Download Evaporation–glow discharge hybrid source for plasma immersion

Surface & Coatings Technology 186 (2004) 165 – 169
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Evaporation–glow discharge hybrid source for plasma
immersion ion implantation
L.H. Li
a,b,c
, Ricky K.Y. Fu b, R.W.Y. Poon b, S.C.H. Kwok b, P.K. Chu b,*, Y.Q. Wu c,
Y.H. Zhang c, X. Cai a, Q.L. Chen a, M. Xu a
a
School of Material Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China
Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China
c
Department of Materials Processing and Controlling (702), School of Mechanical Engineering, Beijing University of Aeronautics and Astronautics,
Beijing 100083, China
b
Available online 25 May 2004
Abstract
The plasma ion sources play a very important role in the plasma immersion ion implantation (PIII) process. In this paper, we report on our
newly designed evaporation – glow discharge hybrid ion source for PIII. The high negative substrate bias not only acts as the plasma producer
but also provides the implantation voltage. The sulfur vapor gas glow discharge shows that the electrons in the plasma are focused to the
orifice of the inlet tube, thereby helping the ionization of the fleeing vapor gases. The sulfur depth profile confirms that this evaporation –
glow discharge hybrid source is effective for materials with a low melting point and high vapor pressure.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Plasma immersion ion implantation; Sulfur; Glow discharge
1. Introduction
Plasma immersion ion implantation (PIII) is a useful
niche technology for the modification of surface properties
of materials and industrial components that are large or
have irregular shapes. In the PIII technique, the substrate is
usually immersed in the plasma and then biased by the
high negative-pulsed voltage. The electrons are immediately repelled and the sheath dynamically expanded during
the pulse. Ions are accelerated through the sheath and
implanted into the surface of the substrate. PIII eliminates
the need of beam rastering because the entire target is
immersed in the plasma [1,2]. Hence, the plasma source
usually plays a very important role in PIII. Many kinds of
plasma production methods, such as radio frequency (RF)
microwave, hot filament and vacuum arc are used as ion
sources in the PIII device [3– 9]. However, up to now,
there is no single method that can satisfactorily produce
ions of a wide spectrum of elements, let alone all the
elements in the periodic table. For example, it is still
* Corresponding author. Tel.: +852-2788-7724; fax: +852-2788-9549.
E-mail address: [email protected] (P.K. Chu).
0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2004.04.025
difficult to ionize solid materials with poor electrical
conductivity, such as sulfur, phosphorus and boron; semiconductor materials such as silicon, germanium; and elements with low melt point and/or high chemical activity
like the group IA and IIA elements.
In this article, we report an evaporation – glow discharge
hybrid source for plasma immersion ion implantation. It is
designed for elements with a low melting point and high
vapor pressure such as sulfur and phosphorus that are very
important elements in the surface modification of biomaterials. In this method, the material is initially vaporized in a
source chamber and then introduced via a 6-mm-diameter
gas inlet tube into a small implantation chamber with an
internal glass shield to reduce contamination. The source
chamber has a large surface area relative to that of the
orifice (inner diameter of the gas inlet tube) through which
the evaporated species escapes. This provides quasi-equilibrium and yields a steady and easily controllable implantation process. The high negative pulses exerted on the
substrate play dual roles on creating plasma and implantation. The plasma is composed of a neutral evaporated gas
introduced into the glass-surrounded chamber. The neutral
evaporated gas is ionized such that a constant source of
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L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169
plasma is provided to surround the sample during the
implantation process.
the implantation chamber was kept at 0.032 –0.01 Pa and
0.014– 0.01 Pa.
2. Experimental details
3. Results and discussion
The plasma immersion ion implanter is schematically
illustrated in Fig. 1. In addition to the evaporation –glow
discharge plasma source, the PIII equipment has some other
plasma producing systems—the pulse arc plasma ion
source, the RF plasma source, and the hot filament system,
which are not shown here. The evaporation – glow discharge
hybrid plasma ion source comprises a sulfur evaporation
source container, argon feed tube and valve, gas inlet tube,
and the implantation chamber. The substrate holder is placed
in the implantation chamber with a diameter of 150 mm and
height of 240 mm. The distance between the vapor outlet
and substrate is 220 mm. A 100-mm silicon wafer was used
as the substrate and placed on the substrate holder. Sulfur
was vaporized in a quartz evaporation container and fed into
the implantation chamber through a grounded gas inlet tube
with an inner diameter of 6 mm. The sulfur vapor was
introduced by two means: (1) Sulfur vapor was used as the
source and fed into the implantation chamber directly. (2)
The sulfur vapor was first mixed with argon, the carrier gas,
and then fed into the implantation chamber. The sulfur
vapor and sulfur + argon vapor were fed in separately, and
high negative pulses of 10 kV for 1 h or 15 kV for 1 h, and
25 kV for 2 h were applied to the substrate. The pressure in
Fig. 2 is a picture depicting the glow discharge of the
hybrid plasma source. Some of the typical characters of
glow discharge are noticeable. The brightest part of the
discharge is different from the normal parallel plane electrode glow discharge where the brightest part is usually the
negative glow. The electrodes used here can be considered
as a tube anode and a plate cathode. The brightest part in
this glow discharge is the anode glow where the electrons
are obviously focused. Under this field, the electrons highly
interact with the outlet vapor gas atoms and assist in the
vapor atom ionization. The tube anode can be considered as
a hollow anode. The second brightest part is the negative
glow, which is separated from the cathode by the cathode
dark space. The negative glow and the positive column are
separated by the Faraday dark space. A focused light at the
cathode can also been observed. The diameter of the
negative glow is obviously less than that of the Si substrate,
and this is because the conductance of the Si substrate is
much less than that of the copper substrate holder whose
diameter is less than that of the Si substrate. It can be
concluded that to achieve uniform implantation, the diameter of the substrate holder should not be less than the
substrate. Because the electrons in the plasma are focused to
Fig. 1. Schematic of the plasma immersion ion implantation equipment with the evaporation – glow discharge hybrid source (the ball shape feature near the
capacitive RF antenna is the exit of the third pulsed arc plasma source).
L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169
Fig. 2. Photograph showing the glow discharge of the sulfur vapor.
the tube outlet orifice where the evaporated atoms flee out,
we believe that this will help the ionization of the gasses.
This is being further studied by using computer simulation
in our laboratory.
Fig. 3a,b displays the implantation current and voltage
waveforms at 25 and 15 kV, respectively. The current
167
waveforms exhibit two spikes at the beginning and at the
end of the pulse. These spikes are the sums of the true
implanted ion current, secondary electron current, as well
as system capacitance. To subtract the system capacitance,
Fig. 3c,d shows the spike current waveforms attributable
to the system capacitance without plasma production at
25 and 15 kV, respectively. From Fig. 3a,b, it can be
observed that the implantation current does not show a
continuously decreasing trend, which is typical of the
expansion of the plasma sheath at low pressure [10,11]. It
is also different from high-pressure, high-voltage implantation in which the current waveform usually exhibits an
obvious breakdown current increase [12]. The implantation current increases obviously only at the beginning.
Once the glow discharge is established, the current
increase slowly. This is true for at least in the given
pulse duration and the current increase may not reach arc
breakdown. As a result, this system is safe and steady
with control of the sulfur evaporation rate. A further
discussion can be found elsewhere [13].
The sulfur depth profiles acquired by sputtering X-ray
photoelectron spectroscopy are shown in Fig. 4. The
simulation results by TRIM are also given. The sulfur
depth distribution for 10 kV implantation reveals a
roughly Gaussian distribution. The peak position is at
15 nm and consistent with the TRIM simulation. The
width of the S distribution (Gaussian peak) profile is less
Fig. 3. Glow discharge and spike current and voltage waveforms. (The high voltage was operated at a frequency of 100 Hz with a pulse duration of 250 As,
corresponding to a duty factor of 2.5%.): (a) 25 kV bias, (b)15 kV, (c) spike for 25 kV, and (d) spike for 15 kV.
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L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169
Fig. 4. Sulfur depth profiles acquired from (a) 15 + 25 kV with sulfur + argon vapor and (b) 10 kV with sulfur vapor and the simulation results using the TRIM.
than that of the simulated one. This may be due to the
measurement error by XPS and other factors. However,
by considering that the width of the 15 + 25 kV implantation sample is also less than that of the simulation one,
we believe that the sputtering depth per minute is underestimated. The outline shape of the sulfur depth distribution implanted for 15 + 25 kV biases seems also to be like
the TRIM calculated one without considering the peak
position. The peak position of the XPS result is nearer to
the surface than that of the calculated one. The reason is
that during the TRIM simulation, Ar is neglected. During
the 15 + 25 kV process, Ar was used as the carrying gas.
Because Ar has a higher sputtering yield, surface etching
of sample B should be more serious, thereby resulting in
an apparently shallower distribution. Anyway, from Fig.
4, it can be concluded that the evaporation – glow discharge hybrid source is suitable for plasma immersion ion
implantation.
atoms implanted by this method exhibit a typical Gaussian
distribution. The XPS depth profile results show that the
evaporation –glow discharge hybrid source is an effective
method for elements possessing a low melting point and
high vapor pressure. If Ar is mixed in the feeding vapor
gases, it can be ionized at the same time and will etch the
surface of the substrate seriously.
4. Conclusion
References
The high negative voltage exerted on the substrate
plays two roles in PIII as the plasma producer as well
as implantation. The grounded gas inlet tube is the anode
for the glow discharge and the electrons in the plasma are
focused to the end of the inlet tube where the vapor gas
escapes. This helps the ionization of the vapor gases and
the anode glow becomes the brightest discharge area. The
current also reveals spikes at the beginning and end of the
negative pulse. The slight increase in the implantation
current shows that arc breakdown can be avoided. The S
Acknowledgements
This work was jointly supported by the National Natural
Science Foundation of China, No. 50271004, Hong Kong
Research Grants Council (RGC) Competitive Earmarked
Research Grant (CERG) #CityU1137/03E, Germany/Hong
Kong RGC Joint Research Scheme #G_HK001/02 (CityU
designation 9050165) and RGC/NSFC Joint Scheme
N_CityU101/03.
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