24P127-P
Damage of a Hot Filament Discharge Cathode Operated with High Frequency
Alternating Current
高周波電流による熱陰極放電フィラメントの損耗
Yuki Takahashi, Naoki Miyamoto, Toshirou Kasuya, Motoi Wada
高橋侑輝, 宮本直樹, 粕谷俊郎, 和田元
Graduate School of Engineering, Doshisha University,
1-3 Tatara Miyakodani, Kyotanabe-shi, Kyoto 610-0321,Japan
同志社大学大学院工学研究科 〒610-0321 京田辺市多々羅都谷1-3
To extend the time between maintenance services of a hot cathode arc discharge ion source, a tungsten
filament cathode has been operated with an AC heating. Operation with an AC heating can mitigate theconcentration of filament erosion on the negative terminal of the cathode operated by a DC heating current,
and the erosion rate for an AC heating was reduced to 70 % of that for a DC heating. Both the discharge
current and the ion saturation current in plasma exhibited fluctuations. The discharge current fluctuatedwith the amplitude of 21 % of the average, and the fluctuation amplitude of the ion saturation current was
about 7 % of the average.

1. Introduction
In the field of semiconductor manufacturing,the ion source plasma operated with hot cathodes
has an advantage of power efficiency and stability
over other producing ions and their beams.
However, hot cathodes have finite lifetimes due
to evaporation and sputtering of cathode materials
immersed in plasma. To extend the lifetime of acathode, mechanisms causing the accelerated
erosion of cathodes were investigated by severalauthors. [1]The negative potential end of the filament isheated up more than the positive potential end,
and the erosion of hot cathode concentrates at thenegative potential end in the case of DC heatingof the cathode [2]. This localized erosion makes
the cathode lifetime shorter. An induction ofalternating heating current can separate one spotof largest erosion for DC heating current into two
spots at both ends of the filament to elongate thefilament lifetime. However the fluctuation ofplasma arises due to the change in position of thepoint which emits the thermal electron from thehot cathode. The stability of plasma and thelifetime of a cathode operated by alternating
heating current have been investigated bychanging the AC frequency.
2. Experimental setup
An ion source whish has been developed for amedium current ion implanter for semiconductor
manufacturing process [3] has been tested. The
schematic diagram of the ion source chamber isshown in Fig.1. A cylindrical vacuum chamber
attached to the source is 200 mm inner diameter,and 400 mm length.
The molybdenum arc
chamber produces and confines plasma in a volumeof 90 mm height, 36 mm width, and 29 mm depth.
Gas inlet

Heating current
conductors

Probe10mm

Hall piece
Arc chamber
Insulator

Electromagnets

Fig.1. Schematic diagram of ion source chamber.

To reduce inductance for supplying power athigher AC frequency, a low inductance bus bar linewith the cross-section area of each conductor aslarge as 8 x 2 mm2 has been employed. The lengthof each conductor is 265 mm and the spacing
between them is 1 mm. The test tungsten filament
was 0.4 mm diameter. The total length of thetungsten was 60 mm, but only the top 30 mm has
been exposed to the plasma. The radius ofcurvature at the end has been set to 1.5 mm.
Discharge gas is introduced into the arc chamber
through a hole opened on the sidewall of arcchamber.
An external magnetic field in thedirection parallel to the filament is produced by apair of electromagnets set on both sides of thevacuum chamber. Magnetic field intensity at thecenter of the ion source can be independently
controlled to minimize the warpage of produced
magnetic field lines of force.
3. Result3.1 Oscillation of discharge current
To investigate fluctuation of plasma caused byoperation of hot cathode with AC heating power,
we observed the discharge current with a highspeed data recorder (KEYENCE GR-7000) coupled
to a current probe (IWATSU SS-250) by keeping
the filament heating power constant at 100 W. A
typical waveform of discharge current of Ar plasma
excited by 15 Hz AC is shown in Fig.2. The
frequency of discharge current is twice thefrequency of filament heating current and thefluctuation amplitude is as large as 21 % for theaverage value of the discharge current.
Discharge voltage : 80 V, Filament power : 100 W
Magnetic flux density : 4 mT, Ar gas pressure : 6 x 10 -2 PaFig.2. Waveforms of discharge current and filament
voltage.
Discharge voltage 80 V, Filament power : 100 W
Magnetic flux density : 4 mT, Ar gas pressure : 6 x 10 -2 Pa

Fig.3. Oscilloscope waveforms of the ion saturation
current and filament voltage.

3.2 Ion saturation current
Fluctuation in ion saturation current of plasma wasmeasured with a Langmuir probe. The tungsten
probe is 0.3 mm diameter, 2.0 mm length andplaced at the position 30 mm from the center of thearc chamber, which is about 10 mm away from theion extraction hole. Figure 3 shows a typical
waveform of the ion saturation current operatedwith 15 Hz AC current. The ion saturation current
oscillated with the amplitude 7 % of average.
3.3 Distribution of erosion on a filament
Figure 4 shows the distribution of erosion along afilament after driving with an AC and DC power
supply for 1.5 hours, the source was operated with 1A discharge current. For 0.4 mm diameter tungsten
filament of 60 mm length, about 14 A heating
current was necessary to produce 1 A discharge
current. By operating the filament with DC power
supply, the erosion point has concentrated at thenegative side of the filament. The AC operation
seems to reduce and homogenize erosion of thefilament.
Discharge voltage : 80 V, Filament power : 100 W
Magnetic flux density : 6 mT, Ar gas pressure : 6 x 10-2 Pa

Fig.4. Erosion distributions of filaments.

4. Summary
A hot filament cathode was operated with AC
heating current. Both the discharge current andthe ion saturation current have shown fluctuations
when AC current has heated the cathode. The AC
operation has shown the filament erosion can behomogenized compared to the case of DC operation,with which the filament has exhibited enhanced
erosion at the side of the filament connected to thenegative terminal of the heater power supply.
References[1] T. N. Horsky, Rev. Sci. Instrum, 69, 4, 1688, (1998).
[2] K. W. Ehlers and K. N. Leung: Rev. Sci. Instrum. 50
3, (1979), 356.
[3] N. Miyamoto, et al. : Rev. Sci. Instrum. 73 2, (2002),
818.