 9.5 mm in horizontal and 17mm in vertical, respectively. The beam current of 1.8eµA was measured with DC beam current monitor andwas confirmed with the Faraday-cup 2 (FC2). We
confirmed that analysing power of the BM and beam
optics of the whole system is a good for testing the Hi
ECR ion source.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

REACTION CHAMBER
A reaction chamber was considered in order to irradiate
the beam on the high temperature superconductor Bi2223.
This equipment consists of a beam guide, gate valve,
deceleration electrode, deflection electrode, vacuum
pump, sample holder, Faraday cup, etc. In order to act as
the monitor of the intensity of irradiation current on-line,
the DC-SQUID monitor is installed in front of thereaction chamber. The reactive-gas spraying equipment
and the introductory equipment for promoting the etchingrate on the surface of the sample in the reaction chamber
again attach gas recovery instrument outside. A test standof the reaction chamber is shown in Fig.3. The planed
reaction chamber with designed vacuum pressure of 10-8
Torr is attained by the 500 L/s TMP system.

examples, PMM electron-resist provides etching rate of840 Å/min and AZ1350 photo-resist shows etching rate of600 Å/min as shown in Table 4. The advanced ion-beamresist such as PMMA (Poly (methyl methacrylate)) could
be applicable in the ion beam lithography as shown in Fig.
4 [7] in order to obtain large aspect ratio.
We will try to apply these resists to make a goodsolution for minute ion beam machining of the hightemperature super conducting materials *.
Table 4. Etching rates (Ar ions at normal incident angle)
TargetÅ/min Target
Å/min
Silicon360
Aluminum
444
GaAs
2600
Zirconium
320
SiO2420
Niobium300
Fe oxide660
Iron320
Gold1600
Chromium200Silver2000
Aluminum130
PMM e-resist 840
AZ1350 photo resist 660

Fig. 3. Test stand of the reaction chamber.
XeF2 as a neutral gas and a toxic gas Cl2 were alreadyexamined elsewhere about reaction gas. In Si, only in thecase of an Ar+ ion beam, XeF2 has an etching rate
remarkably early at a part for 70 Å/min compared with apart for 8 Å/min in the case of only Ar+ ion beam [4].
Although Cl2 is toxic gas, it resembles markedly a part forincrease etching yield with increase of potential energy.
The plan was carried out using 10 kV Arq+ ions atpressure of ~1x10-5 Torr of Cl2, since enforcement
improves the etching yield of Gallium-Arsenide (GaAs) at
high potential energy (q ≥ 7) [5]. We will follow thoseresults in order to etch the thick film of Bi2223. For
comparison, the typical examples of etching rate for Ar
ions with normal incident angle are tabulated in Table 4[6]. These etching rates are obtained at operating
conditions of ion energy 1.0 keV, current density of 1.0
mA/cm2, and pressure of target chamber is 5 x 10 –5 Torr.
The reaction rate of a mask for preventing the ion beam
bombardment plays a role to make minutes of pattern ofBi2223. The ratio of etching rates of the mask andBi2223 is to be considered from 5 to 10 because of large
aspect ratio. The mask for the reactive ion beam etching
is being developed for various ion beam lithography. ForFig. 4. Trajectories of 60 keV H+ ions in PMMA on
substrates of gold and silicon calculated by PIBER
program. [From L. Karapiperis, et al. J. Vac. Sci. Tecnol.
19, (1981) 1260]

REFERENCES[1] T.Watanabe et al., CNS-REP-59 (2002) 71.
[2] S. Watanabe et al.,”CNS-REP-59 (2002) 58.
[3] S. Watanabe, CNS-REP-36 (2000) 58-59.
[4] J.W.Coburn and H.F.Winters, J. Appl. Phys. 50,(1979) 3189.
[5] T.Meguro and Y.Aoyagi, RIKEN Review 31, (Nov.
2000) 48-51.
[6] R. E. Lee , VLSI Electronics, Vol. 8 (1984) 365-409.
[7] Benjamin M. Siegel, VLSI Electronics, Vol. 16(1987)
147-227.
*This research was partially supported by the Ministry of Education,
Science, Sports and Culture, Grant-in-Aid for Exploratory Research,
15654031, 2003