24P129-P
Diagnostics of low Energy Ion Beams Formed by Wire Extraction Electrode
タングステンメッシュ電極による低エネルギ-イオンビームの調査
Shinnosuke Tokumura1, M.Vasquez Jr1., Toshiro Kasuya1, Syuuichi Maeno2, and Motoi Wada1
徳村真ノ介1, M.Vasquez Jr.1，粕谷俊郎1，前野修一2, 和田元1
1

Graduate School of Engineering, Doshisha University
1-3 Tatara Miyakodani, Kyotanabe-shi, Kyoto 610-321, Japan
2
Novelion Systems Co.Ltd.,D-Egg
1,Jizoudani, Koudo, Kyotanabe-shi, Kyoto 610-0332, Japan1
同志社大学大学院工学研究科 〒610-321 京田辺市多々羅都谷1-3
2
ノベリオンシステムズ株式会社 〒610-0332 京田辺市興戸地蔵谷1番地
To investigate argon ion beam formed by thin tungsten wire electrodes with less than 50 eV energy, ion
energy distribution functions with respect to extraction potential from 5 V to 50 V at 0.019 Pa at constantdischarge 50 V have been measured and analyzed using a retarding potential analyzer. Beam energy spread
was typically below 2 V. At the 40 V extraction potential, the energy spread was 1.27 V, which was 2.7 %of the extraction beam energy. Ion beam profile measured from the emittance equipment with collinear
apertures was also investigated.

1. Introduction
In the field of semiconductor processes low
energy ion beam has gained considerable
attention [1]. However, transport of low energy
ion beam is difficult due to space charge effects.
One way to achieve efficient transport of low
energy ion beam less than 50 eV is let ion beam
propagate through a low density plasma. Basedon this idea, a pair of thin wire mesh electrodes
has been designed [2] and put into the operation.
High transparency of these electrodes enable
electrons in the upstream region to penetrate into
the downstream region. These neutralizing
electrons in downstream region can mitigate thespace charge effect caused by positive ion beams.
As the effectiveness of the thin mesh wire
electrodes has been already reported briefly in theprevious paper, [2] more precise transportcharacteristics of low energy argon ion beam lessthan 50 eV has been investigated.

realize with the section set parallel with 1 mm
spacing across the area of 23 mm aperture. The
spacing distance between the electrodes wasmaintained at 0.7 mm. These two W wire electrodes
separate the ion source plasma from thedownstream vacuum chamber. In normal operation,discharge voltage was maintained 50 V, while
extraction potential was changed from 5 V to 50 V.
Discharge current was 0.05 A and gas pressure was
kept constant at 0.019 Pa.

2. Experimental set up

Figure 1. Experimental set-up with the beam emittancemeasuring system.

2.1 Ion source
A 56 mm diameter and 80 mm long stainless steel
cylinder was used as the ion source chamber. Eightpermanent magnets were attached on the outersurface of the side wall. Discharge gas wasintroduced from a port attached on the side wall.
The plasma was produced by thermal electrons
from two hairpin-type shaped of φ 0.3 mm
diameter and 80 mm long tungsten(W) wires.
A 0.1 mm diameter W wire was pulled parallel ofa wire extraction geometry. It was kept in tension to2.2 Ion beam measurement set up
In order to measure ion energy distributionfunction (IEDF), a retarding potential analyzer(RPA) [2] was inserted from the bottom of vacuum
chamber as the diagnostic tool. The distance ofRPA from the ground electrode was maintained
22.5 mm. Figure1 shows the experimental systemwith the emittance measurement set-up. The firstplate was fixed at 70 mm from the ground electrode.
On the first plate eleven 1 mm diameter apertureswere opened with equal distance. Each distance
between apertures was 7 mm. The second plate,with 0.2 mm wide 20 mm long slit, was placed 20mm downstream from the first plate. The second
plate was connected to the motor drive mechanismthat moves the second plate in the direction
perpendicular to the ion beam direction. A Faraday
cup was attached behind the second plate tomeasure the ion current.

40 V, the energy spread decreased linearly against
increase in extraction potential. For the case of 40
V extraction potential, the energy spread was 1.27
V which was 2.7 % of the extraction beam energy.
The ion beam current density was 2.9 cm2 atthat time. For the lowest extraction potential of 5 V,
ion beam current density was 0.231cm2, andenergy spread was 2.12 V or 17.3% of theextraction beam energy.

3. Results

3.2 Measurement of beam broadening
Figure 4 shows the data of beam profile with theslit measurement system. For the case of 0.05 A
discharge current and 20 mm distance between firstand second plates, the measured widths of beamlets
were below 3 mm. Thus, the broadening of thebeam does not severely affect the shapes of beamlet
with low current density operation.

3.1 Ion energy distribution functions

Figure 2. IEDF obtained from –dI/dV peak for different
potentials.

Ion beam current [A]

1.5x10-8

Discharge Voltage: 50 V
Discharge Current: 0.05 AExtraction Potential: 50 VGas Pressure: 0.019 Pa

1.00.50.0103.02030405060Position [mm]2.5Figure 4. Beam profile measured from the beamemittance measuring system.

Energy Spread [V]2.01.51.00.5

Discharge Voltage: 50 V
Discharge Current: 0.05 A
Gas Pressure:0.019 Pa

0.001020304050

Extraction Potential [V]Figure 3. Energy spread of ion beam with respect toextraction potential.

Figure 2 shows –dI/dV or the IEDF obtained fromI-V trace measured the RPA for different extraction
potentials. The peak value of –dI/dV increased inaccordance with extraction potential from 5 V to 40
V. After 40 V, the peak decreased. The energy
spread of the IEDF can be also obtained from Fig. 2by taking the full width half maximum (FWHM) ofthe IEDF peak. Figure 3 shows the energy spreadwith respect to extraction potential. From 20 V to4. Summary
Peak height and the width of IEDF of low-energy
Ar ion beam extracted by W mesh wire electrodes
have been studied. Typical energy spread wasbelow 2 V. In the case of lower extraction potential,
beam divergence was larger with respect to beamenergy as seen from the energy spread measurement.
At low extraction potential, space-charge seemed toaffect beam strongly. Accordingly, spatial
broadening of the beam did not appear severe in thepresent investigation.

References[1] M. Graf, T. Prill: Semiconductor International,27(2004),4,40[2] M. Vasquez Jr., T. Kasuya, S. Maeno, N. Miyamoto,
M. Wada: AIP Conference Proceedings, 1321(2010),
468