Jpn. J. Appl. Phys.

BRIEF NOTE

Modification of film structure by plasma potential control using triode high power
pulsed magnetron sputtering
Takeo Nakano ∗ , Takuya Umahashi, and Shigeru Baba
Department of Materials and Life Science, Seikei University, Musashino, Tokyo 180-8633, Japan

We have designed a new triode configuration in a magnetron sputtering apparatus to control the plasma
potential of the discharge. An additional chimney electrode was introduced above the conventional sputter
gun to apply a positive voltage. The discharge power was provided by a pulse power source to achieve
high power pulsed magnetron sputtering operation. We confirmed that the plasma potential increased withincreasing positive electrode voltage. Copper films with substantially flatter surfaces could be obtained ona water-cooled and electrically grounded substrate at an Ar gas pressure of 5 Pa.

High-power pulsed (or “impulse”) magnetron sputtering (HPPMS/HiPIMS) is an ionized physicalvapor deposition (IPVD) technique that has recently attracted significant attention.1–4) This technique
is based on the application of target power as pulses with low repetition frequency and very small
duty ratio. By this technique, it is possible to generate the high-density plasma required for IPVD
while avoiding heavy thermal load to the target. The HPPMS technique was initially proposed byKouznetsov et al.5) in 1999. By using this technique, they obtained a good trench-filling performance
without voids, which was ascribed to the high ionization fraction of sputtered particles. Alami andcoworkers have also shown that HPPMS is eﬀective in obtaining densified structures of Ta6) and CrN7)
films.
It is well known that the structure of sputtered films depends on the preparation conditions. A
widely accepted zone diagram by Thornton8) includes substrate temperature and sputtering gas pressure as parameters. More recently, Anders9) has proposed another structure zone diagram that consistsof a “generalized energy”, brought by the impinging particles on growing surfaces, instead of the gaspressure. According to these diagrams, a structure with voided defects and a rough surface (Zone I
structure) grows at a low substrate temperature and a low kinetic energy (high gas pressure). Whena denser film structure with a smoother film surface (Zone T structure) is required, a high substratetemperature and/or a high particle kinetic energy are necessary. When a low substrate temperature isstrongly desirable (e.g., deposition on plastic substrates), the particle energy should be high enough.
∗E-mail: nakano@st.seikei.ac.jp

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BRIEF NOTEThis can be achieved if appropriate acceleration potential is introduced in conjunction with the highionization rate of IPVD/HPPMS.
In our previous study,10) we have attempted to modify the target voltage during the pulse-oﬀ period(Vb ) in HPPMS. When films were deposited on water-cooled and electrically grounded substrates, ahigher Vb (+50 to +100 V) resulted in more flattened film surfaces and compact cross-section SEM
images. With the plasma diagnostic that made use of a Langmuir probe, it was observed that the plasma
potential became higher when the negative target voltage was switched to positive at the end of thepulse-on period. It was considered that the space potential of the afterglow plasma increased so thatit could shield the plasma from the positive target voltage, and the ionized species within the plasma
impinged onto the substrate with higher kinetic energy, reflecting the potential diﬀerence between theplasma and the grounded substrate. However, this method has a drawback in that the plasma potential
during the pulse-on period is not high. Therefore, the advantage of the high ionization rate of HPPMS
is not fully harnessed.
In this study, we have attempted to introduce an additional electrode in the sputtering apparatus
in order to configure a triode system. The additional electrode was positively biased to increase theplasma potential for both on and oﬀ periods of the HPPMS operation to promote further energetic impingement of ionized species. To understand the function of this additional electrode, a DC discharge
experiment was performed as well. By applying diﬀerent electrode potentials, Cu films were preparedby the DC and HPPMS discharges. We also conducted probe measurements to confirm whether theplasma potential increased following the introduction of the additional electrode, for both pulse-on
and pulse-oﬀ periods. Usually, a negative bias voltage is applied to the substrate to generate a potential diﬀerence between the plasma and the substrate. However, if the positive plasma potential isachieved, the substrate can be grounded. We believe that doing so increases the design flexibility ofthe sputtering system.
The experiment was conducted with the same sputtering chamber as that used in our previous
study;10) the chamber was a cylinder of 21 cm diameter and 25 cm height. The chamber was evacuatedby a turbo molecular pump to a pressure of 4 × 10−5 Pa. The sputter gun was located along thesymmetric axis of the chamber. A copper disk target (purity of 4N) with a diameter of 5 cm was used.
An additional chimney electrode, 1.5 cm high, was attached on top of the anode cap of the sputter gun,as shown in Fig. 1. For the deposition experiments, Si substrates were set on a water-cooled substrateholder. For plasma diagnostics, the Langmuir probe (Scientific Systems SmartProbe) was introduced,
replacing the substrate holder. After the evacuation, 20 sccm of Ar gas was introduced and the pressure
was set to 5 Pa by throttling the evacuation valve. A DC source (Advanced Energy MDX-1.5k) anda custom-built pulse power source (Heiwa Dengen)11) were used as discharge power sources. In this2/8Jpn. J. Appl. Phys.

BRIEF NOTE

Coolingwater

Si substrates

50mm

Chimney
ElectrodeIC

Cu targetA

Anode Cap

Fig. 1.

VC

Experimental setup that includes the chimney electrode above the sputter gun. Left: before andafter the introduction of the chimney electrode. Right: cross section of the electrode configuration.

experiment, the pulse power source was driven with a repetition frequency and a duty ratio of 200
Hz and 5%, respectively. The pulse-oﬀ target voltage was 0 V. During the pulse power discharge, thewaveforms of target current and voltage were monitored and recorded using an oscilloscope (Agilent
DSO 3062A). The discharge sources were driven in a constant current mode. Initially, the target powersource was set to apply the (time-averaged) power of 100 W with VC = 0, and later the voltage VC
was increased up to +40 V. By doing so, the power that was applied to the discharge volume (fromthe target source and the VC source) was nearly the same under all conditions. With the DC and
HPPMS discharges, copper films were deposited on HF-treated low-resistivity Si(111) substrates. The
substrate holder was electrically grounded and water-cooled to maintain it at room temperature. Afterthe deposition, the films were observed using a secondary electron microscope (SEM; JEOL JSM6510). In another experiment, in which the Langmuir probe was used, current-voltage characteristics
were obtained and analyzed using the method described by Chen,12) and plasma parameters wereobtained. When applied to HPPMS, the Langmuir probe was operated in a boxcar mode for timeresolved measurements triggered by a timing signal from the pulse power source.
Plan-view SEM images of the Cu films deposited on Si substrates are shown in Fig. 2. The scale

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DC

HPPMS
(200 Hz, 5%)

1μm

Vc = 0 V
Fig. 2.

Vc = 20 V

Vc = 40 V

Plan-view SEM images of deposited Cu films with different discharge sources (DC/HPPMS) anddifferent chimney electrode voltages (VC ) at an Ar gas pressure of 5 Pa. In HPPMS deposition, the repetition
frequency and duty ratio were 200 Hz and 5%, respectively. The target power was applied in a constantcurrent mode to achieve 100 W (see text for the detailed procedure). The scale bar at the right bottom
corner of each figure represents 1 µm.

bar at the right bottom corner of each figure represents 1 µm. These samples were deposited for 18 min.
The deposition rate was measured prior to the actual deposition using a quartz crystal microbalancemonitor and found to be 2.0 nm/s, independent of VC and of whether the power was DC or pulse.
Therefore, the thickness of these films was expected to be approximately 2 µm. The target voltage in
DC sputtering was −390 V, and the pulse-on voltage in HPPMS was −550 V (both are at VC = 0).
Cu films that were deposited by a DC discharge at VC = 0 and 20 V appeared to have very roughsurfaces with large grains and voids, which reflected the columnar structure of Zone I. This structure is
specific to high gas pressures (low-energy particles) and low substrate temperatures. At VC = 40 V fora DC discharge, the voids were rather subtle and the film roughness was slightly reduced. However,the films still appeared to contain grains that were larger than 100 nm. On the other hand, in the caseof HPPMS, the film surface was already much smoother at VC = 0 than in the DC cases. The size ofa typical grain was reduced to 30–50 nanometers. We consider that this occurred because the HPPMS
yielded a higher ionization fraction, retaining a kinetic energy as high as 5–7 electron volts provided
by a plasma sheath. By increasing VC to 20 and 40 V, grains became finer and the film surface wassignificantly flattened.
Figure 3 shows the results of the Langmuir probe measurement. In this measurement, the substrateholder was removed and the probe was introduced along the symmetric axis. The end of the probe tip

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BRIEF NOTE(a) 40(b) 1019

30

3
-

)V
(p 20
V

Fig. 3.

VC= 0V

20V
40V

18

ne

pulse on

10

0
)A0.2
(C0.1
I
00

)
m
(10

1017
40
)V30
(p 20
V10
0
0 100 200 300 400 500

10 20 30 40 50
VC (V)

Time (µs )(a) Dependence of plasma potential V p and chimney electrode current IC on the chimney elec-

trode voltage VC in DC discharge. (b) Time resolved plasma parameters (electron density ne and plasma
potential V p ) in HPPMS discharge with VC as a parameter.

was located 50 mm away from the target surface. The dependence of the plasma potential V p on VC
in DC discharge is shown in Fig. 3(a) along with the dependence of the chimney electrode current IC
on VC . It can be seen that IC increases gradually and saturates at VC = 20 V. Before the saturation of
IC , the increase in V p was rather modest, while past the saturation point it increased with a slope ofunity. This suggests that the chimney electrode becomes the anode of the discharge at VC > 20 V. The
electron density was (3 − 4) × 1016 m−3 and was almost independent of VC .
The results of the time-resolved probe measurement in HPPMS discharge are shown in Fig. 3(b).
The origin of the horizontal axis denotes the ignition of the pulse voltage, which continues for 250 µs,
reflecting the pulse frequency of 200 Hz and the duty ratio of 5%. The electron density ne was mostly
independent of VC ; it increased initially and peaked at a density as high as 3×1018 m−3 at approximately
100 µs, and then decreased. The occurence of a peak in ne corresponded to that in target current,
which is commonly observed in HPPMS with modest pulse power.3, 10) On the other hand, the plasma
potential V p showed a diﬀerent dependence on VC . It increased as high as VC in both pulse-on andpulse-oﬀ periods as expected. It increased during the initial stage of the pulse-on period and becamealmost stable after 100 µs, at which point the electron density became maximal. The smooth surface of
the Cu films that was obtained using the HPPMS with a positive VC is ascribed to the positive plasma
potential and eﬀective bombardment of positive ions on the grounded substrate surface.

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The mechanism by which the plasma potential increases up to the most positive electrode potential is understood to be the dragging of electrons by the positive electrode that causes the plasma to“charge up”, resulting in a positive space potential.13) In this viewpoint, the slight increase in V p at
the end of the pulse [Fig. 3(b)] can be ascribed to the termination of the supply of secondary electrons
from the target. The phenomenon can also be explained by the plasma-wall interaction.14) When thewall voltage Vw is much lower than the floating potential V f , the current is mainly delivered by ionsand is not sensitive to Vw . On the other hand, when Vw > V f , the electron current dominates and it
exponentially depends on the diﬀerence V p − Vw of potentials. Therefore, in order for the electrodes
to satisfy the current continuity conditions, the floating potential and hence the plasma potential must
approach the voltage of the most positive electrode unless a very large diﬀerence between electrodeareas exists.
The triode sputtering concept was proposed during the very early years of the sputtering method(see Chopra’s textbook15) for examples). However, in those systems, the third electrode was athermionic electron source that helped the low-gas-pressure discharge. More recently, a triode sputtering system for discharge plasma control has been proposed by Fontana and Muzart,16) which was
fabricated by inserting a grid between the target and the substrate; however, this system does not directly aim to control the plasma potential. An increase in the plasma potential achieved by introducing
an additional grid electrode was reported by Jung et al., but the eﬀect was rather limited because
it utilized a DC sputtering apparatus.17) The plasma potential control using the triode configurationand the high ionization rate of HPPMS were crucial for the eﬀective bombardment and film structure
modification.
Note that the V p in this experiment was considerably lower than that used in our previous study.10)
In our previous experiment, the plasma potential was raised by icontrolling the target voltage only
after the high power discharge was cut oﬀ. Hence, a pulse-oﬀ period target voltage of +100 V wasnecessary to obtain a smooth surface such as that obtained by HPPMS and a VC of +20 V in Fig. 2. In
the present experiment, therefore, it is considered that a large part of ion bombardment occurred during
the pulse-on period and that the energy deposition on the growing surface by accelerated ions became
more eﬀective following the addition of the chimney electrode. Another approach to preparing a Zone
T structure at low substrate temperatures is to reduce the discharge gas pressure so as to prevent thedeceleration of the initial energy of sputtered particles by collisions with ambient gases.8) However, thereduction in gas pressure promotes negative side eﬀects, e.g. super high-energy reflected neutrals.18)
At least in some applications, it is desirable to stop those high-energy particles while bombarding thegrowing film surface at controlled energy.
In summary, we have developed a triode HPPMS system and studied the eﬀect of an additional

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electrode on the plasma and growing film structure. In accordance with the increase in the additional
electrode voltage, the plasma potential increased and a flatter surface structure was obtained. Thissuggests a more eﬀective impingement of ionized species onto the growing film surface.
Acknowledgement

This work was supported by JSPS KAKENHI Grant Number 24560063.

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